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Study of quench effect on heavy metal uptake efficiency by an aluminosilicate-based material
Accepted Manuscript Study of Quench Effect on Heavy Metal Uptake Efficiency by an Aluminosilicate-based Material Chao Ning, Meng Xu, David Chi-Wai HUI, Carol LIN Sze Ki, Gordon McKay PII: DOI: Reference:
S1385-8947(16)31640-0 http://dx.doi.org/10.1016/j.cej.2016.11.078 CEJ 16079
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
22 September 2016 8 November 2016 9 November 2016
Please cite this article as: C. Ning, M. Xu, D. Chi-Wai HUI, C. LIN Sze Ki, G. McKay, Study of Quench Effect on Heavy Metal Uptake Efficiency by an Aluminosilicate-based Material, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.11.078
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Study of Quench Effect on Heavy Metal Uptake Efficiency by an Aluminosilicate-based Material
Chao Ning 1, Meng Xu1, David Chi-Wai HUI1, Carol LIN Sze Ki2, Gordon McKay1,3*
1
Chemical and Biomolecular Engineering Department, The Hong Kong University of
Science and Technology, Hong Kong SAR 2
School of Energy and Environment, The City University of Hong Kong, Tat Chee
Avenue, Hong Kong SAR 3
Division of Sustainability, College of Science and Engineering, Hamad Bin Khalifa
University, Qatar Foundation, Doha, Qatar *
Corresponding Author: Tel: +852 23588412, Fax: +852 23580054, E-mail:
[email protected]
1
Abstract Quench is a widely used technique in organic synthesis or inorganic material treatment but only limited studies have been performed on aluminosilicate. In this work, the quench effect on the textural properties and surface functional groups has been studied by the functionalization of a waste-derived aluminosilicate material. The porous structure has been changed in the presence of a quench reagent. We consider that this difference originated due to the prevention of replacement of potassium atoms in surface ion-exchange sites by hydrogen atoms. Surface characterization including SEM, XPS and FTIR further confirmed the preservation of − − functional groups. Meanwhile, due to the differences between the electronegativities of potassium atoms and hydrogen atoms the ion-exchange ability for heavy metals of the quenched samples was significantly enhanced and this has been validated by adsorption experiments with various kinds of metals. The mass balance for leaching calcium and potassium atoms also revealed that the enhanced ion-exchange capacity was caused by the improved liberation of potassium and calcium atoms from the aluminosilicate network. Moreover, the leaching test for metal doped material shows a limited amount of metal leaching under acidic conditions, indicating the potential application of this material in industrial wastewater treatment or catalysis applications.
Key words: Aluminosilicate, quench, ion-exchange, leaching test 2
1. Introduction Aluminosilicate is generally the combination of aluminum and silicon atoms with oxygen atoms to form various structures including chains, rings, cages, layers and three-dimensional arrays[1]. Besides, calcium or other cations often present as charge compensators in order to compensate the excessive negative charge carried by the unit when a silicon atom is replaced by an aluminum atom[2]. Aluminosilicate is commonly found in the earth in large amounts as zeolite, clay, kaolin, or clinoptilolite[3-6]. The usages of natural aluminosilicate have a long history dating back to ancient times. Currently, the natural and synthetic aluminosilicates are still widely used due to their excellent thermal stability, large surface area, extraordinary ion-exchange ability as well as their low cost and they find a wide array of applications
in
catalysis[7-9],
biology[10],
electrochemistry[11],
organic
synthesis[12], wastewater treatment[13], dehumidification[14], drug delivery[15], fertilizer[16] and sorbents[17]. Moreover, aluminosilicates can form hybrid materials with graphene, chitosan or Nafion due to their alterable size and morphology, convenience for intercalation of various organic species along with stability derived from low permeability[18-20]. The immobilization of organic compounds in the aluminosilicate matrix can provide additional properties including conductivity or catalytic ability[21,22]. Also, doping with transitional metals including iron or yttrium and metal oxide such as TiO2 or MnFe2O4 is another alternative for aluminosilicate
3
modification because of the feasibility of aluminosilicate fabrication to involve atoms or molecules into the structures[23-26]. The high specific surface area and functional group density favor the modification process by hosting nanosized particles to form homogenous/heterogeneous products. Among all the advantages, the capability of structure modification and surface functionalization of aluminosilicate that can be tailored accordingly to demand allows a wide range of applications since the textural properties and surface functional groups can be manipulated to meet selective requirements. Indeed, applications for various purposes will require distinctly different types of aluminosilicate with specific structures and surface functional groups because these differences will have effects on properties that finally result in the differences in catalytic efficiency or ion-exchange capacity. For example, the change of the pore size for ion-exchange atoms can cause different selectivity and diffusion rates for gas or small molecules. Trinh et al. [27]studied the removal of dilute N2O from gas streams using Ca 13X (Ca2+ exchanged Zeolite 13X) and increased the N2O removal capacity from 3.5×10-6 mol g-1 to as high as 80×10-6 mol g-1. It was claimed that the stronger interaction of N2O with Ca2+ cations compared to Na+ cations contributed to the increase in N2O removal capacity. Also, for catalytic applications normally the atoms on catalytic sites will be selected after comprehensive consideration. Therefore, attempts such as the strategies of reaction variables optimization including temperature, impregnation ratio and duration time, the use of templates to assemble at desired or design drying condition 4
like rapid ambient pressure drying, have been tested to develop various patterns of aluminosilicate structure as well as surface functional groups with the purpose of achieving a controllable structure tuning[28-30]. However, to date a quench technique has been rarely used in aluminosilicate modification. In contrast, it is widely used in metal treatment and organic synthesis[31-34]. Quench generally means to end a reaction or change the reaction condition suddenly, normally by cooling. The products obtained will be prevented from undergoing the undesired low-temperature processes[35]. This can be achieved by a wide range of quenching reagents including water, ice, oil or even gas[36-38]. In the organic synthesis process, quenching reagents are added to rapidly stop the reaction, in order to improve the density of favorable functional groups that will fade in a normal cooling process[39]. Recent works[40] show that the quench technique can also be used for inorganic materials like the application in nanoparticle synthesis in order to avoid self-agglomeration. The efforts of quench in inorganic material preparation and modification indicate the possibility of its application on siliceous material functionalization. The preservation of surface functional groups of siliceous material has the possibility to be achieved by the addition of quenching reagents. However, these attempts still remain inadequate and the quench effect on aluminosilicate is not well studied except for some studies on aluminosilicate glasses which have certain differences with functionalized aluminosilicate[41,42].
5
In this work, the quench effects on aluminosilicate structure and surface functional groups were first studied by a waste-derived aluminosilicate-based material though surface characterization and textural properties measurements. Functionalized samples were prepared under different quenching conditions and heavy metal uptake experiments based on the mass balance were performed to explore their potential application on wastewater treatment or as a catalyst support. Furthermore, leaching tests were conducted to evaluate the stability of adsorbent-adsorbate bonding during a harsh environment.
2. Experimental 2.1 Preparation of Quenched Aluminosilicate The non-metallic fraction (NMF) of waste printed circuit board was obtained from a local company in the form of fine powder with an average particle size of 2 µm. It is used as a highly economical and easily available aluminosilicate source. The NMF was mixed with 1 M potassium hydroxide solution at an impregnation ratio of 2 (weight of potassium hydroxide/weight of NMF) at room temperature for 1h until a homogeneous dispersion was achieved. Afterwards, the mixed slurry will undergo thermal functionalization in a muffle furnace for 1 hour at 300 °C under an inert nitrogen atmosphere. The detailed procedure was illustrated in our previous work[43]. Subsequently, the quench process was conducted by adding water as quenching 6
reagent at a range of temperature (110-150°C) after thermal functionalization. The samples obtained at quenching temperatures of 110°C, 130°C and 150°C were named QANMF110, QANMF130 an QANMF150, respectively. Reference samples (ANMF) were prepared as reported in previously published work without the quench procedure[44]. All the samples were washed with deionized water to remove unreacted potassium hydroxide and any impurities attached until the pH value of supernatant is below 8. The solid products were vacuum-dried in an oven at 110°C for 2 hours and kept in a desiccator for later tests. The obtained samples were pale grey powders with high hydrophilicity. Notably, a minimum of three duplicated samples were prepared to increase the credibility of the results.
2.2 Textural Properties Nitrogen adsorption−desorption measurements were performed by a Beckman Coulter SA3100 surface area analyzer over a relative pressure range (p/p0) from 0.01 to 0.99 at 77 K. Prior to analysis the samples were outgassed at 150 °C for 90 min in order to remove moisture and any other impurities attaching on the samples. The specific surface area was calculated using the linear relative pressure range from 0.05 to 0.35 using the Brunauer−Emmett−Teller (BET) equation while the total pore volume (Vt) was determined at a relative pressure of 0.98. The pore size distribution was obtained using the Barrett−Joyner−Halenda (BJH) equation based on the
7
adsorption branch of the isotherm. The average pore diameter (d p) was estimated by assuming the cylindrical pore geometry of the samples structure.
2.3 Surface Characterization The surface characterization of aluminosilicate-based material is important since it reflects the morphology and functional groups distribution of the material. The surface morphologies of samples were investigated by a scanning electron microscopy (JEOL JSM-6390 instrument) coupled with energy-dispersive X-ray spectroscopy (EDX) with the assistance of gold coating. The study areas were selected randomly for EDX elementary mapping. The surface functional groups of all samples were studied by Fourier transform infrared spectroscopy (FTIR) using a FTS 6000 FTIR spectrometer over a range of 4000−400 cm−1. Also, both low-resolution and high-resolution X-ray photoelectron spectroscopy (XPS) data was recorded using a XPS model PHI5600 to investigate the chemical state of the samples.
2.4 Adsorption Experiments The batch adsorption tests were conducted by adding 50 mg functionalized sample into HDPE bottles filled with 50 mL of 5 mmol L−1 metal nitrate solution (copper nitrate, cobalt nitrate or lead nitrate). The pH value was adjusted to 4 by adding 0.5M HNO3 and/or NaOH solutions to minimum the effect of acidity and/or basicity. The 8
solutions were put in a thermostatic orbital shaker at 25 °C with a rotating rate of 150 rpm for 4 days until equilibrium was reached. An inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, PerkinElmer) was used to measure the initial and final metal concentrations. Thus the metal uptake capacities of the functionalized materials can be calculated by simple mass balance between the liquid and solid phases shown in equation (1):
=
( − )
(1)
where C0 and Ce represent the initial and final metal concentrations of the solution, respectively, V is the volume of the solution and m is the mass of functionalized samples added. Since previous works have proved ion-exchange to be the main mechanism of heavy metal uptake, the final pH of all solutions were measured by a pH meter (Bluelab pH meter, Bluelab Corporation Ltd) to ensure that no precipitation occurred[45].
2.5 Leaching Test The leaching test are designed according to USEPA Method 1311 (Toxicity Characteristic Leaching Procedure) and other works[46][47]. After adsorption experiments, the materials with deposited metals, named ANMF/M or QANMF/M (M
9
is the name of the metal) were vacuum-dried in an oven at 110°C for 10 minutes. The leaching agent 1#（ Adding 5.7 mL glacial acid to 500 mL of reagent water, then adding 64.3 mL of 1N NaOH and dilute to a volume of 1 liter. The pH of this fluid was 4.95) was used considering the pH range of the solution. 1g of leaching agent 1# was added to the leaching vessel to mix with 0.05g ANMF/M or QANMF/M. The samples were rotated at 30 rpm for 18h in a thermostatic orbital shaker at 25 °C. The final metal concentrations in the solution indicate the leaching potential of metal ions from ANMF/M or QANMF/M which were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, PerkinElmer).
3. Results and Discussion 3.1 Quench Effect The composition of NMF is randomly assembled calcium aluminosilicate fibers with carbonaceous material mainly consisting of brominated flame retardants or dyes as partially cover. The dominant structural species in NMF network is Q4 units which implies a high content of BOs (bridging oxygens), indicating a weak ability of ion-exchange as our previous work has shown[48]. The functionalization of NMF can be achieved by hydroxylation though the cleavage of bridging oxygens ( − − ) to form functional groups like − − or − − . This is called the tunneling effect and was illustrated in our previous work whereas simplified scheme 10
for the tunneling effect was developed and is shown in Figure 1(a). Also, during the functionalization process calcium atoms, which play the role of network charge compensator in this phase of the process can also be liberated from the structure and change their character to become a network modifier. The hydroxylation process can significantly change the properties of NMF to assign it with efficient ion-exchangeability.
Figure 1. Schematic illustration of (a) tunneling effect and (b) the parallel reaction (potassium atoms are replaced by hydrogen atoms) in the cooling process
However, a parallel reaction of functional group conversion is also expected to happen at the same time as shown in Figure 1(b). The potassium atoms in − − 11
functional groups can be replaced by hydrogen atoms and thus form more − − functional groups. This reaction will move towards the right side of the equation during the cooling process when the hydroxylation process already ended due to the lack of intensive energy input. Therefore, the final product will have a higher content of − − functional groups instead of − − functional groups due to this conversion during the cooling process. Quench can effectively prevent the undesired reaction in cooling process by quickly reducing the temperature to ambient condition through the addition of a quenching reagent. Thus quenching the functionalized aluminosilicate at a relatively high temperature can preserve the − − functional groups in a significantly high percentage and higher quenching temperature which will better favor the preservation. The replacement of potassium atoms by hydrogen atoms in the − − functional groups will lead to the change of both surface and textural properties by the formation of a different size of surface functional groups[49]. The atomic radius for the hydrogen atom and the potassium atoms are 53 pm and 280 pm, respectively. Conversion of − − functional groups to − − functional groups will cause the increase of pore volume and specific surface area due to the formation of a more porous structure. The results of nitrogen adsorption/desorption confirm the decrease of porosity after quench, which is possibly caused by the conversion of − − functional groups to − − as shown in Table 1. The specific surface area drops from 237 m2 g-1 to 180 m2 g-1 and the pore volume decrease from 12
0.5725 cm3 g-1 to 0.5221 cm3 g-1 when even only a mild quench on 110 ℃ was applied, probably due to the filling phenomenon caused by the enhanced density of − − functional groups which have a much larger size compared to the − − functional groups. Further increasing the quench temperature will cause exacerbated loss of specific surface area and pore volume to as low as 86 m2 g-1 and 0.3491 cm3 g-1, respectively. Also the micropore surface area drops from 12 m2 g-1(ANMF) to 5 m2 g-1(QANMF110) and completely disappears at higher quenching temperatures, which is consistent to the surmise that the micropore will be filled first due to their small pore size.
Table 1 Textural properties of unquenched and quenched samples
Sample
SBET (m2 g-1)
Smic(m2 g-1)
Vt (cm3 g-1)
dp (nm)
ANMF
237
12
0.5725
9.66
QANMF110
180
5
0.5221
11.61
QANMF130
118
0
0.4677
15.85
QANMF150
86
0
0.3491
16.23
Detailed nitrogen adsorption/desorption isotherms and pore size distribution curves are shown in Figure 2. Despite the differences in shape and area shown in Figure 2(a), all the isotherm curves are assigned to mesoporous material due to the type IV isotherms with H3-type hysteresis loops they produce according to the IUPAC 13
classification. However, the differences in hysteresis loops revealed the discrepancies in their textural properties.
From ANMF to QANMF the hysteresis loop was
converted to form a spindly shape, indicating the loss of mesoporous surface area. Further increasing the quenching temperature will cause an increase in the disappearance of mesoporous surface area, which is consistent with the fact that the higher quenching temperatures will preserve more − − functional groups with a larger size and thus reinforce the pore blockage effect. The pore size distribution curves in Figure 2(b) show that with the increase in quenching temperature, there is a formation of big holes and/or the disappearance of small holes confirmed by a peak shift from small pore diameter to high pore diameter. Details shown in Table 1 confirms that the average pore diameter calculated by the formula dp=4V/S BET almost doubled from 9.66 nm to 16.23 nm with the increase in quenching temperature. It is obvious that the more − − functional groups are preserved, the less − − functional groups will be created and thus the small pores have more chance to become blocked by the abundance of − − . This would cause the decrease in specific surface area as well as the pore volume and an increase in the average pore diameter.
14
Volume (cm3/g)
a
QANMF150 QANMF130 QANMF110 ANMF
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
(Vp)/d(Dp)((ml/g*nm))
b
QANMF150 QANMF130 QANMF110 ANMF
0
10
20
30
40
50
60
70
Pore diameter (nm)
Figure 2. (a) Nitrogen adsorption desorption isotherm and (b) pore size distribution for the reference sample and the samples quenched at different temperatures.
15
3.2 Surface Characterization Figure S1 shows the scanning electron microscopy (SEM) images of ANMF and quenched ANMF. ANMF is composed of the porous aluminosilicate structure and quenched ANMF shows a similar structure with ANMF, revealing the influence of the quench effect does not occur in the hundred nanometer scale. The EDX elementary mappings were conducted for the same scanning time. The density of oxygen and calcium in the sample surface remain almost the same under the different quench conditions, indicating that there is no additional formation of hydrogen or calcium ion-exchange sites. However, the EDX elementary mappings of potassium show some differences. The higher quenching temperature caused a higher density of potassium in the material surface, which signifies the reinforcement of − − functional groups. Considering the stabilization of oxygen and calcium atoms, it can be concluded that the increase in potassium atoms does not originate from cleavage of − − ′ bonding but is due to the replacement of other silanol groups. The FTIR spectra of ANMF and quenched samples in the frequency region 4000 – 400 cm-1 were investigated to study the surface functionalization of quenched and unquenched samples as shown in Figure 3(a). All the spectra are normalized according to the reference peak in 470 cm-1 representing the bending mode of − − (T= Si or Al) highlighted in Figure 3(a)[50]. The spectra of all samples showed similar shapes and relative peak intensity in the studied region. The peak in the region around 1001 cm-1 stands for the stretching mode of − − (T= Si or 16
Al) has no significant change which can be seen from the dashed line[51]. However, the band within the range 3800 – 2600 cm-1 associated with the stretching mode of the − − (X = K or H) functional groups showed a shift to a lower wavenumber thus giving a prolonged tail for this broad peak, emphasized in the figure using a dashed line. This is possibly due to the replacement of hydrogen atoms by potassium atoms in − − functional groups since the much higher atomic weight of potassium compared with hydrogen will cause the peak shift to a low wavenumber considering the decrease of stretching frequency. A detailed comparison in the region of 1001 cm-1 and 3800 – 2600 cm-1 was conducted only between QANMF150 and ANMF for representativeness. As shown in Figure 3(b) the peaks around 1001 cm-1 which stands for the stretching mode of − − (T = Si or Al), has no significant change in both peak position and intensity. It means that the total amount of − − has limited change, denying the possibility that there was more conversion of − − to − − by hydrolysis, as a result no additional − − functional groups were formed. The spectra region from 3800 – 2600 cm-1, on the other hand, confirmed the replacement of hydrogen atoms by potassium atoms in − − functional groups as shown in Figure 3(c). A comparison of the high resolution spectra of QANMF150 and ANMF in the studied region shows that QANMF150 has an obvious prolonged tail. The higher atoms in a functional group will lead to a decrease of vibration frequency, thus causing the shift toward the lower wavenumber. However, the 17
integration of these two peaks is very similar and it shows that it is the replacement of hydrogen atoms by potassium atoms instead of increasing of − − groups in − − (X = K or H) functional groups.
Figure 3. (a) FTIR spectra for (I)ANMF (II)QANMF110 (III)QANMF130 (IV)QANMF150 and (b) spectra comparison of ANMF and QANMF around 1100 cm-1 and (c) spectra comparison of ANMF and QANMF around 3400 cm-1
X-ray photoelectron spectroscopy (XPS) was used to investigate the quench effect on the surface atomic composition and environment by comparing quenched and
18
unquenched samples. As shown in Figure 4, the low resolution spectra show limited differences among quenched samples so the comparison of high resolution spectra will be conducted only between ANMF and QANMF150, that is, the representative of the quenched samples. Several differences were observed in terms of peak position or intensity, indicating the change of surface chemical composition between QANMF and ANMF. The intensity of the K 2p peak at 293 eV was significantly strengthened in QANMF150 due to the increasing doping of potassium atoms, implying the possibility of − − formation which will improve the ion-exchange ability[52]. However, the intensity of the O 1s peak of QANMF150 at 531.4 eV has no obvious increase. This indicates that the amount of oxygen atoms in QANMF surface did not increase, which means the increase of potassium atoms does not originate from increasing the − − cleavage but the conversion of − − to − − functional groups. The shift of O 1s peak position from 530.9 eV (ANMF) to 531.4 eV (QANMF150) also confirms this option since the replacement of hydrogen atoms by heavier potassium atoms will cause the binding energy to shift to a higher position. In another work of our group, the metal doped material ion-exchanged with copper atoms showed a shift of O 1s peak to 531.8 eV[48]. Considering the fact that copper atoms have a heavier atomic weight than potassium atoms, it is consistent with the hypothesis that the replacement of heavier atoms will cause the shift of the O 1s peak towards the high binding energy side[53]. Moreover, the Ca 2p peak has no obvious change both in peak position and intensity, revealing 19
the fact that due to the difficulty in Ca replacement the conversion of calcium functional groups is limited.
Figure 4 XPS spectra for the ANMF and quenched QANMF. The high resolution spectra of O1s, K 2p and Ca 2p were provided for detailed comparison.
3.3 Ion Exchange Efficiency The object of adsorption isothermal tests is to study the metal ion uptake capacity and set the mass balance between the ions exchanged out and ions adsorbed in order to prove that the increase in ion-exchange capacity is because of the prevention of replacement of potassium atoms in − − functional groups by hydrogen atoms. 20
In this case, copper, cobalt, and lead were employed to evaluate the ion-exchange efficiency of the sample. The initial pH of metal solutions was adjusted to 4 with the purpose to ensure no precipitation was involved in the experiment. The adsorption isothermal test results are shown in Table 2. Table 2 Comparison of ion exchange capacities of ANMF with quenched samples (unit: mmol g-1)
Metal ions Cu 2+
Co2+
Pb2+
ANMF
2.79
3.03
3.19
QANMF110
3.05
3.28
3.19
QANMF130
3.21
3.36
3.22
QANMF150
3.50
3.55
3.19
Sample Names
From the results it can be concluded that quench effect significantly improved the adsorption capacities of Cu2+ and Co2+ from 2.79 mmol g-1 and 3.03 mmol g-1 to 3.50 mmol g-1 and 3.55 mmol g-1, respectively. However, the metal uptake capacity of Pb2+ did not improved obviously and stayed around 3.2 mmol g-1. This can be explained by the electronegativity differences shown in Table 3. It can be seen from the table that the electronegativity of hydrogen is 2.20, which is far beyond the electronegativity of potassium (0.82). Therefore, when − − functional groups do ion-exchange, the driving force for it to exchange with Co2+ (electronegativity 1.88) and Cu 2+ 21
(electronegativity 1.90) is not sufficient[54-56]. However, Pb2+ can still be efficiently ion-exchanged by − − functional groups since the electronegativity of Pb is 2.33, greatly exceeding those of Cu and Co and exceeding that of hydrogen. Therefore, the replacements of hydrogen atoms by potassium atoms cause little effect on the ion-exchange capacities of Pb2+, since Pb2+ has the ability to exploit the majority of the available sites even without the surface modification. Other than electronegativity, the radius of hydrated metal ion also matters in ion-exchange process and final uptake capacity. Therefore, the different capacities between copper and cobalt, which have similar electronegativities, can be explained by the disparity between the radius of hydrated metal ion of copper and cobalt. The radius of hydrated Co 2+ ion is 2.82 Å while for hydrated Cu2+ ion the radius is 2.58 Å in the equatorial positions and 3.22 Å in the axial positions due to Jahn–Teller Effect[57]. Since Cu 2+ and Co2+ have the same hydrated ion charge, Co 2+ with smaller hydrated ion radius will possess higher ion-exchange capacity like shown in Table 2. Moreover, Pb2+ ions with further larger hydrated ion radius (3.74 Å), showed even lower ion-exchange capacities.
22
Table 3 Electronegativities of common elements of Pauling’s scale
Elements
Electronegativity
H
2.20
K
0.82
Ca
1.00
Co
1.88
Cu
1.90
Pb
2.33
The explanation can be proved by the fitting figure of Ca2+ + K+/2 with the metal uptake capacity as shown in Figure 5. Stoichiometrically one calcium atom can exchange with one metal ion that has two positive charges while two potassium atoms are necessary to exchange with only one two plus charged metal ion due to the requirement of the charge balances. The Ca2+ + K+/2 fitting results of Cu 2+ and Co 2+ showed that the improvement in metal uptake capacities were consistent with the increase of potassium leaching, indicating that the capacity increase was just coming from the replacement of hydrogen atoms by potassium atoms. It was achieved by enhancing the ion-exchange capacity through increasing the percentage of − − functional groups. The enhancement of − − groups is mainly caused by preventing the replacement of the potassium atoms in − − by hydrogen atoms, and this was confirmed by the increase in leaching of potassium atoms. The
23
gaps between the real and simulated capacities for lead are related to ion-exchange with − − functional groups that failed to be traced by the mass balance of calcium and potassium leaching. With the increase in quenching temperature, the gap is shrinking because of the enhancement of − − functional groups density. However, the capacities in the Pb2+ ion-exchange had no obvious change, indicating that lead ions have the ability to ion-exchange with both − − or − − functional groups, and this is attributed to the electronegativity of Pb (2.33) being higher than that of H (2.20).
24
Figure 5. Ion-exchange capacities for ANMF and quenched samples. The simulations of Ca2++K+/2 were also provided as well as the K+ leached out.
Also, it can be concluded from the pH profile after ion-exchange shown in Table S1 that the potassium content in quenched samples is higher than ANMF since the pH values of quenched samples after ion-exchange is generally higher than ANMF due to the increase amount of potassium ions into the solution instead of hydrogen ions. The pH value of the Pb2+ solution after ion-exchange has a significant difference between ANMF and the quenched samples because even in ANMF all the hydrogen atoms in − − functional groups were exchanged out due to the high electronegativity of Pb. Therefore, the pH drop of Pb is more obvious than Cu and Co. Meanwhile, after hydrogen atoms were replaced by potassium atoms, the pH value of the quenched samples after ion-exchange with Pb shows a significant enhancement in basicity. Also according to previous works, the potassium atoms in aluminosilicate will compete with calcium atoms for the role of charge compensator[58]. Therefore, calcium atoms will change their roles from network charge compensator to network modifiers after they were replaced by potassium atoms in the competition. Subsequently, calcium atoms have much higher chance to be ion-exchanged. Therefore, although a quench process only increased the density of potassium atoms in the surface of the material and left no change in the density of calcium atoms, it can 25
still enhance the ion-exchange performance of calcium atoms by displacing them from the position of network charge compensator. Excess of potassium atoms will further facilitate this enhancement and this was validated by the ICP test showing the amount of leaching calcium atoms after ion-exchange had also increased slightly.
3.4 Leaching Test Leaching after metal uptake is a critical point for metal deposition, both in the applications of wastewater treatment and catalyst preparation. The leaching of metal ions back to the aquatic system will cause unwanted secondary pollution in wastewater treatment and catalyst deactivation in catalyst process[59-62]. Thus, conducting a leaching test after metal deposition in order to ensure the minimum leaching of heavy metals is indispensable for adsorbent preparation. The leaching tests of Cu 2+, Co2+ and Pb2+ were conducted in this work and the leaching results compared with their metal uptake capacities are shown in Figure 6.
Figure 6. Comparative results of leaching tests 26
The results of leaching tests revealed that the leaching of the sorbed metal ions for both quenched and unquenched ANMF is acceptable. Less than 4% leaching of all the metal ions were observed for all the investigated samples. QANMF150 showed good resistance to acidic condition (pH=4.95) with the leaching percentages of 2.205% (Cu), 1.879% (Co) and 1.937% (Pb), respectively. Also, generally for all samples the leaching percentage of copper and cobalt are higher than lead. This is due to the high electronegativity of Pb (electronegativity 2.33) compared to Co (electronegativity 1.88) and Cu (electronegativity 1.90), therefore, when the materials deposited with heavy metals were put back into acidic solution, hydrogen (electronegativity 2.20) will have more difficulty in exchanging Pb 2+ out compared to Co2+ and Cu2+. The results of the leaching tests showed excellent metal immobilization ability on QANMF[63,64]. Thus, it can be concluded that after taking heavy metal ions from solution, the bonds between adsorbent and adsorbate are very strong and stable. Hence, the results provide potential applications for these aluminosilicate-based quenched materials in wastewater treatment by the removal of heavy metal ions or catalyst preparation by depositing with transition metals on QANMF. The metal doped on the surface of the material can provide an open shell orbit for electron transfer in catalysis reactions[65] . The high stability will greatly favor the above mentioned applications due to the minimum leaching of heavy metal ions indicating the prevention of the release of metal ions into aquatic systems.
27
4. Conclusion The quench effect on the textural properties and surface functional groups has been studied by the functionalization of a waste-derived aluminosilicate material. The porous structure has been changed in the presence of a quenching reagent and it is supposed that this difference originated from the prevention of the replacement of potassium atoms in surface ion-exchange sites by hydrogen atoms. The parallel potassium replacing reaction in cooling process was effectively stopped by this quench technique and thus left the surface of functionalized aluminosilicate with high content of − − functional groups. Surface characterization including SEM, XPS and FTIR further confirmed the preservation of − − functional groups. Meanwhile, due to the differences between the electronegativities of potassium atoms and hydrogen atoms the ion-exchange ability for heavy metals of quench samples are significantly enhanced and this is validated by the adsorption experiment with several metals. The mass balance for leached calcium and potassium atoms also confirmed the fact that the enhanced ion-exchange capacities were caused by the improved liberation of potassium and calcium atoms from the aluminosilicate network. The kinetic tests and adsorption isotherm focusing on the variables of adsorption process will be studied to further establish the adsorption model. Moreover, the leaching tests for metal doped materials showed limited amounts of metal ions had been leached out at acidic conditions, indicating the potential application of this material in industrial wastewater treatment or catalysis. 28
Acknowledgement The authors would like to thank the Hong Kong Research Grant Council for their support of this research.
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38
Supplementary material
Figure S1. SEM images and EDX elementary mapping of O, Ca and K of (a)ANMF (b)QANMF110 (c)QANMF130 and (d)QANMF150
39
Table S1 pH profiles after ion-exchange
Metals Cu
Co
Pb
ANMF
5.1
6.4
4.8
QANMF110
5.2
6.5
5.2
QANMF130
5.3
6.5
5.3
QANMF150
5.5
6.6
5.3
Adsorbents
40
Highlights •
Novel modification technique of aluminosilicate
•
High ion-exchange capacities
•
Trace amount of metal ions leaching after ion-exchange
•
Systematic analysis of quench effect mechanism
41
S1385-8947(16)31640-0 http://dx.doi.org/10.1016/j.cej.2016.11.078 CEJ 16079
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
22 September 2016 8 November 2016 9 November 2016
Please cite this article as: C. Ning, M. Xu, D. Chi-Wai HUI, C. LIN Sze Ki, G. McKay, Study of Quench Effect on Heavy Metal Uptake Efficiency by an Aluminosilicate-based Material, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej.2016.11.078
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Study of Quench Effect on Heavy Metal Uptake Efficiency by an Aluminosilicate-based Material
Chao Ning 1, Meng Xu1, David Chi-Wai HUI1, Carol LIN Sze Ki2, Gordon McKay1,3*
1
Chemical and Biomolecular Engineering Department, The Hong Kong University of
Science and Technology, Hong Kong SAR 2
School of Energy and Environment, The City University of Hong Kong, Tat Chee
Avenue, Hong Kong SAR 3
Division of Sustainability, College of Science and Engineering, Hamad Bin Khalifa
University, Qatar Foundation, Doha, Qatar *
Corresponding Author: Tel: +852 23588412, Fax: +852 23580054, E-mail:
[email protected]
1
Abstract Quench is a widely used technique in organic synthesis or inorganic material treatment but only limited studies have been performed on aluminosilicate. In this work, the quench effect on the textural properties and surface functional groups has been studied by the functionalization of a waste-derived aluminosilicate material. The porous structure has been changed in the presence of a quench reagent. We consider that this difference originated due to the prevention of replacement of potassium atoms in surface ion-exchange sites by hydrogen atoms. Surface characterization including SEM, XPS and FTIR further confirmed the preservation of − − functional groups. Meanwhile, due to the differences between the electronegativities of potassium atoms and hydrogen atoms the ion-exchange ability for heavy metals of the quenched samples was significantly enhanced and this has been validated by adsorption experiments with various kinds of metals. The mass balance for leaching calcium and potassium atoms also revealed that the enhanced ion-exchange capacity was caused by the improved liberation of potassium and calcium atoms from the aluminosilicate network. Moreover, the leaching test for metal doped material shows a limited amount of metal leaching under acidic conditions, indicating the potential application of this material in industrial wastewater treatment or catalysis applications.
Key words: Aluminosilicate, quench, ion-exchange, leaching test 2
1. Introduction Aluminosilicate is generally the combination of aluminum and silicon atoms with oxygen atoms to form various structures including chains, rings, cages, layers and three-dimensional arrays[1]. Besides, calcium or other cations often present as charge compensators in order to compensate the excessive negative charge carried by the unit when a silicon atom is replaced by an aluminum atom[2]. Aluminosilicate is commonly found in the earth in large amounts as zeolite, clay, kaolin, or clinoptilolite[3-6]. The usages of natural aluminosilicate have a long history dating back to ancient times. Currently, the natural and synthetic aluminosilicates are still widely used due to their excellent thermal stability, large surface area, extraordinary ion-exchange ability as well as their low cost and they find a wide array of applications
in
catalysis[7-9],
biology[10],
electrochemistry[11],
organic
synthesis[12], wastewater treatment[13], dehumidification[14], drug delivery[15], fertilizer[16] and sorbents[17]. Moreover, aluminosilicates can form hybrid materials with graphene, chitosan or Nafion due to their alterable size and morphology, convenience for intercalation of various organic species along with stability derived from low permeability[18-20]. The immobilization of organic compounds in the aluminosilicate matrix can provide additional properties including conductivity or catalytic ability[21,22]. Also, doping with transitional metals including iron or yttrium and metal oxide such as TiO2 or MnFe2O4 is another alternative for aluminosilicate
3
modification because of the feasibility of aluminosilicate fabrication to involve atoms or molecules into the structures[23-26]. The high specific surface area and functional group density favor the modification process by hosting nanosized particles to form homogenous/heterogeneous products. Among all the advantages, the capability of structure modification and surface functionalization of aluminosilicate that can be tailored accordingly to demand allows a wide range of applications since the textural properties and surface functional groups can be manipulated to meet selective requirements. Indeed, applications for various purposes will require distinctly different types of aluminosilicate with specific structures and surface functional groups because these differences will have effects on properties that finally result in the differences in catalytic efficiency or ion-exchange capacity. For example, the change of the pore size for ion-exchange atoms can cause different selectivity and diffusion rates for gas or small molecules. Trinh et al. [27]studied the removal of dilute N2O from gas streams using Ca 13X (Ca2+ exchanged Zeolite 13X) and increased the N2O removal capacity from 3.5×10-6 mol g-1 to as high as 80×10-6 mol g-1. It was claimed that the stronger interaction of N2O with Ca2+ cations compared to Na+ cations contributed to the increase in N2O removal capacity. Also, for catalytic applications normally the atoms on catalytic sites will be selected after comprehensive consideration. Therefore, attempts such as the strategies of reaction variables optimization including temperature, impregnation ratio and duration time, the use of templates to assemble at desired or design drying condition 4
like rapid ambient pressure drying, have been tested to develop various patterns of aluminosilicate structure as well as surface functional groups with the purpose of achieving a controllable structure tuning[28-30]. However, to date a quench technique has been rarely used in aluminosilicate modification. In contrast, it is widely used in metal treatment and organic synthesis[31-34]. Quench generally means to end a reaction or change the reaction condition suddenly, normally by cooling. The products obtained will be prevented from undergoing the undesired low-temperature processes[35]. This can be achieved by a wide range of quenching reagents including water, ice, oil or even gas[36-38]. In the organic synthesis process, quenching reagents are added to rapidly stop the reaction, in order to improve the density of favorable functional groups that will fade in a normal cooling process[39]. Recent works[40] show that the quench technique can also be used for inorganic materials like the application in nanoparticle synthesis in order to avoid self-agglomeration. The efforts of quench in inorganic material preparation and modification indicate the possibility of its application on siliceous material functionalization. The preservation of surface functional groups of siliceous material has the possibility to be achieved by the addition of quenching reagents. However, these attempts still remain inadequate and the quench effect on aluminosilicate is not well studied except for some studies on aluminosilicate glasses which have certain differences with functionalized aluminosilicate[41,42].
5
In this work, the quench effects on aluminosilicate structure and surface functional groups were first studied by a waste-derived aluminosilicate-based material though surface characterization and textural properties measurements. Functionalized samples were prepared under different quenching conditions and heavy metal uptake experiments based on the mass balance were performed to explore their potential application on wastewater treatment or as a catalyst support. Furthermore, leaching tests were conducted to evaluate the stability of adsorbent-adsorbate bonding during a harsh environment.
2. Experimental 2.1 Preparation of Quenched Aluminosilicate The non-metallic fraction (NMF) of waste printed circuit board was obtained from a local company in the form of fine powder with an average particle size of 2 µm. It is used as a highly economical and easily available aluminosilicate source. The NMF was mixed with 1 M potassium hydroxide solution at an impregnation ratio of 2 (weight of potassium hydroxide/weight of NMF) at room temperature for 1h until a homogeneous dispersion was achieved. Afterwards, the mixed slurry will undergo thermal functionalization in a muffle furnace for 1 hour at 300 °C under an inert nitrogen atmosphere. The detailed procedure was illustrated in our previous work[43]. Subsequently, the quench process was conducted by adding water as quenching 6
reagent at a range of temperature (110-150°C) after thermal functionalization. The samples obtained at quenching temperatures of 110°C, 130°C and 150°C were named QANMF110, QANMF130 an QANMF150, respectively. Reference samples (ANMF) were prepared as reported in previously published work without the quench procedure[44]. All the samples were washed with deionized water to remove unreacted potassium hydroxide and any impurities attached until the pH value of supernatant is below 8. The solid products were vacuum-dried in an oven at 110°C for 2 hours and kept in a desiccator for later tests. The obtained samples were pale grey powders with high hydrophilicity. Notably, a minimum of three duplicated samples were prepared to increase the credibility of the results.
2.2 Textural Properties Nitrogen adsorption−desorption measurements were performed by a Beckman Coulter SA3100 surface area analyzer over a relative pressure range (p/p0) from 0.01 to 0.99 at 77 K. Prior to analysis the samples were outgassed at 150 °C for 90 min in order to remove moisture and any other impurities attaching on the samples. The specific surface area was calculated using the linear relative pressure range from 0.05 to 0.35 using the Brunauer−Emmett−Teller (BET) equation while the total pore volume (Vt) was determined at a relative pressure of 0.98. The pore size distribution was obtained using the Barrett−Joyner−Halenda (BJH) equation based on the
7
adsorption branch of the isotherm. The average pore diameter (d p) was estimated by assuming the cylindrical pore geometry of the samples structure.
2.3 Surface Characterization The surface characterization of aluminosilicate-based material is important since it reflects the morphology and functional groups distribution of the material. The surface morphologies of samples were investigated by a scanning electron microscopy (JEOL JSM-6390 instrument) coupled with energy-dispersive X-ray spectroscopy (EDX) with the assistance of gold coating. The study areas were selected randomly for EDX elementary mapping. The surface functional groups of all samples were studied by Fourier transform infrared spectroscopy (FTIR) using a FTS 6000 FTIR spectrometer over a range of 4000−400 cm−1. Also, both low-resolution and high-resolution X-ray photoelectron spectroscopy (XPS) data was recorded using a XPS model PHI5600 to investigate the chemical state of the samples.
2.4 Adsorption Experiments The batch adsorption tests were conducted by adding 50 mg functionalized sample into HDPE bottles filled with 50 mL of 5 mmol L−1 metal nitrate solution (copper nitrate, cobalt nitrate or lead nitrate). The pH value was adjusted to 4 by adding 0.5M HNO3 and/or NaOH solutions to minimum the effect of acidity and/or basicity. The 8
solutions were put in a thermostatic orbital shaker at 25 °C with a rotating rate of 150 rpm for 4 days until equilibrium was reached. An inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, PerkinElmer) was used to measure the initial and final metal concentrations. Thus the metal uptake capacities of the functionalized materials can be calculated by simple mass balance between the liquid and solid phases shown in equation (1):
=
( − )
(1)
where C0 and Ce represent the initial and final metal concentrations of the solution, respectively, V is the volume of the solution and m is the mass of functionalized samples added. Since previous works have proved ion-exchange to be the main mechanism of heavy metal uptake, the final pH of all solutions were measured by a pH meter (Bluelab pH meter, Bluelab Corporation Ltd) to ensure that no precipitation occurred[45].
2.5 Leaching Test The leaching test are designed according to USEPA Method 1311 (Toxicity Characteristic Leaching Procedure) and other works[46][47]. After adsorption experiments, the materials with deposited metals, named ANMF/M or QANMF/M (M
9
is the name of the metal) were vacuum-dried in an oven at 110°C for 10 minutes. The leaching agent 1#（ Adding 5.7 mL glacial acid to 500 mL of reagent water, then adding 64.3 mL of 1N NaOH and dilute to a volume of 1 liter. The pH of this fluid was 4.95) was used considering the pH range of the solution. 1g of leaching agent 1# was added to the leaching vessel to mix with 0.05g ANMF/M or QANMF/M. The samples were rotated at 30 rpm for 18h in a thermostatic orbital shaker at 25 °C. The final metal concentrations in the solution indicate the leaching potential of metal ions from ANMF/M or QANMF/M which were determined by an inductively coupled plasma atomic emission spectrometer (ICP-AES, Optima 7300DV, PerkinElmer).
3. Results and Discussion 3.1 Quench Effect The composition of NMF is randomly assembled calcium aluminosilicate fibers with carbonaceous material mainly consisting of brominated flame retardants or dyes as partially cover. The dominant structural species in NMF network is Q4 units which implies a high content of BOs (bridging oxygens), indicating a weak ability of ion-exchange as our previous work has shown[48]. The functionalization of NMF can be achieved by hydroxylation though the cleavage of bridging oxygens ( − − ) to form functional groups like − − or − − . This is called the tunneling effect and was illustrated in our previous work whereas simplified scheme 10
for the tunneling effect was developed and is shown in Figure 1(a). Also, during the functionalization process calcium atoms, which play the role of network charge compensator in this phase of the process can also be liberated from the structure and change their character to become a network modifier. The hydroxylation process can significantly change the properties of NMF to assign it with efficient ion-exchangeability.
Figure 1. Schematic illustration of (a) tunneling effect and (b) the parallel reaction (potassium atoms are replaced by hydrogen atoms) in the cooling process
However, a parallel reaction of functional group conversion is also expected to happen at the same time as shown in Figure 1(b). The potassium atoms in − − 11
functional groups can be replaced by hydrogen atoms and thus form more − − functional groups. This reaction will move towards the right side of the equation during the cooling process when the hydroxylation process already ended due to the lack of intensive energy input. Therefore, the final product will have a higher content of − − functional groups instead of − − functional groups due to this conversion during the cooling process. Quench can effectively prevent the undesired reaction in cooling process by quickly reducing the temperature to ambient condition through the addition of a quenching reagent. Thus quenching the functionalized aluminosilicate at a relatively high temperature can preserve the − − functional groups in a significantly high percentage and higher quenching temperature which will better favor the preservation. The replacement of potassium atoms by hydrogen atoms in the − − functional groups will lead to the change of both surface and textural properties by the formation of a different size of surface functional groups[49]. The atomic radius for the hydrogen atom and the potassium atoms are 53 pm and 280 pm, respectively. Conversion of − − functional groups to − − functional groups will cause the increase of pore volume and specific surface area due to the formation of a more porous structure. The results of nitrogen adsorption/desorption confirm the decrease of porosity after quench, which is possibly caused by the conversion of − − functional groups to − − as shown in Table 1. The specific surface area drops from 237 m2 g-1 to 180 m2 g-1 and the pore volume decrease from 12
0.5725 cm3 g-1 to 0.5221 cm3 g-1 when even only a mild quench on 110 ℃ was applied, probably due to the filling phenomenon caused by the enhanced density of − − functional groups which have a much larger size compared to the − − functional groups. Further increasing the quench temperature will cause exacerbated loss of specific surface area and pore volume to as low as 86 m2 g-1 and 0.3491 cm3 g-1, respectively. Also the micropore surface area drops from 12 m2 g-1(ANMF) to 5 m2 g-1(QANMF110) and completely disappears at higher quenching temperatures, which is consistent to the surmise that the micropore will be filled first due to their small pore size.
Table 1 Textural properties of unquenched and quenched samples
Sample
SBET (m2 g-1)
Smic(m2 g-1)
Vt (cm3 g-1)
dp (nm)
ANMF
237
12
0.5725
9.66
QANMF110
180
5
0.5221
11.61
QANMF130
118
0
0.4677
15.85
QANMF150
86
0
0.3491
16.23
Detailed nitrogen adsorption/desorption isotherms and pore size distribution curves are shown in Figure 2. Despite the differences in shape and area shown in Figure 2(a), all the isotherm curves are assigned to mesoporous material due to the type IV isotherms with H3-type hysteresis loops they produce according to the IUPAC 13
classification. However, the differences in hysteresis loops revealed the discrepancies in their textural properties.
From ANMF to QANMF the hysteresis loop was
converted to form a spindly shape, indicating the loss of mesoporous surface area. Further increasing the quenching temperature will cause an increase in the disappearance of mesoporous surface area, which is consistent with the fact that the higher quenching temperatures will preserve more − − functional groups with a larger size and thus reinforce the pore blockage effect. The pore size distribution curves in Figure 2(b) show that with the increase in quenching temperature, there is a formation of big holes and/or the disappearance of small holes confirmed by a peak shift from small pore diameter to high pore diameter. Details shown in Table 1 confirms that the average pore diameter calculated by the formula dp=4V/S BET almost doubled from 9.66 nm to 16.23 nm with the increase in quenching temperature. It is obvious that the more − − functional groups are preserved, the less − − functional groups will be created and thus the small pores have more chance to become blocked by the abundance of − − . This would cause the decrease in specific surface area as well as the pore volume and an increase in the average pore diameter.
14
Volume (cm3/g)
a
QANMF150 QANMF130 QANMF110 ANMF
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p0)
(Vp)/d(Dp)((ml/g*nm))
b
QANMF150 QANMF130 QANMF110 ANMF
0
10
20
30
40
50
60
70
Pore diameter (nm)
Figure 2. (a) Nitrogen adsorption desorption isotherm and (b) pore size distribution for the reference sample and the samples quenched at different temperatures.
15
3.2 Surface Characterization Figure S1 shows the scanning electron microscopy (SEM) images of ANMF and quenched ANMF. ANMF is composed of the porous aluminosilicate structure and quenched ANMF shows a similar structure with ANMF, revealing the influence of the quench effect does not occur in the hundred nanometer scale. The EDX elementary mappings were conducted for the same scanning time. The density of oxygen and calcium in the sample surface remain almost the same under the different quench conditions, indicating that there is no additional formation of hydrogen or calcium ion-exchange sites. However, the EDX elementary mappings of potassium show some differences. The higher quenching temperature caused a higher density of potassium in the material surface, which signifies the reinforcement of − − functional groups. Considering the stabilization of oxygen and calcium atoms, it can be concluded that the increase in potassium atoms does not originate from cleavage of − − ′ bonding but is due to the replacement of other silanol groups. The FTIR spectra of ANMF and quenched samples in the frequency region 4000 – 400 cm-1 were investigated to study the surface functionalization of quenched and unquenched samples as shown in Figure 3(a). All the spectra are normalized according to the reference peak in 470 cm-1 representing the bending mode of − − (T= Si or Al) highlighted in Figure 3(a)[50]. The spectra of all samples showed similar shapes and relative peak intensity in the studied region. The peak in the region around 1001 cm-1 stands for the stretching mode of − − (T= Si or 16
Al) has no significant change which can be seen from the dashed line[51]. However, the band within the range 3800 – 2600 cm-1 associated with the stretching mode of the − − (X = K or H) functional groups showed a shift to a lower wavenumber thus giving a prolonged tail for this broad peak, emphasized in the figure using a dashed line. This is possibly due to the replacement of hydrogen atoms by potassium atoms in − − functional groups since the much higher atomic weight of potassium compared with hydrogen will cause the peak shift to a low wavenumber considering the decrease of stretching frequency. A detailed comparison in the region of 1001 cm-1 and 3800 – 2600 cm-1 was conducted only between QANMF150 and ANMF for representativeness. As shown in Figure 3(b) the peaks around 1001 cm-1 which stands for the stretching mode of − − (T = Si or Al), has no significant change in both peak position and intensity. It means that the total amount of − − has limited change, denying the possibility that there was more conversion of − − to − − by hydrolysis, as a result no additional − − functional groups were formed. The spectra region from 3800 – 2600 cm-1, on the other hand, confirmed the replacement of hydrogen atoms by potassium atoms in − − functional groups as shown in Figure 3(c). A comparison of the high resolution spectra of QANMF150 and ANMF in the studied region shows that QANMF150 has an obvious prolonged tail. The higher atoms in a functional group will lead to a decrease of vibration frequency, thus causing the shift toward the lower wavenumber. However, the 17
integration of these two peaks is very similar and it shows that it is the replacement of hydrogen atoms by potassium atoms instead of increasing of − − groups in − − (X = K or H) functional groups.
Figure 3. (a) FTIR spectra for (I)ANMF (II)QANMF110 (III)QANMF130 (IV)QANMF150 and (b) spectra comparison of ANMF and QANMF around 1100 cm-1 and (c) spectra comparison of ANMF and QANMF around 3400 cm-1
X-ray photoelectron spectroscopy (XPS) was used to investigate the quench effect on the surface atomic composition and environment by comparing quenched and
18
unquenched samples. As shown in Figure 4, the low resolution spectra show limited differences among quenched samples so the comparison of high resolution spectra will be conducted only between ANMF and QANMF150, that is, the representative of the quenched samples. Several differences were observed in terms of peak position or intensity, indicating the change of surface chemical composition between QANMF and ANMF. The intensity of the K 2p peak at 293 eV was significantly strengthened in QANMF150 due to the increasing doping of potassium atoms, implying the possibility of − − formation which will improve the ion-exchange ability[52]. However, the intensity of the O 1s peak of QANMF150 at 531.4 eV has no obvious increase. This indicates that the amount of oxygen atoms in QANMF surface did not increase, which means the increase of potassium atoms does not originate from increasing the − − cleavage but the conversion of − − to − − functional groups. The shift of O 1s peak position from 530.9 eV (ANMF) to 531.4 eV (QANMF150) also confirms this option since the replacement of hydrogen atoms by heavier potassium atoms will cause the binding energy to shift to a higher position. In another work of our group, the metal doped material ion-exchanged with copper atoms showed a shift of O 1s peak to 531.8 eV[48]. Considering the fact that copper atoms have a heavier atomic weight than potassium atoms, it is consistent with the hypothesis that the replacement of heavier atoms will cause the shift of the O 1s peak towards the high binding energy side[53]. Moreover, the Ca 2p peak has no obvious change both in peak position and intensity, revealing 19
the fact that due to the difficulty in Ca replacement the conversion of calcium functional groups is limited.
Figure 4 XPS spectra for the ANMF and quenched QANMF. The high resolution spectra of O1s, K 2p and Ca 2p were provided for detailed comparison.
3.3 Ion Exchange Efficiency The object of adsorption isothermal tests is to study the metal ion uptake capacity and set the mass balance between the ions exchanged out and ions adsorbed in order to prove that the increase in ion-exchange capacity is because of the prevention of replacement of potassium atoms in − − functional groups by hydrogen atoms. 20
In this case, copper, cobalt, and lead were employed to evaluate the ion-exchange efficiency of the sample. The initial pH of metal solutions was adjusted to 4 with the purpose to ensure no precipitation was involved in the experiment. The adsorption isothermal test results are shown in Table 2. Table 2 Comparison of ion exchange capacities of ANMF with quenched samples (unit: mmol g-1)
Metal ions Cu 2+
Co2+
Pb2+
ANMF
2.79
3.03
3.19
QANMF110
3.05
3.28
3.19
QANMF130
3.21
3.36
3.22
QANMF150
3.50
3.55
3.19
Sample Names
From the results it can be concluded that quench effect significantly improved the adsorption capacities of Cu2+ and Co2+ from 2.79 mmol g-1 and 3.03 mmol g-1 to 3.50 mmol g-1 and 3.55 mmol g-1, respectively. However, the metal uptake capacity of Pb2+ did not improved obviously and stayed around 3.2 mmol g-1. This can be explained by the electronegativity differences shown in Table 3. It can be seen from the table that the electronegativity of hydrogen is 2.20, which is far beyond the electronegativity of potassium (0.82). Therefore, when − − functional groups do ion-exchange, the driving force for it to exchange with Co2+ (electronegativity 1.88) and Cu 2+ 21
(electronegativity 1.90) is not sufficient[54-56]. However, Pb2+ can still be efficiently ion-exchanged by − − functional groups since the electronegativity of Pb is 2.33, greatly exceeding those of Cu and Co and exceeding that of hydrogen. Therefore, the replacements of hydrogen atoms by potassium atoms cause little effect on the ion-exchange capacities of Pb2+, since Pb2+ has the ability to exploit the majority of the available sites even without the surface modification. Other than electronegativity, the radius of hydrated metal ion also matters in ion-exchange process and final uptake capacity. Therefore, the different capacities between copper and cobalt, which have similar electronegativities, can be explained by the disparity between the radius of hydrated metal ion of copper and cobalt. The radius of hydrated Co 2+ ion is 2.82 Å while for hydrated Cu2+ ion the radius is 2.58 Å in the equatorial positions and 3.22 Å in the axial positions due to Jahn–Teller Effect[57]. Since Cu 2+ and Co2+ have the same hydrated ion charge, Co 2+ with smaller hydrated ion radius will possess higher ion-exchange capacity like shown in Table 2. Moreover, Pb2+ ions with further larger hydrated ion radius (3.74 Å), showed even lower ion-exchange capacities.
22
Table 3 Electronegativities of common elements of Pauling’s scale
Elements
Electronegativity
H
2.20
K
0.82
Ca
1.00
Co
1.88
Cu
1.90
Pb
2.33
The explanation can be proved by the fitting figure of Ca2+ + K+/2 with the metal uptake capacity as shown in Figure 5. Stoichiometrically one calcium atom can exchange with one metal ion that has two positive charges while two potassium atoms are necessary to exchange with only one two plus charged metal ion due to the requirement of the charge balances. The Ca2+ + K+/2 fitting results of Cu 2+ and Co 2+ showed that the improvement in metal uptake capacities were consistent with the increase of potassium leaching, indicating that the capacity increase was just coming from the replacement of hydrogen atoms by potassium atoms. It was achieved by enhancing the ion-exchange capacity through increasing the percentage of − − functional groups. The enhancement of − − groups is mainly caused by preventing the replacement of the potassium atoms in − − by hydrogen atoms, and this was confirmed by the increase in leaching of potassium atoms. The
23
gaps between the real and simulated capacities for lead are related to ion-exchange with − − functional groups that failed to be traced by the mass balance of calcium and potassium leaching. With the increase in quenching temperature, the gap is shrinking because of the enhancement of − − functional groups density. However, the capacities in the Pb2+ ion-exchange had no obvious change, indicating that lead ions have the ability to ion-exchange with both − − or − − functional groups, and this is attributed to the electronegativity of Pb (2.33) being higher than that of H (2.20).
24
Figure 5. Ion-exchange capacities for ANMF and quenched samples. The simulations of Ca2++K+/2 were also provided as well as the K+ leached out.
Also, it can be concluded from the pH profile after ion-exchange shown in Table S1 that the potassium content in quenched samples is higher than ANMF since the pH values of quenched samples after ion-exchange is generally higher than ANMF due to the increase amount of potassium ions into the solution instead of hydrogen ions. The pH value of the Pb2+ solution after ion-exchange has a significant difference between ANMF and the quenched samples because even in ANMF all the hydrogen atoms in − − functional groups were exchanged out due to the high electronegativity of Pb. Therefore, the pH drop of Pb is more obvious than Cu and Co. Meanwhile, after hydrogen atoms were replaced by potassium atoms, the pH value of the quenched samples after ion-exchange with Pb shows a significant enhancement in basicity. Also according to previous works, the potassium atoms in aluminosilicate will compete with calcium atoms for the role of charge compensator[58]. Therefore, calcium atoms will change their roles from network charge compensator to network modifiers after they were replaced by potassium atoms in the competition. Subsequently, calcium atoms have much higher chance to be ion-exchanged. Therefore, although a quench process only increased the density of potassium atoms in the surface of the material and left no change in the density of calcium atoms, it can 25
still enhance the ion-exchange performance of calcium atoms by displacing them from the position of network charge compensator. Excess of potassium atoms will further facilitate this enhancement and this was validated by the ICP test showing the amount of leaching calcium atoms after ion-exchange had also increased slightly.
3.4 Leaching Test Leaching after metal uptake is a critical point for metal deposition, both in the applications of wastewater treatment and catalyst preparation. The leaching of metal ions back to the aquatic system will cause unwanted secondary pollution in wastewater treatment and catalyst deactivation in catalyst process[59-62]. Thus, conducting a leaching test after metal deposition in order to ensure the minimum leaching of heavy metals is indispensable for adsorbent preparation. The leaching tests of Cu 2+, Co2+ and Pb2+ were conducted in this work and the leaching results compared with their metal uptake capacities are shown in Figure 6.
Figure 6. Comparative results of leaching tests 26
The results of leaching tests revealed that the leaching of the sorbed metal ions for both quenched and unquenched ANMF is acceptable. Less than 4% leaching of all the metal ions were observed for all the investigated samples. QANMF150 showed good resistance to acidic condition (pH=4.95) with the leaching percentages of 2.205% (Cu), 1.879% (Co) and 1.937% (Pb), respectively. Also, generally for all samples the leaching percentage of copper and cobalt are higher than lead. This is due to the high electronegativity of Pb (electronegativity 2.33) compared to Co (electronegativity 1.88) and Cu (electronegativity 1.90), therefore, when the materials deposited with heavy metals were put back into acidic solution, hydrogen (electronegativity 2.20) will have more difficulty in exchanging Pb 2+ out compared to Co2+ and Cu2+. The results of the leaching tests showed excellent metal immobilization ability on QANMF[63,64]. Thus, it can be concluded that after taking heavy metal ions from solution, the bonds between adsorbent and adsorbate are very strong and stable. Hence, the results provide potential applications for these aluminosilicate-based quenched materials in wastewater treatment by the removal of heavy metal ions or catalyst preparation by depositing with transition metals on QANMF. The metal doped on the surface of the material can provide an open shell orbit for electron transfer in catalysis reactions[65] . The high stability will greatly favor the above mentioned applications due to the minimum leaching of heavy metal ions indicating the prevention of the release of metal ions into aquatic systems.
27
4. Conclusion The quench effect on the textural properties and surface functional groups has been studied by the functionalization of a waste-derived aluminosilicate material. The porous structure has been changed in the presence of a quenching reagent and it is supposed that this difference originated from the prevention of the replacement of potassium atoms in surface ion-exchange sites by hydrogen atoms. The parallel potassium replacing reaction in cooling process was effectively stopped by this quench technique and thus left the surface of functionalized aluminosilicate with high content of − − functional groups. Surface characterization including SEM, XPS and FTIR further confirmed the preservation of − − functional groups. Meanwhile, due to the differences between the electronegativities of potassium atoms and hydrogen atoms the ion-exchange ability for heavy metals of quench samples are significantly enhanced and this is validated by the adsorption experiment with several metals. The mass balance for leached calcium and potassium atoms also confirmed the fact that the enhanced ion-exchange capacities were caused by the improved liberation of potassium and calcium atoms from the aluminosilicate network. The kinetic tests and adsorption isotherm focusing on the variables of adsorption process will be studied to further establish the adsorption model. Moreover, the leaching tests for metal doped materials showed limited amounts of metal ions had been leached out at acidic conditions, indicating the potential application of this material in industrial wastewater treatment or catalysis. 28
Acknowledgement The authors would like to thank the Hong Kong Research Grant Council for their support of this research.
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38
Supplementary material
Figure S1. SEM images and EDX elementary mapping of O, Ca and K of (a)ANMF (b)QANMF110 (c)QANMF130 and (d)QANMF150
39
Table S1 pH profiles after ion-exchange
Metals Cu
Co
Pb
ANMF
5.1
6.4
4.8
QANMF110
5.2
6.5
5.2
QANMF130
5.3
6.5
5.3
QANMF150
5.5
6.6
5.3
Adsorbents
40
Highlights •
Novel modification technique of aluminosilicate
•
High ion-exchange capacities
•
Trace amount of metal ions leaching after ion-exchange
•
Systematic analysis of quench effect mechanism
41