A combined first principles and classical molecular dynamics study of clay-soil organic matters (SOMs) interactions

In this study, clay-soil organic matters (SOMs) interactions were investigated by combining first principles molecular dynamics (FPMD) and classical molecular dynamics (CMD) techniques. FPMD was employed to quantify the binding mechanisms of reactive SOMs groups on clay surfaces and to derive parameters for the bonding of the reactive groups on edge surfaces. By integrating the derived parameters with CVFF-CLAYFF force fields, CMD simulations were carried out to investigate the structures of large models of clay-SOMs associations. All possible reactive SOMs groups, including carboxylate, phosphate, quinone species and ammonium groups were taken into account. FPMD results showed that all these groups are important to the complexation of SOMs on clay surfaces under dry conditions but only chemical bonding of carboxylate and phosphate and cation bridging are of significance under wet conditions. pH dependence of carboxylate and phosphate and Eh-pH dependence of quinone species are presented. CMD simulations showed that SOMs bound on clay surfaces via direct bonding of carboxylate and Ca2+ bridging and revealed the effect of water on structures of the clay-SOMs association. Based on the computational results, a procedure was proposed for constructing realistic molecular models for soils. Possible applications and further improvements of these models are discussed. (C) 2019 Elsevier Ltd. All rights reserved.