Carbonate-Induced Structural Perturbation of Al Hydroxides

Abstract

The chemistry of Al transformation has been well documented, though little is known about the mechanisms of structural perturbation of Al precipitates by carbonates at a molecular level. The purpose of the present study was to investigate the structural perturbation of Al precipitates formed under the influence of carbonates. Initial carbonate/Al molar ratios (MRs) used were 0, 0.1, and 0.5 after aging for 32 days, then the samples were analyzed by X-ray absorption near edge structure spectroscopy (XANES), X-ray diffraction (XRD), Fourier-transform infrared absorption spectroscopy (FTIR), and chemical analysis. The XRD data were in accord with the FTIR results, which revealed that as the carbonate/Al MR was increased from 0 to 0.1, carbonate preferentially retarded the formation of gibbsite and had relatively little effect on the formation of bayerite. As the carbonate/Al MR was increased to 0.5, however, the crystallization of both gibbsite and bayerite was completely inhibited. The impact of carbonate on the nature of Al precipitates was also evident in the increase of adsorbed water and inorganic C contents with increasing carbonate/Al MR. The Al K- and L- edge XANES data provide the first evidence illustrating the change in the coordination number of Al from 6-fold to mixed 6- and 4-fold coordination in the structural network of short-range ordered (SRO) Al precipitates formed under the increasing perturbation of carbonate. The fluorescence yield spectra of the O K-edge show that the intensity of the peak at 534.5 eV assigned to σ* transitions of Al-O and O-H bonding decreased with increasing carbonate/Al MR. The XANES data, along with the evidence from XRD, FTIR, and chemical analysis showed clearly that carbonate caused the alteration of the coordination nature of the Al-O bonding through perturbation of the atomic bonding and structural configuration of Al hydroxides by complexation with Al in the SRO network of Al precipitates. The surface reactivity of an Al-O bond is related to its covalency and coordination geometry. The present findings were, therefore, of fundamental significance in understanding the low-temperature geochemistry of Al and its impacts on the transformation, transport, and fate of nutrients and pollutants in the ecosystem.

Coordination nature of aluminum (oxy)hydroxides formed under the influence of low molecular weight organic acids and a soil humic acid studied by X-ray absorption spectroscopy

“…In the absence of LMW organic acids gibbsite was formed, which is in agreement with other reported studies (Kwong and Huang, 1979b). Some other researchers (Hu et al, 2008;Zhang et al, 2009) have observed bayerite and/or pseudoboehmite in addition to gibbsite, which was attributed to differences in the preparation method, including such factors as the initial Al concentration, titration rate, pH, and rate of precipitation (Huang and Violante, 1986;Violante and Huang, 1993).…”

“…3), the absorption peak characteristics of crystalline Al (oxy)hydroxides virtually disappeared. From 3700 to 3300 cm À1 , a single broad peak was observed, which was attributed to the formation of noncrystalline Al precipitates (Zhang et al, 2009). The absorption peaks at 1409 and 1663 cm À1 for the citrate/Al MR = 0.1 system and the 1407 and 1653 cm À1 for malate/Al MR = 0.1 system were assigned to the c(COO À ) stretching vibration of free carboxylate groups and the c(COO À ) stretching vibration of coordinated carboxylate groups with Al (Liu et al, 2006;Pasilis and Pemberton, 2008 system, the peaks at 1612 and 1582 cm À1 were assigned to a c(C-C) ring vibration, the peak at 1477 cm À1 was the C-C ring bonds of the salicylate complexing with Al, the peak at 1384 cm À1 was assigned to c(COO À ) stretching vibration and the peak at 1267 cm À1 was assigned to C-O stretching of the phenolic OH group (Biber and Stumm, 1994).…”

“…The source of the aluminum reference compounds (gibbsite (c-Al(OH) 3 ), SRO Al(OH) 3 , pseudoboehmite (c-AlOOH), AlPO 4 , and bayerite (a-Al(OH) 3 ) used for this study have been reported previously (Hu et al, 2008;Zhang et al, 2009). All were verified by XRD.…”

“…Only their TEY spectra are shown as the FLY spectra are very similar to the TEY spectra. The Al K-edge spectra of the reference compounds used in this study were reported previously (Hu et al, 2008;Zhang et al, 2009). The reference compounds were selected to represent compounds with known Al coordination numbers (CNs) of 4 and 6, and mixed CNs of 4 and 6; and they were considered to be relevant to the study of the synthesized samples.…”

“…The effectiveness of organic ligands to perturb Al (oxy)hydroxide crystallization depends on their structure and functionality, as well as their concentration (Kwong and Huang, 1978;Martell and Motekaitis, 1989). Therefore, it is essential to investigate the effects of the types of organic acids and their amount on the coordination structure of Al precipitation products. In previous studies the Al K-and L-edge spectra were used to investigate the coordination nature of Al precipitation products in the presence of tannic acid and carbonate, at varying ligand/Al molar ratios (Hu et al, 2008;Zhang et al, 2009). Tannic acid is a large organic molecule (MW = 1701 g) containing carboxyl (COOH) groups like LMW organic acids and they resemble high-molecular mass humic substances such as humic acid that contain phenolic hydroxyl (OH) and ketonic (C@O) groups (Huang, 1995).…”