IT201900015210A1 - SYSTEM FOR THE RESTORATION OF POLLUTING MATRICES USING MICROBIOLOGICAL FUEL CELLS - Google Patents

Description

Description of the invention entitled:

"SYSTEM FOR THE RESTORATION OF POLLUTING MATRICES USING MICROBIOLOGICAL FUEL CELLS"

Description

Technical sector

The present invention refers to the field of chemistry and in particular to the field of electrochemistry in environmental remediation applications. More in detail, the present invention relates to a particular system for the rehabilitation of environmental matrices such as soils, aquifers, sediments, wastewater and the like, from pollutants such as, by way of non-limiting example, hydrocarbons and metals. The said system is characterized by the fact that it uses a particular type of microbiological fuel cells, which involve the use of specific electrodes, and a specific bacterial inoculum for the substrate / matrix to be treated.

Known art

To date, the recovery of polluted environmental matrices can be obtained through chemical-physical and / or biological treatments that can be carried out ex-situ or in situ. The choice of one treatment rather than another is made based on the type of substrate to be restored and the extent of the area to be treated. Ex-situ treatments involve the removal of contaminated soil / sediment from the area affected by the pollution which can be treated on site or off site. In the first case, the soil is removed to be treated in a mobile plant on site; in the second case, the treatment plant is located in a place distant from the area that needs remediation. The ex-situ treatments are based on chemical-physical processes with a waste of resources both in monetary, energy and environmental terms (CO2 emissions, water pollution consumption, etc.). Among the ex-situ treatments are listed:

- Soilwashing: takes place in special plants. It consists in the screening, grinding, homogenization of polluted soils / sediments, with subsequent washing with high pressure water jets (often additives with surfactants). The process water is then subjected to purification treatment. It is usually applied to soils with medium or coarse grain size.

- Soilextration: consists in the extraction of pollutants from the ground. It is a technique suitable for the remediation of soils polluted by volatile organic compounds.

- Inertization: consists in the incorporation of contaminated matrices in inert materials in order to confine them to landfills. It is suitable for soils and sludge contaminated by heavy metals, oils, polycyclic aromatic hydrocarbons, PCBs.

- Vitrification or dual phase extraction: the technique consists in extracting contaminated groundwater present in the saturated area of the soil with rising contaminants present in the soil and subsequent water treatment. It is not indicated in cases of diffuse pollution or deep aquifers.

- Thermal desorption. The technique consists in the vaporization of the compounds without oxidation or destruction of the same. It can be achieved by steam extraction or radio frequency heating.

- Thermal destruction.

Among the in-situ treatments are listed:

- Soil vapor extraction: it is indicated for the reclamation of soils contaminated by volatile organic compounds extracted in the form of steam and treated on the surface.

- Air sparging: through the injection of pressurized air, it allows to carry out the stripping of contaminants present in the land to be reclaimed. - Pump & Treat: pump and treat is a contaminated soil remediation technique that consists of pumping and surface treatment of polluted groundwater. This technique allows the remediation of polluted soils by removing miscible contaminants and also LNAPL (organic compounds lighter than water, such as benzene, diesel, mineral oils, kerosene, BITEX, alkanes) in the free phase and DNAPL (liquid organic substances denser than water, such as chlorinated solvents and halogenated hydrocarbons).

- Chemical oxidation: consists of injecting oxidants of chemical origin into the subsoil, such as hydrogen peroxide or potassium permanganate. - Bioremediation: these techniques, for soils or sediments, are based on the metabolism of microorganisms and plants capable of degrading or sequestering the pollutions that may eventually accumulate in plant tissues (metals, for example) or simply be reduced into simpler compounds that they can be used as a source of energy and carbon. For the purposes of understanding the present invention, described in detail below, it is of interest to indicate that among the various pollutants are hydrocarbons, these substances are typically made up of different fractions, including that of polycyclic aromatic hydrocarbons (PAHs), of particular scientific interest due to their toxicity and mutagenic potential both towards man and other living organisms. More generally, the deleterious effects of hydrocarbons on the environment are related to the density of the mixture, the tendency to evaporate, the viscosity and the pour point, which indicates the lowest temperature at which a fuel can be stored while maintaining the ability to flow. The technology of microbiological fuel cells, hereinafter referred to as "Microbial Fuel Cells (MFCs)", over the last twenty years has shown significant potential in the in situ remediation treatment by increasing the degradation activity of bacteria, in comparisons of these substances, through electrogenesis instead of through oxygenation and the use of solvents. This aspect in itself represents a first gain in terms of energy and resources to be used for bioremediation operations. Another advantage inherent in the use of MFCs is in the production of electricity that can be used to power electronic devices for environmental monitoring or for the management of MFCs. These peculiarities make the technology of microbial cells perfectly insertable among the "green" technologies.

Microbial Fuel Cells (MFCs) are systems in which bacteria, using organic molecules as a source of carbon and energy, exchange electrons with an electrode placed in an environment where there is no oxygen (anode) connected via an external circuit to another electrode, the cathode, placed in an environment in which there is oxygen or, in any case, an electron acceptor. Following the potential difference between the electrodes, an electronic flow is created, an electric current, which can be easily measured by a multimeter. Simultaneously with the external flow of electrons, there is a flow of protons (H + ions) inside the cell, caused by the difference in concentration of these ions between the anodic and cathodic compartment. Finally, on the surface of the cathode the electrons coming from the anode are "captured" by a molecule that acts as an acceptor of electrons and protons, forming a reduced molecule. If the acceptor is oxygen, the final product is water.

MFCs can find different applications, first of all the treatment of waste water, sludge, sediments and soils contaminated by metals and / or hydrocarbons and other organic substances (for example organochlorine compounds). The possibility of coupling the consumption of organic substances dissolved in water with the synthesis of organic compounds starting from carbon dioxide has recently been demonstrated. Given the high versatility of microbial metabolism, MFCs can be virtually fed with any organic compound, even in the presence of heavy metals. In short, bioelectrochemical systems behave like batteries in which there are no chemical catalysts such as metals, but bacteria that operate biocatalysis. The advantages of MFCs technology can be summarized in the following points:

- Energy recovery from clean-up processes in the form of electricity. Currently an energy recovery from microbial cells is estimated in the order of tens of W / m <2> of cathodic / anodic surface used.

- Possibility of coupling the removal of pollutants with the biosynthesis of molecules of industrial interest and with the fixation of CO2 through the biosynthesis of organic compounds.

- MFCs can operate at room temperature.

- Little or no need to add nutrients or oxygen to favor the degradation process of the organic substance.

- Use of inexpensive materials and low environmental impact for the construction of the devices.

- Versatility in the design of the systems that can be designed "ad hoc", according to the foreseen applications.

- The carbon neutral balance process: in a nutshell, there are no net emissions of carbon dioxide.

- The development and commercialization of such a technology can lead to the satisfaction of the need expressed by the market for remediation systems characterized by lower costs.

Compared to micro-electrolytic systems (Microbial Electrolysis Cells-MECs) in which the degradation process is mainly electrochemical in nature, the degradation of the organic substance in MFCs is driven by bacterial metabolism in a process that finds its balance spontaneously. Unlike MECs (Microbial Electrochemical cells) in MFCs there is no need to apply an external potential source. However, in some cases it may be appropriate to act on the electrode potentials in order to facilitate the removal of particularly recalcitrant molecules and / or metals. To this end it is possible to use systems with more electrodes (for example with two or more cathodes) connected to specific resistances, to convey to the electrodes the energy produced by other MFCs fed with the same polluted substrates or with other matrices (waste water, for example ). Alternatively, it is possible to apply an external potential through the use of batteries (1.5 V, for example) or by distributing electrical energy (from mA or A depending on the volumes to be treated) produced by photovoltaic cells.

The purpose of the present patent application is to propose a new type of MFCs with characteristics such as to consistently optimize the degradation process of the polluting organic substance.

The advantages already mentioned are combined with simplified logistics (it is not necessary to move soils / sediments), energy recovery achieved through electrogenesis: from the bioremediation processes it is possible to recover electricity to be reused in the remediation process itself or in other applications. With the development of new electrode materials and systems for managing the generated power, it will be possible to further increase the performance of these systems. These advantages also translate into an increase in the competitiveness of the companies that use them.

Description of the invention

According to the present invention, a new system is described and specifically a system based on a new type of MFCs which, as "eco-compatible" systems with zero emissions and zero use of potentially harmful substances, are in line with the need perceived by the opinion publishes that it has technologies that can improve the quality of the environment and, therefore, reduce the risks to human health and beyond. The possibility of producing and using energy from a renewable source with practically zero CO2 emissions, as the process is completely based on bacterial metabolism, is in fact an aspect of undoubted commercial interest.

The MFCs according to the present invention comprise graphite electrodes and a further carbon fabric coating.

Advantageously, said cells were found to be completely compatible with the spontaneous growth of plants, favoring their development, especially in comparison with untreated soils. In this regard, it is of interest to point out that the growth of plants, through the interactions between bacteria and roots, accelerates the degradation processes of organic substances. The by-products of this degradation can then be assimilated by the plants themselves. In the case of metals, for example, they can change the oxidation state following microbial metabolism and possibly also be assimilated by plants and accumulated in the tissues.

More in detail, the MFCs according to the present invention are systems based on the use of electrodes in combination with a bacterial inoculum composed of at least 10 microbial strains belonging to the families of the bacillaceae, Enterobacteriaceae, Staphilococcaceae, Xanthomonadaceae and Pseudomonadaceae for the treatment of soils, sediments , waste, contaminated by organic substances (including hydrocarbons). As for the electrodes, their dimensions can vary from a few tens of centimeters to a few meters depending on the size of the volumes (solid or liquid) to be treated. As for the electrode materials, they can be chosen from graphite bars coated with carbon fiber fabric, bars or metal meshes containing biochar granules ranging in size from 0.3 to 2.0 cm, but also PLA-graphene conductive and PLA-graphite capable of being processed by 3D printer. The use of the 3D printer allows in this case to explore, for applications in the field, also different electrode conformations such as sheets of square or rectangular geometry with an intact surface or with holes / slits through which the flow of water / wastewater to be treated can be realized. . The bacterial mix used in the cells according to the present invention includes the following microbial strains isolated from heavily polluted dusts by hydrocarbons and identified by analysis of the 16S rDNA: Alcanigenes faecalis, Achromobacter spp., Pseudomonas anguilliseptica I and II, Xantomonas spp., Staphylococcus succinus, Bacillus mojavensis, Enterobacter aerogenes, Arthrobacter misorens, Stenotrophomonas spp. The effectiveness of said bacterial mix in removing hydrocarbons and reducing toxicity in contaminated matrices, with particular regard to polycyclic aromatic hydrocarbons, has been demonstrated for five PAHs among the sixteen priority (naphthalene, anthracene, phenanthrene, pyrene, benzo (a) pyrene ) in tests carried out both in bioreactors containing contaminated soils and in aqueous solution. In the latter case, the tests were carried out both in the presence and in the absence of electrodes.

The microbial mix showed in aqueous solution (after 5 weeks at 20 ° C) a removal of the total PAHs of more than 90%, and of 62% and 73% as regards COD and TOC, in the presence of electrodes. Removal was lower without the help of electrogenesis, with values of about 80% for total PAHs, 18% for COD and 30% for TOC. In bioreactors with contaminated soils, the same consortium recorded, after a month of treatment, a decrease of between about 8% and 79.4% depending on the hydrocarbon considered. In all cases, ecotoxicological tests on Lepidium sativum, Daphnia magna and Ceriodaphnia dubia showed a decrease in acute, but not chronic, toxicity. Both for the suspensions in the aqueous phase and on the ground, a slight stimulating effect was also recorded on Lepidium sativum, with an increase in the germination rate of the plants. With the cell system according to the present invention inoculated with soil polluted by hydrocarbons, the activity of the bacterial mix used led to an acceleration of the degradation of PAHs, in particular of the molecules with 5 and 6 aromatic rings and of anthracene (3 rings aromatics), with degradation rates ranging from 21% of Benzo [k] fluoranthene to 60% of anthracene in 4 weeks of activity at an ambient temperature between 12 and 17 ° C. As for the effect on plants, the wilting rate of the seedlings was reduced compared to controls not content with bacterial inoculation.

On a laboratory scale, the bioremediation MFCs cells according to the present invention may include at least one anode formed by two electrodes connected in parallel by an electrical cable with sheath and by two cathodes also connected in parallel. The electrodes were typically and by way of non-limiting example, constructed as follows:

- A 5 cm long pure graphite bar, 0.5 cm diameter (17 cm <2> of surface) was wrapped in 25 cm of collector wire made of an 80/20 nickel / chromium alloy (0.25 mm in diameter, 4 Ohm of resistance) with turns at an average distance of 1 cm from each other. The carbon fabric was fixed on the graphite bar using another 15 cm of Ni / Cr -80/20 wire (0.25 mm in diameter, 4 Ohm of resistance, turns 1.3 cm apart) .

- The two Ni / Cr collector wires (the internal and the external one) of each electrode have been inserted in aluminum crimps (internal diameter 1 mm, external 2 mm, length 8 mm). The Ni / Cr cables were covered and secured to the crimp by means of a heat shrinkable sheath.

- Finally, an electrical cable was connected with the crimp and the joints covered with electrical tape.

Once ready, the anodes and cathodes can be placed in containers (cells) containing soils or sediments, at varying depths and orientations, subject to a distance between anode and cathode not exceeding 5 cm for electrodes with a diameter of 0, 5 cm. For scaled systems, an approximate ratio between anode-cathode distance and anode diameter between 6 and 10 is provided. Given the absence of a salt bridge capable of connecting the electrodes, the soil must have a water content of no less than at 10% w / w the system may include the set-up of a two-chamber layout connected by a saline bridge formed by a solution of KCl 1M 1.5% agarose made between cylindrical electrodes partially immersed in chambers containing a solution saline (Winogradky's solution).

The application of the MFCs cells according to the present invention, to the remediation of deep layers of soil, sediments or even groundwater, implies the setting of a different layout in which the anode and cathode can be inserted in a linear system connected with the management of the cell and the energy it produces. The management system in scaled systems will also be able to control the potential at the cathode, if necessary.

Description of the figures

The system for the rehabilitation of polluted substrates, typically by the presence of hydrocarbons and metals, using microbiological fuel cells, will be described in detail below also with reference to the attached figures in which: FIGURE 1 and 2 show the profile that the electrodes of the cell / and of the system according to the present invention can take in some embodiments of said system. In more detail, the figure in question shows the architecture of the electrodes when the matrices to be treated are liquid substrates. More in detail, the figure in question shows that in some embodiments the electrodes of the microbiological fuel cell of the bioremediation system in question can be represented by sheets with a square (Fig. 1) or rectangular (Fig. 2) profile with holes / cracks through which the flow of water / wastewater to be treated can be created.

FIGURE 3 shows a schematic representation relating to the variable spatial configuration that the electrodes of the microbiological fuel cell of the system in question can assume according to the specific applications and in particular according to the substrate to be treated.

More in detail, Figures 2 (a), 2 (b) 2 (c) show respectively the 90 °, 45 ° and 30 ° orientation that the anodes can assume with respect to the cathodes of the pile in a system for the treatment of soils / sediment or sludge.

FIGURE 4 shows a front view of a type of electrode 1, represented by a graphite bar, on which the collector wire 1 'can be observed.

Description of the preferred embodiments

Since they are based on microbial metabolism, assisted by electrogenesis, the bioremediation MFCs cells of the system according to the present invention can work both with cathodes exposed to air and immersed in an anoxic / anaerobic environment. This is thanks to the ability of microorganisms to use chemical elements other than oxygen as final electron acceptors. The MFCs bioremediation cells of the system in question are therefore able to operate, or to treat and remediate contaminated soils, at various depths, as well as sediments and polluted aquifers.

As an example of this application, the results obtained during an experiment in which the bioremediation MFCs cells according to the present invention were made in simple plant pots are reported. The cylindrical electrodes are placed in vessels on the bottom of which a layer of expanded clay has been affixed and filled with soil contaminated by hydrocarbons (1 kg) supplied by a company operating in the field of transport and storage of fuels.

An anode formed by two electrodes, connected in parallel, was placed in an inclined position between 30 ° and 45 ° at a depth of about 4 cm below the surface, reaching an approximate depth of about 8 cm on the total 10 cm of soil. available. The cathodes were inserted into the ground for about 2.5 cm, leaving the remaining part exposed to the air. The anode-cathode distance was calculated to be approximately 3 cm. The cells according to the present invention have been conceived as monocode systems capable of becoming polyatodic, as provided in some embodiments according to the present invention, depending on the extent of the volumes to be treated. In this case the cathodes, as well as the anodes, are connected in parallel. Finally, 40 mL of the above bacterial mix suspended in physiological solution (10 <8> CFU / mL) was inoculated in 5 mL aliquots at depths between 2 cm and 4 cm, below the surface of the medium and distributed in eight different points of the cell with a theoretical average concentration of 10 <6> CFU / g of soil. All the cells, as well as the soils used as controls (one with untreated soil and one inoculated with the aforementioned bacterial mix) were irrigated with 40 mL of water, distributed according to the criteria described above, every 48 hours: this to ensure a content of water capable of supporting electrochemical reactions, bacterial metabolism and the eventual germination of seeds.

During the tests of the bio-sanitation cells MFCs of the system according to the present invention, the germination of seeds present in the treated soils was observed, which indicates the compatibility of the system conceived with the spontaneous growth of plants. The application in the field involves the setting up of cell batteries connected in parallel and in series to then be connected to a management system of the batteries and the energy they produce. This system will include an automated management system for the cells in question, maximum power tracking system power converters, energy storage systems. The presence of resistances between the connections between the electrodes allows the potential to be stabilized, while the parallel and series connection of the stacks allows the voltage and current produced to increase and therefore energy recovery from the bio-remediation process. The applicability of the prototypes made, given the positive interaction with plants, will include the potential integration into urban green infrastructures for the treatment of rainwater and, more generally, for the maintenance of green areas and soil quality, with recovery of electrical energy to be reused in situ, for example for the lighting of the same green areas or to power electrical / electronic devices.

Systems according to the present invention can be developed to treat first rain water, leachate, industrial waste water in tanks. In this case, the water to be treated is progressively passed through a series of tanks equipped with electrodes where, gradually, the toxic substances are removed, with the formation of biomass. The inoculum represented by the said bacterial mix can also be used for the treatment of contaminated water.

For applications to wastewater and other liquid matrices, the surface electrodes with ripples and over elevations ranging in size from 0.3 to 1.0 cm depending on the size of the electrodes. The latter may consist of conductive PLA graphite / graphene or other conductive material that can be molded in a 3D printer or otherwise modeled. These electrodes can have a quadrangular geometry, with circular or rectangular slots, with a width / diameter varying between 0.5 and 3 cm to allow the flow of water through the anodes. Another electrode geometry involves the development of carbon fiber screens with mesh sizes ranging from 0.3 mm to 1 cm, filled with granules, with a diameter between 0.6 and 2 cm, formed by biochar or activated carbon . In the screens are inserted metal wires (Ni / Cr 80/20 or other alloy) capable of determining the shape and structure of the anodes and collecting the electrons produced by the bacteria that colonize the granules.

Claims (11)

Claims 1. System for the purification of polluted substrates said system comprising at least one microbiological fuel cell based on the use of electrodes (1) in combination with a bacterial inoculum, said system being characterized in that said bacterial inoculum comprises at least ten microbial strains belonging to the families of the bacillaceae, Enterobacteriaceae, Staphilococcaceae, Xanthomonadaceae, Pseudomondaceae, called microbial strains including at least Alcanigenes faecalis, Achromobacter spp., Pseudomonas anguilliseptica I and II, Xantomonas spp., Staphylocomonensis spp. . 2. System for the purification of polluted substrates according to the preceding claim wherein the electrodes (1) of the microbiological fuel cell comprise materials such as: graphite rods coated with carbon fiber fabric, rods or metal nets containing biochar granules of the size ranging from 0.3 to 2.0 cm, conductive PLA graphene, and PLA graphite. 3. System for the purification of polluted substrates according to the preceding claim in which the electrodes (1) of the microbiological fuel cell are represented by foils having a profile defined by any closed curve or polygonal shape. 4. System for the purification of polluted substrates according to the previous claim in which the electrodes of the microbiological fuel cell have at least one hole (10), the latter being able to allow the flow of polluted substrates such as water / wastewater to be treated. System for the purification of polluted substrates according to any one of the preceding claims in which the electrodes (1) of the microbiological fuel cell are represented by graphite rods 5 cm long and 0.5 cm in diameter, each rod of graphite being also wound in 25 cm of internal wire consisting of a Nikel / Chrome 80/20 alloy, said wire having a diameter of 0.25 mm and 4 Ohm of Resistance, said graphite bar, wound by said wire, being further coated by a carbon fabric fixed on said graphite bar with a further 15 cm of external Nikel / Chrome 80/20 wire of 0.25 cm in diameter and 4 Ohm of resistance, said additional 15 cm of external wire generating on said graphite bar coated by said carbon fabric turns spaced 1.3 cm apart, said inner and outer wires of each electrode being also inserted in aluminum crimps with an internal diameter of 1 mm and an external diameter of 2 mm and the length of 8 mm said internal and external wire being furthermore secured in said crimps by means of a heat-shrink sheath and with joints covered with insulating material. 6. System for the purification of polluted substrates according to the preceding claim in which the electrodes (1) of the microbiological fuel cell are placed in cells, containing soils or sediments, with variable depth and orientation and in which the distance between anode and cathode is no more than 5 cm when said electrodes have a diameter of 0.5 cm. 7. System for the purification of polluted substrates according to the preceding claim wherein the soil or sediment has a water content of at least 10% by weight. 8. System for the purification of polluted substrates according to any of the previous claims in which the cathodes are exposed to air and / or immersed in an anoxic / anaerobic environment. 9. System for the purification of polluted substrates according to any one of the preceding claims in which, in at least one microbiological fuel cell, an anode is formed by two electrodes and is oriented between 30 ° and 45 ° with respect to the ground and in which the distance between anode and cathode is 3 cm. 10. System for the purification of polluted substrates according to any one of claims 1 to 3 wherein the electrodes comprise carbon fiber defining nets with mesh sizes ranging from 0.3 mm to 1 cm, said nets being filled with granules, with a diameter between 0.6 and 2 cm, and formed by biochar or activated carbon in said nets being also inserted metal wires Ni / Cr 80/2. 11. Use of the system according to any one of the preceding claims for the rehabilitation of substrates such as soils, aquifers, wastewater, waters and polluted sediments.