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.