EP4655250A1

EP4655250A1 - Bioelectrochemical method for the remediation of water contaminated with chlorinated aliphatic hydrocarbons

Info

Publication number: EP4655250A1
Application number: EP24705738.3A
Authority: EP
Inventors: Marco ZEPPILLI; Edoardo DELL'ARMI; Mauro Majone; Marco Petrangeli Papini
Original assignee: Universita degli Studi di Roma La Sapienza
Current assignee: Universita degli Studi di Roma La Sapienza
Priority date: 2023-01-24
Filing date: 2024-01-23
Publication date: 2025-12-03
Also published as: WO2024157166A1; IT202300000999A1

Abstract

An electrochemical method is described which stimulates microbial reactions for the removal of chlorinated aliphatic hydrocarbons from water contaminated with chlorinated aliphatic hydrocarbons, which finds use in the field of remediation of groundwater contaminated with chlorinated solvents.

Description

BIOELECTROCHEMICAL METHOD FOR THE REMEDIATION OF WATER CONTAMINATED WITH CHLORINATED ALIPHATIC HYDROCARBONS

FIELD OF THE INVENTION

The present invention relates to an electrochemical method which stimulates microbial reactions for the removal of chlorinated aliphatic hydrocarbons from contaminated water.

BACKGROUND ART

Chlorinated aliphatic hydrocarbons are chemical compounds used as degreasers and as solvents and intermediates in chemical reactions, e.g., for the production of silicones, and are widespread groundwater contaminants worldwide; an important chlorinated aliphatic hydrocarbon by diffusion is trichloroethylene (commonly referred to as TCE). The removal of these compounds to bring their concentration below the allowed limits is extremely complex due to their chemi cal -physical properties, such as low solubility in water and a density higher than that of water.

Biological approaches to the problem have been studied since the early 2000s which are based on the stimulation of micro-organisms present in the contaminated site and capable of degrading the chlorinated aliphatic hydrocarbons into substances which are less polluting or more easily removed from the water. The most commonly used methods involve the injection of fermentable organic substances in aquifers, which gradually release molecular hydrogen to provide the necessary reducing power for the reductive dechlorination reaction. However, these systems can lead to secondary contamination due to the reaction by-products of the organic substances introduced in the aquifer and thus to further degradation of the quality of the water resource.

Around the early 2000s, bioelectrochemical systems were proposed which allow to control the biological reactions taking place at the electrodes; see for example the article “Electron transfer from a solid-state electrode assisted by methyl viologen sustains efficient microbial reductive dechlorination of TCE”, F. Aulenta el al., Environ. Sci. Technol. 41 (2007) 2554-2559, in which the term BEARD (BioEctrochemical Assisted Reductive Dechlorination) was introduced to denote the ability of electrochemical systems to stimulate the reductive dechlorination of chlorinated solvents by microorganisms under appropriate experimental conditions.

The use of these bioelectrochemical systems is not only potentially a more sustainable approach with respect to traditional ones but has also proven to be extremely effective for the complete removal of these pollutants.

This approach has been investigated in numerous scientific papers; for a review of these studies, see the recent article “Bioelectrochemical system for dehalogenation: A review”, Xuemei Zhu et al., Environmental Pollution 293 (2022) 118519.

To date, the mechanism of action by which certain microorganisms succeed in degrading chlorinated aliphatic hydrocarbons is not yet fully elucidated, but there is no scientific evidence reporting the direct interaction of dechlorinating microorganisms with polarised electrodes, thus the commonly accepted theory contemplates the electrolytic production of molecular hydrogen and its subsequent consumption by the microorganisms to carry out reductive dechlorination reactions.

In the studies carried out to date, numerous types of bioelectrochemical systems and contaminants have been proposed and studied, but always operating under electrode potential control conditions, i.e., adjusting the potential of the working electrode to a chosen value (“potentiostatic” control). By setting the electrical potential, the selectivity, yield and speed of the desired reactions can be governed. In these methods, the working potential of the electrode is thus a crucial parameter, as it is directly related to the amount of hydrogen produced and consequently to the theoretical rate of consumption by the different contaminant species.

However, this operating mode has several problems.

The thermodynamic potential at pH 7 for the production of hydrogen is -414 mV with respect to the Standard Hydrogen Electrode (commonly abbreviated SHE), but the presence of electrode biofilms or redox mediators can decrease this potential. Consequently, to increase dechlorination rates, it is convenient to increase the potential to more reducing values; however, this causes greater competition between the micro-organisms with the consequent worsening of their dechlorinating capacity due to the greater availability of hydrogen for competitive mechanisms such as methane formation (methanogenesis), as well as the occurrence of parasitic reactions such as sulphate ion or nitrate ion reduction which exploit the greater reducing power provided by the cell’s cathode. The presence of anions such as sulphate and nitrate also leads to an increase in the circulating current in the electrolysis cells, with a consequent increase in energy expenditure.

It is thus an object of the present invention to provide a bioelectrochemical method for the dechlorination of chlorinated aliphatic hydrocarbons which overcomes the problems of the known systems, and in particular which obtains the desired dechlorination effect with reduced energy expenditure.

SUMMARY OF THE INVENTION

This and other objects are achieved with the present invention with a bioelectrochemical dechlorination method of chlorinated aliphatic hydrocarbons in which bacteria of the genus Dehalococcoides are used, characterised in that the current circulating between the working electrode and the counter electrode is kept constant during the dechlorination operation.

BRIEF DESCRIPTION OF THE DRAWINGS

- Fig. 1 depicts the transformation efficiency of trichloroethylene under potentiostatic (of the known art) and galvanostatic (according to the invention) conditions at two different fixed current values;

- Fig. 2 shows the transformation efficiency of vinyl chloride under potentiostatic (of the known art) and galvanostatic (according to the invention) conditions at two different fixed current values.

DETAILED DESCRIPTION OF THE INVENTION

The following abbreviations are used in the following description and examples:

TCE: trichloroethylene;

VC: vinyl chloride. In the known potentiostatically controlled systems, the current is free to vary spontaneously in accordance with the possible reactions depending on the species present in the treated water; this results in a high energy expenditure of the remediation process.

Instead, the inventors have surprisingly found that operating at constant current, i.e., under galvanostatic conditions, the same results as in the known (potentiostatic) methods in terms of chlorinated aliphatic hydrocarbon degradation are obtained, but with fewer secondary or parasitic reactions, and thus significantly increasing the reaction yield expressed in terms of moles of reacted chlorinated aliphatic hydrocarbons per ampere.

The bacteria used in the method of the invention are of the genus Dehalococcoides, and in particular the type Dehalococcoides mccartyi. For the purposes of the invention, commercial mixtures enriched with these microorganisms with concentrations around 1 x 10¹¹ cells/L can be used.

The microbial strain of choice, Dehalococcoides McCartyi, has extremely low hydrogen values (in the order of 10'⁸ - 10'⁹ atm H2) necessary for the metabolisms associated with the reductive dechlorination reaction, allowing to work at currents in the order of a few milliAmps (generally between about -1 and -10 mA), allowing a cathode (working electrode) potential sufficient to produce molecular hydrogen concentrations adequate for the removal of chlorine atoms from the molecule up to the production of ethylene, an unsaturated C2 compound devoid of chlorine atoms. The value of the current to be set is however adjustable depending on the degree of chlorination and the load of contaminant to be treated in the remediation process.

The above current values are indicated with a negative sign because the system of the invention is an electrolysis cell in which the cathode compartment functions as the working electrode. In an electrolysis cell, energy is supplied from an external source to make non- spontaneous chemical reactions occur, as opposed to what occurs in a battery, in which the chemical potential is used to carry out an electrical work.

The current to be used in the system is typically determined once the amount of chlorinated hydrocarbons to be treated is known, which in turn is estimated based on the volume of water to be treated and the concentration of chlorinated hydrocarbons therein. Knowing the amount of chlorinated hydrocarbons to be removed, the milliequivalents of electrons required for the reaction are calculated, and the value thus obtained is preferably increased by a precautionary value of around 30%; in doing so, the chlorinated hydrocarbons take on the role of the reaction’s “limiting agent”, thus ensuring that their reaction is complete. The maximum current value would theoretically be infinite, but since one of the objects of the invention is to lower the energy consumption required to operate the system, and since high currents, in absolute value, result in a higher energy demand, it is preferable to work with current values close to the minimum required, which is determined as described above.

The invention will be further described by means of the following examples.

The following instruments and materials were used in the experimental tests:

- a self-built micro-pilot system (hereinafter also referred to as a reactor), comprising a first tubular glass cell with a volume of 8.24 L with connectors at both ends for the inlet and outlet of the water being treated, containing a working electrode consisting of a bed of granular graphite (Faima srl, Italy), an internal counter electrode also consisting of granular graphite (Faima srl, Italy), enclosed in a nonwoven fabric casing (polyethylene PE-HD) and an Ag/AgCl reference electrode (Amelchem srl, Italy). In the tests, the reactor was polarised in a three-electrode configuration by means of a VSP 300 potentiostat/galvanostat (BioLogic, France), which allows the polarisation mode of the reactor to be controlled in potentiostatic or galvanostatic mode. The experiment was conducted by feeding a synthetic solution contaminated with TCE at a theoretical concentration of 0.05 mmol/L;

- the reactor was inoculated with a microbial culture grown under laboratory conditions consisting of 75% Dehalococcoides mccarty (16 rRNA sequencing available from DDBJ/ENA/GenBank in project PRJNA705054 (SRA: SRX10172732)).

- A Dani Master gas chromatograph (DANI Instruments SA, Contone, Switzerland) with HeadSpaceSampler and autosampler was used for the analysis of the water samples, before and after the dechlorinating treatment (both of the invention and according to the known art); the detector used to quantify the compounds was a Flame Ionisation Detector coupled to a capillary column for the chromatographic separation of the different compounds.

EXAMPLE 1

The reactor described above was fed continuously for 152 days with contaminated water at an average flow rate of 3 L/d, which corresponded to a hydraulic residence time (HRT) of 2.75 d.

During the first 41 days, the test was conducted under potentiostatic conditions, in order to collect data for comparison with those obtained by the method of the invention, operating with a potential difference between the working electrode (cathode) and the standard hydrogen electrode of -0.45 V (i.e., -0.65 V against the Ag/AgCl reference electrode used); thereafter, up to day 90, operations were carried out under galvanostatic conditions at a fixed current of -10 mA, the current was then set at -1 mA up to day 135, and finally under open circuit conditions.

During the test, water was sampled at the reactor inlet and outlet, the composition of which made it possible to determine the complete transformation of TCE into VC and ethylene, thus highlighting the action of the dechlorinating microorganisms inoculated in the system. The chlorine eliminated by organic molecules passes into solution as chloride ion; the quantity, in the order of micrograms per litre, is not a problem, being significantly less than the amount of chloride ion normally present in environmental waters.

The results of the test are shown graphically in Figures 1 and 2. In Fig. 1, the TCE inlet concentration values are represented by white squares, while those of TCE in output from the reactor are represented by black circles; in Fig. 2, white diamonds and black circles show the concentration values of VC and ethylene in output from the reactor, respectively.

As highlighted in Fig. 1, the removal of TCE is not affected by the transition from potentiostatic to galvanostatic control, and the TCE concentration in output from the system is always zero under all conditions, both the potentiostatic conditions of the known art and those of the invention.

Fig. 2 shows instead the differences in the behaviour of the system in the removal of VC produced by the dechlorination of TCE: operating under potentiostatic conditions (-0.45 V), the removal of VC was poor; in the galvanostatic control, the removal of VC depends on the circulating current, and when the current was set at -10 mA, the main products exiting the system were VC and ethylene with preponderance of the latter, while at -1 mA the conversion efficiency of VC decreases.

Table 1 shows the TCE removal and efficiency of coulombic reductive dechlorination (RD) of the system:

Table 1

By analysing the current, the potential difference between the electrodes and the cathode potential, it was possible to observe a clear reduction in energy consumption due to the galvanostatic control. The cathode potential remains more or less constant over the three periods of the test, decreasing from -0.45 V to -0.42V and -0.35V, while the counterelectrode potential remains approximately similar to that observed in the potentiostatic condition and decreases dramatically in the -1 mA condition, reaching values of -0.17 V. Consequently, the potential difference in the cell is drastically reduced in the -1 mA condition, decreasing by about two orders of magnitude, as shown in Table 2. Table 2

Claims

1. Bioelectrochemical dechlorination method of chlorinated aliphatic hydrocarbons in which bacteria of the genus Dehalococcoides are used, characterised in that the current circulating between the working electrode and the counter electrode is kept constant during the dechlorination operation.

2. Method according to claim 1, wherein said bacteria are of the Dehalococcoides mccartyi type.

3. Method according to any one of claims 1 or 2, wherein the concentration of bacteria of the genus Dehalococcoides is about 1 x 10¹¹ cells/L.

4. Method according to any one of the preceding claims, wherein said current is maintained at a value comprised between -1 and -10 mA.

5. Method according to any one of the preceding claims, wherein the current value is determined by the milliequivalents of electrons necessary for the dechlorination reaction of the chlorinated aliphatic hydrocarbons present.

6. Method according to claim 5, wherein the current value is increased by 30% with respect to the milliequivalents of electrons necessary for the dechlorination reaction of the chlorinated aliphatic hydrocarbons present.