Two types of modeling approaches are illustrated for describing inorganic contaminant adsorption in aqueous environments: (a) the component additivity approach and (b) the generalized composite approach. Each approach is applied to simulate Zn 2+ adsorption by a well-characterized sediment collected from an aquifer at Cape Cod, MA. Zn 2+ adsorption by the sediment was studied in laboratory batch experiments with a range of pH and Zn(II) concentrations selected to encompass conditions observed in the aquifer. In the generalized composite approach, one-and two-site surface complexation model parameters were calibrated with the experimental data using FITEQL. The pH dependence of Zn 2+ adsorption was simulated without explicit representation of electrostatic energy terms. Surface acidity constants and ion pair formation by major electrolyte ions were also not required in the model, thereby minimizing the number of fitted parameters. Predictions of Zn 2+ adsorption with the component additivity modeling approach did not simulate the experimental data adequately without manipulation of surface area or site density parameter values. To apply the component additivity approach to environmental sorbents, further research is needed to better characterize the composition of sediment surface coatings. The generalized composite modeling approach requires less information and can be viewed as more practical for application within solute transport models. With only three adjustable parameters, this approach could simulate Zn 2+ adsorption over a range of chemical conditions that would cause several orders of magnitude variation in the distribution coefficient (K d ) for Zn 2+ within the aquifer.
Abstract. Land disposal of sewage effluent resulted in contamination of a sand and gravel aquifer (Cape Cod, Massachusetts) with zinc (Zn). The distribution of Zn was controlled by pH-dependent adsorption; the Zn extended 15 rn into the 30-m-thick sewage plume within approximately 100 rn of the source but only 2-4 rn into the plume between 100 and 400 rn downgradient. A two-dimensional vertical cross section model coupling groundwater flow with solute transport and equilibrium adsorption is 'used to simulate the influence of pH on Zn transport. Adsorption is described using semiempirical surface complexation models (SCM) by writing chemical reactions between dissolved Zn and mineral surface sites. SCM parameters were determined in independent laboratory experiments. A 59-year simulation with a one-site SCM describes the influence of pH on Zn transport well, with greater mobility at the low pH values near the upper sewage plume boundary than at the higher pH values deeper in the sewage-contaminated zone. Simulation with a two-site SCM describes both the sharpness and approximate location of the leading edge of the Zn-contaminated region. Temporal variations in pH of incoming groundwater can result in large increases in Zn concentration and mobility. The influence of spatial and temporal variability in pH on adsorption and transport of Zn was accomplished much more easily with the semiempirical SCM approach than could be achieved with distribution coefficients or adsorption isotherms.
It is hypothesized that stronger adsorption of Pb 2+ to the aquifer sediments causes the Pb-EDTA complex to disassociate to a greater degree than the Cu-EDTA complex. The mass of dissolved Zn-EDTA increased during the first 175 days of the tracer test to 140% of the mass injected, with the increase due to desorption of sewage-derived Zn. Dissolved Ni-EDTA mass remained nearly constant throughout the tracer test, apparently only participating in reversible adsorption reactions. The results of the field experiment provide a chemically complex data set that can be used in the testing of reactive transport models of flow coupled with chemical reactions.
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