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Approximately 80% of the Cd in an anaerobically digested sewage sludge (initially containing 11.6 mmol Cd kg−1) was removed by acid extraction and dewatering. The acid extracted sludge was treated by (i) neutralization to pH 5.9 with Ca(OH)2, (ii) addition of monocalcium phosphate (MCP) followed by Ca(OH)2neutralization to pH 5.9, and, (iii) addition of rock phosphate (RP) followed by Ca(OH)2neutralization to pH 5.9. The three treated sludges and the non acid‐extracted sludge (aerated prior to application to soil) were applied to Spinks loamy sand (sandy, mixed, mesic, Psammentic Hapludalfs) at rates equivalent to 18.7 and 37.4 µmol Cd kg−1. Swiss chard (Beta vulgaris) was grown in the greenhouse for 56 d. Cadmium, Fe, Ca, and P were measured in saturation extracts of treated soil at 0, 4,18, 39, and 60 d after sludge addition. These data indicated that hydroxyapatite was stable throughout the study in the soil receiving MCP treated sludge but not in other soil treatments. Cadmium concentration in saturation extracts of the soil with MCP sludge decreased below the detection limit of 1.8 × 10−10mol L−1by day 4 at the 37.4 µmol Cd kg−1application rate, while Cd concentrations in saturation extracts of the other sludge treatments were much higher throughout the study. Chard yields were higher in the control than in any of the sludge treatments, and the difference was attributed to greater N availability in the control. Cadmium concentration in Swiss chard tissue at harvest was significantly lower from the MCP sludge than from the other sludges. Cadmium concentration in chard tissue was also higher from the aerated sludge (11.9 mmol Cd kg−1) than from the three acid‐extracted sludges (2.58–3.29 mmol Cd kg−1). No significant difference in the Cd concentration of chard was obtained for the 18.7 and 37.4 µmol Cd kg−1rates of the MCP sludge, while Cd concentrations in chard increased linearly with Cd applied by the other sludges.

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% Na4P2O7extractable (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 KNO3extractable (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.