3.1. Heavy metal leaching concentration of the raw materials
The leaching concentration of heavy metals that passed the toxicity characteristic leaching procedure (TCLP) test is shown in Table 2. In addition, the total leaching concentration of heavy metals was also analyzed, and each material was subjected to a heavy metal leaching test. The concentrations of the leached heavy metals from each material was measured by flame atomic absorption spectrometer (FL-AAS). The analysis results of the total amount of heavy metals show that the TFT-LCD waste glass is dominated by Zn, with a content of 160 mg/kg, and the main source are the wires in the panel etching tank. The predominant heavy metal in the SB waste is mainly Cr, which is caused by the SB waste from the coating treatment of a solar panel, and its content is 90 mg/kg, followed by the contents of Zn and Cu, which are 65 and 25 mg/kg, respectively. The experimental results show that the leaching concentrations of TFT-LCD waste glass and SB waste all met Taiwan’s EPA regulatory limits.
Table 2
Heavy metal leaching concentration tested by TCLP.
Total Metal (mg/kg) | Pb | Cr | Cu | Zn | Cd | Ni |
TFT-LCD Waste Glass | N.D. | N.D. | N.D. | 160 | N.D. | N.D. |
SB waste | N.D. | 90 | 25 | 65 | N.D. | 10 |
TCLP (mg/L) | Pb | Cr | Cu | Zn | Cd | Ni |
TFT-LCD Waste Glass | N.D. | N.D. | N.D. | 0.1 | N.D. | N.D. |
SB waste | N.D. | N.D. | N.D. | 0.8 | N.D. | N.D. |
Regulatory Limits | 5.00 | 5.00 | 15.00 | - | 1.00 | - |
N.D.:Pb༜0.015 mg/L;༛Cd༜0.021 mg/L༛Ni༜0.112 mg/L |
3.2. Crystal phase analysis of Al-MHCM
Figure 2 shows the XRD diffraction patterns of Al-MHCM synthesized at different temperatures and different Si/Al molar ratios. The XRD pattern of Al-MHCM shows that the main diffraction peaks are at 2.34 and 2.42° along with 3.90 and 4.50°, which correspond to the characteristic d(100), d(110) and d(200) peaks of MCM-41, respectively [23]. With the increase in the Si/Al molar ratio, the two higher-order hexagonal structure peaks (d(110) and d(200)) tend to increase. At lower Si/Al ratios, the main characteristic peaks are slightly shifted, which is mainly due to the 2θ angle shift caused by the high aluminum content in the source material; this result means that Al atom doping in the crystal lattice changes the crystallinity of the product and causes poor sorting. Therefore, with increasing aluminum content, the intensity of the diffraction peak gradually decreases, indicating that the order of the structure decreases [24–25]. In addition, it can be found from the change in the hydrothermal temperature that when the temperature increased from 90 to 120°C, the main characteristic peak d(100) has relatively stable crystallinity. The characteristic peaks (d110 and d200) representing the hexagonal structure also have a tendency to gradually form as the hydrothermal temperature increased because when the hydrothermal temperature increased, the balance between the solid and liquid phases of the initial gel is destroyed; this imbalance accelerates the aggregation of silicate species on the micelle surfaces. Therefore, a high hydrothermal temperature is conducive to the formation of a well-ordered MCM-41 structure because the high temperature will accelerate the condensation rate of silicate on the silicon dioxide wall [26].
3.3. Solid-state 27NMR analysis of Al-MHCM
The 27Al NMR spectrum was used to track the change in the position of Al added to Al-MHCM. All the spectra of the current Al-MHCM material are dominated by the signal of the tetrahedral-coordinated aluminum (AlIV) species with 27Al = 53 ppm (Fig. 3), and the weak signal at 27Al = 0 ppm is due to the octahedral-coordinated aluminum (AlVI) species (Fig. 3(c)) [27]. The appearance of AlVI species is usually related to the Lewis acid on the surface, and the strength will increase with an increasing content of AlIV species [28]. However, when the Si/Al molar ratio is lower (Fig. 3 (a)-(b)), more Al atoms (AlVI) are introduced into the silicon dioxide framework, thereby generating and enhancing the AlIV signal. It is well known that AlIV can enhance the acid strength of adjacent SiOH groups, thereby forming Brønsted acid sites (BAS) with high catalytic activity [29]. In addition, from the area content of the tetrahedral-coordinated aluminum (AlIV) species, it can be seen that when the hydrothermal temperature is low (90°C), the occupied area is between 38.50-52.02%, and when the hydrothermal temperature is increased to 105°C, the area occupied by AlIV species is between 52.67–59.61%. In addition, when the hydrothermal temperature is 120°C, the area occupied by species at 53 ppm increased to 54.98–67.17%. These results show that as the hydrothermal temperature increased, AlVI can be more effectively converted to a tetrahedron (Td-Al), which is the form that enters the skeleton.
3.4. SEM analysis of Al-MHCM
In this study, the microstructure changes of Al-MHCM were observed through FE-SEM. The morphology of the Al-MHCM material at a 50,000x magnification is shown in Fig. 4. During the hydrothermal treatment under alkaline conditions (pH = 10), silicate and aluminosilicate are extracted from the TFT-LCD waste glass and SB waste. Then, the extracted silicate and aluminosilicate are mixed with the CTAB surfactant to form a mesoporous silica material similar to MCM-41 [30]. In addition, it can be seen that even after the calcination process (550°C), the macrostructure of Al-MHCM remains intact, thus confirming the high thermal stability of Al-MHCM [31]. The morphology of Al-MHCM is clearly observed as round nanoparticles, and the appearance of the particles is mainly spherical. From the observations in Fig. 4(a)-(c), when the Si/Al ratio is 26 and the hydrothermal temperature is 90–120°C, the size distribution of Al-MHCM crystals is approximately 0.13–0.22 µm. When the Si/Al ratio is increased to 41.8, the crystal size is approximately 0.11–0.40 µm. At the highest Si/Al ratio (56.3), the Al-MHCM crystal size distribution is approximately 0.22–0.60 µm. This phenomenon shows that the increase in hydrothermal temperature causes the crystal size to decrease significantly. According to the study by Grisdanurak et al., this increase in crystal size was mainly due to the increased synthesis temperature and rapid hydrolysis reaction that affected the uniformity of the generated particles [32]. In addition, Ekloff et al. stated that the difference between products obtained by the same synthesis method was due to the formation of delayed structures due to changes in the reaction medium pH or the occurrence of uneven precipitation [33].
3.5. N2 isothermal adsorption-desorption of Al-MHCM
The N2 adsorption-desorption phenomenon provides a technique for determining the surface area, pore volume and pore size distribution. The N2 isotherm adsorption-desorption curve is shown in Fig. 5. All the synthesized Al-MHCM junctions exhibit a type-IV nitrogen adsorption-desorption isotherm. According to the IUPAC nomenclature, this is a typical feature of homogeneous mesoporous materials [34]. The isotherm shows the following three stages: when the low relative pressure is 0.2–0.3, the monolayer adsorption of nitrogen on the mesoporous wall is observed; and when the relative pressure is 0.3–0.4, a sharp increase occurs under the same parameter conditions with different Si/Al ratios. This is characteristic of capillary condensation in mesopores, which shows a narrow hysteresis loop. Finally, when the relative pressure is 0.5–0.9, the synthesized Al-MHCM shows slightly tilted plateau, which is caused by multilayer adsorption on the outer surface of particles [35]. In addition, when the Si/Al ratio is 26, 41.8 and 56.3, the specific surface area ranges from 506–587, 733–795 and 781–1013 m2/g, respectively. The adsorption-desorption characteristics for all samples are typical of mesoporous materials, indicating that mesoporous materials have better adsorption performance.
Figure 6 shows the total pore volume and pore diameter calculated by the N2 adsorption-desorption measurements and the BJH method and are listed in Table 3. The pore size distribution curves of all Al-MHCM shown in Fig. 6 are unimodal and belong to a narrow range of mesopores that are approximately 3–4 nm, indicating the existence of mesopores in the particles. Thus, this result confirms the successful synthesis of a uniform mesoporous material, and is consistent with the BJH method for calculating the pore size (Table 3). In addition, Fig. 6 (a) shows that when the Si/Al ratio is 26, the pore volume of the synthesized Al-MHCM is 0.4–0.5 cm3/g. When the Si/Al ratio is increased to 56.3, the pore volume is increased to 0.9–1.4 cm3/g. Based on the above knowledge, the increase in the Si/Al ratio and hydrothermal temperature can form an ordered mesoporous structure. The pore volume and specific surface area of the synthesized Al-MHCM increase accordingly, with the highest pore volume being 0.97 cm3/g, and the highest specific surface area being 1013 m2/g (Al-MHCM9). The results of Sohrabnezhad and Mooshangaie show that adding metal (such as Ag/AgBr) to an MCM-41 material will increase the pore size and pore volume, which indicated that some of the metal was dispersed on the inner pore surfaces of the MCM-41 material [36].
Table 3
The pore structure characteristics of Al-MHCM with 26, 41.8, and 56.3 Si/Al molar ratio.
Sample | Si/Ala Molar Ratio | Hydrothermal Temperature (℃) | Surface Area (m2/g) | Pore Volume (cm3/g) | Pore Diameter (nm) | d100a (nm) | a0b (nm) | W.tc (nm) |
Al-MHCM1 | 26 | 90 | 511 | 0.51 | 3.52 | 3.67 | 4.24 | 0.72 |
Al-MHCM2 | | 105 | 506 | 0.53 | 3.63 | 3.85 | 4.45 | 0.82 |
Al-MHCM3 | | 120 | 587 | 0.65 | 3.87 | 4.06 | 4.69 | 0.82 |
Al-MHCM4 | 41.8 | 90 | 795 | 0.73 | 3.40 | 3.67 | 4.24 | 0.84 |
Al-MHCM5 | | 105 | 733 | 0.62 | 3.04 | 3.67 | 4.24 | 1.20 |
Al-MHCM6 | | 120 | 780 | 0.71 | 3.28 | 4.06 | 4.69 | 1.41 |
Al-MHCM7 | 56.3 | 90 | 781 | 0.74 | 3.26 | 4.06 | 4.69 | 1.43 |
Al-MHCM8 | | 105 | 870 | 0.78 | 3.18 | 4.06 | 4.69 | 1.51 |
Al-MHCM9 | | 120 | 1013 | 0.97 | 3.05 | 3.85 | 4.45 | 1.40 |
a Unit Cell Distance ; b Unit Cell Constant = 2d100/\(\sqrt{3}\) ; c Wall Thickness (W.t.)= (a0) - (Pore Diamrter) |
3.6. The 24 h equilibrium moisture content curve of Al-MHCM
Figure 7 shows the equilibrium moisture content of Al-MHCM at different relative humidities (RHs). The solid line in Fig. 7 shows the process of water vapor adsorption, and the dotted line is the process of water vapor desorption. The results show that the equilibrium water contents of all samples increase with increasing RH. However, there is a significant difference in the equilibrium moisture content in the material, especially when the relative humidity is high (75 to 95%). Figure 7 shows that when the Si/Al ratio is 26, the RH of the synthesized Al-MHCM is 95%, the equilibrium moisture content is in the range of 65.20–76.60 m3/m3 (Fig. 7 (a)). When the Si/Al ratio is 41.8, the highest equilibrium moisture content is shown to be in the range of 81.34–91.45 m3/m3 (Fig. 7(b)). In addition, when the Si/Al ratio is 56.3 and the RH of the synthesized Al-MHCM is 95%, the equilibrium moisture content is 69.63–75.15 m3/m3 (Fig. 7 (c)). These results show that the adsorption characteristics are closely related to the specific surface area and pore size. In the process of water vapor absorption and desorption, the pore size of Al-MHCM is significantly related to capillary condensation [37]. In addition, Jansen et al. observed that water saturation increased with relative humidity in a relatively nonlinear manner; furthermore, they found that the diffusion coefficient did not depend on the water concentration itself because there was no difference in the diffusion rate between absorption and desorption [38]. In addition, when the low hydrothermal synthesis temperature is 90°C, the equilibrium moisture content of Al-MHCM at 95% RH is 73.19 m3/m3, and when the hydrothermal temperature is increased to 120°C, the equilibrium moisture content increased significantly to 76.60 m3/m3 (Fig. 7(a)). Moreover, the adsorption-desorption curve shows a hysteresis loop. Since the inner wall of the pore is in a wet state after the loss of water during the dehumidification process, the contact angle of the water molecules on the inner wall of the pore is small, thereby delaying the desorption effect. In contrast, in the process of moisture absorption, the inner wall of the dry pore has a better moisture absorption effect with its large water contact angle. The above results show that the change in the Si/Al ratio exhibits no obvious difference in regard to the change in equilibrium moisture content, while an increase in the hydrothermal temperature can destroy the balance between the solid and liquid phases of the initial gel; thus, the concentration of silicate and aluminosilicate in the liquid phase results in the aggregation of silicate species on the microcell surface. Aggregation promotes the crystallization process and increased the pore structure and specific surface area of Al-MHCM; therefore, the equilibrium moisture content gradually increased.
3.7. Feasibility of synthesizing microporous zeolite
In this study, after the alkaline fusion process and extraction of SiO2 and Al2O3, we tried to use the remaining residue as a starting material for a direct hydrothermal synthesis of zeolite with a one-pot hydrothermal method. Through the use of a 3 M NaOH solution as the alkali treatment, the formation and crystal phase influence of the synthesized microporous zeolite at different hydrothermal temperatures (100, 150, 200°C) were discussed; furthermore, this process is expected to achieve the purpose of completely reusing the remaining residue. Figure 8 shows the XRD pattern. It can be seen from Fig. 8 that when the silicon-to-aluminum ratio is fixed, comparing different temperatures will produce different types and crystal phases of zeolite. The results show that it is mainly zeolites such as analcime, sodalite, and cancrinite [39]. When the hydrothermal temperature is 200°C, cancrinite clearly appears at 2θ = 14.22 and 27.18°. Additionally, the alkaline melting process can effectively separate the SiO2 and Al2O3 supernatant liquid and the remaining residue. Thus, the obtained supernatant liquid is mainly silicate and aluminosilicate, which can synthesize valuable Al-MHCM molecular sieves, while the remaining residue can be recycled and activated into microporous zeolite, showing potential for secondary recycling.