Colloidal MnO2 Research Articles

Novel phase transfer catalysis coupled with bifunctional oxidation for enhanced remediation of groundwater polluted with multiple NAPL: Performance and mechanisms

Structural differences among non-aqueous phase liquids (NAPLs) result in varying oxidation rates, limiting mass transfer between NAPLs and oxidants and seriously impairing the effectiveness of remediation via traditional in-situ chemical oxidation. To tackle this challenge, a novel approach is proposed for remediating multi-NAPL-polluted groundwater that leverages phase transfer catalysis (PTC) to enhance heterogeneous mass transfer by transferring oxidants from groundwater to NAPLs. Meanwhile, “oxidation-in-situ activation” is achieved through bifunctional oxidation using permanganate and peroxymonosulfate (PP). The proposed approach is referred to PTC-PP in this study. Herein, trichloroethene (TCE) and benzene serve as a representative multi-NAPL system. Experimental results indicated that PP significantly improved degradation efficiency of benzene in multi-NAPL system by at least 60.8 % compared to single-oxidant systems, and further enhancement (17.6 %) was achieved when PP was combined with PTC compared to PP alone. Dissolved Mn(II) and MnO2 generated by MnO4− reduction effectively activated peroxymonosulfate in PTC-PP system, with colloidal MnO2 being the most effective activator. Consequently, SO4•−, O2•− and 1O2 were formed in both NAPL and aqueous phases, while •OH was formed in aqueous phase, playing a crucial role in benzene oxidation. In phase transfer process of PTC-PP, the proportion of MnO4− transferred to benzene exceeded that to TCE. This finding illustrated that nondirectional phase transfer of oxidants posed a challenge for simultaneous promotion of TCE and benzene degradation. However, TCE and benzene removal efficiencies were both >75.7 % by applying peroxymonosulfate after KMnO4 addition. These findings lay the theoretical groundwork for PTC-PP application in groundwater remediation.

Role of CTAB, and reactive radical species (HO[rad] and O2−[rad]) on the permanganate redox reaction

Permanganate-H2SO4 redox system was used to establish the role of cetyltrimethylammonium bromide (CTAB), and reactive radical species on the oxidative degradation of Congo red. The absorbance at 496 nm increases for ca. 6.0 min, and then decreases. Permanganate-Congo red redox reaction was very slow (degradation efficiency = 46 % in 180 min with kobs = 0.004 min−1 at 496 nm). The degradation efficiency was found to be 85 % in 6.0 min of reaction time with H2SO4. Congo red and permanganate interact with CTAB aggregates due to the electrostatic interactions. The decay of absorbance at 467 nm became very slow with increasing CTAB concentrations. The oxidative treatment of Congo red/MnO4− system follows first-, and complex – order kinetics with permanganate, and Congo red concentrations, respectively. The Congo red degradation efficiency remains unaffected with sodium pyrophosphate and sodium fluoride, which ruled out the possibility of Mn(III) formation as an oxidation state of manganese. Humic acid (HA) enhanced the degradation efficiency of Congo red/MnO4− system, and facilitated the in-situ formation of water soluble colloidal MnO2. The scavenger’s studies revealed that hydroxyl (HO), and superoxide (O2−) radicals were not the reactive species in the Congo red/ MnO4− redox system under our experimental conditions.

Mechanistic Study of Reaction between 5-Sulfosalicylic Acid and Colloidal MnO2

The reaction between 5-sulfosalicylic acid (5-SSA) and colloidal MnO2 has been studied kinetically in acidic medium. During the reaction the concentration of 5-SSA was kept highly excess than the concentration of colloidal MnO2 to maintain the pseudo-first-order condition. The reaction was studied by following decrease in absorbance of colloidal MnO2 at 390 nm and the first order rate constants were determined from linear log (Abs.) against time plots. The linearity of the first-order plots did not involve any autocatalytic part. The oxidation of 5-sulfosalicylic acid (5-SSA) by colloidal MnO2 involves intervention of free radicals and Mn(III). The product of reaction in colloidal MnO2 was p-hydroxy benzene sulfonate ion as confirmed by LCMS-MS analysis. The oxidation by colloidal MnO2 consists of acid dependent and independent mechanisms. The results of added Mn(II), Na4P2O7 and acrylonitrile indicate involvement of Mn(III) and free radical during oxidation by colloidal MnO2. The probable mechanisms for acid dependent and independent paths were predicted and the rate expressions were also obtained. The activation parameters also support the proposed mechanisms.

The relative contributions of Mn(III) and Mn(IV) in manganese dioxide polymorphs to bisphenol A degradation

Polymorphs of MnO2 comprise Mn(III) and Mn(IV), which are both strong oxidants capable of BPA degradation, but their relative contributions are unclear. To advance process understanding, the reactivities of biogenic MnO2 prepared using Roseobacter sp. AzwK-3b and synthetic MnO2 (i.e., hexagonal and triclinic birnessite) toward BPA were compared. Both colloidal and particulate biogenic MnO2, as well as triclinic birnessite, showed insignificant reactivity towards BPA, but degradation did occur when pyrophosphate (PP), a ligand for Mn(III), was present. Despite higher Mn(III) content of triclinic birnessite (38.6 %), only hexagonal birnessite with an Mn(III) content of 30.4 % degraded BPA without PP, and no rate increases were observed following the addition of PP. Similarly, colloidal MnO2 degraded BPA with nearly double the rate measured with particulate MnO2 (i.e., 1.24 ± 0.10 versus 0.73 ± 0.08 h−1), even though the Mn(III) contents were only 10 % different. The Mn(III) release rates from each MnO2 polymorph in the presence of PP correlated more strongly with the observed BPA degradation rates than with Mn(III) content, suggesting that both Mn(III) release rate and Mn(III) content govern MnO2-mediated BPA degradation. In natural settings, Mn(III) generally occurs in complexed form suggesting that laboratory testing should include ligands to derive environmentally relevant information about MnO2-mediated degradation of BPA and other compounds of concern.

Selective abatement of electron-rich organic contaminants by trace complexed Mn(II)-catalyzed periodate via high-valent manganese–oxo species

Mn(II) is among the most efficient catalysts for the periodate (PI)-based oxidation process. In-situ formed colloidal MnO2 simultaneously serves as the catalyst and oxidant during the degradation of organic contaminants by PI. Here, it is revealed that the complexation of Mn(II) by ethylene diamine tetraacetic acid (EDTA) further enhances the performance of PI-based oxidation in the selective degradation of organic contaminants. As evidenced by methyl phenyl sulfoxide probing, 18O-isotope labeling, and mass spectroscopy, EDTA complexation modulates the reaction pathway between Mn(II) and PI, triggering the generation of high-valent manganese–oxo (MnV–oxo) as the dominant reactive species. PI mediates the single-electron oxidation of Mn(II) to Mn(III), which is stabilized by EDTA complexation and then further oxidized by PI via the oxygen-atom transfer step, ultimately producing the MnV–oxo species. Ligands analogous to EDTA, namely, [S,S]-ethylenediaminedisuccinic acid and L-glutamic acid N,N-diacetic acid, also enhances the Mn(II)/PI process and favors MnV–oxo as the dominant species. This study demonstrates that functional ligands can tune the efficiency and reaction pathways of Mn(II)-catalyzed peroxide and peroxyacid-based oxidation processes.

Unraveling the intrinsic mechanism behind the selective oxidation of sulfonamide antibiotics in the Mn(II)/periodate process: The overlooked surface-mediated electron transfer process

Mn(II) exhibits a superb ability in activating periodate (PI) for the efficient degradation of aqueous organic contaminants. Nevertheless, ambiguous conclusions regarding the involved reactive species contributing to the removal of organic contaminants remain unresolved. In this work, we found that the Mn(II)/PI process showed outstanding and selective reactivity for oxidizing sulfonamides with the removal ranging from 57.1% to 100% at pH 6.5. Many lines of evidence suggest that the in-situ formed colloidal MnO2 (cMnO2) served as a catalyst to mediate electron transfer from sulfonamides to PI on its surface via forming cMnO2–PI complex (cMnO2−PI*) for the efficient oxidation of sulfonamides in the Mn(II)/PI process. Experimental results and density functional theory (DFT) calculations verify that the inclusive aniline moiety was the key site determining the electron transfer-dominated oxidation of sulfonamides. Furthermore, DFT calculation results reveal that the discrepancies in the removal of sulfonamides in the Mn(II)/PI process were attributed to different kinetic stability and chemical reactivity of sulfonamides caused by their heterocyclic substituents. In addition, a high utilization efficiency of PI was achieved in the Mn(II)/PI process owing to the surface-mediated electron transfer mechanism. This work provides deep insights into the surface-promoted mechanism in the cMnO2-involved oxidation processes.

In Situ Regulation of MnO2 Structural Characteristics by Oxyanions to Boost Permanganate Autocatalysis for Phenol Removal.

Oxyanions, a class of constituents naturally occurring in water, have been widely demonstrated to enhance permanganate (Mn(VII)) decontamination efficiency. However, the detailed mechanism remains ambiguous, mainly because the role of oxyanions in regulating the structural parameters of colloidal MnO2 to control the autocatalytic activity of Mn(VII) has received little attention. Herein, the origin of oxyanion-induced enhancement is systematically studied using theoretical calculations, electrochemical tests, and structure-activity relation analysis. Using bicarbonate (HCO3-) as an example, the results indicate that HCO3- can accelerate the degradation of phenol by Mn(VII) by improving its autocatalytic process. Specifically, HCO3- plays a significant role in regulating the structure of in situ produced MnO2 colloids, i.e., increasing the surface Mn(III)s content and restricting particle growth. These structural changes in MnO2 facilitate its strong binding to Mn(VII), thereby triggering interfacial electron transfer. The resultant surface-activated Mn(VII)* complexes demonstrate excellent degrading activity via directly seizing one electron from phenol. Further, other oxyanions with appropriate ionic potentials (i.e., borate, acetate, metasilicate, molybdate, and phosphate) exhibit favorable influences on the oxidative capability of Mn(VII) through an activation mechanism similar to that of HCO3-. These findings considerably improve our fundamental understanding of the oxidation behavior of Mn(VII) in actual water environments and provide a theoretical foundation for designing autocatalytically boosted Mn(VII) oxidation systems.

Green Synthesis of Colloidal and Nanostructured MnO2 by Solution Plasma Process

Green Synthesis of Colloidal and Nanostructured MnO2 by Solution Plasma Process

Could manganate be an alternative of permanganate for micropollutant abatement?

Permanganate (MnO4−), an oxidant that has been applied in water treatment, has highly varied reactivity toward pollutants. In this study, we found manganate (MnO42−) could destruct diverse functional groups, with oxidation rates being higher than that of permanganate under acidic and neutral conditions. Mechanistic study revealed manganate rapidly disproportionated to permanganate and colloidal MnO2 in solution. Under acidic conditions, the in-situ formed colloidal MnO2 possess higher reactivity than permanganate and primarily contributed to the degradation of pollutants. The reactivity of in-situ formed colloidal MnO2 is highly sensitive to pH and decreased dramatically with increasing pH. Consequently, the contribution of MnO2 to pollutant removal decreased with elevating pH, which also leads to the decreased degradation efficiency of micropollutants at high pH. Manganate is an intermediate produced during the manufacturing process of permanganate. This study indicates that manganate might be an alternative of permanganate for water purification under acidic and neutral conditions.

Ligands-triggered evolution of catalytic intermediates during periodate activation via soluble Mn(II) for organic contaminants’ abatement

Homogenous catalysis technology could be flexibly combined with existing water treatment facilities, which presents broad applicational prospects and deserves systematic study. This work reinvestigated the periodate (IO4−, PI) activation process mediated by hydrated MnII ions and highlighted the critical involvement of the in-situ formed colloidal MnO2 (MnIV). MnII ions were directly oxidized to colloidal MnIV in the MnII/PI system, and the nascent MnIV could in turn activate PI to form MnIV-PI* complex as the reactive oxidant. Besides, several kinds of ligands were introduced into the MnII/PI oxidation system to explore their influences. Nitrilotriacetic acid (NTA) outperformed other selected ligands and could significantly enhance PI activation to degrade organic contaminants, which could achieve a removal efficiency of ∼99% within seconds. Interestingly, the presence of NTA completely altered the catalytic pathways rather than just accelerating the generation of reactive oxidants. Combining the results of sulfoxide-probe transformation tests and the 18O isotope-tracer experiments, we proposed that the high-valent manganese-oxo (MnV-oxo) species was the dominant reactive oxidant generated in the MnII/NTA/PI system, and the metastable MnIII complex was the precursor of the MnV-oxo species. Notably, maintaining the MnII high-spin d5 center and stabilizing the in-situ formed MnIII intermediates were the crucial premises for the evolution of the catalytic oxidation process triggered by functional ligands. This work clarified the critical involvement of functional ligands during PI activation via soluble MnII and might also inspire the designing of heterogeneous catalyst modification for the water treatment technology.

Characterization, surface morphology and microstructure of water soluble colloidal MnO2 nanoflakes

In the present work, characterization of water soluble colloidal MnO2 nanoflakes which act as an oxidizing agent was carried out using UV–visible spectroscopy. Transmission electron microscopy microstructure of colloidal MnO2 nanoflakes confirms the shape and nature of these particles. Selected area electron diffraction ring indicated that colloidal nanoflakes were amorphous in nature. Surface morphology of synthesized colloidal MnO2 nanostructure was determined by field emission scanning electron microscopy indicating a crumpled net like arrangement.

Trace Mn(II)-catalyzed periodate oxidation of organic contaminants not relying on any transient reactive species: The substrate-dependent dual roles of in-situ formed colloidal MnO2

Traditional advanced oxidation processes (AOPs) generally suffer from the inevitable deactivation of catalysts and the ineffective consumption of transient reactive species (TRSs) that compromise the efficiency in destructing aqueous contaminants. Herein, it was interestingly found that trace Mn(II) could robustly catalyze the oxidation of organic contaminants by periodate (PI), with the performance was much better than the representative TRSs-dominated AOPs (i.e., the Fe(II)-activated hydrogen peroxide, peroxymonosulfate (PMS), peroxydisulfate and PI processes). Multiple lines of evidence excluded the oxidative contributions of TRSs, instead the stoichiometric formation of colloidal MnO2 via the condensation of di-μ-oxo-bridged Mn(IV) cluster was confirmed by UV-vis, X-ray absorption near edge structure spectroscopy and density functional theory calculation. Dependent on the structure of substrate, MnO2 colloids solely or simultaneously served as oxidant and catalyst for the enhanced treatment performance. Benefiting from the non-TRSs-involved oxidation strategy and the catalytic effects of Mn species, the trace-Mn(II)/PI process even outperformed the Co(II)-activated PMS counterpart (i.e., one of the most efficient AOPs known at present) on oxidant utilization efficiency. This study not only elucidated the roles of Mn(II) and colloidal MnO2 in PI-mediated contaminant degradation, but also signified the superiority of trace catalyst-assisted process without TRSs involvement in avoiding undesired side reactions and maximizing oxidation efficiency.

Resource utilization of pyrolytic derivative from petroleum-contaminated soil as heavy metal adsorbent enhanced by colloidal MnO2 pretreatment

Resource utilization of pyrolytic derivative from petroleum-contaminated soil as heavy metal adsorbent enhanced by colloidal MnO2 pretreatment

Enhanced degradation of micropollutants by UV/freshly formed colloidal MnO2: Reactive species, kinetics and pathways

This study reports that freshly formed manganese dioxide (MnO2) is an effective photocatalyst for the abatement of micropollutants, such as nalidixic acid (NDA) and gemfibrozil (GFRZ). The freshly formed MnO2 was flower-like β-MnO2 and showed better efficiency in micropollutant abatement owing to its better adsorption capability than commercial MnO2. Electron paramagnetic resonance (EPR) spectra confirmed that reactive oxygen species (ROS), including hydroxyl radicals (HO•), superoxide radicals (O2•−) and singlet oxygen (1O2), were formed in the UV/MnO2 system. Valence band X-ray photoelectron spectroscopy (VB-XPS) spectra and UV–vis adsorption curves verified the generation of ROS via the reaction between e− and oxygen as the sole pathway. Holes (h+) dominated the degradation of NDA, while both ROS and h+ were important for GFRZ abatement. Side chain cleavage, kentonization and hydroxylation took place in NDA and GFRZ degradation, while demethylation and decarboxylation also occurred during NDA degradation.

Enhanced Degradation of Micropollutants by Uv/Freshly Formed Colloidal Mno2: Reactive Species, Kinetics and Pathways

Enhanced Degradation of Micropollutants by Uv/Freshly Formed Colloidal Mno2: Reactive Species, Kinetics and Pathways

Simultaneous reductions of production loss and environmental burden through the treatment of loose leather with non-toxic manganese dioxide nanoparticles

In this study, the potential of nano-sized manganese dioxdie (MnO2) particles in reconstituting the microstructure of loose collagen fiber that is a structural flaw and an undesirable fact happened during the processing of hides/skins to produce leather causing a significant economic loss was extensively investigated. Rejected leather was successfully impregnated with colloidal MnO2 particles prepared from KMnO4 by a simple soaking method in a slightly acidic solution containing sodium citrate. Incorporation and binding of MnO2 particles with the protein molecule of leather were investigated with Fourier-transform infrared spectroscopic (FT-IR), scanning electron microscopic, x-ray diffraction (XRD) and differential scanning calorimetric techniques. Substantial changes in the morphology of leather were achieved by loading citrate-stabilized MnO2 nanoparticle. The incorporated particle was examined to be poorly crystalline, α polymorph of MnO2. The mechanical properties, e.g., break scale value, tensile strength, tear strength and elongation at break of leather treated with MnO2 nanoparticles increased to the corresponding values of commercially acceptable high quality leather. The binding of negatively charged MnO2 nanoparticles, which were characterized with a dynamic light scattering technique, to the functional groups of collagen fibers was considered to occur through a coordination complex formation. The incorporation of non-toxic, low-cost, environment-friendly MnO2 nanoparticles into the collagen microstructure markedly increases the fiber strength and hence assists to regain the production loss.

Kinetics of oxidation of vanillic acid by colloidal MnO2: correlation of micellar catalysis to the micellar properties of surfactants and mixed surfactants

Kinetics of oxidation of vanillic acid by colloidal MnO2: correlation of micellar catalysis to the micellar properties of surfactants and mixed surfactants

Reducing substances-enhanced degradation of pollutants by permanganate: The role of in situ formed colloidal MnO2

Permanganate is one of common oxidants used for organic pollutant abatement in water treatment. This study showed that the degradation rate of bisphenol A (BPA) by permanganate at pH 5.0 in the presence of aniline is much higher than that in the absence of aniline. 2, 5, and 10 μM of aniline enhanced BPA degradation rate by 104%, 326% and 601%, respectively. Colloidal MnO2 was formed through the reduction of permanganate by aniline and contributed to BPA oxidation considerably. The reactivity of MnO2 is sensitive to pH and is high under acidic conditions, resulting in the observed enhancement of aniline on BPA removal by permanganate at pH < 7.0. The role of MnO2 was further confirmed by the relationship of MnO2 formation and BPA/aniline removal, the inhibitory effect of Ca2+ on the oxidation of BPA in the presence of aniline. Besides the aniline/BPA system, the pollutants which react with permanganate rapidly are likely to enhance the degradation of coexisting pollutants which show high reactivity towards MnO2. Due to the reduction of permanganate and stabilization of the in situ formed colloidal MnO2 by water matrix, the oxidation rate of pollutant in real water is higher than that in pure water.

Bio-inspired metastable intermolecular nanothermite composite based on Manganese dioxide/Polydopamine/Aluminium

Surface engineering of energetic particles can offer significant applications; the emergence of surface modification with polydopamine (PDA) can propose novel material science. Even though, nanothermites (metal oxide/metal) can offer surplus heat output; the reaction rate is comparatively-low as it is limited to the interplaner distance between particles. Intimate mixing between metal oxide/metal could be accomplished via surface modification. This study reports on the facile synthesis of MnO2 nanoparticles (NPs) of 5 nm average particle size. On the other hand, Al NPs in the shape of plates of 100 nm particle size were employed. Surface modification of Al NPs with PDA was conducted via in situ polymerization. Colloidal MnO2 NPs integrated into PDA/Al during the polymerization process. DPA layer was effectively-deposited on the aluminium surface and effectively-anchored MnO2 NPs. SEM micrographs of developed MnO2/PDA/Al nanocomposite demonstrated the successful bonding of MnO2 to Al surface via strong PDA chemical bonding. From FTIR spectra, the sharp peaks at 1544 cm−1 and 1389 cm−1 attributed to the C=O and C–O bonds, respectively, and could be due to bond formation between Mn and oxygen in C=O, and C–O in PDA. The developed MnO2/PDA/Al demonstrated the uniform distribution of MnO2 NPs on Al nano-plates. This work dropped the light toward the green production of novel nanothermite hybrid with durable chemical bonding. MnO2/PDA/Al hybrid can offer superior combustion characteristics with intimate mixing between the oxidizer and metal fuel to the molecular level.

Dissolved Silicate Enhances the Oxidation of Chlorophenols by Permanganate: Important Role of Silicate-Stabilized MnO2 Colloids.

Dissolved silicate is an important background constituent of natural waters, but there is little clarity regarding the effect of silicate on the oxidizing capability of permanganate (Mn(VII)) and on its efficiency for remediation applications. In the present study, we found that dissolved silicate, metasilicate or disilicate (DS), could significantly promote the oxidation of 2,4-dichlorophenol (2,4-DCP) by Mn(VII), and the extent of the promoting effect was even more evident than that of pyrophosphate (PP). The experiments showed that, unlike PP, DS was not capable of coordinating with Mn(III) ions, and the promoting effect of DS was not due to the oxidizing capability of complexed Mn(III). Instead, DS ions, as a weak base, could combine with the hydroxyl groups of MnO2 via hydrogen bonding to limit the growth of colloidal MnO2 particles. The DS-stabilized colloidal MnO2 particles, with hydrodynamic diameters less than 100 nm, could act as catalysts to enhance the oxidation of 2,4-DCP by Mn(VII). The best promoting effect of DS on the performance of Mn(VII) oxidant was achieved at the initial solution pH of 7, and the coexisting bicarbonate ions further improved the oxidation of 2,4-DCP in the Mn(VII)/DS system. Sand column experiments showed that the combined use of Mn(VII) and DS additive could mitigate the problem of permeability reduction of sand associated with the retention of MnO2 particles. This study not only deepens our understanding on the role of dissolved silicate in a Mn(VII) oxidation process but also provides an effective and green method to enhance the oxidizing capacity of Mn(VII)-based treatment systems.