1. Introduction
Water, the source of life, represents the essential raw material for our planet. Consumption of this resource has increased considerably in recent decades due to population growth, industrialization, the intensification of agriculture and changes in consumption patterns. In return, a sharp increase in the volumes of urban, agricultural, and industrial wastewater discharges has accompanied this development [1] [2]. Unfortunately, the medium for discharge of all these types of waste-waters is frequently the environment and particularly watercourses. The increase in anthropogenic activities and textile industries constitute a source of water pollution [3] [4]. This pollution by chemicals is a real health problem for humans and their environment. Simple and inexpensive solutions are therefore strongly required. Among them, we have advanced oxidation processes [5]-[7] which have already shown their potential in the treatment of persistent pollutants [8]. They are essentially based on the production of HO˚ hydroxyl radicals which are very powerful oxidants able of oxidizing and mineralizing all kinds of organic pollutants [9] [10]. One of these oxidation processes is heterogeneous photocatalysis [11] which is based on the excitation by light (natural or artificial) of a semi-conductor (photocatalyst) such as titanium dioxide (TiO2) [12] [13] which generates radical oxidants on its surface. Methyl orange is taken as the target molecule in this work because it has been the subject of studies [14]-[16]. It is used in several sectors of activity such as chemistry, pharmacology, medicine, biology and especially in the textile industry. The abusive use of this substance has harmful consequences on human health (acute inhalation toxicity) and the environment (fauna and flora). This work aims to treat water contaminated by methyl orange with titanium dioxide in suspension under UV radiation. To do this, we have studied the influence of certain physicochemical parameters such as the pH of the solution to be treated, the initial concentration of methyl orange, the volume of the solution and the quantity of TiO2.
2. Material and Methods
2.1. Reactants
All reagents used in this study have not previously undergone purification. Azo dye methyl orange (orange acid 52) or helianthin (C14H14N3NaO3S; 99%) (Figure 1) was supplied by the company SHARLAU. Its peak absorption wavelength is λmax = 465 nm [16]. Sodium hydroxide (0.1 N) and hydrochloric acid (0.1 N) respectively supplied by the SHARLAU and CARLOERBA companies were used to adjust the pH of the solution using a HANNA Instruments brand pH-meter. Crystalline powder titanium dioxide (TiO2 P25; average size 20 nm, purity 97%, surface area 50 m2·g−1 and 80% anatase, 20% rutile) was supplied by the company SHARLAU. Distilled water was used to prepare the solutions.
Figure 1. Structure formula of methyl orange.
2.2. Photocatalytic Device
All experiments were carried out at room temperature in a glass beaker containing the solution of methyl orange (from 10 to 30 mg·L−1) and titanium dioxide (from 0.3 to 1.0 g) in slurry. The solutions, whose volumes varied from 300 to 900 mL with pH ranging from 3.0 to 11.0, were irradiated under four “WAN YUE, 20 W” fluorescent type UV-A lamps positioned horizontally 40 cm above the housing. Magnetic stirring is triggered by a magnetic bar throughout the photochemical reactions to ensure the homogeneity of the irradiated solutions in the reactor. The device is enclosed in a black enclosure to allow proper irradiation of the reaction medium (Figure 2).
Figure 2. Simplified schematic of the photocatalytic device.
2.3. Photocatalytic Tests
The adsorption phase in the dark is carried out for 30 minutes to reach the state of equilibrium between the dye molecules and the TiO2 photocatalyst in suspension. After this phase, the lamps are turned on and the timer is started. It is the instant t = 0 min. At regular time intervals, 5 mL of the solution is taken with a syringe, then filtered using a 0.2 µm millipore filter to separate the liquid and solid phases. The samples are analyzed using a WFJ-762 UV-visible spectrophotometer (λmax = 465 nm). The influence of parameters such as the mass of TiO2, the initial concentration of methyl orange, the volume of the solution and its pH were highlighted. Considering that the degradation follows a pseudo-first order kinetics, the apparent kinetic constant kapp was calculated according to the following equations:
(1)
(2)
with r: reaction rate; C: residual concentration of the pollutant; Co: initial concentration of the pollutant; kapp: apparent kinetic constant and t: UV irradiation time.
The rate of removal of the dye by photocatalytic decolorization was calculated using the following equation:
(3)
with T(%): rate of removal of the dye; C0: initial concentration of the dye; Ct: concentration of the dye at time t.
3. Results and Discussion
3.1. Influence of TiO2 Dosage
To determine the optimal quantity of photocatalyst for which the reaction speed is the greatest, we studied the effect of its mass in a volume of 300 mL of solution containing the dye. This operating parameter is important because the quantity of the photocatalyst depends on the geometric shape of the static flow reactor. The results showed that the discoloration of methyl orange increases with increasing TiO2 concentration from 0.30 g (1.0 g·L−1) up to 0.50 g (1.6 g·L−1) and then decreases (Figure 3). This observation can be explained by the availability of active sites on the TiO2 surface and the possibility of penetration of UV light over the entire surface of the photocatalyst in the solution. This causes an increase in the number of OH• radicals which can participate in the degradation of methyl orange [17]. On the other hand, beyond 0.5 g (1.6 g·L−1) of TiO2, the solution becomes cloudy, and a screening effect occurs, thus preventing the diffusion of UV light in the solution. This causes a reduction in the efficiency of the photocatalytic degradation process of the pollutant. The optimal value for which the dye bleaching rate is maximum is 0.50 g (1.6 g·L−1) of TiO2. This value makes it possible to achieve a degradation rate of around 98.73% and an apparent speed constant of 0.0117 min−1 (Table 1) after four (4) hours of UV-A irradiation. In a static luminous flux photoreactor, Guettaï and Air Amar studied the influence of the quantity of TiO2 P25 (from 0.2 to 1.6 g·L−1) in the presence of methyl orange. The authors showed that at 0.8 g·L−1, the reaction speed was the greatest [18].
![]()
Figure 3. Influence of the TiO2 dosage: 0.30; 0.50; 0.75 and 1.00 g; Co = 10 mg·L−1; pH = 6.4; V = 300 mL; UV-A irradiation; batch mode.
Table 1. Values of apparent rate constants (kapp), half-life times (t1/2) and decolorization rate of methyl orange with different dosages of TiO2.
TiO2 dosage (g) |
kapp (min−1)/R2 |
t1/2 (min) |
Decolorization of Methyl orange (T%) |
0.30 |
0.0034/0.975 |
204 |
63.80 |
0.50 |
0.0117/0.952 |
59 |
98.73 |
0.75 |
0.0076/0.956 |
91 |
91.11 |
1.00 |
0.0046/0.971 |
151 |
73.86 |
3.2. Influence of the Methyl Orange’s Initial Concentration
The concentration of dyes in industrial liquid waste varies with the seasons and modes of use. To this end, we carried out a series of methyl orange degradation tests, considering the optimal operating conditions. The influence of the initial concentration of methyl orange on its photodegradation was studied for a TiO2 mass of 0.50 g. Figure 4 shows that the discoloration of methyl orange decreases as its concentration increases from 10 to 30 mg·L−1. In other words, the degradation of methyl orange is faster at lower concentrations [19]. Beyond this concentration, the degradation kinetics are greatly reduced. The latter has generally been interpreted in terms of the increasing importance of the screening effect of the molecules of the pollutant making the photons unavailable for the TiO2 or by a strong adsorption of the dye which then covers a considerable part of the active sites thus reducing the speed of formation of hydroxide radicals [20]. Table 2 shows that the apparent rate constants kapp and the dye decolorization rate decrease after four (4) hours of treatment, respectively from 0.0110 min−1 to 0.0026 min−1 and 97.5% to 48.00% when the dye concentration increases [21].
![]()
Figure 4. Influence of the methyl orange’s initial concentration: 10; 15; 20; 25; 30 mg/L; m (TiO2) = 0.50 g, pH = 6.4, V = 300 mL; UV irradiation; batch mode.
Table 2. Values of apparent rate constants (kapp), half-life times (t1/2) and decolorization rate of methyl orange with different initial concentrations of the dye.
Co (mg/L) |
kapp (min−1)/R2 |
t1/2 (min) |
Decolorization of Methyl orange (T%) |
10 |
0.0110/0.956 |
63 |
97.50 |
15 |
0.0053/0.969 |
131 |
76.00 |
20 |
0.0040/0.966 |
173 |
66.00 |
25 |
0.0035/0.988 |
198 |
56.00 |
30 |
0.0026/0.990 |
266 |
48.00 |
3.3. Influence the Solution’s Volume
With a view to developing a water depollution system on a larger scale, it is necessary to evaluate our process with larger volumes of effluent to see its influence on the transfer of matter. The experiments carried out correspond to volumes of 300 mL, 500 mL, 700 mL and 900 mL. Figure 5 shows that volume has an influence on the degradation of the pollutant methyl orange. We note that an increase in volume contributes to reducing the rate of degradation of the pollutant with the optimal mass of the catalyst. This is explained by an increase in the quantity of methyl orange molecules around the TiO2. These molecules form a screen to incident UV light. This results in a reduction in the production of radicals in the reaction medium. According to Table 3, the best degradation rate of 82.48% is obtained with a low volume (300 mL). During the photodegradation of Paraquat by N, S-doped TiO2, Zahedi and co-workers have varied the volume of the solution from 600 mL to 150 mL and they arrived at the same conclusions as us. The authors showed that the degradation rate of Paraquat decreased from 92.38% to 25.00% when the volume increased after 8 h of visible irradiation in recirculation in the photocatalytic system [22].
![]()
Figure 5. Influence of the volume of the solution: 300; 500; 700 and 900 mL; m (TiO2) = 0.5 g; Co = 10 mg/L; pH = 6.4; UV irradiation; batch mode.
Table 3. Values of apparent rate constants (kapp), half-life times (t1/2) and decolorization rate of methyl orange with different initial volumes of the dye.
Volume (mL) |
kapp (min−1)/R2 |
t1/2 (min) |
Decolorization of Methyl orange (T%) |
300 |
0.0106/0.95 |
65 |
82.48 |
500 |
0.0049/0.97 |
141 |
76.91 |
700 |
0.0039/0.99 |
178 |
65.03 |
900 |
0.0026/0.98 |
266 |
45.40 |
3.4. Influence of pH
The effect of pH on the removal of methyl orange was examined in five pH media: 3.0; 5.0; 6.4; 9.0 and 11.0. The results obtained (Figure 6) show that at pH = 3.0 the disappearance of methyl orange is faster. This result can be explained by the force of attraction between the dye and the surface charge of the photocatalyst increasing the probability of encounter with the photocatalyst. In a basic medium (pH = 9.0; 11.0), we observe a drop-in photocatalytic activity, this is due to repulsive interactions between the dye and the surface charge of the photocatalyst thus reducing the probability of encounter with the photocatalyst [23]. The apparent rate constant decreases from 0.0171 to 0.0034 min−1 as the pH increases from 3.0 to 11.0 (Table 4). Optimal photodegradation is found at pH = 3.0 during 240 min of UV irradiation [24]. In alkaline solution, the methyl orange molecules are negatively charged, and their adsorption is also expected to be affected by an increase in the density of TiO2 groups on the semiconductor surface. Thus, due to coulombic repulsion, the dye is scarcely adsorbed. At high pH values the hydroxyl radicals are so rapidly scavenged that they do not have the opportunity to react with dyes. For the above reasons the photocatalytic activity of anionic dyes (mainly sulphonated dyes) reached a maximum in acidic conditions followed by a decrease in basic conditions [18].
![]()
Figure 6. Influence du pH: 3.0; 5.0; 6.4; 09; 11.0; m (TiO2) = 0.50 g; Co = 10 mg/L; V= 300 mL; Irradiation UV; batch mode.
Table 4. Values of apparent rate constants (kapp), half-life times (t1/2) and decolorization rate of methyl orange with different initial pH of the dye.
pH |
kapp (min−1)/R2 |
t1/2 (min) |
Decolorization of Methyl orange (T%) |
3.0 |
0.0171/0.92 |
41 |
99.85 |
5.0 |
0.0083/0.97 |
82 |
92.79 |
6.4 |
0.0071/0.89 |
98 |
88.50 |
9.0 |
0.0055/0.98 |
126 |
81.17 |
11.0 |
0.0034/0.99 |
203 |
61.00 |
4. Conclusion
This study falls within the general framework of wastewater decontamination. Its objective was to study the degradation by heterogeneous photocatalysis (TiO2/UV) of methyl orange, which is a synthetic dye present in effluents from textile industries. The degradation of the dye is carried out by the oxidizing action of hydroxyl radicals (OH˚) produced in the medium to be treated. These radicals are capable of degrading almost all persistent organic pollutants because of their very high oxidizing power, their reactivity, and their non-selectivity towards organic substances. To carry out this work, we used the suspended TiO2 photocatalyst and an artificial UV irradiation source by varying some physicochemical parameters. The results showed that the adsorption phenomenon of methyl orange (without irradiation) took place on the TiO2 catalyst before photodegradation. The investigated experimental parameters significantly affect the photocatalytic decolorization of methyl orange. Thus, working under optimal experimental conditions: m (TiO2) = 0.5 g (1.67 g. L−1); pH = 3.0; [MO]o = 10 mg. L−1 and V = 300 mL, we recorded an oxidation of methyl orange of 99.85% during four (4) hours of treatment. The kinetic study allowed us to determine the apparent rate constant which is 0.0171 min−1, or a half-reaction time t1/2 = 41 min. This work could be continued by immobilizing TiO2 onto supports at lower cost to treat liquid effluents from the textile industries.