Waste-to-work photocatalysis: orange-peel-derived CQD@CdS/biochar hybrids enabling efficient visible-light-driven dye mineralisation and mechanistic elucidation

Abstract

The uncontrolled discharge of organic dyes into aquatic systems necessitates the development of efficient and sustainable photocatalysts for environmental remediation. In this work, a green and resource-efficient strategy was employed to synthesize a CQD@CdS/biochar nanocomposite using orange peel waste as a renewable carbon precursor and aloe vera extract as a natural reaction medium. Structural and physicochemical characterization using XRD, FTIR, XPS, FESEM, UV–Vis DRS, PL, and BET confirmed the successful formation of the composite. The photocatalytic activity was evaluated through the degradation of methylene blue under visible-light irradiation. Among the prepared samples, the optimized 5% CQD@CdS/biochar (5CC) composite exhibited superior performance, achieving 96.2% dye removal within 120 min and following pseudo-first-order kinetics with a rate constant of 0.0061 min⁻¹, outperforming the pristine materials and TiO₂-P25. The catalyst also demonstrated good stability, retaining 89.6% of its activity after five reuse cycles. Radical trapping experiments revealed ˙OH radicals as the dominant reactive species responsible for degradation. Furthermore, significant mineralization was confirmed by COD and TOC reductions of 63% and 66%, respectively, supported by HRMS identification of degradation intermediates and a proposed reaction pathway. These findings highlight the potential of waste-derived CQD@CdS/biochar as an efficient photocatalyst for visible-light-driven wastewater treatment.