Equilibrium and kinetic studies of arsenite adsorption to a thiol-functionalized surface as a model for arsenite interactions with natural organic matter

Abstract

Sorption mechanisms for the binding of arsenic (As) to thiol functional groups (-SH) in natural organic matter (NOM) may play an important role in controlling inorganic As mobility and bioavailability in anaerobic environments, including rice paddies. There is great interest in the behavior of As in anaerobic soil-water environments due to concerns over As uptake into rice. Here, we examine arsenite (As(III)) sorption onto the thiol-functionalized resin Ambersep GT74 as a model for thiol binding sites in NOM. Specific objectives of this work are to evaluate the kinetics and reversibility of As(III) adsorption onto thiol sites. We perform adsorption and desorption experiments, and kinetics experiments to evaluate equilibrium and kinetic aspects of As(III) adsorption to a thiol-rich adsorbent. Adsorption kinetics were well described by a piecewise first-order model, with relatively fast adsorption during the first 24 h followed by slow adsorption for an additional 72 h. Langmuir isotherm model fits indicated that affinity and capacity values of Ambersep GT74 for As(III) were not significantly different across a pH range typical for natural soil–water systems. Desorption experiments confirmed that As(III) sorption was irreversible at low aqueous As(III) concentrations, suggesting that As(III) bound to thiol-containing surfaces would remain immobile and most likely unavailable for bio-uptake following changes in soil solution chemistry.