Molecular simulation approach for NO3− -N and NH4+-N sorption and desorption in the pores of palygorskite and sepiolite clay minerals

Molecular simulations methods were applied to determine the developed interactions between (NO ₃ ⁻) or ammonium (NH ₄ ⁺ ) and fibrous clay minerals as adsorbents. Both nitrogenous pollutants can contribute to eutrophication, rendering the research of their remediation crucial. Specifically, the ribbon- like structures of palygorskite and sepiolite, as well as the molecular structure of the nitrogenous ions and water molecules, were built in Materials Studio software and analyzed via the minimum energies by molecular mechanics and dynamics (MD) methods. It was revealed that both nitrogenous ions were mainly interacted with the water molecules presented in the pores of palygorskite and sepiolite, yielding the mutual electrostatic interactions the dominant ones. Nevertheless, the dimensions of both clay pores were crucial parameter for the developed bonds strength, especially in NH ₄ ⁺ case. Based on MD calculation results, the inner size determines the palygorskite and sepiolite removal capacity and subsequently desorption efficiency of NH ₄ ⁺ ions. Specifically, according to obtained binding energies the larger sepiolite pores facilitated the interactions with two NO ₃ ⁻ molecules (−1098 kcal/mol) instead of one NO ₃ ⁻ molecule (−42 kcal/mol) with palygorskite, while deeper NH ₄ ⁺ penetration and stronger bonding could be achieved only on sepiolite, explaining the low and stead NH ₄ ⁺ release rate compared to palygorskite. The calculated results were compared to previously publish experimental data of our research team and presented a good correlation. This study highlights the importance and suitability of molecular simulations contribution to evaluate and enlighten the insight processes of adsorption studies which could not be experimentally proven with the fibrous clay minerals.