Background Soil contamination with petroleum has long been a major environmental problem. Many methods have been developed for its remediation, including bioremediation, one of the most applied procedures. The biodegradation rate of bioremediation depends on the soil environment and can be enhanced by microorganism immobilization. Here, a modified biofuel ash was proposed as a new highly efficient and inexpensive immobilization carrier and tested experimentally in soil systems. Results The degradation rate of petroleum hydrocarbon in soil by inoculated microorganisms immobilized on humic acid-modified biofuel ash reached up to 47.8% during 60 days, which was twice as high as that by free microorganisms. In treatments with immobilized microbes, n-alkanes with C-28 and C31-34 were not detected and the degradation rates of C17-19 were 70.6% (unsterilized soil) and 54.2% (sterilized soil). The change in enzymatic activity reflected the basic respiration rate: it gradually increased in the pre-remediation stage, then decreased, and finally stabilized. Microorganism immobilization carriers had no significant contribution to microbial diversity in unsterilized soil, but they increased in sterilized soil. Conclusions Humic acid-modified biofuel ash inoculated with microorganisms improves the degradability of petroleum and biodiversity in soil without microbes and, through the application of biofuel ash, reuses power plant waste. (c) 2019 Society of Chemical Industry