Molecular dynamics simulation of ammonium ion removal by freezing concentration


 Ammonium wastewater is a serious and common water pollutant that can have harmful effects on the environment. Freeze concentration, as an energy-efficient and environmentally friendly method, is used to treat ammonium wastewater by ice-water phase transition. The simulation results show that most of the ions are retained in the water phase, and it is reported for the first time that the probability of NH4+ (90%) remaining in the water is significantly higher than that of Cl− (67%). We analyzed the influence of ions on ice/water phase structure from the perspective of structure and energy, and explained the reason for the difference in the probability of NH4+ and Cl− remaining in water. We found that the coordination number (CN) of NH4+ decreased from 6 to 4 when one NH4+ permeated the ice layer, indicating that the first hydration layer of ammonium ions underwent significant reorganization during this period. In contrast, a similar reduction in CN was not observed during the entry of Cl− into the ice layer. Moreover, the hydration energy shows that NH4+ prefers to stay in liquid phase than in ice phase because of the higher hydration energy difference compared with that of Cl−. The results of this work indicate that freeze concentration can efficiently remove NH4+ by ice-water phase transition, which greatly reduces the discharge of ammonium wastewater and pave the way for further study of the freezing process for wastewater treatment.