PHREEQEV: The incorporation of a version of Model V for organic complexation in aqueous solutions into the speciation code phreeqe

Abstract

Organic-inorganic phase interactions and the effect of organic material in solution on the transport properties of metal contaminants are of considerable interest to the waste disposal industry. However, the inclusion of organic material in inorganic aqueous speciation calculations is problematic because large organic molecules, such as fulvic and humic acids, contain a large number and range of potential binding sites, and it is difficult to define discrete equilibrium constants for each binding reaction that is taking place. In addition, any such equilibrium constant defined will differ with factors such as the electrostatic charge related to the degree of dissociation of the molecule and interference between sites. Modern approaches for modeling such organic molecules generally are combinations of either a discrete ligand or continuous distribution representation of the binding sites combined with electrostatic theory. A sophisticated, discrete-site electrostatic organic binding model, Model V, developed by Tipping and coworkers at the Institute of Freshwater Ecology (IFE) at Windermere, UK, has been implemented into the established inorganic speciation code, PHREEQE. The cross-verification of the new code, PHREEQEV, with a code developed independently at IFE is presented.