Terahertz Spectra and Hydration Dynamics of KNO₃ and K₂SO₄ Fertilizers: Insights from Molecular Dynamics Simulations

Potassium nitrate and potassium sulfate are two K-type fertilizers commonly used in agriculture that contaminate our water tables. We use classical molecular dynamics simulations with polarizable force field AMOEBA to investigate the solvation dynamics and the spectral properties of aqueous potassium nitrate and potassium sulfate solutions. Our results reveal contrasting solvation behaviors, with the nitrate ions disrupting the hydrogen bond network of the water while sulfate ions strengthening it. The diffusion coefficient of nitrate is higher than that of sulfate, in agreement with experimental values, indicating a more polluting tendency. We further study the solvation dynamics through the calculation of spectra in the terahertz frequency domain. The spectral analysis shows "THz excess", with nitrate causing a red shift and sulfate causing a blue shift in the hydrogen bond stretching mode of pure water. Spatially resolved spectra confirm nitrate's long-range influence in contrast to sulfate's localized effect, which is strong but mostly limited to the first solvation. These findings provide molecular-level insights into the higher and more widespread contaminating tendency of nitrate fertilizers compared to sulfate fertilizers.