Heavy metal immobilization and radish growth improvement using Ca(OH)2-treated cypress biochar in contaminated soil.

Heavy metal contamination poses a significant threat to soil quality, plant growth, and food safety, and directly affects multiple UN SDGs. Addressing this issue and offering a remediation solution are vital for human health. One effective approach for immobilizing heavy metals involves impregnating cypress chips with calcium hydroxide (Ca(OH) 2) to enhance the chemical adsorption capacity of the resulting woody charcoal. In the present study, un-treated cypress biochar (UCBC) and calcium-treated cypress biochar (TCBC), were introduced into pristine and contaminated soil, at rates of 3, 6, and 9% (w/w). Both BCs were alkaline (UCBC pH: 8.9, TCBC pH: 9.7) with high specific surface area, which improved the soil properties (pH, EC, and OM). Radish (Raphanus sativus) cultivated in pots revealed that both UCBC and TCBC demonstrated significant improvements in growth attributes and heavy metal immobilization compared to the control, with TCBC exhibiting superior effects. The TCBC surface showed highly active nanosized precipitated calcium carbonate particles that were active in immobilizing heavy metals. The application of TCBC at a rate of 9% resulted in a substantial reduction in Zn and Cu uptake by radish roots and shoots. In contaminated soil, Zn uptake by radish roots decreased by 55% (68.3–31.0 mg kg−1), and shoots by 37% (49.3–31.0 mg kg−1); Cu uptake decreased by 40% (38.6–23.2 mg kg−1) in roots and 39% (58.2–35.2 mg kg−1) in shoots. Uptake of Pb was undetectable after TCBC application. Principal component analysis (PCA) highlighted the potential of TCBC over UCBC in reducing heavy metal concentrations and promoting radish growth. Future research should consider the long-term effects and microbial interactions of TCBC application. [Display omitted] • To immobilize Zn, Cu, and Pb in soil, a novel biochar (TCBC) was prepared. • TCBC improved soil properties and radish growth attributes. • TCBC resulted in a substantial reduction in Zn, Cu, and Pb in radish roots and shoots. • Highly active nano-sized CaCO 3 particles in TCBC-immobilized heavy metals. • TCBC consistently outperformed UCBC in terms of immobilization properties. [ABSTRACT FROM AUTHOR]