Remediation of Cyanide Contaminated Soil using Soil Washing, Chemical Oxidation and Precipitation

The soil contamination with cyanide has been an environmental concern due to its high toxicity. The chemical oxidation at room temperature, the high temperature and pressure decomposition and the biodegradation have been developed and practiced for the remediation of cyanide contaminated soil. We developed and tested the remedial technology consisting of the soil washing with an alkaline phosphate solution and the waste water treatment by the addition of H2O2 and acid. We also characterized the cyanide species in the soil, which was collected from an abandoned gold mine site and was used as the testing soil for the new technology. Greater than 90 % of the total cyanide of the soil was strong acid dissociable (SAD) cyanide and less than 10% was the weak acid dissociable (WAD) cyanide. Trace amount of free cyanide was contained in the soil. The cyanide extraction rate from the soil increased with increasing the pH and phosphate concentration of the washing solutions. Greater than 99 % of total cyanide was extracted with the pH 12_50 mM Na-pyrophosphate. The free and WAD cyanides in the washing solution were oxidized by the addition of H2O2 at less than 0.1 % under alkaline condition. Almost all of SAD cyanide was precipitated from the washing solution by the acid addition to pH 4. The extraction of cyanide by the alkaline phosphate solution might be due to the increased solubility of SAD cyanide and desorption of cyanide from soil particles. The removal of SAD cyanide from the solution might be due to the decreased solubility of SAD cyanide as decreasing solution pH.