基于可生物降解稳定剂的微米铁表面改性对其修复地下水三氯乙烯污染的影响机制

Nowadays, groundwater has seriously suffered from chlorinated hydrocarbons contamination, which poses a severe threat to drinking water safety and deserves timely treatment and control. Based on stabilizing agent, micro-scale zero valent iron (mZVI) is modified and injected into aquifers to form an in-situ reaction zone, which is a promising alternative remediation technology for chlorinated hydrocarbons contaminated groundwater. Nevertheless, as the impact mechanism for stabilizing agent on mZVI efficiency was completely unknown, no full-fledged selection standards for stabilizing agent and efficiency evaluation system for this technology were established. In our project, with trichloroethylene (TCE) as the model contaminant, biodegradable high molecular polymers are to be employed as stabilizing agent to prepare modified mZVI for TCE contamination remediation. Based on the results of TCE removal kinetic experiments, structure-activity relationship is to be built between stabilizing agent and key pollutant removal parameters (kinetic constants and total amount, hydrochemical parameters, Fe speciation and etc.). Furthermore, seepage column is to be constructed to simulate groundwater aquifers. By measuring the distribution and activity of mZVI along columns, and analyzing composition and valence state of mZVI surface elements, the impact of stabilizing agent on mZVI transport and passivation behaviors is to be clearly investigated. On above basis, the impact mechanism of surface modification with stabilizing agent on remediation performance of mZVI is to be established, which paves the way for further optimization, evaluation and application of this technology.

目前，国内外许多地区的地下水都受到氯代烃污染，严重威胁居民的饮用水安全。采用稳定剂对微米铁进行表面改性，将其注入含水层构建原位反应带，是一项新兴的极具应用前景的污染源修复技术。然而目前稳定剂对微米铁反应带作用效能的影响机制尚不清晰，使得该技术的优化、评估及应用均缺乏依据。本研究以三氯乙烯（trichloroethylene，TCE）为研究对象，选择可生物降解的高聚物作为稳定剂，制备改性的稳定微米铁，在其表面/结构特性表征的基础上，建立稳定剂和TCE去除参数（动力学常数及总量、水化学参数、Fe形态分布等）的构效关系；利用渗流柱模拟实验，监测稳定微米铁的沿程分布、表面元素构成、价态特征和反应活性，考察稳定剂对微米铁在含水介质中迁移和钝化行为的影响，系统解析基于稳定剂的微米铁表面改性对其构建原位反应带修复地下水TCE污染的影响机制，为稳定剂的甄选和稳定微米铁反应带修复系统的优化调控提供理论依据。

利用零价铁材料构建地下水原位反应带是一项具有广泛应用前景的地下水污染修复技术。微米铁（micro-scale zero valent iron，mZVI）依据其低廉的价格、较低的健康风险和良好的反应活性，成为了该技术的理想介质。目前，该技术应用的主要瓶颈是mZVI在地下迁移过程中的重力沉降和活性降低问题。本文通过表面改性手段，制备出稳定化mZVI，系统考察了表面改性对mZVI活性和迁移性的影响及其机制，对因地制宜的构建稳定mZVI原位反应带具有重要指导意义。首先，选取黄原胶（xanthan gum，XG）、瓜尔胶（guar gum，GG）和羧甲基纤维素（carboxymethyl cellulose，CMC）作为受试改性剂，对mZVI进行表面改性，从提高mZVI的稳定性和反应活性两个角度考虑，结果表明XG呈现出最好的改性效果。分别解析吸附和还原对XG-mZVI去除三氯乙烯（trichloroethylene，TCE）的贡献模式，发现表面改性分别抑制和促进了mZVI对TCE的吸附和还原过程。各水化学因子对吸附作用影响不大，但对mZVI的还原作用产生了不同程度的影响，其中Ca(HCO3)2（0.4~2 mM）显著抑制了改性mZVI对TCE的还原。除了影响mZVI活性外，XG还可以有效提高mZVI的悬浮稳定性及其在多孔介质内的迁移性，随着流速和含水介质粒径的增加，改性mZVI的迁移性也相应增加。但离子强度和pH值的变化对改性mZVI迁移性的影响并不显著。在三种水化学条件下，mZVI均有较好的去除TCE的效果，但是在地下咸水和地下淡水中的去除率均高于超纯水；地下咸水体系对mZVI与水的反应的促进作用更加显著，导致在地下咸水中的电子效率最低，影响该技术应用的成本；钝化产物形貌存在明显差异，超纯水、地下淡水与地下咸水中钝化产物分别为Fe3O4、FeCO3、和CaMg(CO3)2-Fe3O4；除了铁活性丧失外，堵塞也是影响该技术运行长效性的关键因素，而导致堵塞的主要原因是气体释放。