Experimental and molecular mechanics and ab initio investigation of activated adsorption and desorption of trichloroethylene in mineral micropores
Experimental and molecular mechanics and ab initio investigation of activated adsorption and desorption of trichloroethylene in mineral micropores
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DOI：
10.1021/es011172e
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发表时间：
2002-04-01
影响因子：
11.4
通讯作者：
Curry, J
中科院分区：
文献类型：
--
作者：
Farrell, J;Luo, J;Curry, J
本研究利用实验和分子模拟技术研究了疏水有机污染物（HOC）在矿物微孔中的活化吸附。采用正面分析色谱法测定了常压和高压下硅胶吸附剂对三氯乙烯（TCE）的吸附。增加流体压力会增加TCE的吸收，而在将压力降低回大气条件时，这种吸收不会被释放。这表明，压力的增加能够迅速诱导抗解吸部分的形成，而之前的研究表明，在大气压下需要数月才能形成。然后使用大规范蒙特卡罗（GCMC）模型来阐明水的性质和二氧化硅微孔中的TCE行为。GCMC模型显示，分子尺度的充填限制导致孔隙流体密度介于体溶液的0.28 ~ 0.78之间。该模型还表明，由于分子尺度的堆积限制，TCE能够取代亲水矿物孔隙中的水。吸附TCE的放热等容热可达-27 kJ/mol，在7和8埃的孔隙中最大。这表明，在最小大小足以容纳TCE分子的孔隙中，TCE的吸附在能量上是最有利的。压力诱导的吸收似乎主要是由于最小孔隙中堆积密度的增加。从头计算表明，TCE分子低能构象的微小畸变需要高活化能。本研究结果表明，需要在吸附质中扭曲键角的活化吸附可能是hoc在微孔固体上缓慢达到吸附平衡的原因。同样，分子级吸附位点的活化解吸可能有助于含水层沉积物中hoc的缓慢释放。
This research investigated activated adsorption of a hydrophobic organic contaminant(HOC) in mineral micropores using experimental and molecular modeling techniques. Adsorption of trichloroethylene (TCE) on a silica gel adsorbent was measured using a frontal analysis chromatography technique at atmospheric and elevated fluid pressures. Increasing the fluid pressure yielded increased TCE uptake that was not released upon lowering the pressure back to atmospheric conditions. This showed that the increase in pressure was able to rapidly induce the formation of a desorption-resistant fraction that previous investigations have shown requires months to develop at atmospheric pressure. Grand Canonical Monte Carlo (GCMC) modeling was then used to elucidate the nature of water and TCE behavior within silica micropores. The GCMC modeling showed that molecular scale packing restrictions resulted in pore fluid densities that ranged from 0.28 to 0.78 of those in the bulk solution. The modeling also showed that TCE was able to displace water from hydrophilic mineral pores due to molecular scale packing restrictions. Exothermic isosteric heats for TCE adsorption up to -27 kJ/mol were observed and were greatest in pores of 7 and 8 Angstrom. This indicated that TCE adsorption was energetically most favorable in pores that were minimally large enough to accommodate a TCE molecule. The pressure-induced uptake appeared to result primarily from an increase in the packing density in the smallest pores. Ab initio calculations showed that small distortions of a TCE molecule from its low energy conformation require high activation energies. Results from this study indicate that activated adsorption requiring bond angle distortions in the adsorbate may be responsible for the slow attainment of adsorptive equilibrium of HOCs on microporous solids. Likewise, activated desorption from molecular-sized adsorption sites may contribute to the slow release of HOCs from aquifer sediments.