Biogenic synthesis of Cu/ZnO functionalized biochar composite: a sustainable approach for visible light-photocatalytic degradation

This study presents the novel synthesis of a biochar-Cu/ZnO composite using Areca catechu L. husk biochar as a carrier for Cu/ZnO photocatalysts derived employing Rubia cordifolia root extracts. The synthesized Cu/ZnO functionalized biochar composite was characterized by SEM, EDX, XRD, and UV-VIS spectroscopy, revealing high crystallinity and a reduced band gap of 2.73 eV, contributing enhanced photocatalytic performance under visible light. Photocatalytic activity was evaluated using Alizarin Red (AZ) and Rhodamine B (RhB) dyes, achieving maximum degradation efficiencies of 97.52 % and 96.83 %, respectively, under optimized conditions. Response Surface Methodology–Central Composite Design (RSM–CCD) was employed to optimize photocatalytic degradation conditions and to develop regression models (R² > 0.90). Scavenger studies using isopropanol indicated that hydroxyl radicals accounted for over 60 % of the degradation mechanism. The degradation kinetics of the composite were evaluated and rate constants exceeds 0.025 min⁻¹ for both dyes. Reusability tests showed less than a 20 % efficiency drop after four consecutive cycles. Treatment of industrial wastewater resulted in 71 % decolorization and significant reductions in COD, BOD, and TDS levels, confirming the material's practical applicability for visible-light-driven wastewater remediation.