Sewage sludge are residues resulting from the treatment of wastewater released from various sources including homes, industries, medical facilities, street run off and businesses. It consists of nutrients and organic matter that can provide soil benefits and are widely used as soil amendments. Over several years, the inability to determine metal species in sewage hampers efforts to understand the mobility, bioavailability and fate of contaminant metals in environmental systems, to assess health risks posed by them and to develop methods to remediate metal contaminated sites. In an attempt to investigate the partitioning of some heavy metals (Cu, Pb, Fe, Cd, Mn and Zn) in sewage, the four stages of the Tessier sequential extraction method were employed. The effect of extractant concentration was also studied and found to affect extractability across the various fractions (reducible, oxidizable and residual). High level of total organic carbon coupled with the neutral pH of the sewage supports their potential agricultural benefits for agricultural land application. Across all samples, the sequence follows the pattern Cu>Fe>Zn>Pb>Mn>Cd. Greater percentage of all the metals occur in the oxidizable and residual fractions. The study shows that the metal ion binding to the biotic surface such as sewage is pH dependent and that there is an optimum pH for bioavailability of metals. The influence of the sludge pH and organic matter was observed only on copper (r = -0.992 at p < 0.01) and cadmium (r = -0.970 at p < 0.05) respectively with both displaying negative correlation. The concept might be a valuable tool in predicting quantitatively the metal ion sorption to plants present in a complex system and to predict the relative change in availability due to environmental changes.
Key words: Sewage sludge, heavy metals, sequential extraction, bioavailability.
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