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Retention of Metals in Sewage Sludge II: Incorporated Radioisotopes¹

ABSTRACT

Samples of digested sewage sludge from Baltimore, Md. and Washington, D.C. were columned and leached for up to 240 hours with 0.06N CaCl₂, as well as with water, to study the release of trace amounts of ⁸⁵Sr and ¹³⁷Cs introduced earlier. In percent of the radioactivity originally present, leaching with 0.06N CaCl₂ removed about 90 and 80% of ⁸⁵Sr, but about 9 and 16% of ¹³⁷Cs from Baltimore and Washington, D.C. sludge, respectively. For leaching with H₂O, the corresponding values were 39 and 34% of ⁸⁵Sr and 4 and 3% of ¹³⁷Cs.

Samples of sludge were subjected to treatments with H₂O₂, and steambath drying, followed by incubating with ¹³⁷Cs and ⁸⁵Sr, and either exhaustive leaching with H₂O (240 hours at 2 cm³ cm^–2 hour^–1), or equilibration for 1 hour with H₂O or 0.01N HCl. In percent of the radioactivity originally present, the pretreatment increased ¹³⁷Cs release by H₂O leaching from 9 to 18%, and from 7 to 10% with Baltimore and Washington, D.C. sludge, respectively, depending on whether the sludge was pretreated with H₂O or with 3% H₂O₂. Corresponding ranges for ⁸⁵Sr were from 47 to 55% and from 41 to 48%.

Incubation of Baltimore and Washington sludge for 44 months at room temperature slightly increased H₂O extractability of ¹³⁷Cs and ⁸⁵Sr when the incubated material was kept dry, rather than moist. This may have been due to smaller concentrations of complexing organic compounds, or to compounds with lesser complexing ability. When sludge was stored moist, the 0.01N HCl extractability of the isotopes sharply increased. This would also point to a greater abundance of organic complexes under moist conditions during storage, since a decrease in pH affects their stability.

Key Words: strontium • cesium • hydrolysis • oxidation • leachability • extractability

¹ Contribution from the Agric. Environ. Qual. Inst., ARS, USDA, Beltsville, MD 20705. This work was supported in part by the U. S. Atomic Energy Commission.

² Research Soil Scientists.

Received for publication August 9, 1974.