Residence time effects on arsenate surface speciation at the aluminum oxide-water interface

Bioavailability of metals/metalloids is often rate limited by contact time (i.e., residence time) in soils and sediments, resulting in irreversible reactions. The fate and transport of the contaminants must be predicted/modeled not only on short-term (<48 h) adsorption/desorption studies but also on long-term (months-years) reactions. However, there is very little information on the long-term effects of metal/metalloid partitioning reactions in soils and soil components. In this study, residence time effects (3 days-1 yr) on As(V) adsorption/desorption reactions and on As(V) surface speciation at the aluminum oxide-water interface were investigated using batch adsorption/desorption experiments coupled with time-resolved Extended X-ray Absorption Fine Structure spectroscopy (EXAFS). Biphasic As(V) adsorption kinetics were observed at pH 4.5 and 7.8, and whereas the reaction at pH 4.5 was nearly completed after 3 days, slow adsorption continued at pH 7.8 after 1 year. The longer the residence time (3 days-1 yr), the greater the decrease in As(V) desorption at both pHs, suggesting nonsingular reactions. EXAFS analyses on aged As(V) reacted aluminum oxide at both pHs showed that As-Al interatomic distances were 3.11 - 3.14 Å (±0.13 Å) in all of the aged samples (3 days to 1 yr) at pH 4.5 and 7.8, suggesting that predominantly bidentate binuclear bonding environments were present. As a point of interest, X-ray Absorption Near Edge Structure spectroscopy (XANES) features suggested some changes in the local chemical structure of adsorbed As(V) with aging. The surface transformations such as (i) a rearrangement of surface complexes and/or (ii) a conversion of surface complexes into aluminum arsenate-like precipitates might be important chemical factors responsible for the decrease in As(V) reversibility with aging.