冻融作用对东北黑土中胶体携带重金属Pb迁移转化的影响机制

Lead has attracted increasing attention due to its universal pollution and serious toxicity. It was previously considered that lead collected on the surface layer, while resently it was found that lead can flow with soil water associated with the soil colloids and increase its mobility. The freeze-thawing changed the content, grain size, composition of the soil colloids, then affected the adsorption capacity of the colloids to lead, and the transportation progress of the colloid-associated metal into the ground water, influenced the behavior of the heavy metal in environment. The intensity of the freeze-thawing is frequent and the lead contamination is serious in the field of the Northeast China. In this project, the lead contaminated soils will be subjected to different freeze/thawing treatments by artificial controlling temperature, indoor experiment. The effect of the freeze-thawing on the migration of the soil colloids and colloid-facilitated lead will be simulated through soil column leaching experiment. The distribution of lead on different size and different type (organic or inorganic colloids) colloids, and formations and the binding energy state of lead on soil colloids will be investigated by combining the ultracentrifugation, AAS, FTIR , XPS and TEM-EDX technologies. The effect mechanism of the freeze-thawing on the soil colloid-facilitated lead migration and transformation will be elucidated through analyzing of the soil colloid contents and other physicochemical properties. The results will provide theoretical basis for scientifically assessing and forecasting the environment behavior of the heavy metal lead in soil and establishing reasonable prevention and controlling measures for the lead pollution in soil in the areas subjected to freeze-thawing.

重金属铅因污染普遍、毒性大而受到广泛关注。通常认为铅不易移动而聚集在土壤表层，但近年来有人研究发现铅可被土壤胶体携带随水流动致使其迁移性大为增加。冻融可增加土壤胶体含量、改变胶体颗粒大小及其化学组成，进而影响胶体对铅的吸附能力和迁移过程，使铅在环境中的行为发生重要改变。本课题针对我国东北黑土区冻融交替频繁和铅污染严重的现状，拟以人工控温方法对铅污染土壤进行不同冻融处理，用土柱淋洗试验模拟土壤胶体和铅的迁移过程，用超速离心技术和原子吸收分光光度法，傅立叶转换红外光谱，X射线光电子能谱及透射电镜-能谱联用技术等测定方法测定重金属铅在不同粒径、不同类型土壤胶体上的分布和铅在土壤胶体上的赋存形态和结合能态，通过比较不同冻融处理土壤的胶体含量及其它理化性质，揭示冻融作用影响土壤胶体携带Pb迁移转化的机制。研究结果可为科学评价和预测冻融地区土壤铅的环境行为、合理制定土壤铅污染防治措施提供理论依据。

重金属Pb很容易与土壤颗粒产生强烈的吸附作用，因此在降水或灌溉的情况下会随着土壤胶体而发生迁移。冻融作用能够影响土壤的理化性质，促进土壤胶体的形成，因而对土壤中胶体结合态铅的迁移过程产生影响。本项目以人为培养不同重金属铅污染程度土壤（250 mg/kg，1000 mg/kg）为研究对象，使土柱经历冻融作用（-20 ℃冻结12h，20 ℃融化12 h为一次冻融循环，分别冻融3次和冻融6次），并通过土柱淋洗试验测定了土壤胶体及不同形态重金属铅的迁移过程。结果表明冻融作用既促进溶解态Pb的迁移也促进胶体结合态Pb的迁移，从而增加土壤中重金属Pb的迁移总量。一方面冻融作用增强了土壤的渗透性，增加了渗滤液的体积，即使在不影响渗滤液中Pb浓度的情况下，也会表现为经冻融处理的土柱重金属Pb的流出量增加；另一方面冻融作用促进了土壤中可移动胶体的增加，从而增加胶体结合态铅的迁移。本研究结果表明淋洗条件下土柱中的重金属Pb主要以胶体结合态的形式迁移。虽然冻融处理后溶解态Pb和胶体结合态Pb的迁移量均有所增加，但是对胶体结合态Pb的增加效果更明显，表现为冻融处理的土柱中Pb迁移过程中胶体结合态Pb所占的比例增加。重金属铅污染程度和土壤含水量也影响铅的迁移。污染程度越重，溶解态铅和胶体结合态铅的迁移量越大。污染程度相同条件下，未冻融时，低含水量土壤中铅的迁移量高于高含水量土壤，但是冻融处理后，低含水量土壤铅的迁移量低于高含水量土壤，冻融作用对高含水量土壤的影响更大。本实验条件下土壤可移动胶体的含量较低，其携带的重金属Pb的含量低于XPS的检测下限，所以未能得出土壤胶体携带重金属Pb的化学形态。淋洗作用下，土壤中的重金属Pb向下迁移，导致重金属Pb在土柱中的垂直空间分布表现为下层土壤中含量增加。本项目的研究结果将为高纬度地区的重金属铅污染土壤在降雨和灌溉情况下的环境风险预测提供科学依据。