Comparison of total and aqua regia extractability of heavy metals in sewage sludge: The case study of a certified reference material

A number of different digestion methods, including aqua regia extraction following two ISO guides were used in an inter-laboratory comparison study. The results obtained showed comparable values for the total and aqua regia extractable content of As, Cu, Fe, Hg, Pb and Zn, while Cd, Co and Cr results were about 10% lower when aqua regia was employed. This small difference was covered by the between-laboratory relative standard deviation of the measurements; therefore in this study no difference in the extraction of the elements by the employed methods was found. The high organic matter content, together with low SiO2 and refractory aluminium and iron oxide amount as well as the small particle size of the sewage sludge material was reputed to have an effect on the extracting capacity of a weaker solvent such as aqua regia, bringing its results close to the total content ones.