ABSTRACT

The nanophotocatalytic process using semiconducting oxides with a nanostructure is one of the technologies used for the destructive oxidation of organic compounds such as dyes. The photocatalytic oxidation of a textile dye—C. I. Basic Blue 41 (BB41) in aqueous solution was assessed by UV ray irradiation in the presence of TiO₂ nanoparticles. The effect of initial dye concentration, pH and TiO₂ loading were investigated and the optimized conditions for maximum amount of degradation were determined. Analysis of the kinetics showed pseudo-first-order model. The mineralization of the dye was reported by measuring the initial and final chemical oxygen demand of the solution that was irradiated under optimized conditions.

1. Introduction

Synthetic dyes comprise an important part of industrial water effluents that are discharged by many manufacturing industries. The impact of these dyes on the environment is a major concern because of the potentially carcinogenic properties of the chemicals [1]. Also, some dyes can undergo anaerobic decolouration to form potential carcinogens [2]. The wastewater which is coloured in the presence of these dyes can block both sunlight penetration and oxygen dissolution that are essential for aquatic life. Consequently, there is a considerable need to treat these coloured effluents before discharging them into various water bodies. Various approaches on handling and decontaminating such effluents have been reported in the literature. Typical techniques include the classical methods such as adsorption [3,4], coagulation [5,6], ion flotation [7] and sedimentation [8]. All these techniques are versatile and useful, but they all end up in producing a secondary waste product that needs to be processed further. Another set of techniques that are relatively newer, more powerful, and very promising called Advanced Oxidation Processes (AOPs) has been developed and employed to treat dye-contaminated wastewater effluents [9].

The semiconductor TiO₂ nanoparticles has been widely utilised as a photocatalyst for inducing a series of reducetive and oxidative reactions on its surface. This is solely attributed to the distinct lone electron characteristic in its outer orbit. When photon energy (hv) of greater than or equal to the bandgap energy of TiO₂ is illuminated onto its surface, usually 3.2 eV (anatase) or 3.0 eV (rutile), the lone electron will be photo-excited to the empty conduction band in femto seconds. The mechanism of the electron hole pair formation when the TiO₂ particle is irradiated with adequate hv is depicted in Figure 1.

Photocatalysis using TiO₂ as photo-catalyst degrade toxic organic compounds [10], reduce metal-ions [11], improve the biodegradability in cellulose effluents [12] and decolour a great variety of dyes in solution [13] or in

solid mixtures [14]. When titania suspensions are irradiated, electrons are excited from the valence to the conduction band, generating positive holes and electrons [9,10]:

(1)

The energy band gap for TiO₂ Degussa P-25, the most used photocatalyst, is around 3.2 eV (390 nm). Electrons and holes can recombine (Equation (2)) or react with other molecules, such as oxygen, generating reactive superoxide anions (Equation (3)).

(2)

(3)

The positive holes can react with electron donors in the solution to produce hydroxyl radicals (Equations (4) and (5)) or would directly oxidise organic molecules (OM) at the semi-conductor surface (Equation (6)).

(4)

(5)

(6)

The hydroxyl radicals generated in the process are strong oxidants and react quickly with the organic matter adsorbed on the catalyst (Equation (7)), forming oxidised intermediates, and if the treatment time is adequate, complete mineralization is achieved.

(7)

The typical mechanism involving hydroxyl oxidative radical reactions go through the steps: electron transfer, hydrogen abstraction and addition to aromatic rings/ double bonds [9,10].

In many cases the macromolecule is completely mineralized into water and carbon dioxide. The AOP technique has drawn considerable attention from various quarters of scientific community as it is easy to handle and produces significantly less residuals as compared to the classical approaches. Amongst the many techniques employed in the AOP approach are the UV photolytic technique [15,16], Fenton process [17,18], photo-Fenton process [19,20], ozonation process [21], sonolysis [22,23], photocatalytic approach [24,25], biodegradation [26,27] and the radiation induced degradation of dyes [28,29].

Various physico-chemical techniques are available for the elimination of dyes from wastewater and in particular photocatalysis is a more promising tool. Moreover, photocatalysis can be used to cause redox transformations and decompose a dye molecule. The use of photosensitive semiconductors such as TiO₂, ZnO, Fe₂O₃, CdS, ZnS and V₂O₅ has been reported in the literature for their use in reducing colour of the dye solutions owing to their environmental-friendly benefits in the saving of resources such as water, energy, chemicals, and other cleaning materials [30,31]. Titanium dioxide (TiO₂) mediated based photodegradation has attracted extensive interest owing to its great advantages in the complete removal of organic pollutants from wastewater [32]. This is mainly because of its various merits, such as optical-electronic properties, low-cost, chemical stability, and non-toxicity [33]. The aim of the research, was to study the photocatalytic process using TiO₂ nanoparticles in the form of slurry for the decolourization and mineralization of C. I. Basic Blue 41 (BB41).

2. Materials and Methods

BB41, commercially called Cationic Blue GRL (C. I. 11105) is a reactive dye bearing an azo group as chromophore, with a molecular weight of 482.57 and molecular formula C₂₀H₂₆N₄O₆S₂ manufactured by Dongwu Dyestuff Co. Ltd. China was obtained from a retail outlet. Titanium dioxide (Degussa P25) was utilized as a photocatalyst. Its main physical data are as follow: average primary particle size around 21 nm, purity ~99.5% and BET surface area 50 ± 15 m²/g. A UV-Vis Spectrophotometer, UV-1700 PharmaSpec, Shimadzu was used to determine the decolourization rate. The photocatalytic activities of the photocatalysts were performed in a 400 ml jacketed glass reactor fitted with a 9W 5" long Philips (PL-S 9W/10/2P Hg) bulb.

The slurry composed of dye solution and catalyst placed in the reactor was stirred magnetically. After photocatalytic treatment, samples were centrifuged (6000 rpm, 10 min) and filtered through a millipore filter (0.45 µm) membrane and analyzed for the concentration of the BB41 in the solution using computer software attached to UV-Vis Spectrophotometer. Photocatalytic degradation processes were performed using a 200 ml solution containing a specified concentration of selected dye. Samples were withdrawn from sample points at certain time intervals and analyzed for decolourization and degradation. Decolourization of dye solutions was checked and controlled by measuring the maximum absorbance of dyes at different time intervals by UV-Vis Spectrophotometer. The maximum absorbance of BB41 in the visible region of UV-Vis spectrum was 608 nm. COD tests were used to assay the mineralization of BB41.

3. Results and Discussion

3.1. Decolourization Kinetics

The variation of of BB41 with irradiation time is shown in Figure 2. It can be observed that decreasing dye concentration is linearly related to the elapse of irradiation time. This means that the pseudofirst-order kinetics relative to dye is operative.

As the Langmuir-Hinshelwood (L-H) model is a surface-area dependent, the reaction rate is expected to increase with irradiation times since less organic substrate will remain after increased irradiation times with higher surface availability. A zero rate of degradation is associated with the total decomposition achieved. Numerous assumptions for the L-H saturation kinetics type exist and for the applicability in the rate of photo-mineralization, any of the four possible situations is valid:

1) Reactions take place between two adsorbed components of radicals and organics;

2) The reactions are between the radicals in water and adsorbed organics;

3) Reactions take place between the radical on the surface and organics in water;

4) Reaction occurs with both radical and organics in water.

Some researchers [34,35] found that simpler zero or first-order kinetics is sufficient to model the photo-mineralization of organic compounds. This was, however only applicable for limited conditions where the solute concentration is inadequately low. In most kinetic studies, a plateau-type of kinetic profile is usually seen where the oxidation rate increases with irradiation time until the rate becomes zero [34]. The appearance of such kinetics regime usually fits the L-H scheme. According to the LH model (Equation (8)), the photocatalytic reaction rate (r) is proportional to the fraction of surface coverage by the organic substrate (θ_x), k_r is the reaction rate constant, C is the concentration of organic species and K is the Langmuir adsorption constant:

(8)

The applicability of Equation (8) depends on several assumptions that include

1) The reaction system is in dynamic equilibrium;

2) The reaction is surface mediated;

3) The competition for the TiO₂ active surface sites by

the intermediates and other reactive oxygen species is not limiting [35].

If these assumptions are valid, the reactor scheme only consists of adsorption surface sites, organic molecules and its intermediates, electron-hole pairs and the reactive oxygen species. The rate constant (k_r) for most of the photocatalytic reaction in water is usually reported to be in the order of 10⁶ - 10⁹ (Ms)^–1 [36]. This k_r-value is the proportionality constant for the intrinsic reactivity of photo-activated surface with C. Others have also related the k_r is proportional to the power law of effective radiant flux (i.e.) during the photomineralization reaction [37]. The K-parameter is the dynamic Langmuir adsorption constant (M^–1) that represents the catalysts adsorption capacity. Equation (8) can be solved explicitly for “t” using discrete change in C from initial concentration to a reference point:

(9)

The K-value can be obtained using a linearized form of Equation (8), where 1/r is plotted against 1/C:

(10)

It was reported that the real K-value obtained from the linearized plot of 1/r against 1/C is significantly smaller [36]. This was explained by the differences in adsorption-desorption phenomena during dark and illuminated period. When the organics concentration is low (in mM), an “apparent” first-order rate constant Equation (11) could be expressed where k′(min^–1) = k_rK:

(11)

Rearranging and integration of Equation (11) yields the typical pseudo-first-order model as in Equations (12) and (13):

(12)

(13)

3.2. Effect of TiO₂ Loading

The optimum concentration of TiO₂ P-25 required for the decolourization of a 50 mg·L^–1 BB41 solution was examined with the slurry method by varying the catalyst amount from 0.0 - 1.0 g·L^–1. The results summarized in Table 1 indicate that the required photons, when the concentration of TiO₂ increased to 0.4 g·L^–1, were thoroughly absorbed. This shows that an increase in the amount of catalyst to the level consistent with the optimized level of light absorption increases the amount of

decolourization [38-40]. Thus, any further increase of the amount of the catalyst will have no effect on the photodegradation efficiency.

Concentration of TiO₂ in the photocatalytic water treatment system affected the overall photocatalysis reaction rate in a true heterogeneous catalytic regime, where the amount of TiO₂ is directly proportional to the overall photocatalytic reaction rate [41]. A linear dependency holds until certain extent when the reaction rate starts to aggravate and becomes independent of TiO₂ concentration.

This is attributed to the geometry and working conditions of the photoreactor where the surface reaction is initiated upon light photon absorption [42]. When the amount of TiO₂ increases above a saturation level (leading to a high turbidity state), the light photon absorption coefficient usually decreases radially. However, such a light attenuation over the radial distance could not be well correlated with the Beer-Lambert Law owing to the strong absorption and scattering of light photons by the TiO₂ particles [43].

The excess TiO₂ particles can create a light screening effect that reduces the surface area of TiO₂ being exposed to light illumination and the photocatalytic efficiency. Therefore, any chosen photoreactor should be operated below the saturation level of TiO₂ photocatalyst used to avoid excess catalyst and ensure efficient photons absorption. In this sense, both catalyst loading and light scattering effect can be considered as a function of optical path length in the reactor.

3.3. Effect of Initial Dye Concentration

To study the effect of dye concentration on the rate of decolourization, the BB41 concentrations were varied from 0, 25, 50, 100 & 150 mg·L^–1 while the other variables were kept constant.

Figure 3 shows the time dependence of unconverted fraction of dye (C/C₀, where C₀ is the initial dye concentration and C is the dye concentration at time t) for the various initial concentrations (0, 25, 50, 100 and 150 mg·L^–1). Table 2 shows that the first-order-kinetic rela-