Ozonation of sulfamethoxazole promoted by MWCNT

Catalysis Communications 35 (2013) 82–87

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Short Communication

Ozonation of sulfamethoxazole promoted by MWCNT Alexandra G. Gonçalves, José J.M. Órfão, Manuel F.R. Pereira ⁎ Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRE/LCM, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

a r t i c l e

i n f o

Article history: Received 28 December 2012 Received in revised form 1 February 2013 Accepted 8 February 2013 Available online 15 February 2013 Keywords: Catalytic ozonation Sulfamethoxazole MWCNT Surface chemistry

a b s t r a c t Multi-walled carbon nanotubes (MWCNTs) with different surface chemical properties were prepared by oxidative and thermal treatments. Samples were characterized by nitrogen adsorption, temperature programmed desorption and pH at the point of zero charge. These catalysts were tested in the ozonation of sulfamethoxazole. Complete conversion of this compound was achieved after approximately 30 min. MWCNTs significantly enhanced the mineralization degree compared to single ozonation. Some oxidation products were quantified. The catalytic process is favored by MWCNTs with basic or neutral properties. Successive experimental runs of SMX degradation show that the catalysts suffer some deactivation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction

2.1. Catalysts preparation

Water contamination by pharmaceuticals represents a rising environmental concern. Catalytic ozonation has emerged as a powerful technology for the treatment of this type of water pollutants. Sulfamethoxazole (SMX) is one of the most widely prescribed antibiotics and is refractory to the conventional treatments employed in the sewage treatment plants [1]. SMX has been frequently detected in the environment [2], namely in sewage treatment plant effluents and wastewater plant effluents [3–7], in natural and surface water [3], in drinking water [7] and even in different aquaculture environments [8,9]. SMX removal by ozonation in the presence of activated carbon has been studied in the literature [10]. More recently we have shown that multi-walled carbon nanotubes (MWCNT) proved to perform more efficiently as ozonation catalysts than activated carbons in the mineralization of oxalic and oxamic acids [11], which are common final refractory products in the non-catalytic ozonation of organic pollutants, and in the ozonation of SMX [12]. The main goal of the present work is to study the relationship between the surface chemical characteristics of MWCNTs and its performance on the removal of SMX. For that purpose, a set of modified MWCNT with different levels of acidity/basicity was prepared and tested in the ozonation of SMX.

A commercial MWCNT sample (Nanocyl 3100), was used as the starting material (MWCNT-original). According to the supplier, these MWCNT have an average diameter of 9.5 nm, an average length of 1.5 μm and a carbon purity higher than 95%. In order to produce carbon nanotubes with high acid character, the original sample was oxidized with HNO3 in the liquid phase (MWCNT-HNO3). This sample was thermally treated under nitrogen for 1 h at T = 400, 600 or 900 °C (MWCNT-HNO3_N2_T). These thermal treatments are intended to remove selectively the groups introduced by the previous oxidation with HNO3, producing materials with different surface chemistries and similar textural properties. Sample MWCNT-original was also oxidized with 5% O2 in N2 for 3 h at 500 °C (MWCNT-O2). Additional details on the procedures are reported in [11].

2. Experimental All chemicals used are of pro analysis grade. Some characteristics of these compounds are presented in Table 1. ⁎ Corresponding author. Tel.: +351 225 081 468; fax: +351 225 081 449. E-mail addresses: [email protected] (A.G. Gonçalves), [email protected] (J.J.M. Órfão), [email protected] (M.F.R. Pereira). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.02.012

2.2. Catalysts characterization The surface areas of the materials were based on the N2 adsorptiondesorption isotherms, determined at −196 °C with a Quantachrome NOVA 4200e apparatus. The surface chemistry was characterized by the determination of pHPZC, in order to express the acid-basic character with a single number [13], and by TPD, which allows the identification and quantification of the oxygenated groups [14,15], as described elsewhere [11]. 2.3. Kinetic experiments The catalysts prepared were tested in the ozonation of SMX. These experiments were carried out in a laboratory scale reactor (ca. 1 L)

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Table 1 Characteristics of the organic compounds mentioned in this work. M (g mol−1)

Compound

Formula

Structure

Sulfamethoxazole (SMX)

C10H11N3O3S

3-Amino-5-methylisoxazole (AMI)

C4H6N2O

p-Benzoquinone (BZQ)

C6H4O2

Oxamic acid

C2H3NO3

89.05

Pyruvic acid

C3H4O3

88.06

Maleic acid

C4H4O4

116.07

253.28

108.09

groups, was characterized by TPD [14,15]. The amounts of CO and CO2 released, the CO/CO2 ratios and the oxygen mass percentages (mO) on the surface of the materials are summarized in Table 2. The pHPZC values of the nanomaterials and the corresponding specific surface areas are also reported in Table 2. These samples were characterized in detail in [11]. Briefly, the sample oxidized with HNO3 (MWCNT-HNO3) is the most acidic (lowest pHPZC). Furthermore, it contains the highest amount of surface oxygen-containing groups, mainly carboxylic acids, as well as a large amount of phenols and carbonyls and, in less extent, carboxylic anhydrides and lactones. The groups introduced by oxidation with HNO3 were selectively removed in the thermally treated samples. Almost all carboxylic acids were removed after heat treatment at 400 °C (MWCNT-HNO3_N2_400). After thermal treatment at 600 °C (MWCNT-HNO3_N2_600), the CO2 releasing groups were almost completely removed (only a few lactones remain), and some CO releasing groups were also removed from the surface of the material, which can be assigned mainly to carboxylic anhydrides. Practically all oxygenated groups are removed at 900 °C (MWCNT-HNO3_N2_900). The acidic nature of the samples decreases with the increase of the treatment temperature. The oxidation treatment with HNO3 leaded to a significant increase of the specific surface area, which was only slightly increased in the thermal treatments, this effect being more significant at higher temperatures because the oxygen containing groups were successively removed, as confirmed by TPD and pHPZC. Thus materials with different surface chemistries and similar textural properties were produced (representing the group B). On the other hand, the oxidation in the gas phase (MWCNT-O2) introduces less acidic surface groups, but large amounts of phenols and carbonyl/quinones and some lactones, in accordance with the literature [11,16]. In addition, a substantial increase of the specific surface was observed. The commercial sample presents almost no oxygenated surface groups and

described in a previous work [11]. In each ozonation experiment the reactor was filled with 700 mL of SMX solution with a concentration of 50 ppm, at the natural pH (around 4.8). Then, 100 mg of catalyst was introduced in the reactor. The experiments were performed at constant flow rate (150 cm 3 min −1) and constant inlet ozone concentration (50 g m −3). The stirring rate was maintained constant at 200 rpm in order to keep the reactor content perfectly mixed. The solution was prepared from ultrapure water obtained in a Milli-Q Millipore system. For comparative purposes, both adsorption on MWCNT and single ozonation experiments were performed in the same system, under identical experimental conditions. In cyclic experiments, the same procedure was followed. After each experiment, the solution was filtered and the MWCNT sample dried in order to be used in another run. This procedure was repeated two times. Textural and surface chemistry characterization of the catalyst was carried out after the first and third runs. All experiments were performed at room pressure and temperature. Samples were collected at selected times using a syringe and were centrifuged before analysis. 2.4. Analytical methods Concentrations of SMX and reaction products (see Table 1) were followed using by HPLC. The degree of mineralization was obtained by total organic carbon (TOC) measurements. Details can be found in reference [12]. 3. Results and discussion 3.1. Characterization results The surface chemical composition of the carbon materials, in particular the nature and amount of the oxygen-containing surface Table 2 Chemical and textural characterization of MWCNT materials. Sample

Group of samples

CO2a (μmol g−1)

COa (μmol g−1)

CO/CO2

mOb (Wt%)

pHPZC

SBET (m2 g−1)

MWCNT-original MWCNT-O2 MWCNT-HNO3 MWCNT-HNO3_N2_400 MWCNT-HNO3_N2_600 MWCNT-HNO3_N2_900

A A B B B B

25 91 1514 561 125 15

478 1339 2435 2350 1494 308

19 15 1.6 4.2 12 20

0.84 2.5 8.7 5.6 2.8 0.54

7.0 5.2 3.0 3.8 5.9 7.3

331 508 476 483 504 529

a b

Amounts of CO and CO2 released, obtained by integration of TPD spectra. Mass percentage of oxygen on the surface, calculated assuming that all the surface oxygen is released as CO and/or CO2.

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experimental conditions. This is observed because ozone selectively attacks activated aromatic rings or double bounds present in the SMX molecule. When the TOC results are compared (see Fig. 2), the presence of MWCNT leads to higher mineralization degrees than single ozonation, except in the case of sample MWCNT-HNO3 (Fig. 2a). In fact, the decrease of acidic groups on the MWCNT surface leads to increasing performances in the mineralization of SMX, which is explained by the negative influence of the oxygen-containing surface groups in the decomposition of ozone, as already explained in a previous work [11]. In the thermal treatments, these surface groups are successively removed and, consequently, their negative influence in the decomposition of ozone decreases. Sample MWCNT-O2 leads to the highest values of TOC removal among group A catalysts (Fig. 2b). Differences relatively to single ozonation are more noticeable during the first 30 min of reaction. Although the higher acidity of MWCNT-O2 compared to MWCNT-original would be unfavorable to the decomposition of ozone on the surface of the catalyst [11], it presents a much higher specific surface where SMX, ozone and organic intermediates and by-products can adsorb and react, justifying the best TOC removal results. Comparing MWCNT-original and MWCNT-HNO3_N2_900 samples, the simultaneous use of the last one and ozone leads to a higher catalytic performance. Taking into account the characterization results both samples present low and similar concentration of oxygenated groups on the surface, but MWCNT-HNO3_N2_900 has a much higher specific surface area. These observations indicate that the textural properties of materials, namely the specific surface area, have an important contribution in the mineralization of SMX. Sample MWCNT-HNO3_N2_900 is the most efficient catalyst, because it presents slightly basic properties and the highest specific surface area. Therefore, this material has higher adsorption capacity than the other samples, and a superior ability to enhance the catalytic decomposition of ozone into surface radical species and/or oxygen-containing radicals, such as OH•, which are very reactive in solution. The oxidation of SMX originated several by-products. In a previous work [12], AMI and BZQ were found to be primary intermediates both during catalytic and non-catalytic ozonation, whereas oxamic, pyruvic and maleic acids were clearly identified as final by-products, which persisted in solution at the end of the reaction period. The evolutions of these compounds were followed (see Fig. 3). Oxalic acid was also detected, but its measurement was not possible because the peak obtained was non-well defined. AMI and BZQ are produced from the beginning of reaction and rapidly removed from solution because ozone reacts quite efficiently with them. Single ozonation produces these aromatic compounds in higher extension than catalytic ozonation. This

neutral properties (pHPZC = 7.0). These two samples represent the group A, in which not only the materials with lower specific surface area is included (MWCNT-original), but also a material with high specific surface area and large amounts of the less acidic oxygenated groups (MWCNT-O2). 3.2. Kinetic results The ozonation of SMX was studied at natural solution pH (around 4.8), in the presence and in the absence of the prepared catalysts. Both SMX and TOC decay were followed during 180 min (Figs. 1 and 2, respectively). Some experiments were carried out in duplicate and the maximum relative deviation obtained was 2%. The analytical measurements were also performed in duplicate with a maximum relative error of ± 0.5%. The pH of the solution with 50 ppm of SMX is between the pKa1 and pKa2 values, being the non-protonated amine the predominant specie. However, during SMX oxidation, several acid compounds are produced and accumulated in solution, which leads to a decrease of pH until values around 3.6 after 3 h. The pH of the solution has a remarkable effect on the reaction mechanism. In fact, the decomposition of ozone catalyzed by MWCNT will depend on the solution pH, because of the amphoteric nature of MWCNT, since its surface might be positively or negatively charged, similarly to what occurs with activated carbon [17]. Therefore, the surface of materials like MWCNT-original, MWCNT-HNO3_N2_600, MWCNT-HNO3_N2_900 and MWCNT-O2, which have a pHPZC higher than the pH of the solution becomes positively charged, enhancing the attraction of hydroxide ions. On the other hand, in the case of the samples MWCNT-HNO3 and MWCNT-HNO3_N2_400, repulsive electrostatic interactions between the surface and OH − ions prevail. Thus, electrostatic interactions play an important role in the catalytic decomposition of ozone on the surface and, consequently, in the mineralization of organic compounds. Adsorption on MWCNT contributes to the removal of SMX (between about 25 and 50 % after 3 h), as it can be confirmed in Fig. 1. Since MWCNT-HNO3_N2_900 has a higher electron density, this sample adsorbs more SMX [18]. Additionally, considering the surface reactions between adsorbed organic compounds and highly reactive species formed by ozone decomposition at the surface, which are part of the complex catalytic action of carbon materials [19,20] there is also a negative influence of the oxygenated groups, since they reduce adsorption as depicted in Fig. 1. Ozonation by itself enables a fast decay of the SMX concentration (Fig. 1), leading to its removal in less than 30 min, under the selected

1.0

O3 O3 + MWCNT-original

0.6

O3 + MWCNT-HNO 3

1.0

O3 + MWCNT-HNO 3_N2_400 0.8

0.4

CSMX/CSMX,0

CSMX/CSMX,0

0.8

0.2

O3 + MWCNT-HNO 3_N2_600 O3 + MWCNT-HNO 3_N2_900

0.6

O3 + MWCNT-O2 O2 + MWCNT-original

0.4

O2 + MWCNT-HNO 3

0.2 0.0

O2 + MWCNT-HNO 3_N2_400 O2 + MWCNT-HNO 3_N2_600 0

10

20

30

O2 + MWCNT-HNO 3_N2_900

t (min)

0.0 0

30

60

90

O2 + MWCNT-O2

120

150

180

t (min) Fig. 1. Evolution of SMX dimensionless concentration during adsorption, catalytic and non-catalytic ozonation at natural pH (C0, 150 cm3 min−1, CO3, feeding = 50 g m−3).

SMX = 50

ppm, catalysts = 0.14 g L−1, Qgas =

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Fig. 2. Evolution of TOC dimensionless concentration during catalytic and non-catalytic ozonation at natural pH of a) group B samples and b) group A samples (C0, SMX = 50 ppm, catalysts = 0.14 g L−1, Qgas = 150 cm3 min−1, CO3, feeding = 50 g m−3).

observation indicates the degradation enhancement of those primary oxidation products when catalysts are present, which can be explained by a faster oxidation via OH• and/or adsorption and/or reaction on the activated carbon surface. Concerning the organic acids identified, oxamic and pyruvic acids are produced from the beginning of reaction, whereas maleic acid is formed at least 5 min after the beginning of reaction. Only the maleic acid evolution presents differences between single and catalytic ozonation. The absence of catalyst leads to a higher production of this acid during the first stages of reaction, followed by a slow but continuous decrease, suggesting that it was principally formed during the first minutes and slowly degraded thereafter. On the contrary, for long times, catalytic ozonation leads to accumulation of maleic acid in solution after ≈1 h. No significant differences between the prepared catalysts are observed in the concentration histories of oxamic and maleic acids, contrarily to pyruvic acid. The concentration of this acid reaches a maximum and decreases thereafter, suggesting its possible complete removal for longer times. In the presence of the commercial sample, pyruvic acid is produced in higher extension than with the remaining catalysts, particularly with that obtained by oxidation with nitric acid as well as those subsequently thermally treated under inert atmosphere (group B samples). Materials oxidized with oxygen lead to formation of pyruvic acid in higher extension than group B samples but in lower extension than the original sample. The carboxylic acids present in solution have low reactivity toward ozone, but may be oxidized by secondary oxidants, such as HO• radicals, produced by ozone decomposition. Even though they are oxidized by hydroxyl radicals, their oxidation rate is usually lower than their formation rate, under the conditions tested. Therefore, they accumulate during the

ozonation processes [21]. Thus, extended reaction times would be needed for complete mineralization of such compounds. Reutilization of MWCNT-original was carried out with the purpose of studying the influence of ozonation on the surface chemistry of the catalyst, and the eventual accompanying deactivation. The kinetic results obtained are depicted in Fig. 4. By comparing the curves of SMX removal, no significant differences in the decay of SMX concentration were observed during catalyst reutilization. This was expected because SMX is easily oxidized by ozone, as it was explained before. However, when TOC results are compared, a slight decrease in the mineralization efficiency is observed. The curves obtained seem to tend to the curve corresponding to sample MWCNT-HNO3, which is the sample with more oxygenated group on its surface. TPD spectra of samples collected during the successive experiments showed a progressive increase in the amount of oxygen-containing surface groups, as observed for oxalic acid removal in a previous study [11]. Then, the observed decrease of the catalyst activity in successive runs is probably due to the introduction of a limited number of oxygenated electron-withdrawing groups, which reduce the electron density on the carbon surface, thus decreasing the catalytic activity of the material for the decomposition of ozone. The surface area of MWCNT-original slightly increases from the first to the last run, meaning that the oxidation by dissolved ozone promotes a weak change in the textural properties of the MWCNT. In conclusion, the surface chemistry plays a significant role in SMX mineralization. However, because of the surface oxidation that occurs during the process, the influence of the surface chemistry tends to be attenuated along successive reutilizations of the catalysts.

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b) 0.025

0.0025

0.020

0.0020

CBZQ (mM)

CAMI (mM)

a)

0.015 0.010 0.005 0.000

0.0015 0.0010 0.0005

0

15

30

0.0000

45

0

15

30

t (min)

c)

60

d) 0.07

0.06

0.06

0.05

Cpyruvic acid (mM)

Coxamic acid (mM)

45

t (min)

0.05 0.04 0.03 0.02

0.03 0.02 0.01

0.01 0.00

0.04

0

30

60

90

120

0.00

180

150

0

30

60

t (min)

e) Cmaleic acid (mM)

90

120

150

180

t (min) O3

0.0020 0.0018 0.0016 0.0014 0.0012 0.0010 0.0008 0.0006 0.0004 0.0002 0.0000

O3 + MWCNT-original O3 + MWCNT-HNO 3 O3 + MWCNT-HNO 3_N2_400 O3 + MWCNT-HNO 3_N2_600 O3 + MWCNT-HNO 3_N2_900 O3 + MWCNT-O 2

0

30

60

90

120

150

180

t (min) Fig. 3. Evolution of primary intermediates and by-products concentrations during catalytic and non-catalytic ozonation of SMX at natural pH: a) AMI, b) BZQ, c) oxamic acid, d) pyruvic acid and e) maleic acid (C0, SMX =50 ppm, catalysts=0.14 g L−1, Qgas =150 cm3 min−1, CO3, feeding =50 g m−3).

b) 1.0

1.0

0.8

0.8

TOC/TOC0

C/C0

a)

0.6 0.4

0.6 O3

0.4

st

O3 + MWCNT-original 1 cycle nd

O3 + MWCNT-original 2 cycle rd

0.2

O3 + MWCNT-original 3 cycle

0.2

O3 + MWCNT-HNO 3

0.0

3

0.0

0

10

20

t (min)

30

0

30

60

90

120

150

180

t (min)

Fig. 4. Evolution of the dimensionless (a) SMX and (b) TOC concentrations during successive catalytic ozonation experiments, at natural pH, in the presence of MWCNT-original (C0, SMX = 50 ppm, catalysts=0.14 g L−1, Qgas =150 cm3 min−1, CO3, feeding =50 g m−3).

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4. Conclusions The presence of MWCNT during ozonation of sulfamethoxazole in aqueous solution improves mineralization. The highest mineralization is achieved in the presence of sample MWCNT-HNO3_N2_900, because it presents slightly basic properties, with no oxygen-containing surface groups, and the largest surface area. The surface chemistry of samples plays an important role in the catalytic ozonation of SMX. In general, catalytic ozonation is favored by MWCNT with basic or neutral properties and high specific surface areas. Successive experimental runs carried out with the original sample show that the MWCNT surface suffers a slight progressive oxidation by exposure to dissolved ozone, with a limited loss of activity. Acknowledgments The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007– 2013) under grant agreement no. 280658. This work was partially supported by project PEst-C/EQB/LA0020/2011, financed by FEDER through COMPETE. A. G. acknowledges the grant received from FCT (BD/45826/2008). References [1] M. Carballa, F. Omil, J.M. Lema, M. Llompart, C. García-Jares, I. Rodríguez, M. Gómez, T. Ternes, Water Research 38 (2004) 2918–2926. [2] K. Kümmerer, Chemosphere 75 (2009) 417–434.

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