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Application of catalytic wet air oxidation for the treatment of H-acid manufacturing process wastewater
Water Research 36 (2002) 1947–1954
Application of catalytic wet air oxidation for the treatment of H-acid manufacturing process wastewater Wanpeng Zhu*, Yuejing Bin, Zhonghe Li, Zhanpeng Jiang, Tong Yin Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China Received 9 January 2001; received in revised form 5 September 2001; accepted 11 September 2001
Abstract Four types of powder catalysts whose main active components are copper (Cu), cerium (Ce), cadmium (Cd) and cobalt-bismuthide (Co-Bi) are prepared with the method of the co-deposition and are evaluated through the catalytic wet air oxidation (CWAO) treatment of H-acid solution. The comparison of the efficiencies of different catalysts shows that Ce3Cu1 (3:1) catalyst is the best one. When the reaction temperature is 2001C, oxygen partial pressure is 3 MPa, pH value is 12, and reaction time is 30 min, the COD removal rate is over 90%. All the H-acid is decomposed in 5 min 2 and is oxidized into NH+ 4 , SO4 , formic acid, acetic acid and other end products. The pH value can greatly affect the COD removal and the production of organic acid. CWAO process not only can get a high reaction rate, but also can oxidize the short-chain organic acid. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Wet air oxidation (WAO); Catalytic wet air oxidation (CWAO); Dye intermediate; H-acid; 1-amino-8-naphthol-3, 6-disulfonic acid
1. Introduction Wet air oxidation (WAO) is a method of oxidizing dissolvable or suspended organic compounds as well as reducible inorganic compounds with oxygen or air under the circumstances of high temperature and high pressure in liquid phase. The application of traditional WAO is limited because of the high operation temperature, high operation pressure and long operation time [1]. Catalytic wet air oxidation (CWAO) was a new technology developed on the basis of WAO in the 1970s. It is a process that can speed the reaction, lower the reaction temperature and pressure with catalyst. It is an available method used in disposing high-concentration effluents, as well as poisonous, detrimental and hardly degradable wastewater. Copper ion is the most effective catalyst that works in the homogeneous state. It was used for the practical treatment of wastewaters discharged from *Corresponding author. Tel.: +86-10-627-84527; fax: +8610-627-91233. E-mail address: [email protected] (W. Zhu).
petrochemical industries [2]. Heterogeneous catalysts that can be easily separated from the reaction medium in batch or continuous processes are better suited than soluble catalysts, which have to be recovered by an additional separation process [3–5]. Different transition metal oxides have been used, but the main drawback is their dissolution in the corrosive reaction mixture [6–9]. Imamura et al. [10] studied the catalytic effect of noble metals on the wet oxidation of phenols and other model pollutant compounds, and found that ruthenium, platinum and rhodium were more active than homogeneous copper catalyst [10]. However, noble metals are of high cost, which severely affects the economy of noble catalysts. H-acid (1-amino-8-naphthol-3, 6-disulfonic acid) is an important dye intermediate. It is widely used in chemical industry for the synthesis of direct, acidic, reactive and azoic dye, as well as in the pharmaceutical industry. Under certain conditions, naphthalene, as a raw material, is used to manufacture the H-acid through a series of chemical processes such as sulfonation, nitration, neutralization, reduction, alkaline dissolving, acid
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precipitation and so on. Since the production process of H-acid is complicated and the utilization ratio of raw materials is low, the wastewater from the manufacturing processes is rich in various substituted derivatives of naphthalene compound and is of dark color and strong acidity. Organic substances in dye intermediate wastewater are often aromatic compounds substituted by some groups, such as aNH2, aNO2, aSO 3 ; etc. They are extremely toxic to organisms. The biological processes cannot effectively degrade these substances and decolorize the H-acid wastewater [11]. As aromatic ring with aSO3H is easily dissolved in water, the general chemical and physical method is very inefficient. Therefore, H-acid wastewater is one of the most hardly treated wastewaters so far [12,13]. In this paper, we studied the CWAO treatment of Hacid solution. The effect of temperature, pH value on the CWAO treatment of H-acid was investigated; support catalysts were prepared and evaluated.
2. Experiments 2.1. Experimental apparatus The GS-0.5 autoclave equipped with a stirrer and a heating device that keeps the constant temperature was used in experiments. It has a cooling coil and two sampling outlets (liquid- and gas-phase outlet). The operation process is presented in Fig. 1.
order to control the pH value, Na2CO3 was added as a precipitation assistant. Four groups of catalysts (Cu group, rare earth group, Cd group and Co/Bi group), 15 species had been prepared in the research processes. The evaluation criteria of catalysts performance are the COD removal of wastewater, the stripping amount of active material of catalyst after 10 or 30 min reaction under given conditions and the lowest pH value during reaction processes. The lowest pH value during reaction processes is a very important factor. The equipment would be easily eroded and the stripping amount of catalytic active metals would increase if the acidity in solution were too high. 2.3. Experimental materials and methods Naphthalene is an important representation of aromatic compounds. It can be used to synthesize many dye intermediates of naphthalene derivatives, such as H-acid, J-acid (5-hydroxy-2-naphthylamine-7-sulfonic acid), chromotropic acid (1,8-dihydroxynaphthalene-3, 6-disulfonic acid) and so on [13]. The H-acid solution was treated with CWAO in our laboratory. In order to study the reaction process and eliminate the disturbance of impurities, reagent H-acid (C.P.) was used to prepare the solution. The molecular structure of H-acid is as follows: H2 N
OH
2.2. Development of catalyst The catalyst was prepared using deposition or codeposition methods [14]. Metal nitrate was employed as the activating material of catalyst in experiments. In
HO3S
Fig. 1. Scheme of WAO process.
SO3Na
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3. Discussion and analysis
CWAO. H-acid wastewater from an actual industry mainly contains intermediates such as H-acid, T-acid (1-naphthylamine-3, 6,8-trisulfonic acid), chromotropic acid, high concentration of salts and so on. The concentrations of H-acid, T-acid, chromotropic acid and Na2SO4 in the simulative wastewater used in experiments were 10, 10, 2, and 150 g/L, respectively. In experiments, the temperature was fixed at 2001C, the amount of catalyst added was 0.2 g/300 mL, and the pH value was 12.0. The results indicate that COD removal can approach 85.0% and 91.3% after 10 and 30 min reaction, respectively, and pH value can be kept at high levels. The lowest pH value is about 5.25. So Ce3Cu1 catalyst has excellent catalyst activity.
3.1. Catalysts selection
3.2. Factors influencing CWAO process
All catalysts within the same group have been compared with each other in this study. The best catalyst was selected from each group that are Cu2Zn1 (weight ratio of Cu and Zn is 2/1, calcination temperature is 6001C), Ce3Cu1 (weight ratio of Ce and Cu is 3/1, calcination temperature is 6001C), Cd2Ce1Cu1 (weight ratio of Cd, Ce and Cu is 2/1/1, calcination temperature is 6001C) and Co1Bi1 (weight ratio of Co and Bi is 1/1, calcination temperature is 6001C). The time dependence of COD removal using different catalysts is shown in Fig. 2. The results reported in Fig. 2 demonstrate that the Ce3Cu1 composite catalyst is the best catalyst among the four catalyst groups. The Ce3Cu1 composite catalyst was selected as the catalyst used in further study on CWAO treatment of dye intermediates. To check the feasibility of practical application of Ce3Cu1 catalyst, simulative wastewater was used in
There are many factors, such as reaction temperature, oxygen pressure, initial pH value of wastewater and dosage of catalyst, which can affect the oxidation of organic materials.
In experiments, H-acid was dissolved in de-ionized water to prepare different concentration solutions. The initial pH value of the solution was adjusted. The experiments were carried out according to the following steps: (1) take 300 mL H-acid solution in the autoclave (model GS-0.5, Weihai, China); (2) add definite amount of catalyst; (3) turn on the heater; (4) fill oxygen into the autoclave. It was the time zero when temperature and pressure were up to the setting values. Samples were taken every 5 min, COD and pH values of the samples were measured.
CODremoval rates /(%)
100 90 80 70 60 50 40 30 20 10 0 0
50
100
150
time (min) Ce3Cu1
Cu2Zn1
Cd2Ce1Cu1
Co1Bi1
Fig. 2. Time dependence of COD removal with different catalysts.
3.2.1. Influence of reaction temperature on CWAO process Ce3Cu1 catalyst was used in CWAO process at the reaction temperatures of 1801C, 2001C and 2201C. Other reaction conditions were fixed: oxygen partial pressure was 3.0 MPa; simulated wastewater concentration was 10 g/L; pH value was 12.0. The experimental results showed that COD removals after 30 min reaction were 75.6%, 92.0% and 92.9% in turn at the reaction temperatures of 1801C, 2001C and 2201C. The corresponding lowest pH values during reaction were 5.76, 6.50 and 7.35, respectively. COD removal reached 92%, the lowest pH value during the reaction was almost neutral. 2001C is a feasible reaction temperature. 3.2.2. Influences of catalyst dosage on catalytic effects H-acid solution (concentration was 10 g/L, initial pH value was 12.0) was treated with CWAO under the following conditions: temperature 2001C; oxygen partial pressure 3.0 MPa. The experimental results showed that COD removals after 30 min reaction were 53.5%, 90.7% and 92.0% when the catalyst dosages were 0.0, 0.1 and 0.2 g/L in turn. The corresponding lowest pH values during the reaction were 3.76, 8.45 and 6.50, respectively. Catalyst dosage has significant influences on COD removal and the lowest pH value of solution. The experimental results illustrate that COD removal in CWAO is 36% higher than that in the process without catalyst, and the lowest pH value is also higher when the catalyst dosage is 0.1 g/300 mL. Without catalyst, the pH value of solution can drop to 3.76. The catalyst is very significant; it can not only accelerate the reaction but
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Table 1 Effects of Ce3Cu1 catalyst on CWAO under different pH value Initial pH of wastewater
8.0 10.0 12.0
Amount of dissolved metal (mg/L) Highest
Lowest
0.91 0.48 0.44
0.18 0.09 0.08
COD removal at 30 min (%)
The lowest pH value in reaction
81.8 71.7 68.3
2.22 3.00 5.00
also retard the equipment corrosion caused by low pH value. The catalyst dosage 0.1–0.2 g/300 mL is acceptable.
12
3.2.3. Influence of initial pH value of H-acid solution When oxygen partial pressure was 3.0 MPa, and Ce3Cu1 catalyst dosage was 0.2 g/300 mL. CWAO experiments of H-acid solution (concentration was 10 g/L) were performed under different initial wastewater pH values. The results are shown in Table 1. With the increase of pH value, the amount of dissolved active metal ions is reduced, and with the increase of the lowest pH value in the reaction, the equipment corrosion is retarded. COD removal decreases rapidly as the pH value increases. COD removal is over 80% when the pH value is 8 and the retention time is 30 min, which is acceptable in practical processes.
8
pH value
10
6 4 2 0 0
30
60
90
120
time (min) without catalyst
Ce3Cu1
Ce1Fe3
Fig. 3. pH value versus operation time in CWAO.
3.3. The CWAO process of H-acid 3.3.1. The variation of pH value in reaction The pH values varying with time have the same regularities in the WAO process and the CWAO process of H-acid. The pH values increase at first, then decrease, and increase at last. Moreover, the reaction is closely correlated to the variety of pH value. The experiment was conducted under the following three operation conditions: (1) without catalyst; (2) with the less efficient Ce1Fe1 (Ce1Fe3 in Fig. 3) catalyst (in the operation); (3) with the more efficient Ce3Cu1 (Ce3Cu1 in Fig. 3) catalyst. The reaction temperature is 2001C. Fig 3 gives the time course of pH. As seen in Fig. 3, the shapes of the pH curve are similar in the three WAO (or CWAO) processes. The pH value decreases rapidly to a nadir point at first, and then increases gradually. The decreasing rate and changing range of the three pH curves differ from each other. The more active the catalyst is, the shorter the time to nadir point is, and the smaller the decreasing range , the higher the final pH value. It indicates that the decomposition rate of short-chain organic acid is higher. Those shortchain organic acids are much more resistant to WAO and CWAO oxidation than the organic compounds
present in the original wastewater [1]. Therefore, the change of pH value in the reaction process can be used to evaluate the activity of the catalyst. Experiments were carried out under two operation conditions, one was an operation process without catalyst [see Fig. 4(a)], and another was an operation process with Ce3Cu1 catalyst [see Fig. 4(b)]. The relation between the pH variety and the COD removal is plotted in Fig. 4. It can be inferred from Fig. 4 that at the beginning of operation, the pH value decreases rapidly and COD removal increases rapidly. COD removal increases slowly after the pH value reaches the nadir point. The pH value reaches the nadir point in 5 min and the decrease range is small if Ce3Cu1 catalyst is used. It indicates that some short-chain organic acids among the intermediate products have been oxidized in CWAO process, but this phenomenon does not occur when there is no catalyst in the reaction system. COD removal can reach 90% within 20 min operation in CWAO process, but the COD removal is still below 80% after 120 min operation in WAO process. The advantage of CWAO over WAO is very obvious.
80
50 40 30 20 10 0 0
30
60
90
60 40 20
pH value
60
90
30
60
90
120
dissolving amount of Cu
Fig. 5. The relation between the pH value and the amount of dissolved Cu.
COD removal rate (%)
pH value
80
time (min)
100 90 80 70 60 50 40 30 20 10 0 120
time (min)
pH value
100
COD removal
10 9 8 7 6 5 4 3 2 1 0 30
120
0
(a) Without catalyst
0
140
0
120
time (min)
pH value
stripping of Cu (mg/L)
60
10 9 8 7 6 5 4 3 2 1 0
160
70
COD removal rate (%)
pH value
10 9 8 7 6 5 4 3 2 1 0
pH value
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COD removal
(b) With Ce3Cul catalyst Fig. 4. The relation between the pH variety and the COD removal.
3.3.2. The relation between pH value and amount of dissolved active component The concentration of the metal ion in the solution, which represents the amount of dissolved active component from catalyst, after CWAO process was measured with an atomic absorption spectrophotometer (Hitachi model 180-80, Japan). The experimental results of CWAO of H-acid with Cu catalyst alone are shown in Fig. 5. The two curves illustrate the relation between pH value and the amount of dissolved Cu2+. As known from Fig. 5, the amount of dissolved catalyst activity components is closely related to the variation of pH value. The amount of dissolved catalyst activity components decreases with the increase of pH. It reaches its highest point at the same time when pH falls to its lowest point and falls to the bottom of the curve when the pH reaches the top of the peak. The results show
that the loss of activity is less when the catalyst is more active. This is understandable as the more active catalyst can shorten the reaction time and restrain the decrease of pH value during the operation process.
3.3.3. Variation of anion in reaction process There are two sulfonic groups and one amino group in the H-acid molecule. In the WAO process, theoretically, aSO3H is easily released from the naphthalene ring and oxidized into SO2 4 . aNH2 can probably be released from the naphthalene ring and either be oxidized into NH+ or be oxidized into NO 4 2 , NO3 or N2. The effluents of CWAO treatment of H-acid solution (prepared with de-ionized water, its initial concentration of SO2 was zero) were measured with an ion 4 chromatograph (Dioner model DX100, USA). It was found that there was a lot of SO2 4 but no NO2 or NO3 in effluents throughout the range of reaction time. The experimental results of SO2 4 concentration varying with operation time are shown in Fig. 6. In Fig. 6, a little SO2 4 appears in the original H-acid solution. It indicates that hydrolysis of sulfonic group takes place during heating process although no oxygen presents. When the pH value in the influent is 8, the initial SO2 concentration is 1084 mg/L, which is the 4 highest among three experimental conditions. Under the other two conditions (pH=12, with catalyst and without catalyst) SO2 concentrations are 475 and 537 mg/L, 4 respectively. According to the results listed above, it is preferred for aSO3H to hydrolyze at lower pH value. In addition, SO2 increase is faster than that of other 4 conditions when pH value is 8 and Ce3Cu1 catalyst is used. It indicates that H-acid is oxidized faster and Hacid oxidation is coupled to COD removal. The calculated results show that most of the aSO3H
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NH4+ concentration(mg/L)
6000 5000 4000 3000 2000 1000
300 250 200 150 100 50 0 0
0 0
30
60
90
30
120
time (min)
3.3.4. Cations in solution in reaction process Neither NO 2 nor NO3 were detected in effluent of CWAO process although H-acid had an amino group. N + is probably oxidized into NH+ 4 or N2. NH4 in effluent was detected with ion spectrometry; the results are shown in Fig. 7 (pH=12, T ¼ 2001C, catalyst was Ce3Cu1). The concentration of NH+ 4 in solution is 474 mg/L (18 10 1000/368) if the amino groups in H-acid molecule in 10 g/L solution are all transformed into + NH+ in the 4 . However, the concentration of NH4 effluent after being treated for 120 min is only 320 mg/L; this means that some amino groups are transformed into other compounds such as N2 and so on.
3.3.5. The variation of short-chain organic acid in solution during reaction Theoretically, organic compounds can finally be degraded into CO2, H2O and other harmless product in WAO or CWAO processes. But actually, there are many short-chain organic acids in effluent that cannot be degraded into CO2, H2O. Formic acid and acetic acid are detected by an ion chromatograph in the effluents of CWAO process. The curves of formic acid concentration, acetic acid concentration and pH value in reaction solution varying with operation time are shown in Fig. 8 (initial concentration of H-acid solution was 10 g/L; pH value was 8.0; operation temperature was 2001C).
organic acid concentration (mg/L)
coalesced with molecular H-acid are oxidized into SO2 4 in the reaction process.
90
120
Fig. 7. NH+ 4 varying with operation time.
pH=8, Ce3Cul catalyst pH=12, without catalyst pH=12, Cu1Fe2 catalyst Fig. 6. SO2 4 concentration varying with operation time.
60
time (min)
700
4.5 4.0
600
3.5 500 3.0 400
2.5
300
2.0 1.5
pH value
SO42- concentration (mg/L)
7000
200 1.0 100
0.5
0
0.0 0
30
60
90
120
time (min) formic acid
acetic acid
pH value
Fig. 8. Formic acid, acetic acid and pH value varying with time in CWAO.
As seen from Fig. 8, formic acid and acetic acid concentrations reach high levels during the reaction. The concentration of formic acid reached its highest point, 245 mg/L, after 10 min of reaction, and the pH value of the solution also reached its lowest point at the same time. Then, formic acid concentration decreased gradually and the concentration was 56.8 mg/L when operation finished. Meanwhile, the pH value increased gradually. This indicated that the variation of formic acid concentration is related to the variation of pH value. However, acetic acid accumulated gradually in the CWAO process and its concentration approached 591.8 mg/L when the operation was finished. Acetic acid was formed very rapidly in the first 30 min of reaction, then the concentration of acetic acid slowly increased, which illuminated that acetic acid was much more resistant to CWAO. It is significant to develop a more
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effective catalyst for speeding the oxidation of acetic acid.
3.3.7. Biodegradability of effluents after catalytic wet oxidation of H-acid When the pH value was 12, the temperature of reaction was 2001C, the catalyst used in the reaction system was Ce3Cu1. The H-acid solution (initial concentration was 10 g/L) was treated with CWAO. Values of BOD5/COD of effluents at different times were measured. The results are shown in Fig. 9. The initial BOD5/COD of H-acid solution was 0.02, which indicated that it had poor biodegradability. The BOD5/COD of effluent at 40 min increased to 0.3, indicating that the effluent was already biodegradable. Therefore, the biodegradability of H-acid solution was improved greatly. CWAO can effectively improve the biodegradability of poisonous, harmful and refractory materials because the intermediates, such as a short-chain carboxylic acid, produced from macromolecule organic compounds in WAO are easily degraded by microorganisms [16]. From the above experiments it is suggested that the biologically toxic amino groups and sulfonic groups on
0.4 BOD5/COD
3.3.6. Ultraviolet absorption spectra of effluent Ultraviolet absorption spectrum is formed due to electronic transition in compound molecule. Therefore, ultraviolet absorption spectrum can illustrate some characteristics of molecular structures. For instance, it can reflect the p–p* electronic transition of conjugated double bond system, and also reflect the n–p* electronic transition of lone electron pair. Therefore, ultraviolet absorption spectrum reflects the conjugated relationship between unsaturated groups. The longer the conjugated bonds in molecule are, the longer the absorption wavelength is [15]. Electrons on the dye intermediate naphthalene ring or on the substituted groups adhering to the naphthalene ring all play roles in the conjugated systems. They interact with each other to form stronger conjugated double bond systems, which are easily detected by ultraviolet absorption analysis. H-acid solution (concentration 10 g/L; pH 12.0) was used in experiments and samples were detected by an ultraviolet absorption spectrometer (Shimadzu Model UV-250, Japan). Experimental results clearly showed that there were two absorption peaks at wavelengths 209.5 and 234.1 on the absorption curve of H-acid (their absorption values are 0.596 and 0.475, respectively) but there was no absorption peak on the absorption curve of effluent after 5 min reaction. Those results indicate that the naphthalene rings of H-acid molecule and benzene rings generated during CWAO process of H-acid were decomposed in 5 min. So the catalytic wet oxidation of H-acid was very fast.
0.5
0.3 0.2 0.1 0 03
0
60 90 time (min)
120
Fig. 9. The variety of BOD5/COD during CWAO process. 2 naphthalene rings were oxidized to aNH+ 4 and SO4 , then naphthalene rings were broken into large amounts of short-chain organic carboxylic acid when H-acid was treated by CWAO. These intermediate products are biodegradable. Therefore, refractory naphthalene dye intermediates wastewater can first be treated by CWAO process in order to remove most COD and improve the biodegradability, then by biochemical methods. The CWAO is an effective pretreatment approach of the biological treatment of H-acid wastewater [17].
4. Conclusions The Ce3Cu1 composite catalyst is a high-performance catalyst in the CWAO process. When temperature is 2001C, oxygen partial pressure is 3.0 MPa, pH value is 12.0, reaction time is 30 min, the COD removal is more than 90%, and the lowest pH value of reaction solution during CWAO of H-acid solution is kept at about 6. There are many factors that can influence on the CWAO process of organic compounds. The most important factors among them are reaction temperature, pH value of wastewater and catalytic conditions to improve treating effects and reduce treating costs. The pH value of solution can affect CWAO reaction significantly. Low pH value is beneficial for speeding the reaction, but at the same time, higher corrosion resistance of reactor and catalysts is required. During CWAO process, amino group and sulfonic groups on the H-acid molecule are converted into NH+ 4 and SO2 4 , respectively. Naphthalene rings are oxidized and broken down into formic acid, acetic acid, etc. During initial periods, the reaction mainly involves the degradation of naphthalene rings into short-chain organic acid that are to be oxidized later. Because the speed of catalyst wet air oxidation of acetic acid is very slow, it is very important to develop a more effective
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catalyst for speeding the oxidation of acetic acid. The Ce3Cu1 composite catalyst has good performance in catalyzing the wet air oxidation of acetic acid. The biodegradability of poisonous, harmful and hardly degradable organic wastewater can be improved greatly after being treated with CWAO. Acknowledgements
[8]
[9]
Funding for this study was provided by the National Natural Science Foundation of China (NSFC).
[10]
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Application of catalytic wet air oxidation for the treatment of H-acid manufacturing process wastewater Wanpeng Zhu*, Yuejing Bin, Zhonghe Li, Zhanpeng Jiang, Tong Yin Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China Received 9 January 2001; received in revised form 5 September 2001; accepted 11 September 2001
Abstract Four types of powder catalysts whose main active components are copper (Cu), cerium (Ce), cadmium (Cd) and cobalt-bismuthide (Co-Bi) are prepared with the method of the co-deposition and are evaluated through the catalytic wet air oxidation (CWAO) treatment of H-acid solution. The comparison of the efficiencies of different catalysts shows that Ce3Cu1 (3:1) catalyst is the best one. When the reaction temperature is 2001C, oxygen partial pressure is 3 MPa, pH value is 12, and reaction time is 30 min, the COD removal rate is over 90%. All the H-acid is decomposed in 5 min 2 and is oxidized into NH+ 4 , SO4 , formic acid, acetic acid and other end products. The pH value can greatly affect the COD removal and the production of organic acid. CWAO process not only can get a high reaction rate, but also can oxidize the short-chain organic acid. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Wet air oxidation (WAO); Catalytic wet air oxidation (CWAO); Dye intermediate; H-acid; 1-amino-8-naphthol-3, 6-disulfonic acid
1. Introduction Wet air oxidation (WAO) is a method of oxidizing dissolvable or suspended organic compounds as well as reducible inorganic compounds with oxygen or air under the circumstances of high temperature and high pressure in liquid phase. The application of traditional WAO is limited because of the high operation temperature, high operation pressure and long operation time [1]. Catalytic wet air oxidation (CWAO) was a new technology developed on the basis of WAO in the 1970s. It is a process that can speed the reaction, lower the reaction temperature and pressure with catalyst. It is an available method used in disposing high-concentration effluents, as well as poisonous, detrimental and hardly degradable wastewater. Copper ion is the most effective catalyst that works in the homogeneous state. It was used for the practical treatment of wastewaters discharged from *Corresponding author. Tel.: +86-10-627-84527; fax: +8610-627-91233. E-mail address: [email protected] (W. Zhu).
petrochemical industries [2]. Heterogeneous catalysts that can be easily separated from the reaction medium in batch or continuous processes are better suited than soluble catalysts, which have to be recovered by an additional separation process [3–5]. Different transition metal oxides have been used, but the main drawback is their dissolution in the corrosive reaction mixture [6–9]. Imamura et al. [10] studied the catalytic effect of noble metals on the wet oxidation of phenols and other model pollutant compounds, and found that ruthenium, platinum and rhodium were more active than homogeneous copper catalyst [10]. However, noble metals are of high cost, which severely affects the economy of noble catalysts. H-acid (1-amino-8-naphthol-3, 6-disulfonic acid) is an important dye intermediate. It is widely used in chemical industry for the synthesis of direct, acidic, reactive and azoic dye, as well as in the pharmaceutical industry. Under certain conditions, naphthalene, as a raw material, is used to manufacture the H-acid through a series of chemical processes such as sulfonation, nitration, neutralization, reduction, alkaline dissolving, acid
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precipitation and so on. Since the production process of H-acid is complicated and the utilization ratio of raw materials is low, the wastewater from the manufacturing processes is rich in various substituted derivatives of naphthalene compound and is of dark color and strong acidity. Organic substances in dye intermediate wastewater are often aromatic compounds substituted by some groups, such as aNH2, aNO2, aSO 3 ; etc. They are extremely toxic to organisms. The biological processes cannot effectively degrade these substances and decolorize the H-acid wastewater [11]. As aromatic ring with aSO3H is easily dissolved in water, the general chemical and physical method is very inefficient. Therefore, H-acid wastewater is one of the most hardly treated wastewaters so far [12,13]. In this paper, we studied the CWAO treatment of Hacid solution. The effect of temperature, pH value on the CWAO treatment of H-acid was investigated; support catalysts were prepared and evaluated.
2. Experiments 2.1. Experimental apparatus The GS-0.5 autoclave equipped with a stirrer and a heating device that keeps the constant temperature was used in experiments. It has a cooling coil and two sampling outlets (liquid- and gas-phase outlet). The operation process is presented in Fig. 1.
order to control the pH value, Na2CO3 was added as a precipitation assistant. Four groups of catalysts (Cu group, rare earth group, Cd group and Co/Bi group), 15 species had been prepared in the research processes. The evaluation criteria of catalysts performance are the COD removal of wastewater, the stripping amount of active material of catalyst after 10 or 30 min reaction under given conditions and the lowest pH value during reaction processes. The lowest pH value during reaction processes is a very important factor. The equipment would be easily eroded and the stripping amount of catalytic active metals would increase if the acidity in solution were too high. 2.3. Experimental materials and methods Naphthalene is an important representation of aromatic compounds. It can be used to synthesize many dye intermediates of naphthalene derivatives, such as H-acid, J-acid (5-hydroxy-2-naphthylamine-7-sulfonic acid), chromotropic acid (1,8-dihydroxynaphthalene-3, 6-disulfonic acid) and so on [13]. The H-acid solution was treated with CWAO in our laboratory. In order to study the reaction process and eliminate the disturbance of impurities, reagent H-acid (C.P.) was used to prepare the solution. The molecular structure of H-acid is as follows: H2 N
OH
2.2. Development of catalyst The catalyst was prepared using deposition or codeposition methods [14]. Metal nitrate was employed as the activating material of catalyst in experiments. In
HO3S
Fig. 1. Scheme of WAO process.
SO3Na
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3. Discussion and analysis
CWAO. H-acid wastewater from an actual industry mainly contains intermediates such as H-acid, T-acid (1-naphthylamine-3, 6,8-trisulfonic acid), chromotropic acid, high concentration of salts and so on. The concentrations of H-acid, T-acid, chromotropic acid and Na2SO4 in the simulative wastewater used in experiments were 10, 10, 2, and 150 g/L, respectively. In experiments, the temperature was fixed at 2001C, the amount of catalyst added was 0.2 g/300 mL, and the pH value was 12.0. The results indicate that COD removal can approach 85.0% and 91.3% after 10 and 30 min reaction, respectively, and pH value can be kept at high levels. The lowest pH value is about 5.25. So Ce3Cu1 catalyst has excellent catalyst activity.
3.1. Catalysts selection
3.2. Factors influencing CWAO process
All catalysts within the same group have been compared with each other in this study. The best catalyst was selected from each group that are Cu2Zn1 (weight ratio of Cu and Zn is 2/1, calcination temperature is 6001C), Ce3Cu1 (weight ratio of Ce and Cu is 3/1, calcination temperature is 6001C), Cd2Ce1Cu1 (weight ratio of Cd, Ce and Cu is 2/1/1, calcination temperature is 6001C) and Co1Bi1 (weight ratio of Co and Bi is 1/1, calcination temperature is 6001C). The time dependence of COD removal using different catalysts is shown in Fig. 2. The results reported in Fig. 2 demonstrate that the Ce3Cu1 composite catalyst is the best catalyst among the four catalyst groups. The Ce3Cu1 composite catalyst was selected as the catalyst used in further study on CWAO treatment of dye intermediates. To check the feasibility of practical application of Ce3Cu1 catalyst, simulative wastewater was used in
There are many factors, such as reaction temperature, oxygen pressure, initial pH value of wastewater and dosage of catalyst, which can affect the oxidation of organic materials.
In experiments, H-acid was dissolved in de-ionized water to prepare different concentration solutions. The initial pH value of the solution was adjusted. The experiments were carried out according to the following steps: (1) take 300 mL H-acid solution in the autoclave (model GS-0.5, Weihai, China); (2) add definite amount of catalyst; (3) turn on the heater; (4) fill oxygen into the autoclave. It was the time zero when temperature and pressure were up to the setting values. Samples were taken every 5 min, COD and pH values of the samples were measured.
CODremoval rates /(%)
100 90 80 70 60 50 40 30 20 10 0 0
50
100
150
time (min) Ce3Cu1
Cu2Zn1
Cd2Ce1Cu1
Co1Bi1
Fig. 2. Time dependence of COD removal with different catalysts.
3.2.1. Influence of reaction temperature on CWAO process Ce3Cu1 catalyst was used in CWAO process at the reaction temperatures of 1801C, 2001C and 2201C. Other reaction conditions were fixed: oxygen partial pressure was 3.0 MPa; simulated wastewater concentration was 10 g/L; pH value was 12.0. The experimental results showed that COD removals after 30 min reaction were 75.6%, 92.0% and 92.9% in turn at the reaction temperatures of 1801C, 2001C and 2201C. The corresponding lowest pH values during reaction were 5.76, 6.50 and 7.35, respectively. COD removal reached 92%, the lowest pH value during the reaction was almost neutral. 2001C is a feasible reaction temperature. 3.2.2. Influences of catalyst dosage on catalytic effects H-acid solution (concentration was 10 g/L, initial pH value was 12.0) was treated with CWAO under the following conditions: temperature 2001C; oxygen partial pressure 3.0 MPa. The experimental results showed that COD removals after 30 min reaction were 53.5%, 90.7% and 92.0% when the catalyst dosages were 0.0, 0.1 and 0.2 g/L in turn. The corresponding lowest pH values during the reaction were 3.76, 8.45 and 6.50, respectively. Catalyst dosage has significant influences on COD removal and the lowest pH value of solution. The experimental results illustrate that COD removal in CWAO is 36% higher than that in the process without catalyst, and the lowest pH value is also higher when the catalyst dosage is 0.1 g/300 mL. Without catalyst, the pH value of solution can drop to 3.76. The catalyst is very significant; it can not only accelerate the reaction but
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Table 1 Effects of Ce3Cu1 catalyst on CWAO under different pH value Initial pH of wastewater
8.0 10.0 12.0
Amount of dissolved metal (mg/L) Highest
Lowest
0.91 0.48 0.44
0.18 0.09 0.08
COD removal at 30 min (%)
The lowest pH value in reaction
81.8 71.7 68.3
2.22 3.00 5.00
also retard the equipment corrosion caused by low pH value. The catalyst dosage 0.1–0.2 g/300 mL is acceptable.
12
3.2.3. Influence of initial pH value of H-acid solution When oxygen partial pressure was 3.0 MPa, and Ce3Cu1 catalyst dosage was 0.2 g/300 mL. CWAO experiments of H-acid solution (concentration was 10 g/L) were performed under different initial wastewater pH values. The results are shown in Table 1. With the increase of pH value, the amount of dissolved active metal ions is reduced, and with the increase of the lowest pH value in the reaction, the equipment corrosion is retarded. COD removal decreases rapidly as the pH value increases. COD removal is over 80% when the pH value is 8 and the retention time is 30 min, which is acceptable in practical processes.
8
pH value
10
6 4 2 0 0
30
60
90
120
time (min) without catalyst
Ce3Cu1
Ce1Fe3
Fig. 3. pH value versus operation time in CWAO.
3.3. The CWAO process of H-acid 3.3.1. The variation of pH value in reaction The pH values varying with time have the same regularities in the WAO process and the CWAO process of H-acid. The pH values increase at first, then decrease, and increase at last. Moreover, the reaction is closely correlated to the variety of pH value. The experiment was conducted under the following three operation conditions: (1) without catalyst; (2) with the less efficient Ce1Fe1 (Ce1Fe3 in Fig. 3) catalyst (in the operation); (3) with the more efficient Ce3Cu1 (Ce3Cu1 in Fig. 3) catalyst. The reaction temperature is 2001C. Fig 3 gives the time course of pH. As seen in Fig. 3, the shapes of the pH curve are similar in the three WAO (or CWAO) processes. The pH value decreases rapidly to a nadir point at first, and then increases gradually. The decreasing rate and changing range of the three pH curves differ from each other. The more active the catalyst is, the shorter the time to nadir point is, and the smaller the decreasing range , the higher the final pH value. It indicates that the decomposition rate of short-chain organic acid is higher. Those shortchain organic acids are much more resistant to WAO and CWAO oxidation than the organic compounds
present in the original wastewater [1]. Therefore, the change of pH value in the reaction process can be used to evaluate the activity of the catalyst. Experiments were carried out under two operation conditions, one was an operation process without catalyst [see Fig. 4(a)], and another was an operation process with Ce3Cu1 catalyst [see Fig. 4(b)]. The relation between the pH variety and the COD removal is plotted in Fig. 4. It can be inferred from Fig. 4 that at the beginning of operation, the pH value decreases rapidly and COD removal increases rapidly. COD removal increases slowly after the pH value reaches the nadir point. The pH value reaches the nadir point in 5 min and the decrease range is small if Ce3Cu1 catalyst is used. It indicates that some short-chain organic acids among the intermediate products have been oxidized in CWAO process, but this phenomenon does not occur when there is no catalyst in the reaction system. COD removal can reach 90% within 20 min operation in CWAO process, but the COD removal is still below 80% after 120 min operation in WAO process. The advantage of CWAO over WAO is very obvious.
80
50 40 30 20 10 0 0
30
60
90
60 40 20
pH value
60
90
30
60
90
120
dissolving amount of Cu
Fig. 5. The relation between the pH value and the amount of dissolved Cu.
COD removal rate (%)
pH value
80
time (min)
100 90 80 70 60 50 40 30 20 10 0 120
time (min)
pH value
100
COD removal
10 9 8 7 6 5 4 3 2 1 0 30
120
0
(a) Without catalyst
0
140
0
120
time (min)
pH value
stripping of Cu (mg/L)
60
10 9 8 7 6 5 4 3 2 1 0
160
70
COD removal rate (%)
pH value
10 9 8 7 6 5 4 3 2 1 0
pH value
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COD removal
(b) With Ce3Cul catalyst Fig. 4. The relation between the pH variety and the COD removal.
3.3.2. The relation between pH value and amount of dissolved active component The concentration of the metal ion in the solution, which represents the amount of dissolved active component from catalyst, after CWAO process was measured with an atomic absorption spectrophotometer (Hitachi model 180-80, Japan). The experimental results of CWAO of H-acid with Cu catalyst alone are shown in Fig. 5. The two curves illustrate the relation between pH value and the amount of dissolved Cu2+. As known from Fig. 5, the amount of dissolved catalyst activity components is closely related to the variation of pH value. The amount of dissolved catalyst activity components decreases with the increase of pH. It reaches its highest point at the same time when pH falls to its lowest point and falls to the bottom of the curve when the pH reaches the top of the peak. The results show
that the loss of activity is less when the catalyst is more active. This is understandable as the more active catalyst can shorten the reaction time and restrain the decrease of pH value during the operation process.
3.3.3. Variation of anion in reaction process There are two sulfonic groups and one amino group in the H-acid molecule. In the WAO process, theoretically, aSO3H is easily released from the naphthalene ring and oxidized into SO2 4 . aNH2 can probably be released from the naphthalene ring and either be oxidized into NH+ or be oxidized into NO 4 2 , NO3 or N2. The effluents of CWAO treatment of H-acid solution (prepared with de-ionized water, its initial concentration of SO2 was zero) were measured with an ion 4 chromatograph (Dioner model DX100, USA). It was found that there was a lot of SO2 4 but no NO2 or NO3 in effluents throughout the range of reaction time. The experimental results of SO2 4 concentration varying with operation time are shown in Fig. 6. In Fig. 6, a little SO2 4 appears in the original H-acid solution. It indicates that hydrolysis of sulfonic group takes place during heating process although no oxygen presents. When the pH value in the influent is 8, the initial SO2 concentration is 1084 mg/L, which is the 4 highest among three experimental conditions. Under the other two conditions (pH=12, with catalyst and without catalyst) SO2 concentrations are 475 and 537 mg/L, 4 respectively. According to the results listed above, it is preferred for aSO3H to hydrolyze at lower pH value. In addition, SO2 increase is faster than that of other 4 conditions when pH value is 8 and Ce3Cu1 catalyst is used. It indicates that H-acid is oxidized faster and Hacid oxidation is coupled to COD removal. The calculated results show that most of the aSO3H
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NH4+ concentration(mg/L)
6000 5000 4000 3000 2000 1000
300 250 200 150 100 50 0 0
0 0
30
60
90
30
120
time (min)
3.3.4. Cations in solution in reaction process Neither NO 2 nor NO3 were detected in effluent of CWAO process although H-acid had an amino group. N + is probably oxidized into NH+ 4 or N2. NH4 in effluent was detected with ion spectrometry; the results are shown in Fig. 7 (pH=12, T ¼ 2001C, catalyst was Ce3Cu1). The concentration of NH+ 4 in solution is 474 mg/L (18 10 1000/368) if the amino groups in H-acid molecule in 10 g/L solution are all transformed into + NH+ in the 4 . However, the concentration of NH4 effluent after being treated for 120 min is only 320 mg/L; this means that some amino groups are transformed into other compounds such as N2 and so on.
3.3.5. The variation of short-chain organic acid in solution during reaction Theoretically, organic compounds can finally be degraded into CO2, H2O and other harmless product in WAO or CWAO processes. But actually, there are many short-chain organic acids in effluent that cannot be degraded into CO2, H2O. Formic acid and acetic acid are detected by an ion chromatograph in the effluents of CWAO process. The curves of formic acid concentration, acetic acid concentration and pH value in reaction solution varying with operation time are shown in Fig. 8 (initial concentration of H-acid solution was 10 g/L; pH value was 8.0; operation temperature was 2001C).
organic acid concentration (mg/L)
coalesced with molecular H-acid are oxidized into SO2 4 in the reaction process.
90
120
Fig. 7. NH+ 4 varying with operation time.
pH=8, Ce3Cul catalyst pH=12, without catalyst pH=12, Cu1Fe2 catalyst Fig. 6. SO2 4 concentration varying with operation time.
60
time (min)
700
4.5 4.0
600
3.5 500 3.0 400
2.5
300
2.0 1.5
pH value
SO42- concentration (mg/L)
7000
200 1.0 100
0.5
0
0.0 0
30
60
90
120
time (min) formic acid
acetic acid
pH value
Fig. 8. Formic acid, acetic acid and pH value varying with time in CWAO.
As seen from Fig. 8, formic acid and acetic acid concentrations reach high levels during the reaction. The concentration of formic acid reached its highest point, 245 mg/L, after 10 min of reaction, and the pH value of the solution also reached its lowest point at the same time. Then, formic acid concentration decreased gradually and the concentration was 56.8 mg/L when operation finished. Meanwhile, the pH value increased gradually. This indicated that the variation of formic acid concentration is related to the variation of pH value. However, acetic acid accumulated gradually in the CWAO process and its concentration approached 591.8 mg/L when the operation was finished. Acetic acid was formed very rapidly in the first 30 min of reaction, then the concentration of acetic acid slowly increased, which illuminated that acetic acid was much more resistant to CWAO. It is significant to develop a more
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effective catalyst for speeding the oxidation of acetic acid.
3.3.7. Biodegradability of effluents after catalytic wet oxidation of H-acid When the pH value was 12, the temperature of reaction was 2001C, the catalyst used in the reaction system was Ce3Cu1. The H-acid solution (initial concentration was 10 g/L) was treated with CWAO. Values of BOD5/COD of effluents at different times were measured. The results are shown in Fig. 9. The initial BOD5/COD of H-acid solution was 0.02, which indicated that it had poor biodegradability. The BOD5/COD of effluent at 40 min increased to 0.3, indicating that the effluent was already biodegradable. Therefore, the biodegradability of H-acid solution was improved greatly. CWAO can effectively improve the biodegradability of poisonous, harmful and refractory materials because the intermediates, such as a short-chain carboxylic acid, produced from macromolecule organic compounds in WAO are easily degraded by microorganisms [16]. From the above experiments it is suggested that the biologically toxic amino groups and sulfonic groups on
0.4 BOD5/COD
3.3.6. Ultraviolet absorption spectra of effluent Ultraviolet absorption spectrum is formed due to electronic transition in compound molecule. Therefore, ultraviolet absorption spectrum can illustrate some characteristics of molecular structures. For instance, it can reflect the p–p* electronic transition of conjugated double bond system, and also reflect the n–p* electronic transition of lone electron pair. Therefore, ultraviolet absorption spectrum reflects the conjugated relationship between unsaturated groups. The longer the conjugated bonds in molecule are, the longer the absorption wavelength is [15]. Electrons on the dye intermediate naphthalene ring or on the substituted groups adhering to the naphthalene ring all play roles in the conjugated systems. They interact with each other to form stronger conjugated double bond systems, which are easily detected by ultraviolet absorption analysis. H-acid solution (concentration 10 g/L; pH 12.0) was used in experiments and samples were detected by an ultraviolet absorption spectrometer (Shimadzu Model UV-250, Japan). Experimental results clearly showed that there were two absorption peaks at wavelengths 209.5 and 234.1 on the absorption curve of H-acid (their absorption values are 0.596 and 0.475, respectively) but there was no absorption peak on the absorption curve of effluent after 5 min reaction. Those results indicate that the naphthalene rings of H-acid molecule and benzene rings generated during CWAO process of H-acid were decomposed in 5 min. So the catalytic wet oxidation of H-acid was very fast.
0.5
0.3 0.2 0.1 0 03
0
60 90 time (min)
120
Fig. 9. The variety of BOD5/COD during CWAO process. 2 naphthalene rings were oxidized to aNH+ 4 and SO4 , then naphthalene rings were broken into large amounts of short-chain organic carboxylic acid when H-acid was treated by CWAO. These intermediate products are biodegradable. Therefore, refractory naphthalene dye intermediates wastewater can first be treated by CWAO process in order to remove most COD and improve the biodegradability, then by biochemical methods. The CWAO is an effective pretreatment approach of the biological treatment of H-acid wastewater [17].
4. Conclusions The Ce3Cu1 composite catalyst is a high-performance catalyst in the CWAO process. When temperature is 2001C, oxygen partial pressure is 3.0 MPa, pH value is 12.0, reaction time is 30 min, the COD removal is more than 90%, and the lowest pH value of reaction solution during CWAO of H-acid solution is kept at about 6. There are many factors that can influence on the CWAO process of organic compounds. The most important factors among them are reaction temperature, pH value of wastewater and catalytic conditions to improve treating effects and reduce treating costs. The pH value of solution can affect CWAO reaction significantly. Low pH value is beneficial for speeding the reaction, but at the same time, higher corrosion resistance of reactor and catalysts is required. During CWAO process, amino group and sulfonic groups on the H-acid molecule are converted into NH+ 4 and SO2 4 , respectively. Naphthalene rings are oxidized and broken down into formic acid, acetic acid, etc. During initial periods, the reaction mainly involves the degradation of naphthalene rings into short-chain organic acid that are to be oxidized later. Because the speed of catalyst wet air oxidation of acetic acid is very slow, it is very important to develop a more effective
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catalyst for speeding the oxidation of acetic acid. The Ce3Cu1 composite catalyst has good performance in catalyzing the wet air oxidation of acetic acid. The biodegradability of poisonous, harmful and hardly degradable organic wastewater can be improved greatly after being treated with CWAO. Acknowledgements
[8]
[9]
Funding for this study was provided by the National Natural Science Foundation of China (NSFC).
[10]
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