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Co-contamination of polycyclic aromatic hydrocarbons (PAHs) and heavy metals in soil is very common worldwide. In this study, free (FE) and immobilized (IE) Escherichia sp. on biochar solidified with alginate were used to reveal the effect of cadmium (Cd) on pyrene biodegradation in solution, spiked soil, and field co-contaminated soils. The results showed that when there was no coexisting Cd, the average removal rates of pyrene in the FE and IE treatments were 99.3% and 92.9%, respectively, in solution within 28 days, and 90.2% and 77.4%, respectively, in spiked soil incubated for 90 days. When the concentration of coexisting Cd was 1 mg L−1 and 50 mg L−1 in solution, negligible biodegradation of pyrene was observed for both FE and IE treatments. In spiked soils, the removal rate of pyrene for FE (88.7%) was significantly higher than that for IE (52.9%) when the content of coexisting Cd was 1 mg kg−1. However, there was negligible biodegradation of pyrene in both FE and IE treatments when the content of coexisting Cd was 50 mg kg−1. In field co-contaminated soil, the biodegradation of PAHs was negligible for all treatments. In comparison to FE, IE did not show superiority in bioremediating co-contaminated soil in terms of PAH removal in this study. • Free/immobilized Escherichia sp. in solution without Cd highly degraded pyrene. • Pyrene degradation blocked by Cd at 1 mg L−1 for free/immobilized cells in solution • Pyrene degradation inhibited by Cd at 1 mg kg−1 for immobilized cell in spiked soil • Degradation of pyrene was higher overall by free cells than by immobilized cells. • Free/immobilized cells did not enhance PAH dissipation in field co-contaminated soil. [ABSTRACT FROM AUTHOR]