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NOx removal of mortar mixed with titania produced from Ti-salt flocculated sludge
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JIEC-1820; No. of Pages 6 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
NOx removal of mortar mixed with titania produced from Ti-salt flocculated sludge Se Min Park a, Laura Chekli b, Jong Beom Kim c, Mohammad Shahid b, Ho Kyong Shon b, Phan Seok Kim d,e, Woon-Seek Lee d, Woong Eui Lee f, Jong-Ho Kim a,c,* a
School of Applied Chemical Engineering & The Research Institute for Catalysis, Chonnam National University, Gwangju 500-757, Republic of Korea Faculty of Engineering and Information Technology, University of Technology, Sydney (UTS), P.O. Box 123, Broadway, NSW 2007, Australia Photo & Environmental Technology Co., Ltd., Gwangju 500-460, Republic of Korea d Division of Systems Management & Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 608-737, Republic of Korea e Hwaseung T&C Corp., 857-2, Eogok-Dong, YangSan, Kyoungnam, Republic of Korea f Department of Polymer and Fiber System Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea b c
A R T I C L E I N F O
Article history: Received 9 May 2013 Accepted 25 December 2013 Available online xxx Keywords: NOx TiO2 Photocatalyst Mortar
A B S T R A C T
Norms for the emissions of NOx are becoming stricter and stricter, leading to an increase in the research interest for the development of novel gas cleaning technologies. In this study, the use of titania (TiO2) produced from Ti-salt flocculated sludge, mixed with mortar, has been investigated as a cost-effective novel solution for the removal of these pollutant gases. This work not only presents an advanced solution for sludge reduction but also proposes a novel production method of TiO2 powders from waste water and investigates the potential use of this material blinded with mortar for a novel application which is air purification. Detailed characterization of the produced TiO2 powders was performed and results showed that the primary particles present a uniform size and spherical shape with a diameter of less than 50 nm. The main constitutive elements were Ti, O, C and P, where the Ti content was found to increase slightly with increasing temperature. The anatase phase was observed at 600 8C and 800 8C and converted to rutile structure at 1000 8C. Two contents (i.e. 3.0 and 5.0 wt%) of TiO2 were tested for mixing with mortar and photocatalytic properties of the mortar containing TiO2 were evaluated for the removal of NOx and were found to be similar to commercial TiO2 (P-25) in terms of photocatalytic activity. Further investigations under direct sunlight were conducted after 28 days of water curing to evaluate the removal of NOx. The NO rejection was about 50% after 5 h. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx), principally consisting of NO and NO2, emitted from the exhausted gas of automobile, combustion of coals and thermal power plants cause many environmental issues. For instance, NOx are responsible for the formation of acid rains and the photochemical pollution results in diseases of the human respiratory system [1]. They are emitted by a wide range of products numbering in the thousands. Examples include: paints, lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings. This leads to the high demand for costeffective and energy-efficient gas cleaning technologies.
* Corresponding author at: School of Applied Chemical Engineering & The Research Institute for Catalysis, Chonnam National University, Gwangju 500-757, Republic of Korea. Tel.: +82 62 530 1888; fax: +82 62 530 1889. E-mail address: [email protected] (J.-H. Kim).
A catalytically active filter element combining NOx removal was developed by Nacken et al. [2]. They found that a TiO2–V2O5–WO3 catalyst system converted 96% of NO at 300 8C. Saracco and Specchia [3] reported a catalytic filter of cylindrical tube effective for the removal of NOx and VOCs. Titania (TiO2) is one of the most widely used photocatalysts that allow the decomposition and the removal of waste organic compounds and harmful gases from water and the atmosphere [4,5]. As the demand for TiO2 increases, the preparation method of TiO2 with a cost-effective and highefficient manner is required. Shon and co-authors [6–8] developed a number of processes that is based on the flocculation of wastewater by using TiCl4 which has to be followed by the calcination of sludge at high temperature. After calcination a useful by-product, TiO2 powder, is generated. TiO2 photocatalyst has recently received high application potential in the construction industry for air purification applications. TiO2 loaded cementitious materials were reported to be efficient for NOx and VOCs removal [9–11]. The combined
1226-086X/$ – see front matter ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.090
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application of cementitious TiO2 results in synergetic effects toward photocatalytic NOx reduction [10]. Japan’s Mitsubishi Materials Corporation [12] has developed an innovative material called Noxer combining TiO2 mounted on a mortar base which demonstrated an 80% NOx removal efficiency. The air purification potential of both dip-coated and sol–gel coated TiO2 enriched concrete sample also shows high removal efficiencies of VOCs (up to 86%), indicating their attractive photocatalytic properties for future application as air purifying building materials [9]. This work has a number of simultaneous environmental and economic benefits, namely: (i) sludge recycling, (ii) novel costeffective method for TiO2 production and (iii) application of TiO2 powder produces from sludge mixed with mortar for air purification application. In this study, the objectives were to mortar containing TiO2 and to investigate its potential for the removal of NOx. Advanced characterization of the novel material was also studied in detail. 2. Experimental 2.1. Preparation of TiO2 powder TiO2 powder produced from Ti-salt flocculated sludge with dye waste water [13,14] was used to fabricate mortar containing TiO2. Dye waste water flocculation using TiCl4 coagulant was carried out to produce TiO2. The characteristics of dye waste water obtained from Daegu dyeing industrial center were chemical oxygen demand (449 mg/L), suspended solid (43 mg/L), total nitrogen (72 mg/L) and total phosphorous (3.2 mg/L). During the rapid mixing time, pH was adjusted and the coagulant was added at 100 rpm. After flocculation, the settled flocs were collected from the sedimentation tank, dewatered in a filter press and further dried in rotary furnace at 300 8C. The dried sludge was calcined at different temperatures (i.e. 400 8C, 600 8C, 800 8C and 1000 8C) to remove volatile organic matter and water. Thereafter, TiO2 produced in this manner is named as S-TiO2_400, S-TiO2_600, STiO2_800, S-TiO2_1000. 2.2. Preparation of mortar containing TiO2 The TiO2 powder was mixed with sand, cement and water to prepare mortar containing TiO2 [15]. Sand (1350 g), cement (450 g) and water (220 g) were mixed with either 3.0 wt% or 5.0 wt% of TiO2 powder and stirred with slow mixing for 60 s and then with rapid mixing for 30 s. After 90 s stay without stirring, 60 s rapid mixing was applied again. Prepared materials were cured for 28 days. The mortar was manufactured with dimension of 100 mm 50 mm 10 mm to investigate photocatalytic activity. 2.3. Characterization of photocatalysts Morphology of the TiO2 powder was analyzed by a scanning electron microscope (SEM, Zeiss Supra 55VP SEM) operating at 15– 25 kV and a transmission electron microscope (TEM, Philips CM200, Netherlands) operating at 200 kV. The elemental composition analyses were carried out using a scanning electron microscope equipped with an energy dispersive X-ray detector (55VP SEM) operating at 15 kV. The crystalline phase of the nanoparticle TiO2 was characterized by X-ray diffraction (XRD) analysis. The XRD patterns were generated on a MDI Jade 5.0 X-ray diffractometer (MaterialsData Inc., USA) with Cu Ka radiation source. The data were measured within the range of scattering angle 2u of 5–80. Brunauer, Emmett and Teller (BET) surface area analyses were performed on an automated surface area analyser (Micromeritics Gemini 2360, USA) by means of nitrogen adsorption–desorption.
The mean pore diameter and the total pore volume of samples were determined from the desorption isotherm via Barret–Joyner– Halenda (BJH) model. 2.4. Decompositions of acetaldehyde In order to investigate the photocatalytic activity of the TiO2 powder, a cuboid stainless steel (Top-face quartz, 220 mm 125 mm 80 mm) airtight reactor was used to study the adsorption and photocatalytic oxidation of acetaldehyde over UV-A illuminated catalysts. The reactor was topped with two 10 W, 352 nm UV-A lamps (Sankyo Denki, F10T8BL, Japan) and had three rubber openings for the injection of acetaldehyde (2000 ppmv), air mixing inside the reactor and for withdrawal of samples. The last opening was connected to a gas chromatograph/flame ionization detector (GC/FID) (HP5890 series II, Wilmington, USA) for measuring variations in acetaldehyde concentration. The reaction took place at room temperature of 24 8C for 200 min. 0.5 g of photocatalysts were uniformly sprinkled on a glass Petri-dish (9 cm of diameter) and placed at a distance of 10 cm from UV-A lamps in the center of the reactor. At least three independent measurements were run per sample and the data presented are averaged values. In general, good agreement was observed between replicates (i.e. standard deviations were less than 5%). 2.5. Removal of NOx under simulated solar light ISO 22197-1 standard based experimental procedure were applied to investigate the NOx removal efficiency using a laboratory scale photocatalytic reactor (Fig. 1). The powder of 5 g was placed on a sample holder (100 mm 50 mm 10 mm), mortar containing TiO2 (100 mm 50 mm 10 mm) and two 10 W UV-A (Sanyo-denki, Japan) lamps with 10 W/m2 were used as pretreatment of irradiation for 5 h. Air conditions were 1 ppmv of NO with 50% relative humidity. Constant flow rate of 3 L/min was used during the experiment. When adsorption was stabilized, the two lamps were switch on TiO2 powder for 1 h and mortar containing TiO2 for 5 h to measure the variation of NOx concentration which was determined using a NOx analyser (CM2041, Casella). Three independent measurements were run per sample and the results presented showed average values. 2.6. Removal of NOx under direct sunlight Following the same method described previously, 5 wt% of photocatalyst (i.e. S-TiO2_600) was mixed with sand, cement and water to prepare mortar containing TiO2. In order to prevent excessive loss of moisture from the concrete, the prepared material was cured for 28 days. The test procedure for the removal of NOx was the same as described in the previous section. However, instead of applying UV-A in a batch reactor, the prepared material was directly exposed to sunlight as shown in Fig. 2. The size of the internal reactor was 100 mm 500 mm 5 mm. The test was conducted for 5 h and the NO concentration was maintained at 0.67 ppmv. 3. Results and discussion 3.1. Characterization of TiO2 powder produced from Ti-salt flocculated sludge Fig. 3 shows the SEM and TEM images of S-TiO2_600 powder. Both images show that most of the particles were in aggregate form. Primary particles were found to be uniform in size and
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Fig. 1. Photocatalytic test chamber for assessing the decomposition of NOx.
Fig. 2. Experimental set-up for assessing the decomposition of NOx under direct sunlight.
Fig. 3. SEM (left side) and TEM (right side) images of S-TiO2_600 powder produced from Ti-salt flocculated sludge.
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Table 1 Composition and surface areas of S-TiO2 series at different incineration temperatures. Temperature (8C)
400 600 800 1000
SBET (m2/g)
Composition (wt%) C
O
P
Ti
14.0 12.0 6.9 5.7
51.0 52.2 51.4 53.0
3.4 3.8 3.4 1.5
31.6 32.0 38.3 39.8
– 76 6 1
presented a spherical shape. The majority of the primary particles were found to be less than 50 nm in diameter. EDX analysis was performed to determine the elemental composition of powder samples calcined at different temperatures (Table 1). The constitutive elements were mainly Ti, O, C and P. The C atoms probably come from the organic residue of the coagulated organic matter as C content decreases with increasing temperature due to decomposition of organic group. The P atom content was found to be very low at all temperatures. The Ti content increases slightly with increasing temperature (i.e. from 31.6% at 400 8C to 39.8% at 1000 8C). The BET surface area was found to be considerably higher for S-TiO2_600 compared to S-TiO2_800, STiO2_1000, but also compared to P-25 TiO2, the most widely used photocatalyst (i.e. 76 m2/g for S-TiO2_600 compared to 42.3 m2/g for P-25) [16]. XRD pattern was used to identify the crystalline structures of the TiO2 powder after incineration at different temperatures from 400 to 1000 8C (Fig. 4). At 400 8C, an amorphous structure was observed which may be due to the remaining organic matter at low temperatures [6]. The anatase structure was observed at 600 8C and 800 8C, and at 1000 8C, the anatase structure changed to rutile. Based on the results of Liao et al. [17], the transformed temperature from the anatase to rutile structure at ambient pressure was found
100
Removal of acetaldehyde (%)
4
80
60 S-TiO2_600
40
S-TiO2_800 S-TiO2_1000
20
P-25
0 0
20
40
60
80
100
120
Time (min) Fig. 5. Removal of acetaldehyde with UV irradiation time over TiO2 powder produced from Ti-salt flocculated sludge by calcination at various temperatures and P-25 (initial concentration = 2000 ppmv; UV irradiation = UV black light three 10 W lamps).
at approximately 550 8C. However, in this study, this transformation occurred at above 800 8C. This may be due to the impurity of the TiO2 produced by the settled floc which was about 10–15% with C and P atoms. Because it is generally assumed that the anatase type has higher photocatalytic activity than rutile and also increasing temperatures involve higher energy consumption and cost, 600 8C was selected as the most effective temperature taking into account energy requirement and photocatalytic activity to produce TiO2 powders use to prepare mortar containing TiO2. 3.2. Photocatalytic decompositions of acetaldehyde by TiO2 powder The photocatalytic property of the Ti-salt flocculated sludge specimens was investigated under irradiation of UV-A for the photocatalytic decomposition of gaseous acetaldehyde (Fig. 5). The decrease in concentration of acetaldehyde was measured by gas chromatography. As expected, the removal of acetaldehyde was much more effective with S-TiO2_600 powder mainly due to the enhanced photocatalytic activity of the anatase type [18,19]. After 75 min of photocatalytic reaction, S-TiO2_600 powder removed the majority of acetaldehydes (i.e. almost 100%), whereas for S-TiO2_800 and S-TiO2_1000 powder show only 50% and 10% removal. S-TiO2_600 powder indicated similar photocatalytic activity compared to the commercially available P-25.
200 cps o
1000 C
o
Intensity
800 C
3.3. Removal of NOx by TiO2 powder and mortar containing TiO2 o
600 C
o
400 C
20
40
60
2 theta (deg.) Fig. 4. XRD patterns of S-TiO2 series at different temperatures.
80
Fig. 6 shows the results of the photocatalytic tests of the STiO2_600 powder and P-25 powder in removing NOx. When the UV lamp was switched on, a rapid decrease of NO concentration with both S-TiO2_600 powder (70%) and P-25 powder (80%) was observed in the very first minutes, probably due to the adsorption of NOx on the surface of TiO2. The difference observed between the two types of powders can be explained by the difference in their apparent density (i.e. 0.4 g/ml for S-TiO2_600 and 0.19 g/ml for P-25) which resulted in larger adsorption sites for P-25. However, NO concentration with S-TiO2_600 powder slowly increased with time to retrieve almost initial concentration at the end of the experiment. An increase in NO concentration over time was also observed with P-25 but this increase was less significant. The increase in NO concentration over time may be due to other reactions which may take place and involve the formation of
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UV lamp on
UV lamp off
solar irradiation
0.7
NO NO2
0.6 0.4 0.2 0.0 (B) 1.0
0.5 30 0.4 20
0.3 0.2
10
NO
0.6
0.0
NO2
0 11
NOx
12
13
14
15
16
Time (hr)
0.2
Fig. 8. Removal behavior of NOx over S-TiO2_600 mixed with mortar under direct sunlight.
0.0 0
10
20
30
40
50
60
Time (min) (A) Fig. 6. Removal behavior of NOx with UV irradiation over S-TiO2_600 (A) and P-25 (B).
(A) 1.0 5 wt%
Concentration (ppmv)
40
NOx
0.1
0.8
0.4
0.8 0.1 0.0 1.0 3 wt%
nitrogenated species including NO, N2O, and HONO, which can be released back to the gas phase [20,21]. Fig. 7 shows NOx removal with 3 wt% and 5 wt% of both STiO2_600 (Fig. 7A) and P-25 (Fig. 7B) mixed with mortar. The NOx removal behavior with TiO2 mixed with mortar is different from that with TiO2 only. When only TiO2 was used, the photocatalytic activity in removing NO decreased with time; however when TiO2 mixed with mortar was applied, a constant photocatalytic activity was maintained throughout the experiments. This suggests that when the final product after photocatalytic reaction is HNO3 from NO gas, HNO3 reacts with Ca(NO3)2 and therefore the photocatalytic activity can be sustained without interference. Both STiO2_600 and P-25 mixed with of mortar showed similar photocatalytic activity. The photocatalytic activity in removing NO increased with higher concentration of TiO2. Therefore, the 5 wt% S-TiO2_600 mixed with mortar was chosen for further investigation under direct sunlight for the removal of NOx. 3.4. Removal of NOx by mortar containing TiO2 under direct sunlight
0.8 0.1 0.0 0
50
100
150
200
250
300
Time (min) (B) 1.0
Concentration (ppmv)
0.6
2
Concentration (ppmv)
Concentration (ppmv)
0.8
dark
UVA intensity (W/m )
1.0
5
5 wt%
0.8
NO NO2
0.1
NOx
Fig. 8 shows the photocatalytic activity of the prepared material (i.e. 5 wt% of S-TiO2_600 mixed with mortar) for the decomposition of NOx. The initial concentration of NO was stable at 0.67 ppmv. When the solar irradiation started, a net decrease in NO concentration was observed within the first minutes (i.e. from 0.67 ppmv to 0.42 ppmv in the first 5 min). The concentration of NO decreased continuously, down to 0.24 ppmv after 2 h which represent a NO rejection rate of about 50% (i.e. calculation based on NO supply of 25.1 mmol/0.05 m2/5 h and NO removal of 12.21 mmol/0.05 m2/5 h). After 3 h, the NO concentration increased, up to 45 ppmv at the end of the experiment, and was related to the decrease in the quantity of light (Fig. 8) due to the presence of clouds. 4. Conclusions
0.0 3 wt%
1.0 0.8 0.1 0.0 0
50
100
150
200
250
300
Fig. 7. Removal behavior of NOx with UV irradiation over TiO2 mixed with mortar. STiO2_600 (A) and P-25 (B).
In summary, treatment method for efficient sludge recycling was developed to produce TiO2 photocatalyst from Ti-salt flocculated sludge. The produced TiO2 were characterized in terms of morphology, structural, chemical and photo-electronic properties. Uniform size and spherical shape of the primary TiO2 particles were obtained and found to be less than 50 nm in diameter. The photocatalytic property of the produced TiO2 was first tested for the decomposition of acetaldehyde and its performance was observed to be similar to the commercialized P-25 TiO2 powder. Two concentrations (i.e. 3.0 and 5.0 wt%) of TiO2 were then selected and tested for mixing with mortar, and photocatalytic
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properties of the mortar containing TiO2 showed great performance for the decomposition of NOx and removal efficiency was again comparable to P-25. Finally, the mortar containing 5.0 wt% of the produced TiO2 was cured for 28 days and its photocatalytic property was tested under direct sunlight. The produced material showed a NO rejection of about 50% although the presence of clouds during the time of the experiments affected its performance. In this study, a novel promising application of the produced TiO2 photocatalyst from Ti-salt flocculated sludge has been investigated. As TiO2 powder can be produced in significant quantities in wastewater treatment plants, it can easily meet the demand for the production of mortar containing TiO2 for application in air purification. Acknowledgement This study was financially supported by Chonnam National University, 2011. References [1] F.L. Toma, et al. Surface and Coatings Technology 200 (20) (2006) 5855.
[2] M. Nacken, et al. Applied Catalysis B: Environmental 70 (1–4) (2007) 370. [3] G. Saracco, V. Specchia, Chemical Engineering Science 55 (5) (2000) 897. [4] D.F. Ollis, H. Al-Ekabi, in: Proceedings of the 1st international conference on TiO2 photocatalytic purification and treatment of water and air, London, Ontario, Canada, November 8–13, 1992, Elsevier Science Ltd., 1993. [5] S.-Y. Lee, S.-J. Park, Journal of Industrial and Engineering Chemistry 19 (6) (2013) 1761. [6] H. Shon, et al. Environmental Science & Technology 41 (4) (2007) 1372. [7] I. El Saliby, et al. Journal of Industrial and Engineering Chemistry 18 (3) (2012) 1033. [8] S.-H. Na, et al. Journal of Industrial and Engineering Chemistry 17 (2) (2011) 277. [9] A.M. Ramirez, et al. Building and Environment 45 (4) (2010) 832. [10] L. Cassar, MRS Bulletin 29 (5) (2004) 328. [11] L. Frazer, Environmental Health Perspectives 109 (4) (2001) A174. [12] MMC. Mitsubishi Materials Corporation – Nitrous Oxide Purification Pavement Block. Available from: http://www.mmc.co.jp/corporate/en/product/construct/ 10.html (27.03.13). [13] J.B. Kim, et al. Journal of Nanoscience and Nanotechnology 10 (5) (2010) 3260. [14] Y. Okour, et al. Bioresource Technology 101 (5) (2010) 1453. [15] ISO, ISO 679 – Cement – Test methods – Determination of strength, 2009. [16] M. Asiltu¨rk, et al. Journal of Hazardous Materials 129 (1) (2006) 164. [17] S. Liao, W. Mayo, K. Pae, Acta Materialia 45 (10) (1997) 4027. [18] F. Toma, et al. Environmental Chemistry Letters 2 (3) (2004) 117. [19] F. Ye, A. Ohmori, Surface and Coatings Technology 160 (1) (2002) 62. [20] A.E. Zein, Y. Bedjanian, Atmospheric Chemistry and Physics 12 (2) (2012) 1013. [21] O. Rosseler, et al. Journal of Physical Chemistry Letters 4 (3) (2013) 536.
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JIEC-1820; No. of Pages 6 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
NOx removal of mortar mixed with titania produced from Ti-salt flocculated sludge Se Min Park a, Laura Chekli b, Jong Beom Kim c, Mohammad Shahid b, Ho Kyong Shon b, Phan Seok Kim d,e, Woon-Seek Lee d, Woong Eui Lee f, Jong-Ho Kim a,c,* a
School of Applied Chemical Engineering & The Research Institute for Catalysis, Chonnam National University, Gwangju 500-757, Republic of Korea Faculty of Engineering and Information Technology, University of Technology, Sydney (UTS), P.O. Box 123, Broadway, NSW 2007, Australia Photo & Environmental Technology Co., Ltd., Gwangju 500-460, Republic of Korea d Division of Systems Management & Engineering, Pukyong National University, 45 Yongso-ro, Nam-gu, Busan 608-737, Republic of Korea e Hwaseung T&C Corp., 857-2, Eogok-Dong, YangSan, Kyoungnam, Republic of Korea f Department of Polymer and Fiber System Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea b c
A R T I C L E I N F O
Article history: Received 9 May 2013 Accepted 25 December 2013 Available online xxx Keywords: NOx TiO2 Photocatalyst Mortar
A B S T R A C T
Norms for the emissions of NOx are becoming stricter and stricter, leading to an increase in the research interest for the development of novel gas cleaning technologies. In this study, the use of titania (TiO2) produced from Ti-salt flocculated sludge, mixed with mortar, has been investigated as a cost-effective novel solution for the removal of these pollutant gases. This work not only presents an advanced solution for sludge reduction but also proposes a novel production method of TiO2 powders from waste water and investigates the potential use of this material blinded with mortar for a novel application which is air purification. Detailed characterization of the produced TiO2 powders was performed and results showed that the primary particles present a uniform size and spherical shape with a diameter of less than 50 nm. The main constitutive elements were Ti, O, C and P, where the Ti content was found to increase slightly with increasing temperature. The anatase phase was observed at 600 8C and 800 8C and converted to rutile structure at 1000 8C. Two contents (i.e. 3.0 and 5.0 wt%) of TiO2 were tested for mixing with mortar and photocatalytic properties of the mortar containing TiO2 were evaluated for the removal of NOx and were found to be similar to commercial TiO2 (P-25) in terms of photocatalytic activity. Further investigations under direct sunlight were conducted after 28 days of water curing to evaluate the removal of NOx. The NO rejection was about 50% after 5 h. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Nitrogen oxides (NOx), principally consisting of NO and NO2, emitted from the exhausted gas of automobile, combustion of coals and thermal power plants cause many environmental issues. For instance, NOx are responsible for the formation of acid rains and the photochemical pollution results in diseases of the human respiratory system [1]. They are emitted by a wide range of products numbering in the thousands. Examples include: paints, lacquers, paint strippers, cleaning supplies, pesticides, building materials and furnishings. This leads to the high demand for costeffective and energy-efficient gas cleaning technologies.
* Corresponding author at: School of Applied Chemical Engineering & The Research Institute for Catalysis, Chonnam National University, Gwangju 500-757, Republic of Korea. Tel.: +82 62 530 1888; fax: +82 62 530 1889. E-mail address: [email protected] (J.-H. Kim).
A catalytically active filter element combining NOx removal was developed by Nacken et al. [2]. They found that a TiO2–V2O5–WO3 catalyst system converted 96% of NO at 300 8C. Saracco and Specchia [3] reported a catalytic filter of cylindrical tube effective for the removal of NOx and VOCs. Titania (TiO2) is one of the most widely used photocatalysts that allow the decomposition and the removal of waste organic compounds and harmful gases from water and the atmosphere [4,5]. As the demand for TiO2 increases, the preparation method of TiO2 with a cost-effective and highefficient manner is required. Shon and co-authors [6–8] developed a number of processes that is based on the flocculation of wastewater by using TiCl4 which has to be followed by the calcination of sludge at high temperature. After calcination a useful by-product, TiO2 powder, is generated. TiO2 photocatalyst has recently received high application potential in the construction industry for air purification applications. TiO2 loaded cementitious materials were reported to be efficient for NOx and VOCs removal [9–11]. The combined
1226-086X/$ – see front matter ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.090
Please cite this article in press as: S.M. Park, et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2013.12.090
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application of cementitious TiO2 results in synergetic effects toward photocatalytic NOx reduction [10]. Japan’s Mitsubishi Materials Corporation [12] has developed an innovative material called Noxer combining TiO2 mounted on a mortar base which demonstrated an 80% NOx removal efficiency. The air purification potential of both dip-coated and sol–gel coated TiO2 enriched concrete sample also shows high removal efficiencies of VOCs (up to 86%), indicating their attractive photocatalytic properties for future application as air purifying building materials [9]. This work has a number of simultaneous environmental and economic benefits, namely: (i) sludge recycling, (ii) novel costeffective method for TiO2 production and (iii) application of TiO2 powder produces from sludge mixed with mortar for air purification application. In this study, the objectives were to mortar containing TiO2 and to investigate its potential for the removal of NOx. Advanced characterization of the novel material was also studied in detail. 2. Experimental 2.1. Preparation of TiO2 powder TiO2 powder produced from Ti-salt flocculated sludge with dye waste water [13,14] was used to fabricate mortar containing TiO2. Dye waste water flocculation using TiCl4 coagulant was carried out to produce TiO2. The characteristics of dye waste water obtained from Daegu dyeing industrial center were chemical oxygen demand (449 mg/L), suspended solid (43 mg/L), total nitrogen (72 mg/L) and total phosphorous (3.2 mg/L). During the rapid mixing time, pH was adjusted and the coagulant was added at 100 rpm. After flocculation, the settled flocs were collected from the sedimentation tank, dewatered in a filter press and further dried in rotary furnace at 300 8C. The dried sludge was calcined at different temperatures (i.e. 400 8C, 600 8C, 800 8C and 1000 8C) to remove volatile organic matter and water. Thereafter, TiO2 produced in this manner is named as S-TiO2_400, S-TiO2_600, STiO2_800, S-TiO2_1000. 2.2. Preparation of mortar containing TiO2 The TiO2 powder was mixed with sand, cement and water to prepare mortar containing TiO2 [15]. Sand (1350 g), cement (450 g) and water (220 g) were mixed with either 3.0 wt% or 5.0 wt% of TiO2 powder and stirred with slow mixing for 60 s and then with rapid mixing for 30 s. After 90 s stay without stirring, 60 s rapid mixing was applied again. Prepared materials were cured for 28 days. The mortar was manufactured with dimension of 100 mm 50 mm 10 mm to investigate photocatalytic activity. 2.3. Characterization of photocatalysts Morphology of the TiO2 powder was analyzed by a scanning electron microscope (SEM, Zeiss Supra 55VP SEM) operating at 15– 25 kV and a transmission electron microscope (TEM, Philips CM200, Netherlands) operating at 200 kV. The elemental composition analyses were carried out using a scanning electron microscope equipped with an energy dispersive X-ray detector (55VP SEM) operating at 15 kV. The crystalline phase of the nanoparticle TiO2 was characterized by X-ray diffraction (XRD) analysis. The XRD patterns were generated on a MDI Jade 5.0 X-ray diffractometer (MaterialsData Inc., USA) with Cu Ka radiation source. The data were measured within the range of scattering angle 2u of 5–80. Brunauer, Emmett and Teller (BET) surface area analyses were performed on an automated surface area analyser (Micromeritics Gemini 2360, USA) by means of nitrogen adsorption–desorption.
The mean pore diameter and the total pore volume of samples were determined from the desorption isotherm via Barret–Joyner– Halenda (BJH) model. 2.4. Decompositions of acetaldehyde In order to investigate the photocatalytic activity of the TiO2 powder, a cuboid stainless steel (Top-face quartz, 220 mm 125 mm 80 mm) airtight reactor was used to study the adsorption and photocatalytic oxidation of acetaldehyde over UV-A illuminated catalysts. The reactor was topped with two 10 W, 352 nm UV-A lamps (Sankyo Denki, F10T8BL, Japan) and had three rubber openings for the injection of acetaldehyde (2000 ppmv), air mixing inside the reactor and for withdrawal of samples. The last opening was connected to a gas chromatograph/flame ionization detector (GC/FID) (HP5890 series II, Wilmington, USA) for measuring variations in acetaldehyde concentration. The reaction took place at room temperature of 24 8C for 200 min. 0.5 g of photocatalysts were uniformly sprinkled on a glass Petri-dish (9 cm of diameter) and placed at a distance of 10 cm from UV-A lamps in the center of the reactor. At least three independent measurements were run per sample and the data presented are averaged values. In general, good agreement was observed between replicates (i.e. standard deviations were less than 5%). 2.5. Removal of NOx under simulated solar light ISO 22197-1 standard based experimental procedure were applied to investigate the NOx removal efficiency using a laboratory scale photocatalytic reactor (Fig. 1). The powder of 5 g was placed on a sample holder (100 mm 50 mm 10 mm), mortar containing TiO2 (100 mm 50 mm 10 mm) and two 10 W UV-A (Sanyo-denki, Japan) lamps with 10 W/m2 were used as pretreatment of irradiation for 5 h. Air conditions were 1 ppmv of NO with 50% relative humidity. Constant flow rate of 3 L/min was used during the experiment. When adsorption was stabilized, the two lamps were switch on TiO2 powder for 1 h and mortar containing TiO2 for 5 h to measure the variation of NOx concentration which was determined using a NOx analyser (CM2041, Casella). Three independent measurements were run per sample and the results presented showed average values. 2.6. Removal of NOx under direct sunlight Following the same method described previously, 5 wt% of photocatalyst (i.e. S-TiO2_600) was mixed with sand, cement and water to prepare mortar containing TiO2. In order to prevent excessive loss of moisture from the concrete, the prepared material was cured for 28 days. The test procedure for the removal of NOx was the same as described in the previous section. However, instead of applying UV-A in a batch reactor, the prepared material was directly exposed to sunlight as shown in Fig. 2. The size of the internal reactor was 100 mm 500 mm 5 mm. The test was conducted for 5 h and the NO concentration was maintained at 0.67 ppmv. 3. Results and discussion 3.1. Characterization of TiO2 powder produced from Ti-salt flocculated sludge Fig. 3 shows the SEM and TEM images of S-TiO2_600 powder. Both images show that most of the particles were in aggregate form. Primary particles were found to be uniform in size and
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Fig. 1. Photocatalytic test chamber for assessing the decomposition of NOx.
Fig. 2. Experimental set-up for assessing the decomposition of NOx under direct sunlight.
Fig. 3. SEM (left side) and TEM (right side) images of S-TiO2_600 powder produced from Ti-salt flocculated sludge.
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Table 1 Composition and surface areas of S-TiO2 series at different incineration temperatures. Temperature (8C)
400 600 800 1000
SBET (m2/g)
Composition (wt%) C
O
P
Ti
14.0 12.0 6.9 5.7
51.0 52.2 51.4 53.0
3.4 3.8 3.4 1.5
31.6 32.0 38.3 39.8
– 76 6 1
presented a spherical shape. The majority of the primary particles were found to be less than 50 nm in diameter. EDX analysis was performed to determine the elemental composition of powder samples calcined at different temperatures (Table 1). The constitutive elements were mainly Ti, O, C and P. The C atoms probably come from the organic residue of the coagulated organic matter as C content decreases with increasing temperature due to decomposition of organic group. The P atom content was found to be very low at all temperatures. The Ti content increases slightly with increasing temperature (i.e. from 31.6% at 400 8C to 39.8% at 1000 8C). The BET surface area was found to be considerably higher for S-TiO2_600 compared to S-TiO2_800, STiO2_1000, but also compared to P-25 TiO2, the most widely used photocatalyst (i.e. 76 m2/g for S-TiO2_600 compared to 42.3 m2/g for P-25) [16]. XRD pattern was used to identify the crystalline structures of the TiO2 powder after incineration at different temperatures from 400 to 1000 8C (Fig. 4). At 400 8C, an amorphous structure was observed which may be due to the remaining organic matter at low temperatures [6]. The anatase structure was observed at 600 8C and 800 8C, and at 1000 8C, the anatase structure changed to rutile. Based on the results of Liao et al. [17], the transformed temperature from the anatase to rutile structure at ambient pressure was found
100
Removal of acetaldehyde (%)
4
80
60 S-TiO2_600
40
S-TiO2_800 S-TiO2_1000
20
P-25
0 0
20
40
60
80
100
120
Time (min) Fig. 5. Removal of acetaldehyde with UV irradiation time over TiO2 powder produced from Ti-salt flocculated sludge by calcination at various temperatures and P-25 (initial concentration = 2000 ppmv; UV irradiation = UV black light three 10 W lamps).
at approximately 550 8C. However, in this study, this transformation occurred at above 800 8C. This may be due to the impurity of the TiO2 produced by the settled floc which was about 10–15% with C and P atoms. Because it is generally assumed that the anatase type has higher photocatalytic activity than rutile and also increasing temperatures involve higher energy consumption and cost, 600 8C was selected as the most effective temperature taking into account energy requirement and photocatalytic activity to produce TiO2 powders use to prepare mortar containing TiO2. 3.2. Photocatalytic decompositions of acetaldehyde by TiO2 powder The photocatalytic property of the Ti-salt flocculated sludge specimens was investigated under irradiation of UV-A for the photocatalytic decomposition of gaseous acetaldehyde (Fig. 5). The decrease in concentration of acetaldehyde was measured by gas chromatography. As expected, the removal of acetaldehyde was much more effective with S-TiO2_600 powder mainly due to the enhanced photocatalytic activity of the anatase type [18,19]. After 75 min of photocatalytic reaction, S-TiO2_600 powder removed the majority of acetaldehydes (i.e. almost 100%), whereas for S-TiO2_800 and S-TiO2_1000 powder show only 50% and 10% removal. S-TiO2_600 powder indicated similar photocatalytic activity compared to the commercially available P-25.
200 cps o
1000 C
o
Intensity
800 C
3.3. Removal of NOx by TiO2 powder and mortar containing TiO2 o
600 C
o
400 C
20
40
60
2 theta (deg.) Fig. 4. XRD patterns of S-TiO2 series at different temperatures.
80
Fig. 6 shows the results of the photocatalytic tests of the STiO2_600 powder and P-25 powder in removing NOx. When the UV lamp was switched on, a rapid decrease of NO concentration with both S-TiO2_600 powder (70%) and P-25 powder (80%) was observed in the very first minutes, probably due to the adsorption of NOx on the surface of TiO2. The difference observed between the two types of powders can be explained by the difference in their apparent density (i.e. 0.4 g/ml for S-TiO2_600 and 0.19 g/ml for P-25) which resulted in larger adsorption sites for P-25. However, NO concentration with S-TiO2_600 powder slowly increased with time to retrieve almost initial concentration at the end of the experiment. An increase in NO concentration over time was also observed with P-25 but this increase was less significant. The increase in NO concentration over time may be due to other reactions which may take place and involve the formation of
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UV lamp on
UV lamp off
solar irradiation
0.7
NO NO2
0.6 0.4 0.2 0.0 (B) 1.0
0.5 30 0.4 20
0.3 0.2
10
NO
0.6
0.0
NO2
0 11
NOx
12
13
14
15
16
Time (hr)
0.2
Fig. 8. Removal behavior of NOx over S-TiO2_600 mixed with mortar under direct sunlight.
0.0 0
10
20
30
40
50
60
Time (min) (A) Fig. 6. Removal behavior of NOx with UV irradiation over S-TiO2_600 (A) and P-25 (B).
(A) 1.0 5 wt%
Concentration (ppmv)
40
NOx
0.1
0.8
0.4
0.8 0.1 0.0 1.0 3 wt%
nitrogenated species including NO, N2O, and HONO, which can be released back to the gas phase [20,21]. Fig. 7 shows NOx removal with 3 wt% and 5 wt% of both STiO2_600 (Fig. 7A) and P-25 (Fig. 7B) mixed with mortar. The NOx removal behavior with TiO2 mixed with mortar is different from that with TiO2 only. When only TiO2 was used, the photocatalytic activity in removing NO decreased with time; however when TiO2 mixed with mortar was applied, a constant photocatalytic activity was maintained throughout the experiments. This suggests that when the final product after photocatalytic reaction is HNO3 from NO gas, HNO3 reacts with Ca(NO3)2 and therefore the photocatalytic activity can be sustained without interference. Both STiO2_600 and P-25 mixed with of mortar showed similar photocatalytic activity. The photocatalytic activity in removing NO increased with higher concentration of TiO2. Therefore, the 5 wt% S-TiO2_600 mixed with mortar was chosen for further investigation under direct sunlight for the removal of NOx. 3.4. Removal of NOx by mortar containing TiO2 under direct sunlight
0.8 0.1 0.0 0
50
100
150
200
250
300
Time (min) (B) 1.0
Concentration (ppmv)
0.6
2
Concentration (ppmv)
Concentration (ppmv)
0.8
dark
UVA intensity (W/m )
1.0
5
5 wt%
0.8
NO NO2
0.1
NOx
Fig. 8 shows the photocatalytic activity of the prepared material (i.e. 5 wt% of S-TiO2_600 mixed with mortar) for the decomposition of NOx. The initial concentration of NO was stable at 0.67 ppmv. When the solar irradiation started, a net decrease in NO concentration was observed within the first minutes (i.e. from 0.67 ppmv to 0.42 ppmv in the first 5 min). The concentration of NO decreased continuously, down to 0.24 ppmv after 2 h which represent a NO rejection rate of about 50% (i.e. calculation based on NO supply of 25.1 mmol/0.05 m2/5 h and NO removal of 12.21 mmol/0.05 m2/5 h). After 3 h, the NO concentration increased, up to 45 ppmv at the end of the experiment, and was related to the decrease in the quantity of light (Fig. 8) due to the presence of clouds. 4. Conclusions
0.0 3 wt%
1.0 0.8 0.1 0.0 0
50
100
150
200
250
300
Fig. 7. Removal behavior of NOx with UV irradiation over TiO2 mixed with mortar. STiO2_600 (A) and P-25 (B).
In summary, treatment method for efficient sludge recycling was developed to produce TiO2 photocatalyst from Ti-salt flocculated sludge. The produced TiO2 were characterized in terms of morphology, structural, chemical and photo-electronic properties. Uniform size and spherical shape of the primary TiO2 particles were obtained and found to be less than 50 nm in diameter. The photocatalytic property of the produced TiO2 was first tested for the decomposition of acetaldehyde and its performance was observed to be similar to the commercialized P-25 TiO2 powder. Two concentrations (i.e. 3.0 and 5.0 wt%) of TiO2 were then selected and tested for mixing with mortar, and photocatalytic
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properties of the mortar containing TiO2 showed great performance for the decomposition of NOx and removal efficiency was again comparable to P-25. Finally, the mortar containing 5.0 wt% of the produced TiO2 was cured for 28 days and its photocatalytic property was tested under direct sunlight. The produced material showed a NO rejection of about 50% although the presence of clouds during the time of the experiments affected its performance. In this study, a novel promising application of the produced TiO2 photocatalyst from Ti-salt flocculated sludge has been investigated. As TiO2 powder can be produced in significant quantities in wastewater treatment plants, it can easily meet the demand for the production of mortar containing TiO2 for application in air purification. Acknowledgement This study was financially supported by Chonnam National University, 2011. References [1] F.L. Toma, et al. Surface and Coatings Technology 200 (20) (2006) 5855.
[2] M. Nacken, et al. Applied Catalysis B: Environmental 70 (1–4) (2007) 370. [3] G. Saracco, V. Specchia, Chemical Engineering Science 55 (5) (2000) 897. [4] D.F. Ollis, H. Al-Ekabi, in: Proceedings of the 1st international conference on TiO2 photocatalytic purification and treatment of water and air, London, Ontario, Canada, November 8–13, 1992, Elsevier Science Ltd., 1993. [5] S.-Y. Lee, S.-J. Park, Journal of Industrial and Engineering Chemistry 19 (6) (2013) 1761. [6] H. Shon, et al. Environmental Science & Technology 41 (4) (2007) 1372. [7] I. El Saliby, et al. Journal of Industrial and Engineering Chemistry 18 (3) (2012) 1033. [8] S.-H. Na, et al. Journal of Industrial and Engineering Chemistry 17 (2) (2011) 277. [9] A.M. Ramirez, et al. Building and Environment 45 (4) (2010) 832. [10] L. Cassar, MRS Bulletin 29 (5) (2004) 328. [11] L. Frazer, Environmental Health Perspectives 109 (4) (2001) A174. [12] MMC. Mitsubishi Materials Corporation – Nitrous Oxide Purification Pavement Block. Available from: http://www.mmc.co.jp/corporate/en/product/construct/ 10.html (27.03.13). [13] J.B. Kim, et al. Journal of Nanoscience and Nanotechnology 10 (5) (2010) 3260. [14] Y. Okour, et al. Bioresource Technology 101 (5) (2010) 1453. [15] ISO, ISO 679 – Cement – Test methods – Determination of strength, 2009. [16] M. Asiltu¨rk, et al. Journal of Hazardous Materials 129 (1) (2006) 164. [17] S. Liao, W. Mayo, K. Pae, Acta Materialia 45 (10) (1997) 4027. [18] F. Toma, et al. Environmental Chemistry Letters 2 (3) (2004) 117. [19] F. Ye, A. Ohmori, Surface and Coatings Technology 160 (1) (2002) 62. [20] A.E. Zein, Y. Bedjanian, Atmospheric Chemistry and Physics 12 (2) (2012) 1013. [21] O. Rosseler, et al. Journal of Physical Chemistry Letters 4 (3) (2013) 536.
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