Avaliação da retenção de microcistinas em solução aquosa através da aplicação de membranas de ultrafiltração

Abstract

Removal of dissolved microcystins in water is a challenge due to their high stability, heat resistance, hydrolysis and oxidation. The use of advanced treatment technologies, such as ultrafiltration, is an alternative for the retention of these substances. In this sense, the objective of this work was to evaluate the retention of dissolved microcystins in water through the application of laboratory synthesized and commercial ultrafiltration membranes. Preliminary tests were performed aiming at the retention of the reactive black 5 dye (20 mg L-1, 120 min, 0.2 MPa) with polymeric membranes produced in the laboratory by the phase inversion technique and composed of different proportions of polyethersulfone (PES ), lithium chloride (LiCl) and N, N-dimethylformamide (DMF), referred to as M1, M2, M3 and M4. It was concluded that the membrane M3 (20% PES / 0% LiCl / 80% DMF) would be the most appropriate to retain microcystin-LR, since permeate flow of 115 ± 5 L m-2 h-1 and retention of 89 ± 4 % were obtained. Then, permeate fluxes and retention of microcystin-LR by M3 membrane at pressures 0.2, 0.4 and 0.6 MPa were evaluated. All the dissolved microcystin-LR retention tests were performed at the initial concentration of 50 μg L-1 for 150 minutes of ultrafiltration. The retention of microcystin-LR, differently from that expected for the M3 membrane, was lower than that of the reactive black 5 dye, obtaining the best removals close to 40% only at pressures of 0.2 and 0.4 MPa. With the objective of improving the retention of M3 membrane, it was subjected to thermal treatments at temperatures of 60, 120 and 180 ºC for 15 min. At the pressure 0.2 MPa, where the best results were obtained, in the membranes treated at 60 °C, the obtained flux was 51 L m-2 h-1 and retention, 80.1%, while in the membranes treated at 180 °C, these values corresponded to 74 L m-2 h-1 and 70.3%. Morphological changes were observed in the filter materials, caused by the heat treatment, which led to differences in flow and retention in relation to untreated membranes. From the best microcystin-LR retention results, its performance was compared to that of two commercial ultrafiltration membranes, composed of polyamide and polyether sulfone, of the GH and GK models. With the GH membrane, permeate flow of 98 ± 5 L m-2 h-1 and retentions of 87.2% were obtained, whereas with the GK membrane flow of 210 ± 8 L m-2 h-1 and retentions of 48.2%, both at 0.9 MPa pressure. In addition, it was noted that, even with the use of high pressure with the GK membrane, the flows and retention of dissolved microcystins were close to those of the M3 membranes at 0.4 MPa pressure. This constitutes an advantage in membrane separation processes, which promotes the reduction of energy consumption. Thus, it is concluded that the membranes synthesized in the laboratory have potential use for the treatment of water containing dissolved microcystins.