Environ Sci Pollut Res DOI 10.1007/s11356-014-3085-3
ENVIRONMENTAL SCIENCE AND POLLUTION SENSING, MONITORING, MODELING AND REMEDIATION
Influence of aqueous environment on agglomeration and dissolution of thiol-functionalised mesoporous silica-coated magnetite nanoparticles Othman Hakami & Yue Zhang & Charles J. Banks
Received: 22 November 2013 / Accepted: 23 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract The purpose of the present research work is to investigate the stability and dissolution of magnetite (Fe3O4) nanoparticles (NPs) and thiol-functionalised mesoporous silica-coated magnetite NPs (TF-SCMNPs). The state of NPs in an aqueous environment was investigated under different pH conditions. Changes in the NPs’ mean diameter due to aggregation were measured over a specific time. The effects of contact time and pH on the dissolution of NPs were also investigated. In order to avoid possible aggregation, Fe3O4 NPs were coated with silica and functionalised further with thiol organic groups. These methods imparted excellent stability to magnetite NPs in an aqueous medium over a wide range of pH values with reasonable hydrodynamic size. The organic group bound magnetite NPs allowed these particles to circulate over a long time in the aqueous system, and particle aggregation and sedimentation did not occur. The trend of decreasing zeta potential was observed after grafting thiol onto the surface of the SCMNPs. The results also revealed that silica exhibited a noteworthy efficient in eliminating the pH dependence and enhancing the NP stability of SCMNPs and SH-SCMNPs in aqueous medium. On the other hand, the dissolution of Fe3O4 NPs was found to be detrimental at pH 2.0 and 4.0 or had a long contact time.
Keywords Nanoparticles . Magnetite . Thiol . Silica . Agglomeration . Dissolution . Dynamic light scattering Responsible editor: Philippe Garrigues O. Hakami (*) : Y. Zhang : C. J. Banks Faculty of Engineering and the Environment, University of Southampton, Highfield, SO17 1BJ Southampton, UK e-mail: [email protected] O. Hakami Faculty of Science, Chemistry Department, University of Jazan, Gizan, Saudi Arabia
Introduction Since its discovery in 1992, micro and mesoporous nanomaterials have been the focus of scientific and technological interest. Due to their high surface area and high porosity, there has been a huge effort to show their potential for applications in many fields, such as materials science, molecular chemistry, medical and biomedical sciences, and bioengineering (Kanel et al. 2005). Nanoporous materials have numerous applications in environmental research, especially in the field of water and wastewater treatment, due to their extremely good catalytic properties and high reactivity. The rapid growth in nanomaterials has yielded numerous achievements in the field of water and wastewater engineering (Diallo et al. 2008), including the removal of heavy metals by adsorption. In the past few years, the special properties of magnetic nanoparticles (NPs) have been realised and utilised in the context of environmental remediation. By utilising the magnetic properties of these adsorbents, magnetic separation has been combined with adsorption for heavy metal removal from contaminated water at laboratory scales (Hu et al. 2004; Shipley et al. 2009). Generally, magnetic separation could solve many of the issues associated with conventional methods, such as filtration, centrifugation, or gravitational separation, as it requires much less energy to achieve a given level of separation. A stable suspension requires a dominant repulsive force to maintain the dispersion of the particles. If the attractive forces are dominant, or if the particles collide with sufficient energy to overcome repulsion, they will begin to flock and eventually form agglomerates that sediment. Typically, particle adhesion is irreversible because a large energy barrier prevents the separation (Elzey and Grassian 2010). In the adsorption process, certain factors might affect the stability of magnetic NPs in water. For example, the concentration of NPs in a water
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sample might influence the stability of the particles. Increasing the concentration of the particles might also lead to an increase in the probability of particle-to-particle interactions and inter-particle collisions (García-Otero et al. 2000). This could cause particle aggregation which would accelerate sedimentation. Time is another factor that could affect the stability of NPs and cause aggregation (Murdock et al. 2008; Mohapatra et al. 2007). Therefore, the stability of NPs in an aqueous suspension is an essential factor for the concept of adsorption, as the NPs must remain dispersed in the solution. The NPs should also stay suspended in the sample long enough for the adsorption process to be completed. Ligands or capping agents are chemicals (such as polymers and surfactants) used in the synthesis of NPs to prevent their aggregation through electrostatic repulsion, steric repulsion, or both (Pankhurst et al. 2003; Schladt et al. 2011). In the case of magnetite (Fe3O4) NPs, the most prevalent capping agents are cetyltrimethylammonium chloride (CTAC) (Wu et al. 2004; Mehendale et al. 2006), oleic acid (OA) (Zhang et al. 2006; Guardia et al. 2007), and long chain thiols (Neamtu et al. 2005). Iron oxides, in general, are compounds with very low solubility (Kraemer 2004) and are readily attacked by acids (Schwertmann 1991). Silicon oxide is relatively water insoluble compared to other minerals, and the solubility of amorphous silicon oxide in water is dependent on the pH range and the relative concentration. The solubility of amorphous silica can be enhanced under alkaline conditions due to the formation of monomeric and multimeric silicate (Stumm et al. 1967; Guthrie and Reardon 2008). Additional factors that influence the rate of dissolution include temperature, composition of the solution phase (e.g. pH, ions, ligands, and the concentration of acids), surface structure, structural modifications, and governing the dissolution of iron (Fe) and silica (Si) (Schwertmann 1991; Pham et al. 2011; Salmimies et al. 2011). The objectives of this study are to investigate the stability and the dissolution of magnetite NPs and thiol-functionalised mesoporous silica-coated magnetite nanoparticles (TFSCMNPs). The state of the NPs in an aqueous environment was investigated under different pH conditions. Changes in the NPs’ mean diameter due to aggregation were measured over a specific time. Moreover, the effects of contact time and pH on the dissolution of NPs were investigated.
Materials and methods Chemicals All the chemicals used in this work were of analytical reagent grade and were obtained from either Fisher Scientific Ltd (UK) or Sigma-Aldrich Co (UK), and were used as received without any further purification.
Synthesis of thiol-functionalised mesoporous silica coated magnetite nanoparticles A typical procedure for the preparation of TF-SCMNPs has been described in our previous work (Hakami et al. 2012) and is schematically shown in Fig. 1.
Instrumentation and analysis A bright field of a scanning electron microscope (SEM) (JSM6500F) was used at 15 keV to observe the morphology and particle size of the prepared NPs. The specimens for the SEM study were prepared by direct deposition of the magnetite NPs on an aluminium holder that was covered by a carbon grid, and then the specimens were sputter coated with gold using gold sputter coating (JEOL, 5010). Moreover, transmission electron microscopy (TEM) images were taken on a JEM 3010 (Japan Electron Co.). The particles were dispersed in ethanol by sonication for 45 min and then transferred to a copper grid (Carbon Films, 200 Mesh Cu Grids, AGAR). The TEM images were obtained using the bright field image mode at an acceleration voltage of 200 keV. The hydrodynamic diameter of the NPs was measured using a Zetasizer Nano ZS (Malvern Instruments Ltd, UK). Measurements were performed at 25 °C and analysed in multiple narrow modes (high resolution). For the data analysis, the refractive index (RI) of iron oxide of 2.30 was used, and the reading was carried out at a 173° angle with respect to the incident beam. NIBS software (V6.01) was used for data analysis. The mean particle diameter is directly calculated by the software from the particle distributions measured, and the resulting polydispersity index (PdI) is a measure of the size ranges present in the solution. The same instrument was also used to measure the zeta potentials (surface charge) of the NPs. The measurements were carried out at pH ranging 2–8, and the pH was adjusted to these levels using 0.1 M of HNO3 and NaOH. The Smoluchowski equation was used to extract the zeta potential ξ from the measured particle electrophoretic mobility μe: ζ¼
η μ ε e
where η and ε are the viscosity and the dielectric constant of the dispersion medium, respectively. A folded capillary cell (DTS1060) filled with 1 ml of the NPs’ suspension was used for the measurement. Dissolved Fe was measured using an inductively coupled plasma–atomic emission spectrometer (Varian Vista Pro ICPOES, UK) (detection limit=0.19 mg l−1), while an inductively coupled plasma–atomic emission spectrometer (Agilent
Environ Sci Pollut Res Fig. 1 Three stages in the production of TF-SCMNPs
7500ce ICP-MS) was used to measure the dissolved Si with a detection limit of 10 μg l−1. Stability and dissolution of TF-SCMNPs The stability of the TF-SCMNPs was examined by dispersing 8 and 32 mg l−1 (these concentrations were used for the removal and recovery of Hg(II) (Hakami et al. 2012)) of TFSCMNPs in deionised water at pH 6.0, and the stability of the suspensions was reviewed by measuring the hydrodynamic size distribution of the NPs over time using a Zetasizer Nano ZS (Malvern Instruments Ltd, UK). The hydrodynamic size distribution of TF-SCMNPs was also examined as a function of pH (2.0–9.0), and the pH was adjusted to the desired value using 0.1 M of HNO3 and NaOH. Low concentrations of salts such as NaCl and KCl were reported to be good solvents to avoid a Coulombic interaction between charged systems (polyelectrolytes) (Meunier et al. 2001; Ito et al. 2004).
Therefore, these solvents were used as a reference to compare with the results obtained using deionised water as the solvent. In this experiment, the pH of the system was adjusted to pH 6.0 and measured directly before transferring the samples into folded capillary cells and mixing via bath sonication and vortexing. The pH of the solution was also investigated as a major factor in the dissolution of Fe and Si: experiments were performed at a pH of 2.0, 4.0, 6.0, and 8.0 by shaking 50 ml of 8 mg l−1 NPs solutions (Fe3O4 NPs and TFSCMNPs) for 2 h at room temperature at a shaking speed of 200 rpm. After the dissolution reached equilibrium, the NPs were separated via an external magnetic field and the supernatant was collected to determine the total amount of Fe and Si in the solution using ICP-OES and ICP-MS, respectively. The effect of contact time on dissolution of Fe from NPs was conducted at pH 2.0 under the shaking speed of 200 rpm at room temperature.
Environ Sci Pollut Res Fig. 2 a TEM micrograph of Fe3O4 NPs and b SEM micrograph of TF-SCMNPs
a
In all cases, 8 mg l−1 of 50 mg ml−1 NPs aqueous dispersion was used as reference material to check how various factors (pH, time, and elution process) affected the dissolution of Fe and Si. All experiments were conducted at least in triplicate and the average values are presented in this paper.
Results and discussion SEM, TEM images, and particle size distribution The morphology and structure of Fe3O4 NPs and TFSCMNPS samples are clearly revealed by examining the SEM and TEM images. Figure 2a shows a typical TEM image for Fe3O4 NPs, where it can be seen that the Fe3O4 NPs are nearly spherical, with some demonstrating a hexagonal structure, as marked by the arrow. The particles appear to be aggregated, which is due to the absence of any stabiliser in the reaction system during the course of formation of Fe3O4 NPs. In the absence of any surface coating, Fe3O4 NPs have a hydrophobic surface, and due to hydrophobic interaction between the particles, the particles tend to agglomerate and form large clusters (Hamley 2003). Figure 2b shows nonFig. 3 DLS plot for the particle size distribution of Fe3O4 NPs, SCMNPs, and TF-SCMNPs
b
aggregated TF-SCMNPs and demonstrates many particles separated from each other. Only one single core is included in each particle, indicating the minimum aggregation during the synthesis of SCMNPs. The aggregation was minimised after using concentration of 75 mmol l−1 of CTAC aqueous solution as a surfactant stabiliser during the preparation process of mesoporous SCMNPs. The addition of CTAC aqueous solution as co-surfactants was used to alter the packing parameter, resulting in elongated CTAC micelles. The CTAC aqueous solution adheres to the surfaces in a substrate specific manner and may cause a reduction in the curvature of the surfactant aggregates. However, these effects are complex and their evaluation is not within the scope of this paper. As Fig. 3 shows, the measured hydrodynamic particle size distribution increased from an average diameter of ~75 nm for Fe3O4 NPs to ~105 nm after silica coating, and was found to be ~111 nm after 3-MPTMS functionalisation. It can be seen from Fig. 3 that the size distribution of the three types of NPs is a unimodal size distribution. The results of the dynamic light scattering (DLS) measurements demonstrate only one peak which suggests that the particles are mostly isolated rather than being aggregated in the solution and that the prepared NPs are highly mono-dispersed in aqueous media. The DLS results for NPs size in various types of solvents are presented in Table 1. Fe3O4 NPs, SCMNPS, and TF-
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Table 1 Particle size and polydispersity index (PdI) of NPs using various types of solvents
100
a
80
Media
DLS Average diameter (nm) (±SD) PdI (±SD)
Fe3O4 NPs
DW
0.1 M NaCl 0.1 M KCl SCMNPs DW 0.1 M NaCl 0.1 M KCl TF-SCMNPs DW 0.1 M NaCl 0.1 M KCl
75.52 (0.59)
0.14 (0.01)
75.60 (0.25) 76.45 (0.52) 105.57 (0.45) 106.23 (0.74) 106.20 (0.40) 111.30 (0.61) 111.27 (0.55) 111.20 (0.46)
0.16 (0.01) 0.18 (0.02) 0.06 (0.02) 0.06 (0.03) 0.05 (0.02) 0.06 (0.01) 0.07 (0.02) 0.06 (0.01)
SCMNPs exhibited nearly the same size when dispersed in water, 0.1 M NaCl, and 0.1 M KCl without any change or agglomeration size. The results of TF-SCMNPs and SCMNPs show nearly mono-dispersed particles (with PdI less than 0.09), whereas Fe3O4 NPs show midrange mono-dispersion. Zeta potential measurement The zeta potential of the Fe3O4 NPs, SCMNPs, and SHSCMNPs were all measured; these show that the magnitude decreased with increasing pH values, Fig. 4. SH-SCMNPs showed good stability at a range of different pH values, with a maximum zeta potential of 43.60 mV. In general, the magnitude decreased with increasing pH values but moderate stability was observed at pH 4.0 when the zeta potential was 20.01 mV. At pH 2.0, however, the magnitude decreased significantly to 2.52 mV, corresponding to a less stable suspension. The surface charge of Fe3O4 NPs remained positive at pH values below 4.8. The point of zero charge (PZC) of Fe3O4 NPs was found to occur at pH 4.8. This value is
40 20 0 120 100
b
0
100
200
300
400
500
80 -1 TF-SCMNPs @ 8 mg l
60 40 20 0 0
100
200
300
400
500
Time (Min.)
Fig. 5 The time-dependent aggregation of TF-SCMNPS: a 8 mg l−1 and b 32 mg l−1
significantly different from the IEP for the SCMNPs and SH-SCMNPs that occurred at pH 3.4 and 2.2, respectively. The negative surface charge of SCMNPs was to be expected due to the increased number of hydroxyl groups present on the surface of Fe3O4 NPs after silica coating; this was previously reported (Vaidya et al. 2011). In addition, the presence of silanol groups on the surface of SCMNPs (derived from TEOS) might play a key role in producing negative charges (Lee et al. 2011). A silanol group contains a hydrogen atom that can dissociate and produce a negative charge. Figure 4 shows that as the pH of the solution was decreased, more hydrogen ions were dissociated and produced negative zeta potentials. It was clearly noticeable that the zeta potential changed after modifying SCMNPs with thiol groups. The obvious change in the zeta potential further confirms the deposition of 3-MPTMS on the surface of the NPs. Similar to the silanol group, the thiol group of SH-SCMNPs contains a hydrogen atom that can dissociate and produce a negative zeta potential.
150 Fe3O4 NPs
20
SCMNPs TF-SCMNPs
0
-20
Average diameter (nm)
40
Zeta potential (mv)
-1 TF-SCMNPs @ 32 mg l
60
Average diameter (nm)
Particles
140
130
120
110 -40
0 2
3
4
5
6
7
pH Fig. 4 Zeta potential of Fe3O4 NPs, SCMNPs, and TF-SCMNPs
8
2
4
6
8
10
pH Fig. 6 Variation of the hydrodynamic sizes of TF-SCMNPs across a range of pH values
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a
1.0 TF-SCMNPs Fe3O4 NPs
6
Fe Conc. (mg L-1)
Fe Conc. (mg l-1)
0.8
0.6
0.4
4
2
0.2
Diss.=10.61%
< 0.19 mg L-1(MDL)
0
0.0 0
50
100
150
200
REF.
250
pH 2.0
Time (Min.)
pH 4.0
pH 6.0
pH 8.0
Samples
Fig. 7 Effect of contact time on the dissolution of Fe from Fe3O4 NPs and TF-SCMNPs at pH 2.0
3.5
b
The hydrodynamic size of TF-SCMNPs was measured against time. As can be seen in Fig. 5a, b, TF-SCMNPs showed a mean hydrodynamic size of ~111 nm with a PdI of less than 0.04; the average particle size distribution remained essentially constant for 8 h at different concentrations without obvious aggregation and any significant deterioration in the hydrodynamic average size. The stabilisation of TF-SCMNPs could be attributed to both the CTAC aqueous solution as a surfactant stabiliser during the preparation process of mesoporous SCMNPs and functionalisation with thiol organic groups; these findings are consistent with the TEM image of Fig. 2b. The hydrodynamic size distribution of SCMNPs and TF-SCMNPS were also measured across different pH ranges (2.0–9.0) as shown in Fig. 6. The average hydrodynamic size of the TF-SCMNPs at pH 2.0 and 3.0 were ~149 and ~136.90 nm, respectively. Changing the pH of the solution from 4.0 to 9.0 did not change the hydrodynamic size of the TF-SCMNPs significantly. Thus, TF-SCMNPs are mostly isolated and are not aggregated across a range of pH values. The agglomeration of particles at pH 2.0–3.0 agrees with the discussion in “Zeta potential measurement” section. The TF-SCMNPs are negatively charged in the pH range (3.0–8.0) and almost stable at neutral to basic pH values, as demonstrated by their zeta potential (Fig. 4). The agglomeration of TF-SCMNPs could be attributed to coagulation that occurs when the zeta potential is above the PZC (Lu et al. 2007). Tombacz et al. (1999) and Cornell and Schwertmann (2003) stated that in oxide suspensions, aggregation occurs at a pH range below or around pHpzc because the charge density is very low in this range. This could be used to describe the agglomeration of the TF-SCMNPs at pH 3.0. However, loose agglomerates are easily broken up by increasing the agitation and re-suspending times of the NPs.
2.5 2.0 1.5 1.0
-1 < 0.19 mg L (MDL)
0.5 0.0
REF.
pH 2.0
pH 4.0
pH 6.0
pH 8.0
Samples Fig. 8 Effect of pH on the dissolution of Fe from a Fe3O4 NPs and b TFSCMNPs
Dissolution of nanoparticles Figure 7 shows the result of the effect of contact time on the dissolution of Fe3O4 NPs and TF-SCMNPs at pH 2.0. The 700 600
Si Conc. (µg L-1)
Stability of TF-SCMNPs
Fe Conc. (mg L-1)
3.0
500 400 300 200 Diss.= 17.12% 100 0 REF.
pH 2.0
pH 4.0
pH 6.0
pH 8.0
Samples Fig. 9 Effect of pH on the dissolution of Si from TF-SCMNPs
Environ Sci Pollut Res FeOH+ FeOH+
Fig. 10 Schematic representation of magnetite NPs dissolution at different pH values
+
H
O
O
H+ HO
Si
Si +
FeOH FeOH+ O O Si Si + FeOH FeOH+ O O Si Si Si Si
At pH 4
H+
O O H+ FeOH+FeOH+
FeOH+ FeOH+ +
H
H+ +
At pH 2 H
H+
O
O
Si
Si
O
H+ FeOH FeOH+
dissolution of the NPs was confirmed by measuring the total residual concentration of Fe in the supernatant after the NPs were isolated using an external magnetic field. The concentration of Fe released from the TF-SCMNPs was below the sample detection limit of the ICP-OES, which is less than 0.19 mg l−1. The initial rate of dissolution of Fe3O4 NPs was low (0.22 mg l−1, ~ 3.69 % of the initial concentration), and it approached the equilibrium condition in 30 min with the dissolution of 0.69 mg l−1 (~11.65 % of the initial concentration). The effect of the contact time on the dissolution of Fe3O4 NPs and TF-SCMNPs at pH 6.0 (data not shown) was negligible. The concentration of iron from Fe3O4 NPs and TF-SCMNPs was less than 0.19 mg l−1. As discussed in our previous work (Hakami et al. 2012), the aim of coating the Fe3O4 NPs with dense liquid silica (DLSi) was to achieve high resistance to leaching in acidic media. The results confirmed that as the concentration of released Fe from the TF-SCMNPs was less than 0.19 mg l−1 at pH ranges from 2.0 to 8.0, there was no protection to the Fe3O4 NPs from dissolution in the acidic media in the absence of any metals dropped on the surface of the NPs. Leaching of sorbent components into the treated water is unfavourable. However, the effluent discharge limit of Fe into municipal sewers is 2 mg l−1 (WHO 2011), which is higher than the concentration of Fe released from the NPs. The effect of pH on the dissolution of Fe3O4 NPs and TFSCMNPs was also investigated and Figs. 8a, b and 9 show the results. The reference concentration of Fe and Si are also
+
FeOH+ FeOH+ O O Si Si + FeOH FeOH+ O O Si Si Si Si HO
+
4FeOH2+
OH
H+
FeOH+ FeOH+ + H O O Si Si FeOH+ FeOH+ + H O O Si Si Si Si O
OH Si
Si
HO Si
OH Si
H Si
OH Si
H Si
OH Si
Si HO
+
8FeOH2+
Si OH
presented in these figures. It was found that the Fe3O4 NPs are sensitive in the acidic media demonstrating the dissolution of 0.63 mg l−1 (10.61 %) of Fe. The dissolution was negligible with increasing pH, and the concentration of Fe was below the sample detection limit of ICP-OES. A small amount of Fe, less than 0.19 mg l−1, was detected over the pH range tested, which confirmed that the pH of the solution does not affect the dissolution of SH-SCMNPs, as Fig. 7b shows. An increasingly small amount of Si was dissolved into the solution as the pH values increased, as Fig. 9 shows. At pH 8.0, the concentration of Si released from the TF-SCMNPs into the solution was 116.50 μg l−1 (17.12 %). This result agrees with the findings of previous researchers (Guthrie and Reardon 2008; Pham et al. 2011) who confirmed that the presence of an alkaline environment enhanced markedly the dissolution rate of silica compared with neutral or acidic environments. The effect of pH on the dissolution was also investigated at lower pH values: 2.0 and 4.0. Figure 10 demonstrates that the dissolution of the supported magnetite depends on the concentration of the hydrogen ion present. At pH 4.0, the hydrogen ions can only interact with the surface of the magnetite leading it to leaching out the surface into the solution. However the magnetite inside the silica pores would not be reached by the hydrogen ions. On the other hand, at pH 2.0, the concentration of hydrogen ions is high enough to reach not only the surface magnetite but also the pores magnetite giving rise to more iron leaching into the solution.
Environ Sci Pollut Res
Conclusions The following conclusions can be drawn from the work carried out in this research work: &
&
&
Fe3O4 NPs were not mono-dispersed in size: their shape was spherical and approximately 75 nm in diameter. Through the co-condensation method, the obtained NPs were capped with nonpolar endgroups (thiol) on their surface that were approximately 111 nm in diameter, spherical in shape, and narrow in their distribution. The NPs obtained via the co-precipitation methods were not mono-dispersed in size: their shape was spherical and approximately 75 nm in diameter. Through the cocondensation method, the obtained NPs were capped with nonpolar endgroups (thiol) on their surface that were approximately 111 nm in diameter, spherical in shape, and narrow in their distribution. In the present work, in order to inhibit the dissolution of Fe3O4 NPs, two approaches for dissolution prevention were developed: (a) using dense liquid silica-coated magnetite NPs (DLSiC-Fe3O4 NPs) and (b) using silica coating Fe3O4 NPs via sol–gel reaction. The increased efficiency of the silica in eliminating the pH dependence and enhancing the NP stability of SCMNPs and SH-SCMNPs in the aqueous medium is noteworthy. On the other hand, the dissolution of Fe3O4 NPs was to found to be detrimental at pH 2.0 or had a long contact time.
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ENVIRONMENTAL SCIENCE AND POLLUTION SENSING, MONITORING, MODELING AND REMEDIATION
Influence of aqueous environment on agglomeration and dissolution of thiol-functionalised mesoporous silica-coated magnetite nanoparticles Othman Hakami & Yue Zhang & Charles J. Banks
Received: 22 November 2013 / Accepted: 23 May 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract The purpose of the present research work is to investigate the stability and dissolution of magnetite (Fe3O4) nanoparticles (NPs) and thiol-functionalised mesoporous silica-coated magnetite NPs (TF-SCMNPs). The state of NPs in an aqueous environment was investigated under different pH conditions. Changes in the NPs’ mean diameter due to aggregation were measured over a specific time. The effects of contact time and pH on the dissolution of NPs were also investigated. In order to avoid possible aggregation, Fe3O4 NPs were coated with silica and functionalised further with thiol organic groups. These methods imparted excellent stability to magnetite NPs in an aqueous medium over a wide range of pH values with reasonable hydrodynamic size. The organic group bound magnetite NPs allowed these particles to circulate over a long time in the aqueous system, and particle aggregation and sedimentation did not occur. The trend of decreasing zeta potential was observed after grafting thiol onto the surface of the SCMNPs. The results also revealed that silica exhibited a noteworthy efficient in eliminating the pH dependence and enhancing the NP stability of SCMNPs and SH-SCMNPs in aqueous medium. On the other hand, the dissolution of Fe3O4 NPs was found to be detrimental at pH 2.0 and 4.0 or had a long contact time.
Keywords Nanoparticles . Magnetite . Thiol . Silica . Agglomeration . Dissolution . Dynamic light scattering Responsible editor: Philippe Garrigues O. Hakami (*) : Y. Zhang : C. J. Banks Faculty of Engineering and the Environment, University of Southampton, Highfield, SO17 1BJ Southampton, UK e-mail: [email protected] O. Hakami Faculty of Science, Chemistry Department, University of Jazan, Gizan, Saudi Arabia
Introduction Since its discovery in 1992, micro and mesoporous nanomaterials have been the focus of scientific and technological interest. Due to their high surface area and high porosity, there has been a huge effort to show their potential for applications in many fields, such as materials science, molecular chemistry, medical and biomedical sciences, and bioengineering (Kanel et al. 2005). Nanoporous materials have numerous applications in environmental research, especially in the field of water and wastewater treatment, due to their extremely good catalytic properties and high reactivity. The rapid growth in nanomaterials has yielded numerous achievements in the field of water and wastewater engineering (Diallo et al. 2008), including the removal of heavy metals by adsorption. In the past few years, the special properties of magnetic nanoparticles (NPs) have been realised and utilised in the context of environmental remediation. By utilising the magnetic properties of these adsorbents, magnetic separation has been combined with adsorption for heavy metal removal from contaminated water at laboratory scales (Hu et al. 2004; Shipley et al. 2009). Generally, magnetic separation could solve many of the issues associated with conventional methods, such as filtration, centrifugation, or gravitational separation, as it requires much less energy to achieve a given level of separation. A stable suspension requires a dominant repulsive force to maintain the dispersion of the particles. If the attractive forces are dominant, or if the particles collide with sufficient energy to overcome repulsion, they will begin to flock and eventually form agglomerates that sediment. Typically, particle adhesion is irreversible because a large energy barrier prevents the separation (Elzey and Grassian 2010). In the adsorption process, certain factors might affect the stability of magnetic NPs in water. For example, the concentration of NPs in a water
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sample might influence the stability of the particles. Increasing the concentration of the particles might also lead to an increase in the probability of particle-to-particle interactions and inter-particle collisions (García-Otero et al. 2000). This could cause particle aggregation which would accelerate sedimentation. Time is another factor that could affect the stability of NPs and cause aggregation (Murdock et al. 2008; Mohapatra et al. 2007). Therefore, the stability of NPs in an aqueous suspension is an essential factor for the concept of adsorption, as the NPs must remain dispersed in the solution. The NPs should also stay suspended in the sample long enough for the adsorption process to be completed. Ligands or capping agents are chemicals (such as polymers and surfactants) used in the synthesis of NPs to prevent their aggregation through electrostatic repulsion, steric repulsion, or both (Pankhurst et al. 2003; Schladt et al. 2011). In the case of magnetite (Fe3O4) NPs, the most prevalent capping agents are cetyltrimethylammonium chloride (CTAC) (Wu et al. 2004; Mehendale et al. 2006), oleic acid (OA) (Zhang et al. 2006; Guardia et al. 2007), and long chain thiols (Neamtu et al. 2005). Iron oxides, in general, are compounds with very low solubility (Kraemer 2004) and are readily attacked by acids (Schwertmann 1991). Silicon oxide is relatively water insoluble compared to other minerals, and the solubility of amorphous silicon oxide in water is dependent on the pH range and the relative concentration. The solubility of amorphous silica can be enhanced under alkaline conditions due to the formation of monomeric and multimeric silicate (Stumm et al. 1967; Guthrie and Reardon 2008). Additional factors that influence the rate of dissolution include temperature, composition of the solution phase (e.g. pH, ions, ligands, and the concentration of acids), surface structure, structural modifications, and governing the dissolution of iron (Fe) and silica (Si) (Schwertmann 1991; Pham et al. 2011; Salmimies et al. 2011). The objectives of this study are to investigate the stability and the dissolution of magnetite NPs and thiol-functionalised mesoporous silica-coated magnetite nanoparticles (TFSCMNPs). The state of the NPs in an aqueous environment was investigated under different pH conditions. Changes in the NPs’ mean diameter due to aggregation were measured over a specific time. Moreover, the effects of contact time and pH on the dissolution of NPs were investigated.
Materials and methods Chemicals All the chemicals used in this work were of analytical reagent grade and were obtained from either Fisher Scientific Ltd (UK) or Sigma-Aldrich Co (UK), and were used as received without any further purification.
Synthesis of thiol-functionalised mesoporous silica coated magnetite nanoparticles A typical procedure for the preparation of TF-SCMNPs has been described in our previous work (Hakami et al. 2012) and is schematically shown in Fig. 1.
Instrumentation and analysis A bright field of a scanning electron microscope (SEM) (JSM6500F) was used at 15 keV to observe the morphology and particle size of the prepared NPs. The specimens for the SEM study were prepared by direct deposition of the magnetite NPs on an aluminium holder that was covered by a carbon grid, and then the specimens were sputter coated with gold using gold sputter coating (JEOL, 5010). Moreover, transmission electron microscopy (TEM) images were taken on a JEM 3010 (Japan Electron Co.). The particles were dispersed in ethanol by sonication for 45 min and then transferred to a copper grid (Carbon Films, 200 Mesh Cu Grids, AGAR). The TEM images were obtained using the bright field image mode at an acceleration voltage of 200 keV. The hydrodynamic diameter of the NPs was measured using a Zetasizer Nano ZS (Malvern Instruments Ltd, UK). Measurements were performed at 25 °C and analysed in multiple narrow modes (high resolution). For the data analysis, the refractive index (RI) of iron oxide of 2.30 was used, and the reading was carried out at a 173° angle with respect to the incident beam. NIBS software (V6.01) was used for data analysis. The mean particle diameter is directly calculated by the software from the particle distributions measured, and the resulting polydispersity index (PdI) is a measure of the size ranges present in the solution. The same instrument was also used to measure the zeta potentials (surface charge) of the NPs. The measurements were carried out at pH ranging 2–8, and the pH was adjusted to these levels using 0.1 M of HNO3 and NaOH. The Smoluchowski equation was used to extract the zeta potential ξ from the measured particle electrophoretic mobility μe: ζ¼
η μ ε e
where η and ε are the viscosity and the dielectric constant of the dispersion medium, respectively. A folded capillary cell (DTS1060) filled with 1 ml of the NPs’ suspension was used for the measurement. Dissolved Fe was measured using an inductively coupled plasma–atomic emission spectrometer (Varian Vista Pro ICPOES, UK) (detection limit=0.19 mg l−1), while an inductively coupled plasma–atomic emission spectrometer (Agilent
Environ Sci Pollut Res Fig. 1 Three stages in the production of TF-SCMNPs
7500ce ICP-MS) was used to measure the dissolved Si with a detection limit of 10 μg l−1. Stability and dissolution of TF-SCMNPs The stability of the TF-SCMNPs was examined by dispersing 8 and 32 mg l−1 (these concentrations were used for the removal and recovery of Hg(II) (Hakami et al. 2012)) of TFSCMNPs in deionised water at pH 6.0, and the stability of the suspensions was reviewed by measuring the hydrodynamic size distribution of the NPs over time using a Zetasizer Nano ZS (Malvern Instruments Ltd, UK). The hydrodynamic size distribution of TF-SCMNPs was also examined as a function of pH (2.0–9.0), and the pH was adjusted to the desired value using 0.1 M of HNO3 and NaOH. Low concentrations of salts such as NaCl and KCl were reported to be good solvents to avoid a Coulombic interaction between charged systems (polyelectrolytes) (Meunier et al. 2001; Ito et al. 2004).
Therefore, these solvents were used as a reference to compare with the results obtained using deionised water as the solvent. In this experiment, the pH of the system was adjusted to pH 6.0 and measured directly before transferring the samples into folded capillary cells and mixing via bath sonication and vortexing. The pH of the solution was also investigated as a major factor in the dissolution of Fe and Si: experiments were performed at a pH of 2.0, 4.0, 6.0, and 8.0 by shaking 50 ml of 8 mg l−1 NPs solutions (Fe3O4 NPs and TFSCMNPs) for 2 h at room temperature at a shaking speed of 200 rpm. After the dissolution reached equilibrium, the NPs were separated via an external magnetic field and the supernatant was collected to determine the total amount of Fe and Si in the solution using ICP-OES and ICP-MS, respectively. The effect of contact time on dissolution of Fe from NPs was conducted at pH 2.0 under the shaking speed of 200 rpm at room temperature.
Environ Sci Pollut Res Fig. 2 a TEM micrograph of Fe3O4 NPs and b SEM micrograph of TF-SCMNPs
a
In all cases, 8 mg l−1 of 50 mg ml−1 NPs aqueous dispersion was used as reference material to check how various factors (pH, time, and elution process) affected the dissolution of Fe and Si. All experiments were conducted at least in triplicate and the average values are presented in this paper.
Results and discussion SEM, TEM images, and particle size distribution The morphology and structure of Fe3O4 NPs and TFSCMNPS samples are clearly revealed by examining the SEM and TEM images. Figure 2a shows a typical TEM image for Fe3O4 NPs, where it can be seen that the Fe3O4 NPs are nearly spherical, with some demonstrating a hexagonal structure, as marked by the arrow. The particles appear to be aggregated, which is due to the absence of any stabiliser in the reaction system during the course of formation of Fe3O4 NPs. In the absence of any surface coating, Fe3O4 NPs have a hydrophobic surface, and due to hydrophobic interaction between the particles, the particles tend to agglomerate and form large clusters (Hamley 2003). Figure 2b shows nonFig. 3 DLS plot for the particle size distribution of Fe3O4 NPs, SCMNPs, and TF-SCMNPs
b
aggregated TF-SCMNPs and demonstrates many particles separated from each other. Only one single core is included in each particle, indicating the minimum aggregation during the synthesis of SCMNPs. The aggregation was minimised after using concentration of 75 mmol l−1 of CTAC aqueous solution as a surfactant stabiliser during the preparation process of mesoporous SCMNPs. The addition of CTAC aqueous solution as co-surfactants was used to alter the packing parameter, resulting in elongated CTAC micelles. The CTAC aqueous solution adheres to the surfaces in a substrate specific manner and may cause a reduction in the curvature of the surfactant aggregates. However, these effects are complex and their evaluation is not within the scope of this paper. As Fig. 3 shows, the measured hydrodynamic particle size distribution increased from an average diameter of ~75 nm for Fe3O4 NPs to ~105 nm after silica coating, and was found to be ~111 nm after 3-MPTMS functionalisation. It can be seen from Fig. 3 that the size distribution of the three types of NPs is a unimodal size distribution. The results of the dynamic light scattering (DLS) measurements demonstrate only one peak which suggests that the particles are mostly isolated rather than being aggregated in the solution and that the prepared NPs are highly mono-dispersed in aqueous media. The DLS results for NPs size in various types of solvents are presented in Table 1. Fe3O4 NPs, SCMNPS, and TF-
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Table 1 Particle size and polydispersity index (PdI) of NPs using various types of solvents
100
a
80
Media
DLS Average diameter (nm) (±SD) PdI (±SD)
Fe3O4 NPs
DW
0.1 M NaCl 0.1 M KCl SCMNPs DW 0.1 M NaCl 0.1 M KCl TF-SCMNPs DW 0.1 M NaCl 0.1 M KCl
75.52 (0.59)
0.14 (0.01)
75.60 (0.25) 76.45 (0.52) 105.57 (0.45) 106.23 (0.74) 106.20 (0.40) 111.30 (0.61) 111.27 (0.55) 111.20 (0.46)
0.16 (0.01) 0.18 (0.02) 0.06 (0.02) 0.06 (0.03) 0.05 (0.02) 0.06 (0.01) 0.07 (0.02) 0.06 (0.01)
SCMNPs exhibited nearly the same size when dispersed in water, 0.1 M NaCl, and 0.1 M KCl without any change or agglomeration size. The results of TF-SCMNPs and SCMNPs show nearly mono-dispersed particles (with PdI less than 0.09), whereas Fe3O4 NPs show midrange mono-dispersion. Zeta potential measurement The zeta potential of the Fe3O4 NPs, SCMNPs, and SHSCMNPs were all measured; these show that the magnitude decreased with increasing pH values, Fig. 4. SH-SCMNPs showed good stability at a range of different pH values, with a maximum zeta potential of 43.60 mV. In general, the magnitude decreased with increasing pH values but moderate stability was observed at pH 4.0 when the zeta potential was 20.01 mV. At pH 2.0, however, the magnitude decreased significantly to 2.52 mV, corresponding to a less stable suspension. The surface charge of Fe3O4 NPs remained positive at pH values below 4.8. The point of zero charge (PZC) of Fe3O4 NPs was found to occur at pH 4.8. This value is
40 20 0 120 100
b
0
100
200
300
400
500
80 -1 TF-SCMNPs @ 8 mg l
60 40 20 0 0
100
200
300
400
500
Time (Min.)
Fig. 5 The time-dependent aggregation of TF-SCMNPS: a 8 mg l−1 and b 32 mg l−1
significantly different from the IEP for the SCMNPs and SH-SCMNPs that occurred at pH 3.4 and 2.2, respectively. The negative surface charge of SCMNPs was to be expected due to the increased number of hydroxyl groups present on the surface of Fe3O4 NPs after silica coating; this was previously reported (Vaidya et al. 2011). In addition, the presence of silanol groups on the surface of SCMNPs (derived from TEOS) might play a key role in producing negative charges (Lee et al. 2011). A silanol group contains a hydrogen atom that can dissociate and produce a negative charge. Figure 4 shows that as the pH of the solution was decreased, more hydrogen ions were dissociated and produced negative zeta potentials. It was clearly noticeable that the zeta potential changed after modifying SCMNPs with thiol groups. The obvious change in the zeta potential further confirms the deposition of 3-MPTMS on the surface of the NPs. Similar to the silanol group, the thiol group of SH-SCMNPs contains a hydrogen atom that can dissociate and produce a negative zeta potential.
150 Fe3O4 NPs
20
SCMNPs TF-SCMNPs
0
-20
Average diameter (nm)
40
Zeta potential (mv)
-1 TF-SCMNPs @ 32 mg l
60
Average diameter (nm)
Particles
140
130
120
110 -40
0 2
3
4
5
6
7
pH Fig. 4 Zeta potential of Fe3O4 NPs, SCMNPs, and TF-SCMNPs
8
2
4
6
8
10
pH Fig. 6 Variation of the hydrodynamic sizes of TF-SCMNPs across a range of pH values
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a
1.0 TF-SCMNPs Fe3O4 NPs
6
Fe Conc. (mg L-1)
Fe Conc. (mg l-1)
0.8
0.6
0.4
4
2
0.2
Diss.=10.61%
< 0.19 mg L-1(MDL)
0
0.0 0
50
100
150
200
REF.
250
pH 2.0
Time (Min.)
pH 4.0
pH 6.0
pH 8.0
Samples
Fig. 7 Effect of contact time on the dissolution of Fe from Fe3O4 NPs and TF-SCMNPs at pH 2.0
3.5
b
The hydrodynamic size of TF-SCMNPs was measured against time. As can be seen in Fig. 5a, b, TF-SCMNPs showed a mean hydrodynamic size of ~111 nm with a PdI of less than 0.04; the average particle size distribution remained essentially constant for 8 h at different concentrations without obvious aggregation and any significant deterioration in the hydrodynamic average size. The stabilisation of TF-SCMNPs could be attributed to both the CTAC aqueous solution as a surfactant stabiliser during the preparation process of mesoporous SCMNPs and functionalisation with thiol organic groups; these findings are consistent with the TEM image of Fig. 2b. The hydrodynamic size distribution of SCMNPs and TF-SCMNPS were also measured across different pH ranges (2.0–9.0) as shown in Fig. 6. The average hydrodynamic size of the TF-SCMNPs at pH 2.0 and 3.0 were ~149 and ~136.90 nm, respectively. Changing the pH of the solution from 4.0 to 9.0 did not change the hydrodynamic size of the TF-SCMNPs significantly. Thus, TF-SCMNPs are mostly isolated and are not aggregated across a range of pH values. The agglomeration of particles at pH 2.0–3.0 agrees with the discussion in “Zeta potential measurement” section. The TF-SCMNPs are negatively charged in the pH range (3.0–8.0) and almost stable at neutral to basic pH values, as demonstrated by their zeta potential (Fig. 4). The agglomeration of TF-SCMNPs could be attributed to coagulation that occurs when the zeta potential is above the PZC (Lu et al. 2007). Tombacz et al. (1999) and Cornell and Schwertmann (2003) stated that in oxide suspensions, aggregation occurs at a pH range below or around pHpzc because the charge density is very low in this range. This could be used to describe the agglomeration of the TF-SCMNPs at pH 3.0. However, loose agglomerates are easily broken up by increasing the agitation and re-suspending times of the NPs.
2.5 2.0 1.5 1.0
-1 < 0.19 mg L (MDL)
0.5 0.0
REF.
pH 2.0
pH 4.0
pH 6.0
pH 8.0
Samples Fig. 8 Effect of pH on the dissolution of Fe from a Fe3O4 NPs and b TFSCMNPs
Dissolution of nanoparticles Figure 7 shows the result of the effect of contact time on the dissolution of Fe3O4 NPs and TF-SCMNPs at pH 2.0. The 700 600
Si Conc. (µg L-1)
Stability of TF-SCMNPs
Fe Conc. (mg L-1)
3.0
500 400 300 200 Diss.= 17.12% 100 0 REF.
pH 2.0
pH 4.0
pH 6.0
pH 8.0
Samples Fig. 9 Effect of pH on the dissolution of Si from TF-SCMNPs
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Fig. 10 Schematic representation of magnetite NPs dissolution at different pH values
+
H
O
O
H+ HO
Si
Si +
FeOH FeOH+ O O Si Si + FeOH FeOH+ O O Si Si Si Si
At pH 4
H+
O O H+ FeOH+FeOH+
FeOH+ FeOH+ +
H
H+ +
At pH 2 H
H+
O
O
Si
Si
O
H+ FeOH FeOH+
dissolution of the NPs was confirmed by measuring the total residual concentration of Fe in the supernatant after the NPs were isolated using an external magnetic field. The concentration of Fe released from the TF-SCMNPs was below the sample detection limit of the ICP-OES, which is less than 0.19 mg l−1. The initial rate of dissolution of Fe3O4 NPs was low (0.22 mg l−1, ~ 3.69 % of the initial concentration), and it approached the equilibrium condition in 30 min with the dissolution of 0.69 mg l−1 (~11.65 % of the initial concentration). The effect of the contact time on the dissolution of Fe3O4 NPs and TF-SCMNPs at pH 6.0 (data not shown) was negligible. The concentration of iron from Fe3O4 NPs and TF-SCMNPs was less than 0.19 mg l−1. As discussed in our previous work (Hakami et al. 2012), the aim of coating the Fe3O4 NPs with dense liquid silica (DLSi) was to achieve high resistance to leaching in acidic media. The results confirmed that as the concentration of released Fe from the TF-SCMNPs was less than 0.19 mg l−1 at pH ranges from 2.0 to 8.0, there was no protection to the Fe3O4 NPs from dissolution in the acidic media in the absence of any metals dropped on the surface of the NPs. Leaching of sorbent components into the treated water is unfavourable. However, the effluent discharge limit of Fe into municipal sewers is 2 mg l−1 (WHO 2011), which is higher than the concentration of Fe released from the NPs. The effect of pH on the dissolution of Fe3O4 NPs and TFSCMNPs was also investigated and Figs. 8a, b and 9 show the results. The reference concentration of Fe and Si are also
+
FeOH+ FeOH+ O O Si Si + FeOH FeOH+ O O Si Si Si Si HO
+
4FeOH2+
OH
H+
FeOH+ FeOH+ + H O O Si Si FeOH+ FeOH+ + H O O Si Si Si Si O
OH Si
Si
HO Si
OH Si
H Si
OH Si
H Si
OH Si
Si HO
+
8FeOH2+
Si OH
presented in these figures. It was found that the Fe3O4 NPs are sensitive in the acidic media demonstrating the dissolution of 0.63 mg l−1 (10.61 %) of Fe. The dissolution was negligible with increasing pH, and the concentration of Fe was below the sample detection limit of ICP-OES. A small amount of Fe, less than 0.19 mg l−1, was detected over the pH range tested, which confirmed that the pH of the solution does not affect the dissolution of SH-SCMNPs, as Fig. 7b shows. An increasingly small amount of Si was dissolved into the solution as the pH values increased, as Fig. 9 shows. At pH 8.0, the concentration of Si released from the TF-SCMNPs into the solution was 116.50 μg l−1 (17.12 %). This result agrees with the findings of previous researchers (Guthrie and Reardon 2008; Pham et al. 2011) who confirmed that the presence of an alkaline environment enhanced markedly the dissolution rate of silica compared with neutral or acidic environments. The effect of pH on the dissolution was also investigated at lower pH values: 2.0 and 4.0. Figure 10 demonstrates that the dissolution of the supported magnetite depends on the concentration of the hydrogen ion present. At pH 4.0, the hydrogen ions can only interact with the surface of the magnetite leading it to leaching out the surface into the solution. However the magnetite inside the silica pores would not be reached by the hydrogen ions. On the other hand, at pH 2.0, the concentration of hydrogen ions is high enough to reach not only the surface magnetite but also the pores magnetite giving rise to more iron leaching into the solution.
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Conclusions The following conclusions can be drawn from the work carried out in this research work: &
&
&
Fe3O4 NPs were not mono-dispersed in size: their shape was spherical and approximately 75 nm in diameter. Through the co-condensation method, the obtained NPs were capped with nonpolar endgroups (thiol) on their surface that were approximately 111 nm in diameter, spherical in shape, and narrow in their distribution. The NPs obtained via the co-precipitation methods were not mono-dispersed in size: their shape was spherical and approximately 75 nm in diameter. Through the cocondensation method, the obtained NPs were capped with nonpolar endgroups (thiol) on their surface that were approximately 111 nm in diameter, spherical in shape, and narrow in their distribution. In the present work, in order to inhibit the dissolution of Fe3O4 NPs, two approaches for dissolution prevention were developed: (a) using dense liquid silica-coated magnetite NPs (DLSiC-Fe3O4 NPs) and (b) using silica coating Fe3O4 NPs via sol–gel reaction. The increased efficiency of the silica in eliminating the pH dependence and enhancing the NP stability of SCMNPs and SH-SCMNPs in the aqueous medium is noteworthy. On the other hand, the dissolution of Fe3O4 NPs was to found to be detrimental at pH 2.0 or had a long contact time.
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