CHEMICAL-STABILITY AND DECOMPOSITION RATE OF IRON CYANIDE COMPLEXES IN SOIL SOLUTIONS

In order to improve the assessment of the bioavailability and behavior of cyanide in the environment, the speciation of dissolved cyanide was studied under pH and redox conditions relevant to soil and groundwater environments. The partition of cyanide over free cyanide [HCN(aq) + CN-] and iron cyanide complexes [or hexacyanoferrates, e.g., Fe(CN)6(3-) and Fe(CN)6(4-)] at thermodynamic equilibrium was calculated as a function of pH and redox potential. These calculations show that the free cyanide form will predominate at chemical equilibrium in the soil. In groundwater from sites contaminated with cyanide, however, only complexed cyanide was found, indicating that the speciation of cyanide is determined not by chemical equilibrium but by decomposition kinetics. In daylight, iron cyanide complexes appeared to decompose rapidly (ca. 8%/h). In the dark, the rate of decomposition appeared to be much slower and was proportional to the fraction of hexacyanoferrate present as HFe(CN)6(3-) and H2Fe(CN)6(2-). The decomposition rates of both species were determined as a function of temperature and used to model decomposition kinetics. Good predictions were made of decomposition rates under various pH and redox conditions.