How permeable could a reverse osmosis membrane be if it was specifically developed for uncharged organic solute rejection?

Abstract

Here, we present both experimental data and modeling results of the removal of four different polar organic solutes by six different commercial nanofiltration (NF) and reverse osmosis (RO) membranes in a natural groundwater matrix. These results are significant for two reasons. First, three of the polar organic solutes are contaminants commonly found in both drinking water aquifers and municipal wastewaters and, hence, are particularly relevant to the protection of public health in drinking water production and potable reuse applications. Second, our membrane transport model can be used (1) to understand the fundamental mechanism governing uncharged, polar organic solute rejection and (2) to make forward predictions about how one might alter NF/RO membrane chemistry and structure to achieve a desired level of rejection. Perhaps a disappointing conclusion is that, for uncharged, polar organic solutes, the only obvious means of improving rejection is to reduce pore size, increase barrier layer thickness, and/or decrease barrier layer porosity, all of which reduce water permeability and energy efficiency. However disappointing this conclusion may be, we believe the fundamental insights that can be derived from this work are significant and important for the drinking water and water reuse scientific communities.

On the Disproportionate Contribution of Membrane Electron Donor Functionality in Membrane Biofouling

“…In contrast to γ LW , γ – of membranes varies greatly for the different polymer chemistries presented in Figure b. PANi (polyaniline) and PAN membranes, which are highly hydrophilic and fouling-resistant, − demonstrate the largest γ – , while poly­(vinyl chloride) (PVC), polysulfone (PSf), and polypropylene (PP) are more hydrophobic materials with lower γ – values. The Lewis acid–base nature of each material is a function of its charged functional groups and moieties possessing unpaired electrons (e.g., R-Ö-R, R- N⃛ -R, etc.)…”

Theoretical Pathway toward Improved Reverse Osmosis Membrane Selectivity for Neutral Solutes: Inspiration from Gas Separations

“…Thus, previous studies have suggested that the size, porosity, thickness, and thermodynamic partitioning coefficient of membranes are all required to accurately predict the transport of such contaminants through RO membranes. 117 Additionally, water clustering can alter the energy of diffusion due to strong intermolecular bonding and the ability of water to be both a double donor and acceptor of hydrogen bonds. 118 Water clusters take the shape that maximizes the number of hydrogen bonds and minimizes geometrical strain.…”

“…The prediction of small neutral solute rejection with RO membranes remains challenging because there is no universal indicator that correlates with the permselectivity of these contaminants. Thus, previous studies have suggested that the size, porosity, thickness, and thermodynamic partitioning coefficient of membranes are all required to accurately predict the transport of such contaminants through RO membranes …”

The Minus Approach Can Redefine the Standard of Practice of Drinking Water Treatment

“… 78 A great deal of research concerns the creation of “fit-for-purpose” NF membranes, 79 which are modified structurally and/or chemically to provide precise solute separation at the subnanometer or subangstrom scale (e.g., the passage of Ca 2+ ions to minimize remineralization needs and scaling propensity while retaining PFAS). 80 Still, some small, polar KUECs, such as 1,4-dioxane and NDMA, can pass through many types of NF and RO membranes (e.g., NDMA removals vary from ∼5–10% with NF to ∼90% with seawater RO), 81 , 82 underscoring the value of multibarrier approaches.…”