sustainability
Article Applicability of TiO2 Penetration Method to Reduce Particulate Matter Precursor for Hardened Concrete Road Structures
Seung Woo Lee 1, Hui Rak Ahn 1, Kyoung Su Kim 2 and Young Kyu Kim 2,*
1 Department of Civil Engineering, Gangneung-Wonju National University, Wonju-City 26403, Korea; [email protected] (S.W.L.); [email protected] (H.R.A.) 2 Institute for Disaster Prevention, Gangneung-Wonju National University, Wonju-City 26403, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-33-640-2419
Abstract: Recently, air pollution is increasing sharply in Korea, caused by fine particulate matter. Nitrogen oxides (NOx) are particulate matter precursors that significantly contribute to air pollution. They are transmitted into the atmosphere by a large amount, especially in high-volume traffic areas. This pollutant is particularly harmful. Therefore, there are increasing efforts focused on NOx
removal from the air. As the photocatalytic reaction of titanium dioxide (TiO2) is the mechanism that eliminates NOx, the ultraviolet (UV) rays in sunlight and TiO2 in existing concrete structure need to be contacted for reaction process. Generally, TiO2 concrete is produced by mixing the concrete binder with TiO2. However, a significant amount of TiO2 in the concrete cannot be exposed to air pollutants or UV. Additionally, this technique may not be applicable to existing structures. Therefore,
an alternative method that utilizes surface penetration agents is used to add TiO2 to the concrete
surface of the structures. This proposed method may not only be economical but also applicable to various types of structures. The applicability of the TiO2 penetration application method to existing
Citation: Lee, S.W.; Ahn, H.R.; Kim, concrete road structures for reducing NOx is the purpose of this study. The penetration depth of TiO particles was measured using scanning electron microscopy (SEM) combined with energy K.S.; Kim, Y.K. Applicability of TiO2 2 Penetration Method to Reduce dispersive analysis of X-rays (EDAX). Additionally, the NOx removal efficiency was evaluated using Particulate Matter Precursor for the NOx analyzing system. The results of this study showed that the TiO2 penetration method was Hardened Concrete Road Structures. advantageous in removing NOx effectively and securing economic feasibility. Sustainability 2021, 13, 3433. https:// doi.org/10.3390/su13063433 Keywords: particulate matter precursor; nitrogen oxides (NOx); titanium dioxide (TiO2); TiO2 penetration method; NOx removal efficiency Academic Editor: José Alvarez
Received: 18 February 2021 Accepted: 17 March 2021 1. Introduction Published: 19 March 2021 A variety of air pollutants give harmful and negative effects on both environmental
Publisher’s Note: MDPI stays neutral and human health. These air pollutants are produced from the combustion processes with regard to jurisdictional claims in involved in space heaters, motor-vehicle traffic, and power generation, mostly in the large published maps and institutional affil- city areas. Most of the air pollution is typically caused by rapid urban development; thus, iations. its negative impacts have become a significant concern for the public. The air pollutants not only contribute to pollution, but they can also travel over vast distances in the atmosphere, and induce acid rain where chemical reactions produce. The principal pollutants which generate from vehicles are nitrogen oxides (NOx), carbon monoxide, fine particulates, and volatile organic compounds (VOCs) [1]. Primarily, Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. NOx is the general term for a highly reactive gases group [2]. Nitrogen oxides are produced This article is an open access article in motor vehicles during the combustion process at high temperatures when fuel is burned. distributed under the terms and NOx causes many problems to the environment and health, such as urban smog due to pho- conditions of the Creative Commons tochemical reactions with hydrocarbons and the production of tropospheric ozone. It is one Attribution (CC BY) license (https:// of the urgent concerns for the environmental problem worldwide [2]. Air pollution from creativecommons.org/licenses/by/ exhaust gases may be effectively mitigated by eliminating the emission source. Therefore, 4.0/). photocatalytic materials have been used to eliminate NOx. These materials can be added
Sustainability 2021, 13, 3433. https://doi.org/10.3390/su13063433 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 3433 2 of 10
to the building and pavement surface. Titanium dioxide (TiO2) is photocatalytic material that is effective in the NOx removal. The photocatalytic reaction of TiO2 is the mechanism that eliminates NOx, and hence the ultraviolet (UV) rays of sunlight are essential for the reaction of TiO2 in concrete roads. Additionally, TiO2 is an excellent photocatalytic ma- terial that imparts biocidal, self-cleaning and smog-abating functionalities when added to cement-based materials [3]. To activate the TiO2 during heterogenic photocatalysis, a wavelength lower than 387 nm of UV light needs to be presented. Moreover, the intensity of the light is vital to optimizing the photocatalytic activity. Fujishima et al. (1999) and Hashimoto et al. (2000) show good effectiveness for NOx removal by combining cementitious with other construction materials [4,5]. Consequently, in modern air-pollution control technologies, TiO2 photocatalysts has been used [6]. It has been shown that paving blocks with TiO2 have been used in Japan to reduce the NOx from automobile exhaust gases. According to Beeldens (2006), in Belgium, separate parking lane construction at the Leien of Antwerp used TiO2 containing pavement blocks. After one year of use, the purification efficiency rate reduced to 20% [2]. Most of the studies have focused on TiO2 concrete produced by mixing the concrete binder with TiO2. As solar photocatalysis of TiO2 is the mechanism that removes NOx, UV rays of sunlight and TiO2 in concrete road need to be contacted for reaction process. A study on TiO2 concrete paving blocks showed that a considerable amount of TiO2 was not exposed to pollutants or UV light. Kim et al. (2014) showed that penetration method using surface penetration agents is an alternative method to produce TiO2 in concrete without mixing with the concrete binder. The purpose of the penetration method is to position the TiO2 into concrete structures’ surface [7]. Replacement of cementitious materials with TiO2 may effectively reduce the automo- bile pollutants. However, a large portion of the photocatalyst reaction may not be activated inside the concrete structure that is unexposed to sunlight. Furthermore, this method can be applied only in upcoming projects that require concrete mixture for their construction. Accordingly, the penetration method may be an effective and economical alternative that applies to various types of new and existing concrete structures. This paper intends to evaluate the applicability of the TiO2 penetration method for existing concrete structures, such as concrete pavement, L-type ditch, interlocking block, and permeable block, to remove NOx, which is a fine-size particulate matter precursor. Accordingly, NOx removal efficiencies were evaluated along with the penetration depth and distribution characteristics of TiO2 particles.
2. Literature Review 2.1. Characteristics of Photocatalytic Reaction of TiO2 NOx is the generic term of a highly reactive gases group. It contains oxygen and nitrogen in varying amounts. Mostly, nitrogen oxides are odorless and colorless. When fuel is burned in the combustion process at high temperature, nitrogen oxides are generated. The primary sources of NOx come from the objects which consume the fuel such as vehicles, commercial, electric utilities, residential and other industrial. NOx is the generic term for nitrogen dioxide (NO2), nitric oxide (NO), and mono-nitrogen oxides. Typically, in large cities with a high volume of traffic areas, nitrogen oxide is transmitted into the atmosphere adding to air pollution with a highly significant amount. Delany et al. (1982) revealed that the nitrogen oxides concentration ranges from 10 to 500 ppb in the unpolluted troposphere, whereas range of 100 ppb in polluted urban air [8]. However, the NOx concentration in the high volume traffic area has increased to approximately 1000 ppb recently [7]. Air pollution gives adverse effects on environmental and human health; therefore, it has become a significant public concern in the rapid urban development of the increasing level of air pollution. Many studies have reported on the modern air-pollution control technologies that use TiO2 photocatalyst for NOx removal in air pollution [6]. TiO2 is a metal and has three different molecular structures, i.e., anatase, rutile, and brookite. It is abundantly present in nature. TiO2 is a natural oxide of the element titanium with Sustainability 2021, 13, x FOR PEER REVIEW 3 of 10
level of air pollution. Many studies have reported on the modern air-pollution control Sustainability 2021, 13, 3433 3 of 10 technologies that use TiO2 photocatalyst for NOx removal in air pollution [6]. TiO2 is a metal and has three different molecular structures, i.e., anatase, rutile, and brookite. It is abundantly present in nature. TiO2 is a natural oxide of the element titanium with insig- nificantinsignificant toxicity, toxicity, and negligible and negligible biological biological effects [9]. effects To active [9]. the To TiO active2 in theheterogenic TiO2 in hetero-pho- tocatalysis,genic photocatalysis, a wavelength a wavelengthlower than 387 lower nm thanof UV 387 light nm needs of UV to light be presented. needs to beMoreover, presented. theMoreover, intensity theof light intensity intensity of light is vital intensity to optimizing is vital to the optimizing photocatalytic the photocatalytic activity. In normal activity. daylight,In normal the daylight,photocatalytic the photocatalytic reaction does reactiontake place. does Currently, take place. most Currently, of the research most of are the focresearchused on are the focused application on the of applicationTiO2 that can of TiObe 2activethat canin the be activevisible in light the visiblerange. lightFigure range. 1 depictsFigure the1 depicts oxidation the- oxidation-reductionreduction process at process active atTiO active2 sites, TiO most2 sites, widely most indicated widely indicated in the literaturein the literature [7]. Hashimoto [7]. Hashimoto et al. (2000) et al. show (2000) that show a free that radical a free of radical O2- is ge ofnerated O2- is generated from a UVfrom-A irradiation a UV-A irradiation of TiO2 in of TiO the 2 presencein the presence of oxygen of oxygen [4]. The [4 ]. UV The-A UV-A has 315~400 has 315~400 nm of nm wavelength.of wavelength. Nitrates Nitrates were wereformed formed by superoxide by superoxide ions which ions which subsequently subsequently react reactwith withni- trogennitrogen oxides oxides [4,6] [.4 ,6].
FigureFigure 1. Photocatalytic 1. Photocatalytic reaction reaction and and NOx NOx removal removal process process of ofTiO TiO2. 2.
2.2. Current Status of TiO2 Application 2.2. Current Status of TiO2 Application In recent years, titanium dioxide (TiO2) has been commonly applied in the production In recent years, titanium dioxide (TiO2) has been commonly applied in the produc- of photoactive concrete materials capable of degrading a wide range of air contaminant tion of photoactive concrete materials capable of degrading a wide range of air contami- such as nitric oxide (NO), VOC, etc. [10]. Fujishima et al. (1999) reveal favorable results nant such as nitric oxide (NO), VOC, etc. [10]. Fujishima et al. (1999) reveal favorable re- of nitrogen oxides removal by combining TiO photocatalyst with cementitious and other sults of nitrogen oxides removal by combining TiO2 2 photocatalyst with cementitious and construction materials [4]. Consequently, many studies show that in modern air-pollution other construction materials [4]. Consequently, many studies show that in modern air- control technologies, TiO2 photocatalyst has been used [6]. TiO2 containing paving blocks pollution control technologies, TiO2 photocatalyst has been used [6]. TiO2 containing pav- were utilized at a roadside in Japan for NOx reduction from automobile exhaust gases. ing blocks were utilized at a roadside in Japan for NOx reduction from automobile ex- In Japan, in 12 h with a UV and wind speed intensity of 0.6 mW/cm2 and 0.036 m/s, haust gases. In Japan, in 12 h with a UV and wind speed intensity of 0.6 mW/cm2 and 0.036 respectively, the paving blocks were eliminating NOx of 45 mg/m2 [6]. At the Leien of m/s, respectively, the paving blocks were eliminating NOx of 45 mg/m2 [6]. At the Leien Antwerp, pavement blocks which contain TiO2 were used for separate parking lanes in of Belgium.Antwerp, The pavement air purifying blocks efficiency which contain and also TiO the2 were durability used for of thisseparate method parking were evaluatedlanes in Belgium.in situ. AfterThe air one-year purifying use, efficiency purification and efficiency also the durability rate was 20% of this [2]. method were evalu- ated in situ. After one-year use, purification efficiency rate was 20% [2]. In summary, many studies overseas reported that TiO2 concrete can be produced by In summary, many studies overseas reported that TiO2 concrete can be produced by mixing the concrete binder with TiO2 material. Otherwise, a solar photocatalysis reaction mixing the concrete binder with TiO2 material. Otherwise, a solar photocatalysis reaction is needed to break down NOxs’ structure. Therefore, the TiO2 in concrete needs exposure is toneeded UV rays to break to be activated,down NOxs which’ structure. are utilized Therefore, for road the structure. TiO2 in concrete However, needs a considerable exposure to UV rays to be activated, which are utilized for road structure. However, a considerable portion of TiO2 in concrete is not in contact with the pollutant or the sunlight. Therefore, portion of TiO2 in concrete is not in contact with the pollutant or the sunlight. Therefore, wasteful quantities of the TiO2 additive are used, which is not economical. The TiO2 2 2 wastefulpenetration quantities method of the is an TiO alternative additive methodare used, besides which mixingis not economical. the TiO2 with The theTiO concrete pen- 2 etrationbinder method [11]. The is penetrationan alternative method method aims besides to attach mixing TiO 2theto TiO the surface with the of concrete the structure binder [11 ]. [11]The. The two penetration methods of method TiO2 penetration aims to attach are: mixingTiO2 to the TiOsurface2 with of the binderstructure and [11] spraying. The twothe methods TiO2 on of the TiO surface2 penetration with a are surface: mixing penetration the TiO2 agent.with the The binder distribution and spraying of TiO the2 in the concrete was measured by using EDAX (energy dispersive X-ray spectroscopy). It distributed up to a 3 mm depth from the specimen surface. In the mixing method, TiO2 particles were evenly distributed, while in the penetration method, a large amount of TiO2 particles were distributed close to the surface. Sustainability 2021, 13, x FOR PEER REVIEW 4 of 10
TiO2 on the surface with a surface penetration agent. The distribution of TiO2 in the con- crete was measured by using EDAX (energy dispersive X-ray spectroscopy). It distributed up to a 3 mm depth from the specimen surface. In the mixing method, TiO2 particles were Sustainability 2021, 13, 3433 4 of 10 evenly distributed, while in the penetration method, a large amount of TiO2 particles were distributed close to the surface.
2.3.2.3. Application Application Methods Methods of TiO of TiO2 Materials2 Materials into into Existing Existing Concrete Concrete Road Road Structures Structures MitigationMitigation methods methods of ofparticulate particulate matter matter precursor precursor include include various various techniques techniques such as as mix substitution,substitution, coating,coating, andand penetrationpenetration into into the the concrete concrete structure. structure. Figure Figure2 represents2 repre- sentsthe the schematic schematic of the of the above above methods. methods. In European In European countries countries and Japan,and Japan, a method a method of mixing of mixingTiO2 isTiO₂ used is toused replace to replace some partssome of parts cement of cement [2,4]. However, [2,4]. However, a large amounta large ofamount particulate of particulatematter precursor matter precursor reduction reduction material shouldmaterial be should used in be the used mixing in the method mixing as m comparedethod as to comparedthe coating to the and coating penetration and penetration methods. Thus, methods. this may Thus, increase this may the initialincrease cost the of initial construction. cost of Ifconstruction. a large amount If a large of material amount is distributedof material onis distributed a surface, NOxon a issurface, better removed.NOx is better In the removed.case of mixIn the substitution, case of mix air substitution, pollutants cannotair pollutants be effectively cannot removedbe effectively because removed they cannot be- causecontact they withcannot the contact atmosphere with the as atmosphere they are farther as they from are the farther surface. from Thus, the surface. this method Thus, is thisless method efficient is less than efficient the coating than the and coating penetration and penetration methods. Additionally, methods. Additionally since mixing, since TiO 2 mixingwith TiO the2 binder with the can binder only becan applied only be in applied upcoming in u structures,pcoming structures, it may not it bemay suitable not be for suitablethe existing for the concreteexisting structures.concrete structures. The coating The methodcoating requiresmethod requires a pre-treatment a pre-treatment process to processimprove to improve the attachment the attachment of the coating of the agentcoating and agent also and the also surface the tosurface be well-developed, to be well- developed,which is disadvantageouswhich is disadvantageous in terms of in the terms economical of the economical aspect. The aspect. coating The method coating can methodbe applied can be only applied on theonly surface, on the surface, and it mayand it be may removed be removed or affected or affected by external by external forces forcessuch such as rain, as rain, wind, wind, or snow. or snow. The penetrationThe penetration method method is applied is applied to concrete to concrete structures struc- by turespenetrating by penetrating the particulate the particulate matter precursorsmatter precursors into the concreteinto the concrete using a surface using a penetration surface penetrationagent, which agent, contains which contains less amount less amount of TiO2 materialof TiO2 material to some to extent. some extent. It can It be can applied be appliedfor NOx for NOx removal removal and isand also is also economical. economical. The The surface surface penetration penetration agent agent penetrates penetrates the theentrained entrained air air through through capillary capillary voids voids by gravityby gravity and and the capillarythe capillary force force acting acting in a vertical in a verticaldirection direction and hence,and hence, pulls pull thes particles the particles into into the concretethe concrete [12]. [12] Since. Since the the size size of theof the TiO 2 TiO₂particle particle is 21is 21 to 260to 260 nm, nm, and and the the entrained entrained air air void void size size is aboutis about 70 70 to 1000to 1000 nm, nm the, the TiO 2 particles can penetrate through the capillary pore. Figure3 shows the ideal penetration TiO2 particles can penetrate through the capillary pore. Figure 3 shows the ideal penetra- and distribution of TiO particles in existing concrete structures. The penetration method tion and distribution of TiO2 2 particles in existing concrete structures. The penetration methodcan ensure can ensure economic economic feasibility feasibility due to due the to reduction the reduction of particulate of particulate matter matter precursors precur- from the surface to a certain depth. It can secure long-term performance and durability since the sors from the surface to a certain depth. It can secure long-term performance and durabil- particle matter penetrates into the concrete structures. ity since the particle matter penetrates into the concrete structures.
(a)
(b)
Figure 2. Cont. Sustainability 2021, 13, 3433 5 of 10 Sustainability 2021, 13, x FOR PEER REVIEW 5 of 10
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(c) (c) FigureFigure 2.2. ApplicationApplication methods methods of of particulate particulate matter matter precursor precursor reduction reduction materials: materials: (a) concrete (a) concrete mix- Figureing with 2. ApplicationTiO2, (b) TiO methods2 coating ofon particulate concrete surface, matter andprecursor (c) TiO reduction2 penetration materials: into concrete (a) concrete structures mix-. mixing with TiO2,(b) TiO2 coating on concrete surface, and (c) TiO2 penetration into concrete ing with TiO2, (b) TiO2 coating on concrete surface, and (c) TiO2 penetration into concrete structures. structures.
Figure 3. Ideal penetration and distribution of TiO2 particles in existing concrete structures. Figure 3. IdealIdeal penetration penetration and distribution of TiO 2 particles in existing concrete structures structures.. 3. Evaluation of Penetration and Distribution of TiO2 3. Evaluation of Penetration and Distribution of TiO 3. Evaluation of Penetration and Distribution of TiO2 3.1.3.1. SpecimenSpecimen PreparationPreparation 3.1. Specimen Preparation TableTable1 1shows shows the the mixture mixture proportioning proportioning of of the the concrete concrete structures structures such such as as concrete concrete Table 1 shows the mixture proportioning of the concrete structures such as concrete pavementpavement andand L-typeL-type sideside ditch.ditch. ConcreteConcrete specimensspecimens usedused forfor pavementpavement andand L-typeL-type sideside pavement and L-type side ditch. Concrete specimens used for pavement and L-type side ditchditch werewere prepared according to to specification specification suggested suggested by by Korea Korea Expressway Expressway Corpora- Corpo- ditchration,tion, wereand and typical prepared typical interlocking interlockingaccording blocks to blocksspecification and and permeable permeable suggested blocks blocksby Koreawere were used Expressway used in this in thisstudy. Corpora- study. Ac- tioAccordingcordingn, and totypical to the the product’sinterlocking product’s specification, specification, blocks and permeablepermeable permeable blocks blocks were contained contained used ain voidthis study. content content Ac- of of cordingaroundaround 20%,20%, to the which which product’s allows allows waterspecification, water to to infiltrate infiltrate permeable directly directly throughblocks through contained the the surface surface a to void the to the subsurface. content subsur- of aroundOnface. the On other20%, the other hand,which hand, theallows interlocking the water interlocking to blocksinfiltrate blocks contained directly contained less through than less 7% thethan voidsurface 7% contentvoid to contentthe similar subsur- sim- to fthatilarace. to ofOn that ordinary the of other ordinary concrete hand, concrete the specimens. interlocking specimens. blocks contained less than 7% void content sim- ilar to that of ordinary concrete specimens. FigureFigure4 4 shows shows the the process process of of specimen specimen preparation preparation for for the the TiO TiO22 penetration method. Figure 4 shows the process of specimen preparation for the TiO2 penetration method. AnAn anatase-typeanatase-type of of TiO TiO2 2mixed mixed with with silane-siloxane silane-siloxane surface surface penetration penetration agent agent in in the the ratio ratio 8 Anto8 to 2 anatase (application2 (application-type amount,of amount, TiO2 mixed 500 500 g/m withg/m2)2 ) wassilan was sprayede sprayed-siloxane on on surface the the existing existing penetration concrete concrete agent structures structures in the suchratio such 8asas to pavement,pavement, 2 (application L-typeL-type amount, sideside ditch,ditch, 500 g/m interlockinginterlocking2) was sprayed block,block, on andand the permeablepermeable existing concrete block.block. AfterAfterstructures curingcuring such forfor as33 days, days,pavement, residual residual L- TiOtype TiO2 particlesside2 par ditch,ticle ons interlockingon the the concrete concrete block, surface surface and were permeable were removed removed byblock. washing by After washing withcuring water with for 3towater days, evaluate to residual evaluate the penetration TiO the2 parpenetrationticle ands distributionon andthe concretedistribution characteristics surface characteristics were of the removed TiO of2 thepenetration by TiO washing2 penetration method. with watermethod to . evaluate the penetration and distribution characteristics of the TiO2 penetration method .
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Table 1. Mixture proportioning of concrete structures used in this study.
3 Type of Concrete GmaxTable 1.fck MixtureSlump proportioningAir of concreteW/C structuresS/a usedContents in this study (kg/m. ) AE Water Structures (mm) (MPa) (mm) (%) (%) (%) WCSG Reducing Agent Gmax fck Slump Air W/C S/a Contents (kg/m3) Type of Concrete Structures AE Water Reducing× Agent Pavement 25(mm) 35 (MPa) 40 (mm)5~7 (%) 45(%) (%) 42 W 148.5C 330S G 759 1067 C 0.3% L-type Pavement 2525 24 35 4040 5~75~7 4545 42 47148.5 166 330 759 370 1067 863 1001 C × 0.3%C × 0.3% side ditch L-type 25 24 40 5~7 45 47 166 370 863 1001 C × 0.3% side ditch
(a) (b) (c)
(d) (e)
Figure 4. Specimen preparation of TiO penetration method: (a) Concrete placement (10 × 10 × 80 cm3), (b) Latency Figure 4. Specimen preparation of TiO2 2penetration method: (a) Concrete placement (10 × 10 × 80 cm3), (b) Latency and 2 anddust dustremoval removal using using high high-pressure-pressure spray, spray, (c) TiO (c)2 TiO sprayed2 sprayed on concrete on concrete (500 (500g/m2 g/m with surfacewith surface penetration penetration agent), agent), (d) Con- (d) Concretecrete pavement pavement and andL-type L-type sideside ditch, ditch, and and(e) Interlocking (e) Interlocking block block and andpermeable permeable block block..
3.2. Test Methods to Evaluate the Penetration and Distribution ofof TiOTiO22
The penetration and distribution characteristics of the TiO22 penetration method was evaluated using scanningscanning electronelectron microscopy microscopy combined combined with with energy energy dispersive dispersive analysis analysis of ofX-rays X-rays (SEM/EDAX). (SEM/EDAX). The The mass mass ratio ratio was was measured measured at different at different depths depth untils until the mass the mass ratio ratioof the of material the material was not was reached. not reached. The higher The higher amount amount of TiO of2 material TiO2 material at penetration at penetration depth depthclose to close the surfaceto the surface is expected is expected in significantly in significantly reducing reducing particulate particulate matter matter precursors. precur- sors. Figure5 represents the measurement process of the field radial injection electron microscope.Figure 5 represents The specimen the measurement is placed in the process SEM/EDAX of the field after radial platinum injection (Pt) electron plating mi- to croscope.place potential The specimen on the specimen. is placed Electronic in the SEM/EDAX microscopes after use platinum electron beams (Pt) plating instead to of place rays potentialto create vacuumon the specimen. conditions Electronic inside the microscopes microscope, use and electron then the beams specimen instead is of analyzed. rays to createComponent vacuum analysis conditions is performed inside theto microscope, check the penetrationand then theand specimen penetration is analyzed. depth. Com- The ponentmass ratio analysis of the is amount performed of Ti to to check the amount the penetration of concrete and was penetration computed. depth. The mass ratio of the amount of Ti to the amount of concrete was computed. Sustainability 2021, 13, x FOR PEER REVIEW 7 of 11
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(a) (b) (c)
(d) (e) (f)
Figure 5. Procedure for evaluating the penetration and distribution of TiO2:(a) SEM/EDAX equip- ment, (b) Pt coating, (c) Setting for test, (d) Convert to image, (e) Image analysis, and (f) Detection of
mass ratio.
3.3. Characteristics of Penetration Depth and Distribution of TiO2 Paticles
The TiO2 mass ratio at 0.1 mm depth from the concrete surface was investigated to evaluate the TiO 2 penetration and distribution characteristics using SEM/EDAX. In addition, the mass ratio at the concrete surface was estimated based on the TiO2 mass ratio
at 0.1 mm depth. The penetration and distribution of TiO2 particles in existing concrete structures are presented in Table2. The mass ratios measured at 0.1 mm depth of TiO 2 applied to concrete pavement, L-type side ditch, interlocking block, and permeable block
are 38%, 43%, 50%, and 15%, respectively. Additionally, the predicted mass ratio of TiO2 on the concrete surface was 40%, 57%, and 55%, respectively. For permeable block, the Figure 5. Procedure for evaluating the penetration and distribution of TiO2: (a) SEM/EDAX equip- predicted mass ratioment, ( ofb) Pt TiO coating,2 could (c) Setting not for be test, estimated (d) Convert sinceto image, the (e) particlesImage analysis, penetrated and (f) Detection the entire cross-sectionof mass due ratio. to the high porosity. These results showed that TiO2 particles, which are particulate matter precursor-reducing material with silane-siloxane type sur- 3.3. Characteristics of Penetration Depth and Distribution of TiO2 Paticles face penetration agent, can be efficiently penetrated and distributed in existing concrete The TiO2 mass ratio at 0.1 mm depth from the concrete surface was investigated to road structures. evaluate the TiO2 penetration and distribution characteristics using SEM/EDAX. In addi- tion, the mass ratio at the concrete surface was estimated based on the TiO2 mass ratio at Table 2. Penetration and distribution of TiO particles in the existing concrete structures. 0.1 mm depth. The2 penetration and distribution of TiO2 particles in existing concrete struc- tures are presented in Table 2. The mass ratios measured at 0.1 mm depth of TiO2 applied Types of Concrete Type of Surface Mixing Ratio of Penetration Mass Ratio at 0.1 mm Predicted Mass Ratio TiO2 Type to concrete pavement, L-type side ditch, interlocking block, and permeable block are 38%, Structure Penetration Agent Agent and TiO2 Depth from Surface (%) at the Surface (%) 43%, 50%, and 15%, respectively. Additionally, the predicted mass ratio of TiO2 on the Concrete pavement concrete surface was 40%, 57%, and 55%, respectively.38 For permeable block, 40 the predicted L-type side ditch 43 57 Anatase mass ratio of TiO8:22 could not be estimated since the particles penetrated the entire cross- Interlocking blockSilane-siloxane 50 55 Permeable block 15 -
4. NOx Removal Efficiency Using TiO2 Penetration Method 4.1. Test Method to Evaluate the NOx Removal Efficiency Figure6 shows the NOx removal evaluation system for photocatalytic concrete fol- lowing the ISO 22197-1 [7,13]. UV-A lamps which have a wavelength ranged from 315 to 400 nm are used in the NOx removal evaluation system since UV rays reaching the surface of the earth generally have a wavelength in the range of 290 to 380 nm. In addition, the NOx concentration in roadside areas is 170 ppb on an average and 854 ppb at a maximum. This concentration is higher than that measured in other areas by 2.5 times. Therefore, the NOx removal efficiency was fixed at 1000 ppb as per ISO 22197-1, which is similar to the highest NO concentration in the roadside areas. SustainabilitySustainability 20220211,, 1313,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1010
ofof thethe earthearth generallygenerally havehave aa wavelengthwavelength inin thethe rangerange ofof 290290 toto 380380 nm.nm. InIn addition,addition, thethe NOxNOx concentrationconcentration inin roadsideroadside areasareas isis 170170 ppbppb onon anan averageaverage andand 854854 ppbppb atat aa maximum.maximum. Sustainability 2021, 13, 3433 ThisThis concentrationconcentration isis higherhigher thanthan thatthat measuredmeasured inin otherother areasareas byby 2.52.5 times.times. Therefore,Therefore,8 of thethe 10 NOxNOx removalremoval efficiencyefficiency waswas fixedfixed atat 10001000 ppbppb asas perper ISOISO 2219722197--1,1, whichwhich isis similarsimilar toto thethe highehighestst NONO concentrationconcentration inin thethe roadsideroadside areas.areas.
FigureFigure 6.6. NOxNOx aanalyzingnalyzing ssystem:ystem: ((aa)) NOxNOx removalremoval efficiencyefficiency tester,tester, ((bb)) SSyntheticynthetic airair andand NONO sourcesource (NO(NO 3030 ppm/Nppm/N22),), andand Figure 6. NOx analyzing system: (a) NOx removal efficiency tester, (b) Synthetic air and NO source (NO 30 ppm/N2), and ((cc)) S SpecimenSpecimenpecimen preparation preparation.preparation..
4.2.4.2. NOxNOx RemovalRemoval EfficiencyEfficiency basedbased onon SurfaceSurface PredictedPredicted MassMass RatioRatio ofof TiOTiO22 4.2. NOx Removal Efficiency Based on Surface Predicted Mass Ratio of TiO2 FigureFigure 77 showsshows thethe NOxNOx removalremoval efficiencyefficiency ofof thethe TiOTiO22 penetrationpenetration methodmethod whenwhen Figure7 shows the NOx removal efficiency of the TiO penetration method when appliedapplied toto variousvarious existingexisting concreteconcrete structuresstructures measuredmeasured usingusing2 thethe NOxNOx analyzinganalyzing system.system. applied to various existing concrete structures measured using the NOx analyzing system. InIn thisthis case,case, thethe applicationapplication amountamount ofof TiOTiO22 mixedmixed withwith surfacesurface penetrationpenetration agentagent waswas 500500 In this case, the application amount of TiO2 mixed with surface penetration agent was g/mg/m22.. TheThe2 initialinitial NOxNOx removalremoval efficiencyefficiency waswas whenwhen thethe residualresidual TiOTiO22 particlesparticles onon thethe sur-sur- 500 g/m . The initial NOx removal efficiency was when the residual TiO2 particles on facefacethe surfacewerewere notnot were removed.removed. not removed. ForFor concreteconcrete For pavement,pavement, concrete pavement, LL--typetype sideside L-type ditch,ditch, side interlockinginterlocking ditch, interlocking block,block, andand permeablepermeableblock, and block,block, permeable thethe initialinitial block, NOxNOx the removalremoval initial NOx efficienciesefficiencies removal werewere efficiencies 52.3,52.3, 55.1,55.1, were 53.3,53.3, 52.3, andand 55.4%, 55.1,55.4%, 53.3, re-re- spectively.spectively.and 55.4%, InIn respectively. additionaddition,, NOxNOx In addition, removalremoval NOx efficienciesefficiencies removal measuredmeasured efficiencies afterafter measured removremovinging after thethe removing residualresidual TiOTiO22 particlesparticles onon thethe concreteconcrete surfacesurface decreaseddecreased inin approximatelyapproximately aa rangerange ofof 5%5% toto 1616%% the residual TiO2 particles on the concrete surface decreased in approximately a range comparedcompared toto initialinitial measurement.measurement. ForFor TiOTiO22 penetrationpenetration method,method, thethe NOxNOx removalremoval effi-effi- of 5% to 16% compared to initial measurement. For TiO2 penetration method, the NOx ciencyciencyremoval ofof above efficiencyabove 50%50% of cancan above bebe achievedachieved 50% can duringduring be achieved thethe initialinitial during periodperiod the of initialof application;application; period of however,however, application; thethe efficiencyefficiency reducesreduces whenwhen thethe residualresidual TiOTiO22 particlesparticles onon thethe surfacesurface areare removedremoved duedue toto however, the efficiency reduces when the residual TiO2 particles on the surface are removed rainfall,rainfall,due to rainfall, snowfall,snowfall, snowfall, wind,wind, etc.etc. wind, Therefore,Therefore, etc. Therefore, longlong--termterm long-term NOxNOx removalremoval NOx removal efficiencyefficiency efficiency cancan bebe securedsecured can be when a large amount of TiO22 particles are penetrated and distributed from the concrete whensecured a large when amount a large of amount TiO particles of TiO2 particlesare penetrated are penetrated and distributed and distributed from the fromconcrete the surfacesurfaceconcrete toto surface aa properproper to depth. adepth. proper FurthermoreFurthermore depth. Furthermore,,, itit waswas shownshown it was thatthat shown thethe NOxNOx that removalremoval the NOx efficiencyefficiency removal increasedincreasedefficiency when increasedwhen thethe TiOTiO when22 massmass the TiO ratioratio2 mass increasedincreased ratio increasedduedue toto thethe due properproper to the penetrationpenetration proper penetration andand distribu-distribu- and tiontiondistribution ofof TiOTiO22 particlesparticles of TiO2 particles inin thethe concreteconcrete in theconcrete structures.structures. structures. ForFor permeablepermeable For permeable block,block, however,however, block, however, aa similarsimilar a trendtrendsimilar forfor trend NOxNOx for removalremoval NOx removal efficiencyefficiency efficiency waswas notnotwas observedobserved not observed...
FigureFigure 7.7. NOxNOxNOx removalremoval efficiencies efficienciesefficiencies using using TiOTiO222 penetrationpenetration methodmethod whenwhen applied appliedapplied to toto various variousvarious existing 2 22 existingexistingconcrete concreteconcrete structures structuresstructures (application (application(application amount amount ofamount TiO2 with ofof TiOTiO surface22 withwith penetrationsurfacesurface penetrationpenetration agent: 500 agentagent g/m:: 500500). g/mg/m ))..
4.3. NOx Removal Efficiency Based on Application Amount of TiO2
The NOx removal efficiency was measured when the application amounts of TiO2 mixed with the penetration agent was varied to 300 g/m2, 500 g/m2, and 700 g/m2, as shown in Figure8. Overall, it was confirmed that a high NOx removal efficiency was measured when the predicted mass ratio at the concrete surface was high. In the case of Sustainability 2021, 13, x FOR PEER REVIEW 9 of 10
4.3. NOx Removal Efficiency based on Application Amount of TiO2
Sustainability 2021, 13, 3433 The NOx removal efficiency was measured when the application amounts of9 TiO of 102 mixed with the penetration agent was varied to 300 g/m2, 500 g/m2, and 700 g/m2, as shown in Figure 8. Overall, it was confirmed that a high NOx removal efficiency was measured when the predicted mass ratio at the concrete surface was high. In the case of concrete pavement,concrete pavement, the NOx theremoval NOx removalefficiencies efficiencies were less were than less 40% than, even 40%, when even the when TiO2 theapplica- TiO2 tionapplication amount amount was increased. was increased. Moreover Moreover,, the predicted the predicted mass ratio mass was ratio relatively was relatively lower lower than thatthan of that the of L- thetype L-type side ditch side ditchand interlocking and interlocking block. block. For the For L- thetype L-type side ditch side and ditch inter- and lockinginterlocking block, block, NOx NOx removal removal efficiencies efficiencies of approximately of approximately 50%50% were were observed observed when when the 2 appliedthe applied amounts amounts of TiO of2TiO with2 withthe surface the surface penetration penetration agent agent was 500 was g/m 5002 or g/m more.or Addi- more. tionally,Additionally, the theNOx NOx removal removal efficienc efficiencyy increased increased significantly significantly when when the the applicationapplication 2 amounts were increased from 500 to 700 g/m2 inin all all experimental experimental cases. cases.
Figure 8. NOx removal efficienciesefficiencies forfor variousvarious applicationapplication amountamount ofof TiOTiO22..
55.. Conclusions Conclusions
This paper intends to evaluate the applicability of the TiO2 TiO2 penetrationpenetration method to reduce NOx, which is the particulate matter precursor emitted into the atmosphere by vehicles. This This method method was applied to existing concrete road structures such as concrete pavement, LL-type-type side ditch, and concrete concrete blocks. blocks. Several important conclusions were achieved and are summarized below. The TiO TiO22 massmass ratios ratios at at 0.1 0.1 mm mm depth depth from from the the concrete concrete surface surface were were investigated investigated to evaluateto evaluate the the TiO TiO2 penetration2 penetration and and distribution distribution characteristics characteristics using using SEM/EDAX. SEM/EDAX. TiO2TiO par-2 ticlesparticles with with silane silane-siloxane-type-siloxane-type surface surface penetration penetration agents agents can can efficiently efficiently penetrate penetrate and and be distributedbe distributed in the in theexisting existing concrete concrete road road structures. structures. LLong-termong-term NOx removal efficiency efficiency can be obtained when a large amountamount ofof TiOTiO22 particles are are penetrated penetrated and and dis distributedtributed from from the the concrete concrete surface surface to a toproper a proper depth. depth. Ad- ditionallyAdditionally,, the theNOx NOx removal removal efficiency efficiency increased increased when when the TiO the TiO2 mass2 mass ratio ratio increased increased due todue the to proper the proper penetration penetration andand distribution distribution of TiO of TiO2 particles2 particles into into thethe concrete concrete structures. structures. A high NOx removal efficiency efficiency was measured when the predicted mass ratio at the concrete surfacesurface waswas high. high. For For the the L-type L-type side side ditch ditch and and interlocking interlocking block, block, a NOx a NOx removal re- movalefficiency efficiency of approximately of approximately 50% was 50% obtained was obtained when when the applied the applied amounts amounts of TiO of2 withTiO2 2 withsurface surface penetration penetration agents agents were were 500 500 g/m g/mor2 or more. more. Additionally, Additionally, the the increaseincrease inin NOx removal efficiency efficiency was insignificantinsignificant when the application amounts were increased from 2 500 to 700 g/m 2. . This study indicates that the TiO2 penetration method can be applied to various existing concrete structures such as concrete pavement, side ditch, and concrete blocks as an alternative method to remove NOx, which is the particulate matter precursor in a high volume traffic area. Sustainability 2021, 13, 3433 10 of 10
Author Contributions: Conceptualization, S.W.L. and Y.K.K.; methodology, H.R.A. and Y.K.K.; investigation, H.R.A. and K.S.K.; original draft preparation, Y.K.K.; writing-review and edition, S.W.L. and Y.K.K. All authors have read and agreed to the published version of the manuscript. Funding: This work is supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (grant number 21POQW- B152342-03). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: The authors would like to thank the members of research team, KAIA and MOLIT for their guidance and supports throughout the project. Conflicts of Interest: The authors declare no conflict of interest.
References 1. Ballari, M.M.; Hunger, M.; Hüsken, G.; Brouwers, H.J.H. NOx photocatalytic degradation employing concrete pavement containing titanium dioxide. Appl. Catal. B Environ. 2010, 95, 245–254. [CrossRef] 2. Beeldens, A. An environmental friendly solution for air purification and self-cleaning effect: The application of TiO2 as photocata- lyst in concrete. In Proceedings of the Transport Research Arena Europe-TRA, Göoteborg, Sweden, 12–15 June 2006. 3. Diamantopoulos, G.; Katsiotis, M.; Fardis, M.; Karatasios, I.; Alhassan, S.; Karagianni, M.; Papavassiliou, G.; Hassan, J. The role of titanium dioxide on the hydration of Portland cement: A combined NMR and ultrasonic study. Molecules 2020, 25, 5364. [CrossRef] 4. Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; Tokyo Bkc: Tokyo, Japan, 1999. 5. Hashimoto, K.; Wasada, K.; Toukai, N.; Komonami, H.; Kera, Y. Photocatalytic oxidation of nitrogen monoxide over titanium (IV) oxide nanocrystals large size areas. J. Photochem. Photobiol A Chem. 2000, 136, 103–109. [CrossRef] 6. Chai-Mei, Y.J. Ambient Air Treatment by Titanium Dioxide (TiO2) Based Photocatalyst in Hong Kong; Technical Report; Environmental Protection Department: Hong Kong Special Administrative Region, China, 2002. 7. Kim, Y.K.; Hong, S.J.; Lee, K.B.; Lee, S.W. Evaluation of NOx removal efficiency of Photocatalytic concrete for road structure. Int. J. Highw. Eng. Korean Soc. Road Eng. 2014, 16, 49–58. [CrossRef] 8. Delany, A.C.; Dickerson, R.R.; Melchior, F.L.; Eartburg, A.F. Modification of a commercial NOx detector for high sensitivity. Rev. Sci. Instrum. 1982, 12, 1899–1902. [CrossRef] 9. Grande, F.; Tucci, P. Titanium Dioxide Nanoparticles: A Risk for Human Health? Mini Rev. Med. Chem. 2016, 16, 762–769. [CrossRef][PubMed] 10. Janus, M.; Zatorska, J.; Zaj ˛ac,K.; Kusiak-Nejman, E.; Czyzewski,˙ A.; Morawski, A.W. Study of nitric oxide degradation properties of photoactive concrete containing nitrogen and/or carbon co-modified titanium dioxide—Preliminary findings. Micro Nano Lett. 2016, 11.[CrossRef] 11. Hong, S.J.; Lee, S.W. An Experimental Study for the Construction of Photocatalytic Method Concrete Road Structure. Int. J. Highw. Eng. Korean Soc. Road Eng. 2013, 15, 1–9. [CrossRef] 12. Oh, H.M. An Assessment of Concrete Durability Using Inorganic and Organic/Inorganic Surface Penetration Agents. Ph.D. Thesis, Sangji University, Wonju, South Korea, 2004. 13. ISO 22197-1:2016, Test Method for Air-Purification Performance of Semiconducting Photocatalytic Materials-Part 1: Removal of Nitric Oxide; International Standard: Geneva, Switzerland, 2016.