geochemistry of prb

7/21/2019 Geochemistry of PRB

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Geochemis try of a PermeableReactive Barrier for Metals and AcidMine Drainage

S . G . B E N N E R , * , D . W . B L O W E S ,

W . D . G O U L D , R . B . H E R B E R T , J R . , , A N DC . J . P T A C E K

Department of Earth Sciences, University of Waterloo,Waterloo, Ontario, N2L 3G1 Canada, and CANMET,Natural Resources Canada, Ottawa, Ontario,K1A 0G1 Canada

A permeable reactive barrier, designed to remove metalsand generate alkalinity by promoting sulfate reductionand metal sulfide precipitation, was installed in August 1995into an aquifer containing effluent from mine tailings.Passage of groundwater through the barrier results in

striking improvement in water quality. Dramatic changesin concentrations of SO4 (decrease of 2000-3000 mg/L), Fe(decrease of 270-1300 mg/L), trace metals (e.g., Nidecreases 30 mg/L), and alkalinity (increase of 800-2700mg/L)are observed.Populations of sulfatereducing bacteriaare 10 000 times greater, and bacterial activity, asmeasured by dehydrogenase activity, is 10 times higherwithin the barrier compared to the up-gradient aquifer.Dissolved sulfide concentrations increase by 0.2-120 mg/L, and the isotope 34S is enriched relative to 32S in thedissolved phase SO42- within the barrier. Water chemistry,coupled with geochemical speciation modeling, indicatesthe pore water in the barrier becomes supersaturated withrespect to amorphous Fe sulfide. Solid phase analysis of

the reactive mixture indicates the accumulation of Femonosulfide precipitates. Shifts in the saturation states ofcarbonate, sulfate, and sulfide minerals and most of theobservedchangesinwaterchemistryinthebarrier anddown-gradient aquifer can be attributed, either directly orindirectly, to bacterially mediated sulfate reduction.

In t roduct ionDischarge of acidic effluent, often containing high concen-trations of toxic trace metals, from mines and mine waste isan intractable, worldwide environmental problem withestimated costs of treatment in the tensof billions of dollars.Theoxidationof residual sulfidemineralsin mines andmine

waste can produce acidic waters containing high concentra-tions of sulfate, Fe(II), and trace metals. The oxidation ofiron sulfide can be expressed as

This effluent often enters underlying and adjacent aquifers

where buffering by mineral dissolution raises the pH to 4-7(1). However, on discharge to the surface, the oxidation ofFe(II) to Fe(III) and the precipitation of ferric oxyhydroxidescan regenerate acidic conditions (pH < 3):

This generated acidity can mobilize toxic trace metals andadversely impact surface water ecosystems. Discharge frommines and mine waste can continue for decades, evencenturies (2). Conventional treatment by lime neutralizationproduces large volumes of metal-rich sludge and ofteninvolves long-term operating costs.

Sulfate reduction and metal sulfide precipitation havethepotential to remediate acidmine drainage (3, 4). In naturalsettings, sulfate reduction is mediated by sulfate reducingbacteria (SRB). Sulfate reducingbacteriaare phylogeneticallydiverse and utilize a variety of biochemical pathways toreduce sulfate by oxidation of organic carbon. Sulfatereducing bacteria utilize short chain organic carbon com-pounds or H2to reduce SO4and are often dependent on aconsortium of microorganisms to provide these substrates

(5). A simplified expression of the reduction of sulfate bylactate can be expressed as

Thereductionof SO4 andthe productionof dissolved sulfidespecies in the presence of dissolved metals can result inprecipitation of metal sulfide mineral phases:

Other potential contaminants, including As, Cd, Cu, Fe, Ni,Pb, and Zn, can also react with sulfide to form sparinglysoluble sulfideminerals. However,acidic waters canconsumegenerated alkalinity and limit the ability of SRB to promotethis reaction sequence (6). This limitation has hamperedattempts to utilize sulfate reduction to treat acidic surfacewater associated with mine drainage (7, 8). We have overcomethis obstacle by promoting sulfate reduction within theaquifer, prior to increased acidification of the effluent upondischarge to the surface. Treatment within the aquifer isaccomplished using a passive in situ permeable reactivebarrier composed of organic material(9). The organic carbonpromotes sulfate reduction and metal sulfide precipitationas the contaminatedgroundwaterflows throughthe structure.

The reactive barrier, containing a mixture of municipalcompost (20vol %),leafmulch (20%),woodchips (9%),gravel(50%),and limestone (1%), waskeyed to underlyingbedrockat thebase andoutcropping bedrockat thesides.The installedbarrier extends 20 m across the aquifer and is 3.5 m deep,

and the barrier is 4 m thick in the direction of flow. Zonesof sand, approximately 1 m thick, were also installed at theup- and down-gradient sides of the organic carbon mixture.These sand zones square off the barrier with the slopingsides of the trench, help to redistribute flow, and facilitateuniform sampling. A protective clay cap (30 cm) was placedover the barrier to minimize oxygen diffusion and flow ofinfiltrating water into the barrier. The barrier installation isdescribed in detail in Benner et al.(10). This barrier is thefirst full-scale reactive barrier in the world designed to treatmetals and acid mine drainage using reductive processes.The installation provides a unique opportunity to evaluate,under controlled conditions, changes in groundwater

* Corresponding author phone: (519)888-4567 ext. 3438; fax:(519)746-3882; e-mail: [email protected].

University of Waterloo. CANMET, Natural Resources Canada. Present address: Department of Geology and Geochemistry,

Stockholm University, S-106 91 Stockholm, Sweden.

2FeS2(s) + 7O2 + 2H2O w4SO42-+ 2Fe2+ + 4H+

4Fe2+ + O2 + 10H2O w4Fe(OH)3(s) + 8H+

2CH3CHOHCOO-+ 3SO4

2-+ 2H+ w6HCO3

-+ 3H2S

H2S + Fe2+wFeS + 2H+

Environ. Sci. Technol. 1999, 33,2793-2799

10.1021/es981040u CCC: $18.00 1999 American Chemical Society VOL. 33, NO. 16, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2793Published on Web 07/13/1999


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geochemistry. Though the barrier is an engineered system,geochemicalchanges closelymatch processes associated withsulfate reduction in natural settings such as sediments andaquifers.This paperdescribes geochemical, microbiological,and mineralogical processes occurring within the permeablereactive barrier and adjoining aquifer.

Site Description.A groundwater plume, containing highconcentrations of SO4(1000-5000 mg/L) and Fe (200-2000mg/L) and a pH of 4-6, flows from the inactive Nickel Rimtailings impoundment intoan adjoining alluvial aquifer (11-13). At the site of the installation, acidic surface water isrecharging the aquifer. Figure 1(a) shows the flow field forthe Nickel Rimaquiferprior to the reactivebarrierinstallation(10-12). The aquifer receives water that has entered theaquifer through the tailings impoundment as well as acidicwater infiltrating directly from the surface above the aquifer.

This flow regime results in water entering the barrier fromtwo distinct sources. The protective clay cap preventsinfiltration directly into the barrier, but acidic surface watercontinues to enter the aquifer on both the up- and down-gradient sides. Treated water exitingthe down-gradient sideof the barrier occupies only the lower half of the aquifer,whereas t he upper half of the aquifer contains acidic waterwhich has not undergone treatment. Groundwater velocityis approximately 15 m/a (10).

MethodsWater Sampling and Analysis. Monitoring wells wereinstalled in nests along a transect roughly parallel to

groundwater flow (Figure 1). Samples of groundwater werecollected in September 1995, June and September 1996, andJuly1997 (1,9, 12,and22 months afterinstallation). A detaileddescription of well installation, water sampling, and fieldand laboratory analysis can be found in Benner et al. (10).Additional analyses include dissolved organic carbon bycombustion and infrared detection (14) and sulfur isotopedetermination by thermal decomposition of barium sulfateand analysis on a mass spectrometer (15).

Solid-Phase Methods.Samples were collected using a 5cm diameter driven coring device. Cores were collected inSeptember 1996 for solid phase and mineralogical analysisand in September 1997 for bacterial analysis (Figure 1b).Cores, collected in aluminumcasing,were sealed withplasticcaps and refrigerated until analyzed.

The sulfate reducing bacteria were grown in a modifiedPostgate mediumC in 20mL serumbottles (16).The mediumhad the following composition in g/L: KH2PO4, 0.5; NH4Cl,1.0; Na2SO4, 4.5; CaCl22H2O, 0.04; MgSO47H2O, 0.06; Nalactate(60%), 2.92; Na acetate, 1.28; yeast extract, 1.0; FeSO47H2O,0.004;Na citrate2H2O, 0.3; andresaurzurin, 0.005 (pHadjusted to 7.5 using NaOH). Serum bottles containingmedium were autoclaved, and 1 g sediment was added toeach of five replicates. Inoculated sampleswere sequentiallydiluted and incubated in an anaerobic glovebox for 30 days.Positive growth of SRB was indicated by precipitation of Fe-sulfides. Values arereportedas mostprobable number (MPN)determinations (17). Triphenyl tetrazolium chloride is asubstrate for a number of nonspecific dehydrogenases

FIGURE1. Cross-sectional profilesofaquiferandreactivebarrier: (a)showsflowlinesfortheNickel Rimaquiferpriortotheinstallationofthereactivebarrierdeterminedbyfield-collecteddataandflowmodelingfromBain(2000),(b)samplewell andcorelocationsinaquiferand reactive barrier, and (c) locationof geochemically distinct zones along groundwater flow path.

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present in microflora and can be generally correlated withrespiratoryactivityand usedas an index of microbial activity(18). To determine dehydrogenase activity (DH), 5 g ofsediment was buffered with CaCO3(to a pH > 6), and 1.75mL of 0.5% 2,3,5-triphenyl tetrazoliumchloride anddistilledwater were added. The samples were incubated for 24 h andextracted sequentially with two 10 mL aliquots of methanoland filtered through Whatman no. 42 paper. The aliquotswere combined in a volumetric flask and made up t o 50 mLwith methanol. Opt ical density of the extract was measuredat 485 m. The enzyme activity assay was performed in

triplicate and averaged.Accumulations of monosulfides (operationally defined as

acidvolatile sulfides,AVS)and the AVSfraction plusdisulfidesand elemental sulfur (total reduced inorganic sulfur, TRS)were determined from core location no. 2 (Figure 1) withinthe organic mixture. The AVS and TRS extraction methodsare described in detail in Herbert et al. (19) and involve thereaction of samples with cold 6 M HCl and hot chromouschloride solutions (20), respectively. Following reaction theevolved H2S is trapped in a zinc acetate solution as ZnS anddeterminedby iodimetrictitration. A fieldemissionscanningelectron microscope employing energy-dispersive X-rayanalysis (SEM-EDX) was used to obtain high-magnificationimages andelemental compositions of black accumulationson a sampling tube from within the permeable reactive

barrier.Geochemical Modeling. The geochemical speciation/

mass transfer computer code MINTEQA2 (21), adjusted tobe consistent with the WATEQ4F database (22), was used toaidin the interpretationof aqueous geochemical data. Inputparameters were Al, Ca, Cl, Fe, K, Mg, Mn, Na, Ni, Si, SO4,Zn,alkalinity,and pH.For water collectedoutsidethe reactivebarrierinstallation,Ehwas input from field platinumelectrodemeasurements. Within the barrier, Ehwas not specified, andmeasured concentrations of dissolved S2- were input tocalculate sulfide mineral saturation indices.

Results and Discussion

A striking improvement in water quality is observed aftergroundwater enters the reactive barrier. Concentrations of

almost all measured dissolved species change(e.g.alkalinityincreases 800-2700 mg/L, SO4decreases 2000-3000 mg/L,Fe decreases 270-1300 mg/L, and Ni decreases 30 mg/L;Figures 2 and 3). These changes indicate that significantgeochemicaltransformationsare occurring withinthe barrier.

Four Distinct Zones. For the purpose of evaluatinggeochemical, mineralogical, and microbiological changesattributed to the barrier, the aquifer can be viewed as beingcomprised of four zones. (Figure 1c).The up-gradient aquifer(up-gradient zone) contains mine-waste effluent water.Thiswater first passes through the up-gradient sand fill (sandzone) andthen the organic carbon mixture(organic mixturezone). Treated water flows intothe lower portion of the down-gradient aquifer (down-gradient zone). Water chemistryanalyses obtained during September1996 from each vertical

well nest along the flow path were averaged (Figure 3).Although these zones are chemically distinct, there is somevariability from well point to well point within each zone(Figure 2).

Up-Gradient Zone. Bain et al. (11, 12) described theaquifer geochemistry priorto the reactivebarrier installation.Geochemical analysis and modeling in this study areconsistent with those results. The groundwater in the up-gradient zone is characterized by high concentrations of SO4(2500-5200 mg/L), Fe (250-1350 mg/L), and Ni (0.12-30mg/L), pH between 2.8 and 5.9 and


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(Figure 5). SEM-EDX analysis of black precipitates thataccumulated on sampling tubes within the barrier indicatean Fe-S composition with a 1:1 stoichiometry, while X-raydefraction (XRD) analysis confirmed that the precipitateswere poorly crystalline mackinawite (Fe(1+x)S)(Figure 6)(19).The precipitation of these sulfide phases provides a sink fordissolved Fe which decreases >85% to an average of 80mg/L. MINTEQA2 modeling indicates that the groundwateris near saturationwith respect to siderite (FeCO3) within theorganic mixture zone. The precipitation of siderite may alsocontribute to the observed decrease in Fe concentrations.

Decreased SO4concentrations result in a shift from nearequilibrium conditions for gypsum in the sand zone toundersaturated conditions in the organic mixture zone(Figure 4). Conversely, high alkalinity results in a shift incalcite stability from undersaturated in the sand zone tosupersaturated conditions in the organic mixture zone.Increased alkalinity also results in supersaturation withrespect to rhodochrosite (MnCO3). Precipitation of rhodo-chrosite, or a less crystalline precursor, can explain theobserved decrease in Mn concentrations. Mn may also beremoved by coprecipitation with FeS or as a discretemanganese sulfide phase (25). The increase in pH to anaverage 6.7 versus 6.2 in the sand zone can be attributed to

buffering between the carbonate and sulfide systems (26)and reflects the ability of this system to regulate pHconditions. Silica concentrationsincrease to an averagevalueof 27 mg/L (Figure 3). This increase may be the result ofenhancedsilicate dissolutionresulting frombacterialactivityon silicate mineral surfaces (27) or may reflect increased Sisolubility due to organic acid complexation (not accountedfor by MINTEQA2 modeling) (28).Both of these mechanismshave been observed in organic-rich groundwater environ-ments.

Concentrations of Ni decline to


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trace metalscan adsorb,coprecipitate, or form discretesulfidemineral phases under sulfate-reducing conditions (6, 25).

Barium concentrations increase from below analytical

detection (1500 mg/L) within the middlehalf than at the top and bottom of the barrier (50 mg/L and generally15 years (10). However, direct transfer of these resultsfrom the laboratory neglects the potential complexities thatexist in a field setting. Factors that may limit barrierperformance in the field include preferential flow or pref-erential mass flux and low temperatures inhibiting bacterialactivity. The long-term treatment capacity for the Nickel Rimreactive barrier will be determined by continued moni-toring.

AcknowledgmentsFunding for this research was provided by the CanadianNaturalSciences and Engineering ResearchCouncil (NSERC),Falconbridge Ltd. and the Ontario Centre for Research inEarthand Space Technology. The authors wouldlike to thankChris Hanton-Fong, Cheryl Sue, Lyne Lortic, Mike Moncur,

and Scott Sale for their assistance with the laboratory andfield work.

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(20) Canfield,D. E.;Raiswell, R.;Westrich,J. T.;Reaves,C. M.;Berner,R. A.Chem. Geol.1986, 54, 149-155.

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FIGURE6. Scanningelectronmicrographofblack precipitatesonsample tube within organic material zone identified by energydispersiveX-ray(EDX) as FeS.

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(22) Ball, J. W.; Nordstrom, D. K.Users manual for WATEQ4F, withrevised thermodynamicdatabase; Open File Report 91-183;U.S.G.S.: 1991.

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Received for review October 7, 1998. Revised manuscriptreceived May 26, 1999. Accepted June 1, 1999.

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geochemistry of prb

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