Exploring the recycling of bioleaching functional bacteria and sulfur substrate using the sulfur-covered biochar particles

Background Bioleaching has been attracting attention in the recent years as an emerging sediment heavy metal pollution remediation technology. However, the use of sulfur powder as sulfur substrate causes the problem of “post-acidification”, and the free bioleaching functional bacteria which are susceptible to environmental impact during reactor operation cannot be used efficiently for multiple rounds. These problems can be solved if the sulfur substrate and the bioleaching functional bacteria can be recycled simultaneously after bioleaching. A new kind of sulfur substrate, the laboratory-made sulfur-covered biochar particles, was used in the bioleaching experiment, compared with sulfur powder and sulfur powder mixed with the surfactant rhamnolipid. Results The sulfur-covered biochar particles exhibited superior bioleaching performance, including faster acidification rate, SO 4 2− production rate and heavy metal bioleaching rate, and higher heavy metal solubilization percentage (Ni 33.76%; Cu 66.16%; Zn 65.494%), which resulted from the acceleration of bioleaching reaction by the bioleaching functional bacteria immobilized on the biochar surface. Otherwise, the sulfur-covered biochar particles could be reused in the second round, and the heavy metal solubilization percentage (Ni 32.84%, Cu 69.93%, Zn 67.17%) was comparable with that of the first round. Nevertheless, the sulfur content became the main limiting factor causing poor bioleaching performance during the third round. Sulfur mixed with the surfactant rhamnolipid did not show significant effect in promoting acidification and heavy metal solubilization due to high levels of organic matter and the impact of the low pH value. Conclusion The research indicated the laboratory-made sulfur-covered biochar particles could realize the dual immobilization of the bioleaching functional bacteria and the sulfur substrate to support their recycling and reuse in the second bioleaching round. In the future research, the way to maintain the reuse of the sulfur-covered biochar particles for more rounds will be explored.