Interfacial structures and acidity constants of goethite from first-principles molecular dynamics simulations

In this paper, we report a first-principles Molecular Dynamics (FPMD) study of interfacial structures and acidity constants of goethite. The pKa values of the groups on (010), (110), and (021) surfaces (space group Pbnm) are derived with the FPMD based vertical energy gap technique. The results indicate that major reactive groups include identical with Fe (sub 2) OH (sub 2) and identical with FeOH (sub 2) on (010), identical with FeOH (sub 2) , identical with Fe (sub 3) O (sub L) H, and identical with Fe (sub 3) O (sub U) H on (110), and identical with FeO (sub h) H (sub 2) and identical with Fe (sub 2) OH on (021). The interfacial structures were characterized in detail with a focus on the hydrogen bonding environment. With the calculated pKa values, the point of zero charges (PZCs) of the three surfaces are derived and the overall PZC range of goethite is found to be consistent with the experiment. We further discuss the potential applications of these results in future studies toward understanding the environmental processes of goethite.