Aplicação de um sistema fotocatalítico associando óxido de nióbio e microalgas no tratamento de efluente contaminado com cromo

Abstract

Bioremediation is an efficient alternative process for Cr removal, which can be achieved with microalgae. Other materials, such as metal oxide-based, which have photocatalytic capacity, also emerging as an option to traditional methods. This work studied the efficiency of Cr (VI) biosorption by the microalgae Chlamydomonas reinhardtii and the process of heterogeneous photocatalysis in two phases: in phase 1, the niobium oxide (Nb2O5) was used and, in phase 2, it was associated the microalgae C. reinhardtii in the removal of Cr as a viable and innovative alternative for the treatment of effluents. In biosorption, the kinetic studies was evaluated at pH 3 and 7; 10 mg/L of Cr (VI); 1 g/L of microalgae; equilibrium time of 180 min and room temperature. At pH 7, Cr (VI) removals of 4.8 % in the absence of light and 4.4 % in the presence of light were obtained. At pH 3, 2.6 % was achieved in the absence of light and 11.7 % in the presence of light. The kinetics of Cr (VI) biosorption followed the Bangham model and intraparticle diffusion. In the biosorption isotherm, evaluated in the range of 10 to 70 mg/L of Cr (VI), a maximum biosorption capacity of 5.8 ± 0.1 mg Cr(VI)/g algae was obtained at pH 3 and in the presence of light, being the adsorption best represented by the Langmuir model. In phase 1 of the photocatalytic process, the conditions were: Cr (VI) solutions at concentrations of 10, 30 and 50 mg/L of Cr (VI); addition of Nb2O5 at concentrations of 0.5 and 1.0 g/L; pH 3; recirculation of the Cr (VI) solution in continuous flow rate of 800 mL/min for 72 h. The reduction results found for 1.0 g/L of Nb2O5 were 51 %, 13 % e 11 % in 10, 30 and 50 mg/L Cr (VI), respectively. Subsequently, in phase 2, microalgae C. reinhardtii was added at a cell density of 1.0 g/L in the effluents provenient from phase 1, with experiments at pH 7 and in a batch process for 120 h. The results of Cr (VI) reduction were 50 % and 18 % for the concentration of 10 mg/L and 50 mg/L of Cr (VI) respectively, regardless of the concentration of Nb2O5 used previously; 81 % for 0.5 g/L and 39 % for 1.0 g/L of Nb2O5, both at a concentration of 30 mg/L of Cr (VI). At the end, the two phases provided, on average, results of 71 %, 31 % and 18 % of removal to the heavy metal for initial concentrations of 10, 30 and 50 mg/L of Cr (VI), respectively. Thus, heterogeneous photocatalysis using Nb2O5 under UV-C light and further application of the microalgae C. reinhardtii, proved to be an adequate process for reducing and removing Cr.