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Residual Cadmium Forms in Acid-extracted Anaerobically Digested Sewage Sludge¹

ABSTRACT

The effects of phosphorus and lime additions after acid extraction on residual Cd solubility and chemical forms in an anaerobically digested sewage sludge were investigated. High Cd content (11 mmol Cd kg^–1) anaerobically digested sewage sludge was aerated and then acidified to pH 2 to solubilize Cd. After 18 h of acidification, the sludge was dewatered and the supernatant and solids separated. Seventy or more percent of the Cd was removed from the solids. Similar amounts of Ni, Mn and Zn were also removed, but Cu removal was only 26% and that of Pb was < 5%. The acid extracted sludge was neutralized to pH 5.9 with Ca(OH)₂. Before liming, the sludge was amended with rock phosphate (RP) or monocalcium phosphate (MCP). The RP was insoluble while MCP dissolved, providing a high level of phosphate ligand for Cd precipitation or coprecipitation. Solution Cd in the limed MCP sludge was 12% of that in the limed RP sludge or in the limed sludge without P addition. Estimated sludge solution solubility products for major Fe, Al and Ca phosphates showed that several of these minerals could have precipitated with P addition, especially with MCP, and Cd may have coprecipitated with these solid phases. Cadmium phosphate may also have been formed in the MCP sludge. Chemical fractionation indicated that 50% of the Cd in the aerated unextracted sludge existed as inorganic precipitates with another 40% Na₄P₂O₇ extractable (organically bound). Acidification solubilized 98% of the inorganic Cd and 86% of the organically bound Cd. Seventy-nine percent of the Cd remaining in the dewatered acidified sludge was in the KNO₃ extractable (exchangeable) fraction. Liming redistributed the Cd with 13 to 19% as inorganic precipitates, 70 to 85% organically bound and < 3% in the exchangeable fraction. Phosphate addition had no significant effect on Cd fractionation.

Key Words: heavy metals • toxic trace elements • waste disposal

¹ Contribution from the Ohio Agricultural Research & Development Center, The Ohio State Univ., Columbus, OH. Published with the permission of the Director as Journal Publication no. 165-84.

² Former graduate teaching associate and professor of Agronomy, respectively. The first author is currently a graduate research associate at the Department of Agronomy, The Pennsylvania State Univ., University Park, PA.

Received for publication September 7, 1984.