DISSOLVED ORGANIC CARBON DIMINISHES MANGANESE OXIDE-DRIVEN OXIDATION OF CHROMIUM

Toxic chromium (Cr) contamination of soils and water from anthropogenic and geogenic sources is a pervasive environmental and health issue. The toxicity of Cr is highly dependent on its oxidation state and interactions in the environment. Chromium (III) is less toxic while hexavalent chromium Cr(VI) is a very mobile and toxic carcinogen. Manganese (Mn) oxides are very ubiquitous, and prolific in facilitating Cr(III) oxidation. Also, in environmental systems, Cr availability and reactivity is dependent on the suite and concentrations of structurally diverse natural dissolved organic carbon (DOC). These confounding factors make it difficult to assess the specific risks of Cr contamination in natural systems. We hypothesize that increasing concentration of DOC will promote Cr solubility. However, DOC moieties may impact Cr(VI) concentration in solution.

The objective of this study is to evaluate the mechanisms of Cr redox dynamics as function of Mn-induced oxidation, DOC concentration and composition. To address these objectives, mixed batch experiments using synthesized Cr hydroxide, manganese oxide (50 nm), citric and gallic acid (CA and GA) as DOC source was conducted for a period of 2 weeks. Dissolved Cr(VI), total Cr [Cr]_T and Mn [Mn]_T and DOC has been quantified in order to elucidate mechanisms of Cr(VI) production across DOC gradients of 0.5 – 10 mM.

Preliminary results at pH 5 show that total Cr, Mn and Cr(VI) production is a function of CA and GA concentration. At 0.5 mM, CA caused rapid Cr(VI) production which peaked at 5.2 µm within 48 hours. However, within the same period, 10 mM CA produced 7.5 times less Cr(VI). This is lesser than the 1.92 µm threshold for [Cr]_T set by the USEPA for drinking water. Similarly, GA enhanced Cr and Mn solubilization, however, at a concentration of 0.5 mM, GA produced approximately 22 times the USEPA threshold for [Cr]_T in drinking water.

Ongoing work on X-ray absorption spectroscopy data seeks to determine the speciation and proportions of Cr in final reaction products of citric and gallic acid batch experiments. Preliminary data show at least 93% of the Cr in the reaction products from CA treatments is Cr(III). These results will show the efficacy of DOC type and concentration in limiting Cr oxidation and help better understand fate of Cr in multi-component systems.