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Degradation of phenol by mechanical activation of a rutile catalyst
Journal of Colloid and Interface Science 339 (2009) 133–139
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Degradation of phenol by mechanical activation of a rutile catalyst M.C. Cotto, A. Emiliano, S. Nieto, J. Duconge, R. Roque-Malherbe * Institute of Physical Chemical Applied Research, School of Science, University of Turabo, P.O. Box 3030, Gurabo, PR 00778-3030, USA
a r t i c l e
i n f o
Article history: Received 23 April 2009 Accepted 7 July 2009 Available online 13 August 2009 Keywords: Mechanochemistry Catalysis Phenol Degradation OH radical mechanism Rutile
a b s t r a c t In the present paper a novel mechanochemical process for the elimination of organic pollutants dissolved in water is proposed. In this regard, phenol aqueous solutions (100 mg L1) were ball-milled for 0, 12, 18, 24, 36, 48, and 72 h with and without a well-characterized (XRD, SEM, and N2 Adsorption), rutile powder catalyst and the reaction products analyzed with UV and GC/MS. It was found that when the catalyst was not included in the process, phenol was not affected, but when it was included, phenol was decomposed. The catalyst itself did not change and the reaction follows a pseudo-first-order kinetics. Besides, intermediates which are characteristic of the OH radical mechanism were found in the reaction products. Then, a mechanism similar to those accepted for other advanced oxidation processes was proposed. The value measured for the pseudo-first-order reaction constant was very low, indicating that the reported process is inefficient. Nevertheless, this problem could be solved by applying catalysts consisting of particles with smaller diameters. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction The conceptual meaning of mechanochemistry was first defined by F.W. Ostwald. He concentrated his works, published by the end of the 19th century and the first years of the 20th century, on the categorization of chemical disciplines by the energy type applied to the chemical processes, including between them mechanical energy [1,2]. Mechanochemistry deals with chemical reactions comprising reagents in any state of aggregation, principally solid-state processes and reactions, either initiated by a mechanical treatment or involving reagents activated by a previous mechanochemical action [2–12]. During these reactions the high energy produced by boundary friction generates pressure and shear [12]. These effects create mechanical stressing, which generates heat and localized flash high temperatures, with a typical duration of 1 ls [11]. Mechanochemical reactions produce changes ranging from polymorph transitions to solid-state reactions [2–5] and catalytic processes [6–10]. A clear benefit of the methodology is the lack of a solvent and the drawbacks related to the recovery of the reaction products from the solution; besides, these reactions are carried out at low temperatures, or do not require any heat [7]. The effects of the mechanical activation on solids in general and catalysts in particular include the increase of the surface area, the creation of crystal lattice defects, free radicals, electron transfer
* Corresponding author. Fax: +1 787 744 5427. E-mail addresses: [email protected], [email protected] (R. Roque-Malherbe). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.07.016
[2,3,6,7], phase transformations [13–20], and other changes. Particularly, the generation of defects in semiconductor catalysts, with the help of different activation processes, is normally associated with modifications in the electronic properties of the catalyst [6,7]. For instance, electronic levels are formed in the forbidden bands, the forbidden bands become narrower, the conductivity electrons appear, and the electron-donor properties are enhanced in semiconductors [21–29]. In particular, some catalytic reactions under the simultaneous action of mechanical treatment have been studied, for example, the pyrolysis of butane over magnesium oxide, the hydrogenation of butadiene over metal hydrides [7], the oxidation of carbon monoxide over rutile [21], the decomposition of formic acid [8], and other processes [6,11]. Besides, it has been shown that simple transition metal oxides, such as NiO, Co3O4, Fe3O4, and Cu2O, catalytically decompose distilled water into H2 and O2 during magnetic stirring in a glass reaction vessel at room temperature [30,31]. The elimination of dangerous organic contaminants from wastewater and groundwater is a very important issue in Pollution Abatement Science and Technology. In this regard, considerable attention has been concentrated on advanced oxidation processes, which are founded on the production of very reactive species, in particular hydroxyl radicals, since it is a strong oxidant having the oxidation potential to totally oxidize organic compounds to CO2 and H2O [32]. In our case the organic compound to be treated, by the mechanical activation of a rutile catalyst, is phenol. This chemical compound is a basic structural constituent for a variety of industrial organic compounds. That is, it is an initial raw material for many
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intermediate and finished products, for example, detergents, adhesives, pesticides, dyes, resins, plasticizers, and additives for rubber chemicals [33]. Subsequently, phenols come from a multiplicity of industrial supplies, such as pesticides, dye and paper industries, coke and resin manufacturing, and textile, plastic, rubber, pharmaceutical, and petroleum production [34]. As a consequence of the toxicity of phenols, their elimination from water is a vital concern. Therefore, during the last years there has been an increasing interest in the creation of processes for the elimination of these compounds from water. In this regard, diverse methodologies, such as oxidation with UV radiation and ozone/ hydrogen peroxide [35], UV radiation and hydrogen peroxide [36], membrane filtration [37], reverse osmosis [38], electrochemical oxidation [39], adsorption [40], photocatalytic oxidation [22– 24], sonocatalytic oxidation [25,26], and pulsed corona discharge degradation [32,41], have been used for the removal of organic products, in general, and phenolic compounds, in particular. In advanced oxidation processes, those applying catalysts are widespread. In the applied catalysts rutile and anatase titanium dioxide polymorphs have been widely studied. Titanium dioxide exists, normally, in three polymorphs: rutile, anatase, and brookite [42]. The catalyst applied here, rutile, is a semiconductor with a band gap of 3.10 eV [43]. It has a tetragonal structure; that is, it consists of a cationic framework of a tetragonal body centered structure with the Ti4+ cation located in the lattice sites and the O2 anions arranged as a TiO6 octahedron connected through opposite edges alongside the c-axis [42]. Heterogeneous photocatalysis and sonocatalysis using titanium dioxide as the catalyst start with the production of electron–hole pairs, as follows [23,44–46]: hv
þ
TiO2 ! TiO2 ½e ðCBÞ þ h ðVBÞ In this regard, the mechanism of the photocatalytic and sonocatalytic decomposition of organic compounds is supposed to follow the subsequent steps: electron–hole pair formation; thereafter, the pairs migrate at the surface and are trapped by OH surface groups and other surface sites forming different radicals; and finally these free radicals cause the oxidation of the organic compounds [23,25,43,45]. In particular, during the sonochemical process, it has been proposed that the ultrasonic irradiation is the source of energy for the activation of the TiO2 powders in order to generate electron–hole pairs, with the subsequent generation of OH radicals for the treatment of organic compounds dissolved in water [25,46]. It has been recognized that mechanochemistry and sonochemistry are closely related [28], because both effects occur under equal local conditions, given that during sonical cavitation, mechanical energy is supplied for all ensuing chemical reactions [29]. In this sense, taking into account the similarities between the sonical and the mechanical treatments, the central idea that we are proposing here is that the mechanical activation of a semiconductor oxide catalyst, as the rutile TiO2 polymorph, will provoke the decomposition of organic compounds, since the mechanical activation will generate electron–hole pairs, with the subsequent production of OH radicals for the treatment of organic compounds dissolved in water. Here we are reporting the degradation of phenol by mechanical activation of rutile as a novel application of mechanochemistry, which could evolve to an energetically efficient methodology for the decomposition of organic compounds in wastewater. This process is expected to be less energy consuming than the other processes applied for the removal of organic compounds, since, as a general rule, a benefit of mechanochemistry is the reduction of energy consumption, because of the diminution of temperature and time of interaction of components [6].
2. Experimental 2.1. Materials The titanium oxide employed in the mechanocatalytic test was a powder provided by ALFA AESAR. The provider guaranteed a minimum of 97% of the rutile phase and a specific surface area: S = 3– 6 m2/g. The used phenol was pure for analysis quality provided by Ricca Chemical Company.
2.2. Methods The X-ray diffractograms were obtained in a Siemmens D5000 X-ray diffractometer, in vertical setup: h 2h geometry, in the range 20 < 2h < 60, with CuKa radiation source, Ni filter, and graphite monochromator. The scanning was carried at slow speed, i.e., 0.6 min1. The phase composition of the applied rutile catalyst was determined using the obtained XRD diffraction pattern. The XRD method was also applied for the measurement of the crystallite size of the rutile powder applying the Scherrer–Williamson–Hall methodology [47]. From the recorded XRD pattern, the exact peak position and integrated intensities as well as the fullwidth at half-maxima (FWHM) (b) of each slow scanned diffraction peaks were calculated by fitting them with the Pearson VII amplitude function. The fitting process was carried out with the peak separation and analysis software PeakFit (Seasolve Software Inc., Framingham, MA) based on a least-square procedure [48]. These values were used to calculate the crystallite size of the rutile powder using the Scherrer–Williamson–Hall equation [47,49]. The SEM study was carried out with a JEOL JSM 6360 microscope in secondary electron mode at an accelerating voltage of 20 kV to image the surface of the rutile powders. The sample grains were adhered to the sample holder with an adhesive tape and then coated under vacuum by cathode sputtering with a 30- to 40-nm gold film prior to observation. The surface morphology was revealed from SEM images and the grain size was calculated with the help of the software provided by the microscope. The surface and porosity characterization of the catalyst was carried out by gas adsorption measurements. This methodology is normally applied for determining the surface area, pore volume, and pore size distribution of porous materials. The specific surface area is typically measured by means of the BET method, the micropore volume is usually measured using the t-plot method, the Dubinin adsorption isotherm, and other methods, and the pore size distribution (PSD) is generally calculated at present using the nonlocal density functional theory (NLDFT) methodology [40]. This methodology was applied, in the present research, to experimental nitrogen adsorption isotherms at 77 K data, obtained for the rutile powder sample with a Quantachrome Accelerated Surface Area, and Porosimetry System Autosorb-1. With the help of the N2 adsorption isotherms the specific surface area, S, in m2/g, applying the BET method and the micropore volume, WMP, in cm3/g were measured using the t-plot method [40,49]. For the calculation of the pore size distribution the nonlocal density functional theory methodology was applied, using the silica kernel to obtain the NLDFT-PSD in the range between 1.8 and 100 nm and the NLDFT pore volume, W, in cm3/g [40]. The ball-milling process was performed with the help of a Glen Mills Inc. jar mill. In this milling setup, the 5-L jar, containing four bottles, to carry out the reaction, is positioned between a motor-driven roller and an idler roller, where the roller speeds reach about 300 rpm. The reaction was carried out in 0.25-L Nalgene polypropylene bottles included in the 5-L jar, under batch conditions. The grinding media, i.e., 20 zirconia cylinders (h = 0.014 m, r = 0.0065 m), provided by US Stoneware and 0.1 L of the 100 mg L1 phenol
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solution without catalyst and with 1 g of the TiO2 catalyst powder were placed in the bottles and milled for 0, 12, 18, 36, 24, 48, and 72 h once without the catalyst included in the milling process and two times with the catalyst included. To make the analysis after the milling process, in the case where the catalyst was included, the slurries produced by the dispersion of the catalyst in the phenol solution by the milling process were vacuum-filtered using a membrane vacuum filtering system composed of a 0.02 lm Anopore inorganic membrane disk, and a VWR 47-mm filter system, evacuated with a Cole–Palmer 70 L min1 rotary vane vacuum pump, in order to obtain a clear solution. The polypropylene bottles were carefully washed with a 50 vol.% HF solution, and the glassware and the grinding media were thoroughly cleaned with a sulfochromic mixture previous to the experiments. To study the evolution of phenol mineralization, the solution milled without catalyst for 0, 12, 24, 48, and 72 h was analyzed by ultraviolet spectrometry (UV) with the help of a GENESYS 10 series UV/VIS spectrometer, equipped with a Xe lamp in the range from 200 to 380 nm. Additionally, the mechanism and the kinetics of the catalytic process were also studied in two independent tests, by UV and gas chromatography coupled with mass spectrometry (GC/MS), to analyze the results with two different methods, and replicate the experiment to check its repeatability. In this regard, the two sets of clear solutions obtained by the filtering process after the milling processes with catalyst for 0, 12, 18, 24, 48, and 72 h were analyzed by UV, with the same procedure previously explained, and with the help of the GC profiles obtained with the GC/MS method. To identify the intermediates in the phenol degradation were applied the mass spectra obtained with the GC/MS method. To analyze these samples, with the GC/MS technique, it was necessary to extract the organic components dissolved in the clear water solutions. To carry out this process, 40 mL of the clear solutions, obtained after milling and filtering, was extracted with dichloromethane (3 15 mL); then, the organic phase was combined and passed through anhydrous magnesium sulfate, followed by concentration in a rotary evaporator [50]. The GC/MS analysis was performed on Shimadzu equipment. For GC analysis, samples were injected in splitless mode in a Shimadzu 17A gas chromatograph, equipped with a Rtx-XLB column, 30 m length, 0.25 mm interior diameter, and 0.24 lm thickness; the GC was coupled to a QP5050 mass spectrometer.
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The conditions for the analysis were column temperature 298 K, injector temperature 523 K, and interface temperature 553 K [51]. 3. Results and discussion 3.1. Catalyst characterization The TiO2 sample in the as-supplied state was characterized with XRD. In Fig. 1 is shown the XRD diffraction pattern of the rutile catalyst. It is evident by comparing the obtained pattern with the powder diffraction data contained in the International Center for Diffraction Data, Powder Diffraction File database that it is a highly crystalline material. Additionally, other crystalline phases were not detected. Besides, from this XRD pattern the accurate peak position and the full-width at half-maximum (b) of each diffraction peaks were determined [49]. These values were used to calculate the crystallite size of the as-supplied TiO2 powder with the help of the Scherrer–Williamson–Hall method. In this regard, the crystallite size (/XRD), estimated with the help of the Scherrer–Williamson–Hall method, was /XRD = 0.1 ± 0.05 lm. SEM micrographs of the, as supplied, titanium oxide powder are reported in Fig. 2. With the help of these micrographs it is possible to show that the powder has an average crystallite size /SEM ¼ 0:3 0:1 lm, a value which approximately agrees with the figure calculated with the help of the XRD pattern. With the previously reported crystallite size data it is possible to estimate the catalyst surface area with the help of the following relation
S¼
6 ; /q
ð1Þ
where / is the crystallite size and q is the material density. Since / = 0.3 lm and q = 4.23 g cm3 is the rutile density, the specific surface, S, is 4.6 m2 g1. In order to corroborate the value previously calculated for the specific surface, S, this parameter was measured with the help of the BET method (see Fig. 3). The measured BET specific surface area was 5 m2 g1, in good agreement with the value calculated with the help of Eq. (1). Additionally, with the help of the adsorption data it was also shown that the micropore volume, WMP, is insignificant and the NLDFT pore volume, W, is, as well, negligible. This means that the rutile powder is not porous.
Fig. 1. XRD pattern of the TiO2 powder as supplied.
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Fig. 4. Kinetics of the ball-milling process without catalyst.
Fig. 2. SEM micrography of the as-supplied TiO2 powder: (a) 10,000 and (b) 20,000.
3.2. Mechanism of phenol degradation by mechanical activation of rutile To show that we are in the presence of a catalytic process, we must, initially, test if the reaction occurs in the absence of the catalyst. Subsequently, our first kinetic experiment consists in testing whether a catalysts is needed to decompose phenol, as is the case during sonic irradiation [27]. Thus, a 100 mg L1 phenol solution was ball-milled, without the catalyst, in water for 0, 12, 24, 48, and 72 h and the reaction products were analyzed. In Fig. 4, are reported the ratios between the phenol concentrations at different times, C(t), and for t = 0, C(0) , calculated by measuring the maximum absorption of the overall UV absorption peak at different times, A(t), and this parameter for t = 0, A(0), and applying the Lambert–Beer law, which states that A(t)/A(0) = C(t)/C(0). From these results, it is obvious that phenol was not decomposed.
Fig. 3. BET plot for the N2 adsorption at 77 K in the rutile powder.
The second step is to show whether the reaction takes place in the presence of the catalyst. Thereafter, two sets of mixtures of 1 g of the rutile powder and a 100 mg L1 phenol solution were ballmilled in water for 0, 12, 18, 24, 48, and 72 h, and the reaction products were analyzed, independently, with UV spectrometry and the GC profiles obtained with the GC/MS method. In Fig. 5 is shown the plot of the ratio between phenol concentrations at different times, C(t), and for, t = 0, C(0), i.e., C(t)/C(0) versus t. In this case it is evident that phenol was decomposed. To show that the catalyst itself is not changed in the course of the process, the XRD profiles of the catalysts milled for 0, 24, 48, and 72 h were obtained and the TiO2 powder as supplied (see Fig. 6). A careful analysis of the obtained information did not show any evidence of change in the XRD profiles. Consequently, the global structure, the particle size, the surface area, and more importantly the phase composition of the catalyst were not affected during the milling process. Therefore, since the reaction does not take place in the absence of rutile, while it is not altered in the course of the process, rutile is a catalyst of the process. During the mechanocatalytic decomposition of water into H2 and O2, the formation of reduced metal during the catalytic reaction was established by X-ray diffraction and chemical analysis; the authors of this research then concluded that a redox reaction between the metal and the metal oxide may be responsible for water splitting [31]. The mechanism for phenol decomposition should be different, since in our case it was not detected, by the X-ray diffraction analysis changes in the catalyst. Consequently, it is necessary to propose a different mechanism. In this regard, it was shown that by milling metal oxides, charge transfer is promoted, a fact which can have a significant effect on oxidation– reduction and other reactions requiring the transfer of electrons [52]. A similar effect is produced during the cavitation process generated by ultrasound radiation [25,46,53,54]. That is, if ultrasonic
Fig. 5. Kinetics of the ball-milling process with the rutile catalyst included.
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degradation [32,41,70,71], and other advanced oxidation processes. Then, if some of the first intermediates of the reaction, i.e., hydroquinone, catechol, or benzoquinone, are identified in our case, this fact indicates that the OH radical, is involved in the degradation pathways. In Fig. 7 are shown the MS spectra of the phenol solutions after being milled for 0 h (a), 18 h (b), and 48 h (c). In both spectra of the solution milled with the catalyst for 18 and 48 h a peak at 108 mass charge relation (m/z) is found, which indicates the presence of benzoquinone, whose molar mass is 108.096 g/mol. Consequently, the OH radical is involved in the degradation pathways. 3.3. Reaction kinetics The OH radical oxidation reaction with the phenol molecule, Ph, in the bulk phase can be expressed by the following chemical equation [25,31,62]:
labeled Phþ OH ! Phox : Fig. 6. XRD profiles of: (a) TiO2 powder as-supplied, and the TiO2 powder milled by: (b) 24 h, (c) 48 h, and (d) 72 h.
irradiation energy corresponds or surpasses the semiconductor band-gap energy, an electron of the valence band can be raised to the conduction band producing a hole; thereafter, these electron–hole pairs migrate at the surface creating free radicals which cause the oxidation of the organic compounds [46]. Consequently, since mechanochemistry and sonochemistry are closely related [28,29], a mechanism similar to that proposed during the sonochemical activation of TiO2 could explain the process studied here. Prior studies have shown that the sonochemical decomposition of organic compounds with catalyst [46] proceeds principally through a free radical mechanism in water solution, with the generation of the hydroxyl free radical (OH). Thereafter, we will prove, or disprove, if in the present case the mechanism is also by oxidation with the (OH) free radical. The hydroxyl radical is one of the strongest oxidizers among the oxygen-based oxidizers [55]. In its reaction with organic molecules, the hydroxyl radical reacts by electrophilic addition to unsaturated bonds; it also abstracts hydrogen atoms from organic molecules and also reacts by electron transfer [56]. Consequently, the oxidation of phenol by the hydroxyl free radical (OH) generates an ample variety of products consisting of: polyhydroxybenzenes/quinones, ring-cleavage products, and polymerization products [27,55,57–59]. The reaction with OH produces dihydroxybenzenes generated by the attachment of the hydroxyl radical on the benzene ring, such as catechol, resorcinol, and hydroquinone, and the trihydroxybenzenes pyrogallol, hydroxyhydroquinone, and phloroglucinol [27,41,59,60]. Afterward, as a result of the oxidation of the polyhydroxybenzenes: the following quinones have been found as reaction products: 1,4benzoquinone, 1,2-benzoquinone, and hydroxybenzoquinone [32,61–63]. Subsequently, the persistence of the oxidation process generates ring-cleavage products such as carboxyl, aldehyde, ketone, or alkanol groups, where the alkanol-functional groups are oxidized to aldehyde groups, while aldehydes are oxidized to carboxylic acids and finally completely degrade to CO2 and H2O [41,60,61,63]. The lifetime of the intermediates produced at distinct phases of the reaction are very small, since the intermediates experience additional oxidation [62]. Consequently, this mechanism has been widely investigated during photocatalytic oxidation [22–24,51,46,53,62], oxidation with UV radiation and ozone [64] or hydrogen peroxide [65], sonocatalytic oxidation [25,26,66–69], pulsed corona discharge
ð2Þ
The rate of this second-order reaction can be formulated in this fashion,
dCðtÞ ¼ rPh ¼ k ½ OH CðtÞ; dt
ð3Þ
where k is the second-order reaction rate constant, [OH] is the OH radical concentration, and C(t) is the phenol concentration. It is evident that since the mechanical activation process is a steady process and the amount of the solvent, i.e., water, is very high, then the [OH] concentration can be considered approximately constant. Consequently,
rPh kBulk CðtÞ;
ð4Þ
which represents the rate of a pseudo-first-order reaction, where kBulk is the pseudo-first-order rate constant for the bulk reaction. However, in our case the bulk phase reaction does not take place (see Fig. 4). In the present case the reaction takes place only in the presence of the catalyst. Subsequently, we should have a Langmuir–Hinshelwood (L–H) mechanism for surface-catalyzed reactions, where the reaction takes place between two surface adsorbed species or an
Fig. 7. Mass spectra of the initial solution (a) and the solutions after be milled for 18 h (b) and 48 h (c).
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Eley–Rideal (E–R) mechanism in which the reaction takes place between a surface adsorbed species and a reactant present in the bulk phase. The L–H model has been applied to describe the rates of photocatalytic [62,72] and sonocatalytic [46] degradation of organic compounds; thereafter we will apply it here. In this regard in the absence of external mass transfer limitations, as is the case here, the reaction rate for the L–H mechanism is given by [49].
r L—H ¼ kPh—OH
K Ph K OH CðtÞ½ OH ð1 þ K Ph CðtÞ þ K OH ½ OHÞ2
:
ð5Þ
If it is supposed that both species are in low concentration in the bulk phase, then 1 + KPhC(t) + KOH [OH] 1. Consequently,
r L—H kPh—OH K Ph K OH CðtÞ½ OH:
ð6Þ
Since the mechanical activation process is a steady process, the [OH] concentration can be considered approximately constant. As a result, we have a pseudo-first-order reaction:
dCðtÞ ¼ kCðtÞ: dt
ð7Þ
Consequently,
ln
CðtÞ ¼ kt; Cð0Þ
ð8Þ
where C(0) and C(t) are the initial phenol concentration and its concentration at time, t. Then, the plot ln [C(t)/C(0)] versus t is a line passing through the origin. In Fig. 8 it is seen that the obtained experimental data logarithmically plotted are fitted by Eq. (8), i.e., a line passing through the origin, with a regression coefficient r2 = 0.975. Consequently, the reaction fulfill the pseudo-first-order kinetics, where the reaction constant values is k = 1.6 105 s1. The pseudo-first-order reaction constant value measured is low. This result indicates that the process is inefficient. This fact is produced by the small specific surface area of the applied catalyst, since the reaction mechanism implies the adsorption of both phenol and the radical in the catalyst surface. Thereafter, in order to develop an efficient process it is necessary to obtain catalyst particles with less particle diameter. The particle diameter measured for the applied catalyst was 300 nm, and the specific area was 5 m2 g1. Then for particles of 10 nm, which can relatively easily obtained for other semiconductor catalysts, for example, ZnO and TiO2, the specific area could be around 150 m2 g1. Consequently, the reaction rate constant could be increased, to gain an efficient process. Besides, it is possible that rutile is not the best catalyst for the present reaction. In this regard, we are now in the process of testing the reaction with other semiconductor catalysts, such as those
previously noted, i.e., ZnO and TiO2 (anatase) with specific surface of 100–200 m2 g1. To conclude it is necessary to state that despite the inefficiency of the process, the methodology reported here is an original application of mechanochemistry, which can evolve into an efficient method for the degradation of organic compounds in wastewater, by the increase of the specific surface area of the semiconductor catalyst applied during the mechanical activation. 4. Conclusions In the present paper a novel process for the elimination of organic pollutants dissolved in water is proposed. Phenol aqueous solutions (100 mg L1) were mechanically treated for 0, 12, 18, 24, 36, 48, and 72 h in the presence of the rutile powder catalyst and without catalyst. Thereafter, the reaction products were analyzed with UV and GC/MS. It was shown that when the catalyst was not included in the process the phenol present in the solution was not affected. Conversely, when the catalyst was included in the batch reactor, phenol was degraded and the catalyst itself did not change. In the reaction products were found intermediates which are distinctive of the OH radical mechanism. Subsequently, this mechanism was proposed for the explanation of the obtained results. It was also shown that the reaction follows a pseudo-first-order kinetics, as should be, for the OH radical mechanism. The value measured for the pseudo-first-order reaction constant was very low, indicating that the reported process is inefficient. However, this problem could be solved applying catalysts consisting of particles with lesser diameters. Thereafter independent of this negative aspect, we are reporting a novel application of mechanochemistry, which could be developed as an energetically efficient method for the degradation of organic compounds in wastewater. Acknowledgments The authors gratefully recognize that the present research was, in part, financed by the US Department of Energy through the Massey Chair project at Turabo University. Also, the authors gratefully acknowledge the Materials Characterization Center at the Rio Piedras Campus of the University of Puerto Rico and the Department of Physics of the Cayey Campus of the University of Puerto Rico for the assistance provided in the characterization of the tested catalyst. Finally, we thank Mr. Edgard Mosquerra for his technical support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] Fig. 8. ln [C(t)/C(0)] versus t plot for the phenol degradation.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Degradation of phenol by mechanical activation of a rutile catalyst M.C. Cotto, A. Emiliano, S. Nieto, J. Duconge, R. Roque-Malherbe * Institute of Physical Chemical Applied Research, School of Science, University of Turabo, P.O. Box 3030, Gurabo, PR 00778-3030, USA
a r t i c l e
i n f o
Article history: Received 23 April 2009 Accepted 7 July 2009 Available online 13 August 2009 Keywords: Mechanochemistry Catalysis Phenol Degradation OH radical mechanism Rutile
a b s t r a c t In the present paper a novel mechanochemical process for the elimination of organic pollutants dissolved in water is proposed. In this regard, phenol aqueous solutions (100 mg L1) were ball-milled for 0, 12, 18, 24, 36, 48, and 72 h with and without a well-characterized (XRD, SEM, and N2 Adsorption), rutile powder catalyst and the reaction products analyzed with UV and GC/MS. It was found that when the catalyst was not included in the process, phenol was not affected, but when it was included, phenol was decomposed. The catalyst itself did not change and the reaction follows a pseudo-first-order kinetics. Besides, intermediates which are characteristic of the OH radical mechanism were found in the reaction products. Then, a mechanism similar to those accepted for other advanced oxidation processes was proposed. The value measured for the pseudo-first-order reaction constant was very low, indicating that the reported process is inefficient. Nevertheless, this problem could be solved by applying catalysts consisting of particles with smaller diameters. Ó 2009 Elsevier Inc. All rights reserved.
1. Introduction The conceptual meaning of mechanochemistry was first defined by F.W. Ostwald. He concentrated his works, published by the end of the 19th century and the first years of the 20th century, on the categorization of chemical disciplines by the energy type applied to the chemical processes, including between them mechanical energy [1,2]. Mechanochemistry deals with chemical reactions comprising reagents in any state of aggregation, principally solid-state processes and reactions, either initiated by a mechanical treatment or involving reagents activated by a previous mechanochemical action [2–12]. During these reactions the high energy produced by boundary friction generates pressure and shear [12]. These effects create mechanical stressing, which generates heat and localized flash high temperatures, with a typical duration of 1 ls [11]. Mechanochemical reactions produce changes ranging from polymorph transitions to solid-state reactions [2–5] and catalytic processes [6–10]. A clear benefit of the methodology is the lack of a solvent and the drawbacks related to the recovery of the reaction products from the solution; besides, these reactions are carried out at low temperatures, or do not require any heat [7]. The effects of the mechanical activation on solids in general and catalysts in particular include the increase of the surface area, the creation of crystal lattice defects, free radicals, electron transfer
* Corresponding author. Fax: +1 787 744 5427. E-mail addresses: [email protected], [email protected] (R. Roque-Malherbe). 0021-9797/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2009.07.016
[2,3,6,7], phase transformations [13–20], and other changes. Particularly, the generation of defects in semiconductor catalysts, with the help of different activation processes, is normally associated with modifications in the electronic properties of the catalyst [6,7]. For instance, electronic levels are formed in the forbidden bands, the forbidden bands become narrower, the conductivity electrons appear, and the electron-donor properties are enhanced in semiconductors [21–29]. In particular, some catalytic reactions under the simultaneous action of mechanical treatment have been studied, for example, the pyrolysis of butane over magnesium oxide, the hydrogenation of butadiene over metal hydrides [7], the oxidation of carbon monoxide over rutile [21], the decomposition of formic acid [8], and other processes [6,11]. Besides, it has been shown that simple transition metal oxides, such as NiO, Co3O4, Fe3O4, and Cu2O, catalytically decompose distilled water into H2 and O2 during magnetic stirring in a glass reaction vessel at room temperature [30,31]. The elimination of dangerous organic contaminants from wastewater and groundwater is a very important issue in Pollution Abatement Science and Technology. In this regard, considerable attention has been concentrated on advanced oxidation processes, which are founded on the production of very reactive species, in particular hydroxyl radicals, since it is a strong oxidant having the oxidation potential to totally oxidize organic compounds to CO2 and H2O [32]. In our case the organic compound to be treated, by the mechanical activation of a rutile catalyst, is phenol. This chemical compound is a basic structural constituent for a variety of industrial organic compounds. That is, it is an initial raw material for many
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intermediate and finished products, for example, detergents, adhesives, pesticides, dyes, resins, plasticizers, and additives for rubber chemicals [33]. Subsequently, phenols come from a multiplicity of industrial supplies, such as pesticides, dye and paper industries, coke and resin manufacturing, and textile, plastic, rubber, pharmaceutical, and petroleum production [34]. As a consequence of the toxicity of phenols, their elimination from water is a vital concern. Therefore, during the last years there has been an increasing interest in the creation of processes for the elimination of these compounds from water. In this regard, diverse methodologies, such as oxidation with UV radiation and ozone/ hydrogen peroxide [35], UV radiation and hydrogen peroxide [36], membrane filtration [37], reverse osmosis [38], electrochemical oxidation [39], adsorption [40], photocatalytic oxidation [22– 24], sonocatalytic oxidation [25,26], and pulsed corona discharge degradation [32,41], have been used for the removal of organic products, in general, and phenolic compounds, in particular. In advanced oxidation processes, those applying catalysts are widespread. In the applied catalysts rutile and anatase titanium dioxide polymorphs have been widely studied. Titanium dioxide exists, normally, in three polymorphs: rutile, anatase, and brookite [42]. The catalyst applied here, rutile, is a semiconductor with a band gap of 3.10 eV [43]. It has a tetragonal structure; that is, it consists of a cationic framework of a tetragonal body centered structure with the Ti4+ cation located in the lattice sites and the O2 anions arranged as a TiO6 octahedron connected through opposite edges alongside the c-axis [42]. Heterogeneous photocatalysis and sonocatalysis using titanium dioxide as the catalyst start with the production of electron–hole pairs, as follows [23,44–46]: hv
þ
TiO2 ! TiO2 ½e ðCBÞ þ h ðVBÞ In this regard, the mechanism of the photocatalytic and sonocatalytic decomposition of organic compounds is supposed to follow the subsequent steps: electron–hole pair formation; thereafter, the pairs migrate at the surface and are trapped by OH surface groups and other surface sites forming different radicals; and finally these free radicals cause the oxidation of the organic compounds [23,25,43,45]. In particular, during the sonochemical process, it has been proposed that the ultrasonic irradiation is the source of energy for the activation of the TiO2 powders in order to generate electron–hole pairs, with the subsequent generation of OH radicals for the treatment of organic compounds dissolved in water [25,46]. It has been recognized that mechanochemistry and sonochemistry are closely related [28], because both effects occur under equal local conditions, given that during sonical cavitation, mechanical energy is supplied for all ensuing chemical reactions [29]. In this sense, taking into account the similarities between the sonical and the mechanical treatments, the central idea that we are proposing here is that the mechanical activation of a semiconductor oxide catalyst, as the rutile TiO2 polymorph, will provoke the decomposition of organic compounds, since the mechanical activation will generate electron–hole pairs, with the subsequent production of OH radicals for the treatment of organic compounds dissolved in water. Here we are reporting the degradation of phenol by mechanical activation of rutile as a novel application of mechanochemistry, which could evolve to an energetically efficient methodology for the decomposition of organic compounds in wastewater. This process is expected to be less energy consuming than the other processes applied for the removal of organic compounds, since, as a general rule, a benefit of mechanochemistry is the reduction of energy consumption, because of the diminution of temperature and time of interaction of components [6].
2. Experimental 2.1. Materials The titanium oxide employed in the mechanocatalytic test was a powder provided by ALFA AESAR. The provider guaranteed a minimum of 97% of the rutile phase and a specific surface area: S = 3– 6 m2/g. The used phenol was pure for analysis quality provided by Ricca Chemical Company.
2.2. Methods The X-ray diffractograms were obtained in a Siemmens D5000 X-ray diffractometer, in vertical setup: h 2h geometry, in the range 20 < 2h < 60, with CuKa radiation source, Ni filter, and graphite monochromator. The scanning was carried at slow speed, i.e., 0.6 min1. The phase composition of the applied rutile catalyst was determined using the obtained XRD diffraction pattern. The XRD method was also applied for the measurement of the crystallite size of the rutile powder applying the Scherrer–Williamson–Hall methodology [47]. From the recorded XRD pattern, the exact peak position and integrated intensities as well as the fullwidth at half-maxima (FWHM) (b) of each slow scanned diffraction peaks were calculated by fitting them with the Pearson VII amplitude function. The fitting process was carried out with the peak separation and analysis software PeakFit (Seasolve Software Inc., Framingham, MA) based on a least-square procedure [48]. These values were used to calculate the crystallite size of the rutile powder using the Scherrer–Williamson–Hall equation [47,49]. The SEM study was carried out with a JEOL JSM 6360 microscope in secondary electron mode at an accelerating voltage of 20 kV to image the surface of the rutile powders. The sample grains were adhered to the sample holder with an adhesive tape and then coated under vacuum by cathode sputtering with a 30- to 40-nm gold film prior to observation. The surface morphology was revealed from SEM images and the grain size was calculated with the help of the software provided by the microscope. The surface and porosity characterization of the catalyst was carried out by gas adsorption measurements. This methodology is normally applied for determining the surface area, pore volume, and pore size distribution of porous materials. The specific surface area is typically measured by means of the BET method, the micropore volume is usually measured using the t-plot method, the Dubinin adsorption isotherm, and other methods, and the pore size distribution (PSD) is generally calculated at present using the nonlocal density functional theory (NLDFT) methodology [40]. This methodology was applied, in the present research, to experimental nitrogen adsorption isotherms at 77 K data, obtained for the rutile powder sample with a Quantachrome Accelerated Surface Area, and Porosimetry System Autosorb-1. With the help of the N2 adsorption isotherms the specific surface area, S, in m2/g, applying the BET method and the micropore volume, WMP, in cm3/g were measured using the t-plot method [40,49]. For the calculation of the pore size distribution the nonlocal density functional theory methodology was applied, using the silica kernel to obtain the NLDFT-PSD in the range between 1.8 and 100 nm and the NLDFT pore volume, W, in cm3/g [40]. The ball-milling process was performed with the help of a Glen Mills Inc. jar mill. In this milling setup, the 5-L jar, containing four bottles, to carry out the reaction, is positioned between a motor-driven roller and an idler roller, where the roller speeds reach about 300 rpm. The reaction was carried out in 0.25-L Nalgene polypropylene bottles included in the 5-L jar, under batch conditions. The grinding media, i.e., 20 zirconia cylinders (h = 0.014 m, r = 0.0065 m), provided by US Stoneware and 0.1 L of the 100 mg L1 phenol
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solution without catalyst and with 1 g of the TiO2 catalyst powder were placed in the bottles and milled for 0, 12, 18, 36, 24, 48, and 72 h once without the catalyst included in the milling process and two times with the catalyst included. To make the analysis after the milling process, in the case where the catalyst was included, the slurries produced by the dispersion of the catalyst in the phenol solution by the milling process were vacuum-filtered using a membrane vacuum filtering system composed of a 0.02 lm Anopore inorganic membrane disk, and a VWR 47-mm filter system, evacuated with a Cole–Palmer 70 L min1 rotary vane vacuum pump, in order to obtain a clear solution. The polypropylene bottles were carefully washed with a 50 vol.% HF solution, and the glassware and the grinding media were thoroughly cleaned with a sulfochromic mixture previous to the experiments. To study the evolution of phenol mineralization, the solution milled without catalyst for 0, 12, 24, 48, and 72 h was analyzed by ultraviolet spectrometry (UV) with the help of a GENESYS 10 series UV/VIS spectrometer, equipped with a Xe lamp in the range from 200 to 380 nm. Additionally, the mechanism and the kinetics of the catalytic process were also studied in two independent tests, by UV and gas chromatography coupled with mass spectrometry (GC/MS), to analyze the results with two different methods, and replicate the experiment to check its repeatability. In this regard, the two sets of clear solutions obtained by the filtering process after the milling processes with catalyst for 0, 12, 18, 24, 48, and 72 h were analyzed by UV, with the same procedure previously explained, and with the help of the GC profiles obtained with the GC/MS method. To identify the intermediates in the phenol degradation were applied the mass spectra obtained with the GC/MS method. To analyze these samples, with the GC/MS technique, it was necessary to extract the organic components dissolved in the clear water solutions. To carry out this process, 40 mL of the clear solutions, obtained after milling and filtering, was extracted with dichloromethane (3 15 mL); then, the organic phase was combined and passed through anhydrous magnesium sulfate, followed by concentration in a rotary evaporator [50]. The GC/MS analysis was performed on Shimadzu equipment. For GC analysis, samples were injected in splitless mode in a Shimadzu 17A gas chromatograph, equipped with a Rtx-XLB column, 30 m length, 0.25 mm interior diameter, and 0.24 lm thickness; the GC was coupled to a QP5050 mass spectrometer.
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The conditions for the analysis were column temperature 298 K, injector temperature 523 K, and interface temperature 553 K [51]. 3. Results and discussion 3.1. Catalyst characterization The TiO2 sample in the as-supplied state was characterized with XRD. In Fig. 1 is shown the XRD diffraction pattern of the rutile catalyst. It is evident by comparing the obtained pattern with the powder diffraction data contained in the International Center for Diffraction Data, Powder Diffraction File database that it is a highly crystalline material. Additionally, other crystalline phases were not detected. Besides, from this XRD pattern the accurate peak position and the full-width at half-maximum (b) of each diffraction peaks were determined [49]. These values were used to calculate the crystallite size of the as-supplied TiO2 powder with the help of the Scherrer–Williamson–Hall method. In this regard, the crystallite size (/XRD), estimated with the help of the Scherrer–Williamson–Hall method, was /XRD = 0.1 ± 0.05 lm. SEM micrographs of the, as supplied, titanium oxide powder are reported in Fig. 2. With the help of these micrographs it is possible to show that the powder has an average crystallite size /SEM ¼ 0:3 0:1 lm, a value which approximately agrees with the figure calculated with the help of the XRD pattern. With the previously reported crystallite size data it is possible to estimate the catalyst surface area with the help of the following relation
S¼
6 ; /q
ð1Þ
where / is the crystallite size and q is the material density. Since / = 0.3 lm and q = 4.23 g cm3 is the rutile density, the specific surface, S, is 4.6 m2 g1. In order to corroborate the value previously calculated for the specific surface, S, this parameter was measured with the help of the BET method (see Fig. 3). The measured BET specific surface area was 5 m2 g1, in good agreement with the value calculated with the help of Eq. (1). Additionally, with the help of the adsorption data it was also shown that the micropore volume, WMP, is insignificant and the NLDFT pore volume, W, is, as well, negligible. This means that the rutile powder is not porous.
Fig. 1. XRD pattern of the TiO2 powder as supplied.
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Fig. 4. Kinetics of the ball-milling process without catalyst.
Fig. 2. SEM micrography of the as-supplied TiO2 powder: (a) 10,000 and (b) 20,000.
3.2. Mechanism of phenol degradation by mechanical activation of rutile To show that we are in the presence of a catalytic process, we must, initially, test if the reaction occurs in the absence of the catalyst. Subsequently, our first kinetic experiment consists in testing whether a catalysts is needed to decompose phenol, as is the case during sonic irradiation [27]. Thus, a 100 mg L1 phenol solution was ball-milled, without the catalyst, in water for 0, 12, 24, 48, and 72 h and the reaction products were analyzed. In Fig. 4, are reported the ratios between the phenol concentrations at different times, C(t), and for t = 0, C(0) , calculated by measuring the maximum absorption of the overall UV absorption peak at different times, A(t), and this parameter for t = 0, A(0), and applying the Lambert–Beer law, which states that A(t)/A(0) = C(t)/C(0). From these results, it is obvious that phenol was not decomposed.
Fig. 3. BET plot for the N2 adsorption at 77 K in the rutile powder.
The second step is to show whether the reaction takes place in the presence of the catalyst. Thereafter, two sets of mixtures of 1 g of the rutile powder and a 100 mg L1 phenol solution were ballmilled in water for 0, 12, 18, 24, 48, and 72 h, and the reaction products were analyzed, independently, with UV spectrometry and the GC profiles obtained with the GC/MS method. In Fig. 5 is shown the plot of the ratio between phenol concentrations at different times, C(t), and for, t = 0, C(0), i.e., C(t)/C(0) versus t. In this case it is evident that phenol was decomposed. To show that the catalyst itself is not changed in the course of the process, the XRD profiles of the catalysts milled for 0, 24, 48, and 72 h were obtained and the TiO2 powder as supplied (see Fig. 6). A careful analysis of the obtained information did not show any evidence of change in the XRD profiles. Consequently, the global structure, the particle size, the surface area, and more importantly the phase composition of the catalyst were not affected during the milling process. Therefore, since the reaction does not take place in the absence of rutile, while it is not altered in the course of the process, rutile is a catalyst of the process. During the mechanocatalytic decomposition of water into H2 and O2, the formation of reduced metal during the catalytic reaction was established by X-ray diffraction and chemical analysis; the authors of this research then concluded that a redox reaction between the metal and the metal oxide may be responsible for water splitting [31]. The mechanism for phenol decomposition should be different, since in our case it was not detected, by the X-ray diffraction analysis changes in the catalyst. Consequently, it is necessary to propose a different mechanism. In this regard, it was shown that by milling metal oxides, charge transfer is promoted, a fact which can have a significant effect on oxidation– reduction and other reactions requiring the transfer of electrons [52]. A similar effect is produced during the cavitation process generated by ultrasound radiation [25,46,53,54]. That is, if ultrasonic
Fig. 5. Kinetics of the ball-milling process with the rutile catalyst included.
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degradation [32,41,70,71], and other advanced oxidation processes. Then, if some of the first intermediates of the reaction, i.e., hydroquinone, catechol, or benzoquinone, are identified in our case, this fact indicates that the OH radical, is involved in the degradation pathways. In Fig. 7 are shown the MS spectra of the phenol solutions after being milled for 0 h (a), 18 h (b), and 48 h (c). In both spectra of the solution milled with the catalyst for 18 and 48 h a peak at 108 mass charge relation (m/z) is found, which indicates the presence of benzoquinone, whose molar mass is 108.096 g/mol. Consequently, the OH radical is involved in the degradation pathways. 3.3. Reaction kinetics The OH radical oxidation reaction with the phenol molecule, Ph, in the bulk phase can be expressed by the following chemical equation [25,31,62]:
labeled Phþ OH ! Phox : Fig. 6. XRD profiles of: (a) TiO2 powder as-supplied, and the TiO2 powder milled by: (b) 24 h, (c) 48 h, and (d) 72 h.
irradiation energy corresponds or surpasses the semiconductor band-gap energy, an electron of the valence band can be raised to the conduction band producing a hole; thereafter, these electron–hole pairs migrate at the surface creating free radicals which cause the oxidation of the organic compounds [46]. Consequently, since mechanochemistry and sonochemistry are closely related [28,29], a mechanism similar to that proposed during the sonochemical activation of TiO2 could explain the process studied here. Prior studies have shown that the sonochemical decomposition of organic compounds with catalyst [46] proceeds principally through a free radical mechanism in water solution, with the generation of the hydroxyl free radical (OH). Thereafter, we will prove, or disprove, if in the present case the mechanism is also by oxidation with the (OH) free radical. The hydroxyl radical is one of the strongest oxidizers among the oxygen-based oxidizers [55]. In its reaction with organic molecules, the hydroxyl radical reacts by electrophilic addition to unsaturated bonds; it also abstracts hydrogen atoms from organic molecules and also reacts by electron transfer [56]. Consequently, the oxidation of phenol by the hydroxyl free radical (OH) generates an ample variety of products consisting of: polyhydroxybenzenes/quinones, ring-cleavage products, and polymerization products [27,55,57–59]. The reaction with OH produces dihydroxybenzenes generated by the attachment of the hydroxyl radical on the benzene ring, such as catechol, resorcinol, and hydroquinone, and the trihydroxybenzenes pyrogallol, hydroxyhydroquinone, and phloroglucinol [27,41,59,60]. Afterward, as a result of the oxidation of the polyhydroxybenzenes: the following quinones have been found as reaction products: 1,4benzoquinone, 1,2-benzoquinone, and hydroxybenzoquinone [32,61–63]. Subsequently, the persistence of the oxidation process generates ring-cleavage products such as carboxyl, aldehyde, ketone, or alkanol groups, where the alkanol-functional groups are oxidized to aldehyde groups, while aldehydes are oxidized to carboxylic acids and finally completely degrade to CO2 and H2O [41,60,61,63]. The lifetime of the intermediates produced at distinct phases of the reaction are very small, since the intermediates experience additional oxidation [62]. Consequently, this mechanism has been widely investigated during photocatalytic oxidation [22–24,51,46,53,62], oxidation with UV radiation and ozone [64] or hydrogen peroxide [65], sonocatalytic oxidation [25,26,66–69], pulsed corona discharge
ð2Þ
The rate of this second-order reaction can be formulated in this fashion,
dCðtÞ ¼ rPh ¼ k ½ OH CðtÞ; dt
ð3Þ
where k is the second-order reaction rate constant, [OH] is the OH radical concentration, and C(t) is the phenol concentration. It is evident that since the mechanical activation process is a steady process and the amount of the solvent, i.e., water, is very high, then the [OH] concentration can be considered approximately constant. Consequently,
rPh kBulk CðtÞ;
ð4Þ
which represents the rate of a pseudo-first-order reaction, where kBulk is the pseudo-first-order rate constant for the bulk reaction. However, in our case the bulk phase reaction does not take place (see Fig. 4). In the present case the reaction takes place only in the presence of the catalyst. Subsequently, we should have a Langmuir–Hinshelwood (L–H) mechanism for surface-catalyzed reactions, where the reaction takes place between two surface adsorbed species or an
Fig. 7. Mass spectra of the initial solution (a) and the solutions after be milled for 18 h (b) and 48 h (c).
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Eley–Rideal (E–R) mechanism in which the reaction takes place between a surface adsorbed species and a reactant present in the bulk phase. The L–H model has been applied to describe the rates of photocatalytic [62,72] and sonocatalytic [46] degradation of organic compounds; thereafter we will apply it here. In this regard in the absence of external mass transfer limitations, as is the case here, the reaction rate for the L–H mechanism is given by [49].
r L—H ¼ kPh—OH
K Ph K OH CðtÞ½ OH ð1 þ K Ph CðtÞ þ K OH ½ OHÞ2
:
ð5Þ
If it is supposed that both species are in low concentration in the bulk phase, then 1 + KPhC(t) + KOH [OH] 1. Consequently,
r L—H kPh—OH K Ph K OH CðtÞ½ OH:
ð6Þ
Since the mechanical activation process is a steady process, the [OH] concentration can be considered approximately constant. As a result, we have a pseudo-first-order reaction:
dCðtÞ ¼ kCðtÞ: dt
ð7Þ
Consequently,
ln
CðtÞ ¼ kt; Cð0Þ
ð8Þ
where C(0) and C(t) are the initial phenol concentration and its concentration at time, t. Then, the plot ln [C(t)/C(0)] versus t is a line passing through the origin. In Fig. 8 it is seen that the obtained experimental data logarithmically plotted are fitted by Eq. (8), i.e., a line passing through the origin, with a regression coefficient r2 = 0.975. Consequently, the reaction fulfill the pseudo-first-order kinetics, where the reaction constant values is k = 1.6 105 s1. The pseudo-first-order reaction constant value measured is low. This result indicates that the process is inefficient. This fact is produced by the small specific surface area of the applied catalyst, since the reaction mechanism implies the adsorption of both phenol and the radical in the catalyst surface. Thereafter, in order to develop an efficient process it is necessary to obtain catalyst particles with less particle diameter. The particle diameter measured for the applied catalyst was 300 nm, and the specific area was 5 m2 g1. Then for particles of 10 nm, which can relatively easily obtained for other semiconductor catalysts, for example, ZnO and TiO2, the specific area could be around 150 m2 g1. Consequently, the reaction rate constant could be increased, to gain an efficient process. Besides, it is possible that rutile is not the best catalyst for the present reaction. In this regard, we are now in the process of testing the reaction with other semiconductor catalysts, such as those
previously noted, i.e., ZnO and TiO2 (anatase) with specific surface of 100–200 m2 g1. To conclude it is necessary to state that despite the inefficiency of the process, the methodology reported here is an original application of mechanochemistry, which can evolve into an efficient method for the degradation of organic compounds in wastewater, by the increase of the specific surface area of the semiconductor catalyst applied during the mechanical activation. 4. Conclusions In the present paper a novel process for the elimination of organic pollutants dissolved in water is proposed. Phenol aqueous solutions (100 mg L1) were mechanically treated for 0, 12, 18, 24, 36, 48, and 72 h in the presence of the rutile powder catalyst and without catalyst. Thereafter, the reaction products were analyzed with UV and GC/MS. It was shown that when the catalyst was not included in the process the phenol present in the solution was not affected. Conversely, when the catalyst was included in the batch reactor, phenol was degraded and the catalyst itself did not change. In the reaction products were found intermediates which are distinctive of the OH radical mechanism. Subsequently, this mechanism was proposed for the explanation of the obtained results. It was also shown that the reaction follows a pseudo-first-order kinetics, as should be, for the OH radical mechanism. The value measured for the pseudo-first-order reaction constant was very low, indicating that the reported process is inefficient. However, this problem could be solved applying catalysts consisting of particles with lesser diameters. Thereafter independent of this negative aspect, we are reporting a novel application of mechanochemistry, which could be developed as an energetically efficient method for the degradation of organic compounds in wastewater. Acknowledgments The authors gratefully recognize that the present research was, in part, financed by the US Department of Energy through the Massey Chair project at Turabo University. Also, the authors gratefully acknowledge the Materials Characterization Center at the Rio Piedras Campus of the University of Puerto Rico and the Department of Physics of the Cayey Campus of the University of Puerto Rico for the assistance provided in the characterization of the tested catalyst. Finally, we thank Mr. Edgard Mosquerra for his technical support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] Fig. 8. ln [C(t)/C(0)] versus t plot for the phenol degradation.
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