Aqueous Silica in the Environment: Effects on Iron Hydroxide Surface Chemistry and Implications for Natural and Engineered Systems
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A second phase of research was aimed at identifying the practical implications of silica sorption to iron hydroxide in natural and engineered systems. Two types of surfaces were prepared by exposing pre-formed Fe(OH)3 to aqueous silica (0-200 mg/L as SiO2) for periods of 1.5 hours or 50 days. The concentration of pre-formed iron passing through a 0.45 micron pore size filter at pH 6.0-9.5 increased as the solids aged in the presence of silica. Consistent with formation of small, stable colloids, "soluble" iron concentrations exceeded 0.2 mg/L only at zeta potentials < -15 mV. When arsenate was added to iron hydroxide particles equilibrated with silica for 1.5 hours, percentage arsenate removals were high. In contrast, arsenate removals decreased markedly as pH and silica concentrations increased if silica was pre-equilibrated with the iron for 50 days. Trends in percentage removal of humic substances were similar. Competition for sorption sites was the main cause of hindered anionic contaminant removal. However, interference with hydrolysis and precipitation are expected to be important under some circumstances, particularly during water treatment.
- Masters Theses