Chromium(III) Oxidation\nby Three Poorly-Crystalline Manganese(IV) Oxides. 1. Chromium(III)-Oxidizing\nCapacity

Abstract

The Cr­(III)-oxidizing capacity of three layered poorly\ncrystalline Mn­(IV)­O₂ phases, i.e. δ-MnO₂, Random Stacked Birnessite (RSB), and Acid Birnessite (AB), was\ndetermined in real-time and in situ, using Quick X-ray Absorption\nFine Structure Spectroscopy (Q-XAFS). The results obtained with this\ntechnique, which allows the measurement of the total amount of Cr­(VI)\nproduced in the system, indicated that the Cr­(III) oxidation reaction\nhad ceased between 30 min and 1 h under most experimental conditions.\nHowever, this cessation was not observed with a traditional batch\ntechnique, which only allows the measurement of Cr­(VI) present in\nsolution and thus neglects the amount of Cr­(VI) that may be sorbed\nto Mn­(IV)­O₂. This study also demonstrated that the Mn­(IV)­O₂ phase oxidizing the highest amount of Cr­(III), which is positively\ncharged in solution, was the mineral featuring the most negatively\ncharged surface. Also, the results indicated that the presence of\nMn­(II) and/or Mn­(III) impurities inside the Mn­(IV)­O₂ structure\ncould enhance the mineral’s capacity to oxidize Cr­(III). The\ninformation provided in this study will be useful in predicting the\ncapabilities of naturally occurring Mn oxide minerals, which are similar\nto the three synthetic Mn­(IV)­O₂ investigated, to oxidize\nCr­(III) to toxic and mobile Cr­(VI) in the soil of contaminated sites.