Plant and Soil Sciences Department, Delaware Environmental Institute, University of Delaware, 152 Townsend Hall, Newark, Delaware 19716, USA.
Environ Sci Technol. 2012 Nov 6;46(21):11594-600. doi: 10.1021/es302383y. Epub 2012 Oct 19.
The Cr(III)-oxidizing capacity of three layered poorly crystalline Mn(IV)O(2) phases, i.e. δ-MnO(2), Random Stacked Birnessite (RSB), and Acid Birnessite (AB), was determined in real-time and in situ, using Quick X-ray Absorption Fine Structure Spectroscopy (Q-XAFS). The results obtained with this technique, which allows the measurement of the total amount of Cr(VI) produced in the system, indicated that the Cr(III) oxidation reaction had ceased between 30 min and 1 h under most experimental conditions. However, this cessation was not observed with a traditional batch technique, which only allows the measurement of Cr(VI) present in solution and thus neglects the amount of Cr(VI) that may be sorbed to Mn(IV)O(2). This study also demonstrated that the Mn(IV)O(2) phase oxidizing the highest amount of Cr(III), which is positively charged in solution, was the mineral featuring the most negatively charged surface. Also, the results indicated that the presence of Mn(II) and/or Mn(III) impurities inside the Mn(IV)O(2) structure could enhance the mineral's capacity to oxidize Cr(III). The information provided in this study will be useful in predicting the capabilities of naturally occurring Mn oxide minerals, which are similar to the three synthetic Mn(IV)O(2) investigated, to oxidize Cr(III) to toxic and mobile Cr(VI) in the soil of contaminated sites.
采用快速 X 射线吸收精细结构光谱学（Q-XAFS）实时原位法测定了三种层状非晶态 Mn(IV)O2 相（即 δ-MnO2、随机堆叠锰矿（RSB）和酸性锰矿（AB））的 Cr(III)氧化能力。该技术可测量体系中生成的 Cr(VI)总量，其结果表明，在大多数实验条件下，Cr(III)氧化反应在 30 分钟至 1 小时之间停止。然而，传统的分批技术并未观察到这种停止，该技术仅允许测量溶液中存在的 Cr(VI)，从而忽略了可能被 Mn(IV)O2 吸附的 Cr(VI)量。本研究还表明，在溶液中带正电荷的 Cr(III)被氧化量最高的 Mn(IV)O2 相是表面带负电荷最多的矿物。此外，结果表明，Mn(IV)O2 结构中存在 Mn(II)和/或 Mn(III)杂质可能会增强矿物氧化 Cr(III)的能力。本研究提供的信息将有助于预测类似于三种所研究的合成 Mn(IV)O2 的天然存在的 Mn 氧化物矿物在污染场地土壤中将 Cr(III)氧化为毒性和迁移性 Cr(VI)的能力。
