e Pergamon War. Sci. Tech. Vol. 34, No. 1-2, pp. 161-168, 1996.Copyright © 1996 1AWQ. Published by Elsevier Science Ltd

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PHOSPHORUS REMOVAL BYHYDROTALCITE-LIKE COMPOUNDS(HTLcs)

Hang-Sik Shin*, Mi-Joo Kim*, Se-Yong Nam* andHi-Chung Moon**

* Department of Civil Engineering, Korea Advanced Institute ofScience andTechnology, Kusong-Dong 373-1, Yusong-ku, Taejon, Korea** Department of Chemistry, Korea Advanced Institute ofScience and Technology,Kusong-Dong 373-1, Yusong-ku, Taejon, Korea

ABSTRACT

Hydrotalcite-Iike compounds (HTLcs), which are also called anionic clay minerals or Layered DoubleHydroxides (LDHs), can be formulated by M(II)1_xM(lII)x(OH)2(An-)x/nyH20. This chemical, which hasanoin exchange capacity, could be an alternative chemical for phophorus removal. The phosphate uptake byHTLcs containing C 1- as the interlayer anion was found to be 2.35-2.83 meg of Pig. The most seriousobstacle anion against phosphate uptake was bicarbonate because the selectivity of carbonate by LDH is thehighest. To investigate the possibility of LDH's recycle employing the "Memory effect", calcined LDH(CLDH) was repeatedly used for 6 times. From the fifth time, the final phosphate uptake capacity decreased.The consecutive reconstruction mechanism of CLDH is not yet known but it is certain that the possibility ofLDH's recycle is promising. Copyright © 1996 IAWQ. Published by Elsevier Science Ltd.

KEYWORDS

Anion exchange capacity; Hydrotalcite-like compound; LDH's recycle; Memory effect; phosphate uptake.

INTRODUCTION

Eutrophication of surface water by algal bloom is due mainly to the influx containing nitrogen andphosphorus, and it has been one of main issues to remove these compounds efficiently.

Generally, the removal of phosphorus is conducted by chemical and biological methods. Among these, thephosphorus removal by adding chemicals has problems such as high capital cost, chemical sludge handling,etc. In addition, such chemicals (alum, iron salt, and lime) can be used only once and cannot be recoveredfor further use, which makes chemical treatment costly. So, it is required to develop recyclable chemicalswhich keep the original treatability. Hydrotalcite-like compounds (HTLcs), which can keep the originalcapacity of ion exchange, could be used as an alternative chemical for phosphorus removal.

HTLcs, which are also called anionic clay minerals or Layered Double Hydroxides (LDHs), can beformulated by M(II)I_xM(III)x(OH)2(An-)xinyH20.M(H) and M(lII) denote di- and trivalent metals,respectively, An- is an anion of which the valence is n, and x (x = M(lII)/(M(II)+M(lII» ranges from 0.17 to

161

H.-S. SHIN et al.

0.33 (Kwon et al., 1988). HTLcs consist of a positively charged brucite-like octahedral layer and anegatively charged interlayer containing anions and water molecules (Miyata, 1975). The positively chargedlayer is formed by partial substitution of a trivalent metal for a divalent one. The layer can be stacked: ~dthe balancing interlayer anions can be exchanged for other anions. However, in case of HTLcs contal~I~gC0

32- as the interlayer anion, C032- is not readily replaceable by other anions, because the anion sel~~t1Vlty

of HTLcs is the highest for C032-. And such anions as CI', NOf, S042- etc., which have low s~lectIvI,ty byHTLcs, should be intercalated during the synthesis of HTLcs to have high anion exchange capacity (ReIchle,

1986).

Hydrotalcite (HT) containing C032- as the interlayer anion is decomposed into magnesium aluminium oxidesolid solution when heated to 500 - 800°C, as expressed by Eq. 1. The resulting calcined products can reoconstruct the original layered structure with rehydration and ion exchange of various anions, as expressed byEq. 2. Here, HT means Mg6AI2(OH)16C034H20 which has Mg2+ and A13+ as di- and trivalent metals,

respectively, and C032- as the interlayer anion.

Mg1.sAls(OH)2(COJ)S/£yH20 ~ Mg1-sAlsOl+sl2 + (xJ2)C02 + (1 +y)H20

Mgl.sAlxOI+sl2 + (xJn)An- + (1+(x/2)+y)H20 ~ Mgl.sAlx(OHhAm·s1n·yH20 + xOH'

This behavior is called "Memory effect".

(I)

(2)

In this study, we investigated the possibility of phosphate removal with HTLcs containing C 1- as theinterlayer anion by ion exchange mechanism, the possibility of phosphate removal with magnesiumaluminium oxide solid, and also the possibility of this chemical's recycle employing the "Memory effect",

NOMENCLATURE

The following nomenclature will be used in this paper.

HT: Hydrotalcite, Mg6AI2(OH)16C034H20LDH: Synthesized artificial Hydrotalcite for this studyLDH-CI: Synthesized HTLcs containing CI- as the interlayer anionCLDH: magnesium aluminium oxide solid obtained by heating of LDH at 600°CLDHs: LDH and LDH-C ISLDH: HTLcs containing P043, as the interlayer anoim obtained from CLDH by ion exchange.

EXPERIMENTAL

Preparation of LDHs and CLDH

LDH was prepared as follows:

A mixed solution of magnesium and aluminium chloride ([Mg2+] = 1.5 M, [A 13+] =0.5 M, M = molelL),0.2 M Na2C03, and a sodium hydroxide solution (2.0 M) were added dropwise through a ringer bottle to aI-L beaker at 40±0.2°C. A precipitation reaction occurs when the solutions are mixed. The mixture was keptthoroughly .stirre~ for the reaction. Flow rates of sodium hydroxide solution were controlled to adjust the pHof the reactmg mIxture to 1O.0±0.2. After reaction, the beaker containing precipitate was kept quiet for agingof crystals for 8 hr at 60°C, followed by filtration and washing with distilled water until the solution was freeof chloride, and then dried at 80°C for 4 days. CLDH was produced by calcination of LDH at 600°C in airfor 3 hours.

Phosphorus removal by HydrolaJcile-like compounds 163

LDH-C I was synthesized in the same way as that of LDH production except the following Na2C03 solutionwas not used and the pH of the reaction mixture was adjusted to 8.5±O.5. In addition, the precipitate waswashed with dilute hydrochloric acid (10-3 M HCI) for intercalation of Cl- ion as the interlayer anion.Finally, X-ray powder diffraction, Ff-IR, NMR, TGAJDSC, ICP-AES and other methods were employed toanalyze the chemical composition and structure of the synthesized LDHs and CLDH.

Phosphate uptake

Phosphate ion-exchange properties were investigated by a batch-wise method. 300mL of aqueous solution ofdibasic potassium phosphate (K2HP04) was mixed with 0.3g of samples (LDHs and CLDH) and stirred with250rpm at room temperature (18-20°C). Initial concentration of K2HP04 was varied over the range from 30to 300 mgP043--PIL. At established mixing time the mixture was centrifuged immediately, and thephosphate concentration in the final resultant was determined spectrophotometrically by the Stannouschloride method. The experiments of phosphate uptake by LDHs and CLDH were carried out as follows:

Phosphate uptake by such a type of LDH, effect of initial phosphate concentration on the uptake rate,isotherm for phosphate uptake, effect of competitive anion such as soi-, HCOf, NOf on phosphateuptake, phosphate removal by CLDH and LDH's recycle.

RESULTS AND DISCUSSIONS

Characterization of LDHs and CLDH

The chemical analyses of LDHs are listed in Table 1. The chemical formulas of these chemicals, given inTable 2, were estimated on the basis of the chemical analysis data. Metal ratio of LDH depends on the moleratio of initial mixed solution which contains magnesium and aluminium chloride. Synthesized LDHsatisfies the condition of electroneutrality. However, in case of LDH-C I, there is some difference betweenthe metal ratio of final result and the mole ratio of initial mixed solution. This indicates the possibility ofimpurity generation such as aluminium hydroxide, A I(OH)3 die to LDHs partial dissolution. According toOokubo et al.,'study (1993), Hydrotalcite (HT) is unstable below pH 5. In case of LDH-cl, a major cause ofpartial dissolution is that resulting precipitate was washed with dilute hydrochloric acid solution, not withdistilled water during the synthesis.

XRD, Ff-IR, I3C CP-MAS NMR, 27A I-MAS NMR, and TGAJDSC were employed to analyze thestructures of synthesized LDHs and CLDH. In this paper, only the results of XRD and TGAJDSC arepresented.

Table 1. Chemical Analysis of LDH and LDH-C 1 Samples

(wt. %)AJ C H °sample

Mg

LDH* 24.1 8.9 2.0 4.0 60.9

LDH 24.6 8.1 2.4 5.0 59.3

LDH-Cl 18.9 13.7 0.3 4.6 52.6

LDH*: M~AJ2(OH)16C034H20

Na CI

0.3 0.3

0.3 9.5

164 H.-S. SHIN et al.

Table 2. Chemical Formulas of LDH and LDH-C 1 Samples

Sample

LDH*

LDH

Chemical formula AtIMt

] .000

0.999

At/Mt: Equivalent ratio to cation

0.997

LDHs show XRD patterns which are characteristic of the HT phase, as shown in Figures 1 (a) and 1 (c). Thediffraction intensities of basal ref;ections on the (003) and (006) faces are strong and sharp compared withother reflections owing to the layered structure of HT, CLDH has the XRD peaks corresponding to MgO justas Nacl structure (Fig. 1 (b».

O()]

(a) 006

Vl0.()

i·'Vi (b)t:

\.Q) I\.. A--.....s(c)

o 10 20 30 40 50 60 70

28 (Degrees)

Figure I. XRD patterns ofLDHs and CLDH (a) LDH (b) CLDH (c) LDH-Cl.

0100

-2

- ~~ 80-4

0- CO

~-6 U

E60 -8

-10

40 -120 200 400 600 800 1000

Temperature(C)

Figure 2. TGNDSC diagram ofLDH TGA (-), DSC (-).

Phosphorus removal by Hydrotalcite-like compounds 165

TGAIDSC

The behavior of the thermal decomposition of LDHs in air was investigated by TGAIDSC. According to theTGAIDSC thermograms shown in Figure 2, LDHs are generally characterized by two endothermictransitions. The first transition is observed at 100 - 280°C die to dehydration of interlayer water and thesecond transition Occurs at 280 - 500°C due to dehydroxylation and decarbonation.

Phosphate uptake by LDHs and CLDH

Phosphates exist in different ionic states such as monovalent HZP04-, and trivalent P043- ions depending onthe pH of the solution. The dissociation equilibrium of H3P04 can be written as follows (Perrin andDempsey, 1974):

Where pK I =2.15, pKz=7.20 and pK3 =12.33.

Ookubo et ai. (1993) noted that phosphate ion exchange capacity by LDH-C 1 was the highest over the pHrange of 7 to 8. So, in this study, the initial pH value of all the experiments was 7.7±0.2. At this pH, the ionicspecies of phosphates in the solution phase are a mixture of HP04Z-(70%) and HZP04-(30%).

Figure 3 shows the results of phosphate uptake by LDHs and CLDH. As expected, LDH exhibited aninsignificant ion exchange capacity. Phosphate removal efficiency by CLDH was 35% at terminal reactiontime (150 min). CLDH is characterized by high sensitivity to the temperature (Cavani et ai., 1991). Thephosphate uptake by CLDH was not completely terminated after 24 hours at room temperature, however athigher temperature (60°C), it was terminated only within 1 hour. LDH-Cl showed 91 % phosphate uptakecapacity of LDH-Cl was smaller than that of CLDH, however, LDH-Cl is more advantageous since thereaction rate is very fast.

48CQ

',= _ 40

f~=~~ btl 32

~ eQ-u C 24~ .5:.... -==.s:: - 16Q"O"l~Q C-=.- 8~

• CLDH• L.D1i-CI

A L.DH

100 125 150755025o ~_..L.-_-'-_--L.._---'__J.-----'

o

Time (min)

Figure 3. Phosphate uptake rate by LDHs and CLDH.

~ffect of initial phosphate concentration on uptake rate by LDH-Cl

~igure 4 shows effect of increasing initial phosphate concentration on the uptake rate and the equilibrium:oncentration of phosphate. Though the initial phosphate concentration was increased, the reaction to reach:quilibrium was terminated within 1 hour. The final phosphate uptake by LDH-C 1 was found to be 1.35,.58, and 1.65 mmol of Pig when its initial phosphate concentration was 50, 100, and 200mg-PIL,espectively. Since phosphate ion exists as the mixture of HP04Z-(70%) and HZP04-(30%) at pH 7.5±O.2,he ion exchange capacity for phosphate was estimated to be 2.35-2.83meq Pig. This value is close to the

166 H.-S. SHIN et al.

equivalent of chloride ion content (2.45mwq C l-/g) in LDH-C I, suggesting that phosphate ions. are mostlyion exchanged with interlayer chloride ions. The presence of small excess of phosphate uptake can beexplained by phosphate adsorption on the external surface of LDH-C 1 crystals.

Time (min)

o 50mgPIL

o IOOmgPILt:. 200mgPIL

-~~

bIl 60

e--~ 50....aa.cCo", 400.c~... 300....c= 200e< 10"0QJ~.. 00", 0

"0<

30 60 90 120 150

Figure 4. Effect of initial phosphate concentration on phosphorus uptake rate by LDH-C I.

Isotherm for phosphate uptake by LDH-C 1

The isotherm of phosphate uptake by LDH-C 1 at room temperature (18-20·C) and pH of 7.5±O.2 is shown inFigure 5, where Ceq is the equilibrium concentration of phosphate in the solution phase. The isothermfollows a Langmuir type adsorption.

-~ 7.5 -bIl 56 bIl-- ~QJ e....aa 6.0 --.c 48 ~

Co ....", ~

0 .c.c 4.5 CoCo ",

40 0"0 .cQJ Q"

,Q 3.0 "0.. 0 Isoth ..... m0 32 ~", ,Q

"0 • LanR'JIuir plot ..~ 1.5 0

",

~"0

QJ 24 <U

70 140 210 280 350 420

Ceq (mg PIL)

Figue 5. Isotherm for phosphate uptake by LDH-C I at room temperature.

Effect of competitive anions on phosphate uptake

Figure 6 shows the effect of competitive anions on the uptake by LDH-Cl. The experimental results indicatethat bicarbonate ions are the most serious obstacle anions have against phosphate uptake because theselectivity of carbonate by LDH is the highest. Nitrate ions have almost no influence on phosphate ionexchange, but sulfate ions cause approximately 12-13% reduction in the phosphate removal efficiency.

Phosphorus removal by Hydrotalcite-Iike compounds 167

N'hen these three anions, i.e., bicarbonate, nitrate, and sulfate, are present simultaneously, phosphateemoval efficiency decreases further.

Time (min)

o phosphate,Po bicarbon ate+-Pl:> sulrate+-PV nitrate+-Po P+S+N+C

-t2Q.cbI) 35e--~ 30....~.c

25c.~=.c 20Q.c~=.... 15=== 10e<"C 5~.ca. 0=~ 0

"C<

10 20 30 40 50 60

Figure 6. Effect of Competitive anions on the phosphate uptake by LDH-C 1 at pH 7.S±O.2.

Recyclability of LDH

fo test the recyclability of LDH, CLDH was repeatedly used for 6 times for ion exchange and calcining:Figure 7). In recalcining SLDH at 600°C, the intercalated phosphate is removed as gaseous P20S andSLDH is changed into a phase closely similar to thet of CLDH. The initial phosphate uptake rate decreasesLIp to the fourth time of the repetition, but the phosphate uptake capacity increases to some extent.

~Q.c 32bI)e-- 24

Q.c~=....=== 0 81e 0 S3~

"C l:>. 84~ "i1 85.ca. 0 86=~

"C -8< 0 2 3 4 5 6 7

Time (hour)

Figure 7. Recyclability ofLDH Sn: LDH recycled for n time.

From the fifth time, however, the recalcining process does not completely remove phosphate so that theJartially remaining phosphate causes the redischarge of phosphate in the beginning of reaction and theJhosphate uptake capacity decreases. The consecutive resonstruction mechanism of CLDH is not yet known,Jut it is certain that the possibility of LDH's recycle is promising.

1ST 34-1/2-G

168

CONCLUSIONS

H.-S. SHIN et al.

I. The ion exchange capacity for phosphate by LDH-CI is estimated to be 2.35-2.83 meq PIg.

2. The isotherm of phosphate uptake by LDH-C I follows a Langmuir type adsorption.

3. Bicarbonate is the most serious obstacle anion against phosphate uptake because the selectivity ofcarbonate by LDH is the highest.

4. The consecutive reconstruction mechanism of CLDH is not yet known but it is certain that the possibilityof LDH's recycle is promising.

ACKNOWLEDGMENT

We really appreciate Mr Ree Seog Woo of the Department of Chemistry in KAIST, for collaborating with uson the chemical analysis.

REFERENCES

Akira Ookubo. Kenta Ooi and Hiromu Hayashi (1993). Preparation and phosphate ion exchange properties of a Hydrotalcite-Iikecompound. Langmuir 9, pp. 1418-1422.

Cavani, F., Trifiro. F. and Vaccari, A. (1991). Hydrotalcite-Type anionic clays: Preparation, Properties and Applications,Catalysis Today 11, p. 287.

Kwon. T., Tsigdinos, G. A. and Pinnavaia, T. J. (1988). J. Am. Soc. 110. pp. 3653.Miyata, S. (1975). Clays and Clay Minerals 23, pp. 141.Perrin. D. D. and Dempsey, B. (1974). Buffers for pH and Metal 1011 Control, Chapman and Hall, London, pp. 156. 159. 162.Reichle. W. T. (1986). Solid State lonies. 22, p. 135.