Zhang Yinfeng, Li Shehong, Sun Jing, Bostick Benjamin C, Zheng Yan
Yunnan Key Laboratory of Plateau Wetland Conservation, Restoration and Ecological Services, College of Wetlands, Southwest Forestry University, Kunming, 650224, China.
State Key Lab of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China.
Environ Sci Nano. 2021 Oct 1;8(10):2964-2975. doi: 10.1039/D1EN00502B. Epub 2021 Aug 13.
Understanding how natural nanoaggregates of iron (Fe) and organic matter (OM), currently identified in organic rich soil or peat, interact with metals and metalloids is environmentally significant. Coal is also organic-rich and exemplifies anoxic sedimentary environments with Fe usually as pyrite and not oxides. Here, we analyze the local structure of Fe (6880-21 700 mg kg) and As (45-5680 mg kg) in representative Guizhou coal samples using X-ray absorption near-edge structure and extended X-ray absorption fine structure (XANES and EXAFS) to illustrate how Fe(iii) and As(v) are preserved in coal formed from reduced, organic-rich precursors. Arsenic XANES indicates that >80% of As exists as As(v) with <14% of As associated with sulfides in 5 Guizhou coal samples, confirming published but unexplained results. An As-Fe shell at 3.25-3.29 Å in the As EXAFS suggests that this As(v) is adsorbed on Fe(iii) oxyhydroxides as evidenced by Fe EXAFS in these coal samples. Significantly, lower Fe-Fe coordination numbers (CN) of 0.6-1.1 relative to those in 2-line ferrihydrite (CN = 1.6) and goethite (CN = 2.1) suggest that these Fe(iii) oxyhydroxides are likely Fe-OM nanoaggregates protected by OM encapsulation and adsorption of arsenate. Such structurally stabilized composites of As(v)-Fe(iii)-OM may be more widely distributed and allow oxidized As and Fe to persist in other organic-rich, reducing environments.
了解目前在富含有机质的土壤或泥炭中发现的铁（Fe）和有机质（OM）的天然纳米聚集体如何与金属和类金属相互作用具有重要的环境意义。煤也是富含有机质的，并代表了缺氧沉积环境，其中铁通常以黄铁矿形式存在而非氧化物形式。在这里，我们使用X射线吸收近边结构和扩展X射线吸收精细结构（XANES和EXAFS）分析了贵州代表性煤样中Fe（6880 - 21700 mg/kg）和As（45 - 5680 mg/kg）的局部结构，以说明Fe(III)和As(V)如何在由还原的、富含有机质的前体形成的煤中得以保存。砷的XANES表明，在5个贵州煤样中，>80%的As以As(V)形式存在，<14%的As与硫化物相关，这证实了已发表但未解释的结果。As的EXAFS中在3.25 - 3.29 Å处的As - Fe壳层表明，这种As(V)吸附在氢氧化铁上，这些煤样中的Fe EXAFS证明了这一点。值得注意的是，相对于二线水铁矿（配位数 = 1.6）和针铁矿（配位数 = 2.1），较低的Fe - Fe配位数（CN）为0.6 - 1.1，这表明这些氢氧化铁可能是由OM包裹和砷酸盐吸附保护的Fe - OM纳米聚集体。这种结构稳定的As(V) - Fe(III) - OM复合材料可能分布更广泛，并使氧化态的As和Fe在其他富含有机质的还原环境中持续存在。
