Harnessing Waste-Derived Biochar Decorated with ZnO-ZrO₂ Nanoparticles for Economical and High-Efficiency Fluoride Remediation

Groundwater fluoride pollution is a significant public health concern particularly in regions where cost-effective and efficient treatment solutions are not feasible. However, many improved biochar-based adsorbents have been studied, but reaching high removal effectiveness at realistic fluoride concentrations with minimal material consumption still remains a critical problem. In this study, a novel zinc–zirconium oxide-modified biochar composite (BZ²) was prepared from coconut shell biochar to improve the fluoride adsorption performance by the synergistic dual metal oxide modification. XRD, SEM and EDAX studies verified the effective incorporation and homogenous distribution of crystalline ZnO and ZrO₂ nanoparticles in the porous charcoal matrix. Batch adsorption tests were performed to investigate the effects of adsorbent dosage, contact time, pH and temperature at an initial fluoride content of 5 mg L⁻¹, which represents the real ground water conditions. The composite BZ² showed superior removal effectiveness (98%) compared to biochar (88%), ZnO (92%), and ZrO₂ (95%) at an optimum pH of 6 with equilibrium attained within 40 min. Kinetic studies showed that the adsorption process followed a pseudo-first-order model (R² = 0.9705–0.9989) and implied diffusion-controlled physisorption, and isotherm studies showed that the Freundlich model (R² = 0.9700–0.9981) was the best fit to describe the adsorption behavior, implying the presence of heterogeneous and multilayer adsorption. The improved performance of BZ² is due to the synergistic effect of ZnO and ZrO₂ nanoparticles with biochar matrix that creates more active binding sites and enhances the adsorption kinetics. In summary, under relevant conditions, the synthesized Zn–ZrO₂–biochar composite exhibits outstanding efficiency, simple synthesis, and significant promise as a sustainable and affordable adsorbent for fluoride removal from groundwater.