Removal pathway of nitrate by MHC
NO3- was taken as an example to explore the mechanism of dissolved contaminants removal by MHC using MD. First, the adsorption site was determined by the relationship between the total interaction potential (\(V_{\text{total}}\)) and relative positions of MHC and NO3-. NO3- positions were classified into three categories according to \(V_{\text{total}}\): strongly attracted (\(V_{\text{total}}\) < -80 kJ/mol, purple dots in Figure 7a, configuration i in Figure 7b), weakly attracted (-40 kJ/mol <\(V_{\text{total}}\) < 0, green dots in Figure 7a, configuration ii in Figure 7b) and almost no interaction (\(V_{\text{total}}\)= 0, yellow dots in Figure 7a, configuration iii in Figure 7b). It can be seen that NO3-that has strongest interaction with MHC was distributed around quaternary ammonium N atoms. The contact area with quaternary ammonium N atoms was large (\(S_{\text{buried}}\) > 1.5 nm2) and the distance to the N plane was 0 (Figure 7a). However, NO3- have weak interaction with MHC were distributed around carbon chains. The above results show that the N atom of the positively-charged quaternary ammonium group is the main adsorption site, and the carbon chains only have very weak attraction to NO3-.
To investigate NO3- removal pathway and driving force, representative trajectory and time evolution of van der Waals (vdW) and electrostatic interaction energies between NO3- and quaternary ammonium N atom were analyzed. Electrostatic interaction energy changed rapidly with NO3- adsorption compared with vdW interaction energy (fluctuating at ~ 0), indicating that electrostatic attraction is the main driving force for adsorption. At t = 1000 ps, the electrostatic interaction energy decreased, but it immediately increased and fluctuated at ~ 0 (Figure 7c), which suggests that the electrostatic interaction at 1000 ps was weak and unstable. Configuration ⅰ in Figure 7e illustrates that NO3- was outside the methyl groups of quaternary ammonium at 1000 ps, which hindered the interaction of positive N atom and NO3-.
After t = 2000 ps, the electrostatic interaction energy decreased and stabilized at ~ -100 kJ/mol (Figure 7c). At the same time, hydrogen bonds between NO3- and coordinating water molecules decreased from ~ 7.9 (before 2000 ns) to ~ 6.7 (after 2000 ns) (Figure 7d), indicating NO3- dehydrates during adsorption. As shown in Figure 7e, NO3- will first adjust its configuration to enter into the methyl groups (Configuration ⅱ) and then will interact closely with the quaternary ammonium N atom (Configuration ⅲ) to achieve stable adsorption. The above results revealed that during NO3- removal, NO3- will dehydrate and penetrate methyl groups of quaternary ammonium and interact strongly with quaternary ammonium N atom by electrostatic attraction.