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目前,针对土壤有机污染物的传统治理技术,通常难以达到去除低环PAHs的修复要求[1-2]。萘 (naphthalene,Nap) 作为一种具有致畸、致突变与致癌的典型低环半挥发性PAHs[3],研究其在土壤介质中的降解规律,并完善其修复治理技术,对于评估低环PAHs的环境风险和提出相应的污染管控措施具有重要的意义。
近年来,过硫酸盐高级氧化技术在污染控制领域受到越来越多的关注[4-6],PS是一种强氧化剂,可以通过热[7-9]、碱[10-11]、过渡金属离子[12-14]、超声[15-17]和微波[18-21]等能量或材料进行活化。研究表明,提升温度可有效增加过硫酸盐的活性,但常规加热存在能耗高、效率低的问题[22]。微波诱导具有加热快速、能耗相对较低、无二次污染等优点[23-24],但缺少机理研究,且大多数人使用市售微波炉,单磁控管能量密度低,很难达到预期活化效果。
工业上绝大多数情况使用多磁控管微波源提高微波功率,然而微波加热的不均匀性在很大程度上限制了微波技术的发展和应用[25]。微波加热不均匀的主要原因是电磁场强度分布不均匀[26]。目前,改善微波加热不均匀性的措施主要是通过改变谐振腔的尺寸及微波馈口数量[27]和相对位置[28],使谐振腔具有尽可能多的振荡模式[29],不同模式叠加后能够获得更加均匀的能量分布状态。本研究拟应用COMSOL Multiphysics[30-31]多物理场仿真软件优化谐振腔内电磁场分布,参照模拟结论,制备微波反应器,并通过MW诱导PS强化降解土壤中萘的实验研究,考察高浓度萘污染土壤的微波诱导修复效果及影响因素,以期为实现该技术的工程应用提供参考。
微波诱导过硫酸盐修复萘污染土壤的模拟优化
Simulation and optimization of naphthalene polluted soil remediation by microwave induced persulfate
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摘要: 微波 (microwave,MW) 诱导过硫酸盐 (persulfate,PS) 去除土壤中的有机污染物是一种有效的土壤修复手段。以萘为土壤中典型的多环芳烃 (polycyclic aromatic hydrocarbons,PAHs) 有机污染物,利用COMSOL Multiphysics多物理场计算软件模拟,结合电磁耦合理论分析探索了微波电磁辐照过程中土壤有机物消解过程机制,并考察了PS浓度、MW辐照时间、MW温度、土壤含水率等工艺参数对萘去除效果的影响。结果表明,根据仿真优化后,微波反应器具有更好的加热均匀性。在微波谐振腔内,萘的降解过程符合伪一级反应动力学模型,MW诱导PS反应速率常数是常规电加热的1.6倍,这可能是因为极性过硫酸盐分子在微波交变电场的作用下胶体运动趋势增加;同时,多环芳烃有机物在微波场中的土壤传质性能得到优化,降低反应所需的活化能。当PS浓度为1.0 mol·L−1、MW温度为80 ℃、土壤含水率为15%、MW处理时间为60 min时,萘的去除率最高可达到96.5%。本研究结果可为微波诱导过硫酸盐技术在多环芳烃污染土壤修复中的应用提供参考。Abstract: PS (persulfate) induced by MW (microwave) has been proved as an effective strategy for organic polluted soil remediation. In this study, naphthalene was selected as the target soil contaminant, which was a typical kind of PAHs (polycyclic aromatic hydrocarbons). The mechanism of naphthalene removal during microwave electromagnetic irradiation process was explored by COMSOL Multiphysics software simulation together with the theoretical analysis of electromagnetic coupling. Moreover, the effects of PS concentration, MW irradiation time, MW temperature, and soil water content on naphthalene removal performance were investigated. The results indicated that the optimized microwave reactor based on software simulations revealed better heating uniformity. The degradation process of naphthalene conformed to the pseudo-first order kinetic model. Besides, the k value of PS induced by MW reaction were 1.6 times higher than those of conventional electrical heating. These results could be attributed to the polar PS molecules movement under the microwave alternating electric field, thereby optimizing the naphthalene mass transfer process in the soil and reducing the activation energy required for the reaction. The maximum naphthalene removal efficiency could reach 96.5% when the PS concentration, MW irradiation time, MW temperature, and soil water content was 1.0 mol L−1, 80 ℃, 60 min, and 15%, respectively. The results obtained from this study could provide theoretical support for practical applications of PS induced by MW to treat PAHs in soil.
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Key words:
- microwave induced /
- persulfate /
- naphthalene /
- analogue simulation /
- soil remediation
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表 1 土壤样品的基本理化性质
Table 1. Basic physical and chemical properties of soil samples
pH 水分/% 有机质质量分数/% 阳离子交换量/ (cmol·kg−1) 粒径分布/% 砂粒 粉粒 黏粒 8.36 4.37 3.45 5.43 19.37 71.35 9.28 表 2 萘降解的中间产物
Table 2. Intermediate products of naphthalene degradation
序号 化合物 分子式 分子量 结构式 1 1,2-Dihydroxynaphthalene C10H8O2 160.17 2 1,2-Benzenedicarboxylic acid C8H6O4 166.13 3 Salicylic acid C7H6O4 138.12 4 Catechol C6H6O2 110.11 5 phenyl hydroxide C6H6O 94.11 表 3 微波诱导条件下不同温度的反应动力学计算
Table 3. Calculation of reaction kinetics at different temperatures under microwave radiation
温度/k 拟合方程式 相关系数R2 反应速率常数 293 y=0.01540x−0.10868 0.989 2 0.015 40 303 y=0.02106x−0.11731 0.992 1 0.021 06 313 y=0.02895x−0.09313 0.997 0 0.028 95 333 y=0.04074x−0.02565 0.996 0 0.040 74 353 y=0.05232x−0.15931 0.997 3 0.052 32 表 4 常规加热条件下不同温度的反应动力学计算
Table 4. Calculation of reaction kinetics at different temperatures under conventional heating conditions
温度/k 拟合方程式 相关系数R2 反应速率常数 293 y=0.01427x−0.08939 0.963 9 0.014 27 303 y=0.01806x−0.08402 0.993 1 0.018 06 313 y=0.02077x−0.09313 0.998 6 0.020 77 333 y=0.03403x−0.02565 0.995 3 0.034 03 353 y=0.03961x−0.15931 0.997 4 0.039 61 -
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