Biogeochemical Dynamics inZero-Valent Iron Columns:Implications for Permeable ReactiveBarriersB . G U , * , † T . J . P H E L P S , † L . L I A N G , †

M . J . D I C K E Y , ‡ Y . R O H , ‡ B . L . K I N S A L L , ‡

A . V . P A L U M B O , † A N D G . K . J A C O B S †

Environmental Sciences Division, Oak Ridge NationalLaboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, andOak Ridge Institute for Science and Education, P.O. Box 2007,Oak Ridge, Tennessee 37831

The impact of microbiological and geochemical processeshas been a major concern for the long-term performanceof permeable reactive barriers containing zero-valent iron(Fe0). To evaluate potential biogeochemical impacts,laboratory studies were performed over a 5-month periodusing columns containing a diverse microbial community.The conditions chosen for these experiments were designedto simulate high concentrations of bicarbonate (17-33mM HCO3

-) and sulfate (7-20 mM SO42-) containing

groundwater regimes. Groundwater chemistry was foundto significantly affect corrosion rates of Fe0 filings and resultedin the formation of a suite of mineral precipitates. HCO3

-

ions in SO42--containing water were particularly corrosive

to Fe0, resulting in the formation of ferrous carbonateand enhanced H2 gas generation that stimulated the growthof microbial populations and increased SO4

2- reduction.Major mineral precipitates identified included lepidocrocite,akaganeite, mackinawite, magnetite/maghemite, goethite,siderite, and amorphous ferrous sulfide. Sulfide was formedas a result of microbial reduction of SO4

2- that becamesignificant after about 2 months of column operations. Thisstudy demonstrates that biogeochemical influences onthe performance and reaction of Fe0 may be minimal in theshort term (e.g., a few weeks or months), necessitatinglonger-term operations to observe the effects of bio-geochemical reactions on the performance of Fe0 barriers.Although major failures of in-ground treatment barriers havenot been problematic to date, the accumulation of ironoxyhydroxides, carbonates, and sulfides from biogeochemicalprocesses could reduce the reactivity and permeabilityof Fe0 beds, thereby decreasing treatment efficiency.

IntroductionRemediation of groundwater contaminated with chlorinatedorganics, heavy metals, and radionuclides using zero-valentiron (Fe0) filings has received considerable attention in recentyears (1-12). Whereas the mechanisms for degrading orimmobilizing these contaminants by Fe0 are not completelyunderstood (13-15), it has been shown that Fe0 can be very

effective at groundwater remediation. Consequently Fe0-based reactive barrier treatment has been generating sig-nificant interest for passive, long-term applications forgroundwater remediation (5-12, 16, 17).

A potential limitation of the Fe0 technology is thedeterioration of the Fe0 materials by corrosion and thesubsequent precipitation of minerals that may cause ce-mentation and decreased permeability of the Fe0 barrier.Few studies are available concerning long-term performancecharacteristics of Fe0-based barriers (1, 16, cf. ref 12). However,data indicate that flow restriction could occur under certainbiogeochemical conditions (16, 18, 19). Liang et al. (18)observed that flow rate decreased over a 6-month periodthrough a series of Fe0-filled canisters used for treatingtrichloroethylene-contaminated groundwater at the Ports-mouth Gaseous Diffusion Plant (Piketon, OH). Post-analysisof the Fe0 filings showed cementation of the iron grains,possibly as a result of precipitation of iron sulfides, oxyhy-droxides, and carbonates. Clogging has also been reportedin laboratory and pilot-scale studies with Fe0 filings as reactivemedia (16, 20, 21). For example, at the Lowry Air Force Base(AFB) in Denver, CO, and at Elizabeth City, NC, sites, greenrusts (i.e., a mixture of partially reduced/oxidized ironoxyhydroxides and sulfate) were observed in barrier materials(22). At the Hill AFB, UT, site, precipitation of iron and calciumcarbonates was concluded to be responsible for a 14%porosity reduction within a few months of operation (23). Incontrast, mineral precipitation was not observed after 1 yearof operation in a reactive barrier at the Borden, Ontario, site(12).

It is recognized that groundwater chemistry plays asignificant role in determining rates of mineral precipitationand barrier clogging and influencing the rates and extent ofmicrobial impacts (5, 12). The roles of dissolved oxygen andpH in determining Fe0 reactivity and precipitation chemistryare well established. On the other hand, influences of othergroundwater constituents, such as HCO3

- and SO42-, are less

well defined. Because SO42- and HCO3

- are both corrosiveto Fe0 (24-30) and are commonly found in groundwater atcontaminated sites, these anions are of particular significancein influencing biotic and abiotic barrier-clogging processes.Both anions promote corrosion of Fe0 by disrupting theprotective oxide layers (24), thus facilitating continued anodicdissolution of the iron and hydrogen generation (31, 32).Furthermore, HCO3

- and hydrogen may serve as excellentcarbon and energy sources facilitating microbially influencediron corrosion (31-34).

Little is known, however, about the impacts of microbialactivities on Fe0 barrier performance. Field evidence for theenhancement of microbial populations as a result of Fe0

barrier corrosion is lacking. Microbial populations were notobserved to increase in Fe0 barriers in Sunnyvale and MoffettField in California and in an industrial site in New York (17).Similarly, microbial activities were found to be low or notobserved at the Lowry AFB or Somersworth, NH, sites (17).In contrast, biofouling was observed in an Fe0 foam/sandreactive barrier in Newbury Park, CA (17), and biofoulingoccurred rapidly in a filter column at the Portsmouth, OH,site (18). At the Portsmouth site, sulfate-reducing bacteriawere detected in water samples and in Fe0 filings aftertreatment. Microorganisms that can utilize SO4

2- as a terminalelectron acceptor producing sulfide are widely distributed(35-37) and are therefore of particular importance in themicrobially mediated mineral precipitation and clogging ofFe0 barriers. Decreases in groundwater SO4

2- concentrationduring transport through Fe0 barriers have been observed at

* Corresponding author phone: (423)574-7286; fax: (423)576-8543;e-mail: gub1@ornl.gov.

† Oak Ridge National Laboratory.‡ Oak Ridge Institute for Science and Education.

Environ. Sci. Technol. 1999, 33, 2170-2177

2170 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 13, 1999 10.1021/es981077e CCC: $18.00 1999 American Chemical SocietyPublished on Web 05/11/1999

the Moffett Field and Lowry AFB sites; at an installation inOak Ridge TN; and at a U.S. Coast Guard Fe0 barrier atElizabeth City, NC (17).

The objective of the present study was to evaluate thepotential role of microorganisms in conjunction withgeochemical reaction mechanisms in affecting the long-termeffectiveness of Fe0 barriers. Of particular concern were thecombined effects of HCO3

- and SO42- on Fe0 corrosion,

mineral precipitation, and microbial activity in the Fe0

medium. Laboratory Fe0 column studies were conductedunder conditions designed to simulate high concentrationsof HCO3

- - and SO42--containing groundwater regimes.

Results of the experiments were used to provide a rationalefor the potential roles of biogeochemical processes on Fe0

barriers and to identify data gaps for additional study.

Materials and MethodsExperimental Design. Seven glass columns (25 × 450 mm)were constructed containing a sludge/sand mixture at theinlet (bottom) of each column followed by Fe0 filings (PeerlessMetal Powders and Abrasives, Detroit, MI). The sludge wasobtained 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 mMphosphate and 0.05% glucose to promote microbial growthand were cultured for 2 days before use. Following theaddition of the sludge/sand mixture (40 mm in depth to thecolumns), Fe0 filings (sieved to 0.5-1 mm size) were addedin a layer of 350 mm thick, followed by 60 mm of sand (∼1mm in diameter) at the top. Two columns (2 and 4) wereinitially sterilized by autoclaving to better differentiate abioticreactions from microbial processes (Table 1).

Influent solution compositions and other conditionsvaried among the columns (Table 1). Each influent solutionwas either sodium sulfate (20 mM SO4

2-), sodium bicarbonate(∼33mM HCO3

-), or a mixture of Na2SO4 and NaHCO3 (7-10 mM SO4

2- and ∼17 mM HCO3-) solutions. After ∼20 L of

each influent solution was passed through the columns, thepH of each influent solution was adjusted to about neutral(using H2SO4) in an attempt to facilitate microbial growth inthe column. Each influent solution was fed to a column bygravity siphon under a constant head, with flow from bottomto top (Figure 1). In columns 1-5, the influent solutions wereinitially purged with N2 to achieve a low dissolved oxygencondition. Glass tubing was also used to minimize thediffusion of oxygen into the influent and effluent solutions.Although the influent (or reagent) solutions were forcedthrough the columns by gravity with a fixed hydraulic head(∆H), average flow rates of the columns varied from 22.8 to58.4 mL/h. These fluctuations in flow rate were primarily theresult of H2 gas formation in the columns. Flow occasionallystopped but was resumed by applying a low suction at theeffluent outlet. Flow rates were relatively high in comparisonwith most groundwater flow and were intended to simulatethe effects of multiyear flows through a typical reactive barrierwithin several months of column operation (16, 39).

Analyses. Chemical parameters monitored in the effluentincluded pH, ferrous ion (Fe2+), sulfide (S2-), total iron, anddissolved H2. Samples were withdrawn at intervals byattaching 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 pHmeter equipped with an Orion combination electrode (OrionInc., Boston, MA). Ferrous and total iron concentrations weredetermined using the 1,10-phenanthroline method andFerroVer iron reagent, and sulfide concentration was de-termined with the methylene blue method (Hach DR/2000Spectrophotometer Handbook, Loveland, CO). The analyticalprecision for Fe2+, Fe3+, and S2- were (0.006, (0.009, and(0.003 mg/L, respectively. Dissolved H2 was measuredfollowing the method of Istok et al. (40) with a detectionlimit 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 thedetermination of microbial population abundance (includingsulfate reducers, heterotrophs, and acetogens/methanogens).Three-tube most probable number (MPN) determinationswere conducted for microbial groups using 10-fold serialdilutions. Heterotrophs were enumerated using a dilutepeptone-, tryptone-, yeast extract-, and glucose-containingmedium reduced with 0.3 g/L cysteine-HCl and containinga trace metal solution and vitamins and were buffered with2 mM HCO3

- plus 2 mM phosphate (31). Acetogens andmethanogens were enumerated in a medium containing 10mg of yeast extract and 10 mM methanol under a 95%/5%N2/H2 atmosphere. Sulfate reducers were enumerated in amedium containing 10 mg/L yeast extract, 10 mM lactate,and 40 mM SO4

2- under a 95%/5% N2/H2 atmosphere (42).

TABLE 1. Influent Chemical Composition and General Experimental Conditions

column 1 column 2 column 3 column 4 column 5 column 6 column 7

HCO3- (mM)a 16.4 32.8 32.8 16.4 16.4

SO42- (mM) 10.4 ∼7.3 ∼7.3 20.8 20.8 10.4 10.4

autoclaved no yes no yes no no nopurge gas N2 N2 N2 N2 N2 air CO2average flow rate (mL/h) 27.5 23 58.4 31.1 36.1 22.8 ba Added HCO3

- concentration. The actual HCO3- concentration was slightly lower because of an initial purging with N2 at pH ∼9. b Column 7

was not monitored after 2 days of operation because of excessive H2 generation that caused flow to stop frequently.

FIGURE 1. Experimental design of column flow-through systemwith a constant hydraulic head (∆H). A N2 gas flow (open to theatmosphere) and glass tubing were used to minimize oxygen in theinfluent solutions.

VOL. 33, NO. 13, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2171

Microbial population data were reported to the nearest orderof magnitude.

At the completion of the flow-through experiments, thecolumns were disassembled in a glovebox under a N2

atmosphere and sectioned for geochemical and microbio-logical analyses. A subportion of wet Fe0 filings was preparedfor mineralogical analysis by rapid drying using an acetonerinse to minimize oxidation. The residual Fe0 filings werethen examined by scanning electron microscopy (SEM) withenergy-dispersive X-ray (EDX) analyses (JEOL JSM-35CF SEM)for surface morphology and elemental composition. Sampleswere placed on carbon stubs and sputtered with carbon toprevent electrical charging during the SEM and EDX analyses.Additionally, precipitated minerals in Fe0 filings were sepa-rated by sonification in acetone, filtered, and characterizedby X-ray diffraction (XRD) analysis (Scintag XDS-2000 dif-fractometer) (11). Total sulfur and carbonate-C contents inFe0 filings were analyzed by Huffman Laboratories (Golden,CO). Microbiological population densities were analyzedusing MPN techniques on separate subsamples as describedin the previous paragraph.

Results and DiscussionpH and Ferrous Iron Concentration. The effluent pH of allexperimental columns increased to about 8.8-9.8 from itsinitial influent pH of about 7 in the SO4

2--only columns andabout of 9 in the HCO3

- systems (Figure 2). This observationwas anticipated because of corrosion of Fe0. Under anaerobicconditions, Fe0 reacts with water according to the followingreaction (5):

In the presence of oxygen, Fe0 corrodes according to thereaction:

Both of these reactions result in an increased solution pH as2 mol of OH- is formed per mole of Fe0 oxidized.

The influent ion composition appeared to influence theeffluent pH (Figure 2). For those N2-purged columns withHCO3

- in the influent (columns 1-3 and 6), the effluent pH

remained relatively high (between 8.5 and 9.8) throughoutthe experiment. This occurred despite the fact that theinfluent pH of the HCO3

- or HCO3-/SO4

2- solution wasadjusted to about 7 after ∼20 L of the influent solution waspassed through each column (after ∼2-3 weeks). The effluentpH in N2-purged columns with only SO4

2- in solution(columns 4 and 5), however, decreased to between 7 and 8.5after about 2 weeks (Figure 2). These results suggested that,although both SO4

2- 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 ahigher dissolved H2 content in the effluent of these systems.

Although corrosion of Fe0 in aqueous solution results inthe formation of ferrous iron (Fe2+) (reactions 1 and 2), solubleiron species detected in the column effluents were low orbelow detection limit. The Fe2+ concentrations in the effluentsof the HCO3

- -containing systems (columns 1-3 and 6) weregenerally less than 0.05 mg/L and remained at less than 0.4mg/L in columns without HCO3

- in solution (columns 4 and5). The low iron concentrations observed were consistentwith removal of Fe2+ by precipitation of ferrous carbonateand hydroxides within the columns under high-HCO3

- andhigh-pH conditions (25, 26, 41). Both FeCO3 and Fe(OH)2 arerelatively insoluble, with solubility products of 3.07 × 10-11

and 4.87 × 10-17, respectively (41). On the basis of thesesolubility products at pH 8, Fe2+ concentrations would beexpected to be less than 0.01 mg/L in the HCO3

- systems andless than 3 mg/L in SO4

2- systems. Examination of an Eh-pHdiagram (Figure 3) indicated that siderite (FeCO3) would bethe dominant form of Fe2+ precipitates between pH 7.4 andpH 11.1 assuming that only amorphous ferrous and ferrichydroxides [Fe(OH)2 and Fe(OH)3] may coexist in the system.However, as ferrous and ferric hydroxides age and formcrystalline minerals, such as magnetite or hematite, thesiderite region (Figure 3b) would shrink considerably fromsystems in Figure 3a because of the extremely low solubilityof these crystalline minerals. The formation of siderite in thefirst several weeks was easily observed in the columns 1-3and 6 as Fe0 grains turned into white-grayish color. Note thatsiderite was suspected to be the dominant carbonateprecipitate in the systems because no other bi- or trivalentcations were added in the influent solution. Similar observa-tions were reported in a study of the mechanisms of oxidefilm formation on Fe0 by Raman spectroscopy (25, 26).

Hydrogen Gas Release and HCO3- Content. Corrosion

of Fe0 in water generates H2 gas, particularly under anaerobicconditions (reaction 1). In the presence of HCO3

- (columns

FIGURE 2. Effluent pH variations of the Fe0-packed columns (C1-C6) over a period of ∼5 months. The influent solutions containedSO4

2- only for C4 and C5 and a mixture of SO42- and HCO3

- for theother columns (Table 1).

Fe0 + 2H2O f Fe2+ + H2 + 2OH- (1)

Fe0 + H2O + 1/2O2 f Fe2+ + 2OH- (2)

FIGURE 3. Eh-pH diagram (a) in equilibrium with amorphous ferrichydroxide [Fe(OH)3], ferrous carbonate, and ferrous hydroxide[Fe(OH)2] in 17 mM HCO3

- and 10-3 mM Fe and (b) in equilibriumwith crystalline iron oxyhydroxides (such as hematite, goethite, orlepidocrocite) as the system (a) ages. Because the amorphous ferrichydroxide is more soluble than the crystalline iron oxyhydroxides,the siderite region shrinks considerably and the Fe(OH)2 regionmay be substituted by magnetite.

2172 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 13, 1999

1-3 and 6), the corrosion of Fe0 metal was substantiallyenhanced. Within the first 5 weeks (Figure 4), the enhance-ment resulted in 4-10-fold higher levels of dissolved H2 inthe HCO3

- -containing effluents (columns 1-3 and 6) ascompared to the SO4

2--only effluent solutions (columns 4and 5). Apparently the Fe0 corrosion rates differed signifi-cantly in these two systems with different anionic composi-tions. Gui and Devine (24, 28) also observed that HCO3

- ionswere particularly effective at enhancing the corrosion rate ofFe0. They indicated that, regardless of the anions present,the passivated films of iron were similar and consisted of amixture 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)2

2-] with Fe2+ (45), accelerate theremoval of the protective layer on Fe0 by active dissolutionof the passivated oxide film, and therefore increase thecorrosion rate. Similarly, Agrawal and Tratnyek (30, 46)reported an enhanced corrosion of Fe0 in carbonate solutions.However, high levels of carbonate concentration resulted inan accumulation of FeCO3 precipitates, which reduced thedegradation rates of nitro-organic compounds.

Acidic conditions are also known to enhance the corrosionof zero-valent metals and the generation of larger quantitiesof H2 gas (29, 30, 32). This behavior is consistent with increasesin dissolved H2 gas in the HCO3

- systems observed between3 and 5 weeks, following the adjustment of influent solutionpH to ∼7. Carbonic acid (H2CO3) is extremely corrosive toFe0, 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 withcarbon dioxide to generate carbonic acid. As a result, up to600 mg/L of ferrous iron was observed in the column effluent.The accumulation of H2 gas in the column frequently causedthe flow through the column to cease; as a result, this columnwas abandoned after approximately 2 days of operation.

Following initial increases, dissolved H2 levels in non-autoclaved HCO3

-/SO42- systems (columns 1 and 3) de-

creased rapidly (from 6 to 22 weeks, Figure 4). Becausedissolved H2 remained elevated (∼80-90% of saturation) inthe autoclaved HCO3

- /SO42- control (column 2), H2 reduc-

tion in columns 1 and 3 could possibly be attributed to thebiological utilization of H2 (33, 47) as will be discussed.

Microbial Activity, Hydrogen Consumption, and SO42-

Reduction. Enumeration of microorganisms in effluentsamples indicated that the highest frequency of microbialdetection (both heterotrophs and sulfate reducers) occurredin effluent samples from column 3, followed by column 1(data not shown). These columns contained both HCO3

-

and SO42- and were not autoclaved. The frequency of

detection of these microorganisms appeared to correspondto chemical changes in the column effluents. In particular,the disappearance of hydrogen from and the appearance ofsulfide in the effluent from column 3 (Figure 5) appears tobe consistent. A similar pattern occurred for column 1 (datanot shown). In contrast, H2 levels remained relatively high,and low to nondetectable levels of sulfide were observed inthe autoclaved control column 2 (Figure 5). These datasupport the hypothesis that the reduced hydrogen levelsobserved in columns 1 and 3 after approximately 6 weekswere due to microbial utilization. Previous studies alsoindicated that a high dissolved H2 and ferrous iron concen-tration may favor growth of certain microorganisms, such asmethanogens, that can metabolize these substances (33, 47).Weathers et al. (33) and Novak et al. (47) found thatmethanogens could utilize dissolved H2 for accelerating thedegradation of certain environmental contaminants, suchas carbon tetrachloride and chloroform.

Analyses of microbial MPNs in solid materials collectedat the end of the column experiment (Table 2) also suggestthe potential for microbial utilization of H2. Heterotrophsand SO4

2- reducers were present in the initial sludge/sandinoculum at levels >108/g. By the end of the experiment,populations of sulfate reducers were present throughout theunsterilized columns. Levels ranged from 104 to 106/gthroughout columns 1, 3, and 5 and from 103 to 106/g incolumn 6 (Table 2). In contrast, levels measured in autoclavedcontrols were far lower, ranging from <1 to 102/g. Theuniformity of MPN measurements within each column

FIGURE 4. Mean of dissolved H2 concentration in the effluent ofFe0-packed columns between weeks of 0-2, 3-5, and 6-22.Dissolved H2 increased in the first 5 weeks in most of the columnsbut decreased substantially thereafter in columns 1 and 3.

FIGURE 5. Sulfide and dissolved H2 concentrations in the effluentsof columns C3 and C2 (autoclaved). Dissolved H2 content decreasedafter ∼6 weeks with a concomitant increase in effluent sulfideconcentration. A sand/sludge mixture was used to provide adiversified microbial community at the beginning of the experiment.

TABLE 2. Microbial Populations (log10 cfu/g solids)a in IronResidues at the Completion of Column Experiment (SamplesCollected between 11/13/97 and 11/19/97)

positioncolumn

1column

2column

3column

4column

5column

6

Total Heterotrophstop 6 2 5 0 3 5middle 5 2 >6 0 2 0bottom 6 2 >6 4 4 6

SO42- Reducers

top 4 0 >6 2 6 5middle 5 0 >6 0 5 3bottom >6 0 >6 0 >6 5

a Microbial populations are presented as logarithms of the mostprobable number (MPN) of microorganisms/g of column material. Threenumbers represent analyses of sections collected at the top, middle,and bottom of each column, respectively.

VOL. 33, NO. 13, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2173

demonstrates that by the end of the experiment viable sulfate-reducing microbial communities had developed throughoutthe three non-autoclaved columns receiving influent con-taining SO4

2-. Columns 2 and 4 were no longer sterile by theend of the experiment; however, populations of bothheterotrophs and SO4

2- reducers were substantially lowerthan those in columns 1 and 3 by factors of 102-106 andtherefore still represented valid controls for microbial activityin these systems.

As noted previously, the appearance of sulfide in theeffluent of columns 1 and 3 (Figure 6) after approximately6 weeks also provided supporting evidence for microbialSO4

2- reduction in columns containing both HCO3- and

SO42-. These were the only columns that showed relatively

high amounts of sulfide in the effluent; in all other columns,sulfide was virtually nondetectable. Sulfide detected in thecolumn effluents may reflect the formation of relativelyinsoluble ferrous sulfide (FeS; Ksp ) 6.3 × 10-18) precipitates.Black precipitates or colloidal particles were occasionallyobserved in the column effluents; inadvertent inclusion ofone or more of these particles in the effluent aliquot analyzedwould explain the relatively large variations in sulfideconcentration in columns 1 and 3. Analysis of residual Fe0

filings at the end of the experiment (Table 3) indicatedsignificantly higher amounts of sulfide accumulation incolumns 1 and 3 (0.8 and 1.5 mg/g, respectively) than in theautoclaved control column 2 (Table 3). Sulfide-bearingminerals, such as amorphous FeS and mackinawite (Fe9S8),were also identified in columns 1 and 3 by the XRD and EDXanalyses (discussed in the following section).

Relatively high dissolved hydrogen concentrations incolumns treated with HCO3

- appeared to stimulate SO42-

utilization. Columns 4 and 5, which contained only SO42-,

did not show detectable amounts of sulfide in the effluents.One of these SO4

2--only columns had been autoclaved(column 4) and was not expected to generate sulfide. The

other SO42--only column (column 5), however, contained

between 105 and 106/g sulfate-reducing bacteria. In theabsence of sufficient hydrogen, therefore, it appears that theviable sulfate-reducing population was not able to reduceSO4

2- at a sufficient rate to generate detectable levels in theeffluent.

Measurements of SO42- reducers in the oxygenated

column 6 at the end of the experiment were 103-105/g (Table2), suggesting that even in the presence of dissolved oxygena viable SO4

2--reducing community had been established.This is not surprising because dissolved oxygen could berapidly consumed at the entrance to the column by reactingwith Fe0 (reaction 2), permitting the growth of anaerobicmicroorganisms despite the presence of an oxygenatedinfluent solution (48).

It is known that some SO42-reducers, such as Desulfovibrio

desulfuricans and Desulfovibrio gigas, can utilize molecularhydrogen for the reduction of SO4

2- with the consumptionof 4 mol of H2/mol of SO4

2- used as electron acceptor (reaction3) (49, 50):

Thus, microbial action is a potential explanation for the rapiddecrease in dissolved H2 concentration observed in columns1 and 3 between 6 and 21 weeks (Figure 4). This decrease inH2 occurred concomitantly with an increase in effluent sulfide(Figure 6) and observation of SO4

2-reducers in the effluentsfrom these two columns. After about 5 weeks, the bacterialpopulations in columns 1 and 3 were likely sufficient toconsume appreciable amounts of the H2 present. Also, theSO4

2--reducing populations were sufficiently dense to pro-duce measurable sulfide.

Total sulfur accumulated in columns 1 and 3 constitutedonly a small percentage (<0.8%) of total sulfur input (as SO4

2-)based on the highest sulfur content in the lower part ofcolumn 3 (i.e., 1.5 mg/g Fe, Table 3). Because SO4

2-

concentrations in the present study were intentionallyestablished at the upper end of expected groundwaterconcentrations, it is likely that the rate of SO4

2- reduction inthe present study was limited by factors other than SO4

2-

concentration. At more environmentally typical SO42- con-

centrations, the fraction of SO42- reduced would likely be

significantly greater, as would be consistent with observeddeclines of SO4

2- in groundwater during passage throughFe0 barriers or canisters at the Oak Ridge Y-12 Plant, MoffettField, Lowry AFB, Elizabeth City, and Portsmouth facilities(17). For example, SO4

2- concentration decreased more than50% within the Fe0 barrier at the Oak Ridge Y-12 site with aninitial SO4

2- concentration of approximately 100 mg/L(unpublished data). These results imply that these levels ofsulfide production in laboratory columns could have beenachieved or exceeded with much lower influent concentra-tions, depending on the residence time of SO4

2- in the Fe0

barrier. The residence time in the present column studieswas relatively short (on the order of 1.5-4 h) in comparisonwith that of the in situ Fe0 barriers (on the order of days).

Mineral Precipitation in Fe0 Filings. Mineral precipitateswere identified in Fe0 residues at the end of the column

TABLE 3. Total Sulfur and Carbonate-C Contents (in mg/g Fe) in Iron Medium after Completion of the Flow-Through Experiment

column 1 column 2 column 3 column 4 column 5 column 6

total Sa lower middle 0.8 0 1.5 bdb bdb ndc

upper middle bdb 0.2 bdb bdb 0.2 ndc

carbonatecarbona lower middle 2.2 5.6 3.3 <0.2 <0.2 ndc

upper middle 1.6 2.6 4.2 <0.2 <0.2 ndc

a Data reported after correction for sulfur and carbon measured in the unreacted iron filings (∼0.7 mg/g S and <0.2 mg/g C). Analytical precisionwas (0.2 mg/g. b bd, below detection limit. c nd, not determined.

FIGURE 6. Sulfide concentrations in the effluents of columns C1-C6. Only C1 and C3 showed a relatively high S2- concentration after∼6 weeks, whereas other columns showed a low or nondetectableamount of S2-. SO4

2- + 4H2 f S2- + 4H2O (3)

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experiment. Precipitates included iron oxyhydroxides (lepi-docrocite, akaganeite, magnetite, and goethite), green rusts,mackinawite (Fe9S8), and amorphous ferrous sulfide (FeS)and carbonate (FeCO3). Siderite was not detected by XRDanalysis, but visual observation suggested its presence as anamorphous form in the Fe0 filings as discussed previously.In addition, analysis of total carbonate-C in Fe0 filings revealedan accumulation of carbonate precipitates, particularly incolumns 2 and 3, which were treated with the highest HCO3

-

concentrations in the influent solutions (Table 3). Theaccumulated carbonate-C ranged from 2.6 to 5.6 mg/g aftercorrection for C content in unreacted Fe0 filings. A loweramount of carbonate-C (1.6-2.2 mg/g) was observed incolumn 1, which contained a lower HCO3

- concentration inthe influent solution. Again, FeCO3 is believed to be thedominant carbonate precipitate in the system because noother bi- or trivalent cations were added in the influentsolution.

Mackinawite (Fe9S8), lepidocrocite (γ-FeOOH), akaganeite(â-FeOOH), and magnetite/maghemite (Fe3O4) were amongthe most abundant iron oxyhydroxide minerals identified byXRD analysis. The relative abundance of these minerals variedslightly among columns (Table 4). More mackinawite andlepidocrocite were observed in columns 1-3 and 6, withHCO3

- in the influent solution than in columns 4 and 5 (whichcontained only SO4

2- in the influent solution). In contrast,

akaganeite appeared to be more abundant in columns 4 and5. However, lepidocrocite, akaganeite, and magnetite/maghemite could not be distinguished by SEM because ofthe fine crystal structure of these minerals. It is also notedthat lepidocrocite, akaganeite, and goethite have the samechemical composition (or formula) but different crystalstructures. The iron oxyhydroxide layer was about 10-20µm in thickness in general (observed by thin section SEM,data not shown). These results are consistent with Gui andDevine (24), who found that the passivated iron film consistedof a mixture of ferrous and ferric oxyhydroxides and thattheir composition depended on the specific anions presentin the solution. Similarly, Blowes and co-workers (3, 9)reported the presence of lepidocrocite, maghemite, goethite,and probably hematite in the corroded Fe0 filings in a batchstudy. It should be pointed out, however, that samplesprepared by these authors were air-dried and stored for 3months before analysis, which may partially explain thepresence of fully oxidized goethite and hematite minerals(9).

The exact mechanism of the formation of mackinawite(Fe9S8) is not clear because reduction of SO4

2- in theenvironment is generally considered to be microbiologicallymediated (51). To date, there is no direct evidence showingan abiotic reduction of SO4

2- by Fe0 although the reductionof sulfonic acid to sulfide by Fe0 was recently reported (27).

TABLE 4. Mineralogical Composition of Fe0 Corrosion Products in Treated Fe0 Filings (in the Order of Relative Abundance)a

position column 1 column 2 column 3 column 4 column 5 column 6

top Mk, Lp, Mg Mk, Lp Lp, Mk, Ak Ak, Mk, Mg Ak, Gr, Mg Lp, Mk, Mgupper middle Mk, Lp, Mg Mk, Lp Lp, Mk, Ak Ak, Mk, Mg Ak, Mg Lp, Mk, Mglower middle Mk, Lp, Mg Mk, Lp Lp, Mk, Ak Ak, Mk, Mg Ak, Mg Lp, Mk, Mgbottom Mk, Lp, Mg Mk, Lp Lp, Mk, Ak Ak, Mk, Mg Ak, Mg Mk, Goa Mk, mackinawite (Fe9S8); Lp, lepidocrocite (γ-FeOOH); Ak, akaganeite (â-FeOOH); Mg, magnetite (Fe3O4); Gr, green rust II (sulfate); Go, goethite

(R-FeOOH).

FIGURE 7. Amorphous sulfide identified by (a) SEM image of ferrous sulfide precipitates in column 1 and (b) by EDX analysis, whichindicated the presence of only Fe and S in the precipitates (circled). Note: Si was an impurity in the Fe0 filings.

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Relatively large quantities of amorphous sulfide (FeS) wereidentified in unsterilized columns 1 and 3 by SEM and EDXanalyses (Figure 7). The EDX spectrum of these precipitatesindicated the presence of only Fe and S (note that Si is animpurity within Fe0 filings). The adsorbed SO4

2- should bea minor contributor to the observed S content because Fe0

filings were washed with acetone and the pH of the HCO3-

system was relatively high (so that surfaces of iron oxyhy-droxides would have been negatively charged). These ob-servations again support the conclusion that the accumulatedS in columns 1 and 3 may be largely attributed to the microbialreduction of SO4

2- to sulfide (50-52).With exception of column 5, the contents of green rusts

were low in the systems and could not be detected by XRDanalysis. However, green rusts were commonly observed inpatches in most of the columns by SEM imaging. Green rustsare hexagonally shaped minerals (Figure 8) containing amixture of ferrous and ferric iron oxyhydroxides. The SO4

2-

green rust is also referred as green rust II (GR-II) with achemical formulation of [4Fe(OH)2‚2FeOOH‚FeSO4‚4H2O](53). The occurrence of these mixed ferrous and ferricoxyhydroxides is not surprising because corrosion productsof Fe0 could result in the formation of both ferrous and ferricoxyhydroxides. A continuous input of influent solutioncontaining a low but nonzero concentration of dissolvedoxygen would be expected to result in oxidation of someferrous ions to ferric species (even though the influentsolutions were initially purged with N2). Both SO4

2- andcarbonate green rusts were observed. The presence of SO4

2-

green rust in columns 4 and 5 was confirmed by both the

EDX analyses (Table 4). The EDX spectrum was similar tothat of Figure 7b, although a weaker elemental S peak wasidentified. Previous studies (24) showed that in mildly acidicSO4

2- solutions, SO42- ions were not only adsorbed on to the

surface of the passivated iron film but also covalently bondedand thus incorporated in the passivated film (presumably asGR-II minerals). The green rust in column 3 (Figure 8b) isattributed to carbonate green rust because neither S nor Cwere detected by the EDX analysis and the EDX system is notsensitive for C detection. The possible chemical formula ofthe carbonate green rust is [Fe4

II Fe2III(OH)12][CO3‚2H2O] (54).

Similarly, Scherer et al. (21) observed the formation of greenrusts on Fe0 particles in a column exposed to high concen-trations of CCl4 in laboratory. In the absence of dissolved O2,Odziemkowski et al. (26) found that the formation ofcarbonate green rust is thermodynamically unfavorable.

Summary and ImplicationsThe laboratory column studies demonstrated that waterchemistry, and in particular the presence of elevated HCO3

-

ions in SO42--containing water, may have a significant impact

on both geochemical and microbial processes that can occurin Fe0-based treatment barriers. Significant amounts of ironoxyhydroxide and carbonate precipitates were generated inthe high-HCO3

- systems in both the presence and absenceof substantial microbial populations. In addition to causingprecipitation of iron oxyhydroxide mineral and carbonateminerals, high concentrations of HCO3

- (with added SO42-)

enhanced the corrosion of Fe0 and H2 production by 4-10-fold over SO4

2--only influent solutions.Although the amount of sulfur and carbonate retained in

the columns represented only a small fraction of the totalsulfate and carbonate input to the system, the biogeochemi-cally mediated mineral precipitation (iron oxyhydroxides,carbonates, and sulfides) that occurred in the columnsdemonstrated the potential for plugging of the Fe0 material.These results are also consistent with previous studies, whichindicated a plugging and porosity reduction as a result ofprecipitation of Fe(OH)2 and FeCO3 in the iron medium (16,23, 39). Assuming that the carbonate retained in columns1-3 [3.2 mg of C (g of Fe)-1 on average] is in the form ofFeCO3, this mineral alone could account for a void volumereduction of ∼5% (estimated based on a porosity of Fe0 filingsof 0.5 and density of FeCO3 of 3.5 g cm-3). The precipitatesof iron oxyhydroxides and sulfides may account for anadditional ∼10% void volume reduction. Therefore, the totalporosity could be reduced to ∼0.42. Note that the actualporosity reduction could be even more because the abovemineral precipitates were not densely packed (as calculated)and would thus significantly impact the flow through the Fe0

medium. For this reason, the flow restriction could be evenworse when fine Fe0 filings are used, although the initial voidvolumes may be similar when either the coarse (e.g., -8 to25 mesh) or the fine (∼40 mesh) Fe0 filings are used. As anexample, flow inhibition through two Fe0 canisters (40-meshFe0 filings) at the Portsmouth facility were noticeable aftertreatment of ∼800 pore volumes in a less than 2-month period(18).

The present column studies also demonstrated theimportance of experimental duration on effective evaluationof biogeochemical processes that may impact Fe0 barriertreatment systems. Microbial populations required 30-50days to develop even with a heavy initial inoculation. If thestudies had been conducted for a shorter period or microbialinoculum had not been provided, declines in pH anddissolved hydrogen concentration and the appearance ofsulfide in column effluents may not have been detected. Inshort-term experiments, only the abiotic processes resultingfrom iron oxidation that dominated the initial behavior ofthe column systems would have been observed. To evaluate

FIGURE 8. Green rusts: (a) SEM images of hexagonal-shaped SO42--

green rust (GR-II) in column 1 and (b) carbonate green rust in column2.

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the long-term behavior of barrier materials, laboratory testsshould be designed of sufficient length of time and withappropriate biogeochemical challenges to ensure that mi-crobial populations have sufficient opportunity to developand that environmentally significant processes are repre-sented.

AcknowledgmentsWe are grateful to Dr. S. E. Herbes for his valuable contribu-tions to the preparation and editing of this manuscript, Dr.S. Y. Lee for his assistance in SEM and XRD analyses, and T.Pick for conducting part of the laboratory experiment.Funding for this research was supported by the Offices ofScience and Environmental Management (EM) of the U.S.Department of Energy (DOE). We especially thank the staffof the Office of Biological and Environmental Research andthe Subsurface Contaminants Focus Area (EM) for theircontinued support. Oak Ridge National Laboratory (ORNL)is managed by Lockheed Martin Energy Research Corp. forthe DOE under Contract DE-AC05-96OR22464. This isPublication No. 4874 of the Environmental Sciences Division,ORNL.

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Received for review October 19, 1998. Revised manuscriptreceived April 5, 1999. Accepted April 8, 1999.

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