Experimental investigation on separation of inorganic electrolyte solutions by nanofiltration membranes

Permeation experiments of aqueous solutions of six inorganic electrolytes (KCl, NaCl, LiCl, MgCl2, MgSO4 and K2SO4) were carried out by using two commercial nanofiltration (NF) membranes nominated by NF45 (Dow Chemical Corporation) and SU200 (Toray Corporation) membranes. The effects of the type and the concentration of electrolytes as well as the pressure on the separation performance of NF membranes (such as observed rejections) were investigated. The experimental results showed that NF45 and SU200 membranes possess similar separation potential to the inorganic electrolytes of 1-1 type (LiCl, NaCl and KCl) and much different ones to the other electrolytes with bivalent ions (MgCl2, K2SO4 and MgSO4) under various concentrations. The rejections to inorganic electrolytes by the two NF membranes declined with the increase of the electrolyte concentrations and approached some certain values under higher concentrations (more than 500 mol &middot m-3), which implied that the size of the electrolyte-ions could not be ignored by comparison to the pore radii of NF membranes.