Treatment of azo dye-containing wastewater by a Fenton-like process in a continuous packed-bed reactor filled with activated carbon

Journal of Hazardous Materials 237–238 (2012) 30–37

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Treatment of azo dye-containing wastewater by a Fenton-like process in a continuous packed-bed reactor filled with activated carbon Isabel Mesquita a , Luís C. Matos a , Filipa Duarte a,b , F.J. Maldonado-Hódar c , Adélio Mendes a,b , Luis M. Madeira a,b,∗ a

Chemical Engineering Department, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal LEPAE – Laboratory for Process, Environmental and Energy Engineering, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Porto, Portugal c Department of Inorganic Chemistry, Faculty of Sciences, University of Granada, Av. Fuentenueva, 18071 Granada, Spain b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Oxidation with the Fenton’s reagent was carried out in a packed-bed reactor.  The packed-bed was filled with ironimpregnated activated carbon.  The increment of temperature increases the Chicago Sky Blue removal and mineralization.  The values of iron leaching were below 0.4 ppm in the outlet effluent.  It was possible to reach a dye conversion of 88% in steady-state.

a r t i c l e

i n f o

Article history: Received 3 April 2012 Received in revised form 11 June 2012 Accepted 8 July 2012 Available online 24 August 2012 Keywords: Heterogeneous Fenton-like oxidation Packed-bed Activated carbon Chicago Sky Blue Hydrogen peroxide

a b s t r a c t In this work, oxidation with a Fenton-like process of a dye solution was carried out in a packed-bed reactor. Activated carbon Norit RX 3 Extra was impregnated with ferrous sulfate and used as catalyst (7 wt.% of iron). The effect of the main operating conditions in the Chicago Sky Blue (CSB) degradation was analyzed. It was found that the increase in temperature leads to a higher removal of the dye and an increased mineralization. However, it also increases the iron leaching, but the values observed were below 0.4 ppm (thus, far below European Union limits). It was possible to reach, at steady-state, a dye conversion of 88%, with a total organic carbon (TOC) removal of ca. 47%, being the reactor operated at 50 ◦ C, pH 3, Wcat /Q = 4.1 g min mL−1 (Wcat is the mass of catalyst and Q the total feed flow rate) and a H2 O2 feed concentration of 2.25 mM (for a CSB feed concentration of 0.012 mM). The same performance was reached in three consecutive cycles. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Dyes used during the dyeing process do not attach completely onto the fibers, and are therefore present in abundance in textile effluents. These are among the main contributors to the pollution

∗ Corresponding author at: Chemical Engineering Department, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal. Tel.: +351 22 5081519; fax: +351 22 5081449. E-mail address: [email protected] (L.M. Madeira). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.07.066

of watercourses [1], and should thus be submitted to an appropriate treatment before being released into the environment. Among the physical processes commonly used in the discoloration of dyes-containing effluents, adsorption onto activated carbon (AC) is worth mentioning and has been widely used. But the adsorption can be summarized as the simply transfer of the adsorbate from one phase to another, without occurring any degradation. The advanced oxidation processes (AOPs) have shown to be excellent methods for treating dye-containing wastewaters, being a viable alternative to the processes already in use. A particularly promising AOP is the one based in the Fenton’s reagent, which has

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many advantages such as high performance in discoloration, simplicity in the oxidation of a wide range of contaminants, being also simple and inexpensive [2]. It may also be operated at room temperature and atmospheric pressure. Moreover, the oxidant used (hydrogen peroxide – H2 O2 ) breaks down into environmentally safe species like water and oxygen [3]. Oxidation with Fenton’s reagent is based on the generation of hydroxyl radicals (HO• ), highly reactive species that attack non-selectively numerous organic compounds. The radicals are produced by the catalytic decomposition of H2 O2 by a transition metal like Fe (II) ions, in acid medium (Eq. (1)): H2 O2 + Fe2+ → HO• + OH− + Fe3+

(1)

As mentioned, these hydroxyl radicals have a high oxidative potential and are able to oxidize (and ultimately mineralize up to CO2 and H2 O) non-biodegradable organic pollutants, such as the dyes used in the textile industry (Eq. (2)): OH• + organic matter → oxidation products

(2)

Because Fe (II) ions act as catalyst in the reaction, it is important that they exist in large concentrations in the reaction medium, or that their regeneration proceeds at high rate (Eq. (3)): H2 O2 + Fe3+ → Fe2+ + H+ + HO2 •

(3)

An important disadvantage of this homogeneous process is the high concentration of iron that remains in solution at the end of the treatment. Typical concentrations are 50–80 ppm for batch processes, which is clearly above the 2 ppm limit imposed by European Union (EU) directives [4]. However, the use of solid matrices to support and fix the transition metal can minimize these adverse effects. Activated carbons are among the most used supports because they exhibit several important advantages such as relative low cost, high surface areas, high stability, and easy control of the textural properties (either morphological or chemical) [3]. The use of heterogeneous Fenton/Fenton-like processes has been the focus of extensive research. Nevertheless, their use in continuous reactors like packed-beds is not much discussed in the literature compared to batch (slurry) reactors. There are however a few exceptions. For instance, Melero et al. [5] and Martínez et al. [6] have studied the catalytic wet peroxide oxidation in fixed-bed reactors for the degradation of pharmaceutical and phenolic aqueous solutions, respectively, using the same Fe2 O3 /SBA-15 nanocomposite as catalyst. On the other hand, Lücking et al. [7] used a fixed-bed reactor filled with an iron-impregnated activated carbon, but for the degradation of 4-clorophenol aqueous solutions. Up to the authors’ knowledge, the degradation of dyes in fixed-bed reactor containing AC as iron support (heterogeneous Fenton-like process) has not yet been addressed in the open scientific literature. This work was thus focused on the heterogeneous Fenton-like process in a continuous packed-bed reactor filled with activated carbon pellets (Norit RX 3 Extra, impregnated with 7 wt.% of iron). This activated carbon was chosen due to the high fraction of large micropores (>0.7 nm) and the good performance found in previous works, namely in what concerns to activity and improved iron dispersion [3]. Chicago Sky Blue is an anionic azo dye mainly used to color cotton fibers, and was used as model compound. 2. Material and methods 2.1. Catalyst preparation and characterization Norit RX 3 Extra AC was used as support of the active phase. The catalyst was previously used in powder form in a slurry batch reactor for the degradation of another dye (Orange II) [3], and the better performance, as compared to other carbon materials, was found

31

to be due to the higher surface area located on large micropores (0.7–2 nm), which favors both dye adsorption and Fe dispersion. This yields a more active catalyst, although iron leaching might be a detrimental side effect. The AC extrudate was milled to a particle size between 1.41 and 1.68 mm in order to respect the heuristics for packed columns (ratio between internal diameter of the reactor and particle diameter above 9–10 so that wall effects can be neglected). Then, 5 g of AC was wet impregnated at room temperature using an iron sulfate (FeSO4 ·7H2 O) solution as precursor, because it is a relatively inexpensive salt. The solution was prepared with 1.74 g of iron sulfate (the amount required to achieve 7 wt.% of iron in the final catalyst) dissolved in the minimum amount of distilled water (according to the salt solubility), with the help of magnetic stirring. Then, the iron solution was added dropwise and uniformly to the carbon, avoiding the excess of solution added, the process being stopped after a homogeneous wetting of the particles [3,8,9]. Iron was selected because it is described as being the best transition metal to activate the H2 O2 molecule in the Fenton reaction [9], while the content of iron in the catalyst (7 wt.%) was chosen based on other works of the authors on this topic [3,9]. After impregnation, the sample was dried overnight in an oven at 100 ◦ C, and finally treated for 2 h at 400 ◦ C under a He stream. The characterization of the materials was performed by adsorption of N2 (at 77 K) and CO2 (at 273 K) in a Quantachrome Autosorb unit-1. Nitrogen adsorption data were used to obtain the BET surface area (SBET ) and the BJH method [10] was applied to the desorption branch of the N2 adsorption isotherms (software AS1WIN-1.52 of Quantachrome) to obtain the mesopore volume (VBJH ) and the pore size distribution (PSD) curve of the AC. The micropore volume, W0 , and the characteristic adsorption energy, E0 , were obtained by applying the Dubininin–Radushkevich equation to both CO2 and N2 adsorption isotherms [11,12]. Then, the mean micropore width, L0 , was obtained by applying the Stoeckli equation [13]: L0 (nm) =

10.8 E0 (kJ mol−1 ) − 11.4

(4)

Additionally, the volume of meso- (above 3.5 nm) and macropores was obtained by mercury porosimetry in a Poremaster 60 Quantachrome apparatus. The true (solid) density of the support material was obtained by helium pycnometry, whereas the apparent (particle) density was deduced from mercury pycnometry. Metal dispersion on the carbon surface was qualitatively analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques using a Philips CM-20 electron microscope and a Bruker D8 Advance diffractometer, respectively. XRD provided also information about the chemical nature of iron particles. The carbon used for adsorption experiments was boiled in water to remove gas present in the pores. This guarantees that there are no gas bubbles blocking the adsorbate diffusion. 2.2. Catalytic activity Chicago Sky Blue (CSB) was the dye used in this work, supplied by Sigma–Aldrich. It is a big molecule with molar mass of 992.8 g mol−1 . The absorption spectrum of the dye in the visible region (400–700 nm) was obtained in a dual beam Jasco V-530 spectrophotometer, which enabled the determination of 616 nm as the maximum absorbance wavelength. Chemical oxidation studies were performed in a borosilicate glass column with 1.5 cm of internal diameter and 20 cm long, which was filled with the catalyst and inert glass spheres of the same particle size as the AC, above and below the catalytic bed. The glass spheres are absolutely inert and do not catalyze the process.

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Fig. 1. Representative diagram of the experimental setup used.

Over time, samples were taken from the effluent at the reactor outlet and an excess of sodium sulfite added in order to stop the homogeneous reaction by instantaneous consumption of residual H2 O2 . These samples were then stored in the refrigerator, before further analyses. Mineralization was then assessed by analysis of the total organic carbon (TOC) in a Shimadzu TOC-5000A apparatus, while the iron content in those samples was measured in a UNICAM SOLAR 939/959 atomic absorption spectrophotometer. Two mechanisms originate the removal of the Chicago Sky Blue dye from the aqueous solution in the heterogeneous process: adsorption onto the activated carbon and oxidation by the Fenton-driven process. Therefore, these two processes were studied independently. 3. Results and discussion 3.1. Support and catalyst characterization The volume pore size distribution function [Dv (d), in cm3 nm−1 g−1 ] of the AC support, defined as the pore volume per unit interval of diameter, according to Eq. (5), is shown in Fig. 2. Dv (d) =

P dV d dP

(5)

0.020 0.018 0.016 0.014 0.012 0.010 0.008 0.006

Dv(d)/cm 3 .nm -1.g -1

Fig. 1 shows a schematic representation of the experimental setup used. The dye and hydrogen peroxide (H2 O2 ) solutions were fed to the column by using a ISMATEC peristaltic pump (Reglo model) with two heads and eight rolls; solutions were fed upwards, avoiding this way the clogging of the column bottom fritz, the clogging of the column itself and the air retention, which exited the column through the end fritz. The flow rates of dye and hydrogen peroxide were the same (checked in every experiment), each representing half of the total flow rate (Q). Throughout this work, Q = 2.5 mL min−1 ; Wcat /Q stands therefore for the ratio between the mass of catalyst and the total feed flow rate. The concentration of the dye solution used was 0.012 mM, because it is within the range of concentrations of azo dyes commonly found in industrial effluents [14]. The feed concentration of hydrogen peroxide was in the range between 0.15 and 12 mM (for the reasons described below). Consequently, the concentration of the H2 O2 solution that was in the reservoir put in the water bath (cf. Fig. 1) was the double of those values, due to the dilution by the dye solution when entering into the reactor. The hydrogen peroxide solution was prepared according to manufacturer specifications (the flask being stored in the fridge), and occasionally checked as reported elsewhere [15]. The column, filled with activated carbon or Fe-impregnated activated carbon, was provided with a water jacket, and the solutions stored in a thermostatic bath (Hüber, model K12) for a better temperature control (cf. Fig. 1). In most cases, and whenever otherwise indicated, runs were carried out at 30 ◦ C. The pH of the solutions was adjusted to 3.0 by addition of small volumes of a 2 M HCl solution. Such value was chosen due to the well-known fact, widely document in the literature, that the pH range of 3.0–3.5 is the optimum for reaching a maximum performance in Fenton/Fentonlike reactions [e.g., 16, 17]. An operating temperature of 30 ◦ C results from the favorable balance between reasonable reaction rate and low leaching of iron in heterogeneous systems (because both generally increase with temperature), along with high stability of H2 O2 (which decomposes thermally into H2 O and O2 ) [18]. In any case, temperatures in the range 10–70 ◦ C were tested in this work. The absorbance of the effluent at the column outlet was followed continuously by using a flow-through cell in a Jenway 6300 spectrophotometer, at the above-mentioned wavelength of 616 nm; absorbance data were monitored and recorded in real-time using a developed LabVIEW 9.0 interface.

0.004 0.002 10000

1000

100

10

1

0.000

pore size/nm Fig. 2. Volumetric pore size distribution for macro- and meso-pores (above 3.5 nm) of the AC support.

400

1.4

350

1.2

300

1.0

dV(logr) /cm 3.g-1

Vgas /cm 3.g-1

I. Mesquita et al. / Journal of Hazardous Materials 237–238 (2012) 30–37

250 200 AC

150

0.8 0.6 0.4

Fe-AC

100

33

0.2 0.0

50

0.0

2.5

5.0

0

10.0

12.5

15.0

0.20

0.40

0.60

0.80

1.00

P/P0 Fig. 3. N2 adsorption (closed symbols) and desorption (open symbols) isotherms at 77 K of the activated carbon support (AC) and catalyst (Fe-AC) used.

In Eq. (5), P is the intrusion pressure (psia), d the pore diameter (nm), dP and dV the increment of pressure (psia) and volume per mass of sample (cm3 g−1 ), respectively. The volume of macro and mesopores was calculated by integration of the volume pore size distribution between 50 nm and 3.5 nm, for mesopores, and above 50 nm, for macropores. It was not possible to cover all the mesoporosity range (between 50 nm and 2 nm) due to the 3.5 nm detection limit of the mercury porosimetry equipment. The volume of macropores was found to be 0.43 cm3 g−1 , corresponding to 84% of the total Hg volume intrusion, and the volume of mesopores above 3.5 nm 0.08 cm3 g−1 , representing 16% of the total pores volume. The densities obtained were as follows: true = 2.24 g cm−3 and apparent = 0.73 g cm−3 . The textural characterization of the materials was also performed based on their corresponding N2 and CO2 adsorption isotherms. The micropore volume obtained from CO2 adsorption at 273 K yields the volume of narrower micropores [19], whereas the total micropore volume was obtained from N2 adsorption at 77 K. The results obtained for the AC support and AC-Fe catalyst are shown in Table 1. The bed porosity obtained was 0.35 (1 − Vcarbon /Vtotal ), and the carbon porosity 0.67 (1 − apparent /true ). Both the AC support and the Fe-AC catalyst are essentially microporous materials, with BET areas of 1120 and 930 m2 g−1 , respectively (Table 1). The decrease of the BET area and of the W0 (N2 ) value from the support to the catalyst is justified by a partial blocking of the porosity by iron particles [3,20]. However, both materials present wide and heterogeneous micropores, pointed out by the L0 values and the accentuated knee of the nitrogen isotherms (Fig. 3). The volume of micropores accessible to the N2 and CO2 molecules is different, with W0 (N2 ) > W0 (CO2 ) because of the N2 condensation within the wider micropores. The nitrogen isotherm matches mainly to the type I, typical of microporous materials, the adsorption occurring mainly at low pressures as a consequence of the micropores filling. However, in this case, the isotherm presents a large knee, a certain slope in the plateau and even a small hysteresis cycle, indicating the presence of large micropores/mesopores. These results are in accordance with previous studies [3]. Mesopore volume of the AC support was also estimated by the BJH method using the N2 desorption data, providing a pore

Fig. 4. Pore size distribution of the AC support obtained by the BJH method.

volume of 0.37 cm3 g−1 . Pore size distribution (Fig. 4) obtained by this method confirms the existence of mesopores (below 3.5 nm) and large micropores, being both methods (BJH and Hg porosimetry) complementary as they cover different pore dimension ranges. From the X-ray diffraction data of iron sulfate (data not shown) it was possible to conclude that the precursor salt was decomposed, once the characteristic XRD peaks of the raw sulfate were not observed in the catalyst XRD pattern (Fig. 5). XRD diffraction peaks assignation is not an easy task due to the existence of different oxides and allotropic forms. However, the major phase seems to be maghemite (␥-Fe2 O3 ), with the 100% relative intensity peak (I1 0 0 ) corresponding to the crystal plane (2 2 0) with diffraction at around 29◦ . Other peaks also identified the presence of this phase. With less relevance, i.e., with smaller intensity, at 35.5◦ also appears the I1 0 0 for plane (3 1 1) of magnetite (Fe3 O4 ) (JCPDS file 19-629). So, iron (II) and particularly iron (III) species are expected to be presented on the AC-Fe catalyst, both being active for the envisaged Fenton-like process. Regarding metal dispersion, both XRD and HRTEM showed the formation of large metallic particles. It is well known the difficulty to detect particles smaller than ca. 4 nm by XRD in supported metal-catalysts [21,22]. Even so, this does not exclude the possibility that small particles are also present. HRTEM micrographs (Fig. 6) show metallic particles (black spots) up to 200 nm, although small

Intensity / arbitrary units

0.00

7.5

radius /nm

10

20

30

40

Scaering angle 2θ / ο Fig. 5. XRD pattern of the catalyst (Fe-AC).

50

60

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Table 1 Textural characteristics of the support and catalyst obtained from N2 and CO2 adsorption at 77 K and 273 K, respectively.a Material

SBET (m2 g−1 )

W0 (N2 ) (cm3 g−1 )

W0 (CO2 ) (cm3 g−1 )

L0 (CO2 ) (nm)

AC (support) Fe-AC (catalyst)

1120 930

0.47 0.40

0.33 0.34

0.71 0.73

a

W0 refers to the micropore volume and L0 to the mean micropore width.

adsorbed during the whole experiment. In fact, after this period a very long tail is noticed because the carbon has a high volume of micropores, responsible for the higher mass transfer resistance but also for the higher fraction of surface area available for adsorption (cf. Section 3.1). Therefore, in such a microporous adsorbent there is slow dye diffusion due to its high molecular weight, and the material is completely saturated only after ca. 300 min (5 h). The stoichiometric time is approximately 13 min, which reflects an adsorption capacity of 2.88 mg g−1 .

Fig. 6. HRTEM photo of the catalyst (Fe-AC).

particles are also present. This evidences a not very good dispersion of the metal on the AC’s surface. 3.2. Elimination of the Chicago Sky Blue dye from the effluent As above-mentioned, the two mechanism involved in the dye removal (adsorption and Fenton-driven oxidation) were studied independently. The chemical oxidation experiments were therefore performed with a saturated material (at the feed dye concentration) to avoid that its adsorptive behavior affects the results (and to achieve steady-state in a shorter period). 3.2.1. Adsorption The adsorption breakthrough curve for the AC support is shown in Fig. 7. In a short time of less than 2 min, the outlet concentration reaches 90% of the inlet value. However, the amount retained in this period corresponds to a short fraction of the total amount of dye

Fig. 7. Adsorption breakthrough curve for the Norit support (0.012 mM feed dye concentration, Wcat /Q = 0.09 g min mL−1 , pH 3, 30 ◦ C).

3.2.2. Catalytic activity of the activated carbon vs. catalyst As mentioned above, the catalytic experiments were performed with the support saturated; this means that the AC was in equilibrium with the dye solution at the feed concentration. So, after adsorption experiments, the column was quickly flushed with water to remove the dye from the inter-particle voids. Only then catalytic experiments were carried out (by feeding H2 O2 + dye solutions), until steady-state was reached. It is well known that the carbons surface has the capacity to activate H2 O2 molecules in the absence of transition metals [3,23]. Batch experiments with AC and AC-Fe (AC impregnated with iron) and without dye (pH 3, T = 30 ◦ C, Csolid = 0.1 g L−1 and CH2 O2 = 6 mM), showed, using a colorimetric method for the quantification of H2 O2 [24], some differences between the oxidant consumption in both cases, which became more significant along time. The FeAC was more effective in the H2 O2 decomposition, with 58% vs. 48% for the AC after 10 h, and 90% vs. 68% after 24 h of reaction; even so, the support is undoubtedly able to activate the oxidant. Thus, experiments in the column reactor were also carried out with the AC support, in the presence of H2 O2 . Fig. 8 shows the normalized dye concentration histories at the column outlet. For comparison purposes this run was also performed with the activated carbon impregnated with iron (Fe-AC). It can be seen that, at steady-state, the removal of dye increases from 10% to 30% due to the presence of iron in the carbon. For the activated carbon Norit support, an activation of the hydrogen peroxide on the carbon surface can be noticed, leading to some dye degradation, which is clearly improved in the presence of the transition metal (iron), due to the heterogeneous catalytic Fenton-like process.

3.2.3. Catalytic activity of the catalyst: effect of the main parameters Before carrying out tests in a fixed-bed reactor, the dynamic stability of the catalytic bed, i.e., the conversion reached in consecutive experiments, was evaluated. It was found that the conversion of the dye at the steady-state remained nearly constant in three consecutive experiments, performed under the same operating conditions (0.012 and 2.25 mM for dye and H2 O2 feed concentrations, respectively, Wcat /Q = 1.88 g min mL−1 , pH 3, 30 ◦ C). Besides, the leaching of iron from the AC support was very small, providing exit concentrations below 0.13 ppm (corresponding in total of the three experiments, each one lasting ca. 6 h, to a loss of iron <0.11%, as compared to that initially present in the catalytic bed). Apparently, the loss of iron in consecutive cycles does not lead to a decrease in the steady-state dye conversion in the short term. Therefore, it was decided to perform the parametric study described in the following

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1.0

C/C0

0.8 0.6 0.4

AC Fe-Ac

0.2 0.0 0

2000

4000

6000

8000

10000

12000

time / s Fig. 8. Dye concentration at the column outlet for the activated carbon support (AC) and activated carbon impregnated with iron (Fe-AC catalyst); (0.012 mM dye feed concentration – C0 , 2.25 mM hydrogen peroxide feed concentration, Wcat /Q = 1.88 g min mL−1 , pH 3, 30 ◦ C).

3.2.3.1. Effect of feed H2 O2 concentration. One of the most important parameters in the Fenton’s process is the concentration of hydrogen peroxide, as it affects the level of pollutant degradation, whereas the iron concentration influences mostly the kinetics [25]. However, an excess of oxidant (H2 O2 ) might, in some circumstances, reduce the conversion due to the scavenging of the hydroxyl radicals that are required in the oxidation reaction [26]: H2 O2 + HO• → H2 O + HO2 •

(6)

Consequently, the optimization of the H2 O2 concentration is required, being also of relevance from an economic point of view. Therefore, several feed concentrations of H2 O2 were tested, keeping constant the concentration of CSB (0.012 mM). The range of H2 O2 concentration studied was between 0.15 and 12 mM. This range, which includes the stoichiometric concentration of 1.12 mM that is required for complete oxidation (calculated based on the following equation: C34 H24 N6 Na4 O16 S4 + 93H2 O2 → 34CO2 + 100H2 O + 6HNO3 + 4NaHSO4 ), was established due to the well-known existence of undesirable parallel reactions that decrease the utilization efficiency of hydrogen peroxide, as mentioned above. Fig. 9 shows the effect of the H2 O2 solution concentration in the degradation of the dye. It appears that, in all cases, after about 40–50 min (2400–3000 s) the concentration of the dye at the reactor outlet remains approximately constant (i.e., steady-state has been reached). As can be seen, within a certain range of concentrations (from 0.15 up to ca. 2.25 mM), an increase in the feed H2 O2 concentration yields an increase in dye conversion, which is related to the increased formation of hydroxyl radicals (Eq. (1)). The inset of Fig. 9 shows the percentage removal of the dye, at steady-state, as a function of the H2 O2 concentration. It is noticed the existence of an optimal feed concentration of oxidant at 2.25 mM, meaning a H2 O2 /CSB molar ratio of 187.5; for higher concentrations the conversion of the dye decreases. This behavior is due to the scavenger effect of H2 O2 , i.e., above that concentration part of the hydroxyl radicals formed are consumed by parallel undesirable reactions (in this case due to the presence of excess of oxidant) and are no longer available to oxidize the dye (cf. Eq. (6)).

3.2.3.2. Effect of temperature. Although the temperature used for Fenton-like oxidation is typically around 25–30 ◦ C, textile effluents from the dyeing process usually exhibit much higher temperatures [27]. To evaluate the influence of this parameter in the removal of the dye, experiments were performed in the range of 10–70 ◦ C. The optimum oxidant dose of 2.25 mM previously determined was used. Fig. 10 shows the results obtained, in terms of transient dye concentrations at the column outlet (steady-state conversions are shown in the inset). As shown in Fig. 10, the conversion of the dye increases with temperature, reaching a value of 78% at 70 ◦ C. This is due to the higher rates of hydroxyl radicals formation and organics oxidation (Eqs. (1) and (2)), which kinetics are exponentially favored with the increase of temperature (Arrhenius law). Several other studies, usually carried out in batch mode, and thus with the catalyst in suspension (slurry reactors), refer that when the temperature is above 40 ◦ C, the H2 O2 molecule breaks down into oxygen and water, reducing the conversion of the dye [18,27]. In this case, when operating the fixed-bed reactor, some thermal decomposition of the oxidant was also noticed, because it was observed the formation of gas bubbles inside the column at 50–70 ◦ C. These gas

1.0

0.15 mM 0.90 mM

0.8

1.50 mM 2.25 mM

0.6

3.00 mM

C/C0

sections with the same catalytic bed, changing each variable at a time while keeping the others constant.

12.0 mM 0.4 0.2 0.0 0

500

1000

1500

2000

2500

3000

time / s Fig. 9. Dye concentration at the column outlet for different feed hydrogen peroxide concentrations; the inset represents steady-state dye conversion values, where the line was included for a better visualization of the data trend (0.012 mM dye feed concentration – C0 , Wcat /Q = 1.88 g min mL−1 , pH 3, 30 ◦ C).

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Fig. 10. Dye concentration at the column outlet for different temperatures of the reaction; the inset represents steady-state dye conversion values, where the line was included for a better visualization of the data trend (0.012 mM dye feed concentration – C0 , 2.25 mM hydrogen peroxide feed concentration, Wcat /Q = 1.88 g min mL−1 , pH 3).

bubbles affect mass transfer between the liquid and the catalyst phase, but even so this phenomenon was not important enough to affect significantly the dye removal (which increased linearly with temperature). The iron concentration in the final effluent was measured for the different experiments and in all cases the levels were clearly below the 2 ppm limit imposed by the current European legislation (values obtained were ∼0.3 ppm). Fig. 11 shows the history, i.e., the temporal evolution, of the normalized TOC at the column outlet, for the different tests. Within the temperature range studied, it was possible to obtain a steady-state TOC removal between 13 and 31%. The degree of dye mineralization increases with temperature, but in the range of 50–70 ◦ C the conversion of TOC is about the same. This may be related to the thermal decomposition of H2 O2 into water and oxygen at high temperatures, as described previously, so that oxidant utilization efficiency is affected; while not affecting the net removal of the dye, which is easily oxidized, it affects the conversion of smaller (intermediate) molecules that are more refractory toward oxidation [8]. 3.2.3.3. Effect of mass of catalyst and catalyst stability. In an attempt to increase the dye conversion and the degree of mineralization (for a feed containing 0.012 mM of CSB), and taken into account the column dimensions, it was decided to increase the mass of catalyst from 4.7 to 10.25 g (and thus Wcat /Q from 1.88 1.0

TOC / TOC 0

0.8 0.6 T = 10 ºC

0.4

T = 30 ºC T = 50 ºC

0.2

T = 70 ºC

0.0 0

1000

2000

3000

4000

5000

6000

time / s Fig. 11. TOC concentration at the column outlet for different reaction temperatures; the lines were included for a better visualization of the trends (0.012 mM dye feed concentration, 2.25 mM hydrogen peroxide feed concentration, Wcat /Q = 1.88 g min mL−1 , pH 3).

Fig. 12. Dye and TOC steady-state conversions for three consecutive tests in the same operating conditions (0.012 mM dye feed concentration – C0 , 2.25 mM hydrogen peroxide feed concentration, Wcat /Q = 4.1 g min mL−1 , pH 3, 50 ◦ C).

to 4.10 g min mL−1 ). Hydrogen peroxide concentration used was 2.25 mM. It was reached a dye conversion of about 70% at a temperature of 30 ◦ C (which should be compared with ca. 37% for Wcat /Q = 1.88 g min mL−1 – Fig. 10). Longer beds lead to higher residence/contact times, thus dye conversion is raised. To further increase the discoloration of the final effluent, the temperature was increased to 50 ◦ C. Fig. 12 shows the steadystate results for this oxidation run at 50 ◦ C, as well as two more cycles, which were performed in order to assess the catalyst stability/reproducibility in successive experiments. As can be seen from Fig. 12, with a 2.25 mM feed H2 O2 concentration, Wcat /Q = 4.1 g min mL−1 and a temperature of 50 ◦ C, it is possible to obtain a steady-state conversion of about 88% (more 24% than with Wcat /Q = 1.88 g min mL−1 – Fig. 10). Besides, this value is reproducible, at least for three consecutive cycles (lasting ca. 5 h each). As for TOC removal, it was around 47% (the slight oscillations observed are within the experimental error). Data provided show that the catalyst is stable in consecutive cycles. However, and as mentioned above, along all the parametric study performed (using the same catalyst bed), iron was leached out from the support. As it was exposed, an heterogeneous process is carried out on the catalyst surface, and so there are two simultaneous effects that modify the porous texture of the catalyst during reaction: Fe-leaching liberates the blocked porosity of the AC support after impregnation, but this fact is compensated by adsorbed organic compounds (as evidenced by TG measurements, not shown for brevity reasons). It is difficult to estimate the contribution of each process, even more, because variation of the BET surface area is not strong regarding the fresh catalyst. Further study should be carried out to give better insight for the mechanisms (Feleaching/blockage of porosity by adsorbed substances) that, in the long term, could lead to catalyst deactivation; this is the aim of ongoing work. 4. Conclusions This work was focused on the chemical oxidation of the azodye CSB by the heterogeneous Fenton-like process, using a tubular reactor packed with an iron-impregnated activated carbon (Norit RX 3 Extra). Upon impregnation with a Fe(II) salt, the drying and the thermal process employed for precursor decomposition resulted in various oxide forms of Fe2+ and mostly Fe3+ that are reactive with hydrogen peroxide. In fact, it was concluded that, in the steadystate, and when in contact with the activated carbon support, H2 O2

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can only oxidize 10% of the dye present in the feed as compared to 30% with the catalyst (AC impregnated with iron), under identical operating conditions. The optimal concentration of H2 O2 for a feed dye concentration of 0.012 mM was 2.25 mM; higher concentrations lead to a decreased conversion of CSB. The removal of both the dye and total organic carbon (TOC) increased with the temperature (in the range 10–50 ◦ C). The increased temperatures also caused a higher level of iron leaching, but the values in the effluent did not exceed 0.35 ppm, clearly below EU legislation. It was possible to obtain a dye conversion of 88% and a TOC removal of around 47% by operating the reactor at a temperature of 50 ◦ C, pH 3, Wcat /Q = 4.1 g min mL−1 and a feed H2 O2 concentration of 2.25 mM. This conversion was reproducible for at least three cycles, which shows how promising it this type of application for continuous processes. The catalyst used, prepared with an inexpensive iron salt (ferrous sulfate), has evidenced no short term decrease of activity, and was, up to the author’s knowledge, one of the first being reported for application in packed-bed heterogeneous Fenton-like reactors using activated carbon. Acknowledgments FD wishes to express her gratitude to FCT (Fundac¸ão para a Ciência e a Tecnologia) for the PhD grant (ref.: SFRH/BD/44703/2008). FD and LMM are also grateful for funding by the Portugal-Spain Cooperation Project (Integrated Action AI-E/11). FJMH is grateful to the Spanish MCI for the supports of CTM2010-18889 and AIB2010PT00378 projects. The authors wish also to thank Norit for supplying the activated carbons and to the Chemical Engineering Department at FEUP (Faculty of Engineering – Porto University) for the facilities and set-up used. References [1] A. Kunz, P. Peralta-Zamora, New tendencies on textile effluent treatment, Quim. Nova 25 (2002) 78–82. [2] R.J. Bigda, Consider Fenton’s chemistry for wastewater treatment, Chem. Eng. Prog. 91 (1995) 62–66. [3] F. Duarte, F.J. Maldonado-Hódar, L.M. Madeira, Influence of the characteristics of carbon materials on their behaviour as heterogeneous Fenton catalysts for the elimination of the azo dye Orange II from aqueous solutions, Appl. Catal. B 103 (2011) 109–115. [4] S. Sabhi, J. Kiwi, Degradation of 2,4-dichlorophenol by immobilized iron catalysts, Water Res. 35 (2001) 1994–2002.

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[5] J.A. Melero, F. Martínez, J.A. Botas, R. Molina, M.I. Pariente, Heterogeneous catalytic wet peroxide oxidation systems for the treatment of an industrial pharmaceutical wastewater, Water Res. 43 (2009) 4010–4018. [6] F. Martínez, M.I. Pariente, J.A. Melero, J.A. Botas, E. Gómez, Catalytic wet peroxidation of phenol in a fixed-bed reactor, Water Sci. Technol. 55 (2007) 75–81. [7] F. Lücking, H. Köser, M. Jank, A. Ritter, Iron powder, graphite and activated carbon as catalysts for the oxidation of 4-chlorophenol with hydrogen peroxide in aqueous solution, Water Res. 32 (1998) 2607–2614. [8] J.H. Ramirez, F.J. Maldonado-Hódar, A.F. Pérez-Cadenas, C. Moreno-Castilla, C.A. Costa, L.M. Madeira, Azo-dye Orange II degradation by heterogeneous Fentonlike reaction using carbon-Fe catalysts, Appl. Catal. B 75 (2007) 312–323. [9] F. Duarte, F.J. Maldonado-Hódar, A.F. Pérez-Cadenas, L.M. Madeira, Fenton-like degradation of azo-dye Orange II catalyzed by transition metals on carbon aerogels, Appl. Catal. B 85 (2009) 139–147. [10] E.P. Barret, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances. I. Computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–380. [11] R.C. Bansal, J.B. Donnet, F. Stoeckli, Active Carbon, Marcel Dekker, New York, 1988. [12] S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, 2nd ed., Academic Press, London, 1982. [13] F. Stoeckli, A. Guillot, A.M. Slasli, D. Hugi-Cleary, Microporosity in carbon blacks, Carbon 40 (2002) 211–215. [14] C. Corso, A. Maganha de Almeida, Bioremediation of dyes in textile effluents by Aspergillus oryzae, Microb. Ecol. 57 (2009) 384–390. [15] R.M. Sellers, Spectrophotometric determination of hydrogen peroxide using potassium titanium (IV) oxalate, Analyst 150 (1980) 950–954. [16] J. Herney Ramirez, F.M. Duarte, F.G. Martins, C.A. Costa, L.M. Madeira, Modelling of the synthetic dye Orange II degradation using Fenton’s reagent: from batch to continuous reactor operation, Chem. Eng. J. 148 (2009) 394–404. [17] F.J. Rivas, V. Navarrete, F.J. Beltran, F.J. Garcia-Araya, Simazine Fenton’s oxidation in a continuous reactor, Appl. Catal. B 48 (2004) 249–258. [18] J.H. Ramirez, M. Lampinen, M.A. Vicente, C.A. Costa, L.M. Madeira, Experimental design to optimize the oxidation of Orange II dye solution using a clay-based Fenton-like catalyst, Ind. Eng. Chem. Res. 47 (2008) 284–294. ˜ [19] D. Cazorla-Amoros, J. Alcaniz-Monge, M.A. De la Casa-Lillo, A. Linares-Solano, CO2 as an adsorptive to characterize carbon molecular sieves and activated carbons, Langmuir 14 (1998) 4589–4596. [20] A. Rey, M. Faraldos, J.A. Casas, J.A. Zazo, A. Bahamonde, J.J. Rodríguez, Catalytic wet peroxide oxidation of phenol over Fe/AC catalysts: influence of iron precursor and activated carbon surface, Appl. Catal. B 86 (2009) 69–77. [21] D.G. Mustard, C.H. Bartholomew, Determination of metal crystallite size and morphology in supported nickel catalysts, J. Catal. 67 (1981) 186–206. [22] J.W. Niemantsverdriet, Catalyst characterization with spectroscopic techniques, in: B.A. Averill, J.A. Moulijn, R.A. van Santen, P.W.N.M. van Leeuwen (Eds.), Catalysis: An Integrated Approach, Stud. Surf. Sci. Catal., vol. 123, 2nd ed., Elsevier, Amsterdam, 1999, p. 489 (Chapter 11). [23] V.P. Santos, M.F.R. Pereira, P.C.C. Faria, J.J.M. Orfão, Decolourisation of dye solutions by oxidation with H2 O2 in the presence of modified activated carbons, J. Hazard. Mater. 162 (2009) 736–742. [24] R.M. Sellers, Spectrophotometric determination of hydrogen peroxide using potassium titanium (IV) oxalate, Analyst 105 (1980) 950–954. [25] E. Chamarro, A. Marco, S. Esplugas, Use of Fenton reagent to improve organic chemical biodegradability, Water Res. 35 (2001) 1047–1051. [26] W.Z. Tang, C.P. Huang, Stoichiometry of Fenton’s reagent in the oxidation of chlorinated aliphatic organic pollutants, Environ. Technol. 18 (1997) 13–23. [27] R. Idel-aouad, M. Valente, A. Yaacoubi, B. Tanouti, M. López-Mesas, Rapid decolourization and mineralization of the azo dye C.I. Acid Red 14 by heterogeneous Fenton reaction, J. Hazard. Mater. 186 (2011) 745–750.