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

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.