Synthesis of bi-functional lignocellulosic adsorbent: Application for the simultaneous removal of cationic and anionic dyes

Abstract

The treatment of water polluted by mixed harmful compounds through minimal steps for treatment costs and environmental footprint reduction purposes, is an actual concern. In the present work, a bifunctional adsorbent capable of removing both cationic and anionic pollutants in aqueous medium was prepared by controlled grafting of a quaternary ammonium functionalized compound (Glycidyltrimethylammonium chloride (GTAC)) on sawdust. In order to facilitate the controlled grafting, the functionalization was performed in the ionic liquid solvent named (1-butyl-3-methylimidazolium chloride). Adsorbent characterizations by ¹³C NMR, points of zero charge determination and adsorption of model ionic compounds, confirmed the effective grafting of GTAC. The adsorbent was successfully applied to the simultaneous adsorption of a cationic dye (methylene blue (MB)) and an anionic dye (Orange (II) (O (II))). A synergistic effect favors the adsorption of the anionic dye to the detriment of the cationic dye due to the decrease of the negative charge of the lignocellulosic backbone. This was confirmed by thermodynamic studies, indicating stronger adsorption of the cationic dye (ΔH = −13.71 kJ/mol versus −8.42 kJ/mol for O(II)). The maximum adsorption capacities were less significant in simultaneous mode (79.7 µmol g⁻¹ for O(II) and 37.7 µmol g⁻¹ for MB) compared to the single mode (86.4 µmol g⁻¹ for O(II) and 202.3 µmol g⁻¹ for MB). The bifunctional sawdust exhibits high efficiency in real water samples and maintains relatively good performances even after five adsorption/desorption cycles.