Transport of target anions, chromate (Cr (VI)), arsenate (As (V)), and perchlorate, through RO, NF, and UF membranes: Removal mechanisms, transport modeling, and applications

With the recent emergence of three toxic anion contaminations (Chromate (Cr (VI)), Arsenate (As (V)), and Perchlorate (ClO₄− )) as an important parameter in drinking water quality, this study involved a careful assessment of membrane technology for toxic anion removal.; This approach highlighted short-term rejection as a function of water quality and selected operational parameters. The data obtained from the bench-scale crossflow flat-sheet filtration tests were modeled by application of a non-equilibrium thermodynamic transport model, based on hindered diffusion embodying membrane properties.; The significant finding of this research was that known and hypothesized rejection mechanisms (e.g. solution diffusion, steric (size) exclusion, and electrostatic interaction (repulsion)) for solute transport (rejection) of target anions were demonstrated by several measurement techniques. Another significant of this research was that target anion transport was predicted well by a non-equilibrium thermodynamic model.; Changes in membrane surface charge, becoming more negative as pH increases and conductivity (ionic strength) decreases, were found to enhance anion rejection because electrostatic repulsion significantly occurred between the anions and negatively charged membranes. Changes in membrane surface charge were also found to reduce hindered diffusion for negatively charged membranes.; An increase in recovery and J_o/k ratio resulted in a decrease of target anion rejection for RO and NF membranes. Based on results of different membranes at varying recoveries with natural water, calcite (CaCO₃) scale was dominant at the membrane surface and detected by various measurements. Antiscalant, characterized by phophonate groups, were effective in the inhibition of calcite formation.; The concentration of various solutes at the membrane surface can be predicted by a semi-empirical constant (K) related to concentration polarization as influenced by recovery.; Target anion rejections by a UF membrane modified with a cationic surfactant was greater than expected, based mostly on steric/size exclusion as a result of a decrease in the membrane pore size impacted by the surfactant conditioning.; In tests performed to study Cr (VI) as an oxidizing agent, no membrane damage was observed at low concentration of Cr (VI), which was similar contamination level of natural water. However, membranes exposed at high concentration of Cr (VI) were significantly damaged, either in terms of flux and NaCl rejection properties as a function of “CT” exposure value and observed by various surface characterizations (e.g. FTIR and SEM).