Intermediary Products in the Catalytic Wet Air Oxidation of Crystal

Article pubs.acs.org/IECR

Intermediary Products in the Catalytic Wet Air Oxidation of Crystal Violet with Ni/MgAlO as Catalyst Gabriel Ovejero, Araceli Rodríguez, Ana Vallet, and Juan García* Grupo de Catálisis y Procesos de Separación (CyPS), Departamento de Ingeniería Química, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain S Supporting Information *

ABSTRACT: In this study, the performance of Ni supported on MgAlO catalyst to promote oxidation of crystal violet was investigated. Oxidation experiments were carried out at T = 140−200 °C and total pressures up to 50 bar. The catalyst was prepared by incipient wetness impregnation to obtain catalyst with metal loading of 7 wt %. A detailed reaction network for the wet air oxidation and catalytic wet air oxidation of crystal ciolet on Ni/MgAlO catalyst is proposed in this study. The reaction network proposed takes into account all the detected intermediate products during the dye oxidation. This work represents the first determination of the reaction network of this compound in wet air and catalytic wet air oxidation.

1. INTRODUCTION The extraordinary development in the past decades is at the origin of the increasing production of industrial wastewaters and, consequently, a rising concern on process waters recycle and disposal of harmfully polluted wastewaters.1−6 Among industrial effluents, textile wastewaters are a large problem for conventional treatment plants, principally due to the presence of dyes and its derivates. Recent studies indicated that approximately 12% of synthetic dyes are lost during manufacturing and processing operations.7 The traditional treatment techniques applied in textile wastewaters, such as coagulation/flocculation, membrane separation (ultrafiltration, reverse osmosis), or elimination by activated carbon adsorption, only do a phase transfer of the pollutant, and biological treatment is not a complete solution to the problem due to biological resistance of some dyes. Color removal from textile wastewater has been a matter of considerable interest during the past 2 decades, not only because of the potential toxicity of certain dyes but also because of their visibility in receiving waters. The release of these wastewaters in natural environments is very problematic to aquatic life8 and mutagenic to humans.9 Hence, the resource to advanced oxidation processes (AOPs), such O3,10 UV/O3,11 UV/H2O2,12 photo Fenton,13or wet air oxidation (WAO) can be considered as a good alternative. AOPs are processes involving the formation of hydroxyl radicals that aggressively and almost indiscriminately attack all types of inorganic and organic pollutants found in water and wastewater.14,15 Among AOPs, WAO consists of the oxidation of the pollutants at high temperatures and pressures (175−320 °C, 5−200 bar). This technology has already been applied successfully to treat effluents containing a large range of products, in particular, from textile bleaching, printing, and dyeing industries.16,17 However, the performances of the WAO process may be strongly improved if a solid catalyst is employed. The degradation of organic pollutants is then increased and milder conditions of temperature and pressure may be used. This © 2012 American Chemical Society

process appears to have the capacity to completely decolorize and partially mineralize the textile industry dyes in short reaction time, as was related by some studies.18 Triphenylmethane (TPM) dyes are employed in a large variety of technical applications. These kinds of dyes are extensively applied in the textile industry for dyeing nylon, wool, cotton, and silk as well as for coloring oil, fats, waxes, varnish, and plastics. Other industries consuming TPM dyes are the paper, leather, cosmetic, and food industries.19 However, one of the great concerns about these kinds of dyes is that the thyroid peroxidase-catalyzed oxidation of the TPM dyes may led to the production of primary and secondary aromatic amines, with structures similar to carcinogenic aromatic amines.20 Among TPM dyes, crystal violet (CV) is one of the most employed ones. CV degradation by means of catalytic wet air oxidation (CWAO) with Ni/MgAlO catalyst has proved to be an efficient method for dye removal and the influence of reaction conditions has been studied.21 However, the intermediate compounds formed during the WAO and CWAO reaction have not been determined. The aim of this paper is to determine the reaction intermediates for the WAO and CWAO of CV with Ni/MgAlO obtained from an hydrotalcite (HT) precursor, as catalyst, and to propose a reaction pathway. This catalyst has been selected at it has provided good TOC and compound removal rates with other kinds of dyes such as Basic Yellow 11 and Naphtol Blue Black.16 It has been found to be stable in long-term reactions (65 days), no remarkable nickel leachates being found in the final effluents of the reaction.22 Received: Revised: Accepted: Published: 11367

June 11, 2012 July 30, 2012 July 31, 2012 July 31, 2012 dx.doi.org/10.1021/ie301533s | Ind. Eng. Chem. Res. 2012, 51, 11367−11372

Industrial & Engineering Chemistry Research

Article

Figure 1. (a) TOC and (b) CV degradation of a 100 ppm CV solution at 140 °C, 50 bar, 800 rpm.

2. MATERIALS AND METHODS 2.1. Materials. CV was selected as the model pollutant because it is a hardly biodegradable dye by the conventional biological processes, but widely employed in the textile, color solvent, ink, paint, paper, pharmaceutical, and plastic industries. The dye was purchased from Sigma-Aldrich (Steinheim, Germany) and used without further purification. More details can be found elsewhere.21 2.2. Preparation of the Catalyst. HT precursor was used as support in the preparation of nickel catalysts. It was prepared by coprecipitation. Mg(NO3)2·6H2O (0.05 mol) and Al(NO3)2·9H2O (0.01 mol) (Sigma-Aldrich, Steinheim, Germany) were dissolved in 600 mL of water to form solution A. Also, Na2CO3 (0.03 mol) and NaOH (0.07 mol) were mixed in 300 mL of water to form solution B. Solution B was stirred for 1 h at a constant temperature of 65 °C. Then solution A was slowly dropped, forming a precipitate.23 The resulting solution was aged at 60 °C for 18 h. Then they were filtered and washed with distilled water at 40 °C for 2 hours and dried at 100 °C for 12 h. Nickel catalyst was prepared by incipient wetness impregnation of the dried HT precursor employing Ni(NO3)2·6H2O purchased from Panreac (Barcelona, Spain). The Ni(NO3)2·6H2O was calculated to obtain a catalyst load of 7 wt % of nickel in the final analysis. The resulting solid was calcined at 550 °C for 5 h, obtaining the Ni/MgAlO catalyst. 2.3. Characterization. The characterization of the support and catalyst were carried out by physical adsorption of nitrogen at 77 K in a Micromeritics ASAP 2010 apparatus. X-Ray powder diffraction (XRD) patterns of support and catalyst before and after calcination were recorded by a difractometer SIEMENS D-501. The metal loading was determined by means of X-ray fluorescence (XRF) (Broker S4 Explorer). More details can be found elsewhere.21,24 2.4. Catalytic Wet Air Oxidation Experiments. Experiments were conducted in a Hastelloy high-pressure Microreactor C-276 Autoclave Engineers with a volume of 100 mL. The reactor (i.d. 5 cm) was equipped with an electrically heated jacket, a turbine agitator, a variable speed magnetic drive, and a valve for sampling. The temperature and the stirring speed were

controlled by means of a PID controller. The gas inlet, gas release valve, pressure gauge, rupture disk, and cooling water feed line were situated on the top of the reaction vessel, whereas the liquid sample line and the thermocouple were immersed in the reaction mixture. The reactor was first loaded with 100 mL of commercial dye solution with (0.5 g) or without catalyst, and initially pressurized with nitrogen to ensure inert atmosphere. Afterward, the system was heated to the desired temperature. The reactor is then pressurized with air and samples were withdrawn periodically after sufficient flushing of the sample line. Pressure drop was monitored and additional oxygen was charged to maintain a constant total pressure throughout the duration of the test. During the reactions, samples were taken at regular intervals to measure total organic carbon (TOC) and dye content and to evaluate the formation of reaction intermediates. 2.5. Procedures and Analysis. Dye concentration was determined at the wavelength corresponding to its maximum UV−vis absorption (488 nm) which was monitored by a Shimadzu UV−vis spectrophotometer. TOC measurements were carried out in a Shimadzu TOC analyzer, after filtration (pore diameter 10 mm) to assess the degree of total oxidation. For evaluation of catalytic activity, color and TOC removal efficiency were calculated as the ratio between the color and TOC measured at each instant and the values of each parameter for the initial solution. HPLC analyses were performed in a Varian Prostar-230 apparatus with a C18. The flow rate of the mobile phase was set at 1.0 mL min−1. Solvent A was 25 mM aqueous ammonium acetate buffer (pH 6.9), while solvent B was methanol. A linear gradient was set as follows: t = 0, C = 95; t = 20, C = 50; t = 45, C = 10; t = 50, C = 95. The identification of nonpolar intermediate products in the solution was performed employing a gas chromatography− mass spectrometry (GC/MS) system. The GC (6890N Agilent Technologies) was equipped with an Agilent 190915-433 capillary column. The GC column was operated in a temperature-programmed mode with an initial temperature of 45 °C held for 2 min, ramped at 80 °C/min up to 200 °C, and held at that temperature for 5 min. Helium served as the carrier 11368

dx.doi.org/10.1021/ie301533s | Ind. Eng. Chem. Res. 2012, 51, 11367−11372


Article

Figure 2. Absorption spectra of the intermediates formed during the CWAO reaction at (a) 200 °C and (b) 140 °C, 50 bar, 100 ppm, 800 rpm.

gas at the flow rate of 1 mL min−1. The identification of degradation products was done by comparing the GC/MS spectra patterns with those of standard mass spectra in the National Institute of Standards and Technology (NIST) library.

was very similar to the one obtained for the noncatalytic reaction (20.2 and 15.5%, respectively). On the other hand, when the Ni/MgAlO catalyst is present, TOC conversion attained 71.4%. The introduction of the catalyst provides a great enhancement of the TOC removal; however, after the reaction, 28.6% of organic matter is still left in the solution. Toxicity analyses were carried out to check if the process allows a diminution of this parameter. Mother solution was found to have 23 TU15 min, whereas after the catalytic treatment this value drops to 2.7 TU15 min. Figure 1b shows the same difference between the WAO, Ni/ MgAlO, and MgAlO obtained for the TOC removal. It can also be noticed that, for the Ni/MgAlO reaction, the dye degradation is above 98%. The observed difference between TOC and dye degradation indicates the formation of reaction intermediates refractory to oxidation at the employed reaction conditions. It is then important to study the formation of such intermediates to improve the effectiveness of the catalyst. As an example Alejandre et al.26 showed that the catalytic wet air oxidation of phenol at 140 °C with an hydrotalcite coprecipitated with nickel and calcined at 750 °C showed lower conversions (among 40−75%, depending on the nickel amount) and were stable at these reaction conditions. The CWAO of toluene with Pd impregnated over hydrotalcite and calcined at 500 °C showed no conversion for temperatures below 250 °C.27 3.3. Reaction Intermediates. 3.3.1. Absorption Spectra of the Formed Intermediates. The evolution of the absorption spectra with the reaction time are measured and shown in Figure 2a,b. Figure 2a shows the formed intermediates at 200 °C for the CWAO reaction. The obtained data showed that the spectral bands shifted hypsochromically from 588.2 nm, corresponding to CV, to 542.1 nm. On the other hand, the formation of reaction intermediates is evident as new absorption bands are formed at 375.6 nm, which progressively shifts toward lower wavelengths (up to 333.8 nm). Additionally, a new band is formed at 242 nm. On the other hand, it can be seen that, at 140 °C (Figure 2b), the hypsochromical effect at the peak at 588 nm is not as strong as in the 200 °C reaction. The same can be observed for the peak formed at 375.6 nm, as only a diminution of its

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. Detailed results for the solid characterization can be consulted in previous publica-

Figure 3. Absorption spectra of the intermediates formed during the WAO reaction at 200 °C, 140 °C, 50 bar, 100 ppm, 800 rpm.

tions.19,22,25 It was proven that, in the presence of nickel, the BET area of the solid decreased from 182 to 169 m2 g−1. It was also shown that the metallic particles exhibited a high metallic dispersion (27%). No remarkable differences in the XRD patterns were found for the uncalcined catalyst and the support, indicating that the inclusion of the Ni cation by wetness impregnation did not affect the HT structure. After calcination at 550 °C the formation of mixed oxides structures was observed.21,24 3.2. Wet Air Oxidation and Catalytic Wet Air Oxidation in Batch Reactor. Figure 1a shows the results obtained for the WAO and CWAO of a 100 ppm CV solution, at 140 °C, 50 bar, and 800 rpm. It can be seen that in the presence of the catalytic support (MgAlO) the TOC reduction 11369



Article

Figure 4. WAO degradation pathway for CV.

intensity can be noticed. No band shifts are observed at this wavelength. However, the peak at 309.2 nm seems to disappear to form new ones at 252 nm which progressively decreases in intensity as the reaction carries on. From the analysis of the precedent figures it can be seen that the formed intermediates are dependent on the reaction temperature. When the temperature is increased, a wider variety of reaction intermediates are formed, as new peaks are detected in Figure 2a,b, which represent new different intermediate compounds. As can be seen, more new peaks are formed for the highest temperature. Figure 3 shows the spectra for the WAO reaction at 200 °C. It can be seen that there is also a slight hypsochromic effect as far as the main peak at 588 nm is concerned. However, the peak displacement is much less evident than for the CWAO reaction at both temperatures. A progressive diminution of the peak intensity can also be noticed. This shows that the reaction is

slower and the extension of the degradation lower, when no catalyst is employed. The intermediates formation seems also to be less complex in the case of WAO reaction. 3.3.2. WAO Intermediates. The identified products during the WAO reaction are shown in Table S1 (see Supporting Information). The identification of these intermediates always fit a value higher than 70% in the program of the NIST library. It can be observed that, for the noncatalytic reaction, the identified products uniquely correspond to the different products of demethylation. This suggests that the OH radicals produced during the WAO reaction are only able to attack the CV molecule on the N,N-dimethyl groups. On the basis of the identified intermediates compounds, a degradation pathway for the WAO of CV is proposed in Figure 4. Only compounds A, B, C, D, and F were identified, whereas E, G, H, and I (in a square) were deduced. The OH radicals attack the peripheral methyl groups to form a N,N-dimethyl11370



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Figure 5. CWAO degradation pathway for CV.

4. CONCLUSIONS

N′,N′-dimethyl-N′′-methyl pararosaniline (A). Same results were found by Chen et al.25 A is itself attacked to form the series of demethylated byproduct (products B−I) which are shown in the representation of the reaction pathway. Then the demethylation continues until a complete demethylated product (I) is formed. 3.3.3. CWAO Intermediates. The same intermediates as those found for the WAO process and which decomposition pathway was described in 3.2.4 were identified for the CWAO reaction (Figure 5). However, new compounds such as 4aminophenol and 4-(N,N-dimethylamino)-4′-(N′,N′dimethylamino)benzophenone (Table S2). The presence of these compounds indicates that an alternative degradation pathway may be followed. In this case, CV molecule would be attacked by OH radicals on the conjugated structure which may led to the formation of a carbon-centered radical which is then attacked by an OH radical to form 4-aminophenol. There, the degradation of 4-aminophenol can be made toward the formation of phenol with the production of NH4+ and NO3−28 and following the degradation pathway proposed by Santos et al.29 Another option for the 4-aminophenol degradation is the dimerization of those molecules to form 4-(N,N-dinmethylamino)-4′-(N′,N′-dimethylamino)benzophenone. Because of the low amounts of this compound found during the intermediates identification and the detection of high percentages of ring cleavage intermediates such as maleic, oxalic, and acetic acids, it can be said that the this compound also degrades to form low molecular weight acids.

Wet air oxidation and catalytic wet air oxidation of crystal violet was performed with a Ni/MgAlO catalyst. TOC and CV concentration were monitored and a reaction pathway was proposed. It was found that during the WAO reaction OH radicals are only able to attack the CV molecule on the N,N-dimethyl groups whereas for the CWAO reaction the conjugated structure would also be attacked, leading to the formation of a carbon-centered radical which is degradated to 4-aminophenol and can either suffer further degradation or dimerize to form 4-(N,N-dinmethylamino)-4′-(N′,N′-dimethylamino) benzophenone.

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ASSOCIATED CONTENT

* Supporting Information S

Identified WAO intermediates are shown in Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* Tel.: +34-91-394-4117/5207. Fax: +34-91-394-4114. E-mail: [email protected] (G. Ovejero); [email protected] ́ (J. Garcia). Notes

The authors declare no competing financial interest. 11371



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(21) Ovejero, G.; Rodríguez, A.; Vallet, A.; Willerich, S.; García, J. Application of Ni supported over mixed Mg-Al oxides to crystal violet wet air oxidation: the role of the reaction conditions. Appl. Catal., B 2011, 111−112, 586−594. (22) Vallet, A.; Besson, M.; Ovejero, G.; García, J. Treatment of a non-azo dye aqueous solution by CWAO in continuous reactor using a Ni catalyst derived from hydrotalcite-like precursor. J. Hazard. Mater. 2012, 227−228, 410−417. (23) Basile, F.; Benito, P.; Fornasari, G.; Vaccari, A. Hydrotalcite-type precursors of active catalysts for hydrogen production. Appl. Clay Sci. 2010, 48, 250−259. (24) Ovejero, G.; Rodríguez, A.; Vallet, A.; Gómez, P.; García, J. Catalytic Wet Air Oxidation with Ni- and Fe- doped mixed oxides derived from hydrotalcites. Water Sci. Technol. 2011, 63, 2381−2387. (25) Fan, H. J.; Huang, S. T.; Chung, W. H.; Jan, J. L.; Lin, W. Y.; Chen, C. C. Degradation pathways of crystal violet by Fenton and Fenton-like systems: Condition optimization and intermediate separation and identification. J. Hazar. Mater. 2009, 171, 1032−1044. (26) Alejandre, A.; Medina, F.; Rodriguez, X.; Salagre, P.; Cesteros, Y.; Sueiras, J. E. Cu/Ni/Al layered double hydroxides as precursors of catalysts for the wet air oxidation of phenol aqueous solutions. Appl. Catal., B 2001, 26, 195−207. (27) Li, P.; He, C.; Cheng, J.; Ma, C. Y.; Dou, B. J.; Hao, Z. P. Catalytic oxidation of toluene over Pd/Co3AlO catalysts derived from hydrotalcite-like compounds: Effects of preparation methods. Appl. Catal., B 2011, 14, 570−579. (28) Oliviero, L.; Wahyu, H.; Barbier, J., Jr.; Duprez, D.; Ponton, J. W.; Metcalfe, I. S.; Mantzavinos, D. Experimental and predictive approach for determining Wet Air Oxidation pathways in synthetic wastewater. Trans. IChemE 2003, 81, 384−391. (29) Santos, A.; Yustos, P.; Quintanilla, A.; Garcia-Ochoa, F. Lower toxicity route in catalytic wet oxidation of phenol at basic pH by using bicarbonate media. Appl. Catal., B 2004, 53, 181−194.

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from Ministerio de Economiá y Competitividad CTQ2011-27169, by CONSOLIDER Program through TRAGUA Network CSD2006-44, and Comunidad de Madrid through REMTAVARES Network S2009/AMB-1588.

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Intermediary Products in the Catalytic Wet Air Oxidation of Crystal

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