Synergistic effects of CO_2 and MgCl_2 on heavy metals removal and phosphorus recovery in biochar obtained from pyrolysis of swine sludge

摘要

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 N-2. The removal of Zn in N-2 was higher than that in CO2 and increased at elevated temperatures; moreover, it was sensitive to chloride addition at 800 and 900 degrees 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 N-2, which remarkably increased after chloride addition under CO2 at 900 degrees C, reaching 85.96 %. Additionally, the mineral phase compositions and transformations treated under different process conditions were investigated. Ca-5(PO4)(3)Cl and Mg-3(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 degrees C under CO2, in which Mg-3(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.