The benthic sediments in Lake Coeur d Alene, northern Idaho,have been contaminated by metals (primarily Zn, Pb, and Cu) from decadesof upstream mining activities. As part of ongoing research on thebiogeo-chemical cycling of metals in this area, a diffusivereactive-transport model has been developed to simulate metal transportin the lake sediments. The model includes 1-D inorganic diffusivetransport coupled to a biotic reaction network with multiple terminalelectron acceptors under redox disequilibrium conditions. Here, the modelis applied to evaluate the competing effects of heavy-metal mobilizationthrough biotic reductive dissolution of Fe(III) (hydr)oxides, andimmobilization as biogenic sulfide minerals. Results indicate that therelative rates of Fe and sulfate reduction could play an important rolein metal transport through the envi-ronment, and that the formation of(bi)sulfide complexes could significantly enhance metal solubility, aswell as desorption from Fe hydroxides

Ginn, Timothy.R.

Moberly, James

Peyton, B.

Sani, Rajesh.K.

Sengor, Sevinc.S.

Spycher, Nicolas.F.

Spycher, Nicolas F.

Ginn, Timothy R.

Sani, Rajesh K.

UNT Digital Library

1 INTRODUCTION The rich mining history of the Western United States, which began in the late 1880’s, has left many sites contaminated with toxic metals including Zn, Pb, and Cu. One example is Lake Coeur d’Alene, located in northern Idaho, where lake sediments have been impacted by decades of contaminated runoff into the Lake Coeur d’Alene River, mainly from the Bunker Hill Superfund mining site.   The weathering and oxidation of sulfide minerals associated with exposed ore and mine tailings results in acid waters containing high concentrations of dis-solved SO4, Fe, Mn, and other pollutant metals. Upon mixing of this acid mine drainage with near-neutral surface waters, insoluble precipitates are typically observed to form, mainly in the form of Fe and Mn (hydr)oxide phases. In the Coeur d’Alene mining district, it has been shown that dissolved heavy metals sorb onto these newly formed Fe and Mn (hydr)oxide particles, resulting in a significant concentration decrease in surface waters (Tonkin et al., 2002; Balistrieri et al., 2003). Analysis of these particles in the lower Lake Coeur d’Alene River val-ley show that these particles are associated with sig-nificant amounts of amorphous and nanocrystalline phases (Balistrieri et al., 2002). These (hydr)oxide particles are likely to be transported in surface wa-ters over very large distances. This process, along   with the transport of detrital primary sulfide miner-als, could significantly contribute to the transport of metals away from mining areas. In the case of Lake Coeur d’Alene, there has been controversy over whether metals in the sediments are primarily asso-ciated with iron oxides (Horowitz et al., 1995, 1999) or a sulfidic phase (Harrington et al., 1998, 1999). Recent spectroscopic data point to abundant ferrihy-drite and high Fe-to-S ratios in the sediments (Toevs et al., 2006), providing further support that metal transport to the lake could occur primarily as sorbed species onto Fe(III) (hydr)oxides.    We have developed a diffusive reactive transport model to simulate the benthic transport and fate of Zn, Pb, and Cu in benthic Lake Coeur d'Alene sedi-ments (Sengor et al., 2007), in order to evaluate the mobilization of heavy metals sorbed onto hydrous Fe(III) oxides by means of microbial reductive dis-solution (e.g., Zachara et al., 2001), and competing effects of metal precipitation as biogenic sulfide minerals. Here, we further investigate the role of metal (bi)sulfide complexes in potentially enhancing metal solubility and desorption from ferrihydrite.     2 NUMERICAL MODEL  Details on the numerical model and input data are provided in Sengor et al. (2007). The 1-D model was Reductive dissolution and metal transport in Lake Coeur d’Alene sediments S.S. Sengor1, N.F. Spycher2, T.R. Ginn1, J. Moberly3, B. Peyton3, R.K. Sani4  1University of California, Civil and Environmental Engineering Department, Davis, CA, USA 2Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3Montana Sate University, Chemical and Biological Engineering Department, Bozeman, MT, USA 4South Dakota School of Mines and Technology, Chemical and Biological Engineering Department, Rapid City, SD, USA    ABSTRACT: The benthic sediments in Lake Coeur d’Alene, northern Idaho, have been contaminated by met-als (primarily Zn, Pb, and Cu) from decades of upstream mining activities. As part of ongoing research on the biogeochemical cycling of metals in this area, a diffusive reactive-transport model has been developed to simulate metal transport in the lake sediments. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network with multiple terminal electron acceptors under redox disequilibrium conditions. Here, the model is applied to evaluate the competing effects of heavy-metal mobilization through biotic re-ductive dissolution of Fe(III) (hydr)oxides, and immobilization as biogenic sulfide minerals. Results indicate that the relative rates of Fe and sulfate reduction could play an important role in metal transport through the environment, and that the formation of (bi)sulfide complexes could significantly enhance metal solubility, as well as desorption from Fe hydroxides.     developed using PHREEQC (Parkhurst and Appelo, 1999), with key aspects summarized below.    2.1 Inorganic reaction system The modeled inorganic reaction processes include aqueous speciation, surface complexation onto Fe hydroxides (as ferrihydrite), and mineral precipita-tion/dissolution reactions. Reactive minerals consid-ered in the model are ferrihydrite (as Fe(OH)3), as primary mineral, and siderite (FeCO3), disordered mackinawite (FeSm), sphalerite (ZnS), galena (PbS), and chalcocite (Cu2S), as secondary phases.  The model conceptualization is based on the analyses of sediments, pore waters and surface waters from Lake Coeur d’Alene and the Coeur d’Alene River, conducted mostly by Winowiecki, (2002), Toevs et al., (2006), Balistrieri et al., (1998, 1999, 2003), and Cummings et al., (2000).  2.2 Microbially mediated reaction system  The inorganic reaction system is coupled to a micro-bially mediated reaction network with microbial consortium biodegradation kinetics involving multi-ple terminal electron acceptors. The terminal elec-tron acceptors included are O2, NO3-, Fe(III), and SO4-2, and redox is decoupled in the reaction net-work. Note that although the Mn content of sedi-ments is elevated, it is ~10 times lower than the Fe content. For this reason, the effect of Mn reduction is currently neglected and sorption is assumed to be controlled by Fe hydroxides alone. Acetate is used as the ultimate energy source, because it is a com-mon end-product of many fermentation processes and can be utilized by many metal- and sulfate-reducing bacteria in sediments (Lovley 2002). Sin-gle Michaelis-Menten kinetics is used to represent the biodegradation of acetate (non limiting) with concomitant reduction of terminal electron accep-tors. The sequential utilization of the terminal elec-tron acceptors is implemented using inhibition fac-tors (Van Cappellen & Gaillard, 1996) to impede the lower Gibb’s free-energy biotic-redox reactions when higher Gibb’s free-energy electron acceptors are available.  3 MODEL RESULTS  The model simulates the diffusion of oxic lake water into a 1-D sediment column (from the top down), initially under oxic conditions throughout. Sorbed heavy metals are included on the surface of ferrihy-drite. As oxygen and subsequent electron acceptors are consumed, ferrihydrite and aqueous sulfate pro-gressively reduce to aqueous Fe(II) and sulfide spe-cies, respectively. The model is run for periods of 2 and 5 years, after which near-steady-state conditions develop. Additional cases without sulfate-reducing bacteria (SRB) and without metal (bi)sulfide aque-ous species are also run.           3.1 Mobilization of metals and pH trends The model results show that the relative rates of Fe(III) reduction versus sulfate reduction may be a key factor controlling the mobilization of heavy metals, pH, and the relative amounts of Fe(II) min-eral precipitation with depth. It is observed that the reductive dissolution of ferrihydrite promotes the re-lease of sorbed metals into the pore water (Figs. 1 & 2). As the metal sulfide precipitation occurs, dis-solved heavy metal concentrations sharply decrease, yielding computed concentrations closer to meas-ured values (Figs. 1 & 2). Such concentration drop at depth has also been reported by others for iron and heavy metals (e.g., Huerta-Diaz et al., 1998).    Modeled pH values show an initially increasing trend with depth, followed by slowly decreasing val-ues further down in the anoxic zone (Fig. 3). The initial increase with depth is controlled by the reduc-tive dissolution of ferrihydrite, which drives pH up, as shown by: 8Fe(OH)3(s) + CH3COOH + 14H+ <=> 8Fe+2 +2HCO3- + 20H2O. The modeled pH reversal at a depth of about 10 cm can be explained by the precipitation of Fe(II) sulfide, as illustrated by: Fe+2 + 2CH3COOH + SO4-2 <=> 8FeSm(s) + 2HCO3- + 2H2O + 2H+. The competitive effects of these reac-tions on pH, is also illustrated by the model results shown for both “high” and “low” rates of Fe(III) re-duction, using the same rate of sulfate reduction in both cases. When the Fe(III) reduction rate is low-ered, the rise in pH (caused by the 1st reaction) is less pronounced, and the decrease in pH (caused by the 2nd reaction) is more pronounced (Fig. 3). Con-sistent with these reactions, when the model is run without SRB activity, pH shows a further increasing trend (Fig. 3) resulting from the lack of sulfide pro-duction, and hence, lack of Fe(II) sulfide precipita-tion. The pH increase in this case further stabilizes surface complexes and increases the net sorption of heavy metals onto ferrihydrite, despite the ongoing microbial reductive dissolution. The net effect, in this case without SRB, is that the dissolved heavy metal concentrations decrease with time and depth (Figs. 1 & 2), illustrating the complex coupling and competitive nature of processes at play.  3.2 Aqueous complexation with biogenic sulfide Model results show that the majority of the dis-solved heavy metals are in the form of (bi)sulfide complexes, and the majority of the aqueous sulfide consists of Pb, Zn, Cu (bi)sulfide species, and FeS(aq) (Fig. 4). When heavy-metal (bi)sulfide complexes are not allowed to form, the model indicates that FeS(aq) becomes the main aqueous sulfide species,    00.050.10.150.20.250.31.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02Concentration (molal)Depth (m)Zn total-5 yrZn total-2 yrZn total-noSRB-5 yrZn total-noSRB-2 yr12347 Figure 1. Zn concentration profile with depth: comparison of model predictions (lines, 5 and 2-year results, with and without the activity of SRB) with measured concentrations (symbols, data source: 1-6 Winowiecki (2002), Site A, 2001: 1,2Summer, 3,4Spring, 5,6Fall; 7Balistrieri (1998), September 2002, Delta Site; 8Toevs et al., (2006), May 2002, Harlow Point).     00.050.10.150.20.250.31.E-08 1.E-07 1.E-06 1.E-05 1.E-04Concentration (molal)Depth (m)Pb total -5 yrPb total -2 yrPb total -noSRB 5 yrPb total -noSRB 2 yr1234567 Figure 2. Pb concentration profile with depth: comparison of model predictions (lines, 5 and 2-year results, with and without the activity of SRB) with measured concentrations (symbols, see caption of Figure 2 for data sources).        00.050.10.150.20.250.36.5 7 7.5 8 8.5 9pHDepth (m)257 Figure 3. pH profile with depth: comparison of model predic-tions (lines, 5-year results) for “high” (solid line), “low” (dashed line) Fe(III) reduction rate, and “high” Fe(III) reduc-tion without the activity of SRB) (dotted line) with measured concentrations (symbols, see caption of Figure 2 for data sources).     00.050.10.150.20.250.31.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03Concentration (molal)Depth (m)S(-2) totalHS-H2SPb(HS)2FeS(aq)Cu(HS)ZnS(HS)- Figure 4. Calculated distribution of aqueous sulfide species. The bulk of dissolved sulfide is computed to occur as metal (bi)sulfide complexes.      and that the total dissolved metal concentrations de-crease compared to the case with all sulfide species. This indicates that the strong metal (bi)sulfide com-plexes can significantly enhance metal solubility as well as desorption from Fe(III) (hydr)oxides.   3.3  Mineral precipitation The two major sinks for dissolved Fe(II) in the system are FeSm and FeCO3. The biogeochemical system forms a delicate balance, with FeSm compet-ing with FeCO3 precipitation for Fe(II), and the competition of aqueous (bi)sulfide complexes and sulfide mineral precipitation for biogenic sulfide.   4 CONCLUSIONS This study focuses on the hypothesis that the bulk of metals in Lake Coeur d’Alene sediments are initially associated with Fe(III) (hydr)oxides, and their mobi-lization occur primarily through reductive dissolu-tion. Authigenic sulfide mineral formation as a result of the coupling of inorganic and microbiological processes is also considered. The model is congruent with the observations by Cummings et al. (2000), Balistrieri et al. (2002, 2003), Winowiecki (2002), and Toevs et al. (2006). The model does not pre-clude the presence of detrital sulfide minerals, but indicates that the trends of dissolved metal concen-trations can be explained by reductive dissolution of Fe(III) (hydr)oxides and biogenic sulfide mineral formation.   The relative rates of Fe(III) versus sulfate reduc-tion is observed to be a key factor controlling pH and the mobility of heavy metals sorbed onto Fe(III) oxide phases. Microbial reductive dissolution of fer-rihydrite promotes the solubilization of metals, whereas precipitation of heavy-metal sulfides sharply decreases the dissolved metal concentra-tions. Decrease in the rate of sulfate reduction re-sults in an increase in the pH trend, because of the decrease in the precipitation of Fe(II) sulfides with depth, which then results in an increased sorption of heavy metals onto Fe(III) (hydr)oxides and an over-all decrease in mobilization. It is observed that dis-solved metals form strong metal (bi)sulfide com-plexes, which also further enhances their desorption from ferrihydrite. A delicate balance is observed to exist between the effects of FeSm and siderite pre-cipitation. The effects of Fe(III) reduction relative to sulfate reduction are currently being further investi-gated through controlled laboratory experiments. 5 REFERENCES Balistrieri, L.S., Box, S.E., Bookstroom, A.A., Hooper, R.L., & Mahoney, J.B., 2002. Impacts of historical mining in the Coeur d’Alene Basin. In: Balistrieri, L.S. and Stillings, L.L. (Eds.), Pathways of Metal Transfer from Mineralized Sources to Bioreceptors: A Synthesis of the Mineral Re-sources Program’s Past Environmental Studies in the West-ern United States and Future Research Directions, U.S. Geological Survey Bulletin 2191, pp. 1-34 (Chapter 6). Balistrieri, L.S., Box, S.E., & Tonkin, J.W., 2003. Modeling precipitation and sorption of elements during mixing of river water and porewater in the Coeur d’Alene River Ba-sin. Environ. Sci. Technol. 37, 4694-4701. Cummings,D.E., March, A.W., Bostick, B.C., Spring, S., Cac-cavo, F., Jr., Fendorf, S., & Rosenzweig, R.F., 2000. Evi-dence for microbial Fe(III) reduction in anoxic, mining -impacted lake sediments (Lake Coeur d'Alene, Idaho). Appl. Environ. Microbiol. 66, 154-162. Huerta-Diaz,M.A.,Tessier,A., Carignan, R., 1998. Geochemis-try of trace metals associated with reduced sulfur in fresh-water sediments. Appl. Geochem. 13, 213-233. Harrington, J.M., La Force, M.J., Rember, W.C., Fendorf, S. E., & Rosenzweig, R.F., 1998. Phase associations and mo-bilization of iron and trace elements in Coeur d'Alene Lake, Idaho. Environ. Sci. Technol. 32, 650-656. Harrington, J.M., Fendorf, S. E., Wielinga, B.W., & Rosenzweig, R.F., 1999. Response to comments on: “Phase associations and mobilization of iron and trace elements in Coeur d'Alene Lake, Idaho”. Environ. Sci. Technol. 33, 203-204. Horowitz, A.J., Elrick, K.A., Robbins, J.A., & Cook, R.B., 1995. Effect of mining and related activities on sediment trace element geochemistry of Lake Coeur D’Alene, Idaho, USA Part II. Subsurface sediments. Hydrol. Process. 9, 35-54. Horowitz, A.J., Elrick, K.A., & Cook, R.B., 1999. Comments on: “Phase associations and mobilization of iron and trace elements in Coeur d'Alene Lake, Idaho”. Environ. Sci. Technol. 33, 201-202. Lovley D.R., 2002. Dissimilatory metal reduction: from early life to bioremediation. ASM News 68(5):231-237 Parkhurst, D.L. & Appelo, C.A.J., 1999. User’s Guide to PHREEQC (Version 2)-a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geo-chemical calculations. Water-Resources Investigations Re-port  99-4259; U.S. Geological Survey, Denver, CO. Sengor, S.S., Spycher, N.F., Ginn, T.R., Sani, R.K., & Peyton, B., 2007. Biogeochemical reactive-diffusive transport of heavy metals in Lake Coeur d’Alene sediments. Applied Geochemistry (in press). Toevs, G.R., Morra, M.J., Polizzotto, M.L., Strawn, D.G., Bos-tick, B.C., & Fendorf, S., 2006. Metal(loid) diagenesis in mine-impacted sediments of Lake Coeur d'Alene, Idaho. Environ. Sci. Technol. 40, 2537-2543. Tonkin, J.W., Balistrieri, L.S., & Murray, J.W., 2002. Model-ing metal removal onto natural particles formed during mixing of acid rock drainage with ambient surface water. Environ. Sci. Technol. 36, 484-492. Van Cappellen, P. & Gaillard,P., 1996.Biogeochemical dy-namics in aquatic sediments. In: Lichtner, P. C.,Steefel, C. I., Oelkers, E. H. (Eds.), Reactive Transport in Porous Me-dia, vol. 34. Mineralogical Society of America, pp. 335–376 (Chapter 8). Winowiecki, L., 2002. Geochemical cycling of heavy metals in the sediment of Lake Coeur d'Alene, Idaho. Masters Thesis, University of Idaho, Moscow, Idaho.   Zachara, J.M., Fredrickson, J.K., Smith, S.C., Gassman P.L., 2001. Solubilization of Fe(III) oxide-bound trace metals by a dissimilatory Fe(III) reducing bacterium. Geochimica et Cosmochimica Acta 1, 75-93.  

Reductive dissolution and metal transport in lake coeur d alenesediments

The benthic sediments in Lake Coeur d Alene, northern Idaho, have been contaminated by  metals (primarily Zn, Pb, and Cu) from decades of upstream mining activities. As part of ongoing  research on the biogeo-chemical cycling of metals in this area, a diffusive reactive-transport  model has been developed to simulate metal transport in the lake sediments. The model includes 1-D  inorganic diffusive transport coupled to a biotic reaction network with multiple terminal electron  acceptors under redox disequilibrium conditions. Here, the model is applied to evaluate the  competing effects of heavy-metal mobilization through biotic reductive dissolution of Fe(III)  (hydr)oxides, and immobilization as biogenic sulfide minerals. Results indicate that the relative  rates of Fe and sulfate reduction could play an important role in metal transport through the  envi-ronment, and that the formation of (bi)sulfide complexes could significantly enhance metal  solubility, as well as desorption from Fe hydroxides

eScholarship - University of California

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleReductive dissolution and metal transport in lake coeur d alene sedimentsPermalinkhttps://escholarship.org/uc/item/0rp9p68dAuthorsSengor, Sevinc.S.Spycher, Nicolas.F.Ginn, Timothy.R.et al.Publication Date2007-04-27eScholarship.org Powered by the California Digital LibraryUniversity of California1 INTRODUCTION The rich mining history of the Western United States, which began in the late 1880’s, has left many sites contaminated with toxic metals including Zn, Pb, and Cu. One example is Lake Coeur d’Alene, located in northern Idaho, where lake sediments have been impacted by decades of contaminated runoff into the Lake Coeur d’Alene River, mainly from the Bunker Hill Superfund mining site.   The weathering and oxidation of sulfide minerals associated with exposed ore and mine tailings results in acid waters containing high concentrations of dis-solved SO4, Fe, Mn, and other pollutant metals. Upon mixing of this acid mine drainage with near-neutral surface waters, insoluble precipitates are typically observed to form, mainly in the form of Fe and Mn (hydr)oxide phases. In the Coeur d’Alene mining district, it has been shown that dissolved heavy metals sorb onto these newly formed Fe and Mn (hydr)oxide particles, resulting in a significant concentration decrease in surface waters (Tonkin et al., 2002; Balistrieri et al., 2003). Analysis of these particles in the lower Lake Coeur d’Alene River val-ley show that these particles are associated with sig-nificant amounts of amorphous and nanocrystalline phases (Balistrieri et al., 2002). These (hydr)oxide particles are likely to be transported in surface wa-ters over very large distances. This process, along   with the transport of detrital primary sulfide miner-als, could significantly contribute to the transport of metals away from mining areas. In the case of Lake Coeur d’Alene, there has been controversy over whether metals in the sediments are primarily asso-ciated with iron oxides (Horowitz et al., 1995, 1999) or a sulfidic phase (Harrington et al., 1998, 1999). Recent spectroscopic data point to abundant ferrihy-drite and high Fe-to-S ratios in the sediments (Toevs et al., 2006), providing further support that metal transport to the lake could occur primarily as sorbed species onto Fe(III) (hydr)oxides.    We have developed a diffusive reactive transport model to simulate the benthic transport and fate of Zn, Pb, and Cu in benthic Lake Coeur d'Alene sedi-ments (Sengor et al., 2007), in order to evaluate the mobilization of heavy metals sorbed onto hydrous Fe(III) oxides by means of microbial reductive dis-solution (e.g., Zachara et al., 2001), and competing effects of metal precipitation as biogenic sulfide minerals. Here, we further investigate the role of metal (bi)sulfide complexes in potentially enhancing metal solubility and desorption from ferrihydrite.     2 NUMERICAL MODEL  Details on the numerical model and input data are provided in Sengor et al. (2007). The 1-D model was Reductive dissolution and metal transport in Lake Coeur d’Alene sediments S.S. Sengor1, N.F. Spycher2, T.R. Ginn1, J. Moberly3, B. Peyton3, R.K. Sani4  1University of California, Civil and Environmental Engineering Department, Davis, CA, USA 2Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA 3Montana Sate University, Chemical and Biological Engineering Department, Bozeman, MT, USA 4South Dakota School of Mines and Technology, Chemical and Biological Engineering Department, Rapid City, SD, USA    ABSTRACT: The benthic sediments in Lake Coeur d’Alene, northern Idaho, have been contaminated by met-als (primarily Zn, Pb, and Cu) from decades of upstream mining activities. As part of ongoing research on the biogeochemical cycling of metals in this area, a diffusive reactive-transport model has been developed to simulate metal transport in the lake sediments. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network with multiple terminal electron acceptors under redox disequilibrium conditions. Here, the model is applied to evaluate the competing effects of heavy-metal mobilization through biotic re-ductive dissolution of Fe(III) (hydr)oxides, and immobilization as biogenic sulfide minerals. Results indicate that the relative rates of Fe and sulfate reduction could play an important role in metal transport through the environment, and that the formation of (bi)sulfide complexes could significantly enhance metal solubility, as well as desorption from Fe hydroxides.     developed using PHREEQC (Parkhurst and Appelo, 1999), with key aspects summarized below.    2.1 Inorganic reaction system The modeled inorganic reaction processes include aqueous speciation, surface complexation onto Fe hydroxides (as ferrihydrite), and mineral precipita-tion/dissolution reactions. Reactive minerals consid-ered in the model are ferrihydrite (as Fe(OH)3), as primary mineral, and siderite (FeCO3), disordered mackinawite (FeSm), sphalerite (ZnS), galena (PbS), and chalcocite (Cu2S), as secondary phases.  The model conceptualization is based on the analyses of sediments, pore waters and surface waters from Lake Coeur d’Alene and the Coeur d’Alene River, conducted mostly by Winowiecki, (2002), Toevs et al., (2006), Balistrieri et al., (1998, 1999, 2003), and Cummings et al., (2000).  2.2 Microbially mediated reaction system  The inorganic reaction system is coupled to a micro-bially mediated reaction network with microbial consortium biodegradation kinetics involving multi-ple terminal electron acceptors. The terminal elec-tron acceptors included are O2, NO3-, Fe(III), and SO4-2, and redox is decoupled in the reaction net-work. Note that although the Mn content of sedi-ments is elevated, it is ~10 times lower than the Fe content. For this reason, the effect of Mn reduction is currently neglected and sorption is assumed to be controlled by Fe hydroxides alone. Acetate is used as the ultimate energy source, because it is a com-mon end-product of many fermentation processes and can be utilized by many metal- and sulfate-reducing bacteria in sediments (Lovley 2002). Sin-gle Michaelis-Menten kinetics is used to represent the biodegradation of acetate (non limiting) with concomitant reduction of terminal electron accep-tors. The sequential utilization of the terminal elec-tron acceptors is implemented using inhibition fac-tors (Van Cappellen & Gaillard, 1996) to impede the lower Gibb’s free-energy biotic-redox reactions when higher Gibb’s free-energy electron acceptors are available.  3 MODEL RESULTS  The model simulates the diffusion of oxic lake water into a 1-D sediment column (from the top down), initially under oxic conditions throughout. Sorbed heavy metals are included on the surface of ferrihy-drite. As oxygen and subsequent electron acceptors are consumed, ferrihydrite and aqueous sulfate pro-gressively reduce to aqueous Fe(II) and sulfide spe-cies, respectively. The model is run for periods of 2 and 5 years, after which near-steady-state conditions develop. Additional cases without sulfate-reducing bacteria (SRB) and without metal (bi)sulfide aque-ous species are also run.           3.1 Mobilization of metals and pH trends The model results show that the relative rates of Fe(III) reduction versus sulfate reduction may be a key factor controlling the mobilization of heavy metals, pH, and the relative amounts of Fe(II) min-eral precipitation with depth. It is observed that the reductive dissolution of ferrihydrite promotes the re-lease of sorbed metals into the pore water (Figs. 1 & 2). As the metal sulfide precipitation occurs, dis-solved heavy metal concentrations sharply decrease, yielding computed concentrations closer to meas-ured values (Figs. 1 & 2). Such concentration drop at depth has also been reported by others for iron and heavy metals (e.g., Huerta-Diaz et al., 1998).    Modeled pH values show an initially increasing trend with depth, followed by slowly decreasing val-ues further down in the anoxic zone (Fig. 3). The initial increase with depth is controlled by the reduc-tive dissolution of ferrihydrite, which drives pH up, as shown by: 8Fe(OH)3(s) + CH3COOH + 14H+ <=> 8Fe+2 +2HCO3- + 20H2O. The modeled pH reversal at a depth of about 10 cm can be explained by the precipitation of Fe(II) sulfide, as illustrated by: Fe+2 + 2CH3COOH + SO4-2 <=> 8FeSm(s) + 2HCO3- + 2H2O + 2H+. The competitive effects of these reac-tions on pH, is also illustrated by the model results shown for both “high” and “low” rates of Fe(III) re-duction, using the same rate of sulfate reduction in both cases. When the Fe(III) reduction rate is low-ered, the rise in pH (caused by the 1st reaction) is less pronounced, and the decrease in pH (caused by the 2nd reaction) is more pronounced (Fig. 3). Con-sistent with these reactions, when the model is run without SRB activity, pH shows a further increasing trend (Fig. 3) resulting from the lack of sulfide pro-duction, and hence, lack of Fe(II) sulfide precipita-tion. The pH increase in this case further stabilizes surface complexes and increases the net sorption of heavy metals onto ferrihydrite, despite the ongoing microbial reductive dissolution. The net effect, in this case without SRB, is that the dissolved heavy metal concentrations decrease with time and depth (Figs. 1 & 2), illustrating the complex coupling and competitive nature of processes at play.  3.2 Aqueous complexation with biogenic sulfide Model results show that the majority of the dis-solved heavy metals are in the form of (bi)sulfide complexes, and the majority of the aqueous sulfide consists of Pb, Zn, Cu (bi)sulfide species, and FeS(aq) (Fig. 4). When heavy-metal (bi)sulfide complexes are not allowed to form, the model indicates that FeS(aq) becomes the main aqueous sulfide species,    00.050.10.150.20.250.31.00E-06 1.00E-05 1.00E-04 1.00E-03 1.00E-02Concentration (molal)Depth (m)Zn total-5 yrZn total-2 yrZn total-noSRB-5 yrZn total-noSRB-2 yr12347 Figure 1. Zn concentration profile with depth: comparison of model predictions (lines, 5 and 2-year results, with and without the activity of SRB) with measured concentrations (symbols, data source: 1-6 Winowiecki (2002), Site A, 2001: 1,2Summer, 3,4Spring, 5,6Fall; 7Balistrieri (1998), September 2002, Delta Site; 8Toevs et al., (2006), May 2002, Harlow Point).     00.050.10.150.20.250.31.E-08 1.E-07 1.E-06 1.E-05 1.E-04Concentration (molal)Depth (m)Pb total -5 yrPb total -2 yrPb total -noSRB 5 yrPb total -noSRB 2 yr1234567 Figure 2. Pb concentration profile with depth: comparison of model predictions (lines, 5 and 2-year results, with and without the activity of SRB) with measured concentrations (symbols, see caption of Figure 2 for data sources).        00.050.10.150.20.250.36.5 7 7.5 8 8.5 9pHDepth (m)257 Figure 3. pH profile with depth: comparison of model predic-tions (lines, 5-year results) for “high” (solid line), “low” (dashed line) Fe(III) reduction rate, and “high” Fe(III) reduc-tion without the activity of SRB) (dotted line) with measured concentrations (symbols, see caption of Figure 2 for data sources).     00.050.10.150.20.250.31.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03Concentration (molal)Depth (m)S(-2) totalHS-H2SPb(HS)2FeS(aq)Cu(HS)ZnS(HS)- Figure 4. Calculated distribution of aqueous sulfide species. The bulk of dissolved sulfide is computed to occur as metal (bi)sulfide complexes.      and that the total dissolved metal concentrations de-crease compared to the case with all sulfide species. This indicates that the strong metal (bi)sulfide com-plexes can significantly enhance metal solubility as well as desorption from Fe(III) (hydr)oxides.   3.3  Mineral precipitation The two major sinks for dissolved Fe(II) in the system are FeSm and FeCO3. The biogeochemical system forms a delicate balance, with FeSm compet-ing with FeCO3 precipitation for Fe(II), and the competition of aqueous (bi)sulfide complexes and sulfide mineral precipitation for biogenic sulfide.   4 CONCLUSIONS This study focuses on the hypothesis that the bulk of metals in Lake Coeur d’Alene sediments are initially associated with Fe(III) (hydr)oxides, and their mobi-lization occur primarily through reductive dissolu-tion. Authigenic sulfide mineral formation as a result of the coupling of inorganic and microbiological processes is also considered. The model is congruent with the observations by Cummings et al. (2000), Balistrieri et al. (2002, 2003), Winowiecki (2002), and Toevs et al. (2006). The model does not pre-clude the presence of detrital sulfide minerals, but indicates that the trends of dissolved metal concen-trations can be explained by reductive dissolution of Fe(III) (hydr)oxides and biogenic sulfide mineral formation.   The relative rates of Fe(III) versus sulfate reduc-tion is observed to be a key factor controlling pH and the mobility of heavy metals sorbed onto Fe(III) oxide phases. Microbial reductive dissolution of fer-rihydrite promotes the solubilization of metals, whereas precipitation of heavy-metal sulfides sharply decreases the dissolved metal concentra-tions. Decrease in the rate of sulfate reduction re-sults in an increase in the pH trend, because of the decrease in the precipitation of Fe(II) sulfides with depth, which then results in an increased sorption of heavy metals onto Fe(III) (hydr)oxides and an over-all decrease in mobilization. It is observed that dis-solved metals form strong metal (bi)sulfide com-plexes, which also further enhances their desorption from ferrihydrite. A delicate balance is observed to exist between the effects of FeSm and siderite pre-cipitation. The effects of Fe(III) reduction relative to sulfate reduction are currently being further investi-gated through controlled laboratory experiments. 5 REFERENCES Balistrieri, L.S., Box, S.E., Bookstroom, A.A., Hooper, R.L., & Mahoney, J.B., 2002. Impacts of historical mining in the Coeur d’Alene Basin. In: Balistrieri, L.S. and Stillings, L.L. (Eds.), Pathways of Metal Transfer from Mineralized Sources to Bioreceptors: A Synthesis of the Mineral Re-sources Program’s Past Environmental Studies in the West-ern United States and Future Research Directions, U.S. Geological Survey Bulletin 2191, pp. 1-34 (Chapter 6). Balistrieri, L.S., Box, S.E., & Tonkin, J.W., 2003. Modeling precipitation and sorption of elements during mixing of river water and porewater in the Coeur d’Alene River Ba-sin. Environ. Sci. Technol. 37, 4694-4701. Cummings,D.E., March, A.W., Bostick, B.C., Spring, S., Cac-cavo, F., Jr., Fendorf, S., & Rosenzweig, R.F., 2000. Evi-dence for microbial Fe(III) reduction in anoxic, mining -impacted lake sediments (Lake Coeur d'Alene, Idaho). Appl. Environ. Microbiol. 66, 154-162. Huerta-Diaz,M.A.,Tessier,A., Carignan, R., 1998. Geochemis-try of trace metals associated with reduced sulfur in fresh-water sediments. Appl. Geochem. 13, 213-233. Harrington, J.M., La Force, M.J., Rember, W.C., Fendorf, S. E., & Rosenzweig, R.F., 1998. Phase associations and mo-bilization of iron and trace elements in Coeur d'Alene Lake, Idaho. Environ. Sci. Technol. 32, 650-656. Harrington, J.M., Fendorf, S. E., Wielinga, B.W., & Rosenzweig, R.F., 1999. Response to comments on: “Phase associations and mobilization of iron and trace elements in Coeur d'Alene Lake, Idaho”. Environ. Sci. Technol. 33, 203-204. Horowitz, A.J., Elrick, K.A., Robbins, J.A., & Cook, R.B., 1995. Effect of mining and related activities on sediment trace element geochemistry of Lake Coeur D’Alene, Idaho, USA Part II. Subsurface sediments. Hydrol. Process. 9, 35-54. Horowitz, A.J., Elrick, K.A., & Cook, R.B., 1999. Comments on: “Phase associations and mobilization of iron and trace elements in Coeur d'Alene Lake, Idaho”. Environ. Sci. Technol. 33, 201-202. Lovley D.R., 2002. Dissimilatory metal reduction: from early life to bioremediation. ASM News 68(5):231-237 Parkhurst, D.L. & Appelo, C.A.J., 1999. User’s Guide to PHREEQC (Version 2)-a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geo-chemical calculations. Water-Resources Investigations Re-port  99-4259; U.S. Geological Survey, Denver, CO. Sengor, S.S., Spycher, N.F., Ginn, T.R., Sani, R.K., & Peyton, B., 2007. Biogeochemical reactive-diffusive transport of heavy metals in Lake Coeur d’Alene sediments. Applied Geochemistry (in press). Toevs, G.R., Morra, M.J., Polizzotto, M.L., Strawn, D.G., Bos-tick, B.C., & Fendorf, S., 2006. Metal(loid) diagenesis in mine-impacted sediments of Lake Coeur d'Alene, Idaho. Environ. Sci. Technol. 40, 2537-2543. Tonkin, J.W., Balistrieri, L.S., & Murray, J.W., 2002. Model-ing metal removal onto natural particles formed during mixing of acid rock drainage with ambient surface water. Environ. Sci. Technol. 36, 484-492. Van Cappellen, P. & Gaillard,P., 1996.Biogeochemical dy-namics in aquatic sediments. In: Lichtner, P. C.,Steefel, C. I., Oelkers, E. H. (Eds.), Reactive Transport in Porous Me-dia, vol. 34. Mineralogical Society of America, pp. 335–376 (Chapter 8). Winowiecki, L., 2002. Geochemical cycling of heavy metals in the sediment of Lake Coeur d'Alene, Idaho. Masters Thesis, University of Idaho, Moscow, Idaho.   Zachara, J.M., Fredrickson, J.K., Smith, S.C., Gassman P.L., 2001. Solubilization of Fe(III) oxide-bound trace metals by a dissimilatory Fe(III) reducing bacterium. Geochimica et Cosmochimica Acta 1, 75-93.  

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Reductive dissolution and metal transport in lake coeur d alene sediments

The benthic sediments in Lake Coeur d'Alene, northern Idaho, have been contaminated by metals (primarily Zn, Pb, and Cu) from decades of upstream mining activities. As part of ongoing research on the biogeochemical cycling of metals in this area, a diffusive reactive-transport model has been developed to simulate metal transport in the lake sediments. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network with multiple terminal electron acceptors under redox disequilibrium conditions. Here, the model is applied to evaluate the competing effects of heavy-metal mobilization through biotic reductive dissolution of Fe(III) (hydr)oxides, and immobilization as biogenic sulfide minerals. Results indicate that the relative rates of Fe and sulfate reduction could play an important role in metal transport through the environment, and that the formation of (bi)sulfide complexes could significantly enhance metal solubility, aswell as desorption from Fe hydroxides. © 2007 Taylor & Francis Group, London

Reductive dissolution and metal transport in lake coeur d alenesediments

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