RESUMO
The photo-Fenton activity of graphitic carbon nitride (g-C3N4) has been widely studied, nevertheless, its Fenton-like catalytic behavior in the dark has not yet been demonstrated. In the present work, it is shown that oxygenated g-C3N4 obtained at different temperatures (500-600 °C) can degrade indigo carmine with hydrogen peroxide in the dark by a reaction similar to a conventional Fenton's reaction. Based on an extensive characterization of g-C3N4, we conclude that Fenton-like activity is directly related to the oxygenated functional groups on g-C3N4 structure, mainly by -OH functional groups. Oxygenated functional groups (e.g., hydroquinone-like groups) can reduce the H2O2 and generate oxidizing hydroxyl radicals, just like in the Fenton reaction performed by metals. In addition to new information on g-C3N4 surface reactivity revealed by this study, the metal-free oxygenated g-C3N4 catalyst may be an alternative to traditional metal catalysts used in Fenton-like reactions for advanced oxidation.
RESUMO
A voltammetric sensor based on the electropolymerization of cobalt-poly(methionine) (Co-poly(Met)) on a glassy carbon electrode (GCE) was developed and applied for the determination of estriol by differential pulse voltammetry (DPV) for the first time. The electrochemical properties of the Co-poly(Met)/GCE were analysed by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the polymers on the GCE surface. The deposition of the Co-poly(Met) film on the GCE surface enhanced the sensor electronic transfer. CV studies revealed that estriol exhibits an irreversible oxidation peak at +0.58 V for the Co-poly(Met)/GCE (vs. Ag/AgCl reference electrode) in 0.10 mol/L Britton-Robinson buffer solution (pHâ¯=â¯7.0). Different voltammetric scan rates (10-200â¯mV/s) suggested that the estriol oxidation on the Co-poly(Met)/GCE surface is controlled by adsorption and diffusion processes. Based on the optimized DPV conditions, the linear responses for estriol quantification were from 0.596⯵mol/L to 4.76⯵mol/Lâ¯(R2â¯=â¯0.996) and from 5.66⯵mol/L to 9.90⯵mol/Lâ¯(R2â¯=â¯0.994) with a limit of detection (LOD) of 0.0340⯵mol/L and a limit of quantification (LOQ) of 0.113⯵mol/L. The DPV-Co-poly(Met)/GCE method provided good intra-day and inter-day repeatability with RSD values lower than 5%. Also, no interference of real sample matrices was observed on the estriol voltammetric response, making the DPV-Co-poly(Met)/GCE highly selective for estriol. The accuracy test showed that the estriol recovery was in the ranges 96.7%-103% and 98.7%-102% for pharmaceutical tablets and human urine, respectively. The estriol quantification in pharmaceutical tablets performed by the Co-poly(Met)/GCE-assisted DPV method was comparable to the official analytical protocols.
RESUMO
Despite photocatalytic degradation is studied generally focusing the catalyst, its interaction with the contaminant molecule plays a fundamental role in the efficiency of that process. Then, we proposed a comparative study about the photodegradation of two well-known dyes, with different acidity/basicity - Methylene Blue (MB) and Rhodamine B (RhB), catalyzed by TiO2 nanoparticles, varying both dye and photocatalyst concentrations. The results showed that the amphoteric character of MB molecules, even in a range of concentration of 5.0-10.0 mg L(-1) , did not imply in pH variation in solution. Therefore, it did not affect the colloidal behavior of TiO2 nanoparticles, independent of the relative dye/catalyst concentration. The acid-base character of RhB influenced the resultant pH of the solution, implicating in different colloidal behavior of the nanoparticles and consequently, in different degradation conditions according to dye concentration. As the isoelectric point of TiO2 is between the pH range of the RhB solutions used in this study, from 1.0 to 7.5 mg L(-1) , the resultant pH was the key factor for degradation conditions, from a well dispersed to an agglomerated suspension.
RESUMO
A number of reports have been published on use of TiO2 in thin films, magnetic nanocomposites, or heterostructures such as TiO2/Ag and TiO2/SnO2, as catalysts for water decontamination. Hence, semiconductor materials such as SnO2, associated with TiO2 in such nanocomposites, should be assessed in depth for such applications, especially those involving complex structures, such as magnetic photocatalytic nanocomposites. The present study describes the synthesis, characterization and testing of the photocatalytic potential of TiO2 or SnO2 magnetic nanocomposites obtained by the polymeric precursor and the hydrolytic sol-gel methods. The nanocomposites TiO2/CoFe2O4 and SnO2/CoFe2O4 were synthesized from polymeric precursors while TiO2/Fe3O4 and SnO2/Fe3O4 were synthesized by the hydrolytic sol-gel method. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (FEG/SEM) and transmission electron microscopy (TEM). The photocatalytic potentials were evaluated by rhodamine B dye photodegradation under UV-C radiation. Compared to SnO2, the nanocomposites with a coating of TiO2 were found to show better photocatalytic activity, but the SnO2 magnetic nanocomposites showed some photocatalytic activity, even though SnO2 is reported to be inactive for these purposes. As for the synthesis method, the nanocomposites obtained from polymeric precursors had smaller surface areas, but higher photocatalytic activity, than those obtained by the hydrolytic sol-gel method. This observation was attributed to the higher crystallinity and a more active surface resulting from calcination of the polymeric precursor material.