CHELATING RESIN METHOD FOR ESTIMATION OF SLUDGE-CADMIUM BIOAVAILABILITY

A chelating resin procedure was developed to predict the plant uptake of Cd by municipal sewage sludges applied to land. Seventeen anaerobically digested sludges were sampled to give a range of total Cd content of 0.07 to 2.02 mmol/kg. Sludge suspensions [20 g in 100 mL 0.05 M Ca(NO3)2] were equilibrated with 1 g Chelex 100 resin placed in dialysis tubing and shaken at 200 rpm for 16 h. Resin-extractable Cd was compared with sludge solution Cd (CdT and Cd2+) in equilibrium with 0.05 M Ca(NO3)2, and 0.05 M Ca(NO3)2 containing 50-mu-M Na-EDTA (ethylenediaminetetraacetate). Resin extractable Cd was correlated with Cd uptake by sudax, a sorghum/sudangrass hybrid (Sorghum bicolar), grown in Spinks loamy sand (Typic Udipsamment) amended with each of the sludges to give a constant Cd concentration of 22-mu-mol/kg soil. Resin extractable Cd ranged from < 0.1 to 48-mu-mol/kg. Resin extracted between zero and 5.3% of total sludge Cd. Resin extractable Cd was highly correlated with CdT and Cd2+ in 0.05 M Ca(NO3)2 (R2 = 0.97 and 0.98, respectively), and with 0.05 M Ca(NO3)2 containing 50-mu-M NaEDTA (R2 = 0.97 and 0.98, respectively). There was a lower correlation with total sludge Cd and soil solution Cd (R2 = 0.53 and 0.63, respectively). Cadmium concentration in sudax was highly correlated with resin "tractable sludge Cd (R2 = 0.92). When the two sludges with highest total sludge Cd were dropped, the correlation dropped (R2 = 0.57), but resin extractable Cd predicted Cd uptake as effectively as CdT and Cd2+ in Ca(NO3)2 or Ca(NO3)2/EDTA. Resin extraction appears to be a promising method of assessing the potential bioavailability of sludge Cd.