The role of interaction between low molecular weight neutral organic compounds and a polyamide RO membrane in the rejection mechanism

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

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 permeability-selectivity 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.

Making wastewater obsolete: Selective separations to enable circular water treatment

By 2050, the societal needs and innovation drivers of the 21st century will be in full swing: mitigating climate change, minimizing anthropogenic effects on natural ecosystems, navigating scarcity of natural resources, and ensuring equitable access to quality of life will have matured from future needs to exigent realities. Water is one such natural resource, and will need to be treated and transported to maximize resource efficiency. In particular, wastewater will be mined for the valuable product precursors it contains, which will require highly selective separation processes capable of capturing specific target compounds from complex solutions. As a case study, we focus on the nitrogen cycle because it plays a central role in both natural and engineered systems. Nitrogen occurs as several species, including ammonia, a fertilizer and precursor to many nitrogen products, and nitrate, a fertilizer and component of explosives. We describe two applications of selective separations: selective materials and electrochemical processes. Ultimately, this perspective outlines the next thirty years of modular, selective, resource-efficient separations that will play a major role in enabling element-specific circular economies and redefining wastewater as a resource.

Group Contribution Method to Predict the Mass Transfer Coefficients of Organics through Various RO Membranes

Reverse osmosis (RO) is a membrane technology that separates dissolved species from water. RO has been applied for the removal of chemical contaminants from water for potable reuse applications. The presence of a wide variety of influent chemical contaminants and the insufficient rejection of low-molecular-weight neutral organics by RO calls for the need to develop a model that predicts the rejection of various organics. In this study, we develop a group contribution method (GCM) to predict the mass transfer coefficients by fragmenting the structure of low-molecular-weight neutral organics into small parts that interact with the RO membrane. Overall, 54 organics including 26 halogenated and oxygenated alkanes, 8 alkenes, and 20 alkyl and halobenzenes were used to determine 39 parameters to calibrate for 6 different RO membranes, including 4 brackish water and 2 seawater membranes. Through six membranes, approximately 80% of calculated rejection was within an error goal (i.e., ±5%) from the experimental observation. To extend the GCM for a reference RO membrane, ESPA2-LD, 14 additional organics were included from the literature to calibrate nitrogen-containing functional groups of nitrosamine, nitriles, and amide compounds. Overall, 49 organics (72% of 68 compounds) from calibration and 7 compounds (87.5% of 8 compounds) from prediction were within the error goal.