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氯代苯酚类化合物具有高毒性、难降解和生物累积性的特征,是一种常见的持久性有机污染物[1-3],且广泛存在于土壤、沉积物、地表和地下水中。为了降低对受纳水体的污染,氯酚废水的处理至关重要。目前对于氯酚类废水的处理方法主要有生物降解法[4-6]、物理吸附法[7-8]、高级氧化法[9-11]等。近年来,高级氧化技术以其氧化能力强、反应速度快、限制少和处理效率高等优点受到众多学者的青睐[12]。臭氧氧化法是目前最具代表性且最有效的高级氧化法之一。众所周知,臭氧氧化有机物主要的两种方式:一是臭氧分子与有机物的直接反应;二是臭氧分解产生的羟基自由基(OH·)与有机物的间接反应[13]。纳米二氧化锰作为催化剂能有效促进臭氧催化氧化过程中羟基自由基等活性物质的形成,使其对难降解有机污染物的去除更加迅速、彻底,是催化臭氧化中最常使用的催化剂之一[14-17]。
目前,很多学者已经对催化臭氧化的降解效率及降解动力学等[18-19]进行了研究,但鲜有研究其降解过程中的生物毒性变化及其关键的致毒因子。已有研究发现,由于臭氧的部分氧化特性,生成的大量降解中间产物的生物毒性可能大于母体污染物,造成母体污染物浓度降低而总体生物毒性升高的现象[20].
本研究以3-CP作为模型污染物,探究二氧化锰催化臭氧化中生物毒性的变化规律,并识别高毒性的中间产物。在此基础上,研究催化臭氧化体系处理各类有机化合物过程中自由氯浓度的变化,以揭示高毒性中间产物的产生机理。
催化臭氧化3-氯酚中生物毒性的变化
Study on the biological toxicity evolution during catalytic ozonation of 3-chlorophenol
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摘要: 氯酚类化合物具有高毒性、难生物降解的特性,催化臭氧化是处理氯酚类废水的重要方法,但催化臭氧化部分氧化的特性常会导致生物毒性升高。为了识别催化臭氧化3-氯酚(3-CP)过程中的关键致毒因子,研究了不同pH下二氧化锰(MnO2)催化臭氧化降解3-CP过程中总体生物毒性的变化规律,探究了急性生物毒性与常规污染指标3-CP浓度、总有机碳浓度(TOC)以及自由氯(FAC)浓度的相关性。结果表明,MnO2催化臭氧化3-CP时,生物毒性不随母体污染物和TOC浓度同比例降低而是出现了先升高后下降的现象,尤其是当pH=3时,生物毒性最高值约为初始值的26倍。通过生物毒性与常规指标的相关性分析发现,处理过程中生物毒性的大小和自由氯浓度在pH=3—10时显著相关。催化臭氧化溶液中加入自由氯去除剂亚硫酸钠后,生物毒性几乎全部去除,最高点生物毒性去除率达到100%。由此推测,自由氯是MnO2催化臭氧化3-CP中生成的高毒性中间产物,在生物毒性演变中起了关键作用。进一步研究发现,体系中的自由氯是由于臭氧分子氧化形成的,它的生成可能与共存的酚类有机物有关。Abstract: Chlorophenols are highly toxic and difficult to biodegrade. Catalytic ozonation is an critical method to degrade chlorophenols in wastewater, but the partial oxidation of catalytic ozonation often lead to the increased biological toxicity. In order to identify the key toxic factors in the degradation of 3-chlorophenol (3-CP) by catalytic ozonation of manganese dioxide (MnO2), the acute toxicity evolution at different pH and its relationship with conventional pollution parameters including 3-CP concentration, total organic carbon (TOC) concentration and free active chlorine (FAC) concentration were studied. It was found that the acute toxicity of ozonated effluents increased firstly and then decreased during ozonation. The highest toxic unit was about 26 times of the initial value at pH=3. The biological toxicity and the free chlorine concentration during catalytic ozonation of 3-CP were significantly correlated at pH=3—10. After adding the free chlorine quencher sodium sulfite to the catalytic ozonated solution, the biological toxicity was almost completely removed, and the highest removal rate of biological toxicity reached 100%. Therefore, it is speculated that free chlorine is a highly toxic intermediate produced in MnO2-catalyzed ozonation of 3-CP and plays a key role in the evolution of biological toxicity. Further study found that the free chlorine in the system was formed by the oxidation of ozone molecules, and its generation might be related to the coexisted phenolic organics.
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Key words:
- catalytic ozonation /
- chlorophenol degradation /
- biological toxicity /
- free chlorine.
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表 1 急性毒性与各指标的相关性
Table 1. Correlation between acute toxicity and each index
常规指标
Conventional index皮尔森相关系数Pearson correlation coefficient (r) pH=1 pH=3 pH=5 pH=7 pH=10 pH=12 3-CP浓度 0.957** −0.344 0.118 0.232 0.175 0.822** TOC 0.963** 0.267 0.503 0.542 0.600* 0.924** 自由氯 −0.282 0.764** 0.872** 0.759** 0.969** 0.760** 注:* P<0.05;** P<0.01。
Note: * means P<0.05; ** means P<0.01.表 2 催化臭氧化氯离子过程反应方程式及速率常数
Table 2. Reaction equation and rate constant of catalytic ozonation of chloride ion
编号Number 反应方程式Reaction equation 速率常数Rate constant (k) 参考文献Reference (1) ${\rm{OH\cdot + C}}{{\rm{l}}^{\rm{ - }}} \longrightarrow {\rm{ClO}}{{\rm{H}}^{\rm{ - }}}{\rm{\cdot}}$ k1=(4.2±0.2)×109 L·mol−1·s−1 [35] (2) ${\rm{ClO}}{{\rm{H}}^{\rm{ - }}}{\rm{\cdot + }}{{\rm{H}}^{\rm{ + }}} \longrightarrow {\rm{C}}{{\rm{l}}^{\rm{ - }}}{\rm{ + OH\cdot}}$ k2=(2.4±0.4)×1010 L·mol−1·s−1 [35] (3) $ {\rm{Cl\cdot + C}}{{\rm{l}}^{\rm{ - }}} \longrightarrow {\rm{Cl}}_{\rm{2}}^{\rm{ - }}{\rm{\cdot}}$ k3=(7.8±0.8)×109 L·mol−1·s−1 [35] (4) ${\rm{2Cl}}_{\rm{2}}^{\rm{ - }}{\rm{\cdot}} \longrightarrow {\rm{C}}{{\rm{l}}^{\rm{ - }}} + {\rm{C}}{{\rm{l}}_{\rm{2}}} \uparrow $ k4=(3.5±2.7)×109 L·mol−1·s−1 [35] (5) $ {{\rm{O}}_{\rm{3}}}{\rm{ + C}}{{\rm{l}}^{\rm{ - }}} \longrightarrow {\rm{Cl}}{{\rm{O}}^{\rm{ - }}}{\rm{ + }}{{\rm{O}}_{\rm{2}}}$ k5=2.48×10−3 L·mol−1·s−1 [32] (6) $ {\rm{Cl}}{{\rm{O}}^{\rm{ - }}}{\rm{ + }}{{\rm{H}}^{\rm{ + }}} \longrightarrow {\rm{HClO}}$ k6=5.0×1010 L·mol−1·s−1 [36] (7) ${\rm{HClO + }}{{\rm{H}}^{\rm{ + }}}{\rm{ + C}}{{\rm{l}}^{\rm{ - }}} \longrightarrow {\rm{C}}{{\rm{l}}_{\rm{2}}} \uparrow + {{\rm{H}}_{\rm{2}}}{\rm{O}}$ k7=2.14×104 L2·mol−2·s−1 [36] 注:反应(1)—(4)温度为297±2 K,反应(5)—(7)温度为298 K.
Note: The temperature of reactions (1)—(4) is 297 ± 2 K, and reactions (5)—(7) is 298 K. -
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