土壤中取代硝基苯化合物被零价铁还原的机理

以气相色谱-质谱联用仪(GC-MS)分析和密度泛函理论(DFT)计算,探讨了土壤中甲基、氯代硝基苯混合物在常温常压下被零价铁还原的机理.GC-MS数据证明,取代硝基苯在土壤中被零价铁还原的主要产物是苯胺,同时还检测到微量的亚硝基苯、甲基苯二氮烯(对二甲基偶氮苯)、氯苯二氮烯(对二氯偶氮苯)等分解和缩合中间产物.理论计算表明:随还原程度的加深,对硝基甲苯和对氯硝基苯反应体系的能量从高到低,还原反应通过逐步添加质子和电子到硝基而进行,质子亲和力、电子亲和力起主导作用.逐步还原的机理合理地解释了实验中还原反应对土壤pH值的敏感以及微量中间产物的生成过程.苯环上的氯或甲基对还原反应的影响没有明显的差异.

REDUCTION MECHANISMS OF SUBSTITUTED NITROBENZENES BY ZERO-VALENT IRON IN SOILS

The reduction mechanisms of mixed methyl- and chloro-substituted nitrobenzenes(SNBs) in soils at(ambient) temperature and pressure by zero valent iron(ZVI) were investigated using gas chromatography-mass spectrometry(GC-MS) as the analytical method for experiments and computational chemistry especially(density) functional theory(DFT) as the guiding principle.GC-MS data indicated that the major products of SNBs reduction by ZVI were anilines,but trace amounts of nitrosobenzenes,4,4'-dimethylazobenzene,4,4'-dichloroazobenzene and other degradation and condensation compounds were also detected as the intermediate products.DFT calculations showed that the total energies of the intermediates decreased with increasing levels of reduction for the SNBs,and the reduction seemed to be a process of step-wise additions of protons and electrons to the nitro group.This mechanism of step-wise reduction offered a reasonable explanation towards why the reduction reaction was sensitive to the soil pH as well as the formations of the detected trace intermediates.With either methyl- or chloro- substitutions,no significant differences in their impacts on the rates of reduction were observed.