Determination of Heavy Metals Immobilization by Chemical Fractions in Contaminated Soil Amended with Biochar

Biochar is a promising tool to immobilize heavy metals (HMs) in the soil. Biochar's effect on HMs immobilization into acidic soil (pH < 5) and the interaction of plants have been investigated. Three types of feedstocks were used for biochar development via pyrolysis at two temperatures and then applied as soil amendments. A vegetative experiment has been carried out with buckwheat and white mustard to determine the effect of biochar as an HMs immobilizing agent in the presence of sewage sludge. The results show promising biochar properties to immobilize heavy metals and reduce their availability for plants. Biochar incorporation increased soil pH and reduced heavy metal forms available to plants. A sequential extraction procedure was applied to investigate five different forms of six heavy metals (Cd, Cr, Cu, Ni, Pb, Zn) and evaluate their distribution after plants' cultivation. The proportion of the residual fraction (RES) of HMs varied widely and differed from metal to metal and from plant species. RES in the soil after treatment with biochar and buckwheat harvest varied between 68.14 and 96.40% for Zn, 42.39 and 59.48% (Cu), 75.89 and 93.11% (Cr), 81.85 and 92.83% (Ni), and 98.94 and 99.20% (Pb). In comparison, a slightly opposite trend was found in the soil after white mustard cultivation. The proportion of RES was: 0.82-53.44% for Zn, 0.99-52.93% (Cu), 48.87-76.41% (Cr), 10.22-72.63% (Ni), and 98.31-99.32% (Pb). HMs immobilization efficiency in the soil after biochar treatment followed the order Ni > Cr > Pb > Cu > Zn and Ni > Pb > Zn > Cr > Cu after buckwheat and white mustard cultivation, respectively.