Environmental Technology
ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20
Ozonation of bezafibrate over ceria and ceria supported on carbon materials Alexandra G. Gonçalves, José J.M. Órfão & Manuel Fernando R. Pereira To cite this article: Alexandra G. Gonçalves, José J.M. Órfão & Manuel Fernando R. Pereira (2015) Ozonation of bezafibrate over ceria and ceria supported on carbon materials, Environmental Technology, 36:6, 776-785, DOI: 10.1080/09593330.2014.961563 To link to this article: http://dx.doi.org/10.1080/09593330.2014.961563
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Date: 30 October 2015, At: 09:51
Environmental Technology, 2015 Vol. 36, No. 6, 776–785, http://dx.doi.org/10.1080/09593330.2014.961563
Ozonation of bezafibrate over ceria and ceria supported on carbon materials Alexandra G. Gonçalves, José J.M. Órfão and Manuel Fernando R. Pereira ∗ Departamento de Engenharia Química, Faculdade de Engenharia, Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Universidade do Porto, Rua Dr. Roberto Frias, Porto 4200-465, Portugal
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(Received 3 March 2014; final version received 24 May 2014 ) Two catalysts containing ceria dispersed on the surface of multi-walled carbon nanotubes and activated carbon were investigated as ozonation catalysts for the mineralization of bezafibrate (BZF). The results were compared with those obtained in the absence of the catalyst and in the presence of the parent carbon materials, as well as in the presence of ceria (CeO2 ). Carbon materials containing ceria showed an interesting catalytic effect. Both materials enhanced the mineralization of BZF relatively to single ozonation and ozonation catalysed by the corresponding carbon materials. In the catalytic ozonation with these materials, both surface and bulk reactions are supposed to occur. The BZF ozonation catalysed by CeO2 leaded to the highest mineralization degrees, indicating that the reaction mechanism followed in the presence of CeO2 (free radical oxidation in solution) leads to the formation of intermediates more easily degradable, mainly after 120 min of reaction. Some primary products and refractory final oxidation compounds in single and catalytic ozonation of BZF were followed. The original chlorine present on the BZF molecule is completely converted to chloride anion and part of the nitrogen is mainly − + converted to NO− 3 along with smaller amounts of NO2 and NH4 . Microtox tests revealed that simultaneous use of ozone and CeO2 originated lower acute toxicity. Keywords: catalytic ozonation; bezafibrate; multi-walled carbon nanotubes; activated carbon; cerium oxide
1. Introduction Water contamination by pharmaceuticals represents a rising environmental concern as a consequence of the rising consumption of human and veterinary drugs.[1–3] Bezafibrate (BZF), a compound from the group of fibrate drugs, is a lipid regulator largely used for the treatment of hyperlipidaemia. This compound is extensively used throughout the world and consequently has been frequently detected in the environment,[4–6] namely in sewage treatment plant effluents at concentrations up to 4.6 μg L−1 ,[4,7] rivers at concentrations up to 57.15 μg L−1 ,[7,8] streams [7] and surface waters at median concentrations of 3.1 μg L−1 .[4] Catalytic ozonation using carbon materials (such as activated carbon (AC) and multi-walled carbon nanotubes (MWCNTs)) has been proved to be an efficient technology for the treatment of refractory compounds like pharmaceuticals.[9–13] However, in previous studies, AC/cerium oxide composites were tested in the ozonation of organic compounds, and they showed better results than AC or cerium oxide.[14–16] The present work aimed at the assessment of two carbon materials (AC and MWCNT) containing dispersed ceria as catalysts for ozonation of BZF and to investigate a possible synergetic effect between carbon materials and cerium oxide. The BZF removal by ozonation has been
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
reported in the literature [17] and the simultaneous use of ozone and carbon materials in its removal was investigated in a previous work.[18] However, no results have been found for this reaction reporting the use of carbon materials containing ceria as catalysts. In this work, the concentrations of the identified oxidation compounds were monitored and the acute toxicity of the treated solutions was evaluated by Microtox assays. 2. Materials and methods 2.1. Preparation and characterization of the materials A commercial AC, Norit GAC 1240 PLUS, and commercial MWCNTs, Nanocyl 3100, were used as supports. According to the supplier, the MWCNTs have an average diameter of 9.5 nm, an average length of 1.5 μm and a carbon purity higher than 95%. AC was used with 100–300 μm particle sizes. These samples were impregnated with 20 wt.% of CeO2 using adequate solutions of Ce(NO3 )3 ·6H2 O in water (CeO2 /MWCNT and CeO2 /AC samples). After impregnation, they were dried at 110°C overnight. The decomposition of the cerium precursor was accomplished under nitrogen flow (100 cm3 min−1 ) at 400°C for 4 h, with a heating rate of 5°C min−1 . For comparative purposes, cerium oxide was prepared by precipitation with NaOH using an aqueous solution of Ce(NO3 )3 ·6H2 O
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(CeO2 sample), according to the procedure described by Orge et al. [15]. The prepared materials were characterized by N2 adsorption–desorption at − 196°C and thermogravimetric analysis. Samples containing ceria were also characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), as described elsewhere.[19] 2.2. Kinetic experiments The ozonation experiments were carried out at room pressure and temperature in a laboratory-scale reactor (ca. 1 L) equipped with agitation (maintained constant at 200 rpm) and a circulation jacket, using 700 mL of BZF solution with a concentration of 20 ppm, at the natural pH (around 4.4). Ozone was produced from pure oxygen in a BMT 802X ozone generator, and a constant flow rate (150 cm3 min−1 ) and constant inlet ozone concentration (50 g m−3 ) were used, originating an initial concentration of dissolved ozone of 8 mg L−1 . The concentration of ozone in the gas phase was monitored with a BMT 964 ozone analyser. Ozone leaving the reactor was converted into oxygen in a series of gas-washing bottles filled with potassium iodide (KI) solution. Adsorption on carbon materials and single ozonation experiments were performed in the same system, under identical experimental conditions, for comparative purposes. In the experiments carried out in the presence of a radical scavenger, a concentration of tert-butanol 10 times higher than the initial BZF concentration was used.[20] At selected times, samples for analysis were collected using a syringe and centrifuged. Most of the experiments were carried out in duplicate and the average deviation obtained was ± 2%. 2.3.
Analytical methods
Concentrations of BZF and reaction products were followed by HPLC using a Hitachi Elite LaChrom HPLC equipped with a diode array detector. For BZF and some intermediates, the stationary phase used was a LiChroCART Purospher STAR RP-18 column (250 mm × 4.6 mm, 5 μm) working at room temperature (stationary phase A). The compounds were analysed under isocratic elution with acetonitrile and ammonium acetate (10 mM, pH 4.0) in the proportion 50:50%v/v and at a flow rate of 1 mL min−1 , using an injection volume of 50 μL. Concentrations of some organic acids resulting from the BZF degradation were followed by HPLC using a Hitachi Elite LaChrom HPLC equipped with a UV detector. The stationary phase used was an Alltech OA-1000 column (300 mm × 6.5 mm), working at room temperature (stationary phase B), under isocratic elution with a solution of H2 SO4 5 mM at a flow rate of 0.5 mL min−1 and using an injection volume of 15 μL. The concentration of inorganic ions released during ozonation experiments was obtained by ion
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chromatography. The anions (nitrite, nitrate and chloride) were identified and quantified in a Dionex ICS-2100 Ion Chromatography System using a Dionex IonPac AS11HC column (250 mm × 4 mm). For the determination of ammonium, an IonPac CS12A column (250 mm × 4 mm) was used in a Dionex DX-120 Ion Chromatography System. The degree of mineralization of BZF solutions was obtained by total organic carbon (TOC) analysis in a Shimadzu TOC-5000A analyser. Microtox acute toxicity tests were performed using the Microtox basic test (Azure Environmental, USA), according to the procedure described in the standard ISO/DIS 11348-3 (see [21]), as described before in[16,18]).
3.
Results and discussion
3.1. Characterization results The characterization results are reported in a previous work.[19] Briefly, CeO2 /AC presents a higher BET surface area and microporosity than CeO2 /MWCNT (593 m2 g−1 and 0.258 cm3 g−1 vs. 262 m2 g−1 and 0 cm3 g−1 ). Comparing with the values of BET surface area reported in the literature for these carbon materials (SBET = 809 and 331 m2 g−1 for AC and MWCNT, respectively),[10] it is verified that CeO2 impregnation produced a noticeable decrease in the surface areas (reduction of 27 and 21% for AC and MWCNT, respectively), which is also accompanied by a significant reduction in the volume of micropores in the case of AC (24%). The results obtained by the thermogravimetric analysis indicate that the amount of ceria corresponds to approximately 20% in both samples. The XPS results showed that the Ce4+ /Ce3+ redox couple exists on the surface of the prepared catalysts. The CeO2 /AC sample presents the highest percentage of Ce3+ on the surface (50.4% compared to 46.7% obtained in the CeO2 /MWCNT sample).[19] The average crystallite diameter (dp ), obtained by XRD analysis, increases in samples CeO2 /MWCNT and CeO2 /AC (17.1 and 12.6 nm, respectively) compared to the CeO2 sample (10.3 nm). 3.2.
Catalytic ozonation of BZF
3.2.1. BZF degradation In the present work, the ozonation of BZF was investigated at natural pH (around 4.4) in the presence of the carbon materials containing dispersed cerium oxide, in order to investigate a possible synergic effect between the components. For comparison purposes, experimental results obtained by single ozonation and by catalytic ozonation with cerium oxide were included. In order to evaluate the adsorption capacity of the catalysts towards BZF, adsorption experiments were also performed. With the purpose of investigating the mechanism, experiments in the presence of the radical scavenger tert-butanol were also carried out.
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(a)
(b)
Figure 1. Evolution of the dimensionless (a) BZF and (b) TOC concentrations at natural pH ( ∼ 4.4) during adsorption, catalytic and non-catalytic ozonation and effect of tert-butanol (C0,BZF = 20 ppm, C0,tert-butanol = 0.5 mM, catalysts = 0.14 g L−1 ).
All experiments were followed for 5 h. The experimental results corresponding to BZF and TOC decay are depicted in Figure 1. Figure 1(a) shows that the ozonation by itself enables a fast decay of BZF concentration and the removal of this compound is achieved in less than 20 min. This is observed because ozone selectively attacks activated aromatic rings, which are present in BZF. There are no important differences between single and catalytic ozonation. However, when TOC results are compared (see Figure 1(b)), the presence of catalysts leads to higher mineralization degrees than single ozonation. After 300 min, CeO2 /MWCNT and CeO2 /AC were able to promote the mineralization of about
66 and 77% of initial BZF, respectively, showing a slightly better performance than the carbon materials (about 59% and 72% for MWCNT and AC, respectively, see [18]). Although having lower surface area than the corresponding carbon materials, the present catalysts allow higher mineralizations, in accordance with a cooperative action between carbon materials and cerium oxide. The former are able to promote the reduction of Ce4+ to Ce3+ species, due to the presence of delocalized π electrons on the basal planes, enhancing its catalytic properties for the generation of hydroxyl radicals in the solution.[14] With the simultaneous use of ozone and ceria a TOC removal higher than 90% was achieved after 300 min of reaction. This sample
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Environmental Technology presents the best catalytic performance, except for short reaction times, since it surpasses the performance of AC containing ceria after about 150 min of reaction. It should be stressed that the leaching of CeO2 was not detected, which is in agreement with other studies performed in our laboratory using ozonation catalysts containing ceria and carbon.[14,15,22,23] Concerning the adsorption results, CeO2 /AC adsorbs higher amounts of BZF than CeO2 /MWCNT. In fact, the adsorption experiment on CeO2 /AC resulted in the practically total removal of BZF after 300 min (about 96%), compared to 67% with CeO2 /MWCNT. These results are similar to those obtained with the corresponding carbon materials, contrary to what was verified for other studied compounds in [16] in which the adsorption capability of the materials containing ceria decreases due to their lower surfaces areas relatively to MWCNT and AC. Then, BZF adsorption cannot be neglected. These results indicate that the specific surface area does not have an important role in the adsorption of BZF since ceria has a very low specific surface area and adsorption is significant. This may be due to the high affinity between the BZF molecule and the surface of the catalysts. In the case of CeO2 /AC, adsorption even leads to a higher TOC removal than ozonation, which is explained by the easier adsorption of BZF than the respective oxidation products. In an attempt to better understand the mechanism of BZF catalytic ozonation with the prepared materials, a few experiments were performed in the presence of a radical scavenger. Due to its HO• radical scavenging action, tertbutanol was added to the solution, in order to suppress the reactions in the bulk between HO• radicals and BZF and/or the corresponding oxidation products. The results presented in Figure 1 show that the presence of tert-butanol accelerates the BZF degradation, indicating that the scavenging of the radicals in the solution favours the reaction between BZF and ozone, which is more effective than radicals for the degradation of BZF. Nevertheless, as mentioned above, the main advantage of using a catalyst is the mineralization of solutions. Unfortunately, due to the important contribution of tert-butanol to the TOC value of the solution, it was not possible to follow this parameter in this type of experiments. Cyclic experiments using CeO2 /MWCNT were carried out with the purpose of assessing the eventual deactivation during mineralization of oxalic acid, which is a smaller molecule and one of the by-products resultant from BZF oxidation. It was verified that there is only a slight decrease in activity from the first to the second cycle, remaining stable thereafter.[22] A similar performance was observed in the ozonation experiments with the erytromycin (pharmaceutical compound).[23] Therefore, a similar performance should be observed in the cyclic experiments of BZF ozonation.
3.2.2.
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Analysis of BZF oxidation intermediates and by-products As a result of BZF oxidation in the presence of ozone several intermediates and by-products are formed, but their complete identification was not always possible to accomplish. 3-[(4-chlorophenyl)formamido]propanoic acid (BBR), 4-chlorobenzoic acid (pCBA) and 4-chloroN-[2-(4-hydroxyphenyl)ethyl]benzamide (BBV), found as aromatic primary intermediates both during catalytic and non-catalytic ozonation, and oxalic, pyruvic and oxamic acids, identified as final by-products,[18] were followed during catalytic and single ozonation of BZF, in the presence of tert-butanol. The evolutions of these oxidation products are presented in Figure 2. In order to elucidate about the different routes followed in the formation of the BZF degradation products during single and catalytic ozonation, a possible ozonation pathway of BZF, proposed in a previous work,[18] can be observed. BBR, pCBA and BBV are produced from the beginning of reaction, as can be observed in Figure 2(a)–(c). The formation and disappearance of these intermediates followed similar trends in the presence or absence of the tested catalysts, i.e. they are mainly produced during the initial stage of ozonation experiments and are easily removed from the solution (they are not detected after 30 min). In the experiments carried out in the presence of tert-butanol, BBR and pCBA are produced in higher extension from the beginning of reaction and do not disappear or disappear much later, which is indicative that those compounds are resulting mainly from the direct oxidation by ozone of their parent compounds and/or the hydroxyl radicals are the main oxidant involved in their degradation mechanism. BBV is produced only in the absence of tert-butanol, which suggests that it is resultant from BZF decomposition by a radical mechanism.[18] Figure 2(a) and (b) shows that in the experiments performed in the presence of tert-butanol the evolutions of BBR and pCBA are very different depending on the catalysts used, which suggests different reaction pathways. In fact, when the CeO2 sample is used, these compounds are produced in larger amounts and persist in the solution for longer times than when using the remaining catalysts, indicating that the reaction mechanism of BZF and its primary oxidation products degradation occurs essentially in the solution via HO• radicals. On the other hand, these intermediates are produced in lower extension in the presence of other catalysts (mainly, CeO2 /MWCNT), which suggests that the reaction mechanism over these catalysts involves both reaction on the surface of catalyst and reactions with HO• radicals in the liquid phase. The evolution of the carboxylic acid concentrations in the presence of ceria and the materials containing ceria is quite different from those corresponding to single ozonation and ozonation catalysed by carbon materials (see [18]). In the former case, faster and total disappearance
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(b)
(c)
(d)
(e)
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Figure 2. Evolution of intermediates and by-products concentrations during catalytic and non-catalytic ozonation of BZF: (a) BBR, (b) pCBA, (c) BBV, (d) oxalic acid, (e) pyruvic acid and (f) oxamic acid.
of pyruvic acids occurs, mainly in the presence of CeO2 and CeO2 /AC. These catalysts also lead to a faster formation and initial degradation of oxalic acid and, according to TOC data, they correspond to the highest mineralization degrees observed (see Figure 1). Oxalic and oxamic acids are mainly produced by the radical mechanism because they are formed in lower amounts when tert-butanol is
present in the solution, where only molecular ozone was available to react with their parent intermediates. On the other hand, pyruvic acid is produced early in larger amounts and its concentration begins to decrease much later in the experiments with tert-butanol, which is indicative that this compound results mainly from the direct oxidation of the parent compounds by ozone and/or hydroxyl
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0.09 0.11 0.10 0.09 0.18 0.25 0.36 0.37 0.84 1.49 3.08 6.14 1.56 2.61 4.69 6.52 0.51 0.29 0.48 0.56 2.23 1.17 1.58 2.12 0.47 2.17 0 0 0 1.32 0 0 0.54 0.51 0.50 0.49 1.27 0.58 1.29 0.82 3.00 0.57 8.92 2.94 3.82 0.79 7.02 1.07 1.40 0.89 0.92 0.41 0 0 0 0 0 0 0 0 0 0 0 0 0.11 0.00 0.04 0.07 0 0 0 0 0 0 0 0 0 0 0 0 0.73 1.16 1.07 1.03 0 0 0 0 0 0 0 0 0 0 0 0 27.0 23.8 30.1 35.7 22.6 31.9 13.2 16.2 17.7 12.6 1.5 1.1 10.0 6.2 1.0 0.2 3.4 3.0 3.1 2.6 3.7 2.0 3.2 3.3 4.3 4.2 12.0 9.1 5.4 4.7 11.7 7.6 91.1 70.2 93.5 94.4 82.9 63.8 42.6 53.6 76.3 53.2 17.5 10.4 54.1 29.2 11.2 1.3 + CeO2 /MWCNT + CeO2 /AC + CeO2
+ CeO2 /MWCNT + CeO2 /AC + CeO2
+ CeO2 /MWCNT + CeO2 /AC + CeO2
5.5 26.8 3.4 3.0 13.4 34.2 54.2 43.1 19.4 42.6 70.5 80.5 40.5 66.1 77.1 91.1 + CeO2 /MWCNT + CeO2 /AC + CeO2
300 min
180 min
30 min
O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 5 min
Oxamic acid Pyruvic acid Oxalic acid BBV pCBA BBR TOCnon-identified TOCidentified
TOCidentified (%) TOCnon-identified (%) TOCremoved (%) Experiments
Contribution for TOC of each compound identified (%)
TOC removal and present (both non-identified and identified as compounds) achieved after 5, 30, 180 and 300 min of single and catalytic ozonation of BZF.
radicals are the main oxidant involved in their degradation mechanism. Analysing the values of TOC removed and present in the solution (both non-identified and identified as compounds) after 5, 30, 180 and 300 min of reaction (see Table 1), it is verified that the fraction of TOC not identified as specific compounds is significant, but that fraction tends to decrease with the reaction time. The unsaturated compounds produced during the initial stages of BZF are quickly degraded, originating mainly the identified carboxylic acids (oxamic, oxalic and pyruvic acids), which are highly persistent. This fact justifies the decrease in the solution pH (up to approximately 3) during single and catalytic ozonation of BZF. The TOCnon-identified /TOCidentified ratio was determined in order to compare the treated solutions obtained by different ozonation experiments in terms of the identified compounds. Thus, the smaller this ratio the less non-identified compounds will be present in the treated solution. For high reaction times, ozonation with CeO2 gives rise to the lowest TOCnon-identified /TOCidentified ratio, meaning that this sample leads to the highest fraction of identified TOC. On the other hand, single ozonation leads to the highest TOCnon-identified /TOCidentified ratio also for high reaction times, showing that the solutions obtained after the treatment present a high amount of non-identified compounds. Comparing TOC removal for single ozonation after 300 min and catalytic ozonation after 30 min, similar values are observed. However, the TOCnon-identified /TOCidentified ratio obtained for these cases is very different, being the ratio obtained after 30 min ozonation in the presence of CeO2 much higher than that obtained after 300 min of single ozonation. Thus, although both cases present similar mineralization degrees, the compositions of the treated solutions are very different. This observation indicates that the degradation pathways of BZF depend on the presence of the catalyst. The conversion of the initial nitrogen and chlorine contents of BZF to inorganic ions such as nitrate, nitrite, ammonium and chloride was monitored during the reaction period and their evolutions are depicted in Figure 3. All chlorine present in BZF is converted to Cl− during the first stages of the ozonation experiments (see Figure 3b), similarly to what was observed in the presence of carbon materials alone.[18] In fact, the presence of CeO2 and CeO2 /AC during ozonation leads to 100% chlorine conversion to chloride anion within 60 min, whereas in ozonation with CeO2 /MWCNT and without catalyst the complete conversion is achieved after about 180 and 120 min, respectively. Therefore, there are no chlorine-containing organic compounds in solution after this time of ozonation, indicating some lability of the chlorine-containing ring present in the BZF molecule. By comparison with the carbon materials, CeO2 /AC leads to a slightly faster total conversion of chlorine than AC, whereas CeO2 /MWCNT converts it more slowly than MWCNT.[18] Analysing the nitrogen
Table 1.
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released as inorganic ions represented in Figure 3(a), during the first stages of ozonation experiments nitrite ion is present in higher amounts than nitrate and ammonium, achieving a maximum between 15 and 30 min. After this time, NO− 2 concentration decreases until complete removal at 120 min of reaction (for CeO2 and CeO2 /AC) and after 300 min (for CeO2 /MWCNT and single ozonation). This trend originates the peak observed in the curves of the total nitrogen mineralized during the initial stages of the reaction (see Figure 3(b)). Comparing nitrate and ammonium released, NO− 3 is produced in higher extension both in the presence or absence of catalyst. The differ+ ence observed in the released amounts of NO− 3 and NH4 may be related to the special features of the initial Ncontaining structure.[16,18,24] Regarding the formation of NH+ 4 , small differences were observed for the different catalytic systems studied, this ion being released in higher amounts in the presence of CeO2 at high reaction times. Higher differences were observed in the concentrations of NO− 3 . CeO2 /MWCNT led to much higher amounts of nitrate, particularly for longer reaction times, whereas CeO2 and single ozonation released lower concentrations. After 5 h of reaction only nitrate and ammonium are present in solution. At this time, differences in the pro+ portion of N produced as inorganic ions (NO− 3 and NH4 ) for the different catalytic systems were observed. In the + case of CeO2 , NO− 3 and NH4 were released in the proportion of about 1.3/1, whereas in catalytic ozonation in the presence of CeO2 /MWCNT and CeO2 /AC those inorganic ions are produced in the proportion of 5.6/1 and 2.6/1, respectively. Comparing these results with those obtained with the parent carbon materials in,[18] it is verified that + the proportion of NO− 3 and NH4 released in the experiment with CeO2 /MWCNT is similar to that determined when MWCNT was used (6.1/1), whereas in the case of CeO2 /AC this proportion is closer to that obtained for CeO2 instead of AC (8.3/1). These observations indicate that the use of AC and MWCNT as ozonation catalysts for BZF degradation leads to the formation of N-containing organic intermediaries that are oxidized more predominantly to NO− 3 than those produced in the presence of CeO2 and carbon materials containing ceria, suggesting that the preferential degradation pathway of BZF ozonation may be influenced by the type of catalyst. Concerning BZF-derived nitrogen, there is a significant difference between single and catalytic ozonation (see Figure 4). After 5 h of single ozonation, only 22% of N was mineralized, whereas in the presence of the catalyst more than 32% of nitrogen was converted to inorganic ions. On the other hand, ozonation in the absence of catalysts leads to the smallest amount of N-containing oxamic acid ( ∼ 12%). This indicates that the initial nitrogen is converted to other N-containing compounds, not identified, without being further oxidized. During ozonation catalysed by CeO2 /MWCNT much less oxamic acid accumulates in the solution than in the presence of CeO2 /AC (23 vs.
(a)
(b)
− Figure 3. Evolution of the dimensionless (a) NO− 2 , NO3 and + NH4 concentrations normalized by the nitrogen concentration present in BZF and (b) total chlorine and total nitrogen concentrations released as inorganic ions normalized by the chlorine and nitrogen concentrations present in BZF, respectively, during catalytic and non-catalytic ozonation of BZF at natural pH ( ∼ 4.4) (C0,BZF = 20 ppm, catalysts = 0.14 g L−1 ).
44%) but the release of nitrogen-containing inorganic ions is much higher (59 vs. 36%). Moreover, with both catalysts similar percentages of identified nitrogen-containing compounds are achieved. These results show that, after 5 h of reaction, CeO2 /MWCNT promotes higher nitrogen mineralization levels than the remaining catalysts, in spite of leading to lower TOC removals (see Figure 1), which means that the preferential degradation pathway depends on the catalyst. When these results are compared to those obtained with the corresponding carbon materials in,[18] it is observed that CeO2 /MWCNT mineralizes slightly more nitrogen than MWCNT (59 vs. 52%), with the fraction of non-identified nitrogen being lower. On the other hand, CeO2 /AC leads to a lower percentage of nitrogencontaining inorganic ions released than AC (36 vs. 56%) and a much higher accumulation of oxamic acid (44 vs. 17%). For the ozonation catalysed by CeO2 , almost all
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N compounds
N ions
100
5.0 17.7
90
19.5
80 70
23.1 66.2
62.6
N (%)
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50 40 30
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59.2 36.4
32.4
22.1
0
Ozonation experiments
Figure 4. Percentage of BZF nitrogen content converted to inorganic ions and identified N-containing oxidation products identified, after 300 min of single and catalytic ozonation at natural pH ( ∼ 4) (C0,BZF = 20 ppm, catalysts = 0.14 g L−1 ).
products were identified and more than 60% of N is present in the final solution as oxamic acid, and only 32% correspond to nitrogen-containing inorganic ions. Thus, it can be concluded that the BZF degradation pathway followed in the presence of CeO2 is more effective in the degradation of nitrogenated compounds. Moreover, the BZF reaction pathway followed in the case of CeO2 /MWCNT is similar to that followed during MWCNT catalytic ozonation, whereas CeO2 /AC and CeO2 lead to similar pathways. The incomplete nitrogen mass balance indicates that other non-identified refractory nitrogenated compounds are formed, contributing to the remaining TOC in the solution. Additionally, it is also possible that the reaction may lead to the formation of gaseous nitrogenated compounds, such as NOx , and the extension of their production may be dependent on the pathway followed and, consequently, also dependent on the selected catalyst. 3.2.3. Bioassays experiments The acute toxicity of the untreated BZF solution and solutions submitted to ozonation during 30, 180 and 300 min was evaluated by Microtox bioassays using the marine bacteria Vibrio fischeri.[21] The results are presented in Figure 5 and were obtained after 30 min of exposition by determination of the percentage of inhibition in the bacteria luminescence caused by each sample. After 30 min of reaction, the solution treated with CeO2 presents lower acute toxicity towards V. Fischeri than the
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untreated solution, whereas no major differences in the acute toxicity of the solution treated with carbon materials containing ceria are verified. This fact means that the intermediates formed in this period, in the presence of the studied catalysts, have lower or similar acute toxicity than the BZF untreated solution. This trend is contrary to that observed for the ozonation catalysed by carbon materials in,[18] indicating the formation of different intermediaries. After that time, the inhibition percentage of the bacteria luminescence continuously increases, mainly in the presence of catalyst, achieving inhibitions effects of 72, 82 and 59% for CeO2 /MWCNT, CeO2 /AC and CeO2 , respectively, after 300 min of reaction. Comparing the values obtained for carbon materials containing ceria with those reported before for the corresponding carbon materials (53% and 29% for MWCNT and AC, respectively),[18] a significant increase in the inhibition percentage is verified, mainly in the case of CeO2 /AC. Although CeO2 /AC and CeO2 /MWCNT lead to treated solutions with higher acute toxicity than the corresponding carbon materials, they allow achieving higher mineralization degrees, which indicates that the compounds produced have higher acute toxicity, due to the different pathways followed by the catalysts. 3.2.4.
Considerations on the reaction mechanism
The ozonation of organic compounds involves a number of complex reactions and several mechanistic approaches have been presented in the literature. In heterogeneous catalytic ozonation, both surface and liquid bulk reactions can occur, involving molecular ozone, HO• radicals and surface-oxygenated radical species. AC catalyses the decomposition of ozone in the aqueous phase by the formation of surface-oxygenated radical species as well as promoting the formation of HO• radicals in the solution.[12,25] The same was proposed for MWCNT.[10] In the case of ozonation catalysed by metal oxides, the proposed mechanisms generally assume that the adsorption of organic molecules and ozone takes place on the surface of the catalyst.[26] The interaction of ozone with the metal oxide surface results in the formation of free radicals that can initiate a radical chain-type reaction both on the surface of the catalyst and in the liquid phase, leading to the production of HO• radicals.[27,28] For cerium oxide catalysts, it has been shown that the free radical oxidation mainly occurs in the solution.[14,15,25] The presence of cerium oxide on AC is believed to promote the reduction of Ce4+ to Ce3+ species, due the existence of delocalized π electrons on the basal planes of the AC.[14,29] Therefore, the presence of the redox pair Ce4+ /Ce3+ on the catalyst surface is believed to enhance its catalytic properties for the generation of HO• radicals. Ceria/AC composites have been found to be very active catalysts for the ozonation of organic acids and some aromatic compounds.[14,29] In a previous work,
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Figure 5. Results of Microtox tests at 0, 30, 180 and 300 min of reaction, with exposure at bacteria V. fischeri during of 30 min.
AC and MWCNT containing dispersed ceria showed to be very active ozonation catalysts in the mineralization of sulphamethoxazole.[16] In this work, two types of reaction mechanism during ozonation in the presence of AC or carbon nanotubes containing ceria was observed. This may be attributed to a combination of heterogeneous catalytic reactions, occurring preferentially on the surface of the carbon materials, and homogeneous reactions between the solutes and HO• radicals resultant from the catalytic decomposition of ozone, mainly on cerium oxide. Nevertheless, in the case of BZF, and contrary to what occurs with other studied molecules (namely oxalic acid and sulphamethoxazole), CeO2 leads to higher mineralization degrees than CeO2 /MWCNT and CeO2 /AC. The results indicate that the mechanism followed in the presence of CeO2 (free radical oxidation reactions in solution) leads to the formation of intermediates more easily degradable, mainly after 120 min of reaction. 4. Conclusions Two catalysts containing ceria dispersed on the surface of MWCNTs and AC were investigated as ozonation catalysts for the mineralization of BZF. The results were compared with those obtained in the absence of catalyst and in the presence of the parent carbon materials used for the preparation of those catalysts, as well as in the presence of ceria. The mineralization of solutions was enhanced by the addition of the prepared catalysts. Carbon materials containing ceria dispersed on the surface lead to higher TOC removal than the corresponding carbon materials. Ozonation in the presence of CeO2 /AC resulted in a TOC removal higher than that obtained with CeO2 /MWCNT. With the simultaneous use of ozone and ceria a TOC removal of more than 90% was achieved after 300 min under the reaction conditions used. BBR, pCBA and BBV were found as primary products of BZF degradation, whereas oxalic, pyruvic and oxamic
acids were identified as final products. The presence of carbon materials containing ceria or ceria leads to fast and total disappearance of pyruvic acid, contrary to what occurs in single ozonation. Addition of the radical scavenger tert-butanol during catalytic or single ozonation evidenced the participation of HO• radical in the ozonation process. The original chorine in BZF is completely converted to chloride. The original nitrogen of the BZF is mainly con− verted to NO− 3 along with smaller amounts of NO2 and + NH4 in all cases. However, the nitrogen mass balance does not close, which indicates the formation of non-identified refractory nitrogenated organic compounds that contribute to the remaining TOC in the solution. Microtox tests revealed that in the early stages of single and catalytic ozonation (first 30 min) intermediaries with lower or similar acute toxicity towards V. Fischeri than BZF are produced. However, for longer reaction times, the acute toxicity increases. The CeO2 sample led to the best results. The catalysts tested in the BZF ozonation showed very distinct results in terms of TOC removal, intermediates and by-products evolution, degradation of nitrogenated compounds and acute toxicity. The observed differences are due to the predominant degradation pathway followed in the presence of each catalyst, which influences the type and the amounts of organic compounds accumulated in the solution. The BZF ozonation catalysed by the CeO2 sample led to higher mineralization degrees than with CeO2 /MWCNT or CeO2 /AC. This indicates that the reaction mechanism followed with CeO2 (free radical oxidation in solution) involves the formation of intermediates more easily degradable.
Acknowledgements This work was supported by the European Community’s Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 226347; FEDER through COMPETE under Project
Environmental Technology PEst-C/EQB/LA0020/2013; FCT under Grant BD/45826/2008; and QREN, ON2 and FEDER under Project NORTE-07-0124FEDER-0000015.
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References [1] Mompelat S, Le Bot B, Thomas O. Occurrence and fate of pharmaceutical products and by-products, from resource to drinking water. Environ Int. 2009;35:803–814. [2] Cimetiere N, Soutrel I, Lemasle M, Laplanche A, Crocq A. Standard addition method for the determination of pharmaceutical residues in drinking water by SPE–LC–MS/MS. Environ Technol. 2013;34:3031–3041. [3] Dhanirama D, Gronow J, Voulvoulis N. Cosmetics as a potential source of environmental contamination in the UK. Environ Technol. 2012;33:1597–1608. [4] Daughton CG, Ternes TA. Pharmaceuticals and personal care products in the environment: agents of subtle change? Environ Health Perspect. 1999;107(Supplement 6): 907–938. [5] Stumpf M, Ternes TA, Haberer K, Seel P, Baumann W. Determination of drugs in sewage treatment plants and river water. Vom Wasser. 1996;86:291–303. [6] Vieno N, Tuhkanen T, Kronberg L. Removal of pharmaceuticals in drinking water treatment: effect of chemical coagulation. Environ Technol. 2006;27:183–192. [7] Ternes TA. Occurrence of drugs in German sewage treatment plants and rivers. Water Res. 1998;32: 3245–3260. [8] Calamari D, Zuccato E, Castiglioni S, Bagnati R, Fanelli R. Strategic survey of therapeutic drugs in the rivers Po and Lambro in northern Italy. Environ Sci Technol. 2003;37:1241–1248. [9] Faria PCC, Órfão JJM, Pereira MFR. Mineralisation of coloured aqueous solutions by ozonation in the presence of activated carbon. Water Res. 2005;39:1461–1470. [10] Gonçalves AG, Figueiredo JL, Órfão JJM, Pereira MFR. Influence of the surface chemistry of multi-walled carbon nanotubes on their activity as ozonation catalysts. Carbon. 2010;48:4369–4381. [11] Beltrán FJ, García-Araya JF, Giráldez I. Gallic acid water ozonation using activated carbon. Appl Catal, B. 2006;63:249–259. [12] Faria PCC, Órfão JJM, Pereira MFR. Activated carbon catalytic ozonation of oxamic and oxalic acids. Appl Catal, B. 2008;79:237–243. [13] Sánchez-Polo M, von Gunten U, Rivera-Utrilla J. Efficiency of activated carbon to transform ozone into OH radicals: influence of operational parameters. Water Res. 2005;39:3189–3198. [14] Faria PCC, Órfão JJM, Pereira MFR. A novel ceriaactivated carbon composite for the catalytic ozonation of carboxylic acids. Catal Commun. 2008;9:2121–2126.
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[15] Orge CA, Órfão JJM, Pereira MFR. Catalytic ozonation of organic pollutants in the presence of cerium oxide-carbon composites. Appl Catal, B. 2011;102:539–546. [16] Gonçalves AG, Órfão JJM, Pereira MFR. Catalytic ozonation of sulfamethoxazole in the presence of carbon materials: catalytic performance and reaction pathways. J Hazard Mater. 2012;239–240:167–174. [17] Dantas RF, Canterino M, Marotta R, Sans C, Esplugas S, Andreozzi R. Bezafibrate removal by means of ozonation: primary intermediates, kinetics, and toxicity assessment. Water Res. 2007;41:2525–2532. [18] Gonçalves A, Órfão JJM, Pereira MFR. Ozonation of bezafibrate promoted by carbon materials. Appl Catal, B. 2013;140–141:82–91. [19] Gonçalves AG, Órfão JJM, Pereira MFR. Ceria dispersed on carbon materials for the catalytic ozonation of sulfamethoxazole. J Environ Chem Eng. 2013;1:260–269. [20] Béltran FJ, Rivas FJ, Fernandez LA, Alvarez PM, Monterode-Espinosa R. Kinetics of catalytic ozonation of oxalic acid in water with activated carbon. Ind Eng Chem Res. 2002;41:6510–6517. [21] DIN, Water quality – Determination of the inhibitory effect of water samples on the light emission of Vibrio fischeri (Luminescent bacteria test). In Part 3: Method using freezedried bacteria, 2005; Vol. ISO/DIS 11348–3. [22] Gonçalves A, Silvestre-Albero J, Ramos-Fernández EV, Serrano-Ruiz JC, Órfão JJM, Sepúlveda-Escribano A, Pereira MFR. Highly dispersed ceria on activated carbon for the catalyzed ozonation of organic pollutants. Appl Catal, B. 2012;113–114:308–317. [23] Gonçalves AG, Órfão JJM, Pereira MFR. Ozonation of erythromycin over carbon materials and ceria dispersed on carbon materials. Chem Eng J. 2014;250:366–376. [24] Calza P, Medana C, Pazzi M, Baiocchi C, Pelizzetti E. Photocatalytic transformations of sulphonamides on titanium dioxide. Appl Catal, B. 2004;53:63–69. [25] Faria PCC, Órfão JJM, Pereira MFR. Activated carbon and ceria catalysts applied to the catalytic ozonation of dyes and textile effluents. Appl Catal, B. 2009;88:341–350. [26] Logemann FP, Annee JHJ. Water treatment with a fixed bed catalytic ozonation process. Water Sci Technol. 1997;35:353–360. [27] Legube B, Karpel Vel Leitner N. Catalytic ozonation: a promising advanced oxidation technology for water treatment. Catal Today. 1999;53:61–72. [28] Cooper C, Burch R. An investigation of catalytic ozonation for the oxidation of halocarbons in drinking water preparation. Water Res. 1999;33:3695–3700. [29] Faria PCC, Órfão JJM, Pereira MFR. Mineralization of substituted aromatic compounds by ozonation catalyzed by cerium oxide and a cerium oxide-activated carbon composite. Catal Lett. 2009;127:195–203.
ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20
Ozonation of bezafibrate over ceria and ceria supported on carbon materials Alexandra G. Gonçalves, José J.M. Órfão & Manuel Fernando R. Pereira To cite this article: Alexandra G. Gonçalves, José J.M. Órfão & Manuel Fernando R. Pereira (2015) Ozonation of bezafibrate over ceria and ceria supported on carbon materials, Environmental Technology, 36:6, 776-785, DOI: 10.1080/09593330.2014.961563 To link to this article: http://dx.doi.org/10.1080/09593330.2014.961563
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Date: 30 October 2015, At: 09:51
Environmental Technology, 2015 Vol. 36, No. 6, 776–785, http://dx.doi.org/10.1080/09593330.2014.961563
Ozonation of bezafibrate over ceria and ceria supported on carbon materials Alexandra G. Gonçalves, José J.M. Órfão and Manuel Fernando R. Pereira ∗ Departamento de Engenharia Química, Faculdade de Engenharia, Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Universidade do Porto, Rua Dr. Roberto Frias, Porto 4200-465, Portugal
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(Received 3 March 2014; final version received 24 May 2014 ) Two catalysts containing ceria dispersed on the surface of multi-walled carbon nanotubes and activated carbon were investigated as ozonation catalysts for the mineralization of bezafibrate (BZF). The results were compared with those obtained in the absence of the catalyst and in the presence of the parent carbon materials, as well as in the presence of ceria (CeO2 ). Carbon materials containing ceria showed an interesting catalytic effect. Both materials enhanced the mineralization of BZF relatively to single ozonation and ozonation catalysed by the corresponding carbon materials. In the catalytic ozonation with these materials, both surface and bulk reactions are supposed to occur. The BZF ozonation catalysed by CeO2 leaded to the highest mineralization degrees, indicating that the reaction mechanism followed in the presence of CeO2 (free radical oxidation in solution) leads to the formation of intermediates more easily degradable, mainly after 120 min of reaction. Some primary products and refractory final oxidation compounds in single and catalytic ozonation of BZF were followed. The original chlorine present on the BZF molecule is completely converted to chloride anion and part of the nitrogen is mainly − + converted to NO− 3 along with smaller amounts of NO2 and NH4 . Microtox tests revealed that simultaneous use of ozone and CeO2 originated lower acute toxicity. Keywords: catalytic ozonation; bezafibrate; multi-walled carbon nanotubes; activated carbon; cerium oxide
1. Introduction Water contamination by pharmaceuticals represents a rising environmental concern as a consequence of the rising consumption of human and veterinary drugs.[1–3] Bezafibrate (BZF), a compound from the group of fibrate drugs, is a lipid regulator largely used for the treatment of hyperlipidaemia. This compound is extensively used throughout the world and consequently has been frequently detected in the environment,[4–6] namely in sewage treatment plant effluents at concentrations up to 4.6 μg L−1 ,[4,7] rivers at concentrations up to 57.15 μg L−1 ,[7,8] streams [7] and surface waters at median concentrations of 3.1 μg L−1 .[4] Catalytic ozonation using carbon materials (such as activated carbon (AC) and multi-walled carbon nanotubes (MWCNTs)) has been proved to be an efficient technology for the treatment of refractory compounds like pharmaceuticals.[9–13] However, in previous studies, AC/cerium oxide composites were tested in the ozonation of organic compounds, and they showed better results than AC or cerium oxide.[14–16] The present work aimed at the assessment of two carbon materials (AC and MWCNT) containing dispersed ceria as catalysts for ozonation of BZF and to investigate a possible synergetic effect between carbon materials and cerium oxide. The BZF removal by ozonation has been
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
reported in the literature [17] and the simultaneous use of ozone and carbon materials in its removal was investigated in a previous work.[18] However, no results have been found for this reaction reporting the use of carbon materials containing ceria as catalysts. In this work, the concentrations of the identified oxidation compounds were monitored and the acute toxicity of the treated solutions was evaluated by Microtox assays. 2. Materials and methods 2.1. Preparation and characterization of the materials A commercial AC, Norit GAC 1240 PLUS, and commercial MWCNTs, Nanocyl 3100, were used as supports. According to the supplier, the MWCNTs have an average diameter of 9.5 nm, an average length of 1.5 μm and a carbon purity higher than 95%. AC was used with 100–300 μm particle sizes. These samples were impregnated with 20 wt.% of CeO2 using adequate solutions of Ce(NO3 )3 ·6H2 O in water (CeO2 /MWCNT and CeO2 /AC samples). After impregnation, they were dried at 110°C overnight. The decomposition of the cerium precursor was accomplished under nitrogen flow (100 cm3 min−1 ) at 400°C for 4 h, with a heating rate of 5°C min−1 . For comparative purposes, cerium oxide was prepared by precipitation with NaOH using an aqueous solution of Ce(NO3 )3 ·6H2 O
Environmental Technology
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(CeO2 sample), according to the procedure described by Orge et al. [15]. The prepared materials were characterized by N2 adsorption–desorption at − 196°C and thermogravimetric analysis. Samples containing ceria were also characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), as described elsewhere.[19] 2.2. Kinetic experiments The ozonation experiments were carried out at room pressure and temperature in a laboratory-scale reactor (ca. 1 L) equipped with agitation (maintained constant at 200 rpm) and a circulation jacket, using 700 mL of BZF solution with a concentration of 20 ppm, at the natural pH (around 4.4). Ozone was produced from pure oxygen in a BMT 802X ozone generator, and a constant flow rate (150 cm3 min−1 ) and constant inlet ozone concentration (50 g m−3 ) were used, originating an initial concentration of dissolved ozone of 8 mg L−1 . The concentration of ozone in the gas phase was monitored with a BMT 964 ozone analyser. Ozone leaving the reactor was converted into oxygen in a series of gas-washing bottles filled with potassium iodide (KI) solution. Adsorption on carbon materials and single ozonation experiments were performed in the same system, under identical experimental conditions, for comparative purposes. In the experiments carried out in the presence of a radical scavenger, a concentration of tert-butanol 10 times higher than the initial BZF concentration was used.[20] At selected times, samples for analysis were collected using a syringe and centrifuged. Most of the experiments were carried out in duplicate and the average deviation obtained was ± 2%. 2.3.
Analytical methods
Concentrations of BZF and reaction products were followed by HPLC using a Hitachi Elite LaChrom HPLC equipped with a diode array detector. For BZF and some intermediates, the stationary phase used was a LiChroCART Purospher STAR RP-18 column (250 mm × 4.6 mm, 5 μm) working at room temperature (stationary phase A). The compounds were analysed under isocratic elution with acetonitrile and ammonium acetate (10 mM, pH 4.0) in the proportion 50:50%v/v and at a flow rate of 1 mL min−1 , using an injection volume of 50 μL. Concentrations of some organic acids resulting from the BZF degradation were followed by HPLC using a Hitachi Elite LaChrom HPLC equipped with a UV detector. The stationary phase used was an Alltech OA-1000 column (300 mm × 6.5 mm), working at room temperature (stationary phase B), under isocratic elution with a solution of H2 SO4 5 mM at a flow rate of 0.5 mL min−1 and using an injection volume of 15 μL. The concentration of inorganic ions released during ozonation experiments was obtained by ion
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chromatography. The anions (nitrite, nitrate and chloride) were identified and quantified in a Dionex ICS-2100 Ion Chromatography System using a Dionex IonPac AS11HC column (250 mm × 4 mm). For the determination of ammonium, an IonPac CS12A column (250 mm × 4 mm) was used in a Dionex DX-120 Ion Chromatography System. The degree of mineralization of BZF solutions was obtained by total organic carbon (TOC) analysis in a Shimadzu TOC-5000A analyser. Microtox acute toxicity tests were performed using the Microtox basic test (Azure Environmental, USA), according to the procedure described in the standard ISO/DIS 11348-3 (see [21]), as described before in[16,18]).
3.
Results and discussion
3.1. Characterization results The characterization results are reported in a previous work.[19] Briefly, CeO2 /AC presents a higher BET surface area and microporosity than CeO2 /MWCNT (593 m2 g−1 and 0.258 cm3 g−1 vs. 262 m2 g−1 and 0 cm3 g−1 ). Comparing with the values of BET surface area reported in the literature for these carbon materials (SBET = 809 and 331 m2 g−1 for AC and MWCNT, respectively),[10] it is verified that CeO2 impregnation produced a noticeable decrease in the surface areas (reduction of 27 and 21% for AC and MWCNT, respectively), which is also accompanied by a significant reduction in the volume of micropores in the case of AC (24%). The results obtained by the thermogravimetric analysis indicate that the amount of ceria corresponds to approximately 20% in both samples. The XPS results showed that the Ce4+ /Ce3+ redox couple exists on the surface of the prepared catalysts. The CeO2 /AC sample presents the highest percentage of Ce3+ on the surface (50.4% compared to 46.7% obtained in the CeO2 /MWCNT sample).[19] The average crystallite diameter (dp ), obtained by XRD analysis, increases in samples CeO2 /MWCNT and CeO2 /AC (17.1 and 12.6 nm, respectively) compared to the CeO2 sample (10.3 nm). 3.2.
Catalytic ozonation of BZF
3.2.1. BZF degradation In the present work, the ozonation of BZF was investigated at natural pH (around 4.4) in the presence of the carbon materials containing dispersed cerium oxide, in order to investigate a possible synergic effect between the components. For comparison purposes, experimental results obtained by single ozonation and by catalytic ozonation with cerium oxide were included. In order to evaluate the adsorption capacity of the catalysts towards BZF, adsorption experiments were also performed. With the purpose of investigating the mechanism, experiments in the presence of the radical scavenger tert-butanol were also carried out.
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(a)
(b)
Figure 1. Evolution of the dimensionless (a) BZF and (b) TOC concentrations at natural pH ( ∼ 4.4) during adsorption, catalytic and non-catalytic ozonation and effect of tert-butanol (C0,BZF = 20 ppm, C0,tert-butanol = 0.5 mM, catalysts = 0.14 g L−1 ).
All experiments were followed for 5 h. The experimental results corresponding to BZF and TOC decay are depicted in Figure 1. Figure 1(a) shows that the ozonation by itself enables a fast decay of BZF concentration and the removal of this compound is achieved in less than 20 min. This is observed because ozone selectively attacks activated aromatic rings, which are present in BZF. There are no important differences between single and catalytic ozonation. However, when TOC results are compared (see Figure 1(b)), the presence of catalysts leads to higher mineralization degrees than single ozonation. After 300 min, CeO2 /MWCNT and CeO2 /AC were able to promote the mineralization of about
66 and 77% of initial BZF, respectively, showing a slightly better performance than the carbon materials (about 59% and 72% for MWCNT and AC, respectively, see [18]). Although having lower surface area than the corresponding carbon materials, the present catalysts allow higher mineralizations, in accordance with a cooperative action between carbon materials and cerium oxide. The former are able to promote the reduction of Ce4+ to Ce3+ species, due to the presence of delocalized π electrons on the basal planes, enhancing its catalytic properties for the generation of hydroxyl radicals in the solution.[14] With the simultaneous use of ozone and ceria a TOC removal higher than 90% was achieved after 300 min of reaction. This sample
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Environmental Technology presents the best catalytic performance, except for short reaction times, since it surpasses the performance of AC containing ceria after about 150 min of reaction. It should be stressed that the leaching of CeO2 was not detected, which is in agreement with other studies performed in our laboratory using ozonation catalysts containing ceria and carbon.[14,15,22,23] Concerning the adsorption results, CeO2 /AC adsorbs higher amounts of BZF than CeO2 /MWCNT. In fact, the adsorption experiment on CeO2 /AC resulted in the practically total removal of BZF after 300 min (about 96%), compared to 67% with CeO2 /MWCNT. These results are similar to those obtained with the corresponding carbon materials, contrary to what was verified for other studied compounds in [16] in which the adsorption capability of the materials containing ceria decreases due to their lower surfaces areas relatively to MWCNT and AC. Then, BZF adsorption cannot be neglected. These results indicate that the specific surface area does not have an important role in the adsorption of BZF since ceria has a very low specific surface area and adsorption is significant. This may be due to the high affinity between the BZF molecule and the surface of the catalysts. In the case of CeO2 /AC, adsorption even leads to a higher TOC removal than ozonation, which is explained by the easier adsorption of BZF than the respective oxidation products. In an attempt to better understand the mechanism of BZF catalytic ozonation with the prepared materials, a few experiments were performed in the presence of a radical scavenger. Due to its HO• radical scavenging action, tertbutanol was added to the solution, in order to suppress the reactions in the bulk between HO• radicals and BZF and/or the corresponding oxidation products. The results presented in Figure 1 show that the presence of tert-butanol accelerates the BZF degradation, indicating that the scavenging of the radicals in the solution favours the reaction between BZF and ozone, which is more effective than radicals for the degradation of BZF. Nevertheless, as mentioned above, the main advantage of using a catalyst is the mineralization of solutions. Unfortunately, due to the important contribution of tert-butanol to the TOC value of the solution, it was not possible to follow this parameter in this type of experiments. Cyclic experiments using CeO2 /MWCNT were carried out with the purpose of assessing the eventual deactivation during mineralization of oxalic acid, which is a smaller molecule and one of the by-products resultant from BZF oxidation. It was verified that there is only a slight decrease in activity from the first to the second cycle, remaining stable thereafter.[22] A similar performance was observed in the ozonation experiments with the erytromycin (pharmaceutical compound).[23] Therefore, a similar performance should be observed in the cyclic experiments of BZF ozonation.
3.2.2.
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Analysis of BZF oxidation intermediates and by-products As a result of BZF oxidation in the presence of ozone several intermediates and by-products are formed, but their complete identification was not always possible to accomplish. 3-[(4-chlorophenyl)formamido]propanoic acid (BBR), 4-chlorobenzoic acid (pCBA) and 4-chloroN-[2-(4-hydroxyphenyl)ethyl]benzamide (BBV), found as aromatic primary intermediates both during catalytic and non-catalytic ozonation, and oxalic, pyruvic and oxamic acids, identified as final by-products,[18] were followed during catalytic and single ozonation of BZF, in the presence of tert-butanol. The evolutions of these oxidation products are presented in Figure 2. In order to elucidate about the different routes followed in the formation of the BZF degradation products during single and catalytic ozonation, a possible ozonation pathway of BZF, proposed in a previous work,[18] can be observed. BBR, pCBA and BBV are produced from the beginning of reaction, as can be observed in Figure 2(a)–(c). The formation and disappearance of these intermediates followed similar trends in the presence or absence of the tested catalysts, i.e. they are mainly produced during the initial stage of ozonation experiments and are easily removed from the solution (they are not detected after 30 min). In the experiments carried out in the presence of tert-butanol, BBR and pCBA are produced in higher extension from the beginning of reaction and do not disappear or disappear much later, which is indicative that those compounds are resulting mainly from the direct oxidation by ozone of their parent compounds and/or the hydroxyl radicals are the main oxidant involved in their degradation mechanism. BBV is produced only in the absence of tert-butanol, which suggests that it is resultant from BZF decomposition by a radical mechanism.[18] Figure 2(a) and (b) shows that in the experiments performed in the presence of tert-butanol the evolutions of BBR and pCBA are very different depending on the catalysts used, which suggests different reaction pathways. In fact, when the CeO2 sample is used, these compounds are produced in larger amounts and persist in the solution for longer times than when using the remaining catalysts, indicating that the reaction mechanism of BZF and its primary oxidation products degradation occurs essentially in the solution via HO• radicals. On the other hand, these intermediates are produced in lower extension in the presence of other catalysts (mainly, CeO2 /MWCNT), which suggests that the reaction mechanism over these catalysts involves both reaction on the surface of catalyst and reactions with HO• radicals in the liquid phase. The evolution of the carboxylic acid concentrations in the presence of ceria and the materials containing ceria is quite different from those corresponding to single ozonation and ozonation catalysed by carbon materials (see [18]). In the former case, faster and total disappearance
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(b)
(c)
(d)
(e)
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Figure 2. Evolution of intermediates and by-products concentrations during catalytic and non-catalytic ozonation of BZF: (a) BBR, (b) pCBA, (c) BBV, (d) oxalic acid, (e) pyruvic acid and (f) oxamic acid.
of pyruvic acids occurs, mainly in the presence of CeO2 and CeO2 /AC. These catalysts also lead to a faster formation and initial degradation of oxalic acid and, according to TOC data, they correspond to the highest mineralization degrees observed (see Figure 1). Oxalic and oxamic acids are mainly produced by the radical mechanism because they are formed in lower amounts when tert-butanol is
present in the solution, where only molecular ozone was available to react with their parent intermediates. On the other hand, pyruvic acid is produced early in larger amounts and its concentration begins to decrease much later in the experiments with tert-butanol, which is indicative that this compound results mainly from the direct oxidation of the parent compounds by ozone and/or hydroxyl
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0.09 0.11 0.10 0.09 0.18 0.25 0.36 0.37 0.84 1.49 3.08 6.14 1.56 2.61 4.69 6.52 0.51 0.29 0.48 0.56 2.23 1.17 1.58 2.12 0.47 2.17 0 0 0 1.32 0 0 0.54 0.51 0.50 0.49 1.27 0.58 1.29 0.82 3.00 0.57 8.92 2.94 3.82 0.79 7.02 1.07 1.40 0.89 0.92 0.41 0 0 0 0 0 0 0 0 0 0 0 0 0.11 0.00 0.04 0.07 0 0 0 0 0 0 0 0 0 0 0 0 0.73 1.16 1.07 1.03 0 0 0 0 0 0 0 0 0 0 0 0 27.0 23.8 30.1 35.7 22.6 31.9 13.2 16.2 17.7 12.6 1.5 1.1 10.0 6.2 1.0 0.2 3.4 3.0 3.1 2.6 3.7 2.0 3.2 3.3 4.3 4.2 12.0 9.1 5.4 4.7 11.7 7.6 91.1 70.2 93.5 94.4 82.9 63.8 42.6 53.6 76.3 53.2 17.5 10.4 54.1 29.2 11.2 1.3 + CeO2 /MWCNT + CeO2 /AC + CeO2
+ CeO2 /MWCNT + CeO2 /AC + CeO2
+ CeO2 /MWCNT + CeO2 /AC + CeO2
5.5 26.8 3.4 3.0 13.4 34.2 54.2 43.1 19.4 42.6 70.5 80.5 40.5 66.1 77.1 91.1 + CeO2 /MWCNT + CeO2 /AC + CeO2
300 min
180 min
30 min
O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 O3 5 min
Oxamic acid Pyruvic acid Oxalic acid BBV pCBA BBR TOCnon-identified TOCidentified
TOCidentified (%) TOCnon-identified (%) TOCremoved (%) Experiments
Contribution for TOC of each compound identified (%)
TOC removal and present (both non-identified and identified as compounds) achieved after 5, 30, 180 and 300 min of single and catalytic ozonation of BZF.
radicals are the main oxidant involved in their degradation mechanism. Analysing the values of TOC removed and present in the solution (both non-identified and identified as compounds) after 5, 30, 180 and 300 min of reaction (see Table 1), it is verified that the fraction of TOC not identified as specific compounds is significant, but that fraction tends to decrease with the reaction time. The unsaturated compounds produced during the initial stages of BZF are quickly degraded, originating mainly the identified carboxylic acids (oxamic, oxalic and pyruvic acids), which are highly persistent. This fact justifies the decrease in the solution pH (up to approximately 3) during single and catalytic ozonation of BZF. The TOCnon-identified /TOCidentified ratio was determined in order to compare the treated solutions obtained by different ozonation experiments in terms of the identified compounds. Thus, the smaller this ratio the less non-identified compounds will be present in the treated solution. For high reaction times, ozonation with CeO2 gives rise to the lowest TOCnon-identified /TOCidentified ratio, meaning that this sample leads to the highest fraction of identified TOC. On the other hand, single ozonation leads to the highest TOCnon-identified /TOCidentified ratio also for high reaction times, showing that the solutions obtained after the treatment present a high amount of non-identified compounds. Comparing TOC removal for single ozonation after 300 min and catalytic ozonation after 30 min, similar values are observed. However, the TOCnon-identified /TOCidentified ratio obtained for these cases is very different, being the ratio obtained after 30 min ozonation in the presence of CeO2 much higher than that obtained after 300 min of single ozonation. Thus, although both cases present similar mineralization degrees, the compositions of the treated solutions are very different. This observation indicates that the degradation pathways of BZF depend on the presence of the catalyst. The conversion of the initial nitrogen and chlorine contents of BZF to inorganic ions such as nitrate, nitrite, ammonium and chloride was monitored during the reaction period and their evolutions are depicted in Figure 3. All chlorine present in BZF is converted to Cl− during the first stages of the ozonation experiments (see Figure 3b), similarly to what was observed in the presence of carbon materials alone.[18] In fact, the presence of CeO2 and CeO2 /AC during ozonation leads to 100% chlorine conversion to chloride anion within 60 min, whereas in ozonation with CeO2 /MWCNT and without catalyst the complete conversion is achieved after about 180 and 120 min, respectively. Therefore, there are no chlorine-containing organic compounds in solution after this time of ozonation, indicating some lability of the chlorine-containing ring present in the BZF molecule. By comparison with the carbon materials, CeO2 /AC leads to a slightly faster total conversion of chlorine than AC, whereas CeO2 /MWCNT converts it more slowly than MWCNT.[18] Analysing the nitrogen
Table 1.
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released as inorganic ions represented in Figure 3(a), during the first stages of ozonation experiments nitrite ion is present in higher amounts than nitrate and ammonium, achieving a maximum between 15 and 30 min. After this time, NO− 2 concentration decreases until complete removal at 120 min of reaction (for CeO2 and CeO2 /AC) and after 300 min (for CeO2 /MWCNT and single ozonation). This trend originates the peak observed in the curves of the total nitrogen mineralized during the initial stages of the reaction (see Figure 3(b)). Comparing nitrate and ammonium released, NO− 3 is produced in higher extension both in the presence or absence of catalyst. The differ+ ence observed in the released amounts of NO− 3 and NH4 may be related to the special features of the initial Ncontaining structure.[16,18,24] Regarding the formation of NH+ 4 , small differences were observed for the different catalytic systems studied, this ion being released in higher amounts in the presence of CeO2 at high reaction times. Higher differences were observed in the concentrations of NO− 3 . CeO2 /MWCNT led to much higher amounts of nitrate, particularly for longer reaction times, whereas CeO2 and single ozonation released lower concentrations. After 5 h of reaction only nitrate and ammonium are present in solution. At this time, differences in the pro+ portion of N produced as inorganic ions (NO− 3 and NH4 ) for the different catalytic systems were observed. In the + case of CeO2 , NO− 3 and NH4 were released in the proportion of about 1.3/1, whereas in catalytic ozonation in the presence of CeO2 /MWCNT and CeO2 /AC those inorganic ions are produced in the proportion of 5.6/1 and 2.6/1, respectively. Comparing these results with those obtained with the parent carbon materials in,[18] it is verified that + the proportion of NO− 3 and NH4 released in the experiment with CeO2 /MWCNT is similar to that determined when MWCNT was used (6.1/1), whereas in the case of CeO2 /AC this proportion is closer to that obtained for CeO2 instead of AC (8.3/1). These observations indicate that the use of AC and MWCNT as ozonation catalysts for BZF degradation leads to the formation of N-containing organic intermediaries that are oxidized more predominantly to NO− 3 than those produced in the presence of CeO2 and carbon materials containing ceria, suggesting that the preferential degradation pathway of BZF ozonation may be influenced by the type of catalyst. Concerning BZF-derived nitrogen, there is a significant difference between single and catalytic ozonation (see Figure 4). After 5 h of single ozonation, only 22% of N was mineralized, whereas in the presence of the catalyst more than 32% of nitrogen was converted to inorganic ions. On the other hand, ozonation in the absence of catalysts leads to the smallest amount of N-containing oxamic acid ( ∼ 12%). This indicates that the initial nitrogen is converted to other N-containing compounds, not identified, without being further oxidized. During ozonation catalysed by CeO2 /MWCNT much less oxamic acid accumulates in the solution than in the presence of CeO2 /AC (23 vs.
(a)
(b)
− Figure 3. Evolution of the dimensionless (a) NO− 2 , NO3 and + NH4 concentrations normalized by the nitrogen concentration present in BZF and (b) total chlorine and total nitrogen concentrations released as inorganic ions normalized by the chlorine and nitrogen concentrations present in BZF, respectively, during catalytic and non-catalytic ozonation of BZF at natural pH ( ∼ 4.4) (C0,BZF = 20 ppm, catalysts = 0.14 g L−1 ).
44%) but the release of nitrogen-containing inorganic ions is much higher (59 vs. 36%). Moreover, with both catalysts similar percentages of identified nitrogen-containing compounds are achieved. These results show that, after 5 h of reaction, CeO2 /MWCNT promotes higher nitrogen mineralization levels than the remaining catalysts, in spite of leading to lower TOC removals (see Figure 1), which means that the preferential degradation pathway depends on the catalyst. When these results are compared to those obtained with the corresponding carbon materials in,[18] it is observed that CeO2 /MWCNT mineralizes slightly more nitrogen than MWCNT (59 vs. 52%), with the fraction of non-identified nitrogen being lower. On the other hand, CeO2 /AC leads to a lower percentage of nitrogencontaining inorganic ions released than AC (36 vs. 56%) and a much higher accumulation of oxamic acid (44 vs. 17%). For the ozonation catalysed by CeO2 , almost all
Environmental Technology N non-identified
N compounds
N ions
100
5.0 17.7
90
19.5
80 70
23.1 66.2
62.6
N (%)
60
44.1
50 40 30
11.7
20
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10
59.2 36.4
32.4
22.1
0
Ozonation experiments
Figure 4. Percentage of BZF nitrogen content converted to inorganic ions and identified N-containing oxidation products identified, after 300 min of single and catalytic ozonation at natural pH ( ∼ 4) (C0,BZF = 20 ppm, catalysts = 0.14 g L−1 ).
products were identified and more than 60% of N is present in the final solution as oxamic acid, and only 32% correspond to nitrogen-containing inorganic ions. Thus, it can be concluded that the BZF degradation pathway followed in the presence of CeO2 is more effective in the degradation of nitrogenated compounds. Moreover, the BZF reaction pathway followed in the case of CeO2 /MWCNT is similar to that followed during MWCNT catalytic ozonation, whereas CeO2 /AC and CeO2 lead to similar pathways. The incomplete nitrogen mass balance indicates that other non-identified refractory nitrogenated compounds are formed, contributing to the remaining TOC in the solution. Additionally, it is also possible that the reaction may lead to the formation of gaseous nitrogenated compounds, such as NOx , and the extension of their production may be dependent on the pathway followed and, consequently, also dependent on the selected catalyst. 3.2.3. Bioassays experiments The acute toxicity of the untreated BZF solution and solutions submitted to ozonation during 30, 180 and 300 min was evaluated by Microtox bioassays using the marine bacteria Vibrio fischeri.[21] The results are presented in Figure 5 and were obtained after 30 min of exposition by determination of the percentage of inhibition in the bacteria luminescence caused by each sample. After 30 min of reaction, the solution treated with CeO2 presents lower acute toxicity towards V. Fischeri than the
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untreated solution, whereas no major differences in the acute toxicity of the solution treated with carbon materials containing ceria are verified. This fact means that the intermediates formed in this period, in the presence of the studied catalysts, have lower or similar acute toxicity than the BZF untreated solution. This trend is contrary to that observed for the ozonation catalysed by carbon materials in,[18] indicating the formation of different intermediaries. After that time, the inhibition percentage of the bacteria luminescence continuously increases, mainly in the presence of catalyst, achieving inhibitions effects of 72, 82 and 59% for CeO2 /MWCNT, CeO2 /AC and CeO2 , respectively, after 300 min of reaction. Comparing the values obtained for carbon materials containing ceria with those reported before for the corresponding carbon materials (53% and 29% for MWCNT and AC, respectively),[18] a significant increase in the inhibition percentage is verified, mainly in the case of CeO2 /AC. Although CeO2 /AC and CeO2 /MWCNT lead to treated solutions with higher acute toxicity than the corresponding carbon materials, they allow achieving higher mineralization degrees, which indicates that the compounds produced have higher acute toxicity, due to the different pathways followed by the catalysts. 3.2.4.
Considerations on the reaction mechanism
The ozonation of organic compounds involves a number of complex reactions and several mechanistic approaches have been presented in the literature. In heterogeneous catalytic ozonation, both surface and liquid bulk reactions can occur, involving molecular ozone, HO• radicals and surface-oxygenated radical species. AC catalyses the decomposition of ozone in the aqueous phase by the formation of surface-oxygenated radical species as well as promoting the formation of HO• radicals in the solution.[12,25] The same was proposed for MWCNT.[10] In the case of ozonation catalysed by metal oxides, the proposed mechanisms generally assume that the adsorption of organic molecules and ozone takes place on the surface of the catalyst.[26] The interaction of ozone with the metal oxide surface results in the formation of free radicals that can initiate a radical chain-type reaction both on the surface of the catalyst and in the liquid phase, leading to the production of HO• radicals.[27,28] For cerium oxide catalysts, it has been shown that the free radical oxidation mainly occurs in the solution.[14,15,25] The presence of cerium oxide on AC is believed to promote the reduction of Ce4+ to Ce3+ species, due the existence of delocalized π electrons on the basal planes of the AC.[14,29] Therefore, the presence of the redox pair Ce4+ /Ce3+ on the catalyst surface is believed to enhance its catalytic properties for the generation of HO• radicals. Ceria/AC composites have been found to be very active catalysts for the ozonation of organic acids and some aromatic compounds.[14,29] In a previous work,
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Luminescence inhibition (%)
90 80 70 60 50
O3
40
O3 + CeO2/MWCNT
30
O3 + CeO2/AC
20
O3 + CeO2
10 0 0
30
180
300
Reaction time (min)
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Figure 5. Results of Microtox tests at 0, 30, 180 and 300 min of reaction, with exposure at bacteria V. fischeri during of 30 min.
AC and MWCNT containing dispersed ceria showed to be very active ozonation catalysts in the mineralization of sulphamethoxazole.[16] In this work, two types of reaction mechanism during ozonation in the presence of AC or carbon nanotubes containing ceria was observed. This may be attributed to a combination of heterogeneous catalytic reactions, occurring preferentially on the surface of the carbon materials, and homogeneous reactions between the solutes and HO• radicals resultant from the catalytic decomposition of ozone, mainly on cerium oxide. Nevertheless, in the case of BZF, and contrary to what occurs with other studied molecules (namely oxalic acid and sulphamethoxazole), CeO2 leads to higher mineralization degrees than CeO2 /MWCNT and CeO2 /AC. The results indicate that the mechanism followed in the presence of CeO2 (free radical oxidation reactions in solution) leads to the formation of intermediates more easily degradable, mainly after 120 min of reaction. 4. Conclusions Two catalysts containing ceria dispersed on the surface of MWCNTs and AC were investigated as ozonation catalysts for the mineralization of BZF. The results were compared with those obtained in the absence of catalyst and in the presence of the parent carbon materials used for the preparation of those catalysts, as well as in the presence of ceria. The mineralization of solutions was enhanced by the addition of the prepared catalysts. Carbon materials containing ceria dispersed on the surface lead to higher TOC removal than the corresponding carbon materials. Ozonation in the presence of CeO2 /AC resulted in a TOC removal higher than that obtained with CeO2 /MWCNT. With the simultaneous use of ozone and ceria a TOC removal of more than 90% was achieved after 300 min under the reaction conditions used. BBR, pCBA and BBV were found as primary products of BZF degradation, whereas oxalic, pyruvic and oxamic
acids were identified as final products. The presence of carbon materials containing ceria or ceria leads to fast and total disappearance of pyruvic acid, contrary to what occurs in single ozonation. Addition of the radical scavenger tert-butanol during catalytic or single ozonation evidenced the participation of HO• radical in the ozonation process. The original chorine in BZF is completely converted to chloride. The original nitrogen of the BZF is mainly con− verted to NO− 3 along with smaller amounts of NO2 and + NH4 in all cases. However, the nitrogen mass balance does not close, which indicates the formation of non-identified refractory nitrogenated organic compounds that contribute to the remaining TOC in the solution. Microtox tests revealed that in the early stages of single and catalytic ozonation (first 30 min) intermediaries with lower or similar acute toxicity towards V. Fischeri than BZF are produced. However, for longer reaction times, the acute toxicity increases. The CeO2 sample led to the best results. The catalysts tested in the BZF ozonation showed very distinct results in terms of TOC removal, intermediates and by-products evolution, degradation of nitrogenated compounds and acute toxicity. The observed differences are due to the predominant degradation pathway followed in the presence of each catalyst, which influences the type and the amounts of organic compounds accumulated in the solution. The BZF ozonation catalysed by the CeO2 sample led to higher mineralization degrees than with CeO2 /MWCNT or CeO2 /AC. This indicates that the reaction mechanism followed with CeO2 (free radical oxidation in solution) involves the formation of intermediates more easily degradable.
Acknowledgements This work was supported by the European Community’s Seventh Framework Programme (FP7/2007–2013) under Grant Agreement No. 226347; FEDER through COMPETE under Project
Environmental Technology PEst-C/EQB/LA0020/2013; FCT under Grant BD/45826/2008; and QREN, ON2 and FEDER under Project NORTE-07-0124FEDER-0000015.
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