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

Turning harmful Mn2+ to treasure: In-situ formed ε-MnO2 for removing heavy metals from acid mine drainage

Acid mine drainage (AMD) contains high concentrations of heavy metals, causing serious environmental pollution. Current neutralization techniques fail to recover and utilize valuable heavy metals, and generate large quantities of hazardous sludge. Manganese (Mn) is generally present at high levels in AMD. Therefore, this paper proposed a technology to recover Mn from AMD, by adding KMnO4 to converting Mn into ε-MnO2. Ultra-Violet C (UVC) was used to photolyze the residual KMnO4. The study then evaluated the processes and mechanisms involved in the technology. The photolysis of KMnO4 in strong acidic conditions was determined, and new mechanisms were proposed. MnO2 produced by the photolysis process was formed through the reaction between Mn(III) and KMnO4. In the absence of KMnO4, Mn(III) underwent further photolysis and was reduced to Mn2+. The maximum adsorption capacities of in-situ formed ε-MnO2 for Pb2+, Cd2+, and Fe3+ were 449.80, 122.05, and 779.88 mg/g, respectively. Higher Mn-OH levels and MnO2 regeneration were crucial in improving adsorption performance. Proton exchange and inner-circle complexation were the main pathways for Pb2+ and Cd2+ adsorption by in-situ formed ε-MnO2. A phase transformation occurred when a substantial amount of Fe3+ was adsorbed, leading to the gradual transformation to MnFe binary oxides. When applying in-situ formed ε-MnO2 technology for actual AMD treatment, 98.62 % of Mn in AMD was recovered within 24 h in the presence of ε-MnO2 for possible further reuse in industries, with a final recovery of 0.76 kg/m3. Further, this technique removed other heavy metals and reduced the sludge volume by 20.99 % when used as a pre-treatment step for neutralization. These results demonstrated the broad potential of this treatment technology.

Manganese dioxide decorated kiwi peel powder for efficient removal of lead from aqueous solutions, blood and Traditional Chinese Medicine extracts

For human health and environment safety, it is of great significance to develop novel materials with high effectiveness for removal of lead from not only aqueous solutions but also human body and traditional Chinese medicines. Here, functional kiwi peel composite, manganese dioxide decorated kiwi peel powder (MKPP), is proposed for the removal of Pb2+ effectively. The adsorption of Pb2+ in aqueous solution is a highly selective and endothermic process and kinetically follows a pseudo-second-order model, which can reach equilibrium with the capacity of 192.7 mg/g within 10 min. Comprehensive factors of hydration energy, charge-to-radius ratio and softness of Pb2+ make a stronger affinity between MKPP and Pb2+. The possible adsorption mechanism involves covalent bond, electrostatic force and chelation, etc. MKPP can be efficiently regenerated and reused with high adsorption efficiency after five cycles. Besides, MKPP can remove over 97% of Pb2+ from real water samples. MKPP can also alleviate lead poisoning to a certain extent and make the Pb level of TCM extract meet the safety standard. This work highlights that MKPP is a promising adsorbent for the removal of Pb2+ and provides an efficient strategy for reusing kiwi peel as well as dealing with the problem of Pb pollution.

Functional Ag-EDTA-modified MnO2 nanocoral reef for rapid removal of hazardous copper from wastewater

A novel MnO2@EDTA-Ag nanocoral reef was constructed via a simplified redox reaction followed by EDTA and Ag nanoparticles impregnation to capture hazardous copper (II) from wastewater. A comprehensive characterization of the synthesized materials was conducted. The morphology of MnO2@EDTA-Ag in the form of a nanocoral reef was constructed of two-dimensional nanoplatelets and nanorod-like nanostructures. The optimal adsorption conditions proposed by the Plackett-Burman design (PBD) that would provide a removal % of 99.95 were pH 5.5, a contact time of 32.0 min, a Cu(II) concentration of 11.2 mg L-1, an adsorbent dose of 0.05 g, and a temperature of 40.3 °C. The loading of Ag nanoparticles onto MnO2@EDTA improved the adsorption capability of MnO2@EDTA-Ag. Additionally, the recyclability of MnO2@EDTA-Ag nanocoral reef was maintained at 80% after three adsorption-desorption cycles, and there was no significant change in the XRD analysis before and after the recycling process, implying its stability. It was found that nanocoral reef-assisted EDTA formed a chelation/complexation reaction between COO- groups and C-N bonds of EDTA with Cu(II) ions. In addition, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis proved the synergistic effect of the electrostatic interaction and chelation/complexation was responsible for the removal mechanism of Cu(II). Also, the results demonstrated no significant variation in MnO2@EDTA-Ag removal efficiency for all the tested real water samples, revealing its efficacy in wastewater treatment. Therefore, the current study suggests that MnO2@EDTA-Ag has substantial potential to be used as a feasible adsorbent for probable hazardous metals remediation.

Label-free colorimetric sensor for Pb2+ determination using catalytic activity of MnO2 nanoflowers and elongated aptamer

In this study, a label-free aptasensor utilizing colorimetric properties was developed to detect Pb2+ with high sensitivity. The approach involved applying modified aptamer which enhanced the oxidase-mimicking activity of MnO2 nanoflowers. This innovative method provides an efficient means for monitoring Pb2+ ions without requiring any labeling techniques. The fundamental principle of this aptasensor is based on the adsorption of a modified aptamer onto MnO2 nanoflowers' surface, which in turn increases their affinity for chromogenic substrates and enhances their catalytic activity. The proposed aptasensor exploits the high sensitivity due to the extension of the aptamer sequence length by terminal deoxynucleotidyl transferase (TdT). Under optimum experimental conditions, the developed colorimetric aptasensor indicated a linear detection range from 4 to 80 nM with a limit of detection (LOD) of 1.4 nM. Moreover, the aptasensor successfully monitored Pb2+ in the drinking water, milk and human serum samples. Henceforth, the colorimetric aptasensor exhibited in this study possesses several benefits such as uncomplicated operation, cost-effectiveness, label-free detection and remarkable sensitivity. Thus rendering it a suitable option for analyzing intricate samples.

Epsilon-MnO2 simply prepared by redox precipitation as an efficient catalyst for ciprofloxacin degradation by activating peroxymonosulfate

Four kinds of manganese oxides were successfully prepared by hydrothermal and redox precipitation methods, and the obtained oxides were used for CIP removal from water by activating PMS. The microstructure and surface properties of four oxides were systematically characterized. The results showed that ε-MnO2 prepared by the redox precipitation method had large surface area, low crystallinity, high surface Mn(III)/Mn(Ⅳ) ratio and the highest activation efficiency for PMS, that is, when the concentration of PMS was 0.6 g/L, 0.2 g/L ε-MnO2 could degrade 93% of CIP within 30 min. Multiple active oxygen species, such as sulfate radical, hydroxyl radical and singlet oxygen, were found in CIP degradation, among which sulfate radical was the most important one. The degradation reaction mainly occurred on the surface of the catalyst, and the surface hydroxyl group played an important role in the degradation. The catalyst could be regenerated in situ through the redox reaction between Mn4+ and Mn3+. The ε-MnO2 had the advantages of simple preparation, good stability and excellent performance, which provided the potential for developing new green antibiotic removal technology.

Significance of MnO2 Type and Solution Parameters in Manganese Removal from Water Solution

A very low concentration of manganese (Mn) in water is a critical issue for municipal and industrial water supply systems. Mn removal technology is based on the use of manganese oxides (MnO_x), especially manganese dioxide (MnO₂) polymorphs, under different conditions of pH and ionic strength (water salinity). The statistical significance of the impact of polymorph type (akhtenskite ε-MnO₂, birnessite δ-MnO₂, cryptomelane α-MnO₂ and pyrolusite β-MnO₂), pH (2-9) and ionic strength (1-50 mmol/L) of solution on the adsorption level of Mn was investigated. The analysis of variance and the non-parametric Kruskal-Wallis H test were applied. Before and after Mn adsorption, the tested polymorphs were characterized using X-ray diffraction, scanning electron microscope techniques and gas porosimetry analysis. Here we demonstrated the significant differences in adsorption level between MnO₂ polymorphs' type and pH; however, the statistical analysis proves that the type of MnO₂ has a four times stronger influence. There was no statistical significance for the ionic strength parameter. We showed that the high adsorption of Mn on the poorly crystalline polymorphs leads to the blockage of micropores in akhtenskite and, contrary, causes the development of the surface structure of birnessite. At the same time, no changes in the surfaces of cryptomelane and pyrolusite, the highly crystalline polymorphs, were found due to the very small loading by the adsorbate.

Nanotechnology- A ray of hope for heavy metals removal

Environmental effects of heavy metal pollution are considered as a widespread problem throughout the world, as it jeopardizes human health and also reduces the sustainability of a cleaner environment. Removal of such noxious pollutants from wastewater is pivotal because it provides a propitious solution for a cleaner environment and water scarcity. Adsorption treatment plays a significant role in water remediation due to its potent treatment and low cost of adsorbents. In the last two decades, researchers have been highly focused on the modification of adsorption treatment by functionalized and surface-modified nanomaterials which has spurred intense research. The characteristics of nano adsorbents attract global scientists as it is also economically viable. This review shines its light on the functionalized nanomaterials application for heavy metals removal from wastewater and also highlights the importance of regeneration of nanomaterials in the view of visualizing the economic aspects along with a cleaner environment. The review also focused on the proper disposal of nanomaterials with crucial issues that persist in the adsorption process and also emphasize future research modification at a large-scale application in industries.

Effects of synthesis temperature on ε-MnO2 microstructures and performance: Selective adsorption of heavy metals and the mechanism onto (100) facet compared with (001)

The heavy-metal adsorbent ε-MnO₂ was produced through a simple, one-step oxidation-reduction reaction at three different synthesis temperatures (25 °C, 50 °C and 75 °C) and their morphology and chemical-physical properties were compared. Of the three materials, MnO₂-25 had the largest specific surface area and the highest surface hydroxyl concentration. Its optimal performance was demonstrated by batch adsorption experiments with Pb²⁺, Cd²⁺ and Cu²⁺. Of the three metals, Pb²⁺ was adsorbed best (339.15 mg/g), followed by Cd²⁺ (107.50 mg/g) and Cu²⁺ (86.30 mg/g). When all three metals were present, Pb²⁺ was still absorbed best but now more Cu²⁺ was adsorbed than Cd²⁺. In order to explore the mechanism for the inconsistent adsorption order of Cd²⁺ and Cu²⁺ in single and competitive adsorption, we combined experimental data with density functional theory (DFT) calculations to elucidate the distinct adsorption nature of MnO₂-25 towards these three metals. This revealed that the adsorption affinity of the (100) facet was superior to (001), and since the surface complexes were also more stable on (100), this facet was most likely determining the adsorption order for the single metals. When the metals were present in combination, Pb²⁺ preferentially occupied the active adsorption sites of (100), forcing Cu²⁺ to be adsorbed on the (001) facet where Cd²⁺ was only poorly bound. Thus, the adsorption behavior was affected by MnO₂-25 surface chemistry at a molecular scale. This study provides an in-depth understanding of the adsorption mechanisms of the heavy metals on this adsorbent and offers theoretical guidance for production of adsorbent with improved removal efficiency.

Layered Double Hydroxides Derived from MIL-88A(Fe) as an Efficient Adsorbent for Enhanced Removal of Lead (II) from Water

The efficient removal of lead (II) from aqueous solution remains a big problem and the development of novel nanomaterials as adsorbents by various technologies to solve this problem is promising. This study contributed a novel nanostructure of MIL-88A-layered double hydroxides (LDHs) as the adsorbent for Pb²⁺, which was synthesized by a two-step solvothermal method with MIL-88A(Fe) as the precursor. The as-prepared material featured a chestnut-like core-shell structure, and exhibited excellent removal performance towards Pb²⁺ from water in comparison to MIL-88A(Fe) and LDHs (directly synthesized). The adsorption of Pb²⁺ by the MIL-88A-LDHs conformed to the pseudo-second-order kinetic model and the Langmuir and Freundlich isotherm models. The maximal adsorption capacity was 526.32, 625.00, and 909.09 mg g^-1 at 278, 298, and 318 K, respectively. The thermodynamic parameters suggested that the adsorption was an endothermic, entropy-increasing, and spontaneous reaction. X-ray photoelectron spectroscopy (XPS) analysis indicated that the surface complexation was mostly responsible for Pb²⁺ elimination. The MIL-88A-LDHs can be readily regenerated and showed good cyclic performance towards Pb²⁺. Thus, the as-prepared MIL-88A-LDHs may hold promise for the elimination of aqueous heavy metals.

Manganese oxides activated peroxymonosulfate for ciprofloxacin removal: Effect of oxygen vacancies and chemical states

Ciprofloxacin (CIP) as an anti-inflammatory drug is frequently detected in various water resources. Recently, Sulfate Radical-based advanced oxidation processes with manganese oxides have been recognized as a highly effective method for CIP degradation. Herein, ε-MnO₂ was obtained through a convenient drying process. After different atmospheric treatments, MnO and Mn₂O₃ were fabricated for subsequent degradation experiments. The results show that MnO exhibits better catalytic activity than Mn₂O₃, with high removal efficiency of almost 84.3% for CIP. Quenching test and electron paramagnetic resonance spectra confirm that ¹O₂ is the dominant species during reaction, while ·OH and SO₄^·- play a supporting role. A related discussion about the role of valence states of Mn and oxygen vacancies is presented, which can provide a theoretical basis for further development of Mn/peroxymonosulfate system.

Sodium Borohydride Treatment to Prepare Manganese Oxides with Oxygen Vacancy Defects for Efficient Oxygen Reduction

Manganese oxides are often used as catalysts for oxygen reduction reactions due to their low price and high stability, and they have been extensively studied. However, the poor electrical conductivity and low intrinsic activity of manganese oxides restrict its application in oxygen reduction. In this paper, the manganese oxide octahedral molecular sieve is used as the research object, and the oxygen reduction performance of the material is adjusted by the surface reduction etching treatment of sodium borohydride. After being treated with 8 mmol/L sodium borohydride, the oxygen vacancy content of the manganese oxide octahedral molecular sieve was 26%. The manganese oxide octahedral molecular sieve showed the best performance, and its half-wave potential was 0.821 V. Tests show that the material has excellent electrical conductivity and high oxygen reduction kinetics. The generation of appropriate oxygen vacancies on the surface directly improves the chemical properties of the material surface, regulates the ratio of Mn3+/Mn4+ on the surface of the nanorod, and increases the oxygen reduction adsorption sites on the surface of the material. On the other hand, the electrical conductivity of the material is adjusted to increase the electron transfer rate during the oxygen reduction process, thereby enhancing the oxygen reduction activity.

Highly effective removal of formaldehyde from aqueous solution using mesoporous ɛ-MnO2 crystals at room temperature

Without additional templating agent or surfactant, porous and sparse MnCO3 was synthesized hydrothermally from Mn2+ with a CO2-storage material (CO2SM). Through thermal decomposition of the as-synthesized MnCO3, ɛ-MnO2 crystals with good catalytic performance and stability in HCHO degradation were prepared. The optimum preparation conditions were determined by tuning the preparation conditions and carrying out response surface studies, and the resulting ɛ-MnO2 crystals could degrade 66.1% of a 10 ml 10 mg l−1 HCHO solution. After the HCHO degradation conditions were optimized, the thermodynamic data could be fitted with the Langmuir isotherm and quasi-secondary kinetic models at T = 25–50°C. The degradation mechanism of HCHO is discussed. This work provides a new strategy for the degradation of HCHO at room temperature.

Selective Adsorption of Pb2+ in the Presence of Mg2+ by Layer-by-Layer Self-Assembled MnO2/Mxene Composite Films

A self-assembled MnO2/Mxene composite film was compounded with MXene nanosheets and layered crystalized MnO2 nanosheets using surfactant sodium dodecyl sulfate (SDS) as a soft template. The obtained material was characterized by XRD, SEM, XPS, and FT-IR, which showed that the films have large surface-active functional groups and metal ion flow channels, indicating that the MnO2/Mxene composite films were capable of both the chemical and physical adsorption of the target heavy metal ions. The analysis of adsorption performance showed that the Pb2+ removal rate reached 98.3% at pH 6 and an initial Pb2+ concentration of 30 mg/L, while the maximum adsorption capacity could reach 1235 µmol/g. In addition, the MnO2/Mxene composite film had specific selectivity and recyclability. The reuse study verified that the Pb2+ removal rate reached 96.4% after five cycles, confirming that the MnO2/Mxene composite films had practical application prospects.

Lead adsorption and antibacterial activity using modified magnetic biochar/sodium alginate nanocomposite

Biochar is one of the most promising wastewater treatment materials. As shown in the Scanning Electron Micrograph, the magnetic biochar (BC) cross-linked glutaraldehyde (G) with sodium alginate (SA) (BC-G-SA) nanocomposite formed with uniform particle size without aggregation, and an X-Ray Diffraction study revealed that the BC-G-SA nanocomposite has an amorphous structure. The BC-G-SA nanocomposite enhanced the microwave adsorption process for Pb (II). The maximum metal capacity value was obtained using the microwave adsorption technique at pH 5.0 and contact time 20 s for Pb (II) at medium and low microwave power (940 and 1400 μmol g^-1, respectively). Pb (II) adsorption isotherm follows a pseudo-second-order model. Also, the BC-G-SA nanocomposite effectively inhibited bacterial growth throughout the growth kinetics experiment. BC-G-SA inhibited the growth of S. aureus at a MIC of 200 g mL^-1, whereas L. monocytogenes had a MIC of 200 g mL^-1. The MIC values for E. faecalis and E. faecium were significantly lower (50 and 100 g mL^-1, respectively).

Removal Performance and Mechanism of Benzo(b)Fluorathene Using MnO2 Nanoflower/Graphene Oxide Composites

High-ring polycyclic aromatic hydrocarbons (PAHs, Benzo[b]fluorathene (BbFA), etc.) are difficult to biodegrade in the water environment. To address this issue, an innovative method for the preparation of MnO₂ nanoflower/graphene oxide composite (MnO₂ NF/GO) was proposed for adsorption removal of BbFA. The physicochemical properties of MnO₂ NF/GO were characterized by SEM, TEM, XRD, and N₂ adsorption/desorption and XPS techniques. Results show that the MnO₂ NF/GO had well-developed specific surface area and functional groups. Batch adsorption experiment results showed that adsorption capacity for BbFA was 74.07 mg/g. The pseudo-second-order kinetic model and Freundlich isotherm model are fitted well to the adsorption data. These show electron-donor-acceptor interaction; especially π-π interaction and π complexation played vital roles in BbFA removal onto MnO₂ NF/GO. The study highlights the promising potential adsorbent for removal of PAHs.

MnO2 -Based Materials for Environmental Applications

Manganese dioxide (MnO₂ ) is a promising photo-thermo-electric-responsive semiconductor material for environmental applications, owing to its various favorable properties. However, the unsatisfactory environmental purification efficiency of this material has limited its further applications. Fortunately, in the last few years, significant efforts have been undertaken for improving the environmental purification efficiency of this material and understanding its underlying mechanism. Here, the aim is to summarize the recent experimental and computational research progress in the modification of MnO₂ single species by morphology control, structure construction, facet engineering, and element doping. Moreover, the design and fabrication of MnO₂ -based composites via the construction of homojunctions and MnO₂ /semiconductor/conductor binary/ternary heterojunctions is discussed. Their applications in environmental purification systems, either as an adsorbent material for removing heavy metals, dyes, and microwave (MW) pollution, or as a thermal catalyst, photocatalyst, and electrocatalyst for the degradation of pollutants (water and gas, organic and inorganic) are also highlighted. Finally, the research gaps are summarized and a perspective on the challenges and the direction of future research in nanostructured MnO₂ -based materials in the field of environmental applications is presented. Therefore, basic guidance for rational design and fabrication of high-efficiency MnO₂ -based materials for comprehensive environmental applications is provided.

Proton Insertion Promoted a Polyfurfural/MnO2 Nanocomposite Cathode for a Rechargeable Aqueous Zn-MnO2 Battery

Rechargeable aqueous Zn-MnO₂ batteries using a mild electrolyte have attracted considerable interest because of their high output voltage, high safety, low cost, and environmental friendliness. However, poor cycling stability remains a significant issue for their applications. Equally, the energy storage mechanism involved is still controversial thus far. Herein, porous polyfurfural/MnO₂ (PFM) nanocomposites are prepared via a facile one-step method. When tested in a rechargeable aqueous Zn-MnO₂ cell, the PFM nanocomposites deliver high specific capacity, considerable rate performance, and excellent long-term cyclic stability. Based on the experimental results, the role of the hydrated basic zinc sulfate layer being linked to the cycling stability of the aqueous rechargeable zinc-ion batteries is revealed. The mechanistic details of the insertion reaction based on the H⁺ ion storage mechanism are proposed, which plays a crucial role in maintaining the cycling performance of the rechargeable aqueous Zn-MnO₂ cell. We expect that this work will provide an insight into the energy storage mechanism of MnO₂ in aqueous systems and pave the way for the design of long-term cycling stable electrode materials for rechargeable aqueous Zn-MnO₂ batteries.