Comparison of Two Methods for the Determination of Stability Constants for Metal-NOM Interactions

Authors : Çetin KANTAR

Pages : 297-308

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Publication Date : 0000-00-00

Article Type : Research Paper

Abstract :The inherent polyfunctional and heterogeneous nature of natural organic matter insert ignore into journalissuearticles values(NOM); has made the modeling of metal complexation reactions difficult. Historically, a number of modeling approaches have been developed to describe metal complexation by NOM. These models can be grouped into two: chemical and non-chemical. In the present study, we compare a non-chemical model insert ignore into journalissuearticles values(Schubert`s method); to a chemical model insert ignore into journalissuearticles values(discrete ligand, non-electrostatic approach); to determine the stability constants of metal/NOM complexes. Our analyses of Co/fulvic acid complexation data using Schubert`s equation result in an apparent nonintegral number of ligands binding the Co2+ ion. The model fit was improved assuming a mixture of 1:1/1:2 Coinsert ignore into journalissuearticles values(II);-ligand complexes. Schubert`s method can be used as an effective tool to provide information on reaction stoichiometries and average stability constants over the whole NOM. However, the binding of metals by NOM occurs on specific sites, mostly associated with carboxylic groups. In the discrete ligand approach, NOM is conceptualized as being composed of a suite of monoprotic acids, HLi, of arbitrarily assigned pKa insert ignore into journalissuearticles values(i); values insert ignore into journalissuearticles values(e.g., 4, 6, 8 and 10);. Although the discrete ligand approach is more complex and requires more fitting parameters insert ignore into journalissuearticles values(i.e. usually more than 5); compared to Schubert`s approach, it provides a means of capturing the complexation behavior of metals with specific sites in a framework suitable for use in equilibrium speciation models under variable chemical conditions. In addition, all of the potential mononuclear and polynuclear metal and organic species can be considered within the framework of the chemical model.
Keywords : Schubert`s method, Stability constants, Chemical model, Complexation, NOM