title: Surface loading effects on orthophosphate surface complexation at the goethite/water interface as examined by extended X-ray Absorption Fine Structure (EXAFS) spectroscopy.
abstract: 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 0.8 mmol L(-1) at pH 4.5 for 5 days. 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 2150 eV. 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 ((2)E), bidentate binuclear ((2)C) and monodentate mononuclear ((1)V) 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.
authors.affiliation: Plant & Soil Sciences Department, University of Delaware, Newark, DE 19716, United States. Electronic address: daltonabdala@gmail.com.
authors.forename: Dalton Belchior
authors.initials: DB
authors.lastname: Abdala
authors.affiliation: Stony Brook University, National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973, United States.
authors.forename: Paul Andrew
authors.initials: PA
authors.lastname: Northrup
authors.affiliation: Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, IL 61801, United States.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Arai
authors.affiliation: Department of Plant & Soil Sciences and Delaware Environmental Institute, University of Delaware, Newark, DE 19716, United States.
authors.forename: Donald Lewis
authors.initials: DL
authors.lastname: Sparks
mesh_terms: D058085:Iron Compounds / Q000737:chemistry*; D008903:Minerals / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D013499:Surface Properties; D014867:Water / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy / Q000379:methods*
chemical_list: D058085:Iron Compounds; D008903:Minerals; D010710:Phosphates; D014867:Water; C094886:goethite
keywords: Phosphorus K-edge EXAFS; Phosphorus retention mechanisms; Phosphorus solid-state speciation; Phosphorus surface complexation