Visible-Light-Driven Zr-MOF/BiOBr Heterojunction for the Efficient Synchronous Removal of Hexavalent Chromium and Rhodamine B from Wastewater

A critical review of MXene-based composites in the adsorptive and photocatalysis of hexavalent chromium removal from industrial wastewater

The growing concern of water pollution is a critical issue stemming from industrialization and urbanization. One of the specific concerns within this broader problem is the toxicity associated with chromium (Cr), especially in its Cr (VI) form. Transition metal carbides/nitrides (MXenes) are attractive materials for the treatment of water due to their unique properties such as layered structure, high surface area, conductivity, flexibility, scalable manufacture, and surface functions. Adsorption and photocatalysis reactions are the two promising methods for the removal of Cr (VI) by using MXenes. Still, most of the previous reviews were limited to the single application area. Hence, this review covers recent developments in MXene-based composites, highlighting their dual role as both adsorbents and photocatalysts in the removal of Cr (VI). MXene-based composites are found to be effective in both adsorption and photodegradation of Cr (VI). Most MXene-based composites have demonstrated exceptional removal efficiency for Cr (VI), achieving impressive adsorption capacities ranging from 100 to 1500 mg g-1 and degradation percentages between 80% and 100% in a relatively short period. The active functional groups present on the surface of MXene have a viable impact on the adsorption and photodegradation performance. The mechanism of Cr (VI) removal is explained, with MXenes playing a key role in electrostatic attraction for adsorption and as co-catalysts in photocatalysis. However, MXene-based composites have limitations such as instability, competition with co-existing ions, and regeneration challenges. Further research is needed to address these limitations. Additionally, MXene-based composites hold promise for addressing water contamination, heavy metal removal, hydrogen production, energy storage, gas sensing, and biomedical applications.

Metal organic framework anchored onto biowaste mediated carbon material (rGO) for remediation of chromium (VI) by the photocatalytic process

Groundwater contaminated with hexavalent chromium Cr(VI) causes serious health concerns for the ecosystem. In this study, a hybrid amino functionalized MOF@rGO nanocatalyst was produced by utilization of a biowaste mediated carbon material (reduced graphene oxide; rGO) and its surface was modified by in situ synthesis of a nanocrystalline, mixed ligand octahedral MOF containing iron metal and NH2 functional groups and the prepared composite was investigated for Cr (VI) removal. The photocatalytic degradation of Cr(VI) in aqueous solutions was carried out under UV irradiation. Using a batch mode system, the effect of numerous control variables was examined, and the process design and optimization were carried out by response surface methodology (RSM). The photocatalyst, NH2-MIL(53)-Fe@rGO, was intended to be a stable and highly effective nanocatalyst throughout the recycling tests. XRD, SEM, EDS, FTIR examinations were exploited to discover more about surface carbon embedded with MOF. 2 g/L of NH2-MIL-53(Fe)/rGO was utilized in degrading 200 mg/L of Cr(VI) in just 100 min, implying the selective efficacy of such a MOF-rGO nanocatalyst. Moreover, the Eg determinations well agreed with the predicted range of 2.7 eV, confirming its possibility to be exploited underneath visible light, via the Tauc plot. Thus, MOF anchored onto biowaste derived rGO photo-catalyst was successfully implemented in chromium degradation.

Fabrication of Cu-MOF-Derived Cu/CuxO/C Bifunctional Materials for Light and Dark Catalytic Properties

Metal-organic frameworks (MOFs) are self-assembled constitutive precursors and efficient self-sacrificial templates with metal ions/clusters and organic linkers from which multifunctional materials with carbon nanostructures can be derived. In this study, we synthesized a novel Cu-MOF with Cu(II) as the central metal ion through two ligands, N,N'-bis(pyridin-3-yl)terephthalamide (3-bpta) and fumaric acid (H2FA), which was used as a template for derivatizing carbon-based nanostructured materials of Cu and CuxO through doping with different materials (melamine, urea, and TiO2) in a simple and efficient one-step pyrolysis. The Cu/CuxO-1 catalyst possesses both dark-catalyzed degradation activity and photocatalytic reduction activity during water purification due to the hole-transfer ability between Cu+ and Cu2+ and its inhibition of electron-hole complexation. In the absence of light, force, and cocatalyst, it can also effectively remove azo dyes in water and effectively reduce Cr(VI) under the action of visible light; therefore, Cu/CuxO-1 can be used as a new type of bifunctional material for the removal of pollutants in water, which has a broad prospect.

Using NH2-MIL-125(Ti) for efficient removal of Cr(VI) and RhB from aqueous solutions: Competitive and cooperative behavior in the binary system

The coexistence of inorganic and organic contaminants is a challenge for real-life water treatment applications. Therefore, in this research, we used NH2-MIL-125(Ti) to evaluate the single adsorption of hexavalent chromium (Cr(VI)) or Rhodamine B (RhB) in an aqueous solution and further investigate simultaneous adsorption experiments to compare the adsorption behavior changes. The main influencing factors, for example, reaction time, initial concentration, reaction temperature, and pH were studied in detail. In all reaction systems, the pseudo-second-order kinetic and Langmuir isotherm models were well illuminated the adsorption progress of Cr(VI) and RhB. Thermodynamic studies showed that the adsorption process was spontaneous and endothermic. As compared to the single system, the adsorption capacity of Cr(VI) in the binary system gradually decreased as the additive amount of RhB increased, whereas the adsorption capacity of RhB in the binary system was expanded brilliantly. When the binary reaction system contained 100 mg/L Cr(VI), the removal rate of RhB increased to 97.58%. The formation of Cr(VI)-RhB and Cr(III)-RhB complexes was the cause that provided facilitation for the adsorption of RhB. These findings prove that the interactions during the water treatment process between contaminants may obtain additional benefits, contributing to a better adsorption capacity of co-existing contaminant.

Assembling Ag@CuO/UiO-66-NH2 nanocomposites for efficient photocatalytic degradation of xylene

Achieving efficient and stable photocatalytic degradation of xylene hinges on the advancement of photocatalytic materials with outstanding visible light activity. This low-carbon strategy serves as a promising solution to combat air pollution effectively. In this study, we synthesized a Z-scheme heterojunction Ag@CuO/UiO-66-NH2 nanocomposite by hydrothermal method to investigate its photodegradation properties for xylene gas under visible light conditions. XRD, XPS, SEM, FTIR, and UV-vis analyses were employed to confirm the presence of the Z-scheme heterojunction. The CuO/UiO-66-NH2 (CuU-2) composite has high photocatalytic activity, which is 2.37 times that of the original UiO-66-NH2. The incorporation of Z-scheme heterojunction facilitates efficient charge transfer and separation, leading to a substantial enhancement in photocatalytic activity. The Ag@CuO/UiO-66-NH2 (Ag-1@CuU) composite has the highest photocatalytic activity with a degradation efficiency of 84.12%, which is 3.36 times and 1.41 times that of UiO-66-NH2 and CuO/UiO-66-NH2, respectively. The silver cocatalyst improves the absorption capacity of the composite material to visible light, makes the ultraviolet visible absorption edge redshift, and significantly improves the photocatalytic performance. This study introduces a novel approach for xylene gas degradation and offers a versatile strategy for designing and synthesizing metal-organic framework (MOF)-based photocatalysts with exceptional performance.

Applications of BiOX in the Photocatalytic Reactions

BiOX (X = Cl, Br, I) families are a kind of new type of photocatalysts, which have attracted the attention of more and more researchers. The suitable band gaps and their convenient tunability via the change of X elements enable BiOX to adapt to many photocatalytic reactions. In addition, because of their characteristics of the unique layered structure and indirect bandgap semiconductor, BiOX exhibits excellent separation efficiency of photogenerated electrons and holes. Therefore, BiOX could usually demonstrate fine activity in many photocatalytic reactions. In this review, we will present the various applications and modification strategies of BiOX in photocatalytic reactions. Finally, based on a good understanding of the above issues, we will propose the future directions and feasibilities of the reasonable design of modification strategies of BiOX to obtain better photocatalytic activity toward various photocatalytic applications.