Photo-catalytic degradation is influenced by various operational parameters. The parameters are discussed in the subsequent section. Their importance suggests that preliminary experiments should be conducted in order to find optimal conditions for each parameter during the degradation of pollutants of interest.
2.4.1 Catalyst makeup
The photo-catalytic activity of TiO2 is dependent on the physical and chemical properties of the catalyst. Properties of particular significance include crystal composition, surface area, surface defects, particle size distribution, porosity, the energy of the band-gap, and surface hydroxyl density. In heterogeneous catalysis particle size is critical to consider since it is related to the efficiency of catalyst through its definition of specific surface area [52].
17 Often the difference in photo-catalytic activity is related to the variation of the specific surface area, degree of structural defects in the crystalline framework, impurities, or density of hydroxyl groups on the catalyst surface [56].
2.4.2 Light intensity and wavelength
Light intensity (Wm-2) facilitates the initial rate of electron-hole formation, the initiator step in photo-catalytic reactions and it determines the extent of light absorbed by the catalyst at a particular wavelength [52]. Futjishima et al. [19] investigated the effect of low light intensity on the decomposition of organic pollutants and postulated that TiO2 photo-catalysis depends on the energy of the incident photons rather than on their light intensity. However, in water treatment, a high light intensity is required to sufficiently provide the surface of the nano-particle active site with the necessary photon energy [11]. Kaneco et al. [57] observed a rapid increase of degradation efficiency of bisphenol A when light intensity was increased.
Light wavelength affects the photo-catalytic reaction rate, depending on the catalyst type (crystallinity). The crystal structure, physical and chemical composition of the material influences the band gap energy of the catalyst; hence different kinds will require different wavelengths of light. For instance, Degussa P25 TiO2 is sufficiently photo activated by a light with a wavelength of λ < 380 nm because it is made of anatase and rutile crystal phases in a (80:20) ratio. Though, the rutile TiO2 with band gap energy of 3.02 eV would be sufficiently photo activated by light with a wavelength of up to 411 nm [55, 11]. Figure 2.3 demonstrates the common TiO2 phases and the wavelengths required to initiate excitation.
18 Figure 2. 3 Schematic presentation of wavelength energies required to activate different TiO2
surfaces.
2.4.3 Catalyst loading
In a heterogeneous regime the photo-catalytic reaction rate is directly proportional to the mass of catalyst present within the system of interest [54]. Initially photo-catalytic degradation increases with an increase in the amount of catalyst, but then decreases at higher mass values of catalyst.
This is because of light scattering and screening effects. At higher catalyst loadings the tendency of particles to agglomerate is increased, thus a decrease in the catalyst surface area that is exposed to light absorption. Excess particle concentration compromises light penetration into solution. Therefore optimum catalyst loading must be determined to avoid wasting catalyst and inhibition of photon absorption [54, 11].
2.4.4 pH
The pH of a solution plays a crucial role in photo-catalytic degradation of organic contaminants, since organics exhibit different properties in waters. Organic pollutants exhibit in H2O varying properties which include hydrophobicity, speciation behaviour and solubility in water. The pH of a solution becomes significant in photo-catalytic degradation as it dictates the surface charge of catalyst, agglomeration size of catalyst particles, and the electrostatic interactions between the catalysts surface and the pollutant of interest [52].
19 The point of zero charge (PZC) has been employed to study the effect of pH on TiO2 catalyst during photo-catalytic reactions. PZC refers to the pH at which a surface has a net neutral charge.
Under acidic (pH < PZC) or alkaline (pH > PZC) condition the surface of titania can be protonated or deprotonated respectively according to the following reactions [54, 1, 11]:
TiOH + H+ → TiOH2+ (2.6)
TiOH + OH- → TiO- + H2O (2.7)
In acidic medium (pH < 6.9) the titania surface will remain positively charged and negatively charged in alkaline medium (pH > 6.9). Depending on the pollutant charge, adsorption Coulombic attraction or repulsion can be favoured. For instance, if the pH of solution is lower than the PZC of catalyst, the surface of the catalyst tends to be positively charged and can cause enhanced absorption of a pollutant (e.g. 4-chlrorphenol) onto the surface [53, 58].
2.4.5 Nature and concentration of organic pollutants
Photo-catalytic degradation rate is dependent on pollutant concentration and constituent [4, 1].
When similar parameters are maintained with varied pollutant concentration it becomes necessary to vary the irradiation period in order to archive mineralization. Very high concentrations of pollutants are said to saturate the TiO2 surface and deactivate the catalyst surface by reducing photon efficiency or producing species that can poisen the surface. As the concentration of pollutant increases, more pollutant molecules get absorbed on the catalyst surface, and if the pollutant absorbs in the UV-Vis range that corresponds to the photo-catalysts band-gap energy, decrease the penetration of light onto the surface. Since the intensity of light and irradiation periods are constant, photo-catalytic activity tends to decrease at higher pollutant concentration [59]. Additionally, highly concentrated pollutants can produce intermediates which may adsorb on the catalyst surface and cause catalyst deactivation as they may diffuse slowly from the catalyst surface [11]. The best degradation condition is highly dependent on the kind of pollutant [59].
20 The corresponding nature and substituent on the target pollutant also influences the degradation efficiency under certain concentrations. For instance, pollutants with electron withdrawing groups are most susceptible to direct oxidation than those with electron donating groups [11, 4, 60].