Zinc and lead leaching from contaminated industrial waste sludges using coupled processes

Metal pollution of solid matrices, like soils, sediments and sludge, is widespread across the globe, and the clean-up of these matrices presents many difficulties. The aim of this study was to develop an efficient leaching technique for metal removal from an industrial carbon sludge contaminated with Zn (8600 mg kg(-1)) and Pb (389 mg kg(-1)). To this end, the possibility of coupling processes, such as extraction with mineral acids, EDTA addition, ultrasound and bioleaching, was investigated. Lead, but not zinc, was totally removed by EDTA treatment (1200 mg L(-1), pH 6.0). Ultrasound treatment was ineffective in metal leaching at all tested pH values, either with or without EDTA addition, probably because of the reduction in size of the carbon particles and the concomitant increase in surface area available for metal binding. A ferrous-iron-oxidizing, endogenous microflora lixiviated 90% of Zn at pH 2.5 in seven days, whereas, after the addition of Acidithiobacillus ferrooxidans, 100% of Zn was removed within four days. These results indicated that the clean-up process for the metal-contaminated sludge should combine an initial chelation step with Na-EDTA (pH 6.0) for complete removal of Pb, followed by a second bioleaching step with A. ferrooxidans for total removal of Zn.