Green synthesis of cellulose graft copolymers for anion exchange water purification

A cellulose graft copolymer (cellulose nanoresin) was synthesized by the all-aqueous functionalization of cellouronic acid with poly (vinyl benzyl trimethyl ammonium chloride) (poly(vbTMAC)). Cellulose was oxidized using the highly reported 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO)-mediated selective C-6 oxidation reaction. Fischer–Speier esterification of cellouronic acid was used to graft poly(vbTMAC) to the cellulosic backbone in a facile click-like mechanism. Synthesis of cellulose nanoresin was confirmed using dynamic light scattering and zeta potential measurements. Conductometric titration was used to determine the carboxylate content of cellouronic acid and the percent functionalization of the cellulose nanoresin, which was 1.69 ± 0.03 mmol/g and 61.2 ± 4%, respectively. Using a disodium fluorescein (NaFL) surrogate adsorbate, the maximum adsorption capacity of CNR was measured to be 26.8 ± 1.3 mg NaFL per gram of CNR with a Langmuir equilibrium binding constant of Ks = 10.5 ± 2 ppm⁻¹. When examined as a thin film membrane, a breakthrough study of CNR showed that equilibrium loading was achieved in less than 30 s, and that > 90% of loading occurred in under 5 s. This data suggests that these films can be used as contact resins for anion-exchange water purification. We show in this work that these films maintain > 99% of loading performance over 40 trials of regeneration and reuse, meaning that these films are green and regenerable. Initial testing shows that CNR is effective at the removal of perfluorooctane sulfonate (PFOS) from water to below our limit of detection of 100 ppt.