Optimized enrichment and purification of ferrocyanide for 13/12C and 15/14N isotope analysis of aqueous solutions

The occurrence of ferrocyanide, Fe(CN)64−, in aqueous environments is of concern, since it is potentially hazardous. For tracing the source of ferrocyanide in contaminated water we developed a method that relies on the determination of the stable isotope ratios of 13C/12C and 15N/14N of this complexed cyanide (CN) after precipitating it as cupric ferrocyanide, Cu2[Fe(CN)6]·7H2O. The precipitate was combusted and the isotope ratios were determined by continuous flow isotope ratio mass spectrometry.At first, ferrocyanide enrichment from synthetic water containing cyanide with known isotopic composition was studied by using six commercial anion-exchange resins. Five resins revealed a quick and complete sorption of ferrocyanide. A nearly quantitative desorption was achieved using NaCl solutions of 5 and 10% by weight for four resins. Subsequent determination of the δ13CCN and δ15NCN values of the ferrocyanide revealed that no significant isotope fractionation occurred during this procedure. These results were reproduced even in column experiments using larger water volumes.Potential interferences were also addressed. Sulfate in excess competes for exchange sites but can be precipitated as BaSO4 prior to ferrocyanide enrichment. Non-cyanide carbon compounds may co-precipitate with cupric ferrocyanide, thus possibly modifying the isotope ratios. However, neither dissolved inorganic carbon nor highly soluble organic compounds did interfere with the method. Poorly soluble organics like BTEX and PAH can be eliminated by passing the samples through appropriate adsorber resins in a prior step.The proposed method is an excellent way of precise determination of the stable cyanide–carbon and cyanide–nitrogen isotope ratios in ferrocyanide-containing aqueous samples, which was successfully applied to four contaminated groundwater samples since measured aqueous isotopes ratios match those of corresponding cyanide-bearing solid wastes.