Removal of Organophosphorus Pesticides from Water Using a TiO2/g-C3N4-Based Sequential Adsorption–Photocatalysis System

Abstract

Excessive pesticide residues in natural waters are closely associated with agricultural drainage, and organophosphorus pesticides (OPPs) account for more than 80% of pesticides used in China. Although semiconductor photocatalysis for pesticide removal has been widely studied, adsorption-assisted strategies to reduce photocatalytic treatment loads remain underexplored. In this study, a TiO₂/g-C₃N₄ (CP1:1) heterojunction with adsorption–photocatalytic capability was synthesized via a pyrolysis-calcination method. Diazinon (DAZ) was selected as a representative OPP, and DAZ and its transformation product diethyl thiophosphate (DETP) were defined as pesticide-derived organic phosphorus. A sequential adsorption–photocatalysis system was established, where CP1:1 first removed pesticide-derived organic phosphorus through adsorption, followed by photocatalytic degradation of residual species. CP1:1 exhibited rapid adsorption performance, with a maximum adsorption capacity of 297.82 mg/g (as TP) at pH 7, an initial DAZ concentration of 60 mg/L (as TP), and an adsorbent dosage of 0.1 g/L. The adsorption process was likely associated with electrostatic attraction, π–π interactions, and hydrogen bonding. Under photocatalytic conditions (0.4 g/L catalyst dosage, 5 mg/L DAZ as TP), 93.4% removal of pesticide-derived organic phosphorus was achieved after 6 h of irradiation with 365 nm UV light. The optimized sequential system achieved over 95% overall removal, with adsorption contributing approximately 60%. This study provides a strategy to reduce photocatalytic treatment loads and enhance pesticide-derived organic phosphorus removal through a sequential adsorption–photocatalysis process.