Bioremediation characteristics of PHE-Cd2+ in water by anaerobic respiration of the sulfate-reducing bacterium Klebsiella sp. CW-D3T

Abstract:

[Objective] A facultative anaerobic bacterium Klebsiella sp. CW-D3T utilizing sulfate as the terminal electron acceptor for anaerobic respiration was used for degradation of target pollutants in the system with phenanthrene (PHE)-Cd²⁺ co-contamination. The response mechanism of the strain to different Cd²⁺ concentrations in the sulfate reduction system and the anaerobic metabolic pathways of the strain for degrading PHE were studied. [Methods] A sulfate reduction system with an initial sulfate concentration of 20 mmol/L was developed to enhance the growth and metabolic activity of functional bacteria and improve the bacterial performance for remediating PHE-Cd²⁺ co-contamination. The changes in extracellular polymer secretion and the vibrational characteristics of characteristic peaks were analyzed to explore the cellular responses to different Cd²⁺ concentrations. Furthermore, qualitative and quantitative analyses of the metabolic products of PHE in the sulfate reduction system were conducted by GC-MS and HPLC. [Results] In the presence of 0.5–50.0 mg/L Cd²⁺, the sulfate reduction system of Klebsiella sp. CW-D3T enhanced the remediation efficiency of target compounds, with the PHE and Cd²⁺ removal rates above 70.00% when the initial Cd²⁺ concentration was below 10 mg/L. As the concentration of Cd²⁺ increased, the secretion of extracellular polysaccharides in extracellular polymeric substances (EPS) was more than that of extracellular proteins, and the intensity of characteristic peaks of polysaccharides and protein functional groups on the surface of bacterial cells was enhanced. The initial activation of PHE in the sulfate reduction system tended to favor carboxylation to produce 2-phenanthroic acid under Cd²⁺ stress. When the initial Cd²⁺ concentrations were 10 mg/L and 50 mg/L, the content of 2-phenanthroic acid peaked at 15.56 μg/L and 10.23 μg/L on day 5, respectively, which decreased by 27.56% and 52.37% compared with that of the control group without the addition of Cd²⁺. Cd²⁺ stress significantly affected the 2-phenanthroic acid content within the cycle and at the end of the cycle. [Conclusion] The biodegradation efficiency of PHE by Klebsiella sp. CW-D3T was significantly improved when sulfate was used as an electron acceptor in the presence of Cd²⁺. The extracellular polysaccharides and proteins played a positive role in enhancing the microbial tolerance to Cd²⁺ stress by regulating the detoxification process.