Production of Cr(VI) from CrxFe1–x(OH)3 precipitates and NOM-Cr(III) colloids upon reaction with H2O2 under oxic conditions

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

“…Indeed, a better linear correlation (i.e., r 2 = 0.96) between the oxidation rate of Cr­(OH) 3 by oxygen and pH was observed in a recent study when a longer reaction period (i.e., 88 days) was applied . Similar pH-enhanced Cr­(III) oxidation was previously observed by Mn­(II)-catalytic oxidation, H 2 O 2 , or Fe­(III) in the presence of light, − during which the generation of Cr­(VI) was thought to be a result of the oxidation of aqueous Cr­(III) formed from the dissolution of Cr­(III)–Fe­(III) hydroxides. − Therefore, the promoted Cr­(III) oxidation at a higher pH in these previous studies was generally ascribed to the enhanced dissolution of Cr­(III)–Fe­(III) hydroxides due to the amphoteric behavior of Cr­(III) with an increased solubility at pH > 10. , …”

“…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. 25,28 To date, it is unclear how pH and the Fe/Cr ratio affect the oxidation of Cr(III)−Fe(III) hydroxides by oxygen in dark and alkaline environments.…”

“…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 , , (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, , and (3) oxidation of Cr­(III) accompanied by Fe­(III) reduction via nonradical metal-to-metal charge transfer in the presence of light . However, the individual role of oxygen (i.e., a thermodynamically viable oxidant of Cr­(III) under alkaline conditions) 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 …”

“…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. ,,,, 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. ,,,, 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). , In contrast, the effect of the Fe/Cr ratio in Cr­(III)–Fe­(III) hydroxides on Cr­(III) oxidation depends on the oxidation pathway. ,, For instance, the Cr­(III) oxidation rate via Mn­(II) catalytic oxidation decreased with an increase in Fe/Cr ratio, but increased if the Cr­(III) oxidation was achieved by H 2 O 2 in the dark or Fe­(III) under visible light irradiation. , To date, it is unclear how pH and the Fe/Cr ratio affect the oxidation of Cr­(III)–Fe­(III) hydroxides by oxygen in dark and alkaline environments.…”