Waste-derived CQDs/ZnIn2S4 S-scheme heterostructures with built-in electric fields for visible-light detoxification of wastewaters containing methylene blue and Cr(VI)

Visible-light purification of contaminated waters is often constrained by rapid charge recombination and strong matrix interference, especially in complex real wastewaters. Here, we upcycled rice-noodle waste into carbon quantum dots (CQDs), and integrated them with ZnIn 2 S 4 to construct three-dimensional CQDs/ZnIn 2 S 4 nanoflowers. Optimising the CQDs loading at 3 wt% enabled the composite to remove 50 mg L −1 of pollutants within 120 min under visible irradiation, achieving 99.3% Cr(VI) reduction and complete methylene blue (MB) degradation with apparent first-order rate constants of 0.0192 min −1 (Cr(VI)) and 0.0223 min −1 (MB). The photocatalyst resisted matrix inhibition, with removals reaching 93.1% (Cr(VI)) and 91.6% (MB) in wastewaters, and above 95% in tap, lake and river waters; catalytic activity was retained above 95% after five reuse cycles. Valence-band X-ray photoelectron spectroscopy (VB-XPS) and density functional theory (DFT) band alignment analysis, together with transient photocurrent response, electrochemical impedance spectroscopy (EIS), photoluminescence (PL)and electron paramagnetic resonance (EPR) and radical-trapping experiments collectively verified an S-scheme CQDs/ZnIn 2 S 4 heterojunction with an interfacial built-in electric field that drives vectorial charge separation and preserves highly reductive electrons and oxidative holes at spatially distinct sites. This waste-to-resource strategy couples low-cost carbon precursors with interfacial band engineering to deliver a scalable, visible-light-driven platform for simultaneous detoxification of inorganic (Cr(VI)) and organic dye pollutants in realistic treatment scenarios.