Goethite surface reactivity: II. A microscopic site-density model that describes its surface area-normalized variability.

The model described in this investigation explains the variable macroscopic surface reactivity of different goethite preparations when adsorption data are normalized by surface area, especially the high reactivity of low specific surface area goethites. A simplified model of crystalline face distributions for each of the goethite preparations, in combination with experimental maximum chromate adsorption values previously determined, allowed a crystallographic site-density analysis that would explain the latter values. In addition, a surface complexation modeling approach was coupled to the previous model and provided individual affinity constants for proton and ion binding for singly, doubly, and triply coordinated surface sites. The proposed microscopic model is able to accurately describe the macroscopic adsorption behavior of protons, carbonate, chromate, and lead(II) ions on three goethites of specific surface areas of 50, 70, and 94 m(2)/g, using the same affinity constants. The model results indicate that the surface of high specific surface area nanoparticulate goethites may be described mostly as a combination of (1 0 1) and (0 0 1) faces, with reactive singly and triply coordinated surface oxygen sites; while the model for low specific surface area goethites requires, in addition to one of the above faces, a variable but high degree of (0 1 0)/(2 1 0) faces containing high surface densities of reactive singly and doubly coordinated oxygen groups. The model is potentially very useful and may be applied to any goethite provided its maximum ion adsorption capacity and proton charging behavior are known.