Enhanced visible photocatalytic degradation of diclofen over N-doped TiO2 assisted with H2O2: A kinetic and pathway study

Highly Efficient and Reusable PI/TiO2 Organic-Inorganic Microfibers for Sustainable Photocatalytic Degradation of Multiple Organic Pollutants under Simulated Sunlight

Herein, a series of polyimide (PI)/titanium dioxide (TiO2) organic-inorganic flexible composite microfibers with high photocatalytic performance and good reusability were prepared by combining electrospinning technology and a hydrothermal method. Under simulated sunlight, the photocatalytic characteristics of the as-prepared PI nanofibers, TiO2 nanorods, and PI/TiO2 microfibers were evaluated with photocatalytic degradation of Rhodamine B (RhB) solution. Among the tested samples, PI/TiO2-3 mL hydrochloric acid-160 °C-14 h (PI/TiO-3-160-14) (100%) exhibited a superior photocatalytic degradation rate compared to pure PI (84.0%) and TiO2 (62.2%). The enhancement of the photocatalytic performance was attributed to the Z-scheme heterojunction mechanism. When the interface was irradiated by simulated sunlight, the band edge bending, built-in electric field, and Coulomb interaction synergistically facilitated the separation and transport of electron-hole pairs in the heterojunction. This enhanced the oxidation and reduction abilities of the valence and conduction bands of PI/TiO2. These results were adequately verified by X-ray photoelectron spectroscopy (XPS) analyses and radical trapping experiments. Additionally, PI/TiO2 microfibers also demonstrated excellent photocatalytic activity toward methylene blue (MB, 81.4%), methyl orange (MO, 95.9%), and malachite green (KG, 98.9%), underscoring the versatile applicability of PI/TiO2. Further supplementary investigations illustrated that PI/TiO2 microfibers also possess excellent photostability during our extensive recycling photocatalytic experiments.

Synthesis of a Co/Ni-MOF-74@PDI Z-scheme photocatalyst as a highly efficient photo-assisted Fenton-like catalyst for the removal of chlortetracycline hydrochloride

In this paper, ultra-thin nanofiber PDI was obtained by self-assembly dispersion of commercial PDINH. A novel Co/Ni-MOF-74@PDI Z-scheme heterojunction photocatalyst material was constructed by a simple solvothermal method. XRD, SEM, TEM, FT-IR and other characterization techniques proved the successful preparation of the Co/Ni-MOF-74@PDI Z-scheme heterojunction photocatalyst material. By degrading chlortetracycline hydrochloride, it was found that the photocatalytic activity of Co/Ni-MOF-74@PDI was much higher than that of pure Co/Ni-MOF-74 and PDI. Subsequently, Co/Ni-MOF-74@PDI was used to activate H2O2 to further improve the degradation efficiency of chlortetracycline hydrochloride. It was found that the photocatalytic performance was greatly improved after the addition of 19.6 mM H2O2 to the system, and the degradation rate of chlortetracycline hydrochloride was 87% within 90 min. The electron transfer pathway and H2O2 activation mechanism of the Co/Ni-MOF-74@PDI composite photocatalyst were proved by free radical quenching experiments, electron paramagnetic resonance analysis and X-ray electron spectroscopy. Finally, the easy exfoliation point and degradation pathway of chlortetracycline hydrochloride were studied using density functional theory, UPLC-MS and toxicity evaluation software. It was found that the main active substances were h+, ˙O2, 1O2 and ˙OH, and the toxicity of chlortetracycline hydrochloride and its intermediates was evaluated.

Enhanced degradation of 2,6-dimethylphenol by photocatalytic systems using TiO2 assisted with H2O2 and Fe(III)

In this study, several photocatalytic degradation systems were investigated using 2,6-dimethylphenol (2,6-DMP) as a model compound. Highly reactive species are formed in four systems, Fe(III), TiO₂, TiO₂/H₂O₂ and TiO₂/Fe(III) where complete degradation of 2,6-DMP was achieved under UV radiation. Photodegradation of the 2,6-DMP has been described by pseudo-first order kinetic model in the presence of TiO₂. In UV/TiO₂-H₂O₂ system, the addition of H₂O₂ in the TiO₂ suspension improves the degradation rate of 2,6-DMP from 70% to 100% for a H₂O₂ concentration of 10^-2 M in 3 h. In homogeneous system, HO^• and Fe²⁺ can be generated by the irradiation of Fe(III) solution. The speciation of Fe(III) obtained from Visual MINTEQ soft showed the formation of several species and Fe(OH)²⁺ were the most predominant and active species in a pH range of 2.5-3.5. At a low concentration of TiO₂ (30 mg L^-1), an important positive effect due to the iron addition has been shown in TiO₂/Fe(III) system, the entrance of metallic ions at different concentrations enhanced the photocatalytic activity of TiO₂. A degradation percentage of 90% was achieved in the UV/TiO₂-Fe(III) system under optimal conditions against 57% in UV/TiO₂ system. Strong synergistic effect was observed in the UV/TiO₂-H₂O₂ binary system. On the basis of literature, a pathway for 2,6-DMP degradation was proposed. The mechanism of degradation of the 2,6-DMP did not involve only HO^• radicals, an interaction of Fe(III) in the excited state with 2,6-DMP occurred giving rise to the formation of 2,6-dimethylphenoxyl radical.

Degradation of Organic Methyl Orange (MO) Dye Using a Photocatalyzed Non-Ferrous Fenton Reaction

Removal of recalcitrant organic pollutants by degradation or mineralization from industrial waste streams is continuously being explored to find viable options to apply on the commercial scale. Herein, we propose a titanium nanotube array (based on a non-ferrous Fenton system) for the successful degradation of a model contaminant azo dye, methyl orange, under simulated solar illumination. Titanium nanotube arrays were synthesized by anodizing a titanium film in an electrolyte medium containing water and ethylene glycol. Characterization by SEM, XRD, and profilometry confirmed uniformly distributed tubular arrays with 100 nm width and 400 nm length. The non-ferrous Fenton performance of the titanium nanotube array in a minimal concentration of H₂O₂ showed remarkable degradation kinetics, with a 99.7% reduction in methyl orange dye concentration after a 60 min reaction time when illuminated with simulated solar light (100 mW cm^-2, AM 1.5G). The pseudo-first-order rate constant was 0.407 µmol^-1 min^-1, adhering to the Langmuir-Hinshelwood model. Reaction product analyses by TOC and LC/MS/MS confirmed that the methyl orange was partially fragmented, while the rest was mineralized. The facile withdrawal and regeneration observed in the film-based titanium nanotube array photocatalyst highlight its potential to treat real industrial wastewater streams with a <5% performance drop over 20 reaction cycles.

Integrated full-scale solar CPC/UV-LED–filtration system as a tertiary treatment in a conventional WWTP for agricultural reuse purposes

Today, the emergence of increasingly restrictive treatment and reuse policies make the implementation of full-scale tertiary treatment, capable of improving the quality of water, a priority. Full-scale TiO2 photocatalysis systems are resulting in a promising option, since TiO2 is commercially available. However, questions such as how to work continuously during day/night irradiation cycle, or the removing of TiO2 in outlet flow are still unresolved. In this work, a full-scale system integrating a solar CPC/UV-LED step combined with commercial microfiltration membranes was installed in a conventional WWTP for agricultural reuse purposes. After optimization, 0.5 g/L of catalyst and combined SOLAR + UV-LED showing the highest pharmaceutical removal percentages, while a self-designed UV-LED included in the own reaction tank resulting in higher efficiencies compared with commercial lamps. Longer membrane surface area decreased fouling problems in the system. However, 60 min of irradiation time was necessary to reach the most restrictive water quality values according with (EU 2020/741). After optimization step, total costs were reduced by 45%. However, it was shown that a reduction in operating and maintenance costs, along with the development of more effective and economical commercial filtration membranes is a key factor; therefore, working on these aspects is essential in the treated water cost reduction.

Graphical abstract

TiO2 based Photocatalysis membranes: An efficient strategy for pharmaceutical mineralization

Among the various emerging contaminants, pharmaceuticals (PhACs) seem to have adverse effects on the quality of water. Even the smallest concentration of PhACs in ground water and drinking water is harmful to humans and aquatic species. Among all the deaths reported due to COVID-19, the mortality rate was higher for those patients who consumed antibiotics. Consequently, PhAC in water is a serious concern and their removal needs immediate attention. This study has focused on the PhACs' degradation by collaborating photocatalysis with membrane filtration. TiO₂-based photocatalytic membrane is an innovative strategy which demonstrates mineralization of PhACs as a safer option. To highlight the same, an emphasis on the preparation and reinforcing properties of TiO₂-based nanomembranes has been elaborated in this review. Further, mineralization of antibiotics or cytostatic compounds and their degradation mechanisms is also highlighted using TiO₂ assisted membrane photocatalysis. Experimental reactor configurations have been discussed for commercial implementation of photoreactors for PhAC degradation anchored photocatalytic nanomembranes. Challenges and future perspectives are emphasized in order to design a nanomembrane based prototype in future for wastewater management.

The Evolution of Photocatalytic Membrane Reactors over the Last 20 Years: A State of the Art Perspective

The research on photocatalytic membrane reactors (PMRs) started around the year 2000 with the study of wastewater treatment by degradation reactions of recalcitrant organic pollutants, and since then the evolution of our scientific knowledge has increased significantly, broadening interest in reactions such as the synthesis of organic chemicals. In this paper, we focus on some initial problems and how they have been solved/reduced over time to improve the performance of processes in PMRs. Some know-how gained during these last two decades of research concerns decreasing/avoiding the degradation of the polymeric membranes, improving photocatalyst reuse, decreasing membrane fouling, enhancing visible light photocatalysts, and improving selectivity towards the reaction product(s) in synthesis reactions (partial oxidation and reduction). All these aspects are discussed in detail in this review. This technology seems quite mature in the case of water and wastewater treatment using submerged photocatalytic membrane reactors (SPMRs), while for applications concerning synthesis reactions, additional knowledge is required.

Visible-Light Photocatalysts and Their Perspectives for Building Photocatalytic Membrane Reactors for Various Liquid Phase Chemical Conversions

Photocatalytic organic synthesis/conversions and water treatment under visible light are a challenging task to use renewable energy in chemical transformations. In this review a brief overview on the mainly employed visible light photocatalysts and a discussion on the problems and advantages of Vis-light versus UV-light irradiation is reported. Visible light photocatalysts in the photocatalytic conversion of CO2, conversion of acetophenone to phenylethanol, hydrogenation of nitro compounds, oxidation of cyclohexane, synthesis of vanillin and phenol, as well as hydrogen production and water treatment are discussed. Some applications of these photocatalysts in photocatalytic membrane reactors (PMRs) for carrying out organic synthesis, conversion and/or degradation of organic pollutants are reported. The described cases show that PMRs represent a promising green technology that could shift on applications of industrial interest using visible light (from Sun) active photocatalysts.

Application of Hybrid Membrane Processes Coupling Separation and Biological or Chemical Reaction in Advanced Wastewater Treatment

The rapid urbanization and water shortage impose an urgent need in improving sustainable water management without compromising the socioeconomic development all around the world. In this context, reclaimed wastewater has been recognized as a sustainable water management strategy since it represents an alternative water resource for non-potable or (indirect) potable use. The conventional wastewater remediation approaches for the removal of different emerging contaminants (pharmaceuticals, dyes, metal ions, etc.) are unable to remove/destroy them completely. Hybrid membrane processes (HMPs) are a powerful solution for removing emerging pollutants from wastewater. On this aspect, the present paper focused on HMPs obtained by the synergic coupling of biological and/or chemical reaction driven processes with membrane processes, giving a critical overview and particular emphasis on some case studies reported in the pertinent literature. By using these processes, a satisfactory quality of treated water can be achieved, permitting its sustainable reuse in the hydrologic cycle while minimizing environmental and economic impact.