Cyanide leaching from soil developed from coking plant purifier waste as influenced by citrate

Soils in the vicinity of manufactured gas plants and coal coking plants are often highly contaminated with cyanides in the form of the compound Prussian blue, Fe(III)4[Fe(II)(CN)(6)]3. The objective of this study was to investigate the influence of citrate on the leaching of iron-cyanide complexes from an extremely acidic soil (pH 2.3) developed from gas purifier waste near a former coking plant. The soil contained 63 g kg-1 CN, 148 g kg-1 Fe, 123 g kg-1 S, and 222 g kg-1 total C. Analysis of the soil by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy revealed the presence of Prussian blue, gypsum, elemental sulfur, jarosite, and hematite. For column leaching experiments, air-dried soil was mixed with purified cristabolite sand at a ratio of 1:3 and packed into chromatography columns. The soil was leached with dilute (0.1 or 1 mM) CaCl2 solutions and the effluent was collected and analyzed for total and dissolved CN, Ca, Fe, SO4, pH, and pe. In the absence of citrate, the total dissolved CN concentration in the effluent was always below current drinking water limits (<1.92 micromolar), indicating low leaching potential. Adding citrate at a concentration of 1 mM had little effect on the CN concentrations in the column effluent. Addition of 10 or 100 mM citrate to the influent solution resulted in strong increases in dissolved and colloidal CN concentrations in the effluent, which was due to ligand-controlled dissolution of Prussian blue, desorption of Fe(II)(CN)4-(6) or Fe(III)(CN)3-(6) by sorption competition with citrate, and mobilization of colloidal particles by citrate. However, our results indicate that relatively high concentrations of citrate are necessary to significantly increase CN leaching from the strongly acidic soil.