Role of charge (Donnan) exclusion in removal of arsenic from water by a negatively charged porous nanofiltration membrane

The removal of arsenic from water by a negatively charged "loose" (porous) nanofiltration (NF) membrane was investigated. To better understand the mec hanisms of arsenic removal by the polymeric membrane, its surface charge, p ore size, and separation behavior for several salt solutions (NaCl, CaCl2, and Na2SO4) were first investigated. The ability of the membrane to remove inorganic arsenic species-As(III) and As(V)-from water was further evaluate d. The removal of As(V) increased from 60 to 90% as the arsenic feed water concentration was increased from 10 to 316 mug/L. Compared to As(V), the re jection of the uncharged As(III) species was significantly lower; the rejec tion of As(III) decreased from 28 to 5% as the arsenic feed concentration w as increased from 10 to 316 mug/L In addition, rejection of As(V) decreased sharply from 85% at pH 8.5 to only 8% at pH 4.5. This behavior is mainly a ttributed to changes in As(V) speciation with pH (HAsO42- at pH >6.8, H2AsO 4- at pH <6.8), and to a lesser extent to a decrease in membrane charge wit h decreasing pH. The separation of the uncharged As(III), on the other hand , was independent of pH over the studied pH range (removal less than 10%). The results were analyzed in light of the membrane properties (charge and p ore size) and arsenic speciation and molecular weight, emphasizing the para mount role of charge (Donnan) exclusion in achieving high rejection of As(V ) species, despite the relatively large membrane pore size.