Geochemistry, transport and treatment of cyanide in the subsurface

This dissertation presents the results of experimental and modeling studies of geochemistry, transport and treatment of cyanide in the subsurface. Cyanide contamination in groundwater occurs at various former or active industrial sites, including coal coking and coal gasification plants, heap leaching facilities, electroplating facilities, and aluminum production plants.; A plume of cyanide-contaminated groundwater in a sand/gravel aquifer underlying a manufactured gas plant (MGP) site was studied through field characterization, continuous monitoring, laboratory studies and modeling in order to understand the speciation, fate and transport of cyanide species in the subsurface.; Cyanide was found to exist mostly as iron cyanide complexes in the groundwater at the MGP site, with less than 2% present as weak acid dissociable cyanide. Laboratory column studies indicated that the iron cyanide complexes moved conservatively through the sand/gravel aquifer material from the site. Laboratory studies also indicated that iron cyanide complexes were stable in the groundwater under the in situ, neutral pH conditions.; The movement of the MGP site cyanide plume observed over two years was well described by assuming nonreactive solute transport in the cyanide transport model. This result was consistent with the nonreactive transport in the laboratory column tests of the cyanide (predominantly iron cyanide complexes) in the site groundwater. Based on the modeling and laboratory results, it appears that dilution may often be the only natural attenuation mechanism for cyanide in a sand/gravel aquifer under neutral pH conditions at MGP sites.; Prussian Blue and Turnbull's Blue were found to be the only stable iron cyanide solids in the aqueous pH and pE domain, with Prussian Blue dominating at low pH and high pE and Turnbull's Blue dominating at high pH and low pE. In the presence of excess iron, however, solid solutions formed between the iron cyanide solids and the amorphous precipitate, hydrous ferric oxide. pH versus pE diagrams showing equilibrium dissolved cyanide concentrations were developed for the iron cyanide solids with and without the presence of excess iron(III). These plots show the calculated total dissolved cyanide concentrations in equilibrium with Prussian Blue and Turnbull's Blue under a range of pH and pE conditions, and also in the presence of excess iron. Aqueous solubility of cyanide was enhanced in the excess iron systems, i.e., the solid solutions of iron cyanide and hydrous ferric oxide were more soluble than the pure phase iron cyanide solids.; Finally, the precipitation of iron cyanide as a means of treating cyanide-contaminated groundwater in situ was investigated. Laboratory column experiments indicated that in situ precipitation holds promise as a possible passive treatment approach for groundwater. (Abstract shortened by UMI.)...