Enhanced oxidation of organic contaminants by Mn(VII) in water

Development of a NS-CDs/fluorescein ratiometric fluorescent sensor-integrated smartphone platform for rapid and specific detection of Mn(VII) and captopril

The construction of a ratiometric fluorescent sensor based on nitrogen-doped carbon dots (NS-CDs) and fluorescein has been reported for the detection of Mn(VII) and captopril. The detection mechanisms for Mn(VII) and captopril are based on the inner filter effect (IFE) and a redox reaction, respectively. The sensor showed good linearity in the detection range of 0.12-100 μM for Mn(VII) and 0.52-300 μM for captopril, with limits of detection (LODs) of 0.12 μM and 0.52 μM, respectively. Furthermore, a smartphone-based platform for the detection of Mn(VII) and captopril was developed, based on the dual-signal response of color and fluorescence intensity. The system displayed linear signal changes with increasing Mn(VII) and captopril concentrations in the ranges of 0.32-100 μM and 0.52-300 μM, with LODs of 0.32 μM and 0.52 μM, respectively. Finally, the ratiometric sensor was validated for the determination of Mn(VII) and captopril in herb and captopril tablet samples, demonstrating its broad applicability and versatility as a portable, sensitive, and specific detector for Mn(VII) and captopril in various sample types. Overall, our ratiometric fluorescent sensor has potential as a portable, on-site detector for Mn(VII) and captopril analysis.

Efficient permanganate activation under UV 222 nm irradiation for enhanced pollutant abatement

As an emerging advanced oxidation process (AOP), the ultraviolet (UV)-driven permanganate [Mn(VII)] activation process has drawn increasing attention in water treatment. Hydroxyl radical (HO•) and various reactive manganese species (RMnS) were simultaneously generated, resulting in enhanced pollutant abatement. However, Mn(VII) activation efficiency by the current UV lights was quite limited, with unsatisfactory apparent quantum yields of Mn(VII) (Φapp, Mn(VII) < 0.25 mol/Einstein). Recently, the krypton chloride (KrCl*) excimer lamp, which emits UV light mainly at 222 nm (UV222), has received growing interest due to its superior photon energy, safety, environmental friendliness, and lifespan compared with currently applied UV lamps. Herein, the KrCl* excimer lamp was applied to activate Mn(VII) for the first time. Φapp, Mn(VII) was determined to be as high as 0.65 mol/Einstein in UV222/Mn(VII). The fluence-based pseudo-first-order reaction rate constants of target pollutants (flumequine and 4-hydroxybenzoic acid) in UV222/Mn(VII) were several to thousands of times higher than those in other UV/Mn(VII) AOPs reported in literature, suggesting significantly higher efficiency of UV222 in Mn(VII) activation. Compared with HO•, RMnS played more significant roles in pollutant abatement. The total contributions of RMnS to pollutant abatement were approximately 60-70% under pH 4.0-9.0, while the contributions of HO• were around 10-25%. Moreover, hypomanganate [Mn(V)] was important RMnS responsible for the abatement of two pollutants, while the role of trivalent manganese [Mn(III)] was limited based on experimental and computational results. Pollutant abatement was inhibited with the increase of pH, while was promoted with the increase of light intensity and Mn(VII) concentration. Furthermore, water matrix components (chloride, bicarbonate, and humic acid) showed negligible or slight influences on pollutant abatement due to the selective oxidation features of RMnS. This study demonstrates the superior potential of the UV222/Mn(VII) AOP in water treatment.

New insights into the role of electron shuttles in enhancing permanganate oxidation for the removal of aniline compounds: Free radicals and intermediate manganese

Permanganate/electron shuttle systems for the enhanced transformation of anilines were evaluated. The presence of electron shuttles (2,2'-azino-bis(3-ethylbenzothiazoline)-6-sulfonate (ABTS), 1-hydroxybenzotriazole (HBT) and catechol (CAT) accelerated the oxidative transformation of aniline by Mn(VII). In the Mn(VII)/ABTS process, ABTS•+ was partially consumed through fragmentation or polymerization with aniline, while the second-order rate constants (kapp2, M-1 s-1) of the interaction of ABTS•+ with aniline rose from 86.57 to 140.04 M-1 s-1 with increase in pH from 5 to 8. In the Mn(VII)/HBT system, aniline removal was markedly enhanced under acidic conditions due to the formation of polymerized products via radical cations (HBT•). Furthermore, the oxidative transformation of aniline (82.5 %) by Mn(VII)/CAT was significantly higher than that by Mn(VII) (65.2 %) because of Michael addition reaction of aniline and ortho-benzoquinone (o-BQ). Moreover, the formation of Mn(III) was confirmed by the overall oxidation state (3.13) and the X-ray Photoelectron Spectroscopy (XPS) spectra of the Mn products in the CAT/Mn(VII) process, revealing a synergistic mechanism between o-BQ and Mn(III) for enhanced aniline degradation.

Reusable aminated lignin-based hydrogel biocomposite for effective dye adsorption in wastewater

This study reports enhanced organic dye adsorption capacity from aqueous solutions of a synthesized novel biocomposite hydrogel based on aminated lignin (AL) and poly(3-acryloamidopropyl)-trimethylammonium chloride P(ClAPTA). The biocomposites were characterized via Fourier-transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, and elemental analysis, confirming successful functionalization and polymerization. A factorial design was used to screen the operational parameters, showing that the P(ClAPTA-AL) composition was the most significant factor affecting dye adsorption compared to the amination ratio. Optimization of the adsorption process was achieved using alizarin red S (ARS) as a model dye through a Box-Behnken design, revealing optimal conditions: pH 12.0, 20 °C, 120 min contact time, and a composite-to-ARS mass ratio of 10, resulting in a high adsorption capacity of 102.1 mg g-1 and a maximum adsorption of 3889 mg g-1. Kinetic studies showed an adsorption process followed a pseudo-second-order model, confirming chemisorption as the predominant mechanism, while thermodynamic analysis revealed a spontaneous and endothermic adsorption process. Furthermore, promissory results demonstrated high reusability, with adsorption efficiency remaining at ∼99 % until the fourth cycle and maintaining 81.1 % after seven cycles.

Unveiling the critical roles of nascent MnO2 in accelerating permanganate carbocatalysis

To probe the underlying mechanisms of carbocatalysis in enhanced permanganate (PM) oxidation and identify the exact roles of nascent MnO2, graphene aerogels (GA) were fabricated to activate PM for naproxen (NPX) degradation. All the three GA samples could accelerate NPX oxidation by PM, the rate constants and reaction stoichiometric efficiency (RSE) followed the order of GA900 > GA600 > GA300. Mechanistic studies revealed that Mn(VI), Mn(V) and Mn(III) were not the major reactive species involved in NPX oxidation, but highlighted the essential contribution of electron transfer pathway (ETP) mediated directly by GA and indirectly by nascent MnO2. For GA300 with strong electron-donating capability, it mainly served as the electron donor for PM decomposition, and indirectly oxidized NPX via nascent MnO2 mediated ETP, thereby exhibiting inferior RSE as well as mediocre recycling performance. GA600 and GA900 could serve as the electron shuttle to directly mediate the ETP for NPX degradation, the nascent MnO2 accumulated on GA framework during the reaction would also mediate the ETP from NPX to PM, thus displaying an obvious accelerating recycling performance. This work provides novel insights into the structure-dominated PM carbocatalysis, which contributes better to development of promising carbocatalysts and utilization of nascent MnO2.

Molecular level removal of antibiotic resistant bacteria and genes: A review of interfacial chemical in advanced oxidation processes

As a kind of novel and persistent environmental pollutants, antibiotic resistant bacteria (ARB) and antibiotic resistance genes (ARGs) have been frequently detected in different aquatic environment, posing potential risks to public health and ecosystems, resulting in a biosecurity issue that cannot be ignored. Therefore, in order to control the spread of antibiotic resistance in the environment, advanced oxidation technology (such as Fenton-like, photocatalysis, electrocatalysis) has become an effective weapon for inactivating and eliminating ARB and ARGs. However, in the process of advanced oxidation technology, studying and regulating catalytic active sites at the molecular level and studying the adsorption and surface oxidation reactions between catalysts and ARGs can achieve in-depth exploration of the mechanism of ARGs removal. This review systematically reveals the catalytic sites and related mechanisms of catalytic antagonistic genes in different advanced oxidation processes (AOPs) systems. We also summarize the removal mechanism of ARGs and how to reduce the spread of ARGs in the environment through combining a variety of characterization methods. Importantly, the potential of various catalysts for removing ARGs in practical applications has also been recognized, providing a promising approach for the deep purification of wastewater treatment plants.

The synergistic effect of oxidant-peroxide coupling systems for water and wastewater treatments

With the growing complexity and severity of water pollution, it has become increasingly challenging to effectively remove contaminants or inactivate microorganisms just by traditional chemical oxidants such as O3, chlorine, Fe(VI) and Mn(VII). Up till now, numerous studies have indicated that these oxidants in combination with peroxides (i.e., hydrogen peroxide (H2O2), peroxymonosulfate (PMS), peracetic acid (PAA) and periodate (PI)) exhibited excellent synergistic oxidation. This paper provided a comprehensive review on the combination of aforementioned oxidant-peroxide applied in water and wastewater treatments. From one aspect, the paper thoroughly elucidated the synergy mechanism of each oxidant-peroxide combination in turn. Among these combinations, H2O2 or PMS generally performed as the activator of four traditional oxidants above to accelerate reactive species generation and therein various reaction mechanisms, including electron transfer, O atom abstraction and oxo ligand substitution, were involved. In addition, although neither PAA nor PI was able to directly activate Fe(VI) and Mn(VII), they could act as the stabilizer of intermediate reactive iron/manganese species to improve the latter utilization efficiency. From another aspect, this paper summarized the influence of water quality parameters, such as pH, inorganic ions and natural organic matter (NOM), on the oxidation performance of most combined systems. Finally, this paper highlighted knowledge gaps and identified areas that require further research.

PMS coupled Mn(II) mediated electrochemistry processes (E-Mn(II)-PMS) on the efficient RB19 wastewater treatment: Focus on the regulation and reinforcement of Mn(III)/Mn(II)

Dye wastewater, represented by reactive blue 19 (RB19), severely threatens the aquatic ecological environment and human health, such that an efficient RB19 wastewater treatment technology should be urgently developed. Based on manganese ion-mediated electrochemistry, PMS was introduced to develop a novel electrocatalytic system (E-Mn(II)-PMS) that can efficiently remove and degrade RB19. The synergistic effect between E, Mn(II), and PMS was verified in this study through comparative experiments of a wide variety of systems. The removal efficiency of RB19 reached 95.1% in 50 min under reasonable power consumption (3.29 kWh/m3). Moreover, the effects exerted by different operating conditions (e.g., initial pH, current density, RB19 concentration, Mn(II) concentration, as well as PMS concentration) and water matrix on the degradation efficiency of RB19 were explored through single factor experiments. The active oxidation species (ROS) and their contribution rate for the degrading and removing RB19 were studied through quenching experiments, EPR experiments, TMT-15 metal capture experiments, as well as PP complexation experiments. The role played by non-free radicals took on critical significance in the oxidation removal of RB19, which comprised direct electro oxidation, Mn(III) oxidation, and 1O2 oxidation. The enhancement effect of free radicals (SO4·- and HO∙) was not sufficiently significant, with a low degree of contribution. The oxidation effect of the anode facilitated the conversion of Mn (II) to Mn (III), which was employed in PMS for expediting the production of 1O2. The reduction effect of the cathode blocked the production of Mn (IV) as a side reaction, such that the continuous circulation of manganese ions between divalent and trivalent was promoted. Meanwhile, the cathode reacted with PMS to generate a small part of SO4·- and HO∙. In addition, the reaction active site of RB19 was predicted, and a possible degradation pathway was proposed in accordance with the mass spectrometry results and the DFT calculation. As revealed by the results of the QSAR analysis and the plant culture experiments, the biological toxicity of RB19 was markedly reduced after the sample was administrated with E-Mn(II)-PMS. E-Mn(II)-PMS-mediated electrochemical technology displays several advantages (e.g., high efficiency, low consumption, recyclability, wide pH window, and strong applicability) while showing promising market development and utilization for treating dye wastewater.

Green polyaspartic acid as a novel permanganate activator for enhanced degradation of organic contaminants: Role of reactive Mn(III) species

Attention has been long focused on enhancing permanganate (Mn(VII)) oxidation capacity for eliminating organic contaminants via generating active manganese intermediates (AMnIs). Nevertheless, limited consideration has been given to the unnecessary consumption of Mn(VII) due to the spontaneous disproportionation of AMnIs during their formation. In this work, we innovatively introduced green polyaspartic acid (PASP) as both reducing and chelating agents to activate Mn(VII) to enhance the oxidation capacity and utilization efficiency of Mn(VII). Multiple lines of evidence suggest that Mn(III), existing as Mn(III)-PASP complex, was generated and dominated the degradation of bisphenol A (BPA) in the Mn(VII)/PASP system. The stabilized Mn(III) species enabled Mn(VII) utilization efficiency in the Mn(VII)/PASP system to be higher than that in Mn(VII) alone. Moreover, the electrophilic Mn(III) species was verified to mainly attack the inclusive benzene ring and isopropyl group to realize BPA oxidation and its toxicity reduction in the Mn(VII)/PASP system. In addition, the Mn(VII)/PASP system showed the potential for selectively oxidizing organic contaminants bearing phenol and aniline moieties in real waters without interference from most of coexisting water matrices. This work brightens an overlooked route to both high oxidation capacity and efficient Mn(VII) utilization in the Mn(VII)-based oxidation processes.

PTIO as a redox mediator to enhance organic contaminants oxidation by permanganate

Although permanganate (Mn(VII)) is extensively utilized as a strong oxidizer for the purification of water, the direct reaction rates between some refractory pollutants and Mn(VII) are moderate or relatively low. In this study, we found that 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO), could act as a redox mediator to enhance bisphenol A (BPA) degradation by Mn(VII) at pH 5.0 - 9.0, with a removal higher than 80% over 5 min. Moreover, the Mn(VII)/PTIO system is highly efficient toward a broad spectrum of contaminants. Mechanism was elucidated as following: PTIO was oxidized by Mn(VII) to PTIO+, an oxoammonium cation. As a newly generated reactive species, PTIO+ could oxidize organics and be reduced to PTIOH (PTIO hydroxylamine) or PTIO simultaneously. The redox cycle of PTIO in consecutive runs as an electron shuttle proved its stability and reusability in Mn(VII) oxidation. In addition to being an electron shuttle, PTIO also acts as an activator of Mn(VII) to promote the production of MnO2, which plays a vital role in enhancing BPA abatement at the acidic condition. For the purpose of further understanding the interaction between PTIO and target contaminants, three corresponding degradation pathways for BPA were proposed. Notably, the transformation products of BPA coupling with PTIO were detected, indicating PTIO inhibited the self-coupling of BPA and facilitated the ring-opening pathway. In addition, the ubiquitous humic acid has a positive effect on the Mn(VII)/PTIO system, suggesting a high promise of this system for practical application.

Simultaneous antibiotic removal and mitigation of resistance induction by manganese bio-oxidation process

Microbial degradation to remove residual antibiotics in wastewater is of growing interest. However, biological treatment of antibiotics may cause resistance dissemination by mutations and horizontal gene transfer (HGT) of antibiotic resistance genes (ARGs). In this study, a Mn(Ⅱ)-oxidizing bacterium (MnOB), Pseudomonas aeruginosa MQ2, simultaneously degraded antibiotics, decreased HGT, and mitigated antibiotic resistance mutation. Intracellular Mn(II) levels increased during manganese oxidation, and biogenic manganese oxides (BioMnOx, including Mn(II), Mn(III) and Mn(IV)) tightly coated the cell surface. Mn(II) bio-oxidation mitigated antibiotic resistance acquisition from an E. coli ARG donor and mitigated antibiotic resistance inducement by decreasing conjugative transfer and mutation, respectively. BioMnOx also oxidized ciprofloxacin (1 mg/L) and tetracycline (5 mg/L), respectively removing 93% and 96% within 24 h. Transcriptomic analysis revealed that two new multicopper oxidase and one peroxidase genes are involved in Mn(II) oxidation. Downregulation of SOS response, multidrug resistance and type Ⅳ secretion system related genes explained that Mn(II) and BioMnOx decreased HGT and mitigated resistance mutation by alleviating oxidative stress, which makes recipient cells more vulnerable to ARG acquisition and mutation. A manganese bio-oxidation based reactor was constructed and completely removed tetracycline with environmental concentration within 4-hour hydraulic retention time. Overall, this study suggests that Mn (II) bio-oxidation process could be exploited to control antibiotic contamination and mitigate resistance propagation during water treatment.

Remediation of benzo[a]pyrene contaminated soils by moderate chemical oxidation coupled with microbial degradation

Chemical oxidation is a promising technology for the remediation of organics-contaminated soils. However, residual oxidants and transformation products have adverse effects on microbial activities. This work aimed at moderate chemical oxidation coupled with microbial degradation (MOMD) for the removal of benzo[a]pyrene (BaP) by optimizing the type and dosage of oxidants. Potassium permanganate (KMnO₄), Fe²⁺ + sodium persulfate (Fe²⁺ + PS), Fenton's reagent (Fe²⁺ + H₂O₂), and hydrogen peroxide (H₂O₂) were compared for BaP removal from loam clay and sandy soils. Overall, the removal efficiency of BaP by a moderate dose of oxidant coupled indigenous microorganism was slightly lower than that by a high dose of relevant oxidant. The contributions of microbial degradation to the total removal of BaP varied for different oxidants and soils. The removal efficiency of BaP from loam clay sandy soil by a moderate dose of KMnO₄ (25 mmol/L) was 94.3 ± 1.1 % and 92.5 ± 1.8 %, respectively, which were both relatively higher than those under other conditions. The indirect carbon footprint yielded by the moderate dose of oxidants was 39.2-72.8 % less than that by the complete oxidation. A moderate dose of oxidants also reduced disturbances to soil pH and OC. The microbial communities after MOMD treatment were dominated by Burkholderiaceae, Enterobacteriaceae, Alicyclobacillaceae, and Oxalobacteraceae. These dominant microorganisms promoted the removal of BaP through the expression of polycyclic aromatic hydrocarbon-ring hydroxylated dioxygenase gene. Compared with complete chemical oxidation, MOMD is also a promising technique with the utilization of indigenous microorganism for remediating BaP-contaminated soils.