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 (BNO_x) 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 BNO_x 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 BNO_x 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 BNO_x NPs shows that surface oxygen defects prevent the sorption of MB molecules due to their planar orientation over the BNO_x surface. To enhance the sorption capacity, surface oxygen defects should be eliminated.

Developing regenerable solid sorbents that combine high CO2 uptake, accessible porosity, and mild regeneration remains a key challenge for practical carbon capture. Single-walled carbon nanotubes have been considered an unattractive candidate among carbon substrates due to the high price, despite the benefits of chemical stability, fully accessible surface area, and macroporous structure. Nevertheless, the recent advances in scaling SWCNT production shifted the paradigm, especially in the case of highly reusable sorbents. Here, we introduce a polyethyleneimine (PEI)/SWCNT hybrid sorbent that combines the highly accessible surface area of SWCNTs with localized CO2-affinitive regions provided by PEI. The hybrid was prepared by a simple and scalable method of incipient wet impregnation, with 2 wt.% PEI identified as the optimum loading while preserving the accessible nanotube framework. The resulting material exhibited a dynamic CO2 adsorption capacity of 7.4 mmol/g at 30 °C under 50% CO2/50% N2 and retained a capacity of 1.2 mmol/g under 15% CO2/85% N2. In the presence of 4% moisture, the capacity decreased to 3.5 mmol/g, indicating competitive water adsorption. The PEI–SWCNT hybrid maintained an average regeneration efficiency of around 95% over ten consecutive adsorption–desorption cycles, with effective regeneration at temperatures ≤100 °C. An apparent heat of adsorption of −20.2 ± 3.5 kJ/mol, together with temperature-programmed desorption and density functional theory results, indicates predominantly physisorption accompanied by localized amine-assisted CO2 stabilization. Overall, the results show that the PEI–SWCNT combination provides a regenerable hybrid adsorbent for dynamic CO2 capture.