Adsorption and Desorption Performance of Dichloromethane over Activated Carbons Modified by Metal Ions

Activated carbons (ACs) were modified separately by impregnation with different types of metal ions, Al3+, Mg2+, Li+, Fe3+, and Ag+ in this work. Adsorption breakthrough experiments of dichloromethane (DCM) were carried out for measuring working adsorption capacities (Qm) of modified ACs toward DCM, and temperature programmed desorption (TPD) experiments were conducted for estimating desorption activation energy (Ed) of DCM on these ACs. The effects of metal ions on the adsorption/desorption of DCM on the modified ACs were discussed with the help of hard and soft acid and base (HSAB) theory. Results showed that Qm of DCM on these ACs followed the order: QAl(III)/AC > QLi(I)/AC > QMg(II)/AC > QFe(III)/AC > QAC > QAg(I)/AC, and the Ed of DCM followed the same order. As compared to that of the original AC, the surface local acid hardness of the modified ACs was enhanced with the doping of hard acid metal ions such as Al3+, Li+, Mg2+, and Fe3+, and thus the interactions between DCM and Al(III)/AC, Li(I)/AC, Mg(II)/AC, and Fe(III)/AC surfaces were also enhanced due to the hard base characteristic of DCM. However, when the AC was doped with soft acid Ag+, its surface local acid hardness was decreased. As a result, the interaction between DCM and Ag(I)/AC was weakened.