Solution Plasma-Assisted Synthesis of ZnO Nanoparticle/Activated Hydrochar Hybrids from Mangosteen Peel for Synergistic Dye Removal

This study demonstrates an activator-free route for converting mangosteen peel waste into bifunctional hybrids via hydrothermal carbonization (HTC), solution plasma process (SPP), and rapid thermal refinement. Crucially, SPP is applied for the first time to activate hydrochar (HC) and in situ deposit ZnO nanoparticles (ZnO-NPs), forming a robust ZnO-NPs/activated hydrochar (ZnO-NPs/AHC) framework. Nitrogen adsorption revealed that incorporating AHC effectively mitigated nanoparticle agglomeration, yielding a specific surface area of 34.55 m2·g–1 for the optimized ZnO-NPs/AHC-1.0 composite compared to pristine ZnO-NPs (0.67 m2·g–1). Under sunlight, the hybrid achieved near-complete methylene blue (MB) decolorization (>99%) within 20 min via synergistic adsorption–photocatalysis. Chemical Oxygen Demand (COD) analysis confirmed substantial organic mineralization (∼66.5%), while ICP-OES measurements demonstrated negligible Zn2+ leaching (≤0.0020 mg·L–1), verifying high chemical stability and environmental safety. Reusability tests confirmed structural integrity across multiple cycles. Overall, this green HTC–SPP–calcination route offers a sustainable strategy for transforming agricultural byproducts into functional environmental materials for wastewater remediation.