The increase demand for textile, leather, paints, cosmetics, pharmaceutical and food additives has led to an increase use of dyes and colouring matters. As a consequence, large quantities of industrial wastewater containing dyes have been discharged into the aquatic environment. The contamination of water bodies endangers the aquatic life and disturbs the balance of the ecological system [1]. Moreover, its diffusion in drinking water bodies can results in catastrophic incidents. Methylene blue (MB) is one of the most commonly used organic dyes in dyeing cotton, wood and silk. Effluents containing MB may cause severe health issues including permanent eye burns, nausea, vomiting, profuse sweating and mental confusion [2]. Owing to these adverse effects, industrial effluents must be treated prior to discharge. Several wastewater treatment techniques are available, including biodegradation, coagulation, adsorption, membrane processes, and advanced oxidation processes (AOPs) [1]. Dyes are recalcitrant to biodegradation which makes biological treatment techniques inefficient. Conventional physicochemical treatment techniques, such as adsorption and coagulation, can effectively remove dyes from wastewater. However, these techniques transfer dyes from liquid phase to solid phase leading to another type of pollution. Oxidation techniques, especially the powerful AOPs, seem to provide the most adequate treatment for this wastewater type. AOPs utilize the oxidation power of highly energetic hydroxyl radicals in attacking these refractory organic compounds resulting in their complete degradation into carbon dioxide and water vapor [3].
Heterogeneous photocatalysis has become one of the most pronounced AOPs owing to its advantages including the capability of working under ambient conditions and the probability of achieving complete conversion of organic carbon into carbon dioxide [1]. The mechanism of heterogeneous photocatalysis is initiated on exposing the photocatalyst particles, such as TiO2, to a suitable irradiation. The irradiation energy used must be at least equal to the band gap energy of the photocatalyst to be capable of exciting its valence electrons. The irradiation excites the valence electrons from the valence band to the conduction band leading to the formation of electron-hole pair which are the main charge carriers. Both charge carriers are capable of producing hydroxyl radicals. The positive holes can react directly with water producing OH• radicals, while, the negative electrons produce the OH• radicals through a series of reaction steps. The main reaction mechanism for OH• radicals production are presented in Eqs. (1) through (6) [4].
TiO2 + \(h\nu\) (UV) → TiO2 (e− (CB) + h+ (VB)) (1)
H2O (ads) + h+ (VB) → OH•(ads) + H+(ads) (2)
O2 + e− (CB) → O2−•(ads) (3)
O2−•(ads) + H+ → HO2•(ads) (4)
2 HO2• (ads) → H2O2 (ads) + O2 (5)
H2O2 (ads) → 2 OH•(ads) (6)
Titanium dioxide (TiO2) has been extensively used as the main photocatalyst in research due to its attractive properties of non-toxicity, availability, low price and high chemical stability. The photocatalytic activity of TiO2 greatly depends on its crystal structure, particle size, porosity and surface area [5]. Regarding the crystal structure, TiO2 has four main polymorphs which are anatase, rutile, brookite and metastable TiO2 (B). Other polymorphs may be found under high pressure conditions. TiO2 polymorphs have relatively high band gaps with values of 3.2, 3.02, and 2.96 eV for the anatase, rutile and brookite respectively. Rutile is the thermodynamically stable form while brookite is not commonly used due to its instability and the difficulty in its preparation. Anatase is preferred owing to its high photocatalytic activity and difficult recombination of charge carriers [6, 7]. Tayade et al. [8] synthesized nanocrystalline TiO2 of both phases and found better results when using the anatase for the photocatalytic degradation of several organic compounds such as acetophenone, nitrobenzene, methylene blue, and malachite green.
Slurry reactor systems have the advantage of overcoming mass transfer limitations via providing high catalyst surface area for the reactants [9]. They have shown superior photocatalytic activity when compared to immobilized ones even under solar irradiation. However, an economic method for catalyst separation is required [10]. Li Puma and Yue [11] investigated the laminar falling film slurry (LFFS) photocatalytic reactor for the degradation of salicylic acid and they stated that it was highly efficient. They recommended the flat wall configuration for solar-powered LFFS photocatalytic reactors, and the inner wall configuration for LFFS photocatalytic reactors irradiated by artificial sources of UV radiation. In a later publication, Li Puma and Yue [12] pointed out the advantages of the artificially irradiated falling film reactor including uniform illumination, better utilization of back-scattered light photons, safer design and simple design of high length to radius ratio that saves more floor space. Joseph et al. [3] studied the effect of different irradiation sources, namely UVA, UVB, UVC and solar, on the photolysis and the photocatalysis of MB dye. The UVC lamp showed the most effective performance in both photolysis and photocatalysis.
The efficiency of a photocatalytic system relies on the selection of its three fundamental elements which are: the photocatalyst, the reactor configuration and the irradiation source. Researchers have been studying these elements in order to achieve a practical cost effective system [13]. Van Gerven et al. [9] addressed the main limitations that face the intensification of photocatalytic processes and pointed out the different strategies that could be followed to overcome them. These limitations involved photon transfer limitations, mass transfer limitations and other limitations concerning reactor design.
In the current study, we investigated the synergistic effect of using a slurry of reactive anatase titania as the photocatalyst, a batch recirculating falling film reactor as the reactor configuration and a short wavelength UVC lamp as the irradiation source. The effectiveness of the photocatalytic system was studied by monitoring its performance in degrading methylene blue dye. Operating parameters including feed solution flow rate, initial pH value of the solution, catalyst dose and initial methylene blue concentration were studied. Furthermore, kinetic models were proposed for the photocatalytic degradation process.