XANES determination of adsorbed phosphate distribution between ferrihydrite and boehmite in mixtures
Journal
Soil Science Society of America Journal
Journal Volume
68
Journal Volume
68
Journal Issue
2
Journal Issue
2
Pages
460-469
Start Page
460
End Page
469
ISSN
03615995
Date Issued
2004
Author(s)
Abstract
Iron- and Al-(hydr)oxide minerals are important sorbents for retaining PO4 in soils. Our objective was to determine the distribution of adsorbed PO4 between ferribydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg-1 for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K-XANES (X-ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed-mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single-mineral system, indicating negligible mineral interactive effects on PO4 adsorption in the mixtures. X-ray absorption near edge structure spectra for PO4 adsorbed on ferrihydrite or in ferrihydrite/ boehmite mixtures showed a pre-edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre-edge region of XANES spectra for mixtures with average spectra for PO 4 adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO4 was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO4 in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO4 concentration (100 mmol kg-1 minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO4 kg-1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO4 kg-1).
SDGs
Publisher
Soil Science Society of America
Type
journal article