Visible Light Accelerates Cr(III) Release and Oxidation in Cr–Fe Chromite Residues: An Overlooked Risk of Cr(VI) Reoccurrence

Abstract

The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr(III)−Fe(III) hydroxide, a typical Cr(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr(VI) was produced. While at basic conditions, only Cr(VI) was obtained by photooxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO 2 under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr(III), where photogenerated Fe(II) accelerates Cr(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr(III)−Fe(III) clusters by light leads to the oxidation of Cr(III) into Cr(VI) through the nonradical "metal-to-metal charge transfer" mechanism. Our study provides a new insight into Cr(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.

Enhanced Oxidation of Cr(III)–Fe(III) Hydroxides by Oxygen in Dark and Alkaline Environments: Roles of Fe/Cr Ratio and Siderophore

“…28 And Fe incorporation facilitated this oxidation process via the formation of a Cr(III)−Fe(III) cluster, which was supported by a linear correlation between the amount of produced Cr(VI) under light irradiation and the concentration of aqueous Cr(III) from Cr 1−x Fe x (OH) 3 dissolution in the dark. 28 However, only slight changes (i.e., ±0.1−0.2 μM) in aqueous Cr(III) concentration from Cr 1−x Fe x (OH) 3 dissolution were observed under the experimental conditions applied in this study (Figure S10). Such a minute difference in aqueous Cr(III) concentration was not able to fully explain the positive linear relationship between Cr(VI) production rate and Fe/Cr ratio, especially at pH 8 and 9 (Figure S9B).…”

“…All experiments were conducted in the dark to avoid the photochemical transformation of Cr(III)−Fe(III) hydroxides (i.e., Cr(III) oxidation accompanied by Fe(III) reduction). 28 Results from this study provide in-depth insights into the oxidation potential of Cr(III)−Fe(III) hydroxides under dark and alkaline conditions and contribute to the understanding of toxic Cr(VI) reoccurrence at alkaline Cr-contaminated sites (e.g., rCOPR) and geogenic formation of Cr(VI) in natural environments (e.g., serpentinite soils/groundwater).…”

“…Indeed, recent investigations revealed that in the absence of Mn oxides, Cr(III) oxidation at alkaline pH can occur via the following pathways: (1) direct oxidation by other strong oxidants such as H 2 O 2 , 24,25 (2) catalytic oxidation by Mn(II), where adsorbed Mn(II) on the surface of Cr(III)−Fe(III) hydroxides can be oxidized to Mn oxides, which can oxidize Cr(III) to Cr(VI) subsequently, 26,27 and (3) oxidation of Cr(III) accompanied by Fe(III) reduction via nonradical metal-to-metal charge transfer in the presence of light. 28 However, the individual role of oxygen (i.e., a thermodynamically viable oxidant of Cr(III) under alkaline conditions) 29 is often underestimated or ignored, with one exception in which Cr(VI) production from pure Cr(OH) 3 oxidation by molecular oxygen at pH 9−11 was observed. 17 The oxidation kinetics of Cr(III)−Fe(III) hydroxides via the above pathways are primarily affected by the pH of the solution and Fe/Cr ratio in Cr(III)−Fe(III) hydroxides.…”

“…17 The oxidation kinetics of Cr(III)−Fe(III) hydroxides via the above pathways are primarily affected by the pH of the solution and Fe/Cr ratio in Cr(III)−Fe(III) hydroxides. 17,21,22,27,28 In most cases, Cr(III) dissolution was considered to be a prerequisite for the subsequent Cr(III) oxidation; thus, the effects of pH and Fe/Cr ratio were accomplished via controlling Cr(III) solubility in Cr(III)− Fe(III) hydroxides. 17,21,22,27,28 A higher pH (>9) usually leads to a higher Cr(III) oxidation rate, which is usually ascribed to the increased dissolution rate of Cr(III)−Fe(III) hydroxides at such a pH due to the amphoteric behavior of Cr(III).…”

“…17,21,22,27,28 A higher pH (>9) usually leads to a higher Cr(III) oxidation rate, which is usually ascribed to the increased dissolution rate of Cr(III)−Fe(III) hydroxides at such a pH due to the amphoteric behavior of Cr(III). 17,28 In contrast, the effect of the Fe/Cr ratio in Cr(III)−Fe(III) hydroxides on Cr(III) oxidation depends on the oxidation pathway. 25,27,28 For instance, the Cr(III) oxidation rate via Mn(II) catalytic oxidation decreased with an increase in Fe/Cr ratio, 27 but increased if the Cr(III) oxidation was achieved by H 2 O 2 in the dark or Fe(III) under visible light irradiation.…”