Predicting the Impact of Variable Chemistry On Reactive Transport of Metal Ions in Field Experiments Using Laboratory-Calibrated Adsorption Models.

Scientific understanding of the adsorption of metal- and metalloid-ions on purified synthetic and specimen minerals led to the development of surface complexation models (SCMs) whereby adsorption is described using a set of chemical reactions between dissolved species and functional groups at mineral surfaces.  The impact of variable chemical conditions on adsorption extent can then be described quantitatively by accounting for changes in aqueous chemical speciation and by stoichiometric relationships in the adsorption reactions.  State of the art reactive transport modeling involves coupling a chemical sub-model, which describes equilibrium and rate-limited aqueous speciation, precipitation-dissolution, oxidation-reduction, and ion exchange reactions, with a physical sub-model, which describes solute transport.  The impact of adsorption on transport can be described by incorporating SCMs into the chemical sub-model as long as the SCMs describing adsorption on the soil or sediment can be calibrated.

The structure and calibration of surface complexation models for reactive transport applications have been examined using field experiments conducted in an aquifer with variable chemical conditions.   Surface complexation models are defined and calibrated using laboratory experiments conducted with site-specific materials and then applied to predict results of field experiments and field-scale plume characterization studies.  Accounting for variable pH and dissolved salt concentrations on adsorption is examined using a series of experiments conducted with nickel, zinc, and lead.  Accounting for competitive adsorption is examined in experiments with phosphate and arsenate.  Incorporating surface complexation models into more complex reaction networks is examined in experiments involving denitrification coupled to iron oxidation and its impact on fate and transport of arsenic and other solutes.  These and studies conducted elsewhere demonstrate that surface complexation models provide a practical approach to quantifying the impact of adsorption on transport under variable chemical conditions, as typically observed in field applications.