Enhancing surface interactions between humic surfactants and cupric ion: DFT computations coupled with MD simulations study

Abstract Humics are considered as an ideal surfactant for removing cupric ions from mining wastewater by ion flotation process. In this study, ultra-efficient enhancement of surface interactions of humics with cupric ions were systematically investigated via combining DFT computations and MD simulations. The results confirm that carboxyl groups play a crucial role in surface interactions, and surface complexation capacities of cupric ions are intensively relevant with carboxyl group contents. Therefore, large number of carboxyl groups are introduced into humic surfaces (MHA) through the esterification reaction between phenolic hydroxyl groups and citric acid, which is quite beneficial to enhance surface interactions of humics with cupric ions. The interactions of humics/MHA with cupric ions strengthen gradually as the number of deprotonated carboxyl groups increases. Owing to existing abundant carboxyl groups on MHA surfaces, the surface interactions are much stronger than that of humics. Unlike humics, cupric ions can form strong chelate with carboxyl groups of MHA. Additionally, new insights into the surface interaction mechanisms were explored, which are dominated by surface complexation, combining the effects of electrostatic interactions. The above-mentioned results demonstrate that it is feasible to enhance surface interactions by increasing the number of carboxyl groups of humic surfaces. This study could provide a new idea for designing efficient flotation reagents and improving removal efficiency of metal ions at molecular scales.