SOIL PARTITION-COEFFICIENTS FOR CD BY COLUMN DESORPTION AND COMPARISON TO BATCH ADSORPTION MEASUREMENTS

Desorption of metals from contaminated soils is the initial step in a transport pathway that may ultimately result in contamination of groundwater. Because partition coefficients can be obtained easily from batch adsorption measurements, it is desirable to utilize them if they provide a good estimate of the concentration of metal that can desorb from the soil. Batch adsorption experiments were conducted as a function of pH and metal concentration for a series of eastern United States acidic soils. For comparison purposes, column desorption studies were also carried out as a function of flow rate and metal loading. Eluant samples were collected at several elution volumes for each of several column flow rates. These column eluant data were treated by a double extrapolation technique to evaluate the partition coefficient obtained under desorption conditions. The apparent partition coefficient increased with increasing column velocity and duration of flow. For each flow rate, the results were extrapolated to that for zero time. These zero-time partition coefficients were then extrapolated to zero flow. The zero-time, zero-flow desorption results from the column data were compared to those for the batch adsorption equilibrium. The partition coefficient obtained for batch adsorption provided a good estimate of the value obtained for desorption of the cadmium contaminant in the dynamic system. The slope of the regression of column K-d versus batch K-d values was greater than 0.90.