Molecular weight cut off (MWCO) determination in ultra- and nanofiltration: Review of methods and implications on organic matter removal

Estimation of membrane molecular weight cut off (MWCO) in the range between ultrafiltration (UF) and nanofiltration (NF) is challenging because retention is not controlled only by size exclusion. This review provides an experimental and theoretical overview of the membrane MWCO in the range from UF to loose NF (from 500 to 0.7 kDa) to evaluate the significance of membrane MWCO on predicting retention of organic solutes when approaching NF pore structure. The experimental section includes filtration of: i) organic tracers with different molecular weights (MW) and properties, such as polyethylene glycol (PEG) and oligosaccharides, and ii) natural organic matter (e.g. humic acid, alginic acid, Tanzanian and Australian organic matter) in the MWCO range between UF and NF, at minimal concentration polarization. The role of molecule structure, size exclusion and charge shielding when filtering organic solutes is elucidated. The molecular structure of uncharged organic tracers plays a major role on MWCO estimation, especially for loose NF membranes, where oligosaccharides are retained more effectively compared to PEG tracers of similar MW. The MWCO determined by PEG filtration and estimated from the pore radius distribution are consistent in the UF range from 1 to 500 kDa, indicating major contribution of size exclusion. Conversely, MWCO of loose NF membranes determined with PEG tracers is overestimated. Charged organics, such as humic acid (1.5 kDa < MW < 3 kDa), shows retention between 60 and 80 % for UF membrane MWCO below 30 kDa (pore radius < 14 nm) and full retention by loose NF (pore radius below 1.4 nm). This is explained with an interplay of size exclusion and charge shielding in the pore. This review can assist in the selection of the organic tracer and operating conditions for membrane MWCO determination between UF and NF, elucidating the relevance of membrane MWCO in organic matter retention.