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Distribution and mobility of heavy metals in sewage sludge
Date Issued |
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2011 |
This study compares the distribution of the heavy metals into different chemical fractions in sewage sludge before and after anaerobic processing. The total content of Zn, Cd and Cu in the digested sludge was above the upper permissible limits of standard criteria for on-land sludge application under LAND 20:2005. The following metal fractions were defined using sequential extraction scheme: exchangeable (F1), adsorbed (F2), oxidizable (F3, bound to organic matter), acid soluble (F4, bound to carbonates), and residual (F5, bound to silicates and detrital materials). In terms of their stability in the environment (according to fraction F5) the investigated heavy metals both in primary and digested sludge can be ranked as follows: Cr>>Pb>Ni>Cd.Cu.Zn. Anaerobic sludge treatment had no significant effect on metal mobility row, expressed as sum of fractions F1 and F2: Ni>Zn>Cu>Cd>Pb>Cr. The ranking order of heavy metal potential bioavailability, expressed as (F1-F3)/(F4+F5) ratio, was following: Cu>>Zn.Ni>Cd>>Pb.Cr. Copper, due to its high affinity for organic matter, was the only metal significantly transferred from "stable" fractions to "more mobile" fractions during the sludge digestion process.