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.