Purpose <p>Eco-friendly and low-cost modified biochar urgently needs to be explored and developed for the remediation of heavy metal-contaminated soil.</p> Methods <p>This study synthesized a series of vermiculite-modified biochar (BV) using maize stalks, vermiculite, triple superphosphate, and urea at different pyrolysis temperatures (600℃ and 750℃) and mixture ratios (5:1:1:x and 10:1:1:x). Then, a 60-day laboratory incubation experiment was conducted to investigate their immobilization effects and underlying mechanisms on Cd and Pb in alkaline sandy loam contaminated soils by the application of BV.</p> Results <p>The results indicated that the 3% application rate of BV (vKB10PN_x and vKB5PN_x) contributed to a significant reduction in the availability of Cd and Pb in soil, with the maximum reduction rates of available Cd and Pb reaching 31.29% and 35.43%, respectively. Speciation analysis demonstrated that BV addition facilitated the transformation of Cd and Pb from the bioavailable fractions (exchangeable and reducible fractions) to nonbioavailable fractions (the oxidizable and residual fractions). These effects should be primarily attributed to the enhancement of the microporous adsorption, precipitation, or complexation of soil Cd and Pb, which resulted from the vermiculite expansion process under high temperature, as well as more functional groups after modification.</p> Conclusion <p>The immobilization mechanism of BV on cadmium was primarily dependent on physical adsorption, while both physical adsorption and chemical coprecipitation played an important role in Pb immobilization. Consequently, BV can serve as a promising remediator for alkaline sandy loam soil polluted with Cd and Pb, which could realize a win-win outcome of waste utilization and soil improvement.</p>