Magnetically recyclable Bi/Fe-based hierarchical nanostructures via self-assembly for environmental decontamination

Hierarchical 3D Flower-like Metal Oxides Micro/Nanostructures: Fabrication, Surface Modification, Their Crucial Role in Environmental Decontamination, Mechanistic Insights, and Future Perspectives

Hierarchical micro/nanostructures are constructed by micro-scaled objects with nanoarchitectures belonging to an interesting class of crystalline materials that has significant applications in diverse fields. Featured with a large surface-to-volume ratio, facile mass transportation, high stability against aggregation, structurally enhanced adsorption, and catalytical performances, three dimenisional (3D) hierarchical metal oxides have been considered as versatile functional materials for waste-water treatment. Due to the ineffectiveness of traditional water purification protocols for reclamation of water, lately, the use of hierarchical metal oxides has emerged as an appealing platform for the remediation of water pollution owing to their fascinating and tailorable physiochemical properties. The present review highlights various approaches to the tunable synthesis of hierarchical structures along with their surface modification strategies to enhance their efficiencies for the removal of different noxious substances. Besides, their applications for the eradication of organic and inorganic contaminants have been discussed comprehensively with their plausible mechanistic pathways. Finally, overlooked aspects in this field as well as the major roadblocks to the implementation of these metal oxide architectures for large-scale treatment of wastewater are provided here. Moreover, the potential ways to tackle these issues are also presented which may be useful for the transformation of current water treatment technologies.

Fast photo-Fenton-like oxidation in bismuth catalysis: A novel Fe(III) self-doped sodium bismuthate nanosheet

Sodium bismuthate dihydrate (NaBiO₃.2 H₂O, NBH) nanosheets were successfully prepared in this study using the persulfate oil bath oxidation method. Benefited from the unique layered structure of NBH, the Fe(III) as a variable valence metal ion was explored for enhancing NBH activation of peroxymonosulfate (PMS) to degrade levofloxacin (LVF) in the visible-light catalytic system. Based on results of the reactive oxygen species (ROS) quenching experiments and electron paramagnetic resonance (EPR) analysis, singlet oxygen (¹O₂) and superoxide radical (O₂^·-) were identified as the main ROS. The morphology, chemical structure, and optical properties of NBH were analyzed using various characterization methods. It was confirmed that Fe(III) embedded in the NBH via the ion exchange with Na, resulting in lattice oxygen vacancies on the surface of the NBH, after the formation of oxygen defect sites, reacts with PMS in the solution to produce active oxygen species with oxidizing efficiency. This study expands the technological application of NBH in the catalytic oxidation of variable valence metals, which are essential for the removal of fluoroquinolone antibiotics.

Potassium Ferrite as Heterogeneous Photo-Fenton Catalyst for Highly Efficient Dye Degradation

In this work, hexagon-shaped potassium ferrite (K2Fe4O7) crystals with different sizes were prepared using the hydrothermal method. The crystals showed a narrow band gap of 1.44 eV, revealed by UV-visible diffuse reflectance spectroscopy, and was thus used as a heterogeneous Fenton catalyst to degrade methylene blue (MB) and crystal violet (CV) in the presence of green oxidant H2O2 under visible-light irradiation. Among the investigated crystals, the as-prepared one with an average size of 20 µm (KFO-20) exhibited better photocatalytic activity due to its high surface area. When it was used as a photo-Fenton catalyst, 100% MB and 92% CV were degraded within 35 min. Moreover, the catalyst maintained high photocatalytic activity and was stable after four continuous cycles. The trapping experiments showed that the active hydroxyl radical (·OH) was dominant in the photo-Fenton reaction. Therefore, this new photo-Fenton catalyst has great potential for the photocatalytic degradation of dye contaminants in water.

A comprehensive performance evaluation of heterogeneous Bi2Fe4O9/peroxymonosulfate system for sulfamethoxazole degradation

In this study, a Bi₂Fe₄O₉ catalyst with nanoplate morphology was fabricated using a facile hydrothermal method. It was used as a catalyst to activate peroxymonosulfate (PMS) for aqueous sulfamethoxazole (SMX) removal. A comprehensive performance evaluation of the Bi₂Fe₄O₉/PMS system was conducted by investigating the effects of pH, PMS dosage, catalyst loading, SMX concentration, temperature, and halides (Cl^- and Br^-) on the degradation of SMX. The Bi₂Fe₄O₉/PMS system demonstrated a remarkable catalytic activity with >95% SMX removal within 30 min (conditions: pH 3.8, [Bi₂Fe₄O₉] = 0.1 g L^-1, [SMX]:[PMS] mol ratio =1:20). It was found that both Cl^- and Br^- can lead to the formation of PMS-induced reactive halide species (i.e. HClO, HBrO, and Br₂) which can also react with SMX forming halogenated SMX byproducts. Based on the detected degradation byproducts, the major SMX degradation pathway in the Bi₂Fe₄O₉/PMS system is proposed. The SMX degradation by Bi₂Fe₄O₉/PMS system in the wastewater secondary effluent (SE) was also investigated. The results showed that SMX degradation rate in the SE was relatively slower than in the deionized water due to (i) reactive radical scavenging by water matrix species found in SE (e.g.: dissolved organic matters (DOCs), etc.), and (ii) partial deactivation of the catalyst by DOCs. Nevertheless, the selectivity of the SO₄^•- towards SMX degradation was evidenced from the rapid SMX degradation despite the high background DOCs in the SE. At least four times the dosage of PMS is required for SMX degradation in the SE to achieve a similar SMX removal efficiency to that of the deionized water matrix.

Self-Assembled Triphenylphosphonium-Conjugated Dicyanostilbene Nanoparticles and Their Fluorescence Probes for Reactive Oxygen Species

We report self-assembled novel triphenylphosphonium-conjugated dicyanostilbene-based as selective fluorescence turn-on probes for ¹O₂ and ClO-. Mono- or di-triphenylphosphonium-conjugated dicyanostilbene derivatives 1 and 2 formed spherical structures with diameters of ca. 27 and 56.5 nm, respectively, through π-π interaction between dicyanostilbene groups. Self-assembled 1 showed strong fluorescent emission upon the addition of ¹O₂ and ClO- compared to other ROS (O₂-, •OH, NO, TBHP, H₂O₂, GSH), metal ions (K⁺, Na⁺), and amino acids (cysteine and histidine). Upon addition of ¹O₂ and ClO-, the spherical structure of 1 changed to a fiber structure (8-nm wide; 300-nm long). Upon addition of ¹O₂ and ClO-, the chemical structural conversion of 1 was determined by FAB-Mass, NMR, IR and Zeta potential analysis, and the strong emission of the self-assembled 1 was due to an aggregation-induced emission enhancement. This self-assembled material was the first for selective ROS as a fluorescence turn-on probe. Thus, a nanostructure change-derived turn-on sensing strategy for ¹O₂ or ClO- may offer a new approach to developing methods for specific guest molecules in biological and environmental subjects.

Visible-Light-Mediated Electrocatalytic Activity in Reduced Graphene Oxide-Supported Bismuth Ferrite

Reduced graphene oxide (RGO)-supported bismuth ferrite (BiFeO₃) (RGO-BFO) nanocomposite is synthesized via a two-step chemical route for photoelectrochemical (PEC) water splitting and photocatalytic dye degradation. The detailed structural analysis, chemical coupling, and morphology of BFO- and RGO-supported BFO are established through X-ray diffraction, Raman and X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy studies. The modified band structure in RGO-BFO is obtained from the UV-vis spectroscopy study and supported by density functional theory (DFT). The photocatalytic degradation of Rhodamine B dye achieved under 120 min visible-light illumination is 94% by the RGO-BFO composite with a degradation rate of 1.86 × 10^-2 min^-1, which is 3.8 times faster than the BFO nanoparticles. The chemical oxygen demand (COD) study further confirmed the mineralization of an organic dye in presence of the RGO-BFO catalyst. The RGO-BFO composite shows excellent PEC performance toward water splitting, with a photocurrent density of 10.2 mA·cm^-2, a solar-to-hydrogen conversion efficiency of 3.3%, and a hole injection efficiency of 98% at 1 V (vs Ag/AgCl). The enhanced catalytic activity of RGO-BFO is explained on the basis of the modified band structure and chemical coupling between BFO and RGO, leading to the fast charge transport through the interfacial layers, hindering the recombination of the photogenerated electron-hole pair and ensuring the availability of free charge carriers to assist the catalytic activity.