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With the recent emergence of hexavalent chromium (CrO42-) and perchlorate (ClO4-)contamination, and natural arsenate (H2AsO4-/HAsO42-) occurrence as an importantdrinking water quality issue, a careful assessment of membrane technology for traceanionic contaminants removal was performed.A bench-scale cross-flow flat-sheet filtration unit was used to evaluate short-term(CrO42-, ClO4-, and H2AsO4-/HAsO42-) 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 thatare physically and chemically similar (e.g., molecular mass cut off (MMCO) andelectrostatic interaction by charge) to these three toxic anions.In this study, RO, NF and tight UF membranes have been tested to evaluate theeffects of pH, conductivity (ionic strength), and mono- or divalent co- and counter ionson the transport/rejection of these three toxic anions. This study investigated the transport(rejection) mechanisms of CrO42-, ClO4-, and H2AsO4-/HAsO42- including solutiondiffusion, steric (size) exclusion, and/or electrostatic exclusion for RO, NF and tight UFmembranes.The target anions have been characterized according to molecular mass, hydratedionic radius (size), and diffusion coefficient in water (Dw). A diffusion cell with actualRO, NF, and tight UF membrane specimens was used to estimate hindered diffusion(Dp) embodying various solute-membrane interaction, with measured hindered diffusioncoefficients several orders of magnitude less than corresponding diffusion coefficients inwater, thus demonstrating the importance of electrostatic and steric hindrance in traceanion rejection. Includes 7 references, tables, figures.

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Edition: Vol. - No. Published: 03/05/2003 Number of Pages: 9File Size: 1 file , 240 KB