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

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 x 10(-18)) instead of PbCO3 (K-sp = 1.5 x 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. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The presence of bentonite can enhance the immobilization effect of biochar on lead by altering the pH and promoting the formation of more stable compounds. The interaction between soluble biochar and bentonite can also significantly accelerate the removal rate of lead.