title: Use of X-ray absorption near edge structure (XANES) to identify physisorption and chemisorption of phosphate onto ferrihydrite-modified diatomite.
abstract: This paper presents a novel technique integrating bulk-sensitive and surface-sensitive XANES methods to distinguish between physisorption and chemisorption for phosphate adsorption onto ferrihydrite-modified diatomite (FHMD). XANES P K-edge, L-edge, and Fe M-edge spectra were obtained for reference samples (K(2)HPO(4) and FePO(4)·2H(2)O) and test samples (phosphate adsorbed onto FHMD (FHMD-Ps) and Si-containing ferrihydrite (FHYD-Ps)). A resolvable pre-edge peak in the P K-edge spectra of FHMD-Ps and FHYD-Ps provided direct evidence for the formation of P-O-Fe(III) coordination and the occurrence of chemisorption. The resemblance between the P L-edge spectra of K(2)HPO(4) and FHMD-Ps and the marked difference between the spectra of FHMD-Ps and FePO(4)·2H(2)O indicated the intact existence of the adsorbate and the adsorbent. The similarity between Fe M-edge spectra of FHMD and FHMD-Ps and the difference between the spectra of FHMD-Ps and FePO(4)·2H(2)O confirmed the findings from P L-edge analyses. Therefore, chemisorption and physisorption coexisted during phosphate adsorption onto FHMD. Phosphate chemisorption occurred in the deeper zone of FHMD (from 50 nm to 5 μm); whereas physisorption occurred in the zone of FHMD shallower than 50 nm since the probing depth of XANES P K-edge method is 5 μm and that of P L-edge and Fe M-edge methods is 50 nm.
authors.affiliation: Department of Civil and Geological Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9. wenhui.xiong@usask.ca
authors.forename: Wenhui
authors.initials: W
authors.lastname: Xiong
authors.forename: Jian
authors.initials: J
authors.lastname: Peng
authors.forename: Yongfeng
authors.initials: Y
authors.lastname: Hu
mesh_terms: D000042:Absorption; D011232:Chemical Precipitation; D007692:Diatomaceous Earth / Q000737:chemistry* / Q000378:metabolism; D005290:Ferric Compounds / Q000737:chemistry* / Q000378:metabolism; D010710:Phosphates / Q000737:chemistry* / Q000378:metabolism; D013052:Spectrometry, X-Ray Emission; D056928:X-Ray Absorption Spectroscopy
chemical_list: D005290:Ferric Compounds; D010710:Phosphates; D007692:Diatomaceous Earth; C033787:diatomite; C092844:ferric oxyhydroxide