Iron(III) fate after complexation with soil organic matter in fine silt and clay fractions: An EXAFS spectroscopic approach

Iron (Fe) speciation in soils is highly dependent on environmental conditions, mineralogy, and chemical interactions withsoil organicmatter(SOM).Thefine silt andclay(FSi +Cl)particlesize fraction of soils constitutes a primary organo-mineral fraction and contains SOM with long turnover time. In this study,the FSi+Cl particle size fractions isolated from a coniferous forest,a grassland,a technosol,and an agricultural soil were reacted with Fe(III) at pH7. Unreacted and reacted samples were then investigated by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. Statistical methods were used to determine goodness-of-fit parameters for linear combination fitting (LCF) and wavelettransformation (WT) of the FeK-edge EXAFS data. WT separated spectral contributions from different backscattering atoms in higher coordination shells located at simila rinteratomic distances from the centra labsorbing Fe atom.LCF results paired with WT showed thatt he FSi + Cl particle size fractions consisted of a mixture of Fe phyllosilicates, Fe (hydr)oxides, and organically complexed Fe in different proportions.Our research revealed that afters orption experiments,in which Fe(III) was added to the system, increasing amounts of Fe(III)-SOM complexes were found in the solid phase of grassland and agricultural soils, whereas the precipitation of Fe(III) led to the preferential formation of ferrihydrite in th econiferous forest soil and in the technosol.Although the quantitative Fe-mediated organic carbon stabilization effect after Fe(III) addition is shown int this work, Fe speciation is not clearly related to SOM amount or quality (i.e.,carbon-to-nitrogenratio).The variation of Fe chemical speciation among the soil fractions likely translates into differences in their environmental fate.