Fate of cadmium in activated sludge after changing its physico-chemical properties by thermal treatment

The effect of thermal treatment of activated sludge on cadmium uptake was investigated in respect with potential modifications of floc surface properties. Soluble fraction biochemical composition and floc size measurements evidenced floc destructuration and the release of (in)organic ligands in solution. Characterization of sludge samples by potentiometric titrations and IR spectra showed the transfer of functional groups from particulate to soluble fraction as well as the higher availability of phosphate groups originating from cell membrane phospholipids after thermal treatment. Batch biosorption tests demonstrated that cadmium uptake was highly affected by sludge modifications due to thermal treatment. For temperatures below 95 °C, floc size decrease allowed a better availability of binding sites, resulting in a higher sorption capacity. At temperatures above 95 °C, the effect of released soluble ligands and of the lower total number of surface functional groups limited cadmium uptake. Uptake mechanisms were also affected by sludge thermal treatment as surface complexation involving ion exchange tends to become predominant over precipitation.