Role of oxygen functional groups in Pb2+ adsorption from aqueous solution on carbonaceous surface: A density functional theory study

The adsorption mechanism of Pb2+ from aqueous solution on carbonaceous surface modified with oxygen functional groups was investigated by using density functional theory method. The zigzag model with seven benzene rings and armchair model with four benzene rings were used to simulate the different structures of carbonaceous surfaces. It was found that the adsorption of Pb2+ on the pure zigzag surface was chemisorption with the adsorption energy of - 306.26 to - 322.36 kJ/mol, while that on the armchair surface was physisorption with the adsorption energy of - 32.39 kJ/mol. The introduction of oxygen functional groups significantly enhanced the Pb2+ adsorption on the armchair surface. The physisorption changed to chemisorption after adding carboxyl, phenolic hydroxyl, or carbonyl functional group, indicating the stronger adsorption ability of the carbonaceous surfaces after modification. On the zigzag surface, however, the studied functional groups cannot benefit the Pb2+ adsorption. The results showed that the Pb2+ tended to adsorb on the carbon atoms instead of moving to the oxygen atoms from the introduced functional groups for adsorption, which suggests that the oxygen functional groups promoted the Pb2+ adsorption by increasing the activity of their neighboring carbon atoms.