Metal Speciation and Toxicity in Sewage Sludge

Abstract: Increasingly stringent controls on metal concentrations in sewage sludge destined for agricultural application provided a basis for the study of the relevance of metal speciation in sewage sludge. A comprehensive literature review identified difficulties in relating metal speciation through single chemical extractions to plant uptake and soil toxicity. This research aims to provide a deeper understanding of the binding of metal species to and toxicity of metals in sewage sludge.

Five chemical extractants were applied singly to subsamples of the sludge and metal concentrations in the extracts were analysed. Both Cd and Zn were found to be extracted to a considerable extent when weaker extractants such as ammonium acetate or sodium acetate were used, whereas no chemical extractant removed more that 36% of Pb. Most of the Cu was found in the organic fraction. Sludge samples were tested for toxicity to Microtox before and after extraction and EDTA was identified as the extractant that removed the toxic metal fraction.

Partitioning of metals in sludge into electrochemically available metal, metals bound to colloids and solid metal was performed under varying conditions including pH,sludge/water ratio and time. A pH of 3.0 gave the highest concentrations in the electrochemically available phase, whereas very little metal could be found in this phase at pH 4.5. The concentration in the electrochemically available phase increased with time for Zn, but decreased for Pb. Low sludge/water ratios gave high KD values for the partitioning between electrochemically available and colloidal metal for Cd, Cu and Zn, but not for Pb. The toxicity exerted by addition of metal to a sludge sample exceeded the toxicity anticipated from the metal concentration present in the electrochemically available and colloidal phase.

Early experiments demonstrated difficulties with the use of a toxicity test (Photobacterium phosphoreum) for sludge samples due to alteration of sludge metal speciation by the incubation solution. A range of alternative solutions were tested and sodium perchlorate (NaClO4) was found to be suitable when assessing metal toxicity in sewage sludge.

Copper toxicity in the presence of a range of organic substances was tested. Selected substances were those commonly found in sludge or model substances with known reactions with Cu. A majority of the substances tested affected the toxicity of Cu. Most substances decreased Cu toxicity, but a few, for example nonylphenol, myristic acid and palmitic acid, increased Cu toxicity. Inter-metallic interactions were tested and an antagonistic effect was found for Cu and Zn. However, in the sludge as a whole, synergistic effects dominate and Cu toxicity was higher than would be expected if only ionic Cu was present.

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