A refined draw solute flux model in forward osmosis: Theoretical considerations and experimental validation

A detailed study of water and draw solute transport through FO membranes is presented. Flux tests were performed using CTA and TFC membranes in both orientations using four draw solutes (NaCl, Na2SO4,MgCl2,MgSO4). Fluxes were modeled using a novel model and using an FO-only approach. The membrane permeability coefficients and structural parameter were determined by model fitting for each draw solute and each membrane orientation, allowing a detailed comparison of membrane and draw solute properties. It was found that the CTA membrane water permeability decreased with increasing osmotic pressure, however, draw solute permeabilities were unaffected. The TFC membrane on the other hand showed variability for both water and draw solute permeability. Draw solute flux across the active layer was described using novel equations, taking into account the concentration-dependence of draw solute diffusivity. This yielded improved model convergence, especially for AL-DS tests. Membrane characterization furthermore allowed the calculation of the apparent tortuosity. The apparent tortuosity was dependent on draw solute and membrane orientation. For the CTA membrane, realistic tortuosity values were obtained in AL-FS mode but were overestimated in AL-DS mode. For the TFC membrane, tortuosity was less dependent on membrane orientation but more on draw solute. Electrostatic interactions of the draw solutes with the active layer were considered and found to be small, although they yielded improved predictions of the Jw/Js ratio.