Equilibrium adsorption and mass transfer of protons and organoarsenicals at the oxide-solution interface

Chemical equilibria and mass transfer phenomena for proton and organoarsenical adsorption at the oxide solution-interface are investigated. The interfacial properties of two adsorbents, hydrous ferric oxide and {dollar}gamma{dollar}-alumina, were determined by surface titration. The adsorption of two organoarsenicals, methylarsenate and cacodylate, were measured as a function of pH, ionic strength, and adsorbate-adsorbent ratio in batch equilibrium experiments. The extent of adsorption decreased with increasing pH, but was independent of ionic strength for all adsorbate-adsorbent combinations. The data were interpreted in terms of the triple-layer surface complexation model modified to include sites of two different adsorption energies. Model predictions of methylarsenate removal also compared with those measured in ferric coprecipitation experiments.; The rate of proton and methylarsenate uptake by alumina was measured in batch experiments. The rate of proton uptake was observed to increase with decreasing ionic strength, pH, and particle size; similar observations were recorded for methylarsenate with the rate also increasing with the adsorbate-adsorbent ratio. The rate of methylarsenate desorption from alumina was measured in differential column reactors. The rate of desorption increased slightly with pH in the range of 5 to 9, but increased more rapidly as the pH was increased above 9. Ionic strength exhibited only a minor effect below pH 9. Some fraction of the adsorbed methylarsenate appears to be bound irreversibly; removal of this fraction increased with pH.; A mathematical model was developed to describe the rate of ion adsorption and desorption in terms of film diffusion, coupled ionic intraparticle diffusion, and surface complexation equilibria. The model was able to approximate the effect of ionic strength and particle size on proton uptake and the effect of pH, adsorbate-adsorbent ratio, and particle size on methylarsenate uptake. It was able to predict the effect of pH on proton uptake and the effect of ionic strength on methylarsenate uptake only through ad hoc manipulation of model parameters. The model was also able to predict the effects of pH on desorption during the initial portion of the experiments, but failed to describe the desorption of the irreversibly adsorbed methylarsenate.