Improving Vinyl Chloride Degradation in Groundwater: A Combined Approach of Aerobic Treatment and Bioaugmentation

Vinyl chloride (VC) often accumulates in groundwaters contaminated by chloroethenes (CE) due to slow anaerobic dehalogenation of low-chlorinated compounds. This drawback is mitigated by etheneotrophic aerobic bacteria, which utilize VC as sole carbon and energy source, and by methanotrophic bacteria that dechlorinate VC by co-metabolism (Bertolini et al., 2023). This study investigates the possibility to applying sequential anaerobic and aerobic bio-barriers for the complete decontamination of a Northern Italian aquifer. It was demonstrated that the aerobic treatment constituted by air and nutrient injections, effectively increased VC-degrading bacteria and reduced VC concentrations. Groundwater monitoring of alkene monooxygenase gene etnC evidenced that etheneotrophic bacteria were present in high numbers (108-109 gene copies/L). Illumina sequencing confirmed the presence of VC-degrading genera like Mycolicibacterium, Nocardioides, Rhodoferax Rhodococcus, Pseudomonas, Brevundimonas and of methanotrophic ones. Five etheneotrophic strains, Mycolicibacterium frederiksbergense 1.1, Ralstonia solanacearum M4, Mycolicibacterium sp. D7, Leifsonia flava D2 and D5 were isolated from a highly VC-contaminated piezometer within the aerobic barrier. Kinetic analysis confirmed their ability to utilize VC and ethene as growth substrates and revealed the presence of etnC and etnE genes, within the VC biodegradation pathway. M. frederiksbergense 1.1 and R. solanacearum M4 strains tolerated high VC concentrations (10,000 μg/L). Bioaugmentation experiments with groundwater at low (2 μg/L) and high (10,000 μg/L) VC concentrations demonstrated accelerated VC degradation with respect to natural attenuation. At low concentrations, both strains reduced VC to regulatory limits (0.5 μg/L ) within 5-7 days, compared to 14 days under natural attenuation. R. solanacearum M4 was more effective in the high VC groundwater, if compared to M. frederiksbergense 1.1 and natural attenuation. Degradation kinetics correlated with microbial growth, and qPCR confirmed significantly higher etnC gene abundance in bioaugmented samples. The obtained results demonstrated that sequential anaerobic-aerobic biobarrier significantly enhanced PCE and TCE dechlorination, achieving also substantial VC mineralization. Aerobic treatment stimulated indigenous VC-degrading bacteria, and bioaugmentation with autochthonous strains further accelerated VC removal. A pilot-scale study will provide further insights into in situ bioaugmentation effectiveness.