Pore structure-dependent adsorption of monochloramine by activated carbon

Activated carbon (AC) adsorption is a promising approach to remove monochloramine (MCA), which is a common secondary disinfectant in chloramine disinfection, but also it reacts with natural organic matter in water to produce genotoxic substances. However, the adsorption process of MCA onto AC has not been clearly recognized. In this study, three micro-and meso-porous ACs were selected to investigate the MCA adsorption properties by combining experiments and theoretical calculations. The results indicated that the MCA adsorption was largely dependent on the pore structure of AC. Mesopores in AC facilitated the rapid adsorption of MCA, while microporous diffusion was the main rate-controlling step for the whole MCA adsorption process. Whereas the total specific surface area of AC was the determining factor to the maximum adsorption capacity of MCA, despite that the equilibrium time was prolonged by micropores in AC. The density functional theory calculation further revealed that the mesoporous AC was more favorable for the MCA adsorption, as the adsorption potential energy for the MCA onto the 2.7 nm mesoporous graphene nanosheet was stronger than the 1.4 nm microporous one. Additionally, the MCA adsorption onto AC did not change much with the increasing ionic strength of K+, Na+, and Mg2+, but ions with large hydration radii inhibit MCA adsorption more strongly. These findings provide valuable insights into the selection and design of AC materials for the efficient removal of MCA.