Structural Configuration of Arsenate on Nano Goethite Surface Using EXAFS and DFT Calculation.

For better understand the dynamics of oxyanion in the rhizosphere, it is essential to recognize the structural configuration of complexation at the solid-water interface. Arsenate is the one of problematic because of the toxicity and ubiquity, and many studies have attempted to identify the complexation, but it is still debatable. In this study, we employed EXAFS and DFT calculation to interpret the structural configuration by varying pH (4, 7 and 10) and sorbate sorbent ratio (SSR, 4.8 and 48 μmol m^-2). EXAFS measurement was carried out on 7D beamline of Pohang accelerator laboratory (PLS-II, 3.0 GeV, 360 mA), and DFT calculation was conducted by Gaussian 09. Previous studies confirmed the peak at 2.6-2.7  for multiple scattering (MS) of As-O-O-As, the peak at 2.9-3.0  for bidentate mononuclear (BM) complex, the peak at 3.2-3.3  for bidentate binuclear (BB) complex and the peaks at 3.5-3.6  for monodentate mononuclear (MM) complex. Recent study also reported possibility of tridentate binuclear (TB) complex by quantum chemistry. In the EXAFS measurement for As, the intensity of individual peak was changed by pH and SSR. The increase in pH significantly decreased the peak at 2.6 and 3.5  while the increase in SSR yielded less change in the peak. We also confirmed 2.6 and 3.5  peak by Fe EXAFS. Based on our observation, we assumed the additional peak was overlapped on the MS peaks because the MS peak should be consistent with As-O shell. We calculated TB complex with two ferric hydroxide clusters using DFT, and the atomic distance of As-Fe was 2.51 and 3.47 . Based on the result, we concluded that TB complex would be present with BB, BM and MM complex together, and abundance of TB complex would be increased at low pH and SSR on the nano goethite surface.