<p>Sustainable water detoxification of toxic heavy metals is a global issue that needs urgent attention. Herein, we report a hybrid nanocomposite photocatalyst based on groundnut shell-derived carbon quantum dots (GSCQDs) and iron oxide (Fe₃O₄) nanoparticles prepared using green hydrothermal and co-precipitation procedures. Structural and surface characterizations (PL, UV–vis, FT-IR, XRD, FE-SEM, TEM, zeta potential) affirmed hierarchical structure, rich functional groups, and magnetic separability. Under UV-C irradiation, removal was evaluated from single-metal aqueous systems containing 20&#xa0;mg L⁻¹ of Cr(VI), Pb(II), or Ni(II). Under optimized conditions (pH 6, catalyst dosage 0.1&#xa0;g L⁻¹, irradiation time 200&#xa0;min), the nanocomposite achieved removal efficiencies of 96.5% (Cr(VI)), 84.4% (Pb(II)), and 82.0% (Ni(II)). These efficiencies were obtained from individual metal solutions rather than mixed systems. Adsorption followed the Langmuir isotherm with GSCQD/Fe₃O₄ nanocomposite facilitating magnetic recovery. The catalyst retained 95% Cr(VI) removal after three cycles, showing stability and reusability. This study points out bio-derived carbon quantum-dot/ferrite nanocomposites as effective, magnetically recyclable photocatalysts and as a green and viable approach to heavy metal remediation in water.</p> Graphical abstract <p></p>