Web Release Date: February 5,
Lead Sorption onto Ferrihydrite. 2. Surface Complexation Modeling
and
Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19717, and DuPont Engineering Technology, Brandywine Building, Wilmington, Delaware 19898
Received for review May 16, 2002
Revised manuscript received December 6, 2002
Accepted December 13, 2002
Abstract:
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, (
FeO)2Pb/FeOHPb2+ and (FeO)2Pb/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 for the species pair, (FeO)2Pb/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.
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