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

Abstract

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.

Keywords: ISO 11466; ISO 12914; Metals; Pollutant; Reference material; Sewage sludge; Soil contamination.

Fig. 1

Results of the t-test (α = 0.01) for the total content determined using microwave digestion/spectroscopy (tc₁) and k₀-NAA (tc₂), aqua regia extractable content determined with ISO 11466 (ar₁) and ISO 12914 (ar₂) and combined data for total content (tc) and aqua regia (ar).

Fig. 2

Mean values for each participating laboratory (◆) with the reported uncertainty budget as error bar (|). a) total content using microwave digestion combined to spectroscopic determination (tc₁); b) aqua regia extractable content according to ISO 11466 (ar₁); c) aqua regia extractable content according to ISO 12914 (ar₂); d) total content on solid samples using k₀-NAA (tc₂).