ABSTRACT Water pollution from synthetic dyes poses a serious environmental threat due to their toxicity and resistance to degradation. This study reports the photocatalytic degradation of Rose Bengal (RB), a hazardous xanthene dye, using green‐synthesized cadmium sulfide (CdS) quantum dots (QDs). CdS QDs were prepared with Ocimum sanctum (Tulsi) leaf extract, yielding 3.0–3.6 nm nanocrystals of hexagonal wurtzite structure and a visible‐light‐responsive bandgap of ∼2.6 eV. Photocatalytic activity was evaluated under visible light by varying catalyst dosage, solution pH, and temperature. Smaller CdS QDs displayed superior performance, achieving over 91% degradation within 90 min under optimal conditions. Kinetic studies confirmed a first‐order degradation process, with higher temperatures and catalyst amounts accelerating the reaction, while alkaline pH reduced efficiency. Maximum removal efficiency of ∼99.3% was obtained using 60 mg of catalyst at 70°C and pH 6. Thermodynamic analysis yielded an activation energy of 12.48 kJ/mol, with enthalpy and entropy of activation estimated at 9.79 kJ/mol and –242.85 J/mol K, respectively. Scavenger tests identified hydroxyl radicals as the dominant reactive species, followed by superoxide radicals and photogenerated holes. Reusability studies showed ∼87% efficiency retention after four cycles, confirming catalyst stability and recyclability. This work introduces green‐synthesized CdS QDs as highly efficient photocatalysts for RB degradation, achieving near‐complete dye removal within 90 min under visible light. Unlike earlier reports with longer degradation times, the study provides systematic optimization of particle size, dosage, pH, and temperature, along with mechanistic insights, highlighting the sustainability and reusability of the catalyst for practical water purification.
Kundu et al. (2026) studied this question.