In situ treatment of cyanide-contaminated groundwater by iron cyanide precipitation

Groundwater contamination with cyanide is common at many former or active i ndustrial sites. Metal-cyanide complexes typically dominate aqueous speciat ion of cyanide in groundwater systems, with iron-cyanide complexes often mo st abundant. Typically, metal-cyanide complexes behave as nonadsorbing solu tes in sand-gravel aquifer systems in the neutral pH range, rendering cyani de relatively mobile in groundwater systems. Groundwater pump-and-treat sys tems have often been used to manage cyanide contamination in groundwater. This study examined the feasibility of using in situ precipitation of iron cyanide in a reactive barrier to attenuate the movement of cyanide in groun dwater. Laboratory column experiments were performed in which cyanide solut ions were passed through mixtures of sand and elemental iron filings. Remov al of dissolved cyanide was evaluated in a variety of cyanide-containing in fluents under various flow rates and sand-to-iron weight ratios. Long-term column tests performed with various cyanide-containing influents under both oxic and anoxic conditions. at neutral pH and at flow rates typical of san d-gravel porous media, yielded effluent concentrations of total cyanide as low as 0.5 mg/L. Effluent cyanide concentrations achieved were close to the solubilities of Turnbull's blue-hydrous ferric oxide solid solutions, indi cating co-precipitation of the two solids. Maximum cyanide removal efficien cy was achieved with approximately 10% by weight of iron in the sand-iron m ixtures; higher iron contents did not increase removal efficiency significa ntly. Results obtained indicate that in situ precipitation is a promising p assive treatment approach for cyanide in groundwater.