Fe Doped TiO2

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Fe Doped TiO2 Prepared by Microwave-Assisted HydrothermalProcess for Removal of As(III) and As(V) from WaterIvan Andjelkovic,*, Dalibor Stankovic, Jelena Nesic, Jugoslav Krstic, Predrag Vulic,

Dragan Manojlovic, and Goran Roglic*,

Innovation Center of the Faculty of Chemistry, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis andChemical Engineering, Faculty of Mining and Geology, Department of Crystallography, Faculty of Chemistry, Chair of AnalyticalChemistry, and Faculty of Chemistry, Chair of Applied Chemistry, University of Belgrade, Studentski Trg 12-16, 11000 Belgrade,Serbia

*S Supporting Information

ABSTRACT: Elevated concentrations of arsenic in groundwater, which is used as a source for drinking water, is a worldwideproblem. Use of TiO2and iron doped TiO2synthesized by a microwave-assisted hydrothermal method for As(III) and As(V)removal were examined. Synthesized sorbents were characterized with XRD and nitrogen physisorption. Synthesized sorbents

have predominantly anatase structure, and no peaks for iron could be observed. Doping of iron increases the surface area ofsynthesized sorbents. Sorption experiments show that increase of iron in sorbents increases the sorption capacity for As(III) andAs(V). Increase of pH from 3 to 11 has no inuence on As(III) sorption but decreases the sorption of As(V). Batch isothermstudies were performed to determine the binding capacities of As(III) and As(V). As(III) followed the Freundlich isothermmodel, while for As(V) a better t was with the Langmuir isotherm. The results of competition of SO4

2 and PO43 anions on

adsorption of As(III) indicated that both anions reduced substantially the efficiency of adsorption on both adsorbents while forAs(V) only the presence of PO4

3 anion interfered with adsorption. Testing 10Fe/TiO2 sorbent with arsenic contaminatednatural water showed that this material could be used for removal of arsenic to the level recommended by WHO withoutpretreatment.

1. INTRODUCTION

Pollution of groundwater, as a main source of drinking water,

with arsenic, is a concern of many countries around the world.Elevated concentrations ofarsenicin groundwater werefoundin the United States,13 China,4 Japan,5 Vietnam,6 andAustralia.7 The most severe problem is in Bangladesh8 andWest Bengal,9 where it has been estimated that as many as 100million people are exposed to high levels of arsenic, exceedingthe World Health Organization (WHO) standard of 10 g/L.

Human beings have a important impact on the concentrationof arsenic in nature through mining activity, combustion offossil fuels, use of arsenical pesticides, and use as a additive tolivestock feed. Use of arsenical products has decreased recently,but the impact on the environment will remain for some years.The inuence of anthropogenic activity can be monitored andcontrolled, but most environmental arsenic problems are aresult of mobilization from soil minerals under naturalconditions which are difficult to control.

Arsenic can occur in the environment in several oxidationstates (3, 0, +3, and +5) but in natural waters is mostly foundin inorganic form as oxyanions of trivalent arsenite As(III) orpentavalent arsenate As(V). It is generally recognized that thesoluble inorganic arsenicals are more toxic than the organicones, and that As(III) compounds are more toxic than As(V)compounds.10

The effects of arsenic exposure through drinking water canmanifest through discoloration of skin and intestinal, vascular,and cardiac problems. The relative toxicity of arsenic dependsmainly on its chemical form and is dictated in part by the

valence state. Trivalent arsenic has a high affinity for thiolgroups, as it readily forms kinetically stable bonds to sulfur.

Thus, reaction with As(III) induces enzyme inactivation, asthiol groups are important to the functions of many enzymes.Pentavalent arsenic has a poor affinity toward thiol groups,resulting in more rapid excretion from the body. However, it isa molecular analogue of phosphate and can uncouplemitochondrial oxidative phosphorylation, resulting in failureof the energy metabolism system.11

Adsorption is considered to be one of the best technologiesfor arsenic removal because it can be simple in operation, iscost-effective, and could reduce arsenic concentrations below10 ppb as is required by water quality standards.12 Effectiveremoval of arsenic by adsorption is dependent on many factorssuch as the presence of common competing anions, pH, and Ehwhich inuence the species of arsenic that are present in water,

reaction kinetics, etc. These factors can inuence that anadsorbent effective with one type of groundwater may be lesseffective with groundwater that has a different matrix.

At the near-neutral pH typical for most groundwaters,As(III) exists as a neutral species (H3AsO3), whereas As(V)exists as an anionic species (H2AsO4

or HAsO42).13 Thus,

most methods used for arsenic removal exhibit higher affinityfor As(V) compared to As(III). In recent years micro and nano

Received: March 5, 2014Revised: June 5, 2014

Accepted: June 13, 2014

Article

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sized materials were explored for arsenic adsorption.1416 Also,metal based adsorbents, especially titanium and iron, have beenextensively examined to remove arsenic from water. Titaniumdioxide is a famous photocatalyst that offers a relativelyinexpensive and environmentally safe way to achieveoxidation.1720 Iron oxide minerals are abundant in natureand are relatively inexpensive. The use of iron oxyhydroxides ispromoted by their amorphous structure which gives high

specic surface area values, and their strong affinity andrelatively high selectivity for the most frequently occurringarsenate species under natural pH values of water.2123 Theapplication of composite sorbents containing two or moremetal oxides was extensively investigated in recent years.2426

FeCe bimetal oxide sorbent demonstrated higher As(V)sorption capacity thanFe3O4 and CeO2 oxides prepared withthe same procedure.27

The objectives of this research were to synthesize TiO2nanoparticles doped with two amounts of iron(III) by amicrowavehydrothermal method, characterize them, andexamine their efficiency for removal of arsenic from ground-water. A microwavehydrothermal technique was chosenbecause it was superior to the conventional hydrothermal

process, as it allows rapid heating to the required temperatureand extremely rapid rates of crystallization.28,29 Preparedmaterials were characterized with X-ray diffraction (XRD)and nitrogen physisorption measurements at 77 K, and thepoint of zero charge was determined. Batch experiments wereconducted to examine sorption characteristics of synthesizedmaterials.

2. EXPERIMENTAL SECTION

All chemicals used in synthesis were of analytical reagent grade.Hydrochloric acid and sodium hydroxide were used to adjustthe pH.

Stock solutions containing (1000 ppm) As(V) and As(III)

were prepared by dissolvi ng appropriate quantit ies ofNa2HAsO47H2O and NaAsO2in deionized water, respectively.Solutions of required lower concentrations were prepared bydiluting the stock solutions.

2.1. Sorbent Preparation. Sorbents were prepared usingthe microwavehydrothermal method. In a typical preparationprocedure, TiCl4was added to icy deionized water (TiCl4:H2O(v/v) ratio 1:10) and a homogeneous and transparent solutionwas obtained. Then, the solution was subjected to precipitationby the slow addition of NH4OH (30%) solution under constantstirring at room temperature. The hydrolysis was controlledwith the addition of NH4OH, until the reaction mixtureattained a pH between 7 and 8. Precipitate was obtained, andthe suspension was transferred into a Teon microwave closed

vessel (digestion system ETHOS 1 Milestone, equipped with ahigh pressure rotor, SK-10, Italy), sealed, and heated bymicrowave irradiation. The maximum temperature of 423 Kwas reached in 10 min, and then this temperature wasmaintained for 15 min more for hydrothermal treatment. Theresulting product was separated by centrifugation and washedrepeatedly with deionized water until the precipitate becamefree of chloride ion. Finally, it was dried at 353 K for 5 h andthen calcined at 773 K for 10 h. Doped TiO2 samples wereprepared according to the above procedure including additionof Fe(III) salt in water to give 4 and 10% dopants.

Both dopant concentrations mentioned in this work are theweight percent. Adsorbents TiO2 doped with 4 and 10% ironwere labeled as 4Fe/TiO2and 10Fe/TiO2, respectively.

2.2. Characterization of Synthesized Adsorbents. X-ray powder diffraction (XRPD) was used for the identicationof crystalline phases, quantitative phase analysis, and estimationof crystallite size and strain. The XRPD patterns were collectedwith a Philips diffractometer PW1710 employing Cu Kradiation. Step scanning was performed with 2 ranging from20 to 100, step size of 0.10, and a xed counting time of 5 sper step. The XRPD patterns were used to rene the

crystallographic structure and microstructural parametersusing the procedure implemented in the FullProf computerprogram.30

Scanning electron microscopy (SEM) was used to examinethe morphology of synthesized materials. The SEM analysis wasconducted on a JEOL microscope Model JSM-6610LV.

Adsorptiondesorption isotherms were obtained by nitrogenadsorption at 77 K using a Sorptomatic 1990 Thermo Finnigandevice. Prior to adsorption, the samples were degassed for 1 hat room temperature under vacuum and additionally 16 h at383 K at the same residual pressure. The specic surface area ofsamples (SBET) was calculated by applying the BrunauerEmmetTeller equation, from the linear part of the adsorptionisotherm.31 The total pore volumes (Vtot) were obtained fromthe N2 adsorption, expressed in liquid form, by applyingGurevitschs rule.32 Micropore volumes (Vmic) wereestimatedaccording to the DubininRadushkevich method.33 Mesoporevolumes (Vmes) were estimated according to the Barrett, Joyner,and Halenda (BJH) method from the desorption branch of theisotherm.34

The point of zero charge (pHpzc) was determined inaccordance with procedure described by Babic et al.35

2.3. Arsenic Adsorption Experiments. The adsorptionkinetic study for As(III) and As(V) was carried out with 10mg/L concentration of arsenic in 0.01 M NaCl solution. Plasticasks with 100 mL of arsenic solutions and 50 mg of adsorbentwere mixed for 30, 60, 120, 240, 360, 720, 1440, and 2880 min

at room temperature (20

2

C). The pH was adjusted at 7.0 0.2 by adding the proper amount of hydrochloric acid andsodium hydroxide at the beginning of adsorption. No pHadjustment was conducted in the kinetic study, and the pHvalue change was found to be negligible after the treatment.

For the pH study, plastic asks were lled with 100 mL of 10mg/L As(III) or As(V) in 0.01 M NaCl solution. The requiredinitial pHs of test solutions were adjusted using HCl or NaOHsolutions. A 50 mg sample of adsorbent was added and theplastic asks were shaken for 1 h at room temperature.

Adsorption isotherm experiments were conducted at roomtemperature at pH 3.0 0.2 by a batch adsorption procedure.Experiments were performed by adding different concen-trations of As(III) and As(V) in 0.01 M NaCl solution in plastic

asks with 50 mg of adsorbent. The concentrations of arsenicwere in the range 110 mg/L. The volume of solution was 100mL in all experiments. Plastic asks were shaken for 1 h.

In order to examine the inuence of common anions(phosphate and sulfate) along with arsenic in water on theadsorption capabilities of adsorbents, two concentrations ofanions, 1 and 5 mM, in 100 mL of 1 mg/L arsenic solution in0.01 M NaCl at pH 3 were mixed with 50 mg of sorbent for 1h. The quantities of adsorbed arsenic with and without thesesalts were compared.

The sorption capability of 10Fe/TiO2was tested on arsenicnaturally contaminated water samples, taken from the tap waterof the city of Zrenjanin, Serbia. The arsenic concentration was92 g/L, and the pH value was 8.0. Kinetic experiments were

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conducted with 0.1, 0.5, and 1.0 g/L adsorbent doses,respectively.

The samples were ltered through a 0.45m membrane lterand arsenic concentrations in supernatant were determinedwith inductively coupled plasma atomic emission spectroscopy(iCap 6500Duo, Thermo Scientic, UK).

3. RESULTS AND DISCUSSION

3.1. Characterization. XRPD analysis showed that themost intensive diffraction peaks for adsorbents can be ascribedto the anatase crystal structure (JCPDS card 78-2486) (Figure1). From diffractograms of 4Fe/TiO2and 10Fe/TiO2it can be

seen that no obvious peaks that could be attributed to thedoped Fe are present. The ionic radius of the Fe3+ ion is closeto the ionic radius of Ti4+, and it can be expected that the Fe3+

ion could be incorporated into the TiO2 lattice. Because nocorresponding peaks from iron oxide phase were observed,either Fe3+ ions are highly dispersed to the TiO2 structure or

small iron oxide clusters are formed having sizes and/orconcentrations lower than the detection limit of thediffractometer.

Physical parameters of TiO2, 4Fe/TiO2, and 10Fe/TiO2synthesized by the microwavehydrothermal method arepresented in Table1. Doping TiO2 with 4% Fe increased the

specic surface area and micropore volume while decreasing themesopore volume. Further increase of the amount of dopediron continued the same trend. The specic surface area andmicropore volume of 10Fe/TiO2are twice as big compared toTiO2. A large specic surface area is preferable for providinglarge adsorption capacity. Also, the size of the microporesdetermines the accessibility of adsorbate molecules to theadsorption surface, so an increase of these two properties of thematerial can be benecial for its sorption properties.

Figures2, 3, and4 show SEM images of TiO2, 4Fe/TiO2,and 10Fe/TiO2, respectively. Comparing images of TiO2 withimages of Fe doped TiO2, we can see differences in themorphology and decrease of the average particle size of dopedTiO2. Figures 3 and4 also reveal more porous structures of4Fe/TiO2 and 10Fe/TiO2 compared to TiO2 which arebenecial for the adsorption of arsenic and are in accordancewith the increase of specic surface area of these materials.

3.2. Kinetics Analysis.In order to have a better picture ofthe adsorption capacities of our adsorbents, initial concen-trations of As(III) and As(V) were higher than the averageconcentration of arsenic in groundwater.

From Figures5and6, we can see that adsorbent with higherpercentage of Fe had better adsorption of As(III) and As(V). Intherst hour, from initial As(III) and As(V) concentrations, 45and 46% were adsorbed on 10Fe/TiO2, respectively. For thesame time, 4Fe/TiO2from initial concentrations of As(III) andAs(V) removed 30 and 36%, respectively. A higher specicsurface area of 10Fe/TiO2 compared to 4Fe/TiO2 could beresponsible for faster initial adsorption. We can see similaritiesin the kinetic behavior of As(III) and As(V) on bothadsorbents. More than 73% of the adsorbed As(III) and

As(V) amounts within 48 h was adsorbed in the rst hour, forboth adsorbents.

The rates of As(III) and As(V) adsorption were evaluatedusing pseudo-rst-order and pseudo-second-order kineticmodels.The pseudo-rst-order rate expression of the Lagergrenequation36 is given as

= Q Q Q k t log( ) logte e 1 (1)

whereQeand Qtare the amounts of arsenate adsorbed (mg/g)at equilibrium and at time t(min), respectively; k1(min

1) isthe rate constant of the pseudo-rst-order adsorption. Theadsorption rate constant can be determined from the slope ofthe linear plot of log(Qe Qt) versus t.

The pseudo-second-order rate expression is as follows:37

= +t

Q Q k

t

Q

1

t e2

2 t (2)

where k2 (g mg1 min1) is the pseudo-second-order rate

constant.k2 can be calculated from the slope and intercept ofthe plot of t/Qtversus t.

Kinetics parameters for the pseudo-rst-order and pseudo-second-order equations are presented in Table2.

The presented kinetics adsorption data for As(III) and As(V)on both adsorbents, based on r2 values, were described betterwith the pseudo-second-order model. Close values of k2 forAs(III) and As(V) on 10Fe/TiO2show similar adsorption rates

of these two species. The real test of the validity of theequations used for the description of the rate of adsorptionarises from a comparison of the experimentally determined Qevalues and those obtained from the plots of log(Qe Qt) versustand t/Qtversus tforthe Lagergren and pseudo-second-ordermodels, respectively.38 Comparison ofQevalues, experimentallydetermined and obtained from the plots for the pseudo-rst-order and pseudo-second-order rate models are presented inTable3. As we can see from Table 3, the adsorption processfollows a more complex mechanism than the one on the basisof simple rst-order kinetics and conrms good agreement withthe pseudo-second-order model rate.

The time required for reaching equilibrium was 12 h forAs(III) and As(V) on both adsorbents. For all other batch

Figure 1. XRD diffractograms of (a) TiO2, (b) 4Fe/TiO2, and (c)10Fe/TiO2.

Table 1. Comparison of Physical Properties of TiO2, 4Fe/TiO2, and 10Fe/TiO2

adsorbent

TiO2 4Fe/TiO2 10Fe/TiO2

pore vol (Gurvich) at p/p0 (cm3 g1) 0.236 0.250 0.257

sp surf. area (m2 g1) 66 84 123

mesopore (BJH) desorption stand.Lecloux, cumul pore vol, (cm3 g1)

0.306 0.191 0.189

micropore vol (DR) (cm3 g1) 0.019 0.028 0.037


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experiments 1 h of mixing time was chosen although theequilibrium time was 12 h because the percentage of As(III)and As(V) removal after 1 h increases by less than 0.1% perminute.

3.3. Effect of pH.Arsenic in groundwater mostly occurs intwo common forms: arsenite (AsO3

3) and arsenate (AsO43).

As(V) exists as H3AsO4at pH


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dominant form of As(III) in the pH range from 0 to 9.2 isH3AsO3, and in more alkaline conditions H2AsO3

, HAsO32,

and AsO33 are dominant. The determined pHpzc values were

6.8 0.2 and 6.7 0.2 for 4Fe/TiO2 and 10Fe/TiO2,respectively. These pHpzc values suggest that below pH 7 thesurface of both adsorbents should be predominantly positive.

From pH 3 to 9, the adsorbed amount of As(III) on 10Fe/TiO2slowly increases, reaching a maximum at pH 9. A nearly50% decrease of adsorbed amount was at pH 3 than at pH 9.

Increasing the pH above 9 resulted in a decrease of As(III)adsorption. The same trend was observed for As(III)adsorption on 4Fe/TiO2 (Figure 7). The decrease ofadsorption of As(III) with increase of pH above 9 could bedue to two factors: the repulsion of the negative surface chargeof adsorbents and H2AsO3

anions which are dominant formunder the mentioned pH and competition of hydroxyl anionsfor active sites.

As can be seen in Figure8, adsorption of As(V) is stronglydependent on the pH. The greatest adsorption for both

adsorbents occurred at pH 3. At pH 3 the surfaces ofadsorbents are positively charged and could bind negativelycharged H2AsO4

anions. With increase of pH the positivecharge of adsorbent surfaces decreases gradually up to pH pzc,after which they are negatively charged. Decreasing positivesurface charge and increasing electrostatic repulsion betweenarsenate anions and the surface could be responsible for thedecrease of adsorption with increase of pH. Also, with theincrease of pH, the concentration of OH anions increases andcould compete for active sites on the adsorbent.

3.4. Adsorption Isotherms. For determination of theadsorption capacity of adsorbents, the Freundlich andLangmuir isotherm models were used.

The expression for the Freundlich isotherm model was given

as=Q K C

nf e

1/(3)

whereQ(mg/g) is the amount of adsorbed arsenic per unit ofadsorbent,Kfis a Freundlich constant related to the adsorptioncapacity, Ce (mg/L) is the equilibrium arsenic concentration,and n is a dimensionless Freundlich constant which indicatesthe intensity of adsorption.

The Langmuir isotherm model is expressed as

= +C

Q bQ

C

Q

1e

e m

e

m (4)

whereQe(mg/g) is the amount of adsorbed arsenic per unit of

adsorbent, Ce (mg/L) is the equilibrium concentration ofarsenic,Qm(mg/g) is the amount adsorbed per unit weight ofadsorbent required for monolayer capacity, and b (L/mg) isLangmuir constant.

Freundlich and Langmuir isotherms do not give any ideaabout the adsorption mechanism. In order to understand themechanism of adsorption, the equilibrium data were also ttedwith the DubininRasdushkevich (DR) isotherm. The DRisotherm has the following linearized form:39

= Q Q Kln lnm DR

2(5)

whereKDRis a constant correlated to the adsorption energy, Qmis the adsorption capacity, (Polanyi potential) is [RTln(1 +1/Ce)], R is the gas constant, and T is the temperature.

Table 2. Pseudo-First-Order and Pseudo-Second-OrderKinetic Parameters for As(III) and As(V) onto 4Fe/TiO2and 10Fe/TiO2

pseudo rst order pseudo second order

adsorbent k1(min1) r2 k2(g mg

1 min1) r2

As(III) 4Fe/TiO2 0.0009 0.7918 0.0031 0.9999

As(III) 10Fe/TiO2 0.0012 0.9933 0.0074 0.9993

As(V) 4Fe/TiO2 0.0010 0.9883 0.0063 0.9976As(V) 10Fe/TiO2 0.0009 0.9940 0.0073 0.9993

Table 3. Comparison ofQeValues, ExperimentallyDetermined and Obtained from the Plots for Pseudo-First-Order and Pseudo-Second-Order Rate Models, forAdsorption of As(III) and As(V) onto 4Fe/TiO2and 10Fe/TiO2

Qe(mg/g)

obtained from plot

adsorbent exptl pseudo rst order pseudo second order

As(III) 4Fe/TiO2 8.286 0.885 8.161

As(V) 4Fe/TiO2

8.672 3.011 8.566

As(III) 10Fe/TiO2 11.600 2.898 11.591

As(V) 10Fe/TiO2 11.952 2.977 11.677

Figure 7. Effect of initial pH on As(III) adsorption onto 4Fe/TiO2and 10Fe/TiO2(c0= 10 ppm, adsorbent dose 0.5 g/L).

Figure 8.Effect of initial pH on As(V) adsorption onto 4Fe/TiO2and10Fe/TiO2(c0= 10 ppm, adsorbent dose 0.5 g/L).


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Generally DR is used to distinguish between physical andchemical adsorption. From the slope and intercept of the plotln Qagainst 2, the DR isotherm constants KDRand Qm arecalculated. The mean energy of adsorption (E) is calculatedfrom the KDRvalue using the following equation:

=

E K(2 )DR0.5

(6)

It is known that the magnitude ofE can be used to estimatethe type of adsorption. If this value is in the range 816 kJ/mol, the adsorption type can be explained by chemisorption,and i f E < 8 kJ/mol, then the adsorpt ion type isphysisorption.40

Correlation coefficients (r2),Kf, KDR, n, Qm, and Qml valueswere derived from linear forms of the Freundlich, Langmuir,and DR models, and are shown in Table4.

The Freundlich equation is often considered to be anempirical equation. It assumes multilayer adsorption on anenergetically heterogeneous surface, while the Langmuir modelis based on the assumption that sorption is monolayer onenergetically equivalent sites without interaction betweenadsorbed molecules on neighboring sites. For the adsorptionof As(III) on TiO2, 4Fe/TiO2, and 10Fe/TiO2and As(V) onTiO2, correlation coefficients were better with the Freundlich

isotherm model suggesting multilayer adsorption. Increase ofthe adsorption capacity of doped sorbent is probably because ofthe increase of the surface area with doping, which providesmore active sites for sorption. Data for adsorption of As(V) onboth doped adsorbents were better tted with the Langmuirmodel than with the Freundlich model. A possible explanationcould be the sorption of As(V) to doped Fe instead of TiO2active sites due to the higher affinity of As(V) toward Fesorption centers. Adsorption capacities, derived from theLangmuir model, for As(III) and As(V) on 10Fe/TiO2 were8.61 and 17.35 mg/g, respectively. Comparing these resultswith adsorption capacities of As(III) and As(V) on TiO2, wecan see a improvement of the adsorption capacity with theincrease of the amount of Fe doped for TiO2. As the adsorption

capacity of the sorbent varies with experimental conditions(initial arsenic concentration, solution pH, time of agitation), itmay be difficult to compare the values directly. Comparison ofthe removal capacity of 10Fe/TiO2 sorbent with adsorbentsreported in the literature (commercially available and dopedsorbents, with experimental conditions similar to ours) arepresented in Table S1 in theSupporting Information.

For the adsorption of As(III) and As(V) on TiO2and As(III)on both 4Fe/TiO2 and 10Fe/TiO2, the experimental data donot correlate well with the DR isotherm model (Table 4),giving low values for r2. The reason for the poor-tting resultscould be because adsorption occurs mainly on the surfacerather than in the pores and the DR model assumes volumelling.41 As(V) adsorption on 4Fe/TiO2 and 10Fe/TiO2 gave

good correlation coefficients, and calculatedE values were 5.86and 5.68 kJ/mol, respectively. These E values indicate that theadsorption of As(V) onto both doped adsorbents isphysisorption.

3.5. Competing Anions.The presence of anions, such asPO4

3 and SO42, can inuence arsenic adsorption. These two

anions are common constituents of natural waters, and theirmolecular structures are similar to that of arsenic. The results ofa competing anion study are shown in Table5.

The presence of 1 mM SO42 decreased the adsorption of

As(III) on 4Fe/TiO2 and 10Fe/TiO2 by more than 70 and78%, respectively. Higher concentration of SO4

2 showedsimilar results, suggesting that saturation of active sites wasachieved with 1 mM concentration. Removal of As(III)decreased in the presence of 1 mM PO4

3 for 87 and 90%on 10Fe/TiO2and 4Fe/TiO2, respectively.

Little decrease of adsorption on both adsorbents occurredwith the presence of 5 mM SO4

2while with 1 mM SO42both

adsorbents effectively removed As(V). With 1 mM PO43

present in solution, 55% decrease of adsorption occurred forboth adsorbents. With an increase of the PO4

3 concentrationto 5 mM, a more than 83% decrease in adsorption occurred for

both adsorbents. However, the concentrations of PO43

used inthis study were much higher than that likely to be encounteredin groundwater.

The larger decrease of adsorption of As(III) on bothsorbents in the presence of PO4

3 and SO42 anions suggests a

stronger sorption affinity of these two anions for adsorbentsthan As(III). As(V) showed stronger affinity for adsorption onboth adsorbents compared with SO4

2. These results are inagreement with n constants derived from the Freundlichisotherm model, which represents the strength of adsorption.

3.6. Arsenic Removal in Natural Water Sample. Thecity of Zrenjanin is located in the northeastern part of Serbia, inVojvodina, which has an elevated concentration of arsenic inthe groundwater. This groundwater is pumped from wells and

Table 4. Freundlich and Langmuir Adsorption Parameters for As(III) and As(V) Adsorption onto TiO2, 4Fe/TiO2, and 10Fe/TiO2

Freundlich Langmuir DR

adsorbent r2 n Kf r2 Qml (mg/g) r

2 KDR(mol2/kJ2) E (kJ/mol)

As(III) TiO2 0.9888 3.21 3.66 0.9542 5.52 0.8183 0.0382 3.62

As(V) TiO2 0.9997 5.63 5.34 0.9941 7.39 0.8407 0.0065 8.77

As(III) 4Fe/TiO2 0.9982 1.70 1.35 0.9636 6.17 0.7538 0.1954 1.60

As(III) 10Fe/TiO2 0.9971 1.79 2.28 0.9654 8.61 0.8194 0.1430 1.87As(V) 4Fe/TiO2 0.9264 3.58 10.54 0.9969 13.41 0.9773 0.0147 5.83

As(V) 10Fe/TiO2 0.8421 4.46 16.52 0.9958 17.35 0.9555 0.0155 5.68

Table 5. Effect of PO43 and SO42 Anions on As(III) andAs(V) Adsorption onto 4Fe/TiO2and 10Fe/TiO2Adsorbentsa

concn of As(III) (ppm)after adsorption with

concn of As(V) (ppm) afteradsorption with

anion concn 10Fe/TiO2 4Fe/TiO2 10Fe/TiO2 4Fe/TiO2

0


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with the addition of chlorine is distributed to taps. Thechemical composition of the water sample is shown in Table S2in theSupporting Information.

Kinetics experiments with different loadings of 10Fe/TiO2adsorbent were carried out to investigate the efficiency with thenatural water sample. From Figure9we can see fast adsorption

of arsenic with all adsorbent dosages. With 1 g/L dosage, after 5min contact with adsorbent more than 70% of arsenic wasremoved. After 60 min contact time, with the same dosage, thetotal arsenic concentration was below the WHO limit forarsenic in drinking water.

4. CONCLUSIONS

A novel TiO2doped with 4 and 10% Fe adsorbents for removalof As(III) and As(V) were synthesized using a microwave-assisted hydrothermal method. Using this method for dopingTiO2 with Fe, higher specic surface area and microporevolume than those of the starting material were achieved. Thepseudo-second-order kinetic model was found to best correlatewith the data for As(III) and As(V) adsorption on bothadsorbents. Both adsorbents showed similar pH dependencesof adsorption. The best results were achieved at pH 3 and pH 9for As(V) and As(III), respectively. Isotherm studies showedthat the Freundlich model ts the experimental data for As(III)adsorption while the Langmuir model better describes As(V)adsorption. The presence of SO4

2 anions decreased As(III)adsorption, while it had no inuence on the adsorption of

As(V). Substantial decrease of adsorption on As(III) and As(V)on both adsorbents occurred in the presence of PO4

3 anions.Improvement in the adsorption capabilities for As(III) andAs(V) was achieved with doping TiO2with Fe. Better removalfor As(III) and As(V) was achieved with 10Fe/TiO2adsorbent.The maximal adsorption capacities, derived from the Langmuirisotherm model, were 8.61 and 17.35 mg/g for As(III) andAs(V), respectively. Treatment of a natural water sample thatcontained 92 g/L total arsenic with 1.0 g/L 10Fe/TiO2adsorbent showed very fast adsorption of arsenic with thenal concentration below the WHO limit after 60 min ofcontact time. Simplicity of synthesis, low cost of sorbent, andincreasing adsorption capability of As(III) and As(V) withincreasing amount of doped Fe recommend further inves-

tigation of this method for synthesis of TiO2doped with higheramounts of Fe, for arsenic removal.

ASSOCIATED CONTENT

*S Supporting Information

Tables comparing adsorption capacities of other sorbents withour study and water quality data of real water sample (tap water

of city of Zrenjanin, Serbia). This material is available free ofcharge via the Internet athttp://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected]. Tel.: + 381 64 3702462.*E-mail:[email protected]. Tel.: +381 64 1718149.

Notes

The authors declare no competing nancial interest.

ACKNOWLEDGMENTS

This work has been supported by the Ministry of Educationand Science, Republic of Serbia (Project No. 172030).

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