Synergistic effects of CO2 and MgCl2 on heavy metals removal and phosphorus recovery in biochar obtained from pyrolysis of swine sludge

Abstract

Although swine sludge (SS) contains a high level of phosphorus (P) and has the potential to be used as an ideal source to produce P-rich biochar, it has a high heavy metal content, especially Cu and Zn, which limits its use. Herein, the influences of temperature, MgCl2 addition, and carrier gas on the removal of Cu and Zn and P recovery during SS pyrolysis were studied. The results indicated that the removal efficiencies of Cu and Zn exhibited different tendencies in the presence of CO2 and N2. The removal of Zn in N2 was higher than that in CO2 and increased at elevated temperatures; moreover, it was sensitive to chloride addition at 800 and 900 °C during pyrolysis under CO2. The atmosphere had a significant effect on Cu removal. The maximum removal efficiency was only 27.91 % when SS was treated under N2, which remarkably increased after chloride addition under CO2 at 900 °C, reaching 85.96 %. Additionally, the mineral phase compositions and transformations treated under different process conditions were investigated. Ca5(PO4)3Cl and Mg3(PO4)2 were the main P-bearing phases in the sludge treated with chloride addition, irrespective of the atmosphere, except for the product obtained at 900 °C under CO2, in which Mg3(PO4)2 was replaced by Ca(PO3)2. The gradual transformation from magnesium phosphate to calcium phosphate during pyrolysis affected P recovery and bioavailability. Pyrolysis under CO2 resulted in a higher P recovery rate and lower bioavailability in the products.