Coupling of photocatalytic and biological processes as a contribution to the detoxification of water

This research contributes to the study and development of a new degradation technique that couples solar and biological processes for the treatment of biorecalcitrant, nonbiodegradable, and/or toxic organic substances present in the aqueous medium. Efficient physicochemical pretreatments are necessary to modify the structure of the pollutants, by transforming them into less toxic and biodegradable intermediates, allowing then, a biological procedure to complete the degradation of the pollutant load in a shorter time and in a less expensive way. The strategy of coupling photochemical and biological processes implicates among others, the study of some fundamental physicochemical properties, the optimization of a coupled reactor at laboratory scale (2 litres), and the study of solar photocatalytic treatment efficacy under direct sunlight using parabolic collectors of 40 to 200 litres. The study of the structure effect on the photoreactivity via TiO2 catalysis is studied using several substituted phenols to cover a wide variety of electronic effects, ranging from strong electron-donating (activating) to strong electron-withdrawing (deactivating) groups and herbicides with very similar molecular structures (metobromuron, isoproturon, chlorbromuron, and chlorotoluron). The photoreactivity of these compounds is affected by the electronic nature of the substituents and their positions in the aromatic ring, being higher when there is a greater electronic density. The Hammett constant, which represents the effect that different substituents have on the electronic character of the aromatic studied compounds, appears to give an adequate descriptor of their photocatalytic degradability. One important consideration in the TiO2-photocatalysed reactions is the adsorption of the organic compound on the surface of semiconductor particles. The dark adsorption isotherms for complete p-halophenols series and four herbicides are measured and correlated with their photoreactivity. The results indicate that no direct correlation exists between the extents of adsorption and the initial photodegradation rates of the studied compounds. Concerning the optimization and utilization of an integrated photocatalytic-biological process at laboratory scale, two kinds of combined systems are developed using immobilized biomass for the biological step and either diluted Fe3+/H2O3 (Fenton reaction) or TiO2 supported on glass rings for the photocatalytic pre-treatment. The advantages of the latter system are that the catalyst can be re-used and that the pH of the solution remains at neutral values. The photo-Fenton reaction instead, renders the phototreated solution acidic making neutralization necessary. The photochemical-biological flow reactors mentioned above, are employed to completely mineralize an isoproturon-herbicide solution. Preliminary experiments concerning the chemical and biological characteristics of the phototreated solution, are carried out to determine the moment at which it becomes biocompatible. Two operation modes (continuous or semi-continuous) of the photo-Fenton-biological coupled reactor are compared by studying the efficiency of the photochemical, biological, and overall treatments of p-nitro-o-toluenesulfonic acid (p-NTS) in solution. The two main parameter affecting the performance of the photo-assisted reactor in continuous mode are related to the very low pollutant concentration that characterise this kind of operation mode and to the high residual H2O3 concentration after the pretreatment. Thus, a semi-continuous mode was applied to try to overcome these inconveniences. In these conditions, 50 to 70 litres of polluted water can be treated per day per litre of photoreactor. The last part of this thesis, addresses the study of the solar photocatalytic treatment efficacy under direct sunlight using parabolic concentrating (Helioman reactors) and nonconcentrating (CPC) collectors. This part, carried out at the "Plataforma Solar de Almería" (PSA) in Spain, indicates that the solar photocatalytic treatment is effective for the purification of water contaminated by herbicides and other substances of industrial origin like the p-NTS. It is demonstrated the utility of both homogeneous (based on Fe3+/H2O3 reaction) and heterogeneous photocatalysis (based on TiO2) as pretreatment methods that can be followed by a biological treatment. Using a coupled system, the treatment of 100 to 300 litres of polluted water per square metre of photoreactor can be envisaged in a sunny day. The low manufacturing, installation, and maintenance costs, and easy operation of the CPC, compared with Helioman collectors, suggest that the former ones are, at present, the best way to apply the solar detoxification technology.