Environ. Sci. Technol. 2004, 38, 133-138
Evolution of Toxicity upon Wet Catalytic Oxidation of Phenol A . S A N T O S , * ,† P . Y U S T O S , † A . Q U I N T A N I L L A , † F . G A R C IÄ A - O C H O A , † J . A . C A S A S , ‡ A N D J . J . R O D R IÄ G U E Z ‡ Departamento de Ingenierı´a Quı´mica, Facultad de Ciences Quı´micas, Universidad Complutense de Madrid, Cuidad Universitaria s/n 28040, Madrid, Spain, and Ingenierı´a Quimica, Universidad Auto´noma de Madrid, 28049 Madrid, Spain
This work reports on the evolution of the toxicity of phenol-containing simulated wastewater upon catalytic wet oxidation with a commercial copper-based catalyst (Engelhard Cu-0203T). The results of the study show that this catalyst enhances detoxification, in addition to its effect on the oxidation rate. The EC50 values of the intermediates identified throughout the oxidation route of phenol have been determined and used to predict the evolution of toxicity upon oxidation. The predicted values have been compared with the ones measured directly from the aqueous solution during the oxidation process. To learn about the evolution of toxicity throughout the routes of phenol oxidation, experiments have been performed with simulated wastewaters containing separately phenol, catechol, and hydroquinone as original pollutants. The significant increase of toxicity observed during the early stages of phenol oxidation is not directly related to the development of the brown color that derives mainly from catechol oxidation. This increase of toxicity is caused by the formation of hydroquinone and p-benzoquinone as intermediates, the former showing the highest toxicity. Furthermore, synergistic effects, giving rise to a significant increase of toxicity, have been observed. These effects derive from the interactions among copper leached from the catalyst and catechol, hydroquinone, and p-benzoquinone and demand that close attention be paid to this potential problem in catalytic wet oxidation.
Introduction Aqueous effluents from industries such as pharmaceutical, chemical, petrochemical, etc., contain refractory organics in concentrations far below the limits that would permit recovery processes but high enough to severely curtail the effectiveness of conventional biological processes. The oxidation of these refractory compounds with oxygen using solid catalysts (1-3) offers an alternative to other processes such as noncatalytic wet oxidation, supercritical oxidation, and adsorption. Heterogeneous catalytic oxidation permits significantly lower temperatures and pressures than required for noncatalytic oxidation (4, 5) and avoids the regeneration of the adsorbent commonly associated with the use of adsorption under economically acceptable conditions (6, 7). * Corresponding author phone/fax: 34-91-3944171; e-mail:
[email protected]. † Universidad Complutense de Madrid. ‡ Universidad Auto ´ noma de Madrid. 10.1021/es030476t CCC: $27.50 Published on Web 11/14/2003
2004 American Chemical Society
Because of their toxicity and the frequency of their presence in industrial wastewaters, phenol and phenolics have gained increased attention in the last two decades. Moreover, phenol is considered to be an intermediate in the oxidation route of higher molecular weight aromatics and so usually is taken as a model compound in research studies dealing with advanced wastewater treatments (8). There are a significant number of papers on wet catalytic oxidation of phenol-containing wastewaters (9-17), but the biodegradability of the liquid streams from this treatment has been scarcely analyzed (2). Furthermore, the introduction of the Integrated Pollution Prevention and Control (IPPC) regulations in the European Union (Council Directive 96/61/EC) requires control of the toxicity of industrial liquid effluents, thus making toxicity an important additional parameter in wastewater treatment studies. The most frequently used method to determine the toxicity of an aqueous sample consists of a bioassay using luminescent marine bacteria (Vibrio fischeri, also known as Photobacterium phosphoreum, NRRL B-11177). This test can be performed through three different normalized procedures, ToxAlert (Merck, Darmstadt, Germany) and Mutatox and Microtox (Azur Environmental, United States, formerly Microbics Corp.). The experimental procedure Microtox has been adopted from the official standards of several countries (USA, ASTM method D5660-1995; Germany, DIN 38412-1990; France, AFNOR T90320-1991; and Spain, ISO 11348-3-1998). In wet catalytic oxidation of phenolic wastewater with copper-based catalysts, phenol is not directly oxidized to CO2 but to a number of different organic intermediates, as identified previously (18). Some unidentified compounds are also produced, particularly from catechol oxidation. These changes in composition during the catalytic oxidation of phenol require a detailed study of the evolution of toxicity throughout the process. Thus, the toxicity of the identified and unidentified organic intermediates as well as that resulting from the copper losses due to catalyst leaching and their possible synergistic effects must be examined. The detailed knowledge of this process will provide the information needed for conducting catalytic oxidation in order to better comply with the discharge policies and regulations. There are several references in the literature focusing on the toxicity of phenolic wastewaters, after ozonation and photocatalytic treatment (24-26). Most of these report increased toxicity during the early stages of phenol oxidation. Shang et al. (19-21) showed that this toxicity has been linked to color development (measured by absorbance at 420 nm) at the initial stage of ozonation, and it has been assumed that the colored intermediates give rise to increased toxicity compared to that of the original pollutant. After photocatalytic treatment of wastewaters containing different aromatic compounds, the early intermediates were more toxic and less biodegradable than the original pollutants (24); an improvement in the BOD/TOC ratio and in toxicity were noticed only when mineralization of at least 80% of the initial TOC was reached. A noticeable increase of toxicity has been also reported in the photocatalytic oxidation of trichlorophenol (25), and the toxic effects were confined to the dihydroxilated photolytic intermediate formed 3,5-dichlorocatechol. Therefore, because of the increase of toxicity accompanying the early stages of oxidation treatments, which remains even after the disappearance of the original pollutant, a more rigorous identification of the intermediates involved and an assessment of their toxicities are essential. The aim of this work is to establish the relationship between toxicity and VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of experimental system (basket-stirred tank reactor, BSTR). intermediates generated in the catalytic wet oxidation of phenol, with a commercial catalyst based on copper as active phase (Engelhard Cu-0203T). The oxidation route of phenol has been previously determined (18). Phenol oxidation yielded, first, hydroquinone and catechol as intermediates. Under the conditions used, catechol oxidation did not yield o-benzoquinone but an unidentified condensate intermediate, which was further oxidized to short-chain acids and CO2. On the other hand, p-benzoquinone was a clearly detected reaction product of hydroquinone oxidation. Data on the toxicity of each identified intermediate have been obtained from the literature and have also been determined in this work. The evolution of toxicity throughout the oxidation of several initial pollutants (phenol, catechol, and hydroquinone) has been assessed and related to the intermediates identified. The toxicities of intermediates resulting from hydroquinone and catechol oxidation have been also compared.
Experimental Setup and Procedure Microtox Toxicity Text. The toxicity of the liquid samples at different oxidation stages was determined by means of a bioassay following the standard Microtox test procedure (ISO 11348-3, 1998) (based on the decrease of light emission by P.m phosphoreum resulting from its exposure to a toxicant). A Microtox M500 Analyzer (Azur Environmental) was used. The inhibition of the light emitted by the bacteria was measured after 15 min contact time. The EC50 is defined as the effective nominal concentration of toxicant that reduces the intensity of light emission by 50%. In the literature, the EC50 is also related to the dilution ratio of the wastewater sample that yields this 50% reduction. To avoid confusion, we designate as IC50 the ratio of the initial volume of sample (VS) to the one yielding, after the required dilution, a 50% reduction of the light emitted by the microorganisms (VF). The toxicity units of the wastewater are calculated as
TU )
100 IC50
(1)
Before measuring the toxicity, the pH values of all the samples were readjusted to between 6 and 7 to prevent the pH effect. All the chemicals used were purchased from Sigma-Aldrich, and the microorganisms were Microtox Acute Reagent supplied by I.O. Analytical. Catalytic Wet Oxidation Runs. A commercial catalyst from Engelhard (Cu-0203) with copper as its active component was used. The catalyst consists of 67-77% copper oxide, CuO, 20-30% copper chromite, 2CuO‚Cr2O3, and 1-3% synthetic graphite (Cu and Cr content about 60% and 10%, respectively); it has been used in an extruded form (dp ) 1/8 134
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in.), and its surface area, total pore volume, and packed average bed density were, respectively, Sg ) 10 m2/g, Vp ) 0.10 cm3/g, and FL ) 2 g/cm3. Experiments were carried out in a basket-stirred tank reactor (BSTR), from Autoclave Engineers (500 mL Spectrum Reactor), operating in batch for the liquid phase, while a flow of oxygen was continuously fed to the reactor to ensure oxygen saturation. A wastewater sample (250 mL) was initially placed in the reactor, and the pH was set at 3.5. This pH was selected from a previous study carried out with this catalyst (12). Values of pH about 6-7 produce an increase of the duration of the induction period, after which the pH decreases sharply to a value of 2.5-3.5, due to the formation of acidic compounds. The catalyst was boiled twice in distilled water before its first use and then placed inside the basket of the reactor. The vessel was pressurized with an oxygen stream (0.15 L/min, STP). Constant flow rate and pressure conditions were achieved by means of a mass flow controller and a backpressure control valve, respectively. The reactor was heated to the reaction temperature, which was measured and controlled in the liquid phase, within a deviation of (0.5 K. Agitation was provided by a magnetic stirrer operating at around 700 rpm. When steady-state conditions for temperature and pressure were achieved, the target compound was fed to the reactor by injecting a loop of a concentrated aqueous solution. Phenol, catechol, and hydroquinone were used as target pollutants. At time zero, the initial target concentration is calculated as the product of the concentration in the loop and the ratio of loop volume to reactor volume. A schematic of the experimental device is given in Figure 1. The experiments were carried out at 140 °C, with a catalyst loading of 180 g/L and the oxygen partial pressure set at 16 bar. A blank reaction without catalyst was also carried out, using phenol as pollutant. Liquid samples were periodically withdrawn from the reactor and analyzed. Phenol and organic intermediates were identified and quantified by HPLC (Hewlett-Packard, model 1100) using a diode array detector (HP G1315A). A Nucleosil C18 5 µm column (AV-750, 25 cm long, 4.6 mm diameter) was used as the stationary phase and 4 mM aqueous sulfuric solution as the mobile phase. The mobile phase was passed at a flow rate of 1-1.9 mL/min, and the UV detector was used at 192 (hydroquinone), 210 (catechol and phenol), and 244 (benzoquinone) nm wavelengths. Some organic acids were also analyzed by ionic chromatography (Metrohm, model 761 Compact IC) using a conductivity detector. A column of anion suppression Metrosep A SUPP 5 (25 cm long, 4 cm diameter) was used as the stationary phase and an aqueous solution of 3.2 mM Na2CO3 and 1 mM NaHCO3 as the mobile phase, at a constant flow rate of 0.07 mL/min. Total organic carbon (TOC) values in the liquid phase were determined with a SGE analyzer, setting the pH
TABLE 1. Data from Individual Toxicity Tests for the Intermediates Detected in the Catalytic Wet Oxidation of Phenol (data at 15 min) chemical
EC50 (mg/L)
lit. values
ref
phenol (PhOH)
16.7 ( 4.2
catechol (ctl)
8.32 ( 2.7
hydroquinone (hq)
0.041
1,4-benzoquinone (bq)
0.1
27 28 29 30 31 32 29 30 31 32 29 31 32 31 32
maleic acid (mal) oxalic acid (oxa) formic acid (for) propionic acid (prop) piruvic acid (piru) acetic acid (ace) Cu2+
247 ( 50 >450 162 ( 43 731 ( 27 >450 130 ( 16 0.58 ( 0.26
22.1 29.5 22.0 34.8 21.1-42 28.6 31.8 (5 min) 47.7 29.6-31.8 32 0.08 (5 min) 0.0382-0.0798 0.042-0.079 0.022-0.08 0.0085-1.4
FIGURE 2. Synergistic effects between copper ions and aromatic intermediates from phenol oxidation. (I) catechol (ctl) 25 mg/L + Cu2+ 0.58 mg/L, (II) hydroquinone (hq) 0.05 mg/L+ Cu2+ 0.58 mg/L, (III) benzoquinone (bq) 0.05 mg/L+ Cu2+ 0.583 mg/L. TU predicted ) (TUi + TU Cu2+), i ) ctl, hq, bq.
value of the samples at 3 by adding HClO4. All the liquid samples were analyzed for copper and chromium by atomic absorption, using a Varian SpectrAA 220 spectrometer.
Results and Discussion Toxicities of the Intermediates Detected in the Oxidation of Phenol. To determine the toxicity of each intermediate previously identified in the catalytic oxidation of phenol (18), the effective nominal concentration (EC50) was expressed in mg/L. Some leaching of copper from the catalyst took place at the acidic pH caused by the presence of acid intermediates produced throughout the oxidation process, and this metal ion was itself considered a pollutant, due to its toxicity. Chromium was also checked, and no leaching was detected. The experimental EC50 values are summarized in Table 1, where the values reported in the literature have also been included for ease of comparison. The agreement between our experimental values and those given in the literature is reasonably good, considering that the differences are explainable by the different testing conditions (time, pH and the strain of microorganisms used in each case). It is well-known that catechol and hydroquinone form complexes with divalent metallic ions, and therefore, it is of interest to analyze the potential occurrence of synergistic effects enhancing the toxicity of catechol and hydroquinone in the presence of copper. To this end, aqueous solutions of known concentrations of catechol-copper and hydroquinone-copper were prepared. The toxicities of these mixtures were measured and the corresponding values expressed according to eq 1. Additionally, the toxicity of the mixtures was predicted based on the EC50 values corresponding to the individual components. This was done according to the following expression, which is based on the concept of concentration addition (21, 27)
M)
z1 z2 + EC501 EC502
(2)
in which z1 and z2 are the concentrations, in mg L-1, of individual pollutants in the mixture. Figure 2 compares the experimentally determined toxicity values of the samples and those predicted from their composition and the EC50 of the individual components.
FIGURE 3. Experimental results for catalytic oxidation of phenol. Operating conditions: T ) 140 °C, QO2 ) 150 mL/min(STP), PO2 ) 16 bar. CCAT ) 180 g/L pHo ) 3.5. CPhOHo ) 800 mg/L phenol (Co ) 608 mg/L as carbon). The results shown in Table 1 reveal that hydroquinone and p-benzoquinone resulting from a series reaction within the oxidation route of phenol are, respectively, 3 and 2 orders of magnitude more toxic than phenol, whereas catechol, which results from a reaction occurring in a parallel reaction of hydroquinone, shows only a toxicity value 2-fold that of phenol. However, as can be seen in Figure 2, the synergistic effect observed on toxicity in the case of the catechol-copper mixture is higher than those encountered for the mixtures of hydroquinone and p-benzoquinone with copper. It can therefore be concluded that, from an environmental point of view, it is not sufficient to follow the oxidation of phenol only from the disappearance of this pollutant; one must check VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Experimental results obtained for catalytic oxidation of catechol. Operating conditions: T ) 140 °C, QO2 ) 150 mL/min(STP), PO2 ) 16 bar. CCAT ) 180 g/L pHo ) 3.5. Cctlo ) 400 mg/L cathecol (Co ) 262 mg/L as carbon). FIGURE 4. Experimental results for noncatalyzed phenol oxidation. Operating conditions: T ) 140 °C, QO2 ) 150 mL/min(STP), PO2 ) 16 bar. CCAT ) 0 g/L pHo ) 3.5. CPhOHo ) 800 mg/L phenol (Co ) 608 mg/L as carbon). the evolution of subsequent aromatics compounds in the oxidation route such as catechol, hydroquinone, and pbenzoquinone, considering also the existence of dissolved copper from the catalyst. This enhances the importance of effective control of copper leaching, an aspect that has been usually omitted in the literature. Catalytic Oxidation of Phenol and Dihydroxyl Intermediates. Phenol, catechol, and hydroquinone have been fed as original pollutants into the BSTR reactor. The experiments have been performed under the following conditions: PO2 ) 16 atm, T ) 140 °C, pHo ) 3.5. The initial concentrations, in mg/L, were 800 for phenol and 400 for catechol and hydroquinone; the catalyst was used at 180 g/L doses, and in the case of phenol, an experiment was carried out also without the catalyst. The results obtained for phenol oxidation, with and without catalyst, are given in Figures 3 and 4, respectively. Figures 5 and 6 show the results obtained for catalytic oxidation of catechol and hydroquinone, respectively. All these figures include a comparison of the experimentally measured toxicity values and those predicted according eq 2 using the EC50 values of Table 1. In all the cases, to perform the ecotoxicity test, the copper in solution was previously removed from the samples by filtration with ion exchanger cartridges (Metrohm AG CH-9101, reference 6.1012.010/ 6.1012.100 IC-H). In the experiments carried out, the development of a dark brown color was observed during the early stages of oxidation, which disappeared at longer reaction times. UV-vis spectra of samples withdrawn at short times, when the maximum color intensity occurs, were obtained 136
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using a spectrophotometer. The resulting spectra are shown in Figure 7. The absorbance values at 332 nm correlate well with color intensity; therefore, the absorbance measurements were used to follow the evolution of color. These measurements were made using a dilution ratio of 1:7. As can be seen in Figures 3 and 4, a significant increase of toxicity occurs in the initial stages of phenol oxidation, which corresponds to the formation of hydroquinone and p-benzoquinone. This effect is more important when no catalyst is used. The catalyst results in a decrease in the toxicity values at longer reaction times, up to about one-quarter of the initial ones. Such reduction is not achieved in the absence of the catalyst. Moreover, without the catalyst, the TU values predicted from the intermediates identified do not fit the experimentally measured ones, suggesting that some toxic intermediates remain unidentified. Several samples from the noncatalytic phenol run at short times (when the highest difference between the experimental measured TOC values and the predicted ones were observed) have been analyzed by GC/MS. The unknown intermediate seems to be some isomer of dihydroxydibenzofuran. The presence of this compound has been also confirmed by the following techniques: 1H NMR spectra, 13C CP/mass spectra, and FTIR spectra. The addition of the catalyst also has a positive effect on the phenol and catechol oxidation rate, and the asymptotic level of TOC conversion (about 0.7) is reached at shorter reaction times. On the other hand, when no catalyst is used, the experimentally measured TOC values of the samples are in disagreement with those predicted from the intermediates identified (ΣCi), the highest differences corresponding to the occurrence of the highest values of 332 nm absorbance and after a maxima for hydroquinone, p-benzoquinone, and
FIGURE 6. Experimental results for catalytic oxidation of hydroquinone. Operating conditions: T ) 140 °C, QO2 ) 150 mL/min (STP), PO2 ) 16 bar. CCAT ) 180 g/L pHo ) 3.5. Chqo ) 400 mg/L hydroquinone (Co ) 262 mg/L as carbon).
took place (increase of the absorbance value at 332 nm in Figure 5). These two facts suggest that some unidentified products have been formed from catechol oxidation, most probably some catechol and/or o-benzoquinone condensation products, as previously postulated (18). The occurrence of a copper-catalyzed condensation reaction of catechol can be postulated as well, and in addition, catechol can also form a complex with copper in solution as the result of catalyst leaching. These intermediates seem to be more toxic than catechol but orders of magnitude less toxic than hydroquinone and p-benzoquinone and are responsible for the disagreement between measured and predicted TU values. These intermediates are subsequently oxidized to acids, as can be concluded from the experimental and predicted TOC curves given in Figure 5. As depicted in Figure 6, hydroquinone is oxidized more slowly than catechol, yielding initially p-benzoquinone, which is further oxidized to less toxic organic acid intermediates. Samples obtained at the early stages of hydroquinone oxidation exhibit a light brown color, lighter than that observed for the samples from catechol oxidation, probably due to condensation products. Only small differences are found between the TOC values measured and those calculated from the intermediates identified, which means that only traces of unidentified intermediates are produced. These traces and/or the synergistic effects between hydroquinone and catechol with copper leached from the catalyst must be responsible for the higher TU experimental values relative to that predicted. The absorbance at 332 nm was lower than that obtained from catechol, but the toxicity was substantially higher. This confirms that, under the reaction conditions studied, there is no direct correspondence between color development and toxicity, hydroquinone and p-benzoquinone being the more toxic intermediates produced. The evolution of the TU values in catalytic hydroquinone wet oxidation confirms this assertion. As seen in Figure 6, toxicity always decreases on oxidation, when hydroquinone is the initial pollutant, which establishes that this compound is the most toxic intermediate in the phenol oxidation route. Consequently, under the operating conditions studied, it has been proven that the catechol route leads to a liquid stream that, in principle, should be more amenable to a further biological treatment than that resulting from the hydroquinone route.
Acknowledgments The authors acknowledge financial support for this research from the Spanish MCYT (project PPQ2000-1763-C03 and the Ramon-Cajal Program) and the Consejeria de Educacion of the Comunidad Autonoma de Madrid (project 07M/0094/ 2000). The authors also thank Engelhard for kindly supplying the commercial catalyst used in this work.
FIGURE 7. Absorbance spectra for phenol (runs 1 and 2), catechol (run 3), and hydroquinone (run 4) oxidation experiments. catechol formation. Consequently, the compounds that remain unidentified in the reaction media correspond, most probably, to aromatic condensation products. For both catalytic and noncatalytic phenol oxidation, the maximum of toxicity does not correspond to the maximum of color development, as postulated by others (19-21), but to the maximum observed for hydroquinone and p-benzoquinone concentration, as shown in Figures 3 and 4. The results of Figure 5 show that catechol disappears almost instantaneously, and only acid intermediates are identified, but these do not justify the TOC values measured at short reaction times. Moreover, it was observed that development of a strong brown color at those short times
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Received for review May 22, 2003. Revised manuscript received October 13, 2003. Accepted October 14, 2003. ES030476T
