Removal of lead from two polluted soils by magnetic wheat straw biochars

Due to high sorption capacity for heavy metals, magnetic biochar (MBC) has the potential to adsorb heavy metals in soils, which are then removed together with MBC from soils by a magnetic field. In this study, two magnetic biochars (MBC300 and MBC700) were derived from the magnetization of wheat straw biochars pyrolyzed at 300 and 700 °C. Strong binding of Pb with iron oxide particles deposited on biochar was observed. After the MBCs (7.5%, w/w) were applied to two naturally Pb-polluted soils (named as He-soil and Hu-soil) for 720 h, the removal efficiency of Pb from the soil by MBC300 (26.8-40.1%) was similar (p > 0.05) to that by MBC700 (25.1-42.1%). This is because MBC300 has lower sorption capacity for Pb but higher recovery percentage from soils as a result of lower saturation magnetization. The removal efficiencies of Pb by the two MBCs were 13-17% higher for He-soil than for Hu-soil, which was due to higher proportion of mobile forms of Pb in He-soil (82.3%) than in Hu-soil (51.5%). Spectroscopic analysis indicated that Pb in soils tended to bind onto the surface of MBC in more stable forms. Moreover, removing Pb from soils by MBC could decrease Pb concentration in ryegrass by about 30%. Therefore, it might be a potential method to remedy Pb-polluted soils.