Diffusion and sorption of simple ions in cellulose acetate—semipermeability

Abstract

The diffusion and sorption properties of NaCl, NaI, KCl and KI have been studied in cellulose acetate (CA) membrane. Sorption is not reversible. Residual solute remains which can only be removed by ion exchange. Thus H +, K + and Na + membranes were prepared. The sorption of K was almost twice that of Na in which connection the biological difference between K and Na is of interest. Both K and Na sorption decrease with decreasing pH. The diffusion of NaCl was much less in the CA than in cellulose and decreased with decreasing concentration. At the higher concentrations the diffusion concentration gradient data implies a single barrier across the membrane as with cellulose but this becomes multibarrier at lower concentration. The observations are understandable according to electrokinetic theory. Salt retention properties in reverse osmosis are thus explained including the decreasing of the semipermeability with increasing solute concentrations. The much greater effectiveness of cellulose acetate vs cellulose is evident in terms of the reduced absorption and diffusion in the more compact less swollen molecular chain network of lower average dielectric constant.