Quantum chemical adsorption studies on the (110) surface of the mineral goethite

The interaction between the (110) goethite surface and water, acetic acid, acetate, 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4-dichlorophenoxyacetate (2,4-D^-), and benzene was studied by means of quantum mechanical calculations based on density functional theory (DFT) using the B3LYP approach. Furthermore, Møller -Plesset calculations to second order using the resolution-of-identity approach (RI-MP2) were also performed for the goethite/benzene interaction. Structural and energetic features of the surface complexes were evaluated using cluster models for the goethite surface derived from a periodic slab surface. Our investigations showed that the (110) goethite surface formed by three types of the hydroxyl groups offers a variety of possibilities for hydrogen bond formation with appropriate polar adsorbents. Calculated interaction energies for the water molecule on different sites are on the order of ca. -20 kcal/mol, a number that is in line with the number and type of hydrogen bonds formed. Comparison of the interaction energies for different interaction sites/clusters shows that these sites are energetically more or less equivalent. Slightly larger interaction energies were observed for acetic acid and 2,4-D in comparison with the goethite/water complexes. The deprotonated, anionic form of acetic acid and 2,4-D show even stronger interactions between -30 and -50 kcal/mol. The interaction with the nonpolar, aromatic benzene molecule is significantly weaker (estimated value is in the range of -5 to -9 kcal/mol) but still significant.