Porphyrin-based hydrogen-bonded organic framework for visible light driven photocatalytic removal of U(VI) from real low-level radioactive wastewater

Facilitated electron-hole separation and enhanced uranium(VI) capture via La-doped WO3: Insights into oxygen vacancies and superior recyclability

WO3-based materials are theoretically promising photocatalysts for uranium(VI) removal due to their stability and narrow bandgap (∼2.8 eV), but they are limited by a lack of adsorption sites and a tendency for photogenerated electron-hole pairs to recombine. Herein, La-doped WO3 (W0.9La0.1O3-x) rich in oxygen vacancies (OVs) was prepared via a facile hydrothermal method. La doping and the introduction of OVs provided abundant active sites, optimized the band structure, increased electron density, and inhibited electron-hole pair recombination of W0.9La0.1O3-x, which resulted in a maximum U(VI) extraction capacity of up to 1199.73 mg/g through synergistic adsorption-photocatalytic reduction processes. Interestingly, W0.9La0.1O3-x demonstrated excellent stability and increased recyclability, superior to that of most reported adsorbents and photocatalysts, probably owing to the strong stability of W and La atoms as well as the additional OVs formed from self-reduction of W0.9La0.1O3-x during photocatalytic reaction process. Overall, this study expands the application of WO3-based materials for U(VI) removal from wastewater and provides valuable theoretical insights and technical guidance for addressing the challenge of balancing removal rate and stability of U(VI) extraction materials.

Enhanced Electrochemiluminescence of Porphyrin-Based Hydrogen-Bonded Organic Frameworks at Low Positive Potential via Substituent-Induced Outer-Sphere Microenvironment Modulation

The porphyrin-based hydrogen-bonded organic framework (HOF) offers a superior platform for decoding electrochemiluminescence (ECL) via controlling charge transfer due to its higher solubility, chemical stability, and tunable framework behavior. In this research, three kinds of HOFs including TDPP-HOF, TCPP-HOF, and TCNPP-HOF are synthesized based on a porphyrin tectonic plate decorated with 2,4-diaminotriazinyl (DAT), carboxyl, and nitrile moieties to study their ECL performances. The hydrazine as the coreactant can trigger TDPP-HOF at the low-excited positive potential to generate 15.8- and 112.9-fold enhancement in ECL signal than TCNPP-HOF and TCPP-HOF. Experimental results and density functional theory calculations verify that TDPP-HOF with a lower bandgap and a larger binding energy (ΔE) between coreactant and HOF is beneficial to intrareticular charge transfer (ICT), facilitating the enhancement of ECL performance. These results indicate that the peripheral substituents can establish a specialized outer-sphere microenvironment around the porphyrin center to tune both the HOF activity and the ECL performance. As a proof of concept, a simple TDPP-HOF-based ECL sensor is constructed to sensitively detect phenolic compounds. This research provides a new avenue for improving the ECL performance via modulating the outer-sphere microenvironment of HOFs.

Porous Supramolecular Crystalline Materials for Photocatalysis

Porous supramolecular crystalline materials (PSCMs), such as hydrogen-bonded organic frameworks (HOFs), π frameworks, can be defined as a type of porous supramolecular assemblies stabilized by hydrogen-bonding, π-π stacking and other non-covalent interactions. Benefiting from the unique features of mild synthesis conditions, well-defined and synthetically tailorable structures, easy healing and regeneration, PSCMs have garnered widespread interest in research fields including molecular recognition, sensor, gas storage and separation. Moreover, they have emerged as promising photocatalysts because these PSCMs could be readily endowed with optical function, and the hydrogen-bonding and π-π stacking can offer channels for electron transfer to boost the photocatalytic activity. However, the research on PSCMs for photocatalysis is still at an early stage, and a review on this topic would help to promote the development of supramolecular chemistry. In this Minireview, we first introduce the synthesis methods for PSCMs, and then highlight their advantages in photocatalysis. Subsequently, we summarize the applications of PSCMs in photocatalysis including CO2 reduction, H2 evolution, H2O2 production, organic transformation and pollutant degradation, and we put particular emphasis on delineating the structure-performance relationship. At the end, we discuss the challenges and perspectives in developing high-performance PSCM-based photocatalysts.

Ti3C2Tx/Cd0.8Zn0.2S composites constructed of Schottky heterojunction for efficient photocatalytic reduction of U(VI)

Photocatalysis has emerged as a extremely promising green technology for the treatment of uranium-containing wastewater. This study focuses on the fabrication of Ti3C2Tx/Cd0.8Zn0.2S composites with Schottky junctions through the in-situ growth of Cd0.8Zn0.2S on Ti3C2Tx nanosheets, enabling efficient photoreduction of U(VI) without the requirement of sacrificial agents. The results demonstrate that the Ti3C2Tx/Cd0.8Zn0.2S composites achieve remarkable 99.48 % U(VI) reduction efficiency within 60 min in a 100 ppm uranium solution. Furthermore, the removal rate remains above 90 % after five cycles. The formation of Schottky heterojunctions by Ti3C2Tx and Cd0.8Zn0.2S leads to the generation of an internal electric field that significantly promotes the rapid separation and transfer of photogenerated carriers, thereby enhancing the photocatalytic reduction efficiency of Ti3C2Tx/Cd0.8Zn0.2S-3:100 (TC/CZS-3:100). A considerable amount of electrons accumulate on Ti3C2Tx via the Schottky barrier, effectively facilitating the reduction of U(VI) to U(IV). As a co-catalyst, Ti3C2Tx enhances the photocatalytic performance and stability of Cd0.8Zn0.2S. Moreover, the practical application in the waste liquid of rare earth tailings reveals that the removal rate can be as high as 91.24 %. This research is of significant value in the development of effective photocatalysts for the elimination of uranium from wastewater.

Enhanced Photocatalytic Removal of U(VI) from Real Radioactive Wastewater by Modulating the Surface Charge Microenvironment in Porphyrin-Based Hydrogen-Bonded Organic Framework

Reduction of soluble U(VI) to insoluble U(IV) based on photocatalysts is a simple, environmentally friendly, and efficient method for treating radioactive wastewater. The present study involved the systematic comparison of the photoelectric properties of three metalloporphyrins with different metal centers and the synthesis of a novel porphyrin-based hydrogen-bonded organic framework (Ni-pHOF) photocatalyst by modulating the surface charge microenvironment in porphyrin for enhanced photocatalytic removal of U(VI) from wastewater. Compared to the metal-free HOF, the surface charge microenvironment around the Ni atom in Ni-pHOF accelerated the reduction kinetics of U(VI) under visible light illumination at the initial moment, showing a high removal rate, even in air. The removal rate of U(VI) from aqueous solution by Ni-pHOF can achieve over 98% in the presence of coexisting nonoxidizing cations and only decreased by less than 8% after five cycles, exhibiting high selectivity and good reusability. Furthermore, Ni-pHOF can remove 86.74% of U(VI) from real low-level radioactive wastewater after 120 min of illumination, showcasing practical application potential. Density functional theory (DFT) calculations and electron paramagnetic resonance (EPR) spectra indicated that modulating the surface charge microenvironment in Ni-pHOF through porphyrin metallization is conducive to improving the charge separation efficiency, prompting more e- and •O2- to participate in the reduction reaction of U(VI). This work provides new insights into the metallization of porphyrin-based HOFs and paves a new way for the tailoring of porphyrin-based HOFs/COFs by modulating the surface charge microenvironment to achieve efficient recovery of U(VI) from real radioactive wastewater.

Structural Design and Energy and Environmental Applications of Hydrogen-Bonded Organic Frameworks: A Systematic Review

Hydrogen-bonded organic frameworks (HOFs) are emerging porous materials that show high structural flexibility, mild synthetic conditions, good solution processability, easy healing and regeneration, and good recyclability. Although these properties give them many potential multifunctional applications, their frameworks are unstable due to the presence of only weak and reversible hydrogen bonds. In this work, the development history and synthesis methods of HOFs are reviewed, and categorize their structural design concepts and strategies to improve their stability. More importantly, due to the significant potential of the latest HOF-related research for addressing energy and environmental issues, this work discusses the latest advances in the methods of energy storage and conversion, energy substance generation and isolation, environmental detection and isolation, degradation and transformation, and biological applications. Furthermore, a discussion of the coupling orientation of HOF in the cross-cutting fields of energy and environment is presented for the first time. Finally, current challenges, opportunities, and strategies for the development of HOFs to advance their energy and environmental applications are discussed.

In Situ Construction of CuTCPP/Bi4O5Br2 Hybrids for Improved Photocatalytic CO2 and Cr(VI) Reduction

Global warming and heavy metal pollution pose tremendous challenges to human development, and photocatalysis is considered to be an effective strategy to solve these problems. Herein, copper(II) tetra (4-carboxyphenyl) porphyrin (CuTCPP) molecules were successfully in situ loaded onto Bi4O5Br2 by a hydrothermal approach. A series of experimental results show that the light absorption capacity and photogenerated carrier separation efficiency were synchronously enhanced after the construction of CuTCPP/Bi4O5Br2 composites. Hence, the as-prepared composites possess significantly improved photocatalytic ability for both CO2 and Cr(VI) reduction. Specifically, CuTCPP/Bi4O5Br2-2 achieves a CO generation rate of 17.14 μmol g-1 under 5 h irradiation, which is twice as high as that of Bi4O5Br2 (8.57 μmol g-1). Besides, the optimized CuTCPP/Bi4O5Br2-2 also exhibits a removal rate of 61.87% for Cr(VI) within 100 min under irradiation. Furthermore, the mechanism of CO2 and Cr(VI) photoreduction was explored by in situ Fourier transform infrared spectroscopy and capture experiments, respectively. This work can be a reference toward the construction of photocatalysts with high activity for solar energy conversion.

Exploring recyclable alginate-enhanced GCN-LDO sponge for U(VI) and Cd(II) removal: Insights from batch and column studies

Developing effective water treatment materials, particularly through proven adsorption methods, is crucial for removing heavy metal contaminants. This study synthesizes a cost-effective three-dimensional material encapsulating graphitic carbon nitride-layered double oxide (GCN-LDO) in sodium alginate (SA) through the freeze-drying method. The material is applied to remove uranium (U(VI)) and cadmium (Cd(II)) in real water systems. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analyses conclusively verified the elemental composition and successful encapsulation of GCN-LDO within the SA matrix. Removal effectiveness was tested under various conditions, including adsorbent dose, ionic strength, contact time, temperature, different initial pollutant concentrations, and the impact of co-existing ions. The adsorption of U(VI) and Cd(II) conformed to the pseudo-second-order (PSO) kinetic model, signifying a chemical interaction between the sodium alginate-graphitic carbon nitride-layered double oxide (SA-GCN-LDO) sponge and the metal ions. The Langmuir isotherm indicated monolayer, homogeneous adsorption for U(VI) and Cd(II) with capacities of 158.25 and 165.00 mg/g. SA-GCN-LDO recyclability was found in up to seven adsorption cycles with a removal efficacy of 70%. The temperature effect study depicts the exothermic nature of the U(VI) and Cd(II) ion removal process. Various mechanisms involved in U(VI) and Cd(II) removal were proposed. Further, continuous fixed bed column studies were performed, and Thomas and the Yoon-Nelson model were studied. These insights from this investigation contribute to advancing our knowledge of the material's performance within the context of U(VI) and Cd(II) adsorption, paving the way for optimized and sustainable water treatment solutions.

The construction of a stable hydrogen-bonded organic framework for the photocatalytic reduction and removal of uranium

An imidazolyl hydrogen-bonded organic framework (HOF-T) with outstanding thermal and water stability was constructed by C-H⋯N hydrogen bonding and C-H⋯π interactions. UO22+ can be selectively captured by the imidazole group of HOF-T and rapidly reduced to UO2 under visible light irradiation, realizing exceptional uranium removal with high capacity and fast kinetics.

Bibliometric insights into the evolution of uranium contamination reduction research topics: Focus on microbial reduction of uranium

Confronting the threat of environment uranium pollution, decades of research have yielded advanced and significant findings in uranium bioremediation, resulting in the accumulation of tremendous amount of high-quality literature. In this study, we analyzed over 10,000 uranium reduction-related papers published from 1990 to the present in the Web of Science based on bibliometrics, and revealed some critical information on knowledge structure, thematic evolution and additional attention. Methods including contribution comparison, co-occurrence and temporal evolution analysis are applied. The results of the distribution and impact analysis of authors, sources, and journals indicated that the United States is a leader in this field of research and China is on the rise. The top keywords remained stable, primarily focused on chemicals (uranium, iron, plutonium, nitrat, carbon), characters (divers, surfac, speciat), and microbiology (microbial commun, cytochrome, extracellular polymeric subst). Keywords related to new strains, reduction mechanisms and product characteristics demonstrated the strongest uptrend, while some keywords related to mechanism and performance were clearly emerging in the past 5 years. Furthermore, the evolution of the thematic progression can be categorized into three stages, commencing with the discovery of the enzymatic reduction of hexavalent uranium to tetravalent uranium, developing in the groundwater remediation process at uranium-contaminated sites, and delving into the research on microbial reduction mechanisms of uranium. For future research, enhancing the understanding of mechanisms, improving uranium removal performance, and exploring practical applications can be considered. This study provides unique insights into microbial uranium reduction research, providing valuable references for related studies in this field.

Sorption behavior and mechanism of U(VI) on Tamusu clay in the presence of U(VI)-CO3 complexes

The sorption behavior of U(VI) on Tamusu clay sampled from a pre-selected high-level radioactive waste (HLW) disposal site in Inner Mongolia (China) was studied systematically in the U(VI)-CO3 solution at pH 7.8 by batch experiments. The results demonstrated that the distribution coefficients (Kd) decreased with the increasing values of pHinitial, [U(VI)]initial, and ionic strength, but increased with the extended time and the rising temperature. The sorption was a pH-dependent, heterogeneous, spontaneous, and endothermic chemical process, which could be better described by Freundlich isothermal model and pseudo-second-order kinetic model. The presence of humic acid (HA) or fulvic acid (FA) significantly inhibited the U(VI) sorption, due to the enhanced electrostatic repulsion between the negatively charged HA/FA adsorbed on the clay surface and the negative U(VI) species, as well as the well dispersed HA/FA aggregates in solution wrapping the U(VI) species. The FTIR and XPS spectra indicated that the HCO3- groups on the surface of Tamusu clay after hydroxylation and the ‒OH groups in HA/FA were involved in the U(VI) sorption. The results reported here provide valuable insights into the further understanding of U(VI) migration in geological media.

Modulating conjugated microporous polymers via cyclization as a remarkable photo-enhanced uranium recovery platform

Uranium recovery is of great significance for managing environmental contamination, improving the utilization rate of uranium resources, reducing the pressure of nuclear fuel supply and building a closed-loop nuclear fuel cycle system. However, most of the current adsorbents are limited in practical application due to their poor selectivity in highly acidic environments (pH = 1). Here, we present a powerful uranium recovery strategy with combined ligand complexation, chemical reduction and photoreduction based on metal-free cyclization-modulated conjugated microporous polymers (CMPs). Our well-tailored CMP (CTATP-DHBA) is rich in pyridine and hydroquinone units, forming favorable six-membered chelation motifs to be well suited for selective loading and reduction of UVI, thus exhibiting remarkable uranium removal efficiency (ca. 83.27% removal in 200 ppm solutions, pH = 1). In the dark, CTATP-DHBA can effectively reduce pre-enriched UVI to UIV in situ via hydroquinone units on the skeleton, thus weakening the proton competition and achieving excellent uranium recovery efficiency. Meanwhile, the synergistic effect of the cyclized π-conjugated skeleton and the oxidized benzoquinone units significantly enhances the photocatalytic activity of CTATP-DHBA, and an additional UVI photocatalytic reduction can occur under visible light irradiation, enabling photo-enhanced uranium recovery. ENVIRONMENTAL IMPLICATION: The mineralization of valence-variable radionuclides such as uranium could not only enhance the extraction performance and simplify the subsequent separation procedure, but also effectively weaken the competition of protons for selective sites in the acidic sample matrix. Thus, the construction of robust CMPs with excellent photoreduction activity and affinity may be an ideal material for the selective extraction and in-situ mineralization of strategic nuclide uranium from strongly acidic radioactive wastewater. Herein, a metal-free cyclization-modulated CMP with the favorable six-membered chelation motifs is tailor-made as an efficient platform for efficient extraction and in situ mineralization of valence-variable nuclide uranium.

A High Crystalline Perylene-Based Hydrogen-Bonded Organic Framework for Enhanced Photocatalytic H2O2 Evolution

Hydrogen-bonded organic frameworks (HOFs) are a kind of crystalline porous material that have shown great potential for photocatalysis on account of their mild synthesis conditions and high crystallinity. Perylene-based photocatalysts have great potential for photocatalytic H2O2 production due to their excellent photochemical stability and broad spectral absorption. In this work, we designed and synthesized a high crystalline perylene-based HOF (PTBA) and an amorphous analog sample PTPA for photocatalytic H2O2 evolution. Under visible light irradiation, PTBA shows a higher photocatalytic H2O2 production rate of 2699 μmol g-1 h-1 than PTPA (2176 μmol g-1 h-1) and an apparent quantum yield (AQY) of 2.96% at 500 nm. The enhanced photocatalytic performance of PTBA is attributed to the promotion of the separation and transfer of photocarriers due to its high crystallinity. This work provides a precedent for the application of HOFs in the field of photocatalytic H2O2 generation.