Transport of complexed cyanide in soil

Abstract

Contamination of soil with cyanide, generally in the form of iron cyanide complexes [Fe(CN) 6 3− and Fe(CN) 6 4−], is commonly found at several types of industrial sites. The risks for human health or the environment posed by such sites are largely determined by the chemical behaviour and transport of complexed cyanide in soil. In acidic soil this behaviour is probably dominated by equilibrium with Prussian blue, Fe 4(Fe(CN) 6) 3(s), which is sparingly soluble under acidic conditions and limits dissolved cyanide concentrations and mobility. However, at pH levels higher than about 7 the solubility of this precipitate is extremely high, which allows cyanide to be very mobile under such conditions. Nevertheless, according to field observations, Prussian blue appears to persist for decades in alkaline soils. In this paper multicomponent transport calculations, including equilibrium with Prussian blue, iron hydroxide and relevant redox reactions, are compared with the situation as observed in two contaminated soil profiles. In case of an acidic soil, there appears to be good agreement between the calculations and the field situation. Here the mobility of cyanide is mainly regulated by the pH and redox potential and is relatively low. In case of an alkaline soil, precipitation of Prussian blue will not take place and cyanide will be very mobile once leached from the initial waste material. In this case the dissolution rate of Prussian blue is the process which determines cyanide concentrations in the groundwater. The calculations show that this dissolution rate is determined by the acid neutralizing capacity of the solution infiltrating in the waste material. The buffer capacity depends on the soil composition and can easily be so low that complete dissolution of Prussian blue may take decades. This very likely explains the persistence of this material in alkaline soils.