Role of Charge (Donnan) Exclusion in Removal of Arsenic from Water by a Negatively Charged Porous Nanofiltration Membrane

Abstract

The removal of arsenic from water by a negatively charged "loose" (porous) nanofiltration (NF) membrane was investigated. To better understand the mechanisms of arsenic removal by the polymeric membrane, its surface charge, pore 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 evaluated. The removal of As(V) increased from 60 to 90% as the arsenic feed water concentration was increased from 10 to 316 μg/L. Compared to As(V), the rejection of the uncharged As(III) species was significantly lower; the rejection of As(III) decreased from 28 to 5% as the arsenic feed concentration was increased from 10 to 316 μg/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 attributed to changes in As(V) speciation with pH (HAsO₄^2- at pH >6.8, H₂AsO₄^- at pH <6.8), and to a lesser extent to a decrease in membrane charge with 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 pore size) and arsenic speciation and molecular weight, emphasizing the paramount role of charge (Donnan) exclusion in achieving high rejection of As(V) species, despite the relatively large membrane pore size.