Use of X-ray absorption near edge structure (XANES) to identify physisorption and chemisorption of phosphate onto ferrihydrite-modified diatomite

[Display omitted] ► Chemisorption occurs together with physisorption during phosphorus adsorption onto ferrihydrite-modified diatomite. ► Chemisorption occurs in the bulk of ferrihydrite-modified diatomite (between 50nm and 5μm). ► Phosphorus physisorption occurs at the zone of ferrihydrite-modified diatomite shallower than 50nm. 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 (K2HPO4 and FePO4·2H2O) 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 K2HPO4 and FHMD-Ps and the marked difference between the spectra of FHMD-Ps and FePO4·2H2O 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 FePO4·2H2O 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 50nm to 5μm); whereas physisorption occurred in the zone of FHMD shallower than 50nm since the probing depth of XANES P K-edge method is 5μm and that of P L-edge and Fe M-edge methods is 50nm.