Field of Art

The present invention relates to zerovalent iron nanoparticles (nZVI) stabilized by a polymer layer, and to a method of preparation thereof.

Background Art

Zerovalent iron nanoparticles (nZVI) which are not stabilized are pyrophoric. Pyrophoric nZVI undergo a swift oxidation by oxygen present in the air, the oxidation typically causes a significant increase of the temperature of the nanoparticles, sometimes even resulting in outburst. Due to this oxidative reactivity, any manipulation with pyrophoric nZVI must be carried out under inert atmosphere, and they must be stored under inert atmosphere. nZVI are useful in environmental remediation technologies, typically for remediation of chlorinated volatile organic compounds or heavy metals. Unfortunately, the requirement for inert atmosphere for storage and manipulation significantly hinders the usability of the pyrophoric nZVI in field applications.

To date, several methods based on formation of protective layer on surface of iron particles are known. Formation of inorganic shell (e.g., oxides, hydroxides, carbon) on surface of iron nanoparticles is typical for commercially produced nZVI. Oxide protective layer is produced by partial oxidation of primarily prepared nZVI but this procedure does not allow to control thickness of protective layer in a simple manner, and has several further disadvantages - it reduces the amount of reactive Fe° in the particle and significantly lowers the reactivity of nZVI in aqueous environment. The thus modified nZVI should be activated for at least several hours in water dispersion before being used in a remediation process. Additionally, this type of modification does not improve dispersibility of the nZVI in aqueous environment during field applications. The problem of aggregation of the oxide layer-modified nZVI can be overcome using an additional stabilization by surfactants or polymers. However, polymers are used not only as stabilizing agents for prevention of aggregation in aqueous dispersion but also as stabilizing agents against air oxidation. In previously published processes, the polymer is added to nZVI during their reductive preparation from iron salts, and the thus obtained stabilized nZVI form colloid solutions. It is cumbersome to separate the stabilized nZVI from such colloid solutions. Natural polymers such as chitosan (X. Jin et al., Carbohydr. Polym., 136, p. 1085-1090, 2016), starch (M. R. Beigy et al., Eng. Life Set, 18, p. 187-195, 2018) or soya protein (P. Jiemvarangkul et al., Chem. Eng. ./., 170, p. 482-491, 2011) are used in same way as synthetic polymers (B. Y. Johnson, Coming Incorporated, US 2015/0001155 Al ), typically carboxymethyl cellulose CMC (S. Ambika et al., Chem. Eng. ./., 289, p. 544-553, 2016; L. Zhao et al., Water Air Soil Pollut ., 230, a.n. 113, 2019), poly(acrylic acid) PAA (Y - H. Lin et al., Sci. Total Environ ., 408, p. 2260-2267, 2010; S. Wu et al., Environ. Sci.: Nano , 6, p. 1189-1206, 2019), poly(vinyl pyrrolidone) PVP (H. Tian et al., Chemosphere , 239 a.n. 124807A7, 2020) or polymethyl methacrylate PMMA (W. Wang et al., J. Hazard. Mater ., 173, p. 724-730, 2010). The main problems of this approach include problematic scalability to industrial production of nZVI, due to the system used, and corrosive action of aqueous environment on the prepared nZVI. The problem of swift corrosion appears also in the approaches where aqueous solutions of polymers are applied for modification of nZVI in powder form, prepared primarily by dry processes (E. Kadar et al., Environ. Sci. Technol. , 45, p. 245-3251, 2011).

Another method for preparation of air stable nZVI is based on formation of composites of nZVI with solid porous inorganic materials such as silica, clays, bentonite or zeolites. Furthermore, carbon based solid porous materials or biochar are popular as cost effective material for this type of composites. More sophisticated composites are constructed in research laboratories based on graphene or reduced graphene oxide rGO. The common main disadvantage of these composites is low content of nZVI. The composites are used mainly as catalysts for activation of persulfates or hydrogen peroxide in advanced oxidation technologies. Their application in the same manner as is commonplace for pure nZVI is practically impossible due to above-mentioned low content of Fe and also due to poor economy of their production.

Nearly all the above-mentioned methods are based on application of wet technologies wherein the main solvent is water. Water has a strong corrosive action on the nZVI in comparison with organic solvents.

Therefore, there is a need for providing nZVIs which are stabilized by a polymer layer, well defined, easy to handle, and do not require an additional inorganic stabilizing intermediate layer. During the stabilization process the nZVI should not come into contact with corrosive water which would significantly worsen the applicability of the modified nZVI in environmental technologies. Disclosure of the Invention

The present invention provides stabilized zerovalent iron nanoparticles (nZVI) having a zerovalent iron core and further having a polymer shell, the nanoparticles being free of any stabilizing inorganic oxide, hydroxide or carbon layer on the surface of the nanoparticles (in particular between the zerovalent iron core and the polymer shell). The zerovalent iron core has a size of 5 to 900 nm, preferably 5 to 500 nm, as measured by transmission electron microscopy (TEM). The size corresponds to the size of the nZVI, without the polymer shell. Based on TEM measurements, it can be determined that the polymer shell has a thickness of about 1-5 nm, more typically 1-2 nm.

The present invention further provides a method of preparation of the stabilized zerovalent iron nanoparticles comprising the steps of:

- providing a solution of a polymer in a non-aqueous solvent; and

- contacting the said solution with zerovalent iron nanoparticles.

The non-aqueous solvent is preferably selected from a group consisting of 1 -propanol, 2- propanol, dioxane, and dimethyl sulfoxide. A particularly preferred solvent is 2-propanol.

The polymer is preferably selected from a group consisting of poly(acrylic acid) (PAA), linear or branched polyetheneimine (PEI), poly(vinyl pyrrolidone) (PVP), methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, and hydroxypropyl methylcellulose.

The poly(acrylic acid) preferably has a molecular weight greater than 100 kDa, more preferably 400 kDa to 4000 kDa. The polyethyleneimine preferably has a molecular weight greater than 4 kDa, more preferably 10 kDa to 300 kDa. The poly(vinyl pyrrolidone) preferably has a molecular weight greater than 10 kDa, more preferably 40 kDa to 1500 kDa. Derivatives of cellulose preferably have a molecular weight greater than 10 kDa, more preferably 90 kDa to 10 000 kDa, even more preferably 90 kDa to 2000 kDa. Using polymers with rather high molecular weight ensures sufficient compactness of the polymer layer so that it very reliably prevents pyrophoricity of the stabilized nanoparticles and inhibits diffusion of oxygen from the air through the polymer shell. On the other hand, the preparation of polymer solution is more time consuming with the high molecular weight polymers. The molecular weight used herein is an average relative molecular weight M _w (weight average molecular weight) determined by LS (laser scattering).

The solution of the polymer in the non-aqueous solvent is prepared by dissolving the polymer in the solvent, preferably at a temperature within the range from laboratory temperature (ca 20-25 °C) to about 60 °C, more preferably from 30 up to 50 °C. Vigorous stirring is advantageous to achieve a fast dissolution. Alternatively, or additionally, ultrasound can be employed to speed up and to complete the dissolution of the polymer.

The step of contacting the solution of the polymer with zerovalent iron nanoparticles is preferably carried out by adding the solution to dry powder of pyrophoric zerovalent iron nanoparticles (nZVIs). This step must be carried out under inert atmosphere.

Alternatively, the step of contacting the solution of the polymer with zerovalent iron nanoparticles may be carried out by adding the polymer solution to a dispersion of pyrophoric nZVIs in a non-aqueous solvent (same or different than the solvent present in the polymer solution, but selected from the same list). Dispersion of nZVIs in a non-aqueous solvent should be prepared under inert atmosphere.

Pyrophoric nZVIs herein means nZVIs which are free of any stabilization layer or component, thus, they are substantially composed only of zerovalent iron.

The concentration of the polymer solution is preferably within the range of 0.5 to 5 % w/v, more preferably within the range of 0.5 to 3 % w/v, even more preferably within the range of 1 to 2 % w/v.

In some embodiments, the weight ratio of pyrophoric nZVIs to the polymer solution is 1:5 to 5:1, preferably 1:1 to 5:1, more preferably 2:1 to 4:1, even more preferably about 3:1 to produce solid material (paste or wet powder). Liquid dispersion of polymer stabilized nZVI in organic solvent is produced when a higher ratio in favour of the polymer solution (1:2 and higher, such as 1:2 to 1:5) is used. Higher ratio in favour of the nZVI nanoparticles (6:1 and higher) is not usable due to imperfect coating of the nanoparticles by polymer layer. After contacting the polymer solution with the pyrophoric nZVIs, stabilized nZVIs are formed, i.e. zerovalent iron nanoparticles with polymer shell.

The stabilized nZVIs prepared by the method of the present invention may form a wet powder (for example at mixing ratio 3:1) which can easily be handled.

Due to the stabilizing polymer layer, there is no need for inert atmosphere to store or handle the nZVIs. At the same time, their high reactivity which is beneficial for their environmental remediation activity is maintained. In particular, these stabilized nZVIs are useful for wastewater and soil remediation, more specifically for removal of metallic and organic pollutants from these environments.

Furthermore, the polymer shell significantly decreases nanoparticle aggregation in aqueous dispersions and thus enables an effective utilization of the nanoparticles in environmental remediation technologies which are based on strong reductive properties of the pyrophoric nZVIs. A typical example of such use is remediation of contaminated ground waters wherein the non-aggregated nZVIs can easily diffuse within the soil and ground environment and thus increase and distribute the remediation effect within a larger area, compared to non-stabilized nanoparticles of pyrophoric nZVIs.

The present invention involves a method for stabilizing of zero-valent iron (nZVI) nanoparticles by polymers without the use of an aqueous phase. The proposed method uses a solution of stabilizing polymer in an organic solvent, which is simply mixed with nZVI nanoparticles in an inert atmosphere and, after mixing, nZVI nanoparticles are protected from rapid oxidation by atmospheric oxygen. Thanks to this modification, the modified nZVI nanoparticles can be stored and handled without the need for an inert atmosphere.

Brief Description of Drawings

Fig. 1: TEM image of pyrophoric nZVI nanoparticles.

Fig. 2: Change of atmospheric O2 concentration in an airtight sealed vessel (volume 2 L) with 1 g of PAA (450 kDa) stabilized pyrophoric nZVI nanoparticles. Fig. 3: The course of reaction between thionine in aqueous solution and pyrophoric nZVI nanoparticles stabilized using PAA (1000 kDa) measured by UV-vis spectroscopy.

Fig. 4: The course of reaction between thionine in aqueous solution and pyrophoric nZVI nanoparticles stabilized using PVP (360 kDa) measured by UV-vis spectroscopy.

Fig. 5: Change of atmospheric O2 concentration in an airtight sealed vessel (volume 2 L) with 1 g of nZVI nanoparticles stabilized using PEI (30 kDa).

Examples of carrying out the Invention

Example 1: Pyrophoric nZVIs stabilized by a shell of PAA 450 kDa

2% (w/v) solution of poly(acrylic acid) (PAA) having the relative molecular weight of 450 kDa in dioxane was prepared. The dissolution is rather slow, and it can be sped up by gentle heating (to about 40-50 °C) and vigorous stirring. Complete dissolution occurs in 2-3 hours, the dissolution process can be further accelerated by using an ultrasonic bath. After the complete dissolution of PAA, the solution is brought into contact with pyrophoric nZVIsunder N2 atmosphere in a glovebox, by adding solution of PAA to the dry pyrophoric nZVIs (NANOFER 25P, Nanoiron, s.r.o., Czech Republic; see TEM image on Fig. 1). The weight ratio of nZVIs to PAA used in this example is 3:1. Further weight ratios of nZVIs to PAA were tested, concluding that the ratios 1:1 to 5:1 resulted in good quality product, with the ratio 3:1 being optimal.

After thorough stirring of the mixture, stabilized pyrophoric nZVIs are obtained, having a PAA polymer shell. The particles are not excessively sticky, the product has a consistence of slightly wet powder. The product can be manipulated in the air without any danger of rapid reaction with oxygen.

The very low interaction of the prepared stabilized pyrophoric nZVI nanoparticles with atmospheric oxygen is demonstrated by a simple experiment in which 1 g of the stabilized nanoparticles was sealed airtight in a vessel with a volume of about 2 L, in which the oxygen content in the atmosphere was monitored with an oxygen probe (see Fig. 2). Throughout the experiment (approximately 60 hours), no significant loss of oxygen was observed in the atmosphere of the vessel, which confirms low reactivity of polymer stabilized pyrophoric nZVI nanoparticles with oxygen. Example 2: Pyrophoric nZVIs stabilized by a shell ofPAA 1000 kDa

1% (w/v) solution of poly(acrylic acid) (PAA) having the relative molecular weight of 1000 kDa in 2-propanol was prepared. The dissolution is rather slow, and it can be sped up by gentle heating (to about 40-50 °C) and vigorous stirring. Complete dissolution occurs in 3-4 hours, the dissolution process can be further accelerated by using an ultrasonic bath. After the complete dissolution of PAA, the solution is brought into contact with pyrophoric nZVIs under N2 atmosphere in a glovebox, by adding solution of PAA to the dry pyrophoric nZVIs (NANOFER 25P, Nanoiron, s.r.o., Czech Republic). The weight ratio of nZVIs to PAA used in this example is 3:1. After thorough stirring of the mixture, stabilized pyrophoric nZVIs are obtained, having a PAA polymer shell. The particles are not excessively sticky, the product has a consistence of slightly wet powder. The product can be manipulated in the air without any danger of rapid reaction with oxygen.

Regardless of the existence of stabilizing surface layer, the stabilized nZVIs are reactive in an aqueous environment and therefore they can be used in environmental technologies in the field of reductive processes for degradation of pollutants from natural or wastewater. The reactivity of the stabilized nZVIs (0.2 g) is demonstrated by a model experiment using thionine (50 mL of aqueous solution with concentration of 3.10 ⁵ mol/L) as a model pollutant. The reaction was conducted in acetate buffer at pH=5 (see Fig. 3).

Example 3: Pyrophoric nZVIs stabilized by a shell ofPVP 360 kDa

2% (w/v) solution of polyvinylpyrrolidone (PVP) having the relative molecular weight of 360 kDa in 2-propanol was prepared. The dissolution is rather slow, and it can be sped up by gentle heating (to about 40-50 °C) and vigorous stirring. Complete dissolution occurs in 1-2 hours, the dissolution process can be further accelerated by using an ultrasonic bath. After the complete dissolution ofPVP the solution is brought into contact with pyrophoric nZVIs under N2 atmosphere in a glovebox, by adding solution of PVP to the dry pyrophoric nZVIs (NANOFER 25P, Nanoiron, s.r.o., Czech Republic). The weight ratio of nZVIs to PVP used in this example is 3:1. After thorough stirring of the mixture, stabilized pyrophoric nZVIs are obtained, having a PVP polymer shell. The particles are not excessively sticky, the product has a consistence of slightly wet powder. The product can be manipulated in the air without any danger of rapid reaction with oxygen.

Regardless existence of stabilizing surface layer, the stabilized nZVIs are reactive in an aqueous environment and therefore they can be used in environmental technologies in the field of reductive processes for degradation of pollutants from natural water or wastewater. The reactivity of the stabilized nZVIs (0.2 g) is demonstrated by a model experiment using thionine (50 mL of aqueous solution with concentration of 3.10 ⁵ mol/L) as a model pollutant. The reaction was conducted in acetate buffer at pH=5 (see Fig. 4).

Example 4: Pyrophoric nZVIs stabilized by a shell of PEI 50 kDa

2% (w/v) solution of poly(ethyleneimine) (PEI) having the relative molecular weight of 50 kDa in dimethylsulfoxide was prepared. The dissolution is quick and it is complete during several minutes at laboratory temperature. After the complete dissolution of PEI, the solution is brought into contact with dry pyrophoric nZVIs (NANOFER 25P, Nanoiron, s.r.o., Czech Republic) under N2 atmosphere in a glovebox and thoroughly mixed. The weight ratio of nZVIs to PEI used in this example is 4:1. After thorough stirring of the mixture, stabilized nZVIs are obtained, having a PEI polymer shell. The particles are not excessively sticky, the product has a consistence of slightly wet powder. The product can be manipulated in the air without any danger of rapid reaction with oxygen.

The very low interaction of the prepared stabilized pyrophoric nZVI nanoparticles with atmospheric oxygen is demonstrated by a simple experiment in which 1 g of the stabilized nanoparticles was sealed airtight in a vessel with a volume of about 2 L, in which the oxygen content in the atmosphere was monitored with an oxygen probe (see Fig. 5). Throughout the experiment (approximately 58 hours), no significant loss of oxygen was observed in the atmosphere of the vessel, which confirms low reactivity of polymer stabilized pyrophoric nZVI nanoparticles with oxygen.