A cross-linked 2,3-dicarboxylic acid cellulose nanofibril network: A nanoporous thin-film layer with tailored pore size for composite membranes

Covalent and ionic cross-linkings of cellulose nanofibrils produced by sequential sodium metaperiodate and sodium chlorite oxidations were used for the fabrication of thin nanoporous layers for liquid filtration membranes. The aim was to tailor the pore formation in the cellulose nanofibril layers of the membranes using trisodium trimetaphosphate (Na3P3O9) and calcium chloride (CaCl2) cross-linkers. A notable distinction was observed between the cross-linked membranes in terms of pore size and fouling tendency during liquid filtration. According to the molecular weight cut-off (MWCO) testing and field emission scanning electron microscope (FESEM) images, covalent cross-linking with Na3P3O9 retained more open pore structure (MWCO with 2000 kDa dextran ∼53%), whereas ionic cross-linking with CaCl2 formed tighter networking between the nanofibrils, resulting in smaller pore size (MWCO 2000 kDa >90%). The filtration performance of the cross-linked membranes and commercial synthetic reference membrane was evaluated using process water from a local pulp mill. The performance of CaCl2 cross-linked nanofibril membrane was similar as the commercial reference membrane in terms of both flux and fouling tendency.