Mriganka M. Ghosh

Surface complexation of methylated arsenates by hydrous oxides

Abstract The adsorption of methyl arsenate and dimethyl arsenate on hydrous ferric oxide and activated alumina was measured as a function of pH, ionic strength and sorbate-sorbent ratio by the bottle-point method and the characteristics of the two oxide surfaces were determined by surface titration. Adsorption decreased with increasing pH while it was only weakly dependent on ionic strength. The results were interpreted in terms of the triple-layer surface complexation model by assuming that arsenicals formed inner-sphere complexes with the oxide surface. A satisfactory interpretation of results could be made by modifying the triple-layer model to include surface sites of two different adsorption energies. Attempts to model adsorption considering a homogeneous surface were unsatisfactory.

Slow sand and diatomaceous earth filtration of cysts and other particulates

Abstract The performance of slow sand and diatomaceous earth (DE) filtration was compared in removing Giardia muris, Cryptosporidium, coliform bacteria, turbidity, and particles added to a pristine surface water. When operated correctly, both filtration systems removed 99.9% or more of the cysts applied. The removals of coliform and turbidity were not satisfactory in biologically immature slow sand filters, particularly during winter months. When operated correctly, mature filters exhibited satisfactory removal of these contaminants. Coliform removal was not satisfactory with diatomaceous earth filtration except when the finest grade of DE (C512) was used. However, with polymer preconditioning of DE, a significant increase in coliform removal was observed.