RESUMO
In this study, sugarcane bagasse ash (SCBA), obtained as residue from the sugar mill, was used as an adsorbent for Acid Red 27 (AR27) removal from aqueous solutions. The ash characterization data showed 23.63% of organic compounds and silica (α-SiO2) as the most expressive inorganic compound (confirmed by X-ray diffractogram), the BET surface area had a value of 62.79 m2.g-1 and the pHpzc was 8.45. Regarding the adsorptive tests, the optimal initial pH to the dye removal was 2.0. The adsorption equilibrium reached in about 4 h contact time and optimum SCBA dosage was found to be 4 g.L-1. The pseudo-second order model best represented the adsorption kinetics. The Freundlich equation presented the best fit to the equilibrium data for the removal of AR27 by ash, with maximum adsorption capacity of 15 mg.g-1 at pH 2.0. Thermodynamic study indicate that AR27 adsorption on SCBA occurs through a physisorption mechanism, with ΔHºads < 15 kJ.mol-1. The ΔHºads evaluated by Vant' Hoff equation was explained as a combination of water desorption enthalpy, ΔHºW and isosteric like enthalpy, ΔHºD for the dye adsorption in liquid environment. The ΔHºD = 9.2 kJ.mol-1 was calculated from Clausius-Clapeyron approach. The effects of coexisting anions on the adsorption and regeneration and reuse of the adsorbent were also investigated. This study suggests that SCBA, which was used without any pretreatment, has the potential to be applied as a low-cost adsorbent to mitigate effluents contamination with AR27 dye at low concentrations.
RESUMO
In this work, a simple, low-cost and easy-to-handle analytical procedure based on carbon quantum dots (CQDs) is proposed to check commercially available formulated microbicides that are used to mitigate the transmission of viruses, such as SARS-COV-2, or bacterial diseases. For this purpose, CQDs were synthesized via pyrolysis using citric acid and ethylenediamine as precursors to produce an intense fluorescence that is used to measure the concentration of hypochlorite, an important biocidal agent present in sanitizing mats, by quenching mechanisms. The characterization of the CQDs was performed using IR spectrophotometry, UV-Vis spectrophotometry, spectrofluorometry, thermogravimetric analysis, scanning electron microscopy, dynamic light scattering, X-ray diffraction, energy-dispersive spectroscopy, and zeta potential measurements. For analytical purposes, fluorescence was measured in a UV chamber irradiated using an LED with the maximum emission at 350 nm. A smartphone was coupled to the UV chamber to measure the fluorescence quenching due to the presence of hypochlorite, and further the digital images were decomposed by RGB data using free software. Tests of pH, CQD concentration and stability of the fluorescence emitted were performed. The stability study of the fluorescence emitted by the CQD solution showed a relative standard deviation lower than 5.0%. The fluorescence digital image-based (FDIB) method resulted in a linear range from 17.44 µmol L-1 to 90.0 µmol L-1 with an LOD of 3.30 µmol L-1 for the determination of hypochlorite using a microplate made of PLA (polylactic acid) customized using a 3D printer. Furthermore, the hypochlorite concentration was tested in situ for its compliance in a sanitizing mat, in a real use situation (daily, a group of four people, each one kept their feet on the mat for 30 s). After 2.5 h, the monitored concentration of hypochlorite was 0.04953% (w/v) or 7.63 mmol L-1, and therefore, it was inefficient to act as a sanitizing agent. Thus, for the first time in the literature, an FDIB method with CQDs is used to verify in situ microbicide practices with a fast and low-cost analytical procedure.
