Determination Of A Claywater Distribution Coefficient For Copper

Purpose. To determine the distribution coefficient of a metal on a characterized soil.

Background. Perhaps the most important fate and transport parameter is the distribution coefficient, Kd. The distribution coefficient is a measure of adsorption phenomena between the aqueous and solid phases and is fundamental to understanding processes responsible for the distribution of pollutants in aquatic systems. For applications of the distribution coefficient to fate and transport modeling of groundwater, lakes, and riverine systems, refer to the modeling Chapters 5, 6, and 8. Mathematically, it can be represented as the ratio of the equilibrium pollutant concentration in the solid (sediment or soil) phase to the equilibrium pollutant concentration in the dissolved (aqueous) phase:

The purpose of the distribution coefficient is to quantify the pollutant's relative preferences for the two phases (solid and aqueous) and thus to determine the mass of pollutant present in each phase. The distribution coefficient is used in virtually every fate and transport model for the estimation of pollutant concentrations in aqueous systems. The aqueous phase concentration is important because the free aqueous phase concentration is usually the most toxic form of pollutants, especially in the case of dissolved metal cations. Inorganic and organic colloids and suspended solids in natural waters will increase the apparent water phase concentration, but pollutants adsorbed to these particles are usually not available for biological uptake. These particles can eventually settle out in quiescent regions of the natural water body or in estuaries and remove sufficient amounts of pollutant from the aquatic system.

Distribution coefficients are relatively easy to determine by allowing a pollutant-soil-water mixture of known composition to equilibrate, separating the mixture into solid and aqueous phases, and determining the pollutant concentration in each phase. This technique can be simplified by measuring (or knowing) the total mass of pollutant added to each sample (as determined from a blank sample), measuring the pollutant in the dissolved phase after equilibration, and estimating the mass of pollutant on the solid phase by difference (total mass of pollutant in blank minus aqueous phase mass). The distribution coefficient is then calculated using the equation given above.

The major problem with designing Kd experiments for the laboratory is the variability (and unpredictability) of results that are obtained given the variety of solid phases available, the nature of the pollutant used (ionic metals or hydrophobic organic compounds), and the experimental aqueous conditions used (pH values, ionic strengths, solids concentrations, and pollutant concentrations). Aqueous conditions are especially important when measuring the Kd for ionic pollutants. Without conducting the experiment under the exact experimental conditions to be used in lab, it is difficult to tell whether aqueous solutions will contain sufficient pollutant in the aqueous phase to be measured, or whether all of the pollutant will be present in the aqueous phase. Given these experimental design problems, it is not surprising that this vital experimental parameter (Kd) is not typically taught in environmental chemistry lab courses, but is usually covered in lecture material. This chapter contains a procedure, using standardized materials and conditions, for the determination of a distribution coefficient for copper. The procedure is also environmentally friendly since no (or limited) hazardous waste is generated.

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