Heavy metal binding to anaerobic sludge

Heavy metal interaction with anaerobically digested sludge was investigated using copper, cadmium and nickel chloride salt solutions. The behaviour of the well-known glycine-metal aqueous system was compared with the behaviour of the sludge-metal system. When equilibrium pH values are presented against the initial metal concentration for the glycine-copper system the profile obtained shows a sharp peak. This behaviour is similar to the behaviour of the experimental sludge-copper system. The peak also appears for cadmium and nickel-glycine systems. However, it is not as pronounced as in the glycine-copper system and cannot be observed in the sludge-metal systems. The similar behaviour of both copper systems suggests that the metal binding mechanism of the sludge involves amino acid functional groups which are present in the cell wall. Due to this similarity, an apparent molarity of the sludge can be evaluated by sludge titration with copper (II). The value obtained can be expressed as mmols of equivalent glycine per gram of sludge.