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近年来,随着重污染工业企业的关闭和搬迁,产生了大量土壤和地下水严重污染的工业企业遗留场地,1,2-二氯乙烷(1,2-DCA)是化工污染场地典型污染物之一。1,2-DCA作为一种重要的化工原料,具有致癌、致畸、致突变“三致”效应[1-2],同时其具有易迁移和难降解的特点,会对生态环境产生危害。1,2-DCA污染地下水主要修复方法有异位修复、原位修复、监测自然衰减等。异位修复耗时长费用高,难以彻底降解1,2-DCA。原位修复包括热脱附[3]、蒸汽浸提[4]、原位化学氧化技术[5]。其中高级氧化技术能够在短时间内高效处理地下水中污染物,使其达到可接受的水平。
基于活化过硫酸盐(PS)的高级氧化技术被广泛应用于有机污染土壤和地下水治理中[6]。在碱、热、光、过渡金属等活化下,PS可分解产生氧化性更强的硫酸根自由基(
${\rm{SO}}_4^{ \cdot - } $ ,E0=2.5~3.1 eV)。${\rm{SO}}_4^{ \cdot - } $ 通过吸氢、加成和电子转移与有机分子发生反应[7],从而可降解氯代烃、多环芳烃等多种有机污染物[8-11]。${\rm{SO}}_4^{ \cdot - } $ 可由过硫酸盐阴离子(${{\rm{S}}_2}{\rm{O}}_8^{2 - } $ )与过渡金属的反应生成(式(1)~式(2)),在碱性环境下,还会诱发链式反应产生HO·反应(式(3))。纳米零价铁(nZVI)具有粒径较小、比表面积较大、反应活性强等特点,既是良好的还原剂,又可作为PS活化剂,因此,被广泛应用于土壤和地下水修复工程[12-13]。nZVI活化PS具有较窄的pH适应范围,在碱性环境下活化效果不理想,在酸性环境下有较好的活化效果[14]。但在酸性环境下,nZVI活化PS反应快速,迅速产生大量的Fe2+,而过量的Fe2+易与自由基反应生成Fe3+,对自由基产生清除作用,不利于自由基的生成。有研究表明,螯合剂(2,2'-联吡啶、1,10-二氮菲、乙二胺四乙酸、柠檬酸等)可与Fe2+结合,降低溶液中游离Fe2+浓度,减少自由基的清除,进而提高PS对污染物氧化降解的效率[15-19]。李明等[20]发现,加入柠檬酸后,60 min内nZVI/PS体系对TCE降解率高达94.7%。ZHANG[21]的研究表明,EDTA的强化性能优于柠檬酸,这是因为EDTA与铁的螯合能力高于柠檬酸,Fe2+(EDTA)的高稳定性使得较少的
${\rm{SO}}_4^{ \cdot - } $ 被清除。目前,nZVI/PS降解地下水中1,2-DCA尚存在诸多问题,1,2-DCA去除效果和药剂利用效率有待提高。因此,本研究拟以水中1,2-DCA为目标污染物,以nZVI为活化剂,EDTA为稳定剂,建立nZVI/PS/EDTA还原/氧化体系,以提高1,2-DCA降解效果和药剂有效利用率,考察了EDTA对nZVI/PS体系降解1,2-DCA强化效果、作用机制及影响因素,以期为1,2-DCA污染地下水化学修复提供参考。
EDTA强化nZVI/PS降解地下水中1,2-二氯乙烷的作用机制及影响因素
Mechanism and influencing factors of 1,2-dichloroethane degradation in groundwater by EDTA-enhanced nZVI/PS
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摘要: 采用乙二胺四乙酸(EDTA)强化纳米零价铁(nZVI)活化过硫酸钠(PS)降解地下水中1,2-二氯乙烷(1,2-DCA)。通过分析1,2-DCA比降解率(Sq)、矿化度、1,2-DCA降解动力学及游离Fe2+和Fe3+质量浓度变化规律,阐明了EDTA强化效果及作用机制;考察了EDTA投加量、pH、阴阳离子对EDTA强化效果的影响。结果表明:经EDTA强化后,1,2-DCA的Sq和矿化度分别提高了21%和190%,反应速率常数提高了10.06%~22.95%,活化能降低了6.98~8.91 kJ·mol−1;EDTA为反应提供了弱酸性环境,促进Fe2+生成,EDTA与Fe2+生成螯合物,降低了Fe2+与自由基反应的机率,提高了nZVI/PS的持续有效性;Sq随EDTA投加量的加大先增加后减小,EDTA投加量为1.8 mmol时Sq达到最大值214.80 mg·g−1。酸性环境下EDTA的强化效果更好。除
${\rm{NH}}_4^ + $ 外,Cl−、K+和${\rm{HCO}}_3^ - $ 均抑制EDTA的强化作用,说明EDTA在实际地下水修复中可能因多种离子存在而难以达到理想的强化效果。以上研究结果可为EDTA强化nZVI/PS降解地下水中1,2-DCA提供参考。Abstract: Ethylenediaminetetraacetic acid (EDTA) was used to strengthen nano-zero-valent iron (nZVI) activated sodium persulfate (PS) for 1,2-dichloroethane (1,2-DCA) degradation in groundwater. Through analyzing the specific degradation rate (Sq), mineralization, degradation kinetics of 1,2-DCA and mass concentration change rules of Fe2+ and Fe3+, the enhancement effect of EDTA and its mechanism were clarified. The effects of EDTA dosage, pH, anion and cation on the strengthening effect of EDTA were investigated. The results showed that after EDTA strengthening, Sq and the degree of mineralization of 1,2-DCA increased by 21% and 190%, respectively. Reaction rate increased by about 10.06%~22.95% and activation energy decreased by 6.98~8.91 kJ·mol−1. EDTA provided a weak acid environment for the promotion of Fe2+ formation. Then EDTA formed a chelate with Fe2+ which reduced the probability of the reaction between Fe2+ and free radicals, and improved the sustained effectiveness of nZVI/PS. Sq first increased and then decreased with the increase of EDTA dosage. Sq reached the maximum value of 214.80 mg·g−1 at EDTA dosage of 1.8 mmol. The strengthening effect of EDTA was better in acidic environment. Except${\rm{NH}}_4^ + $ had a promoting effect, Cl−, K+ and${\rm{HCO}}_3^ - $ all inhibited the strengthening effect of EDTA, indicating that EDTA might be difficult to achieve an ideal strengthening effect in remediation of actual groundwater due to the existence of multiple ions. The research results provide basic parameters and technical supports for degradation of 1,2-DCA in groundwater by EDTA-enhanced nZVI/PS.-
Key words:
- groundwater /
- 1,2-dichloroethane /
- PS advanced oxidation /
- EDTA enhancement /
- specific degradation rate
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表 1 动力学参数
Table 1. Kinetic parameters
体系 温度/℃ k/min−1 R2 Ea
/(kJ·mol−1)
nZVI/PS20 0.003 2 0.992 8 64.22 30 0.012 2 0.988 0 63.04 40 0.016 9 0.990 8 62.27
nZVI/PS/EDTA20 0.003 8 0.990 6 57.24 30 0.015 0 0.963 3 54.13 40 0.018 6 0.992 1 53.64 -
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