Retention of organic matter on the surface of illite particle under the influence of different cations: A molecular dynamics simulation study

Retention of natural organic matter (NOM) in soils is of great importance in many physicochemical and biochemical processes as well as carbon cycling, however, the effect of cations on such a retention behavior and the underlying mechanism are still elusive. This issue is difficult to be solved using traditional experimental tools, while it is the advantage of molecular dynamics (MD) simulation comes in. Here, we used MD simulation to investigate the retention of TNB (Temple-Northeastern-Birmingham) molecules on illite surface, considering four cation types (K+, Na+, Mg2+, and Ca2+) and three ionic concentrations (0.1, 0.3, and 0.6 mol/L). Different cations greatly affected the adsorption capacity and dynamics of TNB clusters on illite substrate, and the TNB retention ability decreased as Ca2+ > Mg2+ > Na+ > K+. This was achieved by regulating the TNB-illite inter-action instead of the TNB-cation interaction. TNB molecules interacted with all four types of cations mainly through carboxyl groups, which were also the main functional group participating in the TNB-illite interaction in the presence of monovalent cations. However, the importance of carboxyl groups in interacting with illite surface decreased with increasing ionic concentration in the presence of divalent cations. There were two kinds of mechanisms for the retention of TNB: hydrogen-bond and cation bridge, but their significance was influenced by cation type and concentration. Two kinds of cation bridges were observed: ternary OS - Cation-COO-TNB complexes for K+, Na+, and Mg2+ while quaternary OS -H2O - Cation-COO-TNB complexes for Ca2+. In addition, suitable amounts of Ca2+ were beneficial to the retention of natural organic matter in soils with high charged minerals, while a too high Ca2+ concentration following an unscientific fertilization may lead to a loss of organic matter in these soils. This study provides a new insight in understanding the stability of organic-mineral association and the soil organic carbon sequestration.