WO2019193490A1 - Process for the production of protected oxidizable metal particles and correlated products - Google Patents

Abstract

The invention refers to a process for the production of oxidizable submicronic particles (nanoparticles) comprising one or more zero-valent metals and incorporated inside a crystallized solid matrix. In particular, the invention refers to a two-step process: (1) synthesis of the metal particles in a solution; (2) crystallization by heterogeneous nucleation of the solute onto the produced particles. The invention also relates to oxidizable metal particles in a crystallized solid matrix, obtained by the process of present invention, and to the uses of these metal particles in the industrial and environmental field.

Description

"Process for the production of protected oxidizable metal particles and correlated products”

Field of the Invention

The present invention relates to a process for the production of oxidizable metal particles incorporated inside a solid crystallized matrix and correlated products.

Background of the Invention

The oxidizable particles can be composed of one or more organic substances, one or more inorganic substances or any combination of these in the form of a composite, and they can be easily oxidized. The oxidizable metal particles are metallic particles composed of an inorganic substance, preferably consisting of a single metal element, preferably zero-valent, and may contain a limited percentage by weight of their oxides and hydroxides, preferably less than 5% and preferably located in the outer part of the structure, strongly capable of being oxidized and therefore reducing other substances by contact[l]. The metal particles are useful in various fields of industry, as a non- exhaustive example, pharmaceutical, electronics, optics, dyes, in medicine and in the environmental field [2-5]. The metal particles of industrial interest are those of metals of the first, second, third, fourth, fifth and sixth groups and those of transition metals, as a non- exhaustive example: Fe, Cd, Ni, Ag, Al, Au, Cr, Zn, Mn, Co, Cu, Pd, Pt, Rh, Ir, Ru, Os, W, Tc, Re, Nb, Mo, V, Ta, Zr, Y, Hf and Sc.

Metal particles may be synthesized through physical, chemical, biological processes or a combination thereof. The physical processes are usually based on one of the following methods: top-down production methodologies starting from blocks of a metal precursor through a dimensional reduction in mills; condensation of a vapor phase of the atomized metal on a condensing surface; by thermal decomposition, in flame, in plasma or inside an electric arc [6]. The chemical process is based on the chemical reduction of a metallic precursor, in ionic form and in liquid solution as dissolved solute, in the zero-valent state, and subsequent chemical precipitation [7] The biological process is based on extra or intracellular synthesis of non-toxic metal particles by bacteria, fungi or other microorganisms placed in liquid solution, in which the metallic precursor is present in a ionic form [8,9].

At present, the most used production process is the one based on chemical reduction in a liquid solution, preferably in an aqueous solution, this being the simplest, cheaper and quicker method with respect to other chemical processes, physical and biological ones [6]. An example of the production of nanoparticles, in particular of zero-valent iron particles, is reported by Ponder et al. 2000 [10], which use sodium borohydride (NaBH₄) in basic solution as a reducing agent, and ferro- sulphate heptahydrate (FeSC^ TFhO) as a precursor of Fe and source of ferrous ions in hydro alcoholic solution (70% water V/V and 30% ethanol V/V). The nanomaterial thus produced is in a colloidal solution, and to avoid a rapid oxidation and deactivation by the oxidizing agents of the environment, it must be protected by substances denominated "of caping" in a solution, in a protected atmosphere or by means of a coating. In the literature there are several works dealing with particle coatings used in the biomedical, industrial and environmental fields [11]. Among the various organic compounds used for coating, some polymers and biopolymers have been known for some time, such as, for example cellulose carboxy-methylated [12], polyvinylpyrrolidone (PVP) [13], gum arabic [14], sodium alginate [15] The main purpose of these coatings is to make the colloidal dispersion of the produced particles more stable, demonstrating, however, only partial protective capacity against oxidation. Some of these compounds are also useful to reduce the toxicity of certain particles against certain classes of microorganisms present in the respective fields of use. The cellulose carboxy-methylate biopolymer, for example, acts as a steric/electrostatic dispersant through the formation of branched carbon chains deposited on the surface of the nanoparticles [16]. The nanoparticles inserted in this structure show a reduced contact with the cellular barriers of microorganisms, preserving their integrity [16]. On the other hand, this type of structure cannot prevent the spread of oxidants and therefore does not protect the nanoparticles from their deactivation. Moreover, the coating is difficult to remove and accordingly, the action of the metal nanoparticles is strongly inhibited since their action can only manifest by means of diffusion processes through the protective layer.

Since the coatings developed till now not able to effectively isolate particles from the external environment, other authors have proposed the use of some compounds in solution for the protection of the suspended nanoparticles (caping). For example, it has been proposed the use of some natural compounds, such as polyphenols from tea leaves (Moulton et al [17]), useful both in production and in the oxidation protection of nanoparticles of zero-valent silver. By this way, the produced nanoparticles are chemically stable and active, without undergoing oxidation by oxygen dissolved in the solution. In another study [16] the polyphenols extracted from tea leaves and coffee were used to produce and protect zero-valent iron nanoparticles; such nanoparticles were then used for the dehalogenation of organic compounds in synthetic liquid waste, but showing lower efficiency compared to similar particles produced with inorganic reducing agents (NaBH₄ and Na2S₂0₄). Although this protective method proves to be effective in the short term, it does not guarantee a long she If- life.

Finally, the applicative use of the metal particles involves the simultaneous introduction of "caping" substances into the applicative system, which would also cause an increase of the amount of organic carbon in the reaction medium.

Another study has developed a protection of zero-iron nanoparticles in a gel state [18], using a sodium alginate bed, which is subsequently used for the removal of nitrates from groundwater. The encapsulation process is the subject of patent application US 20160031766.

Other authors have used inorganic protective coatings by chemical reaction. As an example, one of these is the tetraethyl orthosilicate, which is used to coat numerous nanoparticles [19] and is able to stabilize them, making them however inert from the chemical point of view.

For their applicative use, it is necessary to employ disruptive chemical agents that are able to dissolve the silicate coating.

These agents consist mainly of mixtures of concentrated hydrofluoric and nitric acid, which require additional costs and precautions when used. Different types of clays, from the less plastic and less active ones such as kaolin [20], to the more active ones such as bentonite [21], have been used, for example, to support and stabilize zero-valent nanoparticles. In both cases, the suspension was stable, but this did not make the nanoparticles protected from the attack of the oxygen dissolved in solution.

Therefore, the need is felt to obtain oxidizable metal particles, which are protected by external oxidizing agents, which can be stored for a prolonged period of time and readily available for use.

Summary of the Invention

It is therefore a first object of the present invention a process for the production of oxidizable zero-valent metal particles incorporated inside a crystallized solid matrix. The process according to the invention is characterized by two successive steps: (1) synthesis of the metal particles in a solution; (2) crystallization by heterogeneous nucleation of the solute on the produced particles, by a seeding function.

By the process of the invention it is possible to obtain a protective matrix, which maintains unaltered the oxidability of the particles, providing them with a protection throughout the storage period up to their use. The particles can be preserved for more than 200 days, and can be quickly made available through the simple dissolution of the protective matrix: there is no difference in the oxidability of said particles, measured immediately after their production or after 210 days of exposure to air.

Therefore, another object of the present invention are submicronic particles and oxidizable metallic nanoparticles comprising one or more zero-valent metals and incorporated inside a crystallized solid matrix, directly obtained by means of the process of the invention.

It is also another object of the invention the use of said particles in industrial and/or environmental fields.

Further aspects and advantages of the claimed process will be apparent from the detailed description of the invention.

Brief description of the figures

Figure 1 shows a block diagram of the sequence of the two basic steps (reduction reaction + cristallization) characterizing the process for the production of oxidizable metal particles according to the present invention.

Figure 2a shows the zero-valent iron particles according to the invention produced in a colloidal solution of cellulose carboxymethylate immediately after their production.

Figure 2b shows the particles of Figure 2a stored in an open container 24 hours after their production. Figure 3 shows an image of nFe(O) under Scanning Electron Microscope (SEM).

Figure 4 shows a view by means of an scanned optical microscope of the nFe(O) particles incorporated in the saline matrix of potassium sulfate, after the post-production crystallization.

Figure 5 shows the X-ray diffraction spectrum (XRD) of the particles incorporated in the potassium sulfate matrix.

Figure 6 shows a SEM image of the nFe(O) particles incorporated in the potassium sulfate.

Figure 7 shows the efficiency of reduction of Cr(VI) (solution with initial concentration of 50 mg/L) by the nanoparticles incorporated in potassium sulfate matrix, immediately after their production (7A), and after storing for 30 days (7B), as the nanoparticles concentration is varying.

Figure 8 shows the efficiency of phenol removal (solution with an initial concentration of 25 mg/L) by the nanoparticles incorporated in the potassium sulfate matrix, immediately after their production (A) and after storing for 30 days (B), as the nanoparticles concentration is varying.

Figure 9 shows a view of nZn(0) particles incorporated in a sodium acetate matrix, after the post-production crystallization, taken by optical microscope with the image focus set at 10 pm from the reference plane (microscope slide).

Figure 10 shows the particles of nZn(0) incorporated in the sodium acetate matrix, after the post-production crystallization, taken by an optical microscope with the image focus set at 15 pm from the reference plane (microscope slide).

Figure 11 shows the particles of nZn(0) incorporated in the sodium acetate matrix, after the post-production crystallization, taken by an optical microscope with the image focus set at 19 pm from the reference plane (microscope slide).

Figure 12 shows the particles of nZn(O) incorporated in the sodium acetate matrix, after the post -production crystallization, taken by an optical microscope with the focus of the image set at 24 pm from the reference plane (microscope slide).

Figure 13 shows the particles of nZn(O) incorporated in the sodium acetate matrix, after the post-production crystallization, taken by an optical microscope with the focus of the image set at 28 pm from the reference plane (microscope slide).

Figure 14 shows the particles of nZn(O) incorporated in a sodium chloride matrix, after the post-production crystallization, taken by an optical microscope with the focus of the image set at 15 pm from the reference plane (microscope slide).

Figure 15 shows the particles of nZn(O) incorporated in the sodium chloride matrix, after the post-production crystallization, taken by an optical microscope with the image focus set at 18 pm from the reference plane (microscope slide).

Figure 16 shows the particles of nZn(O) incorporated in the sodium chloride matrix, after the post -production crystallization, taken by an optical microscope with the focus of the image set at 22 pm from the reference plane (microscope slide).

Figure 17 shows the particles of nZn(O) incorporated in the sodium chloride matrix, after the post-production crystallization, taken by an optical microscope with the focus of the image set at 25 pm from the reference plane (microscope slide).

Figure 18 shows the particles of nZn(O) incorporated in the sodium chloride matrix, after the post-production crystallization, taken by an optical microscope with the focus of the image set at 32 pm from the reference plane (microscope slide).

Detailed Description of the Invention

For the purposes of the present invention the following definitions are given:

- submicronic particles are meant the ones having at least a size between 100 nm and 1000 nm;

- nanometric particles or nanoparticles are meant the ones having at least a size between 0 nm and 100 nm in length.

The set of submicronic particles and/or nanoparticles will be here generally referred to as "particles".

Therefore, a first object of the present invention are oxidizable zero-valent metal particles protected by a packaging inside a crystallized solid matrix.

The metal particles according to the invention are metallic particles of a submicronic size (<1000 nm), preferably of a size ranging from 10 nm to 600 nm, more preferably between 15 nm and 99 nm, even more preferably between 20 nm and 49 nm. The shape of these particles can be spherical, needle-like, cubic, plane, pyramidal, prismatic or a combination of these, the preferred shape is the spherical one.

The metal particles according to the invention may comprise one or more metals, preferably belonging to groups II, III, IV, V and VI and to the transition elements of the periodic table, as a non-exhaustive example Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi.

The solid matrix for packaging the metal particles is a matrix obtained by growth of a crystallized solid starting from one or more solutes dissolved in a solution. All the fluorides, chlorides, sulfates, phosphates, iodides, bromides and acetates of metals and semi-metals can be considered as a solute, preferably by way of non exhaustive example of a metal selected from Na, K, Li, Be, Mg, Ca, Sr and Ba. Also all organic solutes in particular, by way of non exhaustive example, fructose, maltose, dextrose can be used as a solute in the process.

According to the invention the mass ratio between the metal particles and the crystallized solid can range between 0.5% and 90%, preferably between 1.0% and 60.0%, more preferably between 2.0% and 30%, even more preferably between 3.5% and 5.0%.

The metal particles remain dispersed and incorporated in the form of non-cohesive agglomerates inside the crystal, and these non- cohesive agglomerates result to be sufficiently dispersed between them.

The metal particles protected according to the invention can be stored under any condition, provided that they do not come into contact with one or more solvents capable of dissolving the crystallized solid matrix.

The crystallized solid matrix can be dissolved by using one or more of solvents, preferably water.

Due to the use of a process for the packaging of metal particles inside a solid crystallized matrix, it is possible to obtain a protective covering able to maintain the oxidability of the particles unaltered, providing them with effective protection throughout the storage period up to their use. The oxidability of the metal particles is preserved and made available through the simple dissolution of the solid matrix.

The zero- valent oxidizable particles according to the present invention remain protected from external oxidizing agents and there is no difference in their level of oxidability, measured immediately after their production and after 210 days of exposure to air. The particles according to the invention find application in the industrial or environmental field (in particular for the depuration of soil, aqueous wastewater and industrial wastewater). By way of an example, the particles according to the invention can be used for the remediation of organic pollutants in the soil by means of ίh-situ techniques, for the elimination of organic or metallic pollutants from wastewater and/or groundwater by means of reduction/adsorption processes on support in contact with the medium to be treated. By way of example, the particles according to the invention were successfully used in the removal of a stable organic pollutant, such as phenol in solution.

The particles may be used in industrial heterogeneous catalysis processes and are further used in paints industry, particularly in the production of protective paints.

It is another object of the present invention a process of the production of zero-valent and oxidizable metal particles. The process of the invention is carried out in a liquid solution by means of two main steps, the second of which consists of packaging the oxidizable metal particles by incorporation thereof into a solid crystallized matrix, the crystallization of which occurs on the surface of said metal particles which, following to their production, act as a trigger in the secondary nucleation process of the solute added during the second phase of the process. The nucleation represents the first step of the crystallization process, consisting in the transition of a solute from a dissolved phase to a solid phase in the solvent. When this transition becomes possible by the operating conditions, the solute spontaneously forms small nuclei giving rise to nucleation. This process is favored in the presence of surfaces or particles available in solution, also introduced by seeding, giving rise to a preferential form of nucleation, denominated heterogeneous nucleation.

The process is therefore characterized by two successive steps: (1) synthesis of the metal particles in a solution; (2) crystallization by heterogeneous nucleation of the solute on the particles produced, with a seeding function.

The crystallization process is an unitary operation through which a crystalline solid phase is formed starting from one or more substances in solution dissolved in liquid phase, which solidify by virtue of their higher concentration compared to the one of equilibrium.

The crystallization according to the invention is exclusively a crystallization from a liquid phase, and not from a melt.

In this type of crystallization the fundamental parameter is the solubility of the selected solute, which depends on the operating temperature T (°C), the considered solute and the selected solvent, which represents the concentration of the solute in the saturated solution, i.e. the equilibrium in function of T. Depending on whether the dissolution of the solute in the solvent is endothermic or exothermic, the solubility may increase or decrease with the temperature. If the concentration of the solute in solution is higher than the equilibrium concentration at the selected T, i.e. is higher than the value of solubility, the solution is defined as "oversaturated".

The relative saturation S is equal to the ratio between the effective concentration of solute in solution and the one of equilibrium at the system temperature. The oversaturated solutions are characterized by S>1.

During step (1) oxidizable metallic particles are synthesized. The metals according to the invention are preferably belonging to groups II, III, IV, V and VI and to the transition elements of the periodic table. By way of non-exhaustive examples, Be, Mg, Ca, Sr, Ba, Sc, Y, Ti , Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg , Al, Ga, In, Tl, Ge, Sn, Pb, Sb, Bi can be mentioned.

In this first step it is necessary to select at least a precursor compound containing at least one metal cation. Metal compounds such as an oxide or a salt are preferably used as precursors: the above mentioned fluorides, chlorides, sulfates, phosphates, iodides, bromides and acetates are preferred.

The selected precursor is dissolved in one or more solvents, preferably a polar solvent, more preferably water, even more preferably degassed ultrapure water. With regard to the solvents, both polar and apolar solvents may be used, in particular inorganic and organic solvents, preferably H₂O, H₂SO₄, H₃PO₄, HBr, HF, HI, HC1, CH₃COOH, CHsOH.

The metal precursor should be in the maximum concentration equal at the equilibrium concentration at the system temperature, preferably below 10% by mass, more preferably between 0.2% and 4.0% by mass. A reducing agent of the metal precursor is added to this solution, preferably at a concentration higher than the stoichiometric one. For instance, a second solution of the reducing agent of the metal precursor is prepared in the same solvent, preferably at a concentration higher than the stoichiometric one. As a reducing agent, sodium borohydride, lithium borohydride, hydrazine monohydrate may be used as a non-exhaustive example.

A dispersing agent, for example cellulose carboxymethylate, or a surfactant such as Tween60, Tween20, sodium lauryl sulfate or sodium laurylether sulfate, can also be added to the precursor solution, in order to further promote the dispersion of the precursor and the particles produced after reaction

This dispersing agent must be soluble in the solvent and is fed at concentrations of from 0.01% by mass to 10.00% by mass, preferably from 0.1% to 1.0% by mass, even more preferably from 0.25% to 0.75% by mass.

The two solutions are contacted inside a reactor having controlled atmosphere, pressure and temperature, at a pressure and temperature such as to guarantee the maintenance of the liquid state of the solution, preferably at room temperature and pressure under mixing conditions, preferably in conditions of micro-mixing.

The micro-mixing is defined as a diffuse mixing on a characteristic scale of Batchelor and below the Kolmogorov scale. Micro-mixing occurs through three main mechanisms: laminar elongation up to complete striation, turbulent erosion and molecular contraction/diffusion. The micro-mixing homogenizes the reacting species and allows chemical reactions on a molecular scale; the reaction takes place preferably in an inert gas atmosphere, preferably nitrogen or a noble gas, to reduce the probability of possible oxidation of the metal particles.

After the contact of the two solutions, a reaction of chemical reduction of the cation occurs, followed by the formation of the particles, preferably nanoparticles, metallic zero-valent, which are oxidizable: such particles, therefore, in the presence of oxidizing agents or in the presence of any electron acceptor increase their oxidation state to a value higher than 0.

The particles are formed at a concentration higher than that of equilibrium in the solvent, with S>1.00, preferably S>100, even more preferably S>1000.

The phase (2) of the process of present invention provides the growth of a crystallized solid matrix starting from a selected solute to achieve the covering to be crystallized onto the metallic particles obtained at the end of step (1).

In order to proceed to step (2) of the process, which takes place by a crystallizer, it is necessary to use a solute having, with respect to the metal precursor, a different cation and the same or a different anion, which is inert with respect to the oxidable metal particles produced, and which is soluble in the solvent of the phase (1), at the operative conditions of the crystallizer. As a solute to be crystallized, all the solutes such as fluorides, chlorides, sulfates, phosphates, iodides, bromides and acetates of metals and semi-metals can be considered, in particular by way of non exhaustive example of metals such as Na, K, Li, Be, Mg, Ca, Sr and Ba, as well as all organic solutes, in particular by way of non-exhaustive example fructose, maltose, dextrose.

During step (2) a crystallizer operates at a controlled pressure, atmosphere and temperature, preferably at the conditions of phase (1), but which can also differ in one or more parameters, due to heterogeneous nucleation, of a solute on the previously produced particles and additions to the crystallizer in solution, which assume the seeding function. The amount of added solute, in the homogenization mixing conditions, must be such as to generate a saturated or undersaturated environment, preferably with a relative saturation S=0.9-1.0, in any case the dissolved solute must be in a concentration at S>=0.00, preferably at SX3.90, even more preferably at S>0.99 at the operating conditions of the crystallizer. Once the solute is completely dissolved in the solvent, the operating conditions of the crystallizer are changed, so as to generate an over-saturated environment, with a value of S falling within the range of the solution metastability, preferably S<10, more preferably S<2, in order to promote the growth of the solute on the seeding.

It is also possible to perform the operation of adding the solute to be crystallized already during step (1), provided that it is inert towards the precursor and the reducing agent, and that it does not crystallize at the operating conditions of step (1). The crystallization according to the invention is a heterogeneous crystallization which can occur by variation of T, by variation of P (by solvent evaporation) or by the solvent removal or by other methods, such as for example, by means of selective membranes, or by more variation operational parameters.

The crystallization process can take place, for example, due to a temperature drop in case of a solute having an endothermic solubility; in this case, the initial temperature T2 is selected and a quantity of solute is dissolved until equilibrium is reached at that T2: the crystallization occurs by cooling the solution from T2 to T3 <T2. At the new temperature T3, the solubility of the solute will be less than that at the temperature T2, so that the excess solute will tend to crystallize.

The crystallization may be also achieved at a constant temperature by reducing the operating pressure until boiling, in such a way as to evaporate the solvent, increasing the solute concentration over that of equilibrium, such as to cause the crystallization on the seeding. This may be also achieved through selective membranes with a high rejection towards the solute, capable of separating the solvent from the solution at constant temperature and pressure. Furthermore, crystallization can be performed by combining the two temperature and pressure methods.

In all the cases reported here, for example, the described crystallization process provides for the and operation of a

heterogeneous nucleation, since the particles constitute the seeding or the trigger for the subsequent growth of the solute crystals. With regard to the ranges of suitable temperature and pressure, a temperature in the range 0-100°C is taken into account when working at atmospheric pressure, and thus by means of cooling crystallization, while a pressure in the range 0-1 atm and a temperature in the range 0-100°C are taken into account, when working by evaporation or adiabatic crystallization. During the adiabatic crystallization the temperature of the solution naturally decreases, as a consequence of solvent evaporation and the consequent removal of heat from the solution as evaporation heat.

Once the crystallization process ended, the metal particles packed inside a solid matrix, according to the teachings of the present invention, can be stored in a solid state, or in the mother solution or other liquid solution, wherein the packaging matrix is not soluble.

The mother solution can be separated from the incorporated crystals by means of separation techniques, preferably selected from continuous centrifugation, discontinuous centrifugation, continuous filtration, discontinuous filtration or sedimentation, depending on the average size of the crystals. The size of the separated crystals is certainly greater than the one of seeding, preferably greater than 100 nm. The obtained product, once separated from the liquid solution, is split in several crystals, whose number and size distribution is a function of the reaction medium, the mixer rotation speed, fluid dynamic conditions, growth time, operating conditions, and type of solute and solvent. The obtained product contains the oxidizable metal particles inside the crystallized matrix, and the metal particles are consequently incorporated and protected by the oxidizing agents of the environment, both inside and outside the reaction medium. The final product can therefore be stored both in a solid state and as a solid in a solution.

Figure 1 shows a block diagram of the sequence of steps characterizing the process for the production of oxidizable metal particles according to the present invention.

A reactor is shown to carry out the reduction step (1) of a precursor compound of oxidizable metal nanoparticles. In addition to the precursor compound, a solvent is fed to this reactor, which is preferably water, an inert gas and a reducing agent, selected on the basis of the above reported teachings. At the exit from the reactor of phase (1) a solution of oxidizable metallic nanoparticles is obtained, which is fed into a crystallizer to carry out step (2) of the process according to the present invention.

This crystallizer is also fed with a solute compound selected from the above-mentioned chemical substances, and through the appropriate regulation and variation of the operating conditions (temperature, pressure) inside the crystallizer the growth of the solute compound around the metallic nanoparticles is adjusted. In this way it is achieved the formation of the solid crystalline matrix which incorporates the oxidizable metallic nanoparticles according to the teachings of the present invention.

Finally, the product coming from the crystallizer is fed to a solid/liquid separation step, which allows to separate the mother liquid (solvent) from the desired product, i.e. the metallic nanoparticles incorporated in a solid crystallized matrix which are a further object of the present invention.

It is therefore an object of the present invention a process for the production of oxidizable metal particles incorporated in a crystallized solid matrix, the process comprising the following steps:

(1) preparing an aqueous solution of at least one precursor compound at temperature and pressure conditions, such that the precursor is close to saturation, i.e. S<1, said precursor containing at least one metal cation and being present in an amount less than 10% by mass, preferably between 0.2% and 4% by mass;

contacting said aqueous solution with a reducing agent preferably added in an amount higher than the stoichiometric one, or with a solution of a reducing agent in the same solvent used to dissolve the precursor compound;

reacting said solution preferably under an inert gas atmosphere, preferably nitrogen, to give rise to zero-valent metal particles. (2) contacting an aliquot of the solution containing zero-valent metal particles coming from step (1) with a solute compound having, with respect to the precursor compound, a different cation and the same or different anion, said solvent being inert with respect to the metal particles and being soluble in the solvent of step (1). The amount of added solute must be such to generate a saturated or undersaturated environment, i.e. S<1;

changing, once achieved the complete dissolution of the solute, the operating conditions to generate an over-saturated environment that promotes the growth of the solute onto the metal particles.

The obtained metal particles are incorporated and packaged in a solid crystallized matrix, thus maintaining their basic chemical- physical characteristics and being completely protected from external oxidizing agents, such as atmospheric or environmental oxidizing gases, and all liquids comprising oxidizing agents and not able to dissolve the solute of packaging.

By virtue of the use of a packaging process based on solutes crystallized onto the metal particles, it is possible to obtain a protective covering that maintains the particles oxidation unaltered, providing them with a protection throughout the storage period, up to their use in the industrial or environmental field. The activity of the metal particles is preserved and made available through the simple dissolution of the crystallized solid matrix encompassing them.

The advantages of the present invention are several and are listed below:

A) The process of packaging the metal particles is simple to implement, and can be carried out in any private or university laboratory, or on an industrial scale, by means of a simple and economical equipment. B) The packaging inside a solid crystallized matrix does not modify the oxidability of the produced particles.

C) This way of packaging preserves the particles from their oxidation during both the storage and transport phase.

D) The metal particles incorporated and packaged by the process of present invention may be stored in the production mother liquid, or further processed at the solid state by means of vacuum evaporation and/or other separation techniques from the mother liquid, without the need of particular operational precautions, except the maintenance of the residual supersaturation during the procedure.

E) The process can be performed avoiding the use of toxic, carcinogenic or mutagenic reagents, and without the use of particular safety protocols.

F) The packaged metal particles can be stored at room temperature in any place, provided they do not come into contact with the solvents of the crystallized solutes.

G) The transport of the metal particles packed in a solid matrix is extremely facilitated, as it is of reduced volume and does not require specific measures to prevent oxidation during the transport from the place of production to the place of use (if these do not coincide) .

P) A reactivation of the particles before their use is not necessary: condition for their release is the presence of one or more solvents able to dissolve the crystallized solid matrix, in most cases this solvent is water.

I) The dissolution of the solid matrix of packaging can represent, in specific application fields, an intake of a value-added substance.

J) The volume of the solid matrix and its dissolution kinetics in the specific solvent determine the release time of the nanoparticles, which may therefore be appropriately set for controlled release applications, both in terms of time and space (carrying). K) The claimed process is overall sustainable from an environmental, technical and economic point of view.

The following examples are enclosed for supporting the present invention and are not to he considered as limiting the relative flow rate.

EXAMPLES

Example 1

Preparation of zero-valent iron nanoparticles incorporated in a potassium sulfate matrix.

Zero-valent iron nanoparticles (hereinafter nFe(O)) are synthesized in a laboratory by chemical reaction between a ferrous ion precursor and a reducing agent (sodium borohydride). Cellulose carboxy methylate (hereinafter CMC) is used as the dispersing agent. These particles are made of a metallic iron core and an outer shell made of mixed valence iron oxides, mainly magnetite.

Figure 2a shows a photo of the nFe(O) particles. The figure, shown here in shades of gray, is supported by a color analysis in terms of RGB on a specific representative point of the photographed sample (RGB, additive type color model: the colors are defined as the sum of the three colors Red, Green and Blue, each in scale 0-255). As it can be seen from the image the nanoparticles are dispersed in the form of colloid and are typically dark black (low RGB values on all three color channels).

After their production the nFe(O) are susceptible to oxidation from the oxygen dissolved in solution and the oxygen dissolvable from atmosphere. The rapid and complete oxidation of nFe(O), once left in a container in contact with air, is clearly evident from Figure 2b. In fact, the typical reddish-brown color of oxidized ferric ions can be observed (RGB values typical of red-brown coloring, with value on the relevant R channel and G value equal to 1/10). Figure 3 shows the SEM (electronic scanning microscope) image of nFe(O). These nanoparticles are initially dispersed by means of the CMC dispersing agent in the form of nanometric aggregates having minimum sizes of 40-50 nm for each single particle.

The process object of the invention applied to the case of nFe(O) has been realized according to the following parameters: concentration of the precursor FeS0₄ 7H2O = 10.9 g/L, concentration of sodium borohydride NaBFL = 2.96 g/L (molar ratio 2:1 borohydridedron sulfate, stoichiometric with respect to reaction (1) taken from [22]), CMC concentration =2.4 g/L, initial temperature T1 =25°C, total volume of solution =100 mL, selected solvent ultrapure water, rotation speed of the turbine (4.5 cm diameter of the "flat six blades" type) equal to 900 rpm.

Depending on these parameters and using a 250 mL volume container, jacketed to maintain constant temperature throughout the process, it is possible to obtain a nanoparticle concentration equal to 2 g/L, being the process yield equal to 91.4% measured according to [23]. As a function of the mass of Fe(0) to be coated (0.2 g), the mass of solute to be crystallized can be calculated. As previously reported, the metal p article s/solute ratio should preferably be in the 3.5-5% mass range. In this case a ratio of 4.5% by mass was selected, from which a mass of solute to be crystallized is obtained equal to about 4.5 g. As a solute for crystallization potassium sulphate was selected, in accordance with what previously reported about the suitable salts to be used. Once calculated the mass of solute necessary for the incorporation of the metal particles, it is possible to determine the temperature (T2) for carrying out the solute dissolution and the concentration of saturation of the same by means of the solubility laws of the solute in aqueous solution (equations 2 and 3),

0,074 + 0,00188 - iTCl, T < 42°C (2) 0,088 + 0.00156 G[ ], T > 42^BC f3>

From equation (2) one can calculate the saturation concentration of K2S04 at 25°C, equal to 121 g/L, so that considering a volume of solution equal to 100 mL, the solubilized mass of solute, equal to 12.1 g, is determined. To this mass are added 4.5 g necessary for the packaging of nFe(0), thus obtaining 16.6 g, i.e. 166 g/L in terms of concentration. By replacing this value in equation (3), T2 = 50°C is obtained. Having identified all the parameters of the process, the experimental test can be performed.

Figure 4 shows the nFe(0) particles incorporated in the solute. From this figure it can be seen that the nFe(0) are incorporated in the crystal in the form of non-cohesive agglomerates in the order of the micron sizes, which are in turn sufficiently dispersed between them.

The crystallization is achieved by means of cooling by reducing the T to 25°C, immediately after the complete solubilization of the solute at 50°C and monitoring the temperature of the solution with a thermocouple: the obtained material was separated from the liquid by filtration on a Whatman 450 nm filter paper and left to air dry. As it can be seen from Figure 5 showing the XRD spectrum of the nFe(0) incorporated by the solute, metal iron is still present in the material following the packaging process according to the present invention, in addition to the potassium sulfate solute and the magnetite (always present on the external surface of the nFe(0)). Figure 6 shows the SEM image of the nFe(0) post-packaging. Efficiency test of nFe(O) particles

Part of the product was then used in the Cr(VI) reduction process in order to test its efficiency. The remaining part was ground to increase the surface exposed to the air, and left at room temperature and pressure in contact with the air. After 30 days the material was used again in the Cr(VI) reduction process to test its efficiency.

Five tests were performed in triplicate (hereinafter the average value of each Cr(VI) measure) for the reduction of Cr(VI). Setting an initial concentration of Cr(VI) equal to 58 mg/L, using K^CrCk potassium chromate as the precursor, and employing a fixed concentration of pure potassium sulfate (blank test) equal to 800 mg/L and four increasing concentrations of nFe(0) equal to 800, 1600, 2000, 2400 mg/L, which, as a function of the ratio of mass Fe(0)/solute selected equal to 4.5% corresponded to 36, 72, 90 and 108 mg/L of Fe(0). The tests, which lasted 4 hours, included a measure of Cr(VI), using the colorimetric method of diphenylcarbazide [24], every 60 min In particular, 300 mL vessels were used with 250 mL of Cr(VI) solution, in which the Fe(0) amounts were dissolved to reach the concentrations previously reported, carrying out the experiments without inert gases. The choice of the quantities of Fe(0) were selected considering the reaction of reduction of Cr(VI) by the Fe(0) reported in equation (4) [25]:

2HCr0 '47 _T 14H ⁺ -f- 3 ¾ 2<¾ + S H_zO_la +- 3 *¾_j (4) according to which the stoichiometric molar ratio Fe(0)/Cr(VI) is equal to 1.5. As a result, two tests were conducted with a molar ratio lower than the stoichiometric ratio (800 and 1600 mg / L), a test with a molar ratio approximately equal to the stoichiometric (2000 mg/L) and one with a molar ratio higher than the stoichiometric ratio (2400mg/L). Figures 7A and 7B show the comparison of the efficiency of reduction of Cr(VI) in aqueous solution of potassium chromium of nFe(O), used immediately after the process and after 30 days of air storage.

As can be seen from the graph the efficiency of removal increases with the increase in the amount of solute dissolved in solution; in particular with a 2000 mg/L test (molar ratio approximately equal to the stoichiometric ratio), a reduction efficiency of over 80% is obtained, while a reduction is obtained with a slightly higher quantity. There are no significant differences in process efficiency by comparing the results obtained with the nFe(0) used immediately after production and with those left 30 days in air. The particles were analyzed again in their activity after 210 days of storage. The results previously found were reconfirmed, as the particles remain protected from external oxidizing agents and there was no difference in their oxidability, which remains unchanged as measured immediately after their production.

Ability to remove nhenol from a solution.

Different concentrations (800, 1600, 2000 or 2400 mg/L) of the product described above have been used for the removal of phenol (CeHsOH) in a concentration of 25 mg/L in aqueous solution. Figures 8(A) and 8(B) show the efficiency data on the phenol removal depending on the concentration of the material just produced, and after 30 days of air storage. The particles are able to remove more than 50% of the phenol concentration at the maximum concentration (2400 mg/L of product) and there are no significant differences in the efficiency of the phenol removal process in solution comparing the results obtained with nFe(0) used immediately after production, with those obtained with nFe(0) subjected to 30 days of exposure to air. Even at low concentrations (1600 mg/L of product) and after 30 days of air storage the particles can remove more than 30% of phenol in solution. Example 2

Preparation of zero-valent zinc nanoparticles in a sodium acetate matrix

Zero-valent zinc nanoparticles (hereinafter nZn(O)) are tested, after their synthesis in a laboratory by chemical reaction from a precursor of bivalent zinc ions and a reducing agent (sodium borohydride). Cellulose carboxymethylate (hereinafter CMC) is used as the dispersing agent. These particles consist of a metallic zinc core and an outer shell of zinc oxide. The process of the present invention in the case of nZn(O) was carried out according to the following parameters: concentration of the precursor ZnS0₄ 71¾0 = 11.249 g / L;

concentration of sodium borohydride NaBH₄ = 2.96 g/L (molar ratio 2:1 borohydride: zinc sulfate, stoichiometric with respect to reaction (5));

CMC concentration = 2.4 g / L;

initial temperature T1 = 25 °C;

total volume of solution = 100 mL;

ultrapure water selected as solvent;

rotation speed of the turbine (4.5 cm diameter of the "flat six blades" type) equal to 900 rpm.

Zn(H_zO) %_q) 4- 2B¾_j ® Zifc i -f 2 B(OH)_a(E + 7¾ T (S)

As a function of the mass of nZn(0) to be incorporated (0.1 g), the mass of solute to be crystallized can be calculated. As previously reported, the metal/solute particle ratio should preferably be in the 3.5- 5% mass range. In this case a ratio of 4.5% is selected, from which a mass of solute to be crystallized is obtained equal to about 2.25 g.

In this example, sodium acetate is used as a solute suitable to generate the crystallization matrix. The mass of solute necessary for the packaging of the particles is determined through the solubility laws of the solutes in aqueous solution (equation 6), the temperature at which the solute dissolution is performed (T³ for sodium acetate), the concentration of saturation of the same.

C ^ ^{k3solu to} ) = 0.0511 · T³ [^CC] + 1.267 T[°C]

'^soluxione'

+ 770.23 (6)

From equation (6) the saturation concentration of sodium acetate is calculated at 25 °C, equal to 833.83 g/L, so that considering a volume of solution equal to 100 mL, the solubilized mass of solute can be determined, equal to 83.38 g of sodium acetate.

To this mass are added 2.25 g necessary for the incorporation of nZn(0), obtaining 85.63 g, ie 856.3 g/L in terms of concentration. By replacing these values in equation (6), a T³ = 31°C is obtained. Having identified all the parameters of the process, the experiment test was performed.

Figures 9-13 show the nanoparticles of nZn(0) incorporated in the crystallized matrix of sodium acetate. In each figure, the same crystal was scanned by an optical microscope with a lOOx lens at the same point, but by changing the focal plane. In this way, it is possible to focus only those nanoparticles, which are in the crystal and on the selected focal plane, while the others will be out of focus. The nanoparticles focused in the selected focal plane are indicated in the figures by a white contour box.

From the comparison of the sequence of Figures 9-13, it is therefore possible to see that the nanoparticles have been incorporated into the crystal at different heights, expressed by their distance from the reference plane (microscope slide). Example 3

Preparation of zero-valent zinc nano articles in a sodium chloride matrix

In this example zero-valent zinc nanoparticles nZn(0) were taken into account, which are synthesized in a laboratory by the same operating conditions indicated in Example 2. Also in this example cellulose carboxymethylate (CMC) is used as a dispersing agent.

These particles are constituted by a metallic zinc core and an outer shell of zinc oxide.

As a function of the mass of nZn(0) to be incorporated (0.1 g), the mass of solute to be crystallized can be calculated. As previously reported, the metal/solute particle ratio should preferably be in the 3.5- 5% mass range. In this case a ratio of 4.5% was selected, from which a mass of solute to be crystallized is obtained equal to about 2.25 g.

In this example, sodium chloride is used as a solute for the crystallization matrix.

The mass of solute necessary for the packaging incorporation of the particles is determined through the solubility laws of the solutes in aqueous solution (equation 7), the temperature at which dissolution of the solute (T² for sodium chloride) is to be carried out, and the saturation concentration of the same. 0 0032 T² [ⁿC] + 0.053 · F[°C]

4 356 89 (7)

From equation (7) it is possible to calculate the saturation concentration of NaCl at 25°C, equal to 360.21 g/L, so that considering a volume of solution equal to 100 mL, the mass of solubilized solute can be determined equal to 36.02 g of sodium chloride.

To this mass are added 2.25 g necessary for the incorporation of nZn(0), obtaining 38.27 g, i.e. 382.7 g/L in terms of concentration. By replacing these values in equation (7), a T² = 82°C is obtained. Having identified all the parameters of the process, the experiment test was performed.

Figures 14-18 show the particles of nZn(O) incorporated in the crystallized matrix of NaCl. In each figure, the same crystal was scanned by an optical microscope with a lOOx lens at the same point, but by changing the focal plane. In this way, it is possible to focus only those nanoparticles that are in the crystal and on the selected focal plane, while the others will be out of focus. The nanoparticles focused in the selected focal plane are indicated in the figures by a white contour box.

From the comparison of the sequence of Figures 4-18 it is therefore possible to see that the nanoparticles of Zn(0) have been incorporated into the crystal at different heights, expressed by their distance from the reference plane (microscope slide).

References

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Claims

1. Process for the production of oxidizable metal particles incorporated in a solid crystallized matrix comprising the following basic stages:

(1) preparing a solution in a solvent of at least one precursor compound under temperature and pressure conditions such that said precursor compound is close to saturation, i.e. relative saturation S<1, said precursor compound containing at least one metal cation and being present in an amount lower than 10% by mass, preferably between 0.2% and 4% by mass;

contacting said solution with a reducing agent added in a quantity greater than the stoichiometric amount;

reacting said solution with said reducing agent in an inert gas atmosphere to generate zero-valent metal particles;

(2) contacting an aliquot of the solution coming from step (1) and comprising said zero-valent metal particles with a solute compound having, with respect to the precursor compound, a different cation and the same or different anion, which is inert with respect to the produced metal particles and that it is soluble in the solvent of step (1); the amount of solute added being such as to generate a saturated or undersaturated environment, i.e. a relative saturation S<1;

after the completed dissolution of the solute, changing the operating conditions to generate an oversaturated environment, suitable to promote the growth of the solute on the metal particles.

2. Process according to claim 1, in which step (1) takes place under micro-mixing conditions.

3. Process according to any one of the previous claims, wherein the crystallization is carried out by at least one or a combination of the following methods: temperature variation, pressure variation and solvent evaporation, filtration through selective membranes, adiabatic evaporation.

4. Process according to any one of the preceding claims, wherein the solvent of step (1) is selected from the group of ¾0, H2SO4, H3PO4, HBr, HF, HI, HC1, CH3COOH, CH3OH.

5. Process according to any one of the preceding claims, wherein the reducing agent of step (1) is selected from sodium borohydride, lithium borohydride or hydrazine monohydrate.

6. Process according to any one of the preceding claims, wherein the metal cation is selected from the group of Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Ga, In, Tl, Ge, Sn, Pb, Sb, and Bi.

7. Process according to any one of the preceding claims, wherein the precursor compound is an oxide or salt selected from fluorides, chlorides, sulfates, phosphates, iodides, bromides or acetates.

8. Process according to any one of the preceding claims, wherein the solution of step (1) comprises at least one dispersing agent or surfactant selected from cellulose carboxy methylate, Tween60, Tween 20, sodium lauryl sulfate and sodium lauryl ether sulfate.

9. Process according to any one of the preceding claims, wherein the solute compound of step (2) is selected from fluorides, chlorides, sulfates, phosphates, iodides, bromides and acetates of metals and semi-metals selected from Na, K, Li, Be, Mg, Ca, Sr and Ba, and organic solutes selected from fructose, maltose, dextrose.

10. Oxidizable metal particles packaged in a solid crystallized matrix, obtained with the process according to claims 1 to 9.

11. Oxidizable metal particles according to claim 10 having a size ranging from 15 nm to 99 nm.

12. Oxidizable metal particles according to claim 10, wherein the mass ratio between said metal particles and said solid crystallized matrix is between 1.0% and 60%.

13. Liquid composition comprising the metal particles according to any one of claims 10 to 12.

14. Paint comprising the metal particles according to any one of claims 10 to 12.

15. Use of the oxidizable metal particles according to claims 10 to 12 for the removal of organic and/or inorganic pollutants from soil, waste waters or groundwater.