Metal-Organic Frameworks Bearing Dense Alkyl Thiol for the Efficient Degradation and Concomitant Removal of Toxic Cr(VI)

Unraveling the adsorption potential of Zr dithiol (MOF-DSH) through experimentation and neural network modeling

In this study, an aqueous-stable metal-organic framework with two thiol groups was synthesized using Zr as the metal centre and dimercaptosuccinic acid as the ligand through a conventional heating method for the removal of Cd(ii) from aqueous solution. Different characterization tools, including XRD, FT-IR, BET, SEM, TGA, and XPS, were employed. XRD results showed a characteristic pattern of a hexa-cluster, which was in agreement with the simulated MOF-801, and the corresponding vibrational peaks were observed in the FT-IR spectra. The synthesized MOF was thermally stable up to 300 °C, as demonstrated by TGA, and exhibited a specific surface area of 290 m2 g-1. Cadmium adsorption studies performed at different pH values showed a maximum adsorption capacity of 91.5 mg g-1 at pH 6. The adsorption behavior was well described by the Langmuir model and the pseudo-second order (PSO) kinetics, confirming the involvement of a monolayer with chemisorption as the dominant mode of adsorption. The synthesized MOF could be reused at least 4 times while retaining ∼80% of its initial adsorption capacity. FT-IR, XPS, and pH studies after Cd(ii) adsorption revealed that the predominant mode of interaction of Cd(ii) with the MOF is an ion-exchange mechanism. An artificial neural network-based (ANN) methodology was employed to model the adsorption capacity of Cd(ii) and predict the adsorption capacity as a function of Cd(ii) concentration, time of contact, and pH of the medium. The model demonstrated excellent results, with an average error of 2.3% and precision of 3.0%. The outcomes of these studies were consistent with the experimental results.

Functionalization of Defective Zr-MOFs for Water Decontamination: Mechanistic Insight into the Competitive Roles of -NH2 and -SH Sites in the Removal of Hg(II) Ions

Functional metal-organic frameworks (MOFs), especially those based on sulfur and nitrogen atoms, were frequently applied for the removal of Hg(II) ions. However, a systematic study on the cooperative or competitive roles of -SH and -NH2 functions in the presence of secondary mechanisms (proton transfer and redox) is still rare. In this work, the UiO-66 framework (Zr6(OH)4O4(BDC)6, BDC2- = benzene-1,4-dicarboxylate) was decorated with functional monocarboxylate linkers including glycine (Gly), mercaptopropionic acid (Mer), and cysteine (Cys). Due to the molecular similarity of these functional linkers, the coordination affinity between the amine and thiol sites with Hg(II) ions can be compared, and the effect of proton transfer and redox mechanisms on the possible thiol···Hg(II) and amine···Hg(II) interactions can be investigated. The results show that the Cys@UiO-66 framework can adsorb 1288 mg g-1 of Hg(II), while Mer@UiO-66 and Gly@UiO-66 can adsorb 593 and 313 mg g-1 at pH = 7 and 500 ppm, respectively. This is due to the facts that both the amine and the thiol functions of the Cys@UiO-66 framework show synergism in Hg(II) removal, and the secondary mechanisms reduce the affinity of thiol in Mer@UiO-66 and amine in Gly@UiO-66 frameworks in the removal process of Hg(II) ions. Free -SH sites in Mer@UiO-66 undergo a redox convert to -SO3H groups, and free protonated -NH2 sites in Gly@UiO-66 do not fully deprotonate during Hg(II) removal. Yet, in the case of Cys@UiO-66, free protonated -NH2 sites are fully deprotonated, and free SH sites did not convert to -SO3H groups during Hg(II) removal. These observations show that the redox and proton transfer mechanisms can negatively affect the adsorption capacity of functional MOFs containing free -SH and -NH2 groups. So, not only the functionalization but also control over secondary mechanisms in the removal process are necessary parameters to improve the affinity between functional MOFs and Hg(II) ions.

Multi-ligand strategy for enhanced removal of heavy metal ions by thiol-functionalized defective Zr-MOFs

Given the significant global concern about heavy metal pollution, the development of effective adsorbents to capture pollutants has become an urgent issue. In this work, thiol-functionalized defective Zr-MSA-DMSA was designed by mixing 2,3-dimercaptosuccinic acid and mercaptosuccinic acid, which was applied for the rapid and efficient removal of M(II) (i.e., Pb(II), Hg(II), Cd(II)) from wastewater. Zr-MSA-DMSA exhibited excellent adsorption performance, and the maximum adsorption capacities for Pb(II), Hg(II), and Cd(II) were 715.2 mg g-1, 862.7 mg g-1, and 450.5 mg g-1. In actual wastewater, Zr-DMSA-MSA exhibited up to 97 % M(II) removal efficiency and excellent anti-interference ability. It also maintained good structural stability after five adsorption/regeneration cycles. Thus, the abundant oxygen vacancies and unsaturated adsorption sites on Zr-MSA-DMSA significantly improved the adsorption performance of M(II). Spectral analysis and DFT calculations confirmed that Zr-MSA-DMSA mainly relied on the coordination of sulfur and oxygen atoms, electrostatic attraction and a large number of defective sites to achieve the adsorption of M(II). Fixed bed experiments showed that Zr-MSA-DMSA exhibited a depletion time of 10500 min and a volume of 7.0 L. In summary, Zr-MSA-DMSA holds significant potential for treating heavy metal wastewater and provides potential applications for defect engineering.

Diamino-functionalized metal-organic framework for selective capture of gold ions

Adsorptive recovery of valuable gold (Au) ions from wastes is vital but still challenged, especially regarding adsorption capacity and selectivity. A novel M - 3,5-DABA metal-organic framework (MOF) adsorbent was prepared via anchoring 3,5-diaminobenzoic acid (3,5-DABA) molecule in the MOF-808 matrix. Benefiting from the positive charge property, dense amino groups (3.2 mmol g-1) and high porosity, the adsorption capacity of M - 3,5-DABA reaches 1391.5 mg g-1 (pH = 2.5) and adsorption equilibrium is attained in 5 min. This amino-based material shows excellent selectivity towards various metal ions, evading the poor selectivity problem of classical thiol groups (e.g. for Ag+, Cu2+, Pb2+ and Hg2+ ions). In addition, the regeneration was easily achieved via using a hydrochloric acid-thiourea eluent. Experimental analysis and density functional theory (DFT) calculation show the amino group works as a reductant for Au(III) ions and meanwhile acts as an active site for adsorbing Au(III) ions together with the μ-OH group. Thus, M - 3,5-DABA can act as a potential adsorbent for Au(III) ions, and our work offers a viable strategy to construct novel MOF-based adsorbents.

Thiol-functionalized black carbon as effective and economical materials for Cr(VI) removal: Simultaneous sorption and reduction

Hazardous Cr(VI) continues to pose critical concerns for environmental and public health, demanding the development of effective remediation methods. In this study, thiol-functionalized black carbon (S-BC) was proposed for Cr(VI) removal by mixing thioglycolic acid (TGA) with black carbon (BC) derived from rice straw residue at 80 °C for 8 h. Using a 1:40 (g mL-1) BC-to-TGA ratio, the resulting S-BC40 sample demonstrated significantly enhanced Cr(VI) sorption capacities of 201.23, 145.78, and 106.60 mg g-1 at pH 3.5, 5.5, and 7.5, surpassing its BC counterpart by 2.0, 2.3, and 2.2 times. Additionally, S-BC40 converted all sorbed Cr into Cr(III) species at pH ≥ 5.5, resulting in an equal distribution of Cr(OH)3 and organic Cr(III) complexes. However, approximately 13% of Cr sorbed on BC remained as Cr(VI) at pH 3.5 and 7.5. Both C-centered and S-centered thiyl radicals might contribute to Cr(VI) reduction; however, sufficient C-S groups replenished via thiol-functionalization was the key for the complete Cr(VI) reduction on S-BC samples as pH ≥ 5.5. Thanks to the exceptional Cr(VI) sorption capacity, affordability, and accessibility, thiol-functionalization stands out as a promising modification method for BC. It presents a distinct opportunity to concurrently achieve the objectives of efficient Cr(VI) remediation and waste recycling.

Thiol and thioether-based metal-organic frameworks: synthesis, structure, and multifaceted applications

Metal-organic frameworks (MOFs) are unique hybrid porous materials formed by combining metal ions or clusters with organic ligands. Thiol and thioether-based MOFs belong to a specific category of MOFs where one or many thiols or thioether groups are present in organic linkers. Depending on the linkers, thiol-thioether MOFs can be divided into three categories: (i) MOFs where both thiol or thioether groups are part of the carboxylic acid ligands, (ii) MOFs where only thiol or thioether groups are present in the organic linker, and (iii) MOFs where both thiol or thioether groups are part of azolate-containing linkers. MOFs containing thiol-thioether-based acid ligands are synthesized through two primary approaches; one is by utilizing thiol and thioether-based carboxylic acid ligands where the bonding pattern of ligands with metal ions plays a vital role in MOF formation (HSAB principle). MOFs synthesized by this approach can be structurally differentiated into two categories: structures without common structural motifs and structures with common structural motifs (related to UiO-66, UiO-67, UiO-68, MIL-53, NU-1100, etc.). The second approach to synthesize thiol and thioether-based MOFs is indirect methods, where thiol or thioether functionality is introduced in MOFs by techniques like post-synthetic modifications (PSM), post-synthetic exchange (PSE) and by forming composite materials. Generally, MOFs containing only thiol-thioether-based ligands are synthesized by interfacial assisted synthesis, forming two-dimensional sheet frameworks, and show significantly high conductivity. A limited study has been done on MOFs containing thiol-thioether-based azolate ligands where both nitrogen- and sulfur-containing functionality are present in the MOF frameworks. These materials exhibit intriguing properties stemming from the interplay between metal centres, organic ligands, and sulfur functionality. As a result, they offer great potential for multifaceted applications, ranging from catalysis, sensing, and conductivity, to adsorption. This perspective is organised through an introduction, schematic representations, and tabular data of the reported thiol and thioether MOFs and concluded with future directions.

Synthesis and properties of thiol-modified CNC via surface tosylation

SH-containing polymers and nanoparticles are a significant direction in the creation of novel materials. The aim of this work is the synthesis of cellulose nanocrystals (CNC) with a surface modified by tosyl functions (CNC-Ts) and their further modification into SH-containing nanocrystals (CNC-SH). CNC-Ts were synthesized in an aqueous-organic emulsion from never-dried particles, while maintaining the size and supramolecular structure of CNC; the content of Ts-functions is up to 2.5 mmol·g-1. Structure of the derivatives was analyzed by TEM, XRD, CP/MAS 13C NMR and FTIR spectroscopies. Nucleophilic substitution and hydrolysis of the obtained thioisouronium salts leads to the production of CNC-SH. To quantify SH-groups we used elemental analysis, potentiometric titration and Folin-Ciocalteu and Ellman's reagents. It is shown that SH-groups on the surface are partially oxidized and are involved in a dense network of hydrogen bonds. Rheological properties of CNC-SH hydrosols are close to those of CNC, addition of H2O2 at acidic pH leads to an increase in viscosity of the system; H2O2 added at neutral pH causes opposite effect - viscosity decreases. CNC-SH have a high capacity for sorption of Cr(VI) in acidic environments and exhibit photoreductive properties under UV irradiation.

Post-synthesis introduction of dual functional groups in metal-organic framework for enhanced adsorption of moxifloxacin antibiotic

Highly effective removal of antibiotics from aqueous solution is of importance while still faces challenge. Herein, we report a novel metal-organic framework (MOF) adsorbent, MOF-808-SIPA (SIPA, 5-sulfoisophthalic acid), constructed via post-synthesis exchange strategy. On the basis, dual active groups including sulfonic acid and carboxyl groups are successfully introduced. The novel MOF-808-SIPA exhibits a high adsorption capacity of 287.1 mg g^-1 for moxifloxacin hydrochloride (MOX·HCl), superior to that (174.6 mg g^-1) of the pristine MOF-808-AA (AA, acetic acid). Besides, MOF-808-SIPA shows rapid adsorption equilibrium of ∼ 30 min, strong anti-interference ability from pH and inorganic ions, and feasible regeneration. The superiority renders MOF-808-SIPA a potential adsorbent for MOX removal. Density function theory (DFT) calculation and experiment confirm that H-bond interaction contributes largely to the excellent adsorption in MOF-808-SIPA. Our work provides a guideline for designing high-efficiency MOF-based adsorbent.

Preparation of a phenylboronic acid and aldehyde bi-functional group modified silica absorbent and applications in removing Cr(vi) and reducing to Cr(iii)

Cr(vi) is a great threat to the ecological environment and human health, so it is urgent to remove Cr(vi) from the environment. In this study, a novel silica gel adsorbent SiO2-CHO-APBA containing phenylboronic acids and aldehyde groups was prepared, evaluated and applied for removing Cr(vi) from water and soil samples. The adsorption conditions including pH, adsorbent dosage, initial concentration of Cr(vi), temperature and time were optimized. Its ability in removing Cr(vi) was investigated and compared with three other common adsorbents, SiO2-NH2, SiO2-SH and SiO2-EDTA. Data showed SiO2-CHO-APBA had the highest adsorption capacity of 58.14 mg g-1 at pH 2 and could reach adsorption equilibrium in about 3 h. When 50 mg SiO2-CHO-APBA was added in 20 mL of 50 mg L-1 Cr(vi) solution, more than 97% of Cr(vi) was removed. A mechanism study revealed that a cooperative interaction of both the aldehyde and boronic acid groups is attributed to Cr(vi) removal. The reducing function was gradually weakened with the consumption of the aldehyde group, which was oxidized to a carboxyl group by Cr(vi). This SiO2-CHO-APBA adsorbent was successfully used for the removal of Cr(vi) from soil samples with satisfactory results which indicates a good potential in agriculture and other fields.

Thiol-decorated defective metal-organic framework for effective removal of mercury(II) ion

Removal of mercury (Hg) ion from water is important while still faces challenges in capacity and adsorption speed. Herein, using thiol-containing mercaptoacetic acid (MA) as the template, we constructed a novel metal-organic framework (MOF) adsorbent, Zr-MSA-MA (MSA, mercaptosuccinic acid). Unlike other monodentate acids such as acetic acid and formic acid, MA benefits to maintain high-content binding sites, in the meantime of defect formation. On the basis, Zr-MSA-MA exhibits a high adsorption capacity of 714.8 mg g^-1 for Hg²⁺ and fast adsorption kinetics, superior to other MOF-based adsorbents. Co-existing metal ions and pH have only slight interference for the adsorption behavior. Besides, the adsorption is proved to an endothermic reaction and the adsorbent can be regenerated based on a simple elution. Further analysis indicates the strong chemical bonding of Hg²⁺ and -SH is the main adsorption mechanism. Thus, our work demonstrates the Zr-MSA-MA can serve as a potential adsorbent for Hg²⁺, and provides a novel strategy to construct defective adsorbent via using active group-containing template.

Robust DUT-67 material for highly efficient removal of the Cr(VI) ion from an aqueous solution

Robust DUT-67 was synthesized by the hydrothermal method and characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). To systematically study the removal of Cr(VI) ion by DUT-67, single-factor, competition ion, material regeneration, kinetic, and thermodynamic experiments were designed. The experimental results show that DUT-67 had a maximum removal rate of 96.1% and a maximum adsorption capacity of 105.42 mg g^-1 with material regeneration and outstanding selective adsorption. In addition, the process of removal of the Cr(VI) ion from an aqueous solution by DUT-67, which accorded with the pseudo-second-order kinetics model and Langmuir model, was studied, and its adsorption mechanism was reasonably explained by the theoretical calculation.

RhB-Embedded Zirconium-Biquinoline-Based MOF Composite for Highly Sensitive Probing Cr(VI) and Photochemical Removal of CrO42-, Cr2O72-, and MO

How to accurately detect and efficiently sweep Cr(VI) from contaminated water has come into focus. Zirconium-based metal-organic frameworks (MOFs) play vital roles in water environmental chemistry due to excellent hydrolysis-resistant stability. However, as photochemical probes and photocatalysts, poor performances in detection sensitivity, selectivity, and photosensitiveness limit sole Zr-MOFs' applications. So, it is urgent to quest valid strategies to break through the dilemmas. Embedding luminous dyes into MOFs has been considered one of the most feasible avenues. Herein, a dual-emissive RhB@Zr-MOF with orange-yellow fluorescence has been assembled by in situ-encapsulating rhodamine B (RhB) into a zirconium-biquinoline-based MOF. Actually, within RhB@Zr-MOF, the aggregation fluorescence quenching (ACQ) effect of RhB molecules was effectively avoided. Notably, RhB@Zr-MOF exhibits a rapid fluorescence quenching response toward Cr(VI) ions with high selectivity, sensitivity, and anti-interference abilities. More interestingly, unlike the most widely reported fluorescence resonance energy transfer (FRET) between MOFs and encapsulated guest modules, photoinduced electron transfer from RhB to Zr-MOF has been confirmed by modeling the ground state and excited states of RhB@Zr-MOF using density functional theory (DFT) and time-dependent DFT (TD-DFT). The effective electron transfer makes RhB@Zr-MOF more sensitive in probing Cr₂O₇^2- and CrO₄^2- ions with ultralow detection limit (DL) values of 6.27 and 5.26 ppb, respectively. Prominently, the detection sensitivity based on DL values has been increased about 6 and 9 times, respectively, compared with pristine Zr-MOF. Moreover, rather negative CB and positive VB potentials make RhB@Zr-MOF have excellent photochemical scavenging ability toward Cr(VI) and MO.

Recent Progress in the Development of MOF-Based Photocatalysts for the Photoreduction of Cr(VI)

There has been a direct correlation between the rate of industrial development and the spread of pollution on Earth, particularly in the last century. The organic and inorganic pollutants generated from industrial activities have created serious risks to human life and the environment. The concept of sustainability has emerged to tackle the environmental issues in developing chemical-based industries. However, pollutants have continued to be discharged to water resources, and finding appropriate techniques for the removal and remedy of wastewater is in high demand. Chromium is one of the high-risk heavy metals in industrial wastewaters that should be removed via physical adsorption and/or transformed into less hazardous chemicals. Photocatalysis as a sustainable process has received considerable attention as it utilizes sunlight irradiation to remedy Cr^(VI) via a cost-effective process. Numerous photocatalytic systems have been developed up to now, but metal-organic frameworks (MOFs) have gained growing attention because of their unique versatilities and facile structural modulations. A variety of MOF-based photocatalysts have been widely employed for the photoreduction of Cr^(VI). Here, we review the recent progress in the design of MOF photocatalysts and summarize their performance in photoreduction reactions.

Preparation of Highly Stable DUT-52 Materials and Adsorption of Dichromate Ions in Aqueous Solution

Highly stable DUT-52 materials were synthesized by the hydrothermal method and well-characterized by X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS). In order to systematically study the adsorption of dichromate ions in aqueous solution by the DUT-52 materials, a single factor experiment, kinetic experiment, thermodynamic experiment, competition ion experiment, and material regeneration experiment were designed. Based on the H-bond interaction between the dichromate ions and the H atoms of a NDC^2- ligand, the DUT-52 materials showed a maximum removal rate of 96.4% and a maximum adsorption capacity of 120.68 mg·g^-1 with excellent selective adsorption and material regeneration. In addition, the process of adsorption of dichromate ions by the DUT-52 materials is in accordance with the pseudo second-order kinetics and Langmuir models, and the adsorption mechanism and the important role of the H-bond interaction were reasonably explained using the XPS pattern and theoretical calculation. Accordingly, DUT-52 can be regarded as a multifunctional material for efficiently removing dichromate ions from the wastewater.

N-Doped MoS2 Nanoflowers for Efficient Cr(VI) Removal

The removal of Cr(VI) has attracted extensive attention since it causes serious harm to public health. Herein, we report a two-step method to synthesize N-doped MoS₂ nanoflowers (NFs) with controllable sizes, which are first utilized for Cr(VI) removal and display outstanding removal performance. The N-MoS₂ NFs with an average size of 40 nm (N-MoS₂ NFs-40 nm) can rapidly remove Cr(VI) in 15 min under optimal conditions. The maximum adsorption capacity of N-MoS₂ NFs-40 nm can reach 787.41 mg·g^-1, which is significantly larger than that of N-MoS₂ NFs-150 and -400 nm (314.46 and 229.88 mg·g^-1). Meanwhile, N-MoS₂ NFs-400 nm have a higher maximum adsorption capacity than pure MoS₂ NFs-400 nm (172.12 mg·g^-1). In this adsorption/reduction process, N-MoS₂ NFs have abundant adsorption sites due to a high surface area. N doping can generate more sulfur vacancy defects in the MoS₂ NF structure to accelerate electron transfer and enhance the reduction of Cr(VI) to low-toxicity Cr(III). This study provides a facile approach to fabricating N-MoS₂ nanoflowers and demonstrates their superior removal ability for Cr(VI).

Multi-functional metal-organic frameworks for detection and removal of water pollutions

Water pollutions have caused serious threats to the aquatic environment and human health, it is of great significance to monitor and control their contents in water. Compared with the traditional...

Porous organic-inorganic hybrid materials for catalysis, energy and environmental applications

Introduction of organic functionalities into the porous inorganic materials make the resulting hybrid porous framework not only more flexible and hydrophobic, but also provide additional scope for further functionalization, which...

Preparation of Recyclable and Versatile Porous Poly(aryl thioether)s by Reversible Pd-Catalyzed C-S/C-S Metathesis

Porous organic materials (polymers and COFs) have shown a number of promising properties; however, the lability of their linkages often limits their robustness and can hamper downstream industrial application. Inspired by the outstanding chemical, mechanical, and thermal resistance of the 1D polymer poly(phenylene sulfide) (PPS), we have designed a new family of porous poly(aryl thioether)s, synthesized via a mild Pd-catalyzed C–S/C–S metathesis-based method, that merges the attractive features common to porous polymers and PPS in a single material. In addition, the method is highly modular, allowing to easily introduce application-oriented functionalities in the materials for a series of environmentally relevant applications including metal capture, metal sensing, and heterogeneous catalysis. Moreover, despite their extreme chemical resistance, the polymers can be easily recycled to recover the original monomers, offering an attractive perspective for their sustainable use. In a broader context, these results clearly demonstrate the untapped potential of emerging single-bond metathesis reactions in the preparation of new, recyclable materials.

Bimetallic Ordered Large-Pore MesoMOFs for Simultaneous Enrichment and Dephosphorylation of Phosphopeptides

Despite the fact that bimetallic metal-organic frameworks (MOFs) could afford multiple functionalities by a synergistic effect of individual metallic centers, their intrinsic microporous structure frequently restricts their wide applications with bulky molecules involved. An urgent need is consequently triggered to design bimetallic hierarchical mesoporous MOFs (mesoMOFs). Herein, Zr/Ce mesoMOFs with a uniform pore size of up to 8 nm was successfully synthesized by a copolymer template strategy with the aid of a Hoffmeister ion. The obtained Zr/Ce mesoMOFs feature high porosity, good chemical and thermal stabilities, and tunable element components, and up to 70% Zr could be incorporated into the mesoporous Ce-based framework without deteriorating its crystallinity. Thanks to the synergistic effect of inherent Ce and Zr as well as the large and open pore channels, a broad range of phosphopeptides with different molecule sizes could be effectively checked out, thanks to their simultaneous enrichment and dephosphorylation capabilities. Such an ability to efficiently concentrate phosphopeptides remained intact even in the presence of abundant non-phosphorylated species. The practical detection of phosphopeptides from human serum was also verified, prefiguring the great potentials of bimetallic large-pore mesoMOFs for the proteome applications.

Self-Assembly of Poly(ethyleneimine)-Modified g-C3N4 Nanosheets with Lysozyme Fibrils for Chromium Detoxification

We disclose a straightforward approach to fabricate nanocomposites for efficient capture of Cr(VI) from an aqueous solution through the self-assembly of poly(ethyleneimine)-modified graphitic carbon nitride nanosheets (PEI-g-C₃N₄ NSs) and lysozyme fibrils (LFs). The as-made PEI-g-C₃N₄ NSs@LFs exhibited mesoporous structures with a high specific surface area of 39.6 m² g^-1, a large pore volume of 0.25 cm³ g^-1, several functional groups (e.g., -N, -NH, -NH₂, and -COOH), and a zero-point charge at pH 9.1. These merits allow the PEI-g-C₃N₄ NSs@LFs to further enhance their physical adsorption and electrostatic attraction with the negatively charged Cr(VI) species of HCrO₄^- and CrO₄^2-, which is beneficial for the uptake of Cr(VI), >80%, from an aqueous solution in a wide pH range. Interestingly, X-ray photoelectron spectra indicate that the PEI-g-C₃N₄ NSs@LFs converted Cr(VI) to Cr(III) through visible-light-induced photoreduction. The adsorption of Cr(VI) on the surface of PEI-g-C₃N₄ NSs@LFs was found to obey the Freundlich isotherm model, signifying that they have a heterogeneous surface for the multilayer uptake of Cr(VI). In contrast, the PEI-g-C₃N₄ NSs and LFs as Cr(VI) adsorbents followed the Langmuir isotherm model. Adsorption kinetic studies showed that the uptake of Cr(VI) through the PEI-g-C₃N₄ NSs@LFs was highly correlated with a pseudo-first-order model, suggesting that physisorption dominates the interaction of Cr(VI) and the PEI-g-C₃N₄ NSs@LFs. In real-life applications, the PEI-g-C₃N₄ NSs@LFs were used for the detoxification of the total chromium in the industrial effluent and sludge samples.

Advanced applications of Zr-based MOFs in the removal of water pollutants

The global water pollution is caused by the increase of industrial and agricultural activities, which have produced various toxic pollutants. Pollutants in water generally consist of metal ions, pharmaceuticals and personal care products (PPCPs), oil spills, organic dyes, and other organic pollutants. Amongst the adsorbents that have been developed to deal with pollutants in water, Zr-based metal-organic frameworks (MOFs) have drawn scientists' great attention due to their excellent stability and adjustable functionalization. Herein, the present review article introduces the synthetic methods of functionalized Zr-based MOFs and summarizes their applications in water pollution treatment. It also clarifies the interactions and removal mechanisms between pollutants and Zr-based MOFs. The use of these MOFs with eminent adsorption ability and recycling performance have been discussed in detail. Zr-based MOFs also face some challenges such as high cost, lack of real water environment applications, selective removal of pollutants, and low ability to remove composite pollutants. Future research should focus on addressing these issues. Although there is still a blank of the practical utility of Zr-based MOFs on a commercial scale, the research reported to date clearly shows that they are very promising materials for the water treatment.

Thiol decorated defective metal–organic frameworks embedded with palladium nanoparticles for efficient Cr(vi) reduction

Incorporation of Pd nanoparticles into thiol decorated defective UiO-66 for the reduction of toxic hexavalent chromium Cr(vi) to Cr(iii) in aqueous medium within a very short time under visible light.

An Enhanced Reduction-Adsorption Strategy for Cr(VI): Fabrication and Application of L-Cysteine-doped Carbon@Polypyrrole with a Core/Shell Composite Structure

Reclamation and recycling of heavy metal ions can offer environmental protection and sustainable development. Here, we report the preparation of L-cysteine (L-cys)-doped glucose carbon sphere (GCS)@polypyrrole (PPy) composites (GCS@PPy/L-cys). The adsorption performance and mechanism of GCS@PPy/L-cys toward Cr(VI) from water were investigated in detail. The chromate enrichment on GCS@PPy is significantly facilitated by doping with L-cys, which prevents the oxidative collapse of the structure. This approach leads to many reduction-adsorption sites that reduce the highly hazardous Cr(VI) into less toxic Cr(III). More significantly, the composite can be reused to fabricate supercapacitors that avoid secondary pollution. This strategy offers high-efficiency treatment and sustainable utilization of hypervalent metals in water.