Reactions of hexavalent chromium with black carbon when co-existing with iron or manganese hydrous oxides

• Main Authors: Ya- Jing Chang, 張雅菁
• Other Authors: 王尚禮
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/11149999038953198983

碩士 === 國立中興大學 === 土壤環境科學系所 === 100 === Cr(VI) is one of the pollutants of major concern in the environment due to its high toxicity and carcinogenicity. In oxic soils, organic matter (OM) and black carbon (BC) are the predominant reductants that can reduce toxic Cr(VI) to less toxic Cr(III). However, Cr(VI) still can be leached to deeper layers of soil or even into groundwater. The low reduction rate of Cr(VI) is presumably attributed to the reactions of other soil constituents with Cr(VI) that inhibit the reduction of Cr(VI) by OM and BC. Thus, in this study, the influences of hydrous oxides of Fe and Mn on the Cr(VI) reduction of BC were investigated. The results showed that the co-existence of goethite with BC inhibited the Cr(VI) reaction of BC due to the fast adsorption rate and strong desorption hysteresis of Cr(VI) on goethite. Nonetheless, the prolonged reaction of Cr(VI) with BC and goethite removed Cr(VI) from solution through the adsorption of Cr(VI) and the subsequent reduction of adsorbed Cr(VI) to Cr(III). The resulting Cr(III) is either released back into solution or bound on BC. When MnO2 coexisted with BC, the aqueous Cr(III) were oxidized to Cr(VI) by MnO2. The Cr(III) oxidation and releasing back into solution occur as coupled reactions. The oxidation of aqueous Cr(III) by MnO2 inhibited the Cr(VI) reduction by BC when MnO2 coexisted with BC. At lower pH, the net positive charge on the surface of BC resulted in electrostatic repulsion between Cr(III) and BC. On the other hand, the oxidation of Cr(III) by Mn oxide led to the formation of Cr(VI) in solution, which was subsequently reduced by BC and consequently enhanced the removal of total Cr by BC at lower pH when BC coexisted with MnO2. In summary, the presence of Fe and Mn oxides inhibited the reduction of Cr(VI) by BC, leading to an increasing risk of Cr(VI) contamination in soil. Thus, the interactions between different soil consitutents need to be considered in order to have better understanding of the environmental risks of Cr contamination in soil.