Enhancing lead immobilization by biochar: Creation of "surface barrier" via bio-treatment.

The long-term effectiveness of heavy metal immobilization is always a concern. This study proposes a completely novel approach to enhance the stability of heavy metals by combined biochar and microbial induced carbonate precipitation (MICP) technologies, to create a "surface barrier" of CaCO layer on biochar after lead (Pb) immobilization. Aqueous sorption studies and chemical and micro-structure tests were used to verify the feasibility. Rice straw biochar (RSB700) was produced at 700 °C, which shows high immobilization capacity of Pb (maximum of 118 mg g). But the stable fraction only accounts for 4.8% of the total immobilized Pb on biochar. After MICP treatment, the stable fraction of Pb significantly increased to a maximum of 92.5%. Microstructural tests confirm the formation of CaCO layer on biochar. The CaCO species are predominantly calcite and vaterite. Higher Ca and urea concentrations in cementation solution resulted in higher CaCO yield but lower Ca utilization efficiency. The main mechanism of the "surface barrier" to enhance Pb stability on biochar was likely the encapsulation effect: it physically blocked the contact between acids and Pb on biochar, and chemically buffer the acidic attack from the environment. The performance of the "surface barrier" depends on both the yield of CaCO and their distribution uniformity on biochar's surface. This study shed lights on the potential application of the "surface barrier" strategy combining biochar and MICP technologies for enhanced heavy metal immobilization.