Co-pyrolysis of sewage sludge and phosphate tailings: Synergistically enhancing heavy metal immobilization and phosphorus availability

Abstract

Phosphate tailings (PT) was used to reduce the release of heavy metals (HMs) during pyrolysis and the leachable rate of residual HMs, and simultaneously improve the bioavailability of phosphorus in the sludge-based biochar. The concentration of heavy metals and the fractions determined by BCR method was used to investigate the release and the transformation of Zn, Pb, Mn, Ni and Cu during pyrolysis involved with the effects of temperature and the addition of PT. The respective pyrolysis experiments shows that the release of Zn and Pb increases with temperature for both sewage sludge (SS) and PT, and the bioavailable fractions (F1 + F2) of Mn, Ni, and Cu increases with temperature for PT. During co-pyrolysis, blended samples released lower quantities of Zn and Pb and presented lower bioavailability of HMs than the individual SS or PT. A synergistic effect of co-pyrolysis was evident for volatile Zn and Pb. The decomposition of CaMg (CO₃)₂ from PT produced CaO, by which the volatile ZnCl₂ and PbCl₂ were transformed into ZnO and PbO with less volatility and higher reactivity with SiO₂ and Al₂O₃ than the chlorides. Then SiO₂ and Al₂O₃ from SS acted as the final stabilizer to immobilize the oxides. The final product combined with SiO₂ and Al₂O₃, such as ZnSiO₄ and ZnAl₂O₄, were detected. The addition of PT also introduced more Ca and P into sludge to produce biochar with higher concentration of apatite phosphorus with higher bioavailability.

Keywords: Bioavailable phosphorus; Co-pyrolysis; Heavy metal stabilization; Phosphate tailings; Sewage sludge.