The effect of co-existing ions and surface characteristics of nanomembranes on the removal of nitrate and fluoride

Abstract

The rejection rates of nitrate and fluoride were investigated by two nanomembranes which had different surface potentials. The surface potential of NTR 7250 and NTR 7450 were approximately - 5mV and - 10 mV, around pH 7, respectively. The effects of calcium, magnesium and sulfate were also investigated since they usually co-existed in groundwater. The experiment indicated that sulfate would be rejected most among other ions in the groundwater. The chloride ions were rejected more than nitrate and fluoride ions. This experiment indicated that the electric repulsion between the nanomembrane and chloride ion was so high that it could even push! some divalent sulfate ions through the membrane. The influence of chloride ions was complicated when magnesium and fluoride ions were in the solution. The experiment showed that when permeated divalent cations cause a strong demand for anion permeation to meet the electroneutrality, the least repulsive anions in the solution could pass a membrane with higher surface potential more than one with lower surface potential. The rejection rates of nitrate and fluoride ions were similar. However, the experimental results indicated that fluoride was less affected by the surface charge of the membrane than nitrate. The hydration effect of nitrate would be stronger at a membrane with lower surface potential. The reductions in rejection rates of monovalent ions were more significant at a membrane with a low surface potential. The 5mV difference in surface potential showed appreciably divergent behavior. However, the electrostatic repulsion between the divalent ion and the membranes seemed to be strong enough so that 5mV of surface charge difference between NTR 7250 and NTR 7450 did not cause any appreciable distinction. Most of the negatively charged groups on the membrane are shielded by cations at high salt concentration. The experiment indicated that calcium ions shielded membrane charges more effectively than magnesium ions. However, despite the charge shielding effect, the rejection rates against the divalent anion were high, and more ions were rejected by the membrane that has a high negative surface potential.