Amine-functionalized silica particles with magnetic core as magnetically removable adsorbents of Ag(I) ions

Abstract

Adsorption of silver ions over amine-functionalized silica particles with magnetic core has been studied. This adsorbent has been synthesized via sol–gel method, in the presence of Fe₃O₄ particles as magnetic cores. As a comparison, magnetic silica adsorbent with non-functionalized surface was also synthesized. As-obtained materials have been characterized by elemental analysis, acid–base titration, nitrogen sorption measurements, X-ray photoelectron spectroscopy and scanning electron microscopy. What is more, parameters of electrical double layer at the silica adsorbent/electrolyte solution interface were also examined. Kinetic of adsorption and isotherms of silver ions were determined. The obtained adsorption of Ag(I) ions for the studied materials was in the range of about 0.2–0.3 mmol/g.

Introduction

Silver has been known from antiquity. On one hand, nanosilver has many applications, in which antibacterial is the most popular; on the other hand, its increasing use in many fields of science and industry introduces this element into the natural environment (Benn and Westerhoff, 2002), contaminating hydro- and geosphere. Here, we present results of silver adsorption onto silica materials with magnetic core. Such materials could be used as magnetically removable adsorbents for metal ions from waters and wastewaters or as controlled delivery systems transporting active substances to a proper place in a living organism.

Experimental

Synthesis of adsorbents

The magnetite nanoparticles, playing role of magnetic core, were synthesized according to precipitation of ferric and ferrous salts in a basic medium, at ambient atmosphere (Ma et al., 2006). In order to create silica layer onto as-synthesized magnetite nanoparticles, sol–gel method was employed (Mel’nik et al., 2012; Melnyk et al., 2016). This method is based on the hydrolysis and co-condensation of tetraethyl orthosilicate and a proper organosilane-containing amine-functional groups ((3-Aminopropyl)triethoxysilane), in the presence of catalyst. In this work, two adsorbents were synthesized: properties of functionalized with amine groups material (A) were compared with its pristine counterpart (S). Material with non-functionalized surface (S) was synthesized similarly like material A, excluding the addition of organosilane-containing amine groups.

Instrumental characterization

The elemental analysis (EA) was carried out using the CHN 2400 analyzer (Perkin Elmer, USA). Nitrogen adsorption/desorption isotherms were measured at –196℃ using Nova 1200e analyzer (Quantachrome Instruments, USA). All samples were degassed at 120℃ in a vacuum, prior to measurement. Brunauer–Emmett–Teller (BET)-specific surface areas (S_BET) were evaluated in the range of relative pressures (p/p₀) of 0.05–0.20. X-ray photoelectron spectroscopy (XPS) spectra were obtained using a multi-chamber Analytical System (Prevac, Poland) with a monochromatic Al-Kα radiation (1486.7 eV) (Gammadata Scienta, Sweden) and an X-ray power of 360 W. Scanning electron microphotographs of the samples were obtained by scanning electron microscope (Quanta 3D FEG; FEI, USA).

In order to determine zeta potential of studied samples, electrophoretic mobility was measured in ZetaSizer Nano ZS (Malvern Instruments, UK). All measurements were performed in diluted suspensions of adsorbents in NaNO₃ solutions, at different ionic strength and at the broad pH range. NaOH and HNO₃ were employed to adjust the pH values of the suspensions. In order to determine the zeta potential, three equations were used: Huckel’s, Henry’s and Smoluchowski’s (Gdula et al., 2016). The previous experiments have shown that in any case κ·a relation was higher than 100. Thus, Smoluchowski’s equation was used to calculate zeta potential.

Potentiometric titration was employed to determine pH_PZC of the synthesized materials. All measurements were carried out in 10⁻³mol/dm³ NaNO₃ solution. To determine pH_PZC, two experiments for each adsorbent were made. In each, different weighted amount of adsorbent was taken. Potentiometric titration measurements were carried out in a thermostated Teflon vessel at 25℃, in the presence of the nitrogen flow preventing the influence of CO₂. pH values were measured using a set of glass REF 451 and calomel pHG201-8 electrodes with the Radiometer assembly.

Adsorption experiments

The adsorption measurements were also performed in the potentiometric titration set by labelled of isotope ¹¹⁰Ag. Radioactivity of the solutions before and after adsorption process was measured using the liquid scintillator counter 2480 Automatic Gamma Counter (Perkin Elmer, USA). Adsorption process was studied in the range of the initial Ag(I) ions (derived from AgNO₃) at concentrations: 0.001, 0.01, 0.1 and 1 mmol/dm³, as a function of pH. Adsorption kinetic was carried out at initial concentration of Ag(I) ions equal 1 mmol/dm³. Based on the changes of the radioactivity before and after adsorption, density of ions adsorption (Γ) on the silica surface was calculated, according to the equations: $\Gamma =\frac{{\text{c}}_0·\text{v}}{\text{m}·{\text{S}}_{\text{BET}}}·(\frac{{\text{N}}_{\text{r}}}{{\text{N}}_0})$ and ${\text{C}}_{\text{r}}{=\text{C}}_0·\frac{{\text{N}}_{\text{r}}}{{\text{N}}_0}$ , where: c₀—initial concentration (mol/dm³); V—solution volume (dm³); m—weighted amount of adsorbent (g); S_BET—specific surface area (m²/g); N_r—number of counting from the source collected during titration; N₀—number of counting from the source collected before adsorption; c_r—equilibrium concentration (mol/dm³). The separation of solution from adsorbent was carried out using a permanent magnet.

Results and discussion

It can be seen that the synthesized magnetite particles are almost spherical and rather monodisperse (Figure 1(a)). The dimension of magnetic particle core is approximately 60 nm. The process of surrounding the magnetic cores by silica shell leads to decrease spherical shape and causes higher polydispersity of the core/shell silica particles (Figure 1(b) and (c)). Such phenomenon could be explained that the generating silica shell on the surface of magnetic cores covers not only single magnetite particle but also its aggregates formed due to strong magnetic interactions between Fe₃O₄ particles.

Figure 1.

SEM microphotographs of the synthesized materials: (a) magnetite, (b) material A and (c) material S (bar length: 400 nm).

The content of nitrogen in the synthesized samples was determined quantitatively by EA. Moreover, the amount of amine groups present on the silica surface was determined by acid–base titration and XPS. Results obtained through above-mentioned methods are presented in Table 1.

Table 1.

Surface characterization of the silica materials.

Sample	Organosilane used in the functionalization	EA (%N)	Acid–base titration (camine gr. (mmol/g))	XPS (%N)		SBET (m2/g)
Fe3O4	–	–	–	–		100
S	–	0.38	0.00	0.0		99
A	APTES	3.66	1.83	5.1	71.1a	144
					28.9pa

a: N in the amine groups; pa: N in the protonated amine groups; EA: elemental analysis; XPS: X-ray photoelectron spectroscopy; S_BET: Brunauer–Emmett–Teller-specific surface areas; APTES: (3-Aminopropyl)triethoxysilane.

It can be seen that the material A exhibits high content of amine-functional groups. The presence of nitrogen in the sample S measured by EA can be explained by the presence of residues of NH₄OH and NH₄F after synthesis.

The changes in porosity of the synthesized silica adsorbents, as well as those for pristine Fe₃O₄ particles, were monitored by nitrogen sorption measurements. Shape of isotherms (not shown here) is similar to the type IV according to the IUPAC classification (Sing et al., 1985). The values of the specific surface area (S_BET) of all synthesized materials are given in Table 1. It can be seen that functionalization process leads to increase the values of S_BET. The surrounding of magnetite particles by silica shell has no effect on S_BET values.

Potentiometric titration and elecrophoretic mobility measurements were employed in order to determine parameters of electrical double layer. Curves of zeta potential as a function of pH for the sample A and S are presented in Figure 2.

Figure 2.

Zeta potential at the magnetic polysiloxane ((a) A and (b) S)/electrolyte solution interface, at different ionic strength of electrolyte (NaNO₃): 10⁻², 10⁻³, 10⁻⁴ and 10⁻⁵mol/dm³.

It can be seen that the concentration of NaNO₃ has almost no influence on the values of zeta potential for the both studied samples. It was found that zeta potential for material A (Figure 2(a)) throughout investigated pH range is positive, unlike to the material S (Figure 2(b)). In the case of material A, the colloid system is stable at pH range from 3 to 8, because values of zeta potential at this pH range were greater than +30 mV. Different situation can be observed in the case of material S. This colloid system has narrower stability range, limited only for pH range from 6 to 9.

Figure 3 presents curves of potentiometric titration of the magnetic polysiloxane, for two different weighted portion of each adsorbent: 15 and 10 m²/g.

Figure 3.

Curves of potentiometric titration of the magnetic polysiloxane A (a) and S (b), of different weighted portion of adsorbent: 15 and 10 m²/g, and for pure NaNO₃.

From the graphs shown in Figures 2 and 3, the values of pH_IEP and pH_PZC for the studied samples could be determined (Table 2). The value of isoelectric point of the silica-coated magnetite nanoparticles (S) is approximately 2.5, very close to that of pure silica (Franks, 2002). The values of pH_PZC and pH_IEP obtained for the material S are characteristic for silica materials, and this fact proves that magnetic cores have been successfully covered by SiO₂. In turn, the values of pH_PZC and pH_IEP for the material A prove of its surface functionalization with amine groups.

Table 2.

Determined values of pH_IEP and pH_PZC.

Sample	pHIEP	pHPZC
A	6.0	6.5
S	2.5	4.2

Increasing pH value results in increasing adsorption of Ag(I) ions, achieving maximum at pH = 8–8.5 (data not shown here). Such phenomenon can be explained, that at such conditions amine functional groups present on the silica surface (A) are in –NH₂ form, while silver is presented as Ag⁺ ions. Thus, electrostatic interactions between adsorbent surface and adsorbate ions can exist. Despite the lack of amine groups at the sample S surface, adsorption of silver ions is also present. At higher pH values, silanol groups are in the form –SiOH (present on both surfaces—material A and S) and can also provide adsorption sites.

Adsorption equilibrium is reached after about 3 and 24 h in the case of sample A and S, respectively (Figure 4(a)). Kinetic adsorption parameters obtained by using pseudo-first- and second-order models are presented in Table 3. In the case of sample A, better fit was obtained by using pseudo-second-order model, in contrast to the sample S.

Figure 4.

Adsorption kinetic curves (a) and adsorption isotherms (b) for the samples: A and S.

Table 3.

Kinetic adsorption parameters obtained by using pseudo-first- and second-order models.

Sample	Pseudo-first-order  $[\frac{1}{min}]$		Pseudo-second-order  $[\frac{1}{min}(\frac{g}{\text{mmol}})]$
	k1	R2	k2	R2
A	0.0011	0.6784	0.2157	0.9983
S	0.0016	0.9674	0.0058	0.7724

Conclusions

Silica particles with magnetic core were synthesized and used in adsorption process of silver(I) ions from water solutions. Pristine and amine-functionalized polysiloxanes were characterized in order to describe their adsorption properties. It was found that functionalization step improve their properties. Material A is characterized by better adsorption properties than material S. Adsorption kinetics for both studied materials were very different, and equilibria were obtained after ca. 3 and 24 h, respectively, for A and S material. Both adsorbents provide big amount of adsorption sites for Ag(I) ions. The obtained results prove the usefulness of silica particles with magnetic core for removing silver ions from waters and wastewaters.