Overlooked involvement of phosphate radicals in the degradation of the atrazine herbicide by sulfate radical-based advanced oxidation

The persistence of recalcitrant pollutants in water is a major health issue calling for advanced and green techniques to clean polluted waters. For instance, direct activation of peroxydisulfate by visible light without any catalyst displays a remarkable potential to degrade recalcitrant pollutants in water, and the involved reactive species in pure conditions with deionized water have been elucidated recently. However, the impact of phosphates, which are commonly abundant in real waters, on the performance of this reaction is still unclear, and possible formation of secondary phosphate radicals has yet to be clarified. Here we studied the effect of phosphate on the degradation of atrazine by peroxydisulfate under visible light. Radicals were studied by electron paramagnetic resonance. Results show that 10 mM of phosphate as H₂PO₄⁻/HPO₄²⁻ decreased the pseudo-first-order rate constant of atrazine degradation by 47.6% in the peroxydisulfate/visible light system at pH 7.0. The degradation kinetics of atrazine correlated linearly with the concentration of phosphate and was pH dependent. Optimal degradation efficiency was achieved at pH 7.0, while higher pH significantly impeded atrazine degradation. These results suggest that the inhibitory effect of phosphate is due to the formation of less reactive phosphate radicals. This hypothesis is supported by the effect of pH on atrazine degradation and the reduced peak intensities of reactive species in electron paramagnetic resonance. Our findings imply that the presence of phosphates could highly decrease the efficiency of pollutant removal in real situations.