Immobilization of potentially toxic elements by grape waste biochar in contaminated soils

Biochar derived from agricultural waste is recognized as an environmentally sustainable method for immobilizing potentially toxic elements (PTEs) in contaminated soils. This study investigated the efficacy of biochar produced from grape residues in immobilizing PTEs such as Pb, Ni, Mn, Cu, and Co in contaminated soils. A total of 110 surface soil samples were collected from three land-use types (agricultural, urban, and industrial) in Arak, Iran. The bioavailable fractions of PTEs were analyzed using the diethylenetriamine penta acetic acid (DTPA) extraction method. The properties of biochar were characterized through X-ray diffraction, field emission scanning electron microscopy, Fourier-transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller (BET) analysis. Biochar was incorporated into the contaminated soils at a rate of 5 % (w/w) and incubated for two months. The results indicated that the biochar application enhanced soil properties, including pH, electrical conductivity, cation exchange capacity, organic matter, and soil microbial respiration. Simultaneously, the DTPA-extractable concentrations of Cu, Ni, Pb, and Co decreased from 7.26, 1.83, 5.82, and 0.25 mg/kg, to 5.54, 0.86, 4.06, and 0.18 mg/kg, respectively, corresponding to reductions of 24 % to 79 % in bioavailability. The reductions were attributed to the functional groups with negative charges and the high specific surface area of the biochar, as identified by FTIR and BET analyses. A random forest analysis further revealed that organic matter and soil microbial respiration were the most influential factors in in reducing the bioavailability of PTEs following biochar amendment. These findings underscore the potential of grape residue-derived biochar as an effective amendment for mitigating PTE contamination in soils.