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. It has been shown that cellulose fibers can act as super-bridging agents offering enhanced turbidity removal when used in conjunction with traditional coagulants and flocculants. Yet, the chemical modification of cellulose fibers and their performance under varying influent conditions remain largely unexplored. In this study, recycled cellulose fibers are modified with quaternary ammonium groups, imparting a positive charge that significantly improves performance. Where conventional treatment (without fibers) reduces turbidity from 62 to 25 NTU, the addition of modified fibers lowers the effluent turbidity to 3 NTU. To assess the robustness of this strategy, influent pH, ionic strength, and turbidity are varied in controlled laboratory experiments. Modified fibers achieve turbidity below 5 NTU across all pHs (7–8.9) and initial turbidities (62–285 NTU) tested, whereas the conventional method is unable to reduce turbidity when pH exceeds 7.7. Furthermore, the modified fibers enhance the removal of metals such as Ni, Mn, Zn, Cr, Fe, and Pb. In total, 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 an interesting strategy for improving the coagulation- flocculation treatment strategy.