Functional Cellulose Nanofiber Filters with Enhanced Flux for the Removal of Humic Acid by Adsorption

Despite great promises of cellulose nanofibers for water treatment, current technologies have lacked the exclusive use of cellulose nanofibers (CNF) in high-flux filters having an affinity for a desired contaminant. To tackle this, we prepared porous and functionalized filters via solvent exchange, supercritical drying, and freeze-drying of cationic CNF and compared them to conventional CNF filters obtained by the paper-making process. Porosity and pore size were evaluated in the dry state qualitatively and quantitatively via scanning electron microscopy and mercury intrusion porosimetry, respectively. The permeance of water and a solution containing a negatively charged model molecule (humic acid) through these filters was measured at various pressures and correlated to the filters' structure. As compared to the CNF filters made via paper-making, the porosity, pore size, and permeance were increased after processing via solvent exchange, supercritical drying, and freeze-drying routes. Those filters which were prepared via freeze-drying displayed the highest permeance reported so far for CNF filters, which is about an order of magnitude higher than the permeance of CNF filters made via paper-making and having the same grammage. While the permeability was clearly affected by the processing technique, the functional filters showed a comparable adsorption capacity for humic acid. The filtration of a humic acid solution provided an initial removal of nearly 100% without noticeable reduction in flow. Considering the diluted concentration of HA in natural waters, we expect that large volumes of HA solution could be treated with the present CNF filters, with the possibility to regenerate these filters for multiple utilizations. The present concept of utilizing functional cellulose nanofibers in highly permeable filters working on the adsorption principle may be extended to encompass removal of other water contaminants for a better supply of clean water.