Co-pyrolysis of sewage sludge with polyvinyl chloride (PVC)/CaO: Effects on heavy metals behavior and ecological risk

Abstract

As an attractive method to recover energy and reduce heavy metals in the resultant biochar, co-pyrolysis of sewage sludge with polyvinyl chloride (PVC) plastics may increase the ecological risk of heavy metals. Considering the sound performance of CaO in dechlorination and immobilization of heavy metals, pyrolysis experiments were performed on four feedstocks and combined with the analysis of heavy metals to investigate the effects of temperature and added PVC/CaO on the behavior of As, Cr, Cu, Ni, Pb, and Zn. The results indicated that, increased temperature and added PVC could reduce the content of As, Pb, and Zn in biochar while CaO had the opposite effect, and the temperature set here had limited effect on the volatilization of Cr, Cu, and Ni with more than 90% of them remaining in biochar. Besides, compared with untreated sewage sludge, higher temperature and extra CaO could reduce the ecological risk of heavy metals in biochar by transforming them into more stable forms while the added PVC inhibited the transformation. When both PVC and CaO were present in pyrolysis system, although the ecological risk of heavy metals in biochar caused by PVC could be partially offset by increasing temperature and introducing CaO, it was still higher than that of other biochar. This study provides valuable insights for the treatment of heavy metals during sludge pyrolysis.