Bioavailability of heavy metals in sewage sludge-amended Thai soils

Plough and subsoil layers of two major agricultural soil series, Rangsit and Tnonburi, found in Bangkok area of Thailand were studied for determining the bioavailability and solubility behavior of heavy metals (Cd, Cu, Zn, Mn, and Fe) following application of sewage sludge from a wastewater treatment plant and a series of laboratory experiments. The soils contained low indigenous heavy metals while the sludge contained higher amounts of heavy metals but in an acceptable range for application as plant nutrient source in agricultural soil. Applications of sewage sludge increased pH of the acid soil and available plant nutrients to the soils. The heavy metal concentration levels in the soils also increased. Most partitioned into easily mobile pools and later into sparingly mobile pools following 12 weeks of incubation time. Bioavailability forms of Cd in soil solution were low but that of Cu, Zn, and Mn remained elevated even at 12 weeks of the reaction time. Applied inorganic Zn depressed Cd sorption capacity of two soils studied but it had no effect on Cd desorption.