胞外聚合物EPS对污水处理系统中活性污泥响应纳米银胁迫的作用机制

Silver nanoparticles (AgNPs) are used in biocidal products for disinfection and microbial inhibition on surfaces. With expanding use of nanosilver products, the release of nanosilver particles into sewage collection systems and wastewater treatment plants (WWTPs) becomes a growing concern. The main environmental exposure route of silver compounds in products will be through wastewater treatment plants. 95% of wastewater is treated with activated sludge process all over the world. Due to their antibacterial property, silver-compounds might have adverse effects on microorganisms involved in WWTPs. Series of research on the impact of nanosilver in wastewater treatment system have been carried out using an integrated approach combining microbial ecology, chemical and environmental engineering methodologies. The specific aims of this project are to determine the distribution and speciation of silver-compounds in the extracellular polymeric substances (EPS) of sludge in wastewater treatment system with nanosilver stress; assess the response of sludge microorganism to the silver-compounds incorporated into EPS of activated sludge; evaluate the effects of the shift of microbial community on the composition and amount of EPS; determine the Ag+-species complexing with EPS and their valence transformation, reduce the toxicity of AgNPs and Ag+ to the microorganisms in activated sludge; identify the key composition and amount of EPS, dominant microorganisms for the production of EPS and the abundance of inducible silver-resistance; optimize process parameters for the restoration of the sludge system under nanosilver stress. The results from this project can provide a theoretical basis for emergency measures aiding functional recovery of wastewater systems under metal-nanoparticles stress and for guideline values of nanoparticles set in freshwater system.

纳米银因优越的抗菌性能而被广泛使用，释放到环境中的纳米银大多进入市政污水处理系统，国内外95%以上的市政污水均采用活性污泥法处理，纳米银及释放的Ag+对污水处理系统中的活性污泥微生物造成明显胁迫，给水体环境带来严重风险。运用微生物生态、环境化学及环境工程等多学科交叉方法研究纳米银进入污水处理系统后，在活性污泥胞外聚合物(EPS)中的赋存形态与数量；EPS中的Ag化合物诱导活性污泥微生物群落结构和功能变动，微生物分泌EPS的数量和组成随之发生变化，与系统中的Ag络合或转化其形态，缓解纳米银及释放的Ag+对系统中微生物的胁迫；查明对Ag胁迫起缓解作用的EPS成分和数量，大量分泌EPS的优势微生物菌群，银抗性基因的丰度；优化污水处理系统的工艺参数；增强纳米银胁迫下污水处理系统的应激自我保护能力。研究结果为纳米银胁迫下污水处理系统功能恢复技术的建立、金属纳米在水体中的环境风险管控限值制定提供基础。

进入市政污水处理设施的纳米银AgNPs会影响活性污泥污水处理系统中微生物活性。以序批式反应器SBR模拟活性污泥污水处理系统，进水中添加10 mg/L AgNPs显著降低SBRs对碳、氮及磷的去除率，抑制效应主要来自于AgNPs本身而不是其释放的Ag+；添加1 mg/L AgNPs对系统去除COD有抑制，但去除率仍高于60 %，出水可满足城镇污水处理厂污染物排放标准(GB18918-2002)一级A标准。目前进入市政污水处理系统AgNPs浓度远低于1 mg/L，对系统的污染物去除几乎没有抑制效应。10 mg/L AgNPs胁迫下，活性污泥微生物硝化和反硝化作用被抑制；Illumina Miseq高通量测序结果表明，AgNPs影响了活性污泥硝化和反硝化细菌的相对丰度；活性污泥微生物分泌AHLs信号分子的数量和种类均有变化；向受10 mg/L AgNPs胁迫的活性污泥系统外源添加混合AHLs，5 d后，总氮平均去除率由69.41%提高至93.04%，并在添加AHLs后的5~10 d内维持较高的总氮去除率。外源添加AHLs可能是提高AgNPs胁迫下污水处理系统生物脱氮效率的有效调控措施。在污水生物处理过程中，性质活泼的AgNPs可发生化学形态转变，对污水处理微生物产生不同的抑制效应。AgNPs及多种形态的Ag如Ag+、Ag2SO4、Ag2CO3、Ag2O和AgCl对N.europaea和P. stutzeri活性均有抑制，氧化胁迫和膜损伤是AgNPs及不同赋存形态的Ag对2株菌株产生毒性的机制。进水中的Ag主要累积在活性污泥中，活性污泥对进水中的Ag存在吸附饱和现象，超出污泥吸附阈值后，反应器出水中总Ag含量持续升高；对活性污泥进行XRD及XPS分析，发现进入活性污泥的AgNPs可以银的氧化物、Ag2S和Ag0等形态存在。含有较高浓度Ag的活性污泥在处理处置过程应评估不同形态Ag的环境影响。硫化银被视为AgNPs在污水处理系统中的最终归趋形态。表面包被PVP或未包被的纳米Ag2S在UV254-过硫酸盐作用下均会溶解，抑制N.europaea和P.stutzeri氮转化能力，菌株体内ROS含量、抗氧化酶活性以及脱氮功能基因表达量均产生不同程度的变化。本项目详细研究了AgNPs在污水生物处理系统分布和赋存形态，对脱氮功能微生物的毒性机制，及AgNPs对系统的胁迫效应和调控策略