Use of magnetic biochars for the immobilization of heavy metals in a multi-contaminated soil

Modified biochars, including magnetic biochars, have been tested in water for the removal of inorganic pollutants. However, at present it is unknown if they possess benefits over conventional biochar materials in relation to land remediation. A paddy soil was collected near Liantang village in Lechang Pb-Zn mine area in Guangdong Province (China). The soil was polluted with Cd, Cu, Zn and Pb, with total contents of 1.4 mg/kg, 80 mg/kg, 1638 mg/kg and 2463 mg/kg, respectively. We prepared magnetic and conventional biochar from two feedstocks (poultry litter and Eucalyptus) at a temperature of 300 and 500 °C. A sequential extraction procedure for the speciation of heavy metals and a phytotoxicity test using rice were performed. Acid-soluble Cd in soils amended with PLB was 8 to 10% lower than in the control polluted soil. This figure was 27 to 29% for acid-soluble Zn and 59 to 63% for acid-soluble Cu. In some cases, differences were found between the heavy metal fractionation in samples amended with magnetic and conventional biochars. Plant biomass was unaffected by most treatments, but increased by 32% in the treatments containing magnetic poultry litter biochar. Our study shows that a careful choice of feedstock is of utmost importance for successful containment of heavy metals in a multi-contaminated mining area soil. An appropriate choice of feedstock (in the case of this study poultry litter vs. eucalyptus) was more determinant with respect to the mobility of pollutants than altering pyrolysis temperature or modifying surface properties through magnetization. However, surface modification through magnetization can have a significant impact on plant yield and offer comparative advantages in the management of some degraded landscapes.