Influence of different ion types and membrane orientations on the forward osmosis performance

This study investigated the influence of different ions on forward osmosis (FO) performance in terms of water flux. NaCl, NaNO3, KCl, KNO3, NH4Cl, NH4NO3, CaCl2 and Ca(NO3)2 were selected as representatives of draw solutions (DSs). Data were obtained from laboratory-scale experiments under well controlled conditions in both membrane orientations operated in FO mode pressure retarded osmosis (PRO) mode. The basic properties of DS (osmotic potential, viscosity, diffusion coefficient) changed the water and reverse salt transmission. The results indicated that the performance of one solution in different modes was different. The behavior of Na+ and K+ salts was quite satisfactory in the PRO mode; specifically NaCl had the greatest water flux (16.96 L/(m2 h)), and the following largest was KCl (16.55 L/(m2 h)). K+ salts had the highest water flux among all the selected positive ions in the FO mode which was KCl (10.30 L/(m2 h)). To the negative ions, Cl− salts showed higher water flux than nitrates in both PRO and FO modes, such as the water flux of NaNO3 was 7.96 L/(m2 h), while that of NaCl was 8.86 L/(m2 h) in the FO mode. Overall, K+ and Cl− were regarded as optimal candidates of cation and anion, respectively.