Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Sciences, ETH Zurich, 8092 Zurich, Switzerland.
Environ Sci Technol. 2011 Nov 15;45(22):9550-7. doi: 10.1021/es202300w. Epub 2011 Oct 24.
Formation of ternary complexes between arsenic (As) oxyanions and ferric iron (Fe) complexes of humic substances (HS) is often hypothesized to represent a major mechanism for As-HS interactions under oxic conditions. However, direct evidence for this potentially important binding mechanism is still lacking. To investigate the molecular-scale interaction between arsenate, As(V), and HS in the presence of Fe(III), we reacted fulvic and humic acids with Fe(III) (1 wt %) and equilibrated the Fe(III)-HS complexes formed with As(V) at pH 7 (molar Fe/As ~10). The local (<5 Å) coordination environments of As and Fe were subsequently studied by means of X-ray absorption spectroscopy. Our results show that 4.5-12.5 μmol As(V)/g HS (25-70% of total As) was associated with Fe(III). At least 70% of this As pool was bound to Fe(III)-HS complexes via inner-sphere complexation. Results obtained from shell fits of As K-edge extended X-ray absorption fine structure (EXAFS) spectra were consistent with a monodentate binuclear ((2)C) and monodentate mononuclear ((1)V) complex stabilized by H-bonds (R(As-Fe) = 3.30 Å). The analysis of Fe K-edge EXAFS spectra revealed that Fe in Fe(III)-HS complexes was predominantly present as oligomeric Fe(III) clusters at neutral pH. Shell-fit results complied with a structural motif in which three corner-sharing Fe(O,OH)(6) octahedra linked by a single μ(3)-O bridge form a planar Fe trimer. In these complexes, the average Fe-C and Fe-Fe bond distances were 2.95 Å and 3.47 Å, respectively. Our study provides the first spectroscopic evidence for ternary complex formation between As(V) and Fe(III)-HS complexes, suggesting that this binding mechanism is of fundamental importance for the cycling of oxyanions such as As(V) in organic-rich, oxic soils and sediments.
砷（As）氧阴离子与腐殖物质（HS）中的铁（Fe）配合物形成三元配合物通常被假设为有氧条件下 As-HS 相互作用的主要机制。然而，这种潜在重要的结合机制的直接证据仍然缺乏。为了研究在 Fe(III)存在下砷酸盐（As(V)）与 HS 之间的分子尺度相互作用，我们用 Fe(III)（1wt%）与富里酸和腐殖酸反应，并在 pH7 下使形成的 Fe(III)-HS 配合物与 As(V)达到平衡（摩尔比 Fe/As~10）。随后，通过 X 射线吸收光谱法研究了 As 和 Fe 的局部（<5Å）配位环境。我们的结果表明，4.5-12.5μmol As(V)/g HS（总 As 的 25-70%）与 Fe(III)结合。该 As 库的至少 70%通过内配位络合与 Fe(III)-HS 配合物结合。从 As K 边扩展 X 射线吸收精细结构（EXAFS）光谱的壳层拟合获得的结果与通过氢键稳定的单核双配位（(2)C）和单核配位（(1)V）配合物一致（R(As-Fe) = 3.30Å）。Fe K 边 EXAFS 光谱分析表明，在中性 pH 下，Fe(III)-HS 配合物中的 Fe 主要以聚合 Fe(III)簇的形式存在。壳层拟合结果符合一种结构模式，其中三个角共享的 Fe(O,OH)(6)八面体通过单个μ(3)-O 桥连接形成平面 Fe 三聚体。在这些配合物中，平均 Fe-C 和 Fe-Fe 键距离分别为 2.95Å 和 3.47Å。我们的研究为 As(V)与 Fe(III)-HS 配合物形成三元配合物提供了首个光谱证据，表明该结合机制对含氧阴离子（如 As(V)）在富含有机质、有氧的土壤和沉积物中的循环具有重要意义。
