Grantee Research Project Results
Final Report: Simultaneous Environmental Monitoring and Purification Through Smart Particles
EPA Grant Number: R829602Title: Simultaneous Environmental Monitoring and Purification Through Smart Particles
Investigators: Sigmund, Wolfgang M. , Wu, Chang-Yu , Mazyck, David
Institution: University of Florida
EPA Project Officer: Aja, Hayley
Project Period: February 10, 2002 through February 9, 2005
Project Amount: $390,000
RFA: Exploratory Research: Nanotechnology (2001) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The objectives of this research project were to: (1) enable the atomic and molecular control of material building blocks; and (2) develop engineering tools to provide the means to assemble and use these tailored building blocks for developing novel smart or multifunctional particles for environmental applications as purifiers and sensors that are environmentally benign. We pursued a multidisciplinary synthesis of technologies, including self-organized structural control, and smart materials with a focus on environmental purification and monitoring to create intelligent surfaces and structures that not only sense and interact with their environment but that can fundamentally alter their own behavior and deactivate themselves as preprogrammed or as desired.
Summary/Accomplishments (Outputs/Outcomes):
We accomplished the following objectives:
- Synthesized multifunctional ferromagnetic photocatalytic particles that increased the specific surface area by two orders of magnitude from less than 0.25 m2/g to 31.7 m2/g.
- Strongly reduced the mass of photocatalyst required for treatment because of the ability of magnetic recycling and an increase in specific surface area from commercially available photocatalysts with 50 m2/g to our novel photocatalyst with 193.5 m2/g.
- Accommodated and optimized multifunctional particle design to air and water reactors.
- Discovered adsorption enhancement mechanisms of silica-titania multifunctional nanocomposites for elemental mercury vapor removal.
- Developed a method to detect presence of mercury in vapor by color change of smart particles from white to yellow.
- Developed a method to indicate adsorption activity of particles in mercury removal by color change. Activated pellets are yellow and inactive ones are white, which is easy to check visually.
- Investigated the effects of flue gas components on the performance of silica-titania nanocomposites for elemental mercury vapor removal.
- Performed quantitative assessment of mutual interference of ozone and mercury on their measurements.
- Improved pollutants mass transfer and exposure to ultraviolet (UV) light through optimization in magnetically agitated fluidization.
- Developed smart particles that can be separated by magnetic removal.
Characterization of Multifunctional Hard Magnetic Composite Photocatalyst -Barium Ferrite/Silica/Titania
The hard magnetic composite photocatalyst, -barium ferrite/silica/titania, was developed successfully during Year 2 of the project. Magnetic properties and photocatalytic properties of the developed composite photocatalyst in the magnetically agitated photocatalytic reactor are characterized in subsequent years of the project.
The magnetic properties of the heat-treated, uncoated barium ferrite particles, H_B/S/T that have been used for 20 hours in a photocatalytic reactor, as well as the uncoated barium ferrite particles that have been used for 20 hours in a photocatalytic reactor were determined by a superconducting quantum interference device (SQUID) magnetometer. The coercivity values of three samples are very similar (0.24 T for the uncoated barium ferrite and 0.23 T for both H_B/S/T and the uncoated barium ferrite after 20 hours use in the photocatalytic reactor). There are, however, reductions in the remanent magnetization values and the saturation magnetization values after photocatalytic reactions for 20 hours.
H_B/S/T after 20 hours use in the photocatalytic reactor shows a decrease of 14.8 percent in the remanent magnetization and a 13.9 percent reduction in the saturation magnetization compared with the uncoated barium ferrite particles. The reduction in the magnetic property of the uncoated barium ferrite particles after 20 hours use in the photocatalytic reactor is worse than that of the used H_B/S/T. They show a 38 percent reduction in the remanent magnetization and a decrease of 39.8 percent in the saturation magnetization compared with the unused bare barium ferrite particles. The reduction in the remanent and saturation magnetization values of H_B/S/T is caused by the silica and titania coating onto the magnetic core. The magnetic property deterioration of the bare barium ferrite particles, however, is ascribed to the photodissolution of barium and iron ions during use in the photocatalytic reactor.
Table 1 presents the results of the photodissolution test. As can be seen, the amount of photo-dissolved ions from H_B/S/T is minimal; the photodissolution of barium and iron ions from the bare barium ferrite particles, however, occurs during use in the photocatalytic reactor, weakening their magnetic property. Therefore, it is deduced that the intermediate silica layer successfully provides the system with an excellent protection layer against the photodissolution of the magnetic core particles.
Table 1. The Amount (%) of Photo Dissolved Ions After 20 Hours of Use in the
Photocatalytic Reactor
Sample | Ba | Fe | Si | Ti |
The uncoated barium | ||||
ferrite | 4.57 | 0.2 | − | − |
H_B/S/T | 0.004 | 0.0004 | 0.006 | 0.001 |
The photocatalytic efficiency of H_B/S/T in the magnetically agitated photocatalytic reactor (MAPR) was compared to the performance in the plain photocatalytic reactor (PR). The photocatalytic performance in MAPR showed an increase of 61 percent in the specific rate constant compared with the photocatalytic performance in PR. Even though the photocatalytic particles are kept in suspension by the vigorous stirring in PR, significantly enhanced photocatalytic efficiency is observed by better mass transfer and increased exposure of the particles to the UV light in MAPR.
Development of TiO2-SnO2 Smart Particle System and Its Enhanced Photocatalytic Activity
TiO2-SnO2 composite nanoparticles are developed to enhance the overall photocatalytic activity by modifying electronic properties of the photoactive shell of the hard magnetic composite photocatalyst. Three different procedures were taken to synthesize TiO2-SnO2 composite nanoparticles. The first method used was the preparation of composite nanoparticles by the co-precipitation technique (one-step method). The second method used is called the two-step method. In the two-step method, TiO2 nanoparticles were precipitated before SnO2 nanoparticles were precipitated onto TiO2 nanoparticles. The third method used was the same procedure as the two-step method except that the heat-treated TiO2 nanoparticles (500 °C, 1 hour) were used instead of the as-prepared TiO2 nanoparticles (modified two-step method).
Table 2. Synthesis Methods, SnO2 Precursors Used, and Ti:Sn Molar Ratios of TiO2-SnO2 Composite Nanoparticles Prepared
Sample | Method | SnO2 precursor | Ti:Sn molar ratio | SnO2 mol % | ||
1S_TS_50 | One-step | Tin tert-butoxide | 1:1 | 50 | ||
2S_TS_50 | Two-step | Tin tert-butoxide | 1:1 | 50 | ||
2S_TS_25 | Two-step | Tin tert-butoxide | 3:1 | 25 | ||
2S_TS_16.7 | Two-step | Tin tert-butoxide | 5:1 | 16.7 | ||
M2S_TS_50 | Modified two-step | Tin tert-butoxide | 1:1 | 50 | ||
M2S_TS_37.5 | Modified two-step | Tin tert-butoxide | 1.67:1 | 37.5 | ||
M2S_TS_25 | Modified two-step | Tin tert-butoxide | 3:1 | 25 | ||
M2S_TS_17.5 | Modified two-step | Tin tert-butoxide | 4.7:1 | 17.5 | ||
M2S_TS_10 | Modified two-step | Tin tert-butoxide | 9:1 | 10 | ||
M2S_TS_25_SnBT | Modified two-step | Tin tert-butoxide | 3:1 | 25 | ||
M2S_TS_25_SnO2 | Modified two-step | SnO2 coll. disp. | 3:1 | 25 | ||
M2S_TS_25_SnC 14 | Modified two-step | SnC 14·5H2O | 3:1 | 25 |
The X-ray diffraction (XRD) result of 1S_TS_50 indicates that 1S_TS_50 forms a rutile-type solid solution. Both TiO2 and SnO2 can crystallize in the tetragonal rutile structure and form solid solutions over the entire range of compositions above a certain temperature. Below the critical temperature, which is a function of the chemical composition of the solution, favorable conditions are created for spinodal or chemical decomposition. 1S_TS_50 forms the rutile-type solid solution even in the temperature range where decomposition is predicted. This can be explained by the fact that the conditions of synthesis are far from equilibrium. It is well known that the anatase TiO2 shows substantially higher photocatalytic activity than the rutile one. Therefore, the rutile-type solid solution of TiO2-SnO2 is not favorable to the photocatalyst. The XRD results of the samples prepared by the two-step method exhibit the peaks from the anatase TiO2 as well as ones from the rutile-type TiO2-SnO2. The intensity of the peak from rutile-type TiO2-SnO2 decreases as the SnO2 mol percent is lowered. The anatase structure of TiO2 in the TiO2-SnO2 nanoparticles prepared by the two-step method is identified clearly in the XRD pattern; however, the peak corresponding to the (101) planes of the anatase phase at 2θ equals 25.4°, which is the most intense peak among the anatase TiO2 peaks, is weak and broad even after the heat-treatment at 550°C for 1 hour. Poor anatase crystallinity and the enhanced anatase to rutile transformation in the TiO2SnO2 system have been previously reported. The addition of SnO 2 probably hinders the anatase crystallization and decreases the temperature of the anatase to rutile transformation to a much lower value. The XRD data of TiO2-SnO2 samples prepared by the two-step method agrees with the previously reported results.
The XRD pattern of M2S_TS_25 indicates that a significant structural variation occurs when compared with that of 2S_TS_25. Furthermore, the phases observed in M2S_TS_25 are very similar to those in H_Titania. The major crystal structure of TiO2 obtained by the precipitation technique is the anatase structure with a minor brookite traces. The absence of a SnO2-related secondary phase is observed in M2S_TS_25. Therefore, the anatase-type TiO2-SnO2 is obtained by the modified two-step method. Because SnO2 precursor is added to the well-grown TiO2 nanoparticles to prepare TiO2-SnO2 system in the modified two-step method, one also can deduce that the anatase-type TiO2-SnO2 occurs in the surface of TiO2.
XRD patterns were taken from the samples prepared by the modified two-step method with the three different SnO2 precursors (i.e., tin tert-butoxide, SnO2 colloidal solution, and SnC 14·5H2O). The XRD data indicate that the structure of the TiO2SnO2 system is affected by the SnO2 precursors used. The samples prepared using tin tert-butoxide and SnO2 colloidal solution form the anatase-type TiO2-SnO2, whereas the sample prepared using SnC 14·5H 2O appears to have the rutile-type structure as well as the anatase-type one.
Photodegradation test results show C/CO (concentration of remaining dye at the time of sample collection/initial concentration of dye) values for MX-5B as a function of irradiation time. Figure 1 presents the results of photodegradation of MX-5B under UV irradiation (302 nm) with the samples prepared by the one-step and two-step method. The sample prepared by the one-step method possesses a very slow photocatalytic activity compared with the samples prepared by the two-step method. 1S_TS_50 destructs only 10 percent of MX-5B during the 80 minute-UV irradiation. The samples synthesized by the two-step method show much faster photocatalytic activity than 1S_TS_50 in the order of 2S_TS_25 greater thaN2S_TS_16.7 greater thaN2S_TS_50. Because the crystal structure of 1S_TS_50 is the rutile-type, the very low photocatalytic activity of 1S_TS_50 is the expected result. 2S_TS_25, however, which shows stronger rutile-type peak thaN2S_TS_16.7, destructs MX-5B faster thaN2S_TS_16.7. This is the contrary result if the crystal structure effect on the photocatalysis is considered. In this case, the addition of SnO2 suppresses the recombination of photo-induced electron-hole pairs and improves the photocatalytic activity. By employing the modified two-step method to prepare the TiO2-SnO2system, the anatase-type structure is strengthened and stabilized. The results of the photodegradation test with the samples synthesized by the modified two-step method are presented in Figure 2. The highest photocatalytic activity is achieved by M2S_TS_25.
Figure 1. Results of Photodegradation of MX-5B Under UV Irradiation (302 nm) With the Samples Prepared by the One-Step and Two-Step Method. (4 mg of sample and 50 ml of 10 mg/L MX-5B solution were used.)
Figure 2 Results of Photodegradation of MX-5B Under UV Irradiation (302 nm) With the Samples Prepared by the Modified Two-Step Method. (2 mg of sample and 50 mL of 10 mg/L MX-5B solution were used.)
The influence of the SnO2 mol percent on the photocatalytic activity of the samples prepared by the modified two-step method is similar to that of the samples prepared by the two-step method. In both cases, the samples with 25 mol percent of SnO2 exhibit the fastest photocatalytic activity resulting from the most efficient electron-hole separation in those samples. The photocatalytic activity of the samples with 25 mol percent of SnO2 prepared by the two-step method and the modified two-step method was compared. The photodegradation rate is enhanced greatly by employing the modified two-step method. The specific rate constant of M2S_TS_25 is higher than that of 2S_TS_25 by 281 percent. This phenomenon is attributed to the enhanced and stabilized anatase-type structure of M2S_TS_25.
The photocatalytic activity of the samples prepared by the modified two-step method is compared in Figure 3. Three different materials as the SnO2 precursor are used in the modified two-step method (Table 2). As discussed previously, M2S_TS_25_SnC 14 showing the rutile structure as well as the anatase structure possess lower photocatalytic activity than M2S_TS_25_SnO2 and M2S_TS_25_SnBT showing only the anatase structure because of the effect of the crystal structure on the photocatalytic activity. Furthermore, the slightly increased anatase structure of M2S_TS_25_SnO2 probably accounts for the higher photocatalytic activity of M2S_TS_25_SnO2 than that of M2S_TS_25_SnBT. The photocatalytic activity of H_Titania (heat-treated TiO2 nanoparticles) and H_E_Titania (heat-treated extra-precipitate TiO2 nanoparticles from the supernatant after magnetically separating B/S/T) also is compared in Figure 3.
Figure 3. Results of Photodegradation of MX-5B Uder UV Irradiation (302 nm) With H_Titania, H_E_Titania, and the Samples Prepared by the Modified Two-Step Method. (2 mg of sample and 50 mL of 10 mg/L MX-5B solution were used.)
Although H_Titania destructs 86.8 percent of MX-5B during 40 minute-UV irradiation, H_E_Titania degrades only 47.6 percent of MX-5B. This result indicates that there is a difference in the property that can affect the photocatalysis between the TiO2 nanoparticles synthesized alone and the TiO2 nanoparticles prepared in the presence of the silica-coated barium ferrite. The energy-dispersive X-ray spectroscopy (EDS) spectrum images of H_Titania and H_E_Titania are presented in Figure 4. The presence of Si in H_E_Titania is detected, whereas the presence of Si is not detected in H_Titania. The approximate content of Si in H_E_Titania is 2.5 atomic %. The presence of Si in H_E_Titania may be the most probable cause of the lower photocatalytic activity of H_E_Titania because the other properties such as the crystal structure, surface morphology, specific surface area, and particle size are very similar.
Enhanced photocatalytic activity per unit surface area of M2S_TS_25_SnO2 clearly shows the effect of SnO2 addition on the photocatalysis. The addition of SnO2 suppresses the recombination of photo-induced electron-hole pairs and improves the photocatalytic activity. The excited electrons generated by the band gap transition of TiO2 flow into SnO2. The conduction band edge of SnO2 (0.41 eV lower than that of TiO2) facilitates interfacial electron transfer and suppresses back electron transfer simultaneously. The valence band edge of TiO2 (1.1 eV higher than that of SnO2) inhibits the interfacial hole transfer from TiO2 to SnO2. The holes left in the valence band of TiO 2 after the interfacial electron transfer participate in the photocatalytic oxidation process.
Figure 4. EDS Spectrum Images of (a) H_Titania and (b) H_E_Titania. The unlabeled peaks are adventitious copper.
The band gap absorption edges of H_Titania, H_E_Titania, and M2S_TS_25_SnO 2 are determined to be 387.5 nm, 397 nm, and 388 nm corresponding to the band gap energy of 3.20 eV, 3.11 eV, and 3.19 eV, respectively. The experimental result for the band gap energy of H_Titania is in excellent agreement with the accepted literature value of the anatase TiO2 (3.2 eV). Si doping and SnO2 addition affects UV-Visual absorption characteristics of TiO2 . The absorption spectra of M2S_TS_25_SnO2 show a stronger absorption in the UV range shorter than 370 nm and a red shift in the band gap transition compared with those of H_Titania. The absorption spectra of H_E_Titania show a red shift in the band gap transition and a weaker absorption in the UV range shorter than 350 nm compared with those of H_Titania. Generally, the rate of the photocatalytic reaction is proportional to (I αφ) n (n=1 for low light intensity and 0.5 for high light intensity), where I α is the photo numbers absorbed by the photocatalyst per second and φ is the efficiency of the band gap transition. Therefore, the enhancement of the photocatalytic activity of M2S_TS_25_SnO 2 compared with that of H_Titania can be explained partly by an increase in I α resulting from a stronger absorption in the UV region and vice versa for H_E_Titania.
M2S_TS_25_SnO2 is tested on the magnetic composite photocatalyst. Figure 5 compares the photocatalytic activity of H_B/S/T with that of H_B/S/M2S_TS_25_SnO 2. (M2S_TS_25_SnO2 is coated on the silica-coated barium ferrite and then the composite particles are heat-treated at 550°C for 1 hour). As expected, the magnetic photocatalyst with the TiO2-SnO2 shell shows faster photocatalytic activity than H_B/S/T.
Figure 5. Rate Constants of Photocatalytic Degradation of MX-5B for H_B/S/T and H_B/S/M2S_TS_25_SnO2 as a Function of Irradiation Time. (The rate constants are normalized to the surface area.)
TiO2-SnO2 composite nanoparticles were prepared with the wet-chemical method to improve the overall photocatalytic activity. The sample preparation procedures used, as well as the SnO2 precursors, significantly affect not only the crystalline phases formed but also the crystallinity of the prepared samples, which also have a critical influence on the photocatalytic activity of the samples. In the prepared TiO2-SnO2 composite nanoparticles, the Ti:Sn molar ratio of 3:1 was found to suppress the photo-induced electron-hole recombination the most, leading to the highest photocatalytic activity. The modified photoactive TiO2-SnO2 shell was prepared and tested on the hard magnetic composite photocatalyst to show the enhanced photocatalytic activity by modifying the electronic property of the photoactive shell.
Magnetically Agitated Photocatalytic Reactor
Initial experiments employed 100 mg of the composite photocatalyst, 1 Amp current, and 60 Hz solenoid frequency. Without the permanent magnet and with one 365 nm lamp, the nDOC concentration decrease was less than the variation in replicate analysis of samples (Figure 6), but the phenol concentration at 7 hours decreased tO29 percent of its original value (Figure 7). When the permanent magnet was present, the phenol concentration after 5 hours was not detected via GC analysis, although close to 70 percent of the nDOC remained. Therefore, a large fraction of the phenol was simply being converted to byproducts. To increase the rate and efficiency of photocatalysis, two 365 nm lamps were added (for a total of 3 lamps or 24 W). Although the decrease in phenol concentration after 7 hours was approximately the same with and without the permanent magnet (Figure 7), the initial pseudo-first order rate constant for phenol oxidation was three times greater (0.80 min -1 vs. 0.27 min -1) with the permanent magnet. Data for nDOC (Figure 6) indicated that after 7 hours 0.6 mg/L C remained with a permanent magnet, whereas when it was not present during the experiment, 3.1 mg/L remained. The improvement in photocatalytic degradation from the fixed magnet is believed to be because of its ability to counteract gravity with a magnetic field gradient. Under the influence of this magnetic field gradient, the catalyst was spread more effectively throughout the reactor thus increasing its exposure to UV.
Figure 6. nDOC Concentration as a Function of Number of Lamps and Presence of Permanent Magnet in the MAPR (WPM = with permanent magnet, NPM = no permanent magnet, 1L = 1-365 nm lamp, 3L = 3-365 nm lamps).
Figure 7. Comparison of Number of UV Lamps and Presence of Permanent Magnet for Degradation of Phenol in the MAPR (WPM = with permanent magnet, NPM = no permanent magnet, 1L = 1-365 nm lamp, 3L = 3-365 nm lamps).
The quantity of catalyst in the reactor was optimized for phenol and nDOC reduction using 2 hours of exposure with one 365 nm lamp. With a 365 nm lamp, which was shown earlier not to cause photolytic mineralization, the phenol removal was greater than nDOC removal for the entire range of catalyst mass. This was most likely an indication that several oxidation byproducts remain in solution after 2 hours of irradiation. The trend in phenol removal showed a slight increase as mass increased up to 400 mg and remained constant at higher mass loadings. The nDOC trend demonstrated an increase in removal with mass until 400 mg where there was a peak and then a descent as the mass increased further. This optimum is believed to be the result of two phenomena. An increase in phenol and nDOC removal is expected with an increase in TiO2 concentration (as a function of catalyst mass increasing) because of an increase in the number of reaction sites and thus the quantity of hydroxyl radicals produced. A competing phenomenon is expected as the catalyst concentration reaches a level where the UV light is blocked, reducing the proportion of catalyst receiving photons of sufficient energy to generate hydroxyl radicals.
The sinusoidal signal frequency fed to the solenoid affected the rate at which the magnetic field gradient, and thus force on the catalyst, alternated direction. This oscillating force caused the catalyst to move within the reactor, increasing the mass transfer of phenol and photodegradation byproducts to the TiO2 on the surface of the composite catalyst. For this experiment, each data point represents a variation in frequency where the current was fixed at 1.25 Amps, and 100 mg of catalyst was irradiated for 2 hours with three 365 nm UV lamps. The data demonstrate that nDOC is eliminated most effectively betweeN20 and 80 Hz where the variation in removal over the frequency range is less than the sample RSD from the methods section. The elimination of oxidation byproducts, which is indicated by nDOC, decreases as frequency increases beyond 80 Hz. This effect may be because of a decrease in the ability of the catalyst to effectively respond to changes in the magnetic field at higher frequencies. As the catalyst motion response is diminished, there will be a corresponding decrease in mass transfer, thus photocatalytic degradation. In contrast to the nDOC result, phenol removal showed a less dramatic decrease at higher frequencies. This discrepancy in oxidation byproduct removal and phenol removal may be because of a higher mass transfer dependence of byproduct oxidation over phenol oxidation. This would mean that the first few steps of phenol oxidation to hydroxylated compounds occur at the same rate for higher frequencies and lower mass transfer but subsequent reaction steps, such as loss of aromaticity and mineralization, occur at a slower rate because they may be more dependent upon mass transfer.
The magnetic flux of the alternating magnetic field generated by the solenoid is directly proportional to current according to the Biot-Savart law. The increase in magnetic flux, therefore, is expected to correlate with an increase in catalyst motion and thus mass transfer of phenol to catalyst. The experimental data agreed with this expected increase from 0 to 1 Amp; there was a corresponding increase in both reduction of nDOC and phenol concentration. As the current increased above 1 Amp, the photocatalytic degradation remained constant. This change in the trend indicates that the system performs optimally with 1 Amp of current, as further increases in current only increase energy requirements and heat generation.
Mutual Interferences of Ozone and Mercury on Their Measurements
Regulating mercury (Hg) emissions and studies on Hg control technologies necessitate accurate Hg measurement. Continuous mercury monitors (CMMs) have the advantage of performing real-time measurement over wet-chemistry Hg measuring methods; atomic absorption spectrometry (AAS)-based CMMs, however, may be subject to interferences by components of the sample gas because of their strong absorption bands near the Hg absorption line. Ozone is one gas that can exert considerable influences on Hg measurement and consequently may affect the risk assessment of human exposure to Hg. On the other hand, investigations have shown that Hg also can have interferences on ozone analyzers based on UV absorption because the UV light source used in such ozone analyzers is usually 254 nm emission line from a discharge Hg lamp.
To test the interference of ozone on Hg measurement, a M146 dynamic gas calibration system (Thermo Electron Instrument) served as the ozone-generating source using its internal ozonator. A RA-915+ Hg analyzer (OhioLumex Co.) was used to measure any interference caused by ozone. To investigate the interference of Hg on ozone measurement, a Hg basin placed in an ice bath was used to introduce saturated Hg vapor into the sample gas, and an M49 UV photometric ozone analyzer (Thermo Electron Instrument) was used to measure any interference caused by Hg. An M49-PS UV photometric ozone calibrator (Thermo Electron Instrument) was applied to calibrate both the ozone generator and analyzer.
The ozone concentration generated in this work ranged from 0 to 120 ppb. The baseline test showed that no interference was detected when no ozone was fed into the gas stream. As ozone concentration increased, the interference on Hg readings was elevated almost linearly. For ozone concentrations at 80 and 120 ppb, the interferences on Hg measurement were approximately 56 and 70 ng/m 3, respectively. The interference can be expressed as:
CHge (ng/m3) =395 .0 × CO3(ppb) + 6.23 wheN20 < CO3 < 120 ppb (Equation 1)
where CHg,e is the equivalent Hg concentration (ng/m3) and CO3 is the ozone concentration (ppb). Equation 1 does not apply to ozone concentration ranging from 0 tO20 ppb because the intercept of the trend line is not zero. When the ozone concentration was lowered below 20 ppb, the Hg readings scattered to a large extent as the Hg analyzer approached its detection limit.
This interference of ozone on Hg measurement can impact the risk assessment of human exposure to Hg. The Reference Concentration (RfC) for elemental Hg specified by the U.S. Environmental Protection Agency (EPA) is 300 ng/m3 based on central nervous system effects in humans. Hg concentrations above this level may result in a further investigation of hazardous exposure. The ozone interference can affect the risk assessment of exposure to Hg because an ozone concentration betweeN20 to 120 ppb can exert an error in Hg measurement equivalent to 10 tO23 percent of the EPA RfC. This is important particularly when the Hg concentration in a condition is close to the RfC. In addition, the use of ozone for the removal of indoor air contaminants has been widely promoted in the United States. In those areas with elevated ozone concentrations, the interference of ozone on Hg measurement may result in a significant overestimate of Hg concentrations. Therefore, eliminating ozone from the sample gas is essential for obtaining accurate Hg concentration and thus is critical to risk assessment of human exposure to Hg.
A designated range of Hg concentrations was fed into the gas stream to investigate the possible Hg interference on ozone measurement. Because of the limitation of the Hg vapor-generating unit used in this work, the minimum Hg concentration introduced was about 2,300 ng/m3. In addition, Hg levels were controlled so that the interferences on ozone readings were within the measurement range of the ozone analyzer (0-500 ppb). As the Hg concentration in the sample gas increases, the corresponding interference reading on the ozone analyzer increases. The trend line of the interference can be expressed as:
CO3,e ( ppb) =117.0 × CHg (ng / m3) (EquatioN2)
where CO3,e is the equivalent ozone concentration (ppb) and CHg is the Hg concentration (ng/m3).
EquatioN2 implies that Hg can exert a significant interference on ozone measurement, which is very likely caused by the strong UV absorption by Hg inside the ozone analyzer. Although the Hg level in the ambient and indoor environment is typically lower than the minimum Hg concentration (2,300 ng/m3) tested in this work, the results obtained can be used as a reference to predict the practical conditions. For example, the highest Hg concentration of 523 ng/m3 measured at 12 indoor sites in New York City would result in an interference on ozone reading of 61 ppb. This value, added up to the normal ambient ozone concentration (0-50 ppb), may have many chances to exceed the National Ambient Air Quality Standards for ozone (80 ppb for maximum 8-hour average). When Hg concentration is equal to the EPA RfC (300 ng/m3), an interference of 35 ppb would be involved, which typically is comparable to the average ambient ozone concentration. Because indoor ozone concentration is typically lower than that outdoors, the interference of Hg for indoor ozone measurement may be even higher. These results indicate that it is essential to eliminate the Hg interference to obtain correct ozone concentration at Hg contaminated places.
Effects of Flue Gas Components on the Performance of Silica-Titania Nanocomposites for Elemental Mercury Vapor Removal
The high surface area silica-titania nanocomposites have exhibited synergistic adsorption and photooxidation of elemental Hg laden in an air stream. In the flue gas from coal-fired boilers, the largest single-known source of anthropogenic mercury emissions in the United States, there are, however, various minor gases such as HC1, SO2, NO, and NO2. Therefore, the goal of this research was to identify whether these minor gases can enhance or inhibit the performance of the silica-titania composites on elemental Hg vapor removal.
The simulated flue gas consisted of three major gases: 84 percent N2, 4 percent O2, and 12 percent CO2. A N2 stream passed through the surface of a liquid Hg reservoir and introduced the saturated Hg vapor into the system. The Hg reservoir was placed in an ice-water bath to maintain a constant Hg vapor pressure. Minor gases including HC1, SO2, NO, and NO2 were introduced into the gas flow individually. The concentrations of minor gases were controlled by adjusting their flow rates. All gas flow rates were controlled by mass-flow controllers (MFC). Downstream of all the gas flows was a packed-bed reactor, where a UV lamp (365 nm, 4 mW/cm 2) is placed in the center. At the bottom of the reactor is a glass frit used to hold the silica-titania pellets within the bed. In this study, 2.5 grams of fresh silica-titania nanocomposites (12% TiO2) were used in each test, which corresponded to an approximately 5 mm bed thickness. A RA-915+ Hg analyzer (OhioLumex) was used to measure the Hg concentration from the outlet of the reactor. Baseline Hg concentration was obtained when the gas bypassed the reactor. Finally, the gas stream passed through a carbon trap before it was exhausted into the fume hood.
A baseline test with no minor gases introduced was performed. Baseline Hg concentration (Hg BL) was 66 µg/m3 at the beginning of the test (2 min). The gas then passed through the reactor and the initial adsorption of Hg by the silica-titania pellets appeared to be insignificant (at 5 min). Hg oxidation was approximately 38 percent after exposure to UV light for 5 min. It should be noted that even though the gas was dry, the pellets may have adsorbed some moisture before being put into the reactor so that the generation of OH radicals and thus photo oxidation can be triggered. UV light then was turned off for another 5 minutes and Hg concentration recovered to its baseline level. A UV on/off cycle then was repeated twice. Total Hg removal (with UV) increased to 48 percent and 59 percent in the second and third cycle, respectively. Hg removal by adsorption only (without UV) also increased to 3 percent and 7 percent in the second and third cycle, respectively. This demonstrates the composite’s synergistiCHg adsorption and photooxidation ability. After the third cycle, the pellets were exposed to UV light for 15 min and the final removal of Hg reached 71 percent. It is possible that the ultimate removal can be higher but it would take a much longer time. At the end of the test, the baseline Hg level was double-checked. Passing though the reactor without UV light gives a final adsorption of 22 percent. This enhanced ability of adsorption is likely because of the deposition of HgO on the composite surface, as reported previously.
The same procedure was applied to the tests with minor gases. The concentrations of the minor gases were designated to be within the range of typical flue gas composition. Table 3 summarizes effects of minor gases on Hg removal by the silica-titania composites. Baseline Hg concentration in each test was relatively constant between 60 and 70 µg/m3 so that it would not be an influence.
Table 3. Percentage of Elemental Hg Removal by Silica-Titania Nanocomposites in the Presence of Minor Gases (Major gas components: 84% N 2, 4% O2, and 12% CO2; Hg BL= 60 ~ 70 µg/m3 ; IA – initial adsorption, FR – final removal, FA – final adsorption)
IA | 1 st cycle with UV | 1 st cycle no UV | 2 nd cycle with UV | 2 nd cycle no UV | 3 rd cycle with UV | 3 rd cycle no UV | FR | FA | |
Baseline test | 0 | 38 | 0 | 48 | 3 | 59 | 7 | 71 | 22 |
HC1=30ppm | 50 | 70 | 54 | 62 | 55 | 60 | 56 | 62 | 60 |
SO2=1200ppm | 2 | 25 | 8 | 31 | 11 | 34 | 16 | 41 | 22 |
NO=300ppm | 2 | 5 | 1 | 6 | 1 | 7 | 1 | 13 | 6 |
NO2=10ppm | 0 | 36 | 3 | 44 | 5 | 52 | 10 | 65 | 19 |
When HC1 concentration was 30 ppm, a significant adsorption (more than 50%) was observed throughout the test when there was no UV light. This may be because HC1 also was adsorbed onto the composite surface and reacted with Hg on the active sites, consequently enhancing the chemisorption of Hg. With exposure to UV light, photooxidation occurred but the oxidation rate seemed to decrease a little over the cycles rather than increase as detected in the baseline test. A possible reason is that some of the generated OH radicals may have reacted with HC1 and thus the oxidation ability by OH radicals has been attenuated.
When SO2 with a concentration of 1,200 ppm was present, the trend of the adsorption/oxidation over cycles was similar to that in the baseline test, except that the adsorption level was a little higher and the oxidation level was lower than the baseline. This indicates that SO2 may be a promoter for Hg adsorption but an inhibitor for Hg oxidation.
NO seemed to be a strong inhibitor for both adsorption and oxidation. When there was 300 ppm of NO, the adsorption almost was zero and the oxidation was less than 10 percent. The attenuation of the oxidation ability may be explained by the scavenging of OH radicals with the reaction: OH + NO + M → HONO + M. NO2 was found to have little effect on the pellet performance. The results obtained when NO2 was 10 ppm were very close to those in baseline.
The Role of Moisture on Hg Adsorption, Photooxidation, and Reemissions
The volume fraction of moisture in the coal-derived flue gas usually accounts for a few percent. Therefore, it is important to investigate the effect of moisture on the performance of the silica-titania nanocomposites. To achieve this, a water bubbler was installed to generate saturated water vapor into the sample gas. A humidity sensor (HX94C, Omega) was used to measure the relative humidity (RH) of the gas.
The experimental procedure was the same as in the baseline test described in the previous section. The comparison of Hg removal efficiency between 75 percent RH and baseline is shown in Table 4.
Table 4. Percentage of Elemental Hg Removal by Silica-Titania Nanocomposites at Different Humidity Levels (Major gas components: 84% N2, 4% O2, and 12% CO2; Hg BL= 60 ~ 70 µg/m3; IA – initial adsorption, FR – final removal, FA – final adsorption)
IA | 1 st cycle with UV | 1 st cycle no UV | 2 nd cycle with UV | 2 nd cycle no UV | 3 rd cycle with UV | 3 rd cycle no UV | FR | FA | |
RH=0 (Baseline) | 0 | 38 | 0 | 48 | 3 | 59 | 7 | 71 | 22 |
RH=75% | 0 | 13 | 0 | 13 | 0 | 14 | 1 | 14 | -3 |
When RH is 75 percent, the final removal efficiency drops to 14 percent compared with 71 percent in the baseline. No enhanced adsorption ability was observed over the three cycles and the final adsorption efficiency drops to –3 percent compared with 22 percent in the baseline. These results show that high humidity (75% RH in this work) can inhibit both the adsorption and oxidation of Hg, although moisture is necessary for generating OH radicals, which are responsible for photocatalytiCHg oxidation. It is likely that high concentration of moisture occupies the active sites on the pellets and hence reduces the adsorption capacity for Hg. Because adsorption may be a necessary step for the following photooxidation, the oxidation ability also can be reduced.
It should be noted that the final adsorption efficiency was negative in the case of 75 percent RH. This implies that there might be a release of Hg from the composites instead of adsorption. To further investigate the effect of moisture on Hg reemission, 2.5 grams of composites were exposed to UV light for 3 hours with Hg vapor (66 µg/m3 ) laden in dry air. During this 3-hour pretreatment, the composites can adsorb a certain amount of elemental Hg as well as oxidized forms of Hg because of the photooxidation. After the pretreatment, a reemission test was carried out when no external Hg vapor was introduced into the sample gas and the results are shown in Figure 8.
Figure 8. Reemission of Hg From Silica-Titania Nanocomposites After 3-Hour Pretreatment
In the first 4 minutes, only dry air (Case I) passed though the pretreated composites and 3µg/m3 of elemental Hg was observed, which shows a natural reemission of elemental Hg from the composites. Further test conditions changed every 4 minutes. When UV light was turned on (Case II), Hg concentration dropped to zero. This indicates that the reemitted elemental Hg has been photooxidized. Immediately after humid air (75% RH) was introduced, however, when UV light was off (Case III), a significant Hg level as high as 25 µg/m3 was observed, which is 38 percent of the Hg fed concentration in the pretreatment (65 µg/m 3). This shows that water vapor can promote the Hg reemission strongly. When UV light was turned on for humid air (Case IV), Hg concentration decreased to approximately 17 µg/m3. When Case III was repeated, the Hg level was found to be different from the previous Case III. This time, Hg concentration further decreased to approximately 10 µg/m3. A following repeat of Case IV caused the Hg concentration to increase to a comparable level as in the previous Case IV. Finally, Cases I to IV were carried out again, and the patterns in the change of Hg concentration almost were repeatable. Major results concerning the reemission of Hg are summarized as follows: (1) reemission of Hg occurs to a small extent in dry air (Case I); (2) UV exposure can prohibit Hg reemission in dry air (Case II); (3) water vapor can promote Hg reemission strongly (Case III); (4) UV exposure in humid air can either prohibit Hg reemission when the reemission level is relatively high (1st III → IV) or promote Hg reemission when the reemission level is relatively low (2nd and 3rd III → IV); and (5) Hg reemission levels decrease as time increases (1st IV → 2nd IV → 3rd IV).
A possible reason for the enhancing ability of water vapor on Hg reemission is that water vapor has stronger affiliation with the active sites on the composite surface than Hg does. As previously discussed, water vapor can compete with Hg vapor for the active sites and thus reduce the composites’ ability to adsorb Hg. In the same manner, it is likely that water vapor can expel adsorbed elemental Hg out of the active sites on the composite surface when there is no more external feeding of Hg vapor.
The effect of UV exposure in humid air can be explained by the competition between photooxidation of reemitted elemental Hg and reduction of oxidized Hg on the composite surface. Reactions 1 to 3 show the generation of OH radicals on the silica-titania composites under UV irradiation.
TiO 2+ hv → e −+ h + (1)
H2O ↔ H ++ OH − (2)
h + + OH −→ OH · (3)
Then OH radicals can react with the reemitted elemental Hg expelled by water vapor to form HgO (Reaction 4).
OH · + Hg (g) → HgO (s) (4)
In the meantime, HgO can dissolve in water on the composite surface to form Hg 2+ (Reaction 5). The surface water comes from the adsorption of water vapor by the composites. Although the solubility of HgO in water is weak, Reaction 5 still can go to the right hand side because there is only a trace amount of HgO compared to H2O. As long as the composites are exposed to UV light, the electrons generated on the composite surface can reduce Hg 2+ to elemental Hg, which is then released into the effluent (Reaction 6).
HgO (s) + H2O ↔ Hg 2+ + 2OH − (5)
Hg 2+ + 2e −→ Hg (g) (6)
When the condition was first changed from III to IV, the reduction of Hg 2+ to elemental Hg (Reaction 6) was not significant. This may be because the amount of water vapor adsorbed was limited because of the short period of time and thus little Hg 2+ was formed by Reaction 5. The oxidation of reemitted elemental Hg (Reaction 4) was then dominant at that time. Therefore, turning the UV on caused a decrease in elemental Hg concentration (i.e., prohibiting the Hg reemission). When condition III was changed to IV for the second and third time, the reduction of Hg 2+ to elemental Hg became dominant. This is because the total available Hg that can be reemitted decreased as time went on and that resulted in a reduced level of Hg reemission (comparing the 2nd III with the 3rd III in Figure 4). Hence, the degree of photooxidation of elemental Hg to HgO (Reaction 4) decreased. On the other hand, the water vapor adsorbed on the composite surface increased along the time and so did the amount of dissolved HgO and that of the elemental Hg generated by photo reduction (Reactions 5 and 6). Therefore, turning the UV on from this time on would cause an increase in elemental Hg concentration (i.e., promoting the Hg reemission).
As Hg reemission levels decrease along the time, there should be a time when all the adsorbed elemental Hg would be depleted by reemission. This was proven by an observation that there was no Hg reemission, even in humid conditions, after the composites have been stored in room conditions for three weeks. It is possible that all the adsorbed Hg has been reemitted naturally during these three weeks. Only if the composites were exposed to UV light and fed with humid air can Hg reemission be observed. This Hg reemission probably is caused by the photo-reduction described in Reactions 5 and 6.
Journal Articles on this Report : 8 Displayed | Download in RIS Format
Other project views: | All 45 publications | 8 publications in selected types | All 8 journal articles |
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Biswas P, Wu CY. Nanoparticles and the environment.Journal of the Air & Waste Management Association 2005;55(6):708-746. |
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Kostedt IV. WL, Drwiega J, Mazyck DW, Lee S-W, Sigmund W, Wu C-Y, Chadik P. Magnetically agitated photocatalytic reactor for photocatalytic oxidation of aqueous phase organic pollutants. Environmental Science & Technology 2005;39(20):8052-8056. |
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Lee SW, Drwiega J, Wu CY, Mazyck D, Sigmund WM. Anatase TiO2 nanoparticle coating on barium ferrite using titanium bis-ammonium lactato dihydroxide and its use as a magnetic photocatalyst. Chemistry of Materials 2004;16(6):1160-1164. |
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Lee SW, Drwiega J, Mazyck D, Wu CY, Sigmund WM. Synthesis and characterization of hard magnetic composite photocatalyst—barium ferrite/silica/titania. Materials Chemistry and Physics 2006;96(2-3):483-488. |
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Lee SW, Sigmund WM. Formation of anatase TiO2 nanoparticles on carbon nanotubes. Chemical Communications 2003;(6):780-781. |
R829602 (2002) R829602 (Final) |
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Li Y, Wu C-Y. Role of moisture in adsorption, photocatalytic oxidation, and reemission of elemental mercury on a SiO2—TiO2 nanocomposite. Environmental Science & Technology 2006;40(20):6444-6448. |
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Pitoniak E, Wu C-Y, Mazyck DW, Powers KW. Adsorption enhancement mechanisms of silica—titania nanocomposites for elemental mercury vapor removal. Environmental Science & Technology 2005;39(5):1269-1274. |
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Wu CY, Biswas P. 2005 Critical review summary—Nanoparticles and the environment. EM: The Magazine for Environmental Managers 2005;(June):33-39. |
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Supplemental Keywords:
air, water, adsorption, heavy metals, innovative technology, remediation, engineering, and industry,, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Sustainable Industry/Business, Sustainable Environment, Environmental Chemistry, Technology for Sustainable Environment, Monitoring/Modeling, New/Innovative technologies, Environmental Engineering, nanosensors, environmental monitoring, monitoring, chemical sensors, nanotechnology, environmental sustainability, environmentally applicable nanoparticles, biomonitoring, analytical chemistry, nanoscale sensors, remediation, sustainability, nano engineering, smart particles, innovative technologiesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.