Titolo della tesi: Electrochemically Assisted Bioremediation Processes
High chlorinated CAHs such as PCE, TCE, or TeCA can be reduced under anaerobic conditions by dehalorespiring microorganisms, which are able to use these molecules as electron acceptors of their metabolism in presence of molecular hydrogen. The so called reductive dechlorination (RD) reaction successively replaces a chlorine atom with a hydrogen atom on the molecule carbon skeleton up to more acceptable molecules (i.e. ethylene or ethane). Usually, non-optimized environmental conditions lead to the accumulation of RD daughter products such as dichloroethylene (DCE) or vinyl chloride (VC), the latter one being more toxic than the parent molecule (Aulenta et al., 2007). To avoid by-products accumulation various fermentable organic substrates have been investigated over the years such as volatile fatty acids, edible oils, humic acids, polyhydroxyalkanoates (Fennell, Gossett and Zinder, 1997; Schollhorn, Andreas; Savary, Catherine; Stuckl, 1997; S.K., 2000; Aulenta et al., 2005; Amanat et al., 2022). Despite the effectiveness of the treatment, direct injection of fermentable organic matter led to possible side effects as the production of methane and secondary contaminations and the mobilization of heavy metal as Fe, Mn, and Cr. Indeed, due to the competition of other microbial consortia many more electron donor is generally needed to achieve complete dehalogenation than would be suggested by the stoichiometric requirement for dehalogenation alone. Indeed, the dehalorespiring microorganism can be outcompeted by other hydrogen Fennell and Gossett, 1998; Yang and McCarty, 1998; Dolfing, 2000, 2003; Aulenta et al., 2008). An innovative approach to the control of biological reactions is offered by the bioelectrochemical systems, that allows to stimulate and control reaction rates and their selectivity by controlling the electrodes potentials. Several studies reported the possibility to use biocathodes for the stimulation of the RD (Aulenta et al., 2008, 2010; Fernández-Verdejo et al., 2021, 2022), as well as the possibility to use a polarized bioanode to stimulate oxidative dechlorination of low chlorinated compounds (Aulenta et al., 2013; Lai et al., 2017). The low chlorinated hydrocarbons couldn’t be reduced at the same rate as the high chlorinated ones, due to thermodynamical limitations, for this reason, the aerobic treatment of DCEs and VC seems to be more effective (Hartmans and De Bont, 1992; Gerritse et al., 1995; Arcangeli et al., 1996).
The main advantage of the utilization of bioelectrochemical systems is the ability to manipulate the redox potential generating a sequential reductive/oxidative zone. Indeed, by the control of the cathodic potential, (Lai et al., 2015) reported the capability to reduce high chlorinated compounds like PCE and TCE in a biocathode to a mixture of less chlorinated by-products (i.e. cisDCE and VC) which were then oxidized in the bioanode through the in situ oxygen production by water splitting.
The following thesis work reports the set up and the optimization of a sequential reductive/oxidative bioelectrochemical process for the treatment of CAHs contaminated groundwaters. This doctoral thesis was focused on the utilization of a new MEC membrane-less configuration equipped with internal graphite granules counter electrode that decrease CAPEX costs and simplify reactor’s scalability. Moreover, the new system was based on two separate MEC for independent reduction and oxidation steps which allowed to separately optimize redox conditions. In the last thesis chapter the scale up of the sequential reductive/oxidative process is reported showing the preliminary results obtained in a field test with real groundwater.
More in detail. Chapter 1 presents an introduction to the chlorinated solvents chemical and physical properties and highlights the basics of a DNAPLs contamination event. The principal conventional technologies for chlorinated solvents remediation are described based on both abiotic and biotic processes.
Chapter 2 Briefly introduces the bioelectrochemical systems and their fundamentals including the electrochemical systems thermodynamics. A literature review about the bioelectrochemical systems utilization for chlorinated aliphatic hydrocarbons removal in soil, water and sediments is also provided. A more specific literature analysis is given on the development of the electrochemically assisted bioremediation processes, mostly investigated by our research group over the last 15 years. At the end of this chapter a conceptual model for the electrostimulated reductive dechlorination reaction is presented.
Chapter 3 Presents the material and methods section that highlights experimental procedures about the process realization and operating conditions, and the calculation and analytical methods utilized for both the laboratory process investigation and the field test.
In Chapter 4 the reductive process is investigated starting from preliminary tests and characterization to the continuous flow run, made by using synthetic medium (PCE in mineral medium) in which the most favourable conditions for the RD evaluated
Chapter 5 is focused on the moving from the simplest mineral medium to the introduction of a different matrix for the feeding solution, more similar to a groundwater. This changeis mostly based on intorducing nitrate and sulphate anions that can compete with the dechlorinating microorganisms for the reducive power. The entity of the competitive reduction reactions is evaluated and the possible countermeasure that can be adopted to promote the RD reaction.
In Chapter 6 the capability of the sequential process to fulfil the total mineralization of the mixture of CAHs contained in a real contaminated groundwater is investigated. The effect of this real matrix is elucidated considering the presence of other organic matter, cations and anions.
Chapter 7 is focused on the scale-up of process to perform a field test. Several preliminary test will show the difference and similarity between the lab scale reactors and the pilot ones. The operation at three different HRTs is described with the different performances and a mesocosm study is used to elucidate the process potential and present limitations.