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Int. J. Curr. Res. Chem. Pharma. Sci. (2016). 3(7): 36-50

© 2016 IJCRCPS. All Rights Reserved 36

INTERNATIONAL JOURNAL OF CURRENT RESEARCH INCHEMISTRY AND PHARMACEUTICAL SCIENCES

(p-ISSN: 2348-5213: e-ISSN: 2348-5221)www.ijcrcps.com

Coden: IJCROO(USA) Volume 3, Issue 7 - 2016

Research Article

Study on fluoride and phosphate removal from water usingLa3+ - doped pyrolusite ore from vietnamNguyen Thi Hue*, Vu Van Tu, Nguyen Hoang Tung

Institute of Environmental Technology, Vietnam Academy of Science and Technology,18 Hoang Quoc Viet Road, Cau Giay District, Hanoi, Vietnam

*Corresponding Author: [email protected]/[email protected]

Abstract

Lanthanum has doped onto the surface of the natural Pyrolusite for simultaneous removal of phosphate and fluoride in aqueoussolution. The adsorbent characterization of the materials was observed by the SEM, BET, and XRD techniques. The dynamics andisotherms models of fluoride and phosphate adsorptions, with respect to pH, pHPZC, adsorbent dose, effect of co-existing ions,were studied. The results showed that lanthanum doped Pyrolusite Ore (LDPO) relatively highly adsorbed amount of phosphateand fluoride from aqueous solution. Phosphate and fluoride removal efficiencies of LDPO are approximately 97% and 95%,respectively. Pseudo-first order for kinetic studies of phosphate and fluoride removal of the LDPO were observed with highcorrelations for fluoride but weak correlations for phosphate. However, pseudo-second order for kinetic studies were highcorrelation for both phosphate and fluoride. The results of Langmuir and Freundlich isotherm for both phosphate and fluorideindicated that the adsorption processes of LDPO were monolayer for fluoride and multilayer for phosphate. The phosphate andfluoride adsorption capacities of the LDPO significantly decreased with the existence of co-ions (sulfate, chloride and nitrate) in theaqueous solution.

Keywords: Pyrolusite ore, phosphate, fluoride, lanthanum, dope, adsorption.

Introduction

Extensive research has been performed to eliminatephosphate or fluoride from aqueous solution due to thehazardous effect of theirs cause to the human health.Several methods such as adsorption [1-5],precipitation [6-9], ion exchange [10, 11], reverseosmosis [12,13], nanofiltration membrane [14,15],Donnan dialysis [16,17], and electrodialysis [18,19]techniques have been used for removal eitherphosphate or fluoride. Amongst mentioned methods,adsorption is a conventional technique which is widelyused for removing phosphate and fluoride of waterbecause it is economical, practicable, environmentallysafe and efficient. Various aluminum salts arecombined along with inorganic or organic compoundsto phosphate- or fluoride-contaminated water to formflocs [20,21]. The flocs in turn remove eitherphosphate or fluoride by adsorption or by co-precipitation. Use of calcite to remove abovementioned compounds from aqueous solution by

precipitation method has been investigated by manyauthors [8, 9,22-24]. Reardon and Wang [8] studiedfluoride removal using a lime stone reactor where thedissolution of calcite occurred to form CaF2 precipitate.Otherwise, various adsorbents, activated alumina [25-27], activated carbon [28,29], low-cost adsorbents[5,30-32], rare earth oxides [33,34], natural products[30,31,35,36] like tree bark, groundnut husk, sawdust,rice husk, etc., have widely been used for removal ofeither phosphate or fluoride.

Lanthanum, a transition metal, is applied in manystudies due to environmentally friendly characteristicand relatively abundant in the earth’s crust [35,37].This element is known to have a high affinity forphosphate and the lanthanum–phosphate complexwas even form in low concentrations of phosphate[38,39]. As a result, considerable attention has beenfocused on the use of lanthanum-containing materials

SOI: http://s-o-i.org/1.15/ijcrcps-2016-3-7-7



for the removal of phosphate in recent years[35,38,39]. Likewise several adsorbents for fluoridealso are lanthanum enriched materials [40-42]. On theother hand, iron is studied for removal phosphate inaqueous solution [1,43]. Therefore, in this study,natural Pyrolusite, a kind of popular ores in theNorthern Vietnam with low cost and high amount ofiron (III) ion, was doped La3+ for removing phosphateand fluoride and also adsorption abilities of its wereestimated. The La3+- doped Pyrolusite ore (LDPO)was expected to develope a new material forsimultaneous removal of both phosphate and fluoridein aqueous solution.

Materials and Methods

Materials

The Pyrolusite ore from Cao Bang province, theNorthern Vietnam contains approximate 4.7 % of ironcalculated from amount of Fe2O3.

All chemical reagents used in the experiments wereanalytical grade purchased from Merck (Germany),mainly including lanthanum nitrate (La(NO3)3.6H2O),sodium hydrate (NaOH), anhydrous sodium phosphate(Na3PO4), sodium fluoride (NaF) and nitric acid(HNO3).

Deionized water from Milli-Q water purification systemwas used throughout all experiment processes of thisstudy.

Preparation of La3+- doped Pyrolusite Ore (LDPO)

The LDPO was synthesized by traditional co-precipitation method of lanthanum and iron in solution.The natural Pyrolusite ore and La(NO3)3.6H2O weresimultaneously stirred in HNO3 solution to dissolvelanthanum and also a part of the iron of Pyrolusite intothe solution. The above dissolved elements were thenco-precipitated in the form of its hydroxides or oxidesby the NaOH + H2O2 mixed solution and thehydrolyzed products are setting onto surface over thetime. By this way, the new fresh layer of pyrolusite wascreated. The material was filtered, dried at thetemperature lower than 70 °C and washed out ofNaCl, and then dried again in the same abovementioned condition.

In order to choose the most optimal LDPO for removalphosphate and fluoride, lanthanum was added withvarious molar ratios between La3+ and the iron (Fe3+),which was dissolved from the Pyrolusite into thesolution. LDPO - 0 was without La3+ doped ontosurface of the Pyrolusite; LDPO 1:3, 1:2, 1:1, 2:1, and3:1 were ratios of molar between La3+ and Fe3+

covered on the surface of the Pyrolusite, respectively.Estimation of amount of phosphate and fluoride ionsadsorbed by LDPO.

The adsorbent was added to a solution containingphosphate (30 mg/ L) and fluoride (10 mg/L). After, thesolution was stirred for 2 hrs at 25 °C and pH of 6. Thesample was then filtrated through a Whatman 0.45 µmmembrane filter, and the filtrate was analyzed theconcentration of phosphate and fluoride by theabsorption spectrophotometer (880 nm for phosphateand 570 nm for fluoride, UV-Vis 2450, Shimadzu,Japan). The analytical procedure of the concentrationof phosphate and fluoride was performed according toStandard Methods for the Examination of Water andWaste Water Fluoride: SMEWW 4500 - F-. B&D:2012.Phosphate: SMEWW 4500 - P.E: 2012. The amount ofphosphate and fluoride ions adsorbed onto theadsorbent at equilibrium was calculated by thefollowing equation:

where qe: the amount adsorbed at equilibrium (mg/g);C0: the initial concentration (mg/l); Ce: the equilibriumconcentration (mg/l); V: the volume of the solution; andW: the weight of the adsorbent (g).

Characterizations of LDPO

This study used 3 methods for estimatingcharacterizations of the adsorbents. The morphologieswere determined by scanning electron microscopy(JEOL JSM-7600F, USA). Energy-dispersive X-rayspectroscopy (EDX) for detecting the elements wasperformed on Oxford Instruments 50 mm2 X-Max (UK).The surface area was measured by the BET surfacearea analyzer (Micromeritics 3 Flex, USA)

Adsorption dynamics studies

In order to study adsorption dynamics, phosphatesolutions of 30, 50, and 100 mg/L and fluoridesolutions of 10, 15, and 20 mg/L concentrations wereinvestigated at the initial pH value of 6.0. 100 mL ofboth phosphate and fluoride containing solution wasstirred with definite amount of adsorbent for differenttime durations ranging from 60 to 300 min at pH of 6.0.After the contact time, the adsorbent was separatedfrom the solution by filtration and the remainingconcentration of phosphate and fluoride in the solutionwas determined.

Adsorption isotherms studies

Phosphate and fluoride adsorption isotherms on LDPOat the pH value of 6.0 were studied at differenttemperatures (20, 30 and 40 °C) by varying the initialconcentrations of both phosphate and fluoride. In eachexperiment, 1 g of the adsorbent was transferred intothe 250 - mL conical flask and 100 mL of solutioncontaining different concentrations of phosphate andfluoride was then added to the flask. After the reaction



period, all samples were filtered by a Whatman 0.45µm membrane filter and analyzed concentrations ofphosphate and fluoride. The amounts of adsorbedphosphate or fluoride were calculated by thedifference of the initial and residual amounts ofphosphate or fluoride in solution divided by the weightof the adsorbent.

Results and Discussion

Phosphate and fluoride removal potential of La3+-doped Pyrolusite Ores (LDPOs)

Possibility of phosphate and fluoride removal byLDPOs was investigated in this study. Fig. 1A

presented the results for simultaneous removal ofphosphate and fluoride from aqueous solutions bydifferent LDPOs (LDPO 1:3, 1:2, 1:1, 2:1, and 3:1)including natural Pyrolusite ore (NPO). It is observedthat LDPO 1:1 displays highest (97.1%) phosphateand (95.3%) fluoride removal from water and in whilethe remaining LDPOs expressed removal result ofphosphate (55.4% - 89.6%) and fluoride (43.2% -88.9%) at 10 mg/L initial fluoride concentration leveland 30 mg/L initial phosphate concentration level.NPO removes approximately 22.2% of phosphate and10.1% of fluoride, indicating significant changes inproperties of LDPOs with respect to NPO.

0

20

40

60

80

100

120

LDPO 0 LDPO 1:3 LDPO 1:2 LDPO 1:1 LDPO 2:1 LDPO 3:1

Percentofadsorption(%)

Ratio of molar between La3+ and Fe 3+

PhosphateFluoride

(A)

0

20

40

60

80

100

120

LDPO 0 LDPO 1:3 LDPO 1:2 LDPO 1:1 LDPO 2:1 LDPO 3:1


Ratio of molar between La3+ and Fe 3+

PhosphateFluoride

(A)

Figure 1. (A) - Percent of fluoride and phosphate removal from aqueous solution by different molar ratios betweenLa3+ and Fe3+; LDPO 0 - without La3+ doped onto surface of the LDPO; LDPO 1:3, LDPO 1:2, LDPO 1:1, LDPO 2:1,and LDPO 3:1 - ratios of molar between La3+ and Fe3+onto surface of the LDPO, respectively. (B) - pH of solution

simultaneously containing phosphate and fluoride treated by various ratios of molar La between La3+ and Fe3+ ontosurface of the LDPO.



In order to understand the mechanism of phosphateand fluoride removal from aqueous solutions byLDPOs, and the role of hydroxide ions, resultant pH oftreated samples were measured. The treated water pHchanged in range from 6.9 to 8.6 (Fig. 1B). The resultsindicated that LDPO 1:1, which removes bothphosphate and fluoride to the highest degree, givespH of treated water at the neutral level (7.5).Therefore, LDPO 1:1 did not significantly releasehydroxide ion into water from in adsorption process ofphosphate and fluoride.

The results shown that LDPO 1:1, denoted as LDPO -G, has good potential of phosphate and fluorideremoval from aqueous solution. Hence,characterization of LDPO - G was studied to

understand the possible adsorption mechanisms andalso useful properties.

Characterization of LDPO – G

SEM analysis was performed to understand themorphology of natural Pyrolusite ore and LDPO - Gand given in Figs. 2A, B, respectively. The surface ofnatural Pyrolusite ore exhibit rough surfacemorphology. Natural Pyrolusite has bulky structurewith no porosity and poor surface area. SEM of LDPO- G gives the changes in morphology of the Pyrolusiteafter lanthanum doped and was shown in Fig. 2B.From SEM images oval shaped particles of lanthanumnitrate dispersed on the surface of LDPO - G is clearlyseen.

.

(A)

(B)

Figure 2. Scanning electron microscopy (SEM) image of Pyrolusite ore, (A) - natural Pyrolusite ore, (B) - La3+- dopedPyrolusite ore at La3+/ Fe3+ molar ratio of 1:1.



The surface area of the natural Pyrolusite ore wasmeasured using the BET surface area analyzer(Micromeritics 3 Flex,USA) and was found to be 25.92m2/g. The cumulative pore volume of the adsorbentwas 0.05 cm3/g. Values of these parameters indicatethe natural Pyrolusite ore to be a suitable adsorbent.Lanthanum modification has changed the surfacemorphology of the natural Pyrolusite and the BETsurface area of LDPO - G was found to be 72.63 m2/gwith a pore volume of 0.34 cm3/g.

Energy dispersive analysis of X - Ray (EDX) wasemployed to analyze the elements of LDPO - G. TheEDX pattern of the LDPO - G was shown in Fig. 3. Itwas observed a component which riches oflanthanum, iron, and etc. The EDX spectrum of theLDPO - G (Fig. 3) evident performed that the presenceof lanthanum along with other principal elements. Theresult provided an evidence for the lanthanum ionsuccessfully doped onto the surface of the LDPO - G.

Figure 3. Energy Dispersive X-ray spectrography (EDX) of LDPO - G.

pH of point of zero charge (pH PZC) of LDPO - G

The pH of point of zero charge (pHPZC) is an importantinterfacial parameter used widely in characterizing theionization behavior of a surface. The pHPZC of theadsorbent was investigated according to experimentsof Teng et al. and Sharma and Upadhyay [44,45]. Forits determination, 0.2 g of the adsorbent wastransferred into 250 - mL conical flasks containing 50mL of 0.01 M NaCl solution and after adjusted tovaried initial pH values (2.0 - 12.0) by 0.1 M HNO3.This setup was kept for 48 hrs and then the adsorbentwas filtered out NaCl solutions by a Whatman 0.45µmmembrane filter and final pH values of the solutionswere measured. The initial and final pH values of thesolutions were measured as pHinitial and pHfinal,respectively. pHPZC is intersection point of lines joiningpH initial and pHfinal in ‘pHfinal vs pHinitial’ graph. ThepHPZC refers to the bulk aqueous phase concentration(or more precisely, activity) of potential-determiningions giving zero surface charge. The pHPZC of LDPO -G was found to be approximate 6.8.

Effect of solution pH on phosphate and fluorideremoval potential of LDPO – G

The role of pH is a major factor which controls theadsorption at the water - adsorbent interface.32 Inorder to estimate the effect of pH to removal ofphosphate and fluoride of LDPO - G, 0.1 g of LDPO -G was added into 500 mL of the solution containing 30mg/L phosphate and 10 mg/L fluoride and after that,the experiments were performed at various initial pH,ranging from 2.0 to 10.0. Fig. 4 shows the adsorptionof both phosphate and fluoride onto LDPO - G asfunctions of pH. Adsorption of phosphate onto LDPO -G increased with increasing pH and reached amaximum range from 4.9 mg/g to 5.1 mg/g at pH 3.0 –6.0 (Fig. 4A). Similarly, the fluoride adsorption ofLDPO - G gently increased with increasing pH andreached a maximum of 1.3 mg/g at pH 6.0 (Fig. 4B).Figs. 4 A, B indicated that maximum adsorption ofLDPO - G is slight difference for pH values betweenphosphate (at pH 5) and fluoride (at pH 6.0).Therefore, in order to simultaneously adsorb bothphosphate and fluoride, the value of 6.0 is consideredto be the optimum pH of LDPO - G for furtherexperimental studies.



2.6

2.9

3.2

3.5

3.8

4.1

4.4

4.7

5.0

5.3

1 3 5 7 9 11

Amountofadsorbed(mg/g)

pH

(A)

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1 3 5 7 9 11

Amountofadsorbed(mg/g)

pH

(B)

Figure 4. Effect of pH to adsorption of fluoride and phosphate on LDPO - G(Initial concentrations of phosphate: 30 mg/L and fluoride: 10 mg/L; contact time: 2 hrs)

Effect of contact time for adsorbing phosphateand fluoride by LDPO - G

In this study, the result indicated that maximumconcentrations of phosphate and fluoride removal wasattained to be approximate 2 hrs and thereafter, italmost remained static for LDPO - G. The period ofcontact time of 2 hrs for further studies was fixed. Thephosphate and fluoride removal capacities at the pHvalue of 6.0 were 5.3 mg/L and 2.1 mg/L, respectively.

Effect of adsorbent dosage on removal ofphosphate and fluoride

In order to find the minimum dosage of LDPO - G formaximum adsorption of phosphate and fluoride, the

effects of adsorbent dosage were measured byvarying the dosages of LDPO - G from 0.01 g to 0.1 gin aqueous solution containing phosphate and fluorideconcentrations to be 30 mg/L and 10 mg/L,respectively. It was observed as shown in Fig. 5 thatthe percentage adsorption increased with respect tothe increased dosage and then remained constantafter 0.08 g of both phosphate and fluoride. Theincrease in adsorption capacity with increase indosage is apparent, because any adsorption processdepends upon the number of active sites [46].Increased adsorbent dosage implied a greater surfacearea and a greater number of binding sites availablefor the constant amounts of both phosphate andfluoride. The adsorbent dosage, therefore, wasoptimized to be 0.1 g for further experimental studies.



80

84

88

92

96

100

0 0.02 0.04 0.06 0.08 0.1


Adsorbent dose (g)

phosphate

fluoride

Figure 5. Effect of dosage to removal of fluoride and phosphate adsorption by LDPO - G.

Adsorption dynamics

To understand the adsorption dynamics of phosphateand fluoride removal, two types of models wereinvestigated to estimate the fitness of the experimentaldata to be pseudo - first order and pseudo - secondorder kinetic models.

Pseudo - first order model

The pseudo - first order kinetic model [47] isrepresented as:

where qt: is the amount (mg/g) of phosphate or fluorideadsorbed on the surface of LDPO - G at time t; k1: theequilibrium rate constant of pseudo - first orderadsorption (min-1). The linearity plots of log(qe - q t)versus t for different experimental conditions will beused to calculate the value of rate constants (k1).

Figs. 6A, B shown the pseudo - first order kinetic plotsof experimental data of phosphate and fluorideadsorption on LDPO - G in aqueous solutions. Table1 presented the summary of parameters of kinetic datamodel fittings. For phosphate adsorption, the analysisindicates that LDPO - G did not follow first - orderkinetics with significantly weak correlation (R2) and lowrate constant (k1) at all phosphate concentrationsinvestigated (Table 1 and Fig. 6A). The values of R2

were found as 0.854, 0.808, and 0.717 and also thevalues of k1 were found as 0.041, 0.037, and 0.016min−1 respectively for the selected initialconcentrations of phosphate. In contrast, LDPO - Gfollows first - order kinetics with good correlation (R2)for fluoride concentrations (Table 1 and Fig. 6B). Thevalues of R2 were found as 0.940, 0.957, and 0.916and also the values of k1 were found as 0.041, 0.048,and 0.051 min−1

, respectively for the selected initialconcentrations of fluoride. The values of k1 were foundto be decreasing with increasing the initialconcentrations of fluoride from 10 to 20 mg/L.

Table 1. The kinetic model parameters for the adsorption of phosphate and fluoride on LDPO

Conc.(mg/L)

Pseudo first order Pseudo second orderk1 (min-1) qe (mg/g) R2 k2 (g/mg.min) qe (mg/g) R2

Phosphate30 0.041 6.32 0.854 0.0025 20.83 0.98550 0.037 22.38 0.808 0.0026 24.39 0.994

100 0.016 10.74 0.717 0.0029 26.32 0.991Fluoride

10 0.041 2.52 0.940 0.006 1.517 0.92815 0.048 1.76 0.957 0.005 1.916 0.93920 0.051 2.72 0.916 0.002 3.436 0.913



-5.0

-3.5

-2.0

-0.5

1.0

2.5

4.0

-15 15 45 75 105 135 165 195

log(qe-q)(min

-1)

time (min)

30mg/L

50mg/L

100mg/L

(A)

-8.0

-6.5

-5.0

-3.5

-2.0

-0.5

1.0

50 100 150 200 250 300

log(qe-q)(min

-1)

time (min)

10 mg/L

15 mg/L

20 mg/L

(B)

Figure 6. Pseudo - first order models of LDPO - G for fluoride and phosphate removal.(A) - removal of phosphate at 30, 50 and 100 mg/L. (B) - removal of fluoride at 10, 15 and 20 mg/L.

Pseudo - second order model

The Pseudo - second order model [48] can beexpressed as

where qt and qe: the amounts (mg/g) of phosphate orfluoride adsorbed onto the surface of LDPO - G at anytime t and at equilibrium; k2: the pseudo - secondorder rate constant obtained from the slope of thelinear plots of t/qt versus t for different experimentalconditions.

From Table 1, it is observed that pseudo - secondorder kinetics shows good correlation (R2) forphosphate removal for all concentrations of phosphate

(Table 1 and Fig. 7A). The values of R2 were found as0.985, 0.994, and 0.991 and also the values of pseudo- second order rate constant (k2) were calculated as0.0025, 0.0026, and 0.0029 mg/g.min at the threeselected phosphate concentrations, respectively. Forfluoride removal, pseudo - second order kineticsshows significantly good correlation (R2) forconcentrations of fluoride (Table 1 and Fig. 7B). Thevalues of R2 were found as 0.928, 0.939, and 0.913and also the values of k2 were calculated as 0.006,0.005, and 0.002 mg/ g.min at the three selectedfluoride concentrations, respectively. Therefore, fromthe good R2 and low k2 obtained from Fig. 6, it can beconcluded that the adsorptions of phosphate andfluoride onto LDPO - G are governed by pseudo -second order model.



-1.0

1.5

4.0

6.5

9.0

11.5

14.0

-15 15 45 75 105 135 165 195 225 255

t/qt(min.g/mg)

time (min)

30mg/L

50mg/L

100mg/L

(A)

45

95

145

195

245

295

345

50 100 150 200 250 300 350 400

t/qt(min.g/mg)

time (min)

10 mg/L

15 mg/L

20 mg/L

(B)

Figure 7. Pseudo - second order models of LDPO - G for fluoride and phosphate removal.(A) - removal of phosphate at 30, 50 and 100 mg/L. (B) - removal of fluoride at 10, 15 and 20 mg/L.

Adsorption isotherms

The equilibrium data have been analyzed by the linearregression of isotherm model equations, Langmuir andFreundlich.

Langmuir isotherm

The Langmuir isotherm model [49], which is based onadsorbent surfaces is

where qe and Ce: the equilibrium adsorption capacity

(mg/g) and the equilibrium adsorbate concentration(mg/L), respectively; q0: the monolayer surfacecoverage (mg/g) and KL: the equilibrium adsorptionconstant (L/mg).

In order to estimate the adsorption capacity and theadsorbate concentration, the above mentionedequation (Eq. 1) can be used as a linear form asfollows:

where the q0 value (mg/g) was calculated from theslope of the linear plot of Ce/qe versus Ce.



The Langmuir isotherm plot in linear form forphosphate removal, as given by Eq. 2 (Ce versusCe/qe) at three temperatures 20, 30 and 40 °C givencorrelation coefficients (R2) as 0.994, 0.990, and0.986, respectively (Fig. 8A). The values of q0 wereobtained as 83.33, 76.92, and 66.67 mg/g (Table 2).The values of KL were 0.026, 0.023, and 0.021 L/mg atthe respective temperatures. The decreasing the KLwith a rise in reaction temperatures indicated that theadsorption process of phosphate is exothermic.Similar effect of temperature on phosphate was also

reported by Tian et al. [50]. On the other hand, R2

values of the Langmuir isotherm plot for fluorideremoval at 20, 30 and 40 °C were 0.993, 0.995, and0.994, respectively (Fig. 8B). The values of q0 wereobtained as 12.82, 10.56, and 9.43 mg/g (Table 2).The values of KL were 0.607, 0.283, and 0.159 L/mg atthe respective temperatures. The increasing the KLwith increasing temperatures indicated that theinteraction between fluoride and LDPO-G is theendothermic adsorption. This result is similar toreports of other researchers [51, 52].

0.0

1.5

3.0

4.5

6.0

7.5

-25 60 145 230 315 400

Ce/qe(g/L)

Ce (mg/L)

20 °C

30 °C

40 °C

(A)

0

0.3

0.6

0.9

1.2

1.5

1.8

0 2 4 6 8 10 12

Ce/qe(g/L)

Ce (mg/L)

20 °C

30 °C

40 °C

(B)

Figure 8. Langmuir isotherm of LDPO - G for fluoride and phosphate removal.(A) - removal of phosphate at 20, 30 and 40 °C. (B) - removal of fluoride at 20, 30 and 40 °C.

Freundlich isotherm

The Freundlich isotherm model equation [53] is asfollows

where qe and Ce have the same meaning as noted

previously, and KF and n are empirical constants thatare dependent on several environmental factors. Thelinear form of the Freundlich equation (Eq. (3)), whichis commonly used to describe adsorption isothermdata, is



The plot of log Ce versus log qe of Eq. (4) should resultin a linearized plot. From the slope and the intercept ofthe plot, the values of n and KF can be obtained.

Fig. 9A indicated the straight line of Freundlichisotherm for phosphate adsorption on LDPO-G. Thecorrelation coefficients (R2) are found as 0.909, 0.932,and 0.951 at the selected temperatures 20, 30 and 40°C, respectively (Table 2). This indicates that theadsorption data of phosphate onto LDPO - G fittedwell with Freundlich isotherm assumptions at thesetemperatures. The values of n were found as 10.8,9.43, and 6.67 at selected temperatures (Table 2). Onthe other hand, the linearized plot of Freundlichisotherm for fluoride adsorption on LDPO - G wasshowed Fig. 9B. The R2 values at the selectedtemperatures 20, 30 and 40 °C were found as 0.941,

0.873, and 0.901, respectively (Table 2). Thisindicates that the adsorption data of fluoride on LDPO- G fitted significantly well with Freundlich isothermassumptions at these temperatures except at 30 °C.The values of n were found as 1.67, 1.79, and 1.43 atselected temperatures (Table 2). In conclusion, theadsorption processes of LDPO were monolayer forfluoride and multilayer for phosphate and the value ofn in between 1 and 10 indicates useful adsorption[54]. Hence, adsorption of phosphate and fluoride onLDPO - G may be called ‘useful’. KF constants werefound as 33.7, 18.8 and 14.1 for phosphate removaland 2.83, 5.83 and 9.53 on these respectivetemperatures (Table 2). The increasing values of KF offluoride removal with temperature indicate thatadsorption is favorable at higher temperatures [45].

1.2

1.3

1.4

1.5

1.6

1.7

1.8

-0.5 0 0.5 1 1.5 2 2.5 3

logqe

log Ce

20 °C

30 °C

40 °C

(A)

0.5

0.8

1.0

1.3

1.5

1.8

2.0

-0.3 -0.1 0.1 0.3 0.5 0.7 0.9 1.1

logqe

log Ce

20 °C

30 °C

40 °C

(B)

Figure 9. Freundlich isotherm of LDPO - G for fluoride and phosphate removal.(A) - removal of phosphate at 20, 30 and 40 °C. (B) - removal of fluoride at 20, 30 and 40 °C.



Table 2. Calculated equilibrium constants using the Langmuir and Freundlich equations for phosphate andfluoride adsorption on the LDPO

Temp.(°C)

Langmuir Freundlichq0 (mg/g) KL(L/mg) R2 n KF (mg/g) R2

Phosphate20 83.33 0.026 0.994 10.8 33.7 0.90930 76.92 0.023 0.990 9.43 18.8 0.93240 66.67 0.021 0.986 6.67 14.1 0.951

Fluoride20 12.82 0.607 0.993 1.67 2.83 0.94130 10.56 0.283 0.995 1.79 5.83 0.87340 9.43 0.159 0.994 1.43 9.53 0.901

Many adsorbents have been tested for their eitherphosphate or fluoride removal potentials from aqueoussolutions and some of the results were summarized inTable 3. Lanthanum was doped or modified in somematerial to have an adsorption capacity of phosphatefrom 6.70 mg/g to 108.7 mg/g [2,55-57]. Otherselements were also used for removal phosphate inrange from 2.5 mg/g to 153 mg/g [1,3,43,58]. WhileLDPO - G gives a phosphate adsorption capacityranging from 66.7 mg/g to 76.9 mg/g with increasing

temperature from 20 °C to 40 °C, as found in thepresent study. In the other hand, for position ofremoval fluoride, Lanthanum was reported to be rangefrom 2.74 mg/g to 23.9 mg/g [59-61] and while othermaterials have range of fluoride removal from 0.19mg/g to 196.1 mg/g [4,34,62-64]. However, in thisstudy, LDPO - G remove a fluoride in aqueous solutionwith ranging from 0.60 mg/g to 1.93 mg/g withincreasing temperature from 20 °C to 40 °C.

Table 3. Comparison of Langmuir and Freundlich adsorption capacities (mg/g) of various materials for removal ofphosphate and fluoride

AdsorbentsLangmuir

adsorptioncapacity (mg/g)

Freundlichadsorption capacity

(mg/g)References

PhosphateFe-Mn binary oxide 33.2 27.0 [1]Hydrous niobium oxide 15.0 2.9 [3]Calcined cobalt hydroxide 2.5 - 153.0 0.02 - 18.2 [58]Nano-structured Fe-Tibimetal oxide 35.4 - 40.9 8.97 - 10.0 [43]

La-doped vesuvianite 6.70 1.96 [55]La-coordinated silicatesmaterial 50.3 - 54.3 20.6 - 32.4 [2]

La(OH)3/ Zeolit 6.05 - 71.9 - [56]La-modified tourmaline 67.1 - 108.7 7.53 - 17. 7 [57]La-doped Pyrolusite ore(LDPO) 66.7 - 76.9 14.1 - 33.7 Present study

FluorideMixed rare earth oxides 196.1 - [34]Low cost materials 0.19 - 4.54 0.023 - 10.5 [4]Ti-based adsorbents 25.8 - 46.6 2.18 - 15.8 [62]MnO2-coated Tamarind Fruit 0.21 - 0.22 - [63]Mn-Ce oxide 137.5 - [64]La-modified activatedalumina 2.74 1.26 [59]

La-loaded bentonite/chitosanbeads 3.70 - 8.55 0.24 - 0.45 [60]

La-Al loaded scoria 23.9 1.25 [61]La-doped Pyrolusite ore(LDPO) 9.43- 12.82 2.83 - 9.53 Present study



Effect of co-exist ions in aqueous solution toremoval of phosphate and fluoride of LDPO – G

Many co-ions such as sulfate, chloride and nitrateusually exist in water and they compete withphosphate and fluoride ions during the adsorptionprocess. Therefore, the effect of these ions inadsorption of both fluoride and phosphate by LDPO -G was carried out. The initial concentration ofphosphate and fluoride were fixed 30 mg/L and 10mg/L, respectively, and while the concentrations (25 -100 mg/L) of sulfate, chloride and nitrate were variedin the adsorption studies. It can be seen that theadsorptions of both fluoride and phosphate decreasewith increase in co-ions concentrations. At 0 mg/L ofthe co-ions, phosphate and fluoride adsorbed 94.1%and 93.2%, respectively. However, the adsorption ofphosphate and fluoride rapidly reduced toapproximately 30% and 25% at 100 mg/L of co-ions.

Conclusion

Transition metal La doped onto the surface of thenatural Pyrolusite ore was successfully synthesized.The surface and structure characteristics were studiedby EDX, BET and SEM analysis. In the studies aboutsimultaneous removal of phosphate and fluoride, theLa3+-doped Pyrolusite ore expresses rapid adsorptionrate and high adsorption capacity. The capacityincreases with the increase in adsorbent dosages. Theoptimal pH for phosphate and fluoride adsorption isapproximate 6.0. Further investigations on adsorptionabilities of this material in wastewater samples areneeded to apply for treating phosphate- or/andfluoride-contaminated waste sources of industrialareas in Vietnam.

Acknowledgments

The authors would like to express special thanks tothe Project of Science and Technology of “Program ofscientific study, and technological application andtransfer for developing environmental industry”performing “The Project of developing environmentalindustry to 2015, and looking into 2025”, Ministry ofIndustry and Trade, Government of Vietnam for thefinancial support to perform this study.

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How to cite this article:Nguyen Thi Hue, Vu Van Tu, Nguyen Hoang Tung. (2016). Study on fluoride and phosphate removal fromwater using La3+ - Doped pyrolusite ore from Vietnam. Int. J. Curr. Res. Chem. Pharma. Sci. 3(7): 36-50.

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