Soil Chemistry

Sludge of two different ages (3 years old; Type 1 sludge, and 3 months old; Type 2 sludge) were applied to the following soil types: two luvisols, an arenosol and a vertisol at different rates to determine whether sludge application had any effect on the partitioning and consequently, bioavailability andmobility of Cu, Pb and Zn. A fivestage sequential extraction procedurewas used to determine the concentration of metals in the different fractions of the soil after sampling and three months after sludge application. Type 1 sludge had higher values forCu, Pb, andZn (115.9, 295.5 and 400 mg kg, respectively) than Type 2 sludge (110.8, 228.6 and 341 mg kg respectively). These valueswere significantly higher than values for the background concentrations of the heavymetals in the different soils. Pseudo-total concentration of Cu, Pb and Zn in the soils therefore increased with increase in sludge application rate. Mobility and bioavailability of Cu, Pb, and Zn were affected by interactions between sludge application rate and soil type with sludge age having no significant effect. Some precaution should be taken when the period between sludge application and planting is relatively short to minimize heavy metal mobility and bioavailability. Agricultural use of sewage sludge, a residue that is derived from the treatment of municipal wastewater is gaining recognition as one way of converting waste into a resource. It is economically and environmentally acceptable [27] and is practiced in several countries [6, 8, 15, 24, 26, 32, 35]. Sewage sludge is a valuable source of nitrogen, phosphorus, and organic carbon to soils, it enhances soil microbial and biological activities [5, 11] and has also been used to improve soil porosity, structure, bulk density, aggregation, and water holding capacity [9, 13]. Sludge has however been documented as one of the sources of potentially toxic heavy metals to soils [1, 31, 35]. The risk presented to the environment by heavy metals in sludge-amended soils depends on the solubility and reactivity of the metal, which itself is determined by its speciation or chemical form. Heavymetals in soils occur in different chemical forms which influence their reactivity and consequently their bioavailability and mobility. P O L I S H J O U R N A L O F S O I L S C I E N C E VOL. XL/2 2007 PL ISSN 0079-2985