Sustainable One-Pot Hydrothermal Engineering of Pine-Needle-Derived Hydrochar Integrated with Mono- and Trimetallic Oxides for Ultrafast Visible-Light-Driven Photocatalytic Degradation of Atrazine: Experimental, Theoretical, and Toxicity Investigations

Abstract

Biowaste-derived multifunctional materials integrated with transition-metal oxides are emerging as sustainable photocatalysts owing to their tunable electronic properties, high catalytic activity, and environmental benignity. Herein, lignocellulosic pine-needle waste was valorized through a one-pot hydrothermal carbonization strategy to fabricate mono- and trimetallic oxide-functionalized hydrochar composites, namely Fe@HC, Co@HC, Ti@HC, and Fe-Co-Ti@HC. Comprehensive physicochemical characterization (UV–vis, FT-IR, Raman, FE-SEM/EDS, P-XRD, XPS, EIS and BET analyses) confirmed the successful incorporation of metal oxide species within the hierarchical porous hydrochar framework. The synthesized materials were assessed as visible-light-responsive adsorbent-photocatalysts for atrazine (ATRZ) degradation in the presence of peroxymonosulfate (PMS) initiator. Among all systems, Fe-Co-Ti@HC exhibited excellent photocatalytic performance, achieving 93.25% ATRZ degradation within 40 min under optimized conditions (20 ppm ATRZ, 0.4 g L⁻¹ catalyst dosage, pH 4), whereas Fe@HC, Co@HC, and Ti@HC afforded comparatively lower degradation efficiencies. The enhanced activity of Fe-Co-Ti@HC is attributed to synergistic metal-metal and metal-carbon interactions, improved charge separation, and increased density of surface-active sites. Kinetic investigations followed pseudo-second-order behavior, indicating dominant surface-mediated photocatalytic processes. The trimetallic photocatalyst also demonstrated excellent recyclability over seven consecutive cycles with negligible loss in activity. In addition, Density functional theory (DFT) calculations revealed stronger ATRZ adsorption and favourable electronic interactions in the Fe-Co-Ti@HC system, corroborating the experimental findings. Toxicity assessment using ECOSAR and brine shrimp lethality bioassay indicated reduced toxicity of degradation products after treatment. Overall, this study presents a sustainable strategy for converting pine-needle waste into efficient multifunctional photocatalysts for environmental remediation.