段階的希釈操作による底質調査方法の全シアン含有量分析の改善  [in Japanese] MODIFIED SEDIMENT TEST METHOD FOR ANALYSIS OF TOTAL CYANIDE CONTENT BY MEANS OF CASCADE SAMPLE BULKING  [in Japanese]

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Abstract

本報告では,都市ガス工場跡地のように鉄シアノ錯体を主体とする土壌の全シアン含有量分析値の信頼性を向上させることを目的に,底質調査方法による全シアン含有量分析の簡易な改善方法として段階的希釈操作の提言を行う.土壌試料を清浄砂で希釈することによってシアン回収率を低減させる要因が相対的に小さくなり,分析値が改善されることが期待される.一連の実験的検討より,段階的な希釈操作によって全シアン含有量の分析値が大幅に改善されることが判明した.通常(無希釈)の分析では234mg/kgという値が,段階的希釈操作によって500~534mg/kgというより信頼性の高い値に改善された.希釈後の分析値が50mg/kg以下の場合はそれ以上の希釈操作は必要がないこと,分析値が50mg/kg以上の場合はさらに希釈操作を行う必要があることがわかった.

 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.<br> 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.<br> 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.<br> It is concluded that the modified sediment test method is effective and necessary for the quantitative evaluation of cleanup of soils contaminated with CN.

Journal

  • Journal of Japan Society of Civil Engineers, Ser. G (Environmental Research)

    Journal of Japan Society of Civil Engineers, Ser. G (Environmental Research) 68(4), 224-233, 2012

    Japan Society of Civil Engineers

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