Environ. Sci. Technol. 2001, 35, 1487-1492
Arsenate and Arsenite Removal by Zerovalent Iron: Kinetics, Redox Transformation, and Implications for in Situ Groundwater Remediation C H U N M I N G S U * ,† A N D R O B E R T W . P U L S ‡ ManTech Environmental Research Services Corporation, 919 Kerr Research Drive, Ada, Oklahoma 74821-1198, and U.S. Environmental Protection Agency, National Risk Management Research Lab, 919 Kerr Research Drive, Ada, Oklahoma 74821-1198
Batch tests were performed utilizing four zerovalent iron (Fe0) filings (Fisher, Peerless, Master Builders, and Aldrich) to remove As(V) and As(III) from water. One gram of metal was reacted headspace-free at 23 °C for up to 5 days in the dark with 41.5 mL of 2 mg L-1 As(V), or As(III) or As(V) + As(III) (1:1) in 0.01 M NaCl. Arsenic removal on a mass basis followed the order: Fisher > Peerless ≈ Master Builders > Aldrich; whereas, on a surface area basis the order became: Fisher > Aldrich > Peerless ≈ Master Builders. Arsenic concentration decreased exponentially with time, and was below 0.01 mg L-1 in 4 days with the exception of Aldrich Fe0. More As(III) was sorbed than As(V) by Peerless Fe0 in the initial As concentration range between 2 and 100 mg L-1. No As(III) was detected by X-ray photoelectron spectroscopy (XPS) on Peerless Fe0 at 5 days when As(V) was the initial arsenic species in the solution. As(III) was detected by XPS at 30 and 60 days present on Peerless Fe0, when As(V) was the initial arsenic species in the solution. Likewise, As(V) was found on Peerless Fe0 when As(III) was added to the solution. A steady distribution of As(V) (73-76%) and As(III) (2225%) was achieved at 30 and 60 days on the Peerless Fe0 when either As(V) or As(III) was the initial added species. The presence of both reducing species (Fe0 and Fe2+) and an oxidizing species (MnO2) in Peerless Fe0 is probably responsible for the coexistence of both As(V) and As(III) on Fe0 surfaces. The desorption of As(V) and As(III) by phosphate extraction decreased as the residence time of interaction between the sorbents and arsenic increased from 1 to 60 days. The results suggest that both As(V) and As(III) formed stronger surface complexes or migrated further inside the interior of the sorbent with increasing time.
Introduction Arsenic, a known carcinogen in humans, is often found in contaminated groundwater as a result of weathering of rocks, industrial waste discharges, agricultural use of arsenical herbicides and pesticides, etc. (1). High naturally occurring As (greater than the proposed EPA maximum contaminant * Corresponding author phone: (580)436-8638, fax: (580)436-8501, e-mail:
[email protected]. † ManTech. ‡ U.S. EPA. 10.1021/es001607i CCC: $20.00 Published on Web 02/13/2001
2001 American Chemical Society
level for As in drinking water of 0.01 mg L-1) in well water supplies has also been reported as a health hazard in Bangladesh (2), Taiwan (3, 4), and elsewhere (5, 6). Since arsenite [As(III)] is more mobile and toxic than arsenate [As(V)], and As(III) is predominant in many groundwaters (5, 6), active remediation of As is often required that may involve conversion of As(III) to As(V), and immobilization of both species by adsorption or coprecipitation. There has been great interest in the in situ remediation of certain organic and inorganic contaminants in groundwater using zerovalent iron (Fe0) as a permeable reactive barrier medium. The Fe0 has been used to effectively destroy numerous chlorinated hydrocarbon compounds via reductive dehalogenation (7-9). Recent studies have also shown that Fe0 effectively removes inorganic contaminants (CrO42-, UO22+, MoO42-, TcO4-, AsO42-, and AsO32-) from aqueous solution (10-17). The removal mechanism appears to be reductive precipitation for these anions except for As(V) and As(III). Surface precipitation or adsorption appears to be the predominant removal mechanism for both As(V) and As(III) by Fe0 (17). Information about redox transformations of As(V) and As(III) in the Fe0 system is lacking, and spectroscopic methods such as X-ray photoelectron spectroscopy (XPS) may be useful for speciation determination of arsenic on the iron surface. Moreover, corrosion products of Fe0 such as magnetite, hematite, and goethite form a passivated layer on the Fe0 surface (18, 19), thus affecting the behavior of the Fe0-As interaction. The objectives of this study were to (1) evaluate the effectiveness of four Fe0 metals in removing As(V) and As(III) from water; (2) examine the redox transformation of sorbed arsenic on the surface of Fe0; and (3) evaluate the potential utilization of Fe0 as permeable reactive subsurface barrier media in remediation of As contamination in groundwater.
Experimental Section Materials Characterization. Four Fe0 materials with a range of surface areas (Table 1) were used including Fisher electrolytic Fe0 (99%, -100 mesh, Fisher Scientific, Fair Lawn, NJ, cat. no. I60-3), Peerless Fe0 (Peerless Metal Powders & Abrasive, Detroit, MI), Master Builders Fe0 (Master Builders Inc., Cleveland, OH), and Aldrich Fe0 (-325 mesh, 97%, hydrogen reduced, cat. no. 20930-9). A synthetic manganese dioxide (birnessite) was made following McKenzie (20) by the addition of concentrated HCl to a boiling KMnO4 solution. It had a surface area of 39.9 m2 g-1. Powder X-ray diffraction data for the metals were collected with a Philips X-ray diffractometer using CuKR radiation generated at 40 kV and 20 mA. All four materials showed pure Fe0 as the predominant phase with minor amounts of magnetite also detected in Peerless and Master Builders Fe0. Surface areas (Table 1) were determined by BET N2 adsorption analysis on a Coulter SA 3100 surface area analyzer (Coulter Co., Hialeah, FL). Batch Tests. Stock solutions (1000 mg of As L-1) were prepared from reagent-grade Na2HAsO4‚7H2O (Aldrich) for As(V) and from NaAsO2 (Baker) for As(III). Working solutions (generally 2 mg of As L-1 in 0.01 M NaCl) were prepared fresh daily for each batch test. One gram of Fe0 was added to a 50 mL polypropylene copolymer centrifuge tube (with a measured, capped volume of 41.64 ( 0.17 mL, n ) 10) that was filled without a headspace with a solution of 2 mg L-1 As(V), As(III), or As(V + III) (1:1) in 0.01 M NaCl. Four replicates were prepared for the Peerless and Master Builders Fe0 and two replicates for the Fisher and Aldrich Fe0. Preliminary experiments showed larger sample heterogeneity and variVOL. 35, NO. 7, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Pseudo-First-Order Rate Constants (k) and Their Surface Area Normalized Rate Constants (kSA) for As Removal by Fe0 iron material
surface area, m2 g-1
Fisher Peerless Master Builders Aldrich
0.091 ( 0.005 2.53 ( 0.44 2.33 ( 0.09 0.192 ( 0.001
As(V)
k × 1000 (h-1) As(III)
As(V + III)
77.8 ( 9.7 34.9 ( 2.0 24.6 ( 1.9 5.31 ( 0.35
149 ( 4 36.2 ( 0.5 36.1 ( 1.9 7.23 ( 0.69
149 ( 7 35.7 ( 0.6 32.7 ( 1.8 6.08 ( 0.24
ability for the Peerless and Master Builders Fe0samples, which were largely in the form of millimeter-sized chips, than for the Fisher and Aldrich Fe0 samples, which were powders. The tubes were covered with aluminum foil to prevent light exposure and placed on a reciprocating shaker at a shaking frequency of 50 oscillations per minute at 23 °C. No attempt was made to control the pH. At time periods (including 0.5 h centrifugation time) preset at 2, 4, 8, 12, 24, 48, 72, 96, and 120 h, the suspension was centrifuged, and 20 mL of supernatant solution was filtered through a 0.1 µm membrane and then analyzed for total As, as well as for As(V) and As(III). The pH and Eh were determined for the remaining supernatant solutions with an Orion ion analyzer (Orion Research Inc., Boston, MA) by using a combination pH electrode and a Pt electrode, respectively. The Eh readings are reported relative to the standard hydrogen electrode. Arsenic sorption capacity was also evaluated for Peerless Fe0. Triplicate samples of 1.0 g of Peerless Fe0 were reacted for 5 days with 41.5 mL (without a headspace) of As(V) or As(III) at initial concentrations of 2, 5, 10, 15, 20, 25, 50, and 100 mg of As L-1 in 0.01 M NaCl. Then the suspensions were centrifuged, and the supernatant solutions were filtered through 0.1 µm membranes before As determination. Phosphate Displacement. To evaluate the reversibility of As retention as influenced by residence time, a known strong adsorbate, phosphate, was used to displace sorbed As from the Fe0 surfaces. Ligand exchange is believed to be the mechanism for the displacement due to the similar chemistry of phosphate and arsenate (21-23). Arsenite can presumably also be displaced by phosphate. Four replicates of the Peerless Fe0 was first reacted in the centrifuge tubes, headspace-free, with either As(V), As(III), or As(V + III) at 2 mg L-1 in 0.01 M NaCl on the shaker in the dark for 1, 30, or 60 days. The suspension was centrifuged, and the supernatant solution was replaced with 5 mM NaH2PO4 in 0.01 NaCl, and the tubes were shaken in the dark for 24 h. The phosphate displacement was repeated. The supernatant solutions were analyzed for total dissolved As, As(V), and As(III). Analytical Methods for As. Total dissolved As, Fe, and Mn were determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Total dissolved As was also determined for samples with total As concentration 0.05 mg L-1, by IC, followed by determination using GFAAS. The IC system separates As(V) from As(III) via a Dionex IonPac AS 11 ion exchange column. The mobile phase was a 20 mM NaOH solution with a flow rate of 1 mL min-1. A total of seven fractions were collected. Void volume fraction, A, was collected from 0.0 to 4.0 min and discarded. Six other fractions were collected at the following time periods: B, 4.0-4.8 min; C, 4.8-5.6 min; D, 5.6-7.5 min; E, 7.5-8.5 min; F, 8.5-9.3 min; and G, 9.3-10.0 min. The B and C fractions contained As(III); the E and F (or F and G) fractions contained As(V). One standard solution was injected after every 10 samples. The results obtained from running the IC standard helped to decide which fraction of samples was to be analyzed by GFAAS, e.g., B, C, E, and F 1488
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As(V)
kSA (h-1 m-2 mL) As(III)
As(V + III)
35.6 ( 4.4 0.572 ( 0.033 0.439 ( 0.034 1.15 ( 0.08
68.2 ( 1.6 0.594 ( 0.008 0.643 ( 0.034 1.56 ( 0.15
68.4 ( 3.2 0.586 ( 0.033 0.582 ( 0.032 1.32 ( 0.05
or B, C, F, and G. The D fraction was not usually analyzed except for confirmation. Concentrations of As in each fraction were determined on a Perkin-Elmer ZL 5100 GFAA spectrometer with a detection limit of 1 µg L-1. Two check standards were run after every 10 samples. X-ray Photoelectron Spectroscopy. The Peerless Fe0 was studied by XPS to evaluate the redox status of sorbed As on the Fe0 surface. Ten grams of Fe0 was reacted for 5, 30, or 60 days with 40.4 mL of 1000 mg L-1 As(V) or As(III) in 0.01 M NaCl in centrifuge tubes without a headspace in the dark on a shaker. The samples were centrifuged, and supernatant solutions were analyzed for pH and Eh. The solids were washed with 20 mL of acetone twice, then quickly transferred with 10 mL of acetone to 25 mL glass serum bottles, and then purged with argon gas. The purging was performed on septacapped serum bottles by means of an argon gas inlet needle and an outlet gas vent needle that were inserted in order to evaporate residual acetone and to keep the iron under an oxygen-free argon gas phase. The bottles were stored in an anaerobic acrylic chamber filled with argon gas. During XPS analysis, samples were prepared within a glovebox purged and maintained with dry nitrogen. Samples were then transferred directly from the glovebox into the ultrahigh vacuum instrument chamber with the aid of an in-vacu transfer vessel in order to minimize atmospheric exposure. Spectra were acquired on a Physical Electronics 5500/5600 ESCA system. The X-ray source on this instrument was monochromatic Al KR radiation at 1486.7 eV. The acceptance angle was 0 ( 7°, and the takeoff angle was 65°. The diameter of the analysis area of the sample was 800 µm, and the depth of samples analyzed ranged from 8 to 10 nm. Chemical states of elements were determined using Physical Electronics’ MultiPak software, version 6, from nonlinear least-squares fitting of the spectra, and the results represent best estimates of the species present, based upon reference data (bonding energies). Minor unidentified peaks were those higher in binding energy than listed for As(3d). These may be loss peaks, i.e., peaks due to As(3d) electrons, which have lost energy to the matrix before escaping the sample surface.
Results and Discussion As Removal Kinetics. Arsenic removal was affected largely by reaction time, by the Fe0 type, and, to a lesser degree, by the initial As species (Figures1 and 2). In all the Fe0 systems, As concentration decreased exponentially with time. Fisher, Peerless, and Master Builders Fe0 decreased concentrations of both As(III) and As(V) to Peerless ≈ Master Builders > Aldrich. This sequence of reactivity was consistent with the results of reductive dechlorination of trichloroethene by these same Fe0 samples from a previous study (24). The surface area normalized rate constants (kSA), however, showed a different order: Fisher > Aldrich > Peerless ≈ Master Builders. The calculated half-lives (t1/2) and normalized halflives (t1/2-N for 1 m2 mL-1) for As removal by Fe0 (Table 2)
FIGURE 1. Kinetics of As removal by Fisher Fe0: (a) As concentration, (b) As(III) in solution, (c) Eh, and (d) pH.
FIGURE 2. Kinetics of As removal by Peerless Fe0: (a) As concentration, (b) As(III) in solution, (c) Eh, and (d) pH.
show the same trend as shown in Table 1. The removal of As was not proportionally related to the surface area of the Fe0 since Fisher Fe0 had the least surface area, but it had the greatest removal rates. Thus, surface area is not the primary factor controlling the interaction of As with Fe0. The effectiveness of Fe0 in removing both As(V) and As(III) has also been demonstrated in a recent laboratory and field column study (17) using three types of Fe0 (Master Builders, Baker, and Connelly). As(III) showed greater removal rates than As(V), with As(V + III) having intermediate removal rates. This may be explained by a difference in the pH effect on adsorption of As(V) versus As(III) by the Fe oxides. Adsorption of As(V) by Fe oxides generally decreases with increasing pH over the pH range from 3 to 10, whereas adsorption of As(III) shows a maximum at about pH 9, and equal amounts of As(V) and As(III) were adsorbed below pH 7. Thus, more As(III) adsorption is expected than As(V) at pH >7 (25, 26). Since Fe oxides are possible Fe0 corrosion products (18, 19), their reaction with As may be more predominant than the reaction with Fe0. The oxidized layer on iron is likely the predominant sorption location for both As species. The pH dependence of As adsorption is usually explained in terms of ionization of both adsorbates and adsorbents (27-34). H3AsO4 has three pKa values: pK1 ) 2.20, pK2 ) 6.97, and pK3 ) 11.53 (34). In the pH range of 3-6, H2AsO4is the predominant species, and, presumably, the major species being adsorbed. The iron oxide surfaces exhibit a net positive charge in this pH range, and adsorption of anionic As(V) is enhanced by Coulombic attractions. A recent study
showed that both As(V) and As(III) have strong affinities for ferrihydrite, and that As(III) is adsorbed in much larger amounts than As(V) at pH >7.5 or at high As concentrations in solution (32). Maximum As(III) adsorption on hematite and amorphous iron hydroxide occurs at pH 7 (35-37). The pH effect is generally explained by the zero point of charge (ZPC) of the adsorbent. A ZPC of 7.1 has been reported for hematite (36). Under low pH conditions, As(III) occurs as neutral H3AsO3, and the iron oxide undergoes surface protonation. Surface protonation diminishes as the pH increases to above 5, and approaches 0 at pH 7, resulting in maximum adsorption. H3AsO3 has three pKa values: pK1 ) 9.22, pK2 ) 12.13, and pK3 ) 13.4 (34). When the pH is above 9, the negatively charged H3AsO3- becomes predominant, whereas the oxide surface also becomes negatively charged; thus, electrostatic repulsion results in decreased adsorption. There is strong evidence for partial oxidation of As(III) to form As(V) in the solutions in all the Fe0 systems (Figures 1b and 2b), whereas there is no evidence for significant reduction of As(V) under the same experimental conditions. Speciation of As in solution was not performed for samples with total As concentration
