Lead sorpion onto ferrihydrite. 2. Surface complexation modeling.

Few studies have combined molecular- and macroscopic-scale investigations with surface complexation model (SCM) development to predict trace metal speciation and partitioning in aqueous systems over a broad range of conditions. In this work, an extensive collection of new macroscopic and spectroscopic data was used to assess the ability of the modified triple-layer model (TLM) to predict single-solute lead(II [Pb(II)] sorption onto 2-line ferrihydrite in NaNO3 solutions as a function of pH, ionic strength, and concentration. Regression of constant-pH isotherm data together with potentiometric titration and pH edge data was a much more rigorous test of the TLM than fitting pH edge data alone. When combined with spectroscopic data, the choices of feasible surface species/site types were limited to a few. In agreement with the spectroscopic data, very good fits of the isotherm data were obtained with a two-species, one-site model using the bidentate-mononuclear/monodentate-mononuclear species pairs, ((is equivalent)FeO)2Pb/(is equivalent)FeOHPb2+ and ((is equivalent)FeO)2Pb/ (is equivalent)FeOPb+-NO3-. Regressing edge data in the absence of isotherm and spectroscopic data resulted in a fair number of surface-species/site-type combinations that provided acceptable fits of the edge data but unacceptable fits of the isotherm data. Surprisingly, best-fit equilibrium "constants" for the Pb(II) surface complexes required adjustment outside the pH range of 4.5-5.5 in order to fit the isotherm data. In addition, a surface activity term was needed to reduce the ionic strength dependence of sorption forthe species pair, ((is equivalent)FeO)2Pb/(is equivalent)FeOHPb2+. In light of this, the ability of existing SCMs to predict Pb(II) sorption onto 2-line ferrihydrite over a wide range of conditions seems questionable. While many advances have been made over the past decade, much work still needs to be done in fine-tuning the thermodynamic framework and databases for the SCMs.