Complexation of quinone species on 2:1 dioctahedral phyllosilicate surfaces

Aiming at the complexation mechanisms of quinone species on clay minerals, we carried out systematic first-principles molecular dynamics simulations by taking 1,4-benzoquinone and its reduced form including semiquinone and hydroquinone as model molecules. The structures and desorption free energies of quinone complexes on both basal surface and edge surfaces ((010) and (110)) were investigated. The pKas of semiquinone and hydroquinone on edge surfaces were also calculated. Results show that semiquinone and hydroquinone can form inner-sphere complexes on edge surfaces, while benzoquinone cannot replace water ligand of edge Al. In contrast, the three quinone species did not form inner-sphere complexes on interlayer surfaces. The pH dependence of semiquinone and hydroquinone complexation was discussed based on the calculated pKas. Complexation of quinone groups onto clay minerals was weak compared with carboxylic groups in natural organic matters (NOM). The microscopic structures and thermodynamic data obtained in the present research can help understand the interactions between NOM and clay minerals in geological environment and provide primary information for developing molecular level clay mineral-NOM models.