RETENTION AND RELEASE OF LEAD BY A VERY FINE SANDY LOAM .1. ISOTHERM MODELING

The retention of lead by a very fine sandy loam was investigated. Aqueous lead concentrations between 10 and 1000 mg/L and soil concentrations ranging from 10 to 167 g/L were used. Lead retention by the soil was a strong function of pH. The width of the pH-adsorption edge decreased with increasing lead concentration. Experimental results were modeled using the Langmuir, Freundlich, and BET isotherms. Only the Langmuir and Freundlich isotherms successfully represented the experimental results. The role of surface precipitation was assumed to be small because of the failure of the BET isotherm to adequately predict metal retention. The Freundlich isotherm provided the best fit because a maximum surface concentration was usually not observed. Langmuir and Freundlich isotherms parameters varied in a way that suggested that the average binding energy and the distribution of bond strengths increased with increasing pH. The isotherm expressions determined in this study can be used as source-sink terms in the generalized mass transport model.