The effect of aging biosolids on the availability of cadmium and zinc in soil

A major concern with the safe re-use of biosolids on land is the potential for release of metals from organic matter in the biosolids, due to decompos ition proceeding as biosolids age. To quantify the effects of biosolid agin g on Cd and Zn bioavailability, two sewage sludges (Lagoon sludge and Filte red sludge) and a garden compost were incubated at 25 degreesC and 35 degre esC for 100 days. Changes in availability of Cd and Zn were determined usin g isotope dilution principles, with the materials being labelled with carri er-free Cd-109 and Zn-65. We determined isotopically exchangeable metal poo ls (E values) and plant available metal pools (L values) by measuring speci fic activities of Cd and Zn in soil extracts and in wheat plants, respectiv ely. Changes in carbon content over time were determined using C-13-NMR spe ctroscopy and chemical extraction methods, and related to changes in availa bility of metal pools as determined by isotopic procedures. Hot-water-extractable carbon content, assumed to represent easily decomposa ble organic matter, decreased during the 100 days by 80-190 mg kg(-1). The Compost and Lagoon sludge showed no change in L values for Cd or Zn with ti me, but in the Filtered sludge the L values for Cd and Zn increased signifi cantly, by 43% and 56%, respectively. The isotopically exchangeable pools o f Cd and Zn did not change with incubation treatment of the biosolids. These data indicate that the potential for metal release from biosolids as organic matter decomposes depends to a large extent on the biosolid composi tion.