Polarization induced covalent/hydrogen bonding adsorption of NH 4 + and K + in soils: comparison study on permanently and variably charged soils

Purpose Exploring the reaction mechanisms of K + and NH 4 + at the soil–water interface is of great scientific and practical importance for fertilizer use efficiency and environmental quality. The purpose of this study is to clarify the adsorption mechanisms, forces and energies of K + and NH 4 + in permanently and variably charged soils. Methods In combination with the theories of orbital asymmetric hybridization and ionic adsorption kinetics, the miscible displacement technique was employed to discover the cation adsorption kinetics, to identify adsorption forces as well as to evaluate adsorption energies. Results The adsorption mechanism and force studies showed that, the zero-order adsorption kinetics with covalent bonding force presented for both cation in the two differently charged soils at the lowest cation concentrations, while the first-order adsorption kinetics with electrostatic force exhibited at higher cation concentrations. The adsorption energies analyses showed that: (1) the adsorption energies with covalent bond for K + was higher than that for NH 4 + , especially in the permanently charged soil; (2) For a given cation type of K + or NH 4 + , the adsorption energy with covalent bond in the permanently charged soil was much larger than that in variably charged soil, especially for K + ; (3) the electrostatic adsorption energy for K + was higher than that for NH 4 + for the two soils, while the electrostatic adsorption energies in the variably charged soil were often larger than that in permanently charged soil. Conclusion Our study indicated that, the covalent bond adsorption between K + and surface O-atom could be referred as “polarization induced covalent bonding”, while the covalent bonding adsorption between NH 4 + and surface O-atom could be referred as “polarization induced hydrogen bonding”. Those findings provide new theoretical basis for studying the effectiveness of potassium and nitrogen.