A New Insight into the Mechanisms Underlying the Discoloration, Sorption, and Photodegradation of Methylene Blue Solutions with and without BNOx Nanocatalysts

Abstract

Methylene blue (MB) is widely used as a test material in photodynamic therapy and photocatalysis. These applications require an accurate determination of the MB concentration as well as the factors affecting the temporal evolution of the MB concentration. Optical absorbance is the most common method used to estimate MB concentration. This paper presents a detailed study of the dependence of the optical absorbance of aqueous methylene blue (MB) solutions in a concentration range of 0.5 to 10 mg·L−1. The nonlinear behavior of optical absorbance as a function of MB concentration is described for the first time. A sharp change in optical absorption is observed in the range of MB concentrations from 3.33 to 4.00 mg·L−1. Based on the analysis of the absorption spectra, it is concluded that this is due to the formation of MB dimers and trimers in the specific concentration range. For the first time, a strong, thermally induced discoloration effect of the MB solution under the influence of visible and sunlight was revealed: the simultaneous illumination and heating of MB solutions from 20 to 80 °C leads to a twofold decrease in the MB concentration in the solution. Exposure to sunlight for 120 min at a temperature of 80 °C led to the discoloration of the MB solution by more than 80%. The thermally induced discoloration of MB solutions should be considered in photocatalytic experiments when tested solutions are not thermally stabilized and heated due to irradiation. We discuss whether MB is a suitable test material for photocatalytic experiments and consider this using the example of a new photocatalytic material—boron oxynitride (BNOx) nanoparticles—with 4.2 and 6.5 at.% of oxygen. It is shown that discoloration is a complex process and includes the following mechanisms: thermally induced MB photodegradation, MB absorption on BNOx NPs, self-sensitizing MB photooxidation, and photocatalytic MB degradation. Careful consideration of all these processes makes it possible to determine the photocatalytic contribution to the discoloration process when using MB as a test material. The photocatalytic activity of BNOx NPs containing 4.2 and 6.5 at.% of oxygen, estimated at ~440 μmol·g−1·h−1. The obtained results are discussed based on the results of DFT calculations considering the effect of MB sorption on its self-sensitizing photooxidation activity. A DFT analysis of the MB sorption capacity with BNOx NPs shows that surface oxygen defects prevent the sorption of MB molecules due to their planar orientation over the BNOx surface. To enhance the sorption capacity, surface oxygen defects should be eliminated.

A Uniform Design Method Can Optimize the Combinatorial Parameters of Antimicrobial Photodynamic Therapy, Including the Concentrations of Methylene Blue and Potassium Iodide, Light Dose, and Methylene Blue’s Incubation Time, to Improve Fungicidal Effects on Candida Species

“…As we know, MB degradation increases as irradiation time is prolonged, along with a decrease in MB concentration. Matveev et al [35] used optical absorbance to estimate MB concentration, and found that aqueous MB solutions have a nonlinear behavior of optical absorbance, suggesting the formation of MB dimers and trimers in the specific concentration range. MB monomers and dimers may be involved in different kinds of photodynamic reactions, and their concentration influences the mechanisms and types of ROS produced [36].…”

Carbon Quantum Dots Accelerating Surface Charge Transfer of 3D PbBiO2I Microspheres with Enhanced Broad Spectrum Photocatalytic Activity—Development and Mechanism Insight

“…Semiconductor photocatalysis is deemed as a promising technique to purify water which is contaminated by various pollutants, such as dyes, antibiotics, endocrine disruptors, and so on [ 1 , 2 , 3 ]. Taking full advantage of solar energy has already been proven as a “green” strategy to settle environmental contamination and energy crunch [ 4 , 5 , 6 ].…”

Editorial for Special Issue “Advanced Materials in Catalysis and Adsorption”

“…Such advanced materials can be also used for electrocatalytic applications involving the oxygen evolution reaction (OER) [ 12 ] and the electrocatalytic reduction of carbon dioxide (CO 2 RR) [ 15 ]. Researchers’ attention has been especially drawn to environmental protection, where advanced materials and adsorbents are highly demanded for the processing of waste components [ 1 , 16 , 17 , 18 ] and pharmaceuticals [ 17 ], the (photo)degradation of dyes [ 16 , 18 ], the catalytic decomposition of chlorinated hydrocarbons [ 1 , 2 , 4 ], wastewater treatment [ 1 , 18 ], and the abatement of CO-containing gases [ 13 ].…”

“…In the research presented in [ 18 ], the authors synthesized zerovalent iron (ZVI) using biological reducing agents isolated from tea-leaves (polyphenolic compounds) and showed the applicability of the designed ZVI system to wastewater treatment. In [ 16 ], the photodegradation of methylene blue over BNO x catalysts (containing 4.2 and 6.5% oxygen) was studied in detail. The authors revealed the important role of surface oxygen defects in the adsorption capacity and photocatalytic activity of BNO x .…”