CADMIUM SORPTION ONTO TWO MINERAL SOILS

The sorption of low range concentrations of cadmium onto two mineral subsoils (< 0.5 mm fraction) of moderate pH was studied by means of laboratory experiments employing a model solute containing 10('-3)M CaCl(,2).;Sorption isotherms determined at pH 6.00 were slightly curvelinear, and the soil did not reach saturation in the concentration ranges investigated (soil contents at equilibrium were less than 6 (mu)g Cd/g and solute equilibrium concentrations were less than 50 (mu)g Cd/ ). A loamy soil exhibited higher cadmium sorption than a sandy soil. The sorption isotherms were expressed by both Langmuir and Freundlich equations.;The sorption capacity of the soils were significantly affected by pH (range: 4-7). The soil cadmium content increased approximately 3 times with every unit of pH increase. Increased calcium concentrations caused decreased cadmium sorption.;Desorption experiments showed that the sandy soil exhibited full reversibility with respect to sorption-desorption, while the loamy soil possessed a partial irreversibility of the order of 1 (mu)g Cd/g soil. However, this irreversibility was not found when the Ca('2+) concentration was increased to 10('-2)M or the pH was decreased to pH 4, indicating that the irreversibility observed was not due to a strong binding of cadmium. Increases in solute concentration of calcium, zinc, organic ligands (EDTA), or a decrease in pH caused significant release of previously sorbed cadmium.;From an environmental point of view sorption of cadmium onto soil is a rapid process. In this study more than 97 percent of the sorption took place within 15 minutes, and equilibrium was reached within 20 hours. No statistically significant long term changes in equilibrium concentrations were found in experiments running 67 weeks.;Aging of the soils in 10('-3)M CaCl(,2) at 1(DEGREES)C for 35 weeks had no effect on the sorption capacity or reversibility of the sorption process in the case of the sandy soil. The loamy soil showed a tendency of decreasing sorption capacity when aged for 35 and 67 weeks prior to exposure to cadmium.;Nine laboratory columns (length = 10 mm; diameter = 18.5 mm) intermittently receiving influent containing 10 or 20 (mu)g Cd/ were monitored with respect to cadmium concentrations in effluent and soil. A simple computer model employing Langmuir sorption parameters estimated from batch experiments resulted in a good prediction of experimentally determined column break-throughs. About 2600 mm of influent (10 (mu)g Cd/ ) percolated through the 10 mm deep columns before break-through corresponding to approximately 700 pore volumes. An unexpected pH-drift caused significant uncertainty at the end of the column break-throughs and limited the evaluation of the computer model.;The results of this study showed that the two soils investigated have an extensive capacity for sorption of free cationic cadmium; however, the sorption process is highly reversible and these soils will not act as a sink for cadmium applied to land. pH control appears to be a primary factor to insure attenuation of cadmium in soil environments.