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View Article OnlineView Journal | View Issue

Temperature effe

aDepartment of Environmental Science and

and Ecology, and Key Laboratory of the

Wetland Ecosystem, Xiamen University, X

Xiamen 361102, China. E-mail: yz3t@xm

+865922185889; Tel: +865922181613; +865bState Key Laboratory of Urban Water Resou

and Environmental Engineering, Harbin In

ChinacCollege of Hydraulic and Environmental En

Yichang 443002, ChinadDepartment of Environmental Engineering

Taiwan

† Electronic supplementary informa10.1039/c4ra08318k

Cite this: RSC Adv., 2014, 4, 51984

Received 7th August 2014Accepted 1st October 2014

DOI: 10.1039/c4ra08318k

www.rsc.org/advances

51984 | RSC Adv., 2014, 4, 51984–5199

cts on arsenate adsorption ontogoethite and its preliminary application to arsenateremoval from simulative geothermal water†

Linlin Hao,ab Tong Ouyang,*a Limin Lai,a Yao-Xing Liu,c Shanshan Chen,a

Hongyou Hu,*a Chang-Tang Changd and Juan-Juan Wanga

Laboratory batch experiments were conducted in order to assess the impacts of temperature on the

performance of goethite in removing arsenate from water. All batch experiments were conducted at

four temperatures (30, 50, 70 and 90 �C) and pH 4.6. The results showed that both the arsenic

uptake rate and capacity were significantly enhanced with increasing temperature from 30 to 90 �C.The adsorption kinetics followed a pseudo-second-order model with coefficients of determination (R2)

all above 0.999. The process followed the Langmuir model, and several thermodynamic parameters

were calculated. Arsenate adsorption was facilitated more under simulative geothermal water

conditions than in RO (reverse osmosis) water. The crystalline structure of goethite was not changed

after adsorption at various temperatures. XPS results showed a decrease in the content of iron

hydroxyl groups, which demonstrated that arsenate adsorption onto goethite may be realised through

the replacement of the iron hydroxyl group to form inner-sphere bidentate/monodentate complexes

at pH 4.6.

1. Introduction

Geothermal water is a kind of natural resource that has beenused by humans for a long time. The importance of geothermalwater has increased over the last decade as demand for non-fossil fuel energy sources has expanded.1 Like a double-edgedsword, signicant environmental changes such as surfacedisturbances, thermal effects and emissions of contaminantsare also generated by geothermal utilisation.2 The mainpotential pollutants in geothermal discharged waters arehydrogen sulphide, mercury, arsenic and other trace metals.3

Arsenic in geothermal water is detected at elevated concen-trations in many places of the world such as the Yangbajaingeothermal elds of Tibet, the southeastern coastal area of the

Technology, College of the Environment

Ministry of Education for Coastal and

iang'an South Road, Xiang'an District,

u.edu.cn; hongyouhu@xmu.edu.cn; Fax:

922880233

rce and Environment, School of Municipal

stitute of Technology, Harbin, 150090,

gineering, China Three Gorges University,

, National ILAN University, I-Lan 26047,

tion (ESI) available. See DOI:

0

Chinese mainland, Taiwan and so on.4 Thompson and Demongereported that geothermal water in Yellowstone National Parkcontained high concentrations of As 1–7800 mg L�1.5 TheRio Loa basin El Tatio geothermal eld of Chile was reportedto have very high arsenic concentration values of up to27.0 mg L�1, and the Los Humeros geothermal eld of Mexicowas even reported to have an arsenic concentration as high as73.6 mg L�1.6 Natural geothermal water (including thermalspring water) is increasingly reported to contain high levels ofarsenic, and this phenomenon is frequently found in south-eastern areas of China and Taiwan. Many of the arsenic-containing geothermal waters are discharged directly into theenvironment without treatment, triggering many environ-mental problems and potential problems.

The distribution of As(III)/As(V) is inuenced by pH and redoxconditions. As(III) is more toxic than As(V), and theWorld HealthOrganization has lowered the maximum contaminant level oftotal arsenic in drinking water to 10 mg L�1. The mobility andtransformation of As-tainted geothermal water have becomesignicant concerns worldwide in environmental health. Thenal fate of arsenic in geothermal water involves its rise to theEarth's surface, and there is concern that it may contaminatethe related groundwater systems, surface-water systems and soilsystems.7

Various kinds of iron oxides and oxyhydroxides exist in soils,sediments and aquatic environments such as goethite(a-FeOOH), lepidocrocite (g-FeOOH), hematite (a-Fe2O3) and soon.8 A great variety of iron oxides and oxyhydroxides usually

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have a strong affinity for arsenic species.9,10 In this study,goethite is selected as the iron oxyhydroxide because it iswidespread in soil systems and is a primary component of soil.11

Investigating the reactions between goethite and differentarsenic species is important to provide insight into the role ofarsenic mobility and transformation in geothermal waters.

So far, many studies have focused on arsenic adsorptiononto goethite.12,13 However, a review of the literature showedthat little has been done to determine the impacts of tempera-ture on the adsorption process. Thus, we conduct a detailedstudy in a batch system in order to gain an understanding of theeffects of temperature on arsenate adsorption onto goethite andthe adsorption performance under simulative geothermal waterconditions.

This paper focuses on the effect of temperature on arsenateadsorption onto goethite because the temperature is the rstconsideration of geothermal water. Previous papers usuallyinvestigated the temperature effect between 20 and 60 �C;14–16

we further consider the comparatively higher temperatures 70and 90 �C because we measured the temperature of thermalspring water near Xiamen city (Fujian province, China) to be88 �C. The pH was adjusted to 4.6 with reference to the acidicpH of the Taiwan Datun volcanic region hot springs reported byChen Bochun et al.;17 the pH of some hot springs in Taiwan areeven as low as 1–2. Usually, the pH values of acidic geothermaldischarged waters fall between 4 to 5.

2. Materials and methods2.1 Materials

The granular goethite (a-FeOOH) used in this study was syn-thesised in the laboratory. A 1 M solution of Fe(NO3)3 wasadjusted to pH 11.0 and stirred in a water bath at 70 � 1 �C for24 h. The suspension was purged with N2 to remove CO2, andthe temperature was then adjusted to 90 �C for 72 h followed byrepeated rinsing of the solids with deionised water. The solu-tion containing a very high concentration of solids was ultra-sonically dispersed for 30 min with the addition of a smallamount of absolute ethyl alcohol. Finally, we obtained thegranular goethite through a freeze drying technique. Theproduct was stored at 4 �C for subsequent use.

The As(V) stock solutions were prepared by Na2HAsO4$7H2O(AR). All the chemicals used in the experiments were AR grade.

2.2 Batch sorption

Adsorption experiments were performed with a backgroundelectrolyte of 0.01 M NaNO3. Suspensions of goethite were madeby adding 0.05 g goethite solids to 100 mL of 0.01 M NaNO3 andmixing continuously with a magnetic stirring apparatus at 30 �Cfor 2 h to make the surface of goethite reach equilibrium. ThepH of the arsenic stock solutions and goethite solutions wereadjusted to 4.6 � 0.2 using dilute HNO3 and NaOH solutions.

Adsorption isotherms experiments were conducted in ashaking water bath with a temperature controller. Batch testswere performed in 200 mL bottles containing 0.5 g L�1 goethiteequilibrated with 1, 2, 5, 10, 15, 20, 30, 40, 50 mg L�1 As(V) under

This journal is © The Royal Society of Chemistry 2014

shaking at 150 rpm for 24 h at 30, 50, 70 and 90 �C. Finally, thesuspensions were ltered through a 0.22 mm membrane lter.

The kinetics experiments were conducted in a closed systemconsisting of a double-layer round glass reactor placed on amagnetic stirring apparatus. The double-layer round glassreactor was connected with a thermostat water bath that couldbe adjusted to different temperatures; the water ows throughthe inside of the double-layer glass reactor to keep thetemperature constant. A pH electrode combined with a ther-mometer was inserted below the surface of the solution todetect the pH change in the reactor. The initial As(V) concen-tration was 1mg L�1, and the goethite suspensions were 0.2 g L�1.The schematic diagram of the experimental apparatus used forkinetic study is shown in Fig. S1.†

2.3 Characterisations

The morphology of goethite was monitored with SEM (scanningelectron microscopy) using a JEOL scanning electronmicroscope model Hitachi S-4800. XRD (powder X-ray diffrac-tion) data were collected from 10� to 70� 2q using Cu Ka

(l ¼ 0.15418 nm) incident radiation in a PANalytical X'pert PROdiffractometer. XPS (X-ray photoelectron spectroscopy) datawere collected on a PHI QUANTUM 2000 spectrometer (PHI,USA) with monochromatic Al Ka radiation (1486.6 eV).

The arsenic analytical method was hydride generationatomic uorescence spectroscopy (HG-AFS), which is capable ofdetecting arsenic as low as 1.0 mg L�1. All the samples were pre-reduced by 5% (w) thiourea – 5% (w) ascorbic acid to ensure thatall arsenic species were converted to detectable As(III).

3. Results and discussion3.1 Granular goethite characterisation

The SEM images are shown in Fig. 1(a); the goethite prepared inthis study formed rod-like nanoparticles that are aggregatedtogether. The specic surface area of the goethite samples wasdetermined by the N2/BET method to be 106.6 � 1 m2 g�1. TheXRD structural analysis (Fig. 1(b)) demonstrates that the gran-ular iron oxyhydroxide is goethite by comparison with thestandard XRD pattern (JCPDS 29-0713) of pure goethite.

3.2 Adsorption kinetics

The effect of time on the arsenate uptake rate at differenttemperatures is shown in Fig. 2, which shows a rapid initialuptake followed by a slow approach to equilibrium. The initialrapid adsorption rate can be attributed to the more adsorptionsites at the initial stage; the arsenic species can interact easilywith these sites. The slower adsorption may be due to slowerdiffusion into the interior of goethite and the decrease of thedriving concentration between bulk solution and the goethitesurface. The adsorption achieves equilibrium gradually within100 min at 30, 50, 70 and 90 �C. As temperature increases from30 �C to 70 �C, the slopes of the kinetic curves (initial rapidstage) gradually become steeper, indicating that highertemperature accelerates the reaction rate. When temperature isincreased from 70 �C to 90 �C, an increasing kinetic trend is also

RSC Adv., 2014, 4, 51984–51990 | 51985

Fig. 1 (a) SEM image and (b) XRD pattern of goethite.

Fig. 2 Kinetics of As(V) adsorption onto goethite at 30, 50, 70, 90 �C(As initial concentration 1 mg L�1, adsorbent dosage 0.2 g L�1, pH 4.6)and the pseudo-second-order fitting curve.

Table 1 The kinetic model fitting parameters for As(V) adsorption ontogoethite at various temperatures and pH 4.6

Kinetic modelPseudo second-orderequation

Intraparticle diffusionfunction

Temperaturek2(g mg�1 min�1) R2

kd(mg L�1 min�1/2) R2

30 �C 0.033 0.999 0.063 0.57650 �C 0.043 0.999 0.055 0.46170 �C 0.156 0.999 0.049 0.33090 �C 0.290 0.999 0.040 0.273

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observed, although the growth rate is smaller compared to the30 to 70 �C temperature increase. This may indicate that arse-nate adsorption onto goethite became less sensitive totemperature within the 70–90 �C range.

Several kinetic models (i.e., pseudo-second-order, Elovichequation, intraparticle diffusion equations) are used to t thekinetic data.18 The calculated parameters of the three kineticmodels are listed in Table 1, and the tting curves of the Elovichequation and intraparticle diffusion model are shown inFig. S2.†

51986 | RSC Adv., 2014, 4, 51984–51990

The pseudo-second-order equation can be written as,

t

qt¼ 1

k2qe2þ 1

qet (1)

and the intraparticle diffusion equation is:

qt ¼ kdt1/2 + C (2)

where t is time, qt is the adsorption capacity at t, qe is theequilibrium adsorption capacity, and k1, k2, and kd are the rateconstants of the pseudo-rst-order, pseudo-second-order andintraparticle diffusion equation, respectively. These parametersare strongly dependent on the applied operating conditionssuch as the initial solute concentration, pH, temperature and soon.

A simple modied Elovich equation is as follows:

qt ¼�1

b

�ln�ab

�þ�1

b

�ln t (3)

where a and b are constants, t is the time, and qt is theadsorption capacity at t. The Elovich equation is frequently usedto describe the initial time period of a sorption process whenthe system is relatively far from equilibrium.19 This model hasbeen proven to be suitable for heterogeneous systems, whichmight exhibit different activation energies for chemicaladsorption on the surface.20

The adsorption process on porous adsorbents is generallydescribed by four stages: bulk diffusion, lm diffusion, intra-particle diffusion and adsorption at a special site on the surface.Bulk and lm diffusion are generally assumed to be rapidbecause of the agitation condition. As can be seen from Table 1,the pseudo-second-order could well describe the experimentaldata with linear regression coefficients (R2) all above 0.999,indicating that arsenate adsorption onto goethite was a second-order chemical adsorption process. To better understand therate-determining step of adsorption, the kinetic data weretested using the intraparticle diffusion equation. As shown inFig. S1,† the tting curves were apparently divided into twostages, which were separately linearly tted well with theintraparticle diffusion model; this indicates that the intra-particle diffusion process is a key rate-limiting step. Accordingto the research of Barrow,21 goethite surfaces are variable andpossibly composed of many crystal defects and micropores; thediffusion process may be attributed to these areas.22 The initial

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rapid stages of the kinetic curves were tted with the Elovichequation, and the values of the correlation coefficients R2 aregreater than 0.93 at four temperatures. The good conformationto the Elovich equation suggests monolayer chemicaladsorption.

3.3 Adsorption isotherms

The adsorption isotherms of arsenate at pH 4.6 at an ionicstrength of 0.01 mol L�1 NaNO3 are presented in Fig. 3. Theadsorption capacity increased with increasing initial arsenicconcentration. Adsorption capacity also increased from 19.84mg g�1 to 25.97 mg g�1 with a rise in temperature from 30 to70 �C. However, a smaller change in adsorption capacity (25.97to 26.60mg g�1) was observed as temperature increased from 70to 90 �C, in agreement with the kinetic results. At pH 4.6, As(V)exists as the negatively charged H2AsO4

�, while the surface ofgoethite is positively charged with the key functional group

Fig. 3 Adsorption isotherm for As(V) adsorption onto goethite at 30,50, 70, 90 �C (initial As concentration of 1, 2, 5, 10, 15, 20, 30, 40, 50mg L�1, respectively, adsorbent dosage 0.5 g L�1, pH 4.6).

Table 2 Adsorption isotherm parameters for As(V) adsorption ontogoethite at pH 4.6

Langmuir isotherm Freundlich isotherm

Temperature(�C)

Qmax

(mg g�1)kL(L mg�1) R2 n

KF

(mg g�1) R2

30 19.84 0.446 0.995 2.958 6.208 0.97450 22.32 0.598 0.995 3.094 7.740 0.98470 25.97 0.992 0.997 3.188 10.15 0.96790 26.60 1.001 0.997 4.301 13.31 0.963

This journal is © The Royal Society of Chemistry 2014

–FeOH2+.23 Thus, anionic arsenate adsorption is probably

enhanced by coulombic attractions.24

The isotherms are tted with the Freundlich and Langmuirequations, and the parameters are summarised in Table 2. Thelinear regression coefficients for the Langmuir model are allabove 0.995, suggesting identical adsorption sites of thegoethite surface andmonolayer adsorption. The isotherms werealso well described by the Freundlich model. According to C.-H.Yang,25 the Freundlich model was created with emphasis on twofactors: the lateral interaction among the adsorbed moleculesand the heterogeneity of the energetic surface. In addition, theFreundlich model is oen applied to situations where the initialconcentration of adsorbate is relatively low.26

The Langmuir isotherm equation is given by:

Ce

qe¼ Ce

qmax

þ 1

qmaxKL

(4)

where qe is the quantity of the species adsorbed at equilibrium(mg g�1), KL is a constant representing the virtual bondingstrength between the target species and adsorber, Ce is theequilibrium concentration of adsorbate in the solution, andqmax is the maximum loading of the adsorbate onto theadsorbent.

The Freundlich isotherm equation was expressed as follows:

ln qe ¼ ln KF + 1/n ln Ce (5)

where qe is the quantity of the species adsorbed at equilibrium(mg g�1), KF is a constant that is a measure of sorption capacity,1/n is a measure of adsorption density, and Ce is the equilibriumconcentration of adsorbate in the solution.

As shown in Table 2, the values of KL for arsenate adsorptionincreased from 0.446 to 1.001 as temperature increased from 30to 90 �C, which is in good agreement with the observation thatadsorption was promoted by increasing temperature. Thevalues of 1/n (0.233–0.338) between 0 and 1 represent thefavourable adsorption of arsenate onto goethite.

3.4 Calculation of thermodynamic parameters

The temperature dependence of arsenic adsorption is associ-ated with changes in several thermodynamic parameters suchas DGo (the standard Gibbs free energy change), DHo (enthalpychange), and DSo (entropy change); these parameters arecalculated using the following equations:

ln(K0) ¼ DS�/R � DH�/RT (6)

DG� ¼ �RT ln(K0) (7)

where R is the universal gas constant, T is temperature (K), andK0 is the thermodynamic equilibrium constant; K0 is deter-mined using the method of Karthikeyan27 by plotting ln(qe/Ce)versus qe and extrapolating ln(qe/Ce) to zero (Fig. S3†).

As shown in Table 3, the values of DG� are calculated fromeqn (7), while the values of DH� and DS� are calculated from theslope and intercept of the Van't Hoff plot, respectively. Thepositive value of DH� (11.29 kJ mol�1) and negative values of

RSC Adv., 2014, 4, 51984–51990 | 51987

Table 3 Thermodynamic parameters for As(V) adsorption ontogoethite at different temperatures and pH 4.6

Temperature(�C) K0

DG�

(kJ mol�1)DS�

(kJ mol�1 K)DH�

(kJ mol�1)

30 3.72 �3.31 0.045 11.2950 4.57 �4.0870 5.40 �4.8190 8.09 �6.31

Table 4 The components of simulative geothermal water

Components Concentration (mg L�1)

Mn2+ 1.0Mg2+ 1.0Al3+ 1.3Ca2+ 2.3Fe2+ 0.5F� 2.0PO4

3� 1.0SiO3

2� 2.6NO3

� 6.2Cl� 4.4SO4

2� 17.5K+ 0.5Na+ 4.8NH4

+ 0.3

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DG� (�3.31 to �6.31 kJ mol�1) conrm the spontaneous andendothermic nature of the adsorption process, and the decreasein DG� with increasing temperature suggests a stronger affinityat higher temperatures. The positive value of DS� implies anincrease in randomness at the solid/solution interface.

3.5 Adsorption under simulative geothermal waterconditions

The components of various types of geothermal waters differfrom each other; thus, modelling of such systems is very chal-lenging. Simulative geothermal water was prepared with refer-ence to the components of the Datun volcanic region hotsprings in Taiwan reported by Chen Bochun et al.17 The detailedcomponents of the simulative geothermal water are listed inTable 4. As can be seen from Fig. 4, the simulative geothermalwater with multiple co-existing components promoted theadsorption of As(V), indicating that certain ionic strengths werebenecial for arsenate adsorption onto goethite. The maximumadsorption capacity increased from 21.7 to 27.1 mg g�1, likelydue to the compression of the double charged layer, allowingthe arsenic species to get closer to the goethite surface. Multi-valent cations such as Ca2+, Mg2+, and Fe3+ likely co-precipitate(CaHAsO4, MgHAsO4, FeAsO4$2H2O) with arsenate, therebyimproving arsenate removal efficiency. Thus, adsorptionexperiments in simulated geothermal water lacking Ca2+, Mg2+,Fe3+, and Al3+ were carried out in this study. The results showthat arsenate adsorption in simulative geothermal water is stillenhanced compared with that in simulative geothermal waterlacking Ca2+, Mg2+, Fe3+, Al3+, indicating that multivalentcations such as Ca2+, Mg2+, and Al3+ promote the As(V)

Fig. 4 As(V) adsorption onto goethite under simulative geothermalwater conditions at 30, 50, 70, and 90 �C (initial As concentration20 mg L�1, goethite concentration 0.5 g L�1, pH 4.6).

51988 | RSC Adv., 2014, 4, 51984–51990

adsorption process. This result is similar to the results of M.Stachowicz28 suggesting that both Ca2+and Mg2+ promote PO4

3�

adsorption onto goethite.Many competitive anions are reported to have adverse effects

on arsenic adsorption onto goethite. Phosphate and uoride,the main single interfering ions, were studied using concen-trations of the competing anions equal to the concentration ofarsenic. The results showed that phosphate had a profoundcompeting effect on arsenic adsorption. This is reasonablebecause phosphorus and arsenic are in the same main group,and PO4

3�, AsO43� have identical chemical structures; both

molecules are tetrahedral oxyanions with similar pKa values.29

Fluoride is frequently detected at high concentrations ingeothermal waters. Our results showed that uoride had littleinterfering effect on arsenate adsorption, as shown in Fig. S4.†

3.6 Analysis of XRD and XPS spectra

The crystalline structures of goethite aer arsenate adsorption(initial arsenic solution of 10 mg L�1) at four differenttemperatures were investigated. Fig. 5 shows the XRD patternsof goethite at four temperatures. All XRD patterns are consistentwith that of the standard XRD card (JCPDS 29-0713),

Fig. 5 XRD patterns of goethite before and after arsenic adsorption atdifferent temperatures.

This journal is © The Royal Society of Chemistry 2014

Fig. 6 The wide scan XPS spectra of As-loaded goethite.

Fig. 7 O(1s) spectra of goethite.

Fig. 8 O(1s) spectra of goethite after As(V) adsorption.

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demonstrating that goethite existed stably even at temperaturesas high as 90 �C, and that the crystalline structure of goethitewas not changed aer arsenic adsorption.

To prove that arsenic adsorbed onto goethite, XPS analysiswas conducted on a sample of goethite reacted with arsenate.Fig. 6 illustrates the wide scan XPS spectrum of goethite aerarsenate adsorption. The occurrence of new arsenate peaks forAs 1 mm and As3d were observed, conrming the adsorption ofarsenate onto goethite. Fig. S5† shows that the binding energiesof the As3d core level in arsenic oxides are ca. 45.5 eV for As(V).Therefore, it can be demonstrated that the valence state of As(V)was not changed during the adsorption process.

To further investigate the surface hydroxyl group of goethite,O(1s) narrow scans of goethite before and aer As(V) adsorptionwere analysed. The O(1s) spectrum is composed of overlappedpeaks of oxide oxygen (O2�), hydroxyl (OH�), and sorbed water(H2O).30 The O(1s) peaks are tted with two components (O2� at529.6 eV and OH� at 530.9 eV) for pure a-FeOOH, and anadditional peak at 531.3 eV can be attributed to the absorbedH2O.31

All of the spectra were tted using a 50 : 50 Gaus-sian : Lorentzian peak shape, and the obtained tting resultsare shown in Fig. 7 and 8. The key reactive group of OH�

occupies 37.4% of the surface oxygen in goethite, as shown inFig. 7. A signicant decrease in the OH� species was observedaer arsenate adsorption at different temperatures, occupying25.6%, 24.5%, 23.9% and 23.6% at 30 �C, 50 �C, 70 �C and 90 �C,respectively (Fig. 8). This result might indicate that singlycoordinated iron hydroxyl (OH�) was involved and replacedarsenate oxyanion; the reaction was probably carried outthrough the formation of the inner-sphere monodentatecomplex FeOAsO2OH and the inner-sphere bidentate complex(FeO)2AsO2, in accordance with the possible reactions shownbelow:28

^FeOH�1/2 + 2H+ + AsO43� /

FeO�1/2+⊿Z0AsO2OH⊿Z1 + H2O (8)

^FeOH�1/2 + 2H+ + AsO43� /

(FeO)2�1/2+⊿Z0AsO2

⊿Z1 + 2H2O (9)

This journal is © The Royal Society of Chemistry 2014

In these reactions, ⊿z0 and ⊿z1 are the interfacial chargedistribution (CD) model coefficients, where ⊿z0 + ⊿z1 is equal tothe charge introduced by arsenate adsorption.

4. Conclusions

The present study showed that the synthesised goethite has astrong affinity for inorganic arsenate in aqueous systems,temperature plays an important role in the adsorption ofarsenic species, and adsorption capacities increase withincreasing temperature from 30–90 �C. Adsorption isothermswere well tted to the Langmuir model, and the kinetic datawere best t by the pseudo-second equation. The adsorption ofarsenate exhibited a better performance under simulativegeothermal water conditions, and co-existing multivalentcations such as Ca2+, Mg2+, and Al3+ facilitated arsenateadsorption. XRD analysis revealed that the crystalline structureof goethite aer arsenate adsorption was not changed withinthe temperature range of 30–90 �C. The content of iron hydroxylgroups decreased from 35.4% in raw goethite to 25.6% at 30 �Cand 23.2% at 90 �C aer adsorption. This indicated that thesingly coordinated iron hydroxyl (OH�) was involved, and thathigher temperature (50–90 �C) geothermal water facilitated

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arsenate to form inner-sphere bidentate or monodentatecomplexes at pH 4.6.

Acknowledgements

This research is nancially supported by the National NaturalScience Foundation of China (Contract no. 21077086). Theauthors would like to thank all the reviewers for their helpfulcomments and valuable suggestions.

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