城市污水和污泥处理过程中甲基汞的生成和降解机理

The global treaty to prevent mercury emission and release will be open for signature in October 2013. Environmental problems induced by methylmercury (MeHg), which is highly toxic and ready for bioaccumulation, will draw more and more attentions from all over the world. The preliminary study suggested that MeHg may widely occurr in the influent and effluent of sewage treatment plants, as well as in the sewage sludge. Environmental and health risks induced by sewage-related MeHg should be assessed cautiously. However, researches on the transport and transformation of MeHg in sewage treatment plants have seldom been carried out. In the proposed study, on-site sampling and laboratory analysis will be conducted. Mercury stable isotope tracer technique will be used in the microcosm sewage treatment and sludge composting experiments, with efforts to 1) illustrate the sources and fate of methylmercury in the sewage treatment plant through mass balance studies, 2) investigate the transport and transformation processes of MeHg in the sewage treatment and sludge composting processes, and 3) identify the controlling factors in these processes. The proposed study will improve our understanding of MeHg transport and transformation mechanisms in both natural and artificial environments. Countermeasures and advices will also be provided to reduce the environmental and health risks induced by sewage-related MeHg.

关于控制和消减汞排放的国际公约签署在即，具有高毒性并易于生物富集的甲基汞引起的环境问题将获得越来越广泛的关注。现有报道和申请人的前期研究均表明，甲基汞很可能广泛存在于城市生活污水厂的进出水和剩余污泥中，其环境和健康风险不容忽视。目前国际学界对甲基汞在生活污水处理厂迁移转化方面的研究甚为少见，国内尚未有相关研究报道。本研究拟以现场采样分析和实验室小试模拟实验相结合，通过物料衡算，明确城市生活污水处理厂各工艺环节甲基汞的来源、去向；在此基础上，通过汞稳定同位素示踪实验，揭示甲基汞在污水厂主要工艺单元中的迁移转化过程及其主要控制因素，进而优化工艺参数，减少甲基汞的生成或促进其降解；开展污泥堆肥的汞稳定同位素示踪实验，研究甲基汞在堆肥过程中的转化机理。项目的开展将加深对于甲基汞在自然和人工环境中迁移转化的认识和理解，为降低城市污水排放和污泥堆肥化利用带来的甲基汞诱发的环境和健康风险提供科学依据。

“水俣公约”的签署，大大推动了汞尤其是甲基汞环境行为与效应的科学研究。已有研究表明，甲基汞广泛存在于城市生活污水chuli厂的进出水和剩余污泥中，其排放和利用带来的环境和健康风险不容忽视。目前学界对该方面的研究甚为少见，对于汞在城市污水处理厂的迁移转化、归趋及其环境效应尚不清楚。本项目针对焦作市两个污水处理厂进行了为期一年的跟踪研究，系统采集了各处理单元进出物料样品，分析了其中汞的含量和形态。研究发现，原污水中THg含量在95.3 - 2.8 ×10^3 ng/L之间，MeHg则为1.5 - 21.3 ng/L，二者均以颗粒态为主要存在形式。二级和三级处理均能够有效去除污水中的总汞和甲基汞，污水厂出水中THg和MeHg分别为1.8 - 56.4 ng/L和0.2 – 1.1 ng/L，溶解态所占比例较污水厂进水有显著升高。在所有的污泥样品中均检测到了总汞和甲基汞，其含量分别为2.0 × 10^3 - 6.9 × 10^3 ng/g 和 3.8 - 10.4 ng/g，说明剩余污泥是一个重要的总汞和甲基汞的污染源。质量平衡计算表明，源自污水的总汞主要转移到了剩余污泥中，大部分甲基汞则在污水处理过程中发生了降解。实验室小试实验表明，污水处理过程中汞的挥发量很小，不构成汞迁移的主要途径。新鲜污泥和灭活污泥均能够降解甲基汞，说明污水处理过程中生物去甲基化和非生物去甲基化可能同时存在。模拟堆肥实验和堆肥工厂现场实验均观察到了堆肥过程中甲基汞含量先增高后降低的现象，并且堆肥产品中的甲基汞含量均高于堆肥原料，表明堆肥过程中发生了汞的甲基化和甲基汞的去甲基化过程。