Application of phosphate and surfactant-modified zeolite for remediation/attenuation of trace elements in soil and coal fly ash
This dissertation presents results of a research work aimed at understanding and addressing trace element contamination sourced by coal fly ash and arsenic trioxide herbicide. Both alkaline and acidic fly ash samples were found to contain significant concentrations of environmentally available trace elements. The treatment of fly ash leachate with surfactant-modified zeolite (SMZ) decreased the mobility of several trace elements. In general, up to 30% of the As, Mo, and V; up to 80% of the Cr; and up to 20% of the Se and Sr were removed from the leachate after SMZ treatment. Batch experiments, surface complexation modeling, and X-ray spectroscopic tools were used to elucidate the kinetics and mechanisms of arsenate (As(V)) and phosphate (Pi) adsorption on ferric hydroxide. Both oxyanions showed similar adsorptions during single-ion adsorption experiments; however, As(V) was preferentially adsorbed during competitive adsorption experiments. Similarly, more As(V) was adsorbed when it was loaded in sequence in Pi-equilibrated system than vice versa. Both oxyanions competed for adsorption on ferric-hydroxide and each of them showed a limited capacity to desorb the other, and relatively, more pre-equilibrated Pi was desorbed by sequentially added As(V) than vice versa. The As K-edge EXAFS analysis indicated the presence mononuclear and binuclear bidentate As(V) surface complexes. The Fe coordination numbers (CN) of these complexes increased with increasing time and decreased with addition of Pi into the system. Finally, an arsenic-contaminated soil collected from an industrial site located in the southeastern United States was amended with Pi and Ca to precipitate the arsenic as As-bearing apatite-like minerals. Phosphoric acid amendment of the soil with simultaneous addition of Ca dramatically decreased the mobility of soil As to near zero at pH > 6. Characterization of precipitate separated from the Ca-Pi treated soil by X-ray diffraction indicated that a carbonate-apatite mineral was formed in the soil and likely incorporated As(V) into its structure. The low solubilities of many of the Ca-Pi-As(V) minerals suggest that Ca-Pi treatment has promise as an effective, long-term method for in situ chemical fixation of As in contaminated soils and wastewaters.