Surfactant-free synthesis of octahedral ZnO/ZnFe2O4 heterostructure with ultrahigh and selective adsorption capacity of malachite green

Selective Adsorption of Hazardous Substances from Wastewater by Hierarchical Oxide Composites: A Review

Large volumes of wastewater containing toxic contaminants (e.g., heavy metal ions, organic dyes, etc.) are produced from industrial processes including electroplating, mining, petroleum exploitation, metal smelting, etc., and proper treatment prior to their discharge is mandatory in order to alleviate the impacts on aquatic ecosystems. Adsorption is one of the most effective and practical methods for removing toxic substances from wastewater due to its simplicity, flexibility, and economics. Recently, hierarchical oxide composites with diverse morphologies at the micro/nanometer scale, and the combination advantages of oxides and composite components have been received wide concern in the field of adsorption due to their multi-level structures, easy functionalization characteristic resulting in their large transport passages, high surface areas, full exposure of active sites, and good stability. This review summarizes the recent progress on their typical preparation methods, mainly including the hydrothermal/solvothermal method, coprecipitation method, template method, polymerization method, etc., in the field of selective adsorption and competitive adsorption of hazardous substances from wastewater. Their formation processes and different selective adsorption mechanisms, mainly including molecular/ion imprinting technology, surface charge effect, hard-soft acid-base theory, synergistic effect, and special functionalization, were critically reviewed. The key to hierarchical oxide composites research in the future is the development of facile, repeatable, efficient, and scale preparation methods and their dynamic adsorption with excellent cyclic regeneration adsorption performance instead of static adsorption for actual wastewater. This review is beneficial to broaden a new horizon for rational design and preparation of hierarchical oxide materials with selective adsorption of hazardous substances for wastewater treatment.

Optimization by Box-Behnken design for environmental contaminants removal using magnetic nanocomposite

In this study, a CoO-Fe2O3/SiO2/TiO2 (CIST) nanocomposite was synthesized and utilized as an adsorbent to remove methylene blue (MB), malachite green (MG), and copper (Cu) from aqueous environments. The synthesized nanocomposite was characterized using field emission scanning electron microscopy (FE-SEM), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD). Input parameters included pH (3-10), contact time (10-30 min), adsorbent amount (0.01-0.03 g), and pollutant concentration (20-60 mg L-1). The effects of these parameters on the removal process efficiency were modeled and optimized using the response surface methodology (RSM) based on the Box-Behnken design (BBD). The RSM-BBD method demonstrated the capability to develop a second-degree polynomial model with high validity (R2 ˃ 0.99) for the removal process. The optimization results using the RSM-BBD method revealed a removal efficiency of 98.01%, 93.06%, and 88.26% for MB, MG, and Cu, respectively, under optimal conditions. These conditions were a pH of 6, contact time of 10 min, adsorbent amount of 0.025 g, and concentration of 20 mg L-1. The synthesized adsorbent was recovered through five consecutive adsorption-desorption cycles using hydrochloric acid. The results showed an approximately 12% reduction from the first to the seventh cycle. Also, MB, MG, and Cu removal from real water samples in optimal conditions was achieved in the range of 81.69-98.18%. This study demonstrates the potential use of CIST nanocomposite as an accessible and reusable option for removing MB, MG, and Cu pollutants from aquatic environments.

High-performance metal-oxide gas sensors based on hierarchical core-shell ZnFe2O4 microspheres for detecting 2-chloroethyl ethyl sulfide

Mustard gas, an erosive chemical agent, is primarily used as a chemical weapon, which seriously threatens human life and health. Therefore, detecting mustard gas and its simulant, 2-chloroethyl ethyl sulfide (2-CEES), is a very important task. As a binary metal oxide with a spinel structure, ZnFe₂O₄ is widely used for fabricating gas sensors because of its stable chemical structure and abundant oxygen vacancies. In this study, gas-sensing ZnFe₂O₄ microspheres with a hierarchical core-shell nanosheet structure were prepared via a simple one-step solvothermal method. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and N₂ adsorption analyses were performed to characterize the morphology, structure, and chemical composition of these microspheres. A gas sensor was fabricated from the as-synthesized material, and its gas sensing performance was evaluated, using 2-CEES as a target gas. The obtained ZnFe₂O₄-based sensor exhibited a high sensitivity of 9.07 to 1 ppm 2-CEES at the optimal working temperature of 250 °C. The sensor response and recovery times were 18 and 546 s, respectively, and its detection sensitivity of 2.87 achieved at a 2-CEES concentration of 0.01 ppm was within an acceptable range. Additionally, the sensor demonstrated sufficiently high 2-CEES selectivity, repeatability, and long-term stability.

Synthesis, Characterization and Dye Removal Capability of Conducting Polypyrrole/Mn0.8Zn0.2Fe2O4/Graphite Oxide Ternary Composites

Herein, ternary composites from polypyrrole (PPy), Mn0.8Zn0.2Fe2O4 (MZF), and graphite oxide (GO) were prepared to remove acid red dye (AR1) from wastewater. MZF was synthesized using spent Zn–C batteries, acid leaching, and sucrose auto-combustion processes; GO was prepared via oxidation and exfoliation of graphite. The composites were prepared by adding MZF and GO during the in-situ polymerization of pyrrole. Different PPy/MZF/GO (PMG) composites were prepared by changing the weight ratios of the PPy, MZF, and GO. We investigated the prepared composites’ structural, magnetic, and electrical/dielectric properties. We evaluated different experimental conditions’ influences on dye removal performance, such as pH, dosage, dye concentration, temperature, and contact time. XRD, FT-IR, and magnetic properties indicated that PPy completely coated the other contents. The electrical/dielectric properties improved while increasing the GO ratio. The PMG at GO content 50 wt.% (PMG50) showed the most efficient ratio for better removing AR1 from wastewater.

Magnetic visible-light activated photocatalyst ZnFe2O4/BiVO4/g-C3N4 for decomposition of antibiotic lomefloxacin: Photocatalytic mechanism, degradation pathway, and toxicity assessment

Magnetic ZnFe₂O₄/BiVO₄/g-C₃N₄ (ZBC) composites were prepared via a facile hydrothermal and calcination method for the degradation of a representative antibiotics lomefloxacin (LFX) under visible light irradiation. The optimal photocatalyst ZBC-10 with a ZnFe₂O₄:BiVO₄:g-C₃N₄ mass ratio of 1:8:10 performed 96.1% removal of LFX after 105 min of illumination. The excellent performance is ascribed to the effective construction of heterojunctions and its capacity to form a double Z-scheme charge transmission pathway among the hosts in ZBC-10. The composite enhanced the separation and migration of photoexcited charge carriers and the effective generation of multiple active radicals including ·OH, ·O₂^-, and ¹O₂. The LFX degradation process, identified based on an integrated HPLC-Q-TOF-MS analysis and density functional theory computation of the Fukui indices, comprised of three pathways initiated by the opening of the piperazinyl ring, separation of piperazinyl and quinoline moieties, and cleavage of the pyridine ring on the quinoline moieties. Ecotoxicological evaluation confirmed the reduced toxicity of transformation intermediates over photocatalysis. Convenient magnetic recovery, high performance, and high recyclability made ZBC-10 a promising visible-light-activated photocatalyst for practical implementation in eliminating antibiotics from wastewater.

Ammonia removal from industrial effluent using zirconium oxide and graphene-oxide nanocomposites

The present study developed and evaluated nano-adsorbents based on zirconium oxide and graphene oxide (ZrO₂/GO) as a novel adsorbent for the efficient removal of ammonia from industrial effluents. Fourier transform infrared (FTIR) spectroscopy, Field Emission Scanning Electron Microscope, Energy-dispersive X-ray Spectroscopy, and X-ray diffraction were used to evaluate and identify the novel adsorbent in terms of morphology, crystallography, and chemical composition. The pH (7), adsorbent quantities (20 mg), adsorbent contact time (30 min) with the sample, and initial ammonia concentration were all tuned for ammonia uptake. To validate ammonia adsorption on the ZrO₂/GO adsorbent, several kinetic models and adsorption isotherms were also utilized. The results showed that the kinetics of ammonia adsorption are of the pseudo-second order due to high R² (>0.99) value as compared first-order (R² = 0.52). The chemical behavior and equilibrium isotherm were analyzed using the isotherm models and Langmuir model provided high R² (>0.98) as compared Freundlich (>0.96). Hence, yielding a maximum uniform equilibrium adsorption capacity of 84.47 mg g^-1. The presence of functional groups on the surface of graphene oxide and ZrO₂ nanoparticles, which interact efficiently with ammonia species and provide an efficient surface for good ammonia removal, is most likely to be responsible.

Facile Fabrication of ZnO-ZnFe2O4 Hollow Nanostructure by a One-Needle Syringe Electrospinning Method for a High-Selective H2S Gas Sensor

Herein, a facile fabrication process of ZnO-ZnFe₂O₄ hollow nanofibers through one-needle syringe electrospinning and the following calcination process is presented. The various compositions of the ZnO-ZnFe₂O₄ nanofibers are simply created by controlling the metal precursor ratios of Zn and Fe. Moreover, the different diffusion rates of the metal oxides and metal precursors generate a hollow nanostructure during calcination. The hollow structure of the ZnO-ZnFe₂O₄ enables an enlarged surface area and increased gas sensing sites. In addition, the interface of ZnO and ZnFe₂O₄ forms a p-n junction to improve gas response and to lower operation temperature. The optimized ZnO-ZnFe₂O₄ has shown good H₂S gas sensing properties of 84.5 (S = R_a/R_g) at 10 ppm at 250 °C with excellent selectivity. This study shows the good potential of p-n junction ZnO-ZnFe₂O₄ on H₂S detection and affords a promising sensor design for a high-performance gas sensor.

Fast and efficient adsorption of palladium from aqueous solution by magnetic metal-organic framework nanocomposite modified with poly(propylene imine) dendrimer

In this study, a magnetic metal-organic framework (MMOF) was synthesized and post-modified with poly(propyleneimine) dendrimer to fabricate a novel functional porous nanocomposite for adsorption and recovery of palladium (Pd(II)) from aqueous solution. The morphological and structural characteristics of the prepared material were identified by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmet-Teller (BET) isotherm, and vibrating sample magnetometer (VSM). The results confirmed the successful synthesis and post-modification of MMOF. Semispherical shape particles (20-50 nm) with appropriate magnetic properties and a high specific surface area of 120 m²/g were obtained. An experimental design approach was performed to show the effect of adsorption conditions on Pd(II) uptake efficiency of the dendrimer-modified magnetic adsorbent. The study showed that the Pd(II) uptake on dendrimer-modified MMOF was well described by the Langmuir isotherm model with the highest uptake capacity of 291 mg/g under optimal condition (adsorbent content of 12.5 mg, Pd ion concentration of 80 ppm, pH = 4, and contact time of 40 min). The adsorption kinetics of Pd(II) ions was suggested to be a pseudo-first-order model. The results revealed a faster adsorption rate and higher adsorption capacity (about 43%) for dendrimer-modified MMOF. Finally, the reusability of the provided adsorbent was evaluated. This work provides a valuable strategy for designing and developing efficient magnetic adsorbents based on MOFs for the adsorption and recovery of precious metals.

Facile Microwave Hydrothermal Synthesis of ZnFe2O4/rGO Nanocomposites and Their Ultra-Fast Adsorption of Methylene Blue Dye

The industry development in the last 200 years has led to to environmental pollution. Dyes emitted by pharmaceutical and other industries are major organic pollutants. Organic dyes are a pollutant that must be removed from the environment. In this work, we adopt a facile microwave hydrothermal method to synthesize ZnFe2O4/rGO (ZFG) adsorbents and investigate the effect of synthesis temperature. The crystal structure, morphology, chemical state, and magnetic property of the nanocomposite are investigated by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy, and a vibrating sample magnetometer. Furthermore, the synthesized ZFGs are used to remove methylene blue (MB) dye, and the adsorption kinetics, isotherm, mechanism, and reusability of this nanomaterial are studied. The optimal ZFG nanocomposite had a dye removal percentage of almost 100%. The fitting model of adsorption kinetics followed the pseudo-second-order model. The isotherm model followed the Langmuir isotherm and the theoretical maximum adsorption capacity of optimal ZFG calculated by this model was 212.77 mg/g. The π-π stacking and electrostatic interaction resulted in a high adsorption efficiency of ZFG for MB adsorption. In addition, this nanocomposite could be separated by a magnet and maintain its dye removal percentage at almost 100% removal after eight cycles, which indicates its high suitability for utilization in water treatment.

A high-efficiency mediator-free Z-scheme Bi2MoO6/AgI heterojunction with enhanced photocatalytic performance

A high-efficiency Z-scheme Bi₂MoO₆/AgI heterojunction was designed and fabricated via in situ growth of AgI on Bi₂MoO₆. Its photocatalytic activity was investigated with the degradation of malachite green (MG). After 40 min of visible light irradiation, near complete degradation of MG (20 mg/L) occurred when BA11 (Bi₂MoO₆:AgI = 1:1, 2.0 g/L) was present, while only 29.0% and 49.7% of the MG could be degraded in the presence of Bi₂MoO₆ and AgI, respectively. The excellent photocatalytic activity of BA11 results from strong visible light absorption and the low recombination efficiency of photogenerated electron-hole pairs induced by the formation of heterojunction. Density function theory (DFT) calculations revealed that the formation of built-in electric field at the interface between Bi₂MoO₆ and AgI facilitates the effective separation and transfer of photogenerated charge carriers. Results of reuse experiments indicated that the heterostructured photocatalyst has excellent stability. Radical scavenging experiments and electron spin resonance spectra showed that superoxide radicals (O₂^-) and hydroxyl radicals (OH) were the major reactive oxygen species in the photocatalytic system. The photocatalytic degradation pathway of MG was proposed based on the organic degradation intermediates detected. These findings demonstrate that the mediator-free Z-scheme Bi₂MoO₆/AgI heterojunction could serve as a promising photocatalyst in photocatalytic treatment of organic pollutants.

Green Synthesis and Applications of ZnO and TiO2 Nanostructures

Over the last two decades, oxide nanostructures have been continuously evaluated and used in many technological applications. The advancement of the controlled synthesis approach to design desired morphology is a fundamental key to the discipline of material science and nanotechnology. These nanostructures can be prepared via different physical and chemical methods; however, a green and ecofriendly synthesis approach is a promising way to produce these nanostructures with desired properties with less risk of hazardous chemicals. In this regard, ZnO and TiO2 nanostructures are prominent candidates for various applications. Moreover, they are more efficient, non-toxic, and cost-effective. This review mainly focuses on the recent state-of-the-art advancements in the green synthesis approach for ZnO and TiO2 nanostructures and their applications. The first section summarizes the green synthesis approach to synthesize ZnO and TiO2 nanostructures via different routes such as solvothermal, hydrothermal, co-precipitation, and sol-gel using biological systems that are based on the principles of green chemistry. The second section demonstrates the application of ZnO and TiO2 nanostructures. The review also discusses the problems and future perspectives of green synthesis methods and the related issues posed and overlooked by the scientific community on the green approach to nanostructure oxides.

PPI-Dendrimer-Functionalized Magnetic Metal-Organic Framework (Fe3O4@MOF@PPI) with High Adsorption Capacity for Sustainable Wastewater Treatment

Herein, a magnetic zirconium-based metal-organic framework nanocomposite was synthesized by a simple solvothermal method and used as an adsorbent for the removal of direct and acid dyes from aqueous solution. To enhance its adsorption performance, poly(propyleneimine) dendrimer was used to functionalize the as-synthesized magnetic porous nanocomposite. The dendrimer-functionalized magnetic nanocomposite was characterized by field-emission scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, nitrogen adsorption/desorption isotherms, and vibration sample magnetometer. The obtained results revealed the successful synthesis and functionalization of the magnetic nanocomposite. The adsorbents exhibited good magnetic properties with high saturation magnetization and high specific surface area. The adsorption isotherms and kinetics of anionic dyes were described by the Freundlich and pseudo-second-order models, respectively. It was found that the kinetics of adsorption of both the investigated dyes by the dendrimer-functionalized magnetic composite is considerably faster than the magnetic composite under the same condition. The adsorption capacity of the dendrimer-functionalized magnetic composite for investigated direct and acid dyes was 173.7 and 122.5 mg/g, respectively, which was higher than those of the existing magnetic adsorbents. This work provides new insights into the synthesis and application of hybrid magnetic adsorbents with synergistic properties of nanoporous metal-organic frameworks and dendrimer with a large number of functional groups for the removal of organic dyes.

A New Class of Zn1 -x Fex -Oxyselenide and Zn1- x Fex -LDH Nanostructured Material with Remarkable Bifunctional Oxygen and Hydrogen Evolution Electrocatalytic Activities for Overall Water Splitting

The scalable and cost-effective H₂ fuel production via electrolysis demands an efficient earth-abundant oxygen and hydrogen evolution reaction (OER, and HER, respectively) catalysts. In this work, for the first time, the synthesis of a sheet-like Zn_{1- x} Fe_x -oxyselenide and Zn_{1- x} Fe_x -LDH on Ni-foam is reported. The hydrothermally synthesized Zn_{1- x} Fe_x -LDH/Ni-foam is successfully converted into Zn_{1- x} Fe_x -oxyselenide/Ni-foam through an ethylene glycol-assisted solvothermal method. The anionic regulation of electrocatalysts modulates the electronic properties, and thereby augments the electrocatalytic activities. The as-prepared Zn_{1- x} Fe_x -LDH/Ni-foam shows very low OER and HER overpotentials of 263 mV at a current density of 20 mA cm^-2 and 221 mV at 10 mA cm^-2 , respectively. Interestingly, this OER overpotential is decreased to 256 mV after selenization and the HER overpotential of Zn_{1- x} Fe_x -oxyselenide/Ni-foam is decreased from 238 to 202 mV at 10 mA cm^-2 after a stability test. Thus, the Zn_{1- x} Fe_x -oxyselenide/Ni-foam shows superior bifunctional catalytic activities and excellent durability at a very high current density of 50 mA cm^-2 . More importantly, when the Zn_{1- x} Fe_x -oxyselenide/Ni-foam is used as the anode and cathode in an electrolyzer for overall water splitting, Zn_{1- x} Fe_x -oxyselenide/Ni-foam(+)ǁZn_{1- x} Fe_x -oxyselenide/Ni-foam(-) shows an appealing potential of 1.62 V at 10 mA cm^-2 . The anionic doping/substitution methodology is new and serves as an effective strategy to develop highly efficient bifunctional electrocatalysts.

Synthesis and Absorption Properties of Hollow-spherical Dy2Cu2O5 via a Coordination Compound Method with [DyCu(3,4-pdc)2(OAc)(H2O)2]•10.5H2O Precursor

Dy2Cu2O5 nanoparticles with perovskite structures were synthesized via a simple solution method (SSM) and a coordination compound method (CCM) using [DyCu(3,4-pdc)2(OAc)(H2O)2]•10.5H2O (pdc = 3,4-pyridinedicarboxylic acid) as precursor. The as-prepared samples were structurally characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS) and standard Brunauer-Emmett-Teller (BET) methods. Compared to the aggregated hexahedral particles prepared by SSM, the Dy2Cu2O5 of CCM showed hollow spherical morphology composed of nanoparticles with average diameters of 100-150 nm and a larger special surface area up to 36.5 m2/g. The maximum adsorption capacity (Q m ) of CCM for malachite green (MG) determined by the adsorption isotherms with different adsorbent dosages of 0.03-0.07 g, reached 5.54 g/g at room temperature. The thermodynamic parameters of adsorption process were estimated by the fittings of the isotherms at 298, 318, and 338 K, and the kinetic parameters were obtained from the time-dependent adsorption isotherms. The results revealed that the adsorption process followed a pseudo-second-order reaction. Finally, the adsorption mechanism was studied using a competitive ion (CI) experiments, and the highly efficient selective adsorption was achieved due to strong O-Cu and O-Dy coordination bonds between Dy2Cu2O5 and MG.

Adsorption removal of malachite green dye from aqueous solution

In this review, the state of the art on the removal of malachite green dye from aqueous solution using adsorption technique is presented. The objective is to critically analyze different adsorbents available for malachite green dye removal. Hence, the available recent literature in the area is categorized according to the cost, feasibility, and availability of adsorbents. An extensive survey of the adsorbents, derived from various sources such as low cost biological materials, waste material from industry, agricultural waste, polymers, clays, nanomaterials, and magnetic materials, has been carried out. The review studies on different adsorption factors, such as pH, concentration, adsorbent dose, and temperature. The fitting of the adsorption data to various models, isotherms, and kinetic regimes is also reported.

ZnO/ZnFe2O4 nanocomposite as a broad-spectrum photo-Fenton-like photocatalyst with near-infrared activity

ZnO/ZnFe₂O₄ nanocomposite/H₂O₂ shows NIR activity due to the absorption of NIR, electron–hole pair separation by p–n junction promoted charge transfer, and reaction of electrons with H₂O₂.