Photocatalytic degradation employing metal oxides, such as TiO₂ nanoparticles, as catalysts is an important technique for the removal of synthetic dyes from wastewater under light irradiation. The basic principles of photocatalysis of dyes, the effects of the intrinsic photoactivity of a catalyst, and the conventional non-fundamental factors are well established. Recently reported photocatalysis studies of dyes in single, binary, and ternary solute solutions opened up a new perspective on competitive photocatalytic degradation of the dyes. There has not been a review on the photocatalytic behavior of binary or ternary solutions of dyes. In this regard, this current review article summarizes the photocatalytic behavior of methylene, rhodamine B, and methyl orange in their binary or ternary solutions. This brief overview introduces the importance of the dynamics of immobilization and reactivity of the dyes, the vital roles of molecular conformation and functional groups on their diffusion onto the catalyst surface, and photocatalytic degradation, and provides an understanding of the simultaneous photocatalytic processes of multiple dyes in aqueous systems.

Amongst the environmental issues throughout the world, organic synthetic dyes continue to be one of the most important subjects in wastewater remediation. In this paper, the photocatalytic degradation of the dimethylmethane fluorescent dye, Auramine O (AO), was investigated in a heterogeneous aqueous solution with 100 nm anatase TiO₂ nanoparticles (NPs) under 365 nm light irradiation. The effect of irradiation time was systematically studied, and photolysis and adsorption of AO on TiO₂ NPs were also evaluated using the same experimental conditions. The kinetics of AO photocatalytic degradation were pseudo-first order, according to the Langmuir–Hinshelwood model, with a rate constant of 0.048 ± 0.002 min⁻¹. A maximum photocatalytic efficiency, as high as 96.2 ± 0.9%, was achieved from a colloidal mixture of 20 mL (17.78 μmol L⁻³) AO solution in the presence of 5 mg of TiO₂ NPs. The efficiency of AO photocatalysis decreased nonlinearly with the initial concentration and catalyst dosage. Based on the effect of temperature, the activation energy of AO photocatalytic degradation was estimated to be 4.63 kJ mol⁻¹. The effect of pH, additional scavengers, and H₂O₂ on the photocatalytic degradation of AO was assessed. No photocatalytic degradation products of AO were observed using UV–visible and Fourier transform infrared spectroscopy, confirming that the final products are volatile small molecules.

In this study, adsorption characteristics of a negatively charged dye, Acid Blue 25 (AB25), on pomelo pith (PP) was studied by varying the adsorption parameters, with the aim of evaluating the adsorption mechanism and establishing the role of hydrogen bonding interactions of AB25 on agricultural wastes. The kinetics, intraparticle diffusion, mechanism, and thermodynamics of the AB25 adsorption were systematically evaluated and analyzed by pseudo-first-order and pseudo-second-order kinetic models, the Weber–Morris intraparticle and Boyd mass transfer models, the Langmuir, Freundlich, Dubinin–Radushkevich, and Temkin isotherm models, and the Van’t Hoff equation. It was found that AB25 adsorption followed pseudo-second-order kinetics, governed by a two-step pore-volume intraparticle diffusion of external mass transfer of AB25 onto the PP surface. The adsorption process occurred spontaneously. The adsorption mechanism could be explained by the Langmuir isotherm model, and the maximum adsorption capacity was estimated to be 26.9 mg g⁻¹, which is comparable to many reported adsorbents derived from agricultural wastes. Changes in the vibrational spectra of the adsorbent before and after dye adsorption suggested that AB25 molecules are bound to the PP surface via electrostatic and hydrogen bonding interactions. The results demonstrated that the use of pomelo pith, similar to other agricultural wastes, would provide a basis to design a simple energy-saving, sustainable, and cost-effective approach to remove negatively charged synthetic dyes from wastewater.

In undergraduate courses, kinetics and thermodynamics are often taught as separate modules. It is because equilibrium data from thermodynamics do not enlighten us about the rate of attainment of equilibrium, which is kinetics. It is true that even if a chemical reaction is thermodynamically favorable, it may never happen due to kinetic considerations. However, this separation of kinetics and thermodynamics is unfortunate in some respects. In this work, the link between chemical kinetics and thermodynamics is explored based on them both being defined by a single potential energy diagram. A common misconception caused by undergraduate courses on chemical kinetics is a claim that the Arrhenius equation is deficient because it does not offer a precise meaning for the pre-exponential term A. Undergraduate courses often go on to proffer more sophisticated theories in the form of collision theory CT and transition state theory TST resulting in the Eyring equation. These latter two theories are required in order to formally show that the pre-exponential term contains information on the entropy requirements of the reaction. In this work, it will be shown that by considering the link between thermodynamics and kinetics it can easily be shown that A was already implicitly linked to the product of the entropy of activation of the reaction and the natural frequency of the reaction. This work makes use of previously published and unpublished results on photochromic naphthoxazine-spiro-indolines to compare different theories.

Time is an enigmatic dimension. People assign somewhat mystical properties to time, especially the prospect of time travel. Nearly everyone has considered the possibility of a time machine at some point in their life, and this isn’t just because of the author Jules Verne. When you consider wanting a time machine, do you want to travel forwards or backwards in time? Since we are always apparently moving forwards in time, I would assume that the obsession is going backwards. The reason for this is that all people make mistakes and have regrets, and they would like to do some things differently in hindsight. But these are all humanistic desires, and it is this humanistic desire that incites people to give a mystical and magical property to time. It is probable that most people would wish to correct a past mistake at some point in their lives. The truth is that people can atone for their mistakes, but they can never absolutely undo them as if they never happened. The desire and ability to reverse time is currently only scientific on a psychological level. In fact, in the absence of some as-yet undiscovered physics the arrow of time is decided by Bolzmann’s entropy, therefore if a person can appreciate the meaning of entropy then they will fundamentally understand the arrow of time. Hence, chemistry students are in a good position when it comes to understanding time.