活性炭载纳米零价铁/钯体系对土壤中多氯联苯的修复机理研究

Granular activated carbon (GAC) samples are impregnated with nanoscale zero-valent iron and palladium (GAC/ZVI/Pd) to function as reactive activated carbon (RAC) for removing polychlorinated biphenyls (PCBs) from soil. Due to GAC's huge surface area and pore structure, it still has a very large adsorptive capacity for PCBs after impregnation with nanoscale metal particles. The objectives of this project are: to employ RAC as a novel material for adsorption/dechlorination of PCBs in soil remediation; to investigate its effect on PCBs desorption from soil, especially the role of soil organic matter with different chemical composition and physical structure in this process; to elucidate the phase distribution and transformation of PCBs of various molecular structure in the soil-RAC system; to observe the aging and oxidation of iron embedded in RAC and quantify the adsorption/dechlorination of PCBs during the long-term RAC aging process. The effects of RAC internal factors (the type and particle size of GAC, iron and palladium content, etc.) and external environmental factors (pH and redox potential of soil, concentration of chlorine ion and/or nitrate anion, etc.) on the long-term life of RAC and its performance of PCBs removal from soil will also be investigated to optimize the RAC synthesis method and to establish the suitable field conditions for PCBs soil remediation application. The expected achievements of this project can provide not only innovative technical support for remediation of PCBs polluted soil, but also the basic data of migration and transformation of PCBs in porous media-soil system for further studies.

活性炭载纳米零价铁/钯体系（RAC）利用了活性炭巨大的比表面积和丰富的孔隙结构，在固定纳米金属颗粒的同时，又对多氯联苯（PCBs）具备很强的吸附性能。本课题旨在将RAC这种新颖的PCBs吸附/脱氯材料引入到土壤体系中，探讨其对于土壤中PCBs解吸动力学的影响，观察土壤有机质含量、化学组成和物理构象在此过程中的贡献。研究各类分子结构的PCBs在土壤-RAC体系各相中分配和转化规律，探讨RAC在土壤环境下的长期老化机理，并对老化过程中PCBs的吸附/脱氯机制进行定量化研究。考察RAC内部因素（活性炭载体类型、铁钯含量等）和外部环境因素（土壤pH、氧化还原电位等）对RAC长期寿命和去除土壤PCBs性能的影响，以便优化RAC制备条件，找到适宜使用RAC进行PCBs土壤修复的场地条件。为环境土壤PCBs修复提供创新技术支持的同时，也为今后土壤中PCBs在类似多孔介质中的迁移转化规律研究提供基础数据。

多氯联苯（PCBs）是目前国际上最受关注的持久性污染物之一，而土壤是PCBs在环境中主要的储存场所。本课题针对活性炭载纳米零价铁/钯体系（RAC-Pd）的制备进行了优化，并开展了其对于土壤PCBs的去除机理和动力学、有机质等土壤环境影响因素和RAC-Pd材料老化等方面研究。主要成果如下：.1.本课题选择了“液相沉淀法”作为制备工艺进行优化，最终采用Norit HD 3000作为载体炭，以聚乙二醇作为分散剂，经硼氢化钠还原后经过700℃热处理，最终制成RAC-Pd。.2.RAC-Pd对于土壤中各类PCBs均有较好的去除效果，在7天内总体去除率可以达到66.93%-82.11%，其中脱氯作用贡献率达到45.57-81.52%。PCBs的氯代程度增加会导致其在反应初期不易从土壤有机质上解吸，但随着处理时间的延长，各种土壤PCBs之间总去除率差异逐渐减少。具有邻位氯原子的非共平面PCBs从土壤中解吸附的速度和在水中传质速率要快于共平面PCBs。RAC-Pd对水相PCBs的去除过程也观察到了相似的情况。高氯代PCBs在RAC-Pd中更容易脱氯降解，而在氯代程度相同的情况下，PCBs上氯原子的脱氯难度符合邻位>间位>对位的规律，且相同取代位的氯原子若处于PCBs不同苯环上将更加容易脱氯。RAC-Pd可以将土壤PCBs及其副产物牢固吸附在其上进行脱氯降解。.3.土壤中有机质会对RAC-Pd去除土壤PCBs起到一定的抑制作用。但土壤有机物的软碳组分对于RAC-Pd的长期脱氯性能有促进作用。由于物理构象的不同，溶解性腐殖酸（DHA）相比固体腐殖酸（SHA）对PCBs具有更高的吸附性能，在土壤颗粒存在的条件下可以使得水相PCBs浓度大幅降低。另外，DHA对于RAC-Pd孔隙的阻塞能力也小于SHA。Cl-、NO3-、CO32-、SO32-等土壤阴离子的存在会降低RAC-Pd对于土壤PCBs的去除能力，其中SO32-对于RAC-Pd的脱氯性能影响最为明显。.4.RAC-Pd在各种环境下的长期老化（200天）使其对于土壤PCBs的去除和脱氯性能均逐渐降低。接触空气对RAC-Pd脱氯性能产生的负面影响最显著，而单纯的土壤环境对RAC-Pd的老化作用并不显著。.以上成果为RAC材料在土壤PCBs修复领域的应用起到了推动作用，也为类似纳米零价铁负载材料对于氯代有机物的去除研究提供了基础。