Chemical speciation and behaviour of cyanide in contaminated soils

Cyanide is present as a contaminant of the soil on several hundred (former) industrial sites in the Netherlands. The risk for the occurrence of adverse effects on human health and the environment strongly depends on the chemical form in which cyanide is present and on the behaviour of this cyanide in soils.The research reported in this thesis aimed to elucidate the predominant forms of cyanide in contaminated soils and the main processes which govern the behaviour of this cyanide. First an automated method of chemical analysis was developed which differentiates the two main forms of cyanide: free cyanide and hexacyanoferrate, and corrects for the presence of thiocyanate. Using this method it was established that the groundwater on former gasworks sites only contains hexacyanoferrate, although this form of cyanide is not thermodynamically stable. The decomposition kinetics of hexacyanoferrate to free cyanide were studied. In daylight complete decomposition appears to proceed within hours. In the dark it proceeds very slowly, with a half-life of hexacyanoferrate ranging from years to hundreds of years, depending on pH and redox potential. These findings were used to improve the separation of free cyanide, hexacyanoferrate and thiocyanate in chemical analysis.The behaviour of hexacyanoferrate in soils was found to be dominated by the precipitation and dissolution of Prussian blue, a mineral occurring in several chemical forms. The exact composition and the solubility product of this mineral were determined, which enabled calculation of hexacyanoferrate concentrations in equilibrium with Prussian blue as afunction of pH and redox potential. Calculations revealed that the solubility of Prussian blue ranges from completely soluble in soils with a pH>ca. 6.5 to slightly soluble in more acid soils. This causes a large difference between the mobility of cyanide in alkaline and in acid soils which was also observed in the field situation.