Sulfur-Doped SnO₂ Synthesized via Garlic Extract for Efficient Visible-Light Cr(VI) Reduction

The photocatalytic reduction of persistent heavy-metal pollutants remains a pressing challenge in environmental remediation. In this study, sulfur-doped SnO₂ (GE-SnO₂) photocatalysts were synthesized via a hydrothermal route using garlic extract (Allium sativum) as a green sulfur source that simultaneously facilitated nucleation and doping. The synthesized catalyst was comprehensively characterized, and the performance of Cr-(VI) reduction was systematically evaluated. Sulfur doping effectively narrowed the band gap from 3.08 to 2.50 eV, thereby enhancing visible-light absorption and charge-carrier transfer across the catalyst-solution interface. Among the synthesized samples, optimally doped 0.5 GE-SnO₂ exhibited the highest photocatalytic activity, achieving 100% Cr-(VI) reduction (10 mg L^-1 initial concentration, 0.375 g L^-1 dosage) within 10 min under visible light, with the apparent rate constant (k_obs) increasing from 0.034 min^-1 for pristine SnO₂ to 0.207 min^-1 for 0.5 GE-SnO₂. The catalyst maintained >94% efficiency after four reuse cycles. An analysis of the intermediate products and reactive radicals (^•OH and ^•O₂ ^-) indicates an enhanced charge separation pathway, which promotes efficient charge separation and enhances the photocatalytic performance of the catalyst. Complementary data-driven analyses, including correlation and Random Forest models (R ² = 0.985 ± 0.007), confirmed the optimum photocatalytic parameters such as reaction time and catalyst dosage. This environmentally benign and scalable synthesis strategy integrates experimental and machine learning insights, offering a rational framework for designing high-performance photocatalysts for Cr-(VI) management and aims to broaden environmental application.