随着药物和护理产品（PPCPs）的广泛使用，多种抗生素在天然水体中不断被检出，而水体中过量的抗生素能够使生物产生严重的耐药性，因此，引起了人们的普遍关注。抗生素在水环境中的主要降解过程是光化学转化，为揭示天然水体中有机质对抗生素光化学转化的影响机制，本研究以6种抗生素为研究对象，提取了北京不同水体中的天然胶体，探究不同类型的天然胶体颗粒、低分子量有机酸（LMWOAs）及两者复合体系对典型抗生素光化学转化的影响机制。

（1）不同类型的天然胶体颗粒对典型抗生素光化学转化的影响机制

在模拟太阳光照射下（氙灯），纯水中“纯化学态”抗生素能发生直接光解，抗生素光化学转化符合一级反应动力学方程，得到四环素（TC）和土霉素（OTC）的光化学转化速率常数分别为0.39 h^-1和0.48 h^-1，半衰期分别为1.76 h和1.54 h，磺胺嘧啶（SDZ）与磺胺二甲基嘧啶（SMZ）的光化学转化速率常数分别为0.17 h^-1和0.08 h^-1，半衰期分别为4.15 h和8.35 h。不同类型的天然胶体对典型抗生素光化学转化的影响存在双重作用，既可以促进其光化学转化，又可以抑制其光化学转化。这主要与目标化合物的内部结构和天然胶体自身的理化性质有关，是胶体颗粒含量、有机碳含量、胶体表面有机物结构以及胶体颗粒的光化学性质等多种因素综合作用的结果。

天然胶体颗粒不仅影响TC与OTC的直接光化学转化，同时也会产生³DOM*影响抗生素的间接光化学转化。对于磺胺类抗生素，天然胶体颗粒中有机质产生的¹O₂和³DOM*，对SDZ与SMZ间接光化学转化产生了重要影响。在天然胶体的作用下，TC与OTC主要发生了四条降解途径：羟基化、去甲基化、脱氨基和脱水，SDZ与SMZ主要发生三种降解途径：脱-SO₂、S-N键断裂与羟基化。与纯水相比，天然胶体颗粒不仅影响典型抗生素光化学转化动力学，同时也显著影响了抗生素光化学转化路径。

（2）LMWOAs影响典型抗生素光化学转化的定量化模型

不同结构的LMWOAs由于取代基的不同而表现出不同程度的抑制作用，除丙酸（PA）和丁酸（BA）外，其他9种脂肪酸抑制TC光化学转化，但差异并不显著，而7种芳香酸抑制TC光化学转化的差异性明显。通过分析不同结构的LMWOAs（11种脂肪酸和7种芳香酸）对TC光化学转化动力学的影响，基于PLS分析方法基础上构建了LMWOAs结构与TC光化学转化速率常数的定量构效关系（QSAR）模型，通过分析和验证，该模型具有较好的拟合性和较强的稳健性，E_LUMO-E_HOMO、E_LUMO、CCR 和 Q_max 四个描述符对log k_obs 影响最大，log k_obs与E_LUMO-E_HOMO, E_LUMO, CCR 呈正相关关系，与Q_max呈负相关关系。该模型具有较好的准确性和可行性，为估算TC在不同结构LMWOAs中的光化学转化提供了一种稳健的预测方法。

（3）天然胶体颗粒与LMWOAs的复合体系对典型抗生素光化学转化的影响机制

不同结构的LMWOAs对典型抗生素光化学转化的影响差异性明显，抑制作用由小到大依次是苯甲酸、丙酸、草酸、柠檬酸和没食子酸。除没食子酸对喹诺酮类抗生素光化学转化的抑制作用明显外，其他LMWOAs的影响不明显，主要由于没食子酸对光吸收最大，直接光解被抑制的作用最明显，导致抗生素竞争光的能力弱化。淬灭实验进一步证明了活性氧化物对氧氟沙星（OFL）间接光化学转化的影响较小。苯甲酸对磺胺类抗生素光化学转化的促进作用较明显，主要是由于苯甲酸与胶体复合体系产生的¹O₂与³DOM*对SDZ与SMZ的光化学转化产生了非常重要的影响，而产生的·OH仅仅对SMZ的光化学转化起作用。

（4）环境因素对抗生素在复合体系中光化学转化的影响

        以抗生素光化学转化速率常数为参考指标，通过单因素实验研究了pH、Fe(III)与Cl^-的浓度三种因素对抗生素在复合体系中的光化学转化的影响，单因素实验确定了三种因素的浓度。基于响应曲面法，通过设计三因素三水平的多因素交互作用的Box-Behnken (BBD) 实验，建立抗生素在不同因素影响下的光解动力学模型。根据对各项F 值与P 值的分析得出单因素对抗生素光化学转化速率常数的影响显著性由大到小为pH、Fe(III)的浓度与Cl^-的浓度；通过对模型进行方差分析，得出该模型是合理可行的，通过分析各因素交互作用的相应曲面图和等高线图，得出pH值与Fe(III)的浓度的交互作用对抗生素光化学转化的影响最大。

As antibiotics extensively used to prevent or treat human and animal diseases, lots of antibiotics have been discharged directly and indirectly into natural water. The drug resistance phenomenons caused by the widespread use of antibiotics have been paid more attention. The main degradation process of antibiotics were the photochemical transformations. In order to reveal the mechanism of organic matter in natural water on the photochemical transformations of antibiotics, six antibiotics were selected as the research object, and extracting natural colloids from the different municipal waters in Beijing. The effects of natural colloids, low-molecular-weight organic acids (LMWOAs) and their composite system on the photochemical transformations of antibiotics were revealed.

(1) The mechanism of natural colloids from different waters on photolysis of typical antibiotics

Under simulated sunlight irradiation, "purified state" antibiotics could undergo direct photolysis. Photochemical transformations of antibiotics fited to the first-order reaction kinetics. The photochemical transformations rate constants of tetracycline (TC) and oxytetracycline (OTC) in pure water were 0.3922 and 0.475 h^-1, respectively. The rate constants of sulfadiazine (SDZ) and sulfamethazine (SMZ) were 0.167 and 0.083 h^-1, respectively. Different natural colloidal particles have dual effects on photochemical transformations of typical antibiotics. These were mainly related to the structures of the target compounds and the physical and chemical properties of the natural colloids. The comprehensive effects of the colloidal content, the colloidal organic carbon content, the organic matter content and the colloidal surface structure and the surface morphology of the natural colloidal particles played a certain role in photochemical transformations of antibiotics.

In the presence of natural colloidal particles, tetracycline antibiotics could undergo not only directly photolysis, but also undergo indirect photochemical transformation involving ³DOM*. The four main reaction pathways of TC and OTC in the presence of natural colloids were hydroxylation, demethylation, deamination and dehydration. For sulfanilamide antibiotics, the production of ¹O₂ and ³DOM* by the organic components in the colloidal particles have a very important influence on the photochemical transformations of SDZ and SMZ. The three main pathways of SDZ and SMZ were the loss -SO₂, S-N bond breakage and hydroxylation. Compared with pure water, natural colloid particles not only affected the photochemical transformation kinetics of typical antibiotics, but also significantly affected the pathways of the photochemical transformation of antibiotics.

(2) Quantitative model of the photochemical transformation of typical antibiotics by LMWOAs

The different structures of LMWOAs have different inhibition effects due to different substituents. Except for PA and BA, nine aliphatic acids inhibited TC photochemical transformation, but the difference was not significant. While seven aromatic acids were significantly inhibited TC photochemical transformation. The quantitative structure-activity relationship (QSAR) model of LMWOAs structure and TC photolysis rate constant was established by PLS analysis method, which was based on the effects of eleven aliphatic acids and seven aromatic acids on TC photochemical transformation kinetics. This QSAR model exhibited satisfactory goodness-of-fit, robustness and good accuracy. The constructed QSAR model revealed that the structure of LMWOAs with high E_LUMO-E_HOMO, E_LUMO, CCR and low Q_max have great influence on TC degradation. The model was accurate and feasible, which provides a robust prediction method for the photochemical transformation of TC in the presence of LMWOAs.

(3) The combination influences of natural colloidal particles and LMWOAs on the photochemical transformation of typical antibiotics

The inhibition effects of LMWOAs on photochemical transformation of typical antibiotics were obvious, and the order of inhibition effects were benzoic acid, propionic acid, oxalic acid, citric acid and gallic acid. Except for the largest inhibition of gallic acid, other LMWOAs have no obvious effects. This mainly because gallic acid has the largest absorption of light, which lead to the decrease of competitive light ability of antibiotics, and then affect the direct photolysis. The quenching experiment further proved that the effects of active oxygen species on indirect photochemical transformation of ofloxacin (OFL) were neglected. Benzoic acid has a significant effect on the photochemical transformations of sulfonamide antibiotics, which mainly due to ¹O₂ and ³DOM* produced by the combination system of benzoic acid and natural colloids in the photochemical transformations of SDZ and SMZ.

(4) The effects of environmental factors on photochemical transformations of antibiotics in the complex system of LMWOAs and natural colloids

The effects of pH, Fe (III) and Cl^- concentration on the photochemical transformation of antibiotics were studied by using single factor experiment and response-surface methodology (RSM). The concentration of three factors were determined by single factor experiment. The photodegradation model of antibiotics under different factors was obtained by using the Box-Behnken (BBD) experiment to reveal the influence of multi factor interaction. BBD experiment design with three factors and three levels was adopted. According to the F value and P value, three factors have a significant influence on the photochemical transformation rate constants of antibiotics, and the order of significant influence were pH, Fe (III) and Cl^- concentration. The variance analysis of the model suggested that the model was suitable, reasonable and feasible for this study. The results showed that the interaction between pH and Fe (III) have the greatest influence on the photochemical transformation of antibiotics.