Removal of Hg(II) from mining-impacted wastewater using ZnO-impregnated coconut fiber: Optimization, kinetics, and thermodynamic studies

Hg(II) contamination in mining-impacted water requires effective and environmentally responsible treatment materials. ZnO-impregnated coconut fiber (CF–ZnO) was developed as a low-cost hybrid adsorbent and characterized by SEM, TEM, UV–Vis, FTIR, XRD, SEM–EDS, and TGA/DTG. Hg(II) removal was optimized by response surface methodology (RSM) using a central composite design (CCD). The quadratic model explained 97% of the response variability and, at the fixed pH of 6, identified 55 min, 0.17 g of adsorbent, and 6.2 mg L⁻¹ Hg(II) as favorable conditions, yielding 99.36% experimental removal. Equilibrium data were best described by the Freundlich model (R² = 0.9851), while the Langmuir model estimated a maximum adsorption capacity of 62.17 mg g⁻¹. Kinetic data followed the pseudo-second-order model (R² = 0.9802), and the apparent activation energy was 27.20 kJ mol⁻¹. Thermodynamic parameters indicated spontaneous, endothermic, and entropy-favored adsorption. CF–ZnO retained 71.19% Hg(II) removal after five adsorption–desorption cycles. Dissolved Zn release occurred mainly during acid regeneration, decreasing from 28.40 to 10.10 mg L⁻¹ across the cycles, whereas substantially lower concentrations were detected in the adsorption filtrates. In fortified mining water, Hg(II) removal ranged from 63.67% to 87.04%. These findings position CF–ZnO as a moderate-capacity adsorbent with practical potential, while highlighting the need to optimize regeneration to reduce Zn mobilization and improve long-term stability.