Moreover, it could be found that the retention for sulphates (MgSO4 and Na2SO4) was much smaller than that of chlorides (MgCl2 and NaCl). This was likely due to the presence of counter-ions, which could bind part of the surface charge, may weaken the repulsive fore. As shown in Fig. 16, the zeta potential of A-powder decreased with the presence of salts. Comparing with the chlorides, the sulphates made a much more significant decrease on the zeta potential of g-alumina. With the effect of sulphates, the g-alumina only showed a very weak zeta potential, which leading to a poor retention for MgSO4 and Na2SO4.
In general, a relatively high retentionwas reached at a lower salt concentration and higher transmembrane pressure. For different salts, a retention sequence of R (MgCl2) > R (CaCl2) > R (LiCl) > R (NaCl) > R (MgSO4) > R (Na2SO4) was achieved, of which the retention rates were 95.2%, 93.5%, 67.0%, 64.4%, 8.4% and 6.1%, respectively. Additionally, due to the similar pore size and zeta potential, similar salts retention performance was obtained for Nmembrane.