MSN-NH2

PTEC-11924; No of Pages 10 Powder Technology xxx (2016) xxx–xxx

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Biphasic synthesis of amine-functionalized mesoporous silica nanospheres (MSN-NH2) and its application for removal of ferrous (Fe2 +) and copper (Cu2 +) ions Wittawinwit Naowanon a,b, Romteera Chueachot c,e, Sujitra Klinsrisuk d, Sittipong Amnuaypanich b,d,⁎ a

Center for Alternative Energy Research and Development, Khon Kaen University, Khon Kaen, 40002, Thailand Applied Chemistry Division, Department of Chemistry and Center of Excellence for Innovations in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand c Program of Chemistry, Faculty of Science, Ubonratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand d Materials Chemistry Research Center (MCRC-KKU), Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand e Functional Nanomaterials and Electrospinning Research Laboratory, Faculty of Science, Ubonratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand b

a r t i c l e

i n f o

Article history: Received 4 January 2016 Received in revised form 2 September 2016 Accepted 6 September 2016 Available online xxxx Keywords: Mesoporous silica Nanospheres Biphasic Adsorption Amine-functionalization

a b s t r a c t Amine-functionalized mesoporous silica nanoparticles (MSN-NH2) were synthesized under cyclohexane-water biphasic system using l-arginine as the weak base catalyst and CTAB as the template. The synthesized MSNNH2 particles were highly monodispersed nanospheres with a particle diameter lower than 50 nm and a pore size approximately 2–4 nm. MSN-NH2 particles were utilized as the adsorbent for a removal of Cu2+ and Fe2+ ions from aqueous solutions. MSN-NH2 possessed higher adsorption efficiency towards Cu2+ than Fe2 ions with the removal efficiency of Cu2+ as high as 99%. Adsorption isotherm showed that Langmuir model was suitably described the adsorption behaviors of Cu2+ and Fe2+ on MSN-NH2. Thermodynamic study indicated that the adsorptions of Cu2+ and Fe2+ on MSN-NH2 were spontaneous process and were primarily governed by a physical adsorption. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Contamination of heavy metals into natural water resources has been a serious threat to human health. Released heavy metals, most frequently in a form of metal ions, can dissolve into rivers and lakes and eventually may contaminate municipal water supply system. In other way, heavy metals as water insoluble forms can precipitate in river sediments and subsequently are collected by plants and animals thus ultimately contaminate humans' food chains [1,2]. Among hazardous metals involved in mining and chemical industries, Copper can leak into natural surrounding along with discharges from productions. Although copper is considered as the nutrient for human's metabolic system, over dose of copper leads to several health problems such as kidney or liver failures and digestive disorders [3]. Despite being nonhazardous to human health, dissolved iron in the form of ferrous ion (Fe2+) causes unpleasant effects to potable and drinking waters including foul odors, rusty taste and color also it can promote the growth of iron bacteria. As recommended by World Health Organization (WHO), iron content should not exceed 0.3 ppm for a safe drinking water [4]. ⁎ Corresponding author at: Applied Chemistry Division, Department of Chemistry and Center of Excellence for Innovations in Chemistry (PERCH-CIC), Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. E-mail address: [email protected] (S. Amnuaypanich).

Therefore with above reasons, it is imperative to have the effective decontamination of Cu2+ and Fe2+ ions in natural water recourses before supplying to communities. Various techniques are applied to eliminate heavy metals from water such as reverse osmosis or nanofiltration [5], ion exchange [6], coagulation and flocculation [7], and adsorption [8–13]. Among them, the adsorption is one effective method for a removal of metal ions from aqueous solutions particularly when porous materials are utilized as the adsorbents. For instances, zeolites were employed as the adsorbents for a removal of arsenic, zinc and copper from wastewater [9,10]. Zinc, nickel, lead and cadmium ions had been removed from aqueous solutions using activated carbon as the adsorbent [11,12]. Due to their high surface area and porosity, mesoporous silica nanoparticles (MSNs) with a pore size in meso-range (2–50 nm) are among the most effective adsorbent [8,13]. MCM-41, one class of mesoporous silica, was employed as the absorbent for the removal of cobalt, nickel and copper ions from wastewater [8]. Mesoporous silica can be modified with some functional groups, e.g. amine [14] or thiol [15] to enhance its metal ion adsorption capability. MSNs were successfully synthesized by the modified StÖber process whereby the pore generating agents, e.g. cationic surfactants or block copolymers was introduced to the mixture of Tetraethyl orthosilicate (TEOS), water, alcohol and ammonia [16,17]. This synthesizing method, however, produces MSNs with low colloidal stability due to relatively

http://dx.doi.org/10.1016/j.powtec.2016.09.014 0032-5910/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: W. Naowanon, et al., Biphasic synthesis of amine-functionalized mesoporous silica nanospheres (MSN-NH2) and its application for removal of ferrous (Fe2..., Powder Technol. (2016), http://dx.doi.org/10.1016/j.powtec.2016.09.014

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W. Naowanon et al. / Powder Technology xxx (2016) xxx–xxx

large particle sizes and broad particle size distributions [18]. A formation of MSNs involves the sol-gel process of silica via the hydrolysis and condensation of TEOS under basic environment [19]. Therefore a manipulation of the hydrolysis and condensation reactions would gain control of MSNs size and size distribution [20]. Synthesizing under low hydrolysis rate of TEOS in the modified StÖber process would lead to significant decreasing of particle size as well as size distribution of MSNs. As demonstrated by Yokoi et.al. [21], very uniform-sized MSNs with average particle size as small as 20 nm were successfully synthesized under low hydrolysis rate of TEOS by maintaining a mild basic condition of the reaction mixture using basic amino acid (l-arginine). Later Wang et.al. [22] synthesized MSNs under low hydrolysis rate of TEOS using TEOS-water biphasic system and obtained MSNs with an average diameter in the range of 28–54 nm and narrow size distribution. A formation of immiscible TEOS-water bi-layer in biphasic synthesis limits TEOS to be hydrolyzed only at liquid–liquid interface. In present study, the synthesis of MSNs functionalized with amine (MSN-NH2) is demonstrated. The co-condensation of TEOS and amino silane (APTES) in biphasic system under mild basic condition was employed to obtain highly monodispersed MSN-NH2 with a particle diameter smaller than 50 nm. The synthesized MSN-NH2 was applied as the adsorbent for Fe2 + and Cu2 + removal from aqueous solution under various conditions. The adsorption equilibrium studies were conducted to evaluate the adsorption behaviors of MSN-NH2. 2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS), 3-Aminopropyl triethoxysilane (APTES) and l-arginine were supplied by Sigma-Aldrich (St. Louis, USA). Cetyltrimethylammonium bromide (CTAB) was purchased from Ajax Finechem (NSW, Australia). Cyclohexane (CHX) was provided by Qrëc (New Zealand). Copper (ΙΙ) sulfate and iron (II) sulfate heptahydrate were supplied by Carlo Erba (Reuil, France). 2.2. Biphasic synthesis of amine-functionalized mesoporous silica nanospheres (MSN-NH2) MSN-NH2 was prepared by co-condensation method in CHX-water biphasic system. CTAB (0.25 g) and l-arginine (0.18 g) were dissolved in 110 mL of deionized water at room temperature. Subsequently CHX phase was formed afloat on the aqueous phase by slow adding of CHX. CHX-water bi-phase was then heated at 60 °C for 30 min. Next the mixture of TEOS (9.2 mL) and APTES (4.4 mL) was added drop-wise into CHX phase to start the reaction and allowed the reaction to proceed for 20 h at 60 °C under mild stirring to maintain undisturbed CHX phase. The reaction was terminated by quickly quenching the reaction vial in iced water. CHX phase was decanted using a syringe prior to centrifugation the aqueous phase at 6000 rpm to collect MSN-NH2 particles. The MSN-NH2 particles were washed respectively with ethanol and water before drying in an oven. CTAB template was removed by extraction with 5 vol.% acetic acid in ethanol for 24 h. Finally, the extracted MSN-NH2 particles were dried in the oven at 60 °C for overnight. For sake of comparison, mesoporous silica nanospheres (MSN) without functionalization with amine was also synthesized using the same method described above except that TEOS was used as sole silica precursor and to eliminate CTAB template, the calcination at 550 °C was employed.

Fig. 1. FT-IR spectra of MSN and MSN-NH2.

Weight loss profile of MSN-NH2 particles was monitored by thermal gravimetric analysis (TGA) using Perkin-Elmer TG/DTA thermal analyzer (Pyris Diamond) at temperatures from 50 to 800 °C with the heating rate of 5 °C min−1 under dried air. The amount of NH2 on MSN-NH2 was determined by the elemental analysis using CHN analyzer (Perkin Elmer, PE-2400II). Transmission electron microscopy (TEM) images were obtained from FEI Tecnai G2 operated at an accelerating voltage of 200 kV. MSN and MSN-NH2 particles were dispersed in ethanol and the dispersion was subjected to ultra-sonification for approximately 10 mins. Dispersion of the particles was deposited on a 200 mesh carbon-coated copper grid and then dried in a desiccator. Average particle size and particle size distributions were determined from at least 200 particles in TEM images using Image J program. Pore characteristic of the synthesized MSN and MSN-NH2 was examined by Nitrogen adsorption-desorption method performing on Autosorb-1, Quantachrome Instruments. Before the measurement, the samples were degasses at 300 °C for 2 h. Pore size and pore-size distribution was calculated from an adsorption branch of the isotherms by Barrett–Joyner–Halenda (BJH) method. Pore volume and specific surface area were determined according to the standard BrunauerEmmett-Teller (BET) method.

2.3. Characterization Functionalization of NH2 on MSN-NH2 was confirmed by Fourier transform infrared spectroscopy (FT-IR) using Bruker FT-IR Spectrometer, Tensor 27.

Fig. 2. Weight loss (TG) curves of MSN and MSN-NH2.



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Fig. 3. TEM analysis: (a) particle size and particle size distributions of MSN and MSN-NH2 particles and (b) MSN and MSN-NH2 particles and their corresponding EDX spectra after adsorption of Cu2+ and Fe2+ ions.


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2.4. Ferrous (Fe2+) and copper (Cu2+) ions adsorption study Copper (ΙΙ) sulfate or Iron (II) sulfate heptahydrate were dissolved in deionized water to prepare metal-ion solutions with initial concentrations (C0) from 5 to 50 mg/L. MSN-NH2 particles were added to 25 mL of each metalion solution. The mixture was then sonicated for 1 h prior to allowing the adsorption to take place under the controlled conditions. After a completion of the adsorption, MSN-NH2 particles were removed from metal ion solution by repeated centrifugation followed by filtration of the supernatant. Final concentration of metal ion solution (Ce) was determined by AAS spectroscopy (Atomic Absorption Spectrometer 3110, Perkin Elmer). The capability of the adsorbent to remove metal ions is justified by the removal percentage (%) calculated from a percentage of (C0 - Ce) to C0. Adsorption of the metal ions was investigated under different adsorption conditions namely initial concentration, pH, contact time and temperature. The pH of metal ion solutions was adjusted by adding either 0.1 M of HCl or NaOH. Equilibrium amount of metal ions adsorbed by the adsorbents (qe, mg metal ion/g adsorbent) was calculated from Eq. (1): qe ¼

ðC 0 −C e ÞV w

ð1Þ

where C0 and Ce are metal ion concentrations (mg/L) at initial and equilibrium conditions, respectively, V is volume (L) of solution and w is mass (g) of adsorbent. The equilibrium adsorption isotherms were described by Langmuir and Freundlich models expressed in Eqs. (2) and (3), respectively [23]. From Langmuir isotherm, separation factor or equilibrium parameter (RL) which justifies the favorable or unfavorable adsorption process was calculated from Eq. (4) [23]. 1 1 1 1 ¼ þ qe K L qm C e qm lnqe ¼

RL ¼

1 ln C e þ lnK F n

1 1 þ K L C0

ð2Þ

ð3Þ

ð4Þ

where qm is the maximum amount of adsorbed metal ion (mg metal ion/g adsorbent), KL is the Langmuir constant (L/g), KF is the Freundrich constant (dimensionless) and n is the Freundrich exponent which is related to the adsorption intensity. 3. Results and discussion 3.1. Characterization of MSN and MSN-NH2 Fig. 1 shows IR spectra of the synthesized MSN and MSN-NH2 particles. Characteristic peaks of silica were observed for MSN at 1100 cm−1 and 806 cm−1 belonging to asymmetric vibrations and symmetric stretching of Si-O-Si [24]. While additional IR peak at 1620 cm−1 was apparent for MSN-NH2 corresponding to bending vibrations of aliphatic amine (-NH2) [25,26]. This confirms the presence of amine functional group in MSN-NH2 particles. Weight loss profiles of MSN and MSN-NH2 was presented in Fig. 2. TG curve of MSN has distinctively single weight loss around 100 °C which is due to a loss of physical adsorbed water on the particle surface. Differently, TG profile of MSN-NH2 exhibited two steps of weight loss. The first weight loss occurred below 200 °C is attributed to the evaporation of physically adsorbed water including bound water that losses weight at temperature higher than 100 °C. The second loss of weight at temperature between 200 and 800 °C is related to the degradation of amine-containing functional group (-C3H6NH2) of MSN-NH2 particles. To be more accurate, the amount of amine in MSN-NH2 was

Fig. 4. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of MSN and MSN-NH2.

estimated from the elemental analysis (CHN) and the result was 3.8 × 10−3 mmol NH2/g particle with 1:1.9 of N:H element ratio. TEM images of MSN and MSN-NH2 particles are illustrated in Fig. 3a. As can be noticed, both TEM images of MSN and MSN-NH2 reveal the porous structure with a pore size in meso-range. MSN-NH2 possessed spherical particles with an average particle size of 43.9 nm however some particles showed irregular shapes due to particle coalescence during the synthesis. More discrete version of the particles was observed for MSN with particle size decreased to 24.6 nm. Both MSN and MSNNH2 particles were highly monodispersed with the particle size deviations being 1.70 nm and 3.52 nm, respectively. Also revealed in Fig. 3b are TEM images of MSN and MSN-NH2 particles after adsorptions of Cu2+ and Fe2+ ions. As can be seen, the TEM images were indistinguishable to those before adsorption (Fig. 3a) therefore EDX was performed which indicated the presence of adsorbed Cu2+ and Fe2+ ions on both MSN and MSN-NH2. However the quantification of adsorbed metal ions by EDX was not attempted due to the minuscule amount of adsorbed metal ions. Table 1 Textural properties of MSN and MSN-NH2 obtained from N2 adsorption-desorption isotherms. Adsorbent Average particle diameter from TEM

MSN MSN-NH2

Specific surface area

Pore volume

Mean pore diameter

(nm)

(m2·g−1)

(cm3·g−1) (nm)

24.6 43.9

509.8 411.2

1.4 0.8

2.1 2.6



5

Fig. 5. Schematic picture showing adsorption sites of MSN and MSN-NH2.

Unlike the homogeneous StÖber process [27], the hydrolysis and condensation reactions in the biphasic system primarily occur at different locations [22]. Since the silica precursors (TEOS or APTES) are dissolved in CHX, the hydrolysis takes place as the precursors reach CHXwater interface then producing silicate species into the aqueous phase. As a consequence, the condensation of silicate species will be undergoing in the aqueous phase which results in a formation of MSN and MSNNH2 via the nucleation and growth mechanism [21]. Mechanistically on a formation of MSN and MSN-NH2, negatively-charged silicate species under weak basic condition (l-arginine) are initially attracted to micelles via the electrostatic interaction and form the silica - micelle composites. The incorporation of silicate species on the micelles weakens the inter-micelle repulsions which result in the aggregation of silica micelle composites giving the particle nuclei. After the nucleation, a

growth of the particles proceeds simultaneously with the micelle aggregating via the condensation of silicate species within the gaps between micelles [28]. Since the surface silanol groups of the silica particles are deprotonated under basic condition, the particle is ceased to grow when the net negative surface-charge is high enough to exclude silicate anions in reacting onto the particle surface [20]. Fig. 4 presents nitrogen adsorption-desorption isotherms (Fig. 4a) and pore size distributions (Fig. 4b) of MSN and MSN-NH2 particles. Isotherms of MSN and MSN-NH2 were typical reversible type IV isotherm without noticeable hysteresis loops except only at relative pressure nearly 1.0 [29]. This suggests that the pores of both particles are cylindrical with a diameter b 4.0 nm [30]. Calculated results obtained from the adsorption-desorption isotherms are summarized in Table 1. Mean pore diameters of MSN and MSN-NH2 were 2.1 nm and 2.6 nm,

Fig. 6. Effect of contact time on Cu2+ and Fe2+ removal efficiencies of MSN and MSN-NH2; C0 = 25 mg/L, absorbent doge = 0.01 g, T = 303 K and pH = 7.0.

Fig. 7. Effect of initial Cu2+ and Fe2+ ions concentrations on removal efficiencies of MSN and MSN-NH2; absorbent doge = 0.01 g, T = 303 K, pH = 7.0 and contact time = 60 min.


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respectively which are in meso-size range. However pore volume of MSN-NH2 was lower than that of MSN implying that despite its larger pore size, MSN-NH2 possessed less number of pores. Therefore with lesser pore numbers and larger particle size, specific surface area of MSN-NH2 was less than that of MSN. 3.2. Adsorptions of Cu2+ and Fe2+ by MSN and MSN-NH2 Expectedly both MSN and MSN-NH2 possess the ability to remove Cu2+ and Fe2+ ions from aqueous solutions by adsorption onto the specific surface functional groups namely silanol group (-SiOH) in MSN and additional amine (-NH2) group in MSN-NH2. MSN and MSN-NH2 contain accessible pore area for metal ions adsorption because their pore sizes are in a range of 1–3 nm which is large compared with 0.73 Å and 0.78 Å of Cu2+ and Fe2+ ion radii [31]. Thus with adsorption taking place both internal and external pore areas, the adsorption capacity of MSN and MSN-NH2 are much higher than non-porous particles. Schematic picture showing the adsorption sites of MSN and MSN-NH2 is illustrated in Fig. 5. 3.2.1. Effects of contact time, initial concentration and pH Fig. 6 shows the effect of contact time on removal efficiency of Cu2+ and Fe2+ ions on MSN and MSN-NH2. The rates of Cu2+ and Fe2+ removal for both adsorbents were instant initially and became slower at longer contact time until reached constants within 60 min. Apart from specific interactions between metal ions and surface functional groups of the adsorbents, the adsorption of metal ions onto the adsorbents is affected by the diffusion of metal ions from bulk solution to adsorbent surfaces [32]. At the early stage of adsorption, the driving force of diffusion is large due to high concentration gradient of metal ions between the bulk phase and the adsorption-sites on adsorbent surfaces resulting in an initial fast adsorption. As longer contact time, adsorption sites are highly occupied from binding with metal ions in addition to a reduction in the metal ion concentration gradient thus slowing down the adsorption rate. Fig. 7 shows a plot of the removal efficiency at different initial concentrations of Cu2+ and Fe2+. It can be seen that both MSN and MSNNH2 adsorbed the metal ions increasingly with their initial concentrations. This is because the driving force for mass transfer induced by the concentration gradient of metal ions between bulk phase and surfaces of the adsorbents becomes more intensive at higher initial concentration [32]. Amine functional group plays an important role in improving of metal ions adsorption since MSN-NH2 revealed higher removal efficiency for both Cu2 + and Fe2+ ions compared with that of MSN for the entire range of initial metal ions concentrations. When considering the adsorption between Cu2+ and Fe2 + ions, it is evidenced that Cu2 + ions were adsorbed preferentially by both MSN and MSNNH2. In addition, the removal efficiency of Cu2 + by MSN and MSNNH2 quickly reached the saturation at very dilute initial Cu2+ concentration (≥ 5 mg/L) with the plateau values being as high as 95% and 99%, respectively. However the removal efficiency of Fe2 + showed lower rate of increasing with initial concentration and attained the plateaus after reaching the initial concentration at 50 mg/L. Higher sensitivity of both MSN and MSN-NH2 towards adsorption of Cu2+ rather than Fe2+ was experimentally proven by performing the competitive adsorption in a mixture of Cu2+ and Fe2+ ions. As shown in Fig. 8, both MSN and MSN-NH2 show selective adsorption for Cu2+ as the removal efficiency of Cu2+ being larger than that of Fe2+. Since the adsorption of metal cations occurs via interactions with electron donor groups, i.e., –SiOH and –NH2 surface groups, the ability of metal ions to coordinate with these functional groups depends on the charge to radius ratio (Z/r). Metal cation with high Z/r ratio will be susceptible to bind with electron donor functional groups and form stable metal complexes [33,34]. Therefore as comparing Z/r of Cu2 + which has Z/ r = 2.74 Å−1 with Z/r of Fe2+ = 2.56 Å−1, it is obvious that MSN and MSN-NH2 are more selective for Cu2+ adsorption than Fe2+ adsorption.

Fig. 8. Competition adsorption of Cu2+ and Fe2+ ions on MSN and MSN-NH2; [Cu2+] = [Fe2+] = 25 mg/L, absorbent doge = 0.01 g, T = 303 K, pH = 7.0 and contact time = 60 min.

Effect of pH on adsorptions of Cu2+ and Fe2+ is demonstrated in Fig. 9. As can be seen, the removal efficiencies of Cu2+ and Fe2+ under acidic and basic conditions were distinguished. While the metal ions adsorption efficiencies of both MSN and MSN-NH2 were low under acidic condition, they increased almost five folds under neutral and basic conditions. However at pH N 9.0, the Cu2+ and Fe2+ started to precipitate due to a formation of water insoluble metal hydroxides [35,36]. Noticeably, at pH = 4.0 the removal efficiency was 7.7% and 7.1% for Cu2+ and Fe2 + ions, respectively and these values increased to 97.7% and 89.2% as pH increased to 7. This is because at pH b 7.0, the solutions contain large amount of hydronium ions (H3O+) with very high ion mobility and consequently, H3O+ will compete with Cu2+ and Fe2+ ions in binding to the adsorption sites. Also if pH decreases to be lower than 4.0, both –SiOH and –NH2 on MSN and MSN-NH2 will be protonated and become positively charged that limiting the adsorption of metal cations [23]. In case of pH ≥ 7.0, H3O+ is presented in low concentration therefore Cu2+ and Fe2+ ions have higher probability to direct interaction with the active sites of the absorbents. 3.2.2. Adsorption isotherms Equilibrium adsorption behaviors of Cu2 + and Fe2 + ions on MSN and MSN-NH2 adsorbents were described by Langmuir and Freundrich

Fig. 9. Effect of pH on removal efficiencies of Cu2+ and Fe2+ on MSN and MSN-NH2; C0 = 25 mg/L, absorbent doge = 0.01 g, T = 303 K and contact time = 60 min.



Fig. 10. Langmuir adsorption isotherms of Cu2+ and Fe2+ ions on MSN and MSN-NH2.

Fig. 11. Freundrich adsorption isotherms of Cu2+ and Fe2+ ions on MSN and MSN-NH2.


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Table 2 Langmuir and Freundlich parameters for adsorptions of Cu2+ and Fe2+ onto MSN and MSN-NH2. Adsorption

Langmuir

2+

MSN/Fe MSN/Cu2+ MSN-NH2/Fe2+ MSN-NH2/Cu2+

Freundlich

qm

KL

R2

KF

1/n

R2

21.74 25.00 27.03 83.33

0.04 0.12 0.12 0.15

0.999 0.996 0.994 0.998

1.00 9.14 10.28 23.82

0.94 0.82 0.83 0.53

0.998 0.984 0.981 0.995

Table 3 RL value for adsorption of Cu2+ and Fe2+ based on Langmuir model. Initial conc. (mg·L−1) 5 10 20 30 40 50

RL MSN/Fe2+

MSN/Cu2+

MSN-NH2/Fe2+

MSN-NH2/Cu2+

0.83 0.71 0.56 0.45 0.38 0.33

0.63 0.45 0.29 0.22 0.17 0.14

0.63 0.45 0.29 0.22 0.17 0.14

0.57 0.40 0.25 0.18 0.14 0.12

Fig. 12. Plots of Kd versus 1/T for Cu2+ and Fe2+ adsorptions on MSN and MSN-NH2.

adsorption models as presented respectively in Figs. 10 and 11. Also Freundrich and Langmuir parameters for the adsorptions of Cu2+ and Fe2+ on MSN and MSN-NH2 were summarized in Table 2. The adsorptions of Cu2+ and Fe2+ on MSN and MSN-NH2 were satisfactorily described by either Langmuir or Freundrich models. This is because the concentration range of the metal ions in this study was under dilute condition therefore the surface saturation with adsorbed monolayer still had yet fulfilled. However the correlation coefficients (R2) suggests that the adsorption behaviors of Cu2+ and Fe2+ ions is somehow better explained by Langmuir model which is rational due to the fact that only monolayer adsorption of the metal ions is permitted since the metal ions can coordinate only to specific surface functional groups of MSN and MSN-NH2 while they are repelling each other by electrostatic interactions. From Langmuir model (see Table 2), it is seen that qm was highest for the adsorption of Cu2 + by MSN-NH2 and drastically reduced for the Fe2+ adsorption. This is not unexpected since Cu2+ ions form more stable ion-complex with amine groups as discussed earlier in the effect of initial concentration. Furthermore despite its lower specific surface area and pore volume (see Table 1), MSN-NH2 has higher adsorption capacity for Cu2+ and Fe2+ ions than has MSN which emphasizes the pivotal role of amine functional group in improving of the metal ions adsorption. The MSN, however, revealed comparable qm for both Cu2+ and Fe2+ adsorptions. RL value based on the Langmuir equation for adsorptions of Cu2+ and Fe2+ was present in Table 3. RL value indicates the nature of adsorption whether it is unfavorable if RL N 1, linear

favorable if RL = 1, favorable if 0 b RL b 1 and irreversible if RL = 0 [37]. Evidently for all initial concentrations of Cu2+ and Fe2+ ions, RLs of both MSN and MSN-NH2 were in the range 0 b RL b 1 indicating the favorable adsorption of Cu2+ and Fe2+ in both adsorbents. Table 4 compares the adsorption capacity of Cu2+ and Fe2+ ions on MSN and MSN-NH2 from this study with those on mesoporous silica adsorbents from literatures. Compared with the previous studies, MSN and MSN-NH2 in the present study possessed large specific surface areas. Consequently, the adsorption capacity of Cu2+ on MSN-NH2 in this study was significantly higher than did other adsorbents from previous studies. The adsorption of Fe2+ on both MSN and MSN-NH2 was less effective however it was comparable to that of previous studies. 3.2.3. Thermodynamics study of Cu2+ and Fe2+ adsorptions on MSN and MSN-NH2 Thermodynamics of Cu2+ and Fe2+ adsorptions on MSN and MSNNH2 was studied by carrying out the adsorption experiments at temperatures from 30 °C to 60 °C. A change in the standard Gibbs free energy (ΔG°) for the adsorption process is expressed by [23] ΔGo ¼ −RT ln K d

ð5Þ

where R is the gas constant (8.314 J/mol·K), T is absolute temperature and Kd is the distribution coefficient for adsorption which is equal to a ratio of qe to Ce at specific initial concentration of metal ions. Following ΔG°, changes in the enthalpy (ΔH°) as well as the entropy (ΔS°) of

Table 4 Comparison of adsorption capacity of Cu2+ and Fe2+ on mesoporous silica adsorbents. Absorbent

Specific surface area (m2·g−1)

Metal ion

qm (mg·g−1)

Normalized qm (mg.m−2)

Ref.

MCM-48-NH2 MSN WMSM-NH2 AMS MSN-NH2 MSN MSN MSN-NH2 MSN

511.0 421.9 243.0 689.0 411.2 509.8 421.9 411.2 509.8

Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Fe2+ Fe2+ Fe2+

27.70 24.78 20.45 53.30 83.33 25.00 22.45 27.03 21.74

0.054 0.059 0.084 0.077 0.203 0.049 0.053 0.067 0.043

[38] [23] [37] [39] This study This study [23] This study This study

WMSM: worm-like mesoporous silica monolithic. AMS:Amine functionalized silica.



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Table 5 Thermodynamics parameters for adsorptions of Fe2+ and Cu2+ on MSN and MSN-NH2. Absorbent/metal

MSN/Fe2+ MSN/Cu2+ MSN-NH2/Fe2+ MSN-NH2/Cu2+

ΔH° (KJ/mol)

ΔS° (J/mol.K)

303 K ΔG° (KJ/mol)

Kd

ΔG° (KJ/mol)

Kd

ΔG° (KJ/mol)

Kd

ΔG° (KJ/mol)

Kd

−4.15 −15.07 −15.97 −29.96

−13.10 −37.53 −45.10 −78.43

−0.17 −3.77 −2.30 −6.19

1.07 4.47 2.41 11.2

−0.05 −3.32 −1.85 −5.41

1.03 3.52 2.16 8.43

0.08 −2.95 −1.40 −4.63

0.97 2.91 1.69 5.49

0.20 −2.57 −0.95 −3.84

0.92 2.62 1.39 3.92

313 K

adsorption are obtained from the plot of lnKd vs. 1/T according to the equation lnK d ¼ −

ΔH o ΔSo þ RT R

ð6Þ

Fig. 12 shows the plot of lnKd vs. 1/T for the adsorptions of Cu2+ and Fe on MSN and MSN-NH2. It is evidenced that the plots were linear following satisfactorily the relationship in Eq. (6). Accordingly, ΔH° and ΔS° were obtained from the slopes and intercepts of the plots, respectively as summarized in Table 5. Noticeably, Kd of MSN and MSNNH2 decreased as increasing adsorption temperature indicating the unfavorable adsorption of Cu2+ and Fe2+ at elevated temperature which is obviously due to the exothermic adsorption process of the metal ions on MSN and MSN-NH2. ΔH° values for adsorptions of Cu2+ and Fe2+ ions on MSN-NH2 were more negative compared with those of MSN suggesting stronger interactions between MSN-NH2 and the metal ions. Furthermore, negative ΔS° for metal ions adsorption on both MSN and MSN-NH2 indicating a less chaotic condition of the adsorbed metal ions on the adsorbents compared with a high mobility condition in bulk phase. ΔG° of the metal ions on MSN and MSN-NH2 were also negative indicating that the adsorptions occur spontaneously. The values of ΔG° were close to 0 kJ/mol suggesting that the adsorptions of Cu2+ and Fe2+ on MSN and MSN-NH2 are primarily physical adsorption [40]. 2+

4. Conclusions Mesoporous silica nanospheres (MSN) and amine-functionalized MSN (MSN-NH2) were synthesized employing the biphasic system under weak basic condition of l-arginine and using CTAB as the template. With low hydrolysis rate of TEOS and APTES, the obtained MSN and MSN-NH2 particles were highly monodispersed with a size b50 nm and the pore sizes of both MSN and MSN-NH2 particles were meso-sized approximately 2–4 nm. MSN and MSN-NH2 were utilized as the adsorbents for a removal of Cu2+ and Fe2+ ions from aqueous solutions. The adsorption capabilities of both particles were satisfactory as the removal efficiency was as high as 99% for Cu2+ and 95% for Fe2+. However due to additional amine functional group, MSN-NH2 had higher efficiency in removal of both Cu2 + and Fe2 + ions than did MSN. Equilibrium adsorption behaviors of Cu2 + and Fe2 + ions on MSN and MSN-NH2 were well described by Langmuir model due to specific interactions between the metal ions and surface functional groups, i.e. silanol and amine. Thermodynamic study indicated that the adsorptions of Cu2+ and Fe2+ on MSN-NH2 and MSN were exothermic and spontaneous. Also the values of ΔG° suggest that the adsorptions were primarily governed by a physical adsorption. Acknowledgments This invited article was presented at the 6th Asian Particle Technology Symposium, Seoul, Korea during September 15-18, 2015. The project was supported by Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission through Advanced Functional Materials Cluster of Khon Kaen University under grant no. NRU57023. The partial research

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support from The Thailand Research Fund under TRF research scholar grant (RSA5980075) is highly appreciated. W. Naowanon would like to acknowledge the graduate research funding from National Research Council of Thailand (NRCT) under contract no. 2558-D51. The graduate scholarships granted to W. Naowanon from Center of Alternative Energy Research and Development, Khon Kaen University (AERD-KKU) (grant no. D01/56) and from Graduate School of Khon Kaen University under grant no. 57242101 are also acknowledged. References [1] M. Roy, L.M. Mcdonald, Metal uptake in plants and health risk assessments in metal – contaminated smelter soils, Land Degrad. Dev. 26 (2015) 785–792. [2] J. Viers, B. Dupre, J. Gaillardet, Chemical composition of suspended sediments in World River: new insights from a new database, Sci. Total Environ. 407 (2009) 853–868. [3] L. Keen Carl, J. Mcardle Harry, A Review: The Impact of Copper on Human Health, International Copper Association, New York, 2013. [4] Guidelines for Drinking-water Quality, Fourth Edition World Health Organisation, 2011. [5] W.J. Lau, A.F. Ismail, Polymeric nanofiltration membranes for textile dye wastewater treatment: preparation, performance evaluation, transport modelling, and fouling control-a review, Desalination 245 (2009) 321–348. [6] J. Gregory, R.V. Dhond, Wastewater treatment by ion exchange, Water Res. 6 (6) (1972) 681–694. [7] E.A. López-Maldonado, M.T. Oropeza-Guzman, J.L. Jurado-Baizavala, A. Ochoa-Terán, Coagulation–flocculation mechanisms in wastewater treatmentplants through zeta potential measurements, J. Hazard. Mater. 279 (2014) 1–10. [8] A. Sayari, S. Hamoudi, Y. Yang, Applications of pore expanded mesoporous silica. 1. Removal of heavy metal cations and organic pollutants from wastewater, Chem. Mater. 17 (2005) 212–216. [9] P. Chutia, S. Kato, T. Kojima, S. Satokawa, Arsenic adsorption from aqueous solution on synthetic zeolite, J. Hazard. Mater. 162 (2009) 440–447. [10] E. Erdem, N. Karapinar, R. Donat, The removal of heavy metal cations by natural zeolites, J. Colloid Interface Sci. 280 (2) (2004) 309–314. [11] S. Cetin, E. Pehlivan, The use of fly ash as a low cost, environmentally friendly alternative to activated carbon for the removal of heavy metals from aqueous solutions, Colloids Surf. A Physicochem. Eng. Asp. 298 (2007) 83–87. [12] H. Ge, X. Fan, Adsorption of Pb2+ and Cd2+ onto a novel activated carbon-chitosan complex, Chem. Eng. Technol. 34 (10) (2011) 1745–1752. [13] H. Yoshitake, T. Yokoi, T. Tatsumi, Adsorption behavior of arsenic at transition metal cations captured by amino-functionalized mesoporous silicas, Chem. Mater. 15 (2003) 1713–1721. [14] N. Fellenz, P. Martin, S. Marchetti, F. Bengoa, Aminopropyl-modified mesoporous silica nanonspheres for the adsorption of Cr(VI) from water, J. Porous. Mater. 22 (2015) 729–738. [15] L. Lin, M. Thirumavalavan, J. Lee, Facile synthesis of thiol-functionalized mesoporous silica-their role for heavy metal removal efficiency, Clean: Soil, Air, Water 43 (5) (2015) 775–785. [16] M. Grün, The synthesis of micrometer- and submicrometer-size spheres of ordered mesoporous oxide MCM-41, Adv. Mater. 9 (3) (2014) 254–257. [17] T. Kim, P. Chung, V. Lin, Facile synthesis of monodisperse spherical MCM-48 mesoporous silica nanoparticles with controlled particle size, Chem. Mater. 22 (2010) 5093–5104. [18] Y. Lin, C.L. Haynes, Impacts of mesoporous silica nanoparticle size, pore ordering, and pore integrity on hemolytic activity, J. Am. Chem. Soc. 132 (2010) 4834–4842. [19] C.J. Brinker, Hydrolysis and condensation of silicates: effect on structure, J. NonCryst. Solids 100 (1988) 31–50. [20] A.Q. Zhen, L. Zhang, M. Guo, Y. Liu, Q. Huo, Synthesis of mesoporous silica nanoparticles via controlled hydrolysis and condensation of silicon alkoxide, Chem. Mater. 21 (2009) 3823–3829. [21] T. Yokoi, T. Karouji, S. Ohta, J.N. Kondo, T. Tatsumi, Synthesis of mesoporous silica nanospheres promoted by basic amino acids and their catalytic application, Chem. Mater. 22 (2010) 3900–3908. [22] J. Wang, A.S. Narutaki, A. Shimojima, T. Okubo, Biphasic synthesis of colloidal mesoporous silica nanoparticles using primary amine catalysts, J. Colloid Interface Sci. 385 (2012) 41–47. [23] E. Da'na, A. Sayari, Adsorption of copper on amine-functionalized SBA-15 prepared by co- condensation: equilibrium properties, Chem. Eng. J. 166 (2011) 445–453.


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[24] K. Nakanishi, M. Tomita, K. Kato, Synthesis of amino-functionalized mesoporous silica sheets and their application for metal ion capture, J. Aust. Ceram. Soc. 3 (2015) 70–76. [25] H. Zhihui, Z. Donghui, W. Jixiao, Direct synthesis of amine-functionalized mesoporous silica for CO2 adsorption, Chin. J. Chem. Eng. 19 (3) (2011) 386–390. [26] H.T. Lu, Synthesis and characterization of amino-functionalized silica nanoparticles, Colloid J. 75 (2013) 311–318. [27] W. Stöber, A. Fink, E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69. [28] M.J. Hollamby, D. Borisova, P. Brown, J. Eastoe, I. Grillo, D. Shchukin, Growth of mesoporous silica nanoparticles monitored by time-resolved small-angle neutron scattering, Langmuir 28 (2012) 4425–4433. [29] A.B.D. Nandiyanto, S. Kim, F. Iskandar, K. Okuyama, Synthesis of spherical mesoporous silica nanoparticles with nanometer-size controllable pores and outer diameters, Microporous Mesoporous Mater. 120 (2009) 447–453. [30] P.L. Llewellyn, Y. Grillet, F. Reichert, K.K. Unger, Effect of pore size on adsorbate condensation and hysteresis within a potential model adsorbent: M41S, Microporous Mater. 3 (1994) 345–349. [31] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A. 32 (1976) 751–767. [32] E. Da'na, N.D. Silva, A. Sayari, Adsorption of copper on amine-functionalized SBA-15 prepared by co-condensation: kinetics properties, Chem. Eng. J. 166 (2011) 454–459.

[33] A. Demirbas, E. Pehlivan, F. Gode, T. Altun, G. Arslan, Adsorption of Cu(II), Zn(II), Ni(II), Pb(II), and Cd(II) from aqueous solution on amberlite IR-120 synthetic resin, J. Colloid Interface Sci. 282 (2005) 20–25. [34] E. Da'na, A. Sayari, Adsorption of heavy metals on amine-functionalized SBA-15 prepared by co-condensation: applications to real water samples, Desalination 285 (2012) 62–67. [35] E. Macingova, A. Luptakova, Recovery of metals from acid mine drainage, Chem. Eng. Trans. 28 (2012) 109–114. [36] Z. Melichová, L. Hromada, Adsorption of Pb2+ and Cu2+ ions from aqueous solutions on natural bentonite, Pol. J. Environ. Stud. 22 (2) (2013) 457–464. [37] S. Cheng-Jun, S. Ling-Zhi, S. Xian-Xiang, Graphical evaluation of the favorability of adsorption processes by using conditional Langmuir constant, Ind. Eng. Chem. Res. 52 (39) (2013) 14251–14260. [38] M. Anbia, K. Kargosha, S. Khoshbooei, Heavy metal ions removal from aqueous media by modified magnetic mesoporous silica MCM-48, Chem. Eng. Res. Des. 93 (2015) 777–788. [39] H. Yang, R. Xu, X. Xue, F. Li, G. Li, Hybrid surfactant-template mesoporous silica formed in ethanol and its application for heavy metal removal, J. Hazard. Mater. 152 (2008) 690–698. [40] M. Emin Argun, S. Dursun, C. Ozdemir, M. Karatas, Heavy metal adsorption by modified oak sawdust: thermodynamics and kinetics, J. Hazard. Mater. 141 (2007) 77–85.
