Green Road Using TiO2
The objective of this study is to test the hypothesis that titanium dioxide (TiO2) can function as a photo catalytic compound when used in the preparation of warm-mix asphalt (WMA). The proposed asphalt mixture would combine the benefits of WMA such as reduced energy consumption and emission during production with the photo catalytic properties of TiO2 to trap and degrade organic and inorganic particles in the air. Two application methods were evaluated, using TiO2 as a modifier to asphalt binder in the preparation of WMA and applying TiO2 to the pavement surface as a water-based solution. Results of the experimental program indicated that the use of TiO2 as a modifier to asphalt binder was not effective in removing NOx pollutants from the air stream; however, the application of TiO2 as part of a water-based spray coating achieved a NOx reduction efficiency ranging from 39 to 52%.
Pollution from exhaust gases is hazardous to people’s health and is one of the major problems in our cities. Pollutants, especially nitrogen oxide (NOx), increase the risk of airway infections and can also impair the function of an airway. The largest quantity of NOx is emitted from road traffic and thus in the direct surrounding of the people. Especially in summer, these nitrogen oxides are the reason for formation of harmful to health ozone in near-ground layers. This nano-crystalline titanium dioxide is a photo catalyst. Activated by daylight the photo catalytic reaction results in oxidizing reagents converting hazardous NOx into harmless NO3 -. During sunny weather, the air can be cleaned by photo catalysis as it leads to elimination of up to 90% of nitrogen oxides, aldehydes, benzenes and chlorinated aromatic compounds. Even when the weather is bad, the sun is not directly shining and the UV radiation is low, up to 70% of the pollutants are still eliminated. The photo catalytic reaction can be repeated at any time without consuming the photo catalyst.
The most important facts about Tio2
· Tio2 is Photo catalytic nano particle, which reduces the level of nitrogen oxides (NOx) by means of an integrated photo catalyst.
· The photo catalytic oxidation of NOx into harmless NO3 is a contact reaction activated by light and thus only taking place on the surface.
· The produced NO3 - is neither toxic nor hazardous to health. It reacts with the calcium hydroxide and washes off with the next rain
· The rate of the photo catalytical oxidation depends on the light intensity and the air flow.
· In laboratory tests, 40% NOx was immediately oxidized to NO3 -.
· Natural daylight is sufficient for initiating the photo catalytic effect.
· The photo catalyst is not consumed during the photo catalytic reaction
· Safe for use in mass processing and providing long lasting protection
· TiO2 is used in many articles of daily life, for example, in cosmetics
· It is an ecologically clean and noncritical solution for humans and the environment.
1. Introduction:
The US faces a significant challenge in controlling air pollution resulting from transportation activities. Although attempts are made to lower vehicle emission standards, a method is needed to remove these pollutants once they are emitted to the atmosphere. The potential of titanium dioxide (TiO2) as an air purifier in urban and metropolitan areas, which suffer from high concentration of air pollutants, has been widely recognized in literature [1, 2]. Evaluation of concrete pavement treated with TiO2 provided promising results as recent research by the authors and others show that a thin surface coating is able to remove a significant portion of nitrogen oxides (NOx) and volatile organic compounds (VOC) pollutants from the atmosphere when placed as close as possible to the source of pollution [2 - 4]. However, with 94% of the US road network covered with asphalt, it appears that widespread use of titanium dioxide in air purification applications can only be achieved by the development of a novel asphalt mixture that does not affect the mechanical properties of the mix while incorporating a photo catalytic compound into current highway construction practices [5]. In addition, the use of Warm Mix Asphalt (WMA) will have the added benefits of reduced energy and the associated pollution emissions during production.
The objective of this study is to test the hypothesis that TiO2 can function as a photo catalytic compound when used in the preparation of WMA. To achieve this objective, a crystallized anatase-based titanium dioxide powder was blended with a WMA asphalt binder classified as PG 64-22 at three percentages by binder weight (3, 5, and 7%). In addition, a second application method was evaluated, especially useful for coating existing pavements, by spraying a water-based solution of TiO2 to the surface at three coverage levels (0.11, 0.21, and 0.31 kg/m2). Prepared blends were characterized using fundamental rheological tests (i.e., dynamic shear rheometer, rotational viscosity, and bending beam rheometer), the semi-circular bend SCB) test for fracture resistance and by measuring the environmental efficiency of the mixture in removing part of the NOx pollutants in the air stream.
2. Background:
The potential of TiO2 as a photo catalyst was discovered by Fujishima and Honda in 1972 [6]. In the presence of UV light, TiO2 produces hydroxyl radicals and superoxides, which are respectively responsible for oxidizing and reducing environmental contaminants including VOC and NOx [7]. A proposed mode of oxidation of NOx via hydroxyl radical intermediates in the presence of the photo catalyst is described by the following equations:
NO + OH à NO2 + H
NO2 + OH à NO3 + H
Based on this heterogeneous photo catalytic oxidation process, NOx are oxidized into water-soluble nitrates; these substances can be washed away by rainfall. Titanium dioxide particles crystallize in three forms: anatase, rutile, and brookite. Anatase is a meta-stable phase that transforms into rutile at high temperatures [8]. Research has shown that TiO2 in the anatase phase is a more powerful photo catalyst than rutile and brookite in environmental purification [9]. Numerous research studies have also reported that the degree of photo catalytic activity depends on the physical properties of TiO2 including the level of crystallization, surface area, particle size, and surface hydroxyls [10]. In pavement applications, it is desirable to prepare a TiO2 coating with hydrophobic properties, which provide for a self-cleaning surface. Through this process, particles of contaminants adhere to water droplets in case of rain and are removed from the surface when the droplets roll off of it.
Use of TiO2 in Pavement Applications:
Available TiO2 technologies have been mostly directed towards concrete pavements in which a fine mixture consisting of cement, sand, TiO2, and water is applied as a thin surface layer or slurry to the surface. Yet few studies are available for asphalt pavements, TiO2 has been incorporated into asphalt pavements by integrating it into the binder and as a thin surface layer that is sprayed on existing pavements [11, 12]. The waterbased emulsion was applied by two different methods, referred to as hot and cold method; distinguished by the spraying of the emulsion during asphalt paving laying operations when the pavement temperature is over 100°C or on existing pavements at ambient temperatures [11]. The study results showed that the reduction efficiencies were highly dependent on the TiO2 nanoparticles used in which efficiencies ranged from 20 to 57% of NOx reductions. Meanwhile, researchers in China mixed TiO2 with an asphalt binder at a 2.5% content of the binder weight to an emulsified asphalt pavement [12]. Evaluation presented in this study showed that a maximum efficiency in removing nitrogen oxide near 40% was achieved. A more efficient approach may be achieved by concentrating the photo catalytic compound at the pavement surface.
3. Experimental Program:
Asphalt cement binder blends were prepared by mixing a conventional WMA binder (WMA additive Evotherm was used at 1% by weight of the binder) classified as PG 64-22 with a commercial crystallized anatase-based TiO2 powder at three percentages 3, 5, and 7% by weight of the binder. The blends were prepared at a mixing temperature of 163°C. While short-term aging was simulated using the rolling-thin film oven (RTFO), long-term aging was simulated using the pressure aging vessel (PAV). The RTFO test simulated construction hardening and asphalt binder aging by subjecting the material to circulating hot air for 85 min. The PAV test simulated long-term oxidative aging for a period ranging from 5- 10 years by subjecting the binder to pressurized air for 20 hrs and a temperature maintained at 100°C. Prepared blends were characterized using fundamental rheological tests (i.e., dynamic shear rheometry, rotational viscosity, and bending beam rheometer) and by comparing the Superpave Performance Grade (PG) of the modified blend to the unmodified WMA binder. To assess the influence of the photo catalytic compound on the binder aging mechanisms and to ensure that TiO2 does not oxidize the binder, both the control and modified prepared blends were subjected to UV light for a period of seven days. The blends were prepared at a mixing temperature of 163°C and compacted by a gyratory compactor. Fracture resistance was assessed using the semi-circular bending (SCB) test. [13]. This test characterizes the fracture resistance of HMA mixtures based on fracture mechanics principals, the critical strain energy release rate, also called the critical value of J-integral, or Jc. To determine the critical value of J-integral (Jc), three notch depths of 25.4, 31.8, and 38 mm were selected based on an a/rd ratio (the notch depth to the radius of the specimen) between 0.5 and 0.75. Test temperature was selected to be 25°C. The semi-circular specimen is loaded monotonically till fracture failure under a constant cross-head deformation rate of 0.5 mm/min in a threepoint bending load configuration. The load and deformation are continuously recorded and the critical value of J-integral (Jc) is determined using the following equation [13]:
Jc = ((U1/b1)-(U2/b2))*(1/(a2-a1))
where,
b = sample thickness;
a = the notch depth; and
U = the strain energy to failure.
A second application method consisting of applying a thin surface coating was also evaluated at three coverage rates (0.11, 0.21, and 0.31 kg/m2). The spray coat used was a mixture of TiO2 anatase nanoparticles suspended in an aqueous liquid at 2% by volume. A thin film was spray coated on each sample in layers using in a cross hatch formation for each of the three defined coverage rates.
Environmental Test Setup: The environmental benefit of the fabricated asphalt blends in trapping and degrading NOx pollutants from the air stream through a photocatalysis mechanism was investigated. A laboratory test setup that is capable of uantifying the photo catalytic efficiency of asphalt and concrete specimens was used, Figure 1. The test setup was adapted from the Japanese standard JIS TR Z 0018 “Photo catalytic materials – air purification test procedure.” The developed experimental setup consists of a pollutant source, zero air source, calibrator, humidifier, photoreactor, and a chemiluminescent NOx analyzer as shown in Figure 1. The setup simulates different environmental conditions by allowing for control of light intensity and air humidity. The pollutants are introduced through an inlet jet stream to the photoreactor, a photo catalytic testing device. A zero air generator is used to supply the air stream, which is passed through a humidifier to simulate the desired humidity level. The photoreactor creates an enclosed controlled environment where the light and the atmosphere can be simulated. Fluorescent lamps, attached to the photo catalytic device, are used to imitate natural sunlight radiation required for photo catalytic activity. The pollutants measured from the recovered air before and after the photoreactor allowed for determination of the absorbed level of pollutants. In this study, NOx and removal efficiency was measured using the Thermo 42i chemiluminescent NOx analyzer. Nitrogen oxides were blown over the surface of the asphalt specimens at a concentration of 450 ppb. All tests were conducted at room temperature while the relative humidity was kept constant at 20%.
4. Results and Analysis: The samples with the TiO2 in the binder were tested at 1 l/min flow rate and 1 mW/cm2. The results presented in Table 1 show low NOx reduction suggesting that the method of incorporation of TiO2 into the asphalt binder mix may not be environmentally-effective. The low efficiencies could be due that only a small amount of TiO2 is actually present at the surface. Other possible explanations could be that the asphalt binder inhibits he photo catalytic reaction at the surface. Future research is underway to support the nderstanding of these results.
For the second application method consisting of applying a surface spray coating, samples were tested using a flow of 1.5 l/min and a luminosity of 2 mW/cm2. Figure 2 illustrates the variation of NOx concentration during the course of the environmental experiment for the sphalt sample treated with a TiO2 surface spray coat with a coverage rate of 0.21 kg/m2. The UV light is turned on 2 hours after the start of the experiment in order to ensure equilibrium condition.
5. Current Apllications:
Pavement in Bergamo, Italy (2006)
◦ TiO2-infused cement paving blocks (12,000 m2 area)
◦ Reduced NOx levels by 40% above road surface
Jubilee Church, Rome (2003)
Repair work to existing road infrastructure (i.e. bridges, tunnels) in Rezzato, Italy (2008)
Minneapolis Twin-Bridges (2008)
Multiple locations throughout Japan
6. Implementation:
· Require all roads within selected area to be paved/capped/ painted with TiO2
· Currently planned construction will also be required to utilize TiO2 material
· Repairs within selected area require use of TiO2 material
· Monitor ozone levels in all Counties to quantify the impacts of TiO2-infused pavement on highways using existing CARB monitoring stations
· Establish NOx monitoring stations along roadsides to compare TiO2-infused pavement to regular asphalt
7. Costs of Implementation
· 10-20% more than traditional paving
· $150,000 per mobile air monitoring station for NOx
Service contracts and maintenance (~$3,000/yr)
Consumables (~$1,500/yr)
8. Stack Holders
· Local Government
◦ Supports reducing NOx and ozone emissions
◦ Improving roadways and public health
◦ All at little to no cost (subsidized)
· Advocacy Groups (including but no limited to)
◦ Now supports the efforts to reduce pollution with novel technologies
◦ Keep supports the reduction of NOx and O3
· Big Business
◦ Cement and construction companies support the proposal as building increases
product demand
◦ TiO2 catalyst is available to cement mixers in ready mix or additive form
◦ TiO2 catalyst is also available from Los Angeles based EcoGreenPlus keeping the stimulus money
· General Public
◦ The public highly support reducing NOx and O3, improving public health via Clean Air
Road repaving/repainting/repair
· Recovery Accountability and Transparency Board
◦ Support the proposed plan to use stimulus dollars for transportation,
9. Summary and Conclusions:
This study evaluated the benefits of incorporating titanium dioxide (TiO2) as an additive to asphalt binder in the preparation of WMA. A commercial crystallized anatase-based titanium dioxide powder was blended with a conventional WMA asphalt binder classified as PG 64-22 at three modification rates (3, 5, and 7%). Prepared blends were characterized using fundamental rheological tests and the SCB test. Two application methods to integrate TiO2 were evaluated, a water-based titanium dioxide solution applied as a thin coating and using TiO2 as a modifier to asphalt binder in the preparation of WMA. Based on the results of the experimental program, the following conclusions may be drawn:
· When used as a modifier to asphalt binder in the preparation of WMA, the photo catalytic compound was not effective in degrading NOx in the air stream. This could be attributed to the fact that only a small amount of TiO2 is present at the surface.
· When used as part of a surface spray coating, TiO2 was effective in removing NOx pollutants from the air stream with an efficiency ranging from 39 to 52%.
· Rheological test results indicated that the addition of TiO2 did not affect the physical properties of the conventional binder. In addition, exposing the binder to UV light did not appear to accelerate the aging mechanisms in the binder.
· The use of TiO2 as a binder modifier improved the mix fracture resistance at 3, and 5% while it did not have a noticeable effect when used at a content of 7.0%.
This technology provides the chance to considerably improve the quality of life in our cities in the near future!
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