Cadmium Availability and Extractability from Sewage Sludge as Affected by Waste Phosphatic Clay

Disposal of sewage sludge containing high levels of heavy metals invokes environmental concerns. Due to previous phosphate rock mining, large reserves of waste phosphatic clay exists in Florida. The objective of this study was to determine the feasibility of disposing contaminated sewage sludge in waste phosphatic clay ponds. In this study, sewage sludge containing 550 mg Cd/kg was utilized as a soil amendment. Application rates corresponding to 0, 6.25, 12.5, 25.0, 50.0, 75.0 and 100 kg Cd/ha were used on two soils in a glasshouse experiment to determine the influence of waste phosphatic clay (SCM) on the uptake and extractability of Cd. The soils were sand tailings and a 4:1 mixture of tailings and waste phosphatic clay (SCM). Alfalfa ( Medicago sativa ) was grown for 155 d and dry matter clippings were taken three times. The DTPA‐extractable Cd was determined and single degree of contrasts was used to compare treatment means, sludge application rates, and plant tissue Cd concentrations. Results indicated that Cd uptake by alfalfa grown on sludge‐amended soils was suppressed by the presence of waste phosphatic clay. Extractable levels of Cd decreased with time on sludge‐amended SCM while remaining relatively the same on sludge‐amended tailings. Waste phosphatic clay suppressed the uptake of Cd added as sewage sludge. Land application of sewage sludges at waste clay disposal sites could improve soil conditions and help solve the problem of sludge disposal in an environmentally sound manner.