Valorization of Cellulose Sludge as a Sodium Alginate-Based Bioadsorbent for Bromazepam Removal from Water

Abstract

The remediation of benzodiazepine residues from industrial discharge remains a critical environmental challenge due to their chemical stability and neurotoxic potential. In this study, cellulose sludge from the pulp-and-paper industry was successfully upcycled into Ca2+-cross-linked sodium alginate hydrogel beads for the targeted removal of bromazepam (BZP). This work represents the first systematic investigation into BZP sequestration using a waste-derived hybrid hydrogel, at deliberately low concentrations (0.5–5.4 mg L–1), kept as close to environmentally relevant levels as the analytical method allows and within the range reported for point-source pharmaceutical effluents. The beads exhibited a rough filamentous surface and an interconnected 3D porous network characterized by a mesoporous texture (SSA: 9.35 m2 g–1; pore volume: 0.04 cm3 g–1) and an amphoteric surface (pHPZC = 8.04). Kinetic studies revealed a rapid initial uptake reaching an experimental capacity of 0.25 mg g–1 at 180 min, with the Elovich model providing the best statistical fit (R2 = 0.994; AIC = −56.9; BIC = −56.7), consistent with adsorption on energetically heterogeneous sites governed by coverage-dependent rate decay. Because the isotherms were measured in this dilute range, no saturation plateau was reached: the equilibrium data were best described by the Freundlich model at 25 °C and by a linear (Henry-type) model at 35 and 45 °C. The hydrogel beads were efficiently regenerated using 0.1 M HCl, maintaining structural integrity and performance across four consecutive cycles (BZP removal of approximately 13–15% in each cycle, with no systematic decline). This study provides a sustainable, zero-waste strategy for the valorization of abundant industrial residues into functional bioadsorbents, directly supporting circular economy goals for emerging contaminant mitigation.