Proton binding by hydrous ferric oxide and aluminum oxide surfaces interpreted using fully optimized continuous pK(a) spectra

A modified regularized least squares pK(a) spectrum approach is proposed to determine proton stability constants and concentrations for binding sites on hydrous ferric oxide (HFO) and aluminum oxide surfaces. Acid-base titrat ion data are fit to a continuous binding site model for the system represen ted as a pK(a) spectrum. The modified parameter fitting method optimizes si multaneously for both smoothness of the pK(a) spectrum and goodness-of-fit, whereas other methods optimize for goodness-of-fit given a fixed smoothnes s factor. The modified method is tested with aluminum oxide and recovers va lues consistent with theoretical values. The regularized pK(a) spectrum met hod optimized for smoothness is applied to prepared samples of two types of HFO. The prepared HFO samples differ only in the total iron concentration of the parent solution. The resultant pK(a) distributions are compared to p roton binding constants from MUSIC model results for crystalline iron oxide s. The types of binding sites in the HFO sample are consistent with theoret ical binding site stability constants for crystalline iron oxides. Overall, the prepared HFO samples have binding constants most consistent with value s for lepidocrocite and goethite.