Synthetic dyes are among the most troublesome pollutants in industrial wastewater. Released by textile, leather, pharmaceutical, food, cosmetic, and printing operations, these compounds feature complex aromatic structures that resist biodegradation and can persist in aquatic ecosystems for long periods. Their presence reduces light penetration and dissolved oxygen in rivers and lakes, harms aquatic organisms, and in some cases produces toxic or potentially carcinogenic breakdown products. Conventional treatments such as coagulation, adsorption, membrane filtration, and biological processing often fall short with recalcitrant contaminants, and they can generate sludge or secondary waste while driving up operating costs.

Photocatalysis offers an appealing alternative because it uses light energy to generate highly reactive species that oxidize organic contaminants under mild conditions. Cuprous oxide, a p-type semiconductor with a narrow bandgap of roughly 1.2 to 2.0 electron volts, has long attracted attention as a visible-light-responsive photocatalyst thanks to its strong visible absorption, low cost, and earth-abundant elements. But pristine Cu₂O suffers from rapid recombination of photogenerated electron–hole pairs, photocorrosion, and poor structural stability during prolonged irradiation—problems that have motivated researchers to pair it with conductive carbon materials that can shuttle charges away before they recombine.

Carbon quantum dots, or CQDs, are zero-dimensional carbon nanomaterials prized for their water dispersibility, tunable photoluminescence, chemical stability, and electron-accepting behavior. Within photocatalytic systems they act as electron reservoirs and conductive bridges, promoting interfacial charge migration and curbing recombination. Unlike heavy-metal quantum dots containing cadmium, lead, or mercury, carbon-based dots exhibit lower toxicity and favorable biocompatibility. Crucially, the new study sourced its CQDs from carrot peel, an abundant food-processing residue rich in hydroxyl and carboxyl groups. The peels required no chemical pretreatment: they were washed, dried at 80 degrees Celsius, ground, dispersed in water, and heated hydrothermally at 150 degrees Celsius for 12 hours to yield a functionalized carbon dot dispersion.

The nanocomposite itself was assembled by a simple mixing-and-drying method, with concentrated CQD solution stirred into Cu₂O powder and dried at 60 degrees Celsius. Oxygen-containing functional groups on the dots anchor to the semiconductor through hydrogen bonding and coordination effects. Fourier transform infrared spectroscopy confirmed the rich surface chemistry—broad hydroxyl stretching near 3400 wavenumbers, carbonyl bands at 1773, and graphitic carbon–carbon stretches around 1620—while X-ray diffraction revealed sharp reflections matching cubic Cu₂O with no detectable CuO, metallic copper, or other impurity phases. A broad peak near 23 degrees confirmed semi-crystalline graphitic carbon domains, and subtle peak broadening pointed to strong interfacial coupling between the two components.

Microscopy reinforced that picture. Scanning electron microscopy showed nearly spherical to polyhedral Cu₂O particles homogeneously distributed without agglomeration, their surfaces roughened by deposited carbon dots. Transmission electron microscopy revealed ultrafine quasi-spherical CQDs intimately anchored to the Cu₂O surface, and high-resolution imaging resolved lattice fringes with a spacing of about 0.24 nanometers corresponding to the (111) plane of cubic Cu₂O—evidence that the synthesis preserved crystal integrity while forming a stable heterointerface.

Optical measurements explained why the hybrid performs so well. UV–visible spectroscopy showed strong ultraviolet absorption from the graphitic π to π* transitions plus a broad tail extending deep into the visible region, and a Tauc plot gave the CQDs a bandgap of about 2.54 electron volts. Most tellingly, photoluminescence intensity dropped sharply in the composite compared with pristine Cu₂O, indicating that far fewer charge carriers recombine radiatively and that more electrons migrate to the carbon dots, where they react with dissolved oxygen to form superoxide radicals. X-ray photoelectron spectroscopy confirmed monovalent copper, abundant surface oxygen species, and binding-energy shifts consistent with interfacial charge redistribution across the heterojunction.

Under a 500-watt tungsten halogen lamp, the composite destroyed 94.67 plus or minus 0.62 percent of PBS dye within 70 minutes at an initial concentration of 5 milligrams per liter, a catalyst dosage of 1.25 grams per liter, and pH 2—conditions optimized across systematic tests of concentration, pH, and dosage. Degradation followed pseudo-first-order kinetics with excellent linearity, and adsorption data fit the Freundlich isotherm best, pointing to a heterogeneous surface with multilayer uptake. Radical scavenging experiments identified hydroxyl radicals as the dominant destructive species, with superoxide radicals playing a secondary role and direct hole oxidation contributing only marginally. The catalyst retained most of its activity over repeated cycles, with the graphitic framework apparently shielding Cu₂O from photocorrosion.

The economic case is equally striking. Electrical energy per order analysis showed favorable energy demand, and the estimated production cost was only about 55 rupees per 100 grams, thanks to essentially free raw material and a one-step synthesis free of hazardous reagents. Operating costs per kilogram of dye removed fell as initial dye concentrations rose, dropping to roughly 962 rupees at 25 milligrams per liter. Fluorescence quenching experiments with human serum lysozyme, yielding a Stern–Volmer constant of 2.654 times 10 to the fourth per mole, further hinted at bioanalytical potential beyond water treatment. The authors caution that validation with real wastewater matrices, broader pH ranges, larger-scale catalyst recovery, and long-term ecotoxicological assessment remain necessary—but the demonstration that juice-shop waste can become a multifunctional, low-cost environmental catalyst marks a compelling step toward sustainable water remediation.

Subject of Research: Visible-light photocatalytic degradation of synthetic dye using a biowaste-derived carbon quantum dot/cuprous oxide nanocomposite

Article Title: Highly efficient visible-light photocatalytic degradation of PBS dye using biowaste–derived CQDs/cu₂O nanocomposite

Article References: Mujahid, M. (2026). Highly efficient visible-light photocatalytic degradation of PBS dye using biowaste–derived CQDs/cu₂O nanocomposite. Results in Chemistry, 31, Article 103912. https://doi.org/10.1016/j.rechem.2026.103912

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103912

Keywords: photocatalysis, carbon quantum dots, cuprous oxide, carrot peel biowaste, dye degradation, wastewater treatment, nanocomposite, visible light, reactive oxygen species, green synthesis, Ponceau BS, water remediation