Avaliação da performance fotocatalítica do óxido modificado SrZrxSn1-xO3 quanto a degradação da microcistina-LR em águas de abastecimento

Abstract

The main cause of eutrophication in rivers and lakes is the accumulation of nutrients produced in different anthropogenic processes. Being eutrophication one of the main factors for the increase of cyanobacterial blooms. These blooms change the water quality establishing a serious health risk. In fact, cyanobacteria produce microcystin and other cyanotoxins that negatively affects water bodies for public supply. Such toxins are very harmful to human and animal health. Microcystins were found to be hepatotoxins by damaging the liver leading to death upon its levels. Usually water treatments used by the treatment facilities are not efficient as they fail in eliminating such toxins dissolved in waters. Thus, the present study aims to evaluate the photocatalytic performance of modified oxides SrZrxSn1-xO3 regarding their ability to degrade LR microcystin from water supply using Box Behnken experimental design. The oxides used in this work were characterized by X-Ray Diffraction, Raman Spectroscopy and Ultraviolet Spectrophotometry. The Microcystis aeruginosa strain, a producer of the microcystin-LR, was cultivated by LAQUISA/EXTRABES laboratory. Experimental design was used for performance evaluation, in which 3 factors varies at 3 levels. Three independent variables were evaluated, structural modification, oxide mass, and time of oxide in solution. The dependent variables tested were the degradation percentage and the haemolysis percentage after the photocatalytic process. Toxin concentrations were analysed before and after each trial by using HPLC-MS. Solution toxicity was measured by haemolytic activity tests. According to the results, XRD patterns showed well-defined peaks related to the main phase of the orthorhombic perovskite (Pbnm) structure. Raman spectra revealed that samples of pure SrZrO3 and SrSnO3 have typical active modes of perovskites of orthorhombic type with 24 active modes. The UV-Vis spectrum was used to calculate the band gap. Results showed the increase of cation A in the SrZrxSn1-xO3 system led to an increase of its energy. The experimental design allowed statistical evaluation of results in order to verify the significance of factors, as well as their interactions. Altogether the Pareto graph, the effects table and the ANOVA experimental design showed significance for the photocatalytic performance of the modified oxides studied for the evaluation of first response, the percentage of degradation, the individual factors of quadratic time and quadratic stoichiometry, as well as the interaction between linear mass with quadratic time, and linear mass with linear time. Similarly, for the evaluation of second response results demonstrated that percentage of haemolysis, individual factors of linear mass, quadratic time, and quadratic stoichiometry, altogether with all quadratic and linear interactions of the main effects were considered significant for the model. In addition, data showed 39.9% of degradation and 13.89% of haemolysis when 0.2 g of SrZr0.75Sn0.25O3 oxide was exposed to radiation of 2 UVC lamps for 5 hours. The best photocatalytic oxide found was SrZr0.75Sn0.25O3, which shows the highest degree of asymmetry. These promising results have shown the use of such oxides are satisfactory for the removal of MC-LR toxin from waters. Furthermore, the photodegradation reaction by-product was found to be non-toxic by having less than 30% of haemolysis.