Simultaneous Redox Conversion of Chromium(VI) and Arsenic(III) under Acidic Conditions

Abstract

Arsenic and chromium are often abundant constituents of acid mine drainage (AMD) and are most harmful as arsenite (As(III)) and hexavalent (Cr(VI)). To simultaneously change their oxidation state from As(III) to As(V), and Cr(VI) to Cr(III), is a potentially effective and attractive strategy for environmental remediation. The coabundance of As(III) and Cr(VI) in natural environments indicates their negligible direct interaction. The addition of H2O2 enables and greatly accelerates the simultaneous oxidation of As(III) and reduction of Cr(VI). These reactions are further enhanced at acidic pH and higher concentrations of Cr(VI). However, the presence of ligands (i.e., oxalate, citrate, pyrophosphate) greatly retards the oxidation of As(III), even though it enhances the reduction of Cr(VI). To explain these results we propose a reaction mechanism where Cr(VI) is primarily reduced to Cr(III) by H2O2, via the intermediate tetraperoxochromate Cr(V). Cr(V) is then involved in the formation of (•)OH radicals. In the presence of ligands, the capacity of Cr(V) to form (•)OH radicals, which are primarily responsible for As(III) oxidation, is practically inhibited. Our findings demonstrate the feasibility for the coconversion of As(III) and Cr(VI) in AMD and real-world constraints to this strategy for environmental remediation.

Redox Conversion of Chromium(VI) and Arsenic(III) with the Intermediates of Chromium(V) and Arsenic(IV) via AuPd/CNTs Electrocatalysis in Acid Aqueous Solution

“…The distinct UV absorption peak at 352 nm and spectrum >500 nm corresponded to the [Cr(VI)O 4 ] 2− and tetraperoxochromate-(V) anion complex, respectively. 16,36 The peak at 352 nm declined, while the absorbance >500 nm increased at 5 min, confirming the abovementioned hypothesis that Cr(V) was initially accumulated (Figure 3b). Moreover, both the decrease of the peak at 352 nm and absorbance >500 nm up to 15 min suggested that the simultaneous reduction of Cr(VI) and Cr(V) by H 2 O 2 was dominant.…”

“…It is possible that the intermediates resulting from the reduction of Cr(VI) gave high priority to the decomposition of H 2 O 2 , and soon after, boosted the oxidation of As(III). 16 However, the measured half-life for the reaction of Cr(VI) with H 2 O 2 is about 0.02 s, 35 thus, the previous results cannot make a convincing explanation for the delay of the conversion of As. Hence, it can be presumed that more intermediates and active species are involved in the Cr(VI)/ As(III) redox conversion via AuPd/CNTs electrocatalysis, which significantly control the conversion pathways of Cr(VI) and As(III).…”

“…This acceleration process greatly shortened the treatment time compared with electrolysis alone and the reported chemical conversion of Cr(VI)/As(III). 13,16 Clearly, this enhancement of Cr(VI)/As(III) redox conversion could be attributed to the contribution of the AuPd/CNTs electrocatalysts, which had been proved to provide sufficient H 2 O 2 to accelerate the simultaneous reduction of Cr(VI) and oxidation of As(III). The contributions of Cr(VI) removal and As(V) generation by AuPd/CNTs electrocatalysis and electrolysis were analyzed separately.…”

Kinetic Studies of Cr(VI) Reduction in an Indigenous Mixed Culture of Bacteria in the Presence of As(III)

“…Equilibrium Equation (2.6-2.7) shows that H2AsO3 -anion is dominant in basic or slightly acidic solution whiles the HAsO3 2-dominate in basic solution (Wagman et al, 1968). The biochemistry of arsenic species has also been demonstrated by researchers (Wang et al, 2013;Sun et al, 2010;Aniruddha and Wang, 2010;Sun et al, 2008). Aniruddha and Wang, (2010) reported that As(III) can be oxidised to As(V) in the presence of oxidizing agent by donating 2 electrons, while generating a considerable amount of energy for cell growth and metabolism Equation (2.8).…”

“…Aniruddha and Wang (2010) reported that about 256 KJ/mol energy is generated during oxidation of As(III) to As(V). Further studies showed that about 467.95KJ of energy could be generated in the process (Wang, 2013 …”