DESCRIPTION

The remediation of contaminated sites is one of the most significant problems which the most industrialized countries must face. In 2018, about 2.8 million sites in the European Union 28 were affected by polluting activities. To date, there are more than 650,000 registered sites where polluting activities were/are underway and of which only 10% have already been reclaimed. In the proposal for a Soil Framework Directive, an overall estimate of the annual cost of remediation of contaminated soil was made. The investment needed by each Member State amounted to €290 million per year for the first 25 EU Member States (EU-25) in the first 5 years and up to €240 million per year in the following 20 years, for an estimated total cost of €119 billion, considering the average remediation costs linked to the size of contaminated sites. In Europe, an average of 42% of total expenditure on the management of contaminated sites comes from public budgets. The management of contaminated sites costs about 0.041% of the national GDP (11 euros per capita) and about 81% of this money is spent on remediation measures while only 15% for on-site surveys and prevention ( estimated at 618 euros per capita ) . The cost for surveys and remediation is quanti fied, as an average of the 27 countries , at €10 per capita/year . Soil contamination is a direct consequence of industrial activities and improper waste disposal which have caused almost two thirds of the contamination to be addressed, with a detrimental ef fect on ecosystem services provided by the soil due to the loss of biodiversity . Many contaminants are transported from the soil to surface water and groundwater, causing enormous environmental damage and direct problems to human health . In October 2017 , the Lancet Commi ssi on on Poll uti on and Heal th documented that diseases caused by air, water and soil pollution were responsible for 16% of global deaths , killing more people than smoking, hunger, natural disasters , war, AIDS and malaria .

The main categories of contaminants include hydrocarbons and heavy metals , homogeneously distributed in both liquid and solid matrices . As for the solid matrices , the most widely adopted strategies for the management of contaminated soils and sediments are currently landfilling and making the contaminated areas of a site safe . The disadvantages of the first method include high costs and signi ficant risks in the excavation, handling, transport of hazardous material and final disposal of the material in new landfills , which counteract the EU perspective of circular economy . Making the sites safe is a temporary solution, as the contamination remains on site , requiring the long-term monitoring and maintenance of isolation barriers to control toxicity and possible trans fer to the organic food chain, with all the associated costs and responsibilities .

Soil treatments , which instead aim to remove soil contamination or trans form contaminants into non-toxic compounds , can be classi fied, based on the type of process on which they are based, into physical , chemical and biological technologies . The treatments can also be distinguished as in si tu and ex si tu, depending on whether they involve the removal of the material to be treated or not .

Among the physical technologies , thermal desorption consists of the removal of contaminants present in soil through the application of high temperatures both in si tu and ex si tu . It is a very ef fective technology but is extremely costly and has an environmental impact . One of the most widely used in situ chemical technologies is chemical oxidation, which consists of the use of strong oxidants for treating organic substances present in the soils and water . Also in this case , it is an application which leads to a rapid solution of the contamination with high costs and with a signi ficant impact on the environmental state of the treated matrix .

The biological technologies (bio-remediation) are instead based on the use of biological processes promoted by living organisms or enzymes aimed at trans forming or reducing the mobility of the contaminants . Most applications are based on the use of microorganisms such as bacteria and fungi , possibly in association with selected plants . The bioremediation technologies rely on human intervention to overcome the limiting factors which negatively af fect the natural biodegradation/biotrans formation rate of the organic and inorganic contaminants present in a given environment . The limiting factors can be lack of inorganic nutrients (nitrogen, phosphorus , microelements ) , low bioavailability of the contaminant , lack of suitable electron donors/acceptors and/or speciali zed microbial populations . In fact , the bioremediation technologies are traditionally classi fied as "biostimulation, " when they consist of the modi fication of inorganic nutrients , surfactants , electron donors/acceptors , and "bioaugmentation" when metabolically speciali zed microbial populations are added . For biostimulation, in particular for in si tu applications , these technologies are currently limited by the energy for providing oxygen as an electron acceptor and organic compounds as an electron donor, when reduction processes must be promoted . In the case of bi oaugmenta ti on, the main challenge is the persistence of the inoculated microorganisms , often overcome by the native microbial communities .

In particular, the most widely used biological treatment methods for the removal of hydrocarbons and other biodegradable contaminants from soils and sediments include bio-pile technology, also commonly called landfarming . In this application, the soil is conditioned with substances which stimulate microbial biodegradation activity such as inorganic nutrients , substances which correct structure (bulking agents') , surfactants and microbial inoculums . The material is then arranged in piles in which optimum moisture and oxygenation are maintained . As a function of how these two parameters are controlled, the biopiles can be called " static" or "dynamic . " In the first case , the biopiles are constructed by overlapping layers of contaminated soil ( for heights up to 4-5 meters ) interspersed with the laying of perforated pipes to distribute air and solutions of water and nutrients ( especially nitrogen and phosphorus ) and air extraction pipes . Inside such reactors , the optimal conditions of nutrient availability, humidity and temperature are arti ficially maintained, while an aeration system provides the oxygen necessary for carrying out the biodegradation processes . To contain the emission of volatile substances , the biopile can included both centrali zed air treatment systems and coverage with plastic sheets and the treatment of vapors with activated carbon . The limits of this type of technology are in the possible formation of preferential paths to air insuf flation and in the non-homogeneous distribution of fluids inside the piles . These limitations can result in an unoptimi zed treatment throughout the volume of the material to be treated .

On the other hand, where the homogeni zation of the material , the oxygenation and the correct degree of humidity are ensured by a periodic tedding, in the absence of fixed structures , the biopile is defined as dynamic . In these cases , the soil is arranged in trapezoidal swaths with maximum heights of about 1 . 5 - 2 m . In this type of approach, the distribution of oxygen and soil conditioners obtained is as homogeneous as possible . However, this treatment has a higher cost with respect to static biopiles and a less ef ficient use of the system surfaces caused by a lower value of the ratio between volume of treated material and system surface .

Background art document US 2015/ 352609 describes a BES (bi oel ectrochemi cal system) system in MFC (Mi crobial Fuel Cell ) configuration for the remediation of contaminated soils , partially immersed in the contaminated matrix, having the cathode in contact with the atmosphere and the anode immersed in the soil , divided by a fiberglass separator .

Summary of the invention

The inventors of the present patent application have surprisingly developed a method for the bioremediation of contaminated soils which utili zes a bioelectrochemical technology which stimulates the biodegradation of the contaminants with a lower energy cost .

Brief description of the figures Figure 1 shows a diagram of the components of the technology of the system of the invention : a ) the electrodes ( anode and cathode ) ; b ) the microbial community adhering to or near the electrode ; c ) the earthy matrix ; d) the container ; e ) the water possibly present in the saturation system .

Figure 2 diagrammatically depicts the system of the invention or a top view thereof in a container filled with soil and in which the electrodes are positioned .

Figure 3 shows the results of the hal f-li fe for hexane by virtue of the technology of the present invention .

Figure 4 shows the current production data in a system according to the present invention .

Figure 5 shows a graph with the statistical processing of the results related to the presence or absence of soil conditioners .

Obj ect of the invention

The present invention relates to a method for the bioremediation of contaminated soils .

Detailed description of the invention

Def ini ti ons

For the purposes of the present invention, the term "pollutants" means biodegradable organic substances comprising aliphatic petroleum hydrocarbons, including for example n-hexadecane, and aromatic, such as benzene, toluene, ethylbenzene, xylene or mixtures thereof (BTEX) , and mixtures of aliphatic and aromatic hydrocarbons; diesel is also included .

The term "soil conditioners" means those compounds added for the purpose of providing inorganic nutrients, electronic mediators, electrolytes, emulsifiers, thermogenic substances. More in particular, these include: Organic and inorganic nutrients

They are added to promote bacterial metabolism. In addition to a carbon source, inorganic compounds containing N, P and other micronutrients such as Fe, S, Mg, Co are added. These compounds and elements are added to the soil to be treated so as to ensure that there are no limitations to bacterial growth. For the compounds containing N and P, it is ensured that the final ratio between the C moles of the contaminants and those of N and P is less than 10 and 1, respectively .

Electronic mediators

The presence of mediators of the transfer of electrons to the anode allows increasing the range of action of the anodic stimulation favoring the biodegradation reactions throughout the volume of treated soil and not only those which occur near the electrode .

For the purposes of the present invention, such electronic mediators include, for example: i) leonhardite; ii) sulfates; iii) biochar .

Leonhardite is a raw material of natural origin rich in humic and fulvic acids.

Sulfates act as an electron acceptor in anaerobic respiration (sulfate reduction) and have been shown to act as an electronic mediator in BES systems .

Biochar contributes to the electronic transfer inside the matrix.

Electrolytes

They are added to ensure the circulation of electric current. Emulsifying substances

They are added to promote the mobility of contaminants. One of the most important factors limiting biodegradation is the low bioavailability of hydrophobic contaminants such as hydrocarbons. Furthermore , in a BES system, the range of action of the electrodes can be increased, in addition to the presence of electronic mediators , also by an increase in the (pseudo ) solubili zation of the contaminants , thus of the mobility thereof . Surfactants , possibly of biological origin, which can be added for the purposes of the present invention include : rhamnolipids or natural emulsi fiers such as soy lecithin .

Thermogeni c substances

They are added to accelerate bacterial metabolism . It is known that the speed of biological processes is strongly influenced by temperature and that the biodegradation of hydrocarbons is very rapid at temperatures compatible with the metabolisms of thermophilic bacteria ( 60- 80 ° C ) . In these conditions , the speed of biodegradative reactions is higher than at ambient temperatures , also by virtue of a higher solubility of hydrocarbons . Easily degradable organic substrates such as green shoots or immature compost can be added to increase soil temperatures during treatment . The thermal increase can be facilitated by the absence of material tedding and air insuf flation . In accordance with a first obj ect of the invention, a method for the bioremediation of soils contaminated by pollutants is described .

The method is preferably actuated on a circumscribed soil .

This means that the portion, area or volume of soil to be treated is isolated, as it has been taken and placed, for example , inside an appropriate container .

To this end, suitable containers or boxes can be used .

According to a preferred aspect of the invention, the volume of soil treated with the described method is about 5- 15 m ³ and preferably about 10 m ³ .

The height of the soil , and thus the thickness thereof , must preferably be at most 1 . 5 m .

According to another aspect of the present invention, the soil can be subj ected to a pretreatment step in order to obtain a homogeneous material .

In this regard, therefore , the soil is dimensionally screened to obtain a particle si ze less than 2 cm . Furthermore , foreign bodies contained in the soil are removed, for example manually, while the screening is performed by rotary dimensional sieve or stellar dimensional sieve .

Optionally, a dimensional reduction of the soil aggregates is performed by impact mill .

By means of appropriate systems , such as a magnetic sorting system, ferromagnetic materials or, for example , by means of aeraulic sorting systems , light foreign materials are removed .

In particular, the method of the invention comprises applying electrodes to the soil to be treated .

The geometry of the electrodes is not a limiting factor .

More in particular, the cathode and anode can be made of biocompatible material , preferably represented by graphite , whole or in granules .

While the anodes are completely fixed or immersed in the soil , the cathodes can be exposed to air, even partially .

I f in contact with the soil , they are preferably coated with protective material , such as perforated steel , but in a manner which allows the contact thereof with the soil . According to a preferred aspect of the invention, said cathode and said anode are positioned at a distance of about 50-70 cm, preferably 60 cm, from each other .

According to a preferred aspect of the invention, the electrodes are positioned for the entire depth of the soil .

According to an aspect of the present invention, a voltage can be applied to said electrodes , so as to increase the potential di f ference between anode and cathode .

In particular, such a voltage , applied by means of a generator or a power supply, can be between about 200 mV and 1 . 2 V .

According to an aspect of the present invention, solid soil conditioners , for example in powder or granular form, such as bi ochar can be added before laying the electrodes .

For the purposes of the present invention, the bi ochar can be added in an amount of about 2 - 10% by weight to the soil mass .

Preferably, the soil conditioners are homogeneously mixed in the soil .

For the purposes of the present invention, such soil conditioners can be represented by one or more of the soil conditioners described above and comprising : inorganic nutrients , electronic mediators , electrolytes , to ensure the circulation of the electric current , emulsi fying substances , thermogenic substances .

After the electrodes are positioned, the soil to be treated can be saturated with water .

Saturation with water is not essential , but it has the advantage of increasing treatment ef ficiency, as it can increase the transport of charges .

In a preferred aspect , the water can have ionic substances added, such as inorganic nutrients , so as to obtain electrical conductivity values of at least 500 pS/cm .

According to an aspect of the present invention, after laying the electrodes , soil conditioners can be added in liquid form .

In particular, such liquid soil conditioners can be added to the water before saturating the soil with water .

For the purposes of the present invention, such soil conditioners can be represented by one or more of the soil conditioners described above and comprising : inorganic nutrients , electronic mediators , electrolytes , to ensure the circulation of the electric current , emulsi fying substances , thermogenic substances .

Subsequently, the method of the invention comprises the step of connecting anodes and cathodes to each other by means of an electrical resistor or a current generator depending on whether the system works as MFC (Mi crobial Fuel Cell ) or MEC (Mi crobial El ectrolysi s Cell ) .

The cables and connections are made with insulating and corrosion-resistant materials .

At the end of the soil preparation, the system can be covered by a waterproof sheet , for example in HDPE , so as to maintain the treatment conditions and reduce the evaporation of water .

According to a particular aspect of the present invention, the soil can have not only soil conditioners added but also a bacterial , homogeneous or mixed population . Bacteria

Bacteria are the biological element which catalyzes the biodegradation reactions of contaminants . Bacteria which possess biodegradative capacity against hydrocarbons belong to numerous species such as Pseudomonas spp . , Rhodococcus spp . ,

Gordonia spp . , Bacill us spp . and possess speci fic enzymes , capable of catalyzing hydrocarbon conversion reactions to intermediate bacterial metabolism compounds . The final result is the conversion of hydrocarbons into CO2 , bacterial biomass and the production of reduced chemical species dependent on the electron acceptor used by the bacteria for respiration . The electron acceptors used by the microorganisms during biodegradation processes can be molecular oxygen ( aerobic respiration) or other compounds such as nitrates , and sul fates in anaerobic respiration . Some microorganisms are also capable of trans ferring the respiration electrons to solid compounds such as ferric iron or a conductive material such as electrodes in bioelectrochemical systems ( Figure 1 , compartment b ) . These microorganisms are also called electrogenic and belong to some bacterial species such as Geobacter spp . , Shewanella spp . The electronic trans fer to the electrode can also occur through a chemical mediator reduced by the microorganisms which subsequently reoxidi zes to the anode . The mediators can be synthesi zed by microorganisms , already present in the soil or added to the soil during the treatment .

The metabolic features of the bacteria which are then utili zed in this treatment system are i ) the ability to degrade polluting organic compounds such as aliphatic hydrocarbons and aromatic hydrocarbons, ii) the ability to transfer the respiration electrons to solid electron acceptors such as electrodes by means of a direct or mediated transfer from other molecules .

Bacteria with these capacities are normally already present in treated soils and are stimulated by the treatment performed.

For the purposes of the present invention, however, it is also possible to add allochthonous microorganisms so as to increase the active microbial biomass inside the treatment system.

The bacterial population can thus comprise the orders Pseudomonadales _r Actinomycetales , Mycobact erl ales, Bacillales , Geobacterales , Alteromonadales .

More in particular, the population of bacteria can comprise: Pseudomonas spp . , Rhodococcus spp . , Gordonia spp., Bacillus spp., Geobacter spp., Shewanella spp.

According to an aspect of the present invention, the described method is conducted under oxygen-free conditions (anoxic conditions) . According to a particular aspect , the method is conducted without the addition of further oxygen from the outside .

According to a particular embodiment of the method of the invention, continuous monitoring of the intensity of the electric current circulating in the circuits is conducted during the treatment .

For the purposes of the present invention, the application of the voltage described above and the detection of the current produced can also occur by means of a single instrument (power supply with integrated datalogger ) .

Since such an intensity is proportional to the speed of the chemical oxidation and reduction reactions which occur at the electrodes , the bioremediation trend can be followed .

Figure 2 diagrammatically shows the system of the invention or a top view of a container filled with soil and in which the electrodes ( anodes and cathodes ) are positioned, spaced apart from each other, for example by 60 cm .

The electrodes are electrically connected two by two and, in turn, each pair of electrodes is connected to a voltage supply and data collection system . Furthermore, chemical and microbiological analyses are performed on soil samples collected with a manual corer to determine residual concentrations of contaminants.

The present invention will hereafter be described by virtue of the following experimental section.

Laboratory scale setup

The system of the invention was tested on a laboratory scale. 3 bioelectrochemical systems were made using a single plexiglass chamber with the dimensions 75 x 30 x 10 cm (LxWxH) . Each system was filled with 20 L of soil specially contaminated with 1000 ppm of hexadecane and saturated with water to impose anoxic conditions. 2 graphite cylindrical electrodes 2 x 20 cm in size were inserted in each chamber, defined as anode and cathode. The electrodes were coated with a perforated stainless steel sheet (thickness 0.4 mm - square hole: 3 x 2.2 mm) . The sheet was closed with a stainless steel wire (4 mm) , used for the electrical connection, protected by a 2.4/1 .2 mm heat-shrink sheath while the entire upper end with a 25.4/12.7 mm sheath. The chambers were closed with a cover of the same material, perforated at the electrodes . The electrodes were positioned at a mutual distance of 60 cm, electrically connected to each other, and a potential di f ference of 800 mV was applied . Reactor 1 thus consisted of a bioelectrochemical system containing 20 L of saturated soil , in which anode and cathode were inserted at a distance of 60 cm from each other . A second reactor was also set up to assess the ef fect of adding a soil conditioner to a system identical to the first . This variable is expected to improve the conductivity of the matrix and support microbial growth . Leonhardite was chosen for this purpose , diluting it 1000 times . Two control reactors were also set up, for a total of 4 systems . The first control consisted of a system with anode and cathode inserted in the soil , but not connected to each other and thus operating with open circuit potential ( OCR ) . This is useful for identi fying the influence of two variables : the presence of the electrodes and the polari zation on the systems . The second control was instead an electrodeless system made with unsaturated soil , which was regularly turned to ensure aeration, thus simulating the most widely used conventional biological technique , aerated biopiles . The test lasted 75 days . Sample preparation

Samples were defined to signi ficantly analyze the area between the two electrodes and evaluate the influence of the distance therefrom . They were performed based on distances of 10 cm, distributed non-homogeneously . A sample was taken near both electrodes ( 0 cm and 60 cm) , 10 cm away therefrom ( 10 and 50 cm) and finally from a central point equidistant from anode and cathode ( 30 cm) . The sampling was performed over the entire depth and homogeni zed .

The samples were taken at 0 , 12 , 25 , 45 and 75 days .

Analytical methodology

Chemical

For the chemical extraction, 2 g of soil was used and mixed with a known amount of anhydrous sodium sul fate suf ficient to sequester the water present and avoid the formation of lumps . The sample was placed in an extraction glass container to which 10 mL of n-hexane containing 50 ppm of o-terphenyl ( tracer and internal standard) was added . The extraction was performed through two sonication cycles of 30 ' each . The supernatant was taken after the first cycle , stored and replaced with another 10 mL of the same solvent to subj ect the sample to the second cycle . The two extracts thus obtained were combined for a total of 20 mL and filtered with wadding and anhydrous sodium sul fate . A gas chromatograph with flame ioni zation detector ( GC-FID) was used for the analysis of the sample , inj ecting 1 pL of the filtered extract . Microbiological

The soil samples taken at the beginning and end of the test were analyzed with biomolecular methodologies to assess the influence of the system on the composition of the microbial community and the evolution thereof , using the gene encoding the 16S rRNA as a phylogenetic molecular marker . In particular, genomic DNA was extracted from the samples in question using the FastDNA® SPIN Kit for Soil (MP Biomedicals , Solon, OH, USA) following the manufacturer ' s protocol . The obtained sample was then prepared for sequencing the V3-V4 hypervariable region of the 16S rRNA gene by means of the I llumina MiSeq platform . Electrical

The currents generated between the electrodes of the electrically connected systems were recorded for the entire duration of the test with a DataLogger Squirrel 2010 , which performs readings every minute and records the averages every hour . Soil characterization

Chemical

The characterization analyses of the soil used were performed by a geopedological laboratory. The main test results are shown in Table 1.

Mi crobi ol ogi cal

As for the microbiological analysis, it can be observed that in system 5 (aerated control) the family Nocardiaceae is particularly enriched. In the bioelectrochemical systems, however, as in the soil used by initial inoculum, the community has significant biodiversity and there are no predominant enrichments, especially of families known to have electrogenic properties. Probably these populations are more present on the surface of the anode than in the bulk soil sample, since it is there that they carry out the function thereof and are thus favored. For this reason, further investigations will be performed which will focus on the analysis of the microbiological communities of samples taken from the surface of the electrodes. However, it is interesting to note in the samples analyzed so far the presence, although not predominantly, of the families of Chi ti nophagaceae and Sphingomonadaceae , found in other work on bioelectrochemical systems associated with biocathode and anode, respectively (Zhang et al., 2012a, 2012b; S. P. Jung et al., 2014) .

Focusing instead on the presence of degraders, families known to have degradative capacities were found, in particular Xanthomonadaceae, Nocardioidaceae (J. Jung et al., 2014) .

On the results of the microbial community, an analysis of the main components (PCA) was also performed, which shows that the initial inoculum and the aerated control are ecologically distant both from each other and from the bioelectrochemical systems. In particular, system 1 and the OCP control are similar to each other, while system 2 with Leonhardite significantly differs from all the others. From this analysis it is therefore evident that the enrichment of the microbial communities of the bioreactors significantly differed from the composition of the initial inoculum community. The significant difference in aeration control with respect to all the other conditions is also interesting, which highlights how the degradation mechanisms stimulated inside an aeration system are different from those found in the absence of aeration. Another element which influences the composition of the microbial community is obviously Leonhardite, but looking at the other results it would seem that this does not lead to an increase in degradation efficiency. Degradation Chemical analyses were performed at day 0, 25 and 75 (respectively tO, t3 and t5) . Analyzing the results at the end of the test, or after 75 days, as for system 1, the highest percentage of contaminant removal (88%) was obtained at 0 cm from the anode, while 69% was reached at a distance greater than 60 cm. In system 2, with Leonhardite added, the major degradation percentage was 59%, obtained at a distance of 60 cm from the anode. Looking at the results of the 3 bioelectrochemical systems as a whole, it can be said that the Leonhardite did not increase the degradation efficiency. As expected, the OCP control instead obtained low degradation yields, confirming the importance of the connection between the two electrodes and the application of an electrical potential. In general, it can be said that system 1 was the one with the best degradation yield, comparable even to the aerated control system, which as expected achieved good results (88%) . Although the two systems are not comparable due to the involvement of different biodegradation mechanisms, one aerobic and the other anaerobic, this result confirms the potential of the bioelectrochemical system set up to compete with the main biological treatments currently in use for the remediation of contaminated soils.

Furthermore, the degradation rate analyzed is described by first-order kinetics, where the halflife tl/2=ln (2 ) /k, indicates the time necessary for the concentration of the pollutant to be reduced to half of the initial concentration. Figure 3 shows how the shortest half-life required for hexadecane is given by the aerated control, suggesting that this technique remains the most efficient. Nevertheless, as described above, the bioelectrochemical system 1 has been shown to achieve a half-life comparable to the aerated control. Furthermore, the half-life of the OCP control is the highest, demonstrating a significant difference between a turned aerobic treatment (the aerated control) and a static anaerobic system (the OCP control) . From these observations, we can conclude that the application of the potential (800 mV) to the bioelectrochemical systems had a positive effect on the degradation of hexadecane, greatly increasing the efficiency of an anaerobic static system, to the point of reaching that of an aerobic treatment.

As for the monitoring of current production (Figure 4) , it can be said that system 2 with Leonhardite achieved the highest peaks, with currents up to 120 mA/m ² . This supports the thesis that the human substances present in Leonhardite increase the conductivity of the treated soil by acting as electron shuttles. Nevertheless, we have described how this factor was not sufficient to increase the degradation ef ficiency, probably due to the preference of microorganisms to use the organic substance present in the soil conditioner as a carbon source , at the expense of the contaminant . Focusing instead on the current density resulting from the system 1 , the presence of peaks can be observed, usually related to the activation of the metabolism of the microorganisms capable of using the anode as the final electron acceptor . This confirms the presence of electroactive microorganisms .

Comparative tests So as to determine the ef fect of di f ferent soil conditioners on the performance of the system of the invention, an experiment was carried out with the aid of a fractional factorial design . This model allows obtaining as much information as possible without examining all the combinations . Speci fically, the addition of bi ochar, sul fates and/or rhamnolipids was evaluated . The current intensity was measured by means of constant monitoring .

The results obtained were statistically processed by providing the graph in Figure 5 , which shows the main ef fects of each individual variable on the concentration of the contaminant in the soil . The biochar showed a significant degradation effect, as well as the addition of sulfates, which had a positive impact on the increase in biodegradation.

From the above description, the advantages linked to the present invention will become immediately apparent to those skilled in the art.

Firstly, the present invention provides a very low- cost method, which thus allows broad application in the remediation and redevelopment projects of polluted areas.

With respect to the already known methods which utilize aerobic microbial metabolisms, that offered by the present invention does not require the treatment of the soil with oxygen, further contributing to reduced costs; in fact, it is not necessary to insufflate oxygen in the soil (or turn it, to allow the aeration thereof) in a continuous manner and with high energy costs.

Furthermore, to conduct the method described, work is carried out on areas or on volumes of confined soil; in fact, the soil to be treated can be contained in a box or in a container. As such boxes or containers can be stacked, the resulting advantage is a much smaller required space. Again, the method of the present invention does not have a limiting factor in the soil type; in fact, the composition of the matrix can be modified by adding organic matter (e.g., compost) or inert matter (e.g., clay, sand, silt) .

In addition to the above, the range of action of the system is 60 cm from the anode, but it can be increased by virtue of the addition of soil conditioners such as electronic mediators or surfactants which improve, respectively, the electrical conductivity and mobility of the pollutants; therefore, the method of the invention can be applied on very large areas of soil.

Finally, the disclosed method can be implemented with simple and widely available technologies.

The advantages described above are in no way detrimental to the efficiency of the suggested method, which is entirely comparable to that of the systems and methods already known, but which require more complex and often much more expensive systems.