Stable carbon (12/13C) and nitrogen (14/15N) isotopes as a tool for identifying the sources of cyanide in wastes and contaminated soils—A method development Original Research Article

The occurrence of iron–cyanide complexes in the environment is of concern, since they are potentially hazardous. In order to determine the source of iron–cyanide complexes in contaminated soils and wastes, we developed a method based on the stable isotope ratios 13C/12C and 15N/14N of the complexed cyanide-ion (CN−). The method was tested on three pure chemicals and two industrials wastes: blast-furnace sludge (BFS) and gas-purifier waste (GPW). The iron–cyanide complexes were converted into the solid cupric ferrocyanide, Cu2[Fe(CN)6]·7H2O, followed by combustion and determination of the isotope-ratios by continuous flow isotope ratio mass spectrometry. Cupric ferrocyanide was obtained from the materials by (i) an alkaline extraction with 1 M NaOH and (ii) a distillate digestion. The [Fe(CN)6]4− of the alkaline extraction was precipitated after adding Cu2+. The CN− of the distillate digestion was at first complexed with Fe2+ under inert conditions and then precipitated after adding Cu2+. The δ13C-values obtained by the two methods differed slightly up to 1–3‰ for standards and BFS. The difference was larger for alkaline-extracted GPW (4–7‰), since non-cyanide C was co-extracted and co-precipitated. Therefore the distillate digestion technique is recommended when determining the C isotope ratios in samples rich in organic carbon. Since the δ13C-values of BFS are in the range of −30 to −24‰ and of −17 to −5‰ for GPW, carbon seems to be a suitable tracer for identifying the source of cyanide in both wastes. However, the δ15N-values overlapped for BFS and GPW, making nitrogen unsuitable as a tracer.