Removal of Phosphate from Eutrophic Lakes through Adsorption byin Situ Formation of Magnesium Hydroxide from DiatomiteFazhi Xie,†,‡,§ Fengchang Wu,†,* Guijian Liu,‡,* Yunsong Mu,† Chenglian Feng,† Huanhua Wang,†

and John P. Giesy∥

†State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences,Beijing 100012, China‡CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space Sciences, University of Science andTechnology of China, Hefei 230026, China§Key Laboratory of Functional Molecule Design and Interface Process, School of Materials Science and Chemical Engineering, AnhuiJianzhu University, Hefei 230022, China∥University of Saskatchewan, Department of Veterinary Biomedical Sciences and Toxicology Centre, 44 Campus Drive, Saskatoon,Saskatchewan, Canada, S7N 5B3

*S Supporting Information

ABSTRACT: Since in situ formation of Mg(OH)2 can efficiently sorb phosphate(PO4) from low concentrations in the environment, a novel dispersed magnesiumoxide nanoflake-modified diatomite adsorbent (MOD) was developed for use inrestoration of eutrophic lakes by removal of excess PO4. Various adsorptionconditions, such as pH, temperature and contact time were investigated. Overall,sorption capacities increased with increasing temperature and contact time, anddecreased with increasing pH. Adsorption of PO4 was well described by both theLangmuir isotherm and pseudo second-order models. Theoretical maximumsorption capacity of MOD for PO4 was 44.44−52.08 mg/g at experimentalconditions. Characterization of PO4 adsorbed to MOD by use of X-ray powderdiffraction (XRD), X-ray photoelectron spectroscopy (XPS) and solid state 31Pnuclear magnetic resonance revealed that electrostatic attraction, surfacecomplexation and chemical conversion in situ were the major forces in adsorptionof PO4. Mg(OH)2 formed in situ had a net positive charge on the surface of theMOD that could adsorb PO4

3‑ and HPO42‑ anion to form surface complex and gradually convert to Mg3(PO4)2 and MgHPO4.

Efficiency of removal of PO4 was 90% when 300 mg MOD/L was added to eutrophic lake water. Results presented heredemonstrated the potential use of the MOD for restoration of eutrophic lakes by removal of excess PO4.

■ INTRODUCTION

Phosphate (PO4) is often the limiting factor for primaryproduction by phytoplankton in lakes and excess PO4 can resultin hazardous algal blooms. Due to widespread development ofindustry and agriculture in China point source wastewater andnonpoint source runoff can transport PO4 to surface waters, suchas lakes and reservoirs and result in cultural eutrophication,which affects wildlife and possibly health of humans and as aresult limit sustainable social development.1,2 To addresseutrophication, limiting PO4 can stop eutrophication fromoccurring or becoming more sever, but reducing loadings alonecannot reverse eutrophication.3Recently, removal of PO4 by useof chemical precipitation, adsorption and biological treatmenthave received attention.4,5 Among the various techniquesavailable to remove PO4 adsorption is simple and economicaland the nutrient-loaded sorbent can be used in agriculture as PO4

fertilizer and as a conditioner of soils.6 Various adsorbents such assands,7−9 fly ash,10,11 steel slag,12 red mud,13 zeolite,14,15

activated alumina,16,17 ferric (hydr)oxides2 and polymer hydro-gels18 for adsorption of PO4 have been investigated .Due to greater control of external inputs of PO4 in the last few

decades, internal PO4 loading by release of PO4 from sedimentscan control trophic status of the lakes.19 Among the large numberof in-lake control measures,3 adsorption of PO4 by use oflimestone, fly ash, gypsum, calcite, bentonite clay, and diatomitehave been used to control eutrophication by removal ofPO4.

2,20−24 But use of these adsorbents has been restrictedbecause they do not have sufficient capacities for adsorption orcontain toxic compounds that can be released to the water duringtreatment. Therefore, these conventional adsorbents might notbe feasible in practical treatment of wastewaters or restoration ofeutrophic lakes.2

Received: August 21, 2013Revised: December 14, 2013Accepted: December 16, 2013Published: December 16, 2013

Article

pubs.acs.org/est

© 2013 American Chemical Society 582 dx.doi.org/10.1021/es4037379 | Environ. Sci. Technol. 2014, 48, 582−590

Diatomite is a natural material formed from the remains ofdiatoms, which grew and were historically deposited in sedimentsof seas or lakes.25 It is abundant in many areas of the world andhas unique physical characteristics, such as low cost, porosity(80−90%), small particle size, low thermal conductivity anddensity26 and it has been approved as a food-grade material bythe U.S. Food and Drug Administration.2 Thus, diatomite can besafely used to remove PO4 from eutrophic lake waters. However,diatomite cannot be used directly as an effective adsorbent ofPO4 due to its relatively small surface area and the fact that atneutral pH, the surface has a net negative charge. That negativecharge would repel the predominant species of PO4 at neutral pHthat are HPO4

2− and H2PO4−. A suitable modification of the

surface of diatomite can dramatically improve the surfacecharacteristics and make it more useful for adsorption of PO4than raw diatomite. Conventional modification methods arepurification in HCl25 and calcination,25,27 which have beenapplied to make treated diatomite more inert for use as a filtersupport. Treatment of diatomite with NaOH and deposition ofmanganese oxide on the surface have been used to make it moreeffective as a scavenger of metal ions. The surface area ofmodified diatomite can be as great as 80 m2/g.28 Diatomite canbe modified with lime and aluminum sulfate (Al2(SO4)3)resulting in aluminum hydroxyl groups on the surface ofdiatomite, which results in greater chemisorption efficiency forPO4. Efficiency of removal of 80% of total PO4 was obtained bythis modified diatomite.29 A ferrihydrite-modified diatomite witha, Brunauer−Emmentt−Teller (BET)-specific surface area of211.1 m2/g and capacity to adsorb PO4 of 37.3 mg/g at pH 4.02

and a total efficiency for removal of PO4 of 85%.30

In situ formation of magnesium hydroxide (known asMAGIC) is an effective method for preconcentration ofnanomolar concentrations of PO4 from fresh and sea watersbefore quantification.31,32 However, reports on removal of anionbyMg(OH)2 formed in situ are limited. Recently, it was reportedthat magnesium oxide (MgO) was useful for removal of As(III),MgO nanoflakes that are porous can form Mg(OH)2 in situ byinteractions with water, which have high affinity for As(III) anionin aqueous environments.33

The objectives of this study were to (1) use MgO anddiatomite, both low cost and environmentally benign materials,to prepare a novel PO4 adsorbent with high surface area andsorption capacity; (2) characterized the mechanism ofadsorption of PO4 to modified diatomite; (3) investigate,under laboratory conditions, the potential of modified diatomiteto adsorb PO4 for removal from eutrophic lake waters.

■ MATERIALS AND METHODSMaterials. Raw diatomite was obtained through Shanghai

Kexi Chemical Co., Ltd., Shanghai, China. The raw diatomite wascomposed of 86.30% SiO2, 4.91% Al2O3, 2.32% Fe2O3, 2.55%CaO, and 0.58% MgO. All stock solutions were prepared usingreagent grade chemicals and deionized water. Stock solutions ofPO4 were prepared by dissolving KH2PO4 (Shanghai ChemicalReagent Company, China Pharmacy Group) in deionized water.Soil humic and fulvic acid were extracted from the soil of JiufengMountain (Beijing, China), and have been characterized indetail.34,35

Preparation of MOD. Raw diatomite (15 g) was added to100mL of 6MNaOH at 353K for 2 h to partially dissolve Si. Themixture was immediately added to 100 mL of 2.5 M MgCl2, andstirred at room temperature for 24 h. The mixture was then agedanother 24 h and centrifuged to remove the supernatant and

washed with deionized water until free fromCl−. The residue wasdried at 323 K for 24 h and then stored at room temperature. Thematerial produced was magnesium hydroxide modified diatomite(MHD). MOD was obtained by calcination of MHD at 723 K.33

Characterization of the Adsorbent. The zero point ofcharge (pHzpc) of adsorbents (raw diatomite, MHD and MOD)were measured by using mass titrations.36 Surface morphologywas determined by use of a JEOL JSM-6700F scanning electronmicroscope (SEM). X-ray diffraction analysis (XRD) toinvestigate crystalline structures was conducted by use of aRigaku TTR-III X-ray diffraction system equipped with Cu Kαradiation (λ = 0.15406 nm). Chemical composition of PO4adsorbed to MOD was probed using X-ray photoelectronspectroscopy (XPS, ESCAL AB250) with Al Kα radiation (1487eV) at a power of 300 W under a vacuum of 1.0 × 10−5 Pa. Datawere analyzed by use of XPSpeak 41 software. N2 adsorption−desorption analysis was performed on each sorbent byASAP2020 M+C system (Micromeritics). Surface areasmeasured by use of the Brunauer−Emmentt−Teller (BET)method. The solid state 31P nuclear magnetic resonancespectrum was collected on a Bruker Avance III 400 spectrometeroperating at 161.984 MHz, by use of a Doty Scientific 4 mm H/X/Y MAS probe.

Effect of pH on Sorption of PO4. Adsorbents (0.20 mg)(raw diatomite, MHD and MOD) and 40 mL of 50 mg/Lstandard KH2PO4 solutions were added to 50 mL centrifugetubes with initial pH adjusted to 5.0−10.0 with dilute NaOH andHCl, and shaken until equilibrated (12h) at a temperature of 298K. Supernatant was filter using a 0.45 μm filter andconcentrations of PO4 in supernatant were determined usingthe method of Murphy and Riley.37 The limit of quantification ofPO4 was 0.01 mg/L and the limit of detection was 1.5 μg/L.Sorption capacity was calculated (eq 1).

=−

qV C C

m( )

e0 e

(1)

Where qe is the sorption capacity at equilibrium(mg/g), C0 is theinitial concentration of PO4 (mg/L), Ce is the equilibriumconcentration of PO4 (mg/L), V is the volume of PO4 solutions,and m is the mass of the sorbent (g).

Sorption Kinetics. A 500 mg aliquant of MOD wasequilibrated with 1000 mL of 50 mg P/L, of which the pH wasadjusted to 5.0 in a glass beaker (2000 mL) at 298 K for a fixedperiod of time (15 min, 30 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h,and 12 h). Approximately 5 mL aliquots were taken from thesuspension was filtered using a 0.45 μm filter and concentrationof PO4 was measured. The amount of PO4 adsorbed wascalculated (eq 1) Data were fitted by using the pseudo first-orderequation (eq 2) and the pseudo second-order equation(eq 3).38

− = ‐q q q k tln( ) lnte e 1 (2)

= +tq k q q

t1 1

t 2 e2

e (3)

Where qt (mg/g) is the PO4 sorption capacity at time t, and k1(1/min) and k2 (g/mg min) are the rate constant of pseudo first-order adsorption and the equilibrium rate constant of pseudosecond-order adsorption, respectively.

Sorption Isotherm and Thermodynamics. Solutions ofPO4 (100 mL) over a concentration range of 4−50 mg/L weremaintained at the pH 5.0 and shaken with MOD (50 mg) at 288,298, 308, and 318 K, respectively. After 12h of mixing, amounts

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of PO4 in the supernatant were determined. The sorptionisotherm was described using eq 4 and eq 5:39

= +*

Cq

CQ b Q

1

e

e e

max max (4)

= +qn

C Klg1

lg lge e F (5)

Where: b is a Langmuir constant. Qmax is the theoreticalmaximum adsorption capacity (mg/g), also known as monolayercoverage of the surface. KF and n are characteristic constants of

the system, where KF is a measurement of adsorption capacityand 1/n of adsorption intensity.The thermodynamic parameters of the adsorption process

were determined from the experimental data obtained at 288,298, 308, and 318 K after 12 h of mixing (eqs 6, 7, and 840):

Δ = −G RT Kln0 0 (6)

=Kq

Cde

e (7)

= Δ − ΔK

RH

RTln

So0

o

(8)

Figure 1. Typical scanning electron micrographs for (a) raw diatomite, (b) Mg(OH)2-modified diatomite(MHD), (c) MgO-modifieddiatomite(MOD).

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Where ΔG0, ΔH0, and ΔS0 are Gibbs free energy change(kJ/mol), enthalpy change(kJ/mol) and entropy change(J/mol·K),respectively. Kd (L/g) is the distribution coefficient for theadsorption, T(K) is the absolute temperature, R(J/mol·K) is gasconstant.Desorption Study. 1000 mL of phosphate solution (50 mg

P/L) was maintained at the pH 5.0 and shaken with MOD (500mg) at 298 K. After 12 h of mixing, the PO4 saturated MOD wasfiltered and washed gently with deionized water to remove anyunadsorbed PO4. Then the PO4 saturated MOD(50 mg) wasmixed with 100 mL of 1 mmol and 10 mmol of Ca2+, CO3

2‑,HCO3

−, OH−, SiO32‑ solutions and deionized water at different

pH values (5.0−10.0). After 12 h of mixing, the desorbed PO4was determined.Removal of PO4 from Eutrophic Lake Water. Eutrophic

lake water was collected in March 2012 from Chaohu Lake, thefifth largest freshwater lake China, located in the lower reach ofthe Yangtze floodplain. The sampling sites were on the west sideof the lake, close to the city of Hefei. Lake water was filtered usinga 0.45 μm filter, and stored in plastic bottles at 277 K. Batchexperiments were carried out in 1000 mL glass beakers using 500mL of eutrophic lake water for measuring PO4 removal efficiency.MODwas added to the lake water at concentrations of 0, 50, 100,200, 300, 400, or 500 mg/L and then mixed at constant stirringfor 12 h at 298 K. Amounts of soluble reactive PO4 in thesupernatant were determined after it was filtered through a 0.45μmmembrane filter. Concentrations of total PO4 were measuredafter acid-persulphate digestion.41 Amounts of Ca and Mg weredetermined by use of a flame atomic absorption spectrometer(WFX-1E2, Beijing, China). Absorption wavelengths of 422.7and 285.2 nmwere used for Ca andMg, respectively. The limit ofquantification of Ca and Mg were 0.02, 0.006 mg/L and the limitof detection was 5, 0.8 μg/L, respectively.Statistical Analysis. All the experimental data were the

averages of triplicate experiments. The relative errors of the datawere less than 5%.The curve fitting and statistical analyses wereperformed using SigmaPlot software (version 10.0, SPSS Inc.,Chicago, IL, USA). The correlation coefficient (R2) obtainedfrom regression was used for comparing the model applicability.

■ RESULTS AND DISCUSSIONCharacterization of Modified Diatomite. Physical

parameters of adsorbents (Raw diatomite, MHD and MOD),such as the BET surface areas, total pore volume, average porediameter and magnesium content, are summarized in SupportingInformation (SI) Table S1. Surface morphology of raw diatomitein the present study was disk-like and many pores can be seen onits surface (Figure 1a). The SEM micrograph of MOD revealsthat the original geometry of the pores is destroyed bypretreatment with NaOH and deposition of magnesium (Figure1b and 1c). After being calcined at 723 K for 2 h, MgO nanoflakeswere formed on the surface of the diatomite (Figure 1c). Thesurface area and the total pore volume of raw diatomite wereimproved significantly. The BET surface area of MOD was 72.53m2/g, which is 113-fold greater than that of raw diatomite. ThepHzpc of raw diatomite, MHD and MOD were experimentallyfound to be at pH 4.5, 10.9 and 9.6, respectively (SI Figure S1).Two diffraction bands were centered at 22.0° and 36.2° (2θ

degree) of the XRD, which is the characteristic peak foramorphous SiO2 (JCPDS 29−0085) (Figure 2a). Four newdiffraction peaks were observed at 18.8°, 38.2°, 51.0° and 58.8° inMHD (Figure 2b), which belong to Mg(OH)2 (JCPDS no. 07−0239). For MOD, the diffraction peaks at 42.9° and 62.3° belong

to MgO (JCPDS no. 65−0476), and no remaining Mg(OH)2peaks could be observed (Figure 2c). This observation suggeststhat Mg(OH)2 was converted to MgO completely after thecalcination at 723 K for 2 h.

Effect of pH. In the pH range of 5.0−10.0, PO4 exhibit weakadsorption on raw diatomite (Figure 3). The sorption capacitydecreased from 4.52 mg/g to 1.09 mg/g as a function of pH.Mg(OH)2 deposited on diatomite resulted in grater sorptioncapacities of 9.10−4.77 mg/g in the pH range of 5.0−10.0 . Aftercalcining, the surface area and sorption capability of adsorbentbecame greater. Sorption capacity can reach 45.72 mg/g with aninitial pH of 5.0, which is 42-fold greater than that of rawdiatomite. This sorption behavior usually can be explained interms of pHZPC of the adsorbent and PO4 speciation occurring insolution.42 The PO4 in aqueous solution exists mainly as H2PO4

−

and HPO42‑ in the pH range of 5.0−10.0 (SI Figure S2).At pH<

pHzpc, the surface of the adsorbent was positively charged,which is favorable for adsorption of the PO4 anion. Weakersorption of PO4 by raw diatomite can be attributed to lowersurface area and the negative charge on its surface underexperimental conditions. Greater soption of PO4 by MOD andMHD can be attributed to the positive charge on its surface in thepH range of 5.0−10.0. The specific surface area of MOD largerthan that of MHD is likely more responsible for the enhancedsorption of PO4

2. Meanwhile, the tendency for sorption of PO4

Figure 2. X-ray diffraction patterns of (a) Raw diatomite, (b) MHD, (c)MOD and (d)MOD after treated with phosphate. * forMg(OH)2 and #for MgO.

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to MOD to be the same as sorption to MHD can be explained bycompetition adsorption between PO4 anion and OH− with atgreater pH.Sorption Kinetics. The time taken for PO4 removal using

MOD is of considerable importance for its possible applicationfor remediation of eutrophic lakes. Sorption of PO4 can reach91% of maximum within 3 h, and then changed slowly until toreach sorption equilibrium (SI Figure S3). The process ofsorption consisted of two distinct steps: an initial rapidadsorption which a rapid diffusion of ions from the solution toexternal adsorbent surfaces, followed by a slower adsorption bydiffusion of ions into pores of inner adsorbent surfaces ofDonnan spaces.43,44 Under the experimental conditions ofcontinuous stirring, it is unlikely that the external diffusion couldcontrol the adsorption process. Chemical reaction might takeplace on the surfaces of MOD. The obtained data were fitted byusing the pseudo first-order equation and the pseudo second-order equation (SI Figure S4, S5, and Table S2). Correlationcoefficients of the pseudo second-order kinetic model (R2 =0.998) was relatively greater than that of the pseudo first-orderkinetic model (R2 = 0.961). The adsorption system obeyspseudo, second-order kinetics for the entire adsorption periodand thus supports the assumption that the adsorption is probablyvia surface reactions until the surface functional sites are fullyoccupied, thereafter PO4 diffuse into the MOD for furthercomplexation interactions.45

Sorption Isotherms. Sorption capacity of MOD for PO4increased with increasing of temperature and initial PO4concentrations and gradually to reach equilibrium (SI FigureS6). Langmuir and Freundlich constants were determined usingthe adsorption data at different temperature. The adsorptionisotherm data fit the Langmuir adsorption model well (Langmuirmodel: R2 >0.997, Freundlich model: R2 <0.946) (SI Figure S7,S8 and Table S3), indicating the adsorption of PO4 on the surfaceof MOD was taking place in a monolayer fashion. According tothe Langmuir equation, the theoretical maximum sorptioncapacity of MOD for P was 44.44−52.08 mg/g at experimentalconditions.Sorption Thermodynamics. Thermodynamic parameters

(ΔGo, ΔSo, and ΔHo) of the adsorption process were calculatedusing the obtained experiment data at several temperatures.Gibbs free energy changes (ΔG°) during sorption of PO4

adsorption on MOD is more negative at higher temperature,which demonstrates that the spontaneity of the adsorptionprocess increases with increasing the temperature (SI Table S4and Figure S9). The positive value of entropy change (ΔS°)implies an increase in the disorderness of the solid-solutionsystem,46 which is probably because of structural changes takeplace in adsorbate and adsorbent during the adsorption. Apositive value of the standard enthalpy change (ΔH°) indicatesthat the adsorption process is endothermic.47

Effects of Coexisting Substance. Many coexisting anions,cations and dissolved organic matter such as Na+,K+,Ca2+,Mg2+,Cl−, NO3

−, SiO32−, CO3

2−‑,SO42‑, fulvic acid, and

humic acid in environmental waters can interfere with theadsorption of PO4 onMOD through competitive adsorption. Forthis purpose, 100 mL of the test solutions containing 5 mg/L ofPO4 and various amounts (1.0 and 10.0 mM for ions; 10, 30, and50 mg C/L for dissolved organic matter) of other coexistingsubstance were equilibrated with MOD (50 mg) at pH 5.0. Thenegligible effects of cations on adsorption PO4 were demon-strated by the experimental data in Figure 4. Cl− and NO3

− had

little negative effect when the concentration of coexisting ionswas increased from 1.0 to 10mM. Fulvic acid and humic acid alsoshow a slight negative effect on PO4 adsorption. When theconcentration of coexisting dissolved organic matter wereincreased from 10 to 50 mg C/L, the sorption efficiencydecreasing from 94.4% and 99.2% to 86.9% and 92.6% for humicacid and fulvic acid, respectively. The presence of CO3

2‑, SO42‑

and SiO32‑ showed a significantly negative influence on

adsorption of PO4 with sorption efficiency decreasing from90.6%, 91.8%, and 89.6% to 78.5%, 81.4%, and 75.3% for CO3

2‑,SO4

2−, and SiO32‑, respectively. The main reason was that the

increased concentration of dissolved organic matter and anionscould completion or blocks the surface sorption sites andreduced the positive charge density on the surface of adsorbent,which decrease the adsorption of the negative charged PO4anions through electrostatic force of attraction. Although some ofthe coexisting anions have negative effects, the experimentalresults showed that larger than 75% of PO4 ions can be removaleven in a solution with a large excess amount of coexisting ions.The results imply that MOD has high sorption selectivity towardPO4 anion and has high potential to removal PO4 from realeutrophic lake water.

Figure 3. Effect of pH on the retention of PO4 by MOD, MHD and rawdiatomite experimental conditions: Vtotal, 40 mL;adsorbent, 20 mg; T,298 K.

Figure 4. Effect of coexisting ions, humic acid and fulvic acid on PO4removal experimental conditions: Vtotal, 100 mL;adsorbent, 50 mg;initial pH 5.0.

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Mechanism of Sorption. In order to clarify the mechanismof sorption of PO4 on MOD, the structure changes of MODbefore and after the saturation sorption of PO4 was examinedwith XRD, and the chemical status of phosphate species after itssorption on MOD was examined by high-resolution XPS scansand solid state 31P NMR.The XRD pattern of MOD after sorption in water solution

with initial concentration PO4 of 50 mg/L was given in Figure2(d). After 12h of mixing, peaks attributed to MgO disappeared.Two new diffraction peaks at the 2θ values of 18.8° and 38.2°could be clearly distinguished (Figure 2(d)). According to theintensity and quantity of the diffraction peaks, it can bespeculated that MgO on the surface of diatomite was graduallyconverted in situ into Mg(OH)2.

33

The high resolution-spectrum of P 2p was divided into twopeaks, 132.9 and 133.8 eV, which corresponded to PO4

3−andHPO4

2−, respectively (Figure 5(a)).48 This result demonstrated

that the surface of MOD was composed of PO43− and HPO4

2−.To confirm this conclusion deconvolution of the XPS spectra ofMg 2p, (Figure 5 (b)) demonstrated the presence of twocompounds. The peak at 52.6 eV probably corresponds toMg3(PO4)2 and the peak at 51.8 eV to MgHPO4.

49

A broad band in the solid state 31P NMR spectrum of MODafter adsorption of PO4 appeared at 1.54 ppm (SI Figure S10).

The chemical shift of physical adsorbed PO4 and the noncrystalstructure of Mg3(PO4)2·8H2O and MgHPO4·3H2O appeared at6.4 ppm,50 0.5 ppm and −2.4 ppm,51,52 respectively. The peakpresent here attributed to the mixture of physical adsorbed PO4,the noncrystal structure of Mg3(PO4)2·8H2O and MgHPO4·3H2O. The solid state

31P NMR results showed great accordancewith the results of the XPS analysis and imply that new bondformation dominated the sorption process.From the above analysis, the removal of PO4 by MOD is not a

simple adsorption process. The whole removal process involvesthe in situ formation of Mg(OH)2 from MgO on the surface ofdiatomite; the adsorption of PO4 anion onto in situ formedpositive charged Mg(OH)2 on the surface of diatomite throughelectrostatic attraction to form surface complex; and the reactionbetween surface Mg(OH)2 and sorbed PO4 to conversionMg3(PO4)2 and MgHPO4.MOD with high specific surface area were added into the PO4

solution, MgO nanoflakes on the surface of diatomite becamehydrated and much more fresh active adsorption site forenhanced PO4 removal was in situ formed (Mg(OH)2) due totheir reaction with water. The in situ formed surface hydroxidemaybe protonated in acidmedia and can deprotonated in alkalinesolutions. Therefore, the density of positive charged surfacegroups of MOD change at different pH values. Under theexperimental conditions, H2PO4

− andHPO42‑ can be formed and

the adsorption process is rapid on the surface of MOD. Withincreasing pH, the number of negatively charged hydroxidegroups increases, and the negatively charged adsorbent surfacesites do not favor the adsorption of PO4 anion due to theelectrostatic repulsion. The pH dependent adsorption indicatesthat the adsorption is dominated by surface complexation,53

which also showed in the sorption kinetic study. At the sametime, surface complexed PO4 species onto MOD could in situreaction with Mg(OH)2 and to conversion the noncrystalstructure compound of Mg3(PO4)2 and MgHPO4, which will bebeneficial to the continuous formation of Mg(OH)2. With theincrease of contact time, highly dispersedMgO nanoflakes on thesurface of diatomite could be in situ converted to Mg(OH)2completely,33 and the reaction between Mg(OH)2 and PO4could remove excess PO4 from water continuously. Theproposed sorption mechanism of MOD for PO4 from aqueoussolution is given (eqs 9-11).

≡ = ⇒ ≡ ‐ ‐ +DS Mg O DS Mg O OH2 (9)

≡ ‐ ‐ + ⇒ ≡+ ‐DS Mg O OH HPO DS MgHPO2 42

4 (10)

≡ ‐ ‐ + ⇒ ≡+ ‐DS Mg O OH PO DS Mg (PO )2 43

2 4 3 (11)

(DS = surface of diatomite)Phosphate Desorption. To assess stability of adsorbed PO4

on the surface ofMOD, the effect of pH and some chemical watercomponents such as Ca2+, CO3

2−, HCO3−, OH−, and SiO3

2‑ ondesorption PO4 from MOD saturated with PO4 was studiedusing batch experimental method. The PO4 desorbability wasdefined as the ratio of the desorbed PO4 to the total adsorbedPO4 by the adsorbent. The desorbability of PO4 depends on thepH of the extraction solutions(5.0−10.0) and chemical watercomponents were shown in Figure.6 and Table 1. Thedesorbabilities of PO4 in the different pH solutions were 3.35−7.67%. Effect of Ca2+, OH− and SiO3

2‑ on PO4 desorption wasnegligible (<11%) for all the concentrations of these chemicalwater components studied. Alkalinity had significant effect on

Figure 5. XPS spectra of MOD after treated with PO4. (a) P 2p and (b)Mg 2p.

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desorption of PO4 from the surface of MOD. Desorbabilities ofPO4 increased from 34.69% and 12.75% to 38.57% and 18.61%with increasing the concentration of CO3

2− and HCO3− from 1.0

to 10 mM, respectively. Low desorbability at different pH and inpresent of Ca2+, OH−, SiO3

2‑ indicates that a small fraction ofPO4 is weakly adsorbed or complexed on the surface of MOD.The results from desorption experiments show that PO4 sorbedon MOD is mostly irreversible and that the bond between theMOD and adsorbed PO4 is strong. It is relatively difficult todesorb PO4 from MOD. These results imply that MOD can be apotential adsorbent of PO4 for some eutrophic lake watertreatment.

Comparison with Other Adsorbents for Removal ofPO4. Increasing numbers of adsorbents have been used in recentyears to remove PO4 from environmental waters. The PO4sorption capacity is usually chosen as the criterion for adsorbentselection. For there are not standard procedures for batchsorption experiments, direct comparison with other adsorbentmaterials is difficult due to the different applied experimentalconditions.54 To illustrate the potential in the use of MOD forremoval of PO4 from waters, a comparative evaluation of theSBET, pHzpc adsorption capacities and experimental conditionsof various PO4 adsorbents were provided (Table 2). Under thesimilar experimental conditions, the PO4 sorption capacity ofMOD is slight greater than that of ferrihydrite-modifieddiatomite (FHMD).2 Some of adsorbent such as biochar,55 Feloaded skin split waste56 and alunite57 have relative high capacityfor PO4, but the initial P concentrations were also higher.Meanwhile, the ratio of material to solution in present study is0.5g/L, which much less than that of reported adsorbents inTable 2. The results indicate that the removal of PO4 by theMODwas efficient and performance of MOD is better than mostof other adsorbents for which data is available. Anotheradvantage of the present adsorbent is its cost-effective and thefact that it leaves no toxic residues or released toxic ions to waterswhich cause a second pollution like some other adsorbents.

Removal of PO4 form Eutrophic Chaohu Lake Water.To test the ability of MOD to remove PO4 from real eutrophicwaters, MOD was added to water from Chaohu Lake as arepresentative eutrophic lake (SI Table S5). The pH value of thewater was 8.12, the concentration of total phosphorus was 0.158mg/L. Lake water parameters such as TP, SRP, pH, Ca, and Mgwere determined after using different dose of MOD (Figure 7and SI Figures S11 and S12). The efficiency of removal of totalphosphorus (TP) was proportional to the amount of MODapplied. Efficiencies of removal of TP and SRP were 87.2% and75% when the concentration of MOD was 250 mg/L. Theconcentration of TP in the treated lake water samples was 19.9μg/L, which is comparable with the amounts of alum andferrihydrite-modified diatomite applied to Lake Sønderby,Denmark 58and Jackfish Lake,Canada.30When the concentrationof MODwas 300 mg/L, efficiency of removal of TP was 90% andleft a residual concentration of TP of less than 15.0 μg/L, whichfalls within the oligotrophic range(3.0−17.7 μg/L).59 The pHvalue of lake water was slight increased from 8.12 to 8.67 due to

Figure 6. Effect of pH on PO4 desorption experimental conditions:Vtotal, 100 mL; PO4 saturated adsorbent, 50 mg.

Table 1. Effect of Chemical Water Composition onDesorbability of PO4 Saturated MOD

desorbability(%)

parameters 1 mmol 10 mmol

Ca2+ 0.13 ± 0.05 0CO3

2‑ 34.69 ± 1.56 38.57 ± 1.94HCO3

− 12.75 ± 0.21 18.61 ± 0.46OH− 3.80 ± 0.15 9.29 ± 0.09SiO3

2‑ 6.68 ± 0.10 11.01 ± 0.34

Table 2. PO4 Adsorption by Different Adsorbentsa

sorbentSBET (m

2/g) pHzpc

adsorption capacity(mg/g) reaction condition reference

MOD 72.53 9.6 45.7 pH5.0,T = 298K,adsorbent=0.5g/L,C 0 = 50 mg/L present studyFe−Mn binary oxide 309 6.6 36 pH5.6, T = 298 ± 1K, adsorbent=0.2g/L, C 0 = 5 mg/L 1FHMD 211.1 10 37.3 pH4.0, T = room temperature, adsorbent=0.5g/L, C0 = 40 mg/L 2ACF-LaOH NA 8.0 16.1 pH NA,T = room temperature, adsorbent=2.5g/L,C0 = 30 mg/L 60milled furnace slag 0.75 NA 43.1 pH7.0−7.2,T = 293K, adsorbent=10g/L,C0 = 500 mg/L 61biochar 336 NA 133 pH7.0,T = 295 ± 0.5K, adsorbent=2g/L,C0 = 640 mg/L 55ZnCl2-activated carbon NA NA 4.2 pH4.0,T = 308K, adsorbent=6g/L,C0 = 40 mg/L 62Fe-EDA-SAMMS 169 NA 43.3 pH5.0,T = room temperature, adsorbent=0.1g/L, C 0 = 18.5 mg/L 63La-loaded orange waste NA NA 14 pH7.5,T = 303K,adsorbent=1.7g/L,C 0 = 40 mg/L 64Fe loaded skin split waste 5.23 NA 68 pH7.0,T = 303,adsorbent=1g/L,C0 = 93 mg/L 56Al pillared bentonite 200 NA 5.05 pH3.0,T = NA,adsorbent=4g/L,C0 = 20 mg/L 65iron impregnated coir pith 155.2 5.8 40.55 pH3.0,T = 303K,adsorbent=2g/L,C0 = 100 mg/L 66alunite 148 NA 106.6 pH5.0,T = 298K,adsorbent=10g/L,C0 = 200 mg/L 57

aC0,initial P concentration NA, Date not available.

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in situ formation of Mg(OH)2. Magnesium ion increased from14.84 to 35.16 mg/L while calcium ion decreased from 178.15 to157.88 mg/L in the eutrophic lake water when concentrations ofMODwere increases from 0 to 300mg/L. The reasonmay be thein situ chemical conversion of magnesium-P compounds tocalcium-P compounds and release magnesium ions into thewaters.

■ ASSOCIATED CONTENT*S Supporting InformationTwelve figures and five tables showing the results of BET data forraw diatomite, MHD and MOD, pHzpc, the data of linearregression, the parameters of sorption kinetic, sorption isothermand thermodynamics, solid state 31P NMR spectroscopy ofMOD after treated with PO4, characteristics of lake water, effectsof MOD dose on Ca, Mg concentration and pH in the lake watersample. This material is available free of charge via the Internet athttp://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +86-10-84915312; fax: +86-10-84931804, e-mail:wufengchang@vip.skleg.cn, lgj@ustc.edu.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the National Basic ResearchProgram of China (No.2008CB418200), the National NaturalScience Foundation of China (No.21107001) and the ChinaPostdoctoral Science Foundation (2013M541842).

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Figure 7. Effects ofMODdose onTP, SRP removal efficiency in the lakewater sample.

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S1

Supporting information for

Removal of Phosphate from Eutrophic Lakes through Adsorption by in situ

Formation of Magnesium Hydroxide from Diatomite

Fazhi Xiea,b,c, Fengchang Wu

a＊, Guijian Liu

b＊, Yunsong Mu

a, Chenglian Feng

a, Huanhua Wang

a and

John P.Giesyd

a State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy

of Environmental Sciences, Beijing 100012, China

b CAS Key Laboratory of Crust-Mantle Materials and Environment, School of Earth and Space

Sciences, University of Science and Technology of China, Hefei 230026, China

c Key Laboratory of Functional Molecule Design and Interface Process, School of Materials Science

and Chemical Engineering, Anhui Jianzhu University, Hefei 230022, China

d University of Saskatchewan, Department of Veterinary Biomedical Sciences and Toxicology

Centre, 44 Campus Drive, Saskatoon, SK, Canada, S7N 5B3

* Corresponding authors: Tel: +86-10-84915312; Fax: +86-10-84931804,

E-mail addresses: wufengchang@vip.skleg.cn, lgj@ustc.edu.cn.

Number SI pages: 18

Number the figures: 12

Number the tables: 5

S2

（Solid Fraction %,w/w)

0 5 10 15 20 25 30

Final pH

2

4

6

8

10

12

MOD

MHD

Raw Diatomite

Fig. S1 Determination of the point of zero charge of MOD, MHD and Raw Diatomite.

Experimental conditions : ionic strength, 0.01mol/L KNO3; T, 298 K.

S3

pH

0 2 4 6 8 10 12 14

（）

Percent×100%

0.0

0.2

0.4

0.6

0.8

1.0

1.2

H3PO

4

H2PO4-

HPO4

2-

PO43-

Fig. S2 PO4 speciation as a function of pH1

S4

Time(h)

0 2 4 6 8 10 12 14

qt（

）mg/g

10

15

20

25

30

35

40

45

50

Fig.S3 Sorption kinetics of PO4 on MOD

Experimental conditions : Vtotal, 1000mL;adsorbent, 500mg; T, 298 K.

S5

Time(h)

0 2 4 6 8 10 12 14

ln(qe-qt)

-10

-8

-6

-4

-2

0

2

4

6

y = -0.9748x + 3.9136

R2 = 0.961

Fig.S4 The linear regression of pseudo first-order kinetic model

S6

Time(h)

0 2 4 6 8 10 12 14

t/qt

0.00

0.05

0.10

0.15

0.20

0.25

0.30

y = 0.0206x + 0.0114

R2 = 0.998

Fig.S5 The linear regression of pseudo second-order kinetic model

S7

Ce（ ）mg/L

0 5 10 15 20 25 30 35

qe（

）mg/L

0

10

20

30

40

50

60

288 K

298 K

308 K

318 K

Fig. S6 Equilibrium isotherms of PO4 on MOD, Experimental conditions : Vtotal, 100mL;adsorbent,

50mg; initial pH 5.0.

S8

ce(mg/L)

0 5 10 15 20 25 30 35

ce/qe

0.0

0.2

0.4

0.6

0.8

288 K

298 K

308 K

318 K

Fig.S7 Experimental data for PO4 absorption fitted to Langmuir isotherm

S9

logce

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

logqe

0.8

1.0

1.2

1.4

1.6

1.8

2.0

288 K

298 K

308 K

318 K

Fig.S8 Experimental data for PO4 absorption fitted to Freundlich isotherm

S10

1000/T

3.10 3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50

lnK0

4.0

4.1

4.2

4.3

4.4

4.5

Fig.S9 Plot of lnK0 versus 1000/T for estimation of thermodynamic parameters

S11

-250-200-150-100-50050100150200250ppm

Fig.10 Solid state 31P NMR spectroscopy of MOD after treated with PO4

S12

Dose of MOD (mg/L)

0 100 200 300 400 500 600

Ca and Mg concentration in lake water(mg/L)

0

20

40

160

180

200

Mg

Ca

Fig. S11 Effects of MOD dose on Ca and Mg concentration in the lake water sample

S13

Dose of MOD (mg/L)

0 100 200 300 400 500 600

pH

7.0

7.5

8.0

8.5

9.0

Fig. S12 Effects of MOD dose on pH in the lake water sample

S14

SBET,Brunauer–Emmentt–Teller (BET)-specific surface area

Sample SBET

(m2/g)

Total pore volume

(cm3/g)

Average pore

diameter (nm)

Magnesium

content (%)

Raw

Diatomite 0.64 0.00079 49.50 0.35

MHD 49.81 0.19 15.66 24.86

MOD 72.53 0.26 14.41 27.49

Table S1 Physical parameters for raw diatomite, MHD and MOD

S15

T qe,exp(mg/g)

Pseudo first-order kinetic model Pseudo second-order kinetic model

k1(h-1) qe,1(mg/g) R

2 k2(g/mg·h) qe,2(mg/g) R

2

298 K 45.72 0.97 50.08 0.961 0.037 48.54 0.998

Table S2 Kinetic parameters for the adsorption of PO4 onto MOD

S16

Correlation

parameters T=288 K T=298 K T=308 K T=318 K

Langmuir

qm(mg/g) 44.44 47.39 51.02 52.08

b（L/mg） 0.69 0.65 0.63 0.81

R2 0.9992 0.9982 0.9972 0.9986

Freundlich

KF（mg1-1/nL1/n/g） 16.28 17.13 18.16 20.27

n 2.95 2.91 2.82 2.99

R2 0.9374 0.9394 0.9458 0.9427

Table S3 The parameters for the Langmuir and Freundlich fitting of PO4 adsorption on MOD

S17

T(K) lnKo △Go(kJ/mol) △So(J/mol·K) △Ho (kJ/mol)

288 4.08 -9.77

62.85 8.40 298 4.15 -10.29

308 4.24 -10.86

318 4.42 -11.69

Table S4 Thermodynamic parameters for the adsorption of PO4 on MOD

S18

Reference

1. Karageorgiou, K.; Paschalis, M.; Anastassakis, G. N., Removal of phosphate species from solution by

adsorption onto calcite used as natural adsorbent. J. Hazard. Mater. 2006, 139, 447-452.

Parameter Value Parameter Value

Na (mg/L) 203 pH 8.12

K (mg/L) 25 TN (mg/L) 2.58

Ca (mg/L) 178 SRP(µg/L) 91

Mg (mg/L) 14.84 TP(µg/L) 156

Table S5 Characteristics of water sampled from Chaohu Lake