Immobilization of lead and zinc in scrubber residues from MSW combustion using soluble phosphates

Abstract

The immobilization of MSWI-scrubber residues with soluble PO 4 3− was studied and compared to the immobilization using cement. The DIN 38414-S4 leaching protocol and pH dependent leaching were used to evaluate the leaching of Pb and Zn. Four different scrubber residues from MSW combustion (Pb concentration: 2.8–4.8 mg/g; Zn concentration: 3.0–12.3 mg/g) were mixed with water and cement or Na 2HPO 4 as source of soluble PO 4 3− at dosages of at least 0, 0.1, 0.2, 0.3 and 0.4 g per g residue. With cement as well as with PO 4 3− a reduction in Pb and Zn leaching was observed. With 0.4 g cement per g residue, the Pb leaching was reduced by a factor ranging from 70 to 100, but still exceeded the Pb landfill limit of 2 mg/l. With PO 4 3− the Pb leaching was reduced with a factor of 100–300 to below 2 mg/l. The Zn landfill limit (10 mg/l) was only exceeded by one untreated residue. Adding 0.2 g cement or 0.1 g PO 4 3− per g of that residue was enough to reduce leaching below 10 mg/l. However, when 0.6 g Na 2HPO 4 per g residue was added to a lime based scrubber residue, an increase in Zn leaching up to 12.5 mg/l was observed due to an increase in pH of up to 13.0. When using NaH 2PO 4 and H 3PO 4 no such increase in Zn leaching was observed. pH dependent leaching performed on one of the four residues showed that in the pH range of 2.5–6, Pb leaching was 100–50 times lower with Na 2HPO 4 treatment than with cement. In the pH range from 7–11, almost equal results were obtained for cement treated and Na 2HPO 4 treated residue. Above a pH of 12, Pb leaching was three times lower for the PO 4 3−-treated residue than for the cement treated residue. With soluble PO 4 3−, Pb leaching below 2 mg/l could be attained at a dosage of 0.27 g PO 4 3−/g residue. With cement, Pb leaching was never below 2 mg/l.