土壤电动力修复中柠檬酸工业废水对重金属的原位协同强化去除及机理研究

Heavy metal pollution of soil is one of the environmental problems in China that need urgent resolution. Electrokinetic remediation is a very promising in-situ soil remediation technology. However, it is practically very difficult to reach a low soil pH environment by electrolysis alone during electrokinetic remediation process. The low mobility of heavy metals in soil has become the bottleneck of electrokientic remediation. To solve this problem, citric acid industrial wastewater after pretreatment is proposed as the enhancement agent in this project to enhance heavy metal extraction from soil through the combination of complexation/chelation, ion exchange, and buffering/neutralization etc. in the normal soil pH range, so as to achieve the purpose of so called "waste treatment by waste". The main research contents include: identification of the components in the citric acid industrial wastewater that are effective in enhancing heavy metal removal; determination of the synergistic reactions mechanisms and kinetics of enhanced heavy metal removal using different components of the wastewater; establishment of kinetic models of heavy metal desorption, physical models of heavy metal sorption, and chemical models of surface complexation; performance of bench-scale electrokinetic remediation experiments using a self-maid two-dimentional electrokinetic extraction apparatus which better simulates the practical remediation process; analysis of the influences of citric acid industrial wastewater on electroosmotic flow, gas generation, electric current, and voltage distribution, and its buffering/neutralization effects on the electrolysis of electrolytes; exploration of the two-dimensional migration and transformation processes and enhanced removal mechanisms of heavy metals under non-uniform electric field; and optimization of the operational conditions in the enhanced electrokinetic remediation using citric acid industrial wastewater. This research might provide theoretical basis and technological support to the resolution of the key problem in the application of electrokinetic remediation of heavy metal-contaminated soils.

土壤重金属污染已成为我国目前亟待解决的环境问题之一。电动力法是一种极具发展潜力的土壤原位修复技术，但在实际修复中单凭电极反应难以形成较低的pH环境，重金属在土壤中的低移动性成为电动力修复的一大瓶颈。针对这一问题，本项目提出利用经预处理的柠檬酸工业废水作为增效剂，在正常土壤pH范围内通过络合/螯合、离子交换、缓冲/中和等作用协同强化重金属的去除，达到"以污治废"的目的。主要研究内容包括：在确定柠檬酸废水中有效组分的基础上，探索各组分对重金属的协同强化去除机理和动力学；建立重金属解吸附动力学模型、物理吸附模型和化学络合模型；采用自制二维电动力装置模拟实际修复过程，阐明柠檬酸废水对电渗析流、气体产生、电流、电压分布等参数的影响及对电极反应的缓冲/中和效果，探索重金属在不均匀电场中的二维迁移转化和强化去除机制，优化废水强化电动力修复的运行条件，为解决土壤电动力修复的关键问题提供理论依据和技术支持。

电动力修复法是一种有效的低渗透土壤原位修复方法。然而仅靠电动力法自身难以达到理想的修复效果，而加入增效剂会大大增加修复成本。本项目选用无毒害成分的柠檬酸工业废水作为增效剂强化重金属镉污染土壤的电动力修复，分别通过批量吸附解吸实验和电动力实验考察其对土壤重金属镉的解吸附机理和动力学及其在电动力修复过程中对电动力参数和镉在土壤中二维迁移转化的影响。.批量实验结果表明：重金属镉在土壤颗粒表面的吸附随着pH值的升高呈“S形”增加；土壤对重金属镉的吸附过程符合Freundlich和Temkin吸附模型；柠檬酸废水可有效促进土壤镉的解吸附，然而随着土壤pH值的增加，镉的解吸附量迅速下降；柠檬酸废水解吸附的镉主要为离子交换态，其次是碳酸盐结合态和铁锰结合态；柠檬酸是废水中最有效的解吸附组分；柠檬酸废水对镉的解吸附在反应的前1个小时非常迅速，在10个小时后基本达到平衡，解吸附动力学拟合最好的是准二级方程，其次是Elovich模型。.电动力实验结果表明：与自来水相比，柠檬酸废水作为电极溶液使得电流降低缓慢；电压分布呈现非线性分布，电压差与土壤溶液的电导率基本呈负相关；电渗析流始终由阳极流向阴极，由于废水可降低土壤的zeta电位，其电渗析流速要比自来水作为电极溶液时高；实验过程中气体生成速率与电流具有很好的相关性，两者之间符合法拉第电解定律。.柠檬酸废水可有效促进镉从土壤中的去除，去除的镉主要富集在阴极溶液中；增大电压和延长修复时间有利于镉的二维迁移，然而修复过程中土壤的pH值对镉去除效率影响更显著；废水从阳极或阴极加入土壤均可以有效强化镉的去除，从阴极加入时可以更好的中和阴极产生的OH-，有利于镉的去除；电极间距增大导致镉在土壤中的不均匀分布，阴极附近镉含量明显升高。.柠檬酸废水成本低廉，无毒害成分，向土壤中施加还可有效改善土壤的理化性质，因此对于原位电动力修复大面积重金属污染土壤是一种十分有前景的增效剂。