Mono-/multiadsorption of chlorobenzene compounds on biochar: influence of the properties of the chlorobenzene molecules and biochar

Purpose This study aimed to explore the adsorption behaviors and mechanisms of chlorobenzene compounds (CBs; monochlorobenzene (MCB); 1,2-dichlorobenzene (1,2-DCB); pentachlorobenzene (PeCB); and hexachlorobenzene (HCB)) on corn straw-based biochar. Materials and methods The adsorption kinetics and isotherms of the four CBs were investigated in mono- or multi-CB adsorption experiments with the corn straw-based biochar which was pyrolyzed at 500 degrees C and modified with 1 mol/L HCl. Moreover, the surface morphology and porosity of the biochar were characterized. The molecular orbitals, electron cloud distribution, and configuration of the CBs were calculated by the quantum chemical method. The relationships were investigated among the adsorption behaviors and quantum chemical parameters of the CBs as well as the morphology characteristics of the biochar to explore the adsorption mechanisms of the CBs on the biochar. Results and discussion The results showed that in both mono- and multi-adsorbate systems, the adsorption of the CBs was well fitted by the pseudo-second-order, Elovich, and the Freundlich model. The adsorption rates, affinities, and amounts indicated that the biochar preferably absorbed highly chlorinated CBs (PeCB and HCB) over low-chlorinated CBs (MCB and 1,2-DCB). The absolute electronegativity (chi), electrophilicity index (omega), polarizability (alpha), and molecular volume (V-m) followed the increasing order MCB < 1,2-DCB < PeCB < HCB. The values of energy gap of molecular frontier orbitals (Delta E-Gap) and molecular hardness (eta) decreased with increasing halogen number of the CBs. The lower energy gap of molecular frontier orbitals, the less molecular stability, and higher deformability could favor the adsorption of highly chlorinated CBs (PeCB and HCB) on the biochar. Conclusions During the adsorption process of the CBs on the biochar, physisorption and pore filling might be the two associated mechanisms. The biochar with properties (aromatization and porosity) matchable to the CBs in energy gap of molecular frontier orbitals, electrophilicity, and polarizability is available to enhance adsorption capacity to the greatest extent. The competitive adsorption might enhance the risk of CB mobility and bioavailability by reducing the adsorption amount and delaying the equilibrium time.

Automated summary

This study explored the adsorption behaviors and mechanisms of chlorobenzene compounds (MCB, 1,2-DCB, PeCB, and HCB) on corn straw-based biochar. The results showed that the biochar preferably absorbed highly chlorinated CBs (PeCB and HCB) over low-chlorinated CBs (MCB and 1,2-DCB). The molecular orbitals, electron cloud distribution, and configuration of the CBs were calculated to investigate the adsorption mechanisms.