  Interactions between organic species and natural minerals are fundamental to
the processes around us. With the aid of molecular dynamics simulations, we
identify key adsorption mechanisms of apigenin on smectite clay minerals. The
mechanism is highly sensitive to the pH -- changing from co-crystallisation in
acidic-to-neutral solutions to the ion-bridging in mild-alkaline. The ionic
species play a significant role in alkaline environments: the deprotonated
apigenin species chelate metals, which, in turn, leads to the formation of a
stable organic-metal-mineral complex and stronger adsorption in the presence of
divalent cations.Smectite clays buffer the solution to mildly alkaline; hence,
the type of exchangeable cations in the clay will be critical in determining
the adsorption mechanism and organic retention capacity. Overall, our study
showcases a computational strategy that can be transferred to a wide variety of
organic-mineral systems in the natural environment.
