Photodegradation of pesticides in water using metal doped multi-component heterojunction photocatalyst

Abstract : For many years now, South Africa has been facing water crisis. Availability of clean water is lacking, and human population increases every year. Available water is contaminated with variety of pollutants such as microbials, organic and inorganic pollutants. This is because of increased industrial and agricultural practices to satisfy the growing population demand. Organic pollutants are prominent due to high agricultural production whereby pesticides and fertilizers are applied to optimize product output. Pesticides are of huge concern due to their persistence and toxic behaviour in the environment. They are found in rivers and ground water streams after application in the field via run-off or seep into ground. Chlorpyrifos is an organophosphate pesticide widely used to control pest. Chlorpyrifos has been detected at high levels in water in Western Cape of South Africa. Department of Agriculture in South Africa banned its use as an ingredient in domestic products and pesticides in 2010 but it is still detected in the environment. Chlorpyrifos is an endocrine disrupting chemical, nerve agent and causes dizziness. This pollutant can enter human body by ingestion, dermal adsorption or inhalation. The WHO limit of chlorpyrifos in water is 30 μg/L. Several methods have been used to remove chlorpyrifos and other pollutants in water such as biological treatment and advanced oxidation processes. Advanced oxidation processes are regarded as safe and efficient pollutant removal, removing a variety of pollutants in water. This is due to utilization of highly reactive species such as hydroxyl and superoxide radicals, known for their rapid and indiscriminative behaviour towards organic compounds resulting in a complete mineralization. Photocatalysis which is a semiconductor-based method emerged as a promising organic pollutant removal method. Tungsten trioxide (WO3) has been extensively studied for degradation of heavy metals and organic pollutants. But suffer from photogenerated charge vi recombination. To curb that, doping has been used with metal/non-metal dopant and by formation of heterojunction with another semiconductor catalyst to name a few. In this study, WO3 was successfully synthesized using a hydrothermal treatment method. It was further modified with Mn2+ ion and SnS2 to improve its photocatalytic activity towards chlorpyrifos degradation by formation of Mn-WO3/SnS2. Mn- WO3/SnS2 was formed with rectangular shapes confirmed through HRTEM and FESEM. A mixture of monoclinic and hexagonal phases was identified using XRD, Raman and SAED indexing. The optical and electrochemical properties of Mn-WO3/SnS2 were studied using EIS, PL and UV-Vis-DRS. Low emission intensity corresponding to less charge recombination, smaller semi-circle diameter corresponding to less charge transfer impedance and high visible light absorbance wavelength (582 nm) were observed in comparison to those of WO3 (466 nm), Mn-WO3 (472 nm), SnS2 (512 nm) and WO3/SnS2 (572 nm). Mn-WO3/SnS2 displayed good stability and increased surface area (77 m2/g) compared to WO3 (6 m2/g). The materials were mesoporous as determined by BET analysis with pore volumes in the range of 0.02 to 0.07. Except SnS2 which displayed porous adsorbent properties. The Mn-doped composite (Mn-WO3/SnS2) nanoparticles displayed high efficiency for the degradation of chlorpyrifos in synthetic water samples within 60 minutes. UHPLC-MS-MS was used to evaluate the concentration of chlorpyrifos and deduce the degradation pathway. Mn-WO3/SnS2 degraded up to 95% chlorpyrifos removal compared to 50, 65, 75, and 85% using WO3, Mn-WO3, SnS2 and WO3/SnS2 respectively. After optimization of conditions such as pH, initial chlorpyrifos concentration and initial photocatalyst loading, 100% chlorpyrifos removal was achieved at pH 7, 1 g of nanoparticles and 1000 ppb of chlorpyrifos concentration. The complete degradation of chlorpyrifos and its major degradation by-product TCP was achieved. vii TCP was completely degraded to innocuous materials. Kinetic studies were deduced to a second order reaction at 209x10-3 M-1s-1.M.Sc. (Chemistry