Structure and spectroscopic study of aqueous Fe(III)-As(V) complexes using UV-Vis, XAS and DFT-TDDFT.

Aqueous complexes between ferric (Fe(III)) and arsenate (As(V)) are indispensable for understanding the mobility of arsenic (As) in Fe(III)-As(V)-rich systems. In this study, aqueous Fe(III)-As(V) complexes, FeH2AsO42+ and FeHAsO4+, were postulated based on the qualitative analysis of UV-Vis spectra in both Fe(III)-As(V)-HClO4 and Fe(III)-As(V)-H2SO4 systems. Subsequently, monodentate structures were evidenced by Fe K-edge EXAFS and modeled as [FeH2AsO4(H2O)5]2+ and [FeHAsO4(H2O)5]+ by DFT. The feature band at ∼280 nm was verified as electron excitation chiefly from Fe-As-bridged O atoms to d-orbital of Fe in [FeH2AsO4(H2O)5]2+ and [FeHAsO4(H2O)5]+. The structural and spectral information of Fe(III)-As(V) complexes will enable future speciation analysis in Fe(III)-As(V)-rich system.