Speciation and surface structure of inorganic arsenic in solid phases: a review

Accurate determination of individual arsenic species is critical because the toxicology, mobility, and adsorptivity of arsenic vary substantially with its chemical forms and oxidation states. Separation techniques together with techniques for chemical identification make it possible to determine the combinational forms and oxidation states of arsenic in solid phases. Selective sequential extraction is often employed to determine operationally defined fractions, but it has a poor precision and selectivity. Direct methods, based on X-ray techniques and vibrational spectroscopy, have been developed to analyze the valence, local coordination, protonation, and other properties of arsenic in solid phases. Extensive research studies in the literature have been performed to elucidate the interfacial reactions between inorganic arsenic and solid surfaces of sulfides, and Fe, Al, and Mn (hydro)oxides. Outer-sphere and inner-sphere complex (monodentate mononuclear, bidentate mononuclear, and bidentate binuclear complex) models have been proposed to interpret the sorption mechanisms. The nature of the surface complexes has been inspected by spectroscopic methods but remains controversial. This paper focuses on the recent advancement in arsenic speciation in solid phases and covers relevant methodological, analytical and modeling aspects. The identification of arsenic species in natural materials, however, is complicated by the presence of multiple species, and the applications of instrumental methods are usually limited due to their comparatively high detection limits. Development of advanced in-situ methods with high sensitivity, therefore, is required.