The Stabilization of Waste Funnel Glass of CRT by SiO 2 Film Coating Technique

: The funnel glass of the CRT monitor contains about 22–28% of lead oxide, of which lead is a highly toxic species and hazardous to the environment. This study proposes a process to form a protective layer of SiO 2 ﬁlm coating on the funnel glass to reduce the hazardous effect of lead leaching to the environment. The ﬁlm coating beneﬁts from the advantages of the sol–gel method. There are two key procedures of the stabilization technique, including the alkaline treatment and the formation of SiO 2 coating from TEOS. The results show that the funnel glass powder treated with 10 M NaOH can produce a mushy layer on the surface. The mushy layer, which comprises OH − and water, can promote the formation of the SiO 2 ﬁlm layer on the surface of funnel glass powder. The conditions of the SiO 2 ﬁlm coating proposed in this study are: alkaline treatment by 10 M NaOH, the addition ratio of TEOS and funnel glass powder 2: 1, reaction temperature 40 ◦ C, and reaction time 1.3 h. The EDS and ESCA results show that the Pb peak intensity on the surface of funnel glass decreases with the ﬁlm coating. In the TCLP test, the leaching amount for Pb of the SiO 2 ﬁlm coated funnel glass powders is 0.7 mg/L, which is far lower than the standard in Taiwan EPA. Based on the experimental results, the formation mechanism of the SiO 2 ﬁlm layer on the surface of waste funnel glass powder is proposed. This study demonstrates that the SiO 2 ﬁlm coating is a potentially effective method to solve the problem of the waste funnel glass. solution followed by the deposition of a layer of SiO 2 ﬁlm via the interaction of TEOS with alkaline-treated funnel glass. The funnel glass powder treated with NaOH alkaline solution can form a mushy layer containing OH and water, which can promote the formation of a SiO 2 ﬁlm from TEOS on the funnel glass. The SEM image indicates that the SiO 2 ﬁlm layer prepared with 10 M NaOH treatment can be well coated on the surface of the funnel glass powder. In addition, the TEOS/funnel glass ratio is an important factor in SiO 2 ﬁlm coating. The Pb content on the surface of funnel glass powder decreases with an increasing amount of TEOS. The coating conditions in this study for the SiO 2 -ﬁlm-funnel glass are alkaline treated with 10 M NaOH and a TEOS/funnel glass ratio equal to 2. The TCLP test shows that the leaching amount for Pb reduces from 4352.5 mg/L for nontreated funnel glass powder to 0.7 mg/L for that treated with a ﬁlm coating. The test result is far lower than the standard in Taiwan EPA. The study provides an effective stabilization technique for the treatment of waste funnel glass.


Introduction
The rapid development of display technology has generated a large amount of discarded cathode ray tube (CRT) monitors. Previous studies indicate that CRT monitors around the world will be phased out from the market in 2022 [1]. As a result, there will be about 3.2 million tons of waste CRT glass needing to be treated annually in the coming years [1,2].
The traditional CRT monitor consists of three types of glass: panel glass, funnel glass, and neck glass. Panel glass contains about 8-10% barium oxide (BaO) and strontium oxide (SrO). The other two types of glass contain high levels of lead oxide (PbO). The Pb content of funnel glass in a CRT monitor is about 22-28% ( Figure 1) [3]. Pb has major toxicological effects for biological systems and environments. Funnel glass is considered to be a hazardous waste [4][5][6][7]. It is estimated that the amount of waste funnel glass generated in Taiwan is about 5000-6000 tons a year. Technology for the treatment of hazardous waste CRT funnel glass, therefore, is needed to solve the environmental problem immediately.
Previous studies indicate that silica can be used to coat the glass surface [8][9][10][11]. In this study, a technology for the treatment of the waste funnel glass by SiO 2 coating is proposed. SiO 2 is selected as the coating material as the major component of funnel glass is SiO 2 . Therefore, a better physical and chemical compatibility between coating Sustainability 2021, 13, 9096 2 of 12 material and funnel glass can be ensured. The preparation of silica material has attracted considerable attention due to its wild applications [12]. Many synthesis and application technologies have been studied for SiO 2 . Among the synthesis techniques of SiO 2 , the sol-gel process is the most popular method to prepare SiO 2 [13][14][15]. Tetraethoxysilane and tetramethoxysilane can be used as silica sources [16]. In a typical sol-gel process, the silica is prepared by dispersion and hydrolysis of Si precursor in an ethanol solution to form silanol, which then undergoes condensation (polymerization) to produce silica [17]. The preparation of SiO 2 is affected by the amounts of TEOS, alkaline, water and alcohol, temperature, reaction time, etc. [18]. Previous studies indicate that silica can be used to coat the glass surface [8][9][10][11]. In this study, a technology for the treatment of the waste funnel glass by SiO2 coating is proposed. SiO2 is selected as the coating material as the major component of funnel glass is SiO2. Therefore, a better physical and chemical compatibility between coating material and funnel glass can be ensured. The preparation of silica material has attracted considerable attention due to its wild applications [12]. Many synthesis and application technologies have been studied for SiO2. Among the synthesis techniques of SiO2, the sol-gel process is the most popular method to prepare SiO2 [13][14][15]. Tetraethoxysilane and tetramethoxysilane can be used as silica sources [16]. In a typical sol-gel process, the silica is prepared by dispersion and hydrolysis of Si precursor in an ethanol solution to form silanol, which then undergoes condensation (polymerization) to produce silica [17]. The preparation of SiO2 is affected by the amounts of TEOS, alkaline, water and alcohol, temperature, reaction time, etc. [18].
In this study, the SiO2 coating on funnel glass was carried out by sol-gel process. Two schemes of SiO2 stabilization coating were investigated and compared, namely SiO2 particle coating and SiO2 film coating. The factors of the coating process were evaluated. The feasibility of application of a SiO2 coating was further demonstrated by TCLP analysis. A mechanism of SiO2 coating was proposed based on the experimental results.

Materials and Methods
The waste funnel glass was provided by E&E recycling, Inc. (Taiwan). The waste fun- In this study, the SiO 2 coating on funnel glass was carried out by sol-gel process. Two schemes of SiO 2 stabilization coating were investigated and compared, namely SiO 2 particle coating and SiO 2 film coating. The factors of the coating process were evaluated. The feasibility of application of a SiO 2 coating was further demonstrated by TCLP analysis. A mechanism of SiO 2 coating was proposed based on the experimental results.

Materials and Methods
The waste funnel glass was provided by E&E recycling, Inc. (Taiwan). The waste funnel glass was crushed to particle size < 75 µm (200 mesh). All the funnel glass powder was washed with 0.1 M NaOH to remove the residue and impurity on the surface. After the pretreatment, the funnel glass powder was risen with deionized water.
In this study, a SiO 2 protection layer on the surface of the waste funnel glass powders was prepared based on the sol-gel technique. All chemicals were analytical grade, tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, 99%) as a precursor, ethanol (C 2 H 5 OH) (Sigma-Aldrich, 99.8%) as solvents, and ammonia solution (NH 4 OH) (Sigma-Aldrich, 28-30%) as a catalyst. The SiO 2 protection layer was covered on the powder surface by two different forms, namely particles and film coating ( Figure 2). mixed with TEOS/ethanol solution. For a typical experiment, the composition of TEOS/ethanol solution was the same as the preparation of SiO2 particles (i.e., TEOS 2.1 mL and C2H5OH 31.7 mL). The reaction temperature was 40 °C and the reaction time was 1.3 h. The funnel glass coated with a layer of SiO2 film (SiO2-film-funnel glass) was dried at 150 °C for 24 h.
The TCLP test procedure followed the standard method (NIEA R201.15C) by EPA, Taiwan. A sample of 100 g (particle size of 1 cm or less) was added to 20 mL of acetic acid (0.1 M) in a container. The container was rotated (30 rotations/min) at room temperature for 20 h. The Pb concentration dissolved in the TCLP test was analyzed by atomic absorption spectrometer (AA, Young Lin Instrument, AAS-8010). The characteristics of the funnel glass powder with SiO2 coating were analyzed by scanning electron microscope (SEM, Hitachi S-4800), energy dispersive X-ray spectrometer (EDS, Bruker, Quad FQ5060), Total Reflection X-ray Fluorescence Spectrometry (TXRF, Bruker, S2 PICOFOX), and electron spectroscopy for chemical analysis (ESCA, JEOL, JPS-9030). For the SiO 2 particles coating process (Figure 2a), the SiO 2 particles were prepared first and deposited on the surface of the funnel glass powder. The conditions for the preparation of the SiO 2 particles followed the previous study [19][20][21]. For the preparation of SiO 2 particles, the TEOS (2.1 mL) and C 2 H 5 OH (31.7 mL) mixture was catalyzed with NH 4 OH (9.1 mL). The reaction temperature was 40 • C and the reaction time was 1 h. The SiO 2 particles were added to the funnel glass powder (2 g) and mixed at 40 • C for 1.3 h. The funnel glass powder coated with SiO 2 particles (SiO 2 -particle-funnel glass) was dried at 150 • C for 24 h.
For the SiO 2 film coating process (Figure 2b), the funnel glass powder (2 g) was first treated with 0.01-10 M NaOH. After alkaline treatment, the funnel glass powder was mixed with TEOS/ethanol solution. For a typical experiment, the composition of TEOS/ethanol solution was the same as the preparation of SiO 2 particles (i.e., TEOS 2.1 mL and C 2 H 5 OH 31.7 mL). The reaction temperature was 40 • C and the reaction time was 1.3 h. The funnel glass coated with a layer of SiO 2 film (SiO 2 -film-funnel glass) was dried at 150 • C for 24 h.
The TCLP test procedure followed the standard method (NIEA R201.15C) by EPA, Taiwan. A sample of 100 g (particle size of 1 cm or less) was added to 20 mL of acetic acid (0.1 M) in a container. The container was rotated (30 rotations/min) at room temperature for 20 h. The Pb concentration dissolved in the TCLP test was analyzed by atomic absorption spectrometer (AA, Young Lin Instrument, AAS-8010).

The Characteristics of Waste Funnel Glass Powder
The morphology of the waste funnel glass powder is shown in Figure 3. The image shows that the powder particle is irregular with sharp edges and a rough surface. The

The Characteristics of Waste Funnel Glass Powder
The morphology of the waste funnel glass powder is shown in Figure 3. The image shows that the powder particle is irregular with sharp edges and a rough surface. The waste funnel glass powder samples were scanned with TXRF. The results indicate that the main components are Si, O, Pb and Al. There are also some trace elements (K, Ca, Fe, Zn, Sr, Ba, etc.) in the composition of funnel glass. According to EDS analysis, the main components include silicon (29.5 atom %), oxygen (59.4 atom %), lead (5.5 atom %), and aluminum (5.6 atom %). The Pb leachability was evaluated based on the toxicity characteristic leaching procedure (TCLP). TCLP is designed to simulate the effect of acidic leaching by the environment in the landfills. Due to the high amount of Pb content, the Pb leachability is about 4352.5 ppm, which is exceedingly higher than the standard of EPA Taiwan (5 mg/L). Therefore, the funnel glass of CRT is considered to be a hazardous waste. In order to reduce the leaching of Pb from funnel glass powder, two schemes of protection coating were proposed in this study, namely SiO2 particle coating and SiO2 film coating.

SiO2 Particles Coating
The SiO2 particles were first prepared by sol-gel synthesis. TEOS and ethanol solution was added with ammonia. The molecules of TEOS in the mixture were hydrolyzed to form silanol groups. The silanol groups were polymerized to form SiO2 particles via nucleation and growth. After the preparation of the SiO2 particles, they were deposited on the surface of the funnel glass powder according to Figure 2a. Figure 4a shows the deposition of SiO2 particles on funnel glass powder with a single deposition. It is clear that SiO2 particles can deposit on the flat surface of glass powder with sparse and uneven distribution. Nevertheless, the surface of the small glass powder is almost completely covered with SiO2 particles. This may be due to high surface energy at the sharp edges of small glass powder. According to EDS analysis, the main components include silicon (29.5 atom %), oxygen (59.4 atom %), lead (5.5 atom %), and aluminum (5.6 atom %). The Pb leachability was evaluated based on the toxicity characteristic leaching procedure (TCLP). TCLP is designed to simulate the effect of acidic leaching by the environment in the landfills. Due to the high amount of Pb content, the Pb leachability is about 4352.5 ppm, which is exceedingly higher than the standard of EPA Taiwan (5 mg/L). Therefore, the funnel glass of CRT is considered to be a hazardous waste. In order to reduce the leaching of Pb from funnel glass powder, two schemes of protection coating were proposed in this study, namely SiO 2 particle coating and SiO 2 film coating.

SiO 2 Particles Coating
The SiO 2 particles were first prepared by sol-gel synthesis. TEOS and ethanol solution was added with ammonia. The molecules of TEOS in the mixture were hydrolyzed to form silanol groups. The silanol groups were polymerized to form SiO 2 particles via nucleation and growth. After the preparation of the SiO 2 particles, they were deposited on the surface of the funnel glass powder according to Figure 2a. Figure 4a shows the deposition of SiO 2 particles on funnel glass powder with a single deposition. It is clear that SiO 2 particles can deposit on the flat surface of glass powder with sparse and uneven distribution. Nevertheless, the surface of the small glass powder is almost completely covered with SiO 2 particles. This may be due to high surface energy at the sharp edges of small glass powder.
In order to obtain complete coverage of the SiO 2 particle layer, the SiO 2 particle deposition procedure was repeated three times. According to the SEM images (Figure 4), the difference of the SiO 2 -particle-funnel glass with various numbers of SiO 2 coating layer can be observed. By comparison, the SiO 2 particle coverage had been significantly improved. For three depositions, the funnel glass powder is almost completely covered with a SiO 2 particle layer. However, it is interesting to note that the arrangement of SiO 2 particles appears to be hexagonal, indicating that the SiO 2 particle is closely packed via the nondirectional physical interaction force. Therefore, experimental observation showed that the coated particles can be easily detached.  In order to obtain complete coverage of the SiO2 particle layer, the SiO2 particle deposition procedure was repeated three times. According to the SEM images (Figure 4), the difference of the SiO2-particle-funnel glass with various numbers of SiO2 coating layer can be observed. By comparison, the SiO2 particle coverage had been significantly improved. For three depositions, the funnel glass powder is almost completely covered with a SiO2 particle layer. However, it is interesting to note that the arrangement of SiO2 particles appears to be hexagonal, indicating that the SiO2 particle is closely packed via the nondirectional physical interaction force. Therefore, experimental observation showed that the coated particles can be easily detached.
The effect of a SiO2 particle coating on the surface of the funnel glass powder was evaluated with surface analysis technique (ESCA), specifically for the Pb component. Figure 5a indicates that the nontreated waste funnel glass powder shows the high intensity of Pb peaks of typical PbO2. The intensity of Pb peaks decreases as the SiO2 particle coating thickness increases. The intensity of Pb peaks of the sample with three layers of the SiO2 coating is significantly reduced (Figure 5b). The effect of a SiO 2 particle coating on the surface of the funnel glass powder was evaluated with surface analysis technique (ESCA), specifically for the Pb component. Figure 5a indicates that the nontreated waste funnel glass powder shows the high intensity of Pb peaks of typical PbO 2 . The intensity of Pb peaks decreases as the SiO 2 particle coating thickness increases. The intensity of Pb peaks of the sample with three layers of the SiO 2 coating is significantly reduced (Figure 5b). In order to obtain complete coverage of the SiO2 particle layer, the SiO2 particle deposition procedure was repeated three times. According to the SEM images (Figure 4), the difference of the SiO2-particle-funnel glass with various numbers of SiO2 coating layer can be observed. By comparison, the SiO2 particle coverage had been significantly improved. For three depositions, the funnel glass powder is almost completely covered with a SiO2 particle layer. However, it is interesting to note that the arrangement of SiO2 particles appears to be hexagonal, indicating that the SiO2 particle is closely packed via the nondirectional physical interaction force. Therefore, experimental observation showed that the coated particles can be easily detached.
The effect of a SiO2 particle coating on the surface of the funnel glass powder was evaluated with surface analysis technique (ESCA), specifically for the Pb component. Figure 5a indicates that the nontreated waste funnel glass powder shows the high intensity of Pb peaks of typical PbO2. The intensity of Pb peaks decreases as the SiO2 particle coating thickness increases. The intensity of Pb peaks of the sample with three layers of the SiO2 coating is significantly reduced (Figure 5b). Evaluation based on the toxicity characteristic leaching procedure (TCLP) shows that the leaching amount of Pb of the SiO 2 -particle-funnel glass with three layers of SiO 2 particle coating is 1838.8 mg/L, which is less than half of that of the nontreated funnel glass. This result indicates that, with sufficient coverage, a SiO 2 particle coating can reduce the leaching of Pb components into the environment. However, the physical deposition of SiO 2 particles is not effective for the further application of SiO 2 -particle-funnel glass.

SiO 2 Film Coating
In order to have an effective SiO 2 layer barrier to prevent the leaching of Pb to the environment, a SiO 2 film coating process was proposed (Figure 2b). This proposed process takes the advantage of the partial dissolution of the surface of funnel glass powder in an alkaline solution followed by the formation of a layer of SiO 2 film via the interaction of TEOS and funnel glass.

Alkaline Treatment
In alkaline treatment, the tested concentrations of NaOH are within 0.01-10 M. The appearance of the waste funnel glass after alkaline treatment is shown in Figure 6. Compared with the nontreated funnel glass powder (Figure 3), the funnel glass surface becomes smooth and the sharp edges of the glass powder disappear after alkaline treatment. This may be due to the dissolution of the funnel glass powder with high surface energy (i.e., fine particle and edge area) in an alkaline solution (Figure 6a,b). After strong alkaline treatment, the surface of funnel glass particles gradually becomes flat and smooth (Figure 6c,d).
Evaluation based on the toxicity characteristic leaching procedure (TCLP) shows that the leaching amount of Pb of the SiO2-particle-funnel glass with three layers of SiO2 particle coating is 1838.8 mg/L, which is less than half of that of the nontreated funnel glass. This result indicates that, with sufficient coverage, a SiO2 particle coating can reduce the leaching of Pb components into the environment. However, the physical deposition of SiO2 particles is not effective for the further application of SiO2-particle-funnel glass.

SiO2 Film Coating
In order to have an effective SiO2 layer barrier to prevent the leaching of Pb to the environment, a SiO2 film coating process was proposed (Figure 2b). This proposed process takes the advantage of the partial dissolution of the surface of funnel glass powder in an alkaline solution followed by the formation of a layer of SiO2 film via the interaction of TEOS and funnel glass.

Alkaline Treatment
In alkaline treatment, the tested concentrations of NaOH are within 0.01-10 M. The appearance of the waste funnel glass after alkaline treatment is shown in Figure 6. Compared with the nontreated funnel glass powder (Figure 3), the funnel glass surface becomes smooth and the sharp edges of the glass powder disappear after alkaline treatment. This may be due to the dissolution of the funnel glass powder with high surface energy (i.e., fine particle and edge area) in an alkaline solution (Figure 6a,b). After strong alkaline treatment, the surface of funnel glass particles gradually becomes flat and smooth ( Figure  6c,d).     (Figure 6a,b). The dissolution of funnel glass is similar to the previous observation for amorphous silica [22]. The dissolution of silica from funnel glass powder in basic solution follows Equations (1) and (2) [23]: where ≡SiO represents a silicon site at the surface, ≡ 2− represents an oxygen vacancy site at the surface.
silica [22]. The dissolution of silica from funnel glass powder in basic solution follows Equations (1) and (2): [23] ≡SiO + OH -→ SiO OH + + ≡ 2- SiO OH + +OH -+H 2 O → H 4 SiO 4 (2) where ≡SiO represents a silicon site at the surface, ≡ 2-represents an oxygen vacancy site at the surface. Nevertheless, for the samples treated by 1 M and 10 M NaOH, it is observed that there is a formation of the viscous mushy layer on the funnel glass surface. This may be due to the combined effects of dissolution and incorporation of the NaOH aqueous solution. After drying at 105 °C for 24 h, the weight differences of the samples treated by 1 M and 10 M NaOH were increased by 0.22% and 3.29%, respectively. The weight increase is due to the viscous mushy layer containing NaOH and some water. The formed mushy layer was difficult to dry, especially with 10 M NaOH. After extensive drying (105 °C, a week) to remove the water for SEM observation, the surface of the sample treated with 10 M NaOH becomes flat and smooth (Figure 6d) due to the resolidification of the mushy layer. The concentration of 5 M NaOH was also tested. However, the 5 M NaOH alkaline treated funnel glass showed a similar physical appearance as that treated with 1 M NaOH.
The atomic composition on the surface of alkaline treated funnel glass was also analyzed by EDS. The result is shown in Table 1. For NaOH concentration of 0.01 M and 0.1 M alkaline treatment, the main components in the funnel glass powder are oxygen and silicon (with a ratio of Si:O = 1:2), which is the same as that of the nontreated funnel glass. For alkaline treatment with 1 M and 10 M NaOH, the sodium content increases to 7.9 and Nevertheless, for the samples treated by 1 M and 10 M NaOH, it is observed that there is a formation of the viscous mushy layer on the funnel glass surface. This may be due to the combined effects of dissolution and incorporation of the NaOH aqueous solution. After drying at 105 • C for 24 h, the weight differences of the samples treated by 1 M and 10 M NaOH were increased by 0.22% and 3.29%, respectively. The weight increase is due to the viscous mushy layer containing NaOH and some water. The formed mushy layer was difficult to dry, especially with 10 M NaOH. After extensive drying (105 • C, a week) to remove the water for SEM observation, the surface of the sample treated with 10 M NaOH becomes flat and smooth (Figure 6d) due to the resolidification of the mushy layer. The concentration of 5 M NaOH was also tested. However, the 5 M NaOH alkaline treated funnel glass showed a similar physical appearance as that treated with 1 M NaOH.
The atomic composition on the surface of alkaline treated funnel glass was also analyzed by EDS. The result is shown in Table 1. For NaOH concentration of 0.01 M and 0.1 M alkaline treatment, the main components in the funnel glass powder are oxygen and silicon (with a ratio of Si:O = 1:2), which is the same as that of the nontreated funnel glass. For alkaline treatment with 1 M and 10 M NaOH, the sodium content increases to 7.9 and 9.2 atom %. This result confirms that the mushy layer is formed due to the combination effects of dissolution and incorporation of NaOH aqueous solution.

Film Coating
In the SiO 2 film coating process, the alkaline (10 M NaOH) treated funnel glass powder was added directly into the TEOS-ethanol solution. As pointed out previously, the surface of the funnel glass powder after strong alkaline treatment has a mushy layer containing NaOH and water. This is important for the formation of the SiO 2 film coating. Figure 8 shows the SEM image of SiO 2 -film-funnel glass. The weight ratio for the addition of TEOS to the funnel glass powder is 1, 2, 5, and 10, respectively. With the increasing amount of TEOS, the surface of SiO 2 -film-funnel glass becomes smoother, indicating the formation of a SiO 2 film from TEOS. Furthermore, some SiO 2 particles appear on the surface. The amount of SiO 2 particles on the surface increased with the TEOS/funnel glass ratio. This may be due to the high amount of TEOS in the solution phase forming SiO 2 particles, which then deposit on the funnel glass surface. In the SiO2 film coating process, the alkaline (10 M NaOH) treated funnel glass pow der was added directly into the TEOS-ethanol solution. As pointed out previously, t surface of the funnel glass powder after strong alkaline treatment has a mushy layer co taining NaOH and water. This is important for the formation of the SiO2 film coating. Figure 8 shows the SEM image of SiO2-film-funnel glass. The weight ratio for t addition of TEOS to the funnel glass powder is 1, 2, 5, and 10, respectively. With the i creasing amount of TEOS, the surface of SiO2-film-funnel glass becomes smoother, ind cating the formation of a SiO2 film from TEOS. Furthermore, some SiO2 particles appe on the surface. The amount of SiO2 particles on the surface increased with the TEOS/funn glass ratio. This may be due to the high amount of TEOS in the solution phase formin SiO2 particles, which then deposit on the funnel glass surface.  The Pb content on the surface of SiO 2 -film-funnel glass analyzed by ESCA is shown in Figure 9. The results show that the peak intensity of Pb was decreased by the coating of the SiO 2 film layer. The coating effect is dependent on the weight ratio of TEOS to the funnel glass powder. In comparison with the nontreated funnel glass powder (Figure 5a), the Pb peak intensity for the sample coated with TEOS/funnel glass = 1 decreased by 75%. The result at TEOS/funnel glass = 5 is the same as TEOS/funnel glass = 1. The Pb peak intensity for the sample coated with TEOS/funnel glass = 10 is not detectable. According to the ESCA analysis, the coating effect of the SiO 2 film is better than the SiO 2 particles. in Figure 9. The results show that the peak intensity of Pb was decreased by the coating of the SiO2 film layer. The coating effect is dependent on the weight ratio of TEOS to the funnel glass powder. In comparison with the nontreated funnel glass powder (Figure 5a), the Pb peak intensity for the sample coated with TEOS/funnel glass = 1 decreased by 75%. The result at TEOS/funnel glass = 5 is the same as TEOS/funnel glass = 1. The Pb peak intensity for the sample coated with TEOS/funnel glass = 10 is not detectable. According to the ESCA analysis, the coating effect of the SiO2 film is better than the SiO2 particles. The content of O, Si, and Pb on the surface of SiO2-film-funnel glass is shown in Table  2. The weight ratios of TEOS to the funnel glass powder are 1, 2, 5, and 10. The results indicate that the Pb content decreases with an increasing amount of TEOS. The Pb content decreases from 5.5 atom % for nontreated funnel glass to 1.0, 0.8, 0.7, and 0.5 atom % for weight ratios of TEOS to the funnel glass powder equal to 1, 2, 5, and 10, respectively, indicating that the effect of the SiO2 film coating is dependent on the additional amount of TEOS. The SiO2-film-funnel glass surface has a better coverage effect of SiO2 film at a high TEOS/funnel glass ratio. When the TEOS/funnel glass = 10, the Pb content further decreased to 0.5 atom %. This may be due to improved coverage and film thickness of the SiO2 film. The SiO2 film coating provides a film layer on the funnel glass surface.

Toxicity Characteristic Leaching Procedure (TCLP) of SiO2-Film-Funnel Glass
For practical application, the sample of SiO2-film-funnel glass was analyzed for the Pb leaching characteristics by the TCLP test. The standard of Pb leachability levels in Taiwan EPA is 5.0 mg/L. The TCLP test results are shown in Table 3. For the nontreated waste funnel glass, the leaching amount of Pb was 4352.5 mg/L. For the SiO2-film-funnel glass, the leaching amount of Pb decreased with an increasing amount of TEOS. The result of Pb dissolution of the SiO2-film-funnel glass with TEOS/funnel glass = 1 was much lower than that of nontreated funnel glass. Nevertheless, the leaching amount of Pb was still  Table 2. The weight ratios of TEOS to the funnel glass powder are 1, 2, 5, and 10. The results indicate that the Pb content decreases with an increasing amount of TEOS. The Pb content decreases from 5.5 atom % for nontreated funnel glass to 1.0, 0.8, 0.7, and 0.5 atom % for weight ratios of TEOS to the funnel glass powder equal to 1, 2, 5, and 10, respectively, indicating that the effect of the SiO 2 film coating is dependent on the additional amount of TEOS. The SiO 2 -film-funnel glass surface has a better coverage effect of SiO 2 film at a high TEOS/funnel glass ratio. When the TEOS/funnel glass = 10, the Pb content further decreased to 0.5 atom %. This may be due to improved coverage and film thickness of the SiO 2 film. The SiO 2 film coating provides a film layer on the funnel glass surface.

Toxicity Characteristic Leaching Procedure (TCLP) of SiO 2 -Film-Funnel Glass
For practical application, the sample of SiO 2 -film-funnel glass was analyzed for the Pb leaching characteristics by the TCLP test. The standard of Pb leachability levels in Taiwan EPA is 5.0 mg/L. The TCLP test results are shown in Table 3. For the nontreated waste funnel glass, the leaching amount of Pb was 4352.5 mg/L. For the SiO 2 -film-funnel glass, the leaching amount of Pb decreased with an increasing amount of TEOS. The result of Pb dissolution of the SiO 2 -film-funnel glass with TEOS/funnel glass = 1 was much lower than that of nontreated funnel glass. Nevertheless, the leaching amount of Pb was still significantly higher than the EPA standard. This may be due to the incomplete coverage of funnel glass by the SiO 2 film. For a TEOS/funnel glass ratio equal to 2 and 10, the leaching amount of Pb reduced to 0.7 and 0.6 mg/L, respectively. The surface cover effect for the funnel glass with the SiO 2 film coating (TEOS/funnel glass = 2) was sufficient to provide a barrier for Pb leaching to the environment. The waste funnel glass after stabilization may become a renewable material for further application in compliance with environmental regulations.

Mechanism of SiO 2 -Film Coating
For the SiO 2 film coating, a mechanism of SiO 2 film coating is proposed (Figure 10) based on the above experimental results. the funnel glass with the SiO2 film coating (TEOS/funnel glass = 2) was sufficient to provide a barrier for Pb leaching to the environment. The waste funnel glass after stabilization may become a renewable material for further application in compliance with environmental regulations.

Mechanism of SiO2-Film Coating
For the SiO2 film coating, a mechanism of SiO2 film coating is proposed (Figure 10) based on the above experimental results.
The steps of the mechanism are proposed as following Equations (3)- (7): Step 1: Alkaline treatment of funnel glass In a basic solution, the dissolution of funnel glass will drastically increase the concentration of the negatively charged surface species, ≡ Si-O - [22]. In contact with water, the negatively charged surface can interact with water according to Equation (3) [24].
where ≡Si-O and ≡Si-OH represent a part of the SiO2 framework of funnel glass. Therefore, the surface of funnel glass will be rich with hydroxyl groups, which can interact with silanol groups from TEOS.
Step 2: Diffusion of TEOS from the outer solution to the mushy layer and formation of silanol group The steps of the mechanism are proposed as following Equations (3)-(7): Step 1: Alkaline treatment of funnel glass In a basic solution, the dissolution of funnel glass will drastically increase the concentration of the negatively charged surface species, ≡ Si-O - [22]. In contact with water, the negatively charged surface can interact with water according to Equation (3) [24].
where ≡ Si − O − and ≡ Si − OH represent a part of the SiO 2 framework of funnel glass. Therefore, the surface of funnel glass will be rich with hydroxyl groups, which can interact with silanol groups from TEOS.
Step 2: Diffusion of TEOS from the outer solution to the mushy layer and formation of silanol group The TEOS diffused from the outer solution undergoes base catalytic hydrolysis with water to form a silanol group in the mushy layer. The formation of the silanol group from TEOS solution follows the mechanism: [20,25] Si(OC 2 Step 3: Interaction of hydrolyzed TEOS with the alkaline-treated funnel glass This procedure is similar to the water condensation in the preparation of SiO 2 particles [20,25]. The proposed mechanism in this step is as Equation (6): Step 4: Growth of SiO 2 film from the addition of TEOS In the final stage, a film of silica is formed. The proposed reaction follows the condensation of TEOS to form SiO 2 [20,25].

Conclusions
Due to the high content of lead (5.5 atom %), the funnel glass in the CRT monitor is considered hazardous waste, based on TCLP. The stabilization of funnel glass is an important issue. This study proposed a process for the stabilization of the waste funnel glass by coating a protective layer of SiO 2 . The results show that the SiO 2 coating layer can effectively reduce the leaching of lead from the funnel glass.
Two schemes of SiO 2 stabilization coating were investigated and compared, namely SiO 2 particle coating and SiO 2 film coating. In SiO 2 particle coating, the SiO 2 particles prepared with TEOS precursor by sol-gel synthesis can physically deposit on the surface of funnel glass. With efficient coverage, the leaching result of the TCLP test shows that the coating of the SiO 2 particles can reduce the Pb releasing. Nevertheless, the physical deposition of SiO 2 particles is not effective for further application of SiO 2 -particle-funnel glass.
SiO 2 film coating on funnel glass takes the advantage of the partial dissolution of the surface of the funnel glass powder in the alkaline solution followed by the deposition of a layer of SiO 2 film via the interaction of TEOS with alkaline-treated funnel glass. The funnel glass powder treated with NaOH alkaline solution can form a mushy layer containing OHand water, which can promote the formation of a SiO 2 film from TEOS on the funnel glass. The SEM image indicates that the SiO 2 film layer prepared with 10 M NaOH treatment can be well coated on the surface of the funnel glass powder. In addition, the TEOS/funnel glass ratio is an important factor in SiO 2 film coating. The Pb content on the surface of funnel glass powder decreases with an increasing amount of TEOS. The coating conditions in this study for the SiO 2 -film-funnel glass are alkaline treated with 10 M NaOH and a TEOS/funnel glass ratio equal to 2. The TCLP test shows that the leaching amount for Pb reduces from 4352.5 mg/L for nontreated funnel glass powder to 0.7 mg/L for that treated with a film coating. The test result is far lower than the standard in Taiwan EPA. The study provides an effective stabilization technique for the treatment of waste funnel glass.