Coating of magnetite with mercapto modified rice hull ash silica in a one-pot process
© Nuryono et al.; licensee Springer. 2014
Received: 7 February 2014
Accepted: 20 August 2014
Published: 11 September 2014
In this research, mercapto-silica coated magnetite (Fe3O4-SiO2-SH) has been prepared in aqueous solution through a simple approach so called a one-pot process. The Fe3O4-SiO2-SH was prepared in nitrogen condition by mixing magnetite, 3-mercaptopropyltrimethoxysilane (MPTMS), and sodium silicate (Na2SiO3) solution extracted from rice hull ash, and adjusting the pH of 7.0 using hydrochloric acid. The residue was washed with deionized water, dried at 150°C and separated with an external magnetic field. In that work, the volume of MPTMS and Na2SiO3 was varied and the total amount of Si represented as silica was kept constant. Characters of the material including the functional group presence, the structure, the porosity, the morphology and stability toward various solvents were identified and evaluated. Results of characterization indicated that mercapto-silica has been coated magnetite particle with a simple one-pot process. Coating mercapto-silica on magnetite increases particle size, surface area, and chemical stability. Additionally, Fe3O4-SiO2-SH also shows high stability toward various organic solvents. The magnetic property of magnetite does not change after coating and the addition of nonmagnetic material still gives high value of maximum saturation magnetization. The presence of mercapto groups effective for interaction with heavy metal ions, the high chemical stability without removing the magnetic property promises the prospective application of Fe3O4-SiO2-SH in the future such as for separation and removal of heavy metal ions from aquatic environments.
KeywordsMagnetite Silica Rice Coating One-pot process
Among the magnetic materials, iron oxides play a major role in many areas of chemistry, physics and material sciences. In particular, magnetic iron oxides such as magnetite (Fe3O4) and maghemite (γ-Fe2O3) have been investigated intensively for environmental and bio-applications (Daniel-da-Silva et al. 2007, 2008; Rebolledo et al. 2008; Hong et al. 2008; Li et al. 2007). Magnetite based materials with high magnetic characteristic are very effective as adsorbents for heavy metal ions removal. Superparamagnetic particles adhered to the target can be removed very quickly from a matrix using a magnetic field, but they do not retain their magnetic properties when the field is removed (Yantasee et al. 2007). However, it should be pointed out that uncoated magnetic nanoparticles are highly susceptible to oxidation when exposed to atmosphere and also susceptible to leaching under acidic conditions (Ren et al. 2008; Mahmoudi et al. 2011). In addition to convenient magnetic properties and low toxicity and price, Fe3O4 exhibit high surface to volume ratios, depending on the particle size, which associated to their ability for surface chemical modification can enhance the capacity for heavy metal adsorption in water treatment processes.
Inorganic polymers, such as silica, have been used as stabilizing agents for iron oxide and the silica coating has attractive properties including high biocompatibility (Mahmoudi et al. 2011), adsorption capacity, acid–base properties, insolubility in most solvents, and chemical and thermal stability (Yang 2003). In addition, silica can be grafted with a variety of functional groups, leading to considerable enhancement of their surface properties. Surface modification achieved by the attachment of inorganic shells or/and organic molecules not only stabilizes the nanoparticles, eventually preventing their oxidation, but also provides specific functionalities that can be selective for ion uptake. This system also has several advantages compared with conventional and other adsorbents in that the process does not generate secondary waste and the materials involved can be recycled and facilely used on an industrial scale. Furthermore, the magnetic particles can be tailored to fix and separate metal species in water, wastes, or slurries (Ren et al. 2008; Ngomsik et al. 2006; Li et al. 2008; Hu et al. 2005; Yavuz et al. 2006; Hai et al. 2005; Chang and Chen 2005; Liu et al. 2008; Zhou et al. 2009).
Modification of silica coated magnetite may be carried out into two steps namely coating silica on magnetite and functionalization on the silica coated magnetite (Lin et al. 2011; Shishehbore et al. 2011). The latter step is conducted by reacting silanol groups on the silica surfaces with organic compounds contaning silane groups at a high temperatur and in water free solvent to prevent hydrogen bonding that may marker the silanol groups. Nowaday, that non-green process has to be avoid. As the silica sources, organosilane agents such as tetraethoxyorthosilane (TEOS) are normally used (Yang et al. 2009; Pang et al. 2012). Using this precursor, two steps (hydrolysis and condensation) are involved in silica gel formation. However, using sodium silicate solution as the precursor, addition of acid to the solution results in the silica gel formation without hydrolysis step as reported by Lin et al. (2011). Sodium silicate that may be produced by treating rice hull ash with sodium hydroxide has been reported as precursor for preparation of ionic imprinting amino modified silica (Sakti et al. 2013), and sulfonato modified silica (Azmiyawati et al. 2012; Sulastri et al. 2011). Rice hull ash is a solid waste of agricultural products potential as row material for preparation of new silica based materials due high content of silica (80–90%) (Sakti et al. 2013).
This paper reports a simple and green approach of coating magnetite with mercapto modified silica in aqueous solution using sodium silicate solution made of rice hull ash as the precursor and 3-mercaptopropyltrimethoxysilane as the mercapto group source. Additionally, chemical reactions that occur in the aqueous solution during coating process are proposed, and the effect of coating on the magnetite properties and stability toward various types of solvents are evaluated.
Materials and methods
Chemicals used included FeCl2.4H2O, FeCl3.6H2O, HCl 37%, and NH4OH 25% supplied from Merck as receipted without any prior treatment for preparation of magnetite (Fe3O4), and commercial Fe3O4 from Aldrich used for a control material. For coating the magnetite was used Na2SiO3 solution (13% SiO2) produced from treatment of rice hull ash and mercaptopropyltrimethoxysilane (MPTMS) from Merck.
Synthesis of magnetite
Magnetite, Fe3O4, was prepared using a simple chemical co-precipitation method reported anywhere. Typically, 2.0 g of FeCl2 · 4H2O and 5.2 g of FeCl3 · 6H2O were dissolved in 1 mL aqueous HCl (37%). The FeCl2 · 4H2O and FeCl3 · 6H2O aqueous solution was then added rapidly with 200 mL of deionized water and the solution was continuously sonificated under nitrogen for 1 h. Upon adding an aqueous NH4OH solution (25%, 15 mL), a distinctive black precipitate of Fe3O4 was formed immediately and the precipitate was kept overnight in a room temperature. The Fe3O4 was isolated and purified by centrifugation and then washed with water three to four times to remove excess NH4OH solution. The magnetite resulted was dried in an oven at 80°C for 2 h. The analog work was carried out with mixing technique of mechanical stirring.
Coating magnetite with mercapto modified silica
Variation of the coating material amount
Volume/amount of coating agents
Fe3O4- SiO2-SH (25:75)
Fe3O4- SiO2-SH (50:50)
Fe3O4- SiO2-SH (75:25)
Characterization of products
Characterization with fourier transform infrared (FT-IR) spectrophotometry
About 0.5 mg of product was homogenized with 200 mg of KBr powder and was converted into a pellet form with 2000 psi in pressure. The pellet was put in a sample cell and the absorbance was measured at a wave number range of 300–4000 cm-1.
The content of elements (C, H, N) was determined with a Yanaco CHN CORDER MT-6 Elemental Analyzer, and a Dionex Ion Chromatography was used to analyze the content of sulfur.
Identification of structure with X-ray diffraction (XRD)
In this characterization, the sample was grounded and put in a sample cell and analyzed with XRD. Cu kα radiation from 40 kV and 30 mA was applied to the sample with a 2θ range of 5–70° (scan speed of 5°/min).
Identification of morphological products
The morphologies of all products were examined using a transmission electron microscopy (TEM) (JEM 1400) with 120 kV power, and frame size 1024 × 1024 pixel.
The Brunauer-Emmett-Teller (BET) surface area analysis was conducted using the nitrogen adsorption-desorption method (GSA. type NOVA 1200) with degassing temperature of 300°C for 3 h.
Measurement of magnetization values
The magnetization values of the products were identified using vibrating sample magnetometer (Oxford) at the maximum external magnetic field of 1.2 Tesla at 25°C.
Evaluation of product stability toward acid and organic solvents
Resulted product (0.5 g) was mixed with 15 mL of HCl solution 1 M, shaked for 5 min and kept in a room temperature. From the mixture, 1 mL of the substrate was collected in one day interval for 5 days, and analyzed the content of dissolved iron with atomic absorption spectrophotometry. Additional work was carried out to examine the stability of the product by dispersing 10 mg of coated silica in 50 mL various solvents and stirring the dispersion for 1 min.
Results and discussion
Effect of mixing techniques on magnetite character
Yield of magnetite synthesized with two stirring techniques
Parameters of magnetite calculated from XRD pattern
Table 3 shows that m-Fe3O4 gives peak intensity lower and the crystallite size represented by smaller than u-Fe3O4. Energy of ultrasonic wave improves effectiveness of stirring, homogenous dispersion and results in the small size crystallite. Higher stirring rate means larger energy transfer to crystallite and resulting in smaller sizes. Yang et al. (2009) reported that single phase formation of magnetite with sonochemical technique takes time only 1 h and 16 h is needed if mechanical technique is applied.
In conclusion, the use of ultrasonic wave energy probable may improve homogeneity of the mixture and effectiveness of contact between reactant particles to form precipitate of Fe(II)/Fe(III) hydroxide, as well as crystalline structure. Therefore, magnetite synthesized with ultrasonic technique was used for the further experiments.
Characters of mercapto-silica coated magnetite
Yield of coated magnetite materials before and after washing with deionized water and fraction of coated material
Weight fraction of coated material (%)
Fe3O4- SiO2-SH (75/25)
Fe3O4- SiO2-SH (50/50)
Fe3O4- SiO2-SH (25/75)
Table 4 shows that washing decreases the weight of products, except for Fe3O4-SH where sodium silicate solution was not used. Sodium chloride may be produced from reaction between sodium silicate and HCl. This byproduct may be trapped in the coated magnetite as impurity. Washing with deionized water may dissolve that salt and may improve the purity of the coated magnetite. Therefore, by using sodium silicate as the silica source, washing of the precipitate is important step to find the coated magnetite with higher purity. By assumption no magnetite loosing during coating, the weight fraction of silica and mercapto group can be calculated and presented in the last column of Table 4.
Result of elemental analysis in magnetite samples
Content of element (%)
Coating mercapto-silica on magnetite results in the larger product weight and the increase is in-line with the amount of MPTMS added (Table 4). With constant weight of magnetite, the capability in mole to bond silicone from both sodium silicate and MPTMS is also constant. Since the molecular weight of MPTMS is larger than that of silica, inclining the MPTMS weight causes the increase of coated magnetite weight.
Before being coated, magnetite was acidified to form the active sites on the magnetite surface facilitating the interaction between the magnetite surface and reagent to be coated. Silica coated magnetite was carried out by adding 1 M HCl solution or 1 M NH4OH solution drop wise on a mixture of sodium silicate and magnetite to reach the pH of 7.0, and insoluble gel is formed. At pH 7.0, silicate anion from sodium silicate solution may form siloxane bonds. At low pH (acidic condition) magnetite can be dissolved and the silica formed was converted from SiO2 to Si(OH)4. At higher pH (alkaline) the siloxane bonds are broken to produce silicate anions (Kalapathy et al. 2002).
Addition of HCl in coating magnetite leads to protonation of oxygen atom on magnetite giving to lower negative charge density on Fe atom. It makes easily to be attracted by electron pair of siloxy group (Si-O-) from silicate anion to form Fe-O-Si group. The reaction of attachment of silica on magnetite can be modeled in Figure 2(b) (Durdureanu-Angheluta et al. 2008).
The presence of silane agent from MPTMS may involve the formation of siloxane bonding with silica bonded to magnetite and it leads to attachment of mercapto modified silica on magnetite. Attachment of mercaptopropyl groups on silica is initiated by hydrolysis of methoxy groups from MPTMS in basic solution to form silanol (Si-OH) groups and these groups may react with silicate anion coated on magnetite to form Fe-O-Si-O-Si-(CH2)3-SH (Figure 2(c)).
Functional groups of coated magnetite
In FT-IR spectra of magnetite (Figure 3(a)) is observed an absorbance band at 586 cm-1 corresponding Fe-O bond and it is attributed to formation of ferrite phase. Different from magnetite, FT-IR spectra of silica coated magnetite, Fe3O4-SiO2, (Figure 3(b)) shows pronounce changes, particularly at region of 1300–700 cm-1, indicating the presence of silica coating. The presence of silica coated on magnetite is shown with characteristic band at 463 cm-1 from bending vibration of Si-O-Si. Asymmetric bending vibration corresponding to Si-O-Si bonding is revealed with absorbance band at 1072 cm-1. Absorbance bands in coated magnetite FT-IR spectra around 1628–1636 cm-1 and 3441–3487 cm-1 come from bending and stretching vibration, respectively, of –OH groups from both Fe-OH and Si-OH. Stretching vibration of Si-O-H bonding results in an absorbance band at 960 cm-1. In FT-IR spectra of magnetite coated only with silica, absorbance of Si-OH vibration does not appear clearly due to overlap with broad band of stretching vibration from Si-O-Si.
In comparison to magnetite and silica coated magnetite, FT-IR spectra of mercapto-silica coated magnetite, Fe3O4-SiO2-SH(50:50), (Figure 3(c)) gives characteristic absorbance of propyl and mercapto groups from MPTMS. The C-H bonding of propyl groups results in absorbance at 2932 cm-1 corresponding to bending and asymmetric vibration of C-H. The presence of mercapto groups is identified by the appearance of bands at 694 and 879 cm-1 that can be assigned to asymmetric stretching of C-S and bending vibration of S-H. Weak bands at 2569 cm-1 in IR spectra of mercapto coated magnetite is additional proof of the presence of -SH groups.
Structure of coated magnetite
Parameters of coated magnetite materials calculated from XRD pattern
Table 6 reveals larger crystallite size of coated magnetite in comparison to that of without coating one indicating that coating on the magnetite surface is proved. Magnetite coated only silica (Fe3O4-SiO2) gives bigger crystallite size than that coated only MPTMS (Fe3O4-SH). It is probable due to the oligomerization of silica before coating on the magnetite surface. In contrast, the presence of mercapto propyl groups in MPTMS inhibits oligomerization reaction and result in the formation of thin one layer MPTMS coated magnetite. Combination coating of silica and MPTMS gives biggest crystallite size among the investigated samples. Oligomerization of silica combined with attachment of MPTMS on the silica is suspected as the factor affecting the crystallite size.
Morphology of coated magnetite
Surface area (m2/g)
Porous total volume (cm3/g)
Porous diameter average (Å)
Magnetic property of coated magnetite
Magnetic parameters of magnetites
Stability of coated magnetite toward various solvents
In this research, synthesis of mercapto-silica coated magnetite using sodium silicate solution prepared from rice hull ash as the silica source has been developed in aqueous solution through a simple and facile preparation approach called one pot process. This approach is rapid and does not require the addition of any surfactant to form mercapto-silica hybrid coated directly to magnetite. The presence of mercapto-silica on magnetite not only improves the stability of magnetite but also gives high potency as active sites effectively for heavy and hazardous metal ions. Material of magnetite coated with mercapto modified silica still shows magnetic property and can be attracted with external magnetic field. Therefore, it is expected that in the future mercapto-silica coated magnetite may be promoted as prospective adsorbent for simple separation of heavy metal ions from industrial waste water.
This work was funded by the Directorate of Research and Community Services, Directorate General of Higher Education (DP2M-DIKTI), Indonesian Ministry of Education and Culture through Research Grant Kerjasama Luar Negeri dan Publikasi Internasional No.180/SP2H/PL/Dit.litabmas/IV/2013.
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