Heavy Metal Contents of Sludge‐Treated Soils as Determined by Three Extraction Procedures

The extraction efficiency of three soil metal extraction procedures (4 M HNO 3 extraction, HClO 4 digestion, and HF decomposition) were compared by determining Cd, Cr, Cu, Ni, Pb, and Zn contents of two sewage sludge‐treated soils. In soils not treated with sludges, approximately 50 and 75% of total metals (HF‐decomposed) were recovered by 4 M HNO 3 extraction and HClO 4 digestion, respectively. For sludge‐treated soils, between 70 and 80% of soil metals were recovered by 4 M HNO 3 extraction. The increase in extraction efficiency indicated that sludge‐borne heavy metals in soils were more extractable with 4 M HNO 3 than metals indigenous to soils. Amounts of Cd, Cr, Cu, and Zn recovered from sludge‐treated soils by HClO 4 were not significantly different from those recovered by HF decomposition. Recoveries of Ni and Pb by HClO 4 digestion, however, were less complete. It appeared that sludge‐borne Ni and Pb were either associated with silicate minerals or converted into silicate‐containing solid phase minerals after land application.