Adsorption and complexation mechanisms of heavy metal uptake in activated sludge

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Abstract
Differences were observed between the uptake of cadmium, copper, manganese and nickel by activated sludge biomass: their removal was studied when a range of metal concentrations was added to resuspended mixed liquor solids in the absence of possibly competitive organic ligands. The predominant mechanism of metal removal was passive uptake with some indication of binding site specificity for copper and possibly manganese. The data obtained were interpreted in the form of adsorption isotherms. The results were used to calculate conditional stability constants (K′) of the complexes formed, and the complexation capacity of the biomass for heavy metal uptake. Studies were conducted over a range of sludge ages from 3 to 12 days. Log10K′ values between 4.6 and 6.7 were found. However, there was no direct relationship between sludge age and K′. EDTA was used to desorb bound metal: this provided a means of differentiating between the surface complexes and the unexchangeable metal such as that taken up intracellularly. There was a difference in the adsorption affinity series of the metals when EDTA was present. This indicated that more than one mechanism for metal uptake was operating, and that the relative importance of physical and chemical absorption varied with each metal.

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