Factors affecting nitrate removal
We next studied influence factors in nitrate removal through MHC
coagulation using synthetic aqueous solutions. The nitrate removal
efficiency remained almost unchanged when initial
NO3--N ranged from 15 to 25 mg/L
(Figure 6a; all are above 69.2%), demonstrating that MHC is suitable
for the treatment of wastewater with varying pollutant concentrations.
Across the pH range of 5–9, there was no substantial effect on the
NO3--N removal efficiency (Figure 6b).
When pH > 10, the removal efficiency decreased
substantially (Figure S4), which can be attributed to the competing
adsorption of OH-.
Using typical anions in natural water (i.e., sulfate and phosphate), we
investigated the effect of competing ions on nitrate removal when using
MHC. As molar ratio of sulfate and phosphate to
NO3- increased, the
NO3--N removal efficiency gradually
decreased (Figure 6c). To further quantify the effect of competition,
the selectivity coefficient \(K_{\text{NO}_{3}^{-}/X^{n-}}\)(calculation formula is in SI) was applied to characterize the affinity
of MHC to NO3- and coexisting ions. K
values greater than one means that the affinity for
NO3- is stronger than \(X^{n-}\),
lower than one means that the affinity for
NO3- is weaker than \(X^{n-}\). When
phosphate and sulfate are present, the selectivity coefficients of MHC
for
NO3-are 3.19 and 0.77 respectively (Figure 6d), revealing that the affinity
order of MHC for the three ions is phosphate < nitrate
< sulfate.
MHC has a stronger affinity for sulfate than nitrate. Sulfate contains
more charges than nitrate, and the hydrated radius for sulfate (0.300
nm)27 is smaller than nitrate (0.335
nm)28, which are both beneficial for electrostatic
interaction with quaternary ammonium. For the secondary biological
effluent, the molar ratio of sulfate to nitrate is mostly lower than
1.5,29-31 and the nitrate removal efficiency can still
reach more than 65% (Figure 6c). Therefore, although sulfate will
reduce nitrate removal to a certain extent when using MHC, sulfate has
little effect on practical applications. MHC has weaker affinity for
phosphate than nitrate, probably because phosphate Gibbs hydration
energy (-1125.3 kJ/mol for HPO42- and
-465 kJ/mol for
H2PO4-, the two main
form of phosphate under neutral pH conditions)32, 33are lower than nitrate Gibbs hydration energy (-306
kJ/mol)33. Thus phosphate only has a slight effect on
the NO3--N removal. In conclusion, MHC
has strong adaptability to the change of water quality.