**5. Characterization**

Characterization of the magnetite can be done by at least five methods, which are vibrating sample magnetometer, powder X-ray diffraction, electron microscopy, elemental analysis, 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 of the surface is successful.

metals like Au [35] and Ag. Another trial is modification using chitosan and EDTA. It shows

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

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

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

Silane compound such as (3-aminopropyl)trimethoxysilane (APTMS) could be used to coat

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.

Characterization of the magnetite can be done by at least five methods, which are vibrating sample magnetometer, powder X-ray diffraction, electron microscopy, elemental analysis,



O4 @SiO<sup>2</sup>




four precious metals, which are Pd(IV), Au(III), Pd(II), and Ag(I), reduces much.

O4 @SiO<sup>2</sup>

**@SiO2**

**−X**

O4 @SiO<sup>2</sup>

O4

O4 @SiO<sup>2</sup>

core-shell to form

was usually coated on the sur-

nanoparticles were used as cores

was separated and washed with

**O4**

more selective for the quasi-precious metal of Cu than Cd and Pb [36].

In reaction (1), we can see the steps of surface modification of Fe<sup>3</sup>

−X, where X is a functional group. The layer of SiO<sup>2</sup>

using the Stöber method. The prepared Fe<sup>3</sup>

**4. Surface functionalization of Fe3**

8 h to perform the silica coating. The produced Fe<sup>3</sup>

since both lauric acid and magnetite are biocompatible.

face modification by alginate gives Fe<sup>3</sup>

**5. Characterization**

magnetite nanoparticles. The product can be described as Fe<sup>3</sup>

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

face of Fe<sup>3</sup>

O4

134 Advanced Surface Engineering Research

deionized water and ethanol [37].

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 usually not of choices for this type of materials.

One major analytical method in the magnetite characterization is powder X-ray diffraction. **Figure 4** shows the XRD patterns of Fe<sup>3</sup> O4 and Fe<sup>3</sup> O4 /SiO<sup>2</sup> solid nanoparticle core-shell. The Fe<sup>3</sup> O4 diffraction patterns have five main peaks at 2θ values of 30.1°, 35.5°, 43.3°, 57.1°, and 62.5°. The Fe<sup>3</sup> O4 has a cubic system as confirmed by JCPDS Card No. 88–0315. The magnetite 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, the FT-IR spectra of Fe<sup>3</sup> O4 and Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell are presented in **Figure 5**. 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

**Figure 4.** XRD patterns of Fe<sup>3</sup> O4 (top) and Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell modified with a thiol group (bottom) [3].

**Figure 5.** FT-IR spectra of resulted Fe<sup>3</sup> O4 (a), Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell (b), and Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle coreshell modified with a thiol group (c) [3].

mode of the silanol group is seen at 964 cm−1. It indicates that the silica has coated well the outer surface of Fe<sup>3</sup> O4 particles [3]. The surface modification of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticles by thiol groups can give a better interaction with [AuCl<sup>4</sup> ]− ions in the solution. The FT-IR spectra of the Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell after modification with thiol group are shown in **Figure 5**. The peak at 686 cm−1 is attributed to the C-S bending vibration mode. The peak at near 2570–2590 cm−1 is attributed to the stretching vibration mode of S-H. The S-H stretching vibration mode is not usually detected [2]. The band at around 2850–2900 cm−1 is due to the stretching vibration of C-H of methylene. This result suggests that the surface modification of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell is successful.

The covering of Fe<sup>3</sup>

**Figure 7.** EDX spectra of Fe<sup>3</sup>

SiO<sup>2</sup>

the TEM images of Fe<sup>3</sup>

O4

results prove that the synthesis of Fe<sup>3</sup>

group is present on the surface of Fe<sup>3</sup>

FT-IR spectra. The modification of Fe<sup>3</sup>

coordination bond with the target ion.

**6. Adsorbent performance**

O4 /SiO<sup>2</sup>

O4 /SiO<sup>2</sup>

has a size of approximately 10–20 nm. The size of Fe<sup>3</sup>

as a capping agent for each Fe<sup>3</sup>

core from dissolution in acid but also control the agglomeration of Fe<sup>3</sup>

O4 /SiO<sup>2</sup>

O4 /SiO<sup>2</sup>

> O4 /SiO<sup>2</sup>

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

nanoparticles modified with thiol group [3].

nanoparticle core-shell. The Fe<sup>3</sup>

In situ analysis of adsorbent is preferable to understand the chemical composition of the product without a change in its nature. **Figure 7** shows the EDX spectra of modified Fe<sup>3</sup>

give free thiol groups on the nanoparticles' surface. The thiols are expected to form covalent

The EDX may also give details of atomic mapping across the sample, especially that of the functional group. For example, sulfur atom in the thiol group is mapped by the EDX method nicely. Functional groups such as amine, thiol, sulfonate, and phosphate may be better

**Table 2** shows a comparison of adsorbent performance for adsorption of precious metals especially gold and palladium [3, 38]. The first two rows show the adsorption performance of

detected by EDX rather than destructive methods such as UV–Vis spectrophotometry.

solid nanoparticle core-shell. The sulfur content is 1.32% (w/w). It suggests that thiol

O4

nanoparticle core-shell by using silica does not only protect the magnetite

O4 /SiO<sup>2</sup>

O4 /SiO<sup>2</sup>

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

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

137

O4 /SiO<sup>2</sup>

nanoparticle core-shell is successful.

O4

nanoparticle core-shell material. It confirms the

solid nanoparticle core-shell by 3-MPTS will

particles. Silica acts

O4 /

core-shell. **Figure 6** shows

core-shell can be observed. These

nanoparticle core-shell

**Figure 6.** TEM image of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell [3].

**Figure 7.** EDX spectra of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticles modified with thiol group [3].

The covering of Fe<sup>3</sup> O4 nanoparticle core-shell by using silica does not only protect the magnetite core from dissolution in acid but also control the agglomeration of Fe<sup>3</sup> O4 particles. Silica acts as a capping agent for each Fe<sup>3</sup> O4 nanoparticle to form Fe<sup>3</sup> O4 /SiO<sup>2</sup> core-shell. **Figure 6** shows the TEM images of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell. The Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell has a size of approximately 10–20 nm. The size of Fe<sup>3</sup> O4 /SiO<sup>2</sup> core-shell can be observed. These results prove that the synthesis of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell is successful.

In situ analysis of adsorbent is preferable to understand the chemical composition of the product without a change in its nature. **Figure 7** shows the EDX spectra of modified Fe<sup>3</sup> O4 / SiO<sup>2</sup> solid nanoparticle core-shell. The sulfur content is 1.32% (w/w). It suggests that thiol group is present on the surface of Fe<sup>3</sup> O4 /SiO<sup>2</sup> nanoparticle core-shell material. It confirms the FT-IR spectra. The modification of Fe<sup>3</sup> O4 /SiO<sup>2</sup> solid nanoparticle core-shell by 3-MPTS will give free thiol groups on the nanoparticles' surface. The thiols are expected to form covalent coordination bond with the target ion.

The EDX may also give details of atomic mapping across the sample, especially that of the functional group. For example, sulfur atom in the thiol group is mapped by the EDX method nicely. Functional groups such as amine, thiol, sulfonate, and phosphate may be better detected by EDX rather than destructive methods such as UV–Vis spectrophotometry.

#### **6. Adsorbent performance**

mode of the silanol group is seen at 964 cm−1. It indicates that the silica has coated well the

**Figure 5**. The peak at 686 cm−1 is attributed to the C-S bending vibration mode. The peak at near 2570–2590 cm−1 is attributed to the stretching vibration mode of S-H. The S-H stretching vibration mode is not usually detected [2]. The band at around 2850–2900 cm−1 is due to the stretching vibration of C-H of methylene. This result suggests that the surface modification of

]−

nanoparticle core-shell after modification with thiol group are shown in

nanoparticle core-shell (b), and Fe<sup>3</sup>

O4 /SiO<sup>2</sup>

ions in the solution. The FT-IR spectra

O4 /SiO<sup>2</sup>

nanoparticles by

nanoparticle core-

particles [3]. The surface modification of Fe<sup>3</sup>

outer surface of Fe<sup>3</sup>

**Figure 5.** FT-IR spectra of resulted Fe<sup>3</sup>

136 Advanced Surface Engineering Research

shell modified with a thiol group (c) [3].

O4 /SiO<sup>2</sup>

**Figure 6.** TEM image of Fe<sup>3</sup>

O4 /SiO<sup>2</sup>

nanoparticle core-shell [3].

of the Fe<sup>3</sup>

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

thiol groups can give a better interaction with [AuCl<sup>4</sup>

O4 (a), Fe<sup>3</sup> O4 /SiO<sup>2</sup>

nanoparticle core-shell is successful.

**Table 2** shows a comparison of adsorbent performance for adsorption of precious metals especially gold and palladium [3, 38]. The first two rows show the adsorption performance of


the adsorbent's surface. People use a complexing agent to release the adsorbed metal cations.

by the material [3]. Thiourea has a better affinity than that of thiol and amine groups. It can

various concentrations following adsorption by the magnetite modified with a thiol group.

ion desorption by HCl/thiourea at various concentration [3].

−

]−

]−

ions bond to the surface. **Figure 9** displays schematic adsorption

ions by magnetite nanoparticles modified with a thiol group [3].

]−

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

ion to dissolve back into the solution. According to the

ion are among the weak bases. A strong coordination

ion by thiourea in 1 M HCl solution at

ion of initially adsorbed 68 mg/g

ions that had been adsorbed

139

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

Thiourea and EDTA are of important environmentally friendly complexing agents.

]− ion.

Thiourea solution in 1 M HCl was employed to liberate [AuCl<sup>4</sup>

]−

]−

form a complex ion with [AuCl<sup>4</sup>

or 60% of the total [AuCl<sup>4</sup>

**Figure 8.** Profile of [AuCl<sup>4</sup>

]−

**Figure 9.** Adsorption and desorption of AuCl<sup>4</sup>

HSAB theory, both thiol and [AuCl<sup>4</sup>

bond forms between thiourea and [AuCl<sup>4</sup>

**Figure 8** depicts the curve of desorption of [AuCl<sup>4</sup>

]−

Dilute thiourea solution can only desorb 43 mg/g [AuCl<sup>4</sup>

**Table 2.** Comparison of adsorption capacities (qmax) of some adsorbents for selected precious metals from aqueous solution. Data presented here are based on the Langmuir isotherm.

magnetic material modified with thiol and dendrimers. It shows that functionalization of the magnetite is very important to increase the adsorption capacity. Thiol group on the surface of magnetite produces high affinity toward Au(III). As we know, thiol can strongly react with gold to form a covalent bond. However, the adsorption capacity of magnetite is still far below that of adsorbent produced by the use of lignin as a precursor.

An adsorbent of magnetic nanoparticles modified by thiourea for effective and selective adsorption of precious metals like gold(III), palladium(II), and platinum(IV) in aqueous acid solution has also been reported. It needs contact time of less than 30 min to reach maximum capacity. Its maximum adsorption capacity of precious metals as determined by Langmuir model was 43.34, 118.46, and 111.58 mg/g for Pt(IV), Au(III), and Pd(II), respectively, at pH 2 and 25°C [40]. The adsorption is selective for Au(III) even in the presence of high concentrations of interfering ion Cu(II). The recycling was achieved by the use of a solution containing 0.7 M thiourea and 2% HCl. The result of the adsorption–desorption test shows that the adsorbent is reusable for the recovery of precious metals.

In general, the adsorption capacity of the magnetite-based adsorbent can reach up to 118.46 mg/g, which is very promising. It may still be less than that of lignin derivatives. For chitosan-modified magnetite, it even can reach the capacity for gold(III) of 707 mg/g [41]. The core-shell-modifier based adsorbent may not have such a high adsorption capacity. The modification step was done through the reaction between chitosan and polymeric Schiff's base of thiourea/glutaraldehyde in the presence of magnetite.

After adsorption test, desorption of the adsorbed cation must also be examined. Complete desorption of the adsorbed cation indicates a better adsorbent performance. In most cases, the acids can desorb adsorbed ion from the surface. The desorption process may use strong acids such as HCl, H<sup>2</sup> SO<sup>4</sup> , and HNO3. The cation is believed to form complex coordination bonds with the surface, and leaching them is difficult.

On the other hand, application of concentrated acid solution may damage the structure of adsorbent. Therefore, mildly acidic solutions are usually employed to release the cation from the adsorbent's surface. People use a complexing agent to release the adsorbed metal cations. Thiourea and EDTA are of important environmentally friendly complexing agents.

Thiourea solution in 1 M HCl was employed to liberate [AuCl<sup>4</sup> ] − ions that had been adsorbed by the material [3]. Thiourea has a better affinity than that of thiol and amine groups. It can form a complex ion with [AuCl<sup>4</sup> ]− ion to dissolve back into the solution. According to the HSAB theory, both thiol and [AuCl<sup>4</sup> ]− ion are among the weak bases. A strong coordination bond forms between thiourea and [AuCl<sup>4</sup> ] − ion.

**Figure 8** depicts the curve of desorption of [AuCl<sup>4</sup> ]− ion by thiourea in 1 M HCl solution at various concentrations following adsorption by the magnetite modified with a thiol group. Dilute thiourea solution can only desorb 43 mg/g [AuCl<sup>4</sup> ]− ion of initially adsorbed 68 mg/g or 60% of the total [AuCl<sup>4</sup> ]− ions bond to the surface. **Figure 9** displays schematic adsorption

**Figure 8.** Profile of [AuCl<sup>4</sup> ]− ion desorption by HCl/thiourea at various concentration [3].

magnetic material modified with thiol and dendrimers. It shows that functionalization of the magnetite is very important to increase the adsorption capacity. Thiol group on the surface of magnetite produces high affinity toward Au(III). As we know, thiol can strongly react with gold to form a covalent bond. However, the adsorption capacity of magnetite is still far below

**Table 2.** Comparison of adsorption capacities (qmax) of some adsorbents for selected precious metals from aqueous

**Adsorbents Adsorption capacity (mg/g) Reference**

Magnetite nanoparticles/thiolated 115 — — [3] Magnetite nanoparticles/dendrimer 3.58 2.71 — [38] Magnetite nanoparticles/thiourea 118.5 111.6 — [40] Magnetite nanoparticles/chitosan 709.2 — 226.5 [41] Magnetite nanoparticles/chitosan 59.5 — — [42] Primary amine-lignin 384 40.43 — [43] Ethylenediamine-lignin 606.76 22.66 — [43]

**Au(III) Pd(II) Ag(I)**

An adsorbent of magnetic nanoparticles modified by thiourea for effective and selective adsorption of precious metals like gold(III), palladium(II), and platinum(IV) in aqueous acid solution has also been reported. It needs contact time of less than 30 min to reach maximum capacity. Its maximum adsorption capacity of precious metals as determined by Langmuir model was 43.34, 118.46, and 111.58 mg/g for Pt(IV), Au(III), and Pd(II), respectively, at pH 2 and 25°C [40]. The adsorption is selective for Au(III) even in the presence of high concentrations of interfering ion Cu(II). The recycling was achieved by the use of a solution containing 0.7 M thiourea and 2% HCl. The result of the adsorption–desorption test shows that the

In general, the adsorption capacity of the magnetite-based adsorbent can reach up to 118.46 mg/g, which is very promising. It may still be less than that of lignin derivatives. For chitosan-modified magnetite, it even can reach the capacity for gold(III) of 707 mg/g [41]. The core-shell-modifier based adsorbent may not have such a high adsorption capacity. The modification step was done through the reaction between chitosan and polymeric Schiff's

After adsorption test, desorption of the adsorbed cation must also be examined. Complete desorption of the adsorbed cation indicates a better adsorbent performance. In most cases, the acids can desorb adsorbed ion from the surface. The desorption process may use strong acids

On the other hand, application of concentrated acid solution may damage the structure of adsorbent. Therefore, mildly acidic solutions are usually employed to release the cation from

, and HNO3. The cation is believed to form complex coordination bonds

that of adsorbent produced by the use of lignin as a precursor.

solution. Data presented here are based on the Langmuir isotherm.

138 Advanced Surface Engineering Research

adsorbent is reusable for the recovery of precious metals.

base of thiourea/glutaraldehyde in the presence of magnetite.

such as HCl, H<sup>2</sup>

SO<sup>4</sup>

with the surface, and leaching them is difficult.

**Figure 9.** Adsorption and desorption of AuCl<sup>4</sup> − ions by magnetite nanoparticles modified with a thiol group [3].

of [AuCl<sup>4</sup> ]− ion by magnetite modified with a thiol group and desorption. The desorption is done by applying thiourea in HCl solution. The concentration of the thiourea is low. The thiol group may form a covalent coordination bond with [AuCl<sup>4</sup> ] − ion on the adsorbent surface.

Purification of the recovered metals may be done through well-known processes. Electrochemical process is the best choice of metallurgy. Other methods by the use of chemical reduction could also be selected. In the large scale, we can consider blast furnace combined with a redox reaction. One important point to consider, the use of environmentally friendly

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

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141

The conventional metal reprocessing uses chemicals that are not environmentally friendly. The magnetite-based adsorbents offer technology that can reduce the application of toxic chemicals. The adsorbents give the possibility to reduce, reuse, and recycle for a few times. The magnetic core of the adsorbent is also readily synthesized with environmentally benign precursors. The coating with silica protects against acid and base media during application and recycle. The silica coating can also facilitate the attachment of the functional groups,

The current advanced electronic devices utilize the precious metals in their important components. The waste of electronic devices grows rapidly along with an increase in smartphone and PC use. Computer parts like processors, memories, motherboards, hard drives, and CD/DVD drives contain gold and other precious metals such as silver, palladium, and so on. The conventional gold recovery process uses cyanide ions for complex ion formation and electrolysis. The current technology attempts to recover gold and other precious metals from computers' and smartphones' components by utilizing magnetite nanoparticles. The new magnetic materials are effective yet environmentally friendly to recover precious metals. The magnetic adsorbents could also be the future of reclaiming precious metals from the waste of the other industries.

In the magnetic adsorbent development, the magnetite core could be possibly substituted with other oxides of transition metals such as manganese, cobalt, or nickel if they maintain strong magnetic characters. However, silica is the main choice for easy coating of the magnetic core, which also helps protect the magnetic core from dissolution in the acidic and basic media. The presence of the ligands on the surface of the magnetite-silica core-shell is critical for adsorption process. The environmentally safe polymers and simple molecules may be used to facilitate coordination bond with the target cations. The desorption process must be done using suitable solutions. The solution for desorption should leave the adsorbent in good shape for further reuse and turnover. The present technology available for purification of the

recovered metals may apply electrochemical, chemical, and thermal processes.

reprocessing of the metals must always be prioritized.

**8. Future recommendation and direction**

which is critical in the modification step.

**Symbols and abbreviations**

AFM atomic force microscopy

APTMS aminopropyl trimethoxysilane
