Permeability of electrolytes through a flat RO membrane in a direct osmosis study

A direct osmosis study was carried out using several 1:1 and 2:1 electrolytes, which diffused in a RO TFC membrane. The solute permeab ility D SMK S/* h as been calculated using the preferential sorption capillary flow model. It was found that to some extent the ions maintain their hydration in the membrane and the permeation is a function of the hydrated radii rather than crystal ionic radii. Moreover, a linear dependence was found between electrolyte permeab ility through the membrane and their average diffusivity in infinite dilute solution. The relatively low permeation of 2:1 chloride salts was attributed to the interaction with the membrane surface charge.

Permeability coefficients for several 1:1 and 1:2 electrolytes through a RO membrane were evaluated in a pure diffusive study. It has been found that for electrolytes with the same cation, the overall rate of permeation is controlled by the anion diffusivity in infinite dilute solution. Thus, all the chloride salts permeated the membrane at a higher rate than the acetates, and the differ- ences were attributed to the higher diffusivity of Cl!1 vs. Ac!1 in water. For electrolytes with the same anion (e.g., KCl and NaCl), the permeation is limited by the cation diffusivity in water. Thus, the order of permeation was given by: LiCl < NaCl < KCl and LiAc < NaAc < KAc MgCl2 < CaC2 and MgAc2 < CaAc2 A very good correlation has been observed between the permeation of electrolytes through the membrane and their overall diffusion coefficient in an infinite dilute solution. This implies that the adsorption of the solute in the membrane dose not play an important role in the process of solute-solvent separation. The relative slow permeation of MgCl2 and CaCl2 was attributed to salt interaction with the membrane surface charge.