EVALUATION OF AN ACTIVATED ALUMINA SORPTION SYSTEM

FOR REMOVAL OF FLUORIDE FROM WATER

by

UKIWO OBASI ONUOHA, B.S. in Ch.E.

A THESIS

IN

CHEMICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

IN

CHEMICAL ENGINEERING

Approved

May, 1983

ACKNOWLEDGMENTS

The author would like to express his sincere gratitude to Dr.

Richard Wm. Tock for his helpful advice and guidance during this study.

Sincere thanks are also due to Dr. Steven R. Beck and Dr. H. R.

Heichelheim, other members of the committee, for their devotion, sugges­

tions and constructive criticisms. Their contributions have been ^er)j

helpful.

I would also like to thank Sue Willis for her diligence in typing

this work.

Special thanks are due to my wi-f'e, Marianne Onuoha, for her inval­

uable steady support and patience throughout the period of the study.

n

TABLE OF CONTENTS

PAGE

ACKNOWLEDGMENTS ii

LIST OF TABLES v

LIST OF FIGURES vi

CHAPTER 1 INTRODUCTION 1

Fluoride Ecology 1

Removal Techniques 3

This Study 5

CHAPTER 2 LITERATURE REVIEW 7

Methods 7

Precipitation Methods 8

Adsorption Methods 9

Earlier Methods Used in Fluoride Removal 9

Alum Coagulation 10

Lime Softening 16

Activated Carbon 23

Zeolite 25

Mi xed Bed Demi neral i zer 26

Bone 27

Bone Char 27

Synthetic Bone Material (Tricalcium Phosphate) 28

Activated Alumina 29

Defluoridation Plant (Bartlett, Texas) 32

Current Trends in Fluoride Removal 36

Regeneration of Activated Alumina with

Aluminum Sulfate 36

Fluoride Removal Using a Fluidized Bed 44

iii

PAGE

CHAPTER 3 EXPERIMENTAL PROCEDURES 57

Procedure for Determination of Fluoride

Concentration i n Water 57

Batch Studies 58

pH 58

Concentrati on Effects 59

Effect of Temperature on Exchange

Capacity During Sorption 60

Batch Thermal Regeneration Study 61

Column Study 62

Column Regeneration Study 63

CHAPTER 4 RESULTS AND DISCUSSION 67

Calibration Curve 67

pH Effects 67

Concentration Effects 74

Isotherm Study, Temperature Effects 74

Batch Thermal Regeneration 90

Packed Column Study 96

Packed Column Regeneration 99

CHAPTER 5 CONCLUSIONS AND RECOMMENDATIONS 104

Conclusions 104

Recommendations 105

BIBLIOGRAPHY 106

APPENDIX 109

TV

LIST OF TABLES

PAGE

TABLE 2-1. Alum Flocculation With and Without Lime

TABLE 2-2. Fluoride Removal by Aluminum Sulfate (21)...,

TABLE 2-3. Fluoride Removal by Lime Softening

TABLE 2-4. Magnesium Removal in Relation to an Effluent Fluoride Concentration of 1.0 ppm

TABLE 2-5. Fluoride Removal Media

TABLE 2-6. Removal of Fluorides By Contact Filter

of Activated Alumina

TABLE 2-7. Efficiency of Activated Alumina Regeneration,

TABLE 2-8. Column Fluidization Characteristics

TABLE 2-9. Fluidized Activated Alumina Fluoride Removal Capacity

TABLE 3-1. List of Apparatus Components (Sizes) and Chemical Reagents Used

TABLE 4-1. Residual Fluoride Concentration in Solution During Contact With 0.1 Grams Activated Alumina

TABLE 4-2. Relationship Between Exchange Capacity of Activated Alumina and the Amount of the Adsorbent at Different Temperatures

TABLE 4-3. Residual Fluoride in Solution Contacted With Varying Amounts of Activated Alumina, pH = 5.0, Temp. = 24.5 + 0.5°C

TABLE 4-4. Results of the Packed Column Regeneration. Influent Fluoride Concentration = 520 ppm...

14

17

19

22

31

33

53

55

56

64

75

87

88

103

LIST OF FIGURES

PAGE

Figure 2- 1. Removal of Fluorides with Aluminum Sulfate pH = 7.2-7.4 (2) f^ = Initial Fluoride Concentration (ppm) 11

Figure 2- 2. Fluoride Removal by Alum Flocculation (Incremental Feeding of Alum During Mixing) (5) 13

Figure 2- 3. Effect of pH on Fluoride Removal (Alum dosage = 250 ppm) (5) 15

Figure 2- 4. Fluoride vs. Magnesium Removal as Predicted by Equation 2-1 (21) 20

Figure 2- 5. Schematic Layout of Defluoridation Unit at Bartlett, TX (15) 35

Figure 2- 6. Schematic Diagram of Apparatus Used in Defluoridation Studies 37

Figure 2- 7. Relation of Exchange Capacity and Regeneration Level (20) 39

Figure 2- 8. Relation of Exchange Capacity and Regeneration Efficiency (20) 40

Figure 2- 9. Exchange Capacity of Activated Alumina vs. Regeneration Time (20) 42

Figure 2-10. Exchange Capacity of Activated Alumina vs. Fluoride Ion Concentration in Influent (20) 43

Figure 2-11. Relation of Exchange Capacity and pH of Influent (20) 45

Figure 2-12. Schematic Diagram of Fluidized Reactor System for Defluoridation Studies (1) 47

Figure 2-13. Effect of Activated Alumina Particle Size on Fluoride Removal Capacity (1) 49

Figure 2-14. Effect of pH on Alumina Fluoride Removal Capacity (1) 50

Figure 2-15. Effect of Influent Fluoride Concentration on Alumina Fluoride Removal Capacity (1) 51

VI

PAGE

Figure 2-16. Exchange Capacity of Activated Alumina vs. Alkalinity of Influent (1) 52

Figure 3- 1. Schematic Diagram of the Apparatus Used in the Defluoridation Study 65

Figure 4- 1. Calibration Curve Showing Fluoride Concentration versus Millivolts at pH 5 and 24.5°C 68

Figure 4- 2. Temperature Dependence of the Fluoride Specific Ion Probe 69

Figure 4- 3. pH Dependence of the Fluoride Specific pH Dependence of the Fluoride Speci Ion Probe. T = 24.5°C 70

Figure 4- 4. Relationship Between Residual Fluoride Concentration in Solution and the pH at Different Contact Times. T = 24.5°C 71

Figure 4- 5. Relationship Between the Exchange Capacity of Activated Alumina and the pH of Influent. T = 24.0 + 0.5°C 72

Figure 4- 6. Relationship Between Exchange Capacity and Initial Fluoride Concentration. T = 24.5°C, pH = 5.0, Contact Time = 10 hrs 76

Figure 4- 7. Residual Fluoride in Solution Using 4.0 ppm Initial Fluoride Concentration at pH 5, 24.5 C and Various Doses of Activated Al umi na 77

Figure 4- 8. Residual Fluoride in Solution Using 8.0 ppm Initial Fluoride Concentration at pH 5, 24.5 C, and Various Doses of Activated Alumina 78

Figure 4- 9. Residual Fluoride in Solution Using 2.0 ppm Initial Fluoride Concentration at pH 4.95, 24.5 C and Various Doses of Acti vated Al umi na 79

Figure 4-10. Residual Fluoride in Solution Using 2.0 ppm Initial Fluoride Concentration at pH 4.90, 56.0 C and Various Doses of Acti vated Al umi na 80

v n

PAGE

Figure 4-11. Residual Fluoride in Solution Using 4.0 ppm Initial Fluoride Concentration at pH 4.90, 55.5°C and Various Doses of Activated Alumina 81

Figure 4-12. Residual Fluoride in Solution Using 8.0 ppm Initial Fluoride Concentration at pH 4.92, 55.0°C and Various Doses of Activated Alumina 82

Figure 4-13. Residual Fluoride in Solution Using 2.0 ppm Initial Fluoride Concentration at pH 4.98, 5.0°C and Various Doses of Activated Alumina 83

Figure 4-14. Residual Fluoride in Solution Using 4.0 ppm Initial Fluoride Concentration at pH 4.98, 5.0°C and Various Doses of Acti vated Al umi na 84

Figure 4-15. Residual Fluoride in Solution Using 8.0 ppm Initial Fluoride Concentration at pH 4.97, 5.0 C and Various Doses of Acti vated Al umi na 85

Figure 4-16. Relationship Between Exchange Capacity and the Ratio of Initial Fluoride Concentration to a Unit Weight of Sorbent. T = 24.5 + 0.5°C, pH = 5.0 T 89

Figure 4-17. Residual Fluoride in Solution at Different Temperatures and Quantity of Activated Alumina 91

Figure 4-18. Residual Fluoride Concentration in Solution Using a Virgin Activated Alumina and the Sorbent Regenerated at Different Temperatures 92

Figure 4-19. Relationship Between Fluoride Removed Per Unit Weight of Activated Alumina, the Virgin Sorbent and Sorbent Regenerated at Different Temperatures 93

Figure 4-20. Relationship Between the Exchange Capacities of Thermally Regenerated Sorbent and Regeneration Temperatures 94

vm

PAGE

Figure 4-21. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Initial Fluoride Concentration of 10 ppm 97

Figure 4-22. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Wastewater from Texas Instruments 520 ppm Fluoride 98

Figure 4-23. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Activated Alumina Regenerated with a 2 Percent Sodi um Hydroxi de 100

Figure 4-24. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Activated Alumina Thermally Regenerated at 800°F 102

IX

CHAPTER 1

INTRODUCTION

Fluoride Ecology

Fluorine, the most electronegative of all elements, rarely occurs

free in nature. It combines chemically to form fluorides which can be

found in varying amounts in soils, water, the atmosphere, vegetation and

in body tissues. A study conducted by the National Academy of Sciences

(17) found inorganic fluoride compounds to constitute the most important

sources of fluoride toxicosis in man and livestock. Organic fluoride

(compounds in which fluorine is bound to carbon) have not yet been

identified as a major source of concern in fluoride toxicosis.

Fluoride ingested in small amounts may be beneficial. Studies (8,

9, 17) have shown that trace quantities of fluoride are needed for the

development of cavity-resistant teeth and may also be necessary in pre­

venting excessive demineralization of bone (osteoporosis) in aged indi­

viduals. However, numerous field studies and research point to the fact

that excessive fluoride ingestion by animal and man can result in

permanent bone and teeth deformities. At a minimum, endemic dental flu­

orosis (fluorotic lesions in permanent dentition) can occur and mottling

of teeth characterized by dull-white, chalk-like patches or striations

of the enamel or by a defective calcification of the enamel described as

hypoplasia can lead to an early loss of teeth. A severe case of dental

fluorosis, often seen as crsamy yellow to brown or black discoloration

of the teeth, results in a rapid erosion of the enamel. Several

standards that have been developed for classification of dental fluoro­

sis have been described in detail (8, 9, 17). Fluorosis generally tends

to occur more frequently in areas with fluoride levels of 1.5 parts per

million or more in the drinking water. Other skeletal problems that

have been linked to fluoride toxicity include stiffness, rheumatism, and

permanent crippling rigidity (4). Some clinical and experimental stud­

ies have suggested a link between kidney defects and long term fluoride

ingestion (17).

In order to alleviate the adverse effects that may result from a

long term ingestion of excess fluoride, the U.S. Public Health Service

Drinking Water Standards have set a mandatory limit on the level of

fluoride allowed in the drinking water of any particular area (1, 13).

These doses vary from location to location depending on the annual aver­

age of the maximum ambient temperature of that particular area (1, 6).

The premise is that people consume more water generally in warmer

climates and therefore reducing the level of fluoride content in the

drinking water also reduces the danger of excessive fluoride intake.

Despite these standards however, reportedly more than 600 water supplies

in the U.S. serving approximately 1.5 million people contain fluoride in

excess of 1.5 mg/liter (exceeding the 1.0 ppm level recommended for

development of healthy teeth) and only very few of these water treatment

plants include fluoride-removal processes (23).

The common sources of fluoride include: a) airborne contamination

from nearby industrial operations; b) feed supplements for livestock

with excessive fluoride such as in fluorite (CaF^) and fluorapatite

^

(3Ca2(P0^)2'Cg^F2); c) water that is excessive in fluoride — which con­

stitutes the single most predominant source of fluoride pollution known

today (3). Fluoride-laden wastewaters are generated from various indus­

trial establishments such as those that manufacture pesticides,

disinfectants, wood preservatives, metals, glasses, and fertilizer.

Aluminum smelting plants, manufacturers of microprocessors, uranium

enrichment plants, and nuclear weapon components production and

engineering facilities also produce wastewater with high levels of

fluorides. A great amount of fluoride deposition into the environment

occurs during mining of phosphate rocks when silicon tetrafluoride is

unearthed and released or leached into nearby waters (23).

These and other dangers posed by pollution to health and welfare

call for the need to develop new and improved technologies to prevent,

treat and manage wastewater systems that otherwise would become hazard­

ous pollutants.

Removal Techniques

Many attempts made in the past to remove fluoride from drinking

water have met with mixed success (13, 15). Most of these methods were

based on precipitation and involved addition of chemicals in large

amounts to form precipitates or coprecipitates of fluorides. Some of

the chemicals used were aluminum sulfate, lime, calcium phosphates, etc.

Other adsorption methods or ion exchange methods used materials such as

bone char," or activated carbon. The major drawback with the use of

these materials were: low exchange capacities of these materials for

fluoride ions, high media costs, and difficult or expensive regeneration

of the media once they become spent. In addition, many of these materi­

als were not specific for removal of fluoride ions (23).

The first break in solving the fluoride problem came in 1936 (1).

In a patent issued to Churchill, the first mention was made that activa­

ted alumina could be used in fluoride removal. Activated alumina is

made by heating aluminum oxide to a temperature of 400 to 500°C in the

presence of alkali metal ions. Since 1936 much effort has and is still

being spent in finding ways of making the use of activated alumina in

fluoride removal efficient and economically viable for water treatment

plants. Several studies have shown that the difference in capacity be­

tween activated alumina and other materials previously used is the

result of the increased surface area per unit volume and the relative

polarity of the small activated alumina particles (1). Fluoride removal

therefore is a surface phenomenon. The other advantages in using acti­

vated alumina besides its high capacity are: low cost, specificity to

fluoride ions and insensitivity to chloride and sulfite ion concentra­

tions, and no loss in effectiveness following regeneration. There are

about three locations in the United States using activated alumina as

the medium for removal of fluoride. There are still major problems with

its use however. This is shown by the lack of public acceptance of

water defluoridation plants (13). Excessive costs of treatment plants

and high operating costs of such plants are the major problems.

Moreover, regeneration of'^spent activated alumina is found to be the

single most expensive item in the overall costs of running defluorida­

tion plants (6, 19).

This Study

In this research project, we evaluated the factors that affect the

exchange capacity of activated alumina for fluoride ions. These factors

include: initial fluoride concentration of wastewater influent, and pH

of influent. To do this, we used a fixed bed of activated alumina as

the adsorbent media. All analyses were performed in accordance with the

procedures outlined by "Standard Methods of the Examination of Water and

Wastewater." An Orion specific ion electrode and pH/mV meter were used

to monitor all fluoride concentrations. Water containing various

amounts of fluoride was passed through the bed. On contact with the

bed, the fluoride ions were adsorbed on the surface of the activated

alumina. The treated wastewater, with the pH adjusted, was then ready

for consumption. Fluoride capacity and fluoride effluent concentration

were the major dependent variables monitored.

In addition to these, we examined the effect of temperature on the

exchange capacity of activated alumina. Our hypothesis was that sorbed

fluoride can be released by increasing the temperature of the sorbent

material. Thermal regeneration of activated alumina could provide a

less expensive means of prolonging the life of the media. This may in

fact eliminate the complexity and costs of using caustic soda solution

for regeneration. The added advantage here would be that precipitation

of fluoride salts would take place outside the bed area eliminating the

usual problems of plugging commonly associated with other regeneration

schemes presently being used (1, 7, 20).

The packed bed process enabled us to evaluate an effective economic

method for the regeneration of spent media and provide a means of recov­

ering fluorides from regenerants. This was necessary because high

concentrations of fluoride recovered along with regenerant solutions

cannot simply be returned to the environment. It should be suitably

disposed of without causing hazard to the environment.

CHAPTER 2

LITERATURE REVIEW

Beginning on June 24, 1977, communities throughout the United

States were required to comply with the Environmental Protection

Agency's (EPA) national interim primary drinking water regulations (18).

Maximum contaminant levels in potable water supplies were established

for ten inorganic chemicals including fluoride. According to the regu­

lations, the maximum allowable contaminant level for fluoride can vary

from 1.4 ppm to 2.4 ppm depending on the annual average of the maximum

daily air temperatures. Since ground water sources can exceed these

limits, some removal techniques are required if the water is to be used

for drinking purposes. In the present chapter, we shall review the

state of the art in fluoride removal technology and examine the changes

that have been made in the methods used in the past. We shall also look

at some of the advantages and disadvantages inherent in most of the flu­

oride removal schemes that have been used thus far.

Methods

There are a variety of methods for removal of fluoride (4, 16, 18).

The most commonly used methods can be divided into two categories, name­

ly, precipitation and adsorption (1, 20). Below is a list of compounds

used in the different methods.

8

Methods of Fluoride Removal from Water

I 1 Precipitation (additive) methods Adsorption (Ion Exchange) Methods

- Alum coagulation - Activated carbon

- Lime softening - Zeolite

- Magnesium sulfate - Mixed bed dimineralizer

- Calcium carbonate - Bone, bone char, and synthetic bone material

- Activated Alumina

Precipitation Methods

These techniques involve addition of chemicals and the formation of

fluoride precipitates or co-precipitates. Among the chemicals used are

lime, calcium chloride, calcium carbonate, aluminum sulfate, and

magnesium sulfate (4, 6, 17, 20). Other substances such as bentonites,

fuller's earth, diatomaceous earth, silica gel, bauxite, sodium

silicate, sodium aluminate and ferric salts have been tried (4, 20).

Precipitation methods usually cannot achieve low fluoride concentration

(4, 6, 23). Fluoride treatment by lime for example has been shown to be

ineffective in dilute (less than 5 ppm) fluoride solutions (23). In

1973, Rabosky and Miller found that fluoride removal by lime precipita­

tion was not effective when the fluoride concentration was below 20 mg

per liter (23). Similarly, alum is required in very high dosage before

chemical removal of fluoride is effected. For the most part, the pH of

precipitation processes must be closely controlled because the solubili­

ties of the inorganic fluoride complexes are normally pH dependent (22).

Some other drawbacks of the precipitation methods include (1) the

necessity for additional reagents and higher costs; (2) higher shipment

and treatment costs; (3) the large volume of sludge produced.

Adsorption Methods

Adsorption methods utilize the passage of fluoride containing water

through a contact bed. Fluoride is removed by ion exchange or by chemi­

cal reaction with the adsorbent. Several types of adsorbents have been

tried. Among these are natural hydroxyapatite, synthetic hydroxyapat-

ite, zeolites, magnesia, trimagnesium phosphate, natural and synthetic

ion resins, activated carbon, activated bauxite, and activated alumina

(20, 22, 23). Superiority of the adsorption methods over those of pre­

cipitation tends to come from the fact that adsorbents such as activated

alumina are both economical and exhibit a good capacity for adsorption

of fluoride ions (14, 20). Moreover, the adsorbent can be readily

regenerated and recycled for reuse (6, 19, 23). Hence adsorption

methods are usually appropriate for removal of relatively low concentra­

tions of fluoride. They may be used as polishing operations after the

removal of fluoride by precipitation techniques to the 10-20 ppm level

(4).

Earlier Methods Used in Fluoride Removal

During the years following the discovery by McKay that fluoride was

the cause of dental fluorosis, various methods and materials for fluor­

ide removal were investigated. Boruff was the first to investigate a

variety of materials for fluoride removal (2). The materials studied

included aluminum sulfate, sodium silicate, ferric fluoride, sodium

aluminate, zeolites, bauxite, silica gel, and lime. Except for lime and

10

aluminum sulfate, none of these materials were found to be yery practi­

cal in fluoride removal (2). These earlier findings led Boruff to

investigate further the removal of fluoride with lime and alum. He

stated that considerable amounts of fluoride could be removed by dosing

with alum and removal of the floe by sedimentation and filtration.

Alum Coagulation

In Boruff's investigation, aluminum sulfate dosages of 8.5 ppm (0.5

gram) to 171 ppm (10 grams) per gallon were added to 2.5 liters of raw

water containing known quantities of fluorides (2). The coagulant and

waters were mixed for half an hour by a mechanical stirrer and allowed

to stand for 18-24 hours before filtering off the floe. In the results

obtained, the author observed that increasing dosages of aluminum

sulfate gave proportionately greater removals of fluoride. Figure 2-1

summarizes the results. The dosage requirement for treating a given

concentration of fluoride were given as:

Cone, of Influent ppm per gal. Cone, of Effluent (2) (ppm) Alum (ppm)

2.5 34.0 1.0

5.0 170.0 1_ 0

In addition to these results, the author (2) observed that chloride and

sulfate concentrations as high as 1000 ppm had no effect on the removal

of fluoride by alum.

In another study on the fluoride removal by alum performed at La

Crosse, Kansas by Culp and Stoltenberg, a method that consisted of

application of 225 ppm of alum in increments during rapid mixing and

11

p E S I 0 u A L F L U 0 1 I 0 £

C 3 N

c i p p .i )

PLUM OCSflGiiPPM)

Figure 2-1. Removal of Fluorides with Aluminum Sulfate pH = 7.2-7.4 (2) "f = I n i t i a l Fluoride Concentration (ppn)

12

flocculation followed by settling and rapid sand filtration was verified

(5). It was desired to reduce the raw water fluoride concentration from

3.6 to 1.0 ppm. The variables studied included effects of a) lime, b)

rate of feed of alum, e) pH, d) mixing time, 3) coagulant aids, and f)

chemical composition of treated water. Their results showed that fluor­

ide removal was proportional to the alum dosage. Figure 2-2. The effi­

ciency appeared to decrease as higher dosages were employed; 3.50 ppm of

alum were needed to reduce 3.6 ppm initial fluoride influent concen­

tration to the required 1.0 ppm. Table 2-1 shows fluoride removal using

alum flocculation with and without addition of lime during rapid mixing.

The authors (5) recommended addition of the lime near the end of the

flocculation period because of the tendency for calcium to slightly in­

terfere with fluoride removal (5). Further results also showed that 10

percent less alum was required for the corresponding reduction in fluor­

ide when the alum was added in increments during rapid mixing. This was

attributed to the better contact between aluminum and fluoride ions dur­

ing floe formation and complete coagulation. Figure 2-3 shows the rela­

tionship between the fluoride removal efficiency of alum and the pH of

solution. This figure indicates the optimum pH range for fluoride

removal to be between 6.5 and 7.5. The authors (5) also found that

residual aluminum in treated water tended to lower the true pH. The

flocculation time did not appear to affect fluoride removal very much.

The authors (5) stressed the need for adequate stirring and slow

addition of alum. The use of coagulant aids such as activated silica,

bentonite, hydroxyethyl cellulose, and fuller's earth improved coagula­

tion but had no significant effect on fluoride removal. Their further

13

3.S0-

0.00-1 1

0 50 100 ISO 200 250 300 350 HOO HSO SOO S50 SOO

ALUM DOSAGE (PPM)

Figure 2-2. Fluoride Removal by Alum Flocculation (Incremental Feeding of Alum During Mixing) (5)

14

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15

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Figure 2-3. Effect of pH on Fluoride Removal (Alum dosage = 250 ppm) (5)

16

use was therefore not recommended. The alum flocculation method

simultaneously reduced the fluoride concentration as well as the

concentration of iron, manganese, color, and turbidity. This removal

process therefore was not particularly specific to fluoride ions.

In another investigation (21) on the removal of fluoride by alum,

well waters containing 1.7, 3.0, 6.0 ppm fluoride were treated with

graded amounts of alum. One liter portions contained in one liter Pyrex

Erlenmeyer flasks were kept tightly stoppered and were shaken 10 times

daily for 7 days, then let stand overnight and the clear supernatant was

decanted off and examined. The results shown on Table 2-2 indicate that

fluoride concentrations of 1.7, 3.0, 6.0 ppm respectively may be lowered

to 1.0 ppm by corresponding dosages of 11.5, 20.0, and 52.0 grains per

gallon. The results of Stoltenberg, et al., are larger than those

obtained by Boruff (2). In the studies by Boruff, initial fluoride

concentrations of 1.5, 3.0, and 5.0 reduced to 1.0 ppm required alum

dosages of 1.3, 2.3 and 10.0 grains per gallon, respectively.

All results however were consistent in that large doses of alum

were required to remove small amounts of fluoride. Because alum doses

required for fluoride removal are much greater than those used for tur­

bidity and color removal, this technique has not been considered a very

practical solution to fluoride removal (22).

Lime Softening

It has been shown that treatment of stock waters with sufficient

calcium hydroxide to cause precipitation of the carbonate and magnesium

hardness, brought about co-precipitation of part of the fluoride present

17

TABLE 2-2

Fluoride Removal by Aluminum Sulfate (21)

Aluminum Sulfate Residual Fluoride (girains per gallon) (ppm)

0 1.7

2 1.6

5 1.4

10 1.1

15 0.8

20 0.6

30 0.4

0 3.0

2 2.6

5 2.2

10 1.7

20 1.0

0 6.0

2 5.4

5 4.5

10 3.5

20 2.4

30 1.9

40 1.5

Alkalinity (ppm)

312

300

275

238

204

168

105

290

278

262

232

165

308

298

278

240

168

105

50

pH

7.2

7.0

6.8

6.6

6.5

6.3

6.2

7.8

7.5

7.2

6.9

6.5

7.5

7.2

7.0

6.8

6.5

5.8

5.6

18

(2). In one test (2), the fluoride content in water containing 5.0 ppm

fluoride was reduced to 2.1 ppm through lime softening processes. Sev­

eral years after this observation was made, Scott and others showed that

the decrease in fluoride concentration was a function of the amount of

magnesium removed during the softening process (21). Upon a routine ex­

amination of raw and treated municipal water supplies, Scott noted that

fluoride concentration in the effluents from lime softening plants show­

ed a substantial reduction when compared to the fluoride in the raw

water. To verify these observations, bottle tests were made using nat­

ural waters with and without the addition of graded quantities of sodium

fluoride. The tests showed a reduction of fluoride by lime and also

that the degree of fluoride removed was a function of the removal of

magnesium. This led the authors (21) to conclude that fluoride was ad­

sorbed by the gelatinous magnesium hydroxide precipitate. So the fluor­

ide removal mechanism during lime softening was determined to be

coprecipitation with magnesium hydroxide. From the results obtained by

Scott, et al. (21), shown on Table 2-3, a relationship was established

that explains the role of magnesium in the lime fluoride removal

process. This relation (eqn. 2-1) and Figure 2-4 appears to hold

between initial fluoride concentrations of 1.5 - 3.5 ppm. Equation

(2-1) says that the fluoride reduction by lime is approximately equal to

7 percent of the initial fluoride multiplied by the square root of the

magnesium removed. In terms of the residual fluoride, the equation is

Y = F - (0.07F)(vJx) (2-1)

19

cn c

•r-C (U

•M M -O

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o«*coo^oo^cvjcr»cvjo^uno^<^oO'— CVJ.— f— O O « ^ C 0 i — f— I— 1— O C O i — r—

r>. vo o^ r>. I— O r - O O CVJ

CT»vo«^cvjcooocy»forooo f— I— I— r— O L O r O i — I CVJr—

t n ^ c v j f ^ i O r - 0 0 0 CVJi— I— O l — .— OCVJ

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20

F = Initial Fluoride (ppm)

Y = Residual Fluoride (ppm)

X = Magnesium Removed (ppm)

TH = Total Hardness

HG = 0.09TH

0.4-3

LEGcNO: p"

MflGNESIUM REMOVED (PPM)

* ¥ m 2 » » » 2. S a o a 3 3.5

Figure 2-4. Fluoride vs. Magnesium Removal as Predicted by Equation 2-1 (21)

21

where Y = Residual fluoride (ppm)

F = Initial fluoride (ppm)

X = Magnesium removed

The amount of magnesium that must be removed to obtain fluoride concen­

tration of 1.0 ppm when the initial fluoride concentration is between

3.5 and 1.5 ppm is shown in Table 2-4. According to the authors, to ac­

complish a high removal percentage of magnesium, essential to obtain a

reasonable reduction in fluoride concentration, requires the addition of

sufficient lime to raise the pH to about 10.5 (21). Under such con­

ditions, a caustic alkalinity of about 30 ppm is required (21). Figure

2-4 is a plot of equation (2-1) when the total concentration of

magnesium in water is about 9 percent of its total hardness. On the

basis of the fluoride reduction formula (eqn. 2-1), it is clear that a

magnesium removal of 100 ppm, secured if necessary, by the addition of a

magnesium salt, limits the initial fluoride content to 3.3 ppm, if the

residual fluoride concentration is to be 1.0 ppm.

On economic grounds, therefore, all efforts must be directed

towards securing a water supply of low fluoride content in the event

that raw waters of high fluoride content are encountered. Clearly

however, if sufficient amount of magnesium is not present in the water,

a magnesium salt must be added in quantities necessary to accomplish the

desired level of fluoride removal. This treatment method therefore is

similar to the alum coagulation method in that it has limited

application (5, 21, 22). Because of the large quantities of chemicals

that would normally be required, the method is adaptable to only low

fluoride-high-magnesium water requiring softening.

22

C L Q.

X>

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o E <U

OC a. CL

CT> CO

LO CO CVJ

c o

u c o C_)

0) "O

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CVJ

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23

Although no extraordinary success was made immediately after

observers linked the causes of dental fluorosis to fluoride (14, 22), it

was clear that definite progress was being made towards finding a means

of reducing the levels of fluoride in potable water to an acceptable

level. In 1947, Meier reported that methods utilizing the fluoride

exchange properties of apatites such as those involving the use of con­

stituents of bone, the ion exchange principle, and those depending on

the sorptive properties of aluminum compounds appeared to show the most

promise for removing excess fluorides from water (14). In the next few

paragraphs, we shall examine some of these ion exchange and adsorption

methods and look at some of the results that were obtained and relate

them to the current changes that have evolved through the years in the

fluoride removal technology.

Activated Carbon

McKee and Johnston conducted experiments with four different types

of carbon (12). The carbons tested were: a) Norit, a commercial carbon

produced from the charcoal of European pine, b) residual carbon discard­

ed by the soda pulp industry, c) same as b) but activated with acid, and

d) anthracite coal activated with hot carbon dioxide gas.

In the results given by the authors (12), a) and d) failed to

remove any fluoride; b) removed about 50 percent of the fluoride at a pH

of less than 3.0. The authors therefore decided to pursue the study of

fluoride removal with residual carbon activated with acid (case c)

because it showed real promise. Their observation (12) was that there

was little removal of fluoride until the pH was between 3.6 and 2.5. In

24

this pH range, fluoride removal increased from approximately 30 percent

to 100 percent. The efficiency of fluoride removal in relation to the

pH of the solution is given as:

pH Percent F Removed (12)

6.9 4

3.57 49

3.16 88

3.01 100

2.53 99

2.38 99

2.25 100

The authors (12) measured the efficiency of fluoride removal by the car­

bon when only 0.08 percent by weight of carbon on the weight of water

was used. Their results are shown below:

(12) Fluoride in (ppm)

10.0

10.0

7.5

5.0

Influent Fl uoride Removed (%)

84.5

75.0

94.0

92.0

In other words, 0.08 percent carbon removed 94 percent of the fluoride

from waters containing up to 7.5 ppm and 80 percent from water with 10

ppm. Other tests conducted by the authors using a continuous system

consisted of a glass tube 40 mm in diameter and 750 mm long and filled

to a height of 620 mm with a 16-30 mesh size activated carbon. Their

findings essentially confirmed their earlier results. It was found that

there was no effective removal of fluoride until a pH of about 3.0 was

reached.

25

From a practical standpoint, therefore, the use of this medium for

removal of fluoride would be expensive because of the need to lower and

raise the pH of the water supply. This conclusion has been reached by

other authors (22). Culp and Stoltenberg found that application of 50

ppm of activated carbon to a well water pH of 8.0 failed to reduce the

fluoride content (5).

Zeolite

In a study made to determine fluoride removal by zeolite (2), a

miniature contact filter was set up in a 55 mm glass tube into which

were added 490 grams of synthetic zeolite. Six liters of stock water

containing 5.0 ppm of fluoride were passed through the filter at a rate

of 300 ml per minute. The effluent showed a concentration of only 0.6

ppm fluoride. Then zeolite was backwashed, regenerated with a 5 percent

sodium chloride solution and washed free of salt with chloride-free

water. Fifty liters of water containing 5.0 ppm fluoride were then pas­

sed through the filter at the rate of 234-165 ml per minute. The 4-5

liter fraction of effluent collected contained only 0.9 ppm fluoride.

This concentration however increased to 2.5 ppm in the 11-14 liter

fraction, 3.4 ppm in the 20-23 liter fraction, 3.6 ppm in the 27-30

liter fraction, 3.7 ppm in the 36-39 liter fraction, and 4.0 ppm in the

44-47 liter fraction.

A second regeneration of the zeolite with a 5 percent sodium chlor­

ide was followed with passage of a 40 liter portion of stock water with

5.0 ppm fluoride. The results showed the following: the 3-5 liter frac­

tion contained 2.9 ppm of fluoride, while the 6-9 liter fraction shov/ed

26

3.8 ppm fluoride. However, subsequent fractions collected showed no re­

moval of fluorides. The zeolite was backwashed, treated for 30 minutes

with 2 percent sodium hydroxide and rinsed free of alkali with fluoride

free water. Again, stock water containing 5.0 ppm fluoride was passed

through the filter. Results showed the 3-5 liters collected to have 3.7

ppm fluoride while the 6-8 liter fraction contained 4.7 ppm. Subsequent

samples showed no evidence of removal of fluoride.

The results obtained in this study (2) led to the conclusion that

zeolite could effectively remove fluoride from water but had wery low

exchange capacities for fluoride ions. This would make the application

of the media economically unattractive for commercial use. The fact

that small quantities of fluoride were removed following regeneration

was explained as a result of preferential adsorption of other ions in

the solution.

Mix€d Bed Demineralizer

Harman and Kalichman (7) reported on the use of a mixed-bed demin-

eralizer for fluoride removal in Death Valley Junction, Southern

California. The mixed-bed demineralizer consisted of sulfonated

polystyrene and quaternary amine polystyrene. In addition to fluoride

removal, this medium also removed excessive concentrations of arsenide,

total dissolved solids and sodium. The 15 gpm demineralization unit was

reported to have sufficient capacity to operate for 4 days at the rate

of 150 gallons per day between regeneration cycles. No data were given

for the exchange capacity of this medium or the fluoride concentration

of raw water treated. Regeneration chemicals were given as caustic soda

and sulfuric acid (7).

27

Bone

The limited application of either alum coagulation, activated car­

bon, or lime softening to fluoride removal led to the employment of

either activated alumina, bone char or granular tricalcium phosphate as

an ion exchange media for defluoridation.

Bone was first suggested for use as an ion exchange media because

of its known affinity for fluoride (22). Bone consists essentially of a

carbonate apatite Cag(PO,)g» CaCO-,. The removal mechanism suggested in

the use of bone was the exchange of the carbonate radical with fluoride

(14, 22). While different studies have shown bone to be an effective

fluoride removal medium, the costs have not allowed its wide use.

Bone Char

Bone char is simply ground animal bones charred to remove all org-

anics. It consists essentially of tricalcium phosphate and carbon. It

has been utilized in full scale defluoridation plants (21, 22). This

material initially developed for decoloring cane syrup (22), is more

economical than bone. The ability to be regenerated by washing with

caustic soda made bone char useful in fluoride removal processes. In

the regeneration of the material with caustic soda, the fluoro-apatite

formed by the adsorbed fluoride probably becomes an hydroxy-apatite and

the fluorides are removed in the form of soluble sodium fluoride. The

hydroxy-apatite subsequently becomes available as an exchange material

by the replacement of its hydroxy radical by fluoride (14).

The first full scale defluoridation plant to use bone char for op­

eration was U.S. PHS plant in Britton, South Dakota, which was in oper­

ation from 1953 to 1971 (22). Also, the defluoridation plant built at

\ '^

28

Fort Irwin, California, was in operation with bone char as its medium

(7). The exchange capacity for fluoride ions before regeneration

obtained in removing 5 ppm of fluoride was 0.102 kg fluoride per cubic

meter of bone char bed (13). Regeneration of bone char has been effect­

ed with solutions of trisodium phosphate and monosodium phosphate, but

the exchange capacity of the medium is reduced by 12 percent after the

first regeneration (14). Alternatively, as previously mentioned, regen­

eration may be accomplished with a 1 percent sodium hydroxide solution.

One pound of caustic soda is required for each cubic foot of bed. The

caustic nature of the bed is then neutralized by thorough washing with

distilled water and with a carbon dioxide solution (14). The use of

carbon dioxide solution greatly increases the life of the bed and

considerably reduces attrition losses. Bone char, however, proved to be

impractical for fluoride waters that also contained arsenide. Arsenide

could simultaneously be removed by bone char but that removal process

could not be reversed. Since arsenide competes with the fluoride ions

and because it cannot be removed by the usual caustic regeneration pro­

cess, the fluoride capacity of bone char tended to decrease rapidly and

had to be frequently replaced thereby making the process economically

less competitive (19, 22).

The second disadvantage with the use of bone char was its

solubility in acid (19, 22). The pH of the raw water and the acid rinse

must therefore be carefully controlled to minimize loss of the media.

Synthetic Bone Material (Tricalcium Phosphate)

Synthetic bone material is produced by reacting phosphoric acid

with solutions of lime (13, 22). It comes in powder or coarse granular

29

form. Granular tricalcium phosphate can be regenerated with caustic

solution and rinsed with dilute acid solution (22). This material was

first commercially employed as a defluoridation medium in Climax, Color­

ado in 1937, and Scoba, Mississippi in 1940 in the fluoride removal

plants built there. However, both plants were abandoned in 1949 (22).

During its use in the defluoridation plant in Britton, South Dakota, in

1948-51, tricalcium phosphate proved to be quite efficient in fluoride

removal. Unfortunately, during the four years it was used, an attrition

loss of 42 percent per year was experienced. The removal capacity

observed was 0.685 kg fluoride removed per cubic meter of medium bed.

It was observed that the presence of 100 ppm sulfate ions in raw water

caused a 3 percent decrease in capacity of tricalcium phosphate (22).

The frequent regeneration needed, coupled with heavy attrition losses,

made synthetic tricalcium phosphate a less attractive medium for

fluoride removal purposes.

Activated Alumina

Activated alumina is made from calcined granules of hydrated alum­

ina. The most well-known and longest operating defluoridation plant in

the U.S. is located in Bartlett, Texas. This process utilized activated

alumina as its exchange medium. Reportedly, the plant continued to op­

erate on its original charge of activated alumina for 25 years (22). The

advantages of activated alumina over most media studied stem from its:

a) specificity for removal of fluoride ions. In an extensive work done

by Kubli he found the anion-alumina adsorption series in order of

decreasing preference to be (11): OH", PO^"^, Cr^O"^, F", S03"^, CrO^"^,

30

NO2', Cl", NO3", MNO^", SO^"^. This affinity preference offers a parti­

cular advantage over synthetic strong-base resins that remove many

anions and whose selectivity series is commonly reported as follows

(22): r , HSO^", NO2", Br", Cl", OH", HCO3", ^ 2 ^ V ' "* ^ ^"'S^

exchange capacity for fluoride ions; c) capacity is not noticeably

affected by the concentration of sulfates or chlorides in the raw water;

d) relatively low cost (about $460 per cubic meter (22)). This is about

10 percent the cost of synthetic anion resin and about 25 percent the

cost of bone char. Table 2-5 compares the 1978 costs of the different

adsorbents for fluoride removal.

The use of caustic (1 percent sodium hydroxide) solution and acid

solution for regeneration is probably the single most expensive item in

the entire defluoridation process (6, 19), and also the major disadvan­

tage to the use of activated alumina, since the reagents will require

careful control and handling and hence the need for well-trained opera­

tors for defluoridation plants.

According to a report by Sorg (22), there are two large-scale acti­

vated alumina defluoridation plants operating. One is at Desert Centre, 3

California with a rated capacity of 5680 m per day of treated water.

The process reduces the fluoride content of water from about 8.0 ppm to

less than 1.0 ppm. The second plant is at X-9 Ranch near Tucson, Ariz­

ona. It operates at 2650 m^ per day and reduces fluoride concentration

in drinking water from 5.0 ppm to below 1.0 ppm. The operating exchange

capacities for the medium used in both plants is reportedly 4.52 kg

fluoride removed per cubic meter of activated alumina bed. Operating

31

to

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32

costs (1978 prices) excluding amortization are estimated at about

$0.15-30.20 per 1000 gallons of treated water.

In an earlier investigation on the use of activated alumina for

fluoride removal, Boruff (2) placed 30 grams of 40 mesh activated alum­

ina in a 35 mm glass tube. This gave a contact bed 305 mm deep. Fifty

liters of test water containing 5.0 ppm fluoride were passed through at

a rate of 200 to 250 ml per minute. The results are summarized in Table

2-6. The fluoride content of the first 21 liters was reduced to less

than 2.0 ppm. The 48-50th liters contained 2.3 ppm. The activated

alumina was then backwashed, regenerated with 50 ml of 2 percent sodium

hydroxide for 30 minutes, rinsed and again treated with water containing

5.0 ppm fluoride at a rate of 200 ml per minute. The results of this

regeneration cycle are shown in Table 2-6. The first 30 liters contain­

ed a maximum of 1.6 ppm fluoride. Regeneration with 500 ml of 5 percent

sodium chloride and 2N hydrochloric acid for 30 minutes gave results

which were approximately the same as those obtained in the sodium hydro­

xide regeneration runs. Regeneration with 2N hydrochloric acid caused

some loss in weight of the activated alumina. The author (2) recommend­

ed the use of sodium chloride for purposes of regeneration of activated

alumina. Further tests were made replacing activated alumina by bauxite

(commercial grade), as the fluoride removal medium. Here, it was

observed that only minor quantities of fluoride were removed (2).

Defluoridation Plant (Bartlett, Texas)

Activated alumina is the adsorbent used at this full-scale defluor­

idation plant. The plant uses 500 cubic feet of media in a standard

33

to

3

CU

>

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< :

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r— O O i — O r - r— o o

o o o o o o o o o L n t n c o c o c o c o c o c o t n

to :iz zn r-c o o o I—

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rt3 CJ CJ o

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34

circular steel filter tank, 11 ft. in diameter and 11-1/2 ft. high. It

is fitted with a steel pipe-grid underdrain, steel wash troughs and a 4

inch plastic acid and caustic distributor. Figure 2-5 shows a schematic

representation of the Bartlett defluoridation plant. There is a 12 inch

layer of graded gravel underlying 60 inches of media. The regeneration

equipment includes a 6,000 gallon caustic solution tank with electric

mixer and pump and a system for measuring, diluting and metering acid

solutions. The plant is capable of treating 400 gpm of Bartlett

well-water and was designed to reduce the fluoride content from 8.0 ppm

to an average of 1.0 ppm. Meier reported that regeneration of the

Bartlett plant's activated alumina bed only became necessary after

treatment of about 450,000 gallons of 8.0 ppm fluoride water (15). Re­

generation of the activated alumina is primarily intended to remove ac­

cumulated fluoride by means of a dilute caustic solution. In the Bart­

lett plant, spent caustic could not be reused because of excessive con­

tamination by other ions. The caustic solution is applied countercur-

rently, which combines in one operation the backwashing step and the

caustic application. The caustic solution is applied at the rate of 235

gallons per minute and is discarded after passing the bed (15). After

regeneration, a dilute acid solution (0.05 Normal) obtained by diluting

66° Baume sulfuric acid to first 15 percent and then to 0.05 N is used

to neutralize the bed. The capacity of the Bartlett plant when reducing

8.0 ppm fluoride water to 1.0 ppm is given to be 1.6 kg fluoride removed

per cubic meter of activated alumina bed (ca 700 grams per cubic foot).

35

t n

to c

• r» P= 3

r— to

" O OJ

+ J fO >

• ^ 4 J U to

CO

-a r—

o J C

Ji£ E 03

4 J

4-> O fO

4-> C

o C J

4 J (U

n ^ 4-> S . (T3

CO

• M fO

+J •r— C

o c; O

4-) to

• o . r -S-

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r— I t -OJ

Q

M-O

••-> 3 O >» to

- J

u • r -4 J ro cz (U

- C

o oo

LO

CVJ

a> i -3 C^

36

Current Trends in Fluoride Removal

In the following paragraphs, we shall present some of the recent

schemes that have studied the removal of fluoride from water utilizing

activated alumina as the adsorbent.

Regeneration of Activated Alumina with Aluminum Sulfate

Figure 2-6 shows the schematic diagram of the apparatus used by

Savinelli and Black (20) to investigate the removal of fluoride via

activated alumina. The purpose of their study was based on the presump­

tion that a dilute solution of aluminum sulfate could be used to regen­

erate exhausted activated alumina. Specifically, they studied the

effects of the following factors on the exchange capacity of activated

alumina: a) quantity of aluminum sulfate used during generation; b)

concentration of aluminum sulfate solution used during regeneration; c)

flow rate of aluminum sulfate solution during regeneration; d) flow rate

of water being treated during exhaustion and; e) concentration of fluor­

ide ion in the raw water.

The apparatus used in this study consisted of one-inch glass tubes

each 24 in. long packed with activated alumina. The bottom of each tube

was drawn and joined to a ground glass stopcock used to control the flow

of water through the column. Above the point where the tube was drawn,

a perforated clay disc supports the bed of activated alumina. The flow

through the column was measured by a rotameter connected to the stopcock

by rubber tubing. The raw water to be treated was prepared in a

55-gallon drum and distributed through a manifold to the 12 columns

attached. The open end at one end of the manifold allowed a slow stream

of water to overflow and produced the effect of a constant head device.

37

CO

O) • I —

3 4-> 00

C

o •r-•l-> to

•r—

o 3

o

CO

Z3

</) 3

+-» «o to Q . CL

E to

cn fl3

• M to E OJ

o oo

vo I

Cvl

OJ

3

cn

38

The overflow returned to the 55 gallon tank. Before collection of any

test data, the authors (20) conditioned all the activated alumina used

by passing it through at least ten complete cycles of regeneration with

aluminum sulfate and exhaustion with water containing 10 ppm of the

fluoride ion. The flow rate during regeneration was 5.5 ml per minute.

Therefore, the 2 liters used for regeneration required approximately 6

hrs. to pass through the column. After regeneration, the columns were

backwashed for 30 minutes with distilled water at such a rate as to

cause 100 percent expansion of the bed based on the original bed volume.

During the treatment phase, the water used to exhaust the beds contained

10.0 +_ 0.1 ppm fluoride ion and was passed through the beds at a flow

rate of 28 ml per minute.

The results obtained in this investigation showed that the capacity

of the alumina for fluoride ion increases rapidly with the amount of

aluminum sulfate used for regeneration up to a maximum of about 4.6 kg 3

per cubic meter of bed (2,000 grains/ft ). Figure 2-7 shows this rela-

3 3

tionship. Beyond a capacity of 2,000 grains/ft (4.6 kg/m ), the

increase in capacity is very slight even with large increases of

aluminum sulfate. Additional findings were that for exchange capacities

of up to 1,500 grains per cubic foot of bed (3.45 kg/m ), the amount of

aluminum sulfate required per kilograin of fluoride removed was

practically constant at about 2 pounds per kilograin. Beyond this

point, the regeneration efficiency decreases. This result is shown

graphically in Figure 2-8. In order to verify the effect of

concentration of regenerant solution on exchange capacity, a constant

amount of filter alum per column but various amounts of volume of water

\

39

2200-1

**>v«^^im«af"^p«^i^«^^i^<^<^^^^^iiPV^^^w«M^M^^v^>^^^M^^^Mi

S 8 10 12 m 16 18

flLUH (La/CU FT)

Note: 434.78 grains/cu f t is 1 kg/m^

Figure 2-7. Relation of Exchange Capacity and Regeneration Level (20)

40

2200-!

3 M 5 S 7

ALUM PER KiLOG* FLUOSIOE-CS

Figure 2-8. Relation of Exchange Capacity and Regeneration Efficiency (20)

41

in which it dissolved was used. The results showed that for solutions

ranging from one to ten percent by weight of aluminum sulfate, the

concentration of the regenerant had little or no effect on the exchange

capacity of the activated alumina.

The test on the effect of the exhaustion flow rate on the exchange

capacity showed that for flow rates of 0.5 to 2.0 liters per hour, there

was little change in the exchange capacity but that when the flow rate

was raised above 2 liters per hour, the exchange capacity of the activa­

ted alumina decreased. Interestingly enough, however, the authors found

that the time of contact of the alumina with aluminum sulfate during re­

generation played a significant part in the exchange capacity of the

medium. Figure 2-9 shows that the exchange capacity of the activated

alumina ranges from about 600 grams per cubic foot of bed (1.38 kg/m )

for a regeneration time of one hour to about 1,750 grams per cubic foot 3

(4.03 kg/m ) for a regeneration time of four hours for the same

regeneration level. For contact times of more than 4 hours, the

increase in exchange capacity was small (20).

The data presented in Figure 2-10 showing the exchange capacity as

a function of the fluoride ion concentration of the influent water, in­

dicate that there is a linear relationship between the two variables.

However, the alkalinity of the influent water also plays an important

part in the fluoride removal process. The study showed that as the al­

kalinity of the influent water increases, the capacity of the medium to

remove fluoride ion decreases. Hence it was inferred that the alkalin­

ity in the water could be competing with the fluoride ion in the

exchange process. This result on alkalinity is confirmed, by a study of

42

2000 H

ISOO-

100-

0-i» i '

C 60 120 ISO 2^0 3QC

RSGENtRfiTlCN TJMECHIM)

420

Figure 2-9. Exchange Capacity of Activated Alumina vs. Regeneration Time (20)

43

1700 H

1600-

500-

U 5 6 7 a

iNlTIflL FLUORiOE CONCENTRflTION (PPM)

JO

Figure 2-10. Exchange Capacity of Activated Alumina vs. Fluoride Ion Concentration in Influent (20)

44

pH effect of the influent water on fluoride exchange capacity of the

alumina. Figure 2-11 summarizes the result of this study. From this

figure, it can be seen that when the pH of the influent water is lowered

to 5.6 with acid, the capacity of the alumina to remove the fluoride ion

increases to about 3,400 grains per cubic foot (7.83 kg/m^). It can be

concluded therefore that neutralization of the alkalinity in the water

is immediately effective in substantially increasing the capacity of the

alumina for the fluoride ion.

The last variable studied was the effect of high concentration of

the sulfate and chloride ions on the exchange capacity of activated

alumina. Very little effect on exchange capacity was found with concen­

trations of either of these ions up to 1,000 ppm.

To summarize, one. might say, that although alum has been shown to

be an effective regenerant of activated alumina, it does have certain

limitations in its applicability (20). The most important of these is

the problem of precipitation in the bed which causes clogging of the

bed. Although the reason for this precipitate is not clearly known, it

has been attributed to the use of aluminum sulfate for regeneration

(20). Clogging is a problem that cannot be overlooked in adsorption

beds because a decrease in flow rate of water through the bed seriously

reduces the efficiency of the adsorbent (2, 20).

Fluoride Removal Using a Fluidized Bed

Another major research work in the field of fluoride removal was

performed by P. Bishop utilizing a fluidized bed of activated alumina

(1). The hypothesis was that fluidization would eliminate the usual

45

• r

5.4 7.2 7.8

PH OF INFLUENT

S.g

Figure 2-11. Relation of Exchange Capacity and pH of Influent (20)

46

problem of bed clogging associated with previous removal techniques (20)

and would enhance the efficiency of the adsorbent. To verify this, the

author conducted a series of batch and continuous flow tests (1). The

batch tests verified the effects of medium mesh size, pH, fluoride con­

centration and alkalinity on fluoride removal capacity of activated

alumina. The continuous studies evaluated fluidization characteristics,

fluoride characteristics, regeneration procedures, and extent of

deterioration of the medium.

The batch studies used one liter reactors. Air was supplied at 28

kpa (4 psig) and regulated so as to circulate the activated alumina

through the reactor. The temperature of the reactors was kept constant

by the use of a constant temperature bath.

A schematic diagram of the apparatus used in the fluidization study

is shown on Figure 2-12. Feed water was pumped from a plastic carboy

through the column by a variable speed peristaltic pump. The acrylic

column is 5.1 cm in diameter, 1.5 m tall and had effluent ports at 15.2

cm intervals. Activated alumina in the column rested on a bed of glass

beads used to achieve a uniform distribution of feed within the column.

Regenerant chemicals were pumped through the column by a peristaltic

pump. A fluoride specific electrode and pH meter monitored all effluent

concentrations.

Preliminary batch studies showed that use of the 14-28 mesh activa­

ted alumina was impractical because of the high fluid velocities requir­

ed to fluidize the medium. Moreover feed water concentration higher

than 8.0 ppm in fluoride could not be reduced to 1.0 ppm at such high

47

AirVent OiSTiHed Water

XapWater

^Ai^ i^gATSupp'v

flecorcier

MiJiivolt

Ion EiectrcdG

Dram U

Hose Clampv,^^ [

4000 mt Er lenmeyer

!

t Beads^

Ordin 1

Figure 2-12. Schematic Diagram of Fluidized Reactor 9v«;tpm for Defluoridation Studies (1) System for Defluor

48

velocities. To obtain the most efficient medium size for fluoride

removal, activated alumina was ground into size ranges of 14-28, 20-28,

28-42 mesh. Two grams of each size were allowed to contact a solution

of known fluoride concentration in the batch reactors until equilibrium

was reached between the alumina and fluoride. Figure 2-13 shows the re­

lationship between exchange capacity of the activated alumina and the

average mesh size of the medium. It shows that capacity increases with

decreasing particle size. The author concluded that the difference in

capacity was the result of the increased surface area per unit volume of

the smaller particles suggesting that fluoride removal by activated

alumina was a surface phenomena. The studies on the effect of pH on the

exchange capacity of activated alumina within the pH range of 5.5-8.0

showed that higher removal capacities were achieved at pH below 6.0 but

that capacities were still reasonably high in the pH range of 6.0 to

8.0. This result is shown in Figure 2-14. The author found a linear re­

lationship. Figure 2-15, between removal capacity of activated alumina

and initial fluoride concentration in the feed water. The result of

this study also showed that increasing the initial alkalinity of the

feed water decreased the capacity of the medium. This relationship is

shown on Figure 2-16.

Regenerant procedures were evaluated using five batch reactors each

containing 3 grams of spent media. Different concentrations of regener­

ant (caustic soda) were added and the systems were allowed to

equilibrate. The results shown in Table 2-7 show that regenerant

concentrations greater than 2 percent sodium hydroxide desorbed all of

the fluoride in the medium. For regenerants less than 2 percent, the

49

^.^6 2.52 2.58 2. CM 2.70 2.76 2.82

€XCHfiHO€ CPPflCITTfKG/CU METER)

2.88 2.9M

Figure 2-13. Effect of Activated Alumina Particle Size on Fluoride Removal Capacity ( l )

2.7H

50

2 . 6 -

l . G -

1.5-

5.0 5.5 ' I • ' ' • ' ' ' ' ' 1 I I • . ,

6.0 6.5 7.0 7,5

PM OF INFLUENT

8.0

Figure 2-14. Effect of pH on Alumina Fluoride Removal Capacity ( l )

51

6 7 8 9

INITIAL FLUOP.IDE CONC. iPPM)

Figure 2-15. Effect of Influent Fluoride Concentration on Alumina Fluoride Removal Capacity ( l )

52

l.JO-f

o.uo-100 125 150 175 200 225 250 275 300 325 350 375 400

INITIRL RLIJflLJNj'TT (PPMJ

Figure 2-16. Exchange Capacity of Activated Alumina vs. Alkalinity of Influent (l)

53

TABLE 2-7 (1)

Efficiency of Activated Alumina Regeneration

Regenerant Percentage Removed

5 percent NaOH 100

2 percent NaOH 100

1 percent NaOH 93

0.5 percent NaOH 86

2.0 percent NaOH-NaS203 90

Note: Regenerant volume = 1 liter; spent activated alumina = 3 gm.;

detention time = 1.0 hr.

54

desorption process was incomplete. The regenerants of lower concentra­

tion would eventually restore the medium to its initial capacity after a

much longer time. The fluidized column provided information on the

general behavior of the medium. Table 2-8 shows the actual and

predicted flows required to achieve a given expansion of the activated

alumina bed. Minimum fluidization velocity was 0.7 x 10 m/s for the _3

35-50 mesh size. A rate of 8 x 10 m/s was necessary to produce a 100

percent bed expansion.

Finally, the column was operated through a series of eleven cycles

of exhaustion and regeneration to investigate the effects of long term

use of the medium, to estimate the exchange capacity of the medium, and

to determine the most effective regeneration procedure. The results of

this investigation are shown in Table 2-9. In all cases, the column 3

contained 500 cm of medium (0.43 kg). Unexpanded medium depth was 25.4

cm (10 in). Initial fluoride concentrations and methods of regeneration

varied during each cycle.

TABLE 2-8 (1)

Column Fluidization Characteristics

55

Flow

10'- m/s

0

2

4

6

8

10

12

16

20

Vel ocity

qpm/sq.

0

3

6

9

12

15

18

24

30

.ft.

Bed Expansion Achieved (percent)

0

25

50

75

100

125

160

210

270

Theoretical Bed Expansion (percent)

0

27

62

85

114

130

150

200

275

-3 Note: Minimum f l u i d i z a t i o n ve loc i ty - 0.7 x 10" m/s (1.0 gpm(sq. f t . )

56

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Q

CHAPTER 3

EXPERIMENTAL PROCEDURES

In this chapter, the defluoridation scheme and method of the exper­

imental procedures will be outlined. This experimental study is organ­

ized very much along the line of work reported by Bishop and Sansoucy

(1). Selection of the experimental apparatus was primarily based on

preliminary design calculations done by C. S. Lee (see Appendix). Final

design was based on data collected during the batch studies.

Procedure for Determination of Fluoride Concentration in Water

All fluoride concentrations were determined by the direct measure­

ment procedure. In this method, samples of fluoride standard solutions

were prepared by serial dilution of the 100 ppm standard. Five ml of

Total Ionic Strength Adjustment Buffer solution (TISAB III) (1, 20) were

added to each sample in order to break fluoride complexes of aluminum

and iron. TISAB III also adjusts the pH and swamps out variation in

total ionic strength. This method allowed sample fluoride concentration

to be determined independent of the level or nature of dissolved miner­

als. The reference electrode, Orion model 90-01-00, and the specific

ion fluoride electrode, Orion model 94-09, were used throughout this

study. All millivolt measurements were made with the aid of the Orion

701A digital pH/mV meter.

To obtain the standard calibration curve (Fig. 4-1), the electrodes

were rinsed, blotted dry and placed in the fluoride standard solution

prepared by the above description. The standard was continuously stir­

red and millivolts readings were taken only after a stable reading was

57

58

observed. The procedure was repeated for each of the fluoride standard

solutions. All readings were duplicated for a check. The duplication

showed -^ 0.5 mV variation. A plot of the millivolt readings against

concentration gave the standard calibration curve from which all subse­

quent concentration values could be directly obtained once the millivolt

reading was available.

Batch Studies

This part of the study was conducted to determine the effects of

pH, influent fluoride concentration, temperature of sorbent during sorp­

tion, and the sorbate-sorbent ratio, on the exchange capacity of activa­

ted alumina for fluoride ions. Regeneration studies were also conducted

to determine the possibility of thermal regeneration for spent alumina.

pH

Aluminum oxide, the main component of activated alumina, is an am­

photeric compound. When the compound is mixed with an acid solution,

the following surface reaction takes place

H"

(Al203)^Al203 + 3H2O ;^=:^ (Al203)^Al(0H)3 (3-1)

The product, aluminum hydroxide complex (Al203)^Al(0H)3, will raise the

pH of the solution. On the other hand, when activated alumina is intro­

duced into an alkali solution, the surface reaction

OH"

^^^2°3^n^^2°3 •" ^2° ^ " ^ ^^^2^3'n^^3^^°3 " " ^

59

occurs. The complex (Al203)^2H3A103, will lower the pn of the soliticn

(23). Hence, the type of complex formed strictly depends upon the acid­

ity and alkalinity of the solution in which adsorption of flt^orice

occurs.

In order to determine the optimum pH for fluoride removal. 130 ml

of 10 ppm fluoride solutions with pH 3, 4. 5. 7, 9, .-d 11 were made up

in plastic beakers. Correction of the pH was Kcomplishea through the

use of 20 percent by weight caustic soda solution or 20 percent by

weight hydrochloric acid solution, whichever applied. The experi'-ents

were conducted at room temperature (23 ^ 0.5°C). The 10 ppm fluoride

solutions were contacted with 0.1 grams of activated alumira under con­

tinuous stirring and constant temperature. The residua] fluoride

concentration in the solution was recorded as a function of time until

adsorption reached equilibrium. The concentraticn-time history or the

10 ppm solutions at different pH's were used to calculate the exchange

capacity of activated alumina for fluoride ions in aqueous solutions at

different pH values. This pH test was useful in estalrlishing the

optimum pH for operation of the packed bed column.

Concentration Effects

In addition to pH, the value of the initial rluoride concentration

would also significantly influence the extent of fljoride rerr.cval.

Therefore, it was decided to study the effect of changing f e imt.i3l

fluoride content of the water being used at the optimum ;)'^.. For the

purpose of this study, 100 ml of laboratory fluoridated water ^ith ini­

tial fluoride concentrations 0.5, 1.0, 2.0. 4.0, 8.0, and 10.0 - were

placed in P|j,?fi^ &iS.^M^^- ^^^^® solutions were contactt^J w^th C l r " 41

Jk

tot ^ } y^ u \v-, Vf" "^ ft'

. J vSi cJ ji? J ^ ^ Hi _ „ a 'xhf—

60

grams of activated alumina under constant stirring and temperature over

a period of 10 hours. After this time, adsorption had reached

equilibrium. During the time of contact, the residual fluoride in

solution was monitored initially after eyery fifteen minutes during the

first two hours. Later, residual fluoride was measured every one hour.

The data collected in this study established, among other things, the

time it took to obtain adsorption equilibrium for the initial

concentrations in question. Also, the variation in exchange capacity as

the initial fluoride concentration changed could be calculated.

Effect of Temperature on Exchange Capacity During Sorption

In this study, it was essential first of all to determine the temp­

erature dependence of the selective ion probe. To do this, solutions

with fluoride concentrations 2.0, 4.0, and 8.0 ppm were prepared from a

sodium fluoride standard solution at temperatures 5 +_ 1.0°C, 24.5 _+

0.5°C, and 54 +_ l.O^C. The low temperature was achieved by immersing

the plastic beaker containing the fluoride solution in an ice bath stor­

ed in the refrigerator at all times. In order to minimize errors aris­

ing from evaporation at the higher temperature, it was essential to pre­

pare the fluoride solution with water at a temperature slightly above 54

°C and use a constant temperature bath during all measurements.

With the fluoride standards all set, the concentration of fluoride

in each solution at the different temperatures was determined using the

pH/mV meter. The variation in the mV readings established the tempera­

ture dependence of the fluoride ion probe.

HP «K"

61

The second part of the temperature stjc: was to establish tne vari­

ation in the rate to adsorption equilibrium jivd contact t^mes at temper­

atures 5.0, 24.0, and 54°C. The data collected in this pa-t of the

study enabled the determination of the rate to equilibrijr. and contact

times for the fixed bed operations. All r..peri:r.onts were conducted with

three different initial fluoride concentrations — :, 4, and 8 ppt;:. Ir,

order to also establish the effect of sorbate/sorbent ratio cn the

exchange capacity of activated alumina, it ^as essential to vary the

amount of sorbent that contacted the given rluoride cor-ontrations. For

this purpose, 1.0, 0.5, 0.25, 0.1, and 0.5 gm of activated alumi--: wrre

used to contact 100 ml, 4.0 ppm fluoridated -vater at 25.0°C. Ml other

experiments in this study used activated alumina doses of 1.0, 0.5 and

0.05 grams to contact 100 ml of initial fluori^.e concentrations of 2.0.

4.0, and 8.0 ppm. The residual fluoride concentration became tne cieprn-

dent variable and time was the independent variable. Two hcurs were

sufficient to achieve adsorption equilib'^ium during this par*- of the

study.

Finally, we needed data that would enable us to specify the quanti­

ty of activated alumina needed to treat a given quantity of ::n*ar.inated

water. To determine this, 0.1 gm activated alumina was adced to 100 rl,

8 ppm fluoridated water in increments of 0.01 gm at ^e three y.fferfirx

temperatures 5.0, 24.0 and 54°C. Residual fluoride concentration in

solution was measured at 10 minute intervals.

3atc^ Theriia] Regeneration Study

T-e r^''?cse '5 Dct^.or c"' z^e st-rv

. - - - f i " - ' • .- c r — - .- "• T C ^ — ^'

62

gms of activated alumina was contacted with 100 ml of 10 ppm fluoridated

water each time until the sorbent was totally saturated with fluoride.

The saturated activated alumina was divided into five parts and each

part was regenerated at temperatures in the range of 0 to 1200°F. The

regeneration was accomplished with the use of a high temperature

furnace. The spent activated alumina was transferred to a porcelain

boat and inserted into the furnace at temperatures 217°F, 517°F, 825°F,

and 1300 F. The heating unit was turned off after a period of 1 hour.

Then the regenerated activated alumina was taken out of the furnace,

allowed to cool, and kept dry in a desicator. Then a residual fluoride

concentration versus time relationship was obtained using 100 ml of 10

ppm fluoridated water and varying amounts of regenerated activated

alumina to study the effect of temperature on regeneration.

Column Study

The column study was designed to analyze the dynamics of the

adsorption process under a constant flow of the fluoride containing

water through the void space of a fixed bed of activated alumina.

Column operations have an advantage over batch operations because rates

of adsorption depend on the concentration of fluoride in solution and,

for column operation, the column is in continuous contact with fresh

solution. While in the batch studies no attempt was made to evaluate

the effect of transport mechanism on adsorption, the column study is

designed to evaluate the transient characteristics of adsorption.

As the fluoride containing water moves down by gravity through the

voids of the alumina bed, the fluoride is adsorbed on the alumina

63

surface. As this process continues, the bed becomes partially saturated

and becomes less effective for further adsorption. The breakthrough

occurs when there is the appearance of treated water in the activated

alumina bed effluent at a concentration greater than 1.0 ppm. The

breakthrough curve is obtained by plotting effluent concentration

against either time or volume of flow. The factors influencing the

breakthrough curve are flow rate, bed depth (or volume), contact time

and other factors influencing the process of adsorption. The break

point, the point at which the effluent concentration of treated water

becomes close to the initial concentration of the fluoride containing

water determines when the process should be discontinued and the bed

regenerated for further use. Table 3-1 summarizes the list of apparatus

and chemicals, and Figure 3-1 shows a schematic diagram of the packed

column used in this study. The column was packed with 283.85 gms of

28-48 mesh activated alumina.

Column Regeneration Study

Different methods for regenerating the spent activated alumina were

studied. The most conventional method of using caustic soda for regen­

eration was investigated. This process consisted of backwashing the

spent bed with 2 liters of distilled water for a period of 30 minutes.

The water entered in an upward flow and was recirculated by the aid of

the pump. Following this backwash, 50 ml of 2 percent sodium hydroxide

was used to regenerate the bed for a period of 30 minutes. This was

followed by a dilute acid neutralization using a 50 ml, 20 percent IN

hydrochloric acid. Finally, the bed was rinsed with 2 liters distilled

64

TABLE 3-1

List of Apparatus Components (Sizes) and Chemical Reagents Used

Apparatus Chemicals

Single junction reference electrode

Fluoride specific ion electrode

4 mm glass beads

1 l i te r Erlenmeyer plastic flask

1/8 in. Tygon tube

Plastic valves

Plastic tubing connectors

Acrylic column (1 f t . x 3 in.)

Pump (1/25 hp)

10-gal. Carboy

Digital pH/mV meter

Media: 28-48 mesh activated alumina

Total ionic strength adjustment buffer solution (TISAB I I I )

Sodium fluoride standard NaF (100 + 0.5 ppm)

Sodium hydroxide solutions

Sulfuric, hydrochloric acid solutions

65

. 5 <D .y p

? ; • ' .• .-••>>

/

•fs

CO

c O

•r-•t-> 03

-o •r—

o 3 <1>

O

4->

c

O) CO

ZD

(A 3 + J 03 im to o. ex o:

x:

03

u C l 03

• I — O

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o

I CO

(U

3 CD

66

water. The bed was ready for use when the effluents' water pH fell back

to 5.0. Briefly summarized below is the regeneration scheme with sodium

hydroxide:

Backwash With Distilled Water

Regeneration With 2% NaOH

Dilute Acid Neutralization

Rinse With Distilled Water

Another regeneration method studied was thermal regeneration; the method

used essentially corresponds to that of the batch studies. The bed was

removed and regenerated at 800°F.

In order to compare the effectiveness of both regeneration modes,

runs were made using the regenerated beds to remove fluoride from waste­

water generated by Texas Instruments. The results of these removal

tests were compared to that obtained using a virgin activated alumina

bed.

CHAPTER 4

RESULTS AND DISCUSSION

Calibration Curve

Figure 4-1 shows a semi-log plot of fluoride concentration in parts

per million and the millivolts recorded on the pH/mV meter. This stan­

dard calibration curve was utilized for the determination of fluoride

concentrations in this study. Concentrations below 0.1 parts per

million were obtained through extrapolation of these experimental

values. The standard calibration was done at room temperature, 24.5 j

0.5 C, and a solution pH of 5.0 +_ 0.2. For conditions different from

the standard, corrections were made using the temperature and pH

dependences of the selective ion probe as shown in Figures 4-2 and 4-3.

pH Effects

The influence of pH on the removal of fluoride by activated alumina

is shown in Figures 4-4 and 4-5. Figure 4-4 shows that the residual

fluoride concentration is a function of both pH and contact time. The

results obtained with initial fluoride concentration of 10 ppm (10

mg/liter) and activated alumina dosage of 0.1 grams per 100 ml solution

clearly showed that the maximum efficiency of fluoride removal occurred

at a pH of 5.0. At pH 5, 81 percent of the initial fluoride concentra­

tion of 10 mg/liter was removed. The exchange capacity (calculated as

shown in the Appendix) under the same conditions as given in Figure 4-4

is graphed in Figure 4-5 as a function of pH. Figure 4-5 shows that the

exchange capacity is pH-dependent. A decrease in exchange capacity was

67

68

10000.0 H

L U 0 R I 0 E

C 0 N C E N T R fl r J 0 N

( P P M )

1000.0 -

100 .0 -

lO.C-

1.0-

0.1

2 0 0

1 8 0

1 6 C

1 4 0

1 2 0

1 0 0

8 0

6 0

4 2 2 0 0 0 0

•1ILuIV0L''S

4 0

6 0

8 0

1 0 c

1 2 0

1 i

4 0

1 6 0

1 8 0

2 0 0

Figure 4-1. Calibration Curve Showing Fluoride Concentration versus Millivolts at pH 5 and 24.5°C

69

10.00-1

1.00-1

L

u 0 R I 0 c

c 0 N c '( p p M C. .'0 )

CC] -• r ^ ^ ^ T ^ ^ ^

PH = 5.08 ^ 0-2 fEMPERflrURE !N OEG.C

!0 !S 20 2S 30

MlLLJVOLrS

• ^ ' I >T ••» M l

40 4'6

Figure 4-2. Temperature Dependence of the Fluoride Specific Ion Probe

•N

70

10.00 H

1.00-F L U 0 R I 0 E

C 0 N C

( P P M ) 0.10

0.01 -^ » i f H ' » l l ' » i f " » » f » TT^ !• • I " I' I —

-20 -10 0 :C 20 30 40 SO 60 7C 80 90 100

MlLLIVOLrS

Figure 4-3. pH Dependence of the Fluoride Specific Ion Probe. T=24.5°C

71

pH=11.0

pH=9.0

pH=7.0

pH=3.0 2pH=4.0

pH=5.0

TIME IN .HOURS

Figure 4-4.

• J .1 ciiinride Concentration Relationship ^flll'^^'T^^^^^^^^^^ Contact Times in Solution and the pH aL T=24.50C

72

7.C-J

6 . 5 -

6 . 0 - "

s.s-E X c M fl S.O-N G E

C 4.5-fl P P C I 4 . 0 -T

r

( K 3 . 5 -G / C U

3 . 0 -M E T E R 2 . 5 -)

2 . 0 -

1 . 5 -

l . C -' I 1 ' ' ' t t > * ' ^ • • p i ^ ^ ym

G 7

PH OF INFLUEN''

10 11

Figure 4-5. Relationship Between the Exchange Capacity of Activated Alumina and the pH of Influent T=24.0 + O.5OC

73

observed as the system pH varied from pH 5. This study showed that the

maximum exchange capacity of fluoride adsorption occured at pH 5. When

the pH either decreased below 4.0 or increased above 6.0, the exchange

capacity decreased sharply.

It has been found elsewhere (4) that anion adsorption sites on min­

erals such as alumina are aquo groups (-M-0H2' ) and hydroxo groups

(-M-OH). The surface chemistry of an oxide in contact with an aqueous

solution is determined to a large extent by deprotonation or a hydroxyl

ion association reaction. For a positively charged surface, the

reaction

and.

•M-OHg' ., ^ -M-OH + H"*" ^^'^^

-M-OH + 0 H " ^ = ± - M-O" + H«0 (4-2)

for a negatively charged surface may be written (6). In this study, as

the pH decreases below 4.0, AI2O3 will be dissolved as Al' '*' and

subsequently, positively charged strong aluminum complexes such as AlF

and AIF2 are formed. The surface of AI2O3 will be further positively

charged with a decrease in pH. Therefore, the sharp decrease in capac­

ity below pH 4 is probably a result of the formation of positively

charged aluminum complexes as well as electrical repulsion between the

positively charged complexes and the positively charged surface. On the

other hand, as the pH increases above pH 6, the hydroxo groups (-M-OH),

as shown by equations 4-1 and 4-2, will gradually disappear forming an

increasingly negatively charged surface. In addition, OH' will compete

74

with fluoride ions for the available sites left on the surface, hence,

contributing to a decrease in the exchange capacity.

Concentration Effects

The effects of the initial fluoride concentration on the perform­

ance of activated alumina in fluoride removal are shown in Table 4-1 and

Figure 4-6. Table 4-1 shows the residual fluoride in solution when sam­

ples containing different levels of fluoride are allowed to contact 0.1

grams of activated alumina during a 10 hour period. Figure 4-6 shows

the dependence of the exchange capacity (the amount of fluoride removed

(kg) per unit volume (m ) of sorbent bed) on the initial fluoride

concentration. The exchange capacity of the adsorbent increased

linearly when the initial fluoride concentrations in the concentration

range studied were increased. The sites present on the interior surface

of a pore may not be as easily available (because of resistance to pore

diffusion) as the sites on the exterior surface. The energetically less

active sites will also be more fully occupied as the activity of

fluoride increases.

Isotherm Study, Temperature Effects

Figures 4-7 to 4-15 show residual fluoride concentrations at temp­

eratures of 5.0 + 1.0°C, 24.5 + 0.5°C, and 54 + 1.0°C. From these fig­

ures, it is seen that, regardless of the temperature of the system, flu­

oride removal generally increased with an increase in the amount of act­

ivated alumina used. It can, therefore, be said that the amount of ad­

sorbent used in a particular removal scheme will be set by the level of

fluoride contained in the raw water, the level of removal desired and

75

TABLE 4-1

Residual Fluoride Concentration in Solution During Contact With 0.1 Grams Activated Alumina

Time (hrs)

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

2.75

3.00

7.00

10.0

10.0

6.80

5.80

5.20

4.70

4.40

4.10

3.90

3.70

W • W «i

3.45

3.30

3.25

1.95

1.90

Residual

8.0

4.40

3.20

2.75

2.50

2.33

2.18

2.10

1.95

1.90

1.88

1.84

1.82

1.50

1.50

Fluoride (

4.0

2.60

1.98

1.70

1.34

1.20

0.99

0.90

0.86

0.76

0.75

0.74

0.68

0.50

0.50

Concentration

2.0

0.78

0.62

0.49

0.46

0.41

0.35

0.33

0.32

0.29

0.28

0.26

0.25

0.18

0.18

(ppm)

1.0

0.56

0.21

0.15

0.13

0.11

0.10

0.09

0.08

0.08

0.07

0.07

0.06

0.06

0.06

0.5

0.12

0.09

0.07

0.06

0.05

0.04

0.03

0.04

0.03

0.03

0.03

0.03

0.02

0.02

76

0.0-

2 3 4 5 6 7

.'NjriflL FLUORIDE CONC. iPPM)

Figure 4-6. Relationship Between Exchange Capacity and Initial Fluoride Concentration. T=24.5°C pH=5.0, Contact Time = 10 hrs

77

10.000 H

0 .010-

0.00! -

INITIAL FLUORIDE CCNC.-4.CPPM TEMPERATURE = 2 4 . 5 OEG.C

PH = 5.0 M = flCTIVflTEO PLUMlNfl (GRflMSJ

i i i i , I l l , I I I ^ I I I I I I 1 1 ' " I

0 10 20 30 40 50 60 70 80 90 100 110 120

TIME IN MINUTES

i.£GEN0: M > • * 0.05 — > ^ i> 0.5 aoa— !

Figure 4-7. Residual Fluoride in Solution Using 4 0 ppm Tniiial Fluoride Concentration at pH 5, 24.5 C "' .!:/.Tnncp<: nf Activated Alumina and Various Doses of Act

78

R E S I 0 u fl L

F L U 0 R I 0 E ( P P M )

INITIAL FLUORIDE C0NC.=8.CPPM TEMPERATURE = 24.Si 0-5 OEG. C

PM = S.O iO^^ M = ACTIVATED ALUMINA fGRAMS1

10.000

1.COO -

0.100-

0.010 -

0.001

0 10 20 30 40 SO 60 70 80 90 100 1! 0 ! 20

TIME IN MINUTES

LEGEND- M > . I C.CS « a » C. S 0 3 3 l-O

Figure 4-8. Residual Fluoride in Solution Using 8.0 ppm Initial Fluoride Concentration at pH 5, 24.5°C, and Various Doses of Activated Alumina

79

INITIAL FLUORIDE C0NC.=2.0PPM TEMPERATURE = 24.5*0.3' OEG.C

PH = 4.95*0.2 M = ACTIVATED ALUMINA (GRAMS)

10.000 H

O.OOI -t . i , . r i i r F i r p prw i i r i p w n M 11 j i M 11 m i ^-nt rf m f [ i i • 11 r f i p M i n 11 *, i i i i T i f * t j n FTTT>TT]I

0 10 20 30 40 SO 60 70 80 90 100 ll 0 120

LEGEND: ri

TIME IN MINUTES

C.CS >—•—»~ O.b a C3 J

Figure 4-9. DoQidiial Fluoride in Solution Using 2.0 ppm Initial Fluiride cincenfration at pH 4.95, 24.5°C and Various Doses of Activated Alumina

80

n = PCTIVATED ALUMINA (GRAMS)

10.000

l.OCO

R £ S I 0 u A L

F L U 0 R I 0 E ( P P M )

0.100

o.cio

c.ooi -' ,0 .0 30 . 0 .0 eo ™ eo so

TIME IN MINUTES

LEGEND: M - '

100 HO '-20

L-0

. in Residual Fluoride in ^''\fll\%'',^iA\r.<i Various Figure 4-10. R"iQua Concentration at pH 4. ".

oises of Activated Alumina

81

INITIAL FLUORIDE C0NC.=4.0PPM TEMPERATURE = 55.StIO OEG.C

PH = 4. 90 « 0-2 M = ACTIVATED ALUMINA (GRAMS)

lO.COC H

0.001 -w T i f T T i r i Tff «"y' f r ^ i 1111 I ' I TTf TT r r ' i n n i ] i t- inRF^ i T [»11 n 111 11 ' 11 • 11 i n 11

30 40 50 60 70 80 90 100 110 120

LEGEND: M

TIME IN MINUTES

-0.C5 —> » -a— 0.5 B O B I-O

Figure 4-11. Residual Fluoride in Solution Using 4.0 ppm Initial Fluoride Concentration at pH 4.90, 55.5 C and Various Doses of Activated Alumina

82

INITIAL FLUORIDE CONC.=a.CPPM TEMPERATURE = SS.OilO OEG.C

PH - 4.92 ±0-2 M = ACTIVATED ALUMINA (GRAMS)

10.000

0.001 -

LEGEND: M

TIME IN MINUTES

> > * O.CS » « > 0.5 a a a 1-0

Figure 4-12. Residual Fluoride in Solution Using 8.0 ppm Initial Fluoride Concentration at pH 4.92, 55.0°C and Various Doses of Activated Alumina

83

INITIAL FLUORIDE C0NC.=2.0PPM TEMPERATURE = 5.0 ±1-0 OEG.C

PH = 4.98*0.2 M = ACTIVATED ALUMINA (GRAMS)

10.000

R E 5 I 0 U A L

L U 0 R I 0 E ( P P M )

l.COO-

0.100-

0.010-

0.00! -

G 10 20 30 40 50 60 70 80 90 100 110 120

TIME IN MINUTES

LEGEND: M I * *i O.CS * ' ' • 0.5 aaa i O

Figure 4-13. Dac-iH.ial Fluoride in Solution Using 2.0 ppm Initial Fluiride cincenfration at pH 4.98, 5.0«C and Various Doses of Activated Alumina

84

R E S I 0 U A L

F L U 0 R I

6 E ( p p M )

INITIAL FLUORIDE C0NC.=4.CPPM TEMPERATURE = 5.C± 1-0 DEC C

PH = 4.98 tOZ M = ACTIVATED ALUMINA (GRAMS)

10.000 -\

1.GOO -

0.100

C.CIO -

0.00! -

LEGEND: M

30 40 50 60 70 80

TIME IN MINUTES

*—*—*—O.CS » » » 0.5

100 no 120

a • n L-0

Figure 4-14. Residual Fluoride in Solution Using 4.0 ppm Initial Fluoride Concentration at pH 4.98, 5.0 C and Various Doses of Activated Alumina

85

INITIAL FLUORIDE C0NC.=8.0PPM TEMPERATURE = 5.0±i-O OEG.C

PH = 4.97 10.2 M = ACTIVATED ALUMINA (GRAMS)

10.000 -I

l.COO-

R E S I 0 U A L

L 0.100 U 0 R I D E ( P P M )

0.010-

0.001 -

LEGEND: M

TIME IN MINUTES

* * * 0.05 . » * i 0.5 a a a 1-0

Figure 4-15. Residual Fluoride in Solution Using 8.0 ppm Initial Fluoride Concentration at pH 4.97, 5.0 C and Various Doses of Activated Alumina

86

economics. Optimum fluoride removal will necessarily involve a

trade-off between these three major factors. Since the increase in the

of fluoride removed (as shown in Figures 4-7 to 4-15) showed no

appreciable increase after 2 hours, the contact time required to est­

ablish adsorption equilibrium was set at 120 minutes. Table 4-2

compares the exchange capacities of activated alumina at the three

different temperatures and various quantities of the adsorbent.

Generally, the capacity did not show any strong dependence on

temperature during sorption. There was, however, a minor change in

capacity when the amount of adsorbent was small enough (0.05 grams) to

accomplish the removal at a much smaller rate. On economical grounds,

it will be necessary to use the minimum amount of activated alumina and

smaller rate of adsorption. As Table 4-2 shows, the capacities did not

show any significant change at the three different temperatures when the

amount of the adsorbent was increased to 0.5 and 1.0 grams per 100 ml of

solution. This can be attributed to the fact that close to total

adsorption occurred in the first 30 minutes of contact (see Figures 4-7

to 4-15).

Table 4-3 and Figure 4-16 show the relationship between fluoride

removal and the ratio of the initial fluoride concentration to a unit

weight of adsorbent used. Table 4-3 shows the residual fluoride in sol­

ution during sorption. It is shown that for the concentration range

studied, adsorption equilibrium was essentially reached in 2 hours.

After a period of 2 hours, no significant removal of fluoride was

observed. Figure 4-16 shows the relationship between the ratio of

initial fluoride concentration to a unit weight of activated alumina (R)

87

CVJ I

CO <3:

<u

c to

to

• r - <U E S-3 3

<— + J

s--a <u <u Q. +-> E to 0) > »—

•r-

o c

O ^m

• I —

O +J (O (TJ Q . to 4->

O C CU

O) ^ CD S-C O fC CO

j = - a o < X

UJ <L> C -M (U <U M-

s o CO C Q . O •r- E x: <x: to c o

- M to

OC

O O

LO

CO

cn j ^

•r— O <o CL to

C_3

<u CD c

x: o X

CJ o

LO

LD

CVJ

CJ o o

in o

00 r>» in

C^sJ

r«» r—

a>

o

ir>

o

If)

o • o

CO •sa-«:f •

< —

o r— P^ •

CVJ

o CT> •

^

uo o

LO

3

s-cu E

LO

o

LO

o

to c •r—

E 3

• O CO <U E O

+J <C • fC S- I— > cn

r«>. LO

CO

LO CO CO

CO

CO CO

^ 00 O

r LO ^

LO CO CO

LO LO CO CO

CO CO LO

«!f 00

o

p«v CO r—

LO CO CO

0 0 cy> LO LO

r— CVJ

p>. CO

LO CO CO

LO

CO

«5r

CO LO

c o

4-> to i-

f— +J to C l

•r- <U U--> O

•-- C E e o Q.

• - • C J C3.

«5r 0 0 o

0 0 LO

LO CO CO

o CVJ

o O

00*

o to

4->

3 o o EO

<X. LO

S- LO

to • CsJ

JC -M II

CO 1

^

LLJ - J OQ < : h -

a. •O E <D <U

•M 1— U to **

4 J O C • O LO

CJ II

c O 31

•r- Q . 4-> 3 *

r— rt3 O C

(>0 -r-E

C 3 • r " r"~

< (U

-O "O •r - 0 ) $- 4-> O fC 3 > — •!—

U_ 4-> O

>— < (O 3

-o •r— to 0) a:

LO O

LO CVJ

o o

o o^

o 00

o 00

o 'd-

o CVJ

o o

o 00

o 00

CVJ

o o

v o LO LO LO LO «;r ^

o CO

o •St

o cr> o o

LO LO

• I —

E

0)

O LO LO CO I— n -

O O ^ S l - C O C V J C V J C V J C V J C V J C v j

o o

LO CVJ

o CO

o CO

o f—

r^ cy»

on 00

00 r>»

i>^ r»

^ CVJ I— r - r— O O O O

O O o

CVJ

CO LO cn cr>

CO

LO r -CO CO CVI CVJ

•tr f— O o o o o o

O L O O L O O O ^ L O O C O r - L O C V J r — O C r i O O C D L O C V J , — r - ^ ^ O O

•— O O O O O O O C D

o Lf)

LO CVJ '—

o <T> O

00 LO o

r«. i n o

LO LO o

LO «>t o

r—

^ o

(-) ^r o

o o o o

o o

l O CO

LO LO

lO co 0 0 CVI

o o

CO CO CVJ CVJ

CVJ CVJ

00 f— o o o o o

o o

LO O CO CVI CVJ 1 — I —

o cy» <— o

00 r>s o o ^ o o o o o o

LO •

o

to c

umi

r—• *m^ « t to

E O "O to ' 0 ) s - <— •M cn ( T J — ' >

•p— + j o

<

o o

• <e:t

O O

^

o o

o CVJ

• o

•d-a> o o

CVJ LO o

CVJ cn o

• o

r>* ^ o o

•5t CVJ o

CVJ LO o

• o

CVJ ro o o

r^ r—

o

00 •51-o

• o

•sa-CVJ o o

CO r—

o

cr> CO o

« o

o CVI o o

o f —

o

LO CO O

• o

r^ r—

o o

cn o o

r— CO o

• o

l O r—

o o

00 o o

00 CVJ o

• o

CO r—

o o

00 o o

CVJ o o o o o o

o o o o o o o lO O LO o r— CO ^ LO

LO O LO o (T> O CVI

89

R^INITIAL FLUORIDE CONC.PER UNIT WEIGHT ACTIVATED ALUMI

0 1 2 3

fVW^'^t • * I I I 'f t* VI I P* • « I f • V V V ^ ^

U 5 6 7 8 9 10 1! 12

R IN MILLIGR.AMS PER GRAM

Figure 4-16. Relationship Between Exchange Cap'-icity and the Ratio of Initial Fluoride Concentration to a Unit Weight of Sorbent. T=24.5 + 0.5^0, pH=5.0

90

in milligrams per gram and the exchange capacity of activated alumina in 3

kg/m . The results showed that the exchange capacity increased with

increase in the ratio R. This result essentially confirms the earlier

findings where it was found that capacity increased linearly with

initial fluoride concentration.

Figure 4-17 shows the relationship between the residual fluoride in

solution and the amount of activated alumina required to treat fluor­

idated water containing 8.0 mg/liter fluoride at temperatures of 5.0,

24.5, and 54°C, respectively. The results showed that the amount of

fluoride removed per unit amount of activated alumina increased substan­

tially only at 54°C. The concentration of fluoride removed at tempera­

tures of 5°C and 24°C was essentially the same. The increased

adsorption at elevated temperature is probably a result of increased

activity and diffusivity of fluoride ions in solution due to increased

thermal activity.

Batch Thermal Regeneration

In this part of the study, attempts were made to thermally

regenerate portions of 0.15 grams of activated alumina which had been

saturated with fluoride. Temperatures of 80°F, 217 F, 517 F, 825 F and

1300°F were used. An evaluation of the effects of the thermal

regeneration efforts was accomplished by determining the subsequent

sorption loading achieved by the regenerated alumina. This operation

involved the exposure of 0.15 grams of the regenerated sorbent to

samples of 100 ml of 10 ppm fluoridated water. Figures 4-18 through

4-20 compare the performance of the media regenerated at these

temperatures to that of the virgin activated alumina.

91

SOLUTION = 8.0PPM FLUORIDE STANDARD T = TEMPERATURE IN DEG '

PH = S.OtO.Z

lO.OH

R E S I 0 U A L

F L U 0 R I 0 r w ( p p M )

1.0-

G. 1-

0.00 0.02 0.04 0.06

OUANTITf OF ACTIVATED ALUMINA(GRAMS)

LEGEND- T ' -p -p 5 — » ••• » 25

0.10

• D C3 S i

Figure 4-17. Residual Fluoride in Solution at Different Temperatures and Quantity of Activated Alumina

92

lO.OH

R E S I 0

u A L F L U 0 R I 0 E ( P P M 1

l.O-J

0.1-

VAA.-VIRGIN ACTIVATED ALUMINA T=REGENERATION TEMPERATURE DEG.!

PH = S.C

• . y I »

^ — . ,

\ N. I 3 a o

\ ^ ,

, , M l I I I 'I I I '•

C 2(3 40 60 80 100 120 140 160 ISO

^IME IN MINUTES

T=80°F

T=217°F

T^SU'^F T=1300°F

T=825"F

VAA

Figure 4-18. Residual Fluoride Concentration in Solution Using a Virgin Activated Alumina and the Sorbent Regenerated at Different Temperatures

93

T-REGENERATION rE.MPERATURE DEG '

X--FLUORJOE REMOVED PER UNlV'wEiGHT S0R8ENT

T=825°F

T=1300°F T=517°F

- T=217T

T=80°F 20 40 60 80

100 J20 mo ISO 180

TIME IN MINUTE.":

Figure 4-19. Relationship Between Fluoride Removed Per Unit Weight of Activated Alumina, the Virgin Sorbent and Sorbent Regenerated at Different Temperatures

94

^.bH

' 4 . 2 -

3 . 9 -

3 . 6 -

E 3 . 3 H X C M A 3 .0-^ N G E

2.7-1 C A P A 2.4-1 C I T r 2.1-J

{ K G 1 . 8 ^ / C u

i.sH M E T E \.2-\ R )

0.9-1

O.G-

0 . 3 -

0 . 0 -' I I I . . . . . — ' ' ' I I ' • •

200 400 600 800

REGENERATION TEMPERAI'URE OEG.F

!CCO 1200

Figure 4-20. Relationship Between the Exchange Capacities of Thermally Regenerated Sorbent and Regeneration Temperatures

95

Figure 4-18 shows the residual fluoride concentration in solution

during a contact period of 180 minutes. It is apparent that increasing

the regeneration temperature improves the adsorption pattern. However,

as shown in Figure 4-19, the maximum removal of fluoride occurred during

the first 90 minutes of contact. After this period, the amount of flu­

oride removed was no longer very significant. The sorbent regenerated

at 825 F achieved the highest removal of fluoride. The capacity of act­

ivated alumina regenerated at 825°F was 4.625 kg/m^ of media compared to

5.10 kg/m , that of the virgin material. The overall effect of the

regeneration temperature on the exchange capacity of activated alumina

in the concentration range and quantity of media studied is shown in

Figure 4-20. The capacity increased with temperature up to 825°F and

then decreased as the temperature increased to 1300°F. It should be

indicated that even at the optimum regeneration temperature, 825°F, the

regenerated alumina did not match the performance of the virgin

activated alumina. Although a host of reasons may be associated with

the drop in exchange capacity at 1300°F, one probable explanation would

be the damage caused by thermal stress to the pore structure of the

activated alumina. Ideally, thermal regeneration of activated alumina

should provide a material for reapplication with the same sorption

affinity and capacity as the parent material. However, this study has

shown that thermally regenerated activated alumina achieved only 84

percent of its original sorption capacity (see Figure 4-20). The

mechanism of thermal regeneration of activated alumina is probably

related to activation since both processes are accomplished by heat

treatment.

96

Packed Column Study

The results of the packed column studies for influent fluoride con­

centrations of 10 ppm and 520 ppm are shown in Figures 4-21 and 4-22.

Figure 4-21 was obtained by suing laboratory fluoridated water while

Figure 4-22 was obtained using process waste water from Texas

Instruments. It was expected that the effluent would approach the

initial concentration when the bed reached saturation. However, as

Figures 4-21 and 4-22 show, the initial concentrations were not reached

by the effluent even though over 50 gallons of the 10 ppm and more than

4 gallons of the water containing 520 ppm fluoride was treated. The

overall adsorption in treating the 10 ppm fluoridated water resulted in

the removal of 0.227 milligrams per gram of activated alumina. Overall

adsorption in the 520 ppm waste water treatment resulted in the removal

of 1.113 milligrams fluoride per gram activated alumina. A breakthrough 3

capacity of 8.327 kg/m was realized when the initial fluoride

concentration was 520 ppm. When the initial fluoride concentration was 3

reduced to 10 ppm, a breakthrough capacity of 4.113 kg/m was obtained.

The exhaustion point (the point where the effluent fluoride concentra­

tion becomes close to the initial concentration) for the 10 ppm

fluoridated water was 9.3 ppm and was obtained after treating a total of

72.7 gallons of contaminated water at an average flow rate of 101.92

ml/min (0.026 gallons per minute). For the 520 ppm treated waste water,

the exhaustion point was reached at an effluent concentration of 490

ppm. A total of 4.04 gallons of waste water was treated at an average

flow rate of 56.6 ml/min (0.015 gallons per minute).

97

E F

L U E N T

F L U 0 R I D E

C 0 N C r u N r R A T r 6 N

( P P M )

'1

8J

7-

6-

5-;

4-1

3-

2J

! -

J

INITIAL FLUORIDE CONCrjC OPPM TEMPERATURE = 24 .5 OEG.C '

r

II ' i [ i r p t F " » ^ f »""'f T'^r • F"* * " ^ T -r 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45

'"IME IN HOURS

higure 4 -21 . Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Initial Fluoride Concentration of 10 ppm

98

INITIAL FLUORIDE CONC.=520.OPPM TEMPERATURE = 24.S OEG.C

PH = S.C

500 H

450-

400 E F F L U 350-E N

r F 300-L U 0 R i 250-C E C 0 200-N C E N T J50-R A r F 0 100-N

SO-

Q J I 1 I I I II I I IT

^i I I I I j I I I ' , I 25 SO 75 100 125 ISC 175 200 225 250 275

TIME IN MINUTES

Figure 4-22. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Wastewater from Texas Instruments 520 ppm Fluoride

99

In summary, it may be said that the factors that influence the

breakthrough curve are the flow rate of the raw water and the bed depth

and initial fluoride concentration. These conditions must be carefully

chosen in order to obtain the best fluoride removal by activated

alumina. One of the biggest problems encountered during the running of

the packed column was that of bed plugging. When the bed plugs, the

contact time is prolonged and inadequate experimental data results. To

reduce this problem, it was essential to half-fill the column first with

water before addition of the sorbent. This method produced a loose bed

that permitted water to flow freely under gravity, thereby enhancing the

bed's performance.

Packed Column Regeneration

Two different regeneration modes, chemical and thermal were

examined in this study. Figure 4-23 shows the breakthrough curve

obtained using the chemically regenerated sorbent. The waste water

treated contained 520 ppm fluoride. A breakthrough capacity of 6.764

kg/m^ was obtained. The exhaustion point reached was 500 ppm fluoride

after treatment of a total of 3.5 gallons of waste water at an average

flow rate of 50 ml/min (0.0132 gallons per minute). An overall fluoride

removal of 0.745 milligrams per gram of sorbent was obtained when the

activated alumina was regenerated with a 2 percent sodium hydroxide

solution. To dispose of the fluoride-laden regenerant solution without

contaminating the environment, it was decided to remove the high level

(300 ppm) of fluoride from the regenerant solution. The pH of the

solution was adjusted from 11.0 to 7.2. Then 65 grams of powdered

SCO-I

100

CHEMICAL REGENERATION INITIAL FLUORIDE C0NC.aS20.CPPM

TEMPERATURE = 2M.S DEG C PM = 5.0

450-

0-4» J J J — * - • — ^ 1 1 1 1 1 1 1 I J ^ 1 1 1 1 1 1 1 1 t I • I i p I ^ ^ •

25 50 75 ICO 125 ISC 175 200 225 250 275

HME JN MINUTES

Figure 4-23. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Activated Alumina Regenerated with a 2 Percent Sodium Hydroxi de

101

calcium chloride were added to one liter of the saturated regenerant.

Fifty ml of 1 percent calcium hydroxide solution was added. The content

was agitated by shaking for 1 hour and left to stand overnight. By this

process, the fluoride concentration of the spent regenerant was reduced

from 300 ppm to 20.1 ppm.

Figure 4-24 shows the breakthrough curve obtained using a sorbent

bed regenerated thermally at 800 ^ 5.0°F. A total of 1.855 gallons of

waste water containing 520 ppm was treated. The exhaustion point 3

obtained was 460 ppm. A breakthrough exchange capacity of 4.077 kg/m

was obtained. An overall fluoride removal of 0.499 milligram per gram

of sorbent was obtained. The average flow rate for this part of the

study was 20.65 ml/min (0.0054 gallons per minute).

Table 4-4 summarizes the results of both regeneration modes

studied. Clearly, the chemical regeneration using a 2 percent sodium

hydroxide provides a sorbent with higher capacity and overall fluoride

removal. Nevertheless, thermal regeneration shows a good promise. It

is therefore an area that necessarily requires further study.

102

THERMALLY REGENERATED SORBENT INITIAL FLUORIDE CONC.-520.OPPM

REGENERATION TEMPERATURE=80C DEG.F

500-1

450-

E F F L U E N T

F L U 0 R I D E

C 0 N C E N T R A T I 0 N

i P P M )

400-

350-

300-

250-

200-

150-

ICO-

50-^

c-b r - r * I f f I ! r r T t r t I' >"T r »• T^-^ - ^ ^ ^ ^ *• r T T | T ^ ' i i ' i •" T » T I -

50 100 150 200

TIME IN MINUTES

250 300 350

Figure 4-24. Breakthrough Curve Showing the Effluent Fluoride Concentration at Different Times Using Activated Alumina Thermally Regenerated at 800 F

103

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CHAPTER 5

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

The results of this study showed that:

1) The use of activated alumina in a gravity flow packed-column,

is a simple and yet effective method for defluoridating water supplies.

Because of the effectiveness of the system in reducing low fluoride

concentration (10 ppm) to below 0.15 ppm, most water supplies would not

require pretreatment.

Fluoride removal by activated alumina is pH dependent. The optimum

pH for fluoride removal occurred at pH 5. The capacity for fluoride re­

moval decreased appreciably at pH values below 4.5 or above 8.5. This

pH range will provide an advantage because most water supplies will have

pH in the optimum range and pH adjustment will not necessarily be

required. Furthermore, at the optimum pH, the exchange capacity of

activated alumina increased linearly with increase in the the initial

fluoride concentration. This is again an added advantage since such a

dependence would permit the convenience of a simplified scale-up of

defluoridation plants.

2) Activated alumina can be thermally and chemically regenerated.

Chemical regeneration with a 2 percent sodium hydroxide solution was

more effective than thermal regeneration in restoring part of the sorp­

tion capacity of the spent sorbent. Fluoride removal after thermal re­

generation was initially rapid and levelled off as time progressed.

104

105

825 F is the optimum regeneration temperature for spent activated alum­

ina. An increase in temperature beyond this optimum temperature results

in a drop in the exchange capacity of activated alumina.

Recommendations

Regeneration of activated alumina is an important part of the cost

of operating defluoridation plants. Activated alumina is an effective

removal medium yet its application is limited and regeneration costs are

relatively high (2, 13, 22). Results presented in this study show that

thermal regeneration of activated alumina is promising. Thermal regen­

eration may offer some advantages over the conventional chemical regen­

eration more frequently used. Regeneration would be outside the bed

area preventing the problem of precipitation and bed plugging commonly

associated with chemical regeneration. Further studies involved with

the thermal regeneration of activated alumina are required. In

addition, since the present study established optimum operating

conditions for low levels of fluoride concentrations, further studies

are required to verify the validity of these findings at fluoride

concentrations and alumina doses outside the range studied. Meanwhile,

it is recommended that water supplies with high levels of fluoride be

pretreated either through lime softening or through alum coagulation in

order to obtain a more efficient removal with activated alumina.

Finally, an investigation of the treatment of fluoride-loaded

regenerant liquid with calcium chloride and calcium hydroxide to form

floes of calcium fluoride is needed. This could be sold to the chemical

companies as a fluoride source to help offset the cost of treatment.

BIBLIOGRAPHY

1. Bishop, P. L., "Fluoride Removal From Drinking Water by Fluidized

Activated Alumina Adsorption," Journal of American Water Works As­

sociation, 70 (October 1978): 554-559.

2. Boruff, C. S., "Removal of Fluorides From Drinking Waters," Indus­

trial and Engineering Chemistry, 26 (January 1934): 69.

3. Bulusu, K. R. and Pathak, B. N., "Water Defluoridation with Activa­

ted Alumina," Journal of Environmental Engineering Division (Ameri­

can Society of Civil Engineers) 106 (April 1980): 466-469.

4. Choi, Won-Wook and Chen, Kenneth, Y., "The Removal of Fluoride From

Waters by Adsorption," Journal of American Water Works Association,

71 (October 1979): 562-569.

5. Culp, R. L. and Stoltenberg, H. A., "Fluoride Reduction At

Lacrosse, Kansas," Journal of American Water Works Association, 50

(March 1958): 427.

6. Frankel, Irwin and Juergens, Eric, "Removal of Fluorides From

Industrial Wastewaters Using Activated Alumina," Technical Report

EPA-600/2-80-058 (March 1980).

7. Harmon, A. J. and Kalichman, S. G., "Defluoridation of Drinking

Water in Southern California," Journal of American Water Works As­

sociation, 57 (February 1965): 245-254.

8. Johnson, M. S., Cooke, J. A., and Andrews, S. M., "Fluoride in

Small Mammals and Their Potential Food Sources in Contaminated

Grasslands," Journal of International Society for Fluoride Re­

search, 15 (April 1982): 56-63.

9. Kaniewski, A., Rydzewska, A., Fiejsierowicz, Z., Chmielnik, M., and

Cyplik, F., "Evaluation of Exposure to Fluoride and Its Compounds

in Children Living in the Area of Aluminum Plants," Journal of In­

ternational Society for Fluoride Research, 15 (January 1982):

21-25.

106

107

10. Kobylanska, M., Limanowska, H., and George, B., "A Study of Factors

Aggravating Dental Fluorosis," Journal of International Society for

Fluoride Research, 15 (April 1982): 70-75.

n . Kubli, H., "Zur Kenntnis von Anionentrennungen Mittels Adsorption

an Tonerde," Helv. Chem. Acta, 3 (1947): 453-463.

12. McKee, R. H. and Johnston, W. S., "Removal of Fluorides From Drink­

ing Water," Journal of Industrial and Engineering Chemistry, 26

(August 1934): 849.

13. Meier, F. J., "Defluoridation of Municipal Water Supplies," Journal

of American Water Works Association, 45 (August 1953): 879.

14. Meier, F. J., "Methods of Removing Fluorides from Water," American

Journal of Public Health, 37 (December 1947): 1559.

15. Meier, F. J., "Partial Defluoridation of Water," Public Works

Journal, 91 (November 1960): 90.

16. Miller, D. G., "Fluoride Precipitation in Metal Finishing Waste Ef­

fluent," AIChE Symposium Series - Water: Industrial Wastewater

Treatment, 70 (1974): 144.

17. National Academy of Sciences. Effects of Fluorides on Animals,

Washington, DC: Printing and Publishing Office (1974).

18. Rao, K. v., Purushottam, D., and Vaidya-Nadham, D., "Uptake of Flu­

oride by Serpentine," Geochim. Cosmochim. Acta, 39 (1975): 1403.

19. Rubel, F. and Woosley, R. D., "Removal of Excess Fluoride From

Drinking Water," Technical Report EPA 570/9-78-001 (January 1978).

20. Savinelli, E. A. and Black, A. P., "Defluoridation of Water With

Activated Alumina," Journal of American Water Works Association, 50

(January 1958): 33-43.

21. Scott, R. D., Kimberly, A. E., Van Horn, A. L., Ey, L. F., and War­

ing, F. H., "Fluoride in Ohio Water Supplies - Its Effect,

Occurrence and Reduction," Journal of American Water Works

Association, 29 (January 1937): 9.

108

22. Sorg, T. J., "Treatment Technology to Meet the Interim Primary

Drinking Water Regulations For Inorganics," Journal of American

Water Works Association, 70 (February 1978): 105-112.

23. Wu, Yeun. C. and Nitya, Anan., "Water Defluoridation With Activated

Alumina," Journal of Environmental Engineering Division, 105 (April

1979): 357-367.

APPENDIX

Preliminary Design Calculations (by C. S. Lee)

1) Pressure drop across a packed bed: The "Carman Kozeny" equation

is used:

u =

K" =

e =

u =

i =

u =

d =

S =

-AP =

rhorl

V ^ ^ l ^ ' f for Re < 2.0

5.0 Kozeny's constant

0.56 (estimated)

1.0 centipoise

25 cm

0.7 X 10"^ - 8 X lO"' m/sec

28-48 mesh

94.1 - 187.5 cm^/cm^

n 1 " -2 (1-e e

.. Re = ^ . ^

)2

-AP = 0.547 psi

^ - 0.974 < 2.0

. * . Assumption of Reynold's number range was correct.

. * . AP = 0.547 psi

Column: Acrylic pipe 1' x 3" (1/8" wall)

V = (1.5 in)^ TT 0.8 f t = 0.04 ft^ (0.3 gal) H/jU

Choose V^3^[,j,y = 10 gallons.

109

no

Power of Pump

Assume U ^ ^ = 8 x lO"' m/sec = 48 cm/min

(Velocity for 100% bed expansion.) (1)

W " ^mx ' ^ " ""- /"i '" = 0-34 gpm

W,,, = mgh = Qgh = l;,\\^l',^h^n ' ^ = 8.7 x lO'^ hp

^real ' ^Vev^^ " - ^ 10"^/0.7 = 0.00012 hp

Choose the smallest power pump = 1/25 hp

A reasonable hold up in column promotes interphase contact. Choose re-

cycle flow rate in desorption process between 0.7 x 10" m/s (loading

point) and 8.0 x 10 m/sec (100 percent expansion) for 28-43 mesh

activated alumina (1). The smaller the packing size, the greater the

pressure drop, so choose glass beads 4 mm diameter for bed support.

I l l

Final Column Design: Determination of bed volume

The f ina l column design was based primarily on the results of the

batch studies.

Column height = 10 3/4 i n . = 0.896 f t .

I.D. = 2 5/16 i n . = 0.193 f t . n2

Volume = TT . - ^ . h = 0.195 gal. = 0.738 l i t e r

For batch studies, 0.15 gram activated alumina was used to reduce 10.0

ppm f luoride solution to 1.0 ppm in 3.5 hours. Solution Volume = 0.1

l i t e r , capacity'obta^ined = 6 . 8 kg/m"

Basis: 50 gallons of contaminated water

(0.738 liter)(Q•]^9/^^^'^^^^V^^"^^'"^)(^QJg^ ""^^ ^^^ ' ) '^0.1 l i t e r raw water '^0.195 gal. '

= 283.85 gms activated alumina

Bulk density of activated alumina = 839.45 kg/m (See Table 1)

Vol. = 3.38 X l O ' V

^\ Height of activated alumina bed = 12.4 cm

TABLE 1

Physical Property Data of Activated Alumina Used in Study

Voidage 0.56 (experimentally estimated)

Bulk density 839.45 (experimentally determined)

(kg/m^)

Part icle Size 28-48 mesh

Sample Calculation for Exchange Capacity

112

In i t ia l Fluoride Concentration = 10 ppm

Equilibrium Fluoride Concentration = 1.90 ppm

Activated Alumina = 0.1 grams

Sample Solution Volume = 100 ml

Bulk Density of Activated Alumina = 839.45 kg/m 3

Capacity = ^^^^'^^PP"^ ' 10^(0.1 grams)

1 kg F' 1000 g F"

1000 grams

1 kg

839.45 kg

m

100 g soln

ppm F

Capacity =6.80 kg/m'

113

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