The photochemical properties of paddy water might be affected by the commonly used amendments in rice fields owing to the associated changes in water chemistry; however, this important aspect has rarely been explored. We examined the effects of agricultural amendments on the photochemistry of paddy water during rice growth. The amendments significantly influenced the photogenerated reactive intermediates (RIs) in paddy water, such as triplet dissolved organic matter (3DOM*), singlet oxygen, and hydroxyl radicals. Compared with control experiments without amendments, 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 in in situ experiments under sunlight. Fluorescence emission–excitation matrix spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry, and structural equation modeling revealed that upon the addition of straw and lime amendments, humified DOM substances contained lignins, proteins, and fulvic acids, which could produce more RIs under irradiation. Moreover, the amendments significantly accelerated the degradation rate of 2,4-dichlorophenol but led to the 3DOM*-mediated formation of more toxic and stable dimeric products. This study provides new insights into the effects of amendments on the photochemistry of paddy water and the pathways of abiotic degradation of organic contaminants in paddy fields.

The concomitant pollution of arsenic and antibiotics has commonly occurred in the environments recently, the behaviors of which may be different from single existence. Colloidal ferric hydroxide (CFH) is ubiquitous, existing in surface waters, seawaters, sediments, and paddy soils. In this study, the interaction between oxytetracycline (OTC) and CFH facilitated OTC degradation and reduced Fe(III) to Fe(II). In oxic conditions, the reactions between dissolved oxygen (DO) and Fe(II) generated reactive oxygen species (ROS). • OH and O 2•− were the main free radicals through electron paramagnetic resonance (EPR) measurement, and 8.1 ± 0.5 μM • OH was detected after 48 h of reaction. Quenching experiments indicated that • OH had dominated the oxidation of As(III). Furthermore, ferric arsenate formed after the oxidation of As(III) to As(V), which greatly decreased As(III) risks in the environment. The transformation of As(III) was also observed with other iron oxides or different antibiotics. The results of this study proved the facilitation of residual antibiotics in the redox process of iron oxides and transformation of co-existing pollutants, which extended our understanding on the roles of organic pollutants in combined contamination.

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