Systematic first-principles molecular dynamics (FPMD) simulations were carried out to study the structures, free energies, and acidity constants of UO22+ surface complexes on montmorillonite in order to elucidate the surface complexation mechanisms of the uranyl ion (UO22+) on clay mineral edges at the atomic scale. Four representative complexing sites were investigated, that is, Al(OH)(2) and AlOHSiO on the (010) surface and AlOHOa and SiOOa on the (110) surface. The results show that uranyl ions form bidentate complexes on these sites. All calculated binding free energies for these complexes are very similar. These bidentate complexes can be hydrolyzed, and their corresponding derived pK a values (around 5.0 and 9.0 for pK(a1), and pK(a2), respectively) indicate that UO2(OH)(+) and UO2(OH)(2) surface groups are the dominant surface species in the environmental pH range. The OH groups of UO2(OH)(2) surface complexes can act as complexing sites for subsequent metals. Additional simulations showed that such multinuclear adsorption is feasible and can be important at high pH. Furthermore, FPMD simulation results served as input parameters for an electrostatic thermodynamic surface complexation model (SCM) that adequately reproduced adsorption data from the literature. Overall, this study provides an improved understanding of UO(2)(2+ )complexation on clay mineral edge surfaces.