Determination of the levels of heavy metals in sewage sludge is necessary prior to application of the sludge to agriculture because of the inherent risk of heavy metal toxicity to soil, plants and humans. The heavy metals (Cu, Cd, Pb and Zn) in sewage sludge were quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES) following extraction with modified-BCR and ultrasonic assisted-BCR extraction methods. The quantities of heavy metals obtained from the two extraction methods were compared. The ICP-OES results indicated that in the exchangeable fraction in both extraction methods the amount of zinc released ranged from 19-227 mg kg-1, and this represented the highest concentration of metals in this fraction. Clarifier sludge contained more zinc (2375 mg kg-1) than raw sludge (310 mg kg-1). An improvement in the amount of zinc (120.8 mg kg-1) and lead (98.7 mg kg-1) released in the reducible fraction in raw sludge samples was obtained with UA-BCR extraction. UA-BCR showed that the raw sludge contained about 17 mg kg-1 of Cd while only about 0.625 mg kg-1 was released using the modified BCR extraction. Some of the metals quantified were higher than the permissible levels, and the sludge not be used for soil enrichment. The results of this study demonstrated that the modified-BCR and the ultrasonic assisted-BCR extraction methods should be used in conjunction since one method alone might not be efficient in matrix breaking and releasing metal ions into solution. This may lead to incorrect conclusions about the presence or absence of metal ions in sewage sludge.
Key words: Heavy metals, ultrasonication, sequential extraction, sludge, clarifier samples
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