Comparison of methods to evaluate heavy metals in organic wastes

The conventional method for the digestion of residues using a mixture of nitric and perchloric acids was compared with the microwave procedure using both aqua regia and nitric acid as extractants, in order to determine which method fits better the routine measurements using inductively coupled plasma emission spectrometry (ICP-AES). Urban domestic wastes, swine and dairy manure and NIES pond sediment certified reference material were used as samples. The microwave digestions were performed on sealed vessels by taking 500 mg of each residue and aqua regia (16 mL) or concentrated nitric acid (10 mL). The conventional nitric-perchloric digestion was performed by heating 500 mg of each sample with 6 mt of HNO3-HClO4 5+1 v/v in Pyrex digestion tubes until complete digestion. All extracts were analysed for Fe, Mn, Zn, Cu, Pb, Ni, Cr and Cd by ICP-AES. Microwave extraction with aqua regia extracted higher amounts of metals from the certified pond sediment reference material than the USEPA-SW 846-method 3051 (nitric acid) and the conventional nitric-perchloric procedure, with a recovery greater than 90% for Fe, Mn, Zn, Cu, Pb and Cd; 76% for Ni and 61% for Cr. The extraction procedure with aqua regia method also presented statistically higher amounts of Fe, Mn, Zn, and Cr for the municipal waste compost, the sewage sludge and for the swine manure, when compared with the two other methods (Tukey test). The amounts of analytes extracted by the USEPA-Method 3051 and the conventional nitric-perchloric techniques were not consistent for all organic materials under investigation.