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

Abstract

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.

Beam-induced oxidation of mixed-valent Fe (oxyhydr)oxides (green rust) monitored by STEM-EELS

“…GR phases can be easily engineered for a wide-range of applications such as catalysis, electrochemistry, and environmental remediation (Bhave and Shejwalkar, 2018;Chen et al, 2018;Huang et al, 2019;Zhang et al, 2018). In particular, they are promising reactants for ground water remediation where they have been shown to remove toxic metal contaminants from water by adsorption (Jönsson and Sherman, 2008;Mitsunobu et al, 2009;Perez et al, 2019), reduction (O'Loughlin et al, 2003;Skovbjerg et al, 2006;Thomas et al, 2018), interlayer intercalation (Refait et al, 2000) and substitution of structural Fe (Ahmed et al, 2008;Refait et al, 1990 Christiansen et al). The structural Fe(II)/Fe(III) ratio of 2 has been determined by Mössbauer spectroscopy, chemical analysis, and X-ray diffraction (Christiansen et al, 2009;Génin et al, 1996;Hansen et al, 1994;Perez et al, 2019;Refait et al, 1999Refait et al, , 1990.…”

“…Cr, Se, U) or its adsorption uptake (e.g. As) (Jönsson and Sherman, 2008;O'Loughlin et al, 2003;Perez et al, 201λ;Skovbjerg et al, 2006;Thomas et al, 2018). One way to check the stability of GR over time is to monitor the Fe(II)/Fe(III) ratio following interaction with metals.…”

“…Analytical transmission electron microscopy (TEM) provides information at high spatial resolution regarding the morphology, crystal structure, chemical composition and oxidation state of a specimen. To date, many TEM studies of GR have used conventional sample preparation techniques (drop-cast and dried) under anoxic conditions (Ahmed et al, 2010;Bach et al, 2014;Géhin et al, 2002;Mann et al, 1989;Perez et al, 2019;Skovbjerg et al, 2006;Thomas et al, 2018). Such methods involve aqueous sample dilution, drop casting onto a TEM grid and drying with alcohol in an anaerobic chamber to minimise the risk of oxidation.…”

Effects of metal cation substitution on hexavalent chromium reduction by green rust

“…Green rust 1 has a narrow interlayer spacing of ~ 8 Å occupied by chloride or carbonate, while green rust 2 has a broad interlayer spacing (~ 11 Å) typically occupied by sulfate, which allows exchange of tetrahedral oxyanions and subsequently reduction and sequestration of these substances in the reaction product's interlayer [13][14][15]. Therefore, it is a promising reagent for exchange and/or reduction of selected groundwater contaminants such as As [16,17], NO 3 − [18,19], U (VI) [15,20], Se (VI) [21,22], Np [23] and Cr(VI) [13,14,[24][25][26][27][28].…”

“…Bond and Fendorf [13] and Skovbjerg et al [14] concluded that these products formed due to exchange of chromate for interlayer sulfate followed by reduction. More recently, our previous study [28] reacted green rust with a series of initial chromium concentrations typical of contaminant plumes and determined that the speciation of chromium in the reaction product is correlated to the initial concentration. Although more goethite was found in the reaction products formed at higher initial concentrations, Cr(III) hydroxide, presumably located on the oxidized green rust particle surfaces, was the primary Cr(III) carrier phase produced.…”