The Research Progress of Environmental Distribution, Behavior and Effects of Cyclic Methylsiloxanes
近年来,挥发性环甲基硅氧烷(cVMS)在生产和生活过程中的广泛使用导致其环境和人体暴露风险日益增加,由于其具有持久性、潜在的生物积累性和毒性而被受关注。目前,人们对cVMS在全球各种环境介质中的赋存、行为及效应有一定的了解。排入环境中的cVMS大部分进入大气,在水体、沉积物、土壤和生物体中也有一定的含量。研究表明,希腊室内空气降尘中总的环硅氧烷含量中位数最高(1380 ng?g￣1),其次为中国(362 ng?g￣1)；中国污水处理厂总的硅氧烷年人均通量(10 g?y￣1)低于英国(D4~D648.3 g?y￣1)和美国(D4~D693.5 g?y￣1),其中大连市一家采用CWSBR工艺的污水处理厂进水中cVMS的总浓度(1.05μg?L￣1)普遍低于希腊(5.14μg?L￣1)、西班牙(9.2μg?L￣1)、加拿大(44μg?L￣1)和一些北欧国家(17μg?L￣1)；我国大部分废水处理厂污泥中甲基硅氧烷的含量(0.1~lμg?g￣1 dw)比一些北欧国家(26μg?g￣1 dw)、希腊(20μg?g￣1 dw)和加拿大(64μg?g￣1 dw)等要低得多。中国普通居民吸入+摄食D4~D6的PELs中位数(173 ng?d￣1)远低于中国普通人群的皮肤暴露(中位数18.5μg?d￣1),更低于英国成人日暴露量(1.875 mg?d￣1)和美国妇女对总硅氧烷的日暴露量(307 mg?d￣1)。环境中cVMS的行为和效应取决于其理化性质和具体的环境条件。进入大气的cVMS会与?NO3、O3和?OH反应,而与?OH反应脱去甲基生成硅醇是其主要的消除机制。污水处理过程中,大部分cVMS被污泥吸附固定,D6吸附污泥的能力最强,其次为D5和D4。挥发、吸附和非生物降解是cVMS在土壤中主要的环境行为。 D4和D5可能存在生物放大作用。评估cVMS的TMF(trophic magnification factor)研究结果相互矛盾,且与BCF、BMF和BSAF的评估结果相反。总之,国内外对污水处理过程中cVMS的赋存状态和迁移、转化行为的研究比较多,且以进、出水和剩余污泥为主,而对整个工艺流程中具体变化的细化研究很少,对其生物积累特征、降解机制和降解产物更缺乏深入研究。因此,今后需要补充对其他环境介质、尤其是和人们居住、工作密切相关环境中cVMS分布规律的研究,深入探索其在实际环境中的降解过程,包括其降解产物或中间产物的环境行为,进一步评估其生态环境效应和人类健康风险。
In recent years, the cyclic volatile methyl siloxanes (cVMS) are widely used in the process of production and people’ s daily life because of their excellent performance, meanwhile leading to increasing environmental and human exposure risks and receiving dramatic concerns because of their persistence, potential biological accumula￣tion and toxicity. At present, researchers have a certain understanding about the occurrence, behavior and effects of cVMS in various environmental media all over the world. Most cVMS were discharged into the atmosphere, and some cVMS were found in water, sediment, soil and organism. Many studies have shown that the highest median of the total cyclic siloxane of indoor air dust was from Greek (1 380 ng?g￣1 ), and followed by China (362 ng?g￣1 ). The total siloxane flux per capita for sewage treatment plant in China (10 g?y￣1 ) was much lower than that of UK (48.3 g?y￣1 for D4￣D6) and the United States (93.5 g?y￣1for D4￣D6). The total cVMS concentration (1.05μg?L￣1) from a sewage treatment plant with CWSBR in Dalian was generally lower than that of Greece (5.14 μg?L￣1 ), Spain (9.2 μg?L￣1), Canada (44μg?L￣1) and some of the Nordic countries (17μg?L￣1). In the aspect of the methyl siloxane of sewage sludge from wastewater treatment plant, China (0.1￣l μg?g￣1 dw) was also much lower than some of the Nordic countries (26μg?g￣1 dw), Greece (20μg?g￣1 dw) and Canada (64μg?g￣1 dw). The PEL (pseu￣do￣exposure level) median (173 ng?d￣1 ) of D4￣D6 by inhaling and feeding for ordinary residents was much lower than the average median (18.5 μg?d￣1) by skin exposure in China, and much lower than the exposure level (1.875 mg?d￣1 ) for adult in the UK and for women (307 mg?d￣1 ) exposed to the total siloxane in the United States. Gener￣ally, the behavior and the effect of cVMS discharged into environment depend on the physical and chemical prop￣erties and the specific environmental condition. The cVMS into the air will react with?NO3, O3,?OH, and demeth￣ylating reaction with?OH to form silanol is its main end mechanism. Most of the cVMS was fixed by the sewage sludge during the sewage treatment process, and their adsorption capacity for D6 was the strongest, followed by D4 and D5. Volatilization, adsorption and biodegradation were the major environmental behaviors for cVMS in the soil. The biological amplification effect for D4 and D5 was possible. The TMF (trophic magnification factor) results of cVMS were inconsistent with each other, and were even contrary to those by BCF ( bioconcentration factor ), BMF (biomagnification factor) and BSAF (biota￣sediment accumulation factor). Conclusively, most of the research of cVMS was on its occurrence, migration, transformation, and behavior in the process of wastewater treatment at home and abroad;furthermore, the research priority was given to the influent, effluent and remaining sludge, while the specific change in the whole process, the characteristics of its biological accumulation, degradation mechanism and degradation products were scarcely studied. Therefore, it needs effort to complement the cVMS distribution in other kinds of environmental media in the future, particularly in living and working environment. Besides, explo￣ring degradation process in real environment, including the environmental behavior of degradation products and in￣termediate products, and further evaluating eco￣environment effect and human health risks are the guidelines of cVMS research in the future.