Treatment of Phenolic Effluents by Catalytic Wet Hydrogen Peroxide

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Treatment of Phenolic Effluents by Catalytic Wet Hydrogen Peroxide Oxidation over Fe2O3/SBA-15 Extruded Catalyst in a Fixed-Bed Reactor Fernando Martıńez,* Juan Antonio Melero, Juan A Ä ngel Botas, M. Isabel Pariente, and Rau´ l Molina Department of Chemical and EnVironmental Technology, ESCET Rey Juan Carlos UniVersity, 28933 Mo´ stoles, Madrid, Spain

Iron oxide supported over mesostructured SBA-15 material (Fe2O3/SBA-15) has been used in a fixed-bed reactor (FBR) for the catalytic wet hydrogen peroxide oxidation (CWHPO) of phenolic aqueous solutions. This catalyst is agglomerated by extrusion with different particle sizes (0.35-2 mm) using bentonite mineral clay and methyl cellulose as inorganic and organic binders, respectively. Several variables, such as the feed flow rate, the weight of the catalyst bed, and the particle size, have been studied in order to optimize the operation conditions for phenol mineralization. Activity of catalyst in terms of phenol degradation and TOC reduction has been monitored. Internal diffusional problems by the increase of the particle size seem not to play a dominant role in the overall catalytic performance. Loadings of catalysts higher than 2.9 g for 1 cm3/ min of 1 g/L phenolic aqueous solution, equivalent to residence times ≈ 3.1-3.8 min, have not substantially affected the phenol and TOC reduction. Under the best operation conditions, complete phenol degradation and remarkable TOC conversion (ca. 66%) have been achieved at steady-state conditions. The role of Fe2O3/ SBA-15 catalyst as compared to a homogeneous catalytic system and the long-term activity of the heterogeneous catalyst have been also demonstrated for the continuous treatment of an aqueous phenolic solution. Introduction The environmental concern is nowadays expressed by more and more stringent governmental regulations imposing lower pollutant discharge limits. In many industrial sectors, pollution prevention, waste minimization, and reutilization are being increasingly integrated in their environmental policies. Because of the high concentration of toxic materials in industrial wastewater, it is necessary to apply specific processes for their separation, transformation, and further degradation. Conventional treatments based on thermal destruction and chemical or biological methods have limitations in applicability, effectiveness, and cost. In this context, some processes such as wet air oxidation (WAO) or wet hydrogen peroxide oxidation (WHPO) are particularly attractive, enabling the complete abatement of hazardous organic pollutants to carbon dioxide and water.1,2 Unlike wet air oxidation (WAO), in which the degradation rate is strongly limited by the mass transfer of molecular oxygen from the gas to the liquid phase, WHPO takes advantage of employing hydrogen peroxide as the liquid oxidant, which avoids gas-liquid mass transfer limitations. The severe operation conditions of WAO (T ) 140-210 °C and P ) 20-80 bar) make it more capital intensive, whereas WHPO demands a lower capital, although it generates higher operating costs arising from the relatively high cost of the hydrogen peroxide.3 Taking into account catalytic applications, hydrogen peroxide is easily decomposed to powerful oxidizing hydroxyl radicals in the presence of transition metals with redox properties. In this sense, the adapted catalytic system based on Fenton’s reagent (FeIII or FeII salts/H2O2), using operation temperatures up to 120 °C under low air pressure, is one of the most important catalytic processes for the oxidation/degradation of dissolved organic pollutants.4 The use of solid catalysts in WHPO processes offers a practical alternative to conventional homogeneous catalysts.5 * To whom correspondence should be addressed. Tel.: 34 91 488 71 82. Fax: 34 91 488 70 68. E-mail: [email protected].

Heterogeneous catalytic systems may, in principle, be easily recovered, regenerated, and reused. For these reasons, several studies have been addressed to the incorporation of active iron or copper species over several supports, such as amorphous silica; zeolitic,6-8 hexagonal mesostructured materialssMCM41, HMS,9 and SBA-15;10 and pillared clays,11,12 for application in WHPO processes. An interesting overview of solid catalysts is reported by Perathoner and Centi,13 in which Al-Fe-clays is shown as one of the most promising catalysts. More recently, the same authors have stated that copper pillared clays are highly effective for WHPO of wastewater streams from olive oil milling production with low loss of copper by leaching.14 Melero et al.15 have also reported a remarkable catalytic performance of a novel Fe2O3/SBA-15 composite catalyst as compared to homogeneous systems, as well as a low dependence on the pH for the treatment of phenolic aqueous solutions. In all these works, the catalyst design is usually a key factor in order to (i) increase its surface area, (ii) minimize the metal sintering, (iii) improve its chemical stability, and (iv) govern the useful lifetime of the catalyst. Most of the studies about catalytic WHPO processes have been carried out in batch operations, and phenol is frequently used as the model pollutant. The results of solid catalysts for WHPO indicates a good perspective of this technology for application in continuous processes, but it still requires large engineering efforts in modeling and optimal designing of industrial reactors. Moreover, their application in continuous systems is normally questioned by the catalyst fouling, in particular for the treatment of wastewater with a complex composition. Therefore, catalyst properties like susceptibility to poisoning and lifetime need to be addressed in experimental installations at the bench scale or the pilot-plant scale. In this sense, a major experience of continuous processes can be found in catalytic wet air oxidation (CWAO). The catalysts used in these processes are basically separated in two main groups: (i) supported noble metals like Pd-Pt/Al2O3 or Pt/TiO216,17 and (ii) mixtures of metallic oxides such as CuO, CoO, Fe2O3, MnO2, or Cr2O3 over γ-alumina or other supports.18-20 The

10.1021/ie070165h CCC: $37.00 © 2007 American Chemical Society Published on Web 05/18/2007

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Figure 1. Flowchart of the catalytic WHPO experimental setup.

deactivation of catalyst seems to be associated to different mechanisms depending on the nature of the metallic species. Thus, catalyst-based noble metals revealed a certain tendency to deactivation by poisoning with carbonaceous deposits over the catalyst surface.21 In contrast, the deactivation of metallic oxides is associated with the metal lixiviation due to local acidic and oxidizing conditions on the catalyst surface.22 Nevertheless, deactivation of metallic oxides by poisoning with carbonaceous deposits has also been demonstrated.23 On the other hand, it must be noted that the catalytic oxidation may proceed following different ways whether the process is carried out in a discontinuous slurry tank reactor or in a continuous fixed-bed reactor.24,25 Several effects, such as mass or heat transfer as well as a different liquid-to-solid ratio, could affect the macrokinetic environment or even modify the reaction mechanism.26 For instance, the formation of phenolic polymers was observed for the phenol degradation using a copper oxide catalyst in a slurry tank reactor, whereas no polymer generation was found with a fixed bed.27 Therefore, the type of three-phase catalytic reactor for catalytic wet oxidation in a continuous mode must be selected carefully.28,29 Whereas WAO processes often operate in downflow using trickle-bed reactors because they take advantage of the partial wetting of the catalyst and enable a better contact of oxygen from the air with the dried catalyst surface, WHPO processes should operate more successfully in upflow based on flooded packed-bed reactors, allowing a better wetting of the particles sizes by the liquid phase with the organic pollutants and hydrogen peroxide. The present paper is mainly addressed to probe the feasibility of agglomerated Fe2O3/SBA-15 catalyst in a packed-bed reactor for the degradation of phenolic aqueous solutions by means of a catalytic WHPO process. The influence of several variables such as the feed flow rate, the particle size of the catalyst, and the volume of the catalyst bed were evaluated. To the best of our knowledge, few works are described in literature dealing with the treatment of organic pollutants by means of a fixedbed reactor using hydrogen peroxide as the oxidant. Finally, the long-term activity of the catalyst was studied in order to ascertain the main factors involved in the partial deactivation of the catalyst. Experimental Section Preparation and Characterization of Catalyst. Powder iron-containing SBA-15 mesostructured material was synthesized as described elsewhere.10 Fe2O3/SBA-15 extrudates were prepared by blending of powder catalyst with sodium bentonite

(75:25 wt %) and synthetic methylcellulose polymer (10 wt % of the previous mixture), which act as binders in the extrusion process.30 Thereafter, deionized water is added and all the components are kneaded under high shear conditions until obtaining a homogeneous paste. After the solid paste was conditioned in a damp atmosphere, it was pushed through a ram extruder of 4.8 mm circular die. The resultant rod-shaped materials were dried for 3 days in a climatic chamber under temperature and relative humidity (RH) control (from 20 °C and 70% RH to 40 °C and 10% RH). Following this, extrudates were calcined in air until 650 °C (2 h) using a slow ramp of temperature, in order to achieve a gradual removal of water and organic content, strengthening the mechanical consistency of the material by sintering of the inorganic binder. The final Fe2O3/ SBA-15 pellets for catalytic reactions were obtained by crushing and sieving the extruded material until particle sizes between 0.35 and 2 mm with spherical-like shapes were reached. Characterization of the samples was performed by different conventional techniques. X-ray powder diffraction (XRD) data were acquired on a Philips X-Pert diffractometer using Cu KR radiation. The data were collected from 2θ ranging from 0.5 to 90° with a resolution of 0.02°. Nitrogen adsorption and desorption isotherms at 77 K were measured using a Micromeritics Tristar 3000 system. Transmission electron microscopy (TEM) microphotographs and energy-dispersive X-ray (EDX) microanalysis were carried out on a Philips Tecnai20 electron microscope operating at 200 kV. Iron content of the synthesized samples was measured by means of atomic emission spectroscopy with induced coupled plasma (ICPAES) analysis collected in a Varian Vista AX system. Carbon content of the fresh and used catalysts was measured in an elemental HCNS analyzer (Vario EL III Elementar Anaysesysteme GMHB). Apparatus, Procedure, and Analysis for Catalytic Oxidation Experiments. Catalytic wet hydrogen peroxide experiments were conducted in a fixed-bed reactor made of a glass tube of 1.2 cm internal diameter and 15 cm length. A schematic flowchart of the experimental setup is outlined in Figure 1. Fe2O3/SBA-15 extrudates were packed between two beds of inert glass pearls placed on the top and bottom sections of the fixed-bed reactor. Typically, the feed aqueous solution of phenol (1 g/L) and hydrogen peroxide (5.1 g/L, stoichiometric amount for the complete mineralization of phenol according to reaction 1) was fed to the reactor using a GILSON 10SC highperformance liquid chromatography (HPLC) pump capable of volumetric flow rates up to 10 cm3/min with high precision

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((0.05 cm3/min). The liquid is fed to the reactor in upflow operation.

C6H5OH + 14 H2O2 f 6 CO2 + 17 H2O

(1)

The temperature in the reactor was measured and set to 80 °C on the top section of the catalyst packing. The fixed-bed reactor is heated by the upstream circulation of hot silicone throughout the annular space of the reactor tube. An external thermal bath keeps the silicone at the needed temperature for the control of temperature. Several catalytic runs at different liquid flow rates (1-5 cm3/ min) and catalyst weights (1.4-3.9 g) were carried out for the treatment of phenolic aqueous solutions. The size of catalyst particles was also assessed between 0.35 and 2 mm. Additional experiments were performed in order to evaluate the role of the solid catalyst and the contribution of the iron species leached from the heterogeneous catalyst. Finally, the long-term activity of Fe2O3/SBA-15 was assessed under the best operation conditions. The residence time of the liquid phase in the packed-bed reactor is calculated as

tR )

Table 1. Physicochemical Properties of Materials

V L LV B ) QL QL

The ratio between the volume of the liquid phase (VL) and the catalyst-bed volume (VB) is the liquid holdup (L). This parameter has been calculated from two characteristic components of the packed bed, the porosity () and the external liquid saturation (βL), as follows,

L ) βL The packing porosity () has been determined as unity minus the ratio between the volume occupied by the catalyst particles (Vp) and the total volume of the packing bed (VB). The volume of particles (Vp) is calculated as the ratio between the mass of the weighted catalyst (W) and the particle density (dp ) 0.48 g/cm3). The latter one is estimated assuming particles with spherical shape and an average diameter for each set of particle size (0.35-0.7; 0.7-1; 1-1.6; and 1.6-2 mm). The external liquid saturation (βL) was calculated using the Excel worksheet simulator for flooded-bed reactor designed by F. Larachi and B. P. A. Grandjean (available at http://www.gch.ulaval.ca/). This worksheet is based on the combination of dimensional analysis and artificial neural networks to identify the most expressive dimensionless groups that allow a reliable prediction of the external liquid saturation, taking into account the fluid physical properties (liquid and gas), operating conditions, and geometrical properties of packings and columns.31 Because all the catalytic runs were carried out at atmospheric pressure without additional air gas supplying, only the carbon dioxide released by the mineralization of the organic compounds is considered for the above-mentioned simulation. The catalytic activity of the oxidation treatment was evaluated by monitoring several parameters in the outlet aqueous solution such as the phenol and TOC removal, the hydrogen peroxide conversion, the pH, and the iron dissolved from the catalyst. Phenol conversion was determined by means of an HPLC chromatograph (Varian Prostar) equipped with a Waters Spherisorb column and a UV detector adjusted at 215 nm. Ultrapure water acidified with H3PO4 up to pH 2.7 was employed as the mobile phase. Total organic carbon (TOC) content of the solutions was analyzed using a combustion/nondispersive

Fe2O3/SBA-15 (powder) Fe2O3/SBA-15 (extruded) Na-bentonite (powder)

average particle size (µm)

SBETb (m2/g)

pore diameterb (nm)

iron contentc (wt %)

iron oxide as hematited

14.4a

495

8.1

19

yes

355-2000

264

7.9

14

yes

6.2a

119

7.2

2

no

a Analyzed by laser-scattering technique. b Determined from adsorption N2 isotherms. c Detected by ICP-AES. d Detected by X-ray diffraction.

infrared gas analyzer model TOC-V Shimadzu. Hydrogen peroxide conversion was determined by iodometric titration. Iron content in the treated solution was measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis collected in a Varian VISTA AX system. Results and Discussion Catalytic Properties of Agglomerated Fe2O3/SBA-15 Materials. Powder Fe2O3/SBA-15 catalyst has been used successfully for the treatment of phenolic aqueous solutions in CWHPO processes.10,15 Its remarkable catalytic performance operating in a discontinuous stirred-tank reactor (STR) makes this material a promising catalyst for application in continuous operations. However, it is well-known that there is an increase of drop pressure in packed-bed reactors when catalyst is used as fine powder. For this reason, Fe2O3/SBA-15 material was extruded following the procedure described in the Experimental Section. In order to ascertain whether the main features of catalyst could be modified by the agglomeration process, the physicochemical properties of powder and extruded materials as well as their catalytic performance for the degradation of a phenolic aqueous solution in a STR operating in batch conditions have been studied. Table 1 summarizes the most relevant properties of iron containing SBA-15 as powder material and after agglomeration, including also those corresponding to the sodium bentonite clay used as inorganic binder. X-ray diffraction patterns at low and high angle for the powder and extruded Fe2O3/SBA-15 catalysts are illustrated in Figure 2. It can be observed that the typical hexagonal arrangement of mesostructured SBA-15 materials (Figure 2a) and the presence of crystalline hematite entities (Figure 2b) were not significantly affected by the agglomeration process. Additionally, the average pore diameter was barely modified in the extruded materials (see Table 1). However, the iron content and the Brunauer-Emmett-Teller (BET) surface area have significantly decreased in the Fe2O3/SBA-15 extrudates, because of the 25 wt % bentonite loading in the extruded particles and the thermal treatment followed by the agglomeration process, which induce the sintering of the inorganic binder and the iron-containing SBA-15 silica support. Figure 3a illustrates a macroscopic picture of Fe2O3/SBA-15 extrudates obtained from the extrusion process. TEM micrographs of the agglomerated material are depicted in parts b-d of Figure 3. The presence of defined crystalline hematite particles (darker areas) supported over the highly ordered SBA15 material (Figure 3 parts b and c) is clearly evident. Lamellar (face-to-face) and delaminated (edge-to-face and edge-to-edge) layers, typical of flocculated clays, can be observed as agglutinating agents of the mesostructured SBA-15 silica. Figure 3d shows a more detailed view of the bentonite used as a binder in the extruded Fe2O3/SBA-15 catalyst.


Figure 2. XRD patterns of Fe2O3/SBA-15 materials: (a) low angle and (b) high angle.

Figure 3. (a) Picture of Fe2O3/SBA-15 rods obtained from the extrusion process. (b), (c), and (d) TEM micrographs of the agglomerated Fe2O3/SBA-15 material.

Figure 4. Catalytic test of powder and extruded Fe2O3/SBA-15 materials for the treatment of phenolic aqueous solutions in a discontinuous STR. Reaction conditions: [Ph-OH]o ) 1 g/L; [H2O2]o ) 5.1 g/L; [cat] ) 0.6 g/L; T ) 80 °C; Pair ) 6 bar; NS pHo ) 5.5.

According to the catalytic results (Figure 4), the extruded catalyst exhibits a significant catalytic reduction in terms of TOC conversion, especially at initial reaction times. This fact could be attributed to several factors such as the reduction of surface area or the lower iron content in the bulk composition of the extruded catalyst. In addition, the higher particle size of agglomerated catalyst (1.3 mm) in comparison to that of the powder catalyst (14.4 µm) should enhance internal diffusional problems in the oxidation process. However, it is remarkable that similar values of TOC conversion are achieved for longer reaction times. Looking at the stability of both samples tested, a slight variation in terms of the iron leaching degree was observed after 60 min of reaction, leading to values between 4

and 6.5% of iron loss with respect to the initial iron content in the solid catalyst. Catalytic Performance of Fe2O3/SBA-15 Catalyst in a Packed-Bed Reactor (PBR) for Continuous Treatment of Phenolic Aqueous Solutions. Once the catalytic activity and stability of extruded Fe2O3/SBA-15 materials have been readily demonstrated, the second part of this work is focused on the treatment of a continuous phenolic aqueous stream by means of CWHPO in a packed-bed reactor (PBR). The influence of several reaction variables, such as the feed flow rate, the size of the catalyst particles, and the packing volume, on the catalytic performance has been evaluated. Moreover, the stability of the catalyst operating in this continuous mode has been also assessed, marking an important effort to understand those factors affecting the feasibility of the process for long catalytic runs. (a) Influence of the Feed Flow Rate. The influence of the residence time has been studied by modifying the liquid flow rate of phenolic aqueous solution for 1, 2.5, and 5 cm3/min using a packed catalyst bed of 11 cm3, corresponding to 2.9 g of catalyst with particle size between 1 and 1.6 mm. These feed flow rates provide residence times for the liquid phase of 3.6, 1.2, and 0.6 min, respectively. The main parameters monitored during the course of the reaction were the TOC and phenol degradation, the pH, and the iron concentration of the aqueous effluent. The values of these parameters after reaching the steady state are depicted in Figure 5. Phenol is almost completely removed at steady state for all the feed flow rates (>95%). However, it is clearly observed that the lower the feed flow rate, which leads to higher residence times, the higher is the TOC degradation. The partial oxidation of phenolic compounds

4400


Figure 5. Influence of the feed flow rate in the degradation of phenolic aqueous solutions at the steady state. Operation conditions, inlet stream: [Ph-OH]o ) 1 g/L and [H2O2]o ) 5.1 g/L; T ) 80 °C, weight of catalyst (W) ) 2.9 g, and particle size ) 1-1.6 mm. Table 2. Parameters Studied for the Evaluation of the Particle Size in the CWHPO of a Phenolic Effluent particle size (mm)

liquid flow rate QL (cm3/min)

bed volume VB (cm3)

bed porosity

liquid saturation βL

liquid holdup L

residence time tR (min)

Weisz-Prater number φWP

0.35-0.7 0.7-1 1-1.6 1.6-2

1 1 1 1

10.0 10.6 11.0 11.5

0.40 0.43 0.45 0.48

0.759 0.730 0.725 0.691

0.306 0.315 0.326 0.330

3.06 3.34 3.59 3.79

6.6 14.5 31.7 57.3

toward refractory carboxylic acids, such as formic, acetic, and oxalic acids, has been detected by HPLC analysis. This fact is in agreement with the decrease of pH for the inlet phenolic aqueous solution (about 5.5) until values between 3 and 4 for the outlet streams. With regards to the catalyst stability, the iron concentration in the treated effluent is significantly increased for intermediate and high residence times (1.2 and 3.6 min, respectively), giving values around 12-14 ppm. From these results, a certain relationship between the extension of phenol mineralization and the iron leached-out from the catalyst can be hypothesized. Thus, operation conditions that have a direct effect on the phenol oxidation, i.e., the increase of the residence time, may have an important influence on the catalyst stability. This fact has been attributed by the interaction of certain aromatic-like compounds to form soluble organometallic complexes, which can promote the extraction of metallic species in the liquid phase.15,22 Others authors have also reported a strong connection with the presence of some byproducts coming from the partial oxidation of phenol such as the oxalic acid, attesting to the soluble nature of iron-oxalate complexes.32,33 Note also that the remarkable TOC degradation for the highest residence time is achieved with an iron concentration in the liquid effluent of ca. 14 ppm, which might contribute to the overall TOC mineralization by homogeneous Fenton-like reactions. Further experiments will be performed to ascertain the homogeneous contribution of dissolved iron species on the overall catalytic performance of the oxidation process. (b) Influence of the Catalyst Particle Size. The study of the particle size of Fe2O3/SBA-15 catalyst has been carried out for the continuous degradation of phenolic aqueous solutions, keeping a constant 1 cm3/min of feed flow rate and 2.9 g of catalyst loading. The extruded Fe2O3/SBA-15 material was crushed and sieved until different sets of particle sizes were obtained in the following intervals: 0.35-0.7, 0.7-1, 1-1.6, and 1.6-2 mm. Although a catalyst with small particle size is more desirable in order to avoid internal diffusion problems, it is also known that fine particles undergo a larger pressure drop across the packed catalyst bed, and eventually, they can be swept away by the liquid stream, producing a loss of catalyst and

additional pollution on the resultant effluent. From another point of view, it must be noted that some features of the packed bed can be affected by the variation of the catalyst particle size such as the porosity, the external liquid saturation, or the liquid holdup, which further modify the residence time of the liquid phase in the catalyst bed. Several features of the packed bed for the different particle sizes are shown in Table 2. The catalytic performance, in terms of TOC mineralization and phenol removal; the stability of the catalyst, in terms of the iron concentration detected from the catalyst leaching; and the pH of the treated effluent at steady state are illustrated in Figure 6. The increase of the particle size leads to higher values of bed porosity (see data in Table 2). This means that the volume of voids occupied by the liquid phase in the packing bed should be bigger as the particle size is increased. However, the right volume available for the liquid phase, called liquid holdup, is estimated by taking into account these values of porosity and the liquid saturation of the catalyst bed. The liquid saturation will be dependent on the carbon dioxide released by the phenol mineralizationsnote values of TOC conversion > 60% in all casessand the operating conditions (physical properties of the liquid stream and the geometrical properties of the packing and column). The liquid holdup has been predicted by the correlations proposed by Bensetiti et al.31 for flooded-bed reactors. The liquid saturation is slightly reduced with the increase of the bed porosity, keeping values between 69 and 76%. Nevertheless, the resultant liquid holdup evidenced an increase with the particle size. Thus, the residence time undergoes a significant enhancement as higher catalyst particles are used, mainly because of the increase of the porosity and the total volume of the packing bed. On the other hand, the extended Weisz-Prater criterion can be useful for the relevance of diffusional limitations based on experiments conducted with different particle sizes. The WeiszPrater modulus (φwp) shown in Table 2 has been calculated according to the following equation,34

φWP )

2 robs A Fp R

DeffCA,S


Figure 6. Influence of the particle size in the degradation of phenolic aqueous solutions at the steady state. Operation conditions, inlet stream: [Ph-OH]o ) 1 g/L, [H2O2]o ) 5.1 g/L, and liquid flow rate ) 1 cm3/min; T ) 80 °C, and weight of catalyst (W) ) 2.9 g. Table 3. Influence of the Packed-Bed Volume at Steady State for Two Different Particle Sizes particle size 1.6-2 mm

particle size 0.7-1 mm

catalyst mass W (g)

bed volume VB (cm3)


XTOC (%)

[Fe]dissolved (ppm)

bed volume VB (cm3)


XTOC (%)

[Fe]dissolved (ppm)

1.4 2.9 3.9

5.6 11.5 15.6

1.90 3.79 5.20

48.1 66.6 63.8

7.0 12.0 10.5

5.2 10.6 14.4

1.67 3.34 4.50

50.6 58.3 60.6

10.8 13.4 17.7

where robs A is the observable TOC degradation rate at steadystate conditions, Fp is the mass of catalyst per available liquid volume (VL), R is the radius of the particle, Deff is the phenol effective diffusivity, and CA,S is the TOC concentration on the catalyst surface (taken as the initial TOC concentration in the liquid phase, assuming a negligible external diffusion limitation). For all the particle sizes tested, φwp > 1, which means that the overall phenol mineralization is partially controlled by internal diffusional constraints. Nevertheless, the mass transport limitations become more important with a packing of larger particle size, as is demonstrated by the increase of the φwp modulus. However, by looking at the TOC and phenol conversion in Figure 6, similar values or even slightly higher for the TOC conversion are attested with the largest particle sizes. These results are not consistent with the extended Weisz-Prater criterion. A possible explanation of this could be related to the mechanism involved in the catalytic wet peroxide oxidation of phenol.15 The use of a liquid oxidant like the hydrogen peroxide enables the degradation of phenol in the bulk liquid phase mediated by its own oxidant12 or reactive radicals coming from the catalytic decomposition of the oxidant. Thus, the oxidation of organic compounds in the liquid phase can have an important role in the overall performance of the CWHPO process. In this sense, the increase of porosity for packed beds with a larger particle size and the resultant increase of the residence time for the liquid phase (see Table 2) can make more relevant the contribution of homogeneous oxidation reactions, which supports the proposed hypothesis. From these results, it can be concluded that internal diffusion problems stated by the Weisz-Prater criterion are offset by the increase of the residence time and the contribution of oxidation reactions in the liquid phase mediated by its own oxidant or reactive free radicals. Another plausible reason that may account for the similar values of TOC conversion for packed beds with different particle sizes could be an excessive amount of catalyst in the reactor (2.9 g of catalyst corresponding to volumes between 10 and 11.5 cm3 depending on the particle size). This fact could overcome the diffusional problems of larger particle sizes. Further catalytic runs will be discussed later in order to figure out this issue.

Since a similar or even a higher catalytic performance was found for packed beds with larger particle sizes, a complete phenol removal, and ca. 65% of TOC reduction, particles between 1.6 and 2 mm are considered an appropriate size for the treatment of phenolic aqueous solutions. Concerning the iron leaching, the results depicted in Figure 6b reveal a poor influence of the particle size on the catalyst stability, which is in fair agreement with the similar levels of TOC degradation obtained for all cases. (c) Influence of the Catalyst-Bed Volume. Several catalytic runs using the feed flow rate of 1 cm3/min and packing beds with different particle size (1.6-2 and 0.7-1 mm) have been studied with catalyst loadings of 1.4, 2.9, and 3.9 g. Table 3 shows values of bed volumes (VB) and residence times for both sets of particle sizes as well as TOC conversion and iron concentration in the outlet effluent at the steady-state conditions. As has been discussed earlier, a larger particle size of catalyst leads to a higher liquid holdup in the catalyst bed and, hence, the increase of the residence time for the same weight of catalyst. This fact is clearly observed again by comparing the residence time of the catalytic runs conducted with different particle sizes. The enhancement of TOC conversion is evident as the catalyst weight is increased from 1.4 to 2.9 g, being more accentuated for the highest particle size. However, nonrelevant variation is observed for catalyst weights > 2.9 g, regardless of the particle size. These results indicate the presence of refractory compounds that are resistant to the mineralization, even for a high amount of catalyst in the studied reaction conditions ([PhOH]o ) 1 g/L, [H2O2]o ) 5.1 g/L, and T ) 80 °C). In fact, different carboxylic acids, such as formic, acetic, and oxalic acids, have been detected by HPLC because some byproducts remained in the liquid effluent after treatment. Finally, it can be concluded that catalyst loadings of 2.9 g turn out more suitable by taking into account the similar values of TOC conversion for packed beds of 2.9 and 3.9 g. Moreover, the use of larger amounts of catalyst should produce a higher loss of iron ions in the liquid effluent. (d) Reproducibility of Experiments. In order to check the reproducibility and the accuracy of the catalytic results, several runs were carried out with the best operation conditions (1 cm3/

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Figure 7. Influence of the iron homogeneous and bentonite agglomerating agent: -9-, extruded Fe2O3/SBA-15; -O-, FeCl3 with glass silica beads; -4-, extruded iron free SBA-15. Profiles of: (a) TOC conversion, (b) phenol removal, (c) hydrogen peroxide conversion, and (d) pH. Operation conditions, inlet stream: [Ph-OH]o ) 1 g/L; [H2O2]o ) 5.1 g/L; liquid flow rate ) 1 cm3/min; T ) 80 °C; weight of catalyst (W) ) 2.9 g, and particle size of any packing ) 1.6-2 mm. Table 4. Reproducibility of Experiments run

XTOC (%)

Xphenol (%)

[Fe]dissolved (ppm)

1 2 3 mean value/SD

66.6 70.6 69.0 68.5 ( 2.4

>99.9 >99.9 >99.9 99.9 ( 0.0

12.2 12.0 15.0 13.1 ( 1.7

min of liquid flow rate and 2.9 g of catalyst bed with a particle size between 1.6 and 2 mm) for the treatment at 80 °C of 1 g/L phenol aqueous solution with a stoichiometric amount of hydrogen peroxide for its complete mineralization. The main reaction parameters monitored in this study, TOC reduction, phenol conversion, and the iron leaching for each run, are shown in Table 4. Standard deviations lower than 2.5% for the TOC conversion and 1.7% for iron leaching are evidence of the low uncertainty of the results in this experimental work. (e) Evaluation of the Homogeneous Catalytic System and the Role of the Bentonite Agglomerating Agent. The loss of iron species from the catalyst has been detected in the outlet aqueous solution, and it could be an important factor in the phenol mineralization by means of homogeneous Fenton-like reactions in the liquid phase. In order to determine the influence of the iron leaching in the overall degradation, an additional experiment was carried out employing a packing of glass silica beads with a similar size to that used for Fe2O3/SBA-15 particles (1.6-2 mm). In this catalytic run, iron(III) chloride was dissolved into the feed phenolic aqueous solution until a concentration of Fe3+ of ca. 20 ppm was achieved. Likewise, in order to check the role of the agglomerating bentonite in the oxidation process, a blank reaction was also conducted with extruded iron-free SBA-15 particles instead of Fe2O3/SBA-15, which were prepared following the same extrusion method. All

these experiments were carried out using the optimal values determined in this study for the feed flow rate and the packing properties (1 cm3/min and 2.9 g of catalyst with a particle size between 1.6 and 2 mm). Figure 7 illustrates the course of the studied parameters along the reaction time. The TOC degradation depicted in Figure 7a is clear evidence that mineralization of phenolic aqueous solutions cannot be completely attributed to the homogeneous contribution of the leached iron species. At steady-state conditions, 42% of TOC reduction was achieved for the homogeneous catalytic system (FeCl3 with glass silica beads) in comparison with ca. 66% for the heterogeneous Fe2O3/SBA-15 catalyst. The initial phenol pollutant was completely removed for both catalytic systems at steady state (Figure 7b). Furthermore, it is noteworthy that the iron concentration detected in the outlet effluent for the heterogeneous Fe2O3/SBA-15 catalytic run was lower (ca. 13 ppm) than that used in the homogeneous one with FeCl3. Another interesting point is the hydrogen peroxide conversion (Figure 7c); whereas the homogeneous catalytic system does not achieve a complete conversion, the heterogeneous one allows the total consumption, which boosts an enhancement of the TOC conversion. Moreover, this total consumption of the oxidant would be an important benefit for possible application in coupling oxidation-biological processes, since hydrogen peroxide is extremely toxic for the microorganisms present in biological treatments. However, the total decomposition of the stochiometric amount of oxidant has led to ca. 66% of TOC reduction. These results are clear evidence that not all the oxidant is efficiently used for the phenol mineralization, because of the formation of carboxylic acid byproducts, more refractory to be oxidized, or scavenging reactions of the hydroxyl radicals


Figure 8. Evolution of the main parameters of reaction for a long-term operation. Profiles of: (a) TOC and phenol conversion, (b) hydrogen peroxide conversion, (c) pH, and (d) iron leached. Operation conditions, inlet stream: [Ph-OH]o ) 1 g/L; [H2O2]o ) 5.1 g/L; liquid flow rate ) 1 cm3/min; T ) 80 °C; weight of catalyst (W) ) 2.9 g, and particle size ) 1.6-2 mm.

generated from catalytic decomposition of hydrogen peroxide. Nevertheless, the heterogeneous catalytic system seems to promote a better use of the oxidant with a remarkable enhancement of the TOC degradation rate as compared with the homogeneous system. The degradation of phenol and other intermediates by oxidation reaction over the catalyst surface with neighboring radicals may account for the increase of the catalytic performance.15 Finally, one must point out the activity of Fe2O3/ SBA-15 at pH conditions of 4.3, a little far of the typical range of 2.5-3.5 considered as optimal for homogeneous Fenton processes (Figure 7d). Note that the heterogeneous catalytic system needs an induction time of 3 h for the preconditioning of the catalyst bed before achieving stable values of catalytic performance at steady state. Iron-free SBA-15 particles coming from the extrusion of powder silica SBA-15 and bentonite show an outstanding decrease of the oxidant decomposition and TOC reduction, whereas approximately 10% of phenol is still present in the outlet effluent at steady state. These results indicate that the contribution of the iron content in the bentonite used for agglomeration (2-3 wt Fe %) is not too relevant in the overall catalytic performance of phenol mineralization. Life-cycle of Catalyst in the Phenol Degradation. The longterm activity of Fe2O3/SBA-15 catalyst has been assessed for the treatment of phenolic aqueous solution. The catalyst was used in successive cycles of continuous operation up to 34 h. The study was carried out with optimal values of residence time and catalyst particle size. The profiles of TOC and phenol degradation, the oxidant consumption, the pH, and the concentration of iron leached-off in the liquid effluent during the successive cycles of reaction are shown in Figure 8The complete phenol removal for the long-term operation is clearly evidenced.

However, a gradual decrease of the TOC conversion with time can be observed, displaying initial values of 65-60% that decrease up to 30-40% for longer times on stream (28-34 h). Interestingly, the gradual reduction of TOC conversion after ca. 12 h on stream is in agreement with the decrease of hydrogen peroxide conversion. Regarding the catalyst stability, the profile of the iron concentration in the outlet effluent shows a maximum concentration between 13 and 16 h, a period of time in which the TOC conversion starts a gradual decrease. These results indicate that there is not a direct relationship between the overall catalytic performance and the loss of iron species in the liquid phase. The increase of the iron concentration for the first 13 h in the outlet effluent could be caused by the decrease of pH (ca. from 6.5 to 3.5). After this period of operation, the pH still goes down until values about 3-2.8, and the decrease of the iron concentration in the treated effluent should be attributed to a normal catalyst deactivation by the oxidation process. The parallel decrease of TOC degradation and hydrogen peroxide conversion is a clear evidence of partial deactivation of the catalyst. The studies found in literature21-23 attribute this deactivation to several factors: (i) formation of carbonaceous deposits that can reduce the specific surface area of the catalyst, (ii) poisoning of the catalyst by complexation of active sites with acid organic compounds, preventing its reactivity with other reactants, and (iii) leaching of iron species within the liquid phase. In order to evaluate the plausible contribution of each one of them in the partial deactivation of catalyst for the treatment of phenol aqueous solutions by the long-term operation, fresh and used catalysts after drying at 110 °C were characterized, with the results shown in Table 5. Elemental HCNS analysis reveals a low increase in the carbon content in the used catalyst, which indicates the absence of carbonaceous

4404


Table 5. Characterization Results of the Fresh and Used Catalyst

Fe2O3/SBA-15 (fresh) Fe2O3/SBA-15 (used) a

iron content (wt %)

SBET (m2/g)

iron oxide as hematite

% Ca

14.0 12.7

264 236

yes yes

0.1

Analyzed by HCNS elemental analysis.

deposits on the catalyst surface by oxidative coupling reactions of phenolic compounds. These results are in contrast with those reported by Hamoudi and co-workers,21,23 who confirmed the presence of carbonaceous substances over the catalyst surface for the wet air oxidation of phenolic aqueous solutions. BET surface areas of the fresh (264 m2/g) and used (236 m2/g) catalysts show that textural properties have not been significantly modified by the acidic and oxidizing conditions of the CWHPO. TEM images and local microanalysis for the fresh and used catalysts have also demonstrated the absence of carbonaceous deposits as well as a similar external appearance of both samples. On the other hand, the low acid pH of the outlet effluent is normally attributed to the formation of refractory carboxylic acids as byproducts, which could be affecting the catalyst properties by adsorption over the active sites, self-poisoning the catalyst by hindering the decomposition of the oxidant and subsequently reducing the TOC conversion. The carbon contents of the used catalyst do not allow one to rule out the plausible self-poisoning by the iron complexation of carboxylic byproducts over supported iron species, because the mild drying step carried out for the catalyst recovering (110 °C) could be responsible for the thermal disappearance of the poisoning carboxylic compounds. Finally, another possible cause of catalyst deactivation is the leaching of iron ions into the aqueous solution. Even though concentrations up to 20 ppm of iron species have been detected in the outlet effluent for the long-term operation, the analysis of the solid catalyst by ICP-AES displays a low variation in the overall iron content of ca. 1.3%. This fact indicates that the amount of iron ions detected in the liquid solution represents a low percentage of the total iron content of the catalyst bed. Therefore, the decrease of the iron concentration in the catalyst seems not to account for the decay of activity along the operation time. Our group has recently proposed an oxidation scheme using heterogeneous catalysts and hydrogen peroxide based on the generation of free radical species (mainly hydroxyl radicals) on the catalyst surface and following oxidation of neighboring organic compounds over the catalyst surface or the surrounded liquid-solid interphase.15 Taking into account this reaction scheme, the efficiency of the active iron species is associated with the oxidant decomposition and the catalyst capacity for the adsorption-desorption of both oxidant and polluted organic compounds. The influence of various types of iron oxides (granular ferrihydrite (Fe5HO84H2O), goethite (R-FeOOH), and hematite (R-Fe2O3)) on the catalytic decomposition of hydrogen peroxide has been studied for the treatment of 2-chlorophenol.35 Crystalline hematite has been reported to be less active in the hydrogen peroxide decomposition although more efficient for the degradation of 2-chlorophenol in comparison to the others iron oxides, because of the pollutant oxidation in the subsurface environment. The catalyst deactivation of Fe2O3/SBA-15 could be attributed to potential alterations of the nature and environment of the crystalline hematite entities to other oxidized iron structures that are less active. From this hypothesis, the decrease of hydrogen peroxide conversion and gradual loss of TOC reduction (Figure 8 parts a and b) could be reasonable. An

ongoing investigation is being carried out in order to know the effect of the reaction conditions on the nature of the hematite particles. On the other hand, the immobilization of iron oxides over mesostructured materials with an extended surface area is established of outstanding relevance with the purpose of enhancing their catalytic properties. Conclusions An extruded catalyst of crystalline hematite supported over a mesoporous silica support (Fe2O3/SBA-15) is shown as a promising catalyst for the treatment of phenolic aqueous solutions in a continuous upflow packed-bed reactor. The increase of the particle size in the range of 0.35-2 mm has not promoted a negative effect on the catalytic performance as expected because of internal diffusional control on the oxidation process. These results have been attributed to the contribution of oxidation reactions in the liquid phase, which takes advantage of the increase of the residence time as larger particle sizes are used. A complete phenol removal and TOC degradation over ca. 66% has been achieved at 80 °C, atmospheric pressure, and hydrogen peroxide concentration of 5.1 g/L (100% of the stoichiometric amount for the complete mineralization of the inlet phenol concentration) with a residence time of 3.8 min (feeding flow rate of 1 cm3/min and 2.9 g of catalyst weight). The role of the heterogeneous catalyst in the oxidation process has been readily demonstrated in comparison with a homogeneous Fenton system based on dissolving iron(III) chloride in the feed phenolic aqueous solution and employing a packing of glass silica beads instead of Fe2O3/SBA-15. It is noteworthy that there is a high stability of the iron oxides supported on the silica support, with only 1.3% of the iron contained in the catalyst bed being leached-off after 34 h of reaction. This catalyst has also evidenced a total degradation of phenol for the long-term operation, even though the activity in terms of TOC conversion gradually decreased. The partial deactivation of catalyst seems to be associated with the chemical alteration of the crystalline iron oxides supported over SBA-15 rather than with poisoning by carbonaceous deposits or iron complexation with acidic organic byproducts. Further research should be focused on the characterization of active iron species in these processes and regeneration strategies. Acknowledgment The authors thank “Ministerio de Ciencia y Tecnologıá” for the financial support through the project CONSOLIDERINGENIO2010 and “Comunidad de Madrid” through the project P-AMB-000395-0505. Nomenclature CA, S (gTOC/cmliq3) ) TOC concentration on the catalyst surface dp (gTOC/cmcat3) ) particle density Deff (mcat2/min) ) phenol effective liquid diffusivity R (m) ) particle radius roA (gTOC / gcatmin) ) observed TOC degradation rate tR (cmliq3/(cmliq3/min)) ) residence time VB (cmbed3) ) volume of the catalyst bed VL (cmliq3) ) available liquid volume in the catalyst bed W (gcat) ) catalyst weight in the reactor βL (cmliq3/cmvoids3) ) external liquid saturation (cmvoids3/cmbed3) ) bed porosity L (cmliq3/cmbed3) ) liquid holdup φwp ) Weisz-Prater criteria Fp (gcat/cmliq3) ) mass of catalyst per available liquid volume


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ReceiVed for reView January 29, 2007 ReVised manuscript receiVed April 3, 2007 Accepted April 12, 2007 IE070165H

Treatment of Phenolic Effluents by Catalytic Wet Hydrogen Peroxide

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