Significant microbial removal of highly chlorinated polychlorinated biphenyls (PCBs) requires the cooperation of anaerobic and aerobic bacteria. During the sequencing process of anaerobic dechlorination and aerobic degradation of PCBs, aerobic degrading bacteria have to undergo anaerobic stress. However, the survival strategy of aerobic degrading bacteria under anaerobic condition is not well-understood. In this study, the culturable cells of Rhodococcus biphenylivorans TG9 decreased from 10 8 CFU/mL to values below the detection limit after 60 days of anaerobic stress while the viable cells remained 10 5 –10 6 cells/mL, indicating that anaerobic condition induced TG9 entering into the viable but nonculturable (VBNC) state. Cell resuscitation was observed when oxygen was supplied further confirming the VBNC state of TG9. The results of single-cell Raman spectroscopy combined with heavy water indicated the significant decrease of metabolic activity after TG9 entering into the VBNC state. Additionally, the degradation ability of TG9 in the VBNC state was also significantly reduced, while it recovered after resuscitation. Our research proved that entering into the VBNC state is a survival strategy of TG9 under anaerobic conditions, and the limited culturability and degrading capacity could be overcome by resuscitation. The present study provides new insights for improving the remediation efficiency of PCBs contamination.
The application of reductants to accelerate the transformation from Fe(III) to Fe(II) has been proposed as a strategy to overcome the rate limitations of Fenton and Fenton-like processes. However, this strategy has been given little consideration to date in ligand-modified homogeneous Fenton-like (LMF) processes. Therefore, this study investigated the roles of ordinary reductants, namely gallic acid (GA), ascorbic acid (AA), acetylacetone (AAc), and three kinds of sulfhydryl compounds (SHs), in a pyrophosphate-modified Fenton-like (PMF) process for degrading polychlorinated biphenyls (PCBs) under circumneutral pH conditions. Interestingly, the inhibitory effects of SHs/AAc, the promoting effect of AA , and the inhibitory following with promoting effects of GA were observed on PCB degradation in the PMF process. The decrease of PP-Fe complex reactivity with decrease of pH and the scavenging effect of SHs/AAc caused their inhibitory effects, while the high reducibility of AA resulted in its promoting effect. The inhibitory pathways of GA in the initial stage were similar to those of SHs/AAc, whereas its promoting effect in the later stage was ascribed to the ligand exchange between GA and PP and the reducibility of GA towards Fe 3+ ions at pH 3.0. Results obtained in aqueous and soil systems under optimal reaction conditions further reveal that the application of suitable reductants, particularly AA, readily facilitates the development of the PMF process for the removal of refractory organic pollutants in natural environments.
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