Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

Hausladen, Debra; Fendorf, Scott

doi:10.1021/acs.est.6b04039

Cited by 140 publications

(85 citation statements)

References 85 publications

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“…Several viable remediation methods exist that are capable of driving this transformation, including Fe 2+ and dithionite [5], zero-valent iron [6][7][8], and green rust, a layered Fe(II)-Fe(III) hydroxide mineral. However, several recent studies [9][10][11] have investigated the regeneration of chromate from Cr(III) and Cr(III)-bearing Fe(III) hydroxides in packed column experiments. These studies revealed that biogenic Mn(IV) oxides are the primary oxidant in these systems and that reductive transformation of chromate in soils may be reversible, depending on the Mn content and redox conditions of the soils as well as the solubility of the Cr(III)-bearing phase.…”

Section: Introductionmentioning

confidence: 99%

Products of Hexavalent Chromium Reduction by Green Rust Sodium Sulfate and Associated Reaction Mechanisms

et al. 2018

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The efficacy of in vitro Cr(VI) reduction by green rust sulfate suggests that this mineral is potentially useful for remediation of Cr-contaminated groundwater. Previous investigations studied this reaction but did not sufficiently characterize the intermediates and end products at chromate (CrO42−) concentrations typical of contaminant plumes, hindering identification of the dominant reaction mechanisms under these conditions. In this study, batch reactions at varying chromate concentrations and suspension densities were performed and the intermediate and final products of this reaction were analyzed using X-ray absorption spectroscopy and electron microscopy. This reaction produces particles that maintain the initial hexagonal morphology of green rust but have been topotactically transformed into a poorly crystalline Fe(III) oxyhydroxysulfate and are coated by a Cr (oxy) hydroxide layer that results from chromate reduction at the surface. Recent studies of the behavior of Cr(III) (oxy) hydroxides in soils have revealed that reductive transformation of CrO42− is reversible in the presence of Mn(IV) oxides, limiting the applicability of green rust for Cr remediation in some soils. The linkage of Cr redox speciation to existing Fe and Mn biogeochemical cycles in soils implies that modification of green rust particles to produce an insoluble, Cr(III)-bearing Fe oxide product may increase the efficacy of this technique.

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Section: Introductionmentioning

confidence: 99%

Products of Hexavalent Chromium Reduction by Green Rust Sodium Sulfate and Associated Reaction Mechanisms

et al. 2018

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“…For neutral to acidic waters, Cr(VI) adsorption may have an important, but, in the present study, undetermined, role in resulting groundwater contamination. Anaerobic conditions including microsites will likely further have pronounced influences on Cr(VI) levels [35], but were not considered in the present simulation. Our results therefore represent likely processes controlling Cr(VI) concentrations in groundwater for aerobic, alkaline environments.…”

Section: Discussionmentioning

confidence: 99%

“…Mn oxides often coat advective boundaries, particularly in fracture rock such as saprolite where an influx of oxygenated water oxidizes Mn(II) catalyzed by mineral surfaces and Mn-oxidizing bacteria [33,34]. As described in Hausladen and Fendorf [35], studies conducted with synthetic aggregates show that in oxic waters with an influx of Mn(II), most biogenic Mn oxides precipitate within ca. 4.5 mm of the advective flow channel.…”

Section: Diffusion Distance Controls Time To a Steady-statementioning

confidence: 98%

“…Mn oxides often coat advective boundaries, particularly in fracture rock such as saprolite where an influx of oxygenated water oxidizes Mn(II) catalyzed by mineral surfaces and Mn-oxidizing bacteria [33,34]. As described in Hausladen and Fendorf [35] As Mn oxides form at weathering fronts or at anoxic-oxic interfaces, there is an ever-changing landscape of Cr and Mn assemblages. Mn oxides often coat advective boundaries, particularly in fracture rock such as saprolite where an influx of oxygenated water oxidizes Mn(II) catalyzed by mineral surfaces and Mn-oxidizing bacteria [33,34].…”

Section: Diffusion Distance Controls Time To a Steady-statementioning

confidence: 99%

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Governing Constraints of Chromium(VI) Formation from Chromium(III)-Bearing Minerals in Soils and Sediments

2019

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The potential for geogenic Cr(VI) contamination is vast, yet it is difficult to predict susceptible environments as groundwater Cr(VI) concentrations vary significantly, even within a given aquifer, due to physical and hydrologic heterogeneity. The physical structure of soils and sediments exerts a dominant control on Cr(VI) production by dictating the separation distance of reactive phases, the diffusion distance from Cr(VI) generation sites to advecting groundwater, and by influencing infiltration rates and porewater velocity. Here, we used a dual-pore domain model to investigate the relative control of these parameters on Cr(VI) production. The reaction distance between Cr(III)-bearing minerals and Mn oxides predominantly controls Cr(VI) export to advecting groundwater, while changes in diffusion distance between sites of Cr(VI) generation and advective flow channels generally have little impact on steady-state Cr(VI) concentrations. Changes in Cr(VI) diffusion distance can, however, increase the time required for groundwater Cr(VI) concentrations to reach a steady-state; thus, under fluctuating hydrologic and biogeochemical conditions, long diffusion distances still have the potential to suppress Cr(VI) supply to advecting water. Furthermore, we show that high porewater flow velocities effectively dilute Cr(VI) diffusing from soil/sediment aggregates, thus minimizing Cr(VI) concentrations relative to lower porewater velocities. The strong control that the physical/hydrologic parameters exert on Cr(VI) production appears to overwhelm the impact of Cr(III)-mineral solubility within soils and sediments.

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“…Hallett et al (2013) also point out that breakdown of soils by dynamic or static mechanical loading yields different fragmentations of soil aggregates. This dependence of the aggregate size distribution on the operational conditions under which it is measured raises the question of whether aggregates exist in soils in their natural state (Young et al, 2001), calling into question the extensive literature that tries to analyze the influence of aggregate size on various processes, e.g., in terms of the sequestration of OM, the distribution of bacteria, a wide range of geochemical processes, or the release of greenhouse gasses (Ranjard and Richaume, 2001; Jasinska et al, 2006; Nunan et al, 2006; Razafimbelo et al, 2008; Goebel et al, 2009; Pallud et al, 2010; Chivenge et al, 2011; Masue-Slowey et al, 2011, 2013; Blaud et al, 2014; Rabbi et al, 2014, 2016; Ebrahimi and Or, 2015; Jiang et al, 2015; San José Martínez et al, 2015; Sheehy et al, 2015; Hausladen and Fendorf, 2017; Rillig et al, 2017; Zhao et al, 2017; Bocking and Blyth, 2018; Li et al, 2018), and explaining perhaps why some authors have failed to observe anticipated correlations between OM content and aggregation (Razafimbelo et al, 2013). Nevertheless, one might argue that this dependence problem can be alleviated somewhat by standardizing methods, and that, in any event, it does not particularly affect attempts to understand at a very local scale in soils the interactions between pore geometry, chemical composition, and microbial activity.…”

Section: Progress On the Physical Frontmentioning

confidence: 99%

Emergent Properties of Microbial Activity in Heterogeneous Soil Microenvironments: Different Research Approaches Are Slowly Converging, Yet Major Challenges Remain

et al. 2018

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Over the last 60 years, soil microbiologists have accumulated a wealth of experimental data showing that the bulk, macroscopic parameters (e.g., granulometry, pH, soil organic matter, and biomass contents) commonly used to characterize soils provide insufficient information to describe quantitatively the activity of soil microorganisms and some of its outcomes, like the emission of greenhouse gasses. Clearly, new, more appropriate macroscopic parameters are needed, which reflect better the spatial heterogeneity of soils at the microscale (i.e., the pore scale) that is commensurate with the habitat of many microorganisms. For a long time, spectroscopic and microscopic tools were lacking to quantify processes at that scale, but major technological advances over the last 15 years have made suitable equipment available to researchers. In this context, the objective of the present article is to review progress achieved to date in the significant research program that has ensued. This program can be rationalized as a sequence of steps, namely the quantification and modeling of the physical-, (bio)chemical-, and microbiological properties of soils, the integration of these different perspectives into a unified theory, its upscaling to the macroscopic scale, and, eventually, the development of new approaches to measure macroscopic soil characteristics. At this stage, significant progress has been achieved on the physical front, and to a lesser extent on the (bio)chemical one as well, both in terms of experiments and modeling. With regard to the microbial aspects, although a lot of work has been devoted to the modeling of bacterial and fungal activity in soils at the pore scale, the appropriateness of model assumptions cannot be readily assessed because of the scarcity of relevant experimental data. For significant progress to be made, it is crucial to make sure that research on the microbial components of soil systems does not keep lagging behind the work on the physical and (bio)chemical characteristics. Concerning the subsequent steps in the program, very little integration of the various disciplinary perspectives has occurred so far, and, as a result, researchers have not yet been able to tackle the scaling up to the macroscopic level. Many challenges, some of them daunting, remain on the path ahead. Fortunately, a number of these challenges may be resolved by brand new measuring equipment that will become commercially available in the very near future.

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Hexavalent Chromium Generation within Naturally Structured Soils and Sediments

Cited by 140 publications

References 85 publications

Products of Hexavalent Chromium Reduction by Green Rust Sodium Sulfate and Associated Reaction Mechanisms

Products of Hexavalent Chromium Reduction by Green Rust Sodium Sulfate and Associated Reaction Mechanisms

Governing Constraints of Chromium(VI) Formation from Chromium(III)-Bearing Minerals in Soils and Sediments

Emergent Properties of Microbial Activity in Heterogeneous Soil Microenvironments: Different Research Approaches Are Slowly Converging, Yet Major Challenges Remain

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