Cross-redox and simultaneous removal of Cr(VI) and As(III): Influences of Fe(II), Fe(III), oxalic acid, and dissolved organic carbon

• Published in: Ecotoxicology and Environmental Safety Vol. 245; p. 114084
• Main Authors: Ng, Kim Hoong, Hsu, Liang-Ching, Liu, Yu-Ting, Hsiao, Chuan-Yen, Chiang, Po-Neng, Teah, Heng Yi, Hung, Jui-Ting, Tzou, Yu-Min
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 15.10.2022; Elsevier BV; Elsevier
• Subjects: Arsenites; As(III) oxidation; Chromium; Cr(VI) reduction; Cross-redox; Dissolved Organic Matter; Ecosystem; Environmental pollution; Environmental sciences; Ferric Compounds; Ferrous Compounds; GE1-350; Humans; Oxalic Acid; Oxidation-Reduction; Soil; TD172-193.5; X-ray absorption spectroscopy; XAS; Cross-redox; As(III) oxidation; X-ray absorption spectroscopy; XAS; Cr(VI) reduction
• ISSN: 0147-6513; 1090-2414; 1090-2414
• DOI: 10.1016/j.ecoenv.2022.114084

Hexavalent chromium [Cr(VI)] and arsenite [As(III)] are hazardous to both human and ecosystem. While their cross-redox reaction decreases both their toxicities, the interferences from ubiquitous substances like Fe (Fe(II) and Fe(III)) and organic compounds (oxalic acid and soil-extracted dissolved organic carbon (DOC)) on such interaction are rarely reported; thence, inspires the investigation in this study. Results showed that the cross-redox, in the absence of interfering substances, only occurred at pH≤2.0, with reaction orders of 0.676 and 0.783 in respect to the concentration of Cr(VI) and As(III). The pseudo-reaction constant, k′, of such reaction was recorded at 0.087 m1.377/(mmol0.459 min). With the addition of Fe(II), the rate of Cr(VI) reduction is promoted in conjunction with suppressed As(III) oxidation. Upon neutralizing to pH 6.0, such reduced Cr can be entirely removed via Fe(II)-assisted adsorption and/or co-precipitation. Meanwhile, the elimination of aqueous As is relatively inferior (36 %), attributed to the largely preserved As(III), which is less susceptible to adsorptive/co-precipitative removal. Unlike Fe(II), Fe(III) did not alter Cr(VI)-As(III) cross-redox path, but triggered high adsorptive and/or co-precipitative removals of Cr and As (90 %). In contrast, both organically-altered systems exhibits plummeted As(III) oxidation, under distinctive mechanisms: oxalic acid competes with As(III) in the redox interactions while DOC reduces As(V) into As(III). Also, DOC would undergo complexion with metals and/or blocked the adsorption or co-precipitation sites, leading to even lower Cr and As precipitation. This study unravelled the interference from ubiquitous species in the co-removal of Cr(VI) and As(III), which provides insightful remediation for heavy metal contaminations. [Display omitted] •Fe(II) improves Cr reduction but retards As oxidation, yet Fe(III) has no impacts.•Oxalate and DOC seldom affects Cr reduction but retards As oxidation.•Fe(II) and Fe(III) could enhance the Cr and As removal respectively at pH 6.•DOC addition into Fe(III) systems deteriorates co-precipitation of Cr and As.