Hierarchical 3D Flower-like Metal Oxides Micro/Nanostructures: Fabrication, Surface Modification, Their Crucial Role in Environmental Decontamination, Mechanistic Insights, and Future Perspectives

Hierarchical micro/nanostructures are constructed by micro-scaled objects with nanoarchitectures belonging to an interesting class of crystalline materials that has significant applications in diverse fields. Featured with a large surface-to-volume ratio, facile mass transportation, high stability against aggregation, structurally enhanced adsorption, and catalytical performances, three dimenisional (3D) hierarchical metal oxides have been considered as versatile functional materials for waste-water treatment. Due to the ineffectiveness of traditional water purification protocols for reclamation of water, lately, the use of hierarchical metal oxides has emerged as an appealing platform for the remediation of water pollution owing to their fascinating and tailorable physiochemical properties. The present review highlights various approaches to the tunable synthesis of hierarchical structures along with their surface modification strategies to enhance their efficiencies for the removal of different noxious substances. Besides, their applications for the eradication of organic and inorganic contaminants have been discussed comprehensively with their plausible mechanistic pathways. Finally, overlooked aspects in this field as well as the major roadblocks to the implementation of these metal oxide architectures for large-scale treatment of wastewater are provided here. Moreover, the potential ways to tackle these issues are also presented which may be useful for the transformation of current water treatment technologies.

Properties and mechanism of Cr(VI) adsorption and reduction by K2FeO4 in presence of Mn(II)

To efficiently treat hexavalent chromium (Cr(VI)) wastewater, K₂FeO₄ was used to remove and reduce Cr(VI) in presence of Mn(II) in this paper. Batch removal experiments were carried out to study the effect of Fe/Mn molar ratios, initial pH, in-situ and ex-situ and co-existing ions on Cr(VI) removal. The results showed the removal efficiency of Cr(VI) was 97.7% for the initial Cr(VI) concentration of 10.0 mg/L at Fe/Mn molar ratio of 2:3 and initial pH 8.0. Meanwhile, the high removal efficiency of Cr(VI) had been maintained throughout the pH range of 3.0-8.0 in the experimental study. Moreover, the removal process was relatively stable regardless of in-situ and ex-situ, and co-existing ions such as Ca²⁺ and low concentration of HCO3- had no intense effect on Cr(VI) removal, while SO42- inhibited Cr(VI) removal in the reaction system. To investigate the removal mechanism of Cr(VI) by K₂FeO₄ in presence of Mn(II), the reaction products were characterized by the Fourier transformed infrared spectrometer, X-ray powder diffraction, Transmission electron microscopy and the high-resolution X-ray photoelectron spectroscopy. The results indicated the ferrate decomposition products of γ-FeOOH/γ-Fe₂O₃ had the ability to adsorb Cr(VI) and react with Mn(II) to form γ-Fe₂O₃-Mn(II) complex to adsorb and reduce Cr(VI).

Recent advances in environmentally benign hierarchical inorganic nano-adsorbents for the removal of poisonous metal ions in water: a review with mechanistic insight into toxicity and adsorption

Recent developments in nanoscience and technology have addressed many of the problems associated with water quality. Accordingly, using the technological outputs of the recent research on nanomaterials, the best solution for the purification of water is highlighted in this review. Herein, the main objective is to provide mechanistic insight into the synthesis of various inorganic nanoadsorbents and their adsorption chemistry for poisonous metal ions present in polluted water. Initially, the toxicity and carcinogenicity of As3+, Pb2+, Cr6+, Cd2+, and Hg2+ metal ions are highlighted. For the removal of these toxic ions, this review focuses on eco-friendly nanoadsorbents. The various preparation procedures utilized for the preparation of nanoadsorbents are briefly discussed. Generally, this is because of the adsorption capacity of nanoadsorbents depends on their morphology, shape, size, surface area, surface active sites, functional groups, and quantization effect. Also, due to the importance of their mechanism of action, the recent developments and challenges of novel nanoadsorbents such as metal oxides, core shell nanoparticles, magnetic nano ferrates, and functionalized core shell magnetic oxides and the processes for the treatment of water contaminated by toxic metal ions such as As3+, Pb2+, Cr6+, Cd2+, and Hg2+ are exclusively reviewed. Further, the adsorption efficiency of inorganic nanoadsorbents is also compared with that of activated carbon derived from various sources for all the above-mentioned metal ions.

Adsorptive-Oxidative Removal of Sulfides from Water by MnO2-Loaded Carboxylic Cation Exchangers

Hybrid ion exchangers (HIX) containing manganese(IV) oxide (MnO₂) based on macroporous and gel-type carboxylic cation exchangers as supporting materials were obtained. The hybrid materials were characterized using scanning electron microscopy with energy dispersive spectrometry (SEM/EDS), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD) and nitrogen adsorption isotherms at 77 K and mercury porosimetry. HIX with introduced MnO₂ (20.0–32.8 wt% Mn) were tested for removal of dissolved sulfides from anoxic aqueous solutions with 100–500 mg S²⁻/dm³ concentrations. The process proceeded effortlessly at pH 10–13 despite unfavorable electrostatic interactions of the reactants. The highest exhibited sorption capacity was 144.3 ± 7.1 mg S²⁻/g. Approximately 65% of dissolved sulfides were oxidized to S₂O₃²⁻ ions and repelled from HIX structure. On average, 13% of sulfide removal products were adsorbed by the MnO₂ surface. The impact of MnO₂ load and the ionic form of HIX functional groups on removal of sulfides and resulting products was examined. The mechanism of the process is suggested.

A facile one-step synthesized epsilon-MnO2 nanoflowers for effective removal of lead ions from wastewater

The increasing contamination of lead ions (Pb(II)) in groundwater has become a serious environmental issue, which provides the impetus for intense research on Pb(II) removal. ε-MnO₂ nanoflowers were successfully fabricated through a simple decomposition reaction. And the obtained ε-MnO₂ nanoflowers were employed to remove Pb(II) from water. The detailed microstructure and surface properties of ε-MnO₂ were systematically characterized. The results indicate that the pure ε-MnO₂ phase was obtained and the specific surface area is 96.33 m² g^-1. Batch adsorption experiments of Pb(II) were carried out, and the ε-MnO₂ nanoflowers exhibited outstanding adsorption performance. The maximum adsorption capacity for Pb(II) and Cd(II) achieved to 239.7 mg g^-1 and 73.6 mg g^-1 at the dosage of 0.2 g L^-1. Besides, the prepared ε-MnO₂ nanoflowers show much higher removal efficiency toward Pb(II) compared with commercial MnO₂. The XRD results reveal the stability of ε-MnO₂ nanoflowers, and the XPS results suggest that both the electrostatic interaction and structural tunnels are responsible for the removal mechanisms of Pb(II). This work finds a facile method to synthesize ε-MnO₂ nanoflowers, showing great potential for Pb(II) removal.

Biomedical application of manganese dioxide nanomaterials

Manganese dioxide nanomaterial is a new type of inorganic nanomaterial offering numerous advantages: simple preparation, low cost, and environmental friendliness. This review summarizes the traditional and novel synthetic methods for manganese dioxide nanomaterials and mainly discusses their potential in biomedical applications. Manganese dioxide nanomaterials are mainly used as drug carriers in tumor therapy. In recent years, the construction of multifunctional nano-platforms using manganese dioxide has gradually improved. The main mechanism is that manganese dioxide nanomaterials can combine with reactive oxygen species in the tumor microenvironment to alleviate tumor hypoxia. Manganese dioxide has also been used to quench fluorescent carbon dots in fluorescent probes. Based on the oxidation ability and catalytic activity of MnO₂, MnO₂ nanosheets are widely used to build biosensors. New research shows that manganese dioxide nanomaterials also have great potential in gene therapy and nuclear magnetic imaging.

Recent trends of MnO2-derived adsorbents for water treatment: a review

Over the years, manganese dioxide (MnO₂) and its different allotropes have gained significant research attention in the field of wastewater treatment because of their exciting physicochemical properties.

Selective heavy metal removal and water purification by microfluidically-generated chitosan microspheres: Characteristics, modeling and application

Many industrial wastewater streams contain heavy metals, posing serious and irreversible damage to humans and living organisms, even at low concentrations due to their high toxicity and persistence in the environment. In this study, high-performance monodispersed chitosan (CS) microspheres were prepared using a simple microfluidic method and evaluated for metal removal from contaminated water. Batch experiments were carried out to evaluate the adsorption characteristics for the removal of copper ions, one representative heavy metal, from aqueous solutions. The inherent advantages of microfluidics enabled a precise control of particle size (CV = 2.3%), while exhibiting outstanding selectivity towards target ions (adsorption capacity 75.52 mg g^-1) and fair regeneration (re-adsorption efficiency 74% after 5 cycles). An integrated adsorption mechanism analytic system was developed based on different adsorption kinetics and isotherms models, providing an excellent adsorption prediction model with pseudo-second order kinetics (R² = 0.999), while the isotherm was fitted best to the Langmuir model (R² = 0.998). The multi-step adsorption process was revealed via quantitative measurements and schematically described. Selective adsorption performance of CS microspheres in the present of other competitive metal ions with different valence states has been demonstrated and studied by both experimental and density functional theory (DFT) analysis.

Simple construction of core–shell MnO2@TiO2 with highly enhanced U(vi) adsorption performance and evaluated adsorption mechanism

Simple construction of core–shell MnO₂@TiO₂ with highly enhanced U(vi) adsorption performance and evaluation of its adsorption mechanism.

Eggshell Membrane-Templated MnO2 Nanoparticles: Facile Synthesis and Tetracycline Hydrochloride Decontamination

Taking advantages of reticular proteins and reductive groups on the surface, eggshell membrane (ESM) was selected to synthesize MnO₂ nanoparticles from potassium permanganate through a super simple way in which ESM acted as both template and reductant. This process avoided harsh reaction conditions or complicated aftertreatments and thus owned the merits of green synthesis, handy operation, low cost, and easy purification. The ESM-templated MnO₂ nanoparticles (MnO₂ NPs/ESM) were characterized, and the content of nanomaterials on the template was tested. MnO₂ NPs/ESM showed a good capacity for decontamination of tetracycline hydrochloride (TCH). The macroscopical materials can be separated easily by taking the membrane out to stop the degradation instead of centrifugation or filtration. It was studied that 72.27% of TCH (50 mg/L) was decontaminated in 20 min by 0.1920 g/L MnO₂ nanoparticles, and removal efficiency could reach 83.10% after 60 min under buffered condition. The kinetics was studied with or without buffer, and it was concluded that the degradation process followed a pseudo-second-order model. The facile synthesis of materials and effective degradation would facilitate the nano-MnO₂-based decontamination applications.

Development of a Nanostructured α-MnO2/Carbon Paper Composite for Removal of Ni2+/Mn2+ Ions by Electrosorption

Toxic metal ions, such as Ni²⁺ and Mn²⁺, in industrial waste streams are nonbiodegradable and can cause damage to the human body. Electrochemical cleaning techniques are attractive as they offer more control and produce less sludge than do chemical/biological approaches without the high pressures needed for membranes. Here, nanoneedle-structured α-MnO₂/carbon fiber paper (CFP) composites were synthesized by a hydrothermal approach and used as electrodes for combined electroadsorption and capacitive deionization removal of nickel and manganese ions from pseudoindustrial waste streams. The specific performance of α-MnO₂/CFP (16.4 mg Ni²⁺ per g of active material) not only shows a great improvement in comparison with its original CFP substrate (0.034 Ni²⁺ mg per g), but also is over 6 times that of activated carbon (2.5 mg Ni²⁺ per g). The high performance of α-MnO₂/CFP composites is attributed to their high surface area, desirable mesoporosity, pore-size distribution that permits the further access of ions, and their property as a pseudocapacitor, which contributes to a more efficient electron/charge transfer in the faradic process. Unfortunately, it was also found that some Mn²⁺ ions are released due to the partial reduction of MnO₂ when operated as a negative electrode. For the removal of Mn²⁺ ions, an asymmetric arrangement, consisting of a MnO₂/CFP positive electrode and an activated carbon negative electrode, was employed. This arrangement reduced the Mn²⁺ concentration from 100 ppm to less than 2 ppm, a vast improvement over the systematical two-activated carbon electrode system that could only reach 42 ppm under the same conditions. It was also observed that as long as the MnO₂/CFP composite was maintained as a positive electrode, it was completely stable. The technique was able to reduce both Ni²⁺ and Mn²⁺ ions to well below the 10 ppm requirement for discharge into public sewers in Singapore.

A Hydrothermal Synthesis of Fe3O4@C Hybrid Nanoparticle and Magnetic Adsorptive Performance to Remove Heavy Metal Ions in Aqueous Solution

Advanced core-shelled material with a high specific area has been considered as an effective material to remove heavy metal from aqueous solutions. A core-shelled Fe3O4@C hybrid nanoparticle aggregates with environmental-friendly channel in the study. Moreover, the higher exposure of adsorption sites can be achieved for the special configuration that higher Brunauer-Emmet-Teller (BET) surface area reaches up to 238.18 m2 g-1. Thus, a more efficiently heavy metal ion removal is obtained, Pb (II), Cd (II), Cu (II), and Cr (VI) up to 100, 99.2, 96.6, and 94.8%, respectively. In addition, the products are easy to be separated from the aqueous solutions after adsorption, due to the relative large submicrometer size and the enhanced external magnetic fields introduced by the iron-based cores. We provide an ideal mode to remove heavy metal ions using core-shelled Fe3O4@C under the water treatment condition. A new approach is clarified that core-shell nano/micro-functional materials can be synthesized well on large scales which are used in many fields such as environmental remediation, catalyst, and energy.

Ultrasonication synthesis of PVA/PVP/α-MnO2-stearic acid blend nanocomposites for adsorbing CdII ion

In this research, the functionalization of α-MnO₂ nanorods (NRs) was conducted with stearic acid (SA) through solvothermal technique. The α-MnO₂-SA NRs were used as nanofiller for the preparation of blend nanocomposites (NCs) based on poly(vinyl alcohol) (PVA) and poly(vinyl pyrrolidone) (PVP) by ultrasonic irradiation. The results exhibit that with increasing in α-MnO₂-SA quantity, the thermal stability of blend NCs was improved. Morphological studies revealed that α-MnO₂-SA with rod structure and a diameter size of 25-80nm was uniformly dispersed in the PVA and PVP matrices. The use of ultrasonic was responsible for these great homogeneities which could not be achieved by mechanical or magnetically stirring. The prepared blend NCs were exploited as an adsorbing for uptake of Cd^II ion from the aqueous system. The results indicated that they can be potentially utilized for the elimination of Cd^II ion from an aqueous system with q_m of 47(mgg^-1).

Isopentyl-Sulfide-Impregnated Nano-MnO₂ for the Selective Sorption of Pd(II) from the Leaching Liquor of Ores

Conventional separation methods are not suitable for recovering palladium present in low concentrations in ore leaching solutions. In this study, a novel isopentyl sulfide (S₂₀₁)-impregnated α-MnO₂ nanorod adsorbent (BISIN) was prepared, characterized, and applied for the selective adsorption and separation of palladium from the leaching liquor of ores. Batch studies were carried out, and the main adsorption parameters were systematically investigated, in addition to the relevant thermodynamic parameters, isotherms, and kinetic models. The thermodynamic parameters reflected the endothermic and spontaneous nature of the adsorption. Moreover, the experimental results indicated that the Langmuir isotherm model fits the palladium adsorption data well and the adsorption was well described by the pseudo-second-order kinetic model. The main adsorption mechanisms of palladium were elucidated at the molecular level by X-ray crystal structure analysis. Thiourea was found to be an excellent desorption agent, and the palladium-thiourea complex was also confirmed by X-ray crystal structure analysis. The results indicated that almost all of the Pd(II) (>99.0%) is adsorbed on BISIN, whereas less than 2% of the adsorbed Pt(IV), Fe³⁺, Cu²⁺, Ni²⁺, and Co²⁺ is observed under the optimum conditions. The proposed method can be used for the efficient adsorption and separation of palladium from the leaching liquor of ores.

Use of PVA/α-MnO2-stearic acid nanocomposite films prepared by sonochemical method as a potential sorbent for adsorption of Cd (II) ion from aqueous solution

Alpha manganese dioxide (α-MnO₂) nanorods have been covalently functionalized with stearic acid by solvothermal method. The α-MnO₂-stearic acid nanorods were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy and transmission electron microscopy. The amount of stearic acid grafted onto α-MnO₂ surface was determined about 41wt% by thermogravimetric analysis. The α-MnO₂-stearic acid nanorods was used as nanofiller for the preparation of poly(vinyl alcohol) (PVA) nanocomposites (NCs). The NCs with 1, 3, and 5wt% of nanofiller were prepared through ultrasound assisted technique as an economical, fast, eco-friendly, and effective method. The PVA/α-MnO₂-stearic acid NCs were characterized by different techniques. The results showed that with increasing the α-MnO₂-stearic acid content, the thermal stability and tensile properties were improved. Besides, the NC 5wt% was used as adsorbent for sorption of Cd (II) ion. The adsorption efficiency of NC 5wt% in different initial concentrations of Cd (II) ion (20-100mgL^-1) was 58-95%. All the isotherm data were well fitted by both Langmuir and Freundlich equations. The adsorption kinetics study demonstrated that pseudo-second-order model was the best fit with the experimental data. The results indicated that prepared NCs are promising adsorbents for the removal of Cd (II) ion from the aqueous solution.

Novel poly(aniline-co-3-amino-4-methoxybenzoic acid) copolymer for the separation and recovery of Pd(ii) from the leaching liquor of automotive catalysts

The PANI–AMB copolymers were prepared and used for separation and recovery palladium from the leaching solutions of the automotive catalysts.

Surface modification of hollow magnetic Fe3O4@NH2-MIL-101(Fe) derived from metal-organic frameworks for enhanced selective removal of phosphates from aqueous solution

Hollow magnetic Fe3O4@NH2-MIL-101(Fe) derived from metal-organic frameworks are fabricated through a general facile strategy. The synthetic parameters are regulated to control the shape of the as-prepared samples. The concentration of phosphates decreased sharply from the initial 0.60 to 0.045 mg.L(-1) with the exposure time in 50 minutes. The correlation between the most significant parameters such as contact time, adsorbent dose, pH, as well as adsorption capacities was optimized, and the effects of these parameters on the removal efficiency of phosphates were investigated. Surface functionalization of magnetic hollow materials is a well-designed way to bridge the gap between high adsorption activity, excellent separation and recovery of phosphates from the water treatment system. Therefore, it exhibits a remarkable selective removal of phosphates from aqueous solution.

Covalent surface modification of α-MnO2 nanorods with l-valine amino acid by solvothermal strategy, preparation of PVA/α-MnO2-l-valine nanocomposite films and study of their morphology, thermal, mechanical, Pb(ii) and Cd(ii) adsorption properties

The surface of α-manganese dioxide (α-MnO₂) nanorods was modified chemically with l-valine amino acid by a solvothermal strategy.

Preparation and characterization of functionalized silica spheres for removal of Cu(ii), Pb(ii), Cr(vi) and Cd(ii) from aqueous solutions

Functionalized silica spheres (SMS) with mesoporous structure were fabricated through a facile and environmentally benign route, and exhibited remarkable removal efficiency for metal ions from aqueous solutions.

Synthesis of highly monodisperse porous manganese oxide spheres using a butyric acid microemulsion

Highly monodisperse manganese oxide spheres are generated by self-assembly using a butyric acid microemulsion.