SORPTION AND RELEASE OF CADMIUM-FULVIC ACID COMPLEXES IN SLUDGE TREATED SOILS

A batch experiment was conducted with ten sludge treated soils wherein soils were equilibrated with various amounts of Cd-115 tagged Cd-fulv ic acid (FA) for time intervals upto 72 h. The equilibrium models, inc luding the Langmuir, two-surface Langmuir and Freundlich models and ki netic models including irreversible zero, first, second, two rate cons tant equation, second approximate and Elovich models, were evaluated f or their ability to describe the retention of Cd-FA in sludge treated soils. The relationship between the amount of Cd-FA retained by soil a nd the concentration of Cd-FA in soil solution was described by either two-surface Langmuir or Freundlich models. The maximum sorption capac ity of Cd-FA in soils at two sites of reaction obtained from the two-s urface Langmuir adsorption isotherm equation was found to be positivel y correlated with the organic matter content and cation exchange capac ity of the soils. In general, none of the equations describing the ord er of reaction could explain the data over the entire time and concent ration ranges studied. The Elovich equation provided the best fit of t he data indicating, thereby, that the sorption of Cd-FA might be due t o complex reactions involving more than one site of reaction with diff erent rates and energetics. The desorption of Cd-FA sequentially throu gh 0.5 M Ca(NO3)(2) and 0.005 M KCl showed that a significant fraction of the Cd-FA complex retained by the soil was released to the solutio n but that some irreversible retention of Cd-FA occurred The possible mechanisms of retention of Cd-FA in soils are discussed. (C) 1998 Else vier Science Ltd. All rights reserved.