Niobium Oxide Catalysts as Emerging Material for Textile Wastewater Reuse: Photocatalytic Decolorization of Azo Dyes

Fe-doped g-C3N4 with duel active sites for ultrafast degradation of organic pollutants via visible-light-driven photo-Fenton reaction: Insight into the performance, kinetics, and mechanism

The photo-Fenton process provides a sustainable and cost-effective strategy for removing refractory organic contaminants in wastewater. Herein, a high-efficient Fe-doped g-C3N4 photocatalyst (Fe@CN10) with a unique 3D porous mesh structure was prepared by one-pot thermal polymerization for ultrafast degradation of azo dyes, antibiotics, and phenolic acids in heterogeneous photo-Fenton systems under visible light irradiation. Fe@CN10 exhibited a synergy between adsorption-degradation processes due to the co-existence of Fe3C and Fe3N active sites. Specifically, Fe3C acted as an adsorption site for pollutant and H2O2 molecules, while Fe3N acted as a photocatalytic active site for the high-efficient degradation of MO. Resultingly, Fe@CN10 showed a photocatalytic degradation rate of MO up to 140.32 mg/L min-1. The dominant ROS contributed to the removal of MO in the photo-Fenton pathway was hydroxyl radical (•OH). Surprisingly, as the key reactive species, singlet oxygen (1O2) generated from superoxide radical (•O2-) also efficiently attacked MO in a photo-self-Fenton pathway. Additionally, sponge/Fe@CN10 was prepared and filled in the continuous flow reactors for nearly 100% degradation of MO over 150 h when treating artificial organic wastewater. This work provided a facile route to prepare highly-active Fe-doped photocatalysts and develop a green photocatalytic system for wastewater treatment in the future.

Fabrication of high performance Ti/SnO2-Nb2O5 electrodes for electrochemical textile wastewater treatment

Textile wastewater contains many organic compounds and colors that affect aquatic life and human health when discharged into the environment. High coloration due to excess dyes entering the wastewater causes coloration to the receiving water stream, affects the photosynthesis process of aquatic species, and adversely affects the landscape. SnO₂-based electrodes have been extensively used in electrochemical water treatment, but their low durability decreases the pollutant treatment ability. Therefore, it is necessary to add another stable oxide to improve the performance and stability of SnO₂ electrodes. This study aims to fabricate Ti/SnO₂-Nb₂O₅ electrodes for the textile wastewater treatment using the electrochemical oxidation method. Different molar ratios of SnO₂:Nb₂O₅ coating were prepared using the sol-gel method and then coated on the Ti substrates for calcination in 60 min at 500 °C. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer Emmett Teller (BET), and cyclic voltammetry (CV) were used to determine the surface and electrochemical properties of Ti/SnO₂-Nb₂O₅ electrodes. The SEM images show that SnO₂-Nb₂O₅ electrode surfaces have the appearance of typical cracking structures of mixed metal oxides electrodes. The XRD spectrum show the SnO₂ peaks of facet (110), (101), (200), (301), (321) and Nb₂O₅ peaks of facet (001), (002), (100), (101), (102) on Ti substrates. Furthermore, the specific surface area of the Ti/SnO₂-Nb₂O₅ electrode ranges from 37.354 m²/g (SnO₂:Nb₂O₅ = 9:1) to 71.885 m²/g (SnO₂:Nb₂O₅ = 1:9). The electrochemical properties of SnO₂:Nb₂O₅ electrodes showed high oxygen, chlorine evolution potential and high organic pollutant degradation in textile wastewater with COD removal at 83 %, decolorization at 74 % and the generation of many free radicals such as HO^•, H₂O₂, O₃, Cl₂. The results demonstrate that the Ti/SnO₂-Nb₂O₅ electrode with the mole ratio of 3:7 is the best in textile wastewater treatment with the longest service life (39 h).

Efficient degradation of Rhodamine B in water by CoFe2O4/H2O2 and CoFe2O4/PMS systems: A comparative study

In this work, a comparative study of efficient degradation of Rhodamine B (RhB) in CoFe₂O₄/H₂O₂ and CoFe₂O₄/PMS systems was performed. Batch experiments indicated that the RhB degradation rate of CoFe₂O₄/H₂O₂ system reached 95.5% at 90 min under the condition of 0.5 g L^-1 of CoFe₂O₄ dosage, 10 mM of H₂O₂ concentration and 3.0 of initial pH. At certain conditions of initial pH = 7.0, 0.3 g L^-1 of CoFe₂O₄ dosage, 7 mM of PMS concentration, CoFe₂O₄/PMS system could completely degrade RhB within 90 min. EPR and quenching experiments indicated that •OH was the main active species of CoFe₂O₄/H₂O₂ system, and •OH, SO₄•^-, •O₂^- and ¹O₂ participated in RhB degradation of CoFe₂O₄/PMS system. The circulate of Co(II)/Co(III) and Fe(II)/Fe(III) on the CoFe₂O₄ surface promoted the formation of free radical species in the two system. In CoFe₂O₄/PMS system, the formed •O₂^- and SO₅•^- realized the generation of non-free radical species (¹O₂). The LC-MS results indicated that N-de-ethylation, chromophore cleavage, opening rings and mineralization were the main steps for the RhB degradation of the two systems. After five cycles of degradation experiment, the CoFe₂O₄/H₂O₂ and CoFe₂O₄/PMS systems still maintained the high degradation rate (85.2% and 92.4%) and low mass loss (2.7% and 3.09%). In addition, CoFe₂O₄/PMS system had better potential value for the actual water and multi-pollutant degradation than CoFe₂O₄/H₂O₂ system. Finally, the toxicity analysis and cost assessment of the two oxidation systems were preliminarily evaluated.

Visible-light-driven 3D Bi5O7I/BiOCl microsphere with enhanced photocatalytic capability: Performance, degradation pathway, antibacterium and mechanism

It is well known that both of the separation efficiency of photogenerated carriers and the response capability to visible light remarkably affect the photocatalytic performance. In the present work, a 3D microsphere of Bi₅O₇I/BiOCl heterojunction catalyst was synthetised. The synergy of Bi₅O₇I and BiOCl not only significantly enhances the transfer rate and separation efficiency of carriers, but also heightens light absorption capacity. As-prepared Bi₅O₇I/BiOCl (40 wt% BiOCl) has a higher degradation efficiency on doxycycline hydrochloride (DC) (90 min, 83.0%) and super high inhibition rate (90 min, 99.92%) on Escherichia coli under visible light, compared to the two monomers. Pollutants DC is finally decomposed into CO₂, H₂O and small molecule intermediates by generated h⁺, •OH and •O₂^-. The effects of reactive radicals follow the order of •OH radicals > h⁺ radicals ≫ •O₂^- and e^- radicals. The possible structures of intermediates and four possible degradation pathways involved were also discussed. In addition, As-synthetised Bi₅O₇I/BiOCl has preferable reusability and excellent chemical stability. Biological toxicity experiments also verify that Bi₅O₇I/BiOCl is a green and environmentally friendly composite material. This strategy provides a green, low-toxic way for the application of traditional type II heterojunction in the fields of environmental remediation and photocatalysis.

Scalable visible light 3D printing and bioprinting using an organic light-emitting diode microdisplay

To address current unmet needs in terms of scalability and material biocompatibility for future photocrosslinking-based additive manufacturing technologies, emergent platform designs are in inexorable demand. In particular, a shift from the present use of cell-damaging UV light sources in light-based three-dimensional (3D) bioprinting methods demands new platforms. We adopted an organic light-emitting diode (OLED) microdisplay as a digital visible light modulator to create a 3D printing platform modality that offers scalability and multi-material capability while forgoing the need for UV photocrosslinking. We formulate biocompatible inks that are visible light-crosslinkable with relatively quick photoinitiation rates. We demonstrated successful attachment and rapid growth of primary human dermal fibroblast-adult (HDF-a) cells on biological substrates fabricated using the OLED platform. This platform incites new possibilities by providing a simple-yet-effective means for low-cost, high-throughput, and multi-material 3D fabrication of functional structures made of polymers, ceramic composites, and biomaterials.

The Surge of Metal-Organic-Framework (MOFs)-Based Electrodes as Key Elements in Electrochemically Driven Processes for the Environment

Metal–organic-frameworks (MOFs) are emerging materials used in the environmental electrochemistry community for Faradaic and non-Faradaic water remediation technologies. It has been concluded that MOF-based materials show improvement in performance compared to traditional (non-)faradaic materials. In particular, this review outlines MOF synthesis and their application in the fields of electron- and photoelectron-Fenton degradation reactions, photoelectrocatalytic degradations, and capacitive deionization physical separations. This work overviews the main electrode materials used for the different environmental remediation processes, discusses the main performance enhancements achieved via the utilization of MOFs compared to traditional materials, and provides perspective and insights for the further development of the utilization of MOF-derived materials in electrified water treatment.

Adsorptive removal and photocatalytic decomposition of cationic dyes on niobium oxide with deformed orthorhombic structure

Nano-rod-shaped niobium oxide with a deformed orthorhombic structure (ortho-Nb₂O₅) is first demonstrated as a selective adsorbent to remove cationic dyes wastewater. Ortho-Nb₂O₅ quickly adsorbs methylene blue (MB) with much greater capacity than reported inorganic adsorbents. Furthermore, ortho-Nb₂O₅ has a stronger affinity to cationic dye than anionic dye because cation exchange is involved in the adsorption process. The dye molecule adsorbed onto ortho-Nb₂O₅ can be degraded entirely under UV light irradiation because of its photocatalytic properties. Moreover, the regenerated ortho-Nb₂O₅ shows high reusability for use in additional adsorption processing. As described herein, new insights into the use of ortho-Nb₂O₅ as a photocatalytically regeneratable adsorbent for wastewater treatment are presented.

Green Synthesis of Flower-Shaped Copper Oxide and Nickel Oxide Nanoparticles via Capparis decidua Leaf Extract for Synergic Adsorption-Photocatalytic Degradation of Pesticides

Green manufacturing of catalysts enables sustainable advanced oxidation processes and water treatment processes for removing trace contaminants such as pesticides. An environmentally friendly biosynthesis process produced high-surface-area CuO and NiO nanocatalysts using phytochemicals in the Capparis decidua leaf extract, which served as a reductant and influenced catalyst shape. Capparis decidua is a bushy shrub, widely distributed in dry and arid regions of Africa, Pakistan, India, Egypt, Jordan, Sudan, Saudi Arabia. The synthesized CuO and NiO nanoparticles were characterized by UV-vis spectroscopy (UV-vis), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) and thermo-gravimetric analysis/differential thermal analysis (TGA/DTA). The produced nanoparticles were spherical and flower-like in shape and have a characteristic face-centered cubic structure of CuO and NiO. Biosynthesized catalysts were photoactive and degraded recalcitrant pesticide Lambda-cyhalothrin (L-CHT). Photocatalytic degradation of L-CHT was affected by the initial L-CHT concentration, solution pH levels between 5 and 9, and photocatalyst concentration. The L-CHT removal percentage attained by CuO photocatalyst (~99%) was higher than for NiO photocatalyst (~89%). The degradation of L-CHT follows a pseudo-first-order kinetic model, and the apparent rate constant (kapp) decreased from 0.033 min−1 for CuO to 0.0084 min−1 for NiO photocatalyst. The novel flower-shaped nanoparticles demonstrated high stability in water and recyclability for removing L-CHT pesticide contamination in water.

Photocatalytic degradation of 2,4-dichlorophenoxyacetic acid from aqueous solutions by Ag3PO4/TiO2 nanoparticles under visible light: kinetic and thermodynamic studies

Between the countless chemical substances applied in agriculture, 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide is considered as a toxic and carcinogenic pollutant which is difficult to remove from water due to its biological and chemical stability and high solubility. The goal of this study was photocatalytic degradation of 2,4-D, using Ag₃PO₄/TiO₂ nanoparticles under visible light. The Ag₃PO₄/TiO₂ nanoparticles were characterized using XRD, FESEM and EDS analysis to investigate its crystal structure and elemental compounds. The effect of operating parameters such as pH, contact time, catalyst dose, and initial concentration of herbicide on the efficiency of the process was studied. Increasing the pH and initial concentration of herbicide led to the reduction of the efficiency of removing the herbicide, while increasing contact time and catalyst dose increased the efficiency. The best result (98.4% removal efficiency) was achieved at pH = 3, 1 g/L catalyst dose, 60 min contact time, and 10 mg/L initial concentration of 2,4-D. According to the results, 2,4-D removal efficiency with Ag₃PO₄/TiO₂ photocatalyst reached 96.1% from 98.4% after 5 cycles of reaction. The pseudo-first-order kinetics was the best fit for the 2,4-D degradation by Ag₃PO₄/TiO₂ with correlation coefficients (R² = 0.9945). The results demonstrated that the photocatalytic process using Ag₃PO₄/TiO₂ nanoparticles in the presence of visible light had a relatively good efficiency in removing 2,4-D. Moreover, Ag₃PO₄/TiO₂ can be used as a reusable photocatalyst for the degradation of such toxins from polluted water and wastewater.

Optical Properties of Porous Alumina Assisted Niobia Nanostructured Films-Designing 2-D Photonic Crystals Based on Hexagonally Arranged Nanocolumns

Three types of niobia nanostructured films (so-called native, planarized, and column-like) were formed on glass substrates by porous alumina assisted anodizing in a 0.2 M aqueous solution of oxalic acid in a potentiostatic mode at a 53 V and then reanodizing in an electrolyte containing 0.5 M boric acid and 0.05 M sodium tetraborate in a potentiodynamic mode by raising the voltage to 230 V, and chemical post-processing. Anodic behaviors, morphology, and optical properties of the films have been investigated. The interference pattern of native film served as the basis for calculating the effective refractive index which varies within 1.75-1.54 in the wavelength range 190-1100 nm. Refractive index spectral characteristics made it possible to distinguish a number of absorbance bands of the native film. Based on the analysis of literature data, the identified oxide absorbance bands were assigned. The effective refractive index of native film was also calculated using the effective-medium models, and was in the range of 1.63-1.68. The reflectance spectra of all films show peaks in short- and long-wave regions. The presence of these peaks is due to the periodically varying refractive index in the layers of films in two dimensions. FDTD simulation was carried out and the morphology of a potential 2-D photonic crystal with 92% (wavelength 462 nm) reflectance, based on the third type of films, was proposed.

Continuous process applied to degradation of triclosan and 2.8-dichlorodibenzene-p-dioxin

This study describes the use of a prototype for the continuous photocatalytic reaction process using Fe/Nb₂O₅-immobilized catalyst for triclosan and 2.8-dichlorodibenzene-p-dioxin (2.8-DCDD)'s degradation. The experiments were carried out with different parameters and matrices in a steady state. In addition, photolysis and photocatalytic tests were performed. The results indicated that the generation of 2.8-DCDD was observed in matrices with Cl^-. The Fe/Nb₂O₅-immobilized catalysts were efficient in the degradation of triclosan and 2.8-dichlorodibenzene-p-dioxin. However, 2.8-DCDD formation was not observed in the ultra-pure water matrix, which indicated influence of ions. The photocatalysis was more efficient than the photolysis when comparing both matrices and radiation. Even with a radiation oscillation, the solar process showed positive results.

Special Issue: New Trends in Photo (Electro)catalysis: From Wastewater Treatment to Energy Production

This Special Issue aimed at focusing on photo- and photo-electrocatalytic processes specifically devoted to present both new catalytic materials and possible applications in environmental and energetic fields [...]

Nb2O5-Based Photocatalysts

Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco-friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Brønsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5-based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5-based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.

Nickel ferrite nanoenabled graphene oxide (NiFe2O4@GO) as photoactive nanocomposites for water treatment

Nanocomposite materials can enhance the capabilities of water treatment processes such as photocatalysis. In this work, novel light-driven nanocatalysts were synthesized by using nickel ferrite (NiFe₂O₄) to nanoenable graphene oxide (GO) substrates. GO is an emerging 2D nanomaterial with high conductivity and adsorption properties. Moreover, the electric properties of GO improve photocatalytic performance by promoting charge carrier separation. Results of the characterization of the nickel ferrite nanoenabled graphene oxide (NiFe₂O₄@GO) nanocomposites demonstrate that homogeneous and stable photocatalysts were produced. The as-synthesized nanocatalysts enabled complete decolorization of the colored water matrix in short irradiation times of 150 min using minimal catalyst loading at 0.5 g L^-1. The selective hook and destroy mechanism reduced the competitive effect of co-existing ions in solution. Furthermore, the use of specific scavengers helped to elucidate the degradation mechanisms of organic dye methylene blue by NiFe₂O₄@GO nanocomposites.

Heterogeneous photocatalytic degradation of pharmaceuticals in synthetic and real matrices using a tube-in-tube membrane reactor with radial addition of H2O2

A tube-in-tube membrane reactor, with radial addition of hydrogen peroxide, was used for the oxidation of four pharmaceuticals, paracetamol (PCT), furosemide (FRS), nimesulide (NMD), and diazepam (DZP), in a continuous-mode operation, using photochemical and photocatalytic processes, driven by UVA or UVC photons. This reactor allows a controlled titration of small H₂O₂ doses (inside-out mode) to the catalyst particles immobilized in the membrane shell side and to the annular space between the membrane inner tubing and the concentric outer quartz tubing, where water to be treated flows. Tests were performed using synthetic (SWW) and real (urban wastewater after secondary treatment) (UWW) matrices, both spiked with the pharmaceutical mix solution (200 μg L^-1 of each). The photochemical and photocatalytic oxidation efficiency was evaluated as a function of H₂O₂ dose (5-20 mg L^-1), oxidant injection mode (radial permeation vs injection upstream from the reactor inlet), light source (UVA vs UVC lamps) and aqueous matrix (synthetic vs real matrix). At steady-state regime, the UVC/H₂O₂/TiO₂ system, with radial H₂O₂ addition (20 mg L^-1), showed the highest pharmaceuticals removal percentage, PCT (27.4%), FRS (35.0%), NMD (24.2%) and DZP (30.0%) in SWW. A substantial decrease in pharmaceuticals elimination was observed for UWW (PCT - 11.5%, FRS - 20.3%, NMD - 8.2% and DZP - 12.6%), in comparison with the SWW matrix. Finally, twelve transformation products (TPs) were identified; most of them showed in their structures hydroxylation in aromatic moiety; all TPs chemical structures were evaluated by BIOWIN software indicating that the TPs are non-biodegradables.

Industrial Textile Wastewater Ozone Treatment: Catalyst Selection

One of the recent trends in textile wastewater treatment has become catalytic ozonation. The necessity of effective color removal in a short treatment time is a standard during industrial implementation. At the same time, efficient chemical oxygen demand (COD), total organic carbon (TOC), and toxic by-product removal are highly expected. This study presents the results of a catalytic ozonation treatment. Three types of catalysts: a metal oxide (TiO2 as P25 by Degussa), activated carbon (nano-powder by Sigma, AC), and metal particles (platinum, 1% wt. supported on AC matrix by Sigma, Pt–AC) have been applied. The investigations were conducted for real industrial wastewater originated in textile dyeing with Reactive Black 5 dye (RB5). The experiments ran for the raw wastewater (without pretreatment), exposed blocking of the catalytic action by all used catalysts. The catalytic effect could be observed when catalytic ozonation was used as a polishing step after electrocoagulation (EC). Although the catalytic effect could be observe for all catalysts then, especially in the removal of colorless by-products, the AC was exposed as the most effective. This contributed to 35% and 40% of TOC and COD removal. While only 18% and 23% of TOC and COD were removed in the same process without AC. The decrease in toxicity was 30%. The results of the study revealed the complexity of the issue and resulted in an extensive discussion devoted to the basis of the catalytic activity of each catalyst.