Enhanced Phosphorus Availability and Heavy Metal Removal by Chlorination during Sewage Sludge Pyrolysis

Abstract

Increasing attention has been paid on the application of sewage sludge-derived biochar as soil amendments, but is always limited by heavy metals. This study conducted experiments on heavy metal removal by adding chlorinating agents (PVC, NaCl, MgCl2, CaCl2) during sludge pyrolysis. The chlorides addition can largely remove heavy metals by achieving the highest removal efficiency with dosage of 80 g(Cl)/kg(dry sludge) at 700 °C. The most effective removal effect was observed for Zn, Mn, Cu and Pb, with removal efficiency from 37.44% to 99.45%, 5.24% to 93.64%, 9.11% to 86.15% and 16.57% to 90.75%, respectively for the sludge before and after chlorination. Furthermore, the P-solubility in neutral ammonium citrate (Pnac) was enhanced after chlorination and the maximum P-solubility can be obtained at 700 °C for each series. After 700 °C pyrolysis, the P-solubility was significantly increased from 40.08% of the sludge biochar to 72.07%, 74.05%, 74.00% and 76.57% of the biochar obtained after adding PVC, NaCl, CaCl2, and MgCl2, respectively. The highest P-solubility was observed in samples with MgCl2 due to the formation of Mg3(PO4)2. This study proposed a novel method to use the sludge biochar as potential P-fertilizer with effective heavy metal removal, finally achieving a “waste-to-resource” strategy for integrated management of sewage sludge.