Adsorption of cations at the illite–water interface and its effect on intrinsic potassium ions

Abstract

AbstractNa+, K+, Cs+ and Ca2+ are common exogenous cations that could be introduced into soils either via long‐term tillage with inorganic fertilizers or the leakage of nuclear waste. However, the manner in which they co‐adsorb and compete with the intrinsic potassium ions in illite, the response of intrinsic potassium (K′+), as well as the underlying mechanisms of these interactions, remain unclear. These aspects were explored using molecular dynamics simulations based on a series of concentrations. Because of the competition between exogenous cations and K′+, the number of adsorbed exogenous cations such as Na+ increased by 11.2, whereas the corresponding number of adsorbed potassium decreased by 14.2, with the increase in concentration from 0.1 to 1.0 mol·L−1, indicating that long‐term application of inorganic fertilizers tends to promote the release of intrinsic potassium. This was further validated from the increased adsorption free energy of IS (inner‐sphere) K′+, which increased from −54.3 kJ·mol−1 (0.1 mol·L−1) to −42.9 kJ·mol−1 (1.0 mol·L−1). The stability of both exogenous cations and intrinsic potassium decreased with the increase in concentration. This was mainly caused by the reduction of illite–cation interactions, as evidenced by the radial distribution function analysis. Nonetheless, intrinsic potassium always dominated the competition, as its abundance was lower than that of exogenous cations only beyond 0.7 mol·L−1. Different exogenous cations exerted diverse effects on the intrinsic potassium and may vary with the concentration, as reflected by the distribution, adsorption number, coordination environment and adsorption stability. Generally, the effect of exogenous cations on intrinsic potassium decreased as Ca2+ > Na+ > Cs+, which may have been caused by the different adsorption sites. These results provide new insight into the environmental effect of the excessive use of inorganic fertilizers and the leakage of radioactive waste in soils.Highlights Increase in exogenous cations tended to promote the release of intrinsic potassium in illite. Intrinsic potassium generally dominated the competition with exogenous cations. Stability of adsorbed mixed cations was reduced with increase of concentration. Exogenous cations exerted a diverse effect on the intrinsic potassium.