A Microcosm Treatability Study for Evaluating Wood Mulch-Based Amendments as Electron Donors for Trichloroethene (TCE) Reductive Dechlorination

Coupling of bioelectrochemical toluene oxidation and trichloroethene reductive dechlorination for single-stage treatment of groundwater containing multiple contaminants

Bioremediation of groundwater contaminated by a mixture of aromatic hydrocarbons and chlorinated solvents is typically challenged because these contaminants are degraded via distinctive oxidative and reductive pathways, thus requiring different amendments and redox conditions. Here, we provided the proof-of-concept of a single-stage treatment of synthetic groundwater containing toluene and trichloroethene (TCE) in a tubular bioelectrochemical reactor, known as a "bioelectric well". Toluene was degraded by a microbial bioanode (up to 150 μmol L^-1 d^-1) with a polarized graphite anode (+0.2 V vs. SHE) serving as the terminal electron acceptor. The electric current deriving from microbially-driven toluene oxidation resulted in (abiotic) hydrogen production (at a stainless-steel cathode), which sustained the reductive dechlorination of TCE to less-chlorinated intermediates (i.e., cis-DCE, VC, and ETH), at a maximum rate of 500 μeq L^-1 d^-1, in the bulk of the reactor. A phylogenetic and functional gene-based analysis of the "bioelectric well" confirmed the establishment of a microbiome harboring the metabolic potential for anaerobic toluene oxidation and TCE reductive dechlorination. However, Toluene degradation and current generation were found to be rate-limited by external mass transport phenomena, thus indicating the existing potential for further process optimization.

Combined Strategies to Prompt the Biological Reduction of Chlorinated Aliphatic Hydrocarbons: New Sustainable Options for Bioremediation Application

Groundwater remediation is one of the main objectives to minimize environmental impacts and health risks. Chlorinated aliphatic hydrocarbons contamination is prevalent and presents particularly challenging scenarios to manage with a single strategy. Different technologies can manage contamination sources and plumes, although they are usually energy-intensive processes. Interesting alternatives involve in-situ bioremediation strategies, which allow the chlorinated contaminant to be converted into non-toxic compounds by indigenous microbial activity. Despite several advantages offered by the bioremediation approaches, some limitations, like the relatively low reaction rates and the difficulty in the management and control of the microbial activity, can affect the effectiveness of a bioremediation approach. However, those issues can be addressed through coupling different strategies to increase the efficiency of the bioremediation strategy. This mini review describes different strategies to induce the reduction dechlorination reaction by the utilization of innovative strategies, which include the increase or the reduction of contaminant mobility as well as the use of innovative strategies of the reductive power supply. Subsequently, three future approaches for a greener and more sustainable intervention are proposed. In particular, two bio-based materials from renewable resources are intended as alternative, long-lasting electron-donor sources (e.g., polyhydroxyalkanoates from mixed microbial cultures) and a low-cost adsorbent (e.g., biochar from bio-waste). Finally, attention is drawn to novel bio-electrochemical systems that use electric current to stimulate biological reactions.