Environ. Sci. Technol. 1999, 33, 2170-2177
Biogeochemical Dynamics in Zero-Valent Iron Columns: Implications for Permeable Reactive Barriers B . G U , * ,† T . J . P H E L P S , † L . L I A N G , † M. J. DICKEY,‡ Y. ROH,‡ B. L. KINSALL,‡ A. V. PALUMBO,† AND G. K. JACOBS† Environmental Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, and Oak Ridge Institute for Science and Education, P.O. Box 2007, Oak Ridge, Tennessee 37831
The impact of microbiological and geochemical processes has been a major concern for the long-term performance of permeable reactive barriers containing zero-valent iron (Fe0). To evaluate potential biogeochemical impacts, laboratory studies were performed over a 5-month period using columns containing a diverse microbial community. The conditions chosen for these experiments were designed to simulate high concentrations of bicarbonate (17-33 mM HCO3-) and sulfate (7-20 mM SO42-) containing groundwater regimes. Groundwater chemistry was found to significantly affect corrosion rates of Fe0 filings and resulted in the formation of a suite of mineral precipitates. HCO3ions in SO42--containing water were particularly corrosive to Fe0, resulting in the formation of ferrous carbonate and enhanced H2 gas generation that stimulated the growth of microbial populations and increased SO42- reduction. Major mineral precipitates identified included lepidocrocite, akaganeite, mackinawite, magnetite/maghemite, goethite, siderite, and amorphous ferrous sulfide. Sulfide was formed as a result of microbial reduction of SO42- that became significant after about 2 months of column operations. This study demonstrates that biogeochemical influences on the performance and reaction of Fe0 may be minimal in the short term (e.g., a few weeks or months), necessitating longer-term operations to observe the effects of biogeochemical reactions on the performance of Fe0 barriers. Although major failures of in-ground treatment barriers have not been problematic to date, the accumulation of iron oxyhydroxides, carbonates, and sulfides from biogeochemical processes could reduce the reactivity and permeability of Fe0 beds, thereby decreasing treatment efficiency.
Introduction Remediation of groundwater contaminated with chlorinated organics, heavy metals, and radionuclides using zero-valent iron (Fe0) filings has received considerable attention in recent years (1-12). Whereas the mechanisms for degrading or immobilizing these contaminants by Fe0 are not completely understood (13-15), it has been shown that Fe0 can be very * Corresponding author phone: (423)574-7286; fax: (423)576-8543; e-mail:
[email protected]. † Oak Ridge National Laboratory. ‡ Oak Ridge Institute for Science and Education. 2170
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effective at groundwater remediation. Consequently Fe0based reactive barrier treatment has been generating significant interest for passive, long-term applications for groundwater remediation (5-12, 16, 17). A potential limitation of the Fe0 technology is the deterioration of the Fe0 materials by corrosion and the subsequent precipitation of minerals that may cause cementation and decreased permeability of the Fe0 barrier. Few studies are available concerning long-term performance characteristics of Fe0-based barriers (1, 16, cf. ref 12). However, data indicate that flow restriction could occur under certain biogeochemical conditions (16, 18, 19). Liang et al. (18) observed that flow rate decreased over a 6-month period through a series of Fe0-filled canisters used for treating trichloroethylene-contaminated groundwater at the Portsmouth Gaseous Diffusion Plant (Piketon, OH). Post-analysis of the Fe0 filings showed cementation of the iron grains, possibly as a result of precipitation of iron sulfides, oxyhydroxides, and carbonates. Clogging has also been reported in laboratory and pilot-scale studies with Fe0 filings as reactive media (16, 20, 21). For example, at the Lowry Air Force Base (AFB) in Denver, CO, and at Elizabeth City, NC, sites, green rusts (i.e., a mixture of partially reduced/oxidized iron oxyhydroxides and sulfate) were observed in barrier materials (22). At the Hill AFB, UT, site, precipitation of iron and calcium carbonates was concluded to be responsible for a 14% porosity reduction within a few months of operation (23). In contrast, mineral precipitation was not observed after 1 year of operation in a reactive barrier at the Borden, Ontario, site (12). It is recognized that groundwater chemistry plays a significant role in determining rates of mineral precipitation and barrier clogging and influencing the rates and extent of microbial impacts (5, 12). The roles of dissolved oxygen and pH in determining Fe0 reactivity and precipitation chemistry are well established. On the other hand, influences of other groundwater constituents, such as HCO3- and SO42-, are less well defined. Because SO42- and HCO3- are both corrosive to Fe0 (24-30) and are commonly found in groundwater at contaminated sites, these anions are of particular significance in influencing biotic and abiotic barrier-clogging processes. Both anions promote corrosion of Fe0 by disrupting the protective oxide layers (24), thus facilitating continued anodic dissolution of the iron and hydrogen generation (31, 32). Furthermore, HCO3- and hydrogen may serve as excellent carbon and energy sources facilitating microbially influenced iron corrosion (31-34). Little is known, however, about the impacts of microbial activities on Fe0 barrier performance. Field evidence for the enhancement of microbial populations as a result of Fe0 barrier corrosion is lacking. Microbial populations were not observed to increase in Fe0 barriers in Sunnyvale and Moffett Field in California and in an industrial site in New York (17). Similarly, microbial activities were found to be low or not observed at the Lowry AFB or Somersworth, NH, sites (17). In contrast, biofouling was observed in an Fe0 foam/sand reactive barrier in Newbury Park, CA (17), and biofouling occurred rapidly in a filter column at the Portsmouth, OH, site (18). At the Portsmouth site, sulfate-reducing bacteria were detected in water samples and in Fe0 filings after treatment. Microorganisms that can utilize SO42- as a terminal electron acceptor producing sulfide are widely distributed (35-37) and are therefore of particular importance in the microbially mediated mineral precipitation and clogging of Fe0 barriers. Decreases in groundwater SO42- concentration during transport through Fe0 barriers have been observed at 10.1021/es981077e CCC: $18.00
1999 American Chemical Society Published on Web 05/11/1999
TABLE 1. Influent Chemical Composition and General Experimental Conditions HCO3- (mM)a SO42- (mM) autoclaved purge gas average flow rate (mL/h)
column 1
column 2
column 3
16.4 10.4 no N2 27.5
32.8 ∼7.3 yes N2 23
32.8 ∼7.3 no N2 58.4
column 4 20.8 yes N2 31.1
column 5
column 6
column 7
20.8 no N2 36.1
16.4 10.4 no air 22.8
16.4 10.4 no CO2 b
a Added HCO - concentration. The actual HCO - concentration was slightly lower because of an initial purging with N at pH ∼9. 3 3 2 was not monitored after 2 days of operation because of excessive H2 generation that caused flow to stop frequently.
b
Column 7
the Moffett Field and Lowry AFB sites; at an installation in Oak Ridge TN; and at a U.S. Coast Guard Fe0 barrier at Elizabeth City, NC (17). The objective of the present study was to evaluate the potential role of microorganisms in conjunction with geochemical reaction mechanisms in affecting the long-term effectiveness of Fe0 barriers. Of particular concern were the combined effects of HCO3- and SO42- on Fe0 corrosion, mineral precipitation, and microbial activity in the Fe0 medium. Laboratory Fe0 column studies were conducted under conditions designed to simulate high concentrations of HCO3- - and SO42--containing groundwater regimes. Results of the experiments were used to provide a rationale for the potential roles of biogeochemical processes on Fe0 barriers and to identify data gaps for additional study.
Materials and Methods Experimental Design. Seven glass columns (25 × 450 mm) were constructed containing a sludge/sand mixture at the inlet (bottom) of each column followed by Fe0 filings (Peerless Metal Powders and Abrasives, Detroit, MI). The sludge was obtained from the Anderson County Waste Treatment Plant (Oak Ridge, TN) to provide a broad microbial inoculum. Approximately 2 L of sludge was combined with 1 L of sand [Unimin Corp., NC (38)] along with additions of 10 mM phosphate and 0.05% glucose to promote microbial growth and were cultured for 2 days before use. Following the addition of the sludge/sand mixture (40 mm in depth to the columns), Fe0 filings (sieved to 0.5-1 mm size) were added in a layer of 350 mm thick, followed by 60 mm of sand (∼1 mm in diameter) at the top. Two columns (2 and 4) were initially sterilized by autoclaving to better differentiate abiotic reactions from microbial processes (Table 1). Influent solution compositions and other conditions varied among the columns (Table 1). Each influent solution was either sodium sulfate (20 mM SO42-), sodium bicarbonate (∼33mM HCO3-), or a mixture of Na2SO4 and NaHCO3 (710 mM SO42- and ∼17 mM HCO3-) solutions. After ∼20 L of each influent solution was passed through the columns, the pH of each influent solution was adjusted to about neutral (using H2SO4) in an attempt to facilitate microbial growth in the column. Each influent solution was fed to a column by gravity siphon under a constant head, with flow from bottom to top (Figure 1). In columns 1-5, the influent solutions were initially purged with N2 to achieve a low dissolved oxygen condition. Glass tubing was also used to minimize the diffusion of oxygen into the influent and effluent solutions. Although the influent (or reagent) solutions were forced through the columns by gravity with a fixed hydraulic head (∆H), average flow rates of the columns varied from 22.8 to 58.4 mL/h. These fluctuations in flow rate were primarily the result of H2 gas formation in the columns. Flow occasionally stopped but was resumed by applying a low suction at the effluent outlet. Flow rates were relatively high in comparison with most groundwater flow and were intended to simulate the effects of multiyear flows through a typical reactive barrier within several months of column operation (16, 39).
FIGURE 1. Experimental design of column flow-through system with a constant hydraulic head (∆H). A N2 gas flow (open to the atmosphere) and glass tubing were used to minimize oxygen in the influent solutions. Analyses. Chemical parameters monitored in the effluent included pH, ferrous ion (Fe2+), sulfide (S2-), total iron, and dissolved H2. Samples were withdrawn at intervals by attaching a 10-mL plastic syringe to the effluent outlet, thereby reducing oxygen exposure during sample collection. Effluent pH was determined using an Orion model 920A pH meter equipped with an Orion combination electrode (Orion Inc., Boston, MA). Ferrous and total iron concentrations were determined using the 1,10-phenanthroline method and FerroVer iron reagent, and sulfide concentration was determined with the methylene blue method (Hach DR/2000 Spectrophotometer Handbook, Loveland, CO). The analytical precision for Fe2+, Fe3+, and S2- were (0.006, (0.009, and (0.003 mg/L, respectively. Dissolved H2 was measured following the method of Istok et al. (40) with a detection limit of about 0.015 mg/L. Dissolved H2 was measured as %H2 saturation at 22 °C (approximately 1.58 mg/L) (41) following calibration with H2-saturated purified water. Effluent samples were also collected every 2 weeks for the determination of microbial population abundance (including sulfate reducers, heterotrophs, and acetogens/methanogens). Three-tube most probable number (MPN) determinations were conducted for microbial groups using 10-fold serial dilutions. Heterotrophs were enumerated using a dilute peptone-, tryptone-, yeast extract-, and glucose-containing medium reduced with 0.3 g/L cysteine-HCl and containing a trace metal solution and vitamins and were buffered with 2 mM HCO3- plus 2 mM phosphate (31). Acetogens and methanogens were enumerated in a medium containing 10 mg of yeast extract and 10 mM methanol under a 95%/5% N2/H2 atmosphere. Sulfate reducers were enumerated in a medium containing 10 mg/L yeast extract, 10 mM lactate, and 40 mM SO42- under a 95%/5% N2/H2 atmosphere (42). VOL. 33, NO. 13, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Effluent pH variations of the Fe0-packed columns (C1C6) over a period of ∼5 months. The influent solutions contained SO42- only for C4 and C5 and a mixture of SO42- and HCO3- for the other columns (Table 1). Microbial population data were reported to the nearest order of magnitude. At the completion of the flow-through experiments, the columns were disassembled in a glovebox under a N2 atmosphere and sectioned for geochemical and microbiological analyses. A subportion of wet Fe0 filings was prepared for mineralogical analysis by rapid drying using an acetone rinse to minimize oxidation. The residual Fe0 filings were then examined by scanning electron microscopy (SEM) with energy-dispersive X-ray (EDX) analyses (JEOL JSM-35CF SEM) for surface morphology and elemental composition. Samples were placed on carbon stubs and sputtered with carbon to prevent electrical charging during the SEM and EDX analyses. Additionally, precipitated minerals in Fe0 filings were separated by sonification in acetone, filtered, and characterized by X-ray diffraction (XRD) analysis (Scintag XDS-2000 diffractometer) (11). Total sulfur and carbonate-C contents in Fe0 filings were analyzed by Huffman Laboratories (Golden, CO). Microbiological population densities were analyzed using MPN techniques on separate subsamples as described in the previous paragraph.
Results and Discussion pH and Ferrous Iron Concentration. The effluent pH of all experimental columns increased to about 8.8-9.8 from its initial influent pH of about 7 in the SO42--only columns and about of 9 in the HCO3- systems (Figure 2). This observation was anticipated because of corrosion of Fe0. Under anaerobic conditions, Fe0 reacts with water according to the following reaction (5):
Fe0 + 2H2O f Fe2+ + H2 + 2OH-
(1)
In the presence of oxygen, Fe0 corrodes according to the reaction:
Fe0 + H2O + 1/2O2 f Fe2+ + 2OH-
(2)
Both of these reactions result in an increased solution pH as 2 mol of OH- is formed per mole of Fe0 oxidized. The influent ion composition appeared to influence the effluent pH (Figure 2). For those N2-purged columns with HCO3- in the influent (columns 1-3 and 6), the effluent pH 2172
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FIGURE 3. Eh-pH diagram (a) in equilibrium with amorphous ferric hydroxide [Fe(OH)3], ferrous carbonate, and ferrous hydroxide [Fe(OH)2] in 17 mM HCO3- and 10-3 mM Fe and (b) in equilibrium with crystalline iron oxyhydroxides (such as hematite, goethite, or lepidocrocite) as the system (a) ages. Because the amorphous ferric hydroxide is more soluble than the crystalline iron oxyhydroxides, the siderite region shrinks considerably and the Fe(OH)2 region may be substituted by magnetite. remained relatively high (between 8.5 and 9.8) throughout the experiment. This occurred despite the fact that the influent pH of the HCO3- or HCO3-/SO42- solution was adjusted to about 7 after ∼20 L of the influent solution was passed through each column (after ∼2-3 weeks). The effluent pH in N2-purged columns with only SO42- in solution (columns 4 and 5), however, decreased to between 7 and 8.5 after about 2 weeks (Figure 2). These results suggested that, although both SO42- and HCO3- are known to be corrosive to Fe0 (24, 29, 43), HCO3- ions appeared to enhance the Fe0 corrosion rate and resulted in an overall higher pH and a higher dissolved H2 content in the effluent of these systems. Although corrosion of Fe0 in aqueous solution results in the formation of ferrous iron (Fe2+) (reactions 1 and 2), soluble iron species detected in the column effluents were low or below detection limit. The Fe2+ concentrations in the effluents of the HCO3- -containing systems (columns 1-3 and 6) were generally less than 0.05 mg/L and remained at less than 0.4 mg/L in columns without HCO3- in solution (columns 4 and 5). The low iron concentrations observed were consistent with removal of Fe2+ by precipitation of ferrous carbonate and hydroxides within the columns under high-HCO3- and high-pH conditions (25, 26, 41). Both FeCO3 and Fe(OH)2 are relatively insoluble, with solubility products of 3.07 × 10-11 and 4.87 × 10-17, respectively (41). On the basis of these solubility products at pH 8, Fe2+ concentrations would be expected to be less than 0.01 mg/L in the HCO3- systems and less than 3 mg/L in SO42- systems. Examination of an Eh-pH diagram (Figure 3) indicated that siderite (FeCO3) would be the dominant form of Fe2+ precipitates between pH 7.4 and pH 11.1 assuming that only amorphous ferrous and ferric hydroxides [Fe(OH)2 and Fe(OH)3] may coexist in the system. However, as ferrous and ferric hydroxides age and form crystalline minerals, such as magnetite or hematite, the siderite region (Figure 3b) would shrink considerably from systems in Figure 3a because of the extremely low solubility of these crystalline minerals. The formation of siderite in the first several weeks was easily observed in the columns 1-3 and 6 as Fe0 grains turned into white-grayish color. Note that siderite was suspected to be the dominant carbonate precipitate in the systems because no other bi- or trivalent cations were added in the influent solution. Similar observations were reported in a study of the mechanisms of oxide film formation on Fe0 by Raman spectroscopy (25, 26). Hydrogen Gas Release and HCO3- Content. Corrosion of Fe0 in water generates H2 gas, particularly under anaerobic conditions (reaction 1). In the presence of HCO3- (columns
FIGURE 4. Mean of dissolved H2 concentration in the effluent of Fe0-packed columns between weeks of 0-2, 3-5, and 6-22. Dissolved H2 increased in the first 5 weeks in most of the columns but decreased substantially thereafter in columns 1 and 3. 1-3 and 6), the corrosion of Fe0 metal was substantially enhanced. Within the first 5 weeks (Figure 4), the enhancement resulted in 4-10-fold higher levels of dissolved H2 in the HCO3- -containing effluents (columns 1-3 and 6) as compared to the SO42--only effluent solutions (columns 4 and 5). Apparently the Fe0 corrosion rates differed significantly in these two systems with different anionic compositions. Gui and Devine (24, 28) also observed that HCO3- ions were particularly effective at enhancing the corrosion rate of Fe0. They indicated that, regardless of the anions present, the passivated films of iron were similar and consisted of a mixture of Fe(OH)2 and Fe3O4/Fe2O3. It was proposed (44) that HCO3- and CO32- ions can form soluble complexes [e.g., FeHCO3+ and Fe(CO3)22-] with Fe2+ (45), accelerate the removal of the protective layer on Fe0 by active dissolution of the passivated oxide film, and therefore increase the corrosion rate. Similarly, Agrawal and Tratnyek (30, 46) reported an enhanced corrosion of Fe0 in carbonate solutions. However, high levels of carbonate concentration resulted in an accumulation of FeCO3 precipitates, which reduced the degradation rates of nitro-organic compounds. Acidic conditions are also known to enhance the corrosion of zero-valent metals and the generation of larger quantities of H2 gas (29, 30, 32). This behavior is consistent with increases in dissolved H2 gas in the HCO3- systems observed between 3 and 5 weeks, following the adjustment of influent solution pH to ∼7. Carbonic acid (H2CO3) is extremely corrosive to Fe0, with H2 gas being one of the byproducts generated (24, 26, 29). Rapid corrosion of Fe0 was observed in column 7 (Table 1) in which the influent solution was purged with carbon dioxide to generate carbonic acid. As a result, up to 600 mg/L of ferrous iron was observed in the column effluent. The accumulation of H2 gas in the column frequently caused the flow through the column to cease; as a result, this column was abandoned after approximately 2 days of operation. Following initial increases, dissolved H2 levels in nonautoclaved HCO3-/SO42- systems (columns 1 and 3) decreased rapidly (from 6 to 22 weeks, Figure 4). Because dissolved H2 remained elevated (∼80-90% of saturation) in the autoclaved HCO3- /SO42- control (column 2), H2 reduction in columns 1 and 3 could possibly be attributed to the biological utilization of H2 (33, 47) as will be discussed. Microbial Activity, Hydrogen Consumption, and SO42Reduction. Enumeration of microorganisms in effluent samples indicated that the highest frequency of microbial detection (both heterotrophs and sulfate reducers) occurred in effluent samples from column 3, followed by column 1 (data not shown). These columns contained both HCO3and SO42- and were not autoclaved. The frequency of
FIGURE 5. Sulfide and dissolved H2 concentrations in the effluents of columns C3 and C2 (autoclaved). Dissolved H2 content decreased after ∼6 weeks with a concomitant increase in effluent sulfide concentration. A sand/sludge mixture was used to provide a diversified microbial community at the beginning of the experiment.
TABLE 2. Microbial Populations (log10 cfu/g solids)a in Iron Residues at the Completion of Column Experiment (Samples Collected between 11/13/97 and 11/19/97) position
column 1
top middle bottom
6 5 6
top middle bottom
4 5 >6
column 2
column 3
column 4
column 5
column 6
Total Heterotrophs 2 5 0 2 >6 0 2 >6 4
3 2 4
5 0 6
SO42- Reducers >6 2 >6 0 >6 0
6 5 >6
5 3 5
0 0 0
a Microbial populations are presented as logarithms of the most probable number (MPN) of microorganisms/g of column material. Three numbers represent analyses of sections collected at the top, middle, and bottom of each column, respectively.
detection of these microorganisms appeared to correspond to chemical changes in the column effluents. In particular, the disappearance of hydrogen from and the appearance of sulfide in the effluent from column 3 (Figure 5) appears to be consistent. A similar pattern occurred for column 1 (data not shown). In contrast, H2 levels remained relatively high, and low to nondetectable levels of sulfide were observed in the autoclaved control column 2 (Figure 5). These data support the hypothesis that the reduced hydrogen levels observed in columns 1 and 3 after approximately 6 weeks were due to microbial utilization. Previous studies also indicated that a high dissolved H2 and ferrous iron concentration may favor growth of certain microorganisms, such as methanogens, that can metabolize these substances (33, 47). Weathers et al. (33) and Novak et al. (47) found that methanogens could utilize dissolved H2 for accelerating the degradation of certain environmental contaminants, such as carbon tetrachloride and chloroform. Analyses of microbial MPNs in solid materials collected at the end of the column experiment (Table 2) also suggest the potential for microbial utilization of H2. Heterotrophs and SO42- reducers were present in the initial sludge/sand inoculum at levels >108/g. By the end of the experiment, populations of sulfate reducers were present throughout the unsterilized columns. Levels ranged from 104 to 106/g throughout columns 1, 3, and 5 and from 103 to 106/g in column 6 (Table 2). In contrast, levels measured in autoclaved controls were far lower, ranging from
