Effects of Sewage Sludge Cadmium Concentration on Chemical Extractability and Plant Uptake

Seventeen anaerobically digested sludges of widely varying total Cd contents were used to test the hypothesis that, at a fixed Cd application rate, plant uptake of the metal is proportional to Cd concentration in the sludge. Proof of this hypothesis would support the concept of a “clean” sludge as a mechanism for reducing risk of food-chain contamination by sludge Cd. The sludges varied in total Cd content from 0.07 to 2.02 mmol/kg. The sludges were extracted with: (i) 0.05 M Ca(NO₃)₂: (ii) 0.05 M Ca(NO₃)₂ plus 50 µM Na-EDTA (ethylene diamine tetraacetate); and (iii) Chelex 100 resin. Total concentrations of Cd and major cations and anions were determined in the equilibrium salt solutions, and the chemical speciation program GEOCHEM was used to calculate Cd²⁺ activities. Uptake of Cd by sudax [Sorghum bicolor (L.) Moench], a hybrid of sorghum [Sorghum vulgare (L.) Moench] and sudangrass (Sorghum vulgare var. sudanense), was measured in a pot study at constant Cd application rates of 11 and 22 µmol/kg soil. Sudax was seeded into acid-washed sand in paper pots with bottoms removed and the sand held in place with cheesecloth. After full root development, the pots were placed on similar containers containing 500 g (oven-dry basis) of Spinks loamy sand (Typic Udipsamment) and grown for 6 wk. The aboveground biomass was weighed and analyzed for Cd. Total Cd (CdT) and Cd²⁺ in Ca(NO₃)₂ and Ca(NO₃)/EDTA, and resin-extractable Cd were all correlated positively with each other and with sludge CdT. Plant uptake of Cd was positively correlated with sludge CdT (R² = 0.33), Ca(NO₃)₂-CdT (R² = 0.91), Ca(NO₃)₂-Cd²⁺ (0.89), Ca(NO₃)/EDTA-CdT (R² = 0.94), Ca(NO₃)/EDTA-Cd²⁺ (0.90), and resin-Cd (R² = 0.92) Uptake was better correlated with Cd/P ratios in sludge than with Cd content alone. The results of this study support the hypothesis that plant uptake is controlled, in part, by sludge Cd chemistry, specifically sludge Cd content, and suggests that “clean” sludges pose less of a risk to the food chain than more contaminated sludges at equal sludge application rates.