Photodegradation of Organic Pollutants Using an Efficient Molybdate Intercalated Mg2+/Fe3+ Layered Double Hydroxide (LDH)

Organic pollutants dyes are the highly toxic major waste products causing severe harmful environmental pollution. From the viewpoint of environmental issues, the removal of harmful organic dye compounds is of great interest and importance1. Traditionally physical and biological methods are generally used to decompose many organic pollutants. However, these methods suffers from certain disadvantages and are time-consuming process. 2,3 Visible-light photocatalysis has been renewable "green" technologies which can harvest solar energy in the environmental remediation capable of removing harmful heavy organic contaminations4. This presentation is focused on the design of a novel kind of photocatalyst that cover entire solar spectrum i.e. from ultraviolet to infrared (IR) regions to decolorize and degrade the organic dye such as rhodamine 6G in an effective way. Now a days, the use of layered double hydroxides (LDHs) as active photo-catalysts has been receiving considerable attention over the layered metal oxides. A number of photocatalysts have been reported for the photocatalytic degradation of organic pollutants. Among the new generation photocatalyst, LDH was very much promising material for pollutant degradation5. However, designing novel visible light active LDH catalysts to meet present technical requirements is a great challenge. Intercalation of different polyoxometalic anionic species into inorganic layered materials like layered double hydroxide (LDH) offers a technique in which altering the properties of the two components are combined into a single modified material. By intercalating different anions, the characteristics of the layered double hydroxide (LDH) can be improved. Layered double hydroxide basically called Hydrotalcite consist of a cationic brucite like sheets with anionic moieties in the interlayer through electrostatic interaction. The unique structure, surface hydroxyl groups, interlayer spaces with intercalated anions, swelling properties, oxo-bridged linkage and high chemical stability are some of the added advantages of this group of materials. To harvest solar energy efficiently a series of Mg/Fe Layered double hydroxide materials has been synthesized by hydrothermal method and modified by intercalating molybdate anion by ion exchange. These materials have been characterized by various techniques and tested for their photocatalytic activity for the pollutant removal. The broad absorption band in case of Mg/Fe LDH was found due to the metal ligand charge transfer band of O2p ?Fe3+ and the metal-metal-charge-transfer spectra of Mg2+-O-Fe3+. The metal to metal charge transfer (MMCT) for an oxo-bridged bimetallic system with different oxidation states was defined to be an excitation transition of an electron from one metal to the other, which is known to absorb visible light and even near-IR light. 6,7 In the case of Mg/Fe/Mo LDH, the absorption edge shifted towards near IR is due to the HOMO-LUMO OMCT of Interlayer Molybdate where the HOMO is mainly derived from the O 2p orbitals and the LUMO is from the Mo 4d orbitals. These materials show enhanced photoactivity for the degradation of organic dyes such as rhodamine 6G. The enhanced photoactivity is due to edge shared metal oxygen octahedron of (MO6) of brucite sheet, visible light absorbing species, low recombination of charge carriers', metal-metal charge transfer spectra (MMCT) of the oxo-bridged bimetallic Mg2+-O-Fe3+ system, long life time of photogenerated charge carriers and HOMO-LUMO oxygen metal charge transfer spectra of intercalated Molybdate anions. These modified photo catalysts can be reused easily with several times without substantial loss of catalytic activity, which is green alternative material for practical applications for degradation of organic dyes like rhodamine 6G.qscienc

A Triphasic Superwetting Catalyst for Photocatalytic Wastewater Treatment

The increasing organic contamination is mainly produced by the widespread industrial, agricultural, and household applications and has become a serious worldwide issue. Therefore, we need to develop sustainable and environmentally friendly technologies to reduce waste detrimental to the environment. A promising approach is known as heterogeneous photocatalysis, inspired by natural photosynthesis. For this purpose, the challenges raised to synthesize appropriate surface nano/microstructured materials with long-term stability and mechanical durability for practical use. The traditional photocatalytic system is diphasic (dependent upon the solid-liquid phase), where the solid-liquid reaction interface depends upon the mass transfer. Especially, the low concentrations of oxygen in water and the slow diffusion rate limit the removal of electrons which decreases the photocatalytic reaction rates even if the presence of high light intensities. Therefore, the work aims to develop novel triphasic superwetting photocatalytic materials where the photocatalytic reaction is carried out at gas-liquid-solid joint interfaces. This triphasic contact line can allow oxygen from the air to this reaction interface and minimize electron-hole recombination even at high light intensities. Herein, we intend to discuss the importance of a novel superwetting triphasic nanoarrays catalyst that will be developed and implemented

Carbon-Based Nanomaterials for Catalytic Wastewater Treatment: A Review

Carbon-based nanomaterials (CBM) have shown great potential for various environmental applications because of their physical and chemical properties. The unique hybridization properties of CBMs allow for the tailored manipulation of their structures and morphologies. However, owing to poor solar light absorption, and the rapid recombination of photogenerated electron-hole pairs, pristine carbon materials typically have unsatisfactory photocatalytic performances and practical applications. The main challenge in this field is the design of economical, environmentally friendly, and effective photocatalysts. Combining carbonaceous materials with carbonaceous semiconductors of different structures results in unique properties in carbon-based catalysts, which offers a promising approach to achieving efficient application. Here, we review the contribution of CBMs with different dimensions, to the catalytic removal of organic pollutants from wastewater by catalyzing the Fenton reaction and photocatalytic processes. This review, therefore, aims to provide an appropriate direction for empowering improvements in ongoing research work, which will boost future applications and contribute to overcoming the existing limitations in this field

Enhanced Photocatalytic Activity of a Molybdate-Intercalated Iron-Based Layered Double Hydroxide

Herein, we report the design and successful fabrication of new Mg/Fe layered double hydroxides (LDHs) with different Mg/Fe molar ratios (2:1, 3:1, and 4:1) by a coprecipitation method. The prepared Mg/Fe (4:1) LDH sample with high crystallinity and phase purity was further modified by intercalation with molybdate anions. The structural, morphological, optical, electrochemical, and photocatalytic properties of the prepared catalysts were subsequently examined. In comparison with that of the Mg/Fe (4:1) LDH, the photocatalytic activity of the Mg/Fe (4:1) material with molybdate anions intercalated in the interlayer for the degradation of methyl orange (MO) was enhanced, and the processed followed apparent first-order kinetics. Radical-trapping experiments demonstrated that h+, OH·radicals, and O2–·radicals are the dominant reactive species for pollutant degradation. The highest photocatalytic activity of molybdate-intercalated LDH is attributed mainly to the combined effects of its layered structure with MgII–O–FeIIIlinkages and intercalated molybdate anions, which act as visible-light absorption centers towards higher wavelengths and can promote charge-carrier trapping as well as hinder the photogenerated electron–hole recombination. The time-resolved photoluminescence (TRPL) spectra demonstrated the prolonged charge separation under visible light

Effect of Co²⁺ Substitution in the Framework of Carbonate Intercalated Cu/Cr LDH on Structural, Electronic, Optical, and Photocatalytic Properties

In the present work, a series of Cu–Co/Cr ternary LDHs containing CO₃^2– in the interlayer was prepared by coprecipitation method. To investigate the effect of divalent metal ions on the catalytic activity, we vaired Cu/Co atomic ratios, keeping constant the atomic ratio of Cu+Co/Cr (2:1). Several characterization tools, such as powder X-ray diffraction (PXRD), Brunauer–Emmett–Teller surface area, Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy, and UV–vis diffuse reflectance spectroscopy, were employed to study the phase structures, textural, and optical properties of the samples. The PXRD of all samples showed the characteristic pattern of the hydrotalcite without any detectable impurity phases. The expected cell parameter variation was calculated assuming the Vegard’s law and proved the ideal atomic arrangement for the cations in the brucite layer. The shifting of the diffraction plane “d110” toward lower angle clearly indicates that Co²⁺ is substituted in the brucite layer. The formation of the highest amount of hydroxyl radicals (OH^•) on the surface of visible-light illuminated LDHs detected by the luminescence technique using terephthalic acid as probe molecules supports the highest activity LDH-4 with Cu/Co atomic ratio 0.033 + 0.1 (i.e., 1:3) toward MG degradation. The degradation of malachite green (MG) followed pseudo-first-order kinetics. The highest photocatalytic activity of LDH4 ascribed to the oxo-bridged system was explained by UV–vis DRS and EPR study.The degradation of MG followed pseudo-first-order kinetics, and the photocatalytic degradation mechanism was also explained in detail

Molybdate/Tungstate Intercalated Oxo-Bridged Zn/Y LDH for Solar Light Induced Photodegradation of Organic Pollutants

MoO₄^2–/WO₄^2– intercalated layered double hydroxide (LDH) was prepared by taking nitrate intercalated Zn/Y LDH (Zn/Y/N) by the ion exchange method. The structure, morphology, texture, optical absorption properties, and photocatalytic activities of all the as-prepared catalysts were studied in detail. Optical difference spectra (ODS) along with electron paramagnetic resonance (EPR) measurement revealed that the absorption in the visible region is attributed to the metal-to-metal charge-transfer (MMCT) excitation of oxo-bridged bimetallic linkage of Zn–O–Y in Zn/Y LDHs and can initiate the degradation of Rhodamine 6G (RhG) upon visible-light irradiation. The enhanced reactivity of tungstate and molybdate intercalated Zn/Y LDHs indicated that the interlayer space is the reaction field. The dye degradation process follows Langmuir–Hinshelwood first order kinetics. The possible photodegradation mechanism was studied by the examination of active species such as OH^•, h_VB⁺, and O₂^–• anions by using appropriate scavengers. The substantial decrease of chemical oxygen demand (COD) during photocatalytic degradation has been established