Plant Uptake of Cadmium from Acid-extracted Anaerobically Digested Sewage Sludge

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)₂ neutralization to pH 5.9, and, (iii) addition of rock phosphate (RP) followed by Ca(OH)₂ neutralization 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 × 10⁻¹⁰ mol L⁻¹ by day 4 at the 37.4 µmol Cd kg⁻¹ application 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⁻¹ rates of the MCP sludge, while Cd concentrations in chard increased linearly with Cd applied by the other sludges.