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Selective adsorption of phosphate from seawater and wastewater by amorphoUS zirconium hydroxide

Journal of Colloid and Interface Science297(2006)

426–433

https://www.doczj.com/doc/066247930.html,/locate/jcis

Selective adsorption of phosphate from seawater and wastewater by

amorphous zirconium hydroxide

Ramesh Chitrakar,Satoko Tezuka,Akinari Sonoda?,Kohji Sakane,Kenta Ooi,Takahiro Hirotsu Health Technology Research Center,National Institute of Advanced Industrial Science and Technology(AIST),2217-14Hayashi-cho,

Takamatsu761-0395,Japan

Received5September2005;accepted8November2005

Available online7December2005

Abstract

Phosphate adsorption from single electrolyte(NaH2PO4),phosphate-enriched seawater,and model wastewater was studied using amorphous zirconium hydroxide,ZrO(OH)2·(Na2O)0.05·1.5H2O,as an adsorbent.Batch experiments were carried out to investigate the adsorption of phos-phate.The effect of pH on phosphate adsorption from seawater showed that the uptake of phosphate increased with an increase in pH up to6,and then decreased sharply with a further increase in pH of the solution.The equilibrium data of phosphate adsorption were followed with a Freundlich isotherm.The uptake of phosphate at the adsorbent/solution ratio0.05g/2L was10and17mg-P/g for the phosphate-enriched seawater and the model wastewater,respectively.A much higher adsorptivity toward phosphate ions in seawater was observed on ZrO(OH)2·(Na2O)0.05·1.5H2O than on other representative adsorbents based on layered double hydroxides of Mg(II)–Al(III),Mg(II)–Fe(III),and Ni(II)–Fe(III).The effective desorption of phosphate ions on ZrO(OH)2·(Na2O)0.05·1.5H2O could be achieved using a0.1M NaOH solution.The usefulness of experimental data for practical applications in removing phosphate in seawater and wastewater is discussed.

Keywords:Zirconium hydroxide;Adsorption;Selectivity;Phosphate;Seawater

1.Introduction

Phosphorus occurs mainly as phosphate in aquasystems. Phosphate is involved in a wide variety of biological and chem-ical processes in natural waters,wastewater,and water treat-ment.The release of phosphate to surface water is of environ-mental concern,because it enhances the growth of organisms in most ecosystems and is therefore the cause of eutrophica-tion,resulting in deterioration of water quality.The need for selective adsorption of phosphate at trace concentration levels has been well recognized.The anion-exchange reaction has at-tracted much interest for the removal of anions.

A number of studies have been carried out on the interaction of phosphate with natural or synthetic minerals:goethite(α-FeOOH)[1,2],manganese nodules(mixed oxides ofδ-MnO2,α-SiO2,FeOOH,and Al2O3)[3],manganese oxides[4],and aluminum-impregnated mesoporous silicates[5].X-ray absorp-*Corresponding author.

E-mail address:a.sonoda@aist.go.jp(A.Sonoda).tion near edge structure spectroscopy(XANES)has recently been used to study phosphate adsorption on mineral oxides[6].

Hydrous oxides of iron(III),aluminum,zirconium,and man-ganese(IV)have been investigated for phosphate adsorption from a single electrolyte[7–10].These adsorbents are gener-ally amphoteric,giving them both cationic and anionic ion-exchange behavior depending on the pH of a contacting so-lution.Adsorption of phosphate from seawater with hydrous oxides of Fe(III),Al,and Mn(IV)has also been studied in de-tail[11–13].However,these adsorbents showed low uptake of phosphate(1–3mg-P/g)from seawater due to the presence of competing Cl?,HCO?

,and SO2?

ions.We have found that calcined Mg(II)–Mn(III)layered double hydroxide shows high selectivity toward phosphate ion in seawater[14].

Studies on the adsorption of phosphate from wastewater be-fore discharge into the environment have been carried out using industrial waste materials and by-products as adsorbents,in-cluding?y ash[15],blast furnace slag(an industrial by-product derived from iron ore processing)[16],and iron oxide tail-ings(iron oxide with sulfate and silicates)[17].A new class

R.Chitrakar et al./Journal of Colloid and Interface Science297(2006)426–433427

of polymeric ion exchangers(chelating resins)was found to be effective for the selective adsorption of phosphate from waste-water[18].

Three types of zirconium oxides are known to exist:cu-bic,tetragonal,and monoclinic.They can be synthesized with different chemical compositions depending on the method of preparation[19].We are interested in amorphous zirconium hydroxide,because of its simplicity of preparation,its large surface area,and its strong af?nity for certain anions such as phosphate or?uoride from a single electrolyte[20].Many stud-ies have been done on the adsorptive properties of cations on zirconium hydroxides(amorphous)[21]and monoclinic types [22]in single or dilute electrolyte solutions.To our knowledge, no study has been done on phosphate adsorption by zirconium hydroxide from seawater or wastewater.This paper describes original work on the effectiveness of amorphous zirconium hy-droxide for the selective adsorption of phosphate from a sodium phosphate solution,seawater and model wastewater.

2.Experimental

2.1.Preparation of amorphous zirconium hydroxide

A0.1M(M=mol/L)NaOH solution was slowly added dropwise to200ml of a0.1M ZrOCl2solution.The latter solution was stirred magnetically during the addition of the sodium hydroxide solution till the pH reached10.The precip-itate was aged overnight,separated by centrifugation,washed with deionized water until free of chloride ions,and?nally dried in air at room temperature.The air-dried sample was ground and sieved to100–200mesh size(150–230μm).

2.2.Preparation of other adsorbents for comparison study

Three types of layered double hydroxides,[Mg0.71Al0.29 (OH)2][Cl0.29·0.59H2O],[Mg0.84Fe0.16(OH)2][Cl0.16·0.68 H2O],and[Ni0.79Fe0.21(OH)2][Cl0.21·0.63H2O],were pre-pared in chloride forms and were designated as MgAl,MgFe, and NiFe,respectively[23].

2.3.Physical properties

Powder X-ray diffraction analysis was carried out using a Rigaku X-ray diffractometer(RINT1200)equipped with Ni-?ltered Cu Kαradiation(λ=1.5404?)and a graphite mono-chromator.TG-DTA experiments were conducted on a MAC Science thermal analyzer(200TG-DTA)at a heating rate of 10?C/min in air.Surface area was determined by nitrogen gas adsorption using Quantachrome Type1-C apparatus after sam-ple was degassed at150?C for4h.

2.4.Chemical analysis

A known amount of each powder sample was dissolved in a

0.5M HCl solution.The dissolved Zr4+ion was analyzed by

a Seiko ICP atomic emission plasma spectrometer(SPS7800), and the Na+,K+,Mg2+,and Ca2+ions were analyzed by a Shimadzu atomic absorption spectrometer(AA-670).The Cl?and SO2?

ions were determined by a Shimadzu liquid chro-matograph(LC-10Ai).

Phosphate was determined with a portable colorimeter, Model DR/700,HACH Company,USA.All the reagents and chemicals were supplied by the HACH Company.The analy-sis was performed according to the procedures described in the manual.

2.5.Preparation of phosphate-enriched seawater and model wastewater solutions

Phosphate-enriched seawater was prepared by the addition of a NaH2PO4solution to fresh seawater up to an appropriate phosphate concentration(0.30mg-P/L)with the pH of the so-lution at7.8.Model wastewater was prepared as described in the literature[24].The following concentrations were?xed in the model wastewater with a pH value of9:phosphate=2mg-P/L,alkalinity=100mg/L(NaHCO3),and Ca=25mg/L (CaCl2).

2.6.Adsorption of phosphate from different solutions

All the adsorption studies were carried out by a batch method at room temperature.The rates of phosphate removal from a NaH2PO4solution,phosphate-enriched seawater,and a model wastewater were determined by stirring a known amount of ZrO(OH)2·(Na2O)0.05·1.5H2O in a known volume of solu-tion.A small amount of supernatant solution was sampled at different intervals of time,and the phosphate concentrations were determined.Adsorption isotherms were obtained by stir-ring known amounts of the sample with2L of solution for 7d to attain satisfactory equilibrium.The phosphate concentra-tions in the supernatant solutions were determined.The uptakes of phosphate were calculated from the decreases of phosphate concentrations relative to those of initial concentration.

The effect of pH on phosphate adsorption from a NaH2PO4 solution and phosphate-enriched seawater were studied at dif-ferent pH values.The pH of the solution was adjusted by the addition of a1M HCl or a1M NaOH solution.

2.7.Uptake of cations and anions from seawater

A quantity of0.3g of ZrO(OH)2·(Na2O)0.05·1.5H2O was stirred in50L of seawater for3d.After the supernatant solution was discarded,the experiment was repeated three times by re-placement with fresh seawater.The solid sample was collected and washed till free of chloride ions.After drying,the solid sample was dissolved in an appropriate acid to elute cations and anions adsorbed in the sample.

2.8.Desorption of phosphate and repetition of the adsorption/desorption cycle

Phosphate desorption was studied using a phosphate-loaded sample,which was prepared by treatment of0.50g of ZrO(OH)2·(Na2O)0.05·1.5H2O with a Na2HPO4solution(2L,phosphate

428R.Chitrakar et al./Journal of Colloid and Interface Science297(2006)426–433

concentration50mg-P/L)for3d.The phosphate-loaded ad-sorbent was immersed in200ml of a0.1M NaOH solution for1d.The amount of phosphate desorbed was determined by the analysis of the phosphate concentration in the solution.The adsorption/desorption cycles were repeated?ve times.

The desorption rate(R des)was calculated as R des=(A des/ Q ad)×100,where A des is the amount of phosphate released after desorption and Q ad is the initial phosphate uptake.The regeneration rate(R reg)was evaluated as R reg=(R ad/Q ad)×100,where R ad is the phosphate uptake by the regenerated ad-sorbent.Q ad,A des,and R ad are expressed in mg-P/g.

3.Results and discussion

3.1.Characterization of ZrO(OH)2·(Na2O)0.05·1.5H2O

Chemical analysis data for the solid sample showed the com-position ZrO2(71.3%),Na2O(2.0%),and H2O(26.2%)on a wt%basis.The water content was obtained from ignition of the solid sample at400?C in air.This corresponds to the for-mulation ZrO2·(Na2O)0.05·2.5H2O or ZrO(OH)2·(Na2O)0.05·1.5H2O.The hydroxide content was formulated on the basis of electrical neutrality.The theoretical ion exchange capacity was evaluated as1.5mmol/g,consistent with the molar content of OH groups in the unit weight of ZrO(OH)2·(Na2O)0.05·1.5H2O. The surface area was228m2/g as determined from the nitrogen adsorption isotherm.

XRD patterns of ZrO(OH)2·(Na2O)0.05·1.5H2O,and its cal-cined product at500?C are shown in Fig.1.This sample could be regarded as an amorphous material.After calcina-tion at500?C for3h in air,the crystallization of the sam-ple occurred,and the diffraction pattern showed a tetragonal phase[25].The TG-DTA results of decomposition of ZrO (OH)2·(Na2O)0.05·1.5H2O are shown in Fig.2.There was weight loss up to400?C showing dehydration.The DTA curve showed one endothermic peak at around70?C,which was at-tributed to the physically adsorbed water.There was a broad exothermic peak around470?C,which was attributed to crys-tallization of ZrO(OH)2·(Na2O)0.05·1.5H2O to a tetragonal phase,while the broad exothermic peak might re?ect a lack of homogeneity in the sample[25].

3.2.Rate of phosphate removal

The removal of phosphate from a NaH2PO4solution, phosphate-enriched seawater,and a model wastewater as a function of time were examined.Fig.3a shows the effect of the initial phosphate concentration of the NaH2PO4solu-tion on phosphate removal at an adsorbent/solution ratio of 0.10g/1L.The phosphate removal from seawater and the model wastewater with initial concentrations of0.3and2mg-P/L were performed at adsorbent/solution ratios of0.05g/2L and0.20g/1L,respectively(Figs.3b and3c).The phosphate removal from the0.3and1.0mg-P/L NaH2PO4solution and the2mg-P/L model wastewater were almost100%in4–6d. To know the exact equilibrium time,phosphate removals were performed at higher concentrations of Na2HPO4solution.

The Fig.1.XRD patterns of ZrO(OH)2·(Na2O)0.05·1.5H2O and its calcined prod-uct at500?

Fig.2.TG-DTA curves of ZrO(OH)2·(Na2O)0.05·1.5H2O.

rate of phosphate removal was found to be slow for all solutions studied,requiring7d to attain equilibrium.

The slow phosphate adsorption on ZrO(OH)2·(Na2O)0.05·1.5H2O could be explained on the basis of the random struc-ture of amorphous zirconium hydroxide proposed by Clear?eld et al.[20],who?rst pointed out that when zirconium hydroxide is precipitated from a solution of ZrOCl2·8H2O by the addi-tion of a base,a polymeric oxohydroxide of the general for-mula[ZrO b(OH)4?2b·x H2O]is formed,most probably based on a tetramer of Zr(IV).The random structure has two types of hydroxyl groups,which are responsible for ion-exchange reac-tions:(i)terminal–OH groups exist on the surface of the solid,

R.Chitrakar et al./Journal of Colloid and Interface Science297(2006)426–433

429

Fig.3.Rate of phosphate adsorption by ZrO(OH)2·(Na2O)0.05·1.5H2O in NaH2PO4solution(a),phosphate-enriched seawater(b),and model wastewa-ter(c).For(a):adsorbent=0.10g,volume=1L,pH5.0;For(b):adsorbent =0.05g,volume=2L,pH7.7;For(c):adsorbent=0.20g,volume=1L, pH8.8.

and(ii)bridging–OH groups lie in the interior part of the solid and are connected by two metal atoms.In the present study, we assume that ZrO(OH)2·(Na2O)0.05·1.5H2O has both types of hydroxyl groups,as shown in Fig.4.Thus,the phosphate adsorption at the surface hydroxyl groups is fast,but the diffu-sion of phosphate ions into the interior is expected to be slow. Accordingly,the slow diffusion may hinder the access of phos-phate ions to the bridging hydroxyl groups,resulting in the slow adsorption rate.In general,adsorption over a period of minutes to hours occurs by anion adsorption on the surface hydroxyl groups of the adsorbent[26,27].Diffusion of anions into the lattice or adsorbent matrix requires a longer time to attain equi-librium[1]

.Fig.4.Structure of amorphous zirconium hydroxide formed by addition of base to aqueous zirconyl solution.

Table1

Ion uptake by ZrO(OH)2·(Na2O)0.05·1.5H2O from seawater

Ions Ion uptake

(mg/g)

Ion concentration in

seawater(mg/ml)

K d

(ml/g) Na+<0.110.8<0.1

K+<0.10.4<0.2

Ca2+<0.10.4<0.2

Cl?9.019.40.4

SO2?

192.77

P8.06×10?51.3×105 Note.K d:distribution coef?cient.K d=ion uptake(mg/g)/ion concentration (mg/ml).

3.3.Uptake of phosphate and other ions from seawater

Seawater contains considerable amounts of Na+,K+,Ca2+, Cl?,and SO2?4ions.The uptake of these ions by ZrO(OH)2·(Na2O)0.05·1.5H2O was determined.The uptakes and the dis-tribution coef?cients of these ions are summarized in Table1. The distribution coef?cient(K d)can be calculated as K d(ml/g) =ion uptake(mg/g)/ion concentration in seawater(mg/ml). The ion uptakes were<0.1mg/g(Na+),<0.1mg/g(K+),

<0.1mg/g(Ca2+),9.0mg/g(Cl?),19mg/g(SO2?

),and 8mg/g(P).A higher K d value was observed for phosphate than for the other ions.The selectivity order from the K d val-ues was Na+,K+,Ca2+

3.4.Effect of pH on phosphate adsorption

It is known that hydrous trivalent and tetravalent metal hy-droxides exhibit amphoteric behavior,exchanging anions in an acidic solution and cations in a basic one.The anion-exchange capacity is strongly governed by the pH of the so-lution,ionic species of phosphate,and surface chemistry of the solids.The effect of pH on phosphate adsorption was demonstrated by measuring the uptake of phosphate by ZrO (OH)2·(Na2O)0.05·1.5H2O as a function of pH in a NaH2PO4

430R.Chitrakar et al./Journal of Colloid and Interface Science297(2006)

426–433

Fig.5.The effect of pH on the uptake of phosphate by ZrO(OH)2·(Na2O)0.05·1.5H2O from NaH2PO4solution(a)and phosphate-enriched seawater(b). For(a):adsorbent=0.10g,conc.=50mg-P/L,volume=100ml.For(b): adsorbent=0.10g,conc.=0.33mg-P/L,volume=2L.Contact time=7d.

solution,and in phosphate-enriched seawater.The results in Fig.5a for the NaH2PO4solution show that the uptake of phos-phate tends to decrease with an increase in pH,from50mg-P/g at pH2to13mg-P/g at pH10.The dissolution of zirconium ions at different pH was about1%.Phosphate can exist in dif-ferent ionic species as monovalent H2PO?4,divalent HPO2?4, and trivalent PO3?4ions,depending on the pH of the solution (p K1=2.15,p K2=7.20,and p K3=12.33)[28].The pH de-pendence of the uptake is likely attributable to the fact that a lower pH causes the ZrO(OH)2·(Na2O)0.05·1.5H2O surface to carry a more positive charge,and thus would more signi?cantly attract the negatively charged monovalent H2PO?4ions in so-lution.The similar pH dependence of the anion adsorption is generally observed in trivalent and tetravalent metal hydrox-ides[29].

In the case of seawater,the uptake of phosphate increases with an increase in pH,reaching a maximum around pH6and then decreasing progressively with an increase in pH up to a value of9(Fig.5b).In pH range2–4,sulfate anions in sea-water may affect the phosphate adsorption greatly,because the adsorbent prefers divalent SO2?4to monovalent HPO?4.The up-take of phosphate was maximal at pH around6.A decrease in the uptake of phosphate in the region pH>7may be due to the fact that the surface charge of the adsorbent becomes more negative at higher pH and causes greater electrostatic repulsion toward the more negatively charged phosphate anions HPO2?4 and PO3?4

.Fig.6.Isotherm of phosphate adsorption by ZrO(OH)2·(Na2O)0.05·1.5H2O in NaH2PO4(a),phosphate-enriched seawater(b),and model wastewater(c). For(a):adsorbent=0.10g,conc.=1–10mg-P/L,volume=1L,pH5.0. For(b):adsorbent=0.012–0.400g,conc.=0.30mg-P/L,volume=2L, pH7.7.For(c):adsorbent=0.04–0.20g,conc.=2mg-P/L,volume=2L, pH8.8.Contact time=7d.

3.5.Adsorption isotherm

Isotherm studies were carried out to determine the con-ditions for maximum uptake of phosphate on ZrO(OH)2·(Na2O)0.05·1.5H2O from a NaH2PO4solution,phosphate-enriched seawater,and a model wastewater.Fig.6shows that the uptake of phosphate increases with the phosphate equilib-rium concentration in all three solutions.The phosphate ad-sorption data were?tted to the Freundlich equation,shown as q e=K F C1/n e,where q e is the amount of phosphate ad-

R.Chitrakar et al./Journal of Colloid and Interface Science297(2006)426–433431

Table2

Freundlich isotherm constants for adsorption of phosphate by ZrO(OH)2·(Na2O)0.05·1.5H2O

Solution Freundlich isotherm

K F n R2 NaH2PO417.430.998 Seawater43.510.956 Wastewater14.9110.981

sorbed on the solid phase(mg-P/g),C e is the equilibrium phosphate concentration in solution phase(mg-P/L),and K F (mg/g)and n(dimensionless)are Freundlich constants.The Freundlich isotherm constants and R2values for different so-lutions are given in Table2.The error bars were±10%for seawater and±3%for NaHPO4solution and wastewater.The constant n refers to the interaction between exchange sites at the adsorbent and phosphate ions.A high value for n>1indicates favorable adsorption.The maximum uptake of phosphate from seawater on ZrO(OH)2·(Na2O)0.05·1.5H2O was10mg-P/g at a phosphate equilibrium concentration of0.2mg-P/https://www.doczj.com/doc/066247930.html,yered double hydroxide is known to have a high phosphate exchange capacity of60mg-P/g at pH8[30],but the maximum uptake of phosphate from seawater was only1mg-P/g.

To summarize the adsorption isotherm data,the uptakes of phosphate from NaH2PO4solution,seawater,and wastewater on ZrO(OH)2·(Na2O)0.05·1.5H2O are30,10,and15mg-P/g at equilibrium pH5,7.7,and8.8,respectively.The uptakes of phosphate are comparable with their respective pH titration data (Figs.5a,5b).But the uptake of phosphate from seawater is too low as compared to that from single electrolyte(NaH2PO4)at pH7.8(Fig.5a).The decrease in uptake may be related to low phosphate concentration and the presence of major and minor seawater components,which strongly reduce the phosphate ad-sorption.

These adsorption isotherms demonstrate that ZrO(OH)2·(Na2O)0.05·1.5H2O may be used as an adsorbent for phosphate from wastewater and seawater,because of its high phosphate uptake.It is noted that the values presented here are high as compared to the reported data in the literature;the maximum uptake is1–3mg-P/g by hydrous oxides of Fe(III),Al,and Mn(IV)in seawater[11–13],and1and7mg-P/g by blast fur-nace slags[16]and alkaline?y ash[15],respectively,in waste-water.

3.6.Desorption of phosphate and repetition of the adsorption/desorption cycle

For reusability,the adsorbed phosphate should be easily desorbed.In the present study,phosphate-loaded ZrO(OH)2·(Na2O)0.05·1.5H2O was desorbed with a0.1M NaOH solu-tion.The desorption and regeneration experimental results are shown in Fig.7.The amount of phosphate desorption was 82%.The phosphate adsorption decreased slightly after the ?rst cycle,but remained almost constant till the?fth repeti-tions of the adsorption/desorption cycle.It is concluded that the adsorption of phosphate on ZrO(OH)2·(Na2O)0.05·1.5H2O is reversible,and the bonding between exchange sites

and Fig.7.Change of regeneration(R reg)and desorption(R des)rates with recycle times.

the adsorbed phosphate is not so strong.This result suggests that ZrO(OH)2·(Na2O)0.05·1.5H2O has practical potential to be used as an adsorbent for phosphate removal.

https://www.doczj.com/doc/066247930.html,parison of phosphate adsorption from

phosphate-enriched seawater with other adsorbents

Phosphate adsorption from phosphate-enriched seawater were studied on layered double hydroxides of MgAl,MgFe, NiFe,and ZrO(OH)2·(Na2O)0.05·1.5H2O at an adsorbent/ seawater ratio of0.05g/2L after stirring for7d.The uptakes of phosphate are summarized as follows;the phosphate adsorptiv-ity increases in the order,MgAl(1mg-P/g)

3.8.Mechanism of adsorption of phosphate on

ZrO(OH)2·(Na2O)0.05·1.5H2O

Cation or anion adsorption on amphoteric metal hydroxides has been extensively studied[20].Functional groups are mostly represented by hydrolyzed species,and acid–base reactions can be represented for ZrO(OH)2·(Na2O)0.05·1.5H2O as follows:≡Zr–OH+H+ ≡ZrOH+2protonation in an acidic

medium(anion exchange),(1)≡Zr–OH ≡ZrO?+H+deprotonation in an alkaline

solution(cation exchange).(2) It is known that certain anions(phosphate,arsenate,selenate, etc.)that are strongly adsorbed do undergo ligand exchange (inner sphere or outer sphere complex),but simple anions(chlo-ride,nitrate,and sulfate)do not.An inner sphere complex is formed when the adsorbed ligand is directly linked to the metal ion by covalent bonding,and an outer complex,which involves

432R.Chitrakar et al./Journal of Colloid and Interface Science297(2006)426–433 electrostatic bonding,is formed when a water molecule is re-

tained between the exchange site and the adsorbed ligand[4].

For a single electrolyte,an increase in pH should cause a de-

crease in the amount of phosphate adsorption.If the adsorption

is only a simple ion-exchange mechanism between the nega-

tively charged phosphate ion and the oxide surface,there should

be no adsorption of phosphate ions at pH above5,because the

value of zero point charge of zirconium hydroxide reported in

the literature was close to4[33].However,a signi?cant amount

of phosphate adsorption occurs at pH>4,as shown in Fig.5.

The formation of an outer-sphere complex is more likely to

occur during phosphate adsorption,because the adsorbed phos-

phate ions are easily desorbed with a0.1M NaOH solution.For

the inner-sphere complex formation,phosphate ions are bound

directly to the metal ion of the adsorbent by the formation of

a covalent bond,and desorption involves only a portion of the

total phosphate[1].

Accordingly,we assume that the phosphate adsorption on

ZrO(OH)2·(Na2O)0.05·1.5H2O is due to the formation of the

outer sphere complex.The reactions are given as

≡Zr–OH+H++H2PO?4 ≡Zr–OH+2–H2PO?4

(at pH2–4),(3)

≡Zr–OH+2H++HPO2?4 ≡Zr–OH+2–HPO2?4+H+

(at pH6–10),(4)

≡Zr–OH+3H++PO3?4 ≡Zr–OH+2–PO3?4+2H+

(at pH>10),(5)

where–OH is a hydroxyl group,and–H2PO?

4,HPO2?

,and

PO3?4are the adsorbed ligands at corresponding pH values.

In seawater,the competition of anions for exchange sites de-

pends on the relative af?nity of the anions for the exchange

sites,the relative concentration of the anions,and the pH of

the solution.At pH<4,phosphate adsorption is low due to the

presence of a high concentration of chloride and sulfate ions in

seawater.Although chloride and sulfate ions may compete for

the available exchange sites on ZrO(OH)2·(Na2O)0.05·1.5H2O with electrostatic interactions,the adsorption of phosphate is

probably followed by outer sphere complex formation,as dis-

cussed above.The uptake of phosphate increases with an in-

crease of the pH value up to7.A decrease in uptake of phos-

phate in the region pH>7is explained by the fact that the

surface charge of the adsorbent changes to be more negative at

higher pH,resulting in electrostatic repulsion of the more nega-

tively charged phosphate anion.A similar outer sphere complex

formation has been reported for the adsorption of phosphate

from seawater on tetravalent manganese oxide[13].Although

ZrO(OH)2·(Na2O)0.05·1.5H2O is regarded as a weak cation ex-changer at high pH[20],the uptake of cations from seawater is negligible at neutral pH,as shown in Table1.

4.Conclusions

The results of Fig.6suggest that the adsorption of phos-

phate is high at low equilibrium concentration of phosphate

(0.2mg-P/L).The maximum uptake of phosphate ions from

seawater and wastewater is10and17mg-P/g,respectively,on ZrO(OH)2·(Na2O)0.05·1.5H2O,much greater than the1–2mg-P/g observed on the layered double hydroxide adsorbents stud-ied so far and reported literature data for seawater[11–13],and 1–7mg-P/g reported for wastewater on blast slag and?y ash [15,16].ZrO(OH)2·(Na2O)0.05·1.5H2O can be effectively used for the removal of phosphate from ef?uents containing phos-phate at wider pH ranges.

The signi?cance of the experimental data in seawater is to demonstrate that ZrO(OH)2·(Na2O)0.05·1.5H2O is highly se-lective for phosphate anions even at extremely low phosphate concentration,and its practical application may be applied to natural waters.The adsorbent may also be used to collect phos-phate ions from seawater,because of increasing industrial de-mand of phosphate.

Adsorption/desorption studies show that phosphate is easily desorbed with a0.1M NaOH solution,indicating that the ad-sorbent can be reused.These results suggest that ZrO(OH)2·(Na2O)0.05·1.5H2O can act as a suitable adsorbent for the safe and economical removal of harmful phosphate ions from sea-water and contaminated ecosystems.

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关于时间管理的英语作文 manage time

How to manage time Time treats everyone fairly that we all have 24 hours per day. Some of us are capable to make good use of time while some find it hard to do so. Knowing how to manage them is essential in our life. Take myself as an example. When I was still a senior high student, I was fully occupied with my studies. Therefore, I hardly had spare time to have fun or develop my hobbies. But things were changed after I entered university. I got more free time than ever before. But ironically, I found it difficult to adjust this kind of brand-new school life and there was no such thing called time management on my mind. It was not until the second year that I realized I had wasted my whole year doing nothing. I could have taken up a Spanish course. I could have read ten books about the stories of successful people. I could have applied for a part-time job to earn some working experiences. B ut I didn’t spend my time on any of them. I felt guilty whenever I looked back to the moments that I just sat around doing nothing. It’s said that better late than never. At least I had the consciousness that I should stop wasting my time. Making up my mind is the first step for me to learn to manage my time. Next, I wrote a timetable, setting some targets that I had to finish each day. For instance, on Monday, I must read two pieces of news and review all the lessons that I have learnt on that day. By the way, the daily plan that I made was flexible. If there’s something unexpected that I had to finish first, I would reduce the time for resting or delay my target to the next day. Also, I would try to achieve those targets ahead of time that I planed so that I could reserve some more time to relax or do something out of my plan. At the beginning, it’s kind of difficult to s tick to the plan. But as time went by, having a plan for time in advance became a part of my life. At the same time, I gradually became a well-organized person. Now I’ve grasped the time management skill and I’m able to use my time efficiently.

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