Photocatalytic Degradation of Azo Dyes. Pilot Plant Investigation

Andronic , L.; Andrasi , D.; Enesca , A.; Visa , M.; Duta , A. The influence of titanium dioxide phase composition on dyes photocatalysis J. Sol–Gel...
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Photocatalytic Degradation of Azo Dyes. Pilot Plant Investigation Andrea Petrella, Giancarlo Boghetich, Mario Petrella, Piero Mastrorilli, Valentina Petruzzelli, and Domenico Petruzzelli* Dipartimento di Ingegneria Civile, Ambientale, del Territorio, Edile e di Chimica (DICATECh), Politecnico di Bari, 4, Via E. Orabona, 70125 Bari, Italy ABSTRACT: The UVB-induced photocatalytic degradation of Methyl Red and Methyl Orange (azo dyes used in the textile industry) containing solutions was carried out by the use of a laboratory-scale pilot plant where the catalyst, TiO2 (anatase), was immobilized at the bottom of a channel through which the liquid was recirculated under UVB irradiation. The plant was preliminarily characterized hydrodynamically, i.e., flow-rate, hydraulic gradients, and residence time. Photodegradation kinetics were followed by UV−vis absorption measurements of the residual dye concentration in the liquid-phase, and the synergistic effects of the catalyst and radiation in promoting the abatement of dyes was demonstrated in the concentration range 0.3−5.0 mg/L. Kinetic data were correlated by the use of first-order (or pseudo-first-order) models up to the concentration range 0.7 mg/ L; at higher concentrations, zero-order models (pure catalytic control) better correlated the experimental data. Photocatalytic degradation of Methyl Red was faster than Methyl Orange, possibly due to the Coulomb repulsion of the negatively charged sulfonate functionalities present on this latter compound. A better hydrodynamic of the liquid recirculating in the channel, i.e., higher flow rate (lower contact time), associated with an improved surface catalyst renovation and a higher frequency of exposition of the substrate to the UVB radiation, together with an improved oxygen dissolution in the liquid-phase, played a positive role in the overall kinetic performance. applied in suspensions in most applications,15−17 and recently, photocatalysts have been immobilized as films deposited onto solid supports to overcome catalyst recovery and recycle when used in large WWTPs, which, otherwise, would impair the quality of the treated water.11,18,19 An innovative laboratory-scale experimental unit was used for the photocatalytic degradation of Methyl Red (MR) [2-(N,Ndimethyl-4-aminophenyl)azobenzenecarboxylic acid] and Methyl Orange (MO) [p-[[p-(dimethylamino)phenyl]azo]benzenesulfonic acid] in the salt form (Na+), two typical, hazardous, organic micropollutants present in the textile industry wastewater. Experimental tests were based on the continuous recirculation of a solution containing the above pollutants in tap water through a leaning channel that was layered with the TiO2 catalyst. A cement mortar including titanium dioxide (50% cement/50% titania w/w) was spread onto the surface of the channel for an average thickness 0.5 mm, determined by the use of a micrometer, for an approximate TiO2 surface density of 10−2 mg/cm2 and an overall TiO2 exposed amount exceeding 23 mg. The flowing liquid was thoroughly irradiated with UVB light of known intensity and wavelength. Photodegradation kinetics was followed by UV−vis absorption measurements of the liquid-phase residual dye concentrations along time, and the obtained data were correlated to credited theoretical models. Comparison of the corresponding kinetic trends of the two substrates was carried out at variable liquid-phase concen-

1. INTRODUCTION Among other advanced oxidation processes (AOPs), considerable attention is lately focused on photocatalysis as a promising technology for the implementation of large-scale removal of persistent organics present at trace levels in water and wastewater.1−3 In this context, conventional wastewater treatment plants (WWTPs) should provide, beside removal of biodegradable organic matter and pathogens, also removal of biorefractory pollutants to minimize their sanitary and environmental impacts. AOPs are primarily based on the reactivity of radicals as redox agents, leading to persistent pollutants mineralization.1−3 Within AOPs, photocatalysis is a promising research area with specific reference to water and wastewater treatment.4−9 In the absence of a radical promoter, such as metals or metal oxides, photolysis may not be economically feasible in large-scale installations, provided that the same catalysts do not influence the quality of the final effluent. UV-enhanced photolysis, i.e., catalyzed by the presence of TiO2 (anatase), is a typical example extensively investigated, at benchtop laboratory scale level, due to the ability of the catalyst in assisting the quantitative oxidation of a variety of organic contaminants.5−7,10−13 When TiO2 absorbs radiation exceeding its energy gap, electron−hole pairs are formed at the liquid−solid interface. The electron−hole pairs induce formation of radicals promoting redox reactions at the TiO2 active surface.5−7,10−14 Anatase and rutile, the most common crystalline forms of TiO2, are used to the purpose. Although rutile (∼3.0 eV) shows a lower band gap as compared to anatase (∼3.2 eV), the latter shows a better photocatalytic activity as a result of a significantly higher exposed surface area to the liquid phase with a corresponding improved number of adsorbed radicals acting as reaction activators. Oxide powders are commonly © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2566

October 17, 2013 January 7, 2014 January 25, 2014 January 26, 2014 dx.doi.org/10.1021/ie403506s | Ind. Eng. Chem. Res. 2014, 53, 2566−2571

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Figure 1. Laboratory-scale pilot plant.

trations and hydrodynamic conditions, i.e., flow-rate, contact time, and water speed of the liquid through the channel.

2. EXPERIMENTAL SECTION 2.1. The Experimental Unit. Figure 1 shows a schematic view of the laboratory scale unit adopted to carry out kinetic experiments. The unit includes (1) a 120 L upper reservoir, equipped with manifolds for coarse control of the hydraulic load of the liquid into the channel, and at constant hydraulic load, fine control of the flow-rate is obtained by proper insertion of a variable thickness plate (1.5−1.0 mm; 11 × 70 cm) in a slot placed at the bottom of the upper reservoir; (2) a 120 L bottom reservoir, provided with an overflow and a piezometric tube, is available for the collection of the solution flowing through the channel, and backup of the recirculating liquid, with demineralized water, is ensured throughout the experiments in case of unsustainable evaporation; (3) a Ushaped (15 cm width, 185 cm length) channel, connecting the reservoirs, layered with a 0.5 mm thick cement mortar (50% cement/50% titania w/w), determined by the use of a micrometer, for an approximate TiO2 surface density of 10−2 mg/cm2 and an overall TiO2 exposed amount exceeding 23 mg (The XRD pattern of commercial TiO2 is shown in the Figure 2A. XRD patterns exhibited strong diffraction peaks at 25° and 48° indicating TiO2 in the anatase phase. All peaks are included in the standard spectrum.); (4) a recirculation pump through reservoirs (model CPm 130, Pedrollo, Milan, Italy; 0.37 kW, 230 V, 50 Hz); (5) three low-pressure UVB lamps (40 W each) from Vilber-Lourmat, France, with a peak wavelength at 312 nm (The emission spectrum of UVB lamp is reported in the inset of Figure 2A.); and (6) positioning bolts for fine control of the hydraulic gradient (leaning) of the channel. Preliminary characterization of the unit was carried out to evaluate the pilot plant operative parameters such as the flow rate, the water speed (contact time), the active irradiated length, and width and thickness of the liquid flowing through the channel. The relationship between flow rate, Q, and the section of water flowing through the channel, A, allows for evaluation of the water speed, v20,21

Figure 2. (A) XRD pattern of TiO2. Inset: Emission spectrum of the UVB lamp. (B) MR and MO UV−vis absorption spectra. CMO, CMR = 5 mg/L.

v = Q /A

(1)

A was evaluated by A = lchw, with lc being the channel length and hw being the average hydraulic load (Table 1). 2.2. Materials and Methods. Titanium dioxide, from Adriatica Legnami s.r.l., with an average grain size of 0.15 μm, a 2567

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Table 1. Kinetic Tests and Experimental Conditions Adopteda

test

influent concn (mg/L)

TiO2 surface amount (mg)

flow rate (L/s)

width of the liquid (cm)

depth of the liquid (cm)

running time (s)

light (120 W)

1 2 3 4 5 6 7

0.3 0.7 1.0 5.0 0.7 0.7 0.7

23 23 23 23 23 0 23

0.066 0.066 0.066 0.066 0.066 0.066 0.147

14 14 14 14 14 14 15

0.65 0.65 0.65 0.65 0.65 0.65 0.79

17.9 17.9 17.9 17.9 17.9 17.9 10.5

on on on on off on on

a

Active length of the channel = 140 cm, Vsol = 60 L, hydraulic load = 13.5 cm.

specific gravity of 3.85 g/cm3, corresponding to an apparent density (pressed) exceeding 0.7 g/cm3, was adopted throughout experiments. Methyl Orange and Methyl Red, pure chemicals from Sigma Aldrich, were used to prepare the synthetic solutions in tap water (0.3−5 mg/L, pH ≈7.2) to simulate industrial wastewater. UV−vis absorption spectroscopy was adopted for determination of the residual dye concentrations in the liquid-phase throughout the kinetic experiments, by the use of a scanning spectrophotometer (model UVIKON 942 from Kontron Instruments).

3. RESULTS AND DISCUSSION MR and MO UV−vis absorption spectra in water are shown in Figure 2. Specifically, MR spectrum shows a 570 nm peak in the more concentrated solutions, a larger band at 428 nm corresponding to π→π* of the dimethylamino electron donors, and a final 260 nm peak corresponding to the π→π* transitions of the aromatic rings. MO shows similar peaks corresponding to the π→π* transitions of the dimethylamino electron donors (470 nm) in the visible range and a secondary peak at 270 nm determined by π→π* transitions of the aromatic rings. Kinetic trends of the degradation reaction, by the evaluation of the residual dye concentrations in the liquid-phase, were monitored as a function of the irradiation time. Half-times of photodegradation were determined from the experimental kinetic curves. Table 1 summarizes experimental kinetics carried out under the indicated conditions. No leaching of TiO2 moieties was detected along the experiments, as titania was tightly immobilized into the cement mortar. Figure 3A shows the kinetic trends respectively carried out under the influence of both irradiation and catalyst (test 2); without irradiation, with catalyst (test 5); and without catalyst under irradiation only (test 6). The synergistic effect of the catalyst and UV radiation in promoting dye removal (photodegradation + sorption onto the catalyst surface) was demonstrated, whereas the effects of simple photolysis and catalysis taken alone are apparently minimal. Removal of the substrates in the absence of UV radiation may only be attributed to surface sorption onto the catalyst surface, reaching a plateau at surface saturation. It is likely that photolysis alone may occur at lower rates (longer contact times) till quantitative degradation. Similar results were obtained for Methyl Orange. As a consequence of the UV radiation, the electrons present in the valence band of titanium dioxide are excited and the

Figure 3. (A) Kinetic trends for photocatalysis (UV + TiO2), photolysis (UV irradiation, no catalyst), catalysis (TiO2, no UV radiation), CMR = 0.7 mg/L; Q = 0.066 L/s. (B) Kinetic trend for test 2. (C) ln C0/C vs t correlation (test 2). Inset: Quenching of the UV− vis MR spectrum during degradation kinetic.

excess energy promotes their jump to the conduction band, thus inducing formation of the electron−hole pairs (e‑, h+). The positive hole (h+) on the titanium dioxide surface breaks apart the water molecule to form hydroxyl radicals, OH•, whereas the surplus electrons (e‑) lead to formation of the super oxide anion radical (O2•−). Both radicals react with the organic molecules until quantitative (partial) mineralization.5,8,9 Photocatalytic kinetics generally follow the Langmuir− Hinshelwood (LH) model:5,13,22,23 − 2568

dC kKC = dt 1 + KC

(2)

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where k (mg/L/min) is the rate constant including various parameters, such as the mass of catalyst and the intensity of the UV radiation. K (L/mg) is the adsorption constant onto the catalyst surface. At low initial concentrations of the substrate, C (mg/L), the term KC may be disregarded with respect to 1 and the rate (eq 2) may be approximated to a simpler pseudo-first order22−24 −

dC = kKC = kappC dt

(3)

where kapp is the apparent rate constant of the resulting pseudofirst-order relationship, which after integration is ⎛C ⎞ −ln⎜ 0 ⎟ = kappt ⎝C⎠

(4)

Figure 3B shows the exponential decay of MR obtained from test 2. The best fit of the above data according to the pseudo first-order model is shown in Figure 3C. The slope of the linear correlation represents kapp. The inset of Figure 3C shows the quenching and blue shift of the UV−vis MR spectrum peak during the kinetic reaction (photobleaching). It is apparent that MR and MO kinetic trends are comparable and in agreement with literature data.23,25−33 It has to be considered, however, that in our case the molar ratio (TiO2/ substrate) is quite low (∼2), as in the literature the above ratio is at least 2 or 3 orders of magnitude higher, with a stronger role played by the catalyst and corresponding faster kinetics. Moreover, reference data were based on nanostructured semiconductor suspensions with an average particle size 1 order of magnitude lower than the particles used in the present work, thus exposing a higher specific surface area to the liquid phase.23,25−33 The influence of the initial dye concentration on the bleaching process was studied in the range 0.3−5.0 mg/L. The fastest photodegradation rate was observed at 0.7 mg/L with a significant decrease at lower (0.3 mg/L) as well as at higher concentrations (1.0−5.0 mg/L) (Figure 4A). On these premises, it is likely that the pseudo-first-order L−H model is applicable till 0.7 mg/L, when, at higher concentrations, a conversion of the reaction kinetics toward a pure catalytic zeroorder mechanism is observed. At higher concentrations (1.0− 5.0 mg/L), indeed, no improvements of the kinetic trends are observed for MR (this is not the case for MO; see the inset in Figure 4A) after a possible saturation of the catalyst active groups present on the TiO2 surface.7,12 In this latter context it may be assumed that the ionic nature of MO as compared to MR may sensibly influence sorption performance on the catalyst surface with consequent kinetic differences between the substrates. Two consecutive phenomena control kinetic performance at the catalyst surface: (a) preliminary formation of the radicals (hydroxyl and superoxide) based on the semiconductor properties and (b) substrate immobilization (sorption onto the catalyst surface) for best efficiency (orientation) of the reactants (radicals) toward reagent (substrate). Needless to say, as the initial dye concentration increases, the hitting frequency of the dye molecules with the reactive radicals (OH•, O2•−) increases in the liquid phase, thus leading to an overall enhancement of the kinetic performance. On the basis of the pseudo-first-order L−H model, an improvement of kinetic performance should be observed at higher substrates concentrations, unless the catalyst active groups would be

Figure 4. (A) Photodegradation kinetics of MR in the concentration range 0.3−5.0 mg/L. (Q = 0.066 L/s). Inset: Photodegradation kinetics of MO in the concentration range 0.3−5.0 mg/L. (Q = 0.066 L/s). (B) Photobleaching of MR sample.

oversaturated by the massive presence of the dye molecules, thus occupying preferentially the active sites in substitution of the radicals, when a pure catalytic control (zero-order kinetic model) would be overcome,22 as observed from the tests at 1.0 and 5.0 mg/L in Figure 4A. Moreover, at higher substrate concentrations, a sensible UV-screening effect operated by the dye molecules onto the catalyst surface may be assumed, and as a consequence, a significant amount of the UV radiation may be absorbed by the molecules themselves rather than by the TiO2 particles for activation. This latter effect sensibly reduces titania catalytic efficiency.24 As the initial concentration of the dye increases, additional catalyst surface is needed for degradation to occur.34 MR degradation is faster than that of MO (Figure 5); this result may be associated with the pH of the solution thus influencing ionic speciation of the substrate as well as that of the catalyst surface. It is known that pH may sensibly affect sorption phenomena of dye molecules at TiO2 active functional groups as the surface charge of titania after hydrolysis of the hydrated hydroxyl groups.19,35 At pH higher than the point of zero charge (PZC), TiO2 shows negatively charged functionalities and vice versa according to the following equilibria:36−38 Ti−OH + H+ ↔ Ti−OH 2+ −

−

Ti−OH + OH ↔ Ti−O + H 2O

(5) (6)

Also speciation of the dye molecule is strongly influenced by the pH, in consideration of the ionic nature of MO as compared to the nonpolar structure of MR. In this context, slower kinetics (lower kapp) showed by MO may be explained in terms of the Coulombic repulsion of the 2569

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that a better hydrodynamic of the system definitely improves oxygen dissolution and the availability of superoxide ions ready for degradation reaction.

4. CONCLUSIONS Examination of the UV-induced photocatalytic degradation kinetics of Methyl Red and Methyl Orange was carried out by an innovative laboratory scale unit. TiO2 (anatase) was immobilized at the bottom of an inclined channel and the dye solution was exposed to the UVB radiation while the liquid was recirculated. The photocatalytic activity of the system was demonstrated. Kinetic data were correlated by the use of firstorder (or pseudo-first-order) models up to 0.7 mg/L; at higher concentrations, zero-order models (pure catalytic control) better correlated the experimental data. Methyl Red degradation was faster than that of Methyl Orange because the pH of the liquid phase induced a better electrostatic surface interaction (sorption) of one substrate (MR) as compared to the other (MO). Higher flow rates, i.e., increased recirculation of the liquid in the channel, has a stronger influence on the overall kinetic performance as compared to, for example, contact time taken alone, due to the simultaneous effects of (a) the larger surface area of the catalyst exposed to the liquid phase, (b) the larger volume of liquid exposed to the UV radiation and the catalyst, and (c) the higher oxygen dissolution and availability of superoxide ions ready for degradation reaction

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +39(0)805963777. Fax: +39(0)805963635. E-mail: d. [email protected].

Figure 5. (A) Comparison of MR and MO photodegradation kinetics. Q = 0.066 L/s, [dye] = 0.7 mg/L. (B) Influence of the flow rate on the photocatalytic degradation for both dyes. [dye] = 0.7 mg/L.

Notes

The authors declare no competing financial interest.

■

sulfonate functionalities to the negatively charged functionalities present on the catalyst surface, and accordingly, the photocatalytic activity of MO reached a maximum in acidic conditions.19,22 MR is likely to be sorbed by weak (van der Waals) interactions with the hydroxyl and amino functional groups present on the dye molecule,19,22 thus resulting in a better interaction and, consequently, in the overall kinetic performance. As a final remark, it has to be considered that at higher flow rates degradation kinetics is faster (tests 2 and 7, Figure 5B; Table 2). Although, in principle, an apparently shorter contact Table 2. MR and MO kapp as a Function of Flow Rate Methyl Red Q(L/s) 0.066 0.147

Methyl Orange −1

kapp (min ) −4

1.53 × 10 2.18 × 10−4

Q(L/s)

kapp (min−1)

0.066 0.147

1.26 × 10−4 1.97 × 10−4

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