EVALUATION OF AEROBIC AND ANAEROBIC BACTERIAL DECHLORINATION POTENTIAL OF A 1,2-DICHLOROETHANE CONTAMINATED AQUIFER

Chlorinated solvents, belonging to the class of chlorinated aliphatic hydrocarbons (CAHs), are synthetic organohalide chemicals frequently found as contaminants of groundwater and soil, due to their widespread use in several industrial processes and improper disposal methods. These compounds pose serious health threats because of their toxic and sometimes carcinogenic effects. Among these compounds, 1,2-dichloroethane (1,2-DCA) is one of the most common aquifer contaminants, considered toxic and classified as a possible human carcinogen. Remediation approaches toward these contaminants include conventional cleanup technologies based on physical/chemical methods, and bioremediation, considered an eco-friendly and low-cost alternative. Bioremediation relies on biodegradation of CAHs, that is carried out by specialized bacteria under anaerobic and aerobic conditions. Biodegradation processes include direct pathways, associated with the growth of specialized microorganisms, and indirect (or cometabolic) pathways, fortuitously mediated by non-specific enzymes produced by microorganisms to grow on other substrates. In contaminated aquifers, chlorinated solvents, including 1,2-DCA, are generally biodegraded by anaerobic specialized bacteria called OHRBs (Organohalide Respiring Bacteria) in a reductive process called dehalorespiration. Biostimulation with appropriate amendments can allow the exploitation of these processes in bioremediation strategies. The aim of this work is to investigate the intrinsic biodegradation potential of a CAHs (mainly 1,2-DCA) contaminated aquifer by evaluating the response of the autochthonous microbial communities to anaerobic and aerobic biostimulation treatments in microcosm, to provide knowledge in the perspective of eventual site-specific bioremediation strategies. The preliminary microbiological characterization of the autochthonous bacterial communities of the aquifer, performed by 16S rRNA Illumina sequencing, revealed a notable difference between the bacterial communities from different sampling sites of the aquifer. Low relative abundances of known anaerobic and aerobic dechlorinating taxa were observed, suggesting a poor intrinsic bioremediation potential of the site. In order to enrich the dechlorinating components of the autochthonous bacterial communities, different anaerobic and aerobic biostimulation treatments in microcosm were tested, using groundwater samples and appropriate culture media amended with 1,2-DCA and additional substrates, if required, to stimulate different biodegradative processes. Chemical monitoring of 1,2-DCA concentration by Gas Chromatography-Mass Spectrometry, revealed high removal efficiency in different anaerobic and aerobic microcosm conditions. The analysis of microcosms bacterial communities, performed by 16S rRNA Illumina sequencing, revealed the enrichment in anaerobic or aerobic dechlorinating taxa. Reductive dehalogenase (dcaA) or haloalkane dehalogenase (dhlA) genes were detected in anaerobic and aerobic microcosms, respectively, by PCR probing the metagenomic DNA extracted from microcosms. Isolation attempts from the aerobic biostimulation treatments led to the achievement of stable aerobic 1,2-DCA dechlorinating oligospecfic consortia, that were analyzed by a Next Generation Sequencing approach. The enriched dechlorinating bacterial communities obtained from biostimulation trials were used in a pilot study to test the feasibility of a polyhydroxybutyrate-based Permeable Reactive Barrier (PRB) and of an aerobic bioremediation approach in two column simulations fed with real contaminated groundwater. Both approaches led to an effective 1,2-DCA removal. Finally, innovative biopolymeric tridimensional scaffolds were tested to evaluate their suitability as carriers either for the immobilization of groundwater autochthonous bacteria or for bioaugmentation with specialized dehalogenating biofilms to be eventually exploited in enhanced bioremediation. Laboratory scale studies and a preliminary field experiment confirmed scaffold could uptake 1,2-DCA and be colonized by a bacterial biofilm from groundwater bacterial communities, also made up of dechlorinating taxa.