sludge samples were introduced in a 50-mL polypropylene centrifuge tubes containing 20 mL of acetic acid (0.1 mol/L) and then shaken for 16 h at room temperature. Subsequently, the suspension was filtered through a 0.45 μm membrane filter and solid residues were preserved for the subsequent fractions. Step two - reducible-fraction (F2): The residues from step one were slurried with a portion of a 20 mL volume of 0.1 mol L−1 hydroxyl ammonium chloride (adjusted to pH 2 with nitric acid) for 16 h. The extraction procedure described above was followed