X-ray Absorption Spectroscopy of Aqueous Aluminum-Organic Complexes

Abstract

Aqueous-phase X-ray absorption near-edge structure (XANES) spectra were collected on dissolved Al complexes with organic ligands, including desferrioxamine B, EDTA, acetohydroxamate, malate, oxalate, and salicylate. Spectral interpretations were made using the density functional theory-based modeling package StoBe. The goals of this work were to study the geometric and electronic structural characteristics of these complexes relative to Al(H(2)O)(6)(3+) and to examine the utility of the aqueous Al XANES technique as a tool for probing Al speciation and structure. In the case of EDTA, aqueous Fourier-transform infrared spectroscopy was also used to corroborate the structures of the Al(EDTA)(-) and AlOH(EDTA)(2-) complexes. Synthetic XANES spectra calculated with StoBe reproduced the observed spectral differences between Al(H(2)O)(6)(3+), Al(dfoB)(+), and Al(EDTA)(-). The narrower XANES feature observed for Al(dfoB)(+) relative to Al(H(2)O)(6)(3+) can be attributed to a weaker splitting of the Al 3p-O 2p interactions in the former, while Al(EDTA)(-) exhibits split Al 3p-ligand interactions that likely result from the mixed O/N coordination. In complexes with mixed aqua/organic-oxygen ligation (Al-acetohydroxamate, Al-malate, Al-oxalate, and Al-salicylate), spectra exhibit linear, systematic changes in peak width as a function of H(2)O to organic ligand ratio in the Al coordination sphere. These results highlight the sensitivity of the aqueous Al K-edge XANES spectrum to coordination environment and demonstrate its utility as an experimental probe for future studies of Al speciation in complex solutions.

Advances in Using Soft X-Ray Spectroscopy for Measurement of Soil Biogeochemical Processes

“…Many reviews for this type of liquid cell identify the limitations for use in environmental studies; they are not adapted to perform XAS, they are limited to the range of solutions that can be studied, and they are not able to study large organic molecules (Winter, 2009;Winter et al, 2008;Winter and Faubel, 2006). More recently, static cells have emerged that allow for the sample to be isolated from the vacuum environment (Hay and Myneni, 2010). The main advantage of this type of apparatus is the ability to use a variety of samples and solvents, since vapor pressure is no longer of concern.…”

Theoretical Studies of the Formation Mechanisms, Thermodynamic Stabilities, and Water-Exchange Reactivities of Aluminum-Salicylate Complexes in Aqueous Solution

“…The ΔG 298,a ‡ for Φ-OH attacking the central Al 3+ from the axial and equatorial directions are 16. is accompanied by the phenol proton dissociation. 13 The proton dissociation pathway is not found in modeling the ring closure process, but the phenol proton is observed to move into adjacent solvent water molecules during the TS optimizations for the phenol deprotonation in Bidentate-II (see S3.3 in Supporting Information). Therefore, it is proposed that Bidentate-III can be formed from Bidentate-II by phenol proton dissociation.…”

Addressing Groundwater Fluoride Contamination Using Inexpensively Processed Bauxite

“…44 Other groups have used Nuclear Magnetic Resonance (NMR, 27Al and 19F) 45 to study the coordination geometry of aqueous fluoroaluminate complexes (AlF 4 -) or have relied on combined spectroscopic studies (e.g., XAS and IR) 46 to characterize the geometric and electronic structural characteristics of aqueous aluminum-organic complexes (e.g., Al-EDTA, Al-oxalate, AlCl 3 , etc.) 47 and determine the impact of varying pH and NOM on aluminum speciation in soils and streams. 46…”