Sistema de ablação de saxitoxina em água de rio.

Abstract

Concerns related to the increasing concentration of cyanobacteria in reservoirs are motivated by the potential production of cyanotoxin, a secondary metabolite that is released in water by cell lysis and may bioaccumulate in the food chain, which can result in serious diseases and even death in the affected population. Saxitoxin (SXT) is a toxin that represents a public health issue when in water even in concentrations within the limits set by regulations. Therefore, the system developed and evaluated in this study focuses on raw water treatment by slow filtration associated with heterogeneous photocatalysis and is proposed to be used as an alternative to public water supply. In the first stage of the study, two slow filters (FL1 and FL2) with filter medium composed of sand and kaolin were evaluated during 90 days. Both models were evaluated based on physicochemical and microbiological parameters and one model was selected after the operation period. In the second stage of the study, balls were produced using kaolin, clay, and TiO2 and characterized by X-ray diffraction (XRD) and ultraviolet visible spectroscopy (UV-vis). Next, the balls were inserted inside the photocatalytic reactor, which was connected to the slow filter. The operation continued for 20 days, and the concentrations of TiO2 used were 15% and 30%. The system was fed in both stages using Piranhas River water. In the first stage, the results obtained for FL1 met the potability standard values established in Ordinance GM/MS Nº 888/21 in 92% (turbidity), 100% (pH), 28% (apparent color), 100% (ammoniacal nitrogen), 16% (E. coli), and 100% (STX) of the samples. FL2 met the standard values in 94% (turbidity), 100% (pH), 34% (apparent color), 100% (ammoniacal nitrogen), 16% (E. coli), and 100% (STX) of the samples. No filter met the requirement for fecal coliforms. In the second stage, the XRD results indicated the main phase as being anatase; from the UV-vis spectroscopy, bandgaps of 3.14 and 3.09 were obtained for balls with 15 and 30% of TiO2, respectively. The results during the operation of the system indicated a more efficient removal of STX in the reactor with balls with 30% TiO2, with maximum removal efficiency of 95% and average efficiency of 39%. Therefore, the system proposed could reduce STX levels and may be used as an alternative water treatment system, particularly in isolated communities. Moreover, clay and kaolin exhibited good capacity in the photocatalytic process mediated by sunlight, increasing degradation/oxidation when compared to other studies in which high sunlight exposure times were adopted.