Degradação fotocatalítica de azul de metileno utilizando pentóxido de nióbio em suspensão e em reator de membranas do tipo fibra oca

Abstract

Niobium pentoxide (Nb2O5) is one of the most promising semiconductors to be used in photocatalysis due to its chemical and physical stability, versatility, non-toxicity and photocatalytic activity. One of the main limiting factors in heterogeneous photocatalysis processes in suspension lies on the difficulty of recovering the catalyst. The use of membranes with catalysts immobilized in their structure has been shown to be an alternative to overcome the limitations of material recovery. Membranes with hollow fiber geometry have a larger area per membrane volume, which guarantees greater efficiency when compared to flat membranes. In this study, Nb2O5 was proposed as a catalyst in photodegradation processes of methylene blue dye using UVC light as a light source. The photocatalytic assays were investigated using Nb2O5 in the amorphous and monoclinic phases, in suspension solution and immobilized in membranes with hollow fiber morphology. A wet ball milling process was carried out in order to reduce the average particle diameter of the powders so membranes with hollow fiber geometry could be formed. The membranes were produced by the extrusion process using flow rates of 10 and 15 mL min-1 for the ceramic suspension and the internal coagulant liquid, respectively. The use of solvent as an internal coagulant produced porous membranes with regions of microchannels. For the photocatalytic tests using the powders in suspension, methylene blue was completely degraded after 120 min using 1 g of amorphous Nb2O5 at an initial concentration of 10 mg L-1 of dye at pH 5. The highest percentage of degradation for the powder in the monoclinic phase (60.1%) was acquired using 1 g of catalyst in 10 mg L-1 of dye after 240 min. The results showed the influence of the material's crystalline structure on the methylene blue degradation rate. In analyses using membranes produced by monoclinic Nb2O5 there was 100% dye degradation after 210 min of experiment for methylene blue concentration of 10 mg L-1 at pH 11 when 7 membranes coated with graphene oxide were used. The membranes produced with the amorphous Nb2O5 powder showed greater degradation of methylene blue (92% after 240 min) when 15 membranes coated with graphene oxide were used for a concentration of 10 mg L-1 of dye at pH 11. The pseudo-first order model was proposed to describe the degradation processes, showing values for the coefficient of determination close to unity. The membranes were submitted to reuse tests for 4 cycles using the same experimental conditions and without any treatment between each cycle. The values obtained showed that there was a reduction in the percentage of dye degradation of about 10% for both membranes, comparing the results obtained at the end of the first and fourth reuse. The observed results evidence the potential application of the produced membranes in systems that need several steps of photodegradation.