|Alternative Title||Mechanism for the enhanced degradation of organic micropollutant by permanganate under the combined pollution condition in water|
|Place of Conferral||北京|
|Keyword||复合污染,高锰酸钾,有机污染物,强化降解,机制 Combined Pollution, Permanganate, Organic Micropollutant, Enhanced Degradation, Mechanism|
饮用水源水中，抗生素、内分泌干扰物和农药等微量有机污染物（organic micropollutants, MPs）以ng L-1–μg L-1的浓度被频繁检出，可能对人体及其它生物产生耐药性、内分泌干扰效应、急性毒性、致突变性或遗传毒性等毒性效应。在当前复合水质污染条件下，水中的共存物质可能严重影响水处理过程中MPs的去除。本文从二级氧化剂生成种类及机理出发，研究了复合水质污染条件下常见MPs在高锰酸钾（KMnO4）氧化过程中的强化降解效果及机制，具体结论如下：
作为饮用水源水中普遍存在的一种天然有机物（NOM），腐植酸（humic acid, HA）与C6H8O6的结构具有共同点，即都含有丰富的羟基。我们发现，HA也同样强化了KMnO4对抗生素左氧氟沙星（levofloxacin, LF）的降解。HA不存在时，pH 7.5下LF被KMnO4降解的二级反应速率常数（k）为3.9 M?1 s?1，并随pH的降低而升高。HA存在时，pH 7.5下LF被KMnO4降解的拟一级速率常数（kobs）在[HA]o:[KMnO4]o = 0.5和1（HATOC和KMnO4的质量浓度比）时分别提高了3.8和2.8倍，酸性条件增强了HA对LF的加速降解作用。二级氧化剂捕获和电子顺磁共振波谱实验等表明，KMnO4与HA反应生成了具有强氧化性的中间产物Mn(III)，其与HA络合[HA-Mn(III)]后进一步产生了O2??和?OH。?OH对LF的加速降解起主要贡献，而[HA-Mn(III)]对其余的加速作用起贡献。KMnO4氧化过程中，LF及其副产物主要通过羟基化、脱氢和羧基化被降解，而HA存在时LF的被破坏程度加剧。上述结果揭示了水处理过程中HA对KMnO4氧化降解MPs的加速机制。
苯醌结构是HA的重要组成部分。我们进一步发现，对苯醌（benzoquinone, BQ）的存在显著强化了KMnO4对LF的降解。pH 7.5时，LF被KMnO4降解的kobs值（0.010 min?1）在[BQ]o:[KMnO4]o = 0.03–0.25（摩尔比）时被提高至0.042–0.443 min?1，酸性条件促进了BQ对LF的加速降解作用。BQ存在时，KMnO4首先被还原至Mn(II)；随后，Mn(II)与BQ反应生成Mn(III)和半醌自由基，酸性条件能增强这两种活性物质的生成。DO存在下，Mn(III)能继续氧化半醌自由基生成单线态氧（1O2）和O2??，并再生BQ。另外，KMnO4还可与Mn(II)反应生成Mn(III)，Mn(III)和半醌自由基的络合反过来促进该反应的发生。因BQ对O2??的淬灭作用，1O2和Mn(III)对LF的加速降解起主要贡献，导致羟基、酮基和内过氧化物结构在降解副产物上显著生成。上述结果证实了水处理过程中NOM的苯醌基团对强化KMnO4氧化降解MPs起重要作用。
In drinking water sources, organic micropollutants (MPs) such as antibiotics, endocrine disrupting chemicals and pesticides are frequently detected at levels of ng L-1–μg L-1, which may induce antibiotic resistance, endocrine disrupting effects, acute toxicity, mutagenicity, genotoxicity or other toxic effects to human and other organisms. Under combined water pollution conditions at current, some coexisting substances in water may considerably affect the removal of MPs during water treatment processes. This study, from the aspects of the formation species and mechanism of secondary oxidants, investigated the enhanced degradation efficiency and mechanism of selected MPs by aqueous permanganate (KMnO4) under combined water pollution conditions. The major conclusions are drawn as follows:
Because of its strong reducing ability, thiosulfate (Na2S2O3) or sulfite (Na2SO3) is often used as an oxidant quencher in the kinetic study of oxidative degradation of various MPs in water. However, they may reduce the oxidation intermediates back to parent compounds, thus underestimating the degradation rate of MPs. Consequently, ascorbic acid (C6H8O6) has been increasingly used as an alternative quencher due to its relatively weaker reducing ability. In this study, we found that C6H8O6 as a quencher enhanced the degradation of phenol and diclofenac by KMnO4. Ascorbyl radical (C6H7O6?), Mn(III), superoxide radical (O2??) and hydroxyl radical (?OH) were produced in the reaction of KMnO4 with C6H8O6, among which ?OH and Mn(III) primarily contributed to enhanced degradation of MPs (about 52–100%). The disproportionation of O2?? in water produced ?OH. Dissolved oxygen (DO) was reduced to O2?? as C6H7O6? was degraded to C6H6O6, which was promoted under alkaline conditions. Moreover, Mn(II) was oxidized to Mn(III) as C6H7O6? was transformed to C6H8O6, which was promoted under acidic conditions. After KMnO4 was quenched by C6H8O6, the intermediates produced from the reaction of Mn(II) with C6H8O6 could continue to degrade MPs. These results reveal that using C6H8O6 as a quencher for residual KMnO4 may overestimate the oxidative degradation rate of MPs.
As a ubiquitous natural organic matter (NOM) in drinking water sources, humic acid (HA) has a structure similar to that of C6H8O6, that is, both contain abundant hydroxyl (–OH) groups. In this study, we found that HA could also enhance the degradation of levofloxacin (LF) by KMnO4. In the absence of HA, the second-order rate constant (k) of LF degradation by KMnO4 was determined to be 3.9 M?1 s?1 at pH 7.5, which increased with decreasing pH. In the presence of HA, the pseudo-first-order rate constant (kobs) of LF degradation by KMnO4 at pH 7.5 was significantly increased by 3.8- and 2.8-fold at [HATOC]o:[KMnO4]o (mass ratio) = 0.5 and 1, respectively. An acidic pH facilitated the HA-accelerated LF degradation. Secondary oxidant scavenging and electron paramagnetic resonance tests indicated that the reaction of KMnO4 with HA could produce a strongly oxidative intermediate, Mn(III), whose subsequent complexation with HA [i.e., HA-Mn(III)] further induced the formation of O2?? and ?OH. The resulting ?OH primarily contributed to the accelerated LF degradation, and the [HA-Mn(III)] complex accounted for the rest of acceleration. During KMnO4 oxidation, LF and its byproducts were mainly degraded through hydroxylation, dehydrogenation and carboxylation, whereas the presence of HA led to a stronger destruction of LF. These results clarify the mechanism for HA-accelerated degradation of MPs by KMnO4 in water treatment.
Quinone group is an important fraction of HA. We further found that the kobs of LF degradation by KMnO4 significantly increased from 0.010 (without benzoquinone, BQ) to 0.042–0.443 min?1 at [BQ]o:[KMnO4]o (molar ratio) = 0.03–0.25 at pH 7.5, and an acidic pH facilitated the BQ-accelerated degradation of LF. In the presence of BQ, MnO4? was first reduced to Mn(II); and then Mn(II) reacted with BQ to produce Mn(III) and semiquinone radical, which was promoted under acidic conditions. With DO available, Mn(III) could further oxidize semiquinone radical to produce singlet oxygen (1O2) and O2?? as well as regenerate BQ. In addition, KMnO4 could also react with Mn(II) to produce Mn(III), whose complexation with semiquinone radical in turn promoted this reaction. Due to the predominant scavenging of O2?? by BQ, 1O2 and Mn(III) mainly contributed to the accelerated degradation of LF, with a notable formation of hydroxyl, ketone and endoperoxide groups in the degradation byproducts. These results demonstrate that the quinone group of NOM plays an important role in the accelerated degradation of MPs by KMnO4 in water treatment.
|许可. 复合水质污染条件下高锰酸钾对微量有机污染物的强化降解机制研究[D]. 北京. 中国科学院生态环境研究中心,2018.|
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