The treatment of effluent can be complex in cases where conventional processes are ineffective, as occurs when trying to decolourise effluent polluted by polyphenolic structures. Advanced oxidation processes, also known as photo-oxidation, are a simple and effective solution for these recalcitrant organic products of industrial origin. Of these, photo-oxidation has one of the best prospects, in either of its two variants: photolysis or photocatalysis.
The aim of advanced oxidation processes is generally to form the hydroxyl radical (OH•), which is highly reactive due to its high oxidation potential. In the presence of organic matter, these radicals trigger a series of chemical reactions that end up in the complete mineralisation of organic compounds, CO2 and water.
These industrial wastewater treatment processes are very attractive due to the number of advantages they offer, such as a high reactivity with most organic compounds, the complete oxidation of both organic and inorganic compounds and the emission of only harmless compounds, since all oxidants are destroyed in the process.
Photolysis is based on irradiating the effluent with ultraviolet light (170-230 nm) so the chemical compounds absorb it and form free radicals. The lower the radiation wavelength, the more energy is absorbed and the greater the efficiency in destroying contaminants.
Radiation causes oxidation reactions by forming free radicals. For these reactions to occur, oxidising species must be present to form these radicals. Among the most effective oxidising agents are ozone and hydrogen peroxide. The combination of ultraviolet radiation and ozone or hydrogen peroxide is very effective in providing a free radical source for non-selective oxidation of most organic molecules. They are also environmentally sustainable compounds as they break down into oxygen and water.
Photocatalytic oxidation is another technique that can be used, which also destroys contaminants by using ultraviolet radiation. The main difference is the use of catalysts to increase the formation of hydroxyl radicals which oxidise the chemical contaminants. The catalysts can be salts of iron, usually chlorides, fluorides or bromides, or, in the case of heterogeneous photocatalysis, semiconducting oxides, such as TiO2, Al2O3 or ZnO.
Titanium dioxide is particularly efficient, due to having another free radical production mechanism for the OH radical. In the presence of ultraviolet radiation and in aqueous medium, the electrons in one valence band of TiO2 migrate to a conduction band, leaving a corresponding hole in the valence band; producing so-called electron-hole pairs (h+- e-). The energy required to excite TiO2 is 3.2V, corresponding to the absorption of ultraviolet light ( < 385nm). Electron-hole pairs can recombine (and thus cancel each other out) or move to the catalyst surface. To prevent the h+- e- pairs from recombining, an oxidant acting as an electron acceptor is required; this is usually oxygen, which forms the superoxide ion (O2-•). An organic molecule (MO) adsorbed in the holes can also be oxidised by electron transfer, as can be seen in the figure.
Thus, the use of TiO2 is one of the most effective and advantageous options for the mineralisation of most organic substances by photocatalytic oxidation.
The destruction of contaminants by photo-oxidation has a number of advantages available to only a few technologies:
- Toxic pollutants are destroyed by converting them into harmless substances (water, CO2 and mineral salts).
- The process is non-selective and can decompose virtually any organic molecule, including complex ones.
- Additional pre- or post-treatment processes are not required.
- Energy consumption is very low, as the process takes place at moderate temperatures (30-80 °C) and the radiation source may be solar energy.
- The chemicals used are relatively low cost and freely available.
Given these advantages, photo-oxidation is an effluent and process water treatment technique of great importance for different sectors, such as the chemical, food, pharmaceutical, textile and electroplating industries, among others: removing species such as cyanide, Zn, Ni, antibiotics, hormones, organochlorides, organic polyphosphates, heterocycloaliphatics and nitrogenous and aromatic organic and heteroaromatic compounds.