Surface Functionalization of Bioactive Hybrid Adsorbents for Enhanced Adsorption of Organic Dyes

In this study, a valuable adsorbent was functionalized using commercial ZnO and a mango seed extract (MS-Ext) as a green approach for synthesis. Fourier-transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, and energy-dispersive X-ray analysis spectraconfirmed the presence of bioactive phenolic compounds and Cu2+ ions on the surface of ZnO. Functionalized Cu-doped ZnO/MS-Ext exhibits high efficacy in acidic, neutral, and alkaline medium, as indicated by 98.3% and 93.7% removal of methylene blue (MB) and crystal violet (CV) dyes, respectively. Cu-doped ZnO/MS-Ext has a zeta potential significantly lower than pristine zinc oxide (p-ZnO), which results in enhanced adsorption of cationic MB and CV dyes. In binary systems, both MB and CV were significantly removed in acidic and alkaline media, with 92% and 87% being removed for CV in acidic and alkaline media, respectively. In contrast, the removal efficiency of methyl orange dye (MO) was 16.4%, 6.6% and 11.2% for p-ZnO, ZnO/Ext and Cu-doped ZnO/Ext, respectively. In general, the adsorption kinetics of MB on Cu-doped ZnO/MS-Ext follow this order: linear pseudo-second-order (PSO) > nonlinear pseudo-second-order (PSO) > nonlinear Elovich model > linear Elovich model. The Langmuir isotherm represents the adsorption process and indicates that MB, CV, and MO are chemisorbed onto the surface of the adsorbent at localized active centers of the MS-extract functional groups. In a binary system consisting of MB and CV, the maximum adsorption capacity (q_m) was 72.49 mg/g and 46.61 mg/g, respectively. The adsorption mechanism is governed by electrostatic attraction and repulsion, coordination bonds, and π-π interactions between cationic and anionic dyes upon Cu-doped ZnO/Ext surfaces.