纳米零价铁降解PCBs污染土壤过程中的氧化作用及机制

Polychlorinated biphenyls (PCBs) are one group of persistent organic pollutants (POPs) that are of international concern. In the past, the degradation of PCBs using nanoscale zero valent iron (NZVI) mainly focused on its reductive dechlorination effect. However, the field environment is difficult to be controlled under anaerobic conditions completely. The corrosion of NZVI by oxygen (O2) can produce strong oxidants capable of transforming recalcitrant contaminants, providing a new approach for the oxidative treatment of contaminated soil. The mechanism through which the NZVI/O2 system produces oxidants involves the two-electron oxidation of NZVI followed by the Fenton reactions. Therefore, the main objective of this study will focus on the oxidation in the process of the degadation of PCBs contaminated soils extracted solutions by solvent in the NZVI/O2 system, and elucidate the role of reactive oxygen species (ROS) generated by NZVI in this process.The related mechanism of oOH generated by NZVI will be studied by investigating the formation of H2O2, Fe(II) and O2o? in this process. The degradation products of PCBs are determined by gas chromatography-mass spectrometry (GC-MS) with different reaction conditions. Electron Paramagnetic Resonance (EPR) technique is used to identify the radical species generated in this process. The effects of environmental factors on the oxidation of PCBs and on the generation of radical species will also investigated in details. These findings will provide a new insight into the understanding of reactivity of NZVI for the transformation of PCBs and possibly other relevant environmental contaminants.

多氯联苯（PCBs）是目前备受关注的持久性有机污染物之一，研究已发现纳米零价铁对PCBs有很好的降解作用，但过去研究主要着重于其还原脱氯过程，而实际环境中由于很难存在绝对的厌氧条件导致纳米零价铁在氧气存在时会传递电子生成超氧自由基，从而诱发类Fenton反应，改变反应路径。本项目针对纳米零价铁修复PCBs污染土壤过程中的氧化作用开展深入研究，拟考察氧气存在下纳米零价铁降解PCBs污染土壤洗脱液中PCBs的氧化作用及其机制，其中，利用电子顺磁共振技术（EPR）检测降解过程中的自由基种类，通过气相色谱-质谱联用仪（GC-MS）鉴定降解产物，并考察自由基对纳米零价铁修复PCBs过程中的氧化贡献比例；同时，模拟实际环境考察不同环境因子对该氧化过程的影响。以期为纳米零价铁修复PCBs污染土壤的应用研究领域提供新的理论和实践。

研究已发现纳米零价铁（NZVI）对多氯联苯（PCBs）有很好的还原脱氯作用，而实际环境由于很难存在绝对的厌氧条件，所以研究有氧气存在的条件下纳米零价铁对多氯联苯的降解过程十分必要。本项目以污染土壤中PCBs为研究对象，对整个反应系统进行pH自动控制，进行有氧条件下纳米零价铁降解水相及土壤中PCBs及其影响因素的研究，并利用电子顺磁共振技术（EPR）对有氧条件下纳米零价铁产生自由基的过程进行分析。.（1）对厌氧条件下纳米零价铁还原脱氯4-氯联苯及小分子有机酸对其影响的研究：反应96 h，NZVI对4-氯联苯的还原脱氯效率为65.0%，去除率为74.0%。添加水杨酸、单宁酸和没食子酸，还原脱氯率分别降低为54.0%，61.2%和54.6%，添加小分子有机酸抑制了纳米零价铁对4-氯联苯的还原脱氯效率。.（2）在pH自动控制下，对有氧条件下纳米零价铁降解2-氯联苯进行研究：pH控制为5.0，有氧条件下反应4 h后，NZVI降解2-氯联苯的去除率达到59.4%，低于厌氧对照处理中65.5%；pH控制为3.0、5.0、7.0、11.0时降解率分别为59.4%、74.5%、42.1%和41.3%；NZVI的添加量为1 g/L、2 g/L、5 g/L和10 g/L时，降解率分别为64.4%、69.5%、74.5%和81.6%，增加纳米零价铁的添加量会加速反应的进行；将腐殖酸、水杨酸、单宁酸和没食子酸作为影响因子的体系中，降解率分别为64.6%，68.5%，63.9%和61.4%，均呈促进作用；利用EPR技术分析发现，体系中有羟基的产生。有氧条件下NZVI降解2-氯联苯的产物分别有联苯、2-氯酚、2-羟基联苯、1-苯乙基-2-酚和2,2'-二羟基联苯等，说明2-氯联苯产生的•OH参与了2-氯联苯的降解。.（3）将pH控制为5.0，对有氧条件下纳米零价铁（NZVI）降解2,4,4’-三氯联苯和土壤提取液进行研究：反应4 h后，有氧状态下降解率为64.2%，厌氧对照中则达到65.1%；添加腐殖酸的体系反应4 h后2,4,4’-三氯联苯的降解率为54.3%，抑制了反应速率。有氧条件下通过pH自动控制可以将土壤提取液中总PCBs量降低35.2%。. 本研究对于多氯联苯污染土壤的修复有一定的理论指导意义和实际应用价值。