Solar light-driven photocatalysis using mixed-phase bismuth ferrite (BiFeO3/Bi25FeO40) nanoparticles for remediation of dye-contaminated water: kinetics and comparison with artificial UV and visible light-mediated photocatalysis

Recent advances in designing and developing efficient sillenite-based materials for photocatalytic applications

Sillenite materials have been the subject of intense investigation for recent years due to their unique characteristics. They possess a distinct structure with space group I23, allowing them to exhibit distinctive features, such as an electronic structure ideal for certain applications such as photocatalysis. The present research delves into the structure, synthesis, and properties of sillenites, highlighting their suitability for photocatalysis. It explores also advanced engineering strategies for designing sillenite-based photocatalysts, including heterojunction formation, morphology modification, doping, and hybrid processes. Each strategy offers advantages and limitations that are critically discussed. The review then lists and discusses the photocatalytic performance of various sillenite-based systems recently developed for common applications, such as removing hazardous organic and inorganic contaminants, and even infrequent applications, such as microbial inactivation, H2 generation, CO2 reduction and N2 fixation. Finally, valuable insights and suggestions are put forward for future research directions in the field of sillenite-based photocatalysis. This comprehensive overview would provide a valuable resource for the development of efficient photocatalytic systems to address environmental and energy challenges.

GO/TiO2-Related Nanocomposites as Photocatalysts for Pollutant Removal in Wastewater Treatment

Water pollution has been a prevalent issue globally for some time. Some pollutants are released into the water system without treatment, making the water not suitable for consumption. This problem may lead to more grave problems in the future including the destruction of the ecosystem along with the organisms inhabiting it, and illness and diseases endangering human health. Conventional methods have been implemented to remove hazardous pollutants such as dyes, heavy metals, and oil but are incapable of doing so due to economic restraints and the inability to degrade the pollutants, leading to secondary pollution. Photocatalysis is a more recently applied concept and is proven to be able to completely remove and degrade pollutants into simpler organic compounds. Titanium dioxide (TiO₂) is a fine example of a photocatalyst owing to its cost-effectiveness and superb efficiency. However, issues such as the high recombination rate of photogenerated electrons along with positive holes while being only limited to UV irradiation need to be addressed. Carbonaceous materials such as graphene oxide (GO) can overcome such issues by reducing the recombination rate and providing a platform for adsorption accompanied by photocatalytic degradation of TiO₂. The history and development of the synthesis of GO will be discussed, followed by the methods used for GO/TiO₂ synthesis. The hybrid of GO/TiO₂ as a photocatalyst has received some attention in the application of wastewater treatment due to its efficiency and it being environmentally benign. This review paper thereby aims to identify the origins of different pollutants followed by the sickness they may potentially inflict. Recent findings, including that GO/TiO₂-related nanocomposites can remove pollutants from the water system, and on the photodegradation mechanism for pollutants including aromatic dyes, heavy metal and crude oil, will be briefly discussed in this review. Moreover, several crucial factors that affect the performance of photocatalysis in pollutant removal will be discussed as well. Therefore, this paper presents a critical review of recent achievements in the use of GO/TiO₂-related nanocomposites and photocatalysis for removing various pollutants in wastewater treatment.

Considering photocatalytic activity of Cu2+/biochar-doped TiO2 using corn straw as sacrificial agent in water decomposition to hydrogen

In this paper, a simple one-pot thermal synthesis method was used to successfully prepare Cu²⁺/biochar-doped TiO₂ composite catalytic materials. The photocatalytic hydrogen production performance of the composites under different environmental conditions (dark, solar, and visible light irradiation) was analyzed in a biomass photocatalytic system using a corn straw suspension as a sacrificial agent. The Cu²⁺/biochar-doped TiO₂ materials were characterized by SEM, TEM, XRD, FT-IR, XPS, and UV analysis. The photoelectric properties of the Cu²⁺/biochar-doped TiO₂ composites were also analyzed, and the charge separation mechanism of photogenerated carriers under different environmental conditions was investigated. Compared with pure TiO₂, the hydrogen production rate of Cu²⁺/biochar-doped TiO₂ is 23.6 times higher under visible light irradiation and 16.8 times higher under simulated solar irradiation. Using density functional theory, a crystal structure model of Cu²⁺/biochar-doped TiO₂ was established to analyze its energy band structure and density of states. An analysis of the mechanism shows that under simulated sunlight irradiation, the synergistic effect of the TiO₂ doped with Cu²⁺ and biochar causes the formation of a potential Schottky heterojunction on the surface and induces interfacial charge transfer. Furthermore, under visible light irradiation, the photocatalytic production of hydrogen by the Cu²⁺/biochar-doped TiO₂ composite is mainly due to the surface plasmon resonance mechanism of Cu ion-doped TiO₂.

Design of Z-scheme g-C3N4/BC/Bi25FeO40 photocatalyst with unique electron transfer channels for efficient degradation of tetracycline hydrochloride waste

High electron transfer rates and a higher number of electron transfer active sites play important roles in inhibiting the recombination of photogenerated electron-hole pairs. In the experiments described in this article, the g-C₃N₄/BC/Bi₂₅FeO₄₀ composite material was prepared to use biochar (BC) as the conductive channel. The presence of BC significantly increases the electron transfer rate due to its excellent electrical conductivity and can provide more electron transfer active sites. At the same time, BC provides a larger surface area and has a loose porous structure, which lead to excellent adsorption performance. Based on various characterization results, it was confirmed that the Z-scheme heterojunction was successfully constructed between g-C₃N₄ and Bi₂₅FeO₄₀. The photocatalytic experiment results showed that the degradation efficiency of g-C₃N₄/BC/Bi₂₅FeO₄₀ on the tetracycline hydrochloride (TCH) could reach 92.2% within 60 min. Parameters such as circulation stability, pH value of the solution and the amount of composite materials were studied. The synthesized composite material has good reusability and high efficiency in a wide pH range of 3-11. Its excellent photocatalytic activity is attributed to the formation of an effective Z-scheme heterostructure, as well as the rapid photoelectron transfer and excellent adsorption capacity of BC. This work provides a way to design new photocatalysts using semiconductor composite materials and BC materials.

The photocatalytic performance and structural characteristics of nickel cobalt ferrite nanocomposites after doping with bismuth

Here, a novel bismuth-doped nickel-cobalt ferrite (Ni_0.5Co_0.5Bi_0.1Fe_1.9O₄) was synthesized using a sol-gel auto-combustion approach. The impact of bismuth substitution on the nickel-cobalt ferrite structural characteristics was investigated relative to the nickel-cobalt ferrite without bismuth substitution (Ni_0.5Co_0.5Fe₂O₄) based on diverse technical options (e.g., scanning electron microscopy-equipped with an energy dispersive X-ray spectrometer, X-ray diffraction, physisorption, and Fourier-transform infrared spectroscopy). Bismuth doping increased the surface area without affecting pore size. The X-ray diffraction pattern confirmed a nano-ferrite cubic spinel structure of the catalyst. Photodegradation of Congo red (CR) was tested using these nickel-cobalt ferrite catalysts under visible light across varying reaction parameters (e.g., pH, catalyst loading, dye concentration, and reaction time). The photo-degradation efficiency for CR in aqueous medium was the highest (98%) at pH 3 with 0.2 g catalyst loading in 100 mL under visible irradiation to reinforce the role of nanostructures as a potent photocatalyst (QY = 2.79 × 10^-7 molecule photon^-1). The kinetic reaction rate of Bi-doped spinel ferrite (3.5 µmol g^-1 h^-1) was1.25 times higher than those undoped materials. This study experimentally proved that the bismuth-doped nickel-cobalt ferrite photocatalyst is an effective option for removing industrial dyes.

Waste tire carbon in synergetic interaction with spent gamma radioactive source for efficient radiocatalytic degradation of organic dye

Enhanced degradation of organic dye was achieved using two different kinds of waste materials: waste tire granules and spent sealed radioactive sources. Waste tire granules were used as raw material for the production of waste tire char (WTC), which was further utilized as an adsorbent matrix for synergetic adsorption/irradiation degradation of organic dye. The spent radioactive sources were radiographic sealed sources that originate from the industry which generate the high energy radiation. Methylene Blue (MB) was used as an organic model compound. Synthesized WTC has turbostratic structure, irregular shaped particles and developed mesoporous surface. Complete degradation of 0.02 dm³ of 100 mg dm^-3 MB solution, having WTC dose of 1.25 g dm^-3, was achieved with delivered doze of only 60 Gy. The applied doses were 100 times smaller than those presented in the literature. Degradation pathway was determined: OH radicals that originate from radiolysis of water and from the surface of WTC played the crucial role in the radiocatalytic degradation of MB. Breakage of the aromatic ring of MB appeared by the scission of the double C‒S⁺˭C bond as a result of the attack of OH species on adsorbed and electronically reorganized MB molecule.

Auto-combustion synthesis of narrow band-gap bismuth ferrite nanoparticles for solar photocatalysis to remediate azo dye containing water

Narrow band gap of ferrites makes it a good photocatalyst, and it plays very prominent role in the level of degradation of organic dyes by photocatalysis. In the current study, bismuth ferrite (BFO) nanoparticles were synthesized by auto-combustion technique. The synthesized BFO particles have the average crystallite size of 33 nm and band gap energy of 1.9 eV. As revealed by microscopic images, uniform, distinct, and hexahedral shaped BFO nanoparticles of 42.7 nm are formed. The BFO nanoparticles exhibited visible and solar light-mediated photocatalytic activity in degrading Acid Yellow-17. The optimum pH and catalyst loading were found to be pH 5 and 0.2 g/L respectively. Around complete degradation under solar and 95% degradation under visible light could be achieved within 135 min of irradiation. Around 85% and 83% chemical oxygen demand (COD) removal could also be achieved under solar and visible light respectively. The degradation followed first-order kinetics in terms of COD removal. The BFO nanoparticles are promising as solar light active catalysts for wastewater treatment.

Facile synthesis of Fe3O4/CuO a core-shell heterostructure for the enhancement of photocatalytic activity under visible light irradiation

A magnetically separable Fe₃O₄/CuO core-shell heterostructure photocatalyst was synthesized by hydrothermal method. The obtained photocatalyst was characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-visible diffuse reflectance (UV-DRS). The obtained photocatalyst was used for the degradation of azo dye Direct Red 89 (DR89), under visible light irradiation provided by fluorescent lamp of 100 W in the presence of 7 mL of H₂O₂ (30%); the results of the photocatalytic activity for Fe₃O₄/CuO photocatalyst showed that in the presence of 0.75 g dispersed in 250 mL of 40 mg/L of DR89 dye at pH 6 the dye was completely removed after 240 min. Moreover, the photocatalytic activity of the prepared Fe₃O₄/CuO was enhanced 11 and 9 times compared with the pure Fe₃O₄ or CuO. The effect of initial dye concentrations on the photocatalytic activity was studied in the range of 20-60 mg/L, and the results showed that the catalyst has a good photocatalytic activity of 89% even at high concentration (60 mg/L). Furthermore, the catalyst maintained its activity after 5 cycles, and its paramagnetic property facilitates its recovery. The excellent photodegradation activity of Fe₃O₄/CuO was attributed to the low band gap of the catalyst equal to 1.54 eV and the enhancement of light absorption in visible range of 330-780 nm, but also to a better charge carriers separation, due to the presence of Fe₃O₄ that reduces electron/hole recombination.

Nanocasting synthesis of BiFeO3 nanoparticles with enhanced visible-light photocatalytic activity

In this work, monodisperse BiFeO₃ nanoparticles with a particle diameter of 5.5 nm were synthesized by a nanocasting technique using mesoporous silica SBA-15 as a hard template and pre-fabricated metal carboxylates as metal precursors. To the best of our knowledge, the synthesized particles are the smallest BiFeO₃ particles ever prepared by any method. The samples were characterized by X-ray powder diffraction, transmission electron microscopy and UV-vis diffuse reflectance spectroscopy. The phase purity of the product depends on the type of carboxylic acid used in the synthesis of the metal precursors, the type of solvent in the wet impregnation process, and the calcination procedure. By using tartaric acid in the synthesis of the metal precursors, acidified 2-methoxyethanol in the wet impregnation process and a calcination procedure with intermediate plateaus, monodisperse 5.5 nm BiFeO₃ nanoparticles were successfully obtained. Furthermore, the nanoparticles were applied in photodegradation reactions of rhodamine B in aqueous solution under visible-light irradiation. Notably, the cast BiFeO₃ nanoparticles demonstrated very high efficiencies and stability under visible-light irradiation, much higher than those of BiFeO₃ nanoparticles synthesized by other synthetic methods. The possible mechanism in the photodegradation process has been deeply discussed on the basis of radical trapping experiments.

Simultaneous Adsorption and Photocatalysis Processes Based on Ternary TiO2–CuxS–Fly Ash Hetero-Structures

Ternary composites of TiO2–CuxS–fly ash were used in simultaneous adsorption and photocatalysis processes for the removal of organic (dye) pollutants. Composites of semiconductor (TiO2, CuxS) nanomaterials hosted within matrices of fly ash, such as film heterostructures, are promising materials for advanced wastewater treatment. The combination of adsorption and photocatalysis processes was investigated in the removal of methylene blue (MB), considered as a standard in photocatalysis. Ternary film heterostructures obtained by doctor blade technique allows overcoming the separation step of particles from treated wastewater. The comparison between the adsorption and photodegradation tests performed with TiO2–CuxS–fly ash showed that in dark conditions, the MB removal was 75% after 360 min, while in the presence of UV radiation, almost total dismissal of MB was achieved in the same treatment period. The degradation rate of MB, when H2O2 is used as an electron acceptor, could reach 90% in adsorption and 99% in simultaneous adsorption/photocatalysis processes. The adsorption isotherm was found to follow the Langmuir and Freundlich models.

Doping TiO2 with CuSO4 enhances visible light photocatalytic activity for organic pollutant degradation

Photocatalysis is one of the most promising advanced oxidation processes due to the capability of solid catalyst to continuously produce oxidant species under light irradiation. The use of conventional UV lamps is high cost intensive, which undermines the possible implementation in developing countries. Visible light active photocatalysts can overcome these challenges and find a market opportunity for competitive technology implementation. This work proposes the synthesis of visible light active catalyst following a facile sol-gel synthesis that introduces CuSO₄ as dopant in TiO₂. Results present complete abatement of methylene blue in 120 min of treatment under 50 mW cm^-2 of blue light (λ = 450 nm), while commercial P25 TiO₂ presented null abatement under identical conditions. Synthesis parameters including dopant level and calcination temperature allowed defining optimum synthesis conditions based on material characteristics modification and catalytic activity enhancement. A doping level of 0.21 mol% CuSO₄ was identified as optimum condition to enable visible light photocatalysis of doped TiO₂ catalysts calcined at 300 °C. Finally, operational parameters were evaluated defining a wide range of pH operation under 3.0 g L^-1 of catalyst dose to treat up to 20 g L^-1 of highly recalcitrant phenothiazine dye. These optimum conditions allowed complete dye removal under visible light after 120 min of treatment.

Hazardous waste treatment technologies

This is a review of the literature published in 2018 on topics related to hazardous waste management in water, soils, sediments, and air. The review covers treatment technologies applying physical, chemical, and biological principles for contaminated water, soils, sediments, and air. PRACTITIONER POINTS: The management of waters, wastewaters, and soils contaminated by various hazardous chemicals including inorganic (e.g., oxyanions, salts, and heavy metals), organic (e.g., halogenated, pharmaceuticals and personal care products, pesticides, and persistent organic chemicals) was reviewed according to the technology applied, namely, physical, chemical and biological methods. Physical methods for the management of hazardous wastes including adsorption, coagulation (conventional and electrochemical), sand filtration, electrosorption (or CDI), electrodialysis, electrokinetics, membrane (RO, NF, MF), photocatalysis, photoelectrochemical oxidation, sonochemical, non-thermal plasma, supercritical fluid, electrochemical oxidation, and electrochemical reduction processes were reviewed. Chemical methods including ozone-based, hydrogen peroxide-based, persulfate-based, Fenton and Fenton-like, and potassium permanganate processes for the management of hazardous were reviewed. Biological methods such as aerobic, anaerobic, bioreactor, constructed wetlands, soil bioremediation and biofilter processes for the management of hazardous wastes, in mode of consortium and pure culture were reviewed.

Towards visible-light photocatalysis for environmental applications: band-gap engineering versus photons absorption-a review

A range of different studies has been performed in order to design and develop photocatalysts that work efficiently under visible (and near-infrared) irradiation as well as to improve photons absorption with improved reactor design. While there is consensus on the importance of photocatalysis for environmental applications and the necessity to utilized solar irradiation (or visible-light) as driving force for these processes, it is not yet clear how to get there. Discussion on the future steps towards visible-light photocatalysis for environmental application is of great interest to scientific and industrial communities and the present paper reviews and discusses the two main approaches, band-gap engineering for efficient solar-activated catalysts and reactor designs for improved photons absorption. Common misconceptions and drawbacks of each technology are also examined together with insights for future progress.