微塑料（MPs）和铬（Cr）是污水处理厂常见的污染物，在污水处理厂的紫外线消毒过程中，MPs可能会被降解，Cr(III)也可能会被光氧化成更危险的Cr(VI)。然而，MPs与Cr的相互作用机制尚不清楚。MPs在光氧化降解过程中释放了溶解性有机质（MPs-DOM），而MPs-DOM如何影响Cr光氧化还原以及自身的微生物矿化仍不清楚。本文选取了四种MPs，包括聚酰胺（PA）、聚丙烯（PP）、聚氯乙烯（PVC）和聚苯乙烯（PS），研究了MPs以及MPs-DOM对共存Cr光氧化还原的影响和MPs-DOM的微生物矿化。本研究的主要结论如下：

首先，对紫外光老化前后的MPs进行了扫描电子显微镜（SEM）、傅里叶变换红外光谱（FTIR）、X射线光电子能谱（XPS）和Zeta电势等表征，表征结果显示MPs在紫外辐照下发生了非常明显的老化降解行为。在老化过程中，MPs的颜色发生了变化，表面出现了裂纹，同时FTIR、XPS显示MPs表面产生了羧基等含氧官能团。Zeta电位的测定结果表明MPs在老化前后，其表面均携带负电荷。四种MPs中，PA的分散性更好，而PP的分散性最差。

在紫外辐照的前40 h内，所有MPs释放的DOM浓度均随辐照时间而升高，DOM的释放速率的相对大小为PA >> PS > PP > PVC。当辐照时间超过40 h时，PA、PS和PVC的DOM浓度呈下降趋，只有PP的DOM浓度进一步升高。辐照PS和PA产生的DOM的SUVA₂₅₄值随辐照时间的延长呈现先增大后减小的趋势，说明随着辐照时间延长，DOM溶液中芳香族光产物开始生成，然后芳香族组分会被进一步分解导致其含量降低。辐照PVC和PP产生的DOM的SUVA₂₅₄值变化不大，这归因于在整个过程中产生了极少量的芳香族光产物。EEM测定显示PS-DOM的荧光组分与蛋白质/酚类物质相关，而PA-DOM的荧光组分与海洋腐殖质类化合物和大分子疏水化合物相关。辐照PS和PA释放的DOM的特征组分的荧光强度均呈现先增大后减小的趋势，其总体趋势与SUVA₂₅₄值的变化趋势一致。

MPs与MPs-DOM的存在会影响Cr的转化，在不添加MPs时，Cr(III)在紫外辐照下会被氧化为Cr(VI)，而当添加了MPs时，Cr(III)在紫外辐照下会先被氧化为Cr(VI)，然后再被还原为Cr(III)。接下来研究了Cr(III)初始浓度和pH对光反应的影响，结果显示初始Cr(III)浓度越高，整个光氧化过程持续时间越长，产生的Cr(VI)浓度降低；当pH降低时，光氧化过程持续时间缩短，且产生的Cr(VI)浓度也降低，说明强酸性条件有利于Cr(VI)的光还原。不同pH条件下对Cr(VI)的有效还原也证实了MPs在酸性pH条件下光还原Cr(VI)的实际应用。光辐照也影响了Cr在MPs上的吸附行为。光辐照老化后的MPs对Cr(III)的吸附发生了明显的改变，可以推测Cr(III)的吸附增加可能影响MPs与Cr(VI)的光氧化还原反应。为了探究MPs和MPs-DOM与Cr的光反应作用机理，进行了电子顺磁共振波谱（ESR）测定，证明了活性氧（ROS）的存在。在不同的照射时刻，MPs和MPs-DOM都产生了类似的ROS，这将影响MPs和MPs-DOM的光降解，以及Cr的光氧化还原。淬灭实验显示?OH的淬灭对Cr(III)的光氧化有较强的抑制作用，但加速了Cr(VI)的光还原。这表明?OH对Cr(III)的光氧化有促进作用，但对Cr(VI)的光还原有较弱的抑制作用。本研究还发现MPs和MPs-DOM都具备产生的ROS的能力，且产量相当，因此不能忽视MPs-DOM在Cr(III)和Cr(VI)之间的光转化中的作用。

MPs-DOM的微生物矿化随MPs类型和接种源的不同而不同。矿化56 d后，MPs-DOM的矿化率依次为PS ≥ PA > PP ≥ PVC > 苏万尼河腐殖酸（SRHA）。双指数模型表明MPs-DOM中不稳定DOM的比例高于SRHA。上述结果表明了MPs-DOM对碳排放的重要贡献。与原始接种相比，不同的来源MPs-DOM对细菌和真菌的丰度和多样性有很大的改变。对于河流接种物的样品，MPs-DOM降低了细菌 16S 基因和真菌 ITS 基因的 ACE、Chao 1、Shannon 和 Simpson 指数。对于带有污泥接种物的样品，除了PA-DOM中细菌16S基因的上述指数和对真菌ITS基因的Shannon和Simpson数外，MPs-DOM也降低了其他相关指数。因此，MPs-DOM降低了微生物的丰度和多样性，其中PVC-DOM对微生物群落的影响最强。

本研究提出了一种光辐照Cr(VI)和MPs修复的双赢解决方案，并强调了MPs-DOM对共存污染物归趋、碳循环和微生物演替的潜在重要作用。

Microplastics (MPs) and chromium (Cr) are common pollutants in WWTPs. During UV disinfection in WWTPs, MPs may be degraded, and Cr(III) may also be photooxidized to the more dangerous Cr(VI). However, the interaction mechanism between MPs and Cr is still unclear. MPs release dissolved organic matter (MPs-DOM) during photooxidative degradation, and how MPs-DOM affects Cr photoredox and its own microbial mineralization remains unclear. In this paper, four kinds of MPs, including polyamide (PA), polypropylene (PP), polyvinyl chloride (PVC) and polystyrene (PS), were selected to study the effects of MPs and MPs-DOM on coexisting Cr photoredox and Microbial mineralization of MPs-DOM. The main conclusions of this study are as follows:

Firstly, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Zeta potential were used to characterize MPs before and after UV irradiation. The characterization results showed that MPs had very obvious aging degradation behavior under UV irradiation. During the aging process, the color of MPs changed, and cracks appeared on the surface. Meanwhile, FTIR and XPS showed that oxygen-containing functional groups such as carboxyl groups were generated on the surface of MPs. The measurement results of Zeta potential showed that the surface of MPs carried negative charges before and after aging. Among the four MPs, PA has better dispersibility, while PP has the worst dispersibility.

During the first 40 h of UV irradiation, the DOM concentrations released by all MPs increased with the irradiation time, and the relative magnitude of the release rates of DOM was PA >> PS > PP > PVC. When the irradiation time exceeded 40 h, the DOM concentrations of PA, PS and PVC showed a downward trend, and only the DOM concentration of PP further increased. The SUVA₂₅₄ value of DOM generated by PS and PA firstly increased and then decreased with the extension of irradiation time, indicating that aromatic light products in DOM solution began to be generated with the irradiation time, and then the aromatic groups were further decomposed, leading to the reduction of their content. The SUVA₂₅₄ value of DOM produced by irradiating PVC and PP has little change, which is attributed to the very small amount of aromatic photoproducts throughout the process. EEM analysis showed that the fluorescence component of PS-DOM was correlated with protein/phenolic substances, while the fluorescence component of PA-DOM was correlated with Marine humus compounds and macromolecular hydrophobic compounds. The fluorescence intensity of the characteristic components of DOM released by PS and PA irradiation first increased and then decreased, and the overall trend was consistent with the change trend of SUVA₂₅₄ value.

The existence of MPs and MPs-DOM will affect the transformation of Cr. Without MPs, Cr(III) will be oxidized to Cr(VI) under UV irradiation, while when MPs is added, Cr(III) will be oxidized to Cr(VI) and then reduced to Cr(III) under UV irradiation. The results show that the higher the initial concentration of Cr(III), the longer the duration of the whole photooxidation process, and the lower the concentration of Cr(VI). With the decrease of pH, the duration of photooxidation process is shortened, and the concentration of Cr(VI) is also reduced, indicating that strong acidic conditions are conducive to the photoreduction of Cr(VI). The effective reduction of Cr(VI) under different pH conditions also confirmed the practical application of MPs in the photoreduction of Cr(VI) under acidic pH conditions. The adsorption behavior of Cr on MPs was also affected by light irradiation. The adsorption of Cr(III) on MPs after irradiation aging has obviously changed, so it can be speculated that the increase of Cr(III) adsorption may affect the photoredox reaction between MPs and Cr(VI). In order to explore the mechanism of the photoreaction of MPs and MPs-DOM with Cr, electron paramagnetic resonance spectroscopy (ESR) was used to determine the existence of reactive oxygen species (ROS). At different irradiation times, both MPs and MPs-DOM produce similar ROS, which will affect the photodegradation of MPs and MPs-DOM and the photoredox of Cr. The quenching experiments showed that the quenching of ?OH had a strong inhibition effect on the photooxidation of Cr(III), but accelerated the photoreduction of Cr(VI). These results indicate that ?OH can promote the photooxidation of Cr(III), but has a weak inhibition effect on Cr(VI). This study also found that both MPs and MPs-DOM have the ability to generate ROS, and the ability is comparable. Therefore, the role of MPs-DOM in the photoconversion between Cr(III) and Cr(VI) cannot be ignored.

The microbial mineralization of MPs-DOM varied with MPs types and inoculum sources. After 56 days of mineralization, the mineralization rate of MPs-DOM was in the order of PS ≥ PA > PP ≥ PVC > Suwannee River humic acid (SRHA). The double exponential model indicated that the proportion of unstable DOM in MPs-DOM was higher than that in SRHA. The above results demonstrate the important contribution of MPs-DOM to carbon emissions. The abundance and diversity of bacteria and fungi were greatly altered by different sources of MPs-DOM compared to the original inoculum. For samples from river inoculum, MPs-DOM reduced the ACE, Chao 1, Shannon and Simpson indices of bacterial 16S genes and fungal ITS genes. In addition to the above indices for bacterial 16S genes in PA-DOM and Shannon and Simpson numbers for fungal ITS genes, MPs-DOM also reduced other relevant indices for the samples with sludge inoculum. Therefore, MPs-DOM reduced the abundance and diversity of microorganisms, with PVC-DOM having the strongest effect on the microbial community.

This study proposes a win-win solution for photo-irradiated Cr(VI) and MPs remediation, and highlights the potentially important role of MPs-DOM on coexisting pollutant fate, carbon cycling, and microbial succession.