LEACHING OF IRON AND TOXIC HEAVY-METALS FROM ANAEROBICALLY-DIGESTED SEWAGE-SLUDGE

Heavy metal-loaded sewage sludge was leached abiotically using FeCl2 and FeCl3 which are applied in waste water treatment plants to eliminate phosphate and for coagulation. Due to the hydrolyzing nature of ferric iron, ferric chloride (100 mmol L(-1)) was able to solubilize more than 90% of copper and zinc and more than 80% of cadmium, with an optimal pulp density of 3% (w/v), after 10 h of exposition at 25 degrees C. Chromium, lead and nickel were solubilized to an extent of 40-70%. With the exception of copper (redoxolysis), all heavy metals monitored were leached following the principle of acidoiysis. Chemical leaching with iron resulted in a secondary contamination of sewage sludge (96 g iron per kg dry weight). The insoluble iron compounds which were precipitated or adsorbed to sludge hocks could be resolubilized with oxalic acid (100 mM, pH <3.3) up to an extent of 90%. Iron was leached by acidolysis and held in solution by complexation with oxalic acid. The pH optimum for the treatment of sewage sludge with 100 mmol L(-1) oxalic acid was similar to pH 3.3. At this pH an excessive solubilization of nutrient elements and compounds (phosphorus, nitrogen, alkaii and alkaii earth elements) could be avoided concomitantly leaching 75% iron. Furthermore the hydrophobicity of the sewage sludge was significantly reduced as a result of treatment with iron chloride. Thiobacillus ferrooxidans (isolated from arsenopyrite and adapted on sewage sludge) utilized ferrous iron as an energy source in the presence of chloride ions (FeCl2) as efficiently as ferrous sulphate. No toxic effects of oxalic acid on T. ferrooxidans were observed at the prevailing concentration.