Experimental investigation on separation performance of nanofiltration membranes for inorganic electrolyte solutions

Permeation experiments of KCl, NaCl, LiCl, MgCl2, MgSO4 and K2SO4 solutions were carried out using two commercial nanofiltration membranes, which were NF45 (Dow Chemical Corporation) and SU200 (Toray Corporation) membranes. The effects of the type and the concentration of electrolytes as well as the pH value of the electrolyte solution on the separation performance (such as 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 containing bivalent ions (MgCl2, K2SO4 and MgSO4) with different concentrations. The rejections to inorganic electrolytes by the two NF membranes declined with the growth of the electrolyte concentrations and approached some certain values when on much high concentrations (more than 400 mol·m–3), which implied that the size of electrolyte-ions could not be ignored by comparison to the pore radii of NF membranes. The two NF membranes behave almost the same rejections to the electrolytes such as LiCl under various pH values of the feed solution from pH=4 to 10 and behaved the similar minimum values near the isoelectric point of pH=6.5.

In this study, the permeation of 6 inorganic electrolytes through NF45 and SU200 membranes were carried out under various conditions by changing the electrolyte concentrations and the pH values of the solutions. We can conclude as follows: 1. The rejections to inorganic electrolytes by the two NF membranes declined with the growth of the electrolyte concentrations and approached some certain values when on much high concentrations (more than 400 mol·m–3), which could be considered as the result of the size effect of electrolyte-ions. 2. NF45 and SU200 membranes showed similar separation potential to the inorganic electrolytes of 1-1 type (LiCl, NaCl and KCl) and much different ones to the other electrolytes containing bivalent ions (MgCl2, K2SO4 and MgSO4) with different concentrations.