Influence of Solute Molecular Diameter on Permeability-Selectivity Tradeoff of Thin-Film Composite Polyamide Membranes in Aqueous Separations

Abstract

Fundamental understanding of the reverse osmosis (RO) transport phenomena is necessary for quantitative prediction of contaminant rejection and development of more selective membranes. The solution-diffusion (S-D) model predicts a tradeoff relationship between permeability and selectivity, and this tradeoff trend was recently reported for RO. But the first principles governing the relationship are not well understood for aqueous separation membranes. This study presents a framework to elucidate the underlying factors of the permeabilityselectivity tradeoff relationship in thin-film composite polyamide (TFC-PA) membranes. Water and solute permeabilities of membranes with a range of selectivities are examined using six nonelectrolyte solutes of various sizes and dimensions. The permeability-selectivity tradeoff trend, as defined by S-D, was observed for all six solutes. Crucially, the slopes of the tradeoff lines, λ, are found to be related to the solute and solvent (i.e., water) diameters, d s and d w , respectively, by λ = (d s /d w ) 2 -1, consistent with the S-D framework established for gas separation membranes. Additionally, the intercepts of the tradeoff lines are shown to be also influenced by d s . These results highlight that solute molecular diameter is a primary influence on the permeability-selectivity tradeoff for the permeants investigated in this study. Furthermore, a transport regime where solute permeation is only very weakly coupled to water transport, in addition to the conventional S-D, is identified for the first time. We demonstrate that the boundary delineating the two transport regimes can be determined by the solute diameter. The relationship between characteristic features of the "additional regime" and solute dimensions are analyzed. The study shows that the general principles of the S-D framework are applicable to TFC-PA membranes and the analysis quantified the principal role of solute size in governing RO transport. The experimental and analytical evidence suggest that nonelectrolyte solute transport can, in principle, be a priori predicted using molecular diameter. Findings of this investigation provide new insights for understanding the transport mechanisms in osmotic membrane processes.

Applying Transition-State Theory to Explore Transport and Selectivity in Salt-Rejecting Membranes: A Critical Review

“…Particularly, the conventional solution-diffusion model, frequently used to study transport in NF and RO membranes, describes the water or solute transport through the membrane using a single parameter (i.e., the water or solute permeability coefficient), which is determined experimentally by dividing the measured water or solute flux by the pressure or concentration gradient over the membrane, respectively . The permeability coefficient is often used to explain transport trends observed as a function of the intrinsic properties of the species (e.g., size and hydration energy) and the membrane (e.g., pore size and charge) or the experimental conditions (e.g., pH and temperature). − However, measuring this coefficient is insufficient to understand more fundamental features of the transport, such as the specific interactions of the transported species with its surrounding molecules.…”

Enthalpic and Entropic Selectivity of Water and Small Ions in Polyamide Membranes

“…Notably, the conventional solution–diffusion model, frequently used to study transport in dense polymeric membranes such as NF and RO, describes water or solute transport through the membrane using a single parameter (i.e., the water or solute permeability coefficient), which is determined experimentally by dividing the measured water or solute flux by the pressure or concentration gradient over the membrane, respectively. The permeability coefficient is often used to qualitatively explain the selectivity between species as a function of the intrinsic properties of the species (e.g., size and hydration energy) and the membrane (e.g., pore size and charge). − However, to explore selectivity beyond measured differences in permeability coefficients and to better understand the molecular aspects driving selectivity, a more fundamental approach to transmembrane permeation modeling is needed.…”

Drivers, challenges, and emerging technologies for desalination of high-salinity brines: A critical review

“…Secondly, the net driving force for water flux, applied hydraulic pressure in excess of transmembrane osmotic pressure, will be relatively low for OMRO, necessitating larger membrane areas compared to HPRO. Membranes with greater water permeabilities will reduce the area requirement, but salt permeation will also correspondingly increase as conventional polymeric membranes are bound by the permeability-selectivity tradeoff [220][221][222], resulting in compromised separation performance. Thirdly, OMRO is still membrane-based and would inescapably be plagued by mineral scaling and other fouling problems; at present, there are no studies on these issues.…”