Sonophotocatalytic degradation of alachlor using crystalline S-doped TiO₂ under visible light

Abstract

Alachlor [2-chloro-2′, 6′-diethyl-N-(methoxymethyl) acetanilide], a widely used herbicide, has been known as a highly toxic endocrine disruptor and recognized as 2B chemical (possible to cause cancer to humans). In this study, a combination of photocatalysis with sonolysis generally called sonophotocatalysis has been used for treating alachlor in synthetic wastewater investigated under S-doped TiO₂ catalyst. The catalyst possesses a strong visible light absorption, small crystallite size (12 nm) and high relative surface area (87 m2 g⁻¹). S-doped TiO₂ showed the decomposition of alachlor higher than TiO₂ by given rate constant of pseudo-first order of 0.00138 min⁻¹. It was tested under the condition: initial concentration and catalyst loading of 10 mg L⁻¹ and 1 g L⁻¹, respectively. The degradation of alachlor on S-doped TiO₂ was also studied under sonophotocatalysis in visible light region. Parameters such as, alachlor initial concentration (5–30 mg L⁻¹) catalyst loading (0.5–2 g L⁻¹) ultrasonic density (0.163–0.538 W mL⁻¹) and oxidizing agent effects (K₂S₂O₈ : 0.0185– 0.111 mM and H₂O₂ : 0.5–50.0 mM), were used for the investigation. Sonophotocatalysis enhanced the reaction rate 11% higher than photocatalysis. The reaction was influenced by OH radicals (through the Fricke reaction) generated from sonication. K₂S₂O₈ solution, an oxidizing agent, affected the degradation of alachlor, while H₂O₂ did not affect. Mineralization of alachlor achieved 65% the optimal condition; alachlor initial concentration 10 mg L⁻¹ catalyst loading 1 g L⁻¹ and ultrasonic density 0.163 W mL⁻¹. In the response surface methodology (Box Behnken model), three parameters were studied simultaneously. The optimum operating conditions were of alachlor concentration, 1.785 g ⁻¹ catalyst loading and 0.55 W mL⁻¹ to achieve 85% alachlor removal in 1 hr. The proposed model was deviated from the observed values by ±2%