The effect of inoculation of iron oxidizing bacteria on elution of Cu from anaerobically digested sewage sludge was investigated without adding ferrous sulfate as a substrate. The inoculation of iron oxidizing bacteria to the sludge brought about the effective elution of Cu at pH 2 and 3. Ferric ion produced by the biological oxidation of ferrous ion which was eluted from the sludge or ferric ion eluted directly from the sludge and iron oxidizing bacteria caused the elution of Cu. Furthermore, both chemical and biological leachings of Cu from CuS were studied at pH 2, 3 and 4. It was found that both ferric ion and iron oxidizing bacteria could elute Cu. The elution rate of Cu with ferric ion decreased as the pH increased and two moles of Fe(III) eluted 1 mole of Cu. The biological leaching by iron oxidizing bacteria eluted Cu more effectively than the chemical one with sulfuric acid or ferric sulfate at pH 2, 3 and 4.

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