The Adsorption Mechanism Of Volatile Organic Compounds On Carbon Nanotubes

Massive production and widespread use of carbon nanotubes (CNTs) would inevitably release into the environment, and it could adsorb the organic pollutants in the environment and affect the migration and transformation of both nanotubes and pollutants. The study of the adsorption of organic pollutants onto CNTs could help explain the fate of the two and assess their ecological risk. Most studies have investigated the mechanism of interactions between CNTs and organic pollutants in the aqueous phase. Only few studies focused on the adsorption of selected organic compounds onto CNTs in the gas phase, the difference of a varied organic compounds adsorbed onto CNTs need farther research. Therefore, this paper will study the adsorption mechanism of VOCs onto carbon nanotubes in the gas phase and calculate Henry constants (KH) that represent absorption strength. By building the linear solvation energy relationship (LSER) model, the paper will quantitatively characterize the relative contribution of each interaction to total adsorption.Inverse Gas Chromatography method was used to determine Henry constant (KH) of35volatile organic compounds (VOCs) onto multi-walled carbon nanotubes (MWNTs), calculated adsorption thermodynamic parameters, and also compared the results with typical12VOCs adsorbed onto single-walled carbon nanotubes (SWNTs) in the gas phase. The results showed that, KH decreased as the temperature increased, thelnKH of alkanes, alcohols, esters had a positive correlation between the numbers of carbon atoms in molecules, thelnKH of chlorinated alkanes had positively correlated with molecular polarizability, thelnKH of benzene and substituted benzenes had positively correlated with dipole moment and molecular polarizability. Thermodynamic analysis showed that the adsorption depends on the Van der Waals interactions and hydrogen bonding. In addition, the KH of VOCs onto SWNTs was about1.5to5times higher than that onto MWNTs. The relationship between KH and temperature was established, which could be used to estimate the adsorption of VOCs onto carbon nanotube under environmental temperature.Linear solvation energy relationship (LSER) was adopted to establish quantitative relationship between lnKH and molecular solvation factors. The results showed that the ability of cavity formation and the dipolarity/polarizability of molecules determined the adsorption onto MWNTs. Based on the adsorption mechanism,20descriptors were selected and calculated, and the theoretical linear solvation energy relationship (TLSER) was created by stepwise regression. According to the result, lnKH is associated with the polarizability of the molecules (a) and the average of the negative potentials on the molecular surface (Vs.).The model was characterized of goodness-of-fit, stability and predictivity and presented as follows: the correlation coefficient (R2) was0.880; the root-mean-square error (RMSE) was0.425; leave-one-out cross-validated R2(Q2Loo) was0.841; and the external validated R2(Q2EXT) was0.820. Based on the results of TLSER model, the adsorption of VOCs onto MWNTs mainly was a result of Van der Waals and hydrogen bonds interactions. The conclusion was consistent with the results of adsorption thermodynamics. The TLSER model was reliable with clear mechanism interpretability, thus could be used for predicting VOCs adsorbed onto MWNTs in gas phase.