DESCRIPTION

State-of-the-art Microbial Electrochemical Technologies (MET) are an attractive but too expensive method for wastewater treatment.

Despite the technological evolution, the ESMs have not yet a large-scale application.

Over time, the MET architecture has however become very flexible, in order to increase the range of potential applications, both in laboratory scale and for field applications.

A limitation on the use of MET is unfortunately represented by the high cost of the materials with which some of their main components are obtained (electrodes, membranes, reactors themselves) .

Only in the last few years attempts have been made to make some types of MET commercially interesting .

Recently, to give a competitive advantage in scale applications of Microbial Electrochemical Technologies (MET) , it has become necessary to develop low-tech architectures and the use of low- cost materials, of natural (biogenic) origin and recyclable . Summary of the invention

In light of the requirements known in the field, the present invention relates to new materials for the production of low-cost MET modules, of biogenic origin and recyclable, to be used as a means for carbon capture and recovery of nutrients in waste water purification and in the remediation of contaminated soils or waste landfills.

Brief description of the drawings

Fig. 1. Starting materials (waste biomass and agroforestry by-products, Arundo donax, bambusoideae, clay) ; pyrolytic process diagram and temperature ramp used; electrodes/separators obtained.

Fig. 2. Diagram of the process phenomena and structure; set-up diagram used for laboratory scale tests. The presence of the connection between the two electrodes through an ad hoc circuit and external resistance can be optional.

Fig. 3. Production trends of electric current (A) and of open circuit potential (B) , on different batch cycles with sodium acetate feed (3 g/L) . The graphs show the experiments in triplicate. Power curves (C) and anodic polarization curves (D) typical of these systems in external circuit mode, Microbial Fuel Cell (MFC) type. Fig. 4 - Characterization (Illumina MiSeq sequencing) of mixed microbial communities that spontaneously form on the electrodes and on the structural elements of the MFCs constructed with the proposed materials. This figure shows an example obtained starting from pig slurry. The microbial activities of oxidation (anode) and reduction (cathode) are the driving force for the functioning of these electrochemical systems.

Fig. 5 - Measurements in the liquid phase of the anodic chamber (waste water) : removal of COD (soluble fraction, 0.45 micron filter), ammonium ion and nitrification and denitrification phenomena.

Fig. 6 - pH Measurements relative to the anode and cathode chamber made during the operating period of the microbial systems. Measurements of pH near the cathode, carried out with pH micro-electrode.

Fig. 7 - Mapping of the Na and Ca distribution through the EDX probe under the SEM (scanning electron microscope) of a section of the cathode after 70 days of operation of the bio-electrochemical system. 3-D tomographic analysis of a cathode analyzed after 70 days. The mineral deposits are highlighted in gray.

Fig. 8 - Elemental content of the main nutrients, in 10 g of separator material/cathode downstream of about 100 working days in two different waste waters: bovine slurry (CM) and pig slurry (SM) .

Fig. 9 - result of control experiments.

Object of the invention

In a first object thereof, the invention describes an MET technology comprising an electrode prepared from biogenic materials (bioelectrode ) .

In a second object thereof, a method for preparing the materials and the structure of the MET and the electrode of the invention is described.

According to another object, the present invention provides application methods which comprise the use of said MET.

According to a further object, the materials deriving from the MET of the invention, at the end of their life, are used entirely to produce mixed fertilizers or soil improvers.

Detailed description of the invention

In a first object, the invention describes a microbial electrochemical technology (MET) and, in particular, a bioelectrode of a microbial electrochemical technology (MET) .

The term "bioelectrode" refers to an electrode that uses electrocatalytic activity developed by electroactive microbial communities.

These microbial communities are preferably ubiquitous in the environment.

For the purposes of the present invention, the bioelectrode comprises a support, which is preferably made from natural or environmentally compatible materials and employs environmentally compatible technologies; advantageously, these features make the bioelectrode of the invention completely recyclable.

In a preferred aspect of the invention, this bioelectrode is a cathode or in alternative aspects it may be an anode.

In a further aspect, the bioelectrode may be a "snorkel" electrode, or an electrode inside which a continuous electrochemical gradient is formed.

For the sake of simplicity of description, hereinafter reference will be made to a bioelectrode (or electrode) .

More in detail, the bioelectrode of the invention is prepared from a biomass.

This biomass can be obtained or can be represented by agroforestry by-products.

In one aspect of the invention, the biomass is represented by a portion of a plant part.

For this purpose, corn (Zea mays), cane (Arundo donax) or bambusoideae may be used.

For example, corn stalks or cane stem portions may be used.

In a preferred aspect of the invention, the biomass used has a naturally cylindrical shape.

In an even more preferred aspect, the biomass has a hollow shape.

In one particular aspect of the invention, the electrode may be in the form of a preparation in which the biomass is mixed with clay.

Alternatively, wood chips from lignocellulosic biomasses or biochar obtained from these materials may be used for mixing with the clay.

The term "biochar" refers to a coal produced by a biomass carbonization process.

Both the wood chips and the biochar must preferably have a minimum size of >0.5 mm and a maximum size of <5 mm; such a size can be obtained by grinding in a mill and subsequent screening.

The mixing preferably occurs in amounts of:

15-60% (weight/weight) of clay when mixed with wood chips,

15-75% (weight/weight) of clay when mixed with biochar .

The mass obtained is subsequently modeled, for example by using ceramic processing molds and ceramic processing techniques, in a suitable shape, which may be cylindrical, spherical, tubular, granular or flat and is preferably of a hollow shape.

In one aspect of the invention, the described bioelectrodes do not contain non-biogenic starting materials .

Biogenic material refers to a non-fossil natural material that is not subjected to subsequent processing which makes it incompatible with the environment .

Materials not contained in the bioelectrodes of the invention are therefore represented by materials selected from the group comprising: plastic polymers, plastics, synthetic binders, Nafion, PTFE, processed carbon fiber, solid metal parts, synthetic resins, non-biodegradable or non-environmentally compatible artificial materials.

According to a second object, it is described a process for preparing the bioelectrodes of the invention .

Such a process comprises the step of providing a bioelectrode in a suitable shape, obtained as described above.

This shape is preferably hollow. Subsequently, the process involves the step of subjecting the bioelectrode to a pyrolysis process, in an inert or reducing atmosphere, during which it is subjected to high temperature.

This makes the material rigid, in its original shape or given by a mold, porous and at the same time electro-conductive (due to a sufficient content of graphite-type carbon crystals) .

For the purposes of the present invention, pyrolysis is carried out according to the following conditions :

- Temperature ramp: up to 900°C with an increase of about 10°C/minute;

Isothermal step: maintenance at 900 °C for about 60 minutes;

- Cooling step: temperature decrease of about 5°C/minute .

According to an optional aspect of the invention, before the pyrolysis step, the materials are subjected to a surface functionalization process, with which they are chemically modified with catalysts or functional groups (for example, oxides, metal oxides) or with treatments that modify the charge and the surface area, the size of the pores, and the surface chemical composition. This has the function of improving the abiotic reduction properties of the oxygen and/or the catalytic properties in the reactions biologically catalyzed by the microorganisms.

These treatments include, for example:

- addition of a biomass of microalgae to the initial mixture (1-10% by weight), which increases the surface presence of heteroatoms of Fe, Mg and N;

- addition to the initial mixture (1-10% by weight) of biomass rich in nitrogen components (for example: animal slurry/manure, food processing waste, etc.), to increase the surface N groups .

The materials made according to the present invention have shown to provide the following performances :

a) electrical conductivity (graphitized carbon) ; b) micro- and meso-porosity (0.1 - 100 nm) ;

c) large surface area (100-1000 m ² /g);

d) rigidity.

According to a further object of the invention, a microbial electrochemical technology (MET) is described, represented by a reactor comprising a plurality of bioelectrodes according to the foregoing .

This reactor comprises one or a plurality of cathode/anode pairs or snorkels, in which the cathode or anode or snorkel are prepared according to the above description.

According to an embodiment of the invention, the anode is positioned around the cathode, optionally adding a layer of insulating material; in the absence of the insulating material, anode and cathode form a single electrode that acts as a snorkel, between the anaerobic environment and the air-water interface, in which oxygen is present as a preferential electron acceptor .

For the purposes of the present invention, there are prepared MET reactors with fixed bed of conductive granular material which acts as an anode, for example based on the materials described above, in which cathodes, for example with a cylindrical shape, are alternated at distances of about 1 - 3 cm from each other.

One or more of the systems or reactors of the invention may be used in a method such as for example: purification or remediation of wastewater or contaminated soils or waste landfills.

These methods represent additional objects of the present patent application.

In particular, said methods comprise the step of contacting a reactor or a plurality of reactors according to the above description with said wastewater, said soils or materials in said landfills for a sufficient time.

According to a preferred aspect of the invention, the inner volume and/or an end of the bioelectrode, and preferably of the cathode or snorkel must be exposed to the air; this is in order to obtain an electrochemical gradient and maximize the deposition of macronutrients as insoluble salts on the material.

According to the present invention, after use, the microbial electrochemical technologies, in a form that we can define as "saturated", still possess an intrinsic novelty.

The described reactors or even portions of said reactors, and the described electrodes themselves, may be used for the preparation of fertilizers and soil improvers for agronomic use and the improvement of soil fertility.

These further uses form further objects of the present invention.

Example 1 Biomass is collected from the stem of Arundo donax L. in the experimental fields of the University of Milan (Cascina Marianna, Landriano, PV) .

The canes were cut into 10 cm long cylindrical trunks and freed of leaf biomass, to allow laboratory scale tests .

For experiments, only samples with an outer diameter of 2.5 cm were selected. The outer layers of the bark were removed with a cutter. The resulting inner and outer surfaces of the biomass were scraped and homogenized with an abrasive paper (P400 grit) .

The canes thus prepared were subjected to pyrolysis, according to a temperature ramp shown in Fig. 1.

Downstream of pyrolysis, the pyrolyzed cylinders were used as air-exposed cathodes in MFCs.

The MFCs constructed in this way were tested in laboratory tests, conducted in triplicate.

As an anode, standard materials (carbon cloth SAATI CI, 40 cm ² ) were used, without any surface pretreatment .

The anodes were electrically connected to an insulated power cable.

The electrical connection was then secured by isolating the contact point from the outside with 4 layers of two-component epoxy resin (PROCHIMA EPOXY GLUE) .

Each anode was wound around the cylindrical cathode, adding a layer of insulating fabric between the two electrodes to avoid short-circuit points, as shown in the diagram in Fig. 2.

The MFC systems were tested in batch mode, at a temperature of 25 ± 1°C, and the potential difference between the two electrodes was monitored with a Data Logger, using an appropriate external resistance. The MFCs were fed with pig slurry (collected in a pig farm near Milan, COD = 6000 mg L ^{^1} ) .

After the acclimatization period, a synthetic medium was used [prepared according to some literature references (Bioelectrochemistry 105, 44-49 (2015) , Biotechnol. Bioeng. 103, 513-523 (2009)] to gradually replace the pig slurry. This served to more effectively control the ion migration in the solution, due to the electro-osmotic forces generated by the MFCs.

Sodium acetate (7.7 g 1 ^_1 ) was selected as the carbon source used, added in batch mode after each cycle end .

Each experiment was conducted in triplicate and, as a control, three identical systems were left in an open circuit (GC-OCP) . This is essential to verify the effect of electro-osmotic forces, compared to other phenomena (e.g. adsorption, diffusion) .

The composition of the indigenous microbial community, which spontaneously forms both on the anodic and cathodic surface was always verified, with adequate microbiological analytics (Illumina MiSeq sequencing) .

Example 2

Results

The current trends generated during the acclimatization period and a feeding cycle with sodium acetate are reported in Fig. 3.

With an external load of 100 Ω, the GC-MFCs promptly produced the first current peak (about 0.4 mA) on day 3. On day 7, a power curve was recorded and the internal resistance of the system was estimated. For this purpose, the external load was changed to 500 Ω, to match the internal resistance and optimize the system. This event caused a current peak. During the second and the subsequent cycle (in which the synthetic medium and sodium acetate feed were used) the current peaks were of similar magnitude (0.4 mA) . The open circuit (GC-OCP) modules were monitored and the potential difference trends show that, in relation to each event (filling of the anode chamber, addition of sodium acetate) , the OCP values increase promptly and then stabilize for a few days, until the next feeding event.

The anodic polarization curves shown in Fig. 3 were recorded on day 0 and day 8. An anodic peak around - 0.375 V (vs Ag/AgCl) was observed already after 8 days, demonstrating the formation of the anodic biofilm. The power curves allowed quantifying the maximum power that can be supplied by this type of system (20 mW nr ² ) , with current density (on a cathodic geometric surface) typically of the order of 200 mA TOT ² .

The presence of the electric field generated by electroactive biofilms (Fig. 4) had a series of effects that contribute to the deposition of inorganic salts on the surface of the cathode; among these effects we list:

Electro-migration of cations in solution towards the cathode .

Oxidation of soluble organic matter (Fig. 5)

Partial oxidation of the ammonium ion and subsequent denitrification, subject to the availability of soluble organic carbon (Fig. 5)

Local pH increase (pH>ll), near the cathode (Fig. 6), due to the incomplete reaction of oxygen reduction, with consequent accumulation of OH ^~ .

Precipitation of salts due to pH and local evaporation of water causes an osmotic gradient, which also attracts anions (Fig. 6,7,8) .

The tests carried out measured the amounts and the distribution of the elements deposited on the cathode, as shown in Fig. 7-8. Through mapping with the EDX probe, the distribution of Na and Ca was mapped, which tend to deposit as carbonates following an increase in pH near the cathode. Micro-tomography (Fig. 7) allowed to reconstruct three-dimensionally a part of the cathode, demonstrating the presence of deposits with higher density than the material forming the cathode, a consequence of the deposition of salts over time.

Through ICP-MS analysis of the cathodes and separators (when present) , whose results are shown in Fig. 8, a depletion of macro and micronutrients from the waste water and a consequent enrichment of cathodes and separators, by the same elements, was confirmed once again.

EXAMPLE 3

Figure 9 shows the results of an experiment conducted to demonstrate the effect of conductivity of the material and of the exposure to air, in bioreactors consisting of fixed beds consisting of a granular and cylindrical composite material.

In order to evaluate the effect of the conductive material, experimental controls were carried out, in which cylinders consisting of 100% mixtures of clay (terracotta) , and therefore not electro-conductive, were placed instead of the cylindrical electrodes. To evaluate the effect of the air-water interface, the tests were repeated with the inner part of the cylinder not exposed to air (submersed in the waste water), as follows:

Electrode air: Main thesis, consisting of cylindrical electrodes, in contact with the granular bed, exposed to the air in the inner and upper part thereof;

Electrode no air: control without the presence of air;

Terracotta air: control with non-conductive material but exposed to air;

Terracotta no air: negative control with non- conductive material and not exposed to air.

From the data of the figure it emerges that when in the presence of conductive material exposed to air, all the macronutrients taken into consideration are deposited as salts on the electrode material.

From reading the above description of the present invention, the numerous advantages offered by the proposed cathodes and microbial electrochemical technologies will become apparent to those skilled in the art .

First of all, being able to avoid production processes that use additives or chemicals, the material is not altered and retains a complete recyclability in the environmental compartments (for example: agrarian soils) .

This is complemented by the complete absence of non-biogenic materials such as, for example: plastic polymers, plastics, synthetic binders, Nafion, PTFE, processed carbon fiber, solid metal parts, synthetic resins, non-biodegradable or non-environmentally compatible artificial materials.

Furthermore, it should not be underestimated that the electrodes of the present invention, due to the highly porous structure, provide a large surface useful for adsorption of macro- and micro-nutrients due to electro-osmotic flows, which contribute to deposit macro- and micro-nutrients inside the pores.

Surprisingly, it was therefore observed that the deposition of inorganic salts and organic matter, already observed in the literature as a problem in the functioning of MES electrodes, does not represent a limit to the optimal functioning of the modules, but rather is the main advantage thereof.

If a portion of a plant is used, the operations of preparation of the cathode are reduced to a minimum and, therefore, a reduction in the costs of preparation is obtained.

Those skilled in the art may make adaptations and changes to the process of the present invention described above and replacements of elements with others, which are functionally equivalent in order to meet specific incidental needs, without departing from the scope of the following claims.