Heterogeneous Photocatalytic Degradation of Dyes in Water/Alcohol Solution Used by the Brazilian Agate Industry

The agate dyeing industry has been commonly seen as a high-pollution industry. Dyeing wastewater treatment is considered one of the most important categories for water-pollution control, because of its intense colour and the high concentration of organic contaminants. Most dyes used in the process present minimal biodegradability due to aromatic organic compounds in their structure. Using a photocatalytic reactor, experiments were carried out to study the decolorization of a water solution containing 8% ethylic alcohol and 200 mg L−1 of the dye Rhodamine B (RhB), the most difficult colorant to degrade among the used by the agate industry. The best conditions were further applied to treat the same agate water/ethyl alcohol solution containing a mixture of 200 mg L−1 of Rhodamine B (RhB), Crystal Violet (CV), Brilliant Green (BG), and Blood Red (BR). All the experiments were performed in a 2 L reactor equipped with ultraviolet (UV) lamps, at a wavelength of 365 nm, with powdered TiO2 or ZnO as the catalyst. The results indicated that the optimal decolorization conditions were attained with 2.5 g L−1 of the catalyst at pH 10 and an irradiation time of 80 min. The process resulted in complete degradation of CV, BG and 80% - 90% degradation of RhB and BR. The catalyst ZnO presented a performance somewhat better than TiO2. It is possible to conclude that the process of heterogeneous photocatalysis is effective for decolorization of water streams from the agate industry.