Perchlorate Reduction by Nanoscale Iron Particles

SDMS DOCID#1142515

Journal of Nanoparticle Research (2005) 7: 499–506 Ó Springer 2005 DOI 10.1007/s11051-005-4412-x

Perchlorate reduction by nanoscale iron particles

Jiasheng Cao, Daniel Elliott and Wei-xian Zhang* Department of Civil and Environmental Engineering, Lehigh University, PA 18015, USA; *Author for correspondence (Tel.: 610-758-5318; Fax: 610-758-6405; E-mail: [email protected])

Received 21 March 2005; accepted in revised form 22 March 2005

Key words: perchlorate, nanoscale iron particles, reduction, activation energy, kinetics, colloids, water quality

Abstract

We report herein the near complete reduction of perchlorate (ClO4 ) to chloride by nanoscale iron particles. ) The iron nanoparticles also reduce chlorate (ClO3 ), chlorite (ClO2 ) and hypochlorite (ClO ) to chloride. No reaction was observed with microscale iron powder under identical conditions. The temperature sen- sitivity of the perchlorate-nanoparticle reaction is evidenced by progressively increasing rate constant values of 0.013, 0.10, 0.64 and 1.52 mg perchlorate per g nanoparticles per hour (mg-g)1-hr)1), respectively, at temperatures of 25, 40, 60 and 75°C. The activation energy of perchlorate-iron reaction was calculated to be 79.02 7.75 kJ/mole. Despite favorable thermodynamics, the relatively large activation energy for this reaction suggests that perchlorate reduction is limited by the slow kinetics. The nanoscale iron particles may represent a potential treatment method for perchlorate-contaminated water.

Introduction tants have been identiﬁed that are capable of reducing perchlorate (Duke & Quinney, 1954; Chemists have long been interested in the unusual King & Garner, 1954; Kallen & Earley, 1971). stability of perchlorate anion (ClO4 ) in aqueous Complexes of tin (II), vanadium (II, III), molyb- solution. It is generally non-reactive and exhibits denum (III), titanium (III), and ruthenium (III, little tendency to serve as a ligand in complexes IV) have been shown to be able to reduce per- (Cotton et al., 1999). Perchlorate is widely used to chlorate under acid-catalyzed conditions. For adjust solution ionic strength and as an inert example, Duke and Quinney (1954) reported the counter ion in numerous applications given that acid-catalyzed reduction of perchlorate to chlor- corrections for the concentration (activity) of ide by Ti (III). Early and co-workers (Kallen & perchlorate complexes are unnecessary. Earley, 1971) rationalized the manner in which From a thermodynamic perspective, perchlorate these reductants form inner-sphere complexes should represent a strong oxidant as evidenced by with perchlorate to cause its reduction to chlor- its relatively high standard potential: ide. Ti (III), V (II and III), and Ru (II) are relatively weak reductants and can slowly reduce þ ClO4 þ 8H þ 8e ¼ Cl þ 4H2O perchlorate with half-lives ranging from 0.83 year o EH ¼ 1:389V ð1Þ for Ti (III) to as long as 11.3 years for V (II). On the other hand, concentrations of these rare- Which exceeds that of molecular chlorine (1.36 V) earth metal elements in natural waters are far and molecular oxygen (1.229 V), two commonly too low to impact the fate of perchlorate in the used oxidants. Nonetheless, relatively few reduc- environment. 500

Over the past few years, perchlorate has emerged 25–75°C. From these experiments, the activation as a high profile contaminant in the U.S. and has energy of the perchlorate reduction was obtained. consequently received considerable regulatory We have previously reported the synthesis and attention (Urbansky & Schock, 1999; Urbansky, applications of nanoscale iron particles for the 2000; Logan, 2001; Stokstad, 2005). The scale of the hydrodechlorination of various chlorinated perchlorate problem became apparent in the late hydrocarbons such as PCB, chlorinated benzenes 1990 s after newly developed analytical techniques and chlorinated aliphatics (Wang & Zhang, 1997; revealed widespread and previously undetected Zhang et al., 1998; Lien & Zhang, 1999). Nanoscale contamination in water supplies. A study in Cali- metallic particles are promising materials with fornia using an improved ion chromatography (IC) potentially significant electronic, magnetic, optical method detected perchlorate in 109 of the 428 wells and chemical properties. They may offer advanta- sampled – 27% of the wells surveyed (California ges over conventional treatment technologies for Department of Health Services, 1997). Perchlorate contaminated soil and water including high reac- has since been detected in both ground and surface tivity, flexible morphologies and ease of deploy- waters. For example, high levels of perchlorate ment. Hydrodechlorination of various chlorinated (>100 ppb) have been found in Nevada’s Lake hydrocarbons can be substantially enhanced Mead, a major drinking water source (Urbansky & through deposition of a small amount (<0.5 %)of Schock, 1999). Although the precise magnitude of a noble metal (Pd, Pt, Ni, Ag etc.) on the iron the problem remains unknown, estimates indicate surface. The feasibility of nanoscale bimetallic that perchlorate has impacted the drinking water particles for the in situ treatment of contaminated supplies of about 15 million residents in several groundwater was also demonstrated (Elliott & western states including California, Arizona, Zhang, 2001). Nevada, and Utah (Urbansky, 2000). Linked to potentially serious thyroid, blood, and kidney disorders, a recent draft toxicological report from Experimental section the U.S. Environmental Protection Agency (EPA) suggests a revised reference dose (RfD) of Chemicals and materials 0.00003 mg of perchlorate per kilogram of body weight per day (mg/kg/day). If converted to a The following chemicals were purchased from drinking water standard, this RfD yields a per- Sigma-Aldrich: sodium borohydride (NaBH4, missible concentration on the order of 1 ppb. Per- 98%), ferrous sulfate (FeSO47H2O, ACS reagent, chlorate has been added to the Federal 99+%), sodium hydroxide (NaOH, ACS reagent, Contaminant Candidate List (CCL) under the Safe 97+%), sodium perchlorate (NaClO4, ACS Drinking Water Act (SDWA) (US EPA, 1998). reagent, 99+%), sodium chlorate (NaClO3, ACS 0 Theoretically, elemental iron (Fe ) should be reagent, 99+%), sodium chlorite (NaClO2,80%), able to serve as a reductant, or electron donor, for sodium hypochlorite (NaOCl, available chlorine the reduction of perchlorate: 10–13%), sodium chloride (NaCl, 99.5+%). Microscale iron powders (<10 micron, 99.9+%) 0 þ 2þ ClO4 þ 4Fe þ 8H ! Cl þ 4Fe were obtained from Aldrich. Average diameter was 4.95 lm according to Aldrich. The iron powders þ 4H2O DG ¼1; 387:49 kJ=mol ð2Þ were pretreated with 1 N HCl for 10 minutes (2 g However, only one research has reported rather Fe/10 mL), and then washed with large volume slow removal of aqueous perchlorate under ambi- (>100 mL/g) of deionized water before use. ent temperature (Moore, et al., 2003). Little or no perchlorate reduction by zero-valent iron was Preparation of nanoparticles observed in several other studies (Espenson, 1997). In the present study, we report the complete The nanoscale iron particles used in this work reduction of perchlorate to chloride by laboratory were synthesized by mixing equal volumes of synthesized nanoscale iron particles over a wide 0.20 M NaBH4 and 0.04 M FeSO4 solutions concentration range. A temperature-dependent according to the following reaction (Wang & study was conducted in the temperature range of Zhang, 1997): 501

(IC) with a suppressed conductivity detector. For 2þ 0 þ 2Fe þ BH4 þ 3H2O ! 2Fe þ H2BO3 þ 4H perchlorate analysis, EPA method 314.0 was þ 2H2ðgÞð3Þ followed. A Dionex AG16 column 4 mm Ion Pac was used. Eluent was 35 mM NaOH. Eluent flow After NaBH4 was added to the FeSO4 solution, the was set at 1.15 ml/min for high concentrations of mixture was stirred for approximately 20 minutes perchlorate (>1 mg/l), and 1.10 ml/min for low or until visible hydrogen evolution had ceased. The concentrations of perchlorate (<1 mg/l). Sample solid particles were separated from the liquid loop volume was 250 ll for high concentrations, reaction mixture using a laboratory vacuum filter. and 1000 ll for low concentrations. A run time of 0 The Fe particles formed have an average diameter 15.5 minutes was utilized for high concentrations at 57 16 nm as observed with a transmission (>1 mg/l) whereas 17.0 minutes was used for low electron microscope (Figure 1). We also tested concentrations (<1 mg/l). The detection limit of various nanoscale bimetallic particles such as Fe/ perchlorate with the 1000lL loop was less than Pd, Fe/Ag and did not observe any performance ) ) 5 lg/l. For ClO3 , ClO2 , ClO ,andCl , a Dionex enhancement compared with the nanoscale iron AG14, 4 mm /AS14, 4 mm Ion Pac column was particles. The surface noble metals have been used. The operational conditions were: eluent at shown to be good catalysts for hydrodechlorina- 3.5 mM Na2CO3/1.0 mM NaHCO3; eluent flow tion of various chlorinated hydrocarbons but at 1.20 ml/min, sample loop volume at 250 ll, apparently yielded little benefit to the reduction of and run time at 10 min. Deionized water was perchlorate. Results reported in this work were used as blank. thus based on the use of non-catalyzed nanoscale iron particles.

Experimental setup Results and discussion

In the experiments, 100 mL of perchlorate-con- Reduction of perchlorate by nanoparticles taminated water with varying concentration of nanoparticles were tracked over time in 150 mL Figure 2 illustrates the changes in normalized batch reactors. Serum bottles sealed with PTFE- perchlorate concentration (C/C0) as a function of lined, butyl rubber septa and aluminum crimp the reaction time, nanoparticle concentration, and caps. Initial perchlorate concentrations ranged reaction temperature. Initial perchlorate concen- from 1–200 mg/l. Parallel batch experiments tration was 200 mg/l for all the experiments shown included controls (no nanoparticles) and reactors in Figure 2. At 25±1°C (Figure 2a), the perchlo- containing microscale (Aldrich, <10lm) iron rate concentration was reduced from 200 mg/l to powder for purposes of comparison. The reactors 81.86 mg/l over 28 days, representing a reduction were agitated throughout the exposure period of 59.07%. Meanwhile, only a slight change (<5%) using a wrist-action shaker (30 rpm). Constant in the perchlorate concentration was observed in temperature experiments were conducted within a the batch reactor containing the microscale iron powder. A small amount (1%) of chlorate Fisher Isotemp incubator (5.0 cu. ft.). The same ) experimental setups were also used for the reac- (ClO 3) was detected in the batch containing the nanoparticles. More importantly, net production tions of chlorate (ClO3 ), chlorite (ClO2 ), hypochlorite (ClO)) with the nanoscale iron particles. or accumulation of chloride was observed in the Stock solutions of perchlorate, chlorate, chlorite, presence of the nanoparticles, but not in the hypochlorite, and chloride were prepared in presence of the microscale iron powder. As further deionized water and purged with nitrogen. discussed later in this paper, chloride is the end product of perchlorate reduction. With the microscale iron powder, no chloride production Analyses was found over an extended period of time (>45 days). The slight decrease of perchlorate ) ) All anions (ClO4 , ClO3 , ClO2 ,ClO,Cl)in observed may have been caused by sorption to the solution were analyzed with a Dionex DX-120 iron or iron oxide surfaces. 502

(a)

(b)

120

100 >- c0 80 Q) j cr f 60 u.. 40

0 25 50 75 100 125 150 175 200 225 300 More Diameter (nm) Figure 1. TEM micrographs of iron nanopartiocles and particle size distribution: (a) a single iron particles, (b) aggregates of particles, (c) normalized Fe nanoparticle size distribution of 442 nanoparticles. Average diameter was 57 16 nm. ( g/ ) p ( g/ )

503

Figure 2. Reactions of perchlorate with nanoscale and microscale iron particles at (a) 25 1°C; (b) 40 1°C; (c) 60 1°C; and (d) 75 1°C. Initial perchlorate concentration was 200 mg/L.

The addition of nanoparticles also caused sig- support microbial growth. Solution pH and stan- nificant changes in water chemistry, particularly dard potential changed rapidly as results of with respect to oxidation-reduction potential nanoparticle reduction and were unfavorable for (ORP) and pH. Standard potentials were typically microbial growth. We also used 20 mg/l sodium reduced from greater than +200 mV to less than azide (NaN3) in a few batch bottles as control to )400 mV within 2–3 min. In other words, the retard microbial growth and did observe similar aqueous solution containing the nanoparticles perchlorate reduction as shown in Figure 2a. represented a highly reducing medium. At the Thus it could be assumed that microbial degra- beginning of the tests, the solution pH was dation in the above described system was unlikely approximately 6.0, increasing quickly to about 8.0 important. where it remained essentially constant. This Experiments were also conducted at 40°C behavior was expected (Equation (2)) and has been (Figure 2b), 60°C (Figure 2c) and 75°C (Figure 2d) verified in our previous tests (Zhang, 2003). The to evaluate the effect of temperature. The rate of data in Figure 2a indicate that the nanoparticle- perchlorate reduction was observed to increase mediated reduction of perchlorate proceeded markedly as a function of temperature. For under room temperature and at neutral pH, example, at 75°C, the perchlorate concentration contrary to previously published reports regarding was reduced from 200 mg/l to 21 mg/l within zero-valent iron (Espenson, 1997). 24 hours, or nearly 90% reduction with a nanop- The potential of microbial growth and reduction article dose of 10 g/l (Figure 2d). For comparison, of perchlorate were also considered. As described the 24-h reduction efficiencies were 54.58%, above, solution was prepared in laboratory with 19.79%,3.27%, at 60, 40, and 25°C, respectively. deionized water. There was little carbon source to It was also evident that the rate of perchlorate 504 reduction was strongly dependent on the nano- þ ClO3 þ 2H þ 2e ! ClO2 þ H2O ð5Þ particle concentration as illustrated in Figures 2b-d. Control tests without iron nanoparticles showed no þ ClO2 þ 2H þ 2e ! ClO þ H2O ð6Þ reduction of perchlorate at the high temperature þ (40°C, 60°Cand75°C). The reduction of perchlo- ClO þ 2H þ 2e ! Cl þ H2O ð7Þ rate by micro iron filings at high temperatures was negligible. In this study, only trace (<1%) amounts of chlo- Data presented in Figures 2 further demon- rate were briefly detected during the reaction of strates that natural logarithmic concentration vs perchlorate with the nanoparticles. Separate tests time plots are generally linear, suggesting that with chlorate, chlorite and hypochlorite as reac- reduction of perchlorate is (pseudo) first-order tants showed that all were reduced to chloride towards both perchlorate and nanoparticle con- within a few minutes to a few hours. Figure 4 centrations. The best fit rate coefficients (k) for shows an example in which chlorate was com- observed perchlorate reduction were calculated to pletely reduced within 2 hours in the presence of be 0.013, 0.10, 0.64, and 1.52 mg perchlorate per 1 g/l of nanoparticles. In all cases, the only stable gram of nanoparticles per hour (mg/g/hr respec- product observed was chloride. tively, at temperatures of 25, 40, 60, and 75°C. A near complete mass balance on chlorine was Rapid perchlorate reduction was observed in obtained. According to the reaction stoichiometry batch reactors with three different initial perchlo- (depicted in Equation 2), the reduction of per- rate concentrations (1, 10 and 100 mg/l) at 75°Cas chlorate should generate equivalent mole of chlo- illustrated in Figure 3. ride. As shown in Figure 5, 1.48 mM chloride was produced from 1.56 mM perchlorate originally Reduction products of perchlorate present, thus representing 94.9% of the theoretical yield. Based upon several recent reports of biologically mediated perchlorate degradation, the reduction of perchlorate is believed to occur via a series of Apparent activation energy of perchlorate reduction step-wise reactions through chlorate (ClO3 ), ) Figure 6 depicts an Arrhenius plot of the natural chlorite (ClO2 ), hypochlorite (ClO ), and finally chloride (Cl)) (Coates et al., 1998; Nzengung et al., logarithm of the first-order rate constant (k) versus 1999, Aken & Schnoor, 2002): 1/T for the nanoparticle-mediated reactions. The data were fitted to the classical Arrhenius þ ClO4 þ 2H þ 2e ! ClO3 þ H2O ð4Þ equation:

Figure 3. Reactions of perchlorate (1 to 100 mg/l) with nanoscale iron particles at 75 1°C. Nanoparticle concen- Figure 4. Reduction of chlorate by nanoscale iron particles tration was 10.0 g/l. at 25°C. Initial chlorate concentration was 180 mg/l. 505

Figure 5. Chlorine mass balance for reduction of perchlorate in a batch reactor with 5.0 g/L nanoscale iron particles at 75°C.

k ¼ A eEa=RT ð8Þ 1999), and 40.5 4.1 kJ/mol for the reduction of hexachloroethane, C2Cl6. During previous experiments with nanoscale iron particles under simi- where k is the rate constant, A is the pre-expo- lar conditions, we determined the Ea to be nential factor, Ea is the activation energy, T is the 44.90 kJ/mol for the reduction of tetrachloro-eth- absolute temperature, and R is the ideal gas ene, C2Cl4 (Lien, 2000). The higher Ea partially constant. The average Ea value was calculated to be explains the sluggish perchlorate reduction with 79.02 7.75 kJ/mole. A survey of literature sug- conventional iron particles. Using Eq. (8), the gests that this represents a rather large Ea for an reaction rate with Ea ¼ 79:02 kJ/mole would be aqueous reaction. By comparison, experiments almost one million times slower than a reaction conducted with microscale iron powders yielded with Ea ¼ 44:90 kJ/mole assuming both two reac- activation energies (Ea) of 32.2–39.4 kJ/mol for the tions have an equivalent A value. reduction of trichloroethene, C2HCl3 (Su & Puls, It should be noted that we limit our interpreta- tion to conventional thermodynamic methods (e.g., apparent activation energy) as the precise reaction mechanisms/steps such as sorption and surface-mediated reaction(s) still remain to be explored. The favorable thermodynamics (equation 2) and high activation energy hurdle represent a classic scenario in which reaction kinetics is limited by the slow rate, likely the reduction of perchlorate to chlorate. Other factors may also contribute to the slow reaction. For example, steric factors probably play a role given that the four large oxygen atoms (diameter 0.126 nm) can eﬀectively shield the much smaller central chlorine (diameter 0.041 nm) from direct interactions with potential reactants. In the tetrahedral struc- ture of the perchlorate anion, a single negative charge is spread over four equivalent oxygens via Figure 6. Arrhenius plots of the natural logarithm of rate delocalized p-bonding, translating into a low constant (k) vs 1/T for the reaction between perchlorate charge density. This could further contribute to (200 mg/l) and nanoscale iron particles (1 – 20 g/l). the slow reaction rates. 506

The fundamental physical and chemical nature Cotton F.A., G. Wilkinson, C.A. Murillo & M. Bochumann, of perchlorate makes it very difficult to remediate, 1999. Advanced Inorganic Chemistry (6th ed.). New York: especially at the low concentrations typically Wiley 560–563. encountered in the environment. Perchlorate is a Duke F.R. & P.R. Quinney, 1954. The kinetics of the reduction of perchlorate ion by Ti(III) in dilute solution. J. Am. Chem. non-volatile contaminant that exhibits appreciable Soc. 76, 3800–3803. aqueous solubility and adsorbs poorly to mineral Elliott D.W. & W. Zhang, 2001. Field assessment of nanoscale surfaces and activated carbons. Conventional bimetallic particles for groundwater treatment. Environ. Sci. water purification methods have generally been Technol. 35(24), 4922–4926. shown to be ineffective and quite expensive insofar Espenson J., 1997. In Proceedings of the Symposium on as perchlorate removal is concerned. The nano- Biological and Chemical Reduction of Perchlorate and scale iron particles may represent a potential Chlorate. Cincinnati, OH: US EPA National Risk Manage- ment Research Laboratory. treatment method for perchlorate-contaminated King W.R. & C.S. Garner, 1954. Kinetics of the oxidation of water. Iron is a ubiquitous and non-toxic species vanadium(II) and vanadium(III) ions by perchlorate. J. Phys. detected in virtually all natural waters. The high Chem. 58, 29–33. reactivity and diminutive size of the iron nano- Kallen T.W. & J.E. Earley, 1971. Reduction of Perchlorate in particles make them effective vehicles for direct by aquoruthenium(II). Inorg. Chem. 10, 1152–1155. subsurface injection. The high reactivity of the Lien H. & W. Zhang, 1999. Transformation of chlorinated nanoparticles towards perchlorate is consistent methanes by nanoscale iron particles. J. Environ. Eng. 125, 1042–1047. with previous observations regarding a wide vari- Lien H., 2000. Nanoscale bimetallic particles for dehalogen- ety of chlorinated hydrocarbons and heavy metal ation of halogenated aliphatic compounds. Lehigh Univer- ions (Wang & Zhang, 1997; Zhang et al., 1998; sity, Bethlehem, PA: Doctoral Dissertation. Lien & Zhang, 1999; Su & Puls, 1999; Lien, 2000). Lien H. & W. Zhang, 2005. Reactions of Chlorinated Methanes The quantitative reduction of perchlorate to with Nanoscale Metal Particles. J. of Environ. Eng. 131(1), chloride in aqueous solution provides strong evi- 4–10. Logan E.B., 2001. Assessing the outlook for perchlorate dence of the high reactivity of nanoparticles in remediation. Environ. Eng. 35, 482A–487A. general. Results presented here may also have Moore A.G., C.H. De Leon & T.M. Young, 2003. Rate and significant implications on the continued use of extent of aqueous perchlorate removal by iron surface. perchlorate as a standard reagent in systems con- Environ. Sci. Technol. 37(14), 3189–3198. taining colloidal and nanoscale iron. Nzengung V.A., Ch-H. Wang & G. Harvey, 1999. Plant- Mediated transformation of perchlorate into chloride. Envi- ron. Sci. Technol. 33(9), 1470–1478. Stokstad E., 2005. Debate Continues Over Safety of Water Acknowledgements Spiked With Rocket Fuel. Science. 307, 507. Su Ch-M. & R.W. Puls, 1999. Kinetics of trichloroethene This work is partially supported by the US Environ- reduction by zero-valent iron and Tin: pretreatment of effect, mental Protection Agency Science to Achieve Results apparent activation energy, and intermediate products. Environ. Sci. Technol. 33(1), 163–168. program (Grants 829625 and 829625), US National U. S. Environmental Protection Agency. 1998. Drinking Water Science Foundation CAREER Award (Grant Num- Contaminant List. EPA Document No. 815-F-98–002; GPO: ber 9983855), and by a grant from Pennsylvania Washington, DC. Infrastructure Technology Alliance (PITA). Urbansky E.T. & M.R. Schock, 1999. Issues in managing the risks associated with perchlorate in drinking water. J. Environ. Manage. 56(2), 79–95. Urbansky E.T., 2000. Perchlorate in the Environment. New References York: Kluwer Academic/Plenum Publishers. Wang C. & W. Zhang, 1997. Synthesis nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs. ) Aken B.V. & J.L. Schnoor, 2002. Evidence of Perchlorate (ClO4 ) Environ. Sci. Technol. 31(7), 2154–2156. Reduction in Plant Tissues (Poplar Tree) Using Radio-Labeled Zhang W., C. Wang & H. Lien, 1998. Treatment of chlorinated 36 ) ClO4 . Environ. Sci. Technol., 36(12), 2783–2788. organic contaminants with nanoscale bimetallic particles. California Department of Health Services. 1997. Perchlorate Catal. Today. 40, 387–395. in California Drinking Water. Zhang W., 2003. Nanoscale Iron Particles for Environmental Coates J.D., R.A. Bruce & J.D. Haddock, 1998. Anoxic Remediation: An Overview. J. of Nanoparticle Research 5, bioremediation of hydrocarbons. Nature 396, 730. 323–332.