Biodegradation of nitro and chlorinated aromatic compounds for bioremediation of a contaminated industrial site

Abstract

When attempting to remediate contaminated sites it is important to have a good understanding of the sites’ physical, chemical, and hydrogeological properties, so that flow of contaminants can be predicted and evaluated. The contribution of biological processes is also of significance and needs to be better understood to improve the remediation strategy and process. Aerobic biotransformations of contaminants are often observed in the shallow layers, while anaerobic biotransformations processes are established in the deeper, more anoxic layers, including groundwater. Aromatic compounds, such as pesticides, dyes, resins, and solvents, are a major group of contaminants originating from a variety of industries. Aiming to combine academic research with remediation projects carried out by the industry partner, this Ph.D. thesis intended to contribute to bioremediation projects considering complex pollutants, such as halogenated aromatic compounds. To evaluate the biodegradability of dichloronitrobenzene (DCNB), dichloroaniline (DCA), monochloroaniline (MCA) and aniline by native microbes from a contaminated site, we first enriched cultures inoculated with soil and groundwater from a contaminated site, under aerobic and anaerobic conditions, amended with the target compounds individually, in the range of 10 and 20 mg/L. In addition to the inoculum and the contaminants, the enrichments contained mineral medium and ethanol and lactate as the electron donors (the anaerobic one). The cultures were kept in Boston glass bottles of 250 mL (150 mL of headspace). By enriching the cultures through multiple re-feedings, we could start to describe the mechanisms of biotransformation and identify organisms potentially involved in the observed reactions. Under aerobic conditions, 2,3-DCA, 3,4-DCA, 2-MCA, 3-MCA, and 4-MCA were potentially mineralized by members of the Pandoraea and Burkholderia-Caballero-Paraburkholderia genera, while chloride and ammonium ions were released. The anaerobic enrichments revealed nitro reduction of the 2,3-, 3,4-, and 2,5-DCNB by fermentative bacteria through a likely co-metabolic process, forming DCA isomers. Desulfitobacterium may be implicated in nitro group reduction in these enrichments, however, further investigation is needed for confirmation. The 2,3- and 3,4- DCA were reductively dehalogenated by organohalide-respiring bacteria, specifically Dehalobacter and Anaeromyxobacter genera. The well-established and active anaerobic community is composed of fermentative bacteria, organohalide-respiring bacteria, sulphate- reducing bacteria, and archaea. In general, from our lab experiments, we can affirm that bioremediation with native microbes from the site is a promising strategy to be applied in the contaminated area. Bioaugmentation seems not to be necessary, but biostimulation through addition of oxygen should be considered since natural attenuation under anaerobic conditions did not result in complete dechlorination and mineralize the contaminants.