AWWA MTC57564

With the recent emergence of hexavalent chromium (CrO4 2-) and perchlorate (ClO4 -) contamination, and natural arsenate (H2AsO4 -/HAsO4 2-) occurrence as an important drinking water quality issue, a careful assessment of membrane technology for trace anionic contaminants removal was performed. A bench-scale cross-flow flat-sheet filtration unit was used to evaluate short-term (CrO4 2-, ClO4 -, and H2AsO4 -/HAsO4 2-) rejection by reverse osmosis (RO), nanofiltration (NF), and tight ultrafiltration (UF) membranes, and a diffusion cell testing unit is being used to investigate anion transport (by hindered diffusion) through membrane pores. Several membrane studies have been performed to evaluate removal of anions that are physically and chemically similar (e.g., molecular mass cut off (MMCO) and electrostatic interaction by charge) to these three toxic anions. In this study, RO, NF and tight UF membranes have been tested to evaluate the effects of pH, conductivity (ionic strength), and mono- or divalent co- and counter ions on the transport/rejection of these three toxic anions. This study investigated the transport (rejection) mechanisms of CrO4 2-, ClO4 -, and H2AsO4 -/HAsO4 2- including solution diffusion, steric (size) exclusion, and/or electrostatic exclusion for RO, NF and tight UF membranes. The target anions have been characterized according to molecular mass, hydrated ionic radius (size), and diffusion coefficient in water (Dw). A diffusion cell with actual RO, NF, and tight UF membrane specimens was used to estimate hindered diffusion (Dp) embodying various solute-membrane interaction, with measured hindered diffusion coefficients several orders of magnitude less than corresponding diffusion coefficients in water, thus demonstrating the importance of electrostatic and steric hindrance in trace anion rejection. Includes 7 references, tables, figures.