Change of the performance properties of nanofiltration cellulose acetate membranes by surface adsorption of polyelectrolyte multilayers

Surface modification of nanofiltration cellulose acetate (CA) membranes was carried out by alternating layer-bylayer deposition of acidic chitosan (CHI) and sodium alginate (ALG) as the cationic and anionic polyelectrolyte, respectively. The supporting CA membranes were obtained by a phase separation process from acetone/formamide (2/1 wt/wt) casting solutions using a 20 wt% polymer concentration. The pore size of the CA substrates monitored by the annealing temperature was determined from the retention values of rigid neutral solutes using the computer simulation program NanoFlux. The influence of the ultrathin multilayered polyelectrolyte surface structure on the permeability performances (water flux and ion rejection) was studied for a series of composite membranes obtained by varying the total number (up to 35) of the adsorbed polyelectrolyte layer pairs. SEM and permeation measurements showed that the modification mainly took place on the substrate surface so that the hydraulic permeability was only little affected by the polyelectrolyte multilayers. The permeation rate of dilute to moderate concentrated NaCl and MgSO4 solutions was found to be higher than that of pure water for some composite membranes. By comparison with the bare support, the rejection of monovalent salt decreased by increasing the number of adsorbed ALG/CHI layer pairs whereas that of divalent salt remained constant.

Surface modification is a versatile process to design membranes with tailor-made filtration properties. With the purpose of examining the different parameters involved in ion rejection, a series of membranes were prepared by adsorption of a different number of ALG/CHI layer pairs on CA supports with pore sizes in the NF range. The modification did not strongly alter the hydraulic permeability of the modified membranes. The change in permeation performances of dilute to moderate concentrated NaCl and MgSO4 solutions was more marked for membranes obtained from the substrates having the larger pore size (rp = 1.37 nm). The main observations were: C The permeation rate of salted solutions was found to be higher than that of pure water. C The rejection of monovalent salt was decreased whereas that of divalent salt remained constant so that the retention ratio increased. C Increasing the concentration of feed solutions enhanced this selectivity effect. Different parameters are involved in the ion partitioning mechanism between PEMs and the solution. More work is needed to distinguish the relative role played by the electrical interactions, the dielectric repulsion and others physicochemical parameters in the ion rejection by PEMs. From our findings, it appears that a thorough examination of the ionic strength influence on the structure of PEMs and on the permeation performances of obtained composite membranes can provide more information on the system studied. However, it has to be kept in mind that this is a complex problem, as operating conditions in which PEMs is formed and further annealing induced upon contacting with salt solutions or changing the pH can completely modify the balance of the different parameters occurring in the ion partitioning.