Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania

Applied Catalysis B: Environmental 39 (2002) 75–90

Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania Hinda Lachheb a,b , Eric Puzenat a , Ammar Houas b , Mohamed Ksibi b , Elimame Elaloui b , Chantal Guillard a , Jean-Marie Herrmann a,∗ a

b

Laboratoire de Photocatalyse, Catalyse et Environnement LPCE, (IFoS UMR CNRS No. 5621); Ecole Centrale de Lyon, B.P. 163, 69131 Ecully Cedex, France Equipe de Catalyse et Environnement, URECAP 99/UR/11-20 Ecole Nationnale d’Ingénieurs de Gabès (ENIG), Gabès, Tunisia Received 8 February 2002; received in revised form 31 March 2002; accepted 31 March 2002

Abstract The photocatalytic degradation of five various dyes has been investigated in TiO2 /UV aqueous suspensions. It was attempted to determine the feasibility of such a degradation by varying the chemical structures, either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or heteropolyaromatic (Methylene Blue (MB)). In addition to a prompt removal of the colors, TiO2 /UV-based photocatalysis was simultaneously able to fully oxidize the dyes, with a complete mineralization of carbon into CO2 . Sulfur heteroatoms were converted into innocuous SO4 2− ions. The mineralization of nitrogen was more complex. Nitrogen atoms in the −3 oxidation state, such as in amino-groups, remain at this reduction degree and produced NH4 + cations, subsequently and very slowly converted into NO3 − ions. For azo-dye (OG, MR, CR) degradation, the complete mass balance in nitrogen indicated that the central –N=N– azo-group was converted in gaseous dinitrogen, which is the ideal issue for the elimination of nitrogen-containing pollutants, not only for environmental photocatalysis but also for any physicochemical method. The aromatic rings were submitted to successive attacks by photogenerated OH• radicals leading to hydroxylated metabolites before the ring opening and the final evolution of CO2 induced by repeated subsequent “photo-Kolbe” reactions with carboxylic intermediates. These results suggest that TiO2 /UV photocatalysis may be envisaged as a method for treatment of diluted colored waste waters not only for decolorization, but also for detoxification, in particular in textile industries in semi-arid countries. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Titania; TiO2 ; Photocatalysis; Photocatalytic degradation; Dyes; Dye removal; Water decolorization; Water purification; Total mineralization; Alizarin S; Crocein Orange G; Methyl Red; Congo Red; Methylene Blue

1. Introduction

∗ Corresponding author. Tel.: +33-4-72-18-64-93; fax: +33-4-78-33-03-37. E-mail address: [email protected] (J.-M. Herrmann).

Waste waters generated by the textile industries, well implanted in Tunisia, are known to contain considerable amounts of non fixed dyes and especially of azo-dyes. It is well known that some azo-dyes and degradation products such as aromatic amines are highly carcinogenic [1]. A total of 15% of the total

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 0 7 8 - 4

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world production of dyes is lost during the dyeing process and is released in textile effluents [2]. The release of those colored waste waters in the ecosystem is a dramatic source of esthetic pollution, of eutrophication and of perturbations in the aquatic life. As international environmental standards are becoming more stringent (ISO 14001, October 1996), technological systems for the removal of organic pollutants, such as dyes have been recently developed. Among them, physical methods, such as adsorption [3], biological methods (biodegradation) [4,5] and chemical methods (chlorination, ozonation [6]) are the most frequently used. The traditional processes for treatment of these effluents prove to be insufficient to purify the important quantity of waste waters after the different operations of textile dyeing and washing. Some methods such as combined coagulation, electrochemical oxidation, active sludge have recently been investigated and proved to be adequate [7]. Other methods such as flocculation, reverse osmosis and adsorption on activated carbon have also been tested [8–10]. The drawbacks of these methods are mainly the creation of a more concentrated pollutant-containing phase. The processes by bacterial beds are less adapted because of the fluctuations of the wastewater composition [11,12]. However, the recent developments of chemical treatment of waste waters gave birth to an improvement of the oxidative degradation of the organic compounds dissolved or dispersed in aqueous media. Among the new oxidation methods called “advanced oxidation processes” (AOP), heterogeneous photocatalysis has appeared as an emerging destructive technology leading to the total mineralization of most of organic pollutants [13–19]. A quasi-exhaustive list of various families of organic pollutants which can be treated by photocatalysis has been given in [20]. In most cases, the degradation is conducted for dissolved compounds in water with UV-illuminated titania. The possible extents of the technique concern the irradiation source and the physical state of the pollutant. Recently, some works have reported the degradation of organic dyes induced by visible light by photosensitization [21–24]. The great present interest is to use solar light which is free and inexhaustible. The photocatalytic degradation pathway with the identification of the main degradation metabolites has already been established in our laboratories for three dyes (Methylene Blue, MB [25], indigo and indigo

carmine [26]). In the present article, it was attempted to determine the feasibility of the total degradation by a UV/TiO2 treatment of some dyes having different chemical structures, either anthraquinonic (Alizarin S (AS)), or azoic (Crocein Orange G (OG), Methyl Red (MR), Congo Red (CR)) or hetero-polyaromatic (MB).

2. Experimental 2.1. Materials Degussa P-25 titanium dioxide was used as the photocatalyst. It is mostly in the anatase form and has a BET surface area of 50 m2 /g corresponding to a mean particle size of ca. 30 nm. The five dyes were purchased from Fluka and used as received without further purification. Their solutions were prepared using water from a Millipore Waters Milli Q purification unit. 2.2. Apparatus Two types of Pyrex reactors opened to air were utilised. Reactor 1 (90 ml) has a bottom optical window of ca. 11 cm2 , through which the suspension was irradiated. Constant agitation of the solution was insured by a magnetic stirrer placed at right angle from the reactor basis. UV-irradiation was provided by a high pressure mercury lamp (Philips HPK-125 W). The IR beams were removed by making the irradiation pass through a 2.2 cm thick circulating-water cuvette equipped with either Pyrex filter transmitting wavelengths >290 nm or a corning glass 0.52 cut-off filter, transmitting wavelengths >340 nm. The photon flux of the UV-radiation reaching the reactor was measured to be 6.94×10−7 mol of photons/s for the Pyrex filter and 2.39 × 10−7 mol of photons/s for the 0.52 corning filter. Reactor 2 (1 l) was equipped with a plunging tube in which a Philips HPK 125 W lamp, identical to that used in reactor 1, was placed vertically. To avoid the heating of the solution, water was circulated through a cylindrical jacket, made of Pyrex and located around the plunging tube. The photon flux of the UV radiation reaching the exposed inner part of the reactor at λ > 290 nm was measured to be around 6 × 10−6 mol of photons/s.

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2.3. Procedure The volume of the aqueous solution of each dye introduced into Reactor 1 was 20 ml, to which 50 mg of powder TiO2 was added (2.5 g/l). In Reactor 2, the suspension contained 750 ml of solution and 375 mg of TiO2 (0.5 g/l). In both cases, the amount of titania has been adjusted for a full absorption of the incident photon flux. The degradations were carried out at 293 K and at different pH’s. The pH was adjusted using either NaOH or HNO3 . The suspension was first stirred in the dark for 60 min before irradiation to reach equilibrated adsorption as deduced from the steady-state concentrations. To determine the adsorption constants, different concentrations of dyes were used. 2.4. Analyses Before analysis, the aqueous samples were filtered through 0.45 ␮m millipores discs to remove

77

TiO2 agglomerates. The UV/VIS spectrophotometer used for the determination of dye disappearance kinetics was a “Safas Monaco 2000” UV/VIS spectrometer recording the spectra over the 190–750 nm. Calibration plots based on Beer–Lambert’s law were established relating the absorbance to the concentration. Each plot was determined at the maximum of absorbance of each dye given in Table 1. Anions and cations were analysed by HPLC using a Waters 501 isocratic pump, a Waters 431 conductivity detector, and an IC-PAK HR anion column (L = 50 mm, ∅ i.d. = 4.6 mm, ∅ particules = 10 ␮m) or a Vydac Cation IC40 (L = 50 mm, ∅ i.d.=4.6 mm). Eluents were, respectively, borate/gluconate at 0.9 ml/min and HNO3 2.5 mM at 1.5 ml/min. Total organic carbon (TOC) was determined by using a Bioritech (model 700) TOC analyzer. Chemical oxygen demand (COD) was made using acidic dichromate method with a Bioblock COD analyzer.

Table 1 Characteristics of the five dyes photocatalytically destroyed MW (g/mol)

λmax (nm)

ε (l/mol cm)

Methylene Blue (MB)

356

660

79.51 × 103

Orange G (OG)

350.33

495

19.61 × 103

Alizarin S (AS)

360.28

520

7.2 × 103

Methyl Red (MR)

269.3

540

5.92 × 103

Congo Red (CR)

696.68

510

18.67 × 103

Dye

Chemical formula

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3. Results and discussion 3.1. Adsorption of the five dyes on titania The kinetics of adsorption for the five dyes (BM, OG, AS, MR, CR) are represented in Fig. 1 for different initial concentrations ranging between 5 and 60 ppm indicated on the right-hand side of each curve. It can be observed that whatever be the dye and its initial concentration, the steady state of adsorption is reached within 1 h. Therefore, this time has been selected for the initial period in the dark previously to UV-irradiation at time tUV = 0 to make sure that the initial degradation initiates at the equilibrium of adsorption. The different values of the quantities adsorbed at steady state have been plotted in a Langmuirian plot 1/n(ads) = f (1/Ce ) (Fig. 2). The linearity of the transforms clearly indicate that the Langmuir isotherm is correctly observed, implying a monolayer adsorption model. From the data in Figs. 1 and 2, one could determine the maximum quantities qmax in mol/gcat of adsorbed dyes, the adsorption constants Kads and the areal density of adsorbed dye molecule in mol/m2 (the adjective “areal” refers to a surface area unit, as defined by Burwell in the IUPAC report in [27]). These values are reported in Table 2. The adsorption constants Kads vary from 2 × 103 to 18 × 103 l/mol. The higher value concerns that of MR. This can be attributed to the carboxylic substituent on one of the benzenic ring. One of us has already studied the photocatalytic degradation of the three isomers of chlorobenzoic acid and it was found that these acidic strongly adsorb at the surface of titania [28]. On the opposite, the weaker adsorption constant concerns CR. This can be explained by the large steric hin-

Table 2 Adsorption characteristics of the dyes

drance due to large aromatic ensembles, including one central biphenyl group and two symetric naphtalenic groups. The areal coverages are given in the last column in Table 2. They vary between 0.1 and 0.2 mol/nm2 . The influence of pH upon adsorption, in the range from 3 to 9, is presented in Fig. 3. For all dyes, except for OG, the increase of the pH favors their adsorption. The pH influences at the same time both the surface state of titania and the ionization state of ionizable organic molecules. For pH’s higher than the pzc of titania, the surface becomes negatively charged and it is the opposite for pH’s < pzc, according to the following equilibria: pH < pzc : Ti-OH + H+ ⇔ TiOH2 +

(1)

pH > pzc : Ti-OH + OH− ⇔ TiO− + H2 O

(2)

Since MB is a cationic dye (see Table 1), it is conceivable that at high pH’s, its adsorption is favored on a negatively charged surface. By contrast, OG has its adsorption inhibited by high pH’s because of its negatively charged sulfonate –␸-SO3 − function. The other dyes, having several functional groups, have a resulting behavior similar to that of MB. 3.2. Photocatalytic degradation of the five dyes After checking that no detectable degradation occurred without titania nor UV-irradiation, the photocatalytic disappearance of the five dyes was performed according to the procedure indicated in the experimental section. The kinetics are given in Fig. 4. All reactions followed an apparent first-order verified by the linear transforms ln C0 /C = f (t) illustrated in the insert in Fig. 4. The slopes give the apparent rate constants listed in Table 3. The initial rates of disappearance in ␮mol/l min are in the following order:

Dye

qmax (␮mol/g)

Kads × 10−3 (l/␮mol)

nads (mol/nm2 )

MR > MB > OG > AS > CR

Methylene Blue Orange G Alizarin S Methyl Red Congo Red

11.7 9.14 12 6.29 18.24

6.65 5.64 4.18 17.715 2.0

0.14 0.11 0.145 0.075 0.22

When the initial rate is expressed in ppm/min, the order is different:

qmax : maximum quantities adsorbed per gcat ; Kads : adsorption constants; nads : areal density of adsorbed dye molecules (in mol/nm2 ).

CR > MB > AS ≈ OG > MR This ponderal classification favors CR because of its smaller molar weight. This observation underlines the

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Fig. 1. Kinetics of adsorption of dyes. Experimental conditions: natural pH; T = 30 ◦ C; m(TiO2 ) = 50 mg; V = 20 ml.

relativity of the expression of the results. The first one is more academic with the right units for kinetics, while the second one is more concerned with the application point of view.

3.2.1. Influence of pH Since dyes to be degraded can be at different pH’s in colored effluents, comparative experiments were performed at three pH values: 3, 6 and 9. The pH had

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Fig. 2. Linear transforms of the Langmuir isotherms deduced from results in Fig. 1. Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 50 mg; V = 20 ml; T = 30 ◦ C; natural pH.

Fig. 3. Effect of pH on the mol number of dyes adsorbed. Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 50 mg; V = 20 ml; T = 30 ◦ C.

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Fig. 4. Kinetics of the photocatalytic degradation of the dyes. In the insert: first-order linear transforms ln C0 /C = f (t). Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 375 mg; V = 750 ml; T = 30 ◦ C; natural pH’s.

a little influence upon the kinetics of disappearance. The results are represented under the form of linear transforms log(kapp ) = f (log[H+ ]) = f (−pH) (Fig. 5). This representation enables one to determine the kinetic partial order with respect to proton concentration by measuring the slopes of the curves. Despite the dilated scale of the y-axis, the slopes actually have tiny values <0.1 in absolute value. This clearly indicates that protons do not intervene in the rate limiting step Table 3 Kinetic data of the photocatalytic degradation of the five dyes Dye

kapp × 102

r0 (␮mol/l min)

r0 (ppm/min)

Methylene Blue Orange G Alizarin S Methyl Red Congo Red

5.30 4.60 4.50 5.74 2.67

3.5 3.05 3.01 3.60 1.90

1.24 1.06 1.08 0.97 1.32

of the photocatalytic system, in agreement with other results on pollutant removal. However, although the partial kinetic orders with respect to [H+ ] are small, their sign are significant and informative. They are all negative, except for OG (Fig. 5). This has to be related to the adsorption data in Fig. 3. OG has only a single substituent, which is a sulfonate function, totally ionized in water. At pH > pzc, the surface is negatively charged (Eq. (2)) and repels R–SO3 − ions. This explains the negative effect of pH both on adsorption and on the sign of the kinetic order with respect to [H+ ]. 3.3. Kinetics of the total mineralization of the five dyes The kinetics of the total mineralization of the five dyes has been followed using two overall techniques, the disappearance of the chemical oxygen demand (COD) and that of the total organic carbon (TOC),

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Fig. 5. The log–log plot of kapp = f [H+ ]. Conditions: C0 = 84.2 ␮mol/l; T = 30 ◦ C; m(TiO2 ) = 375 mg; V = 750 ml.

both techniques being complementary for expressing the detoxification level of water. 3.3.1. Kinetics of COD disappearance The kinetics isotherms of COD disappearance are given in Fig. 6. For all five dyes, it is shown that COD has totally disappeared in <3 h. The initial values plotted in the y-axis are in agreement with the chemical formulae given in Table 1 and with the stoichiometric coefficients expressed in Table 4. AS has the smallest COD, which is the first eliminated because (i) it has the smaller number of carbon atoms (n = 14), (ii) it contains only two heteroatoms (including oxygen yet) and (iii) the other hetero-atom (sulfur) has already its highest final oxidation state (+6). 3.3.2. Kinetics of TOC disappearance The kinetics isotherms of TOC disappearance [TOC] = f (t) are given in Fig. 7. For all five dyes, it is shown that TOC has totally disappeared in <2 h for MR and in <6 h for CR. The initial TOC values plotted in the y-axis are in agreement with the stoichiometries of the molecule degradations expressed in Table 4. AS has the smallest TOC because of its smallest number of carbon atoms (n = 14), but its TOC is not that which is first eliminated. The fastest and most easily eliminated TOC is that of MR. This can be explained

by taking into account that MR molecules contain only two (separated) benzenic rings and a carboxylic group, ready for a first carbon atom elimination via a “photo-Kolbe” reaction (4) resulting from the neutralization of the carboxylic group by a hole [29]: TiO2 + hν → e− + p+

(3)

R–COO− + p+ → R• + CO2

(4)

Such easy reactions have already been observed for polycarboxylic [30] and chloro-benzoic [28] acids. By contrast, all other molecules exhibit no carboxylic substituents and, in addition, possess more complex structures with two or three aromatic rings associated in naphtalenic, bi-phenylic or anthraquinonic groups (Table 1). Both parameters (COD and TOC), which directly evaluate the pollution level of an aqueous solution, do not exhibit similar disappearance patterns (compare Figs. 6 and 7). This could be accounted for by the influence of the different molecular structures of the dyes (Table 1) on their reactivities with OH• radicals which constitute the main oxidizing agents generated in UV-irradiated aqueous suspensions of titania, and which are produced as follows: + H2 O + (TiO2 ) → OH− (ads) + H(ads)

(5)

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Fig. 6. Kinetics of COD disappearance. Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 375 mg; V = 750 ml; natural pH’s.

+ • OH− (ads) + p → OH(ads)

(6)

From the toxicological point of view, TOC analysis seems more accurate and appropriate for evaluating the decontamination of polluted waters containing organics since it takes into account all the residual carbon-containing metabolites. 3.3.3. Kinetics of pH variations during the mineralization reaction According to the stoichiometry of the overall oxidation reactions listed in Table 4, the pH of the reaction medium is expected to decrease. However, pH in

a UV-irradiated titania slurry is a rather complex parameter since it governs (i) water dissociation equilibrium, (ii) the surface charge of titania with respect to its pzc (Eqs. (1) and (2)) and (iii) the ionization state of the organic reactants and of their metabolites. Its temporal variations are given in Fig. 8, including the previous adsorption period in the dark. Two groups can be distinguished, according to their natural initial pH’s: (i) CR, MB, OG with high pH values and (ii) MR and AS with lower values. The first group gives decreasing pH’s as expected from the reactions in Table 4. The second group (MR and AS) with lower pH values give apparent low decrease of pH. However,

Table 4 Stoichiometric equations of the dye total oxidation Methylene Blue (MB)

C16 H18 N3 S+ +

Orange G (OG)

C16 H11 N2 O3 S− + 20O2 → 12CO2 + 2NO3 − + SO4 2− + 3H+ + H2 O

Alizarin S (AS)

C14 H7 O7 S− + 14O2 → 14CO2 + SO4 2− + H+ + H2 O

Methyl Red (MR)

C15 H15 N3 O2 +

Congo Red (CR)

C32 H22 N6 O6 S2 2− +

51 2 O2

43 2 O2

→ 16CO2 +3NO3 − + SO4 2− + 6H+ + 6H2 O

→ 15CO2 + 3NO3 − + 3H+ + 6H2 O 91 2 O2

→ 32CO2 + 6NO3 − + 2SO4 2− + 8H+ + 7H2 O

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Fig. 7. Kinetics of TOC disappearance of five dyes at natural pH. Cdye = 84.2 ␮mol/l; V = 750 ml; [TiO2 P25] = 0.5 g/l; natural pH.

because of the logarithmic scale of pH, the initial decrease for tUV < 20 min appears small. In addition, AS and MR have the lowest stoichiometric coefficients of H+ formation (Table 4). The subsequent increase of

pH for tUV > 30 min could be related to subsequent proton-consuming reactions, mainly the formation of ammonia as ammonium ions arising from the degradation of amino-groups as shown further.

Fig. 8. Kinetics of pH changes during the photocatalytic dye degradations. Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 375 mg; V = 750 ml; T = 30 ◦ C.

H. Lachheb et al. / Applied Catalysis B: Environmental 39 (2002) 75–90

3.3.4. Kinetics of inorganic ions evolution during the mineralization reaction Besides TOC elimination and CO2 evolution, mineralization implies the appearance of inorganic products, mainly anions, since hetero-atoms are generally converted into anions in which they are at their highest oxidation degree [13,16]. 3.3.5. Evolution of sulfate ions The temporal evolution of SO4 2− ions is presented in Fig. 9a for the dyes containing only one S atom (AS, BM and OG) and in Fig. 9b for CR, which contains two S atoms. In Fig. 9a, the relative reactivities of the three dyes are in the order: MB < OG < AS. This can be easily explained from the developed formulae in Table 1. AS has its sulfonyl group located on the same dihydroxybenzene ring, more reactive than the other parts of the molecule. For OG the sulfonyl group is linked to a less reactive naphtalenic ensemble. Eventually, in MB sulfur is involved in a =S+ –aromatic link, yet less reactive. Additionally, it is in the +4 oxidation state and not in the +6 one as in R–SO3 − . In all cases, the initial slopes are positive, indicating that SO4 2− ions are initial products, directly resulting from the initial attack on the sulfonyl group. Some other sulfate ions can be evolved from the degradation of SO3 − -containing intermediates. The release of SO4 2− can be accounted for by an initial attack by a photo-induced OH• radical: OH− + p+ → OH•

(6)

R–SO3 − + OH• → R–OH + SO3 •−

(7)

SO3 •− + OH− → SO4 2− + H•

(8)

The attack of sulfonate groups would be favored if the molecule is adsorbed with its SO3 − group orientated to the surface. The hydrogen atom H• generated in Eq. (8) can subsequently react in different possible ways: (i) release of one electron to the solid to generate a proton necessary to the charge balance − H• + (TiO2 ) → H+ + eTiO 2

(9)

(ii) reaction with other radicals OH•

+ H•

85

(iii) reaction with a neutral functional group such as an amino-group as seen further. Surprisingly, the sulfur-containing dyes did not release the expected stoichiometric quantities of sulfate. It has already been observed that sulfate ions can remain partially adsorbed at the surface of titania [16,26]. However, AS has reached the expected stoichiometric quantity of sulfate (Fig. 9a). This peculiarity has not been yet explained. 3.3.6. Evolution of nitrogen-containing final product The kinetics of NO3 − and NH4 + release in water are given in Fig. 9c for MB and MR and in Fig. 9d for OG. For MB, the nitrogen mass balance, obtained by adding both ion concentrations, almost corresponds to the final expected stoichiometric value. The total mineralization of the nitrogen heteroatoms contained in the formula (Table 1) as inorganic ions (NH4 + and NO3 − ) is not unexpected. The nitrogen atoms in the two amino-groups can lead to NH4 + ions by successive attacks by H• atoms R–NH2 + H• → R• + NH3 +

NH3 + H → NH4

+

(12) (13)

H• atoms can be generated either by other redox reactions such as reaction (8) or by photoreduction of protons (Eq. (14)) H+ + e− → H•

(14)

or by the photo-Kolbe reaction of formic acid indicated further in Eq. (22). In BM, there are two dimethylamino groups. They will generate methyl radicals, which will be oxidized into methanol, then formaldehyde and eventually formic acid decomposed via a photo-Kolbe reaction into CO2 and H• the latter species being involved in ammonium formation (Eqs. (12) and (13)). –␸-N(CH3 )2 + H• → –␸• + H–N(CH3 )2

(15)

H–N(CH3 )2 + H• → CH3• + CH3 –NH2

(16)

CH3 • + OH• → CH3 OH

(17)

CH3 OH + OH• → H2 O + • CH2 OH

(18) (19)

→ H2 O

(10)

• CH

R• + H• → R –H

(11)

or

2 OH

→ H–CHO + H•

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Fig. 9. (a) Kinetics of sulfate ion evolution during the photocatalytic degradation of single S atom containing dyes (MB; OG; AS). Conditions: C0 = 84.2 ␮mol/l; T = 30 ◦ C; m(TiO2 ) = 375 mg; V = 750 ml; (b) kinetics of sulfate ion evolution during the photocatalytic degradation of two S atoms-containing CR dye. Conditions: C0 = 84.2 ␮mol/l; T = 30 ◦ C; m(TiO2 ) = 375 mg; V = 750 ml; (c) kinetic of ammonium and nitrate ion evolution issued from the photocatalytic degradation of MB and MR dyes. Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 375 mg; V = 750 ml; T = 30 ◦ C; (d) kinetics of ammonium and nitrate ion evolution issued from the photocatalytic degradation of OG dye. Conditions: C0 = 84.2 ␮mol/l; m(TiO2 ) = 375 mg; V = 750 ml; T = 30 ◦ C.

H. Lachheb et al. / Applied Catalysis B: Environmental 39 (2002) 75–90

Fig. 9. (Continued).

87

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H. Lachheb et al. / Applied Catalysis B: Environmental 39 (2002) 75–90

H. Lachheb et al. / Applied Catalysis B: Environmental 39 (2002) 75–90 • CH

2 OH

+ OH• → CH2 (OH)2 → H–CHO + H2 O

89

4. Conclusions

(19 ) H–CHO + OH• → H–CO• + H2 O

(20)

H–CO• + OH• → H–COOH

(21)

H–COOH + p+ → H+ + CO2 + H• ( photo-Kolbe reaction [29])

(22)

In addition, the (=N–) atom belonging to the central aromatic ring will follow the same fate. By comparing the initial rates, NH4 + appears as a primary product with respect to NO3 − [25,26,31]. Except BM, the other dyes (Figs. 9c, d and 10) do not provide any nitrogen mass balance in the aqueous phase. This is not unexpected since OG, MR and CR are di-azoic dyes. Various experiments have been done to put in evidence gaseous dinitrogen. Mass-spectrometry analyses of the gas phase in a vacuum-tight static photocell containing a RC-impregnated titania sample in humid oxygen revealed the formation of dinitrogen. Similarly, GC analyses in a closed photoreactor indicated increasing amounts of N2 as a function of UV-irradiation time until reaching the overall mass balance in nitrogen (Fig. 10). The formation of N2 in diazoic dyes can be accounted for by the same processes responsible for NH4 + formation: R–N=N–R + H• → R–N = N• + R –H

(23)

R–N=N•

(24)

→

R•

+ N ≡N

Radicals R• subsequently follow the same degradation process described in Eqs. (17)–(22). There appears that the fate of nitrogen strongly depends on its initial oxidation degree. When present in the −3 state as in amino groups, nitrogen spontaneously evolves as NH4 + cations with the same oxidation degree, before being subsequently and slowly oxidized into nitrate. In azo-dyes, each nitrogen atom is in its +1 oxidation degree. This oxidation degree close to zero combined with the existence of a –N=N– double bond in the initial pollutant molecule favors the evolution of gaseous dinitrogen by the two step reduction process expressed by Eqs. (23) and (24). N2 evolution constitutes the ideal case for a decontamination reaction involving totally innocuous nitrogen-containing final product.

Five different dyes containing three different types of aromatic structures (anthraquinoic, azoic and heteropolyaromatic) were successfully not only decolorized, but also totally degraded and mineralized. The organic part was totally converted into CO2 as testified by the elimination of both COD and TOC. Heteroatoms were released in innocuous diluted inorganic final products. Sulfur, originating either from an aromatic =S+ -link in MB or from sulfonic groups in AS, OG and CR, is released as sulfate. Nitrogen has a more complex behavior. It is first released as ammonium when having the −3 oxidation state before its slow oxidation into nitrate. Interestingly, when present in an azo-link, it evolves as N2 , which represent an ideal case as in deNOx catalysis. The resulting water is not only decolorized but also detoxified when submitted to such a treatment. Photocatalysis appears as a valuable treatment for purifying and reusing colored aqueous effluents in semi-arid countries such as Tunisia, where part of this work has been done and which is rich both in textile industries and in solar energy. Exploratory experiments with the solar pilot plant in Almeria (Spain) were found particularly promising.

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