Competitive sorption and availability of coexisting heavy metals in mining-contaminated soil: Contrasting effects of mesquite and fishbone biochars

Mesquite and fishbone were pyrolyzed to produce biochar (MBC and FBC, respectively) at different temperatures. The effects of the MBC and FBC on the removal of single and competitive metals (Cd, Pb, Zn, and Cu) from aqueous solutions were evaluated. A greenhouse pot experiment was also conducted using wheat plants with the mining-contaminated soils. In the presence of MBC or FBC (dosages of 15 and 30 g kg⁻¹), the bioavailability of co-existing Cd, Pb, Zn, Cu, Mn, and Fe were assessed. The results clearly indicated competitive adsorption among metals with the highest adsorption preference toward Pb. The removal efficiency and partition coefficient (PC) values of heavy metals for FBCs were higher than those for MBCs. These two values increased with MBC pyrolysis temperature under both single- and multi-metals adsorption conditions. Applying FBC to mining soil resulted in the highest reduction in most NH₄NO₃-extractable heavy metals, reducing their availability to wheat plants. At the highest application dosage of 30 g kg⁻¹, the highest metal immobilization, which accounted for 40.0% and 43.0% for Pb, 61.7% and 66.2% for Cu, 48.3% and 55.6% for Zn, and 32.7% and 33.8% for Cd, was achieved following the application of FBC400 and FBC600, respectively. However, applying MBC lead to a significant reduction in the availability of Cu and Pb but not that of Zn and Cd. FBC is thus more effective in removing heavy metal from aqueous solutions, as well as in immobilizing co-existing heavy metals in contaminated mining soil. It could, therefore, be an effective sorbent and immobilizing agent.