Dimethoate degradation by VUV/UV process: Kinetics, mechanism and economic feasibility

Vacuum ultraviolet/ultraviolet (VUV/UV) process has been applied to water treatment recently, but little is known about its efficacy and mechanism for pesticide degradation. This study investigated the degradation kinetics and mechanism of a typical organophosphorus pesticide, dimethoate (DMT) by VUV/UV, and then the economic feasibility was assessed. DMT degradation followed well the pseudo-first-order reaction kinetics at an initial concentration of ≤5.0 mg L^-1. DMT was degraded by 97.8% after 10 min of VUV/UV exposure (VUV fluence = 12 mJ cm^-2), whereas by only 5.2% after 10 min of UV exposure (UV fluence = 156 mJ cm^-2). The apparent quantum yield of DMT degradation by VUV/UV was determined to be 0.19, and at most 50.7% of hydroxyl radicals (HO^•) generated from VUV photolysis of water could be utilized for DMT degradation. As the pH increased from 5.0 to 9.0, the DMT degradation rate decreased from 0.43 to 0.23 min^-1. DMT degradation pathways in the VUV/UV process were proposed based on identified organic intermediates and inorganic ions. SO₄^2- was first released due to HO^• attack on the SP bond of DMT, which governed the DMT degradation efficiency; while the release of PO₄^3- was pertinent to the DMT mineralization efficiency. DMT solution toxicity was significantly reduced after VUV/UV treatment. An electrical energy-per-order (E_EO) value of 0.57 kWh m^-3 Order^-1 demonstrated the economic feasibility of the VUV/UV process for DMT removal in small-scale drinking water treatment.