Solar TiO2-assisted photocatalytic degradation of IGCC power station effluents using a Fresnel lens

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Chemosphere 71 (2008) 161–167 www.elsevier.com/locate/chemosphere

Technical Note

Solar TiO2-assisted photocatalytic degradation of IGCC power station effluents using a Fresnel lens J.M. Monteagudo a, A. Dura´n a

a,*

, J. Guerra a, F. Garcı´a-Pen˜a b, P. Coca

b

Department of Chemical Engineering, Escuela Te´cnica Superior de Ingenieros Industriales, University of Castilla-La Mancha, Avda. Camilo Jose´ Cela 3, 13071 Ciudad Real, Spain b Elcogas S.A., C.T.GICC Puertollano, Carretera de Calzada km 27, 13500 Puertollano, Ciudad Real, Spain Received 16 October 2007; received in revised form 22 October 2007; accepted 23 October 2007 Available online 19 December 2007

Abstract The heterogeneous TiO2 assisted photocatalytic degradation of wastewater from a thermoelectric power station under concentrated solar light irradiation using a Fresnel lens has been studied. The efficiency of photocatalytic degradation was determined from the analysis of cyanide and formate removal. Firstly, the influence of the initial concentration of H2O2 and TiO2 on the degradation kinetics of cyanides and formates was studied based on a factorial experimental design. Experimental kinetic constants were fitted using neural networks. Results showed that the photocatalytic process was effective for cyanides destruction (mainly following a molecular mechanism), whereas most of formates (degraded mainly via a radical path) remained unaffected. Finally, to improve formates degradation, the effect of lowering pH on their degradation rate was evaluated after complete cyanide destruction. The photooxidation efficiency of formates reaches a maximum at pH around 5–6. Above pH 6, formate anion is subjected to electrostatic repulsion with the negative surface of TiO2. At pH < 4.5, formate adsorption and photon absorption are reduced due to some catalyst agglomeration. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Cyanide; Formate; Photocatalysis; Solar UV radiation; Titanium dioxide

1. Introduction Waste waters used during this investigation come from the basic stripper of the Gasification Unit in an Integrated Gasification Combined Cycle (IGCC) Thermoelectric Power Station. According to the future environmental legislation, the discharge waters from these operations will require a more restrictive destruction of some pollutants such as cyanide and formate. The existing treatment for this wastewater stream consisted of an acid stripping for H2S and HCN removal, a basic stripping for ammonium removal, ozonation for cyanide and chemical oxygen *

Corresponding author. Tel.: +34 926 295300; fax: +34 926 295361. E-mail addresses: [email protected] (J.M. Monteagudo), [email protected] (A. Dura´n), [email protected] (J. Guerra), [email protected] (P. Coca). 0045-6535/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.10.067

demand (COD) decrease, and coagulation–flocculation to remove suspended solids and fluorides. Solar technology can be used as alternative to UV lamps to reduce the degradation process costs. In particular, TiO2-assisted photocatalytic degradation of pollutants using solar light has been successfully used being an economically viable process that can replace artificial light sources which are costly and hazardous (Muruganandham and Swaminathan, (2004). Usually, solar photocatalytic degradation reactions are carried out by using solar illumination directly (Gonc¸alves et al., 2005) or compound parabolic collectors (Augugliaro et al., 2002; Malato et al., 2002). In this study, a new low-cost solar concentrating installation based on a Fresnel lens has been used. It is provided with solar automatic tracking in order to maximize the absorption of solar energy obtaining a high density power

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in its focus. This lens has already been used by the authors with success to remove dyes in textile wastewaters (Monteagudo and Dura´n, 2006; Dura´n and Monteagudo, 2007). The investigation includes: (i) a comparative study of different oxidation systems, including H2O2 alone, solar/ H2O2, solar/TiO2 and solar/H2O2/TiO2, (ii) a study of the solar/H2O2/TiO2 system; to this end, a Multivariate Experimental Design (changing initial H2O2 and TiO2 concentrations) was performed and results from experimental tests were fitted using Neural Networks (NNs), (iii) a study of the effect of lowering pH on the degradation of formates after 90 min of reaction, when complete cyanide destruction has been reached (to avoid the formation of HCN gas which is highly toxic and volatile). This research was made under the optimal conditions selected in the previous step. 2. Materials/methods 2.1. Photodegradation experiments Commercial hydrogen peroxide (30% w/v, Merck) was added to the waste water at the beginning of each test to obtain the required initial concentration. The photocatalyst employed was technical grade TiO2 P-25 Degussa (anatase/ rutile = 3.6/1 wt, surface area 56 m2 g1). For experiments performed with TiO2, the waste water containing the appropriated concentration of the semiconductor powder (from 0.1 to 1.50 g l1) was magnetically stirred in the dark for 30 min to attain adsorption equilibrium between pollutants and TiO2 before irradiation was started. In all the cases, the volume of treated water was 400 ml. Orthophos-

phoric acid (0.1 M) and 6 M sodium hydroxide aqueous solutions were used to adjust pH of the waste water. The Fresnel lens was placed on a fixed support facing South in order to maximize the absorption of solar radiation. The lens is made of an acrylic material with a transmission factor of 0.92 in the range 400–1100 nm and is located in Ciudad Real (Spain), at latitude 38o. More characteristics can be found in literature (Monteagudo and Dura´n, 2006). The maximum solar energy concentration factor of 2644 is achieved at 75.7 cm from the lens. The photo-reactor was situated 2 cm away from the lens focus to avoid high temperatures with the consequent evaporation of the solvent. The temperature of solution during the treatment is related to the distance to the focal plane and was always kept around 40 °C (Fig. 1). The solar experiments were carried out under solar illumination on sunny days, between 12 AM and 2 PM (average solar light intensity over the duration of each experiment: 0.12 W cm2, being 317 W cm2 at the lens focus and 243 W cm2 at the reactor wastewater 2 cm away from the lens focus). To remove the TiO2 particles from the samples, the solution was filtered using a 0.22 lm syringe filter (Millex, Millipore). 2.2. Characterization of waste water and treated water The concentration of formates, fluorides and chlorides were analyzed by high performance ion chromatography using a Metrohm chromatograph fitted with an ASUPP5 250 column and an ionic conductivity detector at 10– 12 MPa of pressure. 0.7 ml min1 of a 50/50 (v/v) solution of NaHCO3 (1 mM) and Na2CO3 (3.2 mM) was used as

Fig. 1. Experimental set-up based on a Fresnel lens.

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mobile phase. 0.33 ml min1 of deionized water and 0.5 ml min1 of sulfuric acid (50 mM) were used for the suppression module. Total cyanide and ammonia concentrations were measured using a continuous flow analyser (FLOWSYS) combining higher performances and low running costs developed by SYSTEA. This method determines the sum of organically bound cyanides, free cyanides ions, complex compounds and simple bound metal cyanides, except cyanides from cobalt complexes. COD analyses were performed by using the ‘‘Closed Reflux Colorimetric Method’’. The oxygen consumed was measured using a calibration curve with a spectrophotometer (HACH, DR-2000). Sulphur content was determined by using a standard iodometric method according to the 77043-83 UNE Norm. pH was determined by using a commercial probe (Crison). Suspended solids were determined in accordance with APHA Standard Methods (1995). The degree of mineralization was determined by TOC analysis using a TOC-5050 Shimazdu analyser. 2.3. Experimental design A factorial design was used to optimize the operation conditions of the solar/H2O2/TiO2 process. Thus, a Central Composite Design was performed according to the methodology of response surface (Box et al., 1978) to analyze the influence of two variables (initial concentration of hydrogen peroxide, [H2O2], and initial catalyst concentration, [TiO2]), on the response functions: kinetic degradation rate constant of cyanide, k  CN , and kinetic rate degradation constant of formate, k  HCOO . The initial rate of degradation of cyanides and formates were found to obey pseudo-first order kinetics. 2.4. Neural network strategy The NN applied in this work is solved with two neurones and uses a simple exponential activation function and the strategy is based on a back propagation calculation. Further details can be found in literature (Dura´n et al., 2006). Input variables in this study are initial concentration of hydrogen peroxide and catalyst; output data are cyanides and formates degradation constant. The use of NNs allows to perform an analysis of the relevance of each variable with respect to the others (expressed as a percentage). To this end, a measure of the saliency of the input variables was made based upon the connection weights of the NNs (Nath et al., 1997). 3. Results and discussion 3.1. Physico-chemical pre-treatment A typical analysis of wastewater from the Gasification Unit of Elcoga´s S.A. GICC plant upstream its treatment at site, showed the following concentrations: [F] =

163

560 mg l1, [HCOO] = 1700 mg l1, [Cl] = 3600 mg l1, [SO42] = 1300 mg l1, [NH3] = 150 mg l1, [S2] = 3 mg l1, [COD] = 1240 mg l1, [TOC] = 980 mg l1; [Suspended solids] = 260 mg l1, [CN] = 10 mg l1 and pH 9.5. Wastewater was treated in a prior optimized step using CaCl2 (10 900 mg l1) as precipitate and a commercial anionic polyelectrolyte (8.9 mg l1) as flocculant to remove fluoride ions (99.8%) and suspended solids (92%). During this pre-treatment, the concentrations of cyanide, formate, sulphur, sulphate and ammonium and COD were not modified. The treated wastewater has good transmission of ultraviolet light, allowing a faster degradation of pollutants as previously reported (Monteagudo et al., 2004). 3.2. Comparison of different degradation processes Several experiments were made comparing different systems (H2O2, solar/H2O2, solar/TiO2, and solar/H2O2/ TiO2) with the objective of studying the viability of a solar TiO2 assisted degradation process for the destruction of cyanide-wastewaters. These tests were made under the following experimental conditions: T = 40 °C, [H2O2] = 13 g l1; [TiO2] = 0.8 g l1 and pH 9.5 to avoid the formation of HCN gas, which is highly toxic and volatile. Results in Fig. 2 show that: (i) cyanide can be oxidized in the presence of hydrogen peroxide alone (75% in 80 min) due to a direct molecular reaction with hydrogen peroxide to produce cyanate: CN +H2 O2 !CNO + H2 O

ð1Þ

(ii) formate degradation with H2O2 alone (without hydroxyl radicals generation) is not effective (10% degradation, Fig. 2b) which indicates that the oxidation reaction of this compound occurs mainly through a radical pathway; (iii) Cyanide and formate degradation under solar irradiation in the presence of TiO2 alone was not effective since no oxidizing agent is present and generation of ÆOH radicals is negligible. On the other hand, the TiO2 surface is negatively charged at pH 9.5 (Zero Point Charge, ZPC, of TiO2 is between 5.6 and 6.4) and anionic pollutants are subjected to electrostatic repulsion with the negative surface of TiO2. (iv) Cyanide degradation reaction under concentrated solar irradiation by a Fresnel lens in the presence of H2O2 was improved (100% degradation in 50 min) when compared with the processes that use hydrogen peroxide alone due to the contribution of the radical pathway. Concentrated solar radiation can be enough to cause the H2O2 photolysis and in consequence also involves the in situ generation of highly potent chemical oxidants such as the hydroxyl radical (ÆOH) and their subsequent reaction with cyanide (Reaction (2)) CN +2 OH !OCN + H2 O

ð2Þ

The resulting cyanate will react under continued photolytic oxidation to produce carbon dioxide (which ultimately

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cyanides and formates is feasible and will be studied deeply below to optimize the operation conditions.

100 H2O2 H 2 O2 So lar/H lar/H2O2 2 O2

%f ormate degradation

80

3.3. Degradation study with the solar/H2O2/TiO2 process

So lar/TiO lar/TiO2 2 So lar/H lar/H2O2/TiO2 2 O2 /TiO 2 60

40

20

0

% cyanide degradation

100

80

60

H2O2 H 2 O2 So lar/H lar/H2O2 2 O2 So lar/TiO lar/TiO2 2

40

So lar/H lar/H2O2/TiO2 2 O2 /TiO 2 20

0 0

20

40

60

80

100

Time (min)

Fig. 2. Degradation of pollutants under different processes: (a) cyanide reduction and (b) formate reduction (experimental conditions: [H2O2] = 13 g l1; [TiO2] = 0.8 g l1; pH 9.5; T = 40 °C).

forms bicarbonate) and either nitrogen gas, nitrite or nitrate (Young and Jordan, 1995; Malhotra et al., 2005) OCN þ 3  OH ! HCO 3 þ 1=2N2 ðgÞ þ H2 O   þ  OCN þ 6 OH ! HCO 3 þ NO2 þ H þ 2H2 O

ð3Þ ð4Þ

 þ OCN þ 8  OH ! HCO 3 þ NO3 þ H þ 3H2 O

ð5Þ

Regarding the heterogeneous catalysis (solar/H2O2/ TiO2 system), it can be seen that the addition of TiO2 (without optimization) slightly decreases the reaction rate of cyanide (although cyanide can be successfully degraded in 90 min) whereas the presence of catalyst increases the degree of degradation of formate. These effects suggest a molecular oxidation mechanism for cyanide destruction and a radical pathway for formate degradation, since the presence of TiO2 produces an extra generation of hydroxyl radicals (Zhan and Tian, 1998; Bandara et al., 1999; Galindo et al., 2000; Daneshvar et al., 2003) that only affects significantly the formate oxidation rates. Therefore, from these results, it can be concluded that solar TiO2 assisted degradation of wastewaters containing

3.3.1. NNs fitting A Central Composite Design was developed for the experimental tests performed with the solar/H2O2/TiO2 system. Initial H2O2 and catalyst concentrations leading to an optimal degradation were found by using this statistical methodology. The pseudo-first order kinetic rate constants of degradation of cyanide and formate calculated from each experiment were taken as the response functions. The Central Composite Design matrix and the values of the responses are shown in Table 1. Experimental results and NNs fittings of these constants are in good agreement, with an average error lower than 2.9% and 6.3% for cyanide and formate degradation constants, respectively. The equation  and parameters for the fitting of k  CN and k HCOO using NNs are shown in Table 2. N1 and N2 are general factors related to the first and the second neuron, respectively. W11 and W12 are the contribution parameters to the first neuron and represent the influence of each of the two variables in the process, [H2O2] and [TiO2]; W21 and W22 are the contributions to the second neuron and are related to the same variables. The results of a saliency analysis on the input variables for each NN (%) are shown in Table 3. It can be concluded that the initial concentration of hydrogen peroxide is the most significant factor affecting the degradation kinetic rate constants of cyanide and formate. In the case of cyanide, this high contribution (87.7%) is due to the major role of the direct molecular reaction with H2O2. In the case of formate, the high contribution of H2O2 (81%) is due to its effect on photocatalytic degradation rate which is limited by the recombination of photogenerated hole–electron pairs: when hydrogen peroxide combines with TiO2 inhibits the electron–hole recombination at the semiconductor surface accepting a photogenerated electron from the

Table 1 Two-factor central composite design matrix and values of the response functions Exp.

H2O2

TiO2

H2O2 (g l1)

TiO2 (g l1)

1 k CN ðmin Þ

1 k HCOO ðmin Þ

1 2 3 4 5 6 7 8 9 10 11

+1 1 +1 1 (+a) (a) 0 0 0 0 0

+1 +1 1 1 0 0 (+a) (a) 0 0 0

21.49 4.51 21.49 4.51 25.00 1.00 13.00 13.00 13.00 13.00 13.00

1.29 1.29 0.31 0.31 0.80 0.80 1.50 0.10 0.80 0.80 0.80

0.1570 0.1263 0.2257 0.1457 0.1660 0.0162 0.1870 0.1985 0.2120 0.2010 0.1993

0.0065 0.0150 0.0079 0.0087 0.0055 0.0038 0.0098 0.0085 0.0062 0.0067 0.0064

(Process: solar/H2O2/TiO2).

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Table 2  Equation and parameters of NNs fitting of k  CN and k HCOO Neural network fitting Equation (see footnote)

k [min1]= N1 (1/(1 + 1/EXP([H2O2]a W11 + [TiO2]a W12))) + N2 (1/(1+1/EXP([H2O2]a W21 + [TiO2]a W22)))

Weight factors

Parameter

Values of neurons and factors to obtain the degradation 1 kinetic rate constant of cyanide, k  CN ðmin Þ

N1 W11 W12

Neuron [H2O2] [TiO2]

0.477 8.706 0.031

0.024 13.457 4.518

N2 W21 W22

Neuron [H2O2] [TiO2]

0.417 0.922 0.293

0.031 18.836 2.770

a

Values of neurons and factors to obtain the degradation 1 kinetic rate constant of formate, k  HCOO ðmin Þ

Parameters values in equations must be previously normalized to the (0.1) interval.

Table 3 Saliency analysis of the input variables for the NN (%) Neural network output

Parameters [H2O2]

[TiO2]

Degradation kinetic rate constant 1 of cyanide, k  CN ðmin Þ

87.76

12.24

Degradation kinetic rate constant 1 of formate, k  HCOO ðmin Þ

81.02

18.98

conduction band and thus promotes the charge separation, according to Eq. (6): e CB +H2 O2 !OH + H

ð6Þ

3.3.2. Effect of [H2O2] and [TiO2] Equation shown in Table 2 allows a simulation analysis of the effect of the studied variables on the value of the degradation constants. Thus, the influence of [H2O2] and [TiO2] on cyanides and formates degradation kinetic constants are shown in the 3-D (Fig. 3). It can be seen that when the concentration of hydrogen peroxide increases, the degradation rate increases in both cases until an optimal H2O2 concentration is achieved (12 g l1 for cyanides and 3.5–6.5 g l1 for formates). The increase in the formate degradation rate with the addition of hydrogen peroxide was attributed to the inhibition of electron–hole recombination at the semiconductor surface and to the increase in the concentration of photocatalytically generated hydroxyl radicals. However, when H2O2 is in excess, it may act as a hole or Æ OH scavenger. Above the optimal concentration of H2O2, the reaction rate of cyanide smoothly decreases which indicates that hydroxyl radicals plays a minor role in the degradation of cyanide. However, the formate degradation rate sharply decreases for H2O2 concentrations higher than optimal value (specially for high catalyst concentration) confirming the scavenger effect and consequently the importance of the radical pathway in the oxidation reaction. Regarding to the effect of TiO2 (Fig. 3a), it can be concluded that it practically does not affect the cyanide degra-

Fig. 3. NNs simulation of the effect of initial H2O2 and TiO2 concentrations on degradation kinetic rate constants under the solar/H2O2/TiO2 system: (a) cyanide degradation and (b) formate degradation (conditions: pH 9.5; T = 40 °C).

dation rate. As expected, the degradation occurs through a molecular route which does not need the presence of the catalyst.

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On the contrary, the addition of TiO2 significantly increases the degradation rate of formates (Fig. 3b) when H2O2 is not in excess, suggesting again that the extra generation of hydroxyl radicals due to the catalyst presence are necessary for formate degradation. Optimum conditions selected from this analysis correspond to the maximum formate degradation rate: [H2O2] = 6.5 g l1 and [TiO2] = 1.5 g l1. Under these operation conditions, cyanides degradation is still very fast (k  CN ¼ 0:18min1 ) and formates removal (limiting step) is highly improved (k  HCOO increases from 0.0098, obtained in the previous study in Section 3.2, to 0.02 min1). However, this kinetic constant is still very low since the initial conditions (pH 9.5) inhibit the degradation rate of formates. 3.3.3. Effect of pH Thus, in order to complete the study, and trying to accelerate the degradation rate of formates, a new set of experiments was performed changing pH conditions from 3 to 7, close to the neutrality point of TiO2 (Fig. 4). pH was lowered after 90 min of reaction to ensure that 100% of cyanides had been previously degraded. The values of initial H2O2 and TiO2 concentrations were 6.5 g l1 and 1.5 g l1, respectively (optimal conditions previously determined by NNs simulation). As already mentioned, the ZPC for TiO2-P25 is at pH between 5.6 and 6.4. Hence, at more acidic pH values, the TiO2 surface is positively charged and favours the adsorption of formates which are negatively charged. As it can be seen from Fig. 4, the photooxidation efficiency of formate reached a maximum (97% in 180 min) at pHs 5–6. Above pH 6, formate anion is subjected to electrostatic repulsion with the negative surface of TiO2, and the degradation efficiency decreases. At pH < 4.5, TiO2 particles agglomeration reduces the formate adsorption as well as photon absorption (Muruganandham and Swaminathan, 2004), decreasing the photodegradation efficiency. To sum it up, it can be concluded that when the pH is lowered from 9.5 to 5–6, the formate degradation kinetic

100 97

Formate degradation, %

97 89

80

78

75

71

60

40

20

0

3

4

5

6

7

9.5

pH Fig. 4. Effect of pH on formate degradation under the solar/H2O2/TiO2 process (conditions: [H2O2] = 6.5 g l1; [TiO2] = 1.5 g l1; T: 40 °C; reaction time: 180 min).

rate constant increases up to 0.42 min1. Thus, wastewaters containing cyanides and formates must be treated according to the following methodology when using a Fresnel lens to concentrate solar energy and activate a photo TiO2 assisted process: (a) destruction of cyanides at pH 9.5 for 90 min, (b) reduce pH to 5–6, and (c) destruction of formates at pH 5–6 for another 90 min. 3.3.4. Treated water final specifications The use of a low-cost catalyst and their activation with solar light can offer an economical and practical alternative for destruction of contaminants from thermoelectric power stations and similar industries. Under the optimum conditions reached in this investigation (pH 9.5; pH after 90 min = 6; T = 40 °C; [H2O2] = 6.5 g l1; [TiO2] = 1.5 g l1), the treated waste water shows the following results (below pouring specifications) after 180 min: [F] = 1 mg l1; [HCOO-] = 51 mg l1; [CN] = 0 mg l1; [NH3] = 22 mg l1; [Suspended solids] = 20 mg l1; 1 1 [COD] = 50 mg l ; [TOC] = 14 mg l . Acknowledgments Financial support from the Programa Ciencia y Tecnologı´as Medioambientales (Ministerio de Educacio´n y Ciencia, Spain, CTM2006-03170/TECNO) is gratefully acknowledged. We thank Elcoga´s S.A. for their collaboration and valuable comments and suggestions. References APHA, 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed., American Public Health Association, Washington DC, USA. Augugliaro, V., Baiocchi, C., Prevot, A.B., Brussinno, M.C., Garcı´aLo´pez, E., Loddo, V., Malato-Rodrı´guez, S., Marci, G., Palmisano, L., Pramauro, E., 2002. Sunlight photocatalytic degradation of azodyes in aqueous suspension of polycrystalline TiO2. Fresen. Environ. Bull. 11, 459–465. Bandara, J., Mielczarski, J.A., Kiwi, J., 1999. Photosensitized degradation of azo dyes on Fe, Ti, and Al oxides. Mechanism of charge transfer during the degradation. Langmuir 15, 7680–7687. Box, G.E.P., Hunter, W.G., Hunter, J.S., 1978. Statistics for Experimenters: An Introduction to Design. In: Data Analysis and Model Building. Wiley, New York. Daneshvar, N., Salari, D., Khataee, A.R., 2003. Photocatalytic degradation of azo dye Acid Red 14 in water: Investigation of the effect of operational parameters. J. Photoch. Photobio. A 157, 111–116. Dura´n, A., Monteagudo, J.M., Mohedano, M., 2006. Neural networks simulation of photo-fenton degradation of reactive blue 4. Appl. Catal. B Environ. 65, 127–134. Dura´n, A., Monteagudo, J.M., 2007. Solar Photocatalytic Degradation of Reactive Blue 4 Using a Fresnel Lens. Water Res. 41, 690–698. Galindo, C., Jacques, P., Kalt, A., 2000. Photodegradation of the aminoazobenzene acid orange 52 by three advanced oxidation processes: UV/H2 O2 UV/TiO2 and VIS/TiO2 – Comparative mechanistic and kinetic investigations. J. Photoch. Photobio. A 130, 35–47. Gonc¸alves, M.S.T., Sameiro, M., Pinto, E.M.S., Nkeonye, P., 2005. Degradation of C.I. reactive Orange 4 and its simulated dyebath wastewater by heterogeneous photocatalysis. Dyes Pigment. 64, 135–139.

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