Bioremediation Potential of Cr(VI) by Lysinibacillus cavernae CR-2 Isolated from Chromite-Polluted Soil: A Promising Approach for Cr(VI) Detoxification

Abstract

The present study focuses on an efficient Cr(VI)-reducing bacterial strain (CR-2) isolated from an abandoned chromate plant in Qinghai Province, China. CR-2 was confirmed as Lysinibacillus cavernae using 16S rRNA gene sequencing. CR-2 could survive at 500 mg L⁻¹ Cr(VI) and effectively reduce Cr(VI) at concentrations of <50 mg L⁻¹, a pH of 5–9, a temperature of 20–40 °C, and a salinity of 5–15 g L⁻¹. According to the Box–Behnken experimental design, the maximum Cr(VI) removal efficiency by L. cavernae CR-2 was 76.21% under optimum conditions, which comprised a pH of 6.68, a temperature of 28.90 °C, and a salinity of 9.85 g L⁻¹. With regard to Cr(VI) reduction mediated by L. cavernae CR-2, enhancement in efficiency was observed in the presence of Cu²⁺ and Ca²⁺, while significant inhibition in the reduction capacity occurred upon exposure to Mg²⁺, Ba²⁺, Ni²⁺, Pb²⁺, or Cd²⁺. Moreover, L. cavernae CR-2 tends to use glucose as an electron donor for the reduction of Cr(VI). Results of cell fraction separation and degeneration indicated that the Cr(VI) removal was primarily due to the reduction of Cr(VI) via chromium reductase in the cytoplasm. In addition, bioanalysis of L. cavernae CR-2 by SEM-EDS and TEM-EDS suggested that Cr was distributed both on the surface and in the cell cytoplasm. FT-IR analyses established that multiple functional groups (hydroxyl, carbonyl, amide, amino, and aldehyde groups) participated in the Cr(VI) biosorption on the cell surface. XPS and HPLC also showed that the Cr(III) end-products could be present as Cr(III) hydroxides or as organic–Cr(III) complexes. This study yields insights into the Cr(VI) bioreduction mechanism of L. cavernae CR-2.

Keywords:

• Bioreduction
• bioremediation
• hexavalent chromium
• Lysinibacillus cavernae CR-2

Acknowledgments

We extend our sincere thanks to Nan Zhan from the Ministry of Natural Resources and National Research Center for Geoanalysis (Beijing, China) for their invaluable assistance with HPLC testing. We would like to express our heartfelt appreciation of all our colleagues and students involved in this research for their dedicated and relentless efforts. Their contributions were instrumental in the successful completion of this work. Additionally, we extend our gratitude to the editors and anonymous reviewers for their insightful comments and constructive feedback, which significantly improved the quality of this manuscript. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

Disclosure statement

No penitential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the National Key Research and Development Project of China [grant no. 2019YFC1805901], the National Natural Science Foundation of China [grant no. 41672332] and the National Basic Research Program of China [grant no. 2014CB846003].