Eco-friendly nanocellulose hybrid composites for Cr (VI) adsorption: comparative insights into chitosan and polyvinyl alcohol systems

Abstract

Wastewater contaminated with hexavalent chromium (Cr(VI)) is a potential health and environmental hazard and hence the need for sustainable and cost-effective adsorbents. Cellulose nanocrystals (CNCs) were isolated from sugarcane bagasse by mild acid hydrolysis (10 % v/v H2SO4) and then used to prepare two different membranes, Membrane 1 (Chitosan-CNC, NaOH-crosslinked) and Membrane 2 (PVA-CNC, crosslinker-free) for comparative Cr(VI) removal from synthetic wastewater. DLS and XRD indicated the CNCs were colloidally stable (zeta potential: −25.7 mV) and were crystalline (cellulose Iβ) and had an average particle size of 90–126 nm. Successful composite formation and superior hydrophilicity of Membrane 2 (37.4°) compared to Membrane 1 (74.2°) was confirmed by FESEM, FTIR and contact angle measurements. The maximum adsorption capacities for Cr(VI) were 67.0 and 80.0 mg g ⁻¹ for the Membrane 1 and Membrane 2, respectively, which were obtained in batch adsorption studies at pH 4 and pH 3, respectively. The adsorption behavior of both membranes was well described by the Langmuir and Freundlich isotherm models, indicating monolayer adsorption on homogeneous sites alongside adsorption on heterogeneous surfaces. The pseudo-second-order kinetic model, together with positive values of Δ H ° and Δ S °, and negative for Δ G ° suggested that the adsorption process was predominantly chemisorption-driven, endothermic, and spontaneous. Moreover, after 5 regeneration cycles, both membranes still showed a good Cr(VI) removal percentage (Membrane 1: 86–70 %, Membrane 2: 94–72 %), proving their reusability. The results indicate that the agro-waste produced nanocellulose composites, with very little use of chemicals, can be useful, mechanically strong, and environmentally friendly heavy-metal adsorbents.