Effect of Concomitant Ions on Nanofiltration Dephosphatation of Water

Abstract

The study of the effect of typical macrocomponents of natural and wastewater on the process of nanofiltration dephosphatation showed that nanofiltration purification of phosphate-containing water is carried out under the combined action of capillary-filtration and diffusion mechanisms of membrane permeability, as well as electrostatic interaction of ions with a charged membrane surface, which leads to the formation of a double electric layer. Differences in the retention capacity of the membrane for phosphate and associated ions are due to the size of these ions and their charge. The presence of chloride, sulfate, and bicarbonate ions in phosphate-containing waters at concentrations 25–500 mg/dm3 does not affect the retention capacity of the phosphate nanofiltration membrane up to a permeate recovery ratio of <50% and decreases this capacity with an increase in the permeate recovery. Magnesium ions present in phosphate-containing waters at a concentration of <50 mg/dm3 with a permeate recovery ratio of <50% does not affect the membrane retention capacity. A further increase in the concentration of phosphates in water to 100 mg/dm3, this indicators of the membrane slightly decrease with their subsequent increase with an increase in the concentration of magnesium ions in water to 250–500 mg/dm3, which is due to a change in the charge of the membrane and the formation of magnesium phosphates on its surface. The presence of calcium ions with a concentration of <100 mg/dm3 in these waters and a permeate recovery ratio of <90% increase the phosphate retention capacity of the membrane. Bioassay of phosphate-containing water samples with the concentrations of phosphate ions in them of 0.21, 0.55, 1.0, 3.5, and 8.0 mg/dm3 showed that water containing <0.55 mg/dm3 of phosphate ions is safe in terms of toxicity, since acute, chronic, or genetic toxicity were not manifested either at the level of ceriodaphnia or at the level of fish. A further increase in the concentration of phosphate anions in water leads to the manifestation of chronic toxicity and genotoxicity in ceriodaphnia and fish, respectively,