**1. Introduction**

Magnetic materials that are paramagnetic, ferrimagnetic, and ferromagnetic have received much attention because of their unique properties especially ready to modify and nontoxic [1, 2]. Magnetite (Fe<sup>3</sup> O4 ), one of many magnetic materials, is widely investigated for possible magnetic resonance imaging, sensor, and adsorbent. Magnetic nanoparticles typically consist of a magnetic core, a coating, and, in some cases, surface active modifiers.

The magnetite nanoparticles have a high surface area that yields numerous active sites. However, preparation of Fe<sup>3</sup> O4 nanoparticles is problematic since it can agglomerate, which

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

leads to the decrease in the active sites. Coating with organic or inorganic surfactants is one way to avoid the particle agglomeration. The organic surfactants act as capping agents, but at times, they can give bigger particle size. Inorganic capping agent such as silica (SiO<sup>2</sup> ) has exceptional physical and chemical properties. SiO<sup>2</sup> is chemically stable in acidic solution and tuneable for modification. Coating of Fe<sup>3</sup> O4 nanoparticles with SiO<sup>2</sup> will also avoid the agglomeration and protect them from dissolution in acidic solution. SiO<sup>2</sup> will cover the surface of each Fe<sup>3</sup> O4 nanoparticle to form Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell system [3].

Morel et al. have coated Fe<sup>3</sup> O4 particles with SiO<sup>2</sup> to form core-shell having nanometer scale with an average diameter of 49 nm [4]. The success of Fe<sup>3</sup> O4 /SiO<sup>2</sup> core-shell nanoparticle formation depends on the size of magnetite. However, stirring with the magnetic bar during the preparation causes condensation and agglomeration of the particles. The nonmagnetic mechanical stirring method was chosen for the preparation of Fe<sup>3</sup> O4 nanoparticle [5].

Many researchers used thiol group as an adsorbent for [AuCl<sup>4</sup> ]− ion with a better performance [2, 3, 6–9]. We have reported on the synthesis of the Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell modified with a thiol group. The Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell preparation was performed by applying nonmagnetic stirring method. For improving adsorption capacity, modification with thiol group has been conducted. The thiol groups are of the soft bases.

On the other hand, the [AuCl<sup>4</sup> ] − ions are classified as weak acid species, thus provided specific interaction with each other based on Pearson's hard-soft acid-based concepts [10]. The adsorption kinetics, adsorption capacity, and interaction model for the adsorption of [AuCl<sup>4</sup> ] − ion in solution by Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell adsorbent are reported. A recent review of the matter can be found elsewhere [11].

Recovery of the magnetic material is key in the process following the adsorption. The spent adsorbent can be separated using a magnetic field. **Figure 2** shows how the used magnetic is separated by the external magnetic field. The magnetic adsorbent that has a high content of rare metals can be subject to dissolution and further separation. It is important to state that in the industrial purpose the powerful electromagnet system can be applied to do the job. In the purification, there are many possible green purification processes of metallurgy, starting from

Surface Modification of Fe3O4 as Magnetic Adsorbents for Recovery of Precious Metals

http://dx.doi.org/10.5772/intechopen.79586

129

**Figure 2.** Image of simplified recovery technique of spent magnetic material after adsorption of precious metals [3].

Magnetite is commonly prepared by co-precipitation of Fe(II) and Fe(III) salts with suitable bases. Sodium hydroxide and ammonia are both commonly used in the preparation of magnetite. There are many bases that can be used to help control the size and the shape of the particles. Magnetite nanoparticles can be prepared in the presence of carboxylate such as laurate, palmitate, linoleate, and so on. The addition of surfactant helps reduce the particle size and control the shape. The required shape is usually spherical with a diameter

For co-precipitation methods, the size, shape, and composition of the resulting nanoparticles

electrochemical separation to blast furnace.

very much depend upon the following [12]:

**O4**

**2. Preparation of Fe3**

of the nanoscale.

**Figure 1** shows the schematic architecture of the magnetite-silica-functional groups. The core is magnetite to function as the important part for separation. The shell is silica, which can protect the magnetite from dissolution especially when it contacts with an acidic environment. Modification of the silica surface can be realized by silanization to give functional groups having an important function to react with the metal cations. The functional group must have a strong bond with the surface via complex formation.

**Figure 1.** Step-by-step of Fe<sup>3</sup> O4 @SiO<sup>2</sup> core-shell preparation and functionalization [3].

Surface Modification of Fe3O4 as Magnetic Adsorbents for Recovery of Precious Metals http://dx.doi.org/10.5772/intechopen.79586 129

**Figure 2.** Image of simplified recovery technique of spent magnetic material after adsorption of precious metals [3].

Recovery of the magnetic material is key in the process following the adsorption. The spent adsorbent can be separated using a magnetic field. **Figure 2** shows how the used magnetic is separated by the external magnetic field. The magnetic adsorbent that has a high content of rare metals can be subject to dissolution and further separation. It is important to state that in the industrial purpose the powerful electromagnet system can be applied to do the job. In the purification, there are many possible green purification processes of metallurgy, starting from electrochemical separation to blast furnace.

#### **2. Preparation of Fe3 O4**

leads to the decrease in the active sites. Coating with organic or inorganic surfactants is one way to avoid the particle agglomeration. The organic surfactants act as capping agents, but at times, they can give bigger particle size. Inorganic capping agent such as silica (SiO<sup>2</sup>

O4

mation depends on the size of magnetite. However, stirring with the magnetic bar during the preparation causes condensation and agglomeration of the particles. The nonmagnetic

by applying nonmagnetic stirring method. For improving adsorption capacity, modification

interaction with each other based on Pearson's hard-soft acid-based concepts [10]. The adsorp-

**Figure 1** shows the schematic architecture of the magnetite-silica-functional groups. The core is magnetite to function as the important part for separation. The shell is silica, which can protect the magnetite from dissolution especially when it contacts with an acidic environment. Modification of the silica surface can be realized by silanization to give functional groups having an important function to react with the metal cations. The functional group must have

core-shell preparation and functionalization [3].

tion kinetics, adsorption capacity, and interaction model for the adsorption of [AuCl<sup>4</sup>

O4 /SiO<sup>2</sup>

particles with SiO<sup>2</sup>

agglomeration and protect them from dissolution in acidic solution. SiO<sup>2</sup>

exceptional physical and chemical properties. SiO<sup>2</sup>

nanoparticle to form Fe<sup>3</sup>

O4

with an average diameter of 49 nm [4]. The success of Fe<sup>3</sup>

mechanical stirring method was chosen for the preparation of Fe<sup>3</sup>

O4 /SiO<sup>2</sup>

with thiol group has been conducted. The thiol groups are of the soft bases.

Many researchers used thiol group as an adsorbent for [AuCl<sup>4</sup>

] −

a strong bond with the surface via complex formation.

[2, 3, 6–9]. We have reported on the synthesis of the Fe<sup>3</sup>

and tuneable for modification. Coating of Fe<sup>3</sup>

O4

fied with a thiol group. The Fe<sup>3</sup>

On the other hand, the [AuCl<sup>4</sup>

O4 /SiO<sup>2</sup>

matter can be found elsewhere [11].

solution by Fe<sup>3</sup>

**Figure 1.** Step-by-step of Fe<sup>3</sup>

O4 @SiO<sup>2</sup>

Morel et al. have coated Fe<sup>3</sup>

128 Advanced Surface Engineering Research

face of each Fe<sup>3</sup>

) has

will also avoid the

will cover the sur-

core-shell nanoparticle for-

nanoparticle [5].

ion with a better performance

nanoparticle core-shell modi-

] − ion in

is chemically stable in acidic solution

to form core-shell having nanometer scale

nanoparticles with SiO<sup>2</sup>

O4 /SiO<sup>2</sup>

O4 /SiO<sup>2</sup>

nanoparticle core-shell adsorbent are reported. A recent review of the

nanoparticle core-shell system [3].

O4

nanoparticle core-shell preparation was performed

]−

ions are classified as weak acid species, thus provided specific

Magnetite is commonly prepared by co-precipitation of Fe(II) and Fe(III) salts with suitable bases. Sodium hydroxide and ammonia are both commonly used in the preparation of magnetite. There are many bases that can be used to help control the size and the shape of the particles. Magnetite nanoparticles can be prepared in the presence of carboxylate such as laurate, palmitate, linoleate, and so on. The addition of surfactant helps reduce the particle size and control the shape. The required shape is usually spherical with a diameter of the nanoscale.

For co-precipitation methods, the size, shape, and composition of the resulting nanoparticles very much depend upon the following [12]:

**a.** The type of precursors' salts used, for example, chloride, sulfate, perchlorate, or nitrate

Microwave irradiation might be a promising method in processing materials due to its thermal and nonthermal effects. Microwave synthesis has the advantages of short reaction time, small particle size, and narrow size distribution. Aging under microwave irradiation in short period

O2

O4

Some researchers use capping agent to control crystal growth during Fe<sup>3</sup>

powders with the size of 8–10 nm. The Fe(OH)<sup>2</sup>

the control of the size of magnetite-silica via sol–gel method has also been reported [21].

tion. Co-precipitation technique was used to prepare magnetite nanoparticles with diameter around 35 nm using 0.90 M NaOH solution as the precipitating agent and trisodium citrate as a capping agent. The precursors are ferric and ferrous chloride salts with predetermined Fe3+/ Fe2+ molar ratio. The diameter of silica-coated magnetite nanoparticles synthesized by Stöber method was about 50 nm. Due to an electrostatic interaction between the Fe3+/Fe2+ ions of the

NPs and trisodium citrate surfactant, a stable magnetic fluid containing dispersed Fe<sup>3</sup>

NPs was produced [22]. A green and facile method for synthesis of magnetite nanoparticles was proposed [23]. Nano-sized polyhedral particles were synthesized by heating an aqueous solution of Fe2+, Fe3+, and urea at 85°C. The use of PVA in the synthesis system gives spherical magnetite nanoparticles with loose structure, unaggregated. The size of the microspheres can be tuned by changing the concentration of PVA. Upon addition of acetic acid to the system with PVA, microspheres with looser structure were produced. The size of the microspheres can further be tuned by changing the concentration of acetic acid. The co-precipitation of Fe2+

a narrow size distribution (4–8 nm) than that produced without ultrasound irradiation [4]. Diethylene glycol (DEG) is also possibly used to control the particle size as reported earlier. This surfactant takes an important role in the preparation of magnetite/zinc oxide hybrid

reaction. The functional group that is ready to bond to iron oxide is methoxy silane (CH<sup>3</sup>

leaving the end group remains free. The leaving group is methane and ethane. The reaction is better to be done in an organic solvent. The silane group may have a spacer of long ethylene

For example, modification by the use of 3-mercaptopropyltrimethoxysilane (3-MPTS) [3] reaction is depicted as a chemical reaction (2). For further surface modification, we can use

<sup>3</sup>Si(CH2)3SH Fe3O4@SiO2 [(O)3]nSi(CH2)3SH <sup>+</sup> 3nCH3OH

on. The surface of the silica has different accesses to the organic functional groups [25].

). The ending of the silane may be carboxylic, an amine group, hydroxyl, and so

)

and Fe3+ in aqueous solutions under ultrasound irradiation results in smaller Fe<sup>3</sup>

nanoparticles with complete crystalline structure than those aged for 7 days at

Surface Modification of Fe3O4 as Magnetic Adsorbents for Recovery of Precious Metals

as an oxidizing agent to construct nano-sized super-

O4

nanoparticles in the presence of surfactants [17]. Work on

precipitates were partially

http://dx.doi.org/10.5772/intechopen.79586

nanoparticle forma-

O4

). It is usually performed via silanization


NPs with


(2)

O4

131

yields Fe<sup>3</sup>

Fe<sup>3</sup> O4

material [24].

chain (-CH<sup>2</sup>

**3. Coating of Fe3**

O-Si-) or ethoxy silane (CH<sup>3</sup>

−

other silanization compounds [3]:

**O4**

The next step is coating magnetite with silica (SiO<sup>2</sup>

 **with SiO2**


Fe3O4@SiO2 [(OH)3]n <sup>+</sup> <sup>n</sup> (CH3O

O4

paramagnetic Fe<sup>3</sup>

room temperature [16]. Yu et al. used H<sup>2</sup>

O4

oxidized to generate ultrafine Fe<sup>3</sup>


The remaining issue is that magnetite nanoparticles are easily oxidized to maghemite, so this method is often used to obtain nanoparticles of magnetite and maghemite with the small size of 4–20 nm. Grüttner et al. have listed the size, coating, heating behavior, and magnetic properties of some iron oxide nanoparticles produced by this method [13]. Nanoparticles are produced by this method range in size from 4 to 45 nm. For fixed-synthesis conditions, the quality of the magnetite nanoparticles is very reproducible. Although co-precipitation is unquestionably the easiest process and highly scalable, it is not without issues. Controlling the shape is not easy, and the nanoparticles can be more varied in size than that produced in some other methods [12].

Precursors for the Fe(II) include ferrous sulfate, ferrous nitrate, and ferrous chloride. Some use ferrous acetate and ferrous oxalate. The most used precursor is ferrous sulfate. For the Fe(III), we can use ferric chloride, ferric nitrate, and so on. Ferric nitrate is used a lot. Ferric acetate and ferric oxalate are also commonly used as Fe(III) precursors.

The Fe(II) to Fe(III) molar ratio must be controlled strictly at 1:2. Therefore, the concentration of the starting material must be fixed. The oxidation of the Fe(II) ion must be contained by controlling the atmosphere by the use of inert atmosphere. During the co-precipitation, the nitrogen gas must be kept flowing to reduce possible oxidation process. Other noble gases could be used, which give a better magnetite product.

There are various ways to prepare Fe<sup>3</sup> O4 nanoparticles, such as hydrothermal synthesis [14], co-precipitation [15], microwave irradiation [16], oxidation of Fe(OH)<sup>2</sup> by H2 O2 [17], and microemulsion [18]. The Fe<sup>3</sup> O4 nanoparticles synthesized by a hydrothermal method in the presence of sodium sulfate have a particle size of 160 nm [14]. Among various ways to prepare Fe<sup>3</sup> O4 , hydrothermal is one of the simple methods because it gives unique characters. The shape and size of nanoparticles have a good homogeneity and high degree of crystallinity [19].

The widespread method to produce Fe<sup>3</sup> O4 is by co-precipitation of Fe2+/Fe3+ solution mixture with a molar ratio of 1:2 in alkaline solution [15, 20]. The reaction for Fe<sup>3</sup> O4 formation by coprecipitation method is shown in chemical Eq. (1). Although this method is well known for synthesis Fe<sup>3</sup> O4 , the molar ratio of reactant, pH, and temperature still need attention to get the proper size and morphology [15]:

$$\mathrm{Fe^{2+} + 2Fe^{3+} + 8OH^{-} \rightarrow Fe\_3O\_4 + 4H\_2O} \tag{1}$$

Microwave irradiation might be a promising method in processing materials due to its thermal and nonthermal effects. Microwave synthesis has the advantages of short reaction time, small particle size, and narrow size distribution. Aging under microwave irradiation in short period yields Fe<sup>3</sup> O4 nanoparticles with complete crystalline structure than those aged for 7 days at room temperature [16]. Yu et al. used H<sup>2</sup> O2 as an oxidizing agent to construct nano-sized superparamagnetic Fe<sup>3</sup> O4 powders with the size of 8–10 nm. The Fe(OH)<sup>2</sup> precipitates were partially oxidized to generate ultrafine Fe<sup>3</sup> O4 nanoparticles in the presence of surfactants [17]. Work on the control of the size of magnetite-silica via sol–gel method has also been reported [21].

Some researchers use capping agent to control crystal growth during Fe<sup>3</sup> O4 nanoparticle formation. Co-precipitation technique was used to prepare magnetite nanoparticles with diameter around 35 nm using 0.90 M NaOH solution as the precipitating agent and trisodium citrate as a capping agent. The precursors are ferric and ferrous chloride salts with predetermined Fe3+/ Fe2+ molar ratio. The diameter of silica-coated magnetite nanoparticles synthesized by Stöber method was about 50 nm. Due to an electrostatic interaction between the Fe3+/Fe2+ ions of the Fe<sup>3</sup> O4 NPs and trisodium citrate surfactant, a stable magnetic fluid containing dispersed Fe<sup>3</sup> O4 NPs was produced [22]. A green and facile method for synthesis of magnetite nanoparticles was proposed [23]. Nano-sized polyhedral particles were synthesized by heating an aqueous solution of Fe2+, Fe3+, and urea at 85°C. The use of PVA in the synthesis system gives spherical magnetite nanoparticles with loose structure, unaggregated. The size of the microspheres can be tuned by changing the concentration of PVA. Upon addition of acetic acid to the system with PVA, microspheres with looser structure were produced. The size of the microspheres can further be tuned by changing the concentration of acetic acid. The co-precipitation of Fe2+ and Fe3+ in aqueous solutions under ultrasound irradiation results in smaller Fe<sup>3</sup> O4 NPs with a narrow size distribution (4–8 nm) than that produced without ultrasound irradiation [4]. Diethylene glycol (DEG) is also possibly used to control the particle size as reported earlier. This surfactant takes an important role in the preparation of magnetite/zinc oxide hybrid material [24].

#### **3. Coating of Fe3 O4 with SiO2**

**a.** The type of precursors' salts used, for example, chloride, sulfate, perchlorate, or nitrate

The remaining issue is that magnetite nanoparticles are easily oxidized to maghemite, so this method is often used to obtain nanoparticles of magnetite and maghemite with the small size of 4–20 nm. Grüttner et al. have listed the size, coating, heating behavior, and magnetic properties of some iron oxide nanoparticles produced by this method [13]. Nanoparticles are produced by this method range in size from 4 to 45 nm. For fixed-synthesis conditions, the quality of the magnetite nanoparticles is very reproducible. Although co-precipitation is unquestionably the easiest process and highly scalable, it is not without issues. Controlling the shape is not easy, and the nanoparticles can be more varied in size than that produced in

Precursors for the Fe(II) include ferrous sulfate, ferrous nitrate, and ferrous chloride. Some use ferrous acetate and ferrous oxalate. The most used precursor is ferrous sulfate. For the Fe(III), we can use ferric chloride, ferric nitrate, and so on. Ferric nitrate is used a lot. Ferric

The Fe(II) to Fe(III) molar ratio must be controlled strictly at 1:2. Therefore, the concentration of the starting material must be fixed. The oxidation of the Fe(II) ion must be contained by controlling the atmosphere by the use of inert atmosphere. During the co-precipitation, the nitrogen gas must be kept flowing to reduce possible oxidation process. Other noble gases

in the presence of sodium sulfate have a particle size of 160 nm [14]. Among various ways

acters. The shape and size of nanoparticles have a good homogeneity and high degree of

precipitation method is shown in chemical Eq. (1). Although this method is well known for

Fe2+ + 2Fe3+ + 8OH<sup>−</sup> Fe<sup>3</sup> O4 + 4H2 O (1)

, hydrothermal is one of the simple methods because it gives unique char-

, the molar ratio of reactant, pH, and temperature still need attention to get the

nanoparticles, such as hydrothermal synthesis

is by co-precipitation of Fe2+/Fe3+ solution mixture

O4

nanoparticles synthesized by a hydrothermal method

 by H2 O2 [17],

formation by co-

O4

O4

with a molar ratio of 1:2 in alkaline solution [15, 20]. The reaction for Fe<sup>3</sup>

[14], co-precipitation [15], microwave irradiation [16], oxidation of Fe(OH)<sup>2</sup>

O4

acetate and ferric oxalate are also commonly used as Fe(III) precursors.

could be used, which give a better magnetite product.

There are various ways to prepare Fe<sup>3</sup>

The widespread method to produce Fe<sup>3</sup>

and microemulsion [18]. The Fe<sup>3</sup>

O4

O4

proper size and morphology [15]:

to prepare Fe<sup>3</sup>

synthesis Fe<sup>3</sup>

crystallinity [19].

**b.** The Fe2+/Fe3+ ratio **c.** The mixing orders **d.** The mixing rates

130 Advanced Surface Engineering Research

**f.** The pH value

**e.** The reaction temperature

some other methods [12].

**g.** The ionic strength of the media

The next step is coating magnetite with silica (SiO<sup>2</sup> ). It is usually performed via silanization reaction. The functional group that is ready to bond to iron oxide is methoxy silane (CH<sup>3</sup> - O-Si-) or ethoxy silane (CH<sup>3</sup> -CH<sup>2</sup> -O-Si-). After the reaction, it forms a covalent bond of Fe-O-Si leaving the end group remains free. The leaving group is methane and ethane. The reaction is better to be done in an organic solvent. The silane group may have a spacer of long ethylene chain (-CH<sup>2</sup> − ). The ending of the silane may be carboxylic, an amine group, hydroxyl, and so on. The surface of the silica has different accesses to the organic functional groups [25].

For example, modification by the use of 3-mercaptopropyltrimethoxysilane (3-MPTS) [3] reaction is depicted as a chemical reaction (2). For further surface modification, we can use other silanization compounds [3]:

Recalls us \*\*euptue\*\* as a \*\*ulembar\*\* neuron (2). For \*\*unhuer sumate\*\* mominant\*\*, we can use other silaniization compounds [3]:

$$\text{Fe}\_3\text{O}\_4\text{@SiO}\_2[(\text{OH})\_3]\_a + \text{n(CH}\_3\text{O})\_3\text{Si(CH}\_3)\_3\text{SH}+\text{}$$

$$\text{Fe}\_3\text{O}\_4\text{@SiO}\_2[(\text{O})\_3]\_a\text{Si(CH}\_3)\_3\text{SH}+\text{3nCH}\_3\text{OH}$$

Iron oxide is not stable in acidic condition. After coating with silica, the magnetite core is usually stable in the acidic solution. It will come readily with a proton to give its corresponding ion either Fe2+ or Fe3+. Dissolution of the magnetite will make the core-shell system unstable and break the bond between Fe-O-Si. To make sure that the magnetite is not dissolved in the acidic solution, we can test it by the use of an acid such as hydrochloric acid or nitric acid. The concentration of total iron as Fe2+ or Fe3+ can be an indicator if the magnetite is still strong. If high concentration of Fe3+ is found in the solution, we can say that the magnetite structure is collapsed and even broken down.

The visual indication can be seen from the color of the dispersion of Fe<sup>3</sup> O4 @SiO<sup>2</sup> core-shell. The solution of Fe3+ in the solution is pale yellow. If the dispersion color is pale yellow, it can be concluded that magnetite does dissolve. The color is getting dark when the more magnetite dissolved in the dispersion. A combination of atomic absorption spectrometry and visual observation helps us make sure the magnetite core is still strongly intact.

Fe<sup>3</sup> O4 /SiO<sup>2</sup> core-shell nanoparticles were obtained by dispersing Fe<sup>3</sup> O4 into the mixture solution of 80 mL of ethanol, 20 mL of deionized water, and 1.0 mL of concentrated aqueous ammonia solution (28 wt.%). After this, the mixture solution was homogenized by ultrasonication for 30 min to form a uniform dispersion. Subsequently, a certain amount of tetraethoxysilane (TEOS) was added dropwise into the solution with vigorous stirring. After stirring at room temperature for 6 h, the product was separated with a magnet bar, washed with deionized water for three times, and dried in vacuum at 50°C for 12 h. A series of SiO<sup>2</sup> @Fe<sup>3</sup> O4 particles were prepared with 1–9% SiO<sup>2</sup> content [26].

The silica coating used on a core particle has several advantages. The essential advantages of the silica coating compared with another inorganic (metal or metal oxide) or organic coating are as follows: It reduces the bulk conductivity and increases the suspension stability of the core particles. Also, silica is the most chemically inert material available; it can block the core surface without interfering in the redox reaction at the core surface [27]. There are two methods for coating Fe<sup>3</sup> O4 with silica, by acidic hydrolysis of silicate in aqueous solutions and the modified Stöber process [4, 28].

The Stöber method consists of the alkaline hydrolysis of tetraethyl orthosilicate (TEOS) in ethanol/water mixture in the presence of Fe<sup>3</sup> O4 NPs [28]. The Stöber process is applied to the classic sol–gel process [29]. The Stöber synthesis has the advantage of being easy to scale up for commercial applications and the possibility to effortlessly transfer the nanoparticles into aqueous solutions [30]. Some methods that lead to the synthesis of Fe<sup>3</sup> O4 /SiO<sup>2</sup> are shown in **Table 1**.

interaction is shown in **Figure 3**. Another study shows the success of recovery of gold from copper anode slime by means of magnetite nanoparticles [33]. The surface of magnetite was also modified with oleic acid, methyl methacrylate, and ethylenediamine (EDA-MMA-OA@

@Fe<sup>3</sup> O4

it is applied as a magnetic nano-adsorbent for recovery of precious metal nanoparticles by contacting the nano-adsorbent with Au, Ag, Pd, or Pt hydrosols [35]. The magnetic particles are very efficient for Au due to strong affinity of sulfur-containing groups at the magnetite surfaces with this metal. Since Au colloids are used in laboratory and industrial contexts, the material could have an impact on the development of nanotechnology to recover precious

O4

and au(III) ions [31].

is by the use of dithiocarbamate, and

) giving the adsorption of indium of about 54 mg/g [34].

An interesting example of functionalization of Fe<sup>3</sup>

**Figure 3.** Possible interaction between RS-SR-NH-SiO<sup>2</sup>

**Core Shell**

Wet chemical reaction FeCl<sup>3</sup>

Wet chemical reaction FeCl<sup>3</sup>

Wet chemical reaction FeCl<sup>3</sup>

**Table 1.** Methods for synthesis of Fe<sup>3</sup>

A chemical reaction in microemulsion

Method Precursors Method Basic reagent

.

H4 Sol–gel TEOS

Surface Modification of Fe3O4 as Magnetic Adsorbents for Recovery of Precious Metals

, FeSO<sup>4</sup> Hydrolysis Commercial SiO<sup>2</sup>

SiO<sup>3</sup>

133

http://dx.doi.org/10.5772/intechopen.79586

, FeSO<sup>4</sup> Hydrolysis Na<sup>2</sup>

, FeSO<sup>4</sup> Sol–gel reaction in microemulsion TEOS

, N<sup>2</sup>

coated with SiO<sup>2</sup>

FeCl<sup>3</sup>

O4

Fe<sup>3</sup> O4

About the modification and application of magnetic materials, a coating of Fe<sup>3</sup> O4 by the use of various materials has been reported elsewhere. TiO<sup>2</sup> @Fe<sup>3</sup> O4 , TiO<sup>2</sup> @Fe<sup>3</sup> O4 @chitosan, and methyl pyrazolone-functionalized TiO<sup>2</sup> @Fe<sup>3</sup> O4 @chitosan were prepared for photocatalytic degradation of dyes [32]. They found that the core is important for separation and the shell is an active catalyst. The degradation of dye using these catalysts can reach up to 98–99%.

Thioctic acid-modified silica-coated magnetite nanoparticles, RS-SR-NH-SiO<sup>2</sup> @Fe<sup>3</sup> O4 , have been prepared, and its ability for the recovery of Au(III) in aqueous solutions was evaluated [31]. The Au(III) adsorption capacity of the produced adsorbent is about 25 mg/g. The possible


**Table 1.** Methods for synthesis of Fe<sup>3</sup> O4 coated with SiO<sup>2</sup> .

Iron oxide is not stable in acidic condition. After coating with silica, the magnetite core is usually stable in the acidic solution. It will come readily with a proton to give its corresponding ion either Fe2+ or Fe3+. Dissolution of the magnetite will make the core-shell system unstable and break the bond between Fe-O-Si. To make sure that the magnetite is not dissolved in the acidic solution, we can test it by the use of an acid such as hydrochloric acid or nitric acid. The concentration of total iron as Fe2+ or Fe3+ can be an indicator if the magnetite is still strong. If high concentration of Fe3+ is found in the solution, we can say that the magnetite structure is

The solution of Fe3+ in the solution is pale yellow. If the dispersion color is pale yellow, it can be concluded that magnetite does dissolve. The color is getting dark when the more magnetite dissolved in the dispersion. A combination of atomic absorption spectrometry and visual

tion of 80 mL of ethanol, 20 mL of deionized water, and 1.0 mL of concentrated aqueous ammonia solution (28 wt.%). After this, the mixture solution was homogenized by ultrasonication for 30 min to form a uniform dispersion. Subsequently, a certain amount of tetraethoxysilane (TEOS) was added dropwise into the solution with vigorous stirring. After stirring at room temperature for 6 h, the product was separated with a magnet bar, washed with

deionized water for three times, and dried in vacuum at 50°C for 12 h. A series of SiO<sup>2</sup>

 content [26]. The silica coating used on a core particle has several advantages. The essential advantages of the silica coating compared with another inorganic (metal or metal oxide) or organic coating are as follows: It reduces the bulk conductivity and increases the suspension stability of the core particles. Also, silica is the most chemically inert material available; it can block the core surface without interfering in the redox reaction at the core surface [27]. There are two

The Stöber method consists of the alkaline hydrolysis of tetraethyl orthosilicate (TEOS) in

classic sol–gel process [29]. The Stöber synthesis has the advantage of being easy to scale up for commercial applications and the possibility to effortlessly transfer the nanoparticles into aque-

degradation of dyes [32]. They found that the core is important for separation and the shell is an active catalyst. The degradation of dye using these catalysts can reach up to 98–99%.

been prepared, and its ability for the recovery of Au(III) in aqueous solutions was evaluated [31]. The Au(III) adsorption capacity of the produced adsorbent is about 25 mg/g. The possible

O4

About the modification and application of magnetic materials, a coating of Fe<sup>3</sup>

Thioctic acid-modified silica-coated magnetite nanoparticles, RS-SR-NH-SiO<sup>2</sup>

@Fe<sup>3</sup> O4

with silica, by acidic hydrolysis of silicate in aqueous solutions and

@Fe<sup>3</sup> O4 , TiO<sup>2</sup> O4 @SiO<sup>2</sup>

O4

NPs [28]. The Stöber process is applied to the

@Fe<sup>3</sup> O4

@chitosan were prepared for photocatalytic

O4 /SiO<sup>2</sup> core-shell.

@Fe<sup>3</sup> O4

into the mixture solu-

are shown in **Table 1**.

by the use

@chitosan, and

O4

@Fe<sup>3</sup> O4 , have

The visual indication can be seen from the color of the dispersion of Fe<sup>3</sup>

observation helps us make sure the magnetite core is still strongly intact.

core-shell nanoparticles were obtained by dispersing Fe<sup>3</sup>

collapsed and even broken down.

132 Advanced Surface Engineering Research

particles were prepared with 1–9% SiO<sup>2</sup>

O4

ous solutions [30]. Some methods that lead to the synthesis of Fe<sup>3</sup>

of various materials has been reported elsewhere. TiO<sup>2</sup>

ethanol/water mixture in the presence of Fe<sup>3</sup>

methyl pyrazolone-functionalized TiO<sup>2</sup>

methods for coating Fe<sup>3</sup>

the modified Stöber process [4, 28].

Fe<sup>3</sup> O4 /SiO<sup>2</sup>

**Figure 3.** Possible interaction between RS-SR-NH-SiO<sup>2</sup> @Fe<sup>3</sup> O4 and au(III) ions [31].

interaction is shown in **Figure 3**. Another study shows the success of recovery of gold from copper anode slime by means of magnetite nanoparticles [33]. The surface of magnetite was also modified with oleic acid, methyl methacrylate, and ethylenediamine (EDA-MMA-OA@ Fe<sup>3</sup> O4 ) giving the adsorption of indium of about 54 mg/g [34].

An interesting example of functionalization of Fe<sup>3</sup> O4 is by the use of dithiocarbamate, and it is applied as a magnetic nano-adsorbent for recovery of precious metal nanoparticles by contacting the nano-adsorbent with Au, Ag, Pd, or Pt hydrosols [35]. The magnetic particles are very efficient for Au due to strong affinity of sulfur-containing groups at the magnetite surfaces with this metal. Since Au colloids are used in laboratory and industrial contexts, the material could have an impact on the development of nanotechnology to recover precious metals like Au [35] and Ag. Another trial is modification using chitosan and EDTA. It shows more selective for the quasi-precious metal of Cu than Cd and Pb [36].

and infrared (IR) spectroscopy. The vibrating sample magnetometer (VSM) can reveal the magnetic properties of the magnetic materials. The microscopic images can be obtained commonly by the use of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Nanomaterials are best to characterize by TEM and AFM. The IR spectroscopy is useful to detect the functional groups present on the magnetite surface. The IR spectroscopy is one of the methods to make sure that functionalization

Surface Modification of Fe3O4 as Magnetic Adsorbents for Recovery of Precious Metals

For elemental analysis, a nondestructive energy dispersive X-ray spectrometry (EDS) can be used to estimate the chemical composition. The SEM is usually equipped with EDS system. The EDS results may lack precision and accuracy; however, it can detect the chemical composition in situ. The destructive methods such as atomic absorption spectrometry, UV–Vis spectrometry, and so forth may be used in place of EDS method. The destructive methods are

One major analytical method in the magnetite characterization is powder X-ray diffraction.

and Fe<sup>3</sup>

tite phase can be detected with certainty by XRD. However, when it is coated with silica, the intensity of the XRD peaks will be much diminished since silica is an amorphous solid. Further decrease in the XRD is expected after organic modification on top of the silica layer. The FT-IR could also be useful for more characterization of magnetic materials. It can offer details of the bond between the core, the shells, and the surface modifiers. Here is the example,

Both spectra have a broad peak at 586 cm−1. The peak is attributed to the Fe-O stretching mode of magnetite. The peak at 3400 cm−1 is due to the O-H stretching mode. The sharp peak at 1100 cm−1 can be attributed to the Si-O-Si stretching mode. The Si-O bending vibration

diffraction patterns have five main peaks at 2θ values of 30.1°, 35.5°, 43.3°, 57.1°, and

O4 /SiO<sup>2</sup>

has a cubic system as confirmed by JCPDS Card No. 88–0315. The magne-

solid nanoparticle core-shell. The

http://dx.doi.org/10.5772/intechopen.79586

135

nanoparticle core-shell are presented in **Figure 5**.

nanoparticle core-shell modified with a thiol group (bottom) [3].

O4

of the surface is successful.

Fe<sup>3</sup> O4

62.5°. The Fe<sup>3</sup>

usually not of choices for this type of materials.

O4

and Fe<sup>3</sup>

O4 /SiO<sup>2</sup>

**Figure 4** shows the XRD patterns of Fe<sup>3</sup>

O4

the FT-IR spectra of Fe<sup>3</sup>

**Figure 4.** XRD patterns of Fe<sup>3</sup>

O4

(top) and Fe<sup>3</sup>

O4 /SiO<sup>2</sup>

#### **4. Surface functionalization of Fe3 O4 @SiO2 −X**

In reaction (1), we can see the steps of surface modification of Fe<sup>3</sup> O4 @SiO<sup>2</sup> core-shell to form Fe<sup>3</sup> O4 @SiO<sup>2</sup> −X, where X is a functional group. The layer of SiO<sup>2</sup> was usually coated on the surface of Fe<sup>3</sup> O4 using the Stöber method. The prepared Fe<sup>3</sup> O4 nanoparticles were used as cores and dispersed in ethanol, and it was added to a three-neck round-bottom flask in ethanol and deionized water. The solution of the concentrated ammonia solution was added. After 15 min, TEOS was added dropwise in 10 min. The mixture was allowed for mechanical stirring for 8 h to perform the silica coating. The produced Fe<sup>3</sup> O4 @SiO<sup>2</sup> was separated and washed with deionized water and ethanol [37].

Direct modification by the use of organic compound is also studied, without first modification by silica. Magnetic nanoparticles modified with third-generation dendrimers followed by ethylenediaminetetraacetic acid (EDTA) were prepared and tested for their performance for recovery of precious metals that are Pd(IV), Au(III), Pd(II), and Ag(I) in the aqueous system [38]. It is interesting that high valence Pd(IV) and Au(III) exhibit relatively better adsorption efficiency than that of Pd(II) and Ag(I) with lower valence. It suggests that the adsorption of precious metals by this type of materials modified with EDTA is a function of valence. When the competing ion such as Zn(II) presents, the adsorption efficiency of the adsorbent for all four precious metals, which are Pd(IV), Au(III), Pd(II), and Ag(I), reduces much.

Magnetite nanoparticles could be directly modified with an organic compound of oleic acid. Iron oxide surface possibly bonds to carboxylic end of lauric acid [21]. However, this method of functionalization might not produce an acid-resistive magnetic adsorbent. The bond between lauric acid and magnetite could be easily damaged when the acidic solution is used. Therefore, the magnetite modified with lauric acid may find application in biological systems since both lauric acid and magnetite are biocompatible.

Silane compound such as (3-aminopropyl)trimethoxysilane (APTMS) could be used to coat magnetite nanoparticles. The product can be described as Fe<sup>3</sup> O4 @SiO<sup>2</sup> -CH<sup>3</sup> -NH2. Further surface modification by alginate gives Fe<sup>3</sup> O4 @SiO<sup>2</sup> -CH<sup>3</sup> -NH<sup>2</sup> -AA. The alginate forms the outer shell of the magnetite by electrostatic interaction with amine [39]. **Figure 1** shows a step-by-step extension of magnetite coating and functionalization. Silica coating will allow further functionalization via silanization, extension (additional of the spacer), and functional group attachment. The functional groups should be free to bond the metal ion either by an ionic or covalent coordination bond. Directed functional groups allow selective reaction with certain cation.
