Sustainable Photocatalytic Asphalt Pavements for Mitigation of Nitrogen Oxide and Sulfur Dioxide Vehicle Emissions

Sustainable Photocatalytic Asphalt Pavements forMitigation of Nitrogen Oxide and Sulfur Dioxide

Vehicle EmissionsMarwa Hassan, Ph.D., P.E., M.ASCE1; Louay N. Mohammad, Ph.D., M.ASCE2; Somayeh Asadi3;

Heather Dylla4; and Sam Cooper III5

Abstract: The ability of titanium dioxide (TiO2) photocatalytic nanoparticles to trap and decompose organic and inorganic air pollutantsrender them a promising technology as a pavement coating to mitigate the harmful effects of vehicle emissions. This technology may revo-lutionize construction and production practices of hot-mix asphalt by introducing a new class of mixtures with superior environmental per-formance. The objective of this study was to assess the benefits of incorporating TiO2 into asphalt pavements. To achieve this objective, thephotocatalytic effectiveness and durability of a water-based spray coating of TiO2 was evaluated in the laboratory. This study also presents thefield performance of the country’s first air-purifying photocatalytic asphalt pavement, located on the campus of Louisiana State University.Laboratory evaluation showed that TiO2 was effective in removing NOx and SO2 pollutants from the air stream, with an efficiency rangingfrom 31–55% for NOx pollutants and 4–20% for SO2 pollutants. The maximum NOx and SO2 removal efficiencies were achieved at anapplication rate of 0.05 L=m2. The efficiency of NOx reduction is affected by the flow rate of the pollutant, relative humidity, and ultraviolet(UV) light intensity. In the field, NOx concentrations were monitored for both the coated and uncoated sections to directly measure photo-catalytic degradation. Furthermore, nitrates were collected from the coated and uncoated areas for evidence of photocatalytic NOx reduction.Results from both approaches show evidence of photocatalytic NOx reduction. Further field evaluation is needed to determine the durabilityof the surface coating. DOI: 10.1061/(ASCE)MT.1943-5533.0000613. © 2013 American Society of Civil Engineers.

CE Database subject headings: Sustainable development; Asphalt pavements; Nitrogen; Emissions; Sulfur dioxide.

Author keywords: Sustainable; Titanium dioxide; Photocatalytic; Nitrogen oxide; Hot-mix asphalt (HMA).

Introduction

The importance of the national transportation network to the UnitedStates economy is indisputable; however, there is a growingrecognition that highway construction and maintenance havemajor environmental impacts (U.S. EPA 1994; World Bank1996). Road microenvironments contribute 29% of the volatile or-ganic compounds (VOCs), 35% of the nitrogen oxides (NOx), and58% of the carbon monoxide (CO) emitted in the United States

(U.S. EPA 1994; Kuhns et al. 2004). These concentrations are oftenhigher in cities, where urban development trends and traffic volumeincreases add to emissions. Street canyon conditions inhibit emis-sions dispersal, resulting in high ground level pollutant concentra-tions. The emissions of harmful air pollutants associated withhighway operations often surpass the concentrations from indus-trial sources, rendering traffic emissions the primary source of ur-ban air pollution (Baldauf et al. 2008; Thomas et al. 2008; Chenet al. 2008; Berkowicz et al. 2006). Consequently, many adversehealth effects are linked to areas within 100 m from roads wherepollution is not fully diluted. This affects more than 35 millionAmericans who live within this proximity limit, thus renderingtransportation pollution a major concern (Thomas et al. 2008).

Many organic compounds and air pollutants, including nitrogenoxides and sulfur oxides, can be decomposed by ultraviolet (UV)radiation, but this process is extremely slow. Photocatalytic com-pounds such as titanium dioxide (TiO2) can accelerate this processby trapping and degrading organic and inorganic particles from theair while removing harmful air pollutants, such as NOx, sulfuroxides (SOx), and VOCs in the presence of UV light (sunlight).In addition, their hydrophobic or hydrophilic properties allow themto self-clean in the presence of rain. The concentrations of water-soluble nitrates and sulfates produced as a result of the photocata-lytic oxidation reach a level 10 times inferior to the first pollutionlevel (Kaegi et al. 2008). In spite of these promising findings, thistechnology has only been applied to concrete in the laboratory;concrete pavements only represent 6% of the national road net-work. Approximately 94% of the road network in the United Statesis surfaced with asphalt, which supports directing future researchtowards the use of TiO2 in asphalt pavements.

1Performance Contractors Assistant Professor, Dept. of ConstructionManagement and Industrial Engineering, Louisiana State Univ., 128PFT Hall, Baton Rouge, LA 70803 (corresponding author). E-mail:[email protected]

2Irma Louise Rush Stewart Distinguished Professor, Louisiana Trans-portation Research Center, Louisiana State Univ., 4101 Gourrier Ave.,Baton Rouge, LA 70808. E-mail: [email protected]

3Graduate Research Assistant, Dept. of Construction Management andIndustrial Engineering, Louisiana State Univ., 3128 PFT Hall, BatonRouge, LA 70803. E-mail: [email protected]

4Graduate Research Assistant, Dept. of Construction Management andIndustrial Engineering, Louisiana State Univ., 3128 PFT Hall, BatonRouge, LA 70803. E-mail: [email protected]

5Graduate Research Assistant, Dept. of Construction Management andIndustrial Engineering, Louisiana State Univ., 3128 PFT Hall, BatonRouge, LA 70803. E-mail: [email protected]

Note. This manuscript was submitted on November 28, 2011; approvedon June 7, 2012; published online on August 27, 2012. Discussion periodopen until August 1, 2013; separate discussions must be submitted for in-dividual papers. This paper is part of the Journal of Materials in CivilEngineering, Vol. 25, No. 3, March 1, 2013. © ASCE, ISSN 0899-1561/2013/3-365-371/$25.00.

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Study Objectives

The objective of this study was to determine the benefits of incor-porating TiO2 in asphalt pavements. Development and validation ofthis new class of sustainable pavements has the potential to mitigatethe high levels of pollution that are attributable to traffic. To achievethis objective, the photocatalytic effectiveness and durability of awater-based spray coating of TiO2 was evaluated in the laboratoryand in the field.

Background

Initial interest in environmental photocatalysis began in the1970s, initiated by Fujishima and Honda’s research in photo-electrochemical solar energy conversion. Through biomimicry ofplant photosynthesis, the researchers attempted to replicate thephoto-induced redox reactions by oxidizing water and reducingcarbon dioxide, using a semiconductor irradiated by UV light(Fujishima and Zhang 2006). Since then, increased interest in envi-ronmental photocatalysis has been observed, which has led to theapplication of TiO2 to glass, tile, paper, and pavements for self-cleaning materials, water purification, air purification, steriliza-tion, and oil spill remediation. From these studies, it has beenshown that organic and inorganic compounds can be completelydecomposed and that the TiO2 surface has the ability to self-regenerate (Fujishima et al. 2000). Therefore, rather than absorbingpollutants, which is common to traditional air purification methods,heterogeneous photocatalysis can decompose pollutants to nonhaz-ardous waste products with little energy requirements (Zhao andYang 2003). In the presence of UV light, TiO2 produces hydroxylradicals and superoxides, which are responsible for oxidizingand reducing environmental contaminants, including VOCs andNOx (Fujishima et al. 2000). A proposed mode of NOx oxidationthrough hydroxyl radical intermediates in the presence of the pho-tocatalyst is described by the following equations:

NOþ ˙OH→TiO2

NO2 þ H˙ ð1Þ

NO2 þ ˙OH→TiO2

NO3 þ H˙ ð2ÞBased on this heterogeneous photocatalytic oxidation process,

NOx are oxidized into water-soluble nitrates; these substances canbe washed away by rainfall.

Use of TiO 2 in Pavement Applications

The mechanism described in Eqs. (1) and (2), through which TiO2

accelerates the decomposition of air pollutants such as NOx fromair in the presence of UV light, is shown in Fig. 1. TiO2 photoca-talytic technologies in pavement applications have been primarilydirected towards concrete pavements by applying a photocatalyticconcrete overlay, a thin exterior film of suspended TiO2 nanopar-ticles in a binding agent (cement), or by sprinkling TiO2 nano-particles on curing concrete (Dylla et al. 2010; Chen et al. 2007).Researchers prefer photocatalytic overlays because of their greaterdurability (Diamanti et al. 2008; Beeldens 2008). Nonetheless, thespray coating application has the advantages of being easy to con-struct and potentially cheaper to apply to existing pavements.

Recent research by Hassan and co-workers has measured theimpact of common roadway contaminants, including motor oil,dirt, and de-icing salt, on the effectiveness of photocatalytic road-ways’ ability to remove NOx from the atmosphere (Dylla et al.2011a). The three contaminant types had a strong negative impact

on the photocatalytic NOx removal efficiency. The impact of thecontaminants’ coverage was largely dependent on the soilure type,with oil exerting the largest negative impact. An increase in theflow rate and air relative humidity also resulted in lower NOx effi-ciencies. Hassan and co-workers evaluated the environmental effec-tiveness of a TiO2 coating in photodegrading mixed NO2 and NOgases from the atmosphere (Dylla et al. 2011b). Increasing the flowrate and NO2=NOx ratio negatively affected the effectiveness of thephotocatalytic process. However, the extent of this impact de-pended on many other factors, including flow rate.

Few studies have attempted to use TiO2 in asphalt pavements(Venturini and Bacchi 2009; Li and Qian 2009). In Italy, TiO2

has been incorporated into asphalt pavements as a thin surface layerthat is sprayed onto existing pavements (Venturini and Bacchi2009). The water-based emulsion has been applied by two differentmethods, termed the hot and cold methods, distinguished by spray-ing of the emulsion during asphalt paving laying operations whenthe pavement temperature is over 100°C or on existing pavementsat ambient temperatures, respectively (Venturini and Bacchi 2009).The reduction efficiencies were highly dependent on the type ofTiO2 nanoparticles used, with NOx reduction efficiency rangingfrom 20–57%. Researchers in China have mixed TiO2 with an as-phalt binder at a 2.5% content of the binder weight to an emulsifiedasphalt pavement (Li and Qian 2009). A maximum efficiency inremoving nitrogen oxide near 40% was achieved. A more efficientapproach may be achieved by concentrating the photocatalyticcompound at the pavement surface.

Methodology

Laboratory Study: Specimen Preparation

The application method consisted of applying a water-based sur-face spray coating of TiO2 at three coverage rates (0.026, 0.05,and 0.074 L=m2), as shown in Fig. 2. The spray coating was a mix-ture of TiO2 anatase nanoparticles, with an average size of 6 nm,suspended in an aqueous liquid at 2% by volume. A thin film wasspray coated onto each sample in layers using a crosshatch forma-tion for each of the three defined coverage rates.

The asphalt mix substrates were prepared according to super-pave and Louisiana Department of Transportation and Develop-ment (LADOTD) specifications (Ninitial ¼ 8−, Ndesign ¼ 100,Nfinal ¼ 160 gyrations) and were designed according to AASHTOTP28, “Standard Practice for Designing Superpave HMA,” andSection 502 of the 2006 Louisiana Standard Specifications for

H2O

*OH +NO NO2 NO3-

Hot Mix Asphalt Pavement

NO

Photocatalytic layer TiO2

Fig. 1. Illustration of the photocatalytic process

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Roads and Bridges. Asphalt binders graded as PG 64-22 orPG 70-22 were used in the preparation of the hot-mix asphalt(HMA): see Table 1. The optimum asphalt cement content wasdetermined based on volumetric criteria [voids total mixðVTMÞ ¼2.5–4.5%, voids in mineral aggregateðVMAÞ ≥ 12%, voids filledwith asphaltðVFAÞ ¼ 68–78%] and densification (%Gmm atNinitial ≤ 89, %Gmm at Nfinal ≤ 98) requirements. Siliceous lime-stone aggregates and coarse natural sand were used in the mixdesign.

Laboratory Study: Durability Testing

A laboratory accelerated loading test was used to measure thedurability of TiO2 surface-coated HMA samples in the laboratory.The Hamburg-type loaded wheel tester (LWT), which employs ascaled dynamic wheel passing back and forth over the specimen,was used in this study to simulate loading and wear of the appliedcoating. A steel wheel applied a load of 702 N at a frequency of56 passes=min. This test is considered a torture test that producessignificant damage at the surface as the tested slab is submerged in50°C water. After 20,000 cycles, the test was stopped and the wornsamples were obtained to measure the environmental efficiencyafter durability testing. Two replicates were tested for each condi-tion considered in the experimental program.

Laboratory Study: Environmental Efficiency

The environmental performance of the prepared samples was quan-tified in the laboratory by measuring the concentrations of NO,NO2, NOx, and SO2. The test setup used was modified from theJapanese standard JIS TR Z 0018 to accommodate larger samples(JIS 2004). This setup consists of a pollutant source (gas cylinder ofNO), zero air source, adjustable valves, humidifier, calibrator, pho-toreactor, and chemilumiscent NOx and SO2 analyzer, as shownin Fig. 3.

The calibrator, requiring a pollutant and zero air source, controlsthe inlet flow of NOx and SO2 concentrations introduced to thesample through the photoreactor. Before entering the photoreactor,the inlet jet stream can continue through the humidifier or bypass

the humidifier to simulate various humidity levels. A tee-connection before the photoreactor was connected to the NOx ana-lyzer to verify the inlet concentrations. As a result, the jet streamentered the photoreactor, flowing over the sample at a controlledhumidity, flow, and NOx concentration. The photoreactor maintainsthis controlled environment for the sample. All of the tests wereconducted at room temperature of 24°C (�2°C). UV lights areabove the photoreactor to simulate UV lighting for the photocata-lytic reaction to occur. The outlet jet stream of the photoreactor wasconnected to the NOx and SO2 analyzers to measure any changes inpollutant concentration after exposure to the sample.

Field Study

To date, field evaluation of photocatalytic pavement technologyhas been limited. Hassan and co-workers laid the country’s firstair-purifying photocatalytic asphalt and concrete pavements onDecember 20, 2010. The test area is a pavement site located onthe Louisiana State University (LSU) campus: see Fig. 4. A cus-tomized distributor truck was used in the application of a TiO2

water-based solution. Two parameters were necessary to remainconstant during the application: speed and pressure. To maintainan accurate and steady speed of 50 ft=min, a magnet was placedon the drive shaft that counted each revolution. The distancecovered per revolution was calibrated by counting the numberof rotations, as the truck drove 100 ft. Knowing the revolutionsper minute and the distance per revolution, the speed was calculatedand digitally displayed to the driver to maintain a constant speed.

Fig. 2. Spray coating of TiO2 on HMA samples

Table 1. Descriptions of the Evaluated Samples

Mixture ID Description

HMA64 HMA prepared with PG 64-22 and no TiO2

HMA70 HMA prepared with PG 70-22 and no TiO2

SC-64L1 HMAwith PG 64-22 and a TiO2 surface coating—0.026 L=m2

SC-64L2 HMAwith PG 64-22 and a TiO2 surface coating—0.050 L=m2

SC-64L3 HMAwith PG 64-22 and a TiO2 surface coating—0.074 L=m2

SC-70L1 HMAwith PG 70-22 and a TiO2 surface coating—0.026 L=m2

SC-70L2 HMAwith PG 70-22 and a TiO2 surface coating—0.050 L=m2

SC-70L3 HMAwith PG 70-22 and a TiO2 surface coating—0.074 L=m2

Hum

idifier

Calibrator 146i

Analyzer 42i

Data recorder

Photoreactor

Air Source

NO

cylinder

Fig. 3. Experimental setup flow diagram

Fig. 4. Field application of photocatalytic pavement coating

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With the speed constant, the pressure was adjusted such that a sur-face layer of 1 mL=ft2 was applied. Mounted on the back of thetruck, a spray bar fitted with nozzles distributed the TiO2 water-based solution. To ensure full coverage, the nozzles were spacedaccordingly to coat the entire width of the road with at least1 mL=ft2. A surface coat of 1 mL=ft2 creates a slightly damp ap-pearance that is not visible in Fig. 4. Thus, the visible streaks seenin Fig. 4 are areas with excessive coating. Additional methods ofapplication may need to be developed to optimize material use. Thespray coating used was a mixture of TiO2 anatase nanoparticleswith an average size of 6 nm suspended in an aqueous liquid at2% by volume.

On-site measurements were conducted to collect air traffic emis-sions’ data on the photocatalytic section and from a nearby,untreated section. Traffic count, weather parameters, and NO,NO2, and NOx concentrations at the pavement level were contin-uously monitored and recorded using a Davis 6152 WirelessVantage Pro weather station and a Thermo NOx analyzer. Theequipment used for monitoring is housed in a movable air-conditioned trailer such that the trailer can be moved betweenthe coated and uncoated areas. Data collected from the field siteswere used to identify the range of various NOx concentrations andoperating and environmental parameters specific to the field sites.The data were supplemented with measurements from a nearbyDepartment of Environmental Quality (DEQ) air monitoring stationand historical TMY-2 weather data files. The LSU DEQ station pro-vides ambient air pollution hourly averaged concentrations forNOx, SO2, and ozone.

Results and Analyses

Laboratory Experiments: NOx Reduction Efficiency

Laboratory samples were tested using a flow rate of 1.5 L=min anda luminosity of 2 mW=cm2. Fig. 5 illustrates the variation of NOxconcentration during the course of the environmental experimentfor the asphalt specimen treated with a TiO2 surface spray coatingwith a coverage rate of 0.050 L=m2. The UV light was turned on1.5 h after the beginning of the experiment to ensure equilibriumcondition. The inlet concentration reached equilibrium at 430 ppbbefore the light was turned on. After the light was turned on, a fastdrop of NO concentration in the outlet air stream was exhibited andNO2 was created from the NO oxidation. During the photocatalyticexperiment, NOx concentration slightly increased. After 5.5 h of

testing, the light and gas supply was turned off, allowing for anydesorption to occur. For the test condition shown in Fig. 5, the useof TiO2 photocatalyst coating had an NO removal efficiency andoverall NOX reduction of 83 and 56%, respectively.

Results for all of test conditions are shown in Table 2. Table 2also presents the measured NO efficiency for the asphalt samplethat was not treated with TiO2. The efficiency of the sample withoutTiO2 was negligible, validating the efficiency of the photocatalyticcompound in removing some of the NOx pollutants in the air streamwhen used as a spray coating. By comparing the effect of the TiO2

coverage rate, the improvement of NOx reduction is apparentlynonlinear. The maximum environmental performance was achievedat 0.050 L=m2 coverage rate. The increase in TiO2 application ratebeyond an optimum coverage rate may block nanoparticles’ accessto light and contaminants, and therefore decrease NOx removalefficiency.

Laboratory Experiments: Durability of TiO2 SurfaceCoating

The durability of the TiO2 surface-coated HMA samples wasdetermined using an accelerated loading test by measuring NOxremoval efficiency before and after durability testing. Fig. 6presents the average NOx removal efficiencies for the originaland worn samples (loaded-wheel test samples) treated with TiO2

surface spray coating. The wearing of the samples resulted in a sig-nificant decrease in the coating NOx removal efficiency. An average68% decrease in NOx reduction efficiency was observed after LWTtesting. Whereas there was a significant decrease in NOx reductionefficiency, the HMA specimens were significantly distorted afterLWT testing. Such excessive distortion will probably not occurin the field. Nonetheless, these results indicate that the durabilityof the TiO2 surface coating should be closely monitored in fieldapplications of this technology.

Fig. 5. Variation of NOx concentration during the environmentalexperiment (TiO2 applied at a 0.050 L=m2 coverage rate)

Table 2. Average NOx Reduction and NO Reduction for TiO2 Used as aThin Surface Coating

Sample type

Coveragerate

(L=m2)Bindertype

NOxreduction

(%)

NOreduction

(%)

Control PG 64-22 0 64-22 2.6 5.0Control PG 70-22 0 70-22 8.5 9.6PG (64-22)—I 0.026 64-22 38.9 51.2PG (64-22)—II 0.050 64-22 53.2 70.3PG (64-22)—III 0.074 64-22 40 52.6PG (70-22)—I 0.026 70-22 51 73PG (70-22)—II 0.050 70-22 66.2 76.6PG (70-22)—III 0.074 70-22 54.3 56

0

10

20

30

40

50

60

70

SC-64L1 SC-64L2 SC-64L3 SC-70L1 SC-70L2 SC-70L3

NO

x R

educ

tion

Eff

icie

ncy

Sample ID

Before LWT

After LWT

Fig. 6. NOx removal efficiency before and after loading in the LWT

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Laboratory Experiments: Effects of EnvironmentalFactors

Fig. 7 presents the effects of air relative humidity on NOx reductionefficiency for the HMA sample treated with TiO2 surface coating(Level 2—0.05 L=m2). Humidity had a negative impact on NOxreduction efficiency, given that the increase in relative humidity re-sulted in a decrease in NOx removal efficiency. It is possible that athigh relative humidity, the water molecules interfere with NOx con-tact to the TiO2 active sites on the surface. Fig. 8 presents the effectsof the pollutants’ flow rate on NOx removal efficiency for the HMAsample treated with TiO2 surface coating (Level 2—0.050 L=m2).The increase in flow rate had a significant impact on NOx reductionefficiencies. With faster flow rates, there is less contact time for thephotocatalytic reaction to occur, which results in lower reductionefficiencies.

Fig. 9 presents the effects of UV light intensity on NOx removalefficiency. Whereas UV light is necessary to initiate the photoca-talytic reaction, the required light intensity is not known. Therewas a direct correlation between light intensity and NOx removalefficiency, with greater pollutant abatement at higher UV lightintensity.

Laboratory Experiments: Fig. 10 illustrates the variation of SO2

concentration during the course of the environmental experimentfor the asphalt specimen treated with a TiO2 surface spray coatingwith a coverage rate of 0.050 L=m2. The UV light was turned on2.5 h after the start of the experiment to ensure equilibrium con-dition. The inlet concentration reached equilibrium at 175 ppb be-fore the light was turned on. After the light was turned on, a drop ofSO2 concentration in the outlet air stream was observed. After 5.0 hof testing, the light and gas supply was turned off, allowing for any

desorption to occur. For the test conditions shown in Fig. 10, theuse of a TiO2 photocatalyst coating had an SO2 removal efficiencyof 23%.

The results for all of test conditions are shown in Table 3.Whereas SO2 reduction efficiency was inferior to that observedfor NOx, the removal of SO2 is crucial as it is a major air pollutantand has significant impacts upon human health. Analogously toNOx reduction efficiency, the maximum environmental perfor-mance was achieved at 0.050 L=m2 coverage rate, with an averageSO2 reduction efficiency of 19.8%.

Field Experiments

Two techniques were used to measure photocatalytic degradation inthe field study. The first method measured the reduction directly bymeasuring the ambient air pollution concentration and the secondmethod measured the reduction indirectly by measuring thebyproducts (i.e., nitrates) created from the degradation process.

Direct Measurements of NOx ReductionMeasurements of pollution reduction in the field present a chal-lenge because of a large number of influencing parameters, time,and costs. In the described field experiment, NO concentration wasmeasured at the pavement level by placing a perforated pipe at thesurface in the middle of the lane. To determine NO reduction effi-ciency in the field, 10 days of data were compared immediatelybefore and after TiO2 applications. Fig. 11 compares the measurednitrogen oxide (NO) data collected immediately before andimmediately after application of TiO2 for several days duringthe winter season. The measured NO concentrations were signifi-cantly reduced immediately after the application of the TiO2 sur-face coating on the asphalt pavement. However, measurements ofNO concentrations in the field showed high variability because ofthe influence of numerous factors, including wind speed, vehicletype, humidity, and temperature.

Indirect Measurements of NOx Reduction through NitrateAnalysisFor further evidence of a photocatalytic reduction, the nitrate thataccumulated on the pavement surface was measured at three loca-tions in the coated area and three locations in the uncoated area.Measurements were collected for seven consecutive days duringthe study to identify evidence of photocatalytic degradation ofNOx. Because water was used to dissolve nitrate salts at the pave-ment surface, a new sampling location was used on each day, giventhat repetitive applications of water would clean the surface andmay introduce error to the measurements. Therefore, a total of14 sampling locations (seven each on the treated and untreated

0%

10%

20%

30%

40%

50%

60%

70%

80%

20 50 80

Rem

oval

Eff

icie

ncy

Relative Humidty

NOx Removal Efficiency

NO Removal Efficiency

Fig. 7. Effect of relative humidity on NOx reduction efficiency

0%

10%

20%

30%

40%

50%

60%

70%

80%

1.5 2.0 3.0

Rem

oval

Eff

icie

ncy

Pollutants Flow Rate (l/min)

NOx Removal Efficiency

NO Removal Efficiency

Fig. 8. Effect of pollutant flow rate on NOx reduction efficiency

0%

10%

20%

30%

40%

50%

60%

70%

80%

2.4 1.2 0.5

Rem

oval

Eff

icie

ncy

UV Light Intensity (mW/cm2)

NOx NO

Fig. 9. Effect of UV light intensity on NOx reduction efficiency

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sections) were used. Nitrates that accumulated on the pavementsurface were measured by dissolving them in deionized water: seeFig. 12. To collect the nitrate on the pavement surface, 40 mL ofdeionized (DI) water was poured into a 100 × 150 mm rectangleopening in a wooden device sealed with plumber putty. After5 min, the solution was collected with a syringe and filtered througha 0.45 μm filter into a polyethylene jar. An increase in nitrates onthe coated pavement would demonstrate evidence of photocatalyticreduction of NOx.

Fig. 13 presents the measured nitrate concentrations throughoutthe 7-day collection period. There is a definite indication that pho-tocatalytic degradation of nitrogen oxide occurred in the treatedsection. The photocatalytic process was very active during the first

Table 3. Average SO2 Reduction for TiO2 Used as a Thin Surface Coating

Sample typeCoverage rate

(L=m2)Bindertype

SO2 reduction(%)

Control PG 64-22 0 64-22 3.6Control PG 70-22 0 70-22 5.4PG (64-22)—I 0.026 64-22 10.4PG (64-22—II 0.050 64-22 19.8PG (64-22)—III 0.074 64-22 12.0PG (70-22)—I 0.026 70-22 13.0PG (70-22)—II 0.050 70-22 14.0PG (70-22)—III 0.074 70-22 12.3

Fig. 11. NO concentration reduction efficiency in the field

Fig. 12. Nitrate dissolution in water

Ave

rage

Nit

rate

(mg/

l)

0

0.1

0.2

0.3

0.4

0.5

1 2

Treated Section

3 4Day

Untreated Section

5 6 7

Fig. 13. Nitrate concentrations (mg=L) during the 7-day collectioncycle

Fig. 10. Variation of SO2 concentration during the environmental experiment (TiO2 applied at a 0.050 L=m2 coverage rate)

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5 days and the fifth day had the most nitrates removed. In the sixthday, a slight decrease of nitrate concentration can be seen, which isattributable to rainfall. An increase in nitrate concentration was ob-served on the seventh day.

Cost AnalysisThe cost of the photocatalytic coating is USD1.88=yd2 (in year2012 dollars) and the cost of 3.5 in. of asphalt paving surface isUSD16.39=yd2. Utilization of photocatalytic coating in asphaltpaving will increase the cost by 11%; however, the cost will de-crease significantly once TiO2 is mass produced in the UnitedStates.

Summary and Conclusions

The use of titanium dioxide coating for pavements has receivedconsiderable attention in recent years to improve air quality nearlarge metropolitan areas. However, the proper method of applyingtitanium dioxide to asphalt pavements is still unclear. The objectiveof this study was to assess the benefits of incorporating titaniumdioxide into asphalt pavements. To achieve this objective, the pho-tocatalytic effectiveness and durability of a water-based spray coat-ing of TiO2 was evaluated in the laboratory and the field. The testarea is a pavement site located on the LSU campus. Based on theresults of this study, we draw the following conclusions:• As part of a surface spray coating, TiO2 was effective in

removing NOx pollutants from the air stream, with an efficiencyranging from 31–55% in the laboratory. The maximum NOx re-moval efficiency was achieved at a coverage rate of 0.05 L=m2.However, the durability of the surface spray coating requiresfurther evaluation and monitoring in field applications.

• The increase in flow rate and relative humidity negativelyaffected the effectiveness of the NOx reduction efficiency.However, the increase in UV light intensity improved the NOxremoval efficiency of the surface coating.

• TiO2 was effective in removing SO2 pollutants from the airstream. The maximum environmental performance wasachieved at a 0.05 L=m2 coverage rate with an SO2 reductionefficiency of 19.8%.

• Results of the direct method and indirect methods of measuringphotocatalytic degradation of NOx show that there is evidenceof a photocatalytic reaction occurring in the field.This study represents a step towards better evaluation of photo-

catalytic field studies and implementation of photocatalytic pave-ments as a feasible solution to mitigate near roadway air pollutionproblems. Based on the results presented in this study, further re-search is needed to identify additional significant factors, includingthe impacts of vehicle classification and vehicular activity. In ad-dition, the long-term effectiveness and durability of the photocata-lytic coating in the field should be evaluated.

Acknowledgments

This work was funded through a grant from the Gulf CoastResearch Center for Evacuation and Transportation Resiliency.The authors acknowledge PURETI for donating the materialsneeded for the construction of the field study and the LouisianaTransportation Research Center (LTRC) for granting access to theirlaboratory.

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