SOLUBILITY OF CYANIDE IN CONTAMINATED SOILS

At more than 200 sites in the Netherlands, the soil is contaminated with cyanide as a result of industrial activities. The mobility of cyanide, which mainly occurs in the form of iron cyanide complexes [Fe(CN)63- + Fe(CN)64-], plays an important role in predicting the possible hazards for the environment and human health. Cyanide mobility was investigated by assessing data on cyanide concentrations in groundwater at contaminated sites, and by examination of three contaminated soils. High cyanide concentrations were found in the groundwater at sites with alkaline soils (pH ca. 7.5), whereas much lower concentrations were found in the groundwater with acidic soils (pH ca. 4). This agreed with the assumption that the behavior of cyanide in these contaminated soils is largely governed by the solubility of Prussian blue [Fe4(Fe(CN)6)3(s)], which is relatively insoluble under acidic conditions. In the acidic soils, the concentrations in the soil water appeared to be close to equilibrium with Prussian blue. In alkaline soils this precipitate is extremely soluble. The presence of this material here was possibly the result of slow dissolution kinetics. Precipitation of Prussian blue in acidic soils does not prevent dissolved iron cyanide concentrations from exceeding Dutch soil quality standards.