Biochars Immobilize Lead and Copper in Naturally Contaminated Soil

Biochar (BC) continues to gain considerable interest for remediating metal-contaminated soils. A laboratory incubation study was conducted to evaluate the comparative efficiency of rapeseed residue and rice straw BCs pyrolyzed at 300 degrees C and 550 degrees C for lead (Pb) and copper (Cu) immobilization in naturally contaminated soil, which was not reported earlier. X-ray diffraction, scanning electron microscopy, and fourier-transform infrared (FT-IR) analysis were performed to study the nature of the BCs. Effectiveness of the amendments for Pb and Cu immobilization was assessed using a modified community bureau of reference extraction procedure, single extraction with CaCl2, and the toxicity characteristic leaching procedure, respectively. Amending the soil with RS550 significantly decreased the acid-extractable portions of Pb and Cu by 63.30% and 66%, respectively, whereas the residual (stable) fractions of Pb and Cu were increased by 40.31% and 52.98%, respectively. Immobilized metals were mainly transformed to the reducible fractions. High reductions in the bioavailable phase of Pb (97.13%) and Cu (93.71%) were recorded, while Pb and Cu solubility were reduced by 92.62% and Cu 72.55%, respectively. Affinity of BCs toward Pb and Cu was enhanced due to the increased negativity with increasing pH, as described by zeta potential and cation exchange capacity, which is one of the mechanisms of Pb and Cu immobilization. BCs produced at high temperature efficiently immobilized Pb and Cu compared to low-temperature BCs. These findings suggested that rapeseed residue and rice straw BCs could be used as Pb and Cu stabilizers in contaminated agricultural soils.