Selective removal or enrichment of targeted solutes including micropollutants, valuable elements, and mineral scalants from complex aqueous matrices is both challenging and pivotal to the success of water purification and resource recovery from unconventional water resources. Membrane separation with precision at the subnanometer or even subangstrom scale is of paramount importance to address those challenges via enabling “fit-for-purpose” water and wastewater treatment. So far, researchers have attempted to develop novel membrane materials with precise and tailored selectivity by tuning membrane structure and chemistry. In this critical review, we first present the environmental challenges and opportunities that necessitate improved solute–solute selectivity in membrane separation. We then discuss the mechanisms and desired membrane properties required for better membrane selectivity. On the basis of the most recent progress reported in the literature, we examine the key principles of material design and fabrication, which create membranes with enhanced and more targeted selectivity. We highlight the important roles of surface engineering, nanotechnology, and molecular-level design in improving membrane selectivity. Finally, we discuss the challenges and prospects of highly selective NF membranes for practical environmental applications, identifying knowledge gaps that will guide future research to promote environmental sustainability through more precise and tunable membrane separation.

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This review article is devoted to bridging the conventional and newly-developed NF membranes with the potential environmental applications by systematically discussing the synthesis–property–performance relationships.

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The mechanism underlying gel layer formation on membrane surfaces from soluble and colloidal microbial products (SCMPs) produced under unfavorable operational conditions for membrane bioreactors (MBRs) has been investigated using supernatants from a bench-scale MBR. SCMPs can be grossly classified into gelling and nongelling SCMPs with the gelling fraction associated mostly with the polysaccharide content. The significant role played by multivalent metals in gel formation through metal-ligand complexation has been confirmed. Functional groups of the gelling SCMPs were determined by pH titration and zeta potential measurement as amine/phenolic sites (pK(a) 9.3 and 8.0), carboxylic sites (pK(a) 6.6, 4.9, and ca. 4.0), and phosphoric sites (pK(a) ca. 2.5). The carboxylic sites were more directly involved with multivalent cation complexation; however, the gelling propensity of the SCMP dispersion was minimally affected by pH change in the circum-neutral pH range, suggesting that the strong carboxylic sites were principally responsible for gel formation. The SCMPs demonstrated a high potential for gel formation given the high density of the strong carboxylic acid groups of about 0.44 mmol/g-TOC and a moderate calcium binding stability constant of about 4.9 x 10(3) M(-1).

The attachment of sludge cake to the membrane surface is the main cause of the fouling problem in the submerged membrane bioreactors (SMBR) used in biological wastewater treatment. In this laboratory study, the sludge cake deposited on the membrane was found to have a specific filtration resistance of the order of 10 14 m/kg, which is much greater than expected for sludge cake formed during the dewatering of activated sludge. The filterability tests showed that the cake sludge removed from the fouled membrane of the SMBR had an average specific filtration resistance of 4.9 × 10 13 m/kg, whereas the sludge cake of the SMBR bulk sludge had an average filtration resistance of only 1.9 × 10 11 m/kg. Detailed chemical analysis showed there was a pool of biopolymer clusters (BPC) that was trapped within the sludge cake on the membrane. These clusters could be readily separated from the cake sludge by stirring it into a suspension. The abundance of non-filterable BPC as measured by the total organic carbon (TOC) in the suspended solids (SS) was about 10.3 mg/g SS for the cake sludge, in comparison to 0.4 mg/g SS for the bulk sludge. When the BPC were removed from the cake sludge, the filtration resistance of the cake sludge could be reduced considerably from 4.9 × 10 13 to 8.4 × 10 12 m/kg. It is argued that the BPC are a special form of organic matter formed by affinity clustering of the free extracellular polymeric substances (EPS) and soluble microbial products (SMP) in the sludge cake deposited on the membrane surface. The accumulation of BPC within the pores of the sludge cake is mostly responsible for the unusually high filtration resistance of the cake sludge during the SMBR operation.