Binding of Iron(III) to Organic Soils:  EXAFS Spectroscopy and Chemical Equilibrium Modeling

Abstract

The complexation of iron(III) to soil organic matter is important for the binding of trace metals in natural environments because of competition effects. In this study, we used extended X-ray absorption fine structure (EXAFS) spectroscopy to characterize the binding mode for iron(III) in two soil samples from organic mor layers, one of which was also treated with iron(III). In most cases the EXAFS spectra had three significant contributions, inner-core Fe-O/N interactions at about 2.02(2) A, Fe-C interactions in the second scattering shell at 3.00(4) A, and a mean Fe-Fe distance at 3.37(3) A. One untreated sample showed features typical for iron (hydr)oxides; however, after treatment of iron(III) the EXAFS spectrum was dominated by organically complexed iron. The presence of a Fe-Fe distance in all samples showed that the major part of the organically complexed iron was hydrolyzed, most likely in a mixture of complexes with an inner core of (O5Fe)2O and (O5Fe)3O. These results were used to constrain a model for metal-humic complexation, the Stockholm Humic Model (SHM). The model was able to describe iron(III) binding verywell at low pH considering only one dimeric iron(III)-humic complex. The competition effect on trace metals was also well described.

Chromium(III) Complexation to Natural Organic Matter: Mechanisms and Modeling

“…with the carboxylic acid sites. 23 When the metal binds to two surface sites, a bidentate complex is formed. For aluminium(III), the reaction is described as follows:…”

“…28 This correction is done automatically in the Visual MINTEQ software. Bidentate complexes may involve both carboxylate and phenolate sites; the combinations of proton-binding sites used in S3 the SHM were detailed in Gustafsson et al 23 A bidentate complex similar to that for aluminium(III) was considered also for chromium(III), see Table S2.…”

“…Also, the combinations of proton-binding sites used in the SHM for tridentate complexes were detailed in Gustafsson et al 23 Note that the assignment of bidentate and tridentate complexes in the SHM does not necessarily mean that the structure of the complexes are in reality bidentate or tridentate. This is because the modeling results are sensitive to the proton exchange stoichiometry but are usually not much dependent on the actual number of ROH groups in equations 4 and 7. are from Gustafsson et al 33 , and those for Cr(OH) 2 + are from this study.…”

“…The trimeric complex with a single oxo group has not yet been observed in natural samples, while the corresponding iron(III) complexes have been reported occasionally. [23][24] The probable reason is that iron(III) is more easily hydrolyzed than chromium(III), pK a values of 2.5 and 3.5, respectively, and that trimeric complexes with a single oxo group require higher pH to form than the hydroxo complexes.…”

Silver(I) Binding Properties of Organic Soil Materials Are Different from Those of Isolated Humic Substances

“…generic values for proton and metal binding constants have been derived from calibration of these models on isolated fulvic and humic acids 21 . This approach has proven to be successful for a range of metal ions, such as Al 3+ , Fe 3+ , Cu 2+ , Zn 2+ and Cd 2+ , [22][23][24][25][26] but has not yet been tested for Ag + . Furthermore, there are just a few published data sets available for silver(I) binding by humic and fulvic acids.…”