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Chemical Engineering Journal 185– 186 (2012) 219– 225

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

j ourna l ho mepage: www.elsev ier .com/ locate /ce j

fficient and controllable phosphate removal on hydrocalumite by multi-stepreatment based on pH-dependent precipitation

i Zhi Zhou ∗, Linlin Feng, Jun Zhao, Jianyong Liu, Qiang Liu, Jia Zhang, Guangren Qianchool of Environmental and Chemical Engineering, Shanghai University, No.333 Nanchen Rd., Shanghai 200444, PR China

r t i c l e i n f o

rticle history:eceived 7 December 2011eceived in revised form 14 January 2012ccepted 17 January 2012

eywords:a2Al-Cl-LDHhosphate

a b s t r a c t

The phosphate removal on hydrocalumite (Ca2Al-Cl-layered double hydroxide, Ca2Al-Cl-LDH) was eval-uated by PHREEQC program, which indicated that a pH-dependent dissolution–reprecipitation processwas responsible for the phosphate elimination. The evaluation results also suppose that in the solu-tion with high phosphate concentration, the formation of CaHPO4·2H2O (DCPD) and AlPO4 at pH < 7.0attributed to more phosphate removed than that in the conventional treatment where only hydroxya-patite (HAP) was precipitated. This hypothesis was confirmed by XRD characterization and the removalefficiency of phosphate in batch experiments. Accordingly, the multi-step treatment process (MST) was

astewaterHREEQCrecipitation

conducted for the removal phosphate in real P-containing wastewater, in which DCPD and AlPO4 wereformed at low pH and HAP at high pH. This resulted in the increasing amount of phosphate removal to9.26 mmol/g Ca2Al-Cl-LDH due to the high consuming efficiency of Ca and Al in the whole process. Theother benefit of MST was 12.7% of mass reduction of the as-obtained sludge, compared to the conven-tional one-step treatment. Our work proposed a potential strategy for the improvement of phosphateremoval by Ca2Al-Cl-LDH.

. Introduction

Removal of phosphate in wastewater from industrial, agricul-ural and household activities has attracted much attention ashosphate is responsible for eutrophication of rivers, lakes andays and damage of the aquatic eco-system [1,2]. For this purpose,any strategies have been developed to eliminate phosphate in

he effluent, such as bio-treatment with granular activated sludge3] and physical/chemical treatment with clay materials [4–6] and

odified ones [7–9], active carbon [10], oxides/hydroxides [11–13],a-modified silica [14,15]. Among these treatments, chemical pre-ipitation with iron, aluminum, and calcium salts is a widely usedethod for the treatment of phosphorus [16].Based on the great concern about the effective removal,

l intercalated Ca2Al-layered double hydroxide (Ca2Al-Cl-LDH,a4Al2(OH)12Cl2·4H2O), a member of anionic clays, exhibited a highfficiency for phosphate removal from wastewater [17–19]. Ourroup has found that Ca2Al-Cl-LDH removed phosphate throughhe dissolution–reprecipitation mechanism [19]. In this process,

hosphate precipitates with Ca2+ dissolved from LDH to form

nsoluble hydroxyapatite (Ca5(PO4)3OH, HAP) due to the muchow solubility product of HAP (Ksp = 2 × 10−58, [20]). In particular,

∗ Corresponding author. Tel.: +86 21 66137746; fax: +86 21 66137758.E-mail address: jizhi.zhou@shu.edu.cn (J.Z. Zhou).

385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.01.086

© 2012 Elsevier B.V. All rights reserved.

the higher removal amount of phosphate, about 135 mg P/g, wasobserved at lower initial pH (5.0). Despite, the treatment of phos-phate on Ca2Al-Cl-LDH is limited by the following issues: (1) theincrease of pH up to 12.0 due to OH− releasing from LDH; (2) lowconsuming efficiency of Ca in P removal as 1.67 of the molar Ca/P inHAP; (3) little contribution of Al(OH)3 to the adsorption of P due tothe conversion of Al(OH)3 to soluble Al(OH)n

−(n − 3) (n > 3) at highpH [11].

On the other hand, phosphate and calcium can be precipitatedin various insoluble species apart from HAP. Under different con-ditions, such as temperature, the level of supersaturation, pH, andinitial concentration of reagents, the following Ca-phosphate pre-cipitates are formed [21,22]: Hydroxyapatite (HAP, Ca5(PO4)3OH),tricalcium phosphate (�-TCP, Ca3(PO4)2), octacalcium phosphate(OCP, Ca8H2(PO4)6·5H2O), monetite (DCPA, CaHPO4), and brushite(DCPD, CaHPO4·2H2O), tetracalcium phosphate (TTCP, Ca4(PO4)2O)and amorphous calcium phosphate (amorphous Ca9(PO4)6·xH2O,ACP). Apparently, both DCPA and DCPD have the lowest Ca/P, giv-ing the highest Ca efficiency in the P removal. Moreover, DCPD is thepredominant precipitate with the pH below 7.0 in aqueous solution[22–24].

Such a pH value allows Al to exist predominantly in the form

of Al(OH)3 with a small portion of Al3+ [11] when Ca2Al-Cl-LDH isused as Ca source for the phosphate precipitation. In the presenceof Al(OH)3 and Al3+, the P removal was attributed to the adsorptionon Al(OH)3 and the precipitation with Al3+ in the form of AlPO4,

220 J.Z. Zhou et al. / Chemical Engineering Journal 185– 186 (2012) 219– 225

Table 1Potential minerals in the evaluation by PHREEQC model.

Ca/P (molar ratio) Compound Formula Log Ksp (25 ◦C) Reference

1 Brushite (DCPD) CaHPO4·2H2O −6.59 [3]1 Monetite (DCPA) CaHPO4 −6.9 [39]1.33 Octacalciumphosphate (OCP) Ca4H(PO4)3·2.5H2O −48.3 [39]1.5 Tricalcium phosphate (TCP) Ca3(PO4)2 −32.69 [36]1.67 Hydroxyapatite (HAP) Ca5(PO4)3OH −58.52 [20]

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Similar to that in the batch experiment, the phosphate removalsin both Step-1 and Step-2 were performed in a 150-ml sealedconical flask under stirring for 24 h. Whereas, the last step for phos-phate removal was carried out for 0.5 h. The solid after each step

– Gibbsite

– Aluminum Phosphate

– Hydrocalumite

espectively [25,26]. As a result, the P removal on Ca2Al-Cl-LDHould be improved by the formation of DCPD and AlPO4 and the

dsorption of Al(OH)3 at low pH.Herein, the objectives of this study were to: (1) evaluate the

ptimal conditions of DCPD formation by PHREEQC 2.12; (2) inves-igate the phosphate removal behavior on Ca2Al-Cl-LDH at highhosphate concentration, compared to that on CaCl2 and AlCl3;3) develop the DCPD formation strategy in the treatment of realhosphate-containing wastewater by Ca2Al-Cl-LDH. We found thathe multi-step controlling of DCPD crystallization is a facile andost-effective strategy for the improvement of phosphate removalith less sludge production.

. Materials and method

.1. Materials

Ca2Al-Cl-LDH was prepared with a co-precipitation methods reported elsewhere [27]. Typically, the salt mixture solution50 ml) composed of 50 mmol of CaCl2 and 25 mmol of AlCl3·6H2Oas quickly added to the alkaline solution (100 ml) containing

50 mmol of NaOH under vigorous stirring for 1 h with flowing2 stream. The suspension was aged at 25 ◦C for 18 h under stir-

ing. The precipitate was then collected via filtration, thoroughlyashed with deionized water and dried at 100 ◦C in an oven. The as-repared sample was pressed and crushed to the mesh size of 100rior to further experiment. The sample was identified by the ICP,RD and C/H/O element analyses to be a pure Ca2Al-Cl-LDH with

he approximate formula of Ca1.8Al(OH)5.6Cl0.95(CO3)0.025·2.5H2OM.W. = 275 g/mol), with XRD pattern being consistent to thateported elsewhere (JCPDS No 78-1219) with the d003-spacing of.776 nm [27].

.2. PHREEQC program Modeling

PHREEQC (version 2.12.5), a low-temperature aqueous geo-hemical calculation computer program, developed by the USeology Survey can be applied in saturation-index (SI) calcula-

ion and speciation analysis for modeling precipitation–dissolutionhemical equilibrium [28]. Before the calculation, the Ca2Al-Cl-LDHhermodynamic data with log Ksp = −27.1, �Go

r = 154.68 kJ/mol29,30] was input into PHREEQC database.

In current simulation of phosphate removal, the initial param-ters were: phosphate concentration ranging from 2 mg/L to000 mg/L (0.065–32.3 mmol/L); equilibrium pH value from 2.0 to4.0; the temperature at 25 ◦C in all tests; and Ca2Al-Cl-LDH dosagef 1 g/L. The compiled programs were calculated with the WATEQ4Fatabase [28]. Potential minerals with corresponding solubility

roduct (Ksp) in PHREEQC database were shown in Table 1. Theutput data of the activities, the free ionic activities product andaturation-indexes of every species as well as the solution ionictrength were used to evaluate the potential precipitations in thehosphate removal process.

H)3 −32.34 [25]O4 −18.24 [25]Al2(OH)12Cl2·4H2O −27.1 [29,30]

2.3. Batch experiments

The experimental removal of phosphate was performed ina 150-ml sealed conical flask with 100 ml of KH2PO4 solution([P] = 1000 mg/L = 32.3 mmol/L) and 0.10 g of dosage of Ca2Al-Cl-LDH. The removal process was carried out at 25 ± 1 ◦C for 48 hshaking in a water-bath. By dripping diluted HCl or NaOH solution(0.01 and 0.001 mmol/L), the solution pH was kept at 5.0, 7.0 and11.0 ± 0.1, respectively, monitored by the pH sensor with ± 0.01of precision. For comparison, CaCl2 and AlCl3·6H2O were used toremove phosphate in the similar way where the molar dosage ofmetal was as much as that of corresponding one in 0.10 g of LDH.

After removal, the suspension was filtered by 0.22 �m micro-pore membrane. The resultant solid was collected after waterwashing and drying at 100 ◦C in an oven for further charac-terization. The composition of the separated solution was alsodetermined. All experiments were carried out in triplicate with thereproducibility within ±5%.

2.4. Multi-step treatment

A multi-step treatment (MST) of phosphate-containing wastew-ater was performed on Ca2Al-Cl-LDH, which was based on DCPDprecipitation. Briefly, as illustrated in Fig. 1, 1.5 g/L of Ca2Al-Cl-LDHwas added in the first step treatment (Step-1) to increase the finalpH to 4.0–5.0 as the dissolution of Ca2Al-Cl-LDH. After filtering,the separated solution was ready for next step treatment (Step-2)where another 1.5 g/L of Ca2Al-Cl-LDH was added and the final pHwas increased to 5.0–7.0. In the last step (Step-3), the pH was up to11.0 or more by NaOH solution.

Fig. 1. Schematic of the multi-step treatment for P removal from real wastewater.

ering Journal 185– 186 (2012) 219– 225 221

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P concentration (mmol/L)

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P concentration (mmol/L)

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Fig. 2. Amount of potential precipitates by PHREEQC evaluation at fixed pH 5.0 (A),

J.Z. Zhou et al. / Chemical Engine

reatment was collected by filtering while the solution was sep-rated for next step treatment. For comparison, the conventionalreatment of the wastewater [31] was carried out in the similar wayhere 3.0 g/L of LDH and NaOH solution were added together into

00 ml of wastewater.

.5. Chemical and physical characterization

The pH value was monitored with an Elico digital pH meterModel LI-120) using a combined glass electrode (Model CL1). The phosphate concentration was determined in a UNICOV-spectrophotometer (4802UV/VIS), at 700 nm, following molyb-enum orchid colorimetric method (GB11893-89) [32]. The metaloncentrations in solution were determined by Inductively Coupledlasma-Atom Emission Spectrometer (ICP-AES, Prodidy, Leemano.). The C, H, and O components in LDH were detected by Elementnalysis (EA3000, Leeman Co.).

Powder X-ray diffraction (XRD) was carried out on a D\MAX-200 X-ray diffractometer (Rigaku Co.) with Cu K� radiation� = 0.15406 nm) at a scanning rate of 8◦/min from 5◦ to 80◦ (2�).

. Results and discussions

.1. PHREEQC modeling evaluation

.1.1. Phosphate precipitates at various initial concentrationsThe species of potential precipitates was evaluated by PHREEQC

odel as shown in Fig. 2, in which the dissolution of LDH wasevealed as no LDH remaining in the solid phase with all Al precip-tated in gibbsite (Al(OH)3). AlPO4 and various Ca–P solid species

ere formed at different initial [P] in some cases.Fig. 2A illustrates that with the increase of initial [P], most gibb-

ite was converted to AlPO4 at initial [P] > 4 mmol/L in the case ofH 5.0. The amount of Al in AlPO4 was about 3.6 mmol/L, same ashe theoretical Al content in Ca2Al-Cl-LDH (1.0 g/L of LDH dosage).t was found that the dissolved Ca2+ precipitated P to form DCPDt initial [P] > 14 mmol/L. The similar result was obtained in thease of pH 7.0 where the conversion of gibbsite occurred at initialP] from 10 mmol/L to 14 mmol/L and DCPD was formed at initialP] > 23 mmol/L (Fig. 2B). In particular, TCP was formed with initialP] increasing whereas transformed completely to DCPD after initialP] of 27 mmol/L. Obviously, high initial P concentration improvedhe DCPD formation at pH ≤ 7.0. Different from that at lower pH,he predominant P precipitate was HAP at higher pH. In Fig. 2C,nly gibbsite and HAP were obtained at pH 11.0 regardless of thenitial [P]. The amount of Al in gibbsite was only 3.04–3.07 mmol/ghat is less than 3.63 mmol/g of the theoretical Al content in Ca2Al-l-LDH. This is attributed to the high amount of OH− that convertsome Al(OH)3 to Al(OH)n

−(n − 3).According to the diversity of precipitates above, the phosphate

emoval efficiency at various initial [P] was estimated and shownn Fig. 2D. The HAP formation resulted in the constant P removalmount up to 4.28 mmol/g at pH 11.0. In comparison, the phos-hate removal efficiency at pH 7.0 was considerably increased at

nitial [P] > 10.5 mmol/L due to the formation of AlPO4 and TCP. Fur-hermore, more phosphate was removed at initial [P] > 24.5 mmol/Lue to the conversion of TCP to DCPD. The effect of DCPD forma-ion was also observed in the case of pH 5.0 where the P removalmount was increased gradually at initial [P] > 14.5 mmol/L. There-ore, it is supposed that the formation of DCPD can result in theigher P removal amount, especially at high initial [P].

.1.2. Effect of pHBesides the initial [P], the pH has a significant effect on the phos-

hate removal efficiency as well. Fig. 3 shows the estimation resultf P removal at various pHs but a fixed initial [P] of 32.3 mmol/L.

7.0 (B) and 11.0 (C) as well as the total phosphate removal amount (D) on Ca2Al-Cl-LDH with various initial phosphate concentrations.

222 J.Z. Zhou et al. / Chemical Engineering J

1412108642

0

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AlPO4

DCP D

HAP

TCP

Gibb site

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ig. 3. Potential minerals and phosphate removal amount on Ca2Al-Cl-LDH evalu-ted by PHREEQC at the pH range of 2.0–14.0 with initial [P] = 32.3 mmol/L.

t pH from 2.5 mmol/g to 7.0, 3.6 mmol/g of AlPO4 was formed,ndicating that most of Al in LDH contributed to P removal. In con-rast, most AlPO4 was converted to gibbsite at pH 7.0–7.5 whileibbsite gradually dissolved as Al(OH)n

−(n − 3) formed at pH > 10.0.his suggests that the removal of P with Al was more available atH < 7.0.

Moreover, there were DCPD, TCP and HAP precipitated predom-nantly at pH 4.5–7.5, 7.0–9.0, and 8.5–14, respectively. Excludinghe P in AlPO4, the formation of DCPD resulted in about 7.0 mmol/gf P (=10.6–3.6 mmol/g) removed at pH 6.5–7.0, 1.52–1.64 timesigher than those in cases of TCP and HAP precipitation. It is indi-ated that more P can be removed at pH < 7.0 when DCPD and AlPO4ere formed.

.1.3. Response surface curvesSince the species of precipitates depended on pH and ini-

ial phosphate concentration, the reciprocal effect of these twoactors for phosphate removal efficiency over Ca2Al-Cl-LDH wasstimated by response surface curves. As shown in Fig. 4, at ini-ial [P] > 160 mg/L and pH ranging from 5.0 to 7.0, the P removalfficiency was higher than that under other conditions. This isttributed to the formation of AlPO4, DCPD and TCP. On the con-

rary, at a lower phosphate concentration (2–160 mg/L), high pHabove 9.0) removed approximate 100% of P due to HAP formation.he evaluation result indicates that for P removal in wastewaterith high P concentration, the formation of DCPD and AlPO4 atH < 7.0 is a more efficient way than that of HAP.

ig. 4. Response surface curves of phosphate removal efficiency versus pH (2.0–14.0) and p

ournal 185– 186 (2012) 219– 225

3.2. Phosphate removal in batch experiment

3.2.1. Species of precipitates under various pHsThe experimental P removal was carried out in the batch test.

Fig. 5 shows XRD patterns of the product after the phosphateremoval by Ca2Al-Cl-LDH, CaCl2, AlCl3, and the mixture of CaCl2and AlCl3 (Ca/Al mixture) with different pHs. In the case of Ca2Al-Cl-LDH, Fig. 5A exhibits that complete dissolution of LDH regardlessof various pH values. The DCPD (JCPDS No 72-0713) was identifiedwith low reflections of gibbsite (JCPDS No 74-1775) at pH 5.0 and7.0. This is consistent to the result in Fig. 3 under similar condi-tions. Moreover, HAP (JCPDS No 86-0740) was observed at pH 7.0,indicating partial DCPD were probably transformed to HAP. At pH11.0, a board HAP peak with gibbsite was reflected in XRD pattern.Similarly, in the presence of Ca2+ only, Fig. 5B displays that DCPDand HAP was observed at pH 7.0 while only HAP at pH 11.0. Thisindicates soluble Ca2+ was responsible for the P removal at both pH7.0 and 11.0 in the case of LDH. However, there was no any precip-itate at pH 5.0. The observation suggests that the P removal at pH5.0 was not mainly attributed to the DCPD precipitate although itsXRD pattern indexed in the case of LDH. In addition, the patterns ofOCP (JCPDS No 79-0423) was identified which was contributed tothe predominant HPO4

2− in the solution at pH 7.0 [33].As shown in Fig. 2, the conversion of Al(OH)3 to AlPO4 was con-

sidered at low pH due to the dissolution of Al(OH)3. Fig. 5C displaysthe broad peak at 25–35◦ (2�) in the XRD patterns of both productsat pH 5.0 and pH 7.0, indicating a poor crystallized AlPO4 formedwhen Al3+ was added in the phosphate solution [25]. Due to lessDCPD formed at pH 5.0 (Fig. 5B), it is demonstrated that AlPO4contributed to the P removal predominantly in the case of LDH.Accordingly, poor crystallized AlPO4 was also suggested in Fig. 5Aas the similar broad peak at 25–35◦ (2�) at pH 5.0. Furthermore,the formation of soluble Al(OH)n

−(n − 3) resulted in the dissolutionof Al(OH)3 so that there was no product obtained at pH 11.0 in thecase of AlCl3, demonstrating that most P over Ca2Al-Cl-LDH wasremoved in HAP like the situation in previous work [19], which isin good agreement to the result in Fig. 3.

As shown in Fig. 5D, the effect of Ca2+ combined with Al3+

resulted in amorphous AlPO4 at pH 5.0, DCPD crystal and amor-phous AlPO4 (a board peak at 25–35◦ (2�)) at pH 7.0, and HAP crystalat pH 11.0, respectively. This observation was different from theresult in the case of Ca2Al-Cl-LDH (Fig. 5A) at pH 5.0 and 7.0. It sup-poses that the P removal process on LDH was not similar to that inthe presence of free Ca2+ and Al3+.

3.2.2. Practical P removal processFig. 6 shows phosphate removal percentage at various pHs in

the batch experiment. The sum of P removal percentages in cases

hosphate concentration (2–1000 mg/L) over Ca2Al-Cl-LDH by PHREEQC evaluation.

J.Z. Zhou et al. / Chemical Engineering Journal 185– 186 (2012) 219– 225 223

Fig. 5. XRD pattern of the products after the phosphate removal with initial [P] = 32.3 mmmixture.

0

5

10

15

20

25

30

35

40

0 1.2

6

AlCl3

13

.6

31

.1

16

.7

1.2

6

14

.1

11

.1

15

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17

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17

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16

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pH 5.0

pH 7.0

pH 11 .0

Ca/Al mixtureCaCl2

Rem

ova

l (%

)

pH i n solution

Ca2Al-Cl-LDH

19.2

Fig. 6. Removal percentage of phosphate at various pHs in cases of Ca2Al-Cl-LDH,CaCl2, AlCl3 and Ca/Al mixture.

ol/L at pH 5.0, 7.0 and 11.0 on (A) Ca2Al-Cl-LDH, (B) CaCl2, (C) AlCl3 and (D) Ca/Al

of CaCl2 and AlCl3 was close to the P removal percentage in the pres-ence of both CaCl2 and AlCl3. This is consistent to the XRD result inFig. 5, which indicates that the P removal was contributed to theprecipitation of Ca2+ and Al3+ with P. Moreover, the P removal per-centage in the case of Ca/Al mixture was also similar to estimatedvalues in PHREEQC model at pH 7.0 and 9.0 (Fig. 3) where the Premoval percentage was 35.6% and 15.4%, respectively. It is notedthat at pH 5.0, the theoretic removal percentage was 24.1%, higherthan that of 13.6% in the case of Ca/Al mixture. This observation isattributed to the absence of DCPD in the present case as no DCPDpattern in XRD result in Fig. 5D.

On Ca2Al-Cl-LDH, 19.2% of total P was removed at pH 5.0, slightlyhigher than that in the case of Ca/Al mixture. It is due to the forma-tion of DCPD on Ca2Al-Cl-LDH as shown in Fig. 5A. At pH 7.0, the Premoval percentage was 17.3%, about two times lower than that inthe case of Ca/Al mixture. Such difference in P removal percentageis contributed to the conversion of DCPD to HAP and the forma-

tion of Al(OH)3 as the XRD pattern of HAP and Al(OH)3 identifiedin Fig. 5A. Moreover, the formation of HAP at pH 11.0 was respon-sible for the P removal as the 16.7% of P removal percentage on theCa2Al-Cl-LDH, same as that in the case of Ca/Al mixture. Therefore,

224 J.Z. Zhou et al. / Chemical Engineering Journal 185– 186 (2012) 219– 225

Table 2Composition of electroplating effluent after two different removal processes.

(mmol/L) pH [P] [Zn] [Ca] [Al]

Blank 2.47 27.8 17.1 0.800 0.357Step-1 4.56 18.2 7.31 10.6 0.203Step-2 5.66 5.78 0 13.3 0.166

imCa

3

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SttordiP

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Step-3 11.58 0.00314 0 3.82 0.0178One-step 11.21 5.84 0 0.0548 3.21

t is suggested that the P removal efficiency on Ca2Al-Cl-LDH wasuch lower at high pH, compared to that in the presence of free

a2+ and Al3+. The high P removal amount was probably obtainedt low pH on Ca2Al-Cl-LDH.

.3. Multi-step treatment (MST)

.3.1. Controllable phosphate removalTo improve the P removal, MST was performed in the raw

astewater with low initial pH. The pH and aqueous compositionre listed in Table 2. It is noted that 17.1 mmol/L of soluble Zn wasetected in the wastewater apart from P.

Table 2 also shows the final pH and composition of solution afterreatment. In the Step-1 of MST, the final pH was increased from.47 to 4.56. It was attributed to the complete dissolution of LDHhat resulted in the Ca increasing amount of 9.80 mmol/L in thenal solution (10.6–0.800 mmol/L, Table 2), close to 9.82 mmol/Lf total theoretical Ca in 1.5 g/L of LDH. This also reveals that theeleased Ca2+ has less effect on the P reduction from 27.8 mmol/L to8.2 mmol/L. It is consistent to the results in Fig. 3 as well as Fig. 5B.

n comparison, the low aqueous Al concentration in the final solu-ion indicates that the LDH dissolution at low pH did not result inhe releasing of soluble Al. As the formation of AlPO4 was suggestedn Fig. 3 at pH < 7.0, the insoluble Al was probably responsible for the

removal with Al. Besides, the concentration of Zn was decreasedrom 17.1 mmol/L to 7.31 mmol/L, indicating that 9.79 mmol/L ofn reduction is attributed to the precipitation of Zn3(PO4)2·4H2Oith P [34–36]. Therefore, the 9.60 mmol/L of P reduction was com-osed of 6.53 mmol/L of P removed by Zn3(PO4)2·4H2O formationnd 3.07 mmol/L of P uptake in AlPO4.

In the Step-2, the pH increased from 4.56 to 5.66 due to theomplete dissolution of Ca2Al-Cl-LDH. However, the Ca increas-ng amount was only 2.70 mmol/L (13.3–10.6 mmol/L, Table 2) innal solution, indicating that 7.12 mmol/L of Ca was precipitated9.82–2.70 mmol/L, Table 2). This suggests that the precipitation ofa was attributed to 7.12 mmol/L of the P removal to form DCPDt pH 5.66 as DCPD formation in the precipitate at pH 5.0–7.5Fig. 3). This observation illustrates that 57.1% of P removed in Step-

was contributed to the Ca in LDH. Similar to that in Step-1, theemoval of P was also relative to Zn due to 7.31 mmol/L of Zn precip-tated. The low aqueous Al concentration in the final solution waslso shown in Table 2, as AlPO4 was formed (Fig. 3A). Therefore,2.4 mmol/L of P reduction is attributed to the combined effect ofa, Zn and Al.

In the last step, pH increased to 11.58 with NaOH addition.imultaneously, the Ca concentration decreased from 13.3 mmol/Lo 3.82 mmol/L with the P removal amount of 5.78 mmol/L. Notehat the molar ratio of removed Ca to P was 1.64, close to 1.67f the theoretical Ca/P ratio in HAP. It indicates Ca reduction wasesponsible for the P removal as the HAP precipitation was pre-ominant in the basic solution (Fig. 3). After MST, the residual [P]

n the solution was 0.00314 mmol/L, lower than 0.0161 mmol/L of

in its discharge limitation standard [37].

In contrast, the remaining [P] was 5.84 mmol/L in final solutiont pH 11.21 in the case of one-step treatment. This is contributedo the increase of Ca/P in HAP that was formed at higher pH with

Fig. 7. XRD pattern of products after the phosphate removal from the real P-bearingwastewater on Ca2Al-Cl-LDH by two different treatment processes.

the dissolution of LDH (Fig. 3). In addition, as the quick formationof HAP under the high pH and [P], partial Ca2Al-Cl-LDH particlewas gradually wrapped and its dissolution was inhibited [19]. Thisincomplete dissolution of LDH also resulted in the high [P] resid-ual. On the other hand, [Al] in the final solution was much higherthan that in the multi-step treatment. The high residual [Al] in theone-step treatment was attributed to more total Al added in thesystem. There was 10.9 mmol/L of Al in one-step treatment whileonly 0.166 mmol/L of Al left after Step 2 in multi-step treatment(Table 2). As the equilibrium of Al(OH)3 and Al(OH)n

−(n − 3) at basicsolution [19], more Al in the system resulted in more Al converted tosoluble Al(OH)n

−(n − 3). This indicates less P was removed at currentpH in the one-step treatment.

The other benefit of MST was the reduction of sludge mass aftertreatment. In MST, the mass of sludge was 12.7% lower than that inone-step treatment.

3.3.2. Precipitates in MST processThe precipitations of P removal in MST were confirmed by

XRD. As shown in Fig. 7, Zn3(PO4)2·4H2O (JCPDS No 37-0465) wasidentified in the precipitate at Step-1, which demonstrates the Premoval with Zn2+. Due to the poor crystal of AlPO4, it was notobserved in XRD pattern. In the Step-2, the formation of DCPD witha little gibbsite was identified in XRD pattern. Moreover, partialP was removed in CaZn2(PO4)2·2H2O (JCPDS No 71-2275), whichwas observed by Nriagu [38]. These indicate that P was removedin precipitates of DCPD and CaZn2(PO4)2·2H2O and adsorptionof gibbsite, respectively. Take into account hydration constant ofH3PO4 (log Ka2 = 7.20) [40], the species of phosphate adsorbed ongibbsite may be predominately H2PO4

− at pH = 5.66. The poor crys-

tal of AlPO4 was not still observed. In the two steps above, 79.2% ofP has been removed. In the last step, HAP was the only detectablephase, which is consistent to previous result [19]. In comparison,part of Ca2Al-Cl-LDH was still observable in XRD pattern after

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J.Z. Zhou et al. / Chemical Engine

ne-step treatment, indicating low P removal efficiency at basic pHnd high phosphate concentration by dissolution–reprecipitationn the case of Ca2Al-Cl-LDH.

. Conclusions

We investigated phosphate removal process on Ca2Al-Cl-LDH byHREEQC modeling as well as the batch experiment. The PHREEQCodeling assessment indicated that in a wastewater with high P

oncentration, the formation of DCPD and AlPO4 at pH < 7.0 was more efficient way for P removal, compared to the formationf HAP. The higher P removal amount was obtained at low pH5.0–7.0) due to the formation of DCPD and AlPO4. This evaluatedesult was confirmed by batch experiments.

Accordingly, a multi-step treatment was performed for the Pemoval from a real phosphate-containing wastewater by Ca2Al-l-LDH. The results revealed that controlling DCPD and AlPO4

ormation at low pH improved the efficiency of P removal as wells reduced the mass of as-obtained sludge, compared to one-stepreatment with the same dosage of Ca2Al-Cl-LDH.

cknowledgements

This project is financially supported by National special&T project on treatment and control of water pollution No.008ZX07421-002, No. 2009ZX07106-01, National Nature Scienceoundation of China No. 20677037/B07034, No. 20877053/B0703,o. 20907029/B0703 and Shanghai Leading Academic Disciplineroject No. S30109. This work is also supported by the State Schol-rship Fund (No. 2009689027) by China Scholarship Council andhanghai University Innovative Foundation for Higher Degree Stu-ents. We also appreciate the technical support from Instrumentalnalysis & Research Center of Shanghai University.

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