Structure and reactivity of oxalate surface complexes on lepidocrocite derived from infrared spectroscopy, DFT-calculations, adsorption, dissolution and photochemical experiments

Abstract

Oxalate, together with other ligands, plays an important role in the dissolution of iron(hdyr)oxides and the bio-availability of iron. The formation and properties of oxalate surface complexes on lepidocrocite were studied with a combination of infrared spectroscopy (IR), density functional theory (DFT) calculations, dissolution, and photochemical experiments. IR spectra measured as a function of time, concentration, and pH (50-200 µM oxalate, pH 3-7) showed that several surface complexes are formed at different rates and in different proportions. Measured spectra could be separated into three contributions described by Gaussian line shapes, with frequencies that agreed well with the theoretical frequencies of three different surface complexes: an outer-sphere complex (OS), an inner-sphere monodentate mononuclear complex (MM), and a bidentate mononuclear complex (BM) involving one O atom from each carboxylate group. At pH 6, OS was formed at the highest rate. The contribution of BM increased with decreasing pH. In dissolution experiments, lepidocrocite was dissolved at rates proportional to the surface concentration of BM, rather than to the total adsorbed concentration. Under UV-light (365 nm), BM was photolyzed at a higher rate than MM and OS. Although the comparison of measured spectra with calculated frequencies cannot exclude additional possible structures, the combined results allowed the assignment of three main structures with different reactivities consistent with experiments. The results illustrate the importance of the surface speciation of adsorbed ligands in dissolution and photochemical reactions.

Carboxyl-richness controls organic carbon preservation during coprecipitation with iron (oxyhydr)oxides in the natural environment

“…Di-carboxylic But2/4 (succinic) and Hex2/6 (adipic), however, may adsorb via just one of their carboxyl groups, possibly due to steric constraint on formation of a bidentate mononuclear structure 54 , while tri-carboxylic Pro3/6 might adsorb via two carboxyl groups, although the exact adsorption configuration is unknown 56 . Both mono-and polycarboxylic acids may also form outer-sphere complexes (particularly at basic pH) 57,58 . Since our π* C=O carboxyl carbon peak shift is constant for the mono-(−0.1 ± 0.0 eV), di-(−0.2 ± 0.04 eV) and tri-carboxylic acids (−0.4 ± 0.0 eV) (Fig.…”

Factors modifying the structural configuration of oxyanions and organic acids adsorbed on iron (hydr)oxides in soils. A review

“…Kubicki et al (2017) studied the adsorption of oxalate on goethite and the mechanism of reductive dissolution using DFT calculations, and they showed that oxalate is most stable as a bidentate mononuclear complex. Borowski et al (2018) studied oxalate complexation on lepidocrocite using ATR-FTIR spectroscopy and DFT calculations by changing the pH and surface loading, and MM and BM complexes were identified. The increase in pH leads to a decrease in the content of the BM complex, and UV light causes photolysis of the BM complex compared to the MM or outer-sphere complex.…”

Fe(II)-Catalyzed Ligand-Controlled Dissolution of Iron(hydr)oxides

“…For oxalate, detailed DFT calculations allowed us to draw conclusions about the structures of these different surface complexes. 49 DFT calculations with the larger ligand EDTA in the presence of Fe(II) are computationally more demanding and were not attempted in this study.…”