Assessment of Photocatalytic Efficiency of Graphene Oxide–TiO2 Nanocomposite for Removal of Binary Mixtures of Organophosphorus Pesticides from Water

Abstract

Photocatalysis studies were performed using graphene oxide–TiO₂ (GOT) nanocomposite irradiated using 125 W UV and visible irradiation to investigate the effect of various water matrices, i.e., distilled water (DW), secondary treated wastewater (WWE), and lake water (LW) on the removal of organophosphorus pesticides from binary mixtures formulated using a 2² full factorial design. The EC₆₀ and EC₄₀ values of individual pesticides, determined from the dose response profile using the Ellman assay were used as the high and low concentrations, respectively. Photocatalysis was conducted at a GOT dose of 60 mg/L. For both Mixture-I, comprised of dichlorvos and malathion, and Mixture-II, comprised of parathion and phorate, degradation followed the order, DW > WWE > LW. After 80 min, the highest degradation of ∼80% was observed for Mixture I in DW under UV irradiation when the concentration of both pesticides was at EC₄₀. Malathion displayed a higher rate and extent of degradation and mineralization compared to dichlorvos in all of the mixture combinations. Under similar reaction conditions, phorate and parathion demonstrated similar values of the first-order degradation rate constant. Dissolved organic matter (DOM) had a detrimental effect on pesticide degradation by blocking the active sites on the catalyst and by scavenging the oxidative radicals generated during irradiation. A decrease in SUVA₂₅₄ in both WWE and LW during photocatalysis indicated the decomposition of aromatic moieties in DOM. After UV/visible photocatalysis, the lowest residual toxic effect, as measured in the Ellman assay, was observed in mixtures containing low initial concentration of both the pesticides.