Removal of Phosphate from Aqueous Solutions Using a NewModified Bentonite-Derived Hydrogel

Soghra Yaghoobi Rahni & Nourollah Mirghaffari &Behzad Rezaei & Hassan S. Ghaziaskar

Received: 10 November 2013 /Accepted: 20 February 2014# Springer International Publishing Switzerland 2014

Abstract A bentonite-based hydrogel was chemicallymodified to prepare a new effective adsorbent for theremoval of phosphate from aqueous solutions usingbatch equilibrium experiments at the laboratory scale.The efficiency of the phosphate adsorption by the mod-ified adsorbents followed the order: Al-Fe-hydrogel>Al-hydrogel>Fe-hydrogel>Rewoquate surfactant-hydrogel≅Irasoft surfactant-hydrogel>raw hydrogel.The amount of Fe and Al, as determined in proportionto the cation exchange capacity (CEC) of the hydrogel,was the most important parameter for optimizing themodification process by pillaring solutions. The resultsshowed that the phosphate adsorption was rapid and pHindependent. The removal of phosphate reached up to99 % at the optimized conditions. The adsorption datawere well fitted by Langmuir and Freundlich models.According to the Langmuir model, the maximum ad-sorption capacity of the phosphate on the Fe-Al-hydrogel was 14.29 mg L−1. The removal of phosphatefrom an urban wastewater using the modified adsorbentwas more than 99 %. The Fe-Al-hydrogel selectivelyadsorbed the phosphate from the solutions containingsulphate, bicarbonate, chloride, and nitrate. Based on theobtained results, the synthesized adsorbent could be

used effectively to decontaminate the phosphate pollut-ed water.

Keywords Hydrogel . Phosphate . Pillaring solutions .

Adsorption . Adsorption isotherms

1 Introduction

Phosphorus (P) is a common macronutrient present inthe discharged urban wastewater that can cause theeutrophication of aquatic ecosystems. To control theeutrophication, the wastewater treatment facilities mustoften reduce the P concentration to less than 1 mg L−1

(Peleka and Deliyanni 2009). Although the biogeo-chemical cycle of this element naturally has a limitednonrenewable resource, the wastewaters containingphosphorus are generally considered as a renewableresource (Morse et al. 1998; Cheng et al. 2009; Evansand Johnston 2004; Biswas et al. 2007).

Different physicochemical and biological methodshave been used for the removal of phosphate frommunicipal and/or industrial wastewaters. In recent years,the adsorption process has received more attention dueto low cost, high efficiency at low concentrations, lowsludge production, operational simplicity, and the pos-sibility of phosphate recycling from wastewater (Pelekaand Deliyanni 2009; Tian et al. 2009; Namasivayam andPrathap 2005; Y. Zhao et al. 2009; Khadhraoui et al.2002; G. Zhang et al. 2009; Y. Li et al. 2006). The mainproblem of adsorption process for the phosphate remov-al from aqueous solutions is the availability of the cheap

Water Air Soil Pollut (2014) 225:1916DOI 10.1007/s11270-014-1916-y

S. Y. Rahni :N. Mirghaffari (*)Department of Natural Resources, Isfahan University ofTechnology,Isfahan 8415683111, Irane-mail: mnorolah@cc.iut.ac.ir

B. Rezaei :H. S. GhaziaskarDepartment of Chemistry, Isfahan University of Technology,Isfahan 8415683111, Iran

and effective sorbents (Chubar et al. 2005). Clay min-erals are well known as the inexpensive, abundant, andenvironmental friendly materials with characteristicssuch as porosity, specific surface area, as well as highchemical and mechanical stability, all leading to thepreparation of appropriate adsorbents for water andwastewater treatment (Kubilay et al. 2007).

The removal of phosphate from aqueous solutionsusing clay minerals has been widely investigated(González-Pradas et al. 1992; Fontes and Weed 1996;Dimirkou et al. 2002; Kasama et al. 2004; Ma and Zhu2006; Borgnino et al. 2009; Gan et al. 2009;Haghseresht et al. 2009; Kamiyango et al. 2009;Borgnino et al. 2010; Yan et al. 2010; Hamdi andSrasra 2012). Clay minerals are usually modified bydifferent physicochemical methods to improve their ef-ficiency. The inorganic–organic bentonites (IOBs) mod-ified by cetyltrimethyl ammonium bromide andhydroxy-aluminum were used for the simultaneous ad-sorption of organic compounds and phosphate. Thephosphate removal efficiency of IOBs was slightlyhigher than that of the hydroxy-aluminum pillared ben-tonite (Zhu and Zhu 2007). The smectite and mica clayswere treated by Al-pillaring solution and used for thephosphate sorption (Kasama et al. 2004).

A new promising approach toward the preparation ofeffective adsorbents for the removal of contaminants isbased on the hydrogel structure. Hydrogels are hydro-philic cross-linked polymers synthesized from differentorganic and/or inorganic substances. They are known asa superabsorbance of water with many industrial andagricultural applications. In general, the presence ofhigh water content and the porous network structuresin the hydrogels can provide the useful characteristicsfor their possible use as an effective adsorbent for theremoval of contaminants including heavy metals (Bulutet al. 2009), dyes (Shirsath et al. 2011) and nutrients(Dai et al. 2011; Jing et al. 2013) from aqueousenvironments.

In recent years, a number of researches have beenconducted on the use of hybrid hydrogels to overcomethe disadvantages such as poor mechanical properties ofthe hydrogel (Dai et al. 2011; Jing et al. 2013; Ahmed2013). Nevertheless, few studies have been focused onthe efficiency of hydrogel to remove the phosphate fromaqueous solutions. The main objective of the currentresearch was investigating the possibility of using achemically modified bentonite-derived hydrogel by Fe,Al, Fe-Al, and surfactant solutions as a novel adsorbent

for the removal and recovery of phosphate from waterand urban effluents at the laboratory scale. The effect ofdifferent parameters related to the modification processand the phosphate solution on the adsorption wasstudied.

2 Materials and Methods

2.1 Materials

The raw hydrogel, a new modified and commercialbentonite (Hydrogel®), was supplied from SepahanChemical Farzin Co., Isfahan, Iran. All chemicals used,including NaHCO3, NaCl, NaNO3, Na2SO4, NH3,FeCl3.6H2O, AlCl3.6H2O, and KH2PO4, were of ana-lytical grade and obtained from Reidel-de Haen Co.,Germany. The Irasoft-T18 and Rewoquate-WE18 sur-factants were purchased from Nili Padideh Chemistry,Iran and Goldschmidt, Germany, respectively. The rawhydrogel was ground and sieved by a 35 mesh size(0.5 mm).

2.2 Sorbent Characterization

X-ray diffraction analysis (XRD) of the sorbent wasperformed using a diffractometer (Bruker, D8 advance,Germany) with a Cu tube anode at a wavelength of1.5406 Å (Cu Kα). The XRD patterns were recordedin the 2θ range of 3–60○. The major elemental compo-sitions were determined by X-ray fluorescence (XRF)spectrometry analyzer (Bruker, S4 Pioneer, Germany).The surface functional groups of the sorbent were stud-ied by Fourier transform infrared spectroscopy (FT-IR,Jasco 680). The samples were mixed with KBr in anagate mortar with a ratio of 0.1:3 (w/w), and poweredfinely to prepare the KBr pellets. The absorption bandsin the range of 400–4,000 cm−1 were recorded. Thehydrogel samples before and after phosphate adsorptionwere coated with gold under vacuum in an argon atmo-sphere to observe the surface morphology by scanningelectron microscopy (SEM, Philips series XL30,Netherlands).

2.3 Sorbent Modification

The modification process of the raw hydrogel was con-ducted using different inorganic and organic solutions.The pillaring solutions of aluminum, iron, and

1916, Page 2 of 12 Water Air Soil Pollut (2014) 225:1916

aluminum-iron were prepared by dissolving iron andaluminum chloride in deionised water at different con-centrations equivalent to cation exchange capacity(CEC) of the sorbent (118 Cmol kg−1), as determinedby ammonium acetate method (Bashour and Sayegh2007). To prepare the pillared hydrogels, 25 mLpillaring solutions were added to 10 g hydrogel and keptduring 24 h at room temperature (25 °C), and thenfiltered. The filtrate was mixed with 50 mL ammoniacsolution 20 mM and stirred for 30 min at room temper-ature. It was then filtered again and washed by 100 mLammoniac solution 2 mM. Subsequently, the treatedhydrogels were dried at 70–75 °C in the oven for 24 h.The effect of contact time (1, 2. 4, 8, 16, and 24 h), theconcentration of the selected pillaring solution (0.25 to 4times of CEC) as well as the concentration of ammoniac(0, 2.5, 5, 10, and 20 mM) on the hydrogel modificationwas also optimized. For the preparation of organic so-lutions, 2 g surfactant (Irasoft and Rewoquate) wasdissolved in 50mL of 50% v/v acetone/ethanol solution.Then, 50 g hydrogel was added to a 100 mL preparedsurfactant solutions, mixed for 30 min, and dried in theoven at 50–55 °C for 24 h.

2.4 Phosphate Adsorption

The adsorption experiments were designed by a univar-iate method and were carried out at laboratory scaleusing batch equilibrium technique in a 100 mL glassErlenmeyer flask. One gram of the sorbent was added to50 mL phosphate solution at a concentration of25 mg L−1 (calculated as P) and capped by aluminumsheet. The suspension was shaken by an orbitalshaker at 125 rpm for 1 h at room temperature.After centrifugation, the phosphate concentration inthe supernatant was analyzed by the ascorbic acid meth-od (Clesceri et al. 1999).

After optimizing the hydrogel modification, the ef-fect of different parameters such as contact time (5, 15,30, 60, and 120 min), solution pH (2 to 10), the initialphosphate concentration (10, 25, 50, 100, 250, 500, and1,000 mg L−1), and adsorbent dose (0.5, 1, 1.5, and 2 g)on the phosphate adsorption was investigated.Furthermore, the efficiency of the modified hydrogelfor the phosphate removal from urban wastewater wastested using batch experiment (1 g adsorbent, 50 mLwastewater, pH 6.5, 125 rpm, 1 h). All adsorption ex-periments were performed in duplicates, together with acontrol solution (phosphorus solution without the

adsorbent). The efficiency of phosphate adsorptionwas calculated according to the following equations(Eq. 1 and 2):

%Adsorption ¼ Ci � C f

Ci� 100 ð1Þ

Adsorbent amount mg g−1� � ¼ V Ci � C fð Þ

Wð2Þ

where Ci and Cf are the initial and final phosphateconcentrations (mg L−1) in the solution, respectively; Vis the solution volume (L), and W is the absorbentweight (g).

2.5 Adsorption Isotherms

The Langmuir and Freundlich isotherms models wereapplied to evaluate the adsorption process. TheFreundlich model corresponds to a heterogeneous sur-face and the exponential distribution of active sites andtheir energies. It does not predict any saturation of theadsorbent surface and indicates a physiosorption mech-anism on the surface (Namasivayam and Sangeetha2004). The Freundlich isotherm equation is expressedas Eq. (3):

logx

m

� �¼ logK f þ 1

nlogCe ð3Þ

where Ce is the phosphate concentration (mg L−1) atequilibrium, x/m is the adsorbed amount of P (mg g−1),Kf is the Freundlich constant related to the adsorptioncapacity, and n is a constant related to the energy or theintensity of adsorption. The n value less than 1 means apoor adsorption, from 1–2, a moderately difficult ad-sorption, and from 2–10, a good adsorption (Ma et al.2012). The Freundlich exponent Kf and n were deter-mined from the linear plot of log (x/m) vs log Ce(Zhenget al. 2009).

The Langmuir isotherm model describes a homoge-nous and monolayer sorption process and is formulatedas Eq. (4):

Ce

qe¼ 1

qmaxKLþ Ce

qmaxð4Þ

where Ce is the concentration of phosphate (mg L−1) atequilibrium, qe is the amount of adsorbed phosphate(mg g−1) at equilibrium, qmax is the maximum adsorp-tion capacity (mg g−1), and KL is the Langmuir constant(L mg−1) related to the energy of adsorption(Namasivayam and Sangeetha 2004). Plot Ce/qe vs Ce

Water Air Soil Pollut (2014) 225:1916 Page 3 of 12, 1916

is linear. The essential characteristics of the Langmuirisotherm can be expressed by a dimensionless constant,namely, the equilibrium parameter or separation factorRL, which is given by Eq. (5).

RL ¼ 1

1þ bCoð5Þ

where b is the Langmuir constant and C0 (mg L−1) is theinitial phosphate concentration (Namasivayam andSangeetha 2004; Putra et al. 2009;Zheng et al. 2009).The value ofRL indicates the isotherm type that could beeither unfavorable (RL>1), linear (RL=1), favorable (0<RL<1), or irreversible (RL=0) (Ma et al. 2012).

2.6 Selectivity of Phosphate Adsorption

The competitive adsorption experiments were conduct-ed with different anions commonly found in the munic-ipal wastewater to evaluate the sorbent selectivity forphosphate adsorption. The mixed solutions (pH 6.5)containing bicarbonate (HCO3

−), chloride (Cl−), nitrate(NO3

−), and sulfate (SO42−) at two concentrations of 10

and 100 mg/L were prepared by dissolving sodiumhydrogen carbonate(NaHCO3), sodium chloride(NaCl), sodium nitrate (NaNO3), and sodiumsulphate(Na2SO4) in deionised water. The modified hy-drogel (1 g) was added to 50 mL of the mixed solutionsat pH 6.5 and was shaken at 125 rpm for 1 h at 25 °C.The concentration of the anions was examined accord-ing to the standard method (Clesceri et al. 1999).

2.7 Phosphate Desorption

The phosphate desorption tests were performed to in-vestigate the reusability of the modified hydrogel as wellas the possibility of phosphorous recovery. Thephosphate-loaded hydrogel was prepared using batchexperiments by adding 10 g of the sorbent in a 1.5 Lphosphate solution with the initial concentrations of 100mg L−1 during 1 h. Phosphate was desorbed with a0.1 M NaOH solution (5 g phosphate-loaded hydrogel,200 mL NaOH 0.1 M, 1 h). Desorption efficiency of thephosphate was calculated by Eq. (6):

Rdes ¼ Ddes

Aads� 100 ð6Þ

where Ddes and Aads are the amounts of desorbed andadsorbed phosphate, respectively (Chitrakar et al. 2006).

3 Results and Discussion

3.1 Characteristics of Sorbents

The chemical composition of the raw and modifiedhydrogel by Fe-Al was summarized in Table 1.Aluminum and iron silicates were the most prevalentcompounds. Comparison of the raw and modified hy-drogel composition indicated that iron and alumi-num were intercalated in the layers of hydrogel.The X-ray diffraction patterns of the raw hydrogelwere illustrated in Fig 1. The maximum basalspacing was 1.9 nm. The most abundant mineralcompounds in the hydrogel were Troma, albite,cristobalite, silicon oxide, montmorillonite, andgypsum. Also, the results of XRF showed theexistence of sodium compound in the hydrogel whichwas probably related to the presence of troma andalbite.

The FT-IR spectra of the raw hydrogel, and thosemodified by Fe-Al pillaring solution and after loadingby phosphate were presented in Fig 2. The bands around3,627 cm−1 can be attributed to the Al(Mg)–OH vibra-tion (Putra et al. 2009). The relatively weak bandsaround 3,600–3,300 cm−1 are ascribed to the O–Hstretching vibration (Darvishi and Morsali 2010), andthat of 1,638 cm−1 is related to the H–O–H bending(Putra et al. 2009). The sharp absorption peak at1,096 cm−1 corresponds to the stretching vibration ofthe Si–O bond (Yuan et al. 2006). The peaks at 469–520 cm−1 are due to a Si–O bending vibration. The peakat 795 cm−1 may correspond to the stretching vibrationof Al–O–Si (Darvishi and Morsali 2010), and that of917 cm−1 to the stretching vibration of Al–O(OH)–Aland Al–Al–OH deformation (Zhao et al. 2008; Yuanet al. 2006). The peak at 621 cm−1 is attributed to acoupled out of plane Al–O and Si–O (Yuan et al. 2006).The weak bands around 1,325–1,600 cm−1 in the rawhydrogel were removed in the pillared hydrogel. Inaddition, there was no difference in the FT-IR absorptionpeaks between Fe-Al-hydrogel and those obtained afterloading phosphate.

The SEM images were used to compare the surfacemorphology of the modified hydrogel and that afterphosphate adsorption (Fig 3). The raw hydrogel exhib-ited an aggregated morphology and the flakes wereobserved in some particles. The image of Fe-Al hydro-gel showed the presence of the nanoparticles.Furthermore, its morphology was more porous and

1916, Page 4 of 12 Water Air Soil Pollut (2014) 225:1916

fluffy (Gu et al. 2011). After phosphate adsorption, theflakes of the phosphate were observed on the sorbentsurface (Fig 3c). Yan et al. (2010) reported that afterphosphate adsorption by pillared bentonites, the thinlamellas formed a stacking structure and decreased thesize of the intra-particle voids. Yang et al. (2013) used anovel tablet porous material (TPM) for the phosphateadsorption which was synthesized from the clay, cornstarch, and calcium oxide. After phosphate adsorption,the SEM picture of TPM revealed the formation ofmetal-hydroxyl-phosphate ligand and the chemical pre-cipitation of phosphate-metal.

3.2 Optimization of Hydrogel Modification

Table 2 presents the effect of different modificationsmethods on the phosphate adsorption by the hydrogel.The efficiency of the phosphate adsorption by the mod-ified sorbents followed the order: Al-Fe-hydrogel>Al-

hydrogel >Fe-hydrogel >Rewoquate surfactant-hydrogel≅Irasoft surfactant-hydrogel>raw hydrogel.The modification of the hydrogel by the organic solu-tions did not enhance the phosphate adsorption capacity.Several studies have also revealed that the adsorptioncapacity of organic compounds by organoclays werehigher compared to that of the phosphate (Zhu andZhu 2007; Zhu et al. 2007; Zhu et al. 2009b).

As shown, the adsorption capacity of the Fe-Al-hydrogel was greater than those of the single Al andFe-hydrogel. This could be attributed to the decrease inthe crystallization of Fe and Al oxides in the Fe-Al-hydrogel. As reported by Zhu et al. (2009a), thepresence of hydroxyaluminum could interfere with thecrystallization of hydroxyiron on the interlayers of themontmorillonite during aging and drying, resulting inthe higher adsorption capacity of the hydroxyiron/aluminum-montmorillonite complexes. However, Yanet al. (2010) stated that the phosphate adsorption capac-ity of the modified bentonite by pillaring solutionsfollowed the order: Al-bentonite>Fe-bentonite>Fe-Al-bentonite. The BETsurface area of the Al-bentonite wasconsiderably larger than two other samples. In order tooptimize the hydrogel modification by Al-Fe pillaringsolutions, the effect of following parameters wasinvestigated.

3.2.1 Effect of Contact Time

The results showed that the contact time (1, 2. 4,8, 16, and 24 h) of pillaring solution and hydrogelhad a negligible effect on the efficiency of modi-fication process for phosphate adsorption. For hy-drogel modification, the following steps were per-formed during 4 h.

Table 1 XRF analysis of rawhydrogel and Fe-Al-hydrogel (Feand Al=2 CEC)

a(%w/w)bLoss on ignition

Compound Hydrogela Fe-Al-hydrogel Compound Hydrogel Fe-Al-hydrogel

SiO2 48.42 46.03 TiO2 0.208 0.229

Na2O 11.20 2.01 P2O5 0.040 0.045

Al2O3 9.37 15.90 SrO 0.028 0.014

CaO 2.46 1.55 ZrO2 0.017 0.022

MgO 1.30 1.09 MnO 0.012 –

Fe2O3 1.29 11.07 CuO 0.006 0.012

Cl 0.628 3.92 Nb2O5 0.005 –

SO3 0.321 0.906 ZnO – 0.010

K2O 0.243 0.228 LOIb 24.57 17.88

10 20 30 40 50 60

2 (degree)

inte

nsit

y(a.

u.)

TA

C

S

M

G

T:TromaA:AlbiteC: CristobaliteS: Silicon OxideM: MontmorilloniteG: Gysum

Fig. 1 X-ray diffraction patterns for raw hydrogel

Water Air Soil Pollut (2014) 225:1916 Page 5 of 12, 1916

3.2.2 Effect of Al and Fe Dose

The results of hydrogel modification by varying the FeandAl dose, proportional to the CEC of hydrogel, on the

phosphate adsorption were shown in Table 3. The ad-sorption capacity of phosphate was improved with in-creasing Fe and Al amounts. Porous materials and clayminerals surface loaded with Fe and Al can form the Al-

Fig. 2 FT-IR spectrum of raw hydrogel (- - - -), Fe-Al-hydrogel before(− − ) and after (−−−−) phosphate adsorption

a b

c

Fig. 3 SEM images of a raw hydrogel, b Fe-Al-hydrogel before, and c after phosphate adsorption

1916, Page 6 of 12 Water Air Soil Pollut (2014) 225:1916

Fe-OH and/or Al-Fe-H2O functional groups which arethe active sorption sites for phosphate throughligand exchange with OH (Zhu et al. 2007; Zhuet al. 2009a). The optimum rate of Fe and Alfor hydrogel modification was about four times ofits CEC, indicating an adsorption efficiency ofmore than 99 %. Increasing the adsorption capacityalong with the excessive amount of Fe and Al wasprobably due to the surface precipitation of Fe and Alon the hydrogel. In addition, the Fe and Al hy-droxyl could be intercalated in the interlayerspaces and also adsorbed on the external planarsurfaces and/or at the edges of the clays, leadingto the formation of the fixed and nonexchangeablecations (Zhu et al. 2009a). There was no significantdifference between 2Al+2Fe, 3Al+1Fe, and 3Fe+1Altreatments. Therefore, a ratio of 2Al+2Fe was selectedfor the hydogel modification.

3.2.3 Effect of Ammoniac Concentration

The experimental data showed that the use of ammonicsolutions at various concentrations to prepare the favor-able conditions for the formation of hydroxyaluminumand hydroxyiron during the hydrogel modification hadno effect on the phosphate adsorption. This indicatedthat the Al/Fe hydroxides could be formed on the hy-drogel surface without the need to add an alkali agent.Whereas, the Fe and Al pillaring solutions used forbentonite modification were prepared under alkali con-ditions by slowly adding NaOH solution to obtain afixed OH−/Fe+3 and OH−/Al+3 molar ratio (Yan et al.2010; Li et al. 2010).

3.3 Adsorption Experiments

The modified hydrogel under optimized conditions (4 hfor contact time, the amount of 2Fe-2Al up to four timesof CEC, without the adjustment of pH) was prepared,and the effect of different parameters including contacttime, solution pH, sorbent dose, and initial concentra-tion on the phosphate adsorption was examined.

3.3.1 Effect of Contact Time

The phosphate removal was very fast, reaching theequilibrium with a maximum value (> 99 %) in theinitial 5 min. The rapid sorption of phosphate could beattributed to the direct ligand-exchange process togetherwith the electrostatic sorption on the outer surfaces and/or on the wide pores of the adsorbent (Zhu et al. 2009a).However, the adsorption equilibrium of phosphateby some clay minerals was attained in the long time(Yan-kui et al. 2006; Dable et al. 2008).

3.3.2 Effect of pH

To study the influence of the initial solution pH on thephosphate adsorption by Fe-Al-hydrogel, the experi-ments were performed at pH values ranging from 2 to10. It was found that the removal of phosphate did notdepend on the solution pH. This could be an advantagefor using Fe-Al-hydrogel as an adsorbent in the waterand wastewater treatment.

Generally, the pH has a significant effect on the ionadsorption by the surface of solid phases with variablecharge (e.g., Al and Fe oxides). Besides the ligand-

Table 2 The effect of different modifications on the phosphate adsorption by hydrogel (25 mg P/L, 1 g sorbent, 50 ml solution, pH 6.5,125 rpm, 1 h)

Modification Raw Rewoquate surfactant Irasoft surfactant Fe (equivalentof 2CEC)

Al (equivalentof 2CEC)

Fe-Al (Fe and Alequivalent of CEC)

%Adsorption 9 9 8 20 32 56

Table 3 The effect of Fe and Al ratio on the phosphate adsorption using modified hydrogel (25 mg P/L, 1 g sorbent, 50 ml solution, pH 6.5,125 rpm, 1 h)

0.25Al+0.25Fea 0.5Al+0.5Fe 1Al+1Fe 0.5Al+1.5Fe 1.5Al+0.5Fe 2Al+2Fe 3Al+1Fe 1Al+3Fe

%Adsorption 7.37 26.83 55.87 62.36 40.75 99.86 99.81 99.88

a Numbers show the ratio of Al and Fe

Water Air Soil Pollut (2014) 225:1916 Page 7 of 12, 1916

exchange mechanism, phosphate may also be adsorbedpartly by electrostatic force and formed the outer-spherecomplexes. The ligand-exchange mechanism is less de-pendent on the pH than the electrostatic force (Zhu et al.2009a). The phosphate adsorption onto some adsorbentssuch as vesuvianite (Li et al. 2009), hematite, andgoethite (Oh et al. 1999), activated carbon obtainedfrom coir pith (Namasivayam and Sangeetha 2004),

and iron-bentonite (Yan et al. 2010) was also less pHdependent. On the other hand, pH significantly affectedthe phosphate adsorption using both Al-bentonite andFe-Al-bentonite (Yan et al. 2010), Fe-Mn binary oxide(Zhang et al. 2009), and hydroxyl-iron-lanthanumdoped by the activated carbon fiber (Liu et al. 2013).

3.3.3 Effect of Adsorbent Dose

The removal percentage of phosphate was increasedfrom 22 to 82 % with increasing adsorbent dose from10 to 40g L−1, respectively (Fig 4). Similar variationswith the increase of adsorbent dose were usually ob-served in the adsorption processes due to more availablesorpt ion s i tes (Xiong and Mahmood 2010;Namasivayam and Sangeetha 2004).

3.4 Adsorption Isotherms

The liner plots of the Langmuir and Freundlich isothermsmodels for phosphate adsorption at different concentra-tions (10–1,000 mg L−1) were illustrated in Fig 5. Thehigh correlation coefficient obtained by the Langmuir(R2=0.98) and Freundlich (R2=0.95) models indicatedthat the experimental data were fitted by both models(Table 4). However, the Langmuir model was slightlymore favorable, thereby confirming the monolayer ad-sorption of phosphate onto the Fe-Al-hydrogel surface

0

20

40

60

80

100

10 20 30 40

% A

dsor

ptio

n

Adsorbent dose (g/L)

Fig. 4 The effect of adsorbent dose on the removal of phosphate(initial P concentration of 500 mg L−1, 50 ml solution, pH 6.5,125 rpm, 1 h)

y = 0.28x + 0.42R² = 0.95

-0.5

0.0

0.5

1.0

1.5

2.0

-3 -2 -1 0 1 2 3 4

log

q e (m

g/g)

log Ce (mg/L)

prediction

experimental data

y = 0.07x + 1.18R² = 0.98

0

10

20

30

40

50

60

0 200 400 600 800

Ce/q

e (g

/L)

Ce (mg/L)

prediction

experimental data

a

b

Fig. 5 Plots of a Freundlich and b Langmuir isotherm models forthe phosphate adsorption by Fe-Al-hydrogel (initial P concentra-tions of 10, 25, 50, 100, 250, 500, and 1,000 mg L−1, 1 g sorbent,50 ml solution, pH 6.5, 125 rpm, 1 h)

Table 4 Parameters of Langmuir and Freundlich isotherms forphosphate adsorption onto Fe-Al-hydrogel (initial P concentra-tions of 10, 25, 50, 100, 250, 500, and 1,000 mg L−1, 1 g sorbent,50 ml solution, pH 6.5, 125 rpm, 1 h)

Langmuir Freundlich

qmax (mg g−1) KL (L mg−1) R2 1/n Kf (L g−1) R2

14.29 0.06 0.98 0.28 0.42 0.95

0

20

40

60

80

100

% A

dsor

ptio

n

100 mg/L

10 mg/L

PO42- NO3

- SO42 HCO3

- Cl-

Fig. 6 The effect of coexisting anions on the phosphate adsoptionby Fe-Al-hydrogel (initial anions concentrations of 10 and 100mg L−1, 1 g sorbent, 50 ml solution, pH 6.5, 125 rpm, 1 h)

1916, Page 8 of 12 Water Air Soil Pollut (2014) 225:1916

(Ma et al. 2012). The maximum adsorption capacity ofphosphate determined from the Langmuir isotherm was14.29 mg g−1. According to the obtained n value (3.57)for the Freundlich isotherm model, the phosphate ad-sorption onto the Fe-Al-hydrogel was classified as agood adsorption (Ma et al. 2012). The values of RL, ascalculated from the Langmuir isotherm model, werebetween 0 and 1, indicating that the adsorption processwas favorable (Ma et al. 2012).

Similar results have been reported concerning the fit-ness of these two isotherm models for phosphate adsorp-tion by Fe-bentonite, Al-bentonite and Fe-Al-bentonite(Yan et al. 2010), and bentonite-humic acid as a compositematerial (Zamparas et al. 2013).

The Fe-Al-hydrogel used in this study presentedsome advantages compared to similar adsorbents. Forexample, its maximum adsorption capacity (14.29mg g−1) with a low stochiometry of Fe-Al (1.57mmol g−1) was higher than that of Fe-Al-bentonite(10.5 mg g−1) with a high stochiometry of Fe-Al (10mmol g−1) which was reported by Yan et al. (2010).However, further research on pilot scale is necessary forthe feasibility study and comparing the costs of modi-fied hydrogel with other available adsorbents.

3.5 Effect of Coexisting Anions

The results of selectivity experiments indicated that theFe-Al-hydrogel could selectively adsorbed the phos-phate from aqueous solutions containing different an-ions including bicarbonate, chloride, nitrate, and sul-phate under two initial concentrations of10 and100 mg L−1 (Fig 6). In addition, the simultaneous re-moval of sulphate and bicarbonate anions by the sorbentwas effectively occurred. However, the effect ofcoexisting anions must be more considered particularlyfor the removal of bicarbonate ions under freshwaterenvironments. The phosphate adsorption using lantha-num dopedmeosoporous (Zhang et al. 2010), aluminumand lanthanum/aluminum pillared clays (Tian et al.2009), and chitosan hydrogel bead (Dai et al. 2011)was decreased in the presence of coexisting anions dueto the competition between the anions for the adsorptionsites.

3.6 Phosphate Desorption

The recovery of phosphate and reusability of an adsor-bent are very important from environmental and

economical points of view. The phosphate desorptionfrom saturated Fe-Al-hydrogel by alkali solution usingbatch method was obtained to be 88 %. It should benoted that the reuse of Fe-Al-hydrogel during the suc-cessive cycles of sorption/desorption in the columnexperiments was difficult due to the gel formation.Further investigation is needed to improve the hydraulicproperty as well as the possible release of Fe and Alduring aging and the reuse of the hydrogel. De Vicenteet al. (2008) found that the adsorption capacity of alu-minum hydroxide aged in the absence of phosphate insolution was decreased 75 % after three months, com-pared to that of fresh aluminum hydroxide.

3.7 Phosphate Adsorption from Urban Wastewater

The sorption tests were carried out with an urban waste-water sample to examine the possibility of using the Fe-Al-hydrogel for advanced or tertiary wastewater treat-ment. The phosphate concentration of the wastewaterwas reduced from 4.15 to 0.03mg L−1 after treatment bythe sorbent, corresponding to an efficiency of more than99 %. The removal of the phosphate from a real waste-water generated in a yellow phosphorous factory usinglanthanum/aluminum pillared montmorillonite was var-ied between 65 and 91 % depending on the initial pH(Tian et al. 2009).

4 Conclusions

In this study, a bentonite-based hydrogel was modifiedby different chemical methods to prepare an adsorbentfor the removal of phosphate from aqueous solutions.The Fe-Al pillaring solution was proved to be the ap-propriate method for the modification of the hydrogel.The phosphate removal by Fe-Al-hydrogel was fast andpH-independent. The phosphate adsorption data on theFe-Al-hydrogel were well described by the Langmuirand Freundlich isotherm models. According to theLangmuir model, the maximum adsorption capacity ofthe phosphate on the Fe-Al-hydrogel was 14.29 mg g−1.The presence of commonly coexisting anions in thesolution had no effect on the phosphate adsorption,indicating the good selectivity of Fe-Al-hydrogel.Finally, it should be suggested that the phosphate satu-rated hydrogel may be used as a fertilizer as well aswater absorbance in the agriculture or green space,

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particularly in the arid and semiarid regions where thewater supply for the irrigation is a serious challenge.

Acknowledgements The authors wish to thank the ResearchCouncil of Isfahan University of Technology (IUT) for supportingthis work and Prof. H. Shariatmadari from IUT for his scientificcomments.

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