Polymer compound and related production process

Abstract

The present invention relates to a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, wherein the biochar comprises carboxyl functional groups and / or hydroxyl functional groups, said polymer compound being in dry form or in hydrogel form.

Description

POLYMER COMPOUND AND RELATED PRODUCTION PROCESSDescription

[0001] Field of the invention

[0002] The present invention relates to a polymer compound in dry form or in hydrogel form, a formulation comprising such polymer compound, as well as the corresponding production processes and their use in agriculture.

[0003] State of the art

[0004] Superabsorbent polymers (known by the acronym SAP, "Super Absorbent Polymer") are crosslinked polymers bearing hydrophilic functional groups capable of absorbing and retaining significant quantities of water, forming the so-called hydrogels. In particular, they form 3D structures of physically or chemically crosslinked polymer chains, capable of absorbing amounts of water several times their weight.

[0005] The crosslinking points, i.e. the physical or chemical bonds between the polymer chains that constitute the network of said three-dimensional structures, must be such as to ensure the structural integrity of a hydrogel, on the one hand preventing complete solubilisation of the polymer and on the other hand enabling water retention within the network.

[0006] The polymer structure of a hydrogel must therefore have an appropriate degree of crosslinking, such as to allow for great mobility of the polymer chains without, however, degrading the 3D structure thereof and dissolving in water. In particular, when the degree of crosslinking of the polymer structure is too low, the latter risks dissolving in water, whereas when the degree of crosslinking of the polymer structure is too high, the latter risks being too rigid to absorb a significantamount of water.

[0007] It follows that the absorbent capacity and the swelling capacity of said polymer structures are significantly determined by the type of crosslinking agents used. A low crosslinking density generally results in higher absorbent capacity and swelling, with the consequent formation of a softer and stickier gel. Conversely, a high crosslinking density results in lower absorbent capacity and swelling, with the consequent formation of more solid gels.

[0008] Superabsorbent polymers and the respective hydrogels find wide application in agriculture, for example as soil water retention agents and seed coatings.

[0009] However, currently used hydrogels are mostly synthetic acryl-based hydrogels, non-biodegradable and obtained from toxic monomers, with potential risk of environmental accumulation and toxicity to the environment.

[0010] Some natural hydrogels are also known, obtained from renewable sources such as cellulose and its derivatives, which do not pose a threat to the environment either during the production process or during use.

[0011] Hydrogels obtained by crosslinking of alginate are also known. Alginate, unlike cellulose and its derivatives, has renowned biostimulant properties for plants. Alginate acts, in fact, as a promoter of plant germination and growth and promotes the growth of mycorrhizal fungi that help metabolise phosphorus for plants. Alginate also acts beneficially on plants by inducing the activation of defence mechanisms against pathogens such as fungi, bacteria and viruses, offering in certain cases an alternative to or a reduction in the use of pesticides.

[0012] The crosslinking of alginate is usually carried out using so-called "ionic crosslinking agents", with the formation of ionic bonds between the alginate chains.Such crosslinking agents include salts with bivalent ions, in particular calcium or magnesium salts, such as calcium chloride, magnesium chloride, calcium citrate, magnesium citrate, calcium acetate, magnesium acetate.

[0013] Although the use of the aforesaid ionic crosslinking agents allows the creation of hydrogels, the latter are subject to a limited number of swelling and deswelling cycles from water, sometimes only one, due to the rearrangement of the polymer chains of the respective dry compounds, thus proving to be limited for the water retention function.

[0014] In particular, when using salts with bivalent ions, the following phenomena can be observed that are detrimental to the water retention function of the hydrogel.

[0015] Firstly, the bivalent ions can be removed from the structure by common fertilisers used in agriculture, such as citric chelators, humic and fulvic acids (already present in the soil but also used as additives), ammonia-based chelators and EDTA, used to chelate soil micronutrients and make them bioavailable to plants. In such a case, the structure of the compound in hydrogel form would be solubilised in a short time, leaving the alginate chains free in the soil and more quickly susceptible to degradation by microorganisms. The water retention function would thus be lost in a short time, proving to be sufficient only for the germination of the seed.

[0016] Secondly, alginate crosslinked by means of bivalent ions undergoes a structural collapse upon drying and is unable to return to its initial water retention capacity.

[0017] Consequently, alginate-based hydrogels obtained by crosslinking with ionic crosslinking agents are not particularly suitable for use in agriculture, wherein instead hydrogels that possess high stability and high absorbent capacity are required, aswell as hydrogels capable of withstanding numerous cycles of swelling / deswelling from water, thus proving to be capable of supporting plant development throughout the entire vegetative growth and fruiting stage, and not just in the initial stage.

[0018] Therefore, the problem underlying the present invention is to provide an alginate-based hydrogel, as well as the corresponding polymer compound in dry form, capable of acting both as a water retainer and as a biostimulant for plants, which overcomes all the drawbacks of the prior art.

[0019] Summary of the invention

[0020] The above-mentioned problem is solved by a polymer compound, by a formulation comprising such polymer compound, as well as by the respective production processes and their use in agriculture, as defined in the appended claims, the definitions of which form an integral part of the present description.

[0021] A first object of the present invention is a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, wherein the biochar comprises carboxyl functional groups and / or hydroxyl functional groups, said polymer compound being in dry form or in hydrogel form.

[0022] A second object of the present invention is a formulation comprising the aforementioned polymer compound and further comprising: microorganisms, such as bacteria, for example in the form of spores, and / or one or more molecules selected from the group consisting of: nitrogenous molecules, phosphorus-based molecules, potassium-based molecules, magnesium salts, calcium salts.

[0023] A third object of the present invention is the use in agriculture of the aforementioned polymer compound or of the aforementioned formulation as a waterretention and water in-soil release agent.

[0024] A fourth object of the present invention is a process for producing a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, comprising the following steps: a) Mixing alginate with an aqueous solution of biochar, thus obtaining a solution of alginate and biochar, wherein the biochar comprises carboxyl functional groups and / or hydroxyl functional groups; b) Mixing an aqueous solution of at least one polycarboxylic acid with the solution of alginate and biochar resulting from step a); c) Adding an esterification reaction catalyst to the mixture resulting from step b); d) Drying the mixture resulting from step c), thus obtaining the polymer compound.

[0025] A fifth object of the present invention is a process for producing a formulation comprising a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, and further comprising microorganisms, such as bacteria, for example in the form of spores, and / or one or more molecules selected from the group consisting of: nitrogenous molecules, phosphorus-based molecules, potassium-based molecules, magnesium salts, calcium salts, wherein said process comprises the process for producing the polymer compound described above, and further comprises: a step following step d) of the process for producing the polymer compound, wherein the polymer compound is reduced to granules or powder and mixed with microorganisms, such as bacteria, for example in the form of spores, and / or a step of mixing an aqueous solution comprising one or more molecules selected from the group consisting of: nitrogenous molecules and / or phosphorus-based moleculesand / or potassium-based molecules and / or magnesium salts and / or calcium salts, said step being preferably carried out between step a) and step b), or between step b) and step c), or between step c) and step d) of the process for producing the polymer compound.

[0026] Advantageously, the compound and the formulation according to the present invention act both as water retainers and as plant biostimulants for growth and activation of defence mechanisms against pathogens.

[0027] Advantageously, the compound and the formulation according to the present invention exhibit simultaneously high stability and high absorbent and swelling capacity, and they are capable of withstanding a high number of swelling / deswelling cycles. Advantageously, their water-swelling properties remain stable even after deswelling, rendering the compound and the formulation according to the invention capable of supporting plant development throughout the entire vegetative growth and fruiting stage, and not only in the initial stage.

[0028] Thanks to the aforementioned properties, the polymer compound and the formulation according to the present invention have proven to be particularly suitable for applications in agriculture.

[0029] Further features and advantages of the invention will become more apparent from the description of some illustrative embodiments, given hereinafter by way of non-limiting example.

[0030] Brief description of the figures

[0031] Figure 1 illustrates an example of a reaction scheme between alginate, biochar, and citric acid as polycarboxylic acid, resulting in a polymer compound according to the present invention.

[0032] Figure 2 shows the results of a water swelling I water deswelling test of two alginate-based compounds in hydrogel form, one obtained by crosslinking the alginate with Ca++ions and the other one obtained according to the method of the invention; in particular, it shows photographs of the two compounds in the initial state and following successive drying and hydration steps.

[0033] Detailed description of the invention

[0034] An object of the present invention is a polymer compound comprising alginate chains crosslinked with at least one polycarboxylic acid and with biochar. The biochar, acting as a crosslinking agent, comprises hydroxyl functional groups and / or carboxyl functional groups. The biochar and said at least one polycarboxylic acid, acting as crosslinking agents, form covalent bonds with the alginate chains.

[0035] Said covalent bonds are the result of esterification reactions between the hydroxyl groups and the carboxyl groups of the alginate chains, of the biochar, and of the at least one polycarboxylic acid. Figure 1 illustrates, for illustrative purposes only, an example of esterification reaction between said groups. In particular, in the example of Figure 1 , a carboxyl group of an alginate chain reacts with a hydroxyl group of the biochar, which in turn reacts, through one of its carboxyl groups, with a hydroxyl group of another alginate chain. Always in the example of Figure 1 , carboxyl groups of citric acid react with hydroxyl groups of the alginate chains.

[0036] Therefore, the polymer compound according to the present invention is obtained by crosslinking alginate with covalent crosslinking agents, resulting in the formation of covalent bonds between alginate chains. These crosslinking agents include a biochar comprising carboxyl functional groups and / or hydroxyl functional groups and at least one polycarboxylic acid.

[0037] Biochar is a carbonaceous material obtained by thermal degradation, in particular by pyrolysis, of biomass of either animal or plant origin. Biochar is not a narrowly defined material; in fact, due to the use of various raw materials and different pyrolysis conditions, biochar may present different functional groups and different characteristics. As already mentioned, the biochar used as a crosslinking agent for the alginate chains to obtain the polymer compound according to the present invention comprises hydroxyl functional groups and / or carboxyl functional groups.

[0038] According to an embodiment, said at least one polycarboxylic acid is selected from C4-C12 dicarboxylic, tricarboxylic, tetracarboxylic acids.

[0039] According to various embodiments, said at least one polycarboxylic acid is selected from the group consisting of: citric acid, tartaric acid, malonic acid, malic acid, maleic acid, fumaric acid, succinic acid, sulfosuccinic acid, aconitic acid, isocitric acid, itaconic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, o-phthalic acid, tricarboxylic acid, pyromellitic acid, butanetetracarboxylic acid, and cyclopentanetetracarboxylic acid.

[0040] According to an embodiment, the compound of the invention comprises a weight percentage of alginate between 1 and 40%, or between 1 and 30%, or between 1 and 20%, or between 1 and 10%, or between 1 and 5%, relative to the weight of the compound.

[0041] According to an embodiment, the compound of the invention comprises a weight percentage of said at least one polycarboxylic acid between 0.01 and 5%, or between 0.01 and 4%, or between 0.01 and 3%, or between 0.01 and 2%, or between 0.01 and 1 %, or between 0.01 and 0.5%, or between 0.01 and 0.1%, relative to the weight of the compound.

[0042] According to an embodiment, the compound of the invention comprises a weight percentage of biochar between 1 and 60%, or between 1 and 50%, or between 1 and 40%, or between 1 and 30%, or between 1 and 20%, or between 1 and 10%, or between 1 and 5%, relative to the weight of the compound.

[0043] Thanks to its use as a crosslinking agent, the biochar is not dispersed in the polymer network but is an integral part of it. Consequently, the properties of biochar are effectively exploited in the compound according to the invention. These include the ability to supply carbon and increase the availability of water in the soil, the ability to retain nutrients and make them available to crops, the ability to reduce the heavy metals absorbed by plants, as well as the ability to increase crop yield.

[0044] Advantageously, the compound of the invention has a degree of water absorption between 50 and 100. The degree of water absorption of the compound is defined by the following equation:(weight of the compound in hydrogel form) — (weight of the compound in dry form) (weight of the compound in dry form) wherein the compound in hydrogel form is the hydrated polymer compound, that is, swollen with water.

[0045] A further object of the present invention is a formulation comprising the aforementioned polymer compound and further comprising: microorganisms, such as bacteria, for example in the form of spores or other dormant forms, and / or one or more molecules selected from the group consisting of: nitrogenous molecules, phosphorus-based molecules, potassium-based molecules, magnesium salts,calcium salts, for example ammonium nitrates, ammonium sulphates, urea, orthophosphates, phosphoric anhydride, potassium sulphate, potassium silicate, potassium chloride, potassium nitrate.

[0046] Advantageously, the aforesaid molecules represent sources of specific nutrients for plants and crops in general (i.e., nitrogen, phosphorus, potassium, magnesium, calcium).

[0047] A further object of the present invention is the use in agriculture of the compound or of the formulation according to the invention as a water retention and water in-soil release agent.

[0048] In particular, the compound according to the invention is placed in the soil in dry form, absorbs the water present in the soil and hydrates, becoming a hydrogel. Said compound in hydrogel form advantageously acts as a soil water retention agent or as a water in-soil release agent.

[0049] Likewise, the formulation according to the invention is placed in the soil in dry form, absorbs the water present in the soil and hydrates, becoming a hydrogel. The formulation in hydrogel form advantageously acts as a soil water retention agent or as a water in-soil release agent.

[0050] The formulation according to the invention, in addition to retaining and releasing water in the soil, is advantageously capable of retaining nutrients and gradually releasing them, making them directly available to the roots. Therefore, the formulation according to the invention represents a potential reservoir for the excess water and nutrients in agricultural soils.

[0051] However, uses of the compound or of the formulation according to the invention in other fields are not excluded, for example as an absorbent material forthe production of diapers or as an absorbent material or support for molecules or for the removal of substances harmful to the environment.

[0052] Another object of the present invention is a process for producing a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, comprising the following steps: a) Mixing the alginate with an aqueous solution of biochar, thus obtaining a solution of alginate and biochar, wherein the biochar comprises carboxyl functional groups and / or hydroxyl functional groups; b) Mixing an aqueous solution of at least one polycarboxylic acid with the solution of alginate and biochar resulting from step a); c) Adding an esterification reaction catalyst to the mixture resulting from step b); d) Drying the mixture resulting from step c), thus obtaining the polymer compound.

[0053] Advantageously, the polymer compound resulting from step d) is in dry form.

[0054] According to an embodiment, said process further comprises a step following the drying step d), in which the polymer compound is reduced to granules or powder.

[0055] According to an embodiment, the aqueous solution of biochar with which the alginate is mixed during step a) comprises biochar in an amount between 2 and 50% by weight. For example, said solution comprises biochar in an amount between 4 and 40% by weight, or between 5 and 30% by weight, or between 10 and 20% by weight. Preferably, said solution comprises biochar in an amount between 4 and 10% by weight.

[0056] According to an embodiment, said aqueous solution of biochar is obtained by mixing powdered biochar in water.

[0057] According to an embodiment, during said step a), the alginate is mixed in anamount between 1 and 20% by weight relative to the weight of the aqueous solution of biochar. For example, the alginate is mixed in an amount between 2 and 15% by weight, or between 4 and 12% by weight, or between 5 and 10% by weight, relative to the weight of the aqueous solution of biochar. Preferably, the alginate is mixed in an amount between 2 and 5% by weight relative to the weight of the aqueous solution of biochar.

[0058] Preferably, during said step a), the alginate is mixed with the aqueous solution of biochar until complete dissolution of the alginate is achieved, forming a homogeneous solution of alginate and biochar. By way of example only, said mixing step is carried out for a time period between 20 minutes and 3 hours.

[0059] According to an embodiment, the aqueous solution of at least one polycarboxylic acid with which the solution of alginate and biochar is mixed during step b) has a concentration between 0.2% and 10% (w / v), for example between 0.5% and 5% (w / v), or between 1 % and 2% (w / v).

[0060] According to an embodiment, the aqueous solution of at least one polycarboxylic acid is mixed, during said step b), in a ratio between 1 / 5 and 1 / 40 (v / v), for example between 1 / 10 and 1 / 30 (v / v), with respect to the volume of the solution of alginate and biochar.

[0061] The concentration of the polycarboxylic acid solution as well as the ratio of the polycarboxylic acid solution to the solution of alginate and biochar with which it is mixed during step b) are, advantageously, such as to ensure the mixing of the polycarboxylic acid but not an excessive acidification of the solution, which would lead to catalyzing the esterification of the compound before complete mixing.

[0062] Preferably, said at least one polycarboxylic acid is selected from thosedescribed above with reference to the polymer compound according to the invention.

[0063] Preferably, said at least one polycarboxylic acid is selected from citric acid and / or tartaric acid.

[0064] According to an embodiment, during step b), an aqueous solution of sodium hypophosphite is also mixed with the solution of alginate and biochar. Said sodium hypophosphite solution may be mixed with the solution of alginate and biochar before or simultaneously with the addition and respective mixing of the aqueous solution of at least one polycarboxylic acid.

[0065] According to an embodiment, said aqueous solution of sodium hypophosphite has a concentration between 1 and 3% (w / v), for example between 1.5 and 2.5% (w / v).

[0066] According to an embodiment, said aqueous solution of sodium hypophosphite is mixed in a ratio between 1 / 60 and 1 / 150 (v / v), for example between 1 / 80 and 1 / 120, or between 1 / 80 and 1 / 100 (v / v), with respect to the volume of the mixture resulting from step b).

[0067] According to various embodiments, said esterification reaction catalyst is selected from a strong acid, or phosphoric acid (H3PO4), or a zeolite. Preferably, the strong acid is HCI or H2SO4.

[0068] According to an embodiment, the esterification reaction catalyst is a strong acid and the corresponding aqueous solution, which is added during step c), has a concentration between 0.1 % and 4% (v / v), for example between 0.5% and 2% (v / v).

[0069] According to an embodiment, the esterification reaction catalyst is a strong acid and the corresponding aqueous solution is added, during said step c), in a ratio between 1 / 5 and 1 / 30 (v / v), for example between 1 / 10 and 1 / 20 (v / v), with respect tothe volume of the mixture resulting from step b).

[0070] According to an embodiment, said drying step d) is carried out at a temperature between 50 and 120°C, preferably between 80 and 100°C, or between 70 and 90°C.

[0071] Advantageously, said step d) is carried out until complete drying of the compound, thus obtaining the polymer compound of the invention in dry form. By way of example only, said step d) is carried out for a period of time between 1 and 24 hours, for example between 1 and 15 hours, or between 2 and 12 hours, or between 5 and 10 hours.

[0072] Advantageously, during said drying step d), crosslinking of the alginate chains with the biochar and the at least one polycarboxylic acid takes place, that is, the esterification reactions between the hydroxyl groups and the carboxyl groups of the alginate, of the biochar, and of the at least one polycarboxylic acid occur.

[0073] Without being bound by theory, it is believed that the biochar, added as the first ingredient, basifies the alginate solution by increasing its pH and preventing the protonation of the carboxyl groups of the alginate chains. In this way, the biochar ensures the mixing of alginate in greater amounts than when it is not present.

[0074] Without being bound by theory, it is believed that the biochar, acting as a crosslinking agent for the alginate chains, also advantageously acts as a molecular spacer between the chains themselves, as illustrated in the example of Figure 1 , thereby increasing water penetration and absorption. In other words, it is believed that the biochar, acting as a molecular spacer, increases the average distance between the alginate chains and between the respective crosslinking points, improving the ability of the polymer network to expand so as to significantly increase its absorptionand swelling characteristics.

[0075] A further object of the present invention is a process for producing a formulation comprising a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid and further comprising microorganisms, such as bacteria, for example in the form of spores or other dormant forms, and / or one or more molecules selected from the group consisting of: nitrogenous molecules, phosphorus-based molecules, potassium-based molecules, magnesium salts, calcium salts, for example ammonium nitrates, ammonium sulphates, urea, orthophosphates, phosphoric anhydride, potassium sulphate, potassium silicate, potassium chloride, potassium nitrate.

[0076] Said process comprises the process described above for producing the polymer compound and further comprises: a step following step d) of the process for producing the polymer compound, wherein the polymer compound is reduced to granules or powder and mixed with microorganisms, such as bacteria, for example in the form of spores, and / or a step of mixing an aqueous solution comprising one or more of the aforesaid molecules, preferably carried out between step a) and step b), or between step b) and step c), or between step c) and step d) of the process for producing the polymer compound.

[0077] Experimental part

[0078] The degree of water absorption, as defined above, was determined for three polymer compounds prepared according to the invention (Example 1 , Example 2, Example 3).

[0079] The water swelling / water deswelling capacity was also evaluated for twoalginate-based compounds in hydrogel form, one obtained by crosslinking the alginate with Ca++ions (Comparative Example) and the other, according to the invention, obtained by crosslinking the alginate with citric acid and biochar comprising carboxyl functional groups and / or hydroxyl functional groups (Example 4).

[0080] Determination of the degree of water absorption

[0081] Example 1

[0082] A polymer compound in hydrogel form was prepared comprising 2% by weight of alginate, 4% by weight of biochar, and 0.04% by weight of citric acid, using the following procedure:1) 0.2 grams of biochar powder were dissolved in 5 mL of distilled water, obtaining an aqueous solution of biochar.2) 0.1 grams of alginate were added to the aqueous solution of biochar and mixed at room temperature until completely dissolved, obtaining a solution of alginate and biochar.3) 50 pL of a 4% (w / v) sodium hypophosphite solution in distilled water and 250 pL of a 0.8% (w / v) citric acid solution were added to the solution of alginate and biochar. The mixture was stirred until a homogeneous solution was obtained.4) 750 pL of a 1 % (v / v) HCI solution were added, obtaining a gelatinous compound.5) The gelatinous compound was dried at a temperature of 70°C until complete dehydration (approximately 12 hours), obtaining a polymer compound in dry form.6) The resulting compound was ground into granules.7) The resulting granules were rehydrated in distilled water, obtaining a polymer compound in hydrogel form.8) After 24 hours, the degree of water absorption of the compound, as defined above,was measured and found to be 76.16 (weight of the compound in dry form = 0.19 grams; weight of the compound in hydrogel form = 14.66 grams).

[0083] Example 2

[0084] A polymer compound in hydrogel form was prepared comprising 2% by weight of alginate, 4% by weight of biochar, and 0.04% by weight of tartaric acid, using the following procedure:1) 0.2 grams of biochar powder were dissolved in 5 mL of distilled water, obtaining an aqueous solution of biochar.2) 0.1 grams of alginate were added to the aqueous solution of biochar and mixed at room temperature until completely dissolved, obtaining a solution of alginate and biochar.3) 50 pL of a 0.8% (w / v) tartaric acid solution were added to the solution of alginate and biochar. The mixture was stirred until a homogeneous solution was obtained.4) 1500 pL of a 1 % (v / v) HCI solution were added, obtaining a gelatinous compound.5) The gelatinous compound was dried at a temperature of 70°C until complete dehydration (approximately 12 hours), obtaining a polymer compound in dry form.6) The resulting compound was ground into granules.7) The resulting granules were rehydrated in distilled water, obtaining a polymer compound in hydrogel form.8) After 24 hours, the degree of water absorption of the compound, as defined above, was measured and found to be 85.89 (weight of the compound in dry form = 0.18 grams; weight of the compound in hydrogel form = 15.64 grams).

[0085] Example 3

[0086] A polymer compound in hydrogel form was prepared comprising 4% by weightof alginate, 8% by weight of biochar, and 0.08% by weight of citric acid, using the following procedure:1) 0.4 grams of biochar powder were dissolved in 5 mL of distilled water, obtaining an aqueous solution of biochar.2) 0.2 grams of alginate were added to the aqueous solution of biochar and mixed at room temperature until completely dissolved, obtaining a solution of alginate and biochar.3) 100 pL of a 4% (w / v) sodium hypophosphite solution in distilled water and 500 pL of a 0.8% (w / v) citric acid solution were added to the solution of alginate and biochar. The mixture was stirred until a homogeneous solution was obtained.4) 750 pL of a 1 % (v / v) HCI solution were added, obtaining a gelatinous compound.5) The gelatinous compound was dried at a temperature of 70°C until complete dehydration (approximately 12 hours), obtaining a polymer compound in dry form.6) The resulting compound was ground into granules.7) The resulting granules were rehydrated in distilled water, obtaining a polymer compound in hydrogel form.8) After 24 hours, the degree of water absorption of the compound, as defined above, was measured and found to be 60.28 (weight of the compound in dry form = 0.36 grams; weight of the compound in hydrogel form = 22.06 grams).

[0087] The degree of water absorption exhibited by the hydrogels produced in accordance with the above three examples is indicative of a high water absorption capacity. Moreover, the compounds synthesised according to the above procedures showed a high structural stability as well as the ability to withstand numerous cycles of swelling / deswelling from water.

[0088] Evaluation of swell ing / deswel ling capacity

[0089] Comparative Example

[0090] A spherical polymer compound in hydrogel form was prepared comprising 2% by weight of alginate and 4% by weight of biochar, using the following procedure:1) 0.2 grams of biochar powder were dissolved in 5 mL of distilled water, obtaining an aqueous solution of biochar.2) 0.1 grams of alginate were added to the aqueous solution of biochar and mixed at room temperature until completely dissolved, obtaining a solution of alginate and biochar.3) A solution containing 0.2% w / v of calcium chloride in distilled water was prepared in a beaker.4) The solution of alginate and biochar was taken with a pipette with a cut tip, due to the high viscosity of the solution, and poured dropwise into the calcium chloride solution, forming spheres.5) The spheres were left in the solution for 10 minutes, resulting in polymerisation of the alginate chains by calcium ions.6) The spheres were briefly rinsed in distilled water to remove excess calcium ions and photographed prior to the swelling / deswelling test described below.

[0091] Example 4

[0092] A polymer compound in hydrogel form was prepared comprising 2% by weight of alginate, 4% by weight of biochar, and 0.04% by weight of citric acid, using the following procedure:1) 0.2 grams of biochar powder were dissolved in 5 mL of distilled water, obtaining an aqueous solution of biochar.2) 0.1 grams of alginate were added to the aqueous solution of biochar and mixed at room temperature until completely dissolved, obtaining a solution of alginate and biochar.3) 50 pL of a 4% (w / v) sodium hypophosphite solution in distilled water and 250 pL of a 0.8% (w / v) citric acid solution were added to the solution of alginate and biochar. The mixture was stirred until a homogeneous solution was obtained.4) 750 pL of a 1% (v / v) HCI solution were added, obtaining a gelatinous compound.5) The gelatinous compound was cut into cubes and photographed prior to the swelling / deswelling test described below.

[0093] Swelling / deswelling test

[0094] The compounds in hydrogel form of the Comparative Example and Example 4 (according to the invention) were subjected to drying in a desiccator at 70°C for 12 hours, then photographed. The dried compounds were then hydrated by adding excess distilled water for 3 hours and subsequently photographed. The resulting compounds were then subjected again to a cycle of drying, lasting approximately 4 hours, and hydration, again for 3 hours, for two additional times.

[0095] Figure 2 show the photographs of the compounds in the initial state and after each drying and hydration step. From said photographs, it can be seen that the compound obtained by crosslinking alginate with Ca++ions (Comparative Example) undergoes a collapse of the structure already during the first drying step and is no longer able to rehydrate to retain the initial water volumes. On the contrary, the compound obtained by crosslinking alginate with citric acid and biochar comprising carboxyl functional groups and / or hydroxyl functional groups (Example 4) proved capable of swelling with water even after several drying steps (or deswelling steps).The compound of Example 4 thus proved capable of withstanding numerous cycles of swelling / deswelling from water, unlike the compound of the Comparative Example, which was not able to swell back to the initial levels.

[0096] It is evident that those described are only particular embodiments of the present invention. The person skilled in the art will be able to make to the polymer compound, the formulation, and the processes of the invention all those modifications necessary for their adaptation to particular conditions, without however departing from the scope of protection as defined in the appended claims.

Claims

CLAIMS1. A polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, wherein the biochar comprises carboxyl functional groups and / or hydroxyl functional groups, said polymer compound being in dry form or in hydrogel form.

2. A compound according to claim 1 , wherein said at least one polycarboxylic acid is selected from C4-C12 dicarboxylic, tricarboxylic, tetracarboxylic acids, preferably from the group consisting of: citric acid, tartaric acid, malonic acid, malic acid, maleic acid, fumaric acid, succinic acid, sulfosuccinic acid, aconitic acid, isocitric acid, itaconic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, o-phthalic acid, tricarboxylic acid, pyromellitic acid, butanetetracarboxylic acid, and cyclopentanetetracarboxylic acid.

3. A compound according to claim 1 or 2, comprising a weight percentage of alginate between 1 and 40%, preferably between 1 and 20%, and / or a weight percentage of said at least one polycarboxylic acid between 0.01 and 5%, preferably between 0.01 and 2%, and / or a weight percentage of biochar between 1 and 60%, preferably between 1 and 50%, said % being relative to the weight of the compound.

4. A compound according to any one of the preceding claims, having a degree of water absorption between 50 and 100, wherein the degree of water absorption of the compound is defined by the following equation:(weight of the compound in hydrogel form) — (weight of the compound in dry form) (weight of the compound in dry form)5. A formulation comprising the polymer compound according to any one of the preceding claims and further comprising: microorganisms, such as bacteria, for example in the form of spores, and / or one or more molecules selected from the group consisting of: nitrogenous molecules, phosphorous-based molecules, potassium-based molecules, magnesium salts, calcium salts.

6. Use in agriculture of the compound according to any one of claims 1 to 4 or of the formulation according to claim 5, as a water retention and water in-soil release agent.

7. A process for producing a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, comprising the following steps: a) Mixing alginate with an aqueous solution of biochar, thus obtaining a solution of alginate and biochar, wherein the biochar comprises carboxyl functional groups and / or hydroxyl functional groups; b) Mixing an aqueous solution of at least one polycarboxylic acid with the solution of alginate and biochar resulting from step a);c) Adding an esterification reaction catalyst to the mixture resulting from step b); d) Drying the mixture resulting from step c), thus obtaining said polymer compound.

8. A process according to claim 7, comprising a step following the drying step d), in which the polymer compound in dry form is reduced to granules or powder.

9. A process according to claim 7 or 8, wherein said aqueous solution of biochar comprises biochar in an amount between 2 and 50% by weight, preferably between 4 and 10% by weight.

10. A process according to any one of claims 7 to 9, wherein during step a) the alginate is mixed in an amount between 1 and 20% by weight, preferably between 2 and 5% by weight, with respect to the weight of the aqueous solution of biochar.

11. A process according to any one of claims 7 to 10, wherein said at least one polycarboxylic acid is selected from C4-C12 dicarboxylic, tricarboxylic, tetracarboxylic acids, preferably from the group consisting of: citric acid, tartaric acid, malonic acid, malic acid, maleic acid, fumaric acid, succinic acid, sulfosuccinic acid, aconitic acid, isocitric acid, itaconic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, o-phthalic acid, tricarboxylic acid, pyromellitic acid, butanetetracarboxylic acid, and cyclopentanetetracarboxylic acid.

12. A process according to any one of claims 7 to 11 , wherein said aqueous solution of at least one polycarboxylic acid has a concentration between 0.2% and 10% (w / v), preferably said aqueous solution is mixed during step b) in a ratio between 1 / 5 and 1 / 40 (v / v) with respect to the volume of the solution of alginate and biochar.

13. A process according to any one of claims 7 to 12, wherein said esterification reaction catalyst is a strong acid, or phosphoric acid (H3PO4), or a zeolite, preferably the strong acid being HCI or H2SO4.

14. A process according to any one of claims 7 to 13, wherein said esterification reaction catalyst is a strong acid and, during step c), an aqueous solution of strong acid having a concentration between 0.1 and 4% (v / v) is added to the mixture resulting from step b), preferably said solution is added in a ratio between 1 / 5 and 1 / 30 (v / v) with respect to the volume of the mixture resulting from step b).

15. A process according to any one of claims 7 to 14, wherein said drying step d) is carried out at a temperature between 50 and 120°C, preferably between 80 and 100°C, or between 70 and 90°C.

16. A process according to any one of claims 7 to 15, wherein during step b) an aqueous solution of sodium hypophosphite is also mixed with the solution of alginate and biochar, preferably said aqueous solution of sodium hypophosphite has a concentration between 1 and 3% (w / v), preferably said solution is added in a ratiobetween 1 / 60 and 1 / 150 (v / v) with respect to the volume of the solution of alginate and biochar.

17. A process for producing a formulation comprising a polymer compound comprising alginate chains crosslinked with biochar and with at least one polycarboxylic acid, and further comprising microorganisms, such as bacteria, for example in the form of spores, and / or one or more molecules selected from the group consisting of: nitrogenous molecules, phosphorous-based molecules, potassium- based molecules, magnesium salts, calcium salts, wherein said process comprises: the process for producing the polymer compound according to any one of claims 7 to 16 and further comprises: a step following step d) of the process for producing the polymer compound, wherein the polymer compound is reduced to granules or powder and mixed with microorganisms, such as bacteria, for example in the form of spores, and / or a step of mixing an aqueous solution comprising one or more molecules selected from the group consisting of: nitrogenous molecules and / or phosphorus- based molecules and / or potassium-based molecules and / or magnesium salts and / or calcium salts, said step being preferably carried out between step a) and step b), or between step b) and step c), or between step c) and step d) of the process for producing the polymer compound.

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