Desenvolvimento de sensores eletroquímicos miniaturizados para o monitoramento de espécies de relevância ambiental

Abstract

With the large-scale use and contamination of water resources, both domestically and industrially, there is a growing need to monitor and treat water and effluents. Several methodologies have been developed and applied for the monitoring of harmful substances in water bodies and, among these methodologies, electroanalytical methods have attracted great interest due to their advantageous characteristics, such as high sensitivity, simple instrumentation, and high environmental compatibility. Regarding the treatment of effluents, electrochemical advanced oxidative processes (EAOPs) are promising methodologies for the treatment of effluents, at domestic and industrial levels, being an alternative or complement to traditional methods of water treatment. In reactors for EAOPs, a wide range of organic or inorganic contaminants can be degraded by the action of electrochemically generated strong oxidizing species. In these processes, therefore, an adequate monitoring of both the oxidants generated and the pollutants to be degraded is crucial in order to improve the efficiency and reduce the costs of the decontamination process. Thus, this work proposes the use of miniaturized electrochemical sensors for the real-time monitoring of hydrogen peroxide, an important oxidant, generated inside the reactor and, in a second approach, the quantification of the antibiotic isoniazid (an emerging pollutant) in water samples. Initially, an integrated Pt microelectrode (a microelectrode that combines a Pt working microdisk with a quasi-reference silver electrode) was developed for the monitoring of hydrogen peroxide (H2O2) electrogenerated in the reactor. With this microsensor it was possible to obtain a real-time profile of the generation of H2O2 through the electrolysis of a solution of 0.10 mol L-1 HClO4 in a reactor equipped with Nb/BDD anode and Ti cathode under different current densities (30, 60, 90 and 120 mA cm2 ), showing that the rate of H2O2 production is strongly dependent on the applied current density (j) and that it reaches its maximum after 90 min of electrolysis, showing a pseudo zeroorder kinetics. The results obtained by this electrochemical method agree with the spectrophotometric reference method at a confidence level of 95%. For the quantification of isoniazid, initially a set of gold microelectrodes was obtained from obsolete computer microchips. The preliminary electrochemical characterization of the Au microelectrode array proved that the microelectrode array obtained in an alternative way exhibited a characteristic behaviour of a microelectrode array and a voltammetric profile in H2SO4 compatible with that expected for an Au electrode. The microelectrode array was then modified with electrochemically reduced graphene oxide (ERGO) and used for antibiotic quantification by linear scanning voltammetry, showing a good linear correlation (R2 = 0.9926) and detection limit (1.38 μmol L-1) in the concentration range of 5.0 to 100.0 μmol L-1. The sensor was then applied in the determination of the drug in river and lagoon waters. Thus, it is possible to observe that electroanalytical methods employing miniaturized sensors were successfully applied in the determination of inorganic and organic species of environmental relevance in different matrices.