Adsorption and desorption of cadmium by synthetic and natural organo-clay complexes

Abstract

Tracer levels of 109Cd were used to study the adsorption and desorption of Cd by synthetic and natural organo—clay complexes. Synthetic organo—clay complexes were made by adsorbing humic acid extracted from soil to various forms of < 2 μm diameter montmorillonite (Na, Ca, Al, and Fe saturated and Ca-montmorillonite coated with Al or Fe hydroxide). Natural organo—clay complexes were fractionated from the clay fraction of a Captina silt loam by density-gradient centrifugation in a large-scale zonal rotor. To evaluate the influence of humic acid on adsorption of Cd, Cd was adsorbed to the various forms of montmorillonite before and after humic acid adsorption. No appreciable difference in Cd adsorption was noted except in the case where montmorillonite was coated with Al or Fe hydroxides. Cadmium was found to be strongly bonded to clays coated with Al or Fe hydroxides; however, Cd adsorption to these clays after humic acid adsorption was considerably less. Data indicated Cd and humic acid adsorption sites on Al or Fe coated clays were either identical or prior adsorption of humic acid simply covered available Cd sites. Cadmium adsorption to clay density fractions showed that greatest adsorption was to fractions containing high quantities of organic matter or sesquioxides. Desorption of Cd with 0.01 M Ca (NO 3) 2 showed that Cd was adsorbed more tenaciously to the sesquioxides than organo—clay fractions.