Environ. Sci. Technol. 2003, 37, 2582-2587
In Situ Remediation of Arsenic in Simulated Groundwater Using Zerovalent Iron: Laboratory Column Tests on Combined Effects of Phosphate and Silicate CHUNMING SU* AND ROBERT W. PULS National Risk Management Research Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, 919 Kerr Research Drive, Ada, Oklahoma 74820
We performed three column tests to study the behavior of permeable reactive barrier (PRB) materials to remove arsenic under dynamic flow conditions in the absence as well as in the presence of added phosphate and silicate. The column consisted of a 10.3 cm depth of 50:50 (w:w, Peerless iron:sand) in the middle and a 10.3 cm depth of a sediment from Elizabeth City, NC, in both upper and lower portions of the 31-cm-long glass column (2.5 cm in diameter) with three side sampling ports. The flow velocity (upflow mode) was maintained at 4.3 m d-1 during the 3-4month experiments. As expected, dissolved As concentrations in different positions of the column generally followed the order: column influent > bottom port effluent > middle port effluent > top port effluent > column effluent. The steady-state As removal in the middle Peerless iron and sand mixture zone might be attributed to the continuous supply of corroded iron in the form of iron oxides and hydroxides that served as the sorbents for both As(V) and As(III). Consistent with previous batch study findings, dissolved phosphate (0.5 or 1 mg of P L-1) and silicate (10 or 20 mg of Si L-1) showed strong inhibition for As(V) and As(III) (1 mg of As(V) L-1 + 1 mg of As(III) L-1 in 7 mM NaCl + 0.86 mM CaSO4) removal by Peerless iron in the column tests. The presence of combined phosphate and silicate resulted in earlier breakthrough (C ) 0.5C0) and earlier complete breakthrough of dissolved arsenic relative to absence of added phosphate and silicate in the bottom port effluent. Competition between As(V)/As(III) and phosphate/silicate for the sorption sites on the corrosion products of Peerless iron seems to be the cause of the observations. This effect is especially important in the case of silicate for designing a PRB of zerovalent iron for field use because dissolved silicate is ubiquitous in terrestrial waters.
Introduction Zerovalent iron (Fe0) as a new sorption medium for removing both arsenate [As(V)] and arsenite [As(III)] from wastewater has recently attracted attention (1-6). These studies have shown that Fe0 may potentially be used in permeable reactive barrier (PRB) technologies to remediate arsenic (As) in * Corresponding author phone: (580)436-8638; fax: (580)436-8703; e-mail:
[email protected]. 2582 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 11, 2003
groundwater via surface adsorption/complexation or precipitation. This is encouraging because As is a priority contaminant due to its complex chemistry, toxicity, and widespread contamination of groundwater across the globe (7-13). An additional benefit of using Fe0 in a PRB is the ability of Fe0 to intercept and treat a plume of mixed inorganic contaminants including chromate and phosphate (5, 1417); however, phosphate and silicate are known to compete against As(V) and As(III) for sorption sites at mineral surfaces. Phosphate or silicate alone significantly decreases the sorption kinetics of both As(V) and As(III) onto Peerless Fe0 in batch tests as compared to chloride (5). Silicate decreases As(V) and As(III) removal by coprecipitation with ferric chloride (18) and by ferrihydrite (19). Competitive adsorption between silicate and As(V) also occurs in soil (20). Adverse effect of phosphate has been reported to inhibit adsorption of As(V) and As(III) by aluminum and iron oxides (21-23) and As(V) adsorption by clay minerals (24) and soils (2528). Furthermore, it has been shown that, in the presence of silicate, the adverse effect of phosphate on As(V) adsorption by iron hydroxides is magnified (29). It is thus more appropriate to study the combined effects of phosphate and silicate in further tests on As removal using Fe0. Previous studies in our laboratory focused on batch tests (5) that have shown competition between As(V)/As(III) and phosphate or silicate on the surfaces of corroded Peerless Fe0. In the batch tests, the primary mechanism of As removal was sorption to corrosion products that were on the Fe0 surface at the start of experiment. The use of high initial As concentrations in some of the batch tests masked the slow removal of As by corrosion products formed during the course of the experiment. Arsenic removal should continue as long as the Fe0 continues to corrode. A followup study focused on column tests, which allowed long-term corrosion of Fe0. Both column and batch tests are vital to the success of a PRB strategy for in situ remediation of As in groundwater. These laboratory results should provide valuable information about the design and implementation of field pilot- and full-scale remediation scheme using the iron barrier technologies. Consequently, the goal of this study was to investigate the behavior of As in glass columns packed with Fe0, sand, and sediment under saturated flow conditions with and without added phosphate and silicate.
Materials and Methods Materials. Peerless iron (Peerless Metal Powders & Abrasives, Detroit, MI) was used without pretreatment. It had a BET N2 surface area of 2.53 ( 0.44 m2 g -1 (2). The pristine Peerless Fe0 is a low-grade steel that contains 90+% Fe, 2.50% C, 2.0% Si, 0.60% Mn, and 0.20% Cr (data from the manufacturer). A local quartz sample (Oil Creek sand, 0.05-0.5 mm) and a sediment ( middle port effluent > top port effluent > column effluent. Dissolved As concentrations achieved steady state in the center of the middle zone (Peerless Fe0 and Oil Creek sand mixture) at 900 pore volumes and in the center of the top Elizabeth City sediment zone at 1100 pore volumes. Total dissolved As concentrations converged in the middle port and top port effluents, probably due to bypassing (preferential flow) of solution through the channels in the column between these two ports. No As was detected in the column effluent at pore volumes less than 1200, at which a total of 116 mg of As was removed by the whole column. This resulted in an average concentration of 357 mg of As (kg of solids)-1 in the whole column. The steady-state As removal in the middle Peerless Fe0 and sand mixture zone might be attributed to the continuous supply of corroded iron serving as the sorbent for both As(V) and As(III). Previous work (1, 4) also showed other Fe0 specimens (Baker and Master Builders) exhibited steady-state removal of As at the middle and later stages of the column experiments. This behavior is advantageous to a PRB iron wall application for As removal in that continuous generation of reactive sorbents will ensure that the wall is operative in the subsurface for a long time. Of course, the thickness of the Fe0 wall should be made enough to intercept and treat the As-containing plume during its lifetime. The pH of middle port effluent ranged from 8.5 to 9.5, as a result of Peerless Fe0 corrosion that releases hydroxyl ions into the solution. The bottom and top port effluents had pH values ranging from 6.5 to 7.5. The column effluent pH values ranged from 6.0 to 6.5, which was consistent with a separately measured sediment pH of 6.0 (10 g in 20 mL of 0.01 M CaCl2) and a sediment pH of 6.7 (10 g in 20 mL of deionized water). No negative Eh values were observed in any effluent solutions largely because they were all measured in ambient laboratory atmosphere. Generally, the middle port effluent had the lowest Eh values (110-300 mV), followed by the top port effluent. The bottom port effluent and column effluent had the highest Eh values (350-550 mV). The relative distribution of As(V) and As(III) in the effluent solutions changed with increasing pore volumes (Figure 3)
as compared to the column influent. The feeding solution had an As(III)/As(III + V) ratio of 0.5; whereas, this ratio was near 1.0 for the first 500 pore volumes in the bottom port effluent. This means that almost all the As(V) in the feeding solution was adsorbed or that As(III) was partly oxidized to As(V) by the Elizabeth City sediment up to 500 pore volumes. This is consistent with the batch study that clearly shows that As(V) is preferentially sorbed by the Elizabeth City sediment as compared to As(III). The As(III)/As(III + V) ratio in the middle port (Peerless Fe0 and sand mixture) also showed a decrease from a value near 1.0-0.5 when 600 pore volumes were reached and to 0.3-0.4 when 1000 pore volumes were reached. The same trend was also observed to a lesser extent for the top port effluent with ratios showing a reverse trend at pore volumes >800 with values lower than 0.5. These results indicate that added As(III) was partly oxidized to form As(V) in the solution phase by Peerless Fe0. This is consistent with previous studies (2, 34) that show the Fe0 corrosion reaction under aerobic conditions cause As(III) oxidation to As(V). The presence of Mn as an impurity (possibly as manganese oxides from corrosion) in the Fe0 may be acting as an oxidizing agent (2). In addition, high pH as a result of Fe0 corrosion and the presence of oxygen could favor As(III) oxidation reaction. Dissolved iron was below detection limit (0.035 mg L-1). No phosphate was detected in any of the effluent samples. Dissolved Ca increased from ∼13 mg L-1 from the beginning of column test to 34 mg L-1 after 70 pore volumes for all the sampling ports. This suggest that, in the early stage of the column flow, Ca was removed from the feeding solution mostly likely through Ca-Na/K exchange. No calcite was detected in the Elizabeth City sediment before the experiment. Dissolved silicate concentrations ranged from 2.9 mg of Si L-1 in the beginning of column test to 0.2 mg of Si L-1 at the end of column test for the bottom port effluent, from 0.5 mg of Si L-1 in the beginning of column test to 0.3 mg of Si L-1 at the end of column test for the middle port effluent, from 2.2 mg of Si L-1 in the beginning of column test to 0.6 mg of Si L-1 at the end of column test for the top port effluent, and from 4.0 mg of Si L-1 in the beginning of column test to 0.6 mg of Si L-1 at the end of column test for the whole column effluent. The solubility of quartz at 25 °C is 2.8 mg of Si L-1. The dissolved Si concentrations at the beginning of column flow was probably controlled by quartz. After that, quartz and other silicates were slowly dissolving by the Sifree feeding solution. Column Tests with Added Phosphate and Silicate. The second column test (low P and Si) and third column test (high P and Si) results show that the presence of phosphate and silicate significantly changed the dissolved As concen-
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FIGURE 4. Dissolved As concentrations as a function of pore volumes of the whole column. Column influent contained 1 mg of As(V) L-1 + 1 mg of As(III) L-1 in simulated ECGW that also contained 0.5 mg of P L-1 + 10 mg of Si L-1.
FIGURE 5. Dissolved As concentrations as a function of pore volumes of the whole column. Column influent contained 1 mg of As(V) L-1 + 1 mg of As(III) L-1 in simulated ECGW that also contained 1 mg of P L-1 + 20 mg of Si L-1. trations in the effluents (Figures 4 and 5). The bottom port effluent showed breakthrough (C ) 0.5C0) at only 170 pore volumes with 0.5 mg of P L-1 + 10 mg of Si L-1 and at only 70 pore volumes with 1 mg of P L-1 + 20 mg of Si L-1 as compared to 300 pore volumes without added phosphate and silicate. Complete breakthrough in the bottom port effluent was achieved at 400 pore volumes with 0.5 mg of P L-1 + 10 mg of Si L-1 (Figure 4) and at 200 pore volumes (Figure 5) with 1 mg of P L-1 + 20 mg of Si L-1 as compared to 600 pore volumes without added phosphate and silicate (Figure 2). At the higher phosphate and silicate concentrations, dissolved As concentrations achieved steady state in the center of the middle zone (Peerless Fe0 and Oil Creek sand mixture) at 200 pore volumes and in the center of the top Elizabeth City sediment zone at 400 pore volumes. Dissolved As concentrations converged in the middle port, top port, and column effluents at 700 pore volumes (Figure 4). At 1000 pore volumes, concentrations of As in the middle of the Peerless Fe0 zone were greater than in the columns operated without Si and P (Figures 2, 4, and 5). It is possible that the presence of Si in the columns initiated an accelerated corrosion of iron at a later stage, as demonstrated in a recent 4-month experiment showing that higher levels of Si (up to 23.4 mg of Si L-1) caused more iron release from Fe0 to the water and decreased the size of suspended iron particles (35). The increased corrosion may have compensated for the inhibitory effect of phosphate. At pore volumes greater than 1000, the As concentrations in the middle of the Peerless Fe0 zone were similar to those in the column effluent (Figures
4 and 5). This again may be a result of bypassing of solution through the channels in the column. It is not surprising to see the adverse effect of phosphate on As removal by Fe0. The interactions of As with Fe0 is largely through the interactions of As with the corrosion products of Fe0, which include iron oxides and green rusts (mixed ferrous/ferric hydroxides with interlayer anions) (36). Previous studies have demonstrated that phosphate competes with As(V) or As(III) on iron oxides for adsorption (e.g., refs 21-23). In addition, in the presence of silicate, the inhibitory effect of phosphate on As(V) adsorption by freshly prepared iron hydroxides is enhanced relative to the absence of silicate (29). The relative distribution of As(V) and As(III) in the effluent solutions also changed with added phosphate and silicate. The feeding solution had an As(III)/As(III + V) ratio of 0.5; whereas, this ratio in the bottom port effluent was near 1.0 for the first 100 pore volumes with the lower rate of phosphate and silicate and was near 1.0 for the first 50 pore volumes with the higher rate of phosphate and silicate, indicating that almost all the As(V) in the feeding solution was sorbed by the Elizabeth City sediment until 100 or 50 pore volumes (data not shown). The As(III)/As(III + V) ratio in the middle port (Peerless Fe0 and sand mixture) also showed a decrease from a value near 0.9 to 0.5 when 200 pore volumes were reached. The same trend was also observed for the top port and column effluents. The ratios showed a reverse trend at pore volumes >500-800 with values lower than 0.5 (data not shown). In contrast to the pH values observed without added phosphate and silicate, the difference in pH among the Peerless Fe0 and Oil Creek sand mixture zone and the top and bottom zones decreased in their presence. All the measured pH values ranged from 6.0 to 7.7 in the presence of 0.5 mg of P L-1 + 10 mg of Si L-1 and from 6.0 to 7.1 in the presence of 1 mg of P L-1 + 20 mg of Si L-1, both in simulated ECGW. It is likely that added phosphate inhibited the corrosion of iron that produces hydroxyl ions. At an influent concentration of 0.5 mg of P L-1 + 10 mg of Si L-1, phosphate showed breakthrough (C ) 0.5C0) in the bottom port effluent at only 140 pore volumes and complete breakthrough at 300 pore volumes; whereas, at an influent concentration of 1 mg of P L-1 + 20 mg of Si L-1, phosphate showed breakthrough (C ) 0.5C0) in the bottom port effluent at only 100 pore volumes and complete breakthrough at 180 pore volumes (data not shown). Phosphate concentrations in the effluents were all less than 0.17 mg of P L-1 throughout the test for a 0.5 mg of P L-1 influent concentration and all less than 0.30 mg of P L-1 throughout the test for a 1.0 mg of P L-1 influent concentration. At the higher influent phosphate concentration, dissolved P concentrations reached a maximum between 300 and 500 pore volumes in the middle and top effluents. The decrease of dissolved phosphate at pore volumes greater than 500 may be a result of increased adsorption of phosphate as more corroded iron was produced with increasing time. No phosphate was detected in the column effluent until 500 pore volumes. After that, phosphate concentration increased with increasing pore volumes and then reached a steady state at 700 pore volumes. The higher concentrations of phosphate in the column effluent than those in the three side port effluents may also be caused by preferential flow of solution. Silicate quickly exhibited complete breakthrough in the bottom port effluent at only 30-50 pore volumes (Figure 6). Dissolved Si concentrations in the middle, top, and column effluents increased with increasing time in the early stage of the flow. At an influent concentration of 0.5 mg of P L-1 + 10 mg of Si L-1, dissolved Si concentrations reached a steady state at 400 pore volumes; whereas, at an influent concentration of 1 mg of P L-1 + 20 mg of Si L-1, a steady concenVOL. 37, NO. 11, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Dissolved silicate concentrations as a function of pore volumes of the whole column. Column influent contained 1 mg of As(V) L-1 + 1 mg of As(III) L-1 in simulated ECGW that also contained 1 mg of P L-1 + 20 mg of Si L-1. tration of Si seems to be achieved in the Peerless Fe0 and Oil Creek sand mixture zone at 500 pore volumes, in the top port and column effluents at 1000 pore volumes (Figure 6). Dissolved iron was generally less than 0.035 mg L-1. Dissolved Ca increased from 15 mg L-1 from the beginning of column test to 34 mg L-1 after 150 pore volumes. Solution analysis showed no evidence of phosphate precipitation by added CaSO4 in the stock solution. Mechanisms of As Remediation. Specific mechanisms of As removal by Fe0 are in a need for further study. Both aerobic and anaerobic corrosion can occur to Fe0 (eqs 1 and 2). Although the pH measured within the Peerless Fe0 zone is elevated (pH 8.5-9.5), consistent with the production of hydroxl ion by the corrosion of iron (eqs 1 and 2), the reported pH of the effluent is comparable to the influent (pH ∼6.5). This suggests that enough dissolved oxygen is entering the system to oxidize any Fe2+ produced by corrosion of the Fe0, producing acidity to lower the pH, and also producing the Fe3+ that coprecipitates with the As (hydrolysis reaction, eq 3). Since there is no detectable soluble Fe in the effluent, it is presumed that the (soluble) Fe2+ corrosion product is either completely hydrolyzed or else sorbed onto the sediment zone beyond the Peerless Fe0 zone.
Acknowledgments
2H2O + Fe0 )
anaerobic corrosion:
Fe2+ + H2 +2OH- (1)
aerobic corrosion:
O2 + 2Fe0 + 2H2O )
2Fe2+ + 4OH- (2)
hydrolysis:
4Fe2+ + O2 + 10H2O )
4Fe(OH)3(s) + 8H+ (3)
Common anions in the aqueous solution may influence the effectiveness of Fe0 barriers for As remediation through formation of the following new mineral phases and competitive surface sorption/coprecipitation by these solid phases (36):
3Fe2+ + Fe3+ + Cl- + 8H2O ) Fe4(OH)8Cl
(s, chloride green rust) + 8H+ (4)
2+
4Fe
+ 2Fe
3+
2-
+ SO4
+ 12H2O ) Fe6(OH)12SO4
(s, sulfate green rust) + 12H+ (5)
4Fe2+ + 2Fe3+ + CO32- + 12H2O ) Fe6(OH)12CO3
(s, carbonate green rust) + 12H+ (6)
Fe2+ + HS- ) FeS(s) + H+ 2586
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Carbonate green rust, ferrihydrite, magnetite, aragonite, calcite, mackinawite, greigite, and lepidocrcrocite were identified in the fine-grained fractions of PRB samples from the U.S. Coast Guard Support Center (Elizabeth City, NC) and the Denver Federal Center (Lakewood, CO) sites (37). Sulfate or carbonate green rusts were reported to form in zerovalent iron columns fed with sulfate-rich or bicarbonaterich influent solutions (38). The iron corrosion products (oxides and green rusts) are most likely the solid phases that host the sorbed As(V) and As(III). Analysis of As(III) and As(V) adsorption complexes in the Fe0 corrosion products and synthetic iron oxides (goethite, lepidocrocite, maghemite, magnetite, and hematite) by X-ray absorption spectroscopy indicate both As species form innersphere bidentate complexes (34). Other spectroscopic studies also suggest that As(V) predominately forms inner-sphere bidentate surface complexes with ferrihydrite (39, 40). A pressure-jump experiment (41) and spectroscopic studies support inner-sphere complexation of both As(V) and As(III) on goethite (42-47). The stability and redox transformation of sorbed As in these possible phases will determine the fate of As in groundwater in the PRB iron wall. Further work is under way to examine iron corrosion products and the redox transformation of As in solid phases collected from the columns using X-ray diffraction, spectroscopic techniques, and chemical extractions. These results will aid our understanding of the mechanisms of iron corrosion and As removal. Implications for Field Pilot Study. The column studies showed the presence of added phosphate and silicate resulted in earlier breakthrough (C ) 0.5C0) and earlier complete breakthrough of total dissolved As relative to absence of added phosphate and silicate in the bottom port effluent. Phosphate and silicate were also removed by the PRB materials, especially by the Peerless Fe0. Phosphate and silicate decreased the effectiveness of As removal by the Peerless Fe0. The inner-sphere complex-forming phosphate and silicate compete strongly with As(V) and As(III) for sorption sites. The implications from the oxyanion test are that excess amounts of PRB materials may be needed for in situ remediation of arsenic in groundwater that commonly contains large amounts of dissolved silicate and possibly also contains phosphate.
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Although the research described in this paper has been funded wholly by the U.S. Environmental Protection Agency, it has not been subjected to Agency review and, therefore, does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. We wish to acknowledge gratefully the analytical assistance of Mr. Jarrod A. Tollett, Dr. Ning Xu, Dr. Jihua Hong, and Ms. Kelly Bates of ManTech Environmental Research Services Corp.
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Received for review November 21, 2002. Revised manuscript received March 11, 2003. Accepted March 19, 2003. ES026351Q
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