Deciphering adsorption behaviour and mechanisms of enhanced phosphate removal via optimized cetyltrimethylammonium bromide-modified nanofibrillated cellulose.

To combat the persistent environmental issues resulting from eutrophication, it is necessary to scavenge excess phosphorous levels from aquatic ecosystems. In response, a cationic adsorbent was prepared by modifying agrowaste-derived natural biomacromolecule; nanofibrillated cellulose (NFC) using cetyltrimethylammonium bromide (CTAB) surfactant. Comprehensive characterization through XRD, FTIR, HR-SEM, SEM-EDX, BET and XPS demonstrated that quaternizing NFC significantly improved its surface chemistry by introducing substantial quaternary ammonium groups. This modification imparted positive ζ potential across broad pH range, underscoring a strong affinity for negatively charged phosphate ions. Enhanced roughness and improved spatial dispersion led to nearly threefold increase in phosphate removal efficiency compared to pristine NFC, attributable to a higher number of available active sites. The adsorption process followed pseudo-second-order kinetic and Sips isotherm model, with a maximum adsorption capacity of 21.78 mg P/g, reaching equilibrium within 120 min. Besides, the prepared adsorbent demonstrated pH-dependent adsorption and displayed stable adsorption capacity particularly at weakly acidic or neutral pH conditions. Furthermore, it exhibited excellent retention capacity with only 12.61 % desorption rates over three cycles. Both XPS and FTIR results revealed that electrostatic adsorption (based on Lewis acid-base principle) and hydrogen bonding were primary adsorption mechanisms.