Processes affecting the fate and transport of trace metals
in the environment are controlled by sorption to solid phases. Determining sorption
mechanisms in natural samples is difficult because of the heterogeneity of the
solid phase, as well as the various species present in the solution. Thus, to
better understand speciation in soils it is necessary to study simplified
systems in which primary interactions occurring in the environment can be
elucidated. In this project we are investigating the reaction mechanisms that
occur in ternary systems consisting of smectite minerals, organic acids, and
the trace metal Cu using batch studies and X-ray adsorption spectroscopy (XAS).
Variables in the batch studies included Cu concentration, pH and ionic
strength. X-ray adsorption spectroscopy was used to evaluate the molecular
environment of select samples from the batch studies. Results from the isotherm
and pH studies show decreased Cu sorption with all of the organic acids. The inhibited
adsorption was most pronounced at lower ionic strength. The greatest decrease
was observed using citric acid, while acetate appeared to have minimal effect.
Preliminary interpretation of XAS results shows a unique Cu complex in the
presence of citrate, while spectra from the Cu-acetate system are similar to Cu
sorption with no ligand present. Further analysis of the XAS data will provide
us with insights into how the various ligands impact adsorption mechanisms.
Mechanistic understanding of trace metal behavior in soils will allow for
improved models to be developed that explain processes regulating metal
availability for uptake and transport.