title: Chromium(III) complexation to natural organic matter: mechanisms and modeling.
abstract: Chromium is a common soil contaminant, and it often exists as chromium(III). However, limited information exists on the coordination chemistry and stability of chromium(III) complexes with natural organic matter (NOM). Here, the complexation of chromium(III) to mor layer material and to Suwannee River Fulvic Acid (SRFA) was investigated using EXAFS spectroscopy and batch experiments. The EXAFS results showed a predominance of monomeric chromium(III)-NOM complexes at low pH (<5), in which only Cr···C and Cr-O-C interactions were observed in the second coordination shell. At pH > 5 there were polynuclear chromium(III)-NOM complexes with Cr···Cr interactions at 2.98 Å and for SRFA also at 3.57 Å, indicating the presence of dimers (soil) and tetramers (SRFA). The complexation of chromium(III) to NOM was intermediate between that of iron(III) and aluminum(III). Chromium(III) complexation was slow at pH < 4: three months or longer were required to reach equilibrium. The results were used to constrain chromium-NOM complexation in the Stockholm Humic Model (SHM): a monomeric complex dominated at pH < 5, whereas a dimeric complex dominated at higher pH. The optimized constant for the monomeric chromium(III) complex was in between those of the iron(III) and aluminum(III) NOM complexes. Our study suggests that chromium(III)-NOM complexes are important for chromium speciation in many environments.
authors.affiliation: Department of Soil and Environment, Swedish University of Agricultural Sciences , Box 7014, 750 07 Uppsala, Sweden.
authors.forename: Jon Petter
authors.initials: JP
authors.lastname: Gustafsson
authors.forename: Ingmar
authors.initials: I
authors.lastname: Persson
authors.forename: Aidin Geranmayeh
authors.initials: AG
authors.lastname: Oromieh
authors.forename: Joris W J
authors.initials: JW
authors.lastname: van Schaik
authors.forename: Carin
authors.initials: C
authors.lastname: Sjöstedt
authors.forename: Dan Berggren
authors.initials: DB
authors.lastname: Kleja
mesh_terms: D000327:Adsorption; D000535:Aluminum / Q000737:chemistry; D001578:Benzopyrans / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D005290:Ferric Compounds / Q000737:chemistry; D006812:Humic Substances; D007700:Kinetics; D008956:Models, Chemical; D045483:Rivers / Q000737:chemistry; D012989:Soil Pollutants / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D001578:Benzopyrans; D005290:Ferric Compounds; D006812:Humic Substances; D012989:Soil Pollutants; D002857:Chromium; D000535:Aluminum; C005023:fulvic acid