Photochemical characterization of paddy water during rice cultivation: Formation of reactive intermediates for As(III) oxidation

Although the photochemical behavior of surface water and its effects on pollutant transformation have been studied extensively in recent years, the photochemistry of paddy water remains largely unknown. In this study, we examined the photochemical processes involving paddy water samples collected at four different cultivation stages of rice. Triplet dissolved organic matter ((DOM)-D-3*), singlet oxygen (O-1(2)), and hydroxyl radicals ((OH)-O-center dot) were found to be the dominant reactive intermediates (RIs), and their apparent quantum yields and steady-state concentrations were quantified. Compared with the typical surface water, quantum yields of (DOM)-D-3* and (OH)-O-center dot were comparable, while quantum yields of O-1(2) were about 2.4-6.7 times higher than those of surface water. Fluorescence emission-excitation matrix (EEM) spectra, Fourier transform ion cyclotron resonance mass spec-trometry (FTICR-MS), and statistical analysis revealed that DOM properties and nitrite concentration were the main factor influencing RIs generation. The results suggest that DOM with lower molecular weight and humi-fication extent generated more RIs, and nitrite contributed to 23.9%-100% of (OH)-O-center dot generation. EEM and FTICR-MS data showed that DOM with more saturated and less aromatic formulas could produce more 3DOM* under the irradiation, while the polyphenolic components of DOM inhibited the formation of RIs. Moreover, RIs significantly enhanced arsenite (As(III)) oxidation with oxidation rate increased by 1.8-4.1 times in paddy water, and (OH)-O-center dot and (DOM)-D-3* were the main RIs responsible for As(III) oxidation. This study provides new insight into the pathways of arsenite abiotic transformation in paddy soil and water.

Automated summary

This study investigated the photochemical processes in paddy water, identifying DOM-D-3*, O-1(2), and OH· as the main reactive intermediates with quantified quantum yields and steady-state concentrations. The findings demonstrated that DOM properties and nitrite concentration were key factors influencing RIs generation, and RIs significantly enhanced arsenite oxidation in paddy water. The study provides new insights into arsenite abiotic transformation pathways in paddy soil and water.