New insights into the underlying influence of bentonite on Pb immobilization by undissolvable and dissolvable fractions of biochar

Highlights: • Coexistence of bentonite and undissolvable biochar transformed Pb²⁺ into Pb₂CO₃(OH)₂. • Formation of dissolvable biochar-bentonite-Pb²⁺ ternary complex promoted Pb²⁺ removal. • Pb²⁺ and dissolvable biochar-associated Pb²⁺ entered bentonite interlayer space. Biochar as a green amendment has been used to immobilize heavy metals in contaminated soil. Apart from the importance of the amendment itself, the interaction with soil components like clay minerals might also influence the immobilization behavior of biochar. Here, we examined the impact of a typical soil mineral, bentonite, on the immobilization of Pb by barley grass-derived biochar, and elucidated the underlying mechanisms by dividing biochar into dissolvable and undissolvable fractions. Results showed that biochar and bentonite could immobilize Pb through mechanism of electrostatic sorption, complexation, and precipitation. Compared to sole undissolvable biochar, coexistence of bentonite rapidly raised pH of the mixture over 7.0, leading the free Pb²⁺ transformed into more stable Pb₂CO₃(OH)₂ (K_sp = 1.3 × 10⁻¹⁸) instead of PbCO₃ (K_sp = 1.5 × 10⁻¹³), finally increased Pb²⁺ removal rate by 1.47 times. As for the dissolvable biochar, the generation of dissolvable biochar-bentonite-Pb²⁺ ternary complex raised the Pb²⁺ removal rate by 59.6% with the presence of bentonite. Small angel XRD analysis showed that the free Pb²⁺ and dissolvable biochar-associated Pb²⁺ could enter the interlayer space of bentonite and thus expanded the d-spacing from 1.28 nm to 1.36–1.50 nm, which might favor the formation of ternary complex. Findings of this study not only provided a new insight into the immobilization of heavy metals by biochar in soil, but also emphasized the importance of interaction between biochar and soil minerals.