Estudo da remoção de microcistinas por adsorção em nanotubos de titanato de hidrogênio a partir de uma substância modelo

Abstract

One of the problems caused by contamination of water resources is eutrophication and its main consequence is the excessive proliferation of cyanobacteria. These can pose a health risk to the population, due to the production of cyanotoxins, amongthem microcystins (MCs). Several studies have already shown that conventional water treatment processes are not effective in removing the dissolved fraction of MCs, but alternative processes, such as adsorption, have shown efficacy in removing these compounds. Hydrogen titanate nanotubes (TiNT-H) are promising materials, since they have a large specific surface area with essentially mesoporous texture, facilitating pollutants adsorption of a molecular size similar to microcystins. The aim of this work was to evaluate the removal of MCs in water by adsorption on TiNT-H, based in a model substance. In this context, reactive black 5 dye (PR-5) was used; whose molar mass and molecular size have values close to MCs. The adsorbent was characterized using surface area, pore volume, size, and shape. Adsorption tests were performed with PR-5, at 25 °C and initial concentration of 5 mg L-1. Firstly, pH effect was evaluated (range of 2 to 11), with the best results in pH 2, probably due to neutralization of charges with H+ protons, thus avoiding repulsion of charges between PR-5, an anionic dye, and TiNT-H, which have a predominantly negative surface. In the kinetic study, equilibrium time obtained was 90 minutes, and among the pseudofirst order and pseudo-second order models, the latter was more adequate to the system (R2 = 0.9963) providing a constant of speed (k2) equal to 10.83 g mg-1 min-1. Adsorption equilibrium tests were applied to Langmuir and Freundlich isothermal models, and the latter was adequate for the data obtained in the present study, due to its higher linear correlation (R2 = 0.9539), compared to the Langmuir model (R2 = 0.9123). Freundlich model yielded the corresponding monolayer saturated adsorption capacity (qm) of 52.98 mg g-1 at 25 °C and the constant KL equal to 1.75 L mg-1. The parameters provided by Freundlich isotherm were: constant KF = 0.032 (mg g- 1)(L mg-1)1/n, n = 2.444 and 1/n = 0.40. The isotherm is of type S – Spherical, subgroup 4, according to Giles classification. From the positive results with PR-5, adsorption tests were performed with a pre-purified extract of MCs at 25 ºC for 90 minutes, with an initial concentration of 50 µg L-1, varying volume (25 and 50 mL) and pH (2 and 8). Using the volume of 25 mL, removal was equal to 90.8% at pH 2 and 92,2% at pH8. Using the volume of 50 mL at pH 8, removal was equal to 86,4%. Results of adsorption removal were considered satisfactory and it was concluded that pH is not a factor of great influence in the adsorption process, different from the results obtained with PR-5 dye. Further studies with wide pH variation, as well as kinetic and adsorption equilibrium tests, preferably with an analytical standard of MCs, are proposed in order to more accurately assess their interaction with TiNT-H.