M7 AM Metals in the Environment: Chemistry and Fate Issues
Monday, 14 November 2005: 8:00 AM - 11:40 AM in 327-329

(CAR-1117-853756) Investigation of metal-ligand interactions of model ligands for modeling metal-organic carbon binding.

Di Toro, D¹, Carbonaro, R², ¹ University of Delaware, Newark, DE, USA² Manhattan College, Riverdale, NY, USA

ABSTRACT- The Windermere Humic Aqueous Model version V (WHAM V) formulated by Tipping and coworkers is central to the prediction of toxicity of metals in sediments. It is used to compute the concentration of metal on organic carbon that is in equilibrium with the biotic ligand concentration. The WHAM formulation is based on the idea that there is a universal constant, K_MHA, that relates the strength of metal binding to the strength of proton binding. K_MHA represents the equilibrium constant for the following reaction: HL + M²⁺ = ML⁺ + H⁺ where L represents a metal binding site on humic or fulvic acid. We have been investigating QSARs which will allow for the calculation of K_MHA values for which a constant is not currently available. Correlation of K_MHA to the proton-metal exchange constant for lactic acid as suggested by Tipping (1992) and to acetic acid yields reasonable relationships with the exception of the most strongly bound cation (Th⁴⁺). An alternate approach involves correlating the log K_ML (metal-ligand formation constant) versus ligand pKa for a series of monodentate ligands containing oxygen donor atoms. Slopes from linear regression are metal specific, and a correlation of this slope with the log K_MHA for a series of metals is highly linear. This approach is especially appealing because it avoids the use of a single ligand, e.g. lactate or acetate, but rather relies on the global behavior of the metal-proton replacement for a variety of ligands.

Key words: metals, QSAR, organic matter, complexation