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

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.