Upcycling cardboard waste into functional cellulose for water treatment

Abstract

This research focused on the production of bio-based flocculants from cardboard waste to mitigate the environmental issues associated with conventional chemical coagulants while promoting a circular economy. Cellulose fibers were extracted from post-consumer cardboard using NaOH at room temperature and characterized by FTIR and XRD. These fibers subsequently served as feedstock for two bio-based flocculants. Non-ionic soluble cellulose (SOL-Cel) is produced by a freeze-thaw process, combining with sonication of cellulose fibers under alkaline conditions, yielding cellulose with a molecular weight of 345 kDa as the optimal flocculant. Its cationic counterpart (CAT-Cel) was synthesized via the reaction of cellulose fibers with CHPTAC under alkaline conditions at room temperature. Notably, ¹H-NMR analysis confirmed the successful covalent grafting of CHPTAC onto the cellulose backbone, with the final product reaching a molecular weight of 702 kDa. The flocculation process using SOL-Cel and CAT-Cel was optimized through response surface methodology, and the resulting models were subsequently applied to real surface waters. At pH 6-7, SOL-Cel achieved 97.19% turbidity removal for Chao Phraya River water and 97.91% for Chulalongkorn pond water. In comparison, CAT-Cel achieved 88.42% and 73.33% turbidity removal when applied to samples from Chulalongkorn pond and Dharma Center pond at lower dosages. Furthermore, both flocculants outperformed polyaluminum chloride in turbidity reduction while producing less secondary sludge. SOL-Cel works via a polymer-bridging mechanism through hydrogen bonding, whereas CAT-Cel relies on charge neutralization and polymer-bridging. Overall, with a Path2Green score of 0.45, the method aligns with the green chemistry principle by using renewable feedstocks and supports SDG 6 for clean water and sanitation.