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Retention of Metals in Sewage Sludge I: Constituent Heavy Metals¹

ABSTRACT

Sewage sludge from Baltimore, Md. was compared to that of Washington, D.C. in terms of the tendency to release heavy metals under a variety of circumstances. Total and 1.0N HCl-extractable concentrations of Cd, Cu, Pb, and Zn were determined by atomic absorption spectrophotometry. Fractionation of the metals by leaching columned samples of Baltimore sludge, with either a CaCl₂ solution or H₂O over extended periods of time, showed Cd and especially Zn were more leachable than Cu and Pb. Except for Pb, leachability of the metals was much lower in the sludge from Washington, D.C. than in the Baltimore sludge. Regarding the sign of charge of Cu species as a function of the progression of leaching of Baltimore sludge, anionic and, especially, amphoteric species increased, as cationic ones decreased. When aqueous sludge leachates were stored for 13 months at 5C, amphoteric Cu strongly increased, while all other Cu species decreased. In contrast, nearly all Zn species in the stored leachates were cationic. When sludge samples were subjected to simulated weathering by stirring with either H₂O₂ in a number of concentrations, or H₂O, followed by steambath drying, the extractability of heavy metals with H₂O and 0.01N HCl increased. Extractability of Cu increased more than that of Cd or Zn, suggesting that the stability of the amphoteric Cu species depended strongly on acidity. Because of the H₂O₂ treatments, the cation exchange capacity of the sludges diminished by 15 to 25% depending on their origin, although no C was lost.

Compared with Washington, D.C. sewage sludge, Baltimore sludge had a greater heavy metal contaminating potential for soil amended with it.

Key Words: extractability • leachability • hydrolysis • oxidation • speciation • copper • cadmium • lead • zinc • amphoteric

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. We are indebted to Hsin-yuan Tu, Soil Conservation Service, USDA, Beltsville, for X-ray diffraction and differential thermal analyses of various size fractions of the sludge samples we used.

Received for publication August 9, 1974.