arsenate remediation using nanosized modified zerovalent iron particles

Arsenate Remediation UsingNanosized Modified ZerovalentIron ParticlesGautham Jegadeesan,a Kanchan Mondal,a and Shashi B. Lalvaniba Mechanical Engineering & Energy Processes, Southern Illinois University, Carbondale, IL 62901b Paper Science and Engineering, Miami University, Oxford, OH 45056; [email protected] (for correspondence)

Published online 17 March 2005 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10072

Arsenate [As (V)] removal kinetics in aqueous solu-tions using modified nanosized zerovalent iron (Fe0)particles such as NiFe and PdFe was studied. NiFe andPdFe particles were synthesized by the borohydridereduction of nickel and palladium salts on Fe0 parti-cles. Pseudo-first-order rate equations were found tosatisfactorily describe arsenate removal kinetics usingFe0, NiFe, and PdFe particles (solids concentration of2 g L�1) at low arsenate concentrations (1.35 mM). Ascompared to zerovalent iron (kobs of 0.037 min�1), thearsenate removal rate was up to 2.5 times faster (kobs of0.091 min�1) using NiFe particles, whereas it wasthreefold lower in the case of PdFe particles (kobs of0.011 min�1). With increasing contact times, devia-tion from first-order kinetics was observed, presumablyarising from the loss of available sites on the solidsurface. The surface area normalized pseudo-first-or-der rate constant ksa was 0.0089 L m�2 min�1 for NiFe.Additional experiments were performed to study theinfluence of initial arsenate concentration (0.67 and3.38 mM), temperature (25, 45, and 65° C) and com-peting inorganic anions (sulfate, chloride, nitrate,phosphate, and chromate) on arsenate removal usingNiFe particles. Increasing temperatures (25–65° C) in-creased arsenate removal rates, whereas competingsorption of phosphate and sulfate inhibited arsenateremoval. © 2005 American Institute of Chemical EngineersEnviron Prog, 24: 289–296, 2005

Keywords: arsenic removal, Fe/Ni, galvanic cou-ple, competitive sorption

INTRODUCTIONRecognizing the potentially serious health concerns

because of its toxicity to living organisms, the UnitedStates Environmental Protection Agency (US EPA) re-duced the allowable maximum contaminant level ofarsenic (As) from 0.00067 to 0.00013 mM (0.05 to 0.01mg/L) in drinking water [1, 2]. Elevated concentrationsof arsenic, predominantly in ground and surface water,are associated with weathering of rocks and agricul-tural use of arsenical pesticides [3]. The environmentaland toxicological impact of As is largely determined bythe distribution of the element between immobile andmobile forms and by the distribution of valence states,primarily arsenate [As (V)] and arsenite [As (III)] [4, 5].Arsenate (as H2AsO4

� and HAsO42�) is the predominant

form of arsenic in neutral pH, while arsenite occurspredominantly as H3AsO3

0 and H2AsO3� at pH below

9.2. The close association of mineral oxides with bothAs (III) and As (V) has been demonstrated where botharsenic species are adsorbed on the oxides forminginner-sphere surface complexes [6, 7].

There is significant interest recently in the remedia-tion of inorganic compounds such as chromium [8, 9]and arsenic [10–13] and numerous chlorinated hydro-carbon compounds [14–17] using zerovalent iron (Fe0).Surface precipitation or adsorption appears to be thepredominant mechanism for the removal of As (III) andAs (V) by Fe0. The spontaneous oxidation (corrosion)of zerovalent iron (Fe0) in aqueous solutions leads tothe formation of Fe (II) and Fe (III) oxides, with simul-taneous generation of electrons [18]. Arsenate uptakeon iron oxyhydroxides occurs by a ligand-exchangemechanism that replaces the surface-bonded OH� withan irreversible and stable monodentate arsenate or in-ner-sphere bidentate complex [19, 20]. Even though the© 2005 American Institute of Chemical Engineers

Environmental Progress (Vol.24, No.3) October 2005 289

reduction of dissolved As (V) to lower valence states[As (III) and As (0)] by the electrons generated is ther-modynamically feasible, evidence of the reduction pro-cess is lacking in the literature [10, 17]. The key prob-lems associated with the use of iron for arsenicremediation is that the reaction time required for thecomplete removal of the arsenic species is in the orderof days. Within these results, the actual reaction timerequired depends largely on the quantity of Fe0 surfacearea used in the system. Recent studies [15, 20–23]indicated that when zerovalent iron was used in con-junction with a noble metal, the second metal primarilyinduces Fe0 to release electrons at a faster rate as aresult of accelerated galvanic corrosion. Iron is oxi-dized more rapidly when attached to a less active(more noble) metal such as nickel and palladium. Theiron–noble metal couple essentially creates numerousgalvanic cells where iron acts as an anode and is pref-erably oxidized. Several studies have proved the effec-tiveness of the bimetallic particles in the removal ofselenate [20] and dechlorination of N-nitrosodimethyl-amine [15] and trichloroethylene (TCE) [16, 23] by hy-drogenation or reduction processes. In addition, thehigh reactivity of nanosized particles also serves toenhance the reaction rates.

The objective of this research was to (1) investigatethe effectiveness of the nanosized bimetallic particlesof NiFe and PdFe in As (V) removal; and (2) examinethe effect of pH, particle loading, temperature, andinfluence of competing anions on As (V) removal.

EXPERIMENTAL PROCEDURES

MaterialsFerrous chloride (FeCl2�7H2O), nickel chloride

(NiCl2), palladium chloride (PdCl2), and sodium boro-hydride (NaBH4) were obtained and used as sourcesfor the preparation of the bimetallic nanoparticles. So-dium hydrogen arsenate (NaHAsO4�7H2O) was ob-tained for the preparation of synthetic arsenate solu-tions. All the chemicals were of reagent grade andobtained from Fisher Scientific (Chicago, IL).

Bimetallic Particle PreparationZerovalent iron (Fe0) particles were synthesized by

adding a required amount of NaBH4 (0.8 M) toFeCl2�7H2O solution (0.14 M). The solution was stirredvigorously for 5 min and then ultrasonically agitated for30 min. The reduction of ferrous ions to Fe0 occurs asshown below:

3Fe2 � � BH4� � 3H2O 3 3Fe0 � H2BO3

� � 6H � � H2

(1)

Fe0 particles produced were then soaked in ethanolsolution containing 0.02 M palladium chloride (PdCl2)and further agitated ultrasonically for 20 min. Palladiumdeposition onto the iron surface proceeded accordingto the redox reaction shown in Equation 2. Excessborohydride was added for complete reduction of ironto Fe0.

Fe0 � Pd2 � 3 Fe2 � � Pd0 (2)

The precipitation of bimetallic NiFe powders occursaccording to reaction (Eq. 3) involving the simulta-neous reduction of the metal ions in aqueous solutionby sodium borohydride.

Ni2 � � Fe2 � � BH4� � 2H2O 3 Ni0Fe0 � BO2

�

� 4H � � 2H2 (3)

Equimolar metal salt solutions (0.5 M) of nickel andiron were treated with excess borohydride (0.8 M)solution for NiFe synthesis. The precipitated solidswere centrifuged to remove the water, followed bydrying at 85° C in nitrogen atmosphere for 24 h. Thesolids were stored in air-tight vials under nitrogen.

Batch Remediation StudiesA stock solution (13.5 mM) of arsenate was prepared

from sodium hydrogen arsenate. Synthetic solutions(0.67–3.38 mM) were prepared fresh for each batch testby serial dilution of the stock solution. Distilled deion-ized water was used for the preparation of the stockand synthetic solutions. Batch and kinetic studies wereconducted in stirred 50-mL flasks containing desired As(V) concentrations and desired NiFe, PdFe, and Fe0

solids concentration (1–3 g L�1). The solution pH re-ported corresponds to the pH before the addition ofsolids to the flasks (pH � 7.5 � 0.1). The experimentswere conducted for 3 h unless specified otherwise. Asmall volume of the solution (�1 mL) was collected atdesired intervals in 0.45-�m Whatman Autovial syrin-geless filters (Fisher Scientific), filtered, and stored foranalysis. The concentration of As (V) in the treatedsolution was determined using a Dionex DX-500 ionchromatograph (Dionex, Sunnyvale, CA). The detec-tion limit of the chromatograph is 0.003 mM. Replicatearsenic analysis was performed on the stored samples.Calibration curves for arsenate were prepared usingfour to six standards. Straight lines were fitted withcoefficients of determination (r2) of no less than 0.99for arsenate.

Bimetallic Particle CharacterizationThe relative composition of each metal in the bime-

tallic particle was quantified by energy dispersive X-rayspectroscopy. The nickel content in NiFe powder was50 wt %, whereas the palladium content in PdFe was 10wt %. The surface morphology and the particle sizewere characterized using a Hitachi S-500 scanning elec-tron microscope (SEM, Hitachi, Osaka, Japan) and aHitachi H-7100 transmission electron microscope(TEM), respectively. The TEM and SEM images of NiFeand PdFe particles (Figure 1) show that PdFe particleshave the smallest size (10–20 nm), whereas NiFe par-ticles are up to 100 nm in size. The surface morphologyof the particles seen under the SEM showed that thePdFe particles were agglomerated spongelike, whereasNiFe particles exhibited a needlelike structure. Themean size of the particles ranged from 0.2 to 1 �mbecause of agglomeration.

290 October 2005 Environmental Progress (Vol.24, No.3)

RESULTS AND DISCUSSION

Arsenate Removal StudiesBatch studies were performed to study the disap-

pearance of arsenate from contaminated solutions con-taining initial arsenate concentrations of 1.35 � 0.006mM, which were treated with 2.00 � 0.01 g L�1 solidsconcentration of Fe0, NiFe, and PdFe powders. Figure 2is a plot of the arsenate concentration [normalized withrespect to the initial arsenate concentration (C/C0)] at aspecific contact time vs. time (t). The kinetic plots forNiFe and Fe are similar; however, the former reducesarsenate content at a much faster rate than the latter. Inabout 60 min, a near-complete arsenate removal (finalarsenate concentration of 0.006 mM) was obtained withNiFe, whereas only about 88% reduction in arsenatecontent (final arsenate concentration of 0.16 mM) byFe0 was observed during the same time period. It wasalso observed that no significant reduction in arsenateconcentration is obtained at longer contact times usingFe0 particles. The kinetics of arsenate removal in thecase of PdFe is much more complex. Initially, the ar-

Figure 1. Scanning and transmission electron micrographs of bimetallic particles: (a) TEM image of PdFe; (b)SEM image of PdFe; (c) TEM image of NiFe; (d) SEM image of NiFe.

Figure 2. Kinetics of As (V) removal from 1.35 mM ofAs (V) initial concentration using 2 g L�1 of (Œ) Fe0,(f) NiFe, and (�) PdFe particles.


senate concentration decreases gradually and reaches aminimum of 0.43 mM (68% removal) after 120 min.Subsequently, the arsenate concentration is found toincrease with time, indicating arsenate desorption fromthe surface. The kinetics of arsenate removal is gener-ally described by a shifting-order rate expression (shift-ing between zero and first order) [10], as expressed inEq. 4. The integrated expression for the shifting-orderequation fitted poorly using the data obtained experi-mentally, especially for arsenate removal using PdFeand NiFe. At low arsenate concentrations and low rateconstant values, the shifting-order kinetic equation canbe approximated to a pseudo-first-order kinetic expres-sion (Eq. 5) as

dC

dt� �

k1C

1 � k2C(4)

dC

dt� �kobsC (5)

which can be expressed as

ln�C

C0� � �kobst (6)

At the beginning of the experiment, there are abun-dant sites available for adsorption of arsenate species.The removal rate of arsenate is therefore limited by theconcentration of the dissolved As (V) species; thus it isfirst order with respect to the arsenate concentration. Alogarithmic plot of the normalized arsenate concentra-tion (C/C0) in the solution (Eq. 6 in its linearized form)vs. time results in a straight line with r2 ranging from0.90 to 0.96 (Figure 3). The slope of the line provides

the rate constant kobs, which was found to be 0.037min�1 for Fe0. The rate constant was found to increaseby a factor of 2.5 when Ni was added to the iron surface(kobs of 0.091 min�1), whereas it decreased by a factorof 3 in the case of PdFe (kobs of 0.011 min�1). However,with increasing contact time, deviation from first-orderkinetics was observed, presumably as a result of theloss of active surface sites.

Adsorption of As (V) on to the corroded iron surface(iron oxyhydroxide) is probably the main mechanismof arsenate removal. Previous EXAFS data [13, 19] haveprovided evidence that, at low solids loading, arsenateis retained mainly by the formation of monodentatecomplexes on the iron oxide surface. At high solidsconcentration, arsenate binds to the surface mainly bythe formation of bidentate binuclear and bidentatemononuclear complexes. In the case of iron–nickelcouple, given that iron acts as a sacrificial anode, theaccelerated rate of iron oxidation attributed to galvaniccorrosion results in an increased rate of adsorption. Therelatively low arsenate removal using PdFe can beattributed to the mass transport resistance arising fromthe formation of hydrogen gas bubbles on the bimetal-lic surface [21]. The electrons generated (by iron oxi-dation) are used to reduce protons on the palladiumsurface to form hydrogen gas. At the beginning of theexperiment, the arsenate removal rate on vacant palla-dium and iron sites is much faster than the rate of H2generation. However, with increasing contact time, for-mation of gas bubbles over the bimetallic surface couldenhance the resistance to mass transfer of dissolvedarsenic from the solution to the available surface sites.Since poorly dispersed palladium has a greater affinityfor proton adsorption, as is the case here, arsenateadsorption on the palladium surface is probably hin-dered by the excessive formation of H2 gas, therebyinhibiting arsenate remediation. It is also hypothesizedthat the adsorption of arsenate onto palladium sites is areversible process, unlike its adsorption on iron sites,which is irreversible. With increasing contact time, theprotons generated from iron catalyzed water dissocia-tion are adsorbed onto palladium sites (which has ahigh affinity for proton adsorption) by replacing theadsorbed arsenate, resulting in arsenate desorption, asseen in Figure 2. It can thus be seen that NiFe is moreeffective than PdFe and Fe in arsenate removal. Furtherstudies were performed to evaluate the effectiveness ofNiFe particles for As (V) removal at high concentra-tions.

Figure 4 is the plot of the arsenate concentration vs.time for experiments performed using initial As (V)concentration of 0.67 and 3.37 mM and 2 g L�1 NiFe. Allexperiments were performed for 2 h. The data showthat increasing the arsenate concentration resulted in adecrease in removal rates. A near-complete removalwas observed from solution containing 0.67 mM initialAs (V) concentrations. However, upon increasing theconcentration to 3.37 mM, arsenate removal decreasedto 51%. It can also be seen that the arsenate concen-tration decreased rapidly in the first 15 min of operationto 0.006 mM from solutions containing initial concen-tration of 0.67 mM. At lower arsenate concentration, theratio of arsenate to that of the reactive surface active

Figure 3. Logarithmic normalized As (V)concentration in solutions treated with 2 g L�1 (Œ)Fe0, (f) NiFe, and (�) PdFe particles. The solid linesare the model fit to the experimental data using first-order rate equation. The equations in the illustrationprovide values for the first-order rate constant.


sites is low, resulting in higher removals. However, athigher arsenate concentration (high ratio of arsenate tothat of the number of active sites), the rate of arsenateremoval is limited by the availability of reactive sites;which resulted in low removal. The kinetic data at earlycontact times fitted well (r2 � 0.95) to a pseudo first-order kinetic expression. The first-order rate constant(see insert in Figure 4) was found to decrease withincreasing initial concentration. This can be attributedto the loss in the amount of available surface sites foradsorption process. At high concentrations, more ac-tive sites are occupied by the arsenate species, therebyreducing the rate of arsenate removal. Nevertheless, itcan be concluded that the use of NiFe powders isbeneficial for long-term reduction in arsenate concen-tration.

Effect of Solids ConcentrationFigure 5 contains the data on the final As (V) con-

centration (C) at various NiFe solid concentrations.Synthetic solutions containing 1.35 mM of As (V) wastreated with 0.1–3 g L�1 of NiFe for 3 h. The data inFigure 5 show that the rate of disappearance of arsen-ate from the solution was negligible after contactingwith the particles for 2 h, indicating near-equilibriumconditions. The data show that by increasing the bime-tallic particle loading, an increase in arsenate removal isobtained, with the final arsenate concentration decreas-ing from 1.31 mM (3% removal) to non-detect valueswhen the loading was increased from 0.1 to 3 g L�1.Because arsenate removal is dependent on the gener-ation of adsorption sites for arsenate complexation [19],the increase in the reactive active sites on the bimetallicsurface at higher solids loading resulted in higher ar-senate removal. At high loadings, there are sufficient

sites available on the bimetallic surface for arsenateadsorption, resulting in high removals. However, atrelatively low loadings, the rate of arsenate removal isdependent on the generation of such reactive sites. Thepseudo- first-order rate constant kobs can be expressedin terms of the solid concentration of the bimetallicparticles as

dC

dt� �kobsC � �ksaas�mC (7)

where ksa is the surface area normalized first-order rateconstant, as is the surface area (m2 g�1), and �m is thesolid concentration of the particles (g L�1). The surfacearea of NiFe nanosized particles was obtained by aQuantachrome Nova 2000 BET analyzer and was foundto be 42.44 m2 g�1. A plot of kobs vs. as�m (surface areaconcentration) provides an excellent fit (r2 � 0.99). Ascan be seen in the insert (Figure 5), the surface areanormalized rate constant ksa was found to be 0.0084 Lm�2 min�1. However, the change in surface area re-sulting from oxidation of iron particles could affect therate constant of the reaction, albeit in a positive man-ner.

Effect of TemperatureFigure 6 is the plot of kinetic data for arsenate removalat different temperatures. Synthetic arsenate (1.35 mM)solutions at temperatures ranging from 25 to 65° C weretreated with 2 g L�1 of NiFe powders for 3 h. The datashow that the rate of arsenate removal increased withtemperature. At the end of 20 min, the final arsenateconcentration decreased to nondetect levels and 0.32mM for experiments conducted at 65 and 45° C, respec-

Figure 4. Arsenate removal kinetics from 0.67 (�) and 3.37 (Œ) mM As (V) solution using NiFe particles. Insertshows the plot of observed rate constant kobs vs. the initial As (V) concentration.


tively. The increase in removal rate with temperature isexplained by the Arrhenius equation. Assuming thefirst-order kinetics for arsenate removal, a plot of ln kobsvs. 1/T provided a good fit (r2 � 0.99). The activationenergy was calculated to be 455.75 kJ mol�1. Theincrease in temperature has two opposing effects onarsenate removal. The rate of reaction is enhanced withtemperature, whereas the solubility of oxygen (re-quired for the oxidation of iron) decreases with tem-perature. This could lead to the lowering of arsenate

adsorption because a smaller quantity of oxides andoxyhydroxides is expected to form in an oxygen-defi-cient environment.

Effect of Competing AnionsThe effect of various competing anions such as chlo-

ride, sulfate, and nitrate on arsenate removal was stud-ied. Figure 7 shows the effect of impurities on theremoval effectiveness of arsenate by NiFe powders ascharacterized by the arsenate retained in solution. The

Figure 5. Arsenate removal kinetics from 1.35 mM As (V) initial concentration using NiFe. Insert shows the plotof observed rate constant kobs vs. the surface area concentration. The data fit provides the surface areanormalized rate constant ksa for NiFe particles.

Figure 6. Effect of temperature on arsenate removal. Experiments were conducted using 1.35 mM of As (V)solution and a solid concentration of 2 g L�1 NiFe at temperature (in ° C) of 25 (Œ), 45 (�), and 65 (f). Insertshows the plot of observed rate constant kobs vs. the temperature in accordance with Arrhenius equation.


initial concentration of As (V) used in the experimentswas 1.35 mM and the solid loading used was 2 g L�1.Synthetic As (V) solution was spiked with 2.5 g L�1 ofanionic co-solutes (chloride, nitrate, phosphate, sul-fate, chromate, and bicarbonate). It can be seen fromthe figure that phosphate anions had the greatest ad-verse effect on arsenate removal, whereas the presenceof chloride ions in the solution did not affect arsenateremoval. The arsenate removal from the solution wasfound to be nearly complete (100%) in the presence ofchloride, whereas it was 37% in the presence of 2.5 gL�1 (0.08 M) phosphate. It was also observed that thepresence of borate anions also reduced the effective-ness of the bimetallic particles. Phosphate is a knowninner-sphere complex–forming anion that is stronglyadsorbed to mineral surfaces or is coprecipitated toform discrete solid phases on mineral surfaces [11]. Thesuppression of arsenate sorption by phosphates hasbeen reported to occur in soils and iron oxides. Be-cause both arsenate and phosphate compete for thesame reaction sites for complexation, the presence ofphosphate inhibits arsenate removal. Sulfates and ni-trates are nonspecific anions and they could competefor adsorption at the active sites and subsequentlycould undergo reduction at these sites. Nitrate has beenfound to undergo reduction to NH4

� and small amountof NO2

� by zerovalent iron [11]. However, the effect ofaddition of nitrate and sulfate is largely pH dependentand the influence of these anions on arsenate removalis marginal in alkaline pH, which is the case here(pH � 9.2). The influence of arsenite of similar con-centration (1.35 mM) on arsenate removal was alsostudied. The data in Figure 7 show that the inclusion ofAs (III) anions lowered removals to 73.7%. Althougharsenite anions are known to adsorb at the iron sur-faces, the amount of adsorption As (V) was greater thanthat of As (III). Thus, the adverse effect of arsenite onAs (V) removal was lower compared to that of phos-phate. Anions such as chromate and bicarbonate hadpractically no effect on As (V) removal. Thus, it can be

concluded that inner-sphere complex–forming anionsgreatly affected As (V) removal compared to nonspe-cific anions. The increasing order of their effect is:chloride � nitrate � chromate � bicarbonate � sul-fate � arsenite � phosphate.

CONCLUSIONSThis study shows that NiFe is effective in removing

arsenate from aqueous solutions. The ability of NiFe totreat a mixed waste containing high amounts of anionicco-solutes makes it a promising material for in situremediation of contaminated groundwater resources.Possible explanations for the differences in reactionrates between bimetallic NiFe and PdFe involve theinhibition of arsenate adsorption on palladium becauseof its strong affinity for hydrogen adsorption. The re-activity of bimetallic zerovalent iron depends on thespecific properties of the surface and also on the com-pound to be reduced. The inhibition of arsenic removalby phosphate suggests that the primary mechanism forarsenate removal is adsorption or precipitation. Precip-itation reactions are significant at high solute concen-trations, whereas adsorption reactions are predominantat low concentrations. More studies are needed to con-firm the mechanisms involved in arsenate removal.

ACKNOWLEDGMENTSWe gratefully acknowledge the assistance with par-

ticle characterization of Dr. John Bozzola and Dr. SteveSchmitt of Image Microscopy Center, Southern IllinoisUniversity, Carbondale. The authors thank the review-ers for their valuable suggestions in this work.

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Figure 7. Effect of competing inorganic anions on As (V) removal. The initial As (V) concentration used was1.35 mM and a solid concentration of 2 g L�1 NiFe.


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arsenate remediation using nanosized modified zerovalent iron particles

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