Harnessing Biochar in Contaminated Soil for Heavy Metal Immobilization, Soil Health Enhancement, and Carbon Sequestration

Biochar was employed for Cd(II) and Cr(VI) immobilization while simultaneously sequestering carbon in soil. Biochar application also improved soil fertility, raised carbon, and improved chili (Capsicum annum) crop production versus control soil. Tea residue biochar (TRBC) and paddy straw biochar (PSBC) were prepared by pyrolyzing used tea residue biomass (TRBM) and paddy straw biomass (PSBM) at 650°C and 550°C, respectively, under a nitrogen atmosphere. Both pyrolysis processes were carried out under N2. TRBC and PSBC characterization was accomplished by combining X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy, elemental analysis, and SBET surface area determinations. The TRBC and PSBC were applied to contaminated soil to investigate the Cr(VI) and Cd(II) immobilization. Soils amended with tea residue and paddy straw biomass were compared to TRBC- and PSBC-amended soils. Both raw biomass and their corresponding biochars were combined in different amounts in controlled incubations conducted for 168 days to investigate their CO2 emission sequestration potential. In biochar-amended soils, there was a marked increase in both cation exchange capacity and total organic carbon levels, accompanied by a significant reduction in soil CO2 emission flux versus soils amended with tea residue and paddy straw biomass. Sequential extractions were conducted to study Cr(VI) and Cd(II) immobilization in both biochar-amended soils. TRBC and PSBC addition to soil converts substantial amounts of exchangeable Cr(VI) and Cd(II) fractions to immobilized forms (iron–manganese, bound to organic material, and residual fractions). This provides both immobilization and toxicity reduction. Adsorption and metal reduction dominated Cr(VI) and Cd(II) immobilization. TRBC and PSBC application enhanced the chili (C. annum) plant growth (heights, leaf numbers, fresh weights and dry weights). This work highlighted the TRBC and PSBC potential as sustainable amendments that will simultaneously immobilize Cr(VI) and Cd(II), sequester carbon, and enhance soil fertility.