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Complexation of cadmium, copper and methyl mercury to functional groups in natural organic matter

studied by X-ray absorption spectroscopy and binding affinity experiments

Karlsson, Torbjörn (2005). Complexation of cadmium, copper and methyl mercury to functional groups in natural organic matter. Diss. (sammanfattning/summary) Umeå : Sveriges lantbruksuniv., Acta Universitatis agriculturae Sueciae, 1652-6880 ; 2005:117
ISBN 91-576-6916-3
[Doctoral thesis]



Due to various human activities contamination of soils, sediments and waters with trace metals like cadmium (Cd), copper (Cu) and mercury (Hg) are widespread. It is generally accepted that concentrations of specific metal species are of higher significance than total metal concentrations for the bioavailability and toxicity of trace metals. Thus, in order to make proper risk assessments of contaminated sites it is vital to know the speciation of the metals. Of special importance for the speciation are chemical interactions with colloidal and particle surfaces. The associations of trace metals with inorganic surfaces like minerals and metal oxyhydroxides are fairly well described, but less is known about metal associations with natural organic matter (NOM). In this thesis extended X-ray absorption fine structure (EXAFS) spectroscopy was used to determine the coordination chemistry of Cd and Cu in NOM and binding affinity experiments, using competitive complexation and ion selective electrodes (ISE), were used to determine the bonding strength of Cd and methyl mercury (CH3Hg) to functional groups in soil organic matter (SOM). Results from the EXAFS experiments revealed that reduced organic sulphur (S) ligands are involved in the complexation of Cd by NOM. In the first coordination shell, Cd is coordinated by 0.3–2.5 S atoms and 3.0–4.5 O/N atoms. A second shell of 1.7–6.0 C atoms was also found, providing direct evidence for inner-sphere complexation of Cd. Copper on the other hand is complexed to O and/or N containing functional groups. Four O/N atoms was found in the first coordination shell and analysis of the second shell showed that Cu form inner-sphere complexes consisting of one or two 5-membered chelate rings in NOM. Based on results from the EXAFS experiments, and earlier EXAFS studies on CH3Hg, simple models in which Cd2+ and CH3Hg+ associate to thiol (RSH) groups in SOM were successfully used to describe the bonding at native and low metal concentrations. Determined stability constants were in accordance with stability constants for the bonding of these metals to reduced S in well-defined organic compounds. At Cd concentrations ranging from 500–54000 µg g-1, experimental data was best described by a model consisting of one RSH group and one carboxylic group. Speciation calculations based on determined stability constants, in the concentration range 0.23–10000 µg Cd g-1, showed that Cd complexed by SOM dominates the speciation and that the percentage free Cd2+ is only a few % of total Cd at pH values above 4.0. In an organic soil with native concentration of CH3Hg (10 ng g-1), speciation calculations showed that CH3Hg complexed by SOM is the dominating CH3Hg form in the pH range 3–7. Concentrations of free CH3Hg+ and its inorganic complexes, assumed to be the most bioavailable forms, are extremely low in comparison to CH3Hg complexed by SOM.

Authors/Creators:Karlsson, Torbjörn
Title:Complexation of cadmium, copper and methyl mercury to functional groups in natural organic matter
Subtitle:studied by X-ray absorption spectroscopy and binding affinity experiments
Year of publishing :November 2005
Number of Pages:37
ALLI. Karlsson, T., Persson, P. and Skyllberg, U. 2005. Extended X-ray Absorption Fine Structure Spectroscopy Evidence for the Complexation of Cadmium by Reduced Sulfur Groups in Natural Organic Matter. Environmental Science and Technology 39, 3048-3055. II. Karlsson, T., Persson, P. and Skyllberg, U. 2005. Coordination Chemistry of Copper(II) in Organic Soils and in Dissolved Natural Organic Matter - Studied by X-ray Absorption Spectroscopy. Manuscript. III. Karlsson, T., Dalgren, K. E., Björn, E. and Skyllberg, U. 2005. Complexation of Cadmium to Functional Groups in Soil Organic Matter - Determined by Competitive Complexation and ISE Measurements. Manuscript. IV. Karlsson, T. and Skyllberg, U. 2003. Bonding of ppb Levels of Methyl Mercury to Reduced Sulfur Groups in Soil Organic Matter. Environmental Science and Technology 37, 4912-4918.
Place of Publication:Umeå
ISBN for printed version:91-576-6916-3
Publication Type:Doctoral thesis
Full Text Status:Public
Agris subject categories.:P Natural resources > P33 Soil chemistry and physics
Subjects:Not in use, please see Agris categories
Agrovoc terms:cadmium, copper, mercury, organic matter, x ray spectroscopy, complexing, environmental impact
Keywords:cadmium, copper, methyl mercury, natural organic matter, thiol groups, stability constants, speciation, coordination chemistry, X-ray absorption spectroscopy.
Permanent URL:
ID Code:978
Department:(S) > Institutionen för skogsekologi (930701-061231)
Deposited By: Torbjörn Karlsson
Deposited On:14 Nov 2005 00:00
Metadata Last Modified:02 Dec 2014 10:08

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