稻田上覆水中光致活性组分形成过程及其环境效应研究进展

稻田上覆水中富含有机质、亚硝酸盐和硝酸盐等光敏活性物质，在太阳光作用下会产生三重激发态有机质(3CDOM*)、单线态氧(1O2)和羟基自由基(?OH)等活性组分，其对稻田污染物转化和碳氮等元素循环具有重要意义。基于此，综述了稻田上覆水中光致活性组分产生过程和机制，重点介绍了水稻不同生长周期内光活性组分的类型、通量变化趋势；探讨了不同环境因素对自由基产生的影响；阐述了上覆水光致活性组分对稻田中砷和不同有机污染物的非生物转化贡献与机制，并展望了稻田上覆水光化学过程的未来研究方向。

Research Progress of Photochemical Reactive Intermediates Processes and Abiotic Transformation of Pollutants in Paddy Water

Rice is the staple food for about half the world""s population and one of the most widely grown crops. Because long periods of flooding are needed during rice planting, paddy water is becoming an important sink of contaminants such as herbicides, pesticides and heavy metal(loid)s (e.g., arsenic). Compared with surface water, the concentrations of nitrite/nitrate and dissolved organic matter (DOM) in paddy water are usually higher, due to the extensive application of nitrogen fertilizers and release of DOM from microbial degradation of soil organic matter. Owing to long-term flooding during rice cultivation, desorption of soil humic substances, reductive dissolution of mineral–OM complexes, and root exudation release a large amount of DOM into paddy water, which can generate reactive intermediates (RIs) such as triple excited organic matter (3CDOM*), singlet oxygen (1O2) and hydroxyl radicals (?OH) under sunlight. These RIs usually exhibit high reactivity and play an important role in pollutant transformation and element cycling in paddy fields. In this paper, the generation process and mechanism of RIs in paddy water were reviewed and the type and concentration of RIs in different growth stages of rice were emphasized. Compared with the typical surface water, quantum yields of 3CDOM* and ?OH were comparable, while quantum yields of 1O2 were about 2.4-6.7 times higher than those of surface water. DOM in paddy water with lower molecular weight and humification extent generated more RIs, and nitrite contributed to 23.9%–100% of ?OH generation. DOM with more saturated and less aromatic formulas could produce more 3CDOM* under irradiation, while the polyphenolic components of DOM inhibited the formation of RIs. The effects of different environmental factors on the production of RIs were discussed. The application of straw and lime increased the RI concentrations by up to 16.8 and 11.1 times, respectively, while biochar addition had limited effects on RI generation from paddy water. Furthermore, the solar radiation directly affected the quantum yields of RIs in paddy water, and the radiation intensity showed a linear positive correlation with the concentration of free radicals. The mechanism of abiotic transformation of arsenic and different organic pollutants in paddy water was described and the degradation of contaminants, including As(III), 2,4-dichlorophenol (2,4-DCP), polycyclic aromatic hydrocarbons (PAHs), chlorotoluron, diuron, dimethomorph, and propanil, was significantly accelerated by photoinduced RIs generated in paddy water. Most of previous studies were conducted with stimulation experiments in the laboratory and the underlying mechanisms of RIs generation in paddy water have not been fully elucidated. Future studies should pay more attention to the generation of RIs in the paddy field and link global scale factors such as photoactive material flux, different soil properties and improvement measures, as well as the transformation of pollutants and material circulation to form a systematic understanding. In addition, future studies should not only focus on the degradation of these pollutants themselves, but also on the toxicity and ecological risks of the degradation products of these pollutants. More attention should be paid to the transformation of emerging pollutants, such as new pesticides, antibiotics and microplastics, in the paddy system.