Heterogeneous and homogeneous Fenton processes using activated carbon for the removal of the herbicide amitrole from water

Applied Catalysis B: Environmental 101 (2011) 425–430

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Heterogeneous and homogeneous Fenton processes using activated carbon for the removal of the herbicide amitrole from water M.A. Fontecha-Cámara a , M.A. Álvarez-Merino a , F. Carrasco-Marín b , M.V. López-Ramón a , C. Moreno-Castilla b,∗ a b

Departamento de Química Inorgánica y Orgánica, Facultad de Ciencias Experimentales, Universidad de Jaén, 23071 Jaén, Spain Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain

a r t i c l e

i n f o

Article history: Received 14 September 2010 Received in revised form 7 October 2010 Accepted 9 October 2010 Available online 15 October 2010 Keywords: Amitrole oxidation Fenton processes Activated carbon cloth Carbon–Fe catalysts

a b s t r a c t The study objective was to investigate the removal of amitrole (AMT) by oxidation using the decomposition of hydrogen peroxide in heterogeneous and homogeneous Fenton reactions. For this purpose, an activated carbon cloth was used to prepare supported iron catalysts with different iron salts (sulfate, acetate and nitrate) and iron metal loadings. Homogeneous Fenton reactions were carried out by using iron sulfate or acetate in the absence or presence of the activated carbon cloth. In the heterogeneous Fenton reaction, the amounts of TOC (total organic carbon) and AMT removed depended on the iron metal loading and the Fe/H2 O2 molar ratio; iron leaching was very low. The highest AMT degradation (∼90%) was achieved with the homogeneous Fenton reaction using FeSO4 in the presence of the activated carbon cloth, which may be due to a synergic effect between activated carbon and iron salt; with this procedure, ∼60% of the TOC was removed. The synergic effect was not observed when iron acetate was used as homogeneous catalyst. The presence of sulfate ions favored the oxidation of AMT to urazole. Nitrate and ammonium ions were observed but at negligible concentrations. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Amitrole (AMT) is a heterocyclic herbicide derived from triazole(3-amino-1,2,4-triazole); its structure is depicted in Fig. 1. It is widely used for weed control in agriculture and along roadsides and railways and is a non-selective herbicide sometimes used in place of other prohibited herbicides [1–3]. Due to its high solubility (280 g/L at 25 ◦ C), relatively high levels of amitrole can be found in surface water and can contribute to ground-water contamination via leaching. As a result of its high water solubility, AMT adsorption on activated carbons is very low [4], and only 20–25% of an AMT solution (90 mg/L) was adsorbed on the activated carbon cloth (ACC) used in this study. Advanced oxidation processes (AOPs) may be more appropriate than adsorption for the removal of compounds with low adsorbability. AOPs are characterized by the production of OH• radicals, which are potent (2.8 V) and unselective oxidants [5] that can oxidize and mineralize organic pollutants in water, yielding CO2 and other inorganic compounds.

∗ Corresponding author. E-mail address: [email protected] (C. Moreno-Castilla). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.10.012

Hydroxyl radicals can be generated from H2 O2 by the use of activated carbons [6–11] and Fenton’s reagent [12–16], among other decomposition catalysts. Our group recently studied AMT removal by oxidation with carbon/H2 O2 -generated hydroxyl radicals, using the above ACC and its heat-treated derivative to remove surface oxygen complexes [17]. The best results were obtained on basic activated carbon surfaces at pH 7–10, when hydroxyl radical formation is favored, achieving 35–45% AMT removal in comparison to the 20–25% removed by adsorption. Oxygen fixed on the carbon surface during AMT oxidation must be removed by heat treatment to regenerate the surface basicity of the carbon before it is reutilized in another oxidation cycle. Fenton’s reagent uses Fe ions as homogeneous catalysts, producing hydroxyl and perhydroxyl radicals from water according to reactions (1) and (2): Fe2+ + H2 O2 → Fe3+ + OH− + OH•

(1)

Fe3+ + H2 O2 → Fe2+ + H+ + O2 H•

(2)

Fenton processes can also be carried out under heterogeneous conditions by immobilizing the iron catalyst on a support such as activated carbon [18–22]. Limited data have been published on AMT degradation and mineralization by AOP [2,3]. One study used photoexcitation of Fe(III) aquocomplexes by artificial and sunlight irradiations to produce

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H

H

N

N

N

OH

N

N

H2N

N

HO

Amitrole

Urazole Fig. 1. Amitrole and urazole structures.

hydroxyl radicals [2]. Hydroxyl radicals are formed by excitation into the ligand-to-metal charge transfer band [2,16] according to Eq. (3): Fe(OH)2+ + h → Fe2+ + OH•

(3)

The major AMT degradation product was urazole (see Fig. 1). Total degradation was obtained after 10 h of irradiation and total mineralization was observed after 160 h, producing NH4 + ions [2]. Research has also been conducted into AMT removal from water by anodic oxidation and the electro-Fenton method [3]. Hydroxyl radicals are formed at the anode from water oxidation in anodic oxidation, whereas H2 O2 is continuously formed at the cathode due to oxygen reduction in the electro-Fenton process. This H2 O2 produces hydroxyl radicals by Fenton reactions (1) and (2). Both anodic oxidation and electro-Fenton methods completely eliminate AMT from water at pH 3, producing NO3 − and NH4 + ions. The objective of the present study was to analyze the removal of AMT by oxidation with hydroxyl radicals obtained from hydrogen peroxide by Fenton reactions using different iron salts, running experiments under homogeneous and heterogeneous conditions using an activated carbon cloth as support. 2. Experimental An ACC from Kynol Europe was cut into 6 mm diameter circles for use. Aqueous solutions of FeSO4 ·7H2 O, Fe(CH3 COO)2 , and Fe(NO3 )3 ·9H2 O were used to prepare carbon-supported Fe catalysts by an incipient wetness technique, yield nominal Fe metal loadings between 3 and 12 wt.%. The precise metal loading was determined by weighing the ashes left after burning off the supported catalysts in air at 800 ◦ C. Carbon-supported Fe catalysts were heat-treated in N2 flow to decompose the iron salt before their characterization and use in the Fenton reaction. Treatment temperatures were 725 ◦ C for catalysts prepared from iron(II) sulfate and 300 ◦ C for those prepared with the other salts. These treatment temperatures were needed to decompose the iron salts, which were determined by thermogravimetric analysis. The decomposition product in all cases was ferric oxide, as determined by X-ray diffraction (XRD). Supported catalysts will be referred to in the text as ACC followed by the iron percentage and the letter S, A, or N to indicate the iron salt used (sulfate, acetate, or nitrate, respectively). The support and carbon-supported Fe catalysts were characterized by N2 adsorption at −196 ◦ C to determine their BET surface area, SBET , using an Autosorb 1 from Quantachrome after outgassing samples overnight at 110 ◦ C under high vacuum (∼10−6 mbar). Temperature-programmed desorption (TPD) was used to measure the oxygen content of carbon-supported catalysts before and after AMT oxidation. For this purpose, the amounts of CO and CO2

evolved after their heat treatment in He flow to 1000 ◦ C were determined by using a Prisma mass spectrometer from Pfeiffer. Carbon-supported Fe catalysts were further characterized by XRD, using a Bruker D8 Avance diffractometer, and by X-ray photoelectron spectroscopy (XPS) in order to determine the chemical nature of the iron, its dispersion, and its mean particle size. XPS measurements were performed with an ESCA 5701 from Physical Electronics, equipped with MgK␣ X-ray source (h = 1253.6 eV) and hemispherical electron analyzer. Survey and multi-region spectra were recorded at C1s , O1s , N1s , S2p , and Fe2p photoelectron peaks. The internal standard peak for determining binding energies (BEs) was that of carbon C1s (284.6 eV). All oxidation experiments were carried out at pH 3 using 0.1 L of a 1.07 mM AMT solution in thermostated flasks at 25 ◦ C that were shaken at 300 rpm. Each data point of AMT removal kinetics was obtained from a different flask. In the heterogeneous Fenton process, the iron-supported catalyst was added to the AMT solution and the pH was adjusted to 3 followed by the addition of H2 O2 . Oxidation was conducted at different Fe/H2 O2 molar ratios. Iron leaching from the supported catalysts was determined by atomic absorption spectrometry. The homogeneous Fenton process was carried out by dissolving the iron salt in the AMT solution followed by adjustment of the pH to 3 and the addition of H2 O2 solution to obtain a Fe/H2 O2 molar ratio of 0.04. These experiments were run in the absence and presence of activated carbon. Blank experiments were also carried out to determine the percentage of AMT adsorbed by the activated carbon with respect to the total amount of AMT removed from the solution. AMT concentrations were measured by high-performance liquid chromatography (HPLC) using an LC-10A model Shimadzu machine with UV/VIS detector at 202 nm and a Nucleosil C-18 column of 250 mm × 4.6 mm as stationary phase. The mobile phase was a mixture of 70% HPLC grade acetonitrile and 30% ultrapure water with a flow of 1.1 mL/min. In all cases, samples were diluted ten times before their analysis. The amount of AMTdegraded during the Fenton reaction was obtained by subtracting the amount of AMTadsorbed from the total amount of AMT removed determined by HPLC. AMT mineralization was followed by measurement of the TOC with a TOC-5000A model Shimadzu analyzer. TOC results are the average of at least three measurements with an accuracy of ±5%. Specific Merck Spectroquant kits were used to measure concentrations of nitrate and ammonium ions in solution, based on the formation of 4-nitro-2,5-dimethylphenol for nitrate ions (ISO 7890/1) and an indophenol blue derivative for ammonium ions (ISO 7150/1), determining these compounds with a double-beam spectrophotometer at 330 and 690 nm, respectively. 13 C-nuclear magnetic resonance (13 C NMR) was used to determine the nature of the AMT degradation products after reactions. Spectra were determined with a NMR Avance 400 spectrometer from Bruker. Solutions were dried by rotary evaporation at 60 ◦ C, and the solids were redissolved in D2 O with TSP (2,2 -3,3 tetradeutero-trimethylsilylpropionic acid) as internal chemical shift standard.

3. Results and discussion 3.1. Characteristics of the activated carbons and supported catalysts BET surface areas of the support and carbon-supported catalysts are shown in Table 1. These catalysts were heat-treated at the temperature indicated in the experimental section for decomposition of the iron salts, which was previously verified by thermal analyses.

M.A. Fontecha-Cámara et al. / Applied Catalysis B: Environmental 101 (2011) 425–430 Table 1 BET surface area and XPS-determined surface composition of the support and the carbon-supported iron catalysts. Sample

SBET (m2 /g)

N (wt.%)

Oa (wt.%)

Sb (wt.%)

Fe (wt.%)

ACC ACC-3.1S ACC-6.1S ACC-11.6S ACC-6.0N ACC-5.9A

2128 1997 1936 1909 1768 1883

0.4 0.6 0.5 0.5 0.3 0.7

6.9 9.8 (1.6) 19.4 (5.0) 20.1 (5.7) 8.6 (2.7) 19.9 (5.6)

Nil 1.0 (0.4) 3.1 (1.7) 3.2 (2.1) Nil Nil

Nil 1.5 3.5 7.7 4.1 8.2

a b

427

A

Values in parenthesis indicate the wt.% of oxygen from iron oxide. Values in parenthesis indicate the wt.% of sulfate.

SBET of the carbon support decreased when iron was deposited. This decrease was higher with increases in Fe content due to blocking of the microporosity by the metal. XRD patterns of the carbon-supported iron catalysts prepared with iron(II) sulfate are depicted in Fig. 2. These patterns showed diffraction peaks that can be assigned to Fe3 O4 (JCPDS 88-0866) and ␥-Fe2 O3 (JCPDS 13-534). Samples ACC-6.0N and ACC-5.9A showed no diffraction peaks corresponding to an iron oxide, likely because the iron oxide particles were below 4 nm in size and were therefore not detectable by XRD. This may be a consequence of the lower temperature used to decompose the iron(III) nitrate and iron(II) acetate in comparison to the iron(II) sulfate. XPS patterns of the Fe2p , O1s and S2p levels of the catalyst ACC-6.1S are depicted, as an example, in Fig. 3, which also includes the curve-fitted spectra. BEs of the Fe2p3/2 showed components at 710.8 and 712.7 eV that can be assigned to iron(II) and (III), respectively [23]. The BEs of the O1s showed three components at 530.2, 531.3 and 532.9 eV: the first (530.2 eV) is due to Fe–O bonds in the iron oxide [23], while the other two correspond to oxygen bound to the carbon support, with the BE at 531.3 due to C O double bonds and that at 532.9 eV to C–O single bonds [24]. In the case of sulfur, the S2p3/2 components

30.4

35.7

43.3

57.4

727

722

717

712

707

Binding energy (eV)

B

63.0 536

534

532

530

528

Binding energy (eV)

C

Arbitrary units

ACC-11.6S

ACC-6.1S

173

ACC-3.1S

170

167

164

Binding energy (eV) Fig. 3. XPS of A: Fe2p , B: O1s and C: S2p regions of ACC-6.1S catalyst.

20

30

40

50

60

70

2θ Fig. 2. XRD patterns of supported catalysts prepared with iron(II) sulfate as precursor salt. Fe3 O4 ,  and ␥-Fe2 O3 , .

161

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1.0

0.02

0.8

C/C0

(Fe/C)s

0.03

0.01

0.6

0.00 0.00

0.01

0.02

0.4

0.03

0

2

4

(Fe/C)t

6

8

10

t (h)

Fig. 4. Variation of surface atomic ratio, (Fe/C)s, plotted against total atomic ratio, (Fe/C)t, for catalysts. , ACC-xS series and , ACC-6.0N.

appeared at 164.6 and 169.1 eV, corresponding to elemental sulfur and sulfate, respectively [25–27]. Iron, oxygen, sulfur, and nitrogen contents obtained by XPS are compiled in Table 1. Supported iron catalysts show higher oxygen contents than the corresponding supports, due to the oxygen from the iron oxide and the oxygen fixed on the carbon surface after the precursor salt decomposition. Supported catalysts prepared from iron sulfate showed residual sulfur on their surface after decomposition of the precursor salt. A proportion of this sulfur was as sulfate, observing a higher percentage of sulfate with greater iron content. Fig. 4 depicts the relationship between the surface atomic ratio, (Fe/C)s , and the total or bulk atomic ratio, (Fe/C)t , for catalysts prepared with iron sulfate and nitrate. The linear relationship found between these two magnitudes indicates that the metallic phase was uniformly dispersed on the support throughout the concentration range under study [28]. Catalyst ACC-5.9A did not fit the linear relationship found for the other catalysts, with results that showed a surface segregation of Fe. 3.2. AMT removal by heterogeneous and homogeneous Fenton processes Fig. 5 depicts AMT removal kinetics with catalyst ACC-11.6S at different Fe/H2 O2 molar ratios, as an example. These curves were used to obtain the results compiled in Table 2. Solutions were analyzed by 13 C NMR after 10 h of reaction and Fig. 6 shows the results found with some catalysts. When ACC-6.1S was used, AMT signals (at 143.3 and 155.1 ppm) appeared together with a very small signal at 159.9 ppm corresponding to urazole (see Fig. 1), which was not observed when ACC was used. TOCremoved was similar or close to the sum of AMTdegraded and AMTadsorbed , indicating that practically

Fig. 5. Adsorption and degradation kinetics of amitrole with ACC-11.6S at 25 ◦ C. V = 0.1 L, Ccarbon = 0.5 g/L, CAMT = 1.07 mM, pH 3; (♦) without H2 O2 (adsorption); () Fe/H2 O2 molar ratio = 0.09; () Fe/H2 O2 molar ratio = 0.04; () Fe/H2 O2 molar ratio = 0.02.

all AMTdegraded was mineralized. The nitrogen compounds detected in solution after AMT mineralization were nitrate and ammonium ions, although at very low concentrations, indicating that N from AMT mineralization was largely converted to volatile N compounds that escaped from the solution. In all cases, the amount of AMTdegraded was higher than the AMTadsorbed . Increases in the Fe content of the catalysts augmented the AMTdegraded and TOCremoved , due to the greater number of surface active sites for H2 O2 decomposition. Decreases in the Fe/H2 O2 molar ratio also increased the amounts of AMTdegraded and TOCremoved due to the greater formation of hydroxyl radicals. Iron leaching after 10 h of reaction (Table 2) was very low and ranged between 0.7 and 2.8% relative to the iron initially present on the catalysts. The effect on AMT removal of the iron precursor salt used to prepare the Fe catalysts was studied, using a catalyst containing around 6 wt.% Fe with a Fe/H2 O2 molar ratio of 0.04. Results obtained are compiled in Table 3. 13 C NMR analysis of solutions after 10 h of reaction (Fig. 6) showed that AMT and urazole appeared when catalyst ACC-6.1S was used but that urazole was not detected when the other supported catalysts were tested. This suggests that sulfur or sulfate retained on the support may favor the oxidation of AMT to urazole. In all cases, the degraded AMT was almost completely mineralized, and the type of Fe salt used had no influence on TOCremoved . Nitrate and ammonium ions were the only nitrogen derivatives found in solution, although at negligible concentrations. Iron leaching was also very low and similar in the three catalysts used. We also studied AMT removal by homogeneous Fenton reaction in the absence and presence of ACC, using the same Fe/C and Fe/H2 O2 molar ratios as in the heterogeneous Fenton reaction in

Table 2 AMT removal at 25 ◦ C with heterogeneous Fenton systems after 10 h of reaction. V = 0.1 L, Ccarbon = 0.5 g/L, CAMT = 1.07 mM, pH 3. Fe leached is relative to the Fe initially present. Sample

Fe/H2 O2 molar ratio

TOCremoved (%)

AMTdegraded (%)

AMTadsorbed (%)

NO3 − (mM)

NH4 + (mM)

Feleached (%)

ACC-3.1S

0.09 0.04 0.02 0.09 0.04 0.02 0.09 0.04 0.02

22 27 30 25 36 47 39 45 53

15 20 22 18 30 41 26 36 46

6

0.03 0.04 0.06 0.04 0.06 0.07 0.05 0.06 0.08

0.06 0.07 0.08 0.07 0.08 0.09 0.10 0.11 0.16

2.8 2.8 2.8 1.5 1.1 1.6 0.7 0.8 0.7

ACC-6.1S

ACC-11.6S

6

5

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429

Table 3 Effect of the Fe salt used to prepare the supported catalyst on the AMT removal at 25 ◦ C with heterogeneous Fenton systems after 10 h of reaction. V = 0.1 L, Ccarbon = 0.5 g/L, CAMT = 1.07 mM, Fe/H2 O2 molar ratio = 0.04, pH 3. Fe leached is relative to the Fe initially present. TOCremoved (%)

AMTdegraded (%)

AMTadsorbed (%)

NO3 − (mM)

NH4 + (mM)

Feleached (%)

ACC-6.1S ACC-6.0N ACC-5.9A

36 32 34

30 27 26

6 6 11

0.06 0.01 0.06

0.08 0.02 0.05

1.1 1.0 1.0

155.8

143.9

Sample

159.9

155.1

143.3

ACC

155.3

143.5

ACC-6.1S

155.4

143.6

ACC-6.0N

154.4

160.3

142.5

ACC-5.9A

FeSO4+H2O2

order to compare the effectiveness of the two systems. Fig. 7 depicts the adsorption and degradation kinetics obtained, showing that the highest AMT degradation was achieved when FeSO4 was used as homogeneous Fenton catalyst in the presence of ACC. Results obtained after 10 h of reaction are compiled in Table 4. Analysis of solutions after this time (see Fig. 6) revealed the presence of urazole in all systems that used FeSO4 as catalyst. It is noteworthy that urazole was the only product in the solution when the homogeneous Fenton reaction was carried out in the presence of activated carbon, whereas AMT alone was observed after 10 h of reaction using iron acetate. According to these findings, the presence of sulfate ions in solution or on the support favors AMT oxidation to urazole, and this trend is enhanced by the additional presence of activated carbon. TOCremoved values were similar among systems 1, 2, and 4 (see Table 4) and slightly lower than those shown for the corresponding heterogeneous Fenton systems (Table 3), whereas much a higher value (∼60%) was obtained with system 3. The highest amount of AMTdegraded (∼90%) was also obtained with this system, although not all of the degraded AMT was mineralized. These interesting results indicate that FeSO4 is a good catalyst for AMT removal by homogeneous Fenton oxidation when carried out in the presence of activated carbon. Although the role of activated carbon in this process is not clear, it is a good catalyst for hydroxyl radical generation from H2 O2 and may therefore have a synergic effect on FeSO4 activity [6–11]. On the other hand, iron content of solution after 10 h reaction in systems 3 and 4 of Table 4 was slightly lower than the initial iron concentration of the solution. This was due to the iron adsorbed (∼7%) on ACC during reaction. Surface oxygen complexes are fixed on ACC or on the carbonsupported catalysts during AMT oxidation. Table 5 shows the total oxygen content of these solids before and after AMT oxidation, as determined by TPD. There was no difference between the Ofixed on used ACC-5.9A and that fixed on ACC after homogeneous Fen-

160.0

1.0

ACC+FeSO4+H2O2

0.8

144.9

ACC+FeAc2+H2O2

C/C0

0.6

156.6

0.4

160

0.2

155

150

145

140

ppm

0.0 0

2

4

6

8

10

t (h) Fig. 6. 13 C NMR spectra of AMT solutions after 10 h of reaction using different oxidation systems.

Fig. 7. Adsorption and degradation kinetics of amitrole at 25 ◦ C with different systems. V = 0.1 L, Ccarbon = 0.5 g/L, CAMT = 1.07 mM, Fe/H2 O2 molar ratio = 0.04, pH 3; () ACC + AMT (adsorption); (♦) FeSO4 + H2 O2 + AMT; () ACC-6.1S; () ACC + FeSO4 + H2 O2 + AMT.

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Table 4 AMT removal at 25 ◦ C with homogeneous Fenton systems in the absence and presence of carbon after 10 h of reaction. Experimental conditions were the same as in Table 3. System 1 2 3 4 a

TOCremoved (%) FeSO4 + H2 O2 Fe(Ac)2 + H2 O2 FeSO4 + H2 O2 + ACC Fe(Ac)2 + H2 O2 + ACC

24 26 58 (64)a 29

AMTdegraded (%)

AMTadsorbe (%)d

NO3 − (mM)

NH4 + (mM)

24 30 90 29

– – 4 4

0.04 0.08 0.05 0.08

0.02 0.02 0.06 0.02

Values in parenthesis are after 24 h of reaction.

Table 5 Results obtained from TPD experiments on fresh and used ACC and carbonsupported Fe catalysts. Sample

O (mmol/g)

Ofixed (mmol/g)

Fresh ACC-6.1S Used ACC-6.1S Fresh ACC-5.9A Used ACC-5.9A Fresh ACC ACC + FeSO4 + H2 O2 ACC + Fe(Ac)2 + H2 O2

2.82 3.88 5.03 6.33 1.21 2.78 2.54

– 1.06 – 1.30 – 1.57 1.33

ton reaction with iron acetate. In contrast, Ofixed on ACC was much higher after homogeneous Fenton reaction with iron sulfate than after heterogeneous Fenton reaction with ACC-6.1S catalyst. This represents experimental evidence of a higher hydroxyl radical generation in the ACC + AMT + FeSO4 + H2 O2 system than in the other studied systems, especially when iron acetate was used as homogeneous catalyst. This finding may indicate a synergic effect of ACC and FeSO4 on hydroxyl radical generation in this system. Finally, the concentration of nitrate and ammonium ions was negligible after 10 h of homogeneous Fenton reaction in the absence or presence of ACC, as also found with the heterogeneous Fenton systems. 4. Conclusions Results obtained for AMT removal using heterogeneous Fenton processes with carbon-supported iron catalysts prepared from iron sulfate showed increased amounts of AMTdegraded and TOCremoved with higher Fe contents of the catalyst and lower Fe/H2 O2 molar ratios. These findings are attributable to increases in the number of surface active sites for H2 O2 decomposition and in the formation of hydroxyl radicals, respectively. The iron leaching after 10 h of reaction was very low. The type of iron salt used to prepare the supported catalysts (sulfate, acetate, or nitrate) had no influence on TOCremoved . TOCremoved was slightly lower with homogeneous Fenton catalysts in the absence of activated carbon than with the corresponding heterogeneous Fenton systems. The highest AMT degradation (∼90%) was obtained using FeSO4 as homogeneous Fenton catalyst in the presence of ACC, although not all of the degraded AMT was mineralized, and the TOCremoved was ∼60%. Activated carbon and iron sulfate showed a synergic effect on hydroxyl generation from H2 O2 , which was not observed when iron acetate was used as homogeneous catalyst in the presence of ACC. AMT oxidation to urazole was favored by the presence of sulfate ions on the support or in the solution. Nitrate and ammonium ions were detected after 10 h of reaction with both the heterogeneous

and homogeneous Fenton systems, but at very low concentrations. Acknowledgements Authors are grateful to MEC and FEDER Project CTQ-200767792-C02-02 for financial support. References [1] T. Oesterreich, U. Klaus, M. Volk, B. Neidhart, M. Spiteller, Chemosphere 38 (1999) 379–392. [2] C. Catastini, S. Rafqah, G. Mailhot, M. Sarakha, J. Photochem. Photobiol. A 162 (2004) 97–103. [3] A. Da Pozzo, C. Merli, I. Sirés, J.A. Garrido, R.M. Rodríguez, E. Brillas, Environ. Chem. Lett. 3 (2005) 7–11. [4] M.A. Fontecha-Cámara, M.V. López-Ramón, M.A. Álvarez-Merino, C. MorenoCastilla, Langmuir 23 (2007) 1242–1247. [5] A. Zapata, T. Velegraki, J.A. Sánchez-Pérez, D. Mantzavinos, M.I. Maldonado, S. Malato, Appl. Catal. B: Environ. 88 (2009) 448–454. [6] R.C. Bansal, J.B. Donnet, F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988, p. 441. [7] M. Kimura, I. Miyamoto, Bull. Chem. Soc. Jpn. 67 (1994) 2357–2360. [8] L.B. Khalil, B.S. Girgis, T.A.M. Tawfik, J. Chem. Technol. Biotechnol. 76 (2001) 1132–1140. [9] L.C.A. Oliveira, C.N. Silva, M.I. Yoshida, R.M. Lago, Carbon 42 (2004) 2279–2284. [10] A. Georgi, F.D. Kopinke, Appl. Catal. B: Environ. 58 (2005) 9–18. [11] V.P. Santos, M.F.R. Pereira, P.C.C. Faria, J.J.M. Órfao, J. Hazard. Mater. 162 (2009) 736–742. [12] H.J.H. Fenton, J. Chem. Soc. 65 (1894) 899–911. [13] B.G. Kwon, D.S. Lee, N. Kang, J. Yoon, Water Res. 33 (1999) 2110–2118. [14] R.F. Brown, S.E. Jamison, U.K. Pandit, J. Pinkus, G.R. White, H.P. Braendlin, J. Org. Chem. 29 (1964) 146–153. [15] C. Walling, Acc. Chem. Res. 8 (1975) 125–131. [16] J.J. Pignatello, E. Oliveros, A. MacKay, Crit. Rev. Environ. Sci. Technol. 36 (2006) 1–84. [17] C. Moreno-Castilla, M.A. Fontecha-Cámara, M.A. Álvarez-Merino, M.V. LópezRamón, F. Carrasco-Marín, Adsorption, doi:10.1007/s10450-010-9270-x, accepted for publication. [18] F. Lücking, H. Köser, M. Jank, A. Ritter, Water Res. 32 (1998) 2607–2614. [19] J.A. Zazo, J.A. Casas, A.F. Mohedano, J.J. Rodríguez, Appl. Catal. B: Environ. 65 (2006) 261–268. [20] T.L.P. Dantas, H.J. Mendoc¸a, A.E. José, R.F.P.M. Rodrigues, Moreira, Chem. Eng. J. 118 (2006) 77–82. [21] J.H. Ramirez, F.J. Maldonado-Hódar, A.F. Pérez-Cadenas, C. Moreno-Castilla, C.A. Costa, L.M. Madeira, Appl. Catal. B: Environ. 75 (2007) 312–323. [22] F. Duarte, F.J. Maldonado-Hódar, A.F. Pérez-Cadenas, L.M. Madeira, Appl. Catal. B: Environ. 85 (2009) 139–147. [23] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 36 (2004) 1564–1574. [24] M.A. Álvarez-Merino, F. Carrasco-Marín, J.L.G. Fierro, C. Moreno-Castilla, J. Catal. 192 (2000) 363–373. [25] B.J. Lindberg, K. Hamrin, G. Johansson, U. Gelius, A. Fahlmann, C. Nordling, K. Siegbahn, Phys. Scr. 1 (1970) 286–298. [26] C.R. Ginnard, R.S. Swingle, B.M. Monroe, J. Electron Spectrosc. Relat. Phenom. 6 (1975) 77–80. [27] M.M. Chehimi, M. Delamar, J. Electron Spectrosc. Relat. Phenom. 49 (1989) 213–231. [28] F.P.J.M. Kerkhof, J.A. Moulijn, J. Phys. Chem. 83 (1979) 1612–1619.