- Nano Express
- Open Access
Magneto-Sensitive Adsorbents Modified by Functional Nitrogen-Containing Groups
© Melnyk et al. 2016
- Received: 30 November 2015
- Accepted: 21 January 2016
- Published: 3 February 2016
In order to obtain amino-functionalized silica materials with magnetic core, one-step synthesis was carried out. Several materials, differ in number and structure of amino groups, were synthesized on the basis of sol-gel method. The synthesized materials were examined by several analytical techniques. The presence and content of amino groups were measured by using Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy and acid-base titration, respectively. Specific surface areas were measured by nitrogen/adsorption desorption isotherms. It was proved that sol-gel approach leads to obtain materials with high content of amino groups built into their surfaces (in the range 1.6–2.7 mmol/g). As-obtained materials were tested as potential adsorbents for copper(II) ions. The received maximum adsorption capacities were in the range 0.4–0.7 mmol/g.
- Magnetite nanoparticles
- Polysiloxane shells
- Nitrogen-containing functional groups
- Copper(II) ions adsorption
A number of industrial enterprises have increased significantly with the simultaneously development of society. Consequently, a lot of hazardous compounds, such as heavy metal ions or pharmaceuticals, enter into the natural environment. Thus, there is a need to remove those substances from natural environment. Therefore, modern methods which are characterized by high efficiency, environmentally safe and are relatively inexpensive, should be proposed to remove such substances, especially from water and wastewaters. To separate heavy metal ions from aqueous solutions, a lot of methods based on using nanocomposites of magnetite are used [1, 2]. Many papers are devoted to the removal of copper ions [3–5], lead [4–6], cadmium [4, 5], and so on. From the one hand, using the magnetic nanoparticles coated with functionalized silica materials allows to adsorb onto their surface many organic or inorganic compounds; on the other hand, separation of adsorbent with the adsorbed compounds from solution can be easily carried out by using permanent magnet.
Composition of initial reactants and properties of magneto-sensitive adsorbents
Molar ratio of components
C f.gr., mmol/g
S sp., m2/g
Chemicals and Reagents
Iron(II) chloride, FeCl2∙4H2O (Sigma-Aldrich, 99 %); iron(III) chloride, FeCl3∙6H2O (Sigma-Aldrich, 98 %); ammonium hydroxide, NH4OH (Aldrich, 25 % aqueous solution); tetraethoxysilane, Si(OC2H5)4 (TEOS, Aldrich, 98 %); 3-aminopropyltriethoxysilane, (C2H5O)3Si(CH2)3NH2 (APTES, Aldrich, 99 %); methyltriethoxysilane, (C2H5O)3SiCH3 (MTЕS, Aldrich, 99 %); n-propyltriethoxysilane, (C2H5O)3Si(CH2)2CH3 (PTES, Fluka, 97 %); N-[3-trimethoxysilylpropуl]ethylendiamine, (CH3O)3Si(CH2)3NH(CH2)2NH2 (TMPED, Aldrich, 97 %); bis[3-(trimethoxysilylpropyl)]amine [(CH3O)3Si(CH2)3]2NH (BTMPA, Aldrich, 90 %); ethanol, C2H5OH (EtOH, 96 %); and ammonium fluoride, NH4F (analytical grade, Reahim, Ukraine) were used as received, without further purification.
Copper(II) nitrate, Cu(NO3)2∙3Н2О (Merck, 99.5 %). Ammonium chloride, NH4Cl; sodium nitrate, NaNO3; sodium chloride, NaCl (chemically pure, Macrochem, Ukraine). Nitric acid, HNO3; hydrochloric acid, HCl; sodium hydroxide, NaOH; ethylenediaminetetraacetic acid EDTA, C10H16N2O8 (fixanal concentrates, Cherkasy State Chemical Plant, Ukraine). Methyl orange, C14H14N3NaO3S (analytical grade, Reahim, Ukraine); and murexide, C8H8N6O6 (analytical grade, Reahim, Ukraine) were used.
Each of examined amino-functionalized silica adsorbents with magnetic core was synthesized on the basis of sol-gel procedure, which is detailed described in papers [10, 11]. The magnetite nanoparticles were obtained by co-precipitation of iron(II) and (III) salts in a basic medium, reported by . Briefly, iron chlorides(II) and (III) (in molar ratio: Fe2+/Fe3+ = 1/2) were dissolved in 450 mL of distilled water at 80 °C, under nitrogen flow. Next, 50 mL of ammonia solution was slowly added to the mixture. The black magnetite precipitate was produced in a few seconds and kept under 80 °C and mechanical stirring, during 30 min. After this time, the heating was switched off, and in 10 min more, the stirrer was also switched off. After cooled to the room temperature, the magnetic nanoparticles were separated from solution by decantation on permanent neodymium magnet. In order to remove unreacted reactants, the magnetite particles were cleaned by repeated cycles of water, to obtain final pH = 6.
In order to prepare silica layers onto as-prepared magnetite nanoparticles, sol-gel procedure was employed. This method is based on hydrolysis and condensation of TEOS and a proper functional monomer (having amino groups), in the presence of catalyst. In order to prepare various materials differing in both, type and amount of amino groups built in the adsorbent structure, and presence of additional aliphatic chain, five different materials were synthesized (three single- and two bifunctional). The resulting samples were labeled as follows: Fe3O4/TEOS/APTES—A, Fe3O4/TEOS/APTES/MTES—AM, Fe3O4/TEOS/APTES/PTES—AP, Fe3O4/TEOS/TMPED—DA, and Fe3O4/TEOS/BTMPA—BA. Briefly, 0.75 g of magnetite nanoparticles was placed into the three-neck flask and dispersed in 62.5 mL of distilled water. To ensure better particle dispersion, ultrasound treatment was used. In a separate beaker, calculated amount of a proper functional monomer was mixed with ethanol (3 mL) and ammonium fluoride (1.9 mL of 1 % aqueous solution), acting as a catalyst role. After 5 min, the monomer solution was added to magnetite nanoparticles and stirred during 5 min. Next, 11.3 mL of TEOS was added dropwise. The stirring was continued during 6 h. After this time, the reaction was completed. In order to remove unreacted compounds and impurities, the amino-functionalized silicas with magnetic core were cleaned by repeated cycles of decantation on the magnet and redispersion in distilled water, (3 × 50 mL) and ethanol (2 × 50 mL). As-prepared adsorbents were dried overnight in the oven at 100 °C. In the case of bifunctional samples (AM and AP), mixtures of initial monomers were mixed with ethanol in two separate beakers.
The diffuse reflectance infrared Fourier transform (DRIFT) spectra were recorded on the Thermo Nicolet Nexus FT-IR at 4 cm−1 resolution, using the special thermal vacuum adapter “Collector II” at 100 °C. The samples were mixed with KBr (1:20).
The nitrogen adsorption/desorption isotherms for all the samples were measured on the “Kelvin-1042” adsorption analyzer (Costech Microanalytical). Before the measurements, the samples were degassed at 110 °C, in the helium atmosphere. The BET specific surface area  was evaluated in the 0.03–0.35 range of relative pressures.
The content of amino groups was determined by acid-base titration. Batches of the samples (0.05 g) were treated with a 0.1 M HCl solution (20 mL) for 6 h. The precipitates were removed by a magnet, and supernatant was titrated with 0.1 M NaOH, in the presence of indicator (methyl orange) . Concentration of the amino groups was determined from the difference between the content of protons in solution before and after sorption.
Cu(II) Adsorption Experiments
The magnetite nanoparticles (Fe3O4) were obtained by co-precipitation of iron(II) and (III) salts in a basic medium, under a nitrogen atmosphere . Five different magnetic amino-functionalized silica-based materials were synthesized and tested as potential adsorbents for copper(II) ions. Each of examined adsorbents was synthesized on the basis of sol-gel procedure, which is detailed described in papers [10, 11]. In order to compare sorption measurements, three different monomers containing different number and structure of amino groups were used to modify surface of magnetic silica materials (Table 1).
The presence of functional groups was confirmed by IR spectroscopy, and their content was calculated using acid-base titration (Table 1). The results obtained by acid-base titrations are very close to those published previously [10, 11]. The small differences in the content of amino group values could be attributed by the nature of sol-gel procedure, in which a number of synthesis factors can influence on the properties of the final material. Therefore, such small deviations may fit in a limit of error. What is more, the obtained materials were examined by nitrogen adsorption/desorption measurements. In the Table 1, values of specific surface area (S sp) are presented. As before, the values of S sp are very similar, when compared with the previous results [10, 11]. Small deviations in the values of S sp may be caused by measurement error or small differences in the structure of materials, obtained by sol-gel method.
IR results, obtained from IR spectroscopy, clearly show that the surface of the obtained Fe3O4 particles is coated by polysiloxane layers with amino-containing functional groups.
Kinetic adsorption parameters obtained by using pseudo-first- and pseudo-second-order models for Cu(II) adsorption
ln (a eq − a t ) = ln a eq − k 1 t
t/a t = 1/(k 2 ∙ a eq 2) + t/a eq
0.039 ± 0.01
0.381 ± 0.038
0.122 ± 0.039
0.045 ± 0.001
0.268 ± 0.009
0.122 ± 0.048
0.026 ± 0.005
0.476 ± 0.047
0.307 ± 0.047
0.014 ± 0.001
0.54 ± 0.003
1.996 ± 0.599
0.006 ± 0.001
0.73 ± 0.114
0.061 ± 0.031
Parameters of copper(II) adsorption calculated from Langmuir and Freundlich isotherm models
C eq/a eq = 1/(K L ∙ a m) + (1/a m) ∙ C eq
lg a eq = lg K F + (1/n) ∙ lg C eq
a m, mmol/g
K L, L/mmol
In addition, DRIFT spectra were obtained and analyzed before and after adsorption of copper(II) ions (Fig. 1). As it is seen from the IR spectra, at the 3150–3250 cm−1 region, there are visible absorption bands (even when heated sample), characteristic for stretching vibrations of coordinated 3-aminopropyl groups. An intense absorption band at ~1371 cm−1 relating to fluctuations anion NO3 − can be observed . It is worth to noticed that, in the case of samples with adsorbed Cu(II) ions, an absorption band of deformation vibrations of amino groups is shifted toward to the lower frequency region.
It is known that copper(II) ions can form complexes with amino groups as rule 1:2. If in our case we suggest the formation of such complexes, we can calculate, for all samples, that the part of groups is not involved in complexation, but they are still available for protons. It could be connected with the particles specific structure with the surface layers and groups, located on their surface.
Comparative characteristic of the magnetite/amine-containing composites
Adsorption properties of magneto-sensitive materials in relation to copper(II) ions were studied. It was shown that the amount of adsorbed metal ions depends on the number of functional groups present in the structure of the adsorbent, as well as on their nature. The best adsorption properties showed material functionalized with monomer having secondary amine group (BA), while the fast adsorption kinetic shows sample functionalized with ethylenediamine (DA). Thus, the synthesized materials can be used as efficient adsorbents of Cu(II) ions from aqueous solutions.
IM is grateful to Programme SASPRO 3rd call, grant agreement no. 1298/03/01, and KG is grateful to People Programme (Marie Curie Actions) of the FP7/2007-2013/ under REA, grant agreement no. PIRSES-GA-2013-612484 for the financial support of the work.
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