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Characterization of Manganese Oxide Amendments for in Situ Remediation of Mercury-Contaminated Sediments

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Characterization of Manganese Oxide Amendments for in Situ Remediation of Mercury-Contaminated Sediments Environmental Science: Processes & Impacts Characterization of manganese oxide amendments for in situ remediation of mercury-contaminated sediments Journal: Environmental Science: Processes & Impacts Manuscript ID EM-ART-12-2017-000576.R1 Article Type: Paper Date Submitted by the 25-Feb-2018 Author: Complete List of Authors: Leven, Alexander; University of California Merced, Environmental Systems Program Vlassopoulos, Dimitri; Anchor QEA LLC, ; Anchor QEA LLC Kanematsu, Masakazu; Anchor QEA LLC Goin, Jessica; Anchor QEA LLC O'Day, Peggy; University of California Merced School of Natural Sciences, Page 1 of 30 Environmental Science: Processes & Impacts 1 2 3 Characterization of manganese oxide amendments for in situ 4 5 remediation of mercury-contaminated sediments 6 7 Alexander Leven1, Dimitri Vlassopoulos2, Masakazu Kanematsu2, Jessica Goin2, 8 and Peggy A. O’Day1,* 9 10 1 Environmental Systems Program, University of California Merced, CA, USA 95343 11 2 Anchor QEA, LLC, Portland, OR, USA 12 * corresponding author email: [email protected] 13 14 15 16 Environmental Significance Statement 17 18 Methylation of inorganic mercury to bioavailable methylmercury poses a complex and 19 challenging problem for mitigation and remediation. Because contamination in sediments or soils 20 is often spread over wide areas at low concentrations, in situ treatment rather than removal and 21 disposal is desirable. Manganese(IV) oxide minerals are good candidates for a solid-phase 22 remedial amendment to suppress methylation of mercury by poising porewater oxidation state at 23 a thermodynamic potential higher than sulfate reduction and associated mercury methylation. 24 Experimental investigation of Mn(IV)-oxide amendments to mercury-contaminated sediments 25 26 and spectroscopic characterization of reaction products followed changes in mineralogy and 27 oxidation state over time to demonstrate mineral redox buffering by mixed-valent (Mn,Fe)(III,II) 28 oxides. Results provide a mechanistic basis for design and application to mercury-contaminated 29 field sites. 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Environmental Science: Processes & Impacts Page 2 of 30 1 2 3 Characterization of manganese oxide amendments for in situ 4 5 remediation of mercury-contaminated sediments 6 7 Alexander Leven1, Dimitri Vlassopoulos2, Masakazu Kanematsu2, Jessica Goin2, 8 and Peggy A. O’Day1,* 9 10 1 Environmental Systems Program, University of California Merced, CA, USA 95343 11 2 Anchor QEA, LLC, Portland, OR, USA 12 * corresponding author email: [email protected] 13 14 15 16 Abstract 17 18 Addition of Mn(IV)-oxide phases pyrolusite or birnessite was investigated as remedial 19 amendment for Hg-contaminated sediments. Because inorganic Hg methylation is a byproduct of 20 bacterial sulfate reduction, reaction of Mn(IV) oxide with pore water should poise sediment 21 oxidation potential at a level higher than favorable for Hg methylation. Changes in Mn(IV)-oxide 22 mineralogy and oxidation state over time were investigated in sediment tank mesocosm 23 experiments in which Mn(IV)-oxide amendment was either mixed into Hg-contaminated 24 sediment or applied as a thin-layer sand cap on top of sediment. Mesocosms were sampled 25 26 between 4 and 15 months of operation and solid phases were characterized by X-ray absorption 27 spectroscopy (XAS). For pyrolusite-amended sediments, Mn(IV) oxide was altered to a mixture 28 of Mn(III)-oxyhydroxide and Mn,Fe(III,II)-oxide phases, with a progressive increase in the 29 Mn(II)-carbonate fraction over time as mesocosm sediments became more reduced. For 30 birnessite-amended sediments, both Mn(III) oxyhydroxide and Mn(II) carbonate were identified 31 at 4 months, indicating a faster rate of Mn reduction compared to pyrolusite. After 15 months of 32 33 reaction, birnessite was converted completely to Mn(II) carbonate, whereas residual 34 Mn,Fe(III,II)-oxide phases were still present in addition to Mn(II) carbonate in the pyrolusite 35 mesocosm. In the thin-layer sand cap mesocosms, no changes in either pyrolusite or birnessite 36 XAS spectra were observed after 10 months of reaction. Equilibrium phase relationships support 37 the interpretation of mineral redox buffering by mixed-valent (Mn,Fe)(III,II)-oxide phases. 38 Results suggest that amendment longevity for redox buffering can be controlled by adjusting the 39 mass and type of Mn(IV) oxide applied, mineral crystallinity, surface area, and particle size. For 40 41 a given site, amendment capping versus mixing with sediment should be evaluated to determine 42 the optimum treatment approach, which may vary depending on application constraints, rate of 43 Mn(IV) oxide transformation, and frequency of reapplication to maintain desired oxidation state 44 and pH. 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Page 3 of 30 Environmental Science: Processes & Impacts 1 2 3 Introduction 4 5 6 Mercury and methylmercury pose a significant threat to human and environmental health 7 because of methylmercury bioaccumulation in the food chain. It is estimated that several 8 hundred thousand tons of mercury have been released into the environment during the 20th 9 century alone 1 and atmospheric emissions in the early 21st century are estimated to be around 10 2000 tons per year 2. Human and wildlife exposure to mercury is primarily through fish 11 2, 3 12 consumption and mostly in the form of methylmercury . The primary pathway for 13 methylation of inorganic mercury is by bacteria as a side reaction during sulfate reduction in 14 anaerobic conditions 4-7, although other bacteria involved in iron reduction and methanogenesis 15 may also methylate mercury 8-10. In mercury-contaminated environments, however, 16 concentrations of methylmercury are often not correlated with total inorganic mercury 17 concentrations 5, 11, which suggests that either mercury is not available to methylating bacteria, 18 rates of demethylation are high, or environmental conditions do not promote bacterial processes 19 20 resulting in mercury methylation. Overall, the reduction-oxidation (redox) potential of the 21 sediment-water system controls microbial processes associated with mercury methylation and 22 environmental factors that influence mercury concentrations in sediments. 23 24 Methylation of inorganic mercury is prevalent in a variety of environmental settings and poses a 25 widespread, complex, and challenging problem for mitigation and remediation. Very low 26 concentrations of total mercury and methylmercury in sediments (ng/g to μg/g) and water are 27 28 considered hazardous, and replicate measurements at a given site are often variable and 29 seasonally transient. Contamination is often spread over wide areas such that in situ treatments 30 rather than removal and disposal are desirable 12-14. One strategy for reducing methylmercury 31 concentrations is to manipulate the redox state and/or pH of a system to create conditions 32 unfavorable for sulfate or iron reduction, thus limiting Hg methylation rate. This approach has 33 been employed as part of the remediation plan for mercury at Onondaga Lake, New York 34 15, 16 35 through the addition of nitrate to the lake’s hypolimnion . A similar approach using liquid 36 calcium nitrate addition to suppress mercury methylation was tested in a pilot project in a 37 dimictic, mesotrophic lake in Minnesota 17. Monitoring of lake bottom waters showed that 38 nitrate addition raised oxidation potential, suppressed sulfate reduction, and lowered 39 methylmercury concentrations. Once nitrate was depleted, bottom water redox potential 40 decreased as sulfate reduction increased, and methymercury concentrations rose over several 41 weeks 17. Addition of nitrate, either by intentional dosing or from discharge of treated 42 43 wastewater, or addition of dissolved oxygen to anaerobic lake bottom waters, has been shown to 18-21 44 mitigate methylmercury accumulation . These prior studies employed addition to the water 45 overlying anaerobic sediments to suppress mercury methylation, which required repeated dosing 46 or constant discharge to maintain redox conditions at a state higher than favorable for sulfate 47 reduction. An alternative method is the application of a solid phase amendment to mercury- 48 contaminated soil or sediment that similarly poises the local oxidation state at a relatively high 49 level through reaction of the mineral with pore water. 50 51 52 Manganese(IV) oxide minerals are good candidates for a solid-phase remedial amendment to 53 suppress methylation of inorganic mercury. Reduction of Mn(IV) and Mn(III) oxides to Mn(II) 54 occurs at a much higher equilibrium redox potential than that of sulfate reduction 22 and thus 55 creates a thermodynamic "buffer" that energetically disfavors electron transfer among species 56 57 58 2 59 60 Environmental Science: Processes & Impacts Page 4 of 30 1 2 3 with lower equilibrium potentials. These minerals have a variety of polymorphs and related 4 hydrated phases 23, which allows for solids of different surface area, morphology, and reactivity 5 6 to be selected for specific purposes. They are relatively inexpensive to synthesize or purchase 7 commercially. In natural environments, Mn-oxide minerals are widespread and form readily as a 8 result of coupled chemical and biologically catalyzed pathways in which both bacteria and fungi 9 are known to participate 24, 25. Their ability to both surface adsorb and incorporate trace metals 10 into their crystal structures are well studied, such that natural Mn oxides are structurally diverse 11 and typically contain a variety of impurities 23, 25-27. The most common remediation application 12 of Mn oxides has been for removal of dissolved Mn from coal mine drainage waters using 13 28-30 14 aerobic passive flow beds with Mn oxide-coated limestone . Natural attenuation of dissolved 15 Mn and other metals through large-scale precipitation of Mn-oxide minerals was documented in 16 the stream hyporheic zone and aquifer at an acid mine drainage site (Pinal Creek, AZ) 31-33.
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