Tuning the surface charge of cellulose super-bridging agents for improved performance during wastewater treatment

Wastewater treatment is challenged by refractory contaminants and rising water demand, while conventional coagulation-flocculation suffers from low throughput and is sensitive to the influent water conditions. Cellulose fibers can act as super-bridging agents offering enhanced turbidity removal when used in conjunction with traditional coagulants and flocculants. However, the chemical modification of cellulose fibers and their performance under challenging influent conditions remain largely unexplored. In this study, recycled cellulose fibers are modified with cationic quaternary ammonium groups, imparting a positive charge that can improve treatment performance for a wider range of water matrices including varying influent pH, ionic strength, and turbidity. Modified fibers achieve turbidity below 5 NTU across all influent pHs (7.0–8.9) and turbidities (62–285 NTU) tested, while pristine fibers show that the target turbidity of 25 NTU is not reached when the influent pH or turbidity exceed 8.2 or 130 NTU, respectively. Additionally, modified fibers achieve the target turbidity of 25 NTU even in the presence of an additional 20 mM NaCl, while pristine fibers cannot reach this target at an additional 10 mM NaCl. Furthermore, the modified fibers enhance the removal of metals such as Zn, Cr, Fe, and Pb through the incorporation of metal hydroxide precipitates within fiber-based flocs. Overall, pristine and modified fiber-enhanced treatments remove 52 % and 65 % of metals, respectively, compared to 39 % with the conventional method. Therefore, modified cellulose fibers represent a promising strategy for enhancing coagulation-flocculation treatment performance.