Abstract
Soils at Manufactured Gas Plant (MGP) sites are often contaminated by cyanide (CN). Besides free-CN, contaminated soils at MGP sites contain less or little toxic Metal-CN complexes (mostly iron-CN complexes) as a large part of total CN. The total CN is an important index used for the evaluation of soil remediation process such as soil washing.
The total CN content is measured by the sediment test method. After having experiences of soil washing for several MGP sites, it has been noticed that the mass balance was not maintained when the total CN content was measured by the sediment test method.
In order to eliminate the observed inadequacy of the sediment test method, an experimental study was conducted to modify the method. Each Soil sample from three MGP sites was divided into six parts to make a series of cascade bulk samples. Each cascade bulk sample was mixed with clean sand in such a way that the prepared sample had increased its dry weight by 1, 2, 5, 10, 20, and 50 times the divided soil sample. Then the relationship between the rate of bulking and the test value of total CN content was investigated. The same test is repeated for a standard sample comprising ferric ferrocyanide (Fe4[Fe(CN)6]3) and clean sand. As a result of study, it has been observed that; 1) a bulking factor of 10 is enough to get near true value of total CN content for highly contaminated soils, 2) the test with standard samples of varying total CN content showed a maximum CN recovery rate of 80%, and, 3) no bulking of sample is necessary for soils with less than 50mg/kg of CN.
It is concluded that the modified sediment test method is effective and necessary for the quantitative evaluation of cleanup of soils contaminated with CN.
