Surface loading effects on orthophosphate surface complexation at the goethite/water interface as examined by extended X-ray Absorption Fine Structure (EXAFS) spectroscopy

To investigate the effect of P surface loading on the structure of surface complexes formed at the goethite/water interface, goethite was reacted with orthophosphate at P concentrations of 0.1, 0.2, and 0.8mmol L^-1 at pH 4.5 for 5days. The P concentrations were chosen to ensure that P loadings at the surface would allow one to follow the transition between adsorption and surface precipitation. Extended X-ray Absorption Fine Structure (EXAFS) spectra were collected in fluorescence mode at the P K-edge at 2150eV. The structural parameters were obtained through the fits of the sorption data to single and multiple scattering paths using Artemis. EXAFS analysis revealed a continuum among the different surface complexes, with bidentate mononuclear (²E), bidentate binuclear (²C) and monodentate mononuclear (¹V) surface complexes forming at the goethite/water interface under the studied conditions. The distances for P-O (1.51-1.53Å) and P-Fe (3.2-3.3Å for bidentate binuclear and around 3.6Å for mononuclear surface complexes) shells observed in our study were consistent with distances obtained via other spectroscopic techniques. The shortest P-Fe distance of 2.83-2.87Å was indicative of a bidentate mononuclear bonding configuration. The coexistence of different surface complexes or the predominance of one sorption mechanism over others was directly related to surface loading.