Method for obtaining metal nanoparticles of zinc oxide with interstitial zinc and a solid lignocellulose fraction, and zinc oxide with interstitial zinc and solid lignocellulose fraction obtained

Abstract

The invention relates to a method which, using cacao husk, makes it possible to obtain metal nanoparticles of zinc oxide with interstitial zinc – with minimum antioxidant capacity of 2.5 and 5.7 mg eq. Trolox / g ZnO / ZnNPs, respectively – and a solid lignocellulose fraction, stable at temperatures above 250ºC. The method requires grinding the cacao husk, adding alkaline solvent and extracting with microwaves, thereby obtaining a multifunctional supernatant to which a metal precursor is added and irradiated with microwaves, thereby obtaining an intermediate solution that is subjected to solid-liquid separation, the separated solid being calcinated to obtain nanoparticles of zinc oxide with interstitial zinc (ZnO / ZnNPs). Optionally, part of the multifunctional supernatant is used to precipitate a pectin-rich in soluble fraction and antioxidant-rich supernatant. Advantageously, the solid lignocellulose fraction is an adsorbent of heavy metals or a source of fibre.

Description

[0001] PROCEDURE FOR OBTAINING METALLIC ZINC OXIDE NANOPARTICLES WITH INTERSTITIAL ZINC AND A SOLID FRACTION

[0002] LIGNOCELLULOSIC, AND ZINC OXIDE WITH INTERSTITIAL ZINC AND LIGNOCELLULOSIC SOLID FRACTION OBTAINED

[0003] OBJECT OF THE INVENTION

[0004] The present invention relates to a semi-pilot or industrial-scale process for obtaining zinc oxide nanoparticles with interstitial zinc and a lignocellulosic solid fraction, based on sequential microwave-assisted extraction from cocoa husks generated as a waste product in the chocolate industry of high economic interest. Advantageously, the proposed process allows - in addition to the metallic zinc oxide nanoparticles with interstitial zinc - to sequentially obtain additional compounds with high added value, such as water-soluble proteins, antioxidants, and lignocellulosic material.

[0005] The object of the invention is to offer a semi-pilot or industrial scale process that allows reducing the production costs of the nanoparticles obtained and generates added value from the cocoa husk waste used as raw material in this process, obtaining a zero-waste process where all the incoming raw material is transformed into a product.

[0006] Also object of the present invention are the zinc oxide nanoparticles with interstitial zinc obtained by the developed process and the lignocellulosic solid fraction that constitutes an adsorbent for heavy metals or a source of fiber.

[0007] The compounds obtained through this procedure can be used in the chemical, cosmetic, pharmaceutical and food industries.

[0008] BACKGROUND OF THE INVENTION The global production of large quantities of food waste poses a significant challenge due to its rapid accumulation and negative environmental impacts. The economic, social, and environmental implications of this waste are considerable, and strict regulations governing waste treatment generate considerable expenses. Converting waste into high-value products represents an opportunity for companies to reduce treatment costs, increase their profits, and improve their competitiveness. Furthermore, the recovery and valorization of by-products complies with current sustainability and environmental protection standards (He et al. 2023).

[0009] Cocoa husks are the main waste produced during chocolate manufacturing after roasting cocoa beans, constituting 12% of the total weight. The production of chocolate and chocolate derivatives represents a major economic and social impact in Spain, making it necessary to propose environmentally sustainable solutions to manage this type of high-production waste. Cocoa husks are composed of 70% carbohydrates, mainly cellulose and pectin, with low fat, ash, and moisture contents (2%, 4%, and 6%, respectively). Cocoa husks are also rich in antioxidant compounds (theobromine, caffeic acid, epicatechin, ferulic acid, protocatechuic acid, chlorogenic acid) and pigments, with potential applications in sectors such as food, cosmetics, medicine, and advanced materials (Mellinas et al., 2020; Sánchez et al., 2023).

[0010] Mellinas et al. (Mellinas et al., 2020) have demonstrated the high potential of microwave synthesis to obtain high-value compounds from cocoa husks, optimizing laboratory-scale processes and controlling the extraction temperature. However, the laboratory conditions described in this study do not allow for the scalability of the microwave-based process using cocoa husks.

[0011] Considering the volume of husk produced by cocoa companies, there is still no integrated use of this industrial waste, apart from efforts to recover compound vapors through non-sequential processes, mostly using conventional extraction methods such as Soxhlet, maceration, or reflux. These combinations of mixing, agitation, temperature, and / or maceration offer a simple and effective method for extracting these compounds. However, these processes require long extraction times, high solvent consumption, and can degrade the target compounds due to local overheating effects, yielding inaccurate results.

[0012] Patent US20060269633A1 develops a sequential extraction of cocoa husks to obtain the polyphenols and theobromine present in them. To achieve this, a conventional extraction process is proposed, repeated two to five times to obtain the maximum yield of these fractions, using a water-acetone mixture as a solvent and temperatures between 40 and 80°C for four to six hours. Methanol, which is not recovered during the process, is used to separate the different fractions, increasing the use of organic solvents during extraction. The use of water as an extractant is contemplated, obtaining low yields in the process. Finally, the husks are subjected to a degreasing process prior to the extraction process, thus increasing the number of stages in the overall process.

[0013] Patent WO2020245293A1 applies a cascade process to obtain extracts from cocoa beans through hydrothermal extraction using mixtures of different organic solvents. To achieve this, it uses extraction times between 24 and 96 hours at temperatures below 70°C.

[0014] Patent CN110882289A performs a sequential extraction to extract the polyphenols present in cocoa beans, with the aim of reducing the costs of inactivating the polyphenol oxidase (PPO) enzyme. This process is necessary to increase the shelf life of cocoa beans. To achieve this, an initial extraction is carried out with hexane:acetone to defat the cocoa beans, and they are subsequently extracted with inorganic acids at pH values ​​between 2 and 6 and a temperature range between 30 and 85 °C for 2 hours on a pilot scale, obtaining fractions with 54% polyphenols and 41% theobromine.

[0015] Patent W02007138118A1 uses unfermented cocoa beans as raw material in a pilot-scale sequential extraction to obtain lipid fractions I and polyphenols. To achieve this, a hydroalcoholic extraction (30-70%) is carried out at a temperature range of 40-80°C for 1-2 hours. Maximum polyphenol extraction yields of 50% are obtained, with extracts enriched by up to 90% through evaporation.

[0016] Advances in extraction techniques have moved away from hydrothermal processes and promoted the use of more sustainable and faster technologies capable of extracting high-value compounds. Sequential compound extraction has been applied in recent years (Aimone et al., 2023; Moreira et al., 2023; Pérez-Bassart et al., 2023) with the aim of increasing the efficiency and sustainability of extraction processes for different agri-food waste, minimizing their environmental impact.

[0017] There are different investigations published in recent years on the extraction of active compounds present in cocoa husks (Barbosa-Pereira et al., 2018; Bruna et al., 2009; Handojo et al., 2019; Mellinas et al., 2020; Nsor-Atindana et al., 2012; Okiyama et al., 2019; Patricia Isabel et al., 2017; Rojo-Poveda et al., 2020, 2019) where non-conventional extraction techniques such as supercritical fluids or microwaves are used, but their developments are focused on a laboratory scale.

[0018] Patent W02020038905A1 describes a process for extracting cocoa pod husk components and utilizing the resulting soluble extract and its applications. This methodology proposes the use of unconventional techniques such as microwaves and ultrasound, but involves 10-12 stages prior to obtaining the extract. In this case, the company also uses enzymes to maximize extraction yields, although increasing the final cost of the process.

[0019] Patent ES2695874B2 proposes the sequential extraction of different high added value compounds (soluble proteins, lipids, pigments and carbohydrates) from wet biomass through successive extractions with pressurized subcritical water, ethanol expanded with sub- or supercritical CO2 and, finally, Supercritical CO2.

[0020] Grillo et al. propose the use of semi-pilot-scale ultrasound to obtain essential oil and polyphenols from cocoa husks using ternary mixtures of ethanol / hexane and water (25 L) and an energy requirement of 6.82 kW. Conventional technologies for the synthesis of metal nanoparticles employ a top-down approach, generating nanoparticles from starting bulk material and fragmenting them through various processes. This technique presents challenges in terms of homogeneity in nanoparticle shape and size, in addition to requiring bulky equipment with high energy consumption. As an alternative, a bottom-up approach has been developed for nanoparticle biosynthesis. This method is considered more viable as it allows the chemical synthesis of nanoparticles by the reduction of ionic species from a metal salt precursor and a reducing and / or stabilizing agent (Jamkhande et al., 2019).Patents ES2827623 and ES2340122 develop a process for obtaining iron nanoparticles with a high reducing capacity, using wastewater from the extraction of solids from the oil production process and a solution of a nonionic surfactant in an apolar solvent as the oxidizing solution, respectively. Both processes are designed for laboratory scale.

[0021] In recent years, alternative strategies for the synthesis of nanoparticles with a more sustainable approach have been explored, employing natural agents such as plant extracts and microbial enzymes as reducing and / or stabilizing agents (Aswathi et al., 2023). Patent W02022168070 describes the formulation of an antimicrobial product containing metallic nanoparticles or metal oxides synthesized from plant extracts. To do this, they carry out an extraction with organic solvents of different plant products using a temperature range between 20-130 °C and pH values ​​between 2 and 10. Subsequently, these extracts are mixed with another polymer solution and the metallic precursor and heated for 2 hours between 25 and 100 °C, resulting in an antimicrobial formulation. Scaling up the procedure used is not contemplated.

[0022] Patents ES2763230 and ES2580009 explain a process for preparing gold nanoparticles from polysaccharides present in citrus fruits and dragon fruit leaf extract, respectively. In both cases, conventional heating is used for both the extraction and synthesis processes, ranging from 50 to 80°C, proposing a laboratory-scale methodology.

[0023] Patent WO2018205539A1 describes a hydrothermal process based on a precursor compound of lignin and zinc oxide, followed by a second high-temperature calcination step. The resulting composite material has a three-dimensional porous structure but does not contain interstitial zinc.

[0024] Microwave nanoparticle synthesis offers significant advantages over conventional methods, including improved microstructure, increased synthesis yield, energy savings, reduced manufacturing costs, and the production of new materials with diverse properties. Several papers have been reported on the laboratory-scale production of various types of nanoparticles using this technology (Ahammed et al., 2020; Borowska et al., 2023; Kaur et al., 2023; Mellinas et al., 2019; Pauzi et al., 2019; Xiao et al., 2020).

[0025] Patent MX / a / 2014 / 002235 develops a laboratory-scale process for obtaining silver, gold, and zinc oxide nanoparticles using aqueous mucilage from Opuntia amychlaea. The extract is obtained by macerating the plant product in water for 2 hours at temperatures between 25 and 95 °C. It is then mixed with the plant residue and the solution is heated at temperatures between 20 and 80 °C for a minimum of 30 minutes, adjusting the pH to 13 with sodium hydroxide, obtaining nanoparticles with a size of 20 and 100 nm. Purification of these nanoparticles is not contemplated; mixtures of nanoparticles with the original residual material are obtained after synthesis.

[0026] Currently, the synthesis of zinc oxide nanoparticles with interstitial zinc has generated growing interest due to their adaptable band gap, compositional diversity, controllable sizes and shapes, and low toxicity. Furthermore, these nanoparticles have remarkable optical properties, allowing ZnO to emit light throughout the visible region.

[0027] The synthesis of zinc oxide nanoparticles with interstitial zinc has been previously reported at laboratory scale using conventional heating and modified starches, demonstrating the great potential of this type of nanoparticles (Lin et al., 2014). A process for synthesizing these nanoparticles using microwaves at laboratory scale, albeit with a larger number of steps, has recently been published in a doctoral thesis using cocoa husk as a precursor to the reducing agent (Mellinas-Ciller, 2021). However, the conditions described in this work are not applicable at semi-pilot scale due to limitations in the equipment and the applied powers.

[0028] The adaptability of the processes developed from laboratory to semi-pilot or industrial scale presents significant limitations in terms of working conditions. Conditions cannot be directly transferred linearly, as the process is influenced by many factors, such as the power and frequency of microwave radiation, the initial temperature, the dielectric properties of the material, and the design of the microwave equipment. Therefore, these factors must be optimized. The developed processes offer several advantages: adaptability, flexibility, reduced CO2 emissions, speed, use in continuous processes, and high energy efficiency.

[0029] Regarding scalable microwave processes for waste recovery, patents ES2615752T3 and WO2013067896A propose the extraction of polysaccharides from fungi and algae, respectively, on a pilot scale with a pressure range of 20-760 mmHg, using organic and inorganic acids. Patent WO2013150262A1 proposes a microwave-assisted citrus waste biorefinery for obtaining different value-added products, such as essential oil, flavonoids, cellulose and pectin. For this purpose, water-miscible organic solvents are used at an extraction temperature between 80-150 °C and a power between 100 W-10 MW.

[0030] No zero-waste procedure has been reported to date for the synthesis of metallic zinc oxide nanoparticles with interstitial zinc using microwave energy on a semi-pilot scale and from a natural extract from the waste of the chocolate production process (cocoa husk) as a reducing agent.

[0031] DESCRIPTION OF THE INVENTION

[0032] The present invention proposes a low-cost, zero-waste process for obtaining, on a semi-pilot or industrial scale, metallic zinc oxide nanoparticles with interstitial zinc (ZnO / ZnNPs) with a unique allotropic structure from cocoa husks (a byproduct of cocoa processing) by means of microwave-assisted sequential extraction. The described process results in higher extraction yields, fewer steps, and lower energy requirements than those previously reported.

[0033] Furthermore, the proposed microwave-assisted processes utilize an aqueous medium, avoiding the use of organic solvents, improving the sustainability of the processes compared to reported methodologies.

[0034] Advantageously, since it is a non-selective extraction in an aqueous medium, the proposed process also allows for the production of other high-value compounds, such as water-soluble proteins, antioxidants, and lignocellulosic material, resulting in the complete valorization of the cocoa husk, with all the raw material entering the process ultimately becoming a product.

[0035] This approach is fully aligned with the principles of "green chemistry" and the "circular economy," reducing waste generated in the agri-food industry to zero. The products obtained with multifunctional properties offer great potential for use in multiple sectors, such as food, food packaging, cosmetics, and biomedicine.

[0036] Both the non-selective extraction process and microwave-assisted synthesis involve the use of a natural extract rich in antioxidants obtained from chocolate industry waste.

[0037] The synthesis procedure for zinc oxide nanoparticles with interstitial zinc involves the use of a salt as a precursor, preferably in the form of an aqueous solution, together with a natural extract rich in natural antioxidants obtained after extraction in a basic medium of the cocoa husk (liquid extract P1), used as a reducing agent.

[0038] Thus, the first product of this invention is a non-toxic, economically viable, and easily accessible reducing agent for the production of zinc oxide nanoparticles with interstitial zinc, with great potential for use in various fields, such as electronics, agriculture, food packaging, and medicine. This approach not only significantly reduces the costs of the production process but also represents a "green" synthesis method that incorporates the principles of the "circular economy" by reusing materials previously considered waste.

[0039] It is important to note that the biorefinery process proposed in this invention does not require pretreatment, presenting a novel and more sustainable approach to cocoa husk processing.

[0040] Thus, the present invention allows obtaining zinc oxide nanoparticles with interstitial zinc and, optionally, other high added value compounds, from cocoa husk according to the following steps:

[0041] - Grinding of the cocoa husk, where it is preferably ground to a size between 0.5 and 2 mm.

[0042] - Addition of an alkaline solvent to the ground cocoa husks until the pH is between 8 and 12, obtaining a solution with a ratio of 10-70 g of husks / L of alkaline solvent. The alkaline solvent added to the ground cocoa husks is preferably sodium hydroxide or potassium hydroxide.

[0043] - Microwave-assisted extraction of the solution obtained in the previous stage, carried out at an energy density of between 0.3 and 0.7 kW s / mL, generating a lignocellulosic solid fraction (P2) and a multifunctional supernatant (P1). Optionally, the microwave-assisted extraction is preferably carried out for a time interval of between 35 and 100 minutes, working under stirring and atmospheric pressure. Advantageously, in microwave-assisted extraction, it is not necessary to perform any pretreatment on the ground cocoa husk to obtain a first extract rich in water-soluble proteins and other value-added compounds, as well as the lignocellulosic solid fraction. In fact, the lignocellulosic solid fraction can be used as a metal adsorbent or as a dietary fiber without the need for pretreatment, as it is rich in lignocellulosic material.

[0044] - Solid-liquid separation obtaining a lignocellulosic solid fraction (P2) and a multifunctional supernatant (P1), where the multifunctional supernatant (P1) is composed of at least a minimum concentration of 235 mg of Galacturonic Acid / g dry extract and antioxidant compounds with a minimum antioxidant capacity of 18 mg eq. Gallic Acid / g extract and 49 mg eq. Gallic Acid / g extract, measured by FRAP (acronym for the iron-reducing antioxidant power measurement method) and ABTS (acronym for the ABTS+ cation radical reduction measurement method), respectively.

[0045] - Synthesis of zinc oxide nanoparticles with interstitial zinc (ZnO / ZnNPs) by microwave irradiation at an energy density between 0.36 - 0.61 kW s / mL of a solution formed by the multifunctional supernatant (P1) and a metallic precursor in the form of a metal salt with a concentration between 7 and 10 g / L, obtaining an intermediate solution. Thus, P1 is used as a carbon source, reducing agent and stabilizer for the synthesis of zinc oxide nanoparticles with interstitial zinc; while the metallic precursor in the form of the metal salt of ZnO / ZnNPs is preferably zinc chloride. Likewise, it should be noted that preferably, the synthesis of metallic nanoparticles of zinc oxide with interstitial zinc (ZnO / ZnNPs) by microwave irradiation is carried out for a period of between 30 and 60 minutes.

[0046] - Solid-liquid separation of the intermediate solution, where the separated solid is subjected to calcination applying temperatures between 500 and 600 °C, obtaining zinc oxide nanoparticles with interstitial zinc (ZnO / ZnNPs) that present a hexagonal structure, determined with X-ray diffraction (XRD) with a characteristic peak at 46° related to the presence of interstitial zinc and with a size between 30 and 50 nm, presenting a GAP band between 2.9 and 3.2 eV, a stability between -27 and -50 mV and a minimum antioxidant capacity, measured by ABTS and FRAP, between 2.5 and 5.7 mg eq. Trolox / g ZnO / ZnNPs, respectively.

[0047] It should be noted that the lignocellulosic solid fraction (P2) is stable at temperatures greater than 250 °C.

[0048] It is important to note that, based on what is known, the invention proposed herein is not the result of linear MAE scaling; that is, it requires optimization of the extraction parameters to maximize yields without affecting the quality of the products obtained. In this regard, it is worth highlighting that the microwave extraction yield is preferably between 40 and 50% of the initial raw material. To date, such a complete biorefinery that reduces waste to zero has not been developed, so that everything entering the process is transformed into a product.

[0049] Optionally, a portion of the multifunctional supernatant (P1) obtained from the solid-liquid separation is contacted with an acid until the isoelectric point of the proteins that form (P1) is reached, generating the precipitation of an insoluble fraction (P3), and a supernatant (P4) rich in antioxidants; where the precipitated insoluble fraction (P3) is recovered by means of a solid-liquid separation at room temperature, whose extraction yield is greater than 70% with respect to the multifunctional supernatant (P1).

[0050] Thus, the insoluble fraction (P3) is preferably subjected to a drying process, by spray drying, nebulization, lyophilization, or in an oven; while the supernatant (P4) is subjected to a drying process, by spray drying, nebulization, lyophilization, or in an oven.

[0051] The ZnO / ZnNPs obtained have diverse applications in different fields. They are used in sunscreen products to disperse ultraviolet radiation, act as catalysts in chemical reactions due to their high surface area, are used in the manufacture of sensors to detect gases and substances, and find applications in electronic and optoelectronic devices thanks to their semiconductor properties.

[0052] In addition, they are used in medicine for drug delivery systems, contrast agents, and photodynamic therapies. They are also used in coatings to confer antibacterial properties to various surfaces, such as textiles and food packaging.

[0053] The nanoparticles of this invention can be used to obtain food packaging materials with antibacterial and UV-blocking properties, increasing the shelf life of packaged foods. The addition of 0.25-1% ZnO / ZnNPs to the polymeric material reduces the organoleptic deterioration of packaged foods. All these applications highlight the versatility and usefulness of the nanoparticles obtained.

[0054] The present semi-pilot or industrial scale procedure for extracting cocoa husks presents novel extraction conditions that are different from and not proportional to those used on a laboratory scale.

[0055] The use of water as the primary solvent to obtain polyphenol-rich fractions in high yields improves the sustainability of the processes proposed to date. Furthermore, a biorefinery process is provided to isolate different fractions from cocoa husks, including short-time microwave-assisted extraction and microwave synthesis of ZnO / ZnNPs in a cascade and sequential process.

[0056] The products obtained in this invention include proteins, polyphenols, lignocellulosic fraction and ZnO / ZnNPs.

[0057] Specifically, the present invention allows obtaining metallic nanoparticles of zinc oxide with interstitial zinc (ZnO / ZnNPs) that have a hexagonal structure, determined with X-ray diffraction (XRD) with a characteristic peak at 46° related to the presence of interstitial zinc and with a size between 30 and 50 nm, presenting a GAP band between 2.9 and 3.2 eV, a stability between -27 and -50 mV and a minimum antioxidant capacity, measured by ABTS and FRAP, of between 2.5 and 5.7 mg eq. Trolox / g ZnO / ZnNPs, respectively.

[0058] Likewise, the process of the present invention allows obtaining a lignocellulosic solid fraction, with application as an adsorbent of metals, preferably heavy metals such as lead or cadmium, present in wastewater due to its porous structure, with a minimum efficiency between 26-50%, under the conditions studied, without the need to carry out any subsequent treatment after the solid-liquid separation process.

[0059] Likewise, said lignocellulosic solid fraction, obtained by the process of the invention, can be used as a source of soluble and insoluble fiber with a concentration greater than 40%.

[0060] The use of microwaves in the present procedure on a semi-pilot / industrial scale generates higher yields for the main products obtained (proteins and antioxidants) compared to the conventional methods reported to date, given that the extraction occurs as a consequence of changes in the cellular structure of the agri-food waste when interacting with microwave waves, under the conditions previously described.

[0061] The multifunctional extracts obtained (P1) are stable at temperatures above 200 °C, presenting high thermal stability for processing in different industrial applications.

[0062] The production of ZnO / ZnNPs using the liquid extract (P1) as a synthesis medium, obtained in the alkaline extraction, which reacts with the precursor, allows obtaining a reaction intermediate based on a Zn-extract complex by microwave irradiation, giving rise to nanoparticles after the calcination process, with active properties improved by the presence of interstitial zinc in its structure (decreasing the value of the GAP band).

[0063] The ZnO / ZnNPs obtained in the present invention advantageously possess excellent stability according to the results of dynamic light scattering (values ​​between -27 and -50 mV), being of interest for their application in different fields such as electronics, agriculture, medicine, cosmetics and food packaging.

[0064] Table 1 summarizes the properties of the multifunctional liquid extract (P1) obtained after the first microwave-assisted extraction, which are not intended to limit its scope, but rather to be illustrative.

[0065] Table 1. Characterization of the extract obtained after the first extraction using 0.45 kW s / mL, 60 min, 40 g / L, basic pH, 8000 mL. Antioxidant capacity, ABTS (mg-rroiox / g extract 49.6 ± 9.2

[0066] The process of this invention reduces the number of synthesis steps at a semi-pilot / industrial scale compared to processes developed at laboratory scale. Furthermore, the energy density values ​​applied based on the volume and material used make the process innovative compared to processes reported at laboratory scale, given that at semi-pilot / industrial scale there are greater wave penetration depths and significantly higher efficiencies compared to smaller systems. The method proposed in this invention operates at a semi-pilot / industrial scale in an aqueous medium, adjusting the necessary energy density based on the amount of raw material entering the system.

[0067] Advantageously, the cascade steps and the circularity of the process allow for a zero-waste procedure by eliminating sample pretreatment steps proposed by other authors, such as concentrating or drying, between the extraction and synthesis processes, which involve the use of organic solvents and significant time and energy consumption. This invention reduces the carbon footprint along with a significant reduction in total process costs.

[0068] BRIEF DESCRIPTION OF THE DRAWINGS

[0069] To complement the description that follows and in order to help better understand the characteristics of the invention, according to preferred examples of practical implementation thereof, a set of figures is attached as an integral part of said description, in which the following has been represented for illustrative and non-limiting purposes:

[0070] Figure 1.- Shows a flow diagram of the method for obtaining the different compounds mentioned above, object of the invention, where the sequential microwave extractions and the products obtained in the different stages have been represented.

[0071] Figure 2.- TEM (transmission electron microscopy) images showing the size of the ZnO / ZnNPs synthesized according to the process of the invention. PREFERRED EMBODIMENT

[0072] The preferred embodiments detailed in this section constitute non-limiting examples of the invention.

[0073] Thus, the experimental procedure on a semi-pilot / industrial scale is detailed first, in which up to five types of products can be obtained (as detailed in Figure 1), which consists of the following stages:

[0074] 1. Obtaining a multifunctional extract (P1), liqnocellulosic fraction (P2), water-soluble proteins (P3), antioxidant compounds (P4) and zinc oxide nanoparticles with interstitial zinc (P5):

[0075] 1.1. The previously ground residue with a particle size less than 2000 microns, preferably 1000 microns, is introduced into water and the pH of the solution is adjusted to a value between 8-12 (basic medium) preferably using a base, such as sodium or potassium hydroxide. This alkaline solution is subjected to a microwave extraction process (alkaline MAE), under different extraction conditions: energy density between 0.3 - 0.7 kW s / mL for 35-100 minutes and 10 - 70 g husk / L solvent ratio.

[0076] A particular advantage of this extraction step from cocoa husk in the present invention is that it is not necessary to add organic solvents to promote the extraction of proteins and antioxidants, in relatively short times.

[0077] 1.2. After a solid-liquid separation, a lignocellulosic solid fraction (P2) and a multifunctional liquid extract (P1) are obtained, with a microwave extraction yield for this stage between 40 - 50%. The multifunctional extract (P1) is mainly composed of proteins (400 - 600 mg eq. BSA / g extract, Bradford method) and antioxidant compounds with a minimum antioxidant capacity of 49 and 18 mg eq. Gallic Acid / g extract determined by the ABTS and FRAP spectrophotometric methods, respectively. The product (P2) is a lignocellulosic solid fraction that presents a highly porous structure with a thermal stability greater than 250 °C. This type of material can be used to adsorb heavy metals (cadmium, lead or arsenic) for wastewater purification, in addition to other possible applications in sectors such as cosmetics, food or the development of new advanced materials.The adsorbents of this invention can be used for the reduction of metals (such as lead or cadmium) present in wastewater or irrigation water with a minimum efficiency between 26-50, under the conditions studied.

[0078] A particular advantage of this extraction step from cocoa husk in the present invention is the obtaining of two usable fractions, reducing to zero the waste generated in this process of valorizing an agri-food waste, considerably decreasing the carbon footprint of this procedure. Optionally, the multifunctional extract (P1) obtained in the previous step is acidified using an organic or inorganic acid, until reaching the isoelectric point of the proteins, where they precipitate and are recovered by solid-liquid separation at room temperature (P3), whose extraction yield is above 70%. The invention also provides the supernatant corresponding to the antioxidant-rich fraction (P4), which can be applied in different sectors to inhibit oxidative degradation processes.Both fractions can be dried using atomization by nebulization drying, freeze-drying or oven depending on the required humidity percentage and their subsequent storage. Zinc oxide nanoparticles with interstitial zinc (ZnO / ZnNPs): The multifunctional liquid extract (P1) obtained in the previous extraction, without the need for pretreatment, is used as a reducing and stabilizing agent for the synthesis of ZnO / ZnNPs by microwave radiation.

[0079] To do this, it is mixed with a concentration of metal precursor in the form of metal salt between 7 and 10 g / L, where zinc chloride is preferably used, and is subjected to microwave treatment with an energy density between 0.36 - 0.61 kW s / mL for 30-60 minutes.

[0080] Next, a solid-liquid separation is carried out where the obtained solid is subjected to a muffle calcination process between 500-600 °C, for the time necessary for the formation of ZnO / ZnNPs. These nanoparticles present a hexagonal structure, determined by X-ray diffraction (XRD), with a characteristic peak at 46° related to the presence of interstitial zinc within its structure. The particle size is between 30-50 nm (TEM, Figure 3), with a GAP band between 2.9 and 3.2 eV.

[0081] The minimum antioxidant capacity of these nanoparticles, as measured by ABTS and FRAP, is 2.5 ± 0.2 and 5.7 ± 0.2 mg eq. Trolox / g nanoparticles, respectively, with a stability between -27 and -50 mV (measured by dynamic light scattering (DLS) after dispersion in water).

[0082] Two examples of preferred embodiments for obtaining metallic nanoparticles of zinc oxide with interstitial zinc (ZnO / ZnNPs) according to the process proposed in the present invention are detailed below.

[0083] Example 1

[0084] 450 g of cocoa husk, previously ground to a size between 0.5-1 mm, are deposited, a sufficient quantity of water is added to achieve a 40 g / L ratio and the pH is adjusted to a value of 8.5 with sodium hydroxide.

[0085] The mixture is subjected to microwave treatment at atmospheric pressure under the following conditions: 0.3 kW s / mL for 90 minutes.

[0086] The lignocellulosic solid fraction (P2) is then separated by centrifugation at 5300 rpm for the time necessary to separate both phases. The necessary amount of citric acid is added to the supernatant obtained, a multifunctional liquid extract (P1), rich in proteins and antioxidants, to reach the isoelectric point of the proteins and thus obtain a protein fraction (P3) and another rich in antioxidants (P4) that are separated by centrifugation at room temperature and 10000 rpm for the time necessary to separate both phases.

[0087] The lignocellulosic solid fraction (P2) is dried in an oven at 50 °C until the desired humidity percentage is reached and stored under vacuum to be used as a metal adsorbent.

[0088] Following the circularity of the process, the multifunctional liquid extract P1 is used without pretreatment in the synthesis of ZnO / ZnNPs. 8.1 g of zinc chloride per liter of solution is added, and an energy density of 0.31 kW s / mL is used for 50 minutes in a microwave system operating at atmospheric pressure. The solid obtained is subsequently separated by centrifugation at room temperature and 10,000 rpm, the time required to separate both phases.

[0089] This solid is placed in a muffle furnace at 500°C for 6 hours. The resulting ZnO / ZnNPs (P5), with an average size of 40 nm, are stored under vacuum to extend their storage time.

[0090] Example 2

[0091] 300 g of cocoa husk, previously ground to a size between 0.5-1 mm, are deposited, a sufficient amount of water is added to achieve a 40 g / L ratio and the pH is adjusted to a value of 12 with sodium hydroxide.

[0092] The mixture is subjected to microwave treatment at atmospheric pressure under the following conditions: 0.5 kW s / mL for 40 minutes.

[0093] The lignocellulosic solid fraction (P2) is then separated by centrifugation at 5300 rpm for the time necessary to separate both phases. The necessary amount of citric acid is added to the obtained supernatant (P1), rich in proteins and antioxidants, to reach the isoelectric point of the proteins and thus obtain a protein fraction (P3) and another rich in antioxidants (P4), which are separated by centrifugation at room temperature and 10000 rpm for the time necessary to separate both phases. The lignocellulosic solid fraction (P2) is dried in an oven at 50 °C until the desired humidity percentage is reached and is stored under vacuum to be used as a metal adsorbent.

[0094] Following the circularity of the process, extract P1 is used without pretreatment in the synthesis of ZnO / ZnNPs. 6.7 g of zinc chloride per liter of solution is added, and an energy density of 0.51 kW / mL is used for 40 minutes in a microwave system operating at atmospheric pressure.

[0095] The solid obtained is then separated by centrifugation at room temperature and 10,000 rpm, the time required to separate both phases. This solid is placed in a muffle furnace at 500 °C for 6 hours. The ZnO / ZnNPs obtained (P5), with an average size of 37 nm, are stored under vacuum to extend storage time.

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Claims

1. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, comprising the following steps: - Grinding of cocoa husks. - Addition of an alkaline solvent to the ground cocoa husk until pH between 8 and 12 and obtaining a solution with a ratio of 10-70 g husk / L alkaline solvent. - Microwave-assisted extraction of the solution obtained in the previous stage, which is carried out at an energy density of between 0.3-0.7 kW s / mL, generating a solid lignocellulosic fraction (SLC) and a multifunctional supernatant (P1). - Solid-liquid separation obtaining a lignocellulosic solid fraction (P2) and a multifunctional supernatant (P1), where the multifunctional supernatant (P1) is composed of, at least, a minimum concentration of 235 mg of Galacturonic Acid / g dry extract and antioxidant compounds with a minimum antioxidant capacity of 18 mg eq. Gallic Acid / g extract and 49 mg eq. Gallic Acid / g extract, measured by FRAP and ABTS, respectively. - Synthesis of zinc oxide nanoparticles with interstitial zinc (ZnO / ZnNPs) by microwave irradiation at an energy density between 0.36 - 0.61 kW s / mL of a solution formed by the multifunctional supernatant (P1) and a metallic precursor in the form of a metallic salt with a concentration between 7 and 10 g / L, obtaining an intermediate solution. - Solid-liquid separation of the intermediate solution, where the separated solid is subjected to calcination applying temperatures between 500 and 600 °C, obtaining zinc oxide nanoparticles with interstitial zinc (ZnO / ZnNPs) that present a hexagonal structure, determined with X-ray diffraction (XRD) with a characteristic peak at 46° related to the presence of interstitial zinc and with a size between 30 and 50 nm, presenting a GAP band between 2.9 and 3.2 eV, a stability between -27 and -50 mV and a minimum antioxidant capacity, measured by ABTS and FRAP, between 2.5 and 5.7 mg eq. Trolox / g ZnO / ZnNPs, respectively, where the lignocellulosic solid fraction (P2) is stable against temperatures greater than 250 °C.

2. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a, characterized in that in the grinding stage the cocoa husk is ground to a size between 0.5 and 2 mm.

3. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the alkaline solvent is sodium hydroxide or potassium hydroxide.

4. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the microwave-assisted extraction is carried out for between 35 and 100 minutes working under agitation and atmospheric pressure.

5. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized by the fact that the microwave extraction yield is between 40 and 50% compared to the initial raw material.

6. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a, characterized in that a portion of the multifunctional supernatant (P1) obtained from the solid-liquid separation is brought into contact with an acid until the isoelectric point of the proteins that form (P1) is reached, generating the precipitation of an insoluble fraction (P3), and a supernatant (P4) rich in antioxidants; where the precipitated insoluble fraction (P3) is recovered by means of a solid-liquid separation at room temperature, whose extraction yield is greater than 70% with respect to the multifunctional supernatant (P1).

7. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 6 a , characterized in that the insoluble fraction (P3) is subjected to a drying process, by atomization, nebulization, lyophilization or oven.

8. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 6 a , characterized in that the supernatant (P4) is subjected to a drying process, by atomization, nebulization, lyophilization or oven.

9. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a , characterized in that the metallic precursor in the form of a metal salt of ZnO / ZnNPs is zinc chloride.

10. Procedure for obtaining metallic nanoparticles of zinc oxide with interstitial zinc and a lignocellulosic solid fraction from cocoa husk, according to claim 1 a, characterized in that the synthesis of metallic nanoparticles of zinc oxide with interstitial zinc (ZnO / ZnNPs) by microwave irradiation is carried out over a period of between 30 and 60 minutes.

11. Metallic nanoparticles of zinc oxide with interstitial zinc (ZnO / ZnNPs) obtained according to any of the preceding claims, characterized in that they have a hexagonal structure, determined with X-ray diffraction (XRD) with a characteristic peak at 46° related to the presence of interstitial zinc and with a size between 30 and 50 nm, presenting a GAP band of between 2.9 and 3.2 eV, a stability between - 27 and -50 mV and a minimum antioxidant capacity, measured by ABTS and FRAP, of between 2.5 and 5.7 mg eq. Trolox / g ZnO / ZnNPs, respectively.

12. Lignocellulosic solid fraction obtained in the solid-liquid separation of the process according to claim 1 a, for use as a heavy metal adsorbent when brought into contact with compounds containing heavy metals, such as wastewater.

13. Lignocellulosic solid fraction obtained in the solid-liquid separation of the process according to claim 1 a , for use as a source of soluble and insoluble fiber with a concentration greater than 40%.

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