Use of Mesquite Hardwood-Derived Biochar for Stabilization and Solidification of Lead-Contaminated Soil

Soil amendment and carbon sequestration are among the many possible applications for biochars, which are solid carbonaceous materials produced by biomass pyrolysis in an oxygen-deficient environment. Biochars have recently gained popularity as a resource for decontaminating soils polluted with hazardous materials. Primarily, the leaching procedure is used to assess the leachability of heavy metals in biochar-treated soils, but the effect of biochar on the stabilization and solidification (S/S) of heavy metals [e.g., lead (Pb)] in terms of geotechnical properties such as compaction and soil strength is not well understood. Therefore, this study aimed to determine the effect of hardwood biochar ranging from 0 to 10 wt.% on the S/S of soil contaminated with high concentrations of lead (5,000 and 10,000 mg/kg) with a curing period of up to 56 days. A comprehensive experimental study was carried out, including tests such as pH, compaction characteristics, unconfined compressive strength, the toxicity characteristic leaching procedure (TCLP), and the Community Bureau of Reference-sequential extraction procedure. Results showed that decreasing TCLP Pb levels were a function of increasing binder dosage and curing time. For Pb0.5% soil, adding 5 wt.% biochar could increase the immobilization efficiency to 88% after 28 days of curing. Chemical fraction analyses correlated a high degree of Pb immobilization to a decrease in the weak acid-soluble and reducible (F1 and F2) fractions and an increase in the oxidizable (F3) and residual (F4) fractions. The compressive strength of treated soil increased to 3.5-4 times that of untreated soil with an increase in the pH values after 28 days. X-ray diffraction and scanning electron microscopy analyses were carried out to deduce that physical adsorption and lead precipitation into compounds such as cerussite and pyromorphite were the underlying mechanisms for Pb immobilization. Finally, the treated soil was found to be a safe and environmentally friendly construction material, proving that hardwood biochar provides green and sustainable treatment of Pb-contaminated soil.