Adsorption and redox transformation of arsenite (As(III)) and arsenate (As(V)) at water-mineral interfaces

Adsorption and redox transformation are the two main processes controlling the mobility and bioavailability of arsenic in soil and water. In this research both wet chemistry and spectroscopic techniques were applied to study the adsorption and redox transformation of As(III) and As(V) on model minerals as well as in sod and sediment.; The present research is therefore summarized as follows: (1) For the first time, by using FTIR combined with a deuteration method, the singly (A-type), doubly (C-type) and triply (B-type) coordinated OH groups on goethite surfaces have been identified. By using this technique, it is found that major fractions of adsorbed arsenate and arsenite replaced singly coordinated hydroxyls (A-type) and may have formed inner-sphere binuclear complexes on the (110) faces of goethite. (2) Mn and Fe are closely related in chemical properties and often occur together in soils and sediments. Mn can isomorphously substitute Fe in the goethite structure. In this study Mn-substituted goethite was synthesized. The interaction between arsenite As(III) and Mn-substituted goethite was investigated by both wet chemistry and XANES spectroscopy. The results indicated that the oxidation of As(III) could be catalyzed by Mn-substituted goethite. This reaction was more sensitive to temperature than to pH. (3) The aging and chemical extraction experiments indicated that the solvents used did not provide good selectivity for defined “chemical forms” of arsenic in model sod minerals. The results from three typical schemes indicated that even qualitative interpretation of data from these selective sequential extraction methods was difficult. It is suggested that the fractions should be divided according to their affinity to the minerals rather than the “chemical forms”.; The adsorption and redox transformation of arsenic oxyanions on different components of soils and sediments are the major processes for controlling the mobility and bioavailability of arsenic in the terrestrial ecosystem. The results from this study provides important information for understanding these processes and potentially, possible techniques for arsenic contamination control. (Abstract shortened by UMI.)...