Bioleaching of heavy metals from anaerobically digested sewage sludge using FeS₂ as an energy source

The effect of using FeS₂ as an energy source, on the bioleaching of heavy metals (Zn, Cr, Cu, Pb and Ni) and nutrients (nitrogen and phosphorus) from anaerobically digested sludge using isolated indigenous iron-oxidizing bacteria was investigated in this paper. Addition of FeS ₂ in the range of 0.5-4.0 gl^-1 accelerated the acidification of sludge and raised the oxidation-reduction potential of sludge medium with an inoculation of 15% (v/v) of active bacteria, thus resulting in an overall increase in metal dissolution efficiency. After 16 days of bioleaching at 28°C and an initial pH of 3.0, up to 99% of Zn, 65% of Cr, 74% of Cu, 58% of Pb and 84% of Ni can be removed from the sludge. In contrast, only 94% of Zn, 12% of Cr, 21% of Cu, 32% of Pb and 38% of Ni were leached out in the control without inoculation of iron-oxidizing bacteria and the addition of FeS₂. Less than 15% of nitrogen and 6% of phosphorous were lost after 16 days of bioleaching when using FeS₂ as the energy source. Comparing to 39% and 45% loss respectively for these two nutrients when using FeSO₄·7H₂O as the energy source, FeS₂ appears to be a more suitable energy source for preserving nutrients in sludge while removing heavy metals from sludge.