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Thermodynamics of metal cation binding by a solid soil derived humic acid. 2. Binding of Mn(II), Co(NH3)6aq3+ and Hg(II).

Ghabbour EA, Shaker M, El-Toukhy A, Abid IM, Davies G

Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115-5000, USA. e.ghabbour@neu.edu

Metal binding is an important function of humic acids (HAs) in soils, sediments and waters. At pH 2.0, Mn(II) and Co(NH3)6aq3+ bind tightly in one step labeled A to a solid humic acid NHA isolated from a New Hampshire soil. Two consecutive steps are observed for Hg(II) binding. All the binding isotherms fit the Langmuir model in the temperature range 10.0-50.0 degrees C. Stoichiometric site capacities indicate predominant binding by charge-neutralizing HA carboxylate groups for Mn(II) and the second step A of Hg(II) binding. The binding affinity order in step A is Co(NH3)(6)3+>Hg(II)>Mn(II). Metal binding enthalpy and entropy changes fit the linear correlation found previously for binding of other metal cations by solid HAs. Free energy buffering from cooperative enthalpy and entropy changes and lower enthalpies for metal-HA interactions in solution suggest that desolvation of the cations and HA binding sites as well as HA conformational changes to allow for inner-sphere complexation predominate metal binding by hydrated solid HAs.

Published 10 July 2006 in Chemosphere, 64(5): 826-33.
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