Destruction of the organic matter present in effluent from a cellulose and paper industry using photocatalysis

The use of photocatalysis to perform the degradation of the organic matter present in the effluent from a paper and cellulose mill was studied. The best conditions for the photocatalytical process at a laboratory scale were determined using real effluent and an aqueous solution of a pre-degraded lignosulphonate as model, for large-scale studies. The results show that the reaction is little influenced by temperature, the apparent activation energy being between 7.9 and 10.5 kJ/mol, typical of phenol degradation mediated by TiO2. On the other hand, pH, the use of some additives, and the morphology of the photocatalyst exert a considerable influence on the results. P25 proved to be a better photocatalyst than anatase, giving a 60% higher degradation rate. The addition of hydrogen peroxide in the reactions mediated by P25 resulted in an increase of 173.5% in the efficiency of the photocatalytical process. When compared to the reaction induced by anatase, the increase in efficiency was 354%. The association of TiO2 and a photosensitizer (zinc phthalocyanine) gave a small increase (<6%) in the reaction efficiency, under laboratory conditions. This behaviour is expected considering that using a medium-pressure mercury lamp is only possible to excite the Soret band of the photosensitizer (around 360 nm), due to the insignificant emission of these lamps in the absorption region characteristic of the Q-band (around 660 nm). On the other hand, the addition of HPO42− did not result in a significative increase of efficiency to the degradation (<4%). This was attributed to the small amount of cationic and electron-deficient species, compared to the amount of anionic or electron-rich species, between the organic matter to be degraded in the effluent, since the best pH for the degradation are lower than pHzpc. Although the reaction mechanism follows the Langmuir–Hinshelwood model, the degradation rate is at least 100 times faster than the adsorption of the organic matter on the TiO2, showing that the reactions induced by the active species generated on the surface of the photocatalyst and desorbed to the solid–liquid interface must exert an important rule on the degradation rate. Under the best laboratory conditions, more than 80% of the organic matter is mineralised after 60 min of reaction. The best conditions were tested under solar conditions using a compound parabolic collector (CPC) setup and large volumes of effluent, giving excellent results. The experiments were done using small amounts of TiO2 (50 mg/l of effluent).