DISSOLUTION BEHAVIOR OF IRON CYANIDE (PRUSSIAN BLUE) IN CONTAMINATED SOILS

The solubility of iron cyanide complexes [Fe(CN)63- and Fe(CN)64-] in soils was studied to elucidate the possible hazards posed by soils contaminated with iron cyanide. Earlier work had indicated that these complexes probably precipitate as a form of Prussian blue, a mineral containing Fe(CN)64- and iron. The composition and solubility product of this mineral were estimated from experiments in which it was precipitated from solutions containing Fe(CN)64- and iron chelated with EGTA, HEDTA, NTA, or citrate. The composition of the mineral was found to be Fe4(Fe(CN)6)3(s); its log K(sol) was estimated to be ca. -84.5. These results were used to model the dissolution and precipitation behavior of iron cyanide complexes in the soil. The model calculations do agree with experimental results and show that the solubility of Prussian blue strongly depends on the pH. Above a pH of ca. 6 it is very soluble, allowing high iron cyanide concentrations in groundwater or soil solutions. Below this pH iron cyanide concentrations are reduced by precipitation of Prussian blue. Above a pH of ca. 4 iron cyanide concentrations in groundwater may exceed the Dutch maximum tolerated level.