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Decomposition of indoor ammonia with TiO2-loaded cotton woven fabrics prepared by different textile finishing methods
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Atmospheric Environment 41 (2007) 3182–3192 www.elsevier.com/locate/atmosenv
Decomposition of indoor ammonia with TiO2-loaded cotton woven fabrics prepared by different textile finishing methods Yongchun Donga,b,, Zhipeng Baib, Ruihua Liua, Tan Zhub a
Tianjin Municipal Key Laboratory of Fiber Modification and Functional Fiber, School of Material Sciences and Chemical Engineering, Tianjin Polytechnic University, 63 Chenglin Road, Tianjin 300160, China b State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Sciences and Engineering, Nankai University, 94 Weijin Road, Tianjin 300071, China Received 16 March 2006; received in revised form 18 August 2006; accepted 18 August 2006
Abstract Addition of urea-based antifreeze admixtures during cement mixing in construction of buildings has led to increasing indoor air pollution due to continuous transformation and emission of urea to gaseous ammonia in indoor concrete wall. In order to control ammonia pollution from indoor concrete wall, the aqueous dispersion was firstly prepared with nano-scale TiO2 photocatalysts and dispersing agent, and then mixed with some textile additives to establish a treating bath or coating paste. Cotton woven fabrics were used as the support materials owing to their large surface area and large number of hydrophilic groups on their cellulose molecules and finished using padding and coating methods, respectively. Two TiO2loaded fabrics were obtained and characterized by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Moreover, a specifically designed ammonia photocatalytic system consisting of a small environmental chamber and a reactor was used for assessing the performance of these TiO2-loaded fabrics as the wall cloth or curtains used in house rooms in the future and some factors affecting ammonia decomposition are discussed. Furthermore, a design equation of surface catalytic kinetics was developed for describing the decomposition of ammonia in air stream. The results indicated that increasing dosage of the TiO2 aqueous dispersion in treating bath or coating paste improved the ammonia decomposition. And ammonia was effectively removed at low ammonia concentration or gas flow rate. When relative humidity level was 45%, ammonia decomposition was remarkably enhanced. It is the fact that ammonia could be significantly decomposed in the presence of the TiO2-padded cotton fabric. Whereas, the TiO2-coated cotton fabric had the reduced photocatalytic decomposition of ammonia and high adsorption to ammonia owing to their acrylic binder layer. Finally, the reaction rate constant k and the adsorption equilibrium constant K values were determined through a curve-fitting method and the TiO2padded cotton fabric had the higher k value and lower K value than the TiO2-coated cotton fabric. r 2006 Elsevier Ltd. All rights reserved. Keywords: Ammonia decomposition; Photocatalyst; Cotton fabric; Padding; Coating
Corresponding author. Tianjin Municipal Key Laboratory of
Fiber Modification and Functional Fiber, School of Material Sciences and Chemical Engineering, Tianjin Polytechnic University, 63 Chenglin Road, Tianjin 300160, China. E-mail addresses: [email protected], dyefi[email protected] (Y. Dong). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.08.056
1. Introduction Antifreezes based on urea or ammonia compounds were widely applied in construction of concrete buildings in recent years; however this have resulted in the increased indoor air pollution owing to the
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emission of ammonia from indoor concrete wall. Therefore, the standards and regulations to control indoor ammonia pollution have been enacted and developed in China (Bai et al., 2002). Ammonia is harmful to human health and environment. Several researchers investigated indoor ammonia emission sources (Wirtanen et al., 2002, Tuomainen et al., 2000). It is believed that the UV/TiO2 process is a promising alternative for decomposing of various refractory contaminants in gaseous streams in the past decades. Coating of TiO2 by impregnation is widely used by most researchers because the technique is easy and does not require any complicated equipment. Materials including paper (Ichiura et al., 2003) nonwoven textile (Ku et al., 2001) have been developed for possible applications of TiO2 photocatalysis process. Padding and coating are the most common finish methods for application of chemical formulation to textile materials, in continuous processes. Padding consists of contacting the material with the formulation, usually by immersion, and squeezing the formulation out with squeeze rolls (Perkins, 2004). On the other hand, coating is generally based on the knife coater, which is a metering device that continuously spreads viscous liquids onto fabric. It contains a stationary knife blade positioned over a fabric support. The material applied to the fabric is fed to one side of the knife blade as the fabric moves continuously under the spreader. In this study, cotton woven fabrics were used as based materials for TiO2 photocatalysts because of their flexibility in engineering. Additionally, the large surface area and high hydrophilic function of the cellulose chain can provide more active sites for the deposition of TiO2 particles than the inorganic materials. The TiO2-loaded fabrics as the wall cloth or curtains to be used in house rooms in the future were obtained with aqueous dispersion containing TiO2 photocatalysts using padding or coating process. The heterogeneous decomposition of ammonia by UV/TiO2 process in a photoreactor fixed with TiO2-loaded cotton fabric was studied under various experimental conditions and a comparison between their performances of removing ammonia were made. 2. Experimental section 2.1. Preparation of TiO2 aqueous dispersion Degussa P-25 TiO2 powder was employed as the photocatalyst, without further treatment. Aqueous
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dispersion was prepared by mixing 1.5 g of Degussa P-25 TiO2 powder and the appropriate amounts of deionized water in an ultrasonic mixer. After 5 h of ultrasonic mixing, 0.125 g of polyglycol as the dispersing agent was added into aqueous dispersion and mixed under continuous magnetic stirring for 30 min. The TiO2 aqueous dispersion containing 3 wt% of Degussa P-25 TiO2 powder and 0.25 wt% of polyglycol was obtained. The average particle size was measured in the TiO2 aqueous dispersion to be 0.212 mm using a LA-300 particle sizer (Horiba Instrument Ltd., Japan). 2.2. Padding method for TiO2-loaded cotton fabrics Commercially bleached 100% cotton woven fabrics were used as support for TiO2 photocatalysts. And a commercial grade of reactive amino silicone additive AM-200 from Fuzhou Botex Chemicals Company (China) was used for fixing TiO2 particles on the fabrics. Before padding, the fabric samples were first treated with a solution containing 2 g l1 sodium carbonate and 2 g l1 of soap and boiled for 30 min, then thoroughly washed with cold water and dried at ambient temperature. And the fabric samples were padded twice (take up 7571%) with a treating solution containing various amounts of TiO2 aqueous dispersion and 10 g l1 AM-200 on a laboratory padding mangle. After being padded, the samples were immediately dried at 100 1C for 2 min, and then cured at 170 1C for 1 min. The amount of TiO2 loaded on padded fabric sample can be calculated through Q ¼ P C W 104 ,
(1)
where Q is the weight of loaded TiO2 per unit fabric weight (mg g1), P the take up (75%), C the concentration of TiO2 aqueous dispersion (3%), W the weight of TiO2 aqueous dispersion added to 1 l of treating bath (g). 2.3. Coating method for TiO2-loaded cotton fabrics The same fabrics as mentioned above were also used for coating process. Two commercial textile additives including a self-crosslinking acrylic binder TOW (BASF China Ltd.) and a Lutexal HIT thicker (BASF China Ltd.) were employed to form the coating formulation. Various amounts of TiO2 aqueous dispersion, 30 g l1 binder and 10 g l1 thicker were mixed by an electromagnetic stirrer for 20 min to establish a coating paste. Before
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coating, the cotton fabrics were first treated with a solution containing 30 g l1 water repellent agent EC-50 (Nikka Chemicals Co., Japan) by pad-dry process in order to keep the coating paste on the surface. And then the paste was continuously spread onto cotton fabrics on a laboratory knife coater (Werner Mathis AG, Switzerland). It is noted that the weight of coating paste deposited on the surface of fabric was controlled to be about 50% of fabric weight by adjusting the gap between the blade and the fabric. Finally, the coated fabrics were immediately dried at 100 1C for 3.0 min, cured at 170 1C for 1.5 min. The amount of TiO2 loaded on the coated fabric sample can be estimated by Q ¼ 50% C W 104 ,
(2)
where Q is the weight of loaded TiO2 per unit fabric weight (mg g1), C the concentration of TiO2 aqueous dispersion (3%), W the weight of TiO2 aqueous dispersion added to 1 l of coating paste (g). 2.4. Characterization of TiO2-loaded cotton fabrics The composition of TiO2-loaded fabrics was verified by using a Rigaku Xd/Max-2500 X-ray diffractometer (Rigaku Co., Japan) operating with Cu Ka radiation. Scanning electron microscopic (SEM) observations on specimens of TiO2-loaded fabrics was carried out using Hitachi S-670 electron microscope (Hitachi High-Technologies Co., Japan). 2.5. Decomposition of indoor ammonia The photocatalytic system consists mainly of a small stainless steel environmental condition simulated chamber (1 m3) and a glass photoreactor. The schematic diagram of photodegradation system is shown in Fig. 1. Four concrete wall pieces were prepared with 65.5 kg cement, 3.73 kg FDJ antifreeze containing 34 wt% urea compound (Tianjin Bocheng New Type Building Materials company, China) and a large amount of fine sand, gravel and water, and used for simulating the indoor concrete wall as a stable ammonia emitting source. Generally, the similar concentration of antifreeze was also used in construction of actual rooms in China. It is noticed that ratio of wall pieces weight to the volume of environmental chamber is equals to the ratio of actual indoor concrete weight to indoor volume and the proportion of the emitting surface area of wall pieces to the volume of environmental
chamber equals that of the emitting surface area of actual indoor concrete wall and indoor volume in the given rooms by partial covering of plastic film on wall pieces. The specifications of concrete wall pieces are presented in Table 1. The photocatalytic reactor was a 38 cm long glass plug flow tubular reactor with an effective volume 2.5 l. It contained a 2 cm i.d. quartz tube inside cooling water and housed a UV 365 nm lamp (Shanghai Philips-Yaming Company, China). The TiO2-loaded fabric with an area of 0.022 m2 was wound and fixed on the inner wall of the photoreactor. Environmental chamber and photoreactor were thoroughly cleaned with a solution containing 2g l1 sodium carbonate and deionized water, respectively. Gaseous ammonia emitted from wall pieces in environmental chamber and clean air were mixed in a gas mixer and maintained constant for providing ammonia gas with a steady state concentration. The temperature of the mixed gas flow in the photoreactor was kept at 2571 1C and humidity of the mixed gas flow reached steady by adjusting water bubbling humidity regulator and measured through flow meter and CR-052 thermo/humidity meter (Crecer company, Japan). A NH3 gas detector TG-2400KBP (Bionics Instrument Co. Ltd., Japan) was used to determine the concentration of gaseous ammonia in the air stream from the photoreactor. The reaction was not considered to reach balance until the concentration of gaseous ammonia in the photoreactor was kept steady for 5 min. The ammonia concentration in the photoreactor at this time was referring to the balance concentration of ammonia. Photocatalytic decomposition percentage of ammonia (Dp% at constant humidity and temperature was calculated by Dp % ¼
ðC 0 CÞ 100, C0
(3)
where Dp% is the photocatalytic decomposition percentage of ammonia, C0 is the initial concentration of ammonia (mg m3) and C is the residual concentration of ammonia (mg m3). 3. Results and discussion 3.1. Characterization of TiO2-loaded fabrics 3.1.1. XRD The X-ray diffraction (XRD) curves of untreated cotton fabric and TiO2-loaded cotton fabric were presented in Fig. 2.
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Air
1 Ammonia
0
3
5
4
2 10 Cooling water 8
Vent
9
12
7
6
11
Fig. 1. The schematic diagram of photocatalytic and testing system for ammonia in air, (0) wall pieces, (1) environmental chamber, (2) gas mixer, (3) dewater reagent, (4) water bottle, (5) activated carbon filter (6) thermo/humidity meter, (7) flow meter, (8) quartz shelter (inside cooling water) (9) TiO2-loaded fabric, (10) UV lamp, (11) photocatalytic reactor, (12) ammonia detector. Table 1 Specification of concrete wall pieces Dimension of each wall piece
520 mm 300 mm 200 mm
Total weight of four wall pieces Total volume of four wall pieces Total emission area of four wall pieces Total content of urea in four wall pieces Amount of ammonia emitted from four pieces in theory
312 kg 0.1248 m3 1.248 m2 1.114 kg 6.13 105 mg
As seen from Fig. 2, it is found that three major peaks (14.841, 16.381 and 22.681) of cellulose fiber were similar to those published in Weaver (1984). More importantly, a relatively strong reflection peak (25.281, peak a) and three weak peaks (37.841, peak b; 47.981, peak c; and d: 53.90–55.121, peak d) of anatase TiO2 were also
observed in XRD curves of the TiO2-loaded cotton fabrics. TiO2 grain sizes in two samples were determined from the broadening corresponding X-ray spectral peaks by Scherrer’s formula L ¼ 0.90l/(b cos y), where L is the grain size, l is the wavelength of X-ray radiation, and b is the line width at half maximum height. Based on XRD analysis, the average grain sizes of anatase TiO2 in two samples are 30–40 nm. 3.1.2. SEM Fig. 3 illustrates the SEM of untreated (panel a), TiO2-padded (panel b) and TiO2-coated cotton (panel c) fabrics. Fig. 3a (magnification ¼ 200 and 5000 ) showed the surface structure of cotton fiber: a round surface was very smooth to the touch. Fig. 3b (magnification ¼ 200 and 7000 ) depicts the morphological change in appearance of cotton fiber after TiO2-padding. Cotton fiber surface was
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1 Cotton, 2 TiO2-coated cotton, 3 TiO2-paded cotton a b
c
d 3
a
b
c
d 2
1 10
20
30
40
50
60
2 Theta (Deg.) Fig. 2. XRD spectrum of untreated, TiO2-padded and TiO2coated cotton fabrics.
covered with TiO2 particles, and seemed somewhat rough and uneven. Therefore, it is proved that TiO2 particles were loaded on cotton fiber by padding process, but their distribution on fiber surface was not quite even, possibly because of aggregation of some fine TiO2 particles. Fig. 3c demonstrates the quite different morphological surface of TiO2coated cotton fabric from the TiO2-padded cotton fabric, and a few TiO2 particles or their aggregations appeared the rough binder layer overlaid cotton fabric.
in this research. Also Dp% is increased with the prolongation of reaction time, and increasing tendency becomes level after reaction time of 15 min. Moreover, nitrate or nitrite was not found in the aqueous extraction of the TiO2-padded cotton fabric used for the decomposition of ammonia by ion chromatography. This indicated that one of major products of decomposition should be considered as nitrogen, which agreed with the result reported in a previous study (Morranega et al., 1979). It is noticed that increasing the amount of TiO2 aqueous dispersion in the treating bath or coating paste is accompanied with an enhancement in degradation of ammonia especially, in the presence of TiO2-padded cotton fabric. This is due to the fact that the increasing dosage of TiO2 aqueous dispersion leads to more TiO2 particles loaded on the fiber surface, which improved Dp%. Also it was found that TiO2-padded cotton fabric had higher Dp% than TiO2-coated cotton fabric at the same conditions. It is believed that TiO2-padded cotton fabric still keeps an open structure after treating, whereas TiO2-coated cotton fabrics has a close structure after coating owing to binder layer covered on the surface of fabric. The close structure is considered a more powerful limitation to light penetration than the open structure. Another possible explanation is that binder layer enhanced the TiO2 particles aggregation and retarded the TiO2 particles contacting with gaseous substances such as ammonia and oxygen and inhabited UV/ TiO2 process. 3.3. Initial concentration of ammonia
3.2. Dosage of TiO2 aqueous dispersion Experimental results of gaseous ammonia photodecomposition of TiO2-padded and coated cotton fabrics with different dosage of TiO2 aqueous dispersion in treating bath or coating paste are given in Fig. 4. It can be seen from Fig. 4 that the Dp% gradually increased at the beginning stage of reaction in the presence of the cotton fabrics without TiO2 catalysts, but then came back to about initial zero level during a short time. This may be due to the limited adsorption of ammonia onto the cellulose fiber surface, suggesting that ammonia could not be directly decomposed by UV light of 365 nm. The main reason is that the maximum wavelength for breaking the N–H bond is 220 nm (Morranega et al., 1979), which is shorter than 365 nm employed
Experimental results of ammonia photodecomposition under various initial ammonia concentrations in the presence of TiO2-loaded cotton fabrics were presented in Fig. 5. It is apparent that the ammonia concentration decreased significantly with increasing reaction time in the initial 10 min, and then there is a small concentration change after 10 min. This is because increasing ammonia concentration in inlet gas flow decreased the decomposition efficiency of ammonia. For experiment conducted at low initial ammonia concentration more than 80% of ammonia was decomposed, but the efficiency was reduced to less than 50% for experiment conducted for high initial ammonia concentration. This is possibly owing to the fixed amount of active sites on the surface of TiO2 available for the adsorption of ammonia
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Fig. 3. SEM images of untreated (panel a), TiO2-padded (panel b) and TiO2-coated cotton fabrics, (a) (panel a), (b) (panel b), (c) (panel c).
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a
b 100
80
80
60
60
D p%
Dp %
100
40
40
without P-25 30 gl-1 50 gl-1 100 gl-1
20
without P-25 50 gl-1 100 gl-1 150 gl-1
20
0
0 0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
35
40
45
Time (min)
Time (min)
Fig. 4. Effect of TiO2 aqueous dispersion dosage on Dp%, (a) padding, (b) coating. Padding: 30 g l1, Q ¼ 0.675 mg g1; 50 g l1, Q ¼ 1.125 mg g1; 100 g l1, Q ¼ 2.25 mg g1, coating: 50 g l1, Q ¼ 0.750 mg g1; 100 g l1, Q ¼ 1.50 mg g1; 150 g l1, Q ¼ 2.25 mg g1, temperature ¼ 2571 1C, RH ¼ 4572%, gas flow rate ¼ 0.5 l min1.
a
b 50
70 14.3 mgm-3 36.1 mgm-3 51.5 mgm-3 63.8 mgm-3
NH3 (mgm-3)
50
13.9 mgm-3 29.1 mgm-3 33.1 mgm-3 46.4 mgm-3
40 NH3 (mgm-3)
60
40 30
30
20
20 10 10 0
0 0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
35
40
45
Time (min)
Time (min)
Fig. 5. Concentration change of ammonia under different initial concentrations, (a) padding, (b) coating, temperature ¼ 2572 1C, RH ¼ 4572%, gas flow rate ¼ 0.5 l min1.
molecules before the decomposition of ammonia. Similar results were observed and discussed for the decomposition of trichloroethylene (Ku et al., 2001) and ethylene (Obee and Hay, 1997) by UV/TiO2 process with saturated adsorption under fixed active sites. On the other hand, there are also some differences in ammonia concentration decrease levels especially, in the beginning reaction time between two TiO2-loaded cotton fabrics. Moreover, TiO2-padded cotton fabric had more powerful
decomposing performance of ammonia compared with TiO2-coated cotton fabric. 3.4. Relative humidity Fig. 6 showed that Dp% increased with reaction time and reached equilibrium in about 10 min in the case of TiO2-padded cotton fabric. When the humidity level was increased from 15% to 45%, the Dp% was enhanced to the highest stage and then
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b 100
100
80
80
60
60
Dp %
Dp %
a
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40
40 RH 15 % RH 45 %
20
RH 15% RH 45% RH 65%
20
RH 65 %
0
0 0
5
10
15
20
25
30
35
40
45
0
5
10
Time (min)
15
20
25
30
35
40
45
Time (min)
Fig. 6. Effect of relative humidity on Dp%, (a) padding, (b) coating, temperature ¼ 2571 1C, gas flow rate ¼ 0.5 l min1.
b
a 20 min
100
40 min
80
80
60
60
Dp %
Dp %
100
40
40
20
20
0 0.0
0.3
0.6
0.9
1.2
1.5
0 0.0
20 min
0.3
Gas flow rate (Lmin-1)
0.6
0.9
40 min
1.2
1.5
Gas flow rate (Lmin-1)
Fig. 7. Effect of gas flow rate on Dp%, (a) padding, (b) coating, temperature ¼ 2571 C, RH ¼ 4572%.
decreased with the increase in relative humidity. This is similar to formaldehyde investigated by Obee (Obee and Brown, 1995). The main reasons may be that TiO2 surface carries weakly or strongly bound molecular water, and hydroxyl groups from the hydrolysis of TiO2 can combine with ammonia by hydrogen bonding bond formation. On the other hand, both ammonia and water molecules are adsorbed on TiO2 surface through hydrogen bonding, thus resulting in the competition of the adsorption locations, and excessive water vapor on the catalyst surface will lead to a decrease in decomposition of ammonia because water molecules can occupy the active sites of the reactants on
the surface (Zhao and Yang, 2003). In contrast, TiO2-coated cotton fabric had the similar trend in the ammonia decomposition at different humidity level to that of TiO2-padded cotton fabric, and humidity level in a reactor reaching about 45% also led to the highest Dp% value. 3.5. Gas flow rate It can be found from Fig. 7 that two TiO2-loaded cotton fabrics remove ammonia at different gas flow rates in common. The Dp% of TiO2-padded cotton fabric gradually decreased with increasing gas flow rates from 0.3 to 1.2 l min1. On the other hand,
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increased gas flow rates just resulted in a slight decline in the Dp% of TiO2-coated cotton fabric particular in 20 min reaction time, possibly because of improved ammonia adsorption onto the sticky acrylic binder layer and reduced flow of ammonia by their close structure, particularly at the high gas flow rates. The differences of Dp% between 20 and 40 min of reaction time depended upon the level of gas flow rates. In the heterogeneous catalytic reaction, mass transfer and surface reaction are important factors controlling overall reaction rates (Wang et al., 1998). In lower flow rates, decomposition reaction rates of ammonia are significantly affected by mass transfer, but in higher flow rates, surface reaction controls the progress of reactions; and when flow rates become too high and residence time are shortened, an insufficient contact of gas stream with the catalyst layers occurs, lowing decomposition of ammonia. On the contrary, the influence of gas flow rates on decomposition reaction becomes insignificant for lower gas flow rates, and when gas flow rates were less than 0.6 l min1, Dp% was not remarkably enhanced by increasing reaction time in both cases. 3.6. Wash experiment Two TiO2-loaded cotton fabrics were washed with an aqueous solution containing 2 g l1 anionic detergent and 2 g l1 sodium carbonate through SW-12 washing testing machine (Shandong Laizhou Instruments Company, China). And then they were used for ammonia decomposition in photocatalytic system. For comparison, control experiment in which additive was absent in the padding bath or coating paste was carried out, experimental results are presented in Fig. 8. Fig. 8 exhibits that wash cycles caused a significant decrease in the decomposition levels of ammonia in the case of TiO2-padded cotton fabric. In contrast, the Dp% of TiO2-coated cotton fabric was slightly changed by wash cycles. It is believed that TiO2 particles on the surface of cotton fiber in control experiment could be easily removed since they were not strongly fixed without any additives. High adsorption and adhesion of amino silicon additive enhanced the fixation of TiO2 particles on the fiber surface in the case of padding. On the other hand, acrylic binder improved the resistance of TiO2-coated cotton fabric to water wash due to more TiO2 particles being kept with their excellent adhesion and close structure.
Padding Control padding Coating
100 80 Dp %
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60 40 20 0 W0
W2
W5
W10
Wash cycles
Fig. 8. Relationship between wash cycles and Dp%, washing condition: liquid ratio (fabric (g): washing bath (ml)) ¼ 1:50, 20 1C, 15 min/cycle.
3.7. Modeling of the experimental results The heterogeneous photocatalytic kinetics of target pollutants have been frequently described by the Langmuir–Hinshelwood rate equation (Ku et al., 2001; Zhao and Yang, 2003), for a plug flow reactor, the design equation can be written as follows: V 1 ln C 0 =C ¼ þ QðC 0 CÞ k ðC 0 CÞkK
(4)
where C is concentration of ammonia (mg l1), k reaction rate constant (mg l1 min1), K adsorption equilibrium constant (l mg1), V volume of reactor (l), Q gas flow rate (l min1), C0 the initial concentration of ammonia (mg l1). With the necessary operation and reaction parameters, Eq. (4) can be used as the design equation for the photocatalytic decomposition of gaseous ammonia in an annular reactor, and then by regressing Eq. (4) with respect to the experimental data of the effect of initial concentration of ammonia, k and K values can be determined. The regression results of the Fig. 5 by Eq. (4) for the photocatalytic decomposition of ammonia by UV/TiO2 process in both cases of padding and coating are shown as Fig. 9, and also the k and K values can be determined and are listed in Table 2. Table 2 indicats that the TiO2-padded cotton fabric was more than the TiO2-coated cotton fabric with respect to k the reaction rate constant. However with respect to K adsorption equilibrium
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500
(V/Q) (Co-C)-1
400 300 200 100
Padding Coating
0 0
25
50
75
100
ln (Co/C) (Co
125
150
175
200
-C)-1
Fig. 9. The regression results of Fig. 5 by the Eq. (4) for the photocatalytic decomposition of ammonia by UV/TiO2 process.
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significantly decomposed in the presence of TiO2padded cotton fabric at a certain operating conditions. On the other hand, TiO2-coated cotton fabric had the reduced photocatalytic decomposition of ammonia and relatively strong adsorption to ammonia molecules because of the acrylic binder layer covering TiO2 particles on the surface of fiber. Finally, Langmuir–Hinshelwood model was developed and used for formulating the photocatalytic reaction of ammonia in air, and both constants k and K values were obtained from the measured kinetic data through a curve-fitting method. TiO2padded cotton fabric exhibited higher k value and lower K value compared to TiO2-coated cotton fabric. Acknowledgments
Table 2 k and K values for the photocatalytic decomposition of ammonia Methods
k (mg l1 min1)
K (l mg1)
R
SD
N
P
Padding Coating
0.01457 0.00612
35.26 112.9
0.9955 0.9800
15.01 23.56
4 4
0.0045 0.0204
constant we have just reverse of k, TiO2-padded cotton fabric was much less effectively than TiO2-coated cotton fabric in terms of K. A possible explanation is that the coated fabric has a close structure presenting a low photocatalytic power. However, gaseous ammonia was strongly adsorbed onto the acrylic binder layer through hydrogen bonding of carboxyl group with ammonia molecules in the case of TiO2-coated cotton fabric. 4. Conclusion Combination of TiO2-loaded cotton fabrics produced using padding or coating methods with UV irradiation of 365 nm wavelengths can effectively eliminate gaseous ammonia in a photocatalytic reactor. The decomposition efficiency of ammonia was much affected by the dosage of TiO2 aqueous dispersion, initial ammonia concentration, relative humidity and gas flow rate. Furthermore, it is noticed that the ammonia removal depended also on the TiO2-loaded cotton fabric produce by padding or coating methods. It is fact that ammonia could be
The authors are very grateful to the support from the Ministry of Education, P. R. China, through ‘‘Joint Research Grant to Both Nankai University and Tianjin University’’ and ‘‘Trans-Century Training Programme Foundation for the Talents (2002–2048)’’.
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Atmospheric Environment 41 (2007) 3182–3192 www.elsevier.com/locate/atmosenv
Decomposition of indoor ammonia with TiO2-loaded cotton woven fabrics prepared by different textile finishing methods Yongchun Donga,b,, Zhipeng Baib, Ruihua Liua, Tan Zhub a
Tianjin Municipal Key Laboratory of Fiber Modification and Functional Fiber, School of Material Sciences and Chemical Engineering, Tianjin Polytechnic University, 63 Chenglin Road, Tianjin 300160, China b State Environmental Protection Key Laboratory of Urban Ambient Air Particulate Matter Pollution Prevention and Control, College of Environmental Sciences and Engineering, Nankai University, 94 Weijin Road, Tianjin 300071, China Received 16 March 2006; received in revised form 18 August 2006; accepted 18 August 2006
Abstract Addition of urea-based antifreeze admixtures during cement mixing in construction of buildings has led to increasing indoor air pollution due to continuous transformation and emission of urea to gaseous ammonia in indoor concrete wall. In order to control ammonia pollution from indoor concrete wall, the aqueous dispersion was firstly prepared with nano-scale TiO2 photocatalysts and dispersing agent, and then mixed with some textile additives to establish a treating bath or coating paste. Cotton woven fabrics were used as the support materials owing to their large surface area and large number of hydrophilic groups on their cellulose molecules and finished using padding and coating methods, respectively. Two TiO2loaded fabrics were obtained and characterized by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Moreover, a specifically designed ammonia photocatalytic system consisting of a small environmental chamber and a reactor was used for assessing the performance of these TiO2-loaded fabrics as the wall cloth or curtains used in house rooms in the future and some factors affecting ammonia decomposition are discussed. Furthermore, a design equation of surface catalytic kinetics was developed for describing the decomposition of ammonia in air stream. The results indicated that increasing dosage of the TiO2 aqueous dispersion in treating bath or coating paste improved the ammonia decomposition. And ammonia was effectively removed at low ammonia concentration or gas flow rate. When relative humidity level was 45%, ammonia decomposition was remarkably enhanced. It is the fact that ammonia could be significantly decomposed in the presence of the TiO2-padded cotton fabric. Whereas, the TiO2-coated cotton fabric had the reduced photocatalytic decomposition of ammonia and high adsorption to ammonia owing to their acrylic binder layer. Finally, the reaction rate constant k and the adsorption equilibrium constant K values were determined through a curve-fitting method and the TiO2padded cotton fabric had the higher k value and lower K value than the TiO2-coated cotton fabric. r 2006 Elsevier Ltd. All rights reserved. Keywords: Ammonia decomposition; Photocatalyst; Cotton fabric; Padding; Coating
Corresponding author. Tianjin Municipal Key Laboratory of
Fiber Modification and Functional Fiber, School of Material Sciences and Chemical Engineering, Tianjin Polytechnic University, 63 Chenglin Road, Tianjin 300160, China. E-mail addresses: [email protected], dyefi[email protected] (Y. Dong). 1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2006.08.056
1. Introduction Antifreezes based on urea or ammonia compounds were widely applied in construction of concrete buildings in recent years; however this have resulted in the increased indoor air pollution owing to the
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emission of ammonia from indoor concrete wall. Therefore, the standards and regulations to control indoor ammonia pollution have been enacted and developed in China (Bai et al., 2002). Ammonia is harmful to human health and environment. Several researchers investigated indoor ammonia emission sources (Wirtanen et al., 2002, Tuomainen et al., 2000). It is believed that the UV/TiO2 process is a promising alternative for decomposing of various refractory contaminants in gaseous streams in the past decades. Coating of TiO2 by impregnation is widely used by most researchers because the technique is easy and does not require any complicated equipment. Materials including paper (Ichiura et al., 2003) nonwoven textile (Ku et al., 2001) have been developed for possible applications of TiO2 photocatalysis process. Padding and coating are the most common finish methods for application of chemical formulation to textile materials, in continuous processes. Padding consists of contacting the material with the formulation, usually by immersion, and squeezing the formulation out with squeeze rolls (Perkins, 2004). On the other hand, coating is generally based on the knife coater, which is a metering device that continuously spreads viscous liquids onto fabric. It contains a stationary knife blade positioned over a fabric support. The material applied to the fabric is fed to one side of the knife blade as the fabric moves continuously under the spreader. In this study, cotton woven fabrics were used as based materials for TiO2 photocatalysts because of their flexibility in engineering. Additionally, the large surface area and high hydrophilic function of the cellulose chain can provide more active sites for the deposition of TiO2 particles than the inorganic materials. The TiO2-loaded fabrics as the wall cloth or curtains to be used in house rooms in the future were obtained with aqueous dispersion containing TiO2 photocatalysts using padding or coating process. The heterogeneous decomposition of ammonia by UV/TiO2 process in a photoreactor fixed with TiO2-loaded cotton fabric was studied under various experimental conditions and a comparison between their performances of removing ammonia were made. 2. Experimental section 2.1. Preparation of TiO2 aqueous dispersion Degussa P-25 TiO2 powder was employed as the photocatalyst, without further treatment. Aqueous
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dispersion was prepared by mixing 1.5 g of Degussa P-25 TiO2 powder and the appropriate amounts of deionized water in an ultrasonic mixer. After 5 h of ultrasonic mixing, 0.125 g of polyglycol as the dispersing agent was added into aqueous dispersion and mixed under continuous magnetic stirring for 30 min. The TiO2 aqueous dispersion containing 3 wt% of Degussa P-25 TiO2 powder and 0.25 wt% of polyglycol was obtained. The average particle size was measured in the TiO2 aqueous dispersion to be 0.212 mm using a LA-300 particle sizer (Horiba Instrument Ltd., Japan). 2.2. Padding method for TiO2-loaded cotton fabrics Commercially bleached 100% cotton woven fabrics were used as support for TiO2 photocatalysts. And a commercial grade of reactive amino silicone additive AM-200 from Fuzhou Botex Chemicals Company (China) was used for fixing TiO2 particles on the fabrics. Before padding, the fabric samples were first treated with a solution containing 2 g l1 sodium carbonate and 2 g l1 of soap and boiled for 30 min, then thoroughly washed with cold water and dried at ambient temperature. And the fabric samples were padded twice (take up 7571%) with a treating solution containing various amounts of TiO2 aqueous dispersion and 10 g l1 AM-200 on a laboratory padding mangle. After being padded, the samples were immediately dried at 100 1C for 2 min, and then cured at 170 1C for 1 min. The amount of TiO2 loaded on padded fabric sample can be calculated through Q ¼ P C W 104 ,
(1)
where Q is the weight of loaded TiO2 per unit fabric weight (mg g1), P the take up (75%), C the concentration of TiO2 aqueous dispersion (3%), W the weight of TiO2 aqueous dispersion added to 1 l of treating bath (g). 2.3. Coating method for TiO2-loaded cotton fabrics The same fabrics as mentioned above were also used for coating process. Two commercial textile additives including a self-crosslinking acrylic binder TOW (BASF China Ltd.) and a Lutexal HIT thicker (BASF China Ltd.) were employed to form the coating formulation. Various amounts of TiO2 aqueous dispersion, 30 g l1 binder and 10 g l1 thicker were mixed by an electromagnetic stirrer for 20 min to establish a coating paste. Before
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coating, the cotton fabrics were first treated with a solution containing 30 g l1 water repellent agent EC-50 (Nikka Chemicals Co., Japan) by pad-dry process in order to keep the coating paste on the surface. And then the paste was continuously spread onto cotton fabrics on a laboratory knife coater (Werner Mathis AG, Switzerland). It is noted that the weight of coating paste deposited on the surface of fabric was controlled to be about 50% of fabric weight by adjusting the gap between the blade and the fabric. Finally, the coated fabrics were immediately dried at 100 1C for 3.0 min, cured at 170 1C for 1.5 min. The amount of TiO2 loaded on the coated fabric sample can be estimated by Q ¼ 50% C W 104 ,
(2)
where Q is the weight of loaded TiO2 per unit fabric weight (mg g1), C the concentration of TiO2 aqueous dispersion (3%), W the weight of TiO2 aqueous dispersion added to 1 l of coating paste (g). 2.4. Characterization of TiO2-loaded cotton fabrics The composition of TiO2-loaded fabrics was verified by using a Rigaku Xd/Max-2500 X-ray diffractometer (Rigaku Co., Japan) operating with Cu Ka radiation. Scanning electron microscopic (SEM) observations on specimens of TiO2-loaded fabrics was carried out using Hitachi S-670 electron microscope (Hitachi High-Technologies Co., Japan). 2.5. Decomposition of indoor ammonia The photocatalytic system consists mainly of a small stainless steel environmental condition simulated chamber (1 m3) and a glass photoreactor. The schematic diagram of photodegradation system is shown in Fig. 1. Four concrete wall pieces were prepared with 65.5 kg cement, 3.73 kg FDJ antifreeze containing 34 wt% urea compound (Tianjin Bocheng New Type Building Materials company, China) and a large amount of fine sand, gravel and water, and used for simulating the indoor concrete wall as a stable ammonia emitting source. Generally, the similar concentration of antifreeze was also used in construction of actual rooms in China. It is noticed that ratio of wall pieces weight to the volume of environmental chamber is equals to the ratio of actual indoor concrete weight to indoor volume and the proportion of the emitting surface area of wall pieces to the volume of environmental
chamber equals that of the emitting surface area of actual indoor concrete wall and indoor volume in the given rooms by partial covering of plastic film on wall pieces. The specifications of concrete wall pieces are presented in Table 1. The photocatalytic reactor was a 38 cm long glass plug flow tubular reactor with an effective volume 2.5 l. It contained a 2 cm i.d. quartz tube inside cooling water and housed a UV 365 nm lamp (Shanghai Philips-Yaming Company, China). The TiO2-loaded fabric with an area of 0.022 m2 was wound and fixed on the inner wall of the photoreactor. Environmental chamber and photoreactor were thoroughly cleaned with a solution containing 2g l1 sodium carbonate and deionized water, respectively. Gaseous ammonia emitted from wall pieces in environmental chamber and clean air were mixed in a gas mixer and maintained constant for providing ammonia gas with a steady state concentration. The temperature of the mixed gas flow in the photoreactor was kept at 2571 1C and humidity of the mixed gas flow reached steady by adjusting water bubbling humidity regulator and measured through flow meter and CR-052 thermo/humidity meter (Crecer company, Japan). A NH3 gas detector TG-2400KBP (Bionics Instrument Co. Ltd., Japan) was used to determine the concentration of gaseous ammonia in the air stream from the photoreactor. The reaction was not considered to reach balance until the concentration of gaseous ammonia in the photoreactor was kept steady for 5 min. The ammonia concentration in the photoreactor at this time was referring to the balance concentration of ammonia. Photocatalytic decomposition percentage of ammonia (Dp% at constant humidity and temperature was calculated by Dp % ¼
ðC 0 CÞ 100, C0
(3)
where Dp% is the photocatalytic decomposition percentage of ammonia, C0 is the initial concentration of ammonia (mg m3) and C is the residual concentration of ammonia (mg m3). 3. Results and discussion 3.1. Characterization of TiO2-loaded fabrics 3.1.1. XRD The X-ray diffraction (XRD) curves of untreated cotton fabric and TiO2-loaded cotton fabric were presented in Fig. 2.
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Air
1 Ammonia
0
3
5
4
2 10 Cooling water 8
Vent
9
12
7
6
11
Fig. 1. The schematic diagram of photocatalytic and testing system for ammonia in air, (0) wall pieces, (1) environmental chamber, (2) gas mixer, (3) dewater reagent, (4) water bottle, (5) activated carbon filter (6) thermo/humidity meter, (7) flow meter, (8) quartz shelter (inside cooling water) (9) TiO2-loaded fabric, (10) UV lamp, (11) photocatalytic reactor, (12) ammonia detector. Table 1 Specification of concrete wall pieces Dimension of each wall piece
520 mm 300 mm 200 mm
Total weight of four wall pieces Total volume of four wall pieces Total emission area of four wall pieces Total content of urea in four wall pieces Amount of ammonia emitted from four pieces in theory
312 kg 0.1248 m3 1.248 m2 1.114 kg 6.13 105 mg
As seen from Fig. 2, it is found that three major peaks (14.841, 16.381 and 22.681) of cellulose fiber were similar to those published in Weaver (1984). More importantly, a relatively strong reflection peak (25.281, peak a) and three weak peaks (37.841, peak b; 47.981, peak c; and d: 53.90–55.121, peak d) of anatase TiO2 were also
observed in XRD curves of the TiO2-loaded cotton fabrics. TiO2 grain sizes in two samples were determined from the broadening corresponding X-ray spectral peaks by Scherrer’s formula L ¼ 0.90l/(b cos y), where L is the grain size, l is the wavelength of X-ray radiation, and b is the line width at half maximum height. Based on XRD analysis, the average grain sizes of anatase TiO2 in two samples are 30–40 nm. 3.1.2. SEM Fig. 3 illustrates the SEM of untreated (panel a), TiO2-padded (panel b) and TiO2-coated cotton (panel c) fabrics. Fig. 3a (magnification ¼ 200 and 5000 ) showed the surface structure of cotton fiber: a round surface was very smooth to the touch. Fig. 3b (magnification ¼ 200 and 7000 ) depicts the morphological change in appearance of cotton fiber after TiO2-padding. Cotton fiber surface was
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1 Cotton, 2 TiO2-coated cotton, 3 TiO2-paded cotton a b
c
d 3
a
b
c
d 2
1 10
20
30
40
50
60
2 Theta (Deg.) Fig. 2. XRD spectrum of untreated, TiO2-padded and TiO2coated cotton fabrics.
covered with TiO2 particles, and seemed somewhat rough and uneven. Therefore, it is proved that TiO2 particles were loaded on cotton fiber by padding process, but their distribution on fiber surface was not quite even, possibly because of aggregation of some fine TiO2 particles. Fig. 3c demonstrates the quite different morphological surface of TiO2coated cotton fabric from the TiO2-padded cotton fabric, and a few TiO2 particles or their aggregations appeared the rough binder layer overlaid cotton fabric.
in this research. Also Dp% is increased with the prolongation of reaction time, and increasing tendency becomes level after reaction time of 15 min. Moreover, nitrate or nitrite was not found in the aqueous extraction of the TiO2-padded cotton fabric used for the decomposition of ammonia by ion chromatography. This indicated that one of major products of decomposition should be considered as nitrogen, which agreed with the result reported in a previous study (Morranega et al., 1979). It is noticed that increasing the amount of TiO2 aqueous dispersion in the treating bath or coating paste is accompanied with an enhancement in degradation of ammonia especially, in the presence of TiO2-padded cotton fabric. This is due to the fact that the increasing dosage of TiO2 aqueous dispersion leads to more TiO2 particles loaded on the fiber surface, which improved Dp%. Also it was found that TiO2-padded cotton fabric had higher Dp% than TiO2-coated cotton fabric at the same conditions. It is believed that TiO2-padded cotton fabric still keeps an open structure after treating, whereas TiO2-coated cotton fabrics has a close structure after coating owing to binder layer covered on the surface of fabric. The close structure is considered a more powerful limitation to light penetration than the open structure. Another possible explanation is that binder layer enhanced the TiO2 particles aggregation and retarded the TiO2 particles contacting with gaseous substances such as ammonia and oxygen and inhabited UV/ TiO2 process. 3.3. Initial concentration of ammonia
3.2. Dosage of TiO2 aqueous dispersion Experimental results of gaseous ammonia photodecomposition of TiO2-padded and coated cotton fabrics with different dosage of TiO2 aqueous dispersion in treating bath or coating paste are given in Fig. 4. It can be seen from Fig. 4 that the Dp% gradually increased at the beginning stage of reaction in the presence of the cotton fabrics without TiO2 catalysts, but then came back to about initial zero level during a short time. This may be due to the limited adsorption of ammonia onto the cellulose fiber surface, suggesting that ammonia could not be directly decomposed by UV light of 365 nm. The main reason is that the maximum wavelength for breaking the N–H bond is 220 nm (Morranega et al., 1979), which is shorter than 365 nm employed
Experimental results of ammonia photodecomposition under various initial ammonia concentrations in the presence of TiO2-loaded cotton fabrics were presented in Fig. 5. It is apparent that the ammonia concentration decreased significantly with increasing reaction time in the initial 10 min, and then there is a small concentration change after 10 min. This is because increasing ammonia concentration in inlet gas flow decreased the decomposition efficiency of ammonia. For experiment conducted at low initial ammonia concentration more than 80% of ammonia was decomposed, but the efficiency was reduced to less than 50% for experiment conducted for high initial ammonia concentration. This is possibly owing to the fixed amount of active sites on the surface of TiO2 available for the adsorption of ammonia
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Fig. 3. SEM images of untreated (panel a), TiO2-padded (panel b) and TiO2-coated cotton fabrics, (a) (panel a), (b) (panel b), (c) (panel c).
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a
b 100
80
80
60
60
D p%
Dp %
100
40
40
without P-25 30 gl-1 50 gl-1 100 gl-1
20
without P-25 50 gl-1 100 gl-1 150 gl-1
20
0
0 0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
35
40
45
Time (min)
Time (min)
Fig. 4. Effect of TiO2 aqueous dispersion dosage on Dp%, (a) padding, (b) coating. Padding: 30 g l1, Q ¼ 0.675 mg g1; 50 g l1, Q ¼ 1.125 mg g1; 100 g l1, Q ¼ 2.25 mg g1, coating: 50 g l1, Q ¼ 0.750 mg g1; 100 g l1, Q ¼ 1.50 mg g1; 150 g l1, Q ¼ 2.25 mg g1, temperature ¼ 2571 1C, RH ¼ 4572%, gas flow rate ¼ 0.5 l min1.
a
b 50
70 14.3 mgm-3 36.1 mgm-3 51.5 mgm-3 63.8 mgm-3
NH3 (mgm-3)
50
13.9 mgm-3 29.1 mgm-3 33.1 mgm-3 46.4 mgm-3
40 NH3 (mgm-3)
60
40 30
30
20
20 10 10 0
0 0
5
10
15
20
25
30
35
40
45
0
5
10
15
20
25
30
35
40
45
Time (min)
Time (min)
Fig. 5. Concentration change of ammonia under different initial concentrations, (a) padding, (b) coating, temperature ¼ 2572 1C, RH ¼ 4572%, gas flow rate ¼ 0.5 l min1.
molecules before the decomposition of ammonia. Similar results were observed and discussed for the decomposition of trichloroethylene (Ku et al., 2001) and ethylene (Obee and Hay, 1997) by UV/TiO2 process with saturated adsorption under fixed active sites. On the other hand, there are also some differences in ammonia concentration decrease levels especially, in the beginning reaction time between two TiO2-loaded cotton fabrics. Moreover, TiO2-padded cotton fabric had more powerful
decomposing performance of ammonia compared with TiO2-coated cotton fabric. 3.4. Relative humidity Fig. 6 showed that Dp% increased with reaction time and reached equilibrium in about 10 min in the case of TiO2-padded cotton fabric. When the humidity level was increased from 15% to 45%, the Dp% was enhanced to the highest stage and then
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b 100
100
80
80
60
60
Dp %
Dp %
a
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40
40 RH 15 % RH 45 %
20
RH 15% RH 45% RH 65%
20
RH 65 %
0
0 0
5
10
15
20
25
30
35
40
45
0
5
10
Time (min)
15
20
25
30
35
40
45
Time (min)
Fig. 6. Effect of relative humidity on Dp%, (a) padding, (b) coating, temperature ¼ 2571 1C, gas flow rate ¼ 0.5 l min1.
b
a 20 min
100
40 min
80
80
60
60
Dp %
Dp %
100
40
40
20
20
0 0.0
0.3
0.6
0.9
1.2
1.5
0 0.0
20 min
0.3
Gas flow rate (Lmin-1)
0.6
0.9
40 min
1.2
1.5
Gas flow rate (Lmin-1)
Fig. 7. Effect of gas flow rate on Dp%, (a) padding, (b) coating, temperature ¼ 2571 C, RH ¼ 4572%.
decreased with the increase in relative humidity. This is similar to formaldehyde investigated by Obee (Obee and Brown, 1995). The main reasons may be that TiO2 surface carries weakly or strongly bound molecular water, and hydroxyl groups from the hydrolysis of TiO2 can combine with ammonia by hydrogen bonding bond formation. On the other hand, both ammonia and water molecules are adsorbed on TiO2 surface through hydrogen bonding, thus resulting in the competition of the adsorption locations, and excessive water vapor on the catalyst surface will lead to a decrease in decomposition of ammonia because water molecules can occupy the active sites of the reactants on
the surface (Zhao and Yang, 2003). In contrast, TiO2-coated cotton fabric had the similar trend in the ammonia decomposition at different humidity level to that of TiO2-padded cotton fabric, and humidity level in a reactor reaching about 45% also led to the highest Dp% value. 3.5. Gas flow rate It can be found from Fig. 7 that two TiO2-loaded cotton fabrics remove ammonia at different gas flow rates in common. The Dp% of TiO2-padded cotton fabric gradually decreased with increasing gas flow rates from 0.3 to 1.2 l min1. On the other hand,
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increased gas flow rates just resulted in a slight decline in the Dp% of TiO2-coated cotton fabric particular in 20 min reaction time, possibly because of improved ammonia adsorption onto the sticky acrylic binder layer and reduced flow of ammonia by their close structure, particularly at the high gas flow rates. The differences of Dp% between 20 and 40 min of reaction time depended upon the level of gas flow rates. In the heterogeneous catalytic reaction, mass transfer and surface reaction are important factors controlling overall reaction rates (Wang et al., 1998). In lower flow rates, decomposition reaction rates of ammonia are significantly affected by mass transfer, but in higher flow rates, surface reaction controls the progress of reactions; and when flow rates become too high and residence time are shortened, an insufficient contact of gas stream with the catalyst layers occurs, lowing decomposition of ammonia. On the contrary, the influence of gas flow rates on decomposition reaction becomes insignificant for lower gas flow rates, and when gas flow rates were less than 0.6 l min1, Dp% was not remarkably enhanced by increasing reaction time in both cases. 3.6. Wash experiment Two TiO2-loaded cotton fabrics were washed with an aqueous solution containing 2 g l1 anionic detergent and 2 g l1 sodium carbonate through SW-12 washing testing machine (Shandong Laizhou Instruments Company, China). And then they were used for ammonia decomposition in photocatalytic system. For comparison, control experiment in which additive was absent in the padding bath or coating paste was carried out, experimental results are presented in Fig. 8. Fig. 8 exhibits that wash cycles caused a significant decrease in the decomposition levels of ammonia in the case of TiO2-padded cotton fabric. In contrast, the Dp% of TiO2-coated cotton fabric was slightly changed by wash cycles. It is believed that TiO2 particles on the surface of cotton fiber in control experiment could be easily removed since they were not strongly fixed without any additives. High adsorption and adhesion of amino silicon additive enhanced the fixation of TiO2 particles on the fiber surface in the case of padding. On the other hand, acrylic binder improved the resistance of TiO2-coated cotton fabric to water wash due to more TiO2 particles being kept with their excellent adhesion and close structure.
Padding Control padding Coating
100 80 Dp %
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60 40 20 0 W0
W2
W5
W10
Wash cycles
Fig. 8. Relationship between wash cycles and Dp%, washing condition: liquid ratio (fabric (g): washing bath (ml)) ¼ 1:50, 20 1C, 15 min/cycle.
3.7. Modeling of the experimental results The heterogeneous photocatalytic kinetics of target pollutants have been frequently described by the Langmuir–Hinshelwood rate equation (Ku et al., 2001; Zhao and Yang, 2003), for a plug flow reactor, the design equation can be written as follows: V 1 ln C 0 =C ¼ þ QðC 0 CÞ k ðC 0 CÞkK
(4)
where C is concentration of ammonia (mg l1), k reaction rate constant (mg l1 min1), K adsorption equilibrium constant (l mg1), V volume of reactor (l), Q gas flow rate (l min1), C0 the initial concentration of ammonia (mg l1). With the necessary operation and reaction parameters, Eq. (4) can be used as the design equation for the photocatalytic decomposition of gaseous ammonia in an annular reactor, and then by regressing Eq. (4) with respect to the experimental data of the effect of initial concentration of ammonia, k and K values can be determined. The regression results of the Fig. 5 by Eq. (4) for the photocatalytic decomposition of ammonia by UV/TiO2 process in both cases of padding and coating are shown as Fig. 9, and also the k and K values can be determined and are listed in Table 2. Table 2 indicats that the TiO2-padded cotton fabric was more than the TiO2-coated cotton fabric with respect to k the reaction rate constant. However with respect to K adsorption equilibrium
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500
(V/Q) (Co-C)-1
400 300 200 100
Padding Coating
0 0
25
50
75
100
ln (Co/C) (Co
125
150
175
200
-C)-1
Fig. 9. The regression results of Fig. 5 by the Eq. (4) for the photocatalytic decomposition of ammonia by UV/TiO2 process.
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significantly decomposed in the presence of TiO2padded cotton fabric at a certain operating conditions. On the other hand, TiO2-coated cotton fabric had the reduced photocatalytic decomposition of ammonia and relatively strong adsorption to ammonia molecules because of the acrylic binder layer covering TiO2 particles on the surface of fiber. Finally, Langmuir–Hinshelwood model was developed and used for formulating the photocatalytic reaction of ammonia in air, and both constants k and K values were obtained from the measured kinetic data through a curve-fitting method. TiO2padded cotton fabric exhibited higher k value and lower K value compared to TiO2-coated cotton fabric. Acknowledgments
Table 2 k and K values for the photocatalytic decomposition of ammonia Methods
k (mg l1 min1)
K (l mg1)
R
SD
N
P
Padding Coating
0.01457 0.00612
35.26 112.9
0.9955 0.9800
15.01 23.56
4 4
0.0045 0.0204
constant we have just reverse of k, TiO2-padded cotton fabric was much less effectively than TiO2-coated cotton fabric in terms of K. A possible explanation is that the coated fabric has a close structure presenting a low photocatalytic power. However, gaseous ammonia was strongly adsorbed onto the acrylic binder layer through hydrogen bonding of carboxyl group with ammonia molecules in the case of TiO2-coated cotton fabric. 4. Conclusion Combination of TiO2-loaded cotton fabrics produced using padding or coating methods with UV irradiation of 365 nm wavelengths can effectively eliminate gaseous ammonia in a photocatalytic reactor. The decomposition efficiency of ammonia was much affected by the dosage of TiO2 aqueous dispersion, initial ammonia concentration, relative humidity and gas flow rate. Furthermore, it is noticed that the ammonia removal depended also on the TiO2-loaded cotton fabric produce by padding or coating methods. It is fact that ammonia could be
The authors are very grateful to the support from the Ministry of Education, P. R. China, through ‘‘Joint Research Grant to Both Nankai University and Tianjin University’’ and ‘‘Trans-Century Training Programme Foundation for the Talents (2002–2048)’’.
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