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.