EVALUATION OF X-RAY PHOTOELECTRON SPECTROSCOPY FOR COLLOIDAL ADSORPTION STUDIES

There has been considerable interest in elucidating the mechanism by which cationic and anionic species are adsorbed by colloidal particulates in aquatic and soil environments. The adsorption of zinc and phosphate ions by amorphous hydrous ferric oxide (HFO) was chosen as a model system for evaluating the application of the surface specific technique, X-ray photoelectron spectroscopy (XPS), to colloidal adsorption.; Surface and bulk characterization of the amorphous HFO adsorbent were found to be consistent with a FeOOH-like structure. Curve-fitting of the 01s peak and Auger parameter data for the 01s and 0 KVV X-ray excited Auger peak proved useful for identifying surface iron oxides and hydroxides.; The adsorption of Zn on 1 x 10('-3) M HFO was studied as a function of solution pH and initial Zn concentrations. At pHs < 6.5 adsorption was found to involve submonolayer coverages of chemisorbed species. At higher pHs, for Zn concentrations (GREATERTHEQ) 5 x 10('-5) M, surface layers of physisorbed Zn(OH)(,2) formed before solubility product requirements for precipitation in solution were exceeded. Bulk solution and XPS results are consistent with a mechanism involving the adsorption of Zn followed by surface-mediated hydrolysis to form Zn(OH)(,2).; The adsorption of phosphate on HFO was studied as a function of solution pH. The XPS results were found to be consistent with the formation of a new Fe-P surface phase. The XPS-derived distribution curves for the various adsorbed species (FePO(,4)H(,2),(' )FePO(,4)H('-) and Fe(,2)PO(,4)('-))(' )compared reasonably well with theoretical predictions of surface from the literature.; The adsorption of Zn on HFO in the presence of phosphate also was studied as a function of solution pH. The XPS results were found to be consistent with theoretical descriptions of ligand-like adsorption, involving the chemical adsorption of Zn on the new Fe-P surface phase and the adsorption of Zn-P solution complexes.; These XPS results suggest that the analysis of colloidal particulates in an ultrahigh vacuum analytical chamber is feasible. Although the XPS-derived distribution curves for Fe-P surface species may be influenced by the vacuum induced loss of H(,2)O corresponding to the condensation of monodentate surface species to bidenate surface species, there is no evidence that there were any major vacuum-induced surface modifications in the other XPS studies.