| One of the most prevalent inorganic contaminants in groundwaters is arsenic, a human carcinogen. Previous studies indicate the potential of using porous activated alumina as a sorbent for arsenic in Fixed Bed Reactors (FBRs), but have not obtained pertinent information needed to develop a model for the column behavior to advance the state of the technology. This technology has been extensively studied for cadmium and selenium in the Stanford Environmental Engineering Program and provides impetus to extend the matrix to include arsenic. The objective of this thesis was to demonstrate that a highly porous transition alumina (ALCOA DD660) provides media for efficient removal of inorganic arsenic to meet new water quality criteria. The work was divided into three distinct experimental categories, each examined for the physical/chemical processes pertinent to evaluating the FBR treatment technology for arsenic remediation. These categories were classified as batch equilibrium sorption, batch rate of uptake and column studies.; The batch equilibrium studies isolated the effect of solution (pH, ionic strength, arsenic concentration, arsenic oxidation state, and in the presence of sulfate, phosphate, silicate, and carbonate) and surface chemistry on arsenic sorption to the alumina and identified optimum conditions for column studies. Arsenate sorption was not significantly affected by other anions in the lower pH range whilst arsenite sorption decreased across the pH range studied. The Triple Layer Model (TLM) for surface complexation satisfactorily described the data and the surface stoichiometries obtained were applied to interpreting column behavior. Rate of uptake experiments investigated the processes occurring at the grain-scale. A pore diffusion model using an empirical Kd representation for adsorption successfully described arsenite uptake, and indicated that other mechanisms, related to the stronger binding of arsenate and sensitivity to changing chemical conditions due to surface complexation, affected arsenate uptake. Column studies assessed the combined effects of surface chemistry, mass transfer and mass transport in a dynamic system under optimum conditions identified in batch experiments. The column behavior was investigated using arsenic concentrations that gave accelerated saturation and extrapolations to concentrations applicable to actual groundwater concentrations suggest that the technology is extremely feasible. |
