WO2017046252A1

WO2017046252A1 - A core-shell composition for purifying contaminated water and/or biological-medical systems such as tissues, cells or blood

Info

Publication number: WO2017046252A1
Application number: PCT/EP2016/071841
Authority: WO
Inventors: Lluís BALCELLS ARGEMÍ; Konstantinos Symeonidis; Carlos MARTÍNEZ BOUBETA
Original assignee: Consejo Superior De Investigaciones Científicas (Csic)
Priority date: 2015-09-18
Filing date: 2016-09-15
Publication date: 2017-03-23
Also published as: ES2621190B1; ES2621190A1

Abstract

The invention relates to core-shell composition comprising a core of magnetic nanoparticles and a shell of a layered double hydroxide compound, and its procedure of obtainment. Furthermore, the invention refers to the use of said composition for purifying contaminated water, especially useful for the removal of harmful contaminants like arsenic, to make the water suitable for human consumption. Moreover, the invention refers to the use of said composition for purifying biological-medical systems such as tissues, cells or blood.

Description

A CORE-SHELL COMPOSITION FOR PURIFYING CONTAMINATED WATER

AND/OR BIOLOGICAL-MEDICAL SYSTEMS SUCH AS TISSUES, CELLS OR

BLOOD. DESCRIPTION

The invention relates to core-shell composition comprising a core of magnetic nanoparticles and a shell of a layered double hydroxide compound, and its procedure of obtainment. Furthermore, the invention refers to the use of said composition for purifying contaminated water, especially useful for the removal of harmful contaminants like arsenic, to make the water suitable for human consumption. Moreover, the invention refers to the use of said composition for purifying biological-medical systems such as tissues, cells or blood. STATE OF ART

Arsenic presence is considered as one of the most important pollution problems of potable water as long-range consumption is correlated to the risk of cardiovascular diseases, diabetes, as well as skin, lung, bladder cancer and as a consequence to the increase of mortality. Maximum contaminant level of total arsenic was recently set to 10 μg/L in the EU and USA.

Numerous methods have been proposed for efficient heavy metal removal from water, including chemical precipitation, ion exchange, membrane filtration and electrochemical technologies, and so forth. Though, adsorption is the most convenient and popular method because its simplicity, high efficiency and low energy requirements. Nevertheless, the common adsorbents often only have low adsorption capacities, slow adsorption rates and limited ranges of conditions under which they can be applied. Recently, it has been discovered that nano-sized particles like iron, iron oxide or magnesium oxide may be used to remove contaminants such as arsenic from water, that is, the contaminants will be captured and stabilized on the surface of the nanoparticles by forces, including but not necessarily limited to, van der Waals and electrostatic attraction, thereby removing them from water. In practice, however, the adoption of nanoparticles in water technology meets a number of limitations related to technical, economical and safety issues, in becoming competitive. Most importantly, the small particles dimensions constitutes a drawback considering the susceptibility of composition and structural changes during storage, their degradation or dissolution during application and disposal and the difficulty in their complete separation and recovery after contact with water.

DESCRIPTION OF THE INVENTION

The present invention discloses a core-shell composition (also called core-shell nanohybrid herein) comprising a core of magnetic nanoparticles and a very thin shell of a layered double hydroxide (LDH) compound, wherein said very thin LDH shell is completely coating said magnetic core.

This composition (herein LDH@Fe nanoparticles) may be used for purifying contaminated water. The composition is a magnetic adsorbent useful for the removal of harmful contaminants from drinking water, groundwater resources, industrial and mining wastewater, secondary wastes coming from the regeneration of other adsorbents. Harmful contaminants are, for instance, As(V) and As(lll) that appear as negatively or neutrally charged oxy-ions (e.g. HAs0₄ ^2", H₂As0^3") in water. Furthermore, the composition of the invention may be used for the purification of biological/medical systems such as tissues, cells or blood.

During water treatment, the LDH shell, which is completely coating the magnetic nanoparticles core, comes directly in contact with the polluted water providing the maximum possible adsorption efficiency per mass of LDH. The magnetic core is regarded as completely inert to the adsorption process and it is not exposed to any contact with water. This configuration also ensures a high specific surface area for the nanohybrid as defined by the specific surface of the substrate (the core of magnetic nanoparticle) and the porosity of formed LDH layer (LDH shell) itself.

The main advantages of the present invention are the following:

The core-shell composition shows chemical stability for a wide pH range, from 5 to 12.

High arsenic removal efficiency has been obtained in a variety of water systems such as drinking water, groundwater resources, industrial and mining wastewater and secondary wastes coming from the regeneration of other adsorbents.

The process of obtainment of the core-shell composition in a multi-stage continuous flow reactor using low cost and environmental friendly reagents. The core-shell composition (magnetic adsorbent) with the adsorbent pollutants can be separated and recovered by using an external magnetic field.

A first aspect of the present invention relates to a core-shell composition (herein "composition of the invention") characterized in that it comprises:

a core of at least a magnetic nanoparticle;

a shell of a layered double hydroxide (LDH) compound of the formula I

[I] wherein M is selected between Mg or Ca;

wherein x ranges between 0 and 0.3;

wherein n ranges between 0 and 10; and wherein said shell is completely coating said core and wherein said shell has a thickness between 0.1 nm and 10 nm.

In a preferred embodiment, the magnetic nanoparticle is selected from the list consisting of Fe and its iron alloys, Fe₃0₄, Y-Fe₂0₃, CoFe, CoFe₂0₄, NiFe₂0₄, MnFe₂0₄, MgFe₂0₄ and a combination thereof.

In another preferred embodiment, the magnetic iron nanoparticle ranges a size between 3 nm and 100 nm.

A second aspect of the present invention relates to a process of obtainment of the composition of the invention (herein "process of the invention") characterized in that it comprises at least the following steps:

a) depositing a layered double hydroxide of the formula I

M_(1-x)Fe_x(OH)₂(C0₃)n [I] wherein M is selected between Mg or Ca;

wherein x ranges between 0 and 0.3; and

wherein n ranges between 0 and 10; onto magnetic nanoparticles; and b) treating the coated particles obtained in step a) at a temperature range between 20 °C and 95 °C for a time period between 6 h and 36 h.

In a preferred embodiment, the magnetic nanoparticles of step a) are prepared by precipitation of salts at a pH higher than 10. Examples of salts are the following:

Bivalent iron salts selected from the list consisting of FeS0₄, FeCI₂, (NH₄)₂Fe(S0₄)₂.

Trivalent iron salts are selected from the list consisting of Fe₂(S0₄)₃, FeCI₃, Fe(N0₃)₃.

Preferably, the other bivalent metal salts are selected from the list consisting of CoS0₄, CoCI₂, Co(N0₃)₂, NiCI₂, NiS0₄, Ni(N0₃)₂, MgS0₄, MgCI₂, Mg(N0₃)₂, MnS0₄, MnCI₂ Mn(N0₃)₂. Preferably, the magnetic nanoparticles of step a) are prepared by precipitation of salts at pH ranging between 10 and 13.

Preferably the pH of step a) is controlled by the addition of one or more of the reagents NaOH, NaHC0₃, Na₂C0₃, KOH, KHC0₃, and K₂C0₃.

The reducing or oxidation ability of the solution obtained in step a) is measured. Preferably the redox potential of the dispersion of magnetic nanoparticles ranges between -1.2 V and -0.5 V during preparation of magnetic particles. In order to control the redox potential in step a), a reducing environment is used, which is selected from hydrazine, NaBH₄, NaHS0₃, Na₂S₂0₃, Na₂S₂0₄, Na₂S₂0₅, Na₂S and a combination thereof, so that the preparation of magnetic nanoparticles is performed under this reducing environment.

Thus, in another preferred embodiment of the process of the present invention step a) is performed by precipitation of magnesium or calcium salts in the presence of a carbonate reagent of a concentration between 0.01 M and 2 M and under the following conditions:

· at a pH higher than 8; and

• at a temperature range between 50 °C and 95 °C.

Preferably, the magnesium salts are selected from the list consisting of MgS0₄, MgCI₂ and Mg(N0₃)₂.

Preferably, the carbonate reagent is selected from the list consisting of Na₂C0₃, NaHC0₃, K₂C0₃ and KHC0₃.

Preferably, step a) is performed at a pH ranging 8 and 13.

Preferably, the redox potential of step a) ranges between -0.9 V and -0.3 V.

The formation of the composite of the invention is performed under less alkaline, strongly oxidative conditions and heating by the single precipitation, for example, of Mg²⁺ and its incorporation/diffusion to the oxidized surface of nanoparticles, for example of Fe³⁺. The growth of LDH shell with these parameters results in a material with positive surface charge density and anion-exchange ability, properties that enhance, for example, the efficiency in arsenic adsorption and preserve it at significant levels in a wide pH range. In particular, the development of the LDH under such mild alkaline conditions, allow at the same time: i) the incorporation of C0₃ ^2" ions in the structure which enhance the ion-exchange ability; and (ii) the preservation of positive surface charge required for the approach of the negatively charged arsenic oxy-ions.

A further embodiment of the present invention provides a process of the invention, which is performed in a continuous flow reactor, wherein each step is performed in a separate reactor. The core-shell composition is prepared in discrete steps under well- regulated condition of pH, redox potentials and temperature able to be separately optimized in each stage. For instance, the magnetic nanoparticles are prepared in a first reactor, step a) in a second reactor and step b) in a third reactor, each one working at different conditions.

An important advantage of the synthesis of the core-shell composition of the invention is the low-cost procedure as defined by the serial and continuous-flow operation of the described stages. Compared to currently available batch processes, the preparation of the composition of the invention combines the possibility for fully controllable and stable reaction conditions in each stage and the industrial-scale production.

A third aspect of the invention refers to the use of the composition of the invention for purifying biological-medical systems such as tissues, cells or blood.

A fourth aspect of the invention refers to the use of the composition of the invention for purifying contaminated water.

Preferably, the composition of the invention is a magnetic adsorbent of a dispersion/separation device for the removal of harmful contaminants from drinking water, groundwater resources, industrial and mining wastewater and secondary wastes coming from the regeneration of other adsorbents.

In the present invention harmful contaminants are heavy metals such as arsenic, hexavalent chromium, mercury, nickel, lead, antimony, vanadium, cadmium and uranium.

In a preferred embodiment, the composition of the invention is a magnetic adsorbent for the removal of arsenic from drinking water, groundwater resources, industrial and mining wastewater, secondary wastes coming from the regeneration of other adsorbents for a wide working pH range, from 5 to 12.

The last aspect of the invention refers to a process for purifying contaminated water characterized in that it comprises the following steps:

a) contacting the contaminated water with the composition of the invention; and b) separating the insoluble matter from the water by means of the application of a magnetic field of 0.03 Teslas and 3 Teslas.

The magnetism of the core magnetic nanoparticles is another important parameter towards the separation of the adsorbent after its application. The stabilization of core magnetic nanoparticles implies a magnetization value above 200 emu per gram of core magnetic material which is proportional to the separation yield by an external applied magnetic field. The potential for magnetic separation is enhanced in the case of these core-shell nanoparticles as a result of the high percentage of the magnetic core compared to the thin LDH layer. The adsorbent has relatively high magnetism. The absorbent with absorbed pollutants can be separated and recovered by using an external magnetic field method, and the recovery is easy. Therefore, the absorbent, the method and the application have good industrial application values. Another important issue is that isolated core-shell magnetic nanoparticles provide the advantage of the aggregated-induced separation after the initial chain formation when the field is applied. This effect introduces the possibility for complete separation under significantly lower applied field intensities. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to be limiting of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Scheme of the growth mechanism of functional core-shell composite. FIG. 2. Scheme of the procedure of obtainment of the functional core-shell composite. FIG. 3. Adsorption isotherms for As(V) and As(lll) at pH 10 for l_DH@Fe nanoparticles with diameter around 40 nm and various compositions. FIG. 4. Adsorption capacity at residual As(V) 10 μg/L for various adsorption pH for the optimum LDH@Fe nanoparticles in comparison to common arsenic adsorbents consisting of Fe hydroxides, magnetite and zero-valent adsorbents.

EXAMPLES

Preparation of the core-shell composite of the invention

A functional core-shell nanohybrid/composite with an inner magnetic core coated by a Mg_(1-X)Fe_x(OI-l)₂(C0₃)n, wherein 0<x<0.3, 0<n<10, has been prepared [See Figure 1 ]. Its production has been achieved in a continuous flow three-stage reactor following a precipitation route shown in Figure 2. In a first stage of a stirred reactor, magnetic nanoparticles consisting of Fe and iron alloys, Fe₃0₄, Y-Fe₂0₃, CoFe₂0₄, NiFe₂0₄, MnFe₂0₄, or MgFe₂0₄ are first prepared by the precipitation of proper bivalent or trivalent iron salts (FeS0₄, FeCI₂, (NH₄)₂Fe(S0₄)₂, Fe₂(S0₄)₃, FeCI₃, Fe(N0₃)₃) or other bivalent metal salts (M²⁺: CoS0₄, CoCI₂, Co(N0₃)₂, NiCI₂, NiS0₄, Ni(N0₃)₂, MgS0₄, MgCI₂, Mg(N0₃)₂, MnS0₄, MnCI₂ Mn(N0₃)₂) under strongly alkaline (pH>10), at a temperature of 20°C and under reducing environment with a redox potential between - 1 .2 V and -0.5 V during less than 20 minutes (1 ). The particles dispersion coming out from the outflow feeds a second stirred reactor where the precipitation of magnesium salts occur under alkaline environment (pH>8), controlled carbonate concentration (0.01 -2 M), temperature (70 °C) and redox potential between -0.9 V and -0.3 V during an hour resulting in the coating nanoparticles by a magnesium carbonate hydroxide (2). In a third reactor, the produced suspension is aged for several hours (6 h - 36 h) under high temperature (90 °C) to enable interface diffusion between the magnetic phase and the shell for stabilizing the Mg-Fe LDH (3). The product is thoroughly washed (4) and whether dried or stored in the form of an aqueous dispersion (5).

Example 1 : LDH-coated Fe nanoparticles with x=0.1 In the first stage, Fe nanoparticles are prepared in a 0.1 m³ stirred reactor by the precipitation of 0.1 M FeS0₄, under strongly alkaline conditions (pH=10), under reducing environment with a redox potential adjusted to -1 .2 V by the continuous addition of hydrazine solution and at a temperature of 20 °C (1 ). The retention time in the first reactor is 15 minutes. The particles dispersion coming out from the outflow feeds the second stirred reactor (0.4 m³) where the precipitation of 0.02 M MgS0₄ occurs under alkaline environment (pH=10), controlled carbonate concentration (0.05 M) by the addition of Na₂C0₃, temperature (70 °C) and redox potential -0.9 V resulting in the coating nanoparticles by a magnesium carbonate hydroxide (2). The retention time in the second reactor is 1 hour. In a third reactor tank (6 m³), the produced suspension is aged for 20 h under high temperature (90 °C) and slow stirring to enable interface diffusion between the magnetic phase and the shell for stabilizing the Mg-Fe LDH (3). The product is thoroughly washed (4) and thickened and stored in the form of an aqueous dispersion (5). This procedure results in the formation of LDH-coated Fe nanoparticles with a core diameter of 40 nm, a shell thickness of 5 nm and a shell composition Mgo.9Feo.i(OH)₂(C0₃)2-

Example 2: LDH-coated Fe nanoparticles with x=0.3 In the first stage, Fe nanoparticles are prepared in a 0.1 m³ stirred reactor by the precipitation of 0.1 M FeS0₄, under strongly alkaline conditions (pH=10), under reducing environment with a redox potential adjusted to -1 .2 V by the continuous addition of hydrazine solution and at a temperature of 20 °C (1 ). The retention time in the first reactor is 15 minutes. The particles dispersion coming out from the outflow feeds the second stirred reactor (0.4 m³) where the precipitation of 0.08 M MgS0₄ occurs under alkaline environment (pH=10), controlled carbonate concentration (0.05 M) by the addition of Na₂C0₃, temperature (70 °C) and redox potential -0.9 V resulting in the coating nanoparticles by a magnesium carbonate hydroxide (2). The retention time in the second reactor is 1 hour. In a third reactor tank (12 m³), the produced suspension is aged for 30 h under high temperature (90 °C) and slow stirring to enable interface diffusion between the magnetic phase and the shell for stabilizing the Mg-Fe LDH (3). The product is thoroughly washed (4) and thickened and stored in the form of an aqueous dispersion (5). This procedure results in the formation of LDH-coated Fe nanoparticles with a core diameter of 40 nm, a shell thickness of 7 nm and a shell composition Mgo.7Feo.3(OH)₂(C0₃)₄. Example 3: LDH-coated Fe₃0₄ nanoparticles

In the first stage, Fe₃0₄ nanoparticles are prepared in a 0.1 m³ stirred reactor by the coprecipitation of 0.1 M FeS0₄ and 0.2 M Fe₂(S0₄)₃, under strongly alkaline conditions (pH=12), under reducing environment with a redox potential self-adjusted to -0.9 V and at a temperature of 20 °C (1 ). The retention time in the first reactor is 15 minutes. The particles dispersion coming out from the outflow feeds the second stirred reactor (0.4 m³) where the precipitation of 0.02 M MgS0₄ occurs under alkaline environment (pH=10), controlled carbonate concentration (0.05 M) by the addition of Na₂C0₃, temperature (70 °C) and redox potential -0.9 V resulting in the coating nanoparticles by a magnesium carbonate hydroxide (2). The retention time in the second reactor is 1 hour. In a third reactor tank (8 m³), the produced suspension is aged for 20 h under high temperature (90 °C) and slow stirring to enable interface diffusion between the magnetic phase and the shell for stabilizing the Mg-Fe LDH (3). The product is thoroughly washed (4) and thickened and stored in the form of an aqueous dispersion (5). This procedure results in the formation of LDH-coated Fe₃0₄ nanoparticles with a core diameter of 30 nm, a shell thickness of 4 nm and a shell composition Mg₀.83Feo.i7(OH)₂(C0₃)₂.

Example 4: LDH-coated MnFe₂0₄ nanoparticles

In the first stage, MnFe₂0₄ nanoparticles are prepared in a 0.1 m³ stirred reactor by the coprecipitation of 0.1 M FeS0₄ and 0.2 M MnS0₄, under strongly alkaline conditions (pH=12), under reducing environment with a redox potential self-adjusted to -0.9 V and at a temperature of 20 °C (1 ). The retention time in the first reactor is 20 minutes. The particles dispersion coming out from the outflow feeds the second stirred reactor (0.3 m³) where the precipitation of 0.02 M MgS0₄ occurs under alkaline environment (pH=10), controlled carbonate concentration (0.05 M) by the addition of Na₂C0₃, temperature (70 °C) and redox potential -0.9 V resulting in the coating nanoparticles by a magnesium carbonate hydroxide (2). The retention time in the second reactor is 1 hour. In a third reactor tank (6 m³), the produced suspension is aged for 20 h under high temperature (90 °C) and slow stirring to enable interface diffusion between the magnetic phase and the shell for stabilizing the Mg-Fe LDH (3). The product is thoroughly washed (4) and thickened and stored in the form of an aqueous dispersion (5). This procedure results in the formation of LDH-coated MnFe₂0₄ nanoparticles with a core diameter of 20 nm, a shell thickness of 3 nm and a shell composition Mg₀.9Feo.i(OH)₂(C0₃)3. Evaluation of the prepared core-shell composite for the removal of As(V) and As(ll l) from water

The performance of the obtained LDH@Fe nanoparticles to adsorb As(V) and As(ll l) was evaluated by batch adsorption experiments after dispersing a quantity of nanoparticles in 0.2 m³ of water with an initial concentration of arsenic 5 mg/L. Figure 3 shows the adsorption isotherms for As(V) and As(l ll) at water pH 10 for LDH@Fe nanoparticles with diameter around 40 nm and various LDH compositions with x=0, 0.1 , 0.2 and 0.3. Results indicate that nanoparticles coated by LDH with x=0.2 are the most efficient being able to equally remove As(V) and As(l ll) at almost 100 % and reach a residual concentration below the drinking water regulation limit (10 Mg/L).

Adsorption capacity comparison between the core-shell composite of the invention and commercial adsorbents Figure 4 shows the adsorption capacity at residual As(V) 10 g/L for various adsorption pH for the optimum LDH@Fe nanoparticles (40 nm, x = 0.2) in comparison to common arsenic adsorbents consisting of Fe hydroxides, magnetite and zero-valent adsorbents. LDH@Fe nanoparticles indicate a significant and almost constant adsorption capacity independently to the water pH. The widely commercially available iron hydroxides show a higher efficiency at acidic and neutral pH values but become completely inactive at alkaline conditions. Compared to iron and iron oxides, LDH-coated nanoparticles indicate a better performance at any pH value and being the only effective adsorbent at high pH values. Comparison between the core-shell composite of the invention and a composition of magnetite spherical nanoparticles of an average diameter of 50 nm dispersed within Fe-hvdrotalcite described in "arsenic removal from aqueous solution with Fe- hvdrotalcite supported magnetite nanoparticle" T. Turk et al Journal of Industrial and Engineering Chemistry 20 (2014) 732-738. The core-shell nanoparticles of the invention also show a significantly higher efficiency compared to a composition of magnetite spherical nanoparticles of an average diameter of 50 nm dispersed within Fe-hydrotalcite described by T. Turk et al Journal of Industrial and Engineering Chemistry 20 (2014) 732-738. The given adsorption capacity for residual arsenic 10 μg/l is only 0.04 μg As/mg. In addition, the pH dependence of the efficiency is strong given that it is maximized at pH 9. This difference comes out as a result of the preparation acidity and redox as well as the lower specific surface area of the described LDH supported nanoparticles. Application of the core-shell composite in a continuous flow drinking water treatment system

A concentrated dispersion of 40 nm Fe nanoparticles coated by LDH with x = 0.2 (0.5 g/L) is continuously added at a rate 0.1 m³/h in a 1 m³ stirred tank and comes into contact with the contaminated water (50 μg As/L, pH = 7) which is continuously pumped at a rate of 0.5 m³/h. The treated water containing the As-loaded particles, outflows from the tank and passes through a magnetic separator or a nanofiltration system to completely remove the solid. Magnetic separation of the core-shell composite after used for the purification of contaminated water

The magnetic separation system is sequenced in the outflow of the water treatment unit used to treat contaminated water by the core-shell composite nanoparticles. In one case, the magnetic separator consists of a horizontal or vertical tube 1 m long with a diameter of 10 cm. The tube is placed into the magnetic field of between 0.03 and 3 Teslas generated by parallel rectangular permanent magnets consisting of NdFeB. Alternatively, the magnetic field may be generated by an electromagnet. The tube may contain a filling material (wires, wool, glass fiber) so as to enhance the gradient of the applied magnetic field and assist the separation of the nanoparticles. The nanoparticles are separated at a yield of 100 % and treated water free of the contaminant comes out of the separator.

Claims

A core-shell composition characterized in that it comprises:

• a core of at least a magnetic nanoparticle;

• a shell of a layered double hydroxide compound of the formula I

[I] wherein M is selected between Mg or Ca;

wherein x ranges between 0 and 0.3;

wherein n ranges between 0 and 10; and

wherein said shell is completely coating said core and wherein said shell has a thickness between 0.1 nm and 10 nm.

The core-shell composition according to the preceded claim, wherein the magnetic nanoparticle is selected from the list consisting of Fe, Fe₃0₄, Y-Fe₂0₃, CoFe, CoFe₂0₄, NiFe₂0₄, MnFe₂0₄, MgFe₂0₄ and a combination thereof.

The core-shell composition according to any of claims 1 or 2, wherein the magnetic nanoparticle ranges a size between 3 nm and 100 nm.

A process of obtainment of the core-shell composition according to any of claims 1 to 3 characterized in that it comprises at least the following steps:

a) depositing a layered double hydroxide of the formula I

[I] wherein M is selected between Mg or Ca;

wherein x ranges between 0 and 0.3; and

5. The process according to the preceded claim, wherein the magnetic nanoparticles of step a) are prepared by precipitation of salts at a pH higher than 10, preferably at pH ranging between 10 and 13.

6. The process according to the preceded claim, wherein the salts are selected from the list consisting of FeS0₄, FeCI₂, (NH₄)₂Fe(S0₄)₂, Fe₂(S0₄)₃, FeCI₃, Fe(N0₃)₃, CoS0₄, CoCI₂, Co(N0₃)₂, NiCI₂, NiS0₄, Ni(N0₃)₂, MgCI₂, Mg(N0₃)₂, MnS0₄ and

MnCI₂ Mn(N0₃)₂.

7. The process according to any of claims 5 or 6, wherein the redox potential of the dispersion of magnetic nanoparticles ranges between -1.2 V and -0.5 V during preparation of magnetic particles.

8. The process according to any of claims 5 to 7, wherein the preparation of magnetic nanoparticles is performed under a reducing environment selected from hydrazine, NaBH₄, NaHS0₃, Na₂S₂0₃, Na₂S₂0₄, Na₂S₂0₅, Na₂S and a combination thereof.

9. The process according to any of claims 4 to 8, wherein step a) is performed by precipitation of magnesium or calcium salts, preferably selected from the list consisting of MgS0₄, MgCI₂ and Mg(N0₃)_2, in the presence of a carbonate reagent, .of a concentration between 0.01 M and 2 M, preferably the carbonate reagent is selected from the list consisting of Na₂C0₃, NaHC0₃, K₂C0₃ and KHC0₃, and under the following conditions:

• at a pH higher than 8; preferably at a pH ranging between 8 and 13, and

• at a temperature range between 50 °C and 95 °C. 10. The process according to claim 9, wherein the redox potential of step a) ranges between -0.9 V and -0.3 V.

1 1 . The process according to any of claims 4 to 10, which is performed in a continuous flow reactor, wherein each step is performed in a separate reactor.

12. Use of the composition according to any of claims 1 to 3 for purifying biological- medical systems such as tissues, cells or blood.

13. Use of the composition according to any of claims 1 to 3, for purifying contaminated water.

14. A process for purifying contaminated water characterized in that it comprises the following steps:

a) contacting the contaminated water with the composition according to claims 1 to 3; and

b) separating the insoluble matter from the water by means of the application of a magnetic field of 0.03 Teslas and 3 Teslas.