Degradation mechanisms of carbamazepine by δ-MnO2: Role of protonation of degradation intermediates

Insight into the generation of toxic by-products during UV/H2O2 degradation of carbamazepine: Mechanisms, N-transformation and toxicity

Carbamazepine (CBZ) is a widely used anticonvulsant drug that has been detected in aquatic environments. This study investigated the toxicity of its by-products (CBZ-BPs), which may surpass CBZ. Unlike the previous studies, this study offered a more systematic approach to identifying toxic BPs and inferring degradation pathways. Furthermore, quadrupole time-of-flight (QTOF) and density functional theory (DFT) calculations were employed to analyze CBZ-BP structures and degradation pathways. Evaluation of total organic carbon (TOC) and total nitrogen (TN) mineralization rates, revealed carbon (C) greater susceptibility to mineralization compared with nitrogen (N). Furthermore, three rules were established for CBZ decarbonization and N removal during degradation, observing the transformation of aromatic compounds into aliphatic hydrocarbons and stable N-containing organic matter over time. Five potentially highly toxic BPs were screened from 14 identified BPs, with toxicity predictions guiding the selection of commercial standards for quantification and true toxicity testing. Additionally, BP207 emerged as the most toxic, supported by the predictive toxicity accumulation model (PTAM). Notably, highly toxic BPs feature an acridine structure, indicating its significant contribution to toxicity. These findings offered valuable insights into the degradation mechanisms of emerging contaminants and the biosafety of aquatic environments during deep oxidation.

The mechanisms of ·OH formation in MnO2 and oxalate system: Implication for ATZ removal

Manganese oxides (MnO2) are commonly prevalent in groundwater, sediment and soil. In this study, we found that oxalate (H2C2O4) dissolved MnO2, leading to the formation of Mn(II)/(III), CO2(aq) and reactive oxygen species (·CO2-/O2·-/H2O2/·OH). Notably, CO2(aq) played a crucial role in ·OH formation, contributing to the degradation of atrazine (ATZ). To elucidate underneath mechanisms, a series of reactions with different gas-liquid ratios (GLR) were conducted. At the GLR of 0.3, 3.76, and + ∞ 79.4 %, 5.32 %, and 5.28 % of ATZ were eliminated, in which the cumulative ·OH concentration was 39.6 μM, 8.11 μM, and 7.39 μM and the cumulative CO2(aq) concentration was 11.2 mM, 4.7 mM, and 2.8 mM, respectively. The proposed reaction pathway was that CO2(aq) participated in the formation of a ternary complex [C2O4-Mn(II)-HCO4·3 H2O]-, which converted to a transition state (TS) as [C2O4-Mn(II)-CO3-OH·3 H2O]-, then decomposed to a complex radical [C2O4-Mn(II)-CO3·3 H2O]·- and ·OH after electron transfer within TS. It was novel to discover the role of CO2(aq) for ·OH yielding during MnO2 dissolution by H2C2O4. This finding helps revealing the overlooked processes that CO2(aq) influenced the fate of ATZ or other organic compounds in environment and providing us ideas for new technique development in contaminant remediation. ENVIRONMENTAL IMPLICATION: Manganese oxides and oxalate are common in soil, sediment and water. Their interactions could induce the formation of Mn(II)/(III), CO2(aq) and ·CO2-/O2·-/H2O2. This study found that atrazine could be effectively removed due to ·OH radicals under condition of high CO2(aq) concentration. The concentrations of Mn (0.0002-8.34 mg·L-1) and CO2(aq) (15-40 mg·L-1) were high in groundwater, and the surface water or rainfall seeps into groundwater and bring organic acids, which might promote the ·OH formation. The results might explain the missing steps of herbicides transformation in these environments and be helpful in developing new techniques in remediation in future.

Lattice oxygen activation of MnO2 by CeO2 for the improved degradation of bisphenol A in the peroxymonosulfate-based oxidation

The structure of MnO2 was modified by constructing the composites CeO2/ MnO2 via a facile hydrothermal method. The catalytic performance of optimal composite (Mn-Ce10) in peroxymonosulfate (PMS) activation for the degradation of bisphenol A (BPA) is approximately three times higher than that of MnO2 alone. The average valence of manganese in CeO2/MnO2 is lowered compared to MnO2, which induces the generation of more free radicals, such as OH and SO4•-. In addition, the composite exhibits a higher concentration of oxygen vacancies than MnO2, facilitating bondingwith PMS to produce more singlet oxygen (1O2). Moreover, the incorporation of CeO2 activates the lattice oxygen of MnO2, improving its oxidative ability. Consequently, approximately 48% of BPA decomposition in 10min is attributed to direct oxidation in the Mn-Ce10/PMS system, whereas only 36% occurs in 30min for the MnO2/PMS system. Simulation results confirm weakened Mn-O covalency and elongated Mn-O bonds due to the activation of lattice oxygen in CeO2/MnO2, demonstrating that PMS tends to be adsorbed on the composite rather than on MnO2. This work establishes a relationship between lattice oxygen and the degradation pathway, offering a novel approach for the targeted regulation of catalytic oxidation.

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.

Nitrilotriacetic acid-assisted Mn(II) activated periodate for rapid and long-lasting degradation of carbamazepine: The importance of Mn(IV)-oxo species

Periodate-based (PI, IO₄^-) oxidation processes for pollutant elimination have gained increased attention in recent years. This study shows that nitrilotriacetic acid (NTA) can assist trace Mn(II) in activating PI for fast and long-lasting degradation of carbamazepine (CBZ) (100% degradation in 2 min). PI can oxidize Mn(II) to permanganate(MnO₄^-, Mn(VII)) in the presence of NTA, which indicates the important role of transient manganese-oxo species. ¹⁸O isotope labeling experiments using methyl phenyl sulfoxide (PMSO) as a probe further confirmed the formation of manganese-oxo species. The chemical stoichiometric relationship (PI consumption: PMSO₂ generation) and theoretical calculation suggested that Mn(IV)-oxo-NTA species were the main reactive species. The NTA-chelated manganese facilitated direct oxygen transfer from PI to Mn(II)-NTA and prevented hydrolysis and agglomeration of transient manganese-oxo species. PI was transformed completely to stable and nontoxic iodate but not lower-valent toxic iodine species (i.e., HOI, I₂, and I^-). The degradation pathways and mechanisms of CBZ were investigated using mass spectrometry and density functional theory (DFT) calculation. This study provided a steady and highly efficient choice for the quick degradation of organic micropollutants and broadened the perspective on the evolution mechanism of manganese intermediates in the Mn(II)/NTA/PI system.

A pH-sensitive drug delivery using biodegradable succinoglycan/chitosan hydrogels with synergistic antibacterial activity

Since succinoglycan (SG) produced by Sinorhizobium meliloti is an anionic polysaccharide having substituents such as succinate and pyruvate groups, a polyelectrolyte composite hydrogel can be made together with chitosan (CS), a cationic polysaccharide. We fabricated polyelectrolyte SG/CS hydrogels using the semi-dissolving acidified sol-gel transfer (SD-A-SGT) method. The hydrogel showed optimized mechanical strength and thermal stability at an SG:CS weight ratio of 3:1. This optimized SG/CS hydrogel exhibited a high compressive stress of 497.67 kPa at 84.65 % strain and a high tensile strength of 9.14 kPa when stretched to 43.73 %. Additionally, this SG/CS hydrogel showed a pH-controlled drug release pattern for 5-fluorouracil (5-FU), where a change from pH 7.4 to 2.0 increased the release from 60 % to 94 %. In addition, this SG/CS hydrogel not only showed a cell viability of 97.57 %, but also showed synergistic antibacterial activity of 97.75 % and 96.76 % against S. aureus and E. coli, respectively. These results indicate the potential of this hydrogel as a biocompatible and biodegradable hydrogel material for wound healing, tissue engineering, and drug release systems.

Increased photocorrosion resistance of ZnO foams via transition metal doping

ZnO is a widely studied photocatalyst, but practical use is hindered by its low resistance to photocorrosion in water, which leads to metal leaching and loss of performance over time. In this work, highly porous and mechanically stable ZnO foams, called MolFoams, were doped by adding 1% or 2% Co, Ni or Cu salts to the starting Zn salt, followed by air insufflation during a sol-gel rection and sintering. The resulting doped foams showed a major increase in stability, with a 60-85% reduction in Zn2+ leaching after irradiation, albeit with a reduction in photocatalytic activity. A systematic analysis using XRD, Raman, XPS and XANES allowed for the identification of dopant species in the foams revealing the presence of Co3O4, NiO and Cu2O within the ZnO lattice with doping leading to a reduced band gap and significant increases in the resistance to photocorrosion of ZnO while identifying the cause of the reduction in photocatalytic activity to be shifting of the band edge positions. These results provide a pathway to significantly reduce the photocorrosion of ZnO in water, with further work required to maintain the photocatalytic activity of undoped ZnO.

Enhanced MnO2 oxidation of methotrexate through self-sensitized photolysis

MnO₂, which is ubiquitous in soil and sediment in natural water environments, may play an important role in the photolysis of contaminants by sunlight, but the interactions between MnO₂ and contaminants in aqueous environments under sunlight irradiation have not been investigated. In this study, the simultaneous presence of sunlight and MnO₂ significantly enhanced the degradation efficiency of methotrexate (MTX). Accordingly, we hypothesized that the overall enhancement of this synergistic reaction is due to the additional production of Mn(III) via MTX self-sensitized photolysis. The pseudo-first-order kinetic model for the photoreaction of MTX with MnO₂ (Light/MTX+MnO₂) during the initial reaction kinetics (0-2 h) revealed a rate constant of 0.43 h¹ ([MTX] = 20 μM, [MnO₂] = 200 μM, and pH = 7), which is faster than that obtained with sunlight alone (0.14 h¹) or MnO₂ alone; Mn(II) and Mn(III) were formed at concentrations of 24.3 ± 1.0 μM and 14.8 ± 1.4 μM, respectively. Dissolved Mn(III) species were identified as the main oxidant species responsible for the degradation of MTX. Two reaction pathways for the production of Mn(III) through Light/MTX+MnO₂ were proposed; MTX acts as a photosensitizer to produce ³MTX* responsible for the reduction of MnO₂ to Mn(III), whereas O₂^• participates in the oxidation of Mn(Ⅱ) to Mn(Ⅲ). Byproduct analysis demonstrated that the Mn(III) generated in the Light/MTX+MnO₂ system enhances C－N bond cleavage, ketonization, and hydrolysis pathways in the MTX transformation.

Combined Process of Biogenic Manganese Oxide and Manganese-Oxidizing Microalgae for Improved Diclofenac Removal Performance: Two Different Kinds of Synergistic Effects

Biogenic manganese oxides (Bio-MnOx) have attracted considerable attention for removing pharmaceutical contaminants (PhCs) due to their high oxidation capacity and environmental friendliness. Mn-oxidizing microalgae (MnOMs) generate Bio-MnOx with low energy and organic nutrients input and degrade PhCs. The combined process of MnOMs and Bio-MnOx exhibits good prospects for PhCs removal. However, the synergistic effects of MnOMs and Bio-MnOx in PhCs removal are still unclear. The performance of MnOMs/Bio-MnOx towards diclofenac (DCF) removal was evaluated, and the mechanism was revealed. Our results showed that the Bio-MnOx produced by MnOMs were amorphous nanoparticles, and these MnOMs have a good Mn²⁺ tolerance and oxidation efficiency (80–90%) when the Mn²⁺ concentration is below 1.00 mmol/L. MnOMs/Bio-MnOx significantly promotes DCF (1 mg/L) removal rate between 0.167 ± 0.008 mg/L·d (by MnOMs alone) and 0.125 ± 0.024 mg/L·d (by Bio-MnOx alone) to 0.250 ± 0.016 mg/L·d. The superior performance of MnOMs/Bio-MnOx could be attributed to the continuous Bio-MnOx regeneration and the sharing of DCF degradation intermediates between Bio-MnOx and MnOMs. Additionally, the pathways of DCF degradation by Bio-MnOx and MnOMs were proposed. This work could shed light on the synergistic effects of MnOMs and Bio-MnOx in PhCs removal and guide the development of MnOMs/Bio-MnOx processes for removing DCF or other PhCs from wastewater.

Degradation pathways of penthiopyrad by δ-MnO2 mediated processes: a combined density functional theory and experimental study

Penthiopyrad is a widely used succinate dehydrogenase inhibitor (SDHI) fungicide and frequently detected in natural environments. In order to better understand its fate in natural systems, the degradation of penthiopyrad by manganese dioxide (MnO₂) was investigated in this study. The results show that penthiopyrad is rapidly degraded in the δ-MnO₂ system. Moreover, density functional theory (DFT) calculations reveal that the atoms of C18, C12, and S1 in penthiopyrad have relatively high reactive active sites. The degradation products mainly include sulfoxides, sulfones, and diketone. A sulfoxide and sulfone are formed by the oxidation of the thioether group, and diketone is formed by the oxidation of the olefin group, respectively. Based on the DFT calculations and degradation products, the degradation pathway of penthiopyrad by MnO₂ is proposed. This study also reveals that the degradation of penthiopyrad by δ-MnO₂ is affected by various environmental factors. A warm environment, low pH, and co-existing humic acid are beneficial to the degradation of penthiopyrad in the δ-MnO₂ system, whereas, co-existing metal cations inhibit penthiopyrad degradation. This result provides theoretical guidance for predicting the potential fate of penthiopyrad in natural environments.

Carbamazepine removal by the synergistic effect of manganese-oxidizing microalgae and biogenic manganese oxides

Forty strains of Mn-oxidizing microalgae (MnOMs) with different Mn²⁺ oxidation mechanisms were identified from two aquatic environments. Among them, three strains of isolates (Chlamydomonas sp. WH1-1, Chlamydomonas sp. WH1-4, and Chlorella sp. WH2-5) oxidize Mn²⁺ by increasing the ambient pH and by secreting Mn oxidation factors (e.g., superoxide-production enzymes and/or other Mn oxidases) into the extracellular environment at the same time. In carbamazepine (CBZ) removal by MnOMs and/or Bio-MnOx, the combination of MnOMs and Bio-MnOx significantly increased the CBZ (1 mg/L) removal efficiency from 36.05% (by MnOMs alone) and 20.11% (by Bio-MnOx alone) to 80.13% by two synergistic mechanisms. One of the synergistic mechanisms was confirmed as that the Mn²⁺ was re-oxidized by MnOMs to Bio-MnOx, which can promote the CBZ removal, and another was the mutual exchange of degradation products of CBZ as shared reactants between MnOMs and Bio-MnOx. The degradation intermediates of CBZ were analyzed using high-performance liquid chromatography-tandem mass spectrometry, based on which the CBZ degradation pathway by MnOMs and Bio-MnOx was proposed. These findings expand existing knowledge on the Mn²⁺ oxidation mechanisms of MnOMs, and indicate that MnOMs and their generated Bio-MnOx are promising for the removal of CBZ or other pharmaceutical contaminants from wastewater.

Removal of acetaminophen through direct electron transfer by reactive Mn2O3: Efficiency, mechanism and pathway

Mn₂O₃ with certain oxidative reactivity, was fabricated via sol-gel method and applied for the removal of acetaminophen (APAP). The mechanism of APAP oxidation was revealed in depth through electrochemical tests and X-ray photoelectron spectroscopy (XPS). Moreover, the selective abatement of various organic contaminants contained different functional groups by Mn₂O₃ was investigated through linear free energy relationship (LFER) estimated with peak potentials (E_op) of these organic contaminants. Under acidic condition, APAP could be effectively eliminated by Mn₂O₃. The open circuit potential (OCP) and zeta potential tests illustrate that the oxidative reactivity of Mn₂O₃ is associated to the surface acid-base behavior of Mn₂O₃ and its surface charge situation. The XPS experiments and Mn leaching results imply that Mn(III) could capture electron from APAP and release Mn²⁺ to aqueous phase. The intermediates could be ascribed to fragmentation of acetamido radicals and phenoxy radicals, both of which were formed through electron transfer from APAP to Mn₂O₃. The reactive Mn₂O₃ shows selective oxidation of different contaminants in the electron transfer process. LFER analysis indicates good negative linear correlation between lnk₁ and E_op of various pollutants. The efficiency of Mn₂O₃ in the elimination of APAP and selective oxidation of different contaminants suggest some new insights for transformation of APAP and other electron-rich pollutants in the environments.

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.

A new insight into the mechanism of carbamazepine oxidation by MnO2: Crystalline structure versus Mn(III)

Mn(III) has been regarded as the origin of oxidative reactivity of MnO₂ recently, however this remains controvertible. Herein, carbamazepine (CBZ), a typical refractory pharmaceutical, was treated by δ-, α-, β-, and γ-MnO₂ and the role of Mn(III) was investigated. After the removal of Mn(III) by pyrophosphate washing, the δ-MnO₂ exhibited a higher kinetics rate (0.180 min^-1) than the sample before washing (0.075 min^-1). Dissolved Mn(III) in the forms of acetate-complex Mn(III), newly acid-dissolved Mn(III) from MnO₂ solid, and in-situ generated Mn(III) showed negligible oxidative reactivity towards the oxidation of CBZ. These evidenced that Mn(III) did not play a critical role in the oxidation of CBZ. The oxidative reactivity of MnO₂ with different structures for the oxidation of CBZ followed the order: δ-MnO₂ >> > α-MnO₂ ≈ γ-MnO₂ > β-MnO₂. Density functional theory calculations suggested that the crystalline plane of δ-MnO₂ significantly contributed to the oxidation of CBZ, thus leading to the superior performance of δ-MnO₂. A new surface reaction dominated mechanism was proposed, which implies that the oxidative reactivity of MnO₂ may not result from Mn(III) as previously believed. These findings could shed light on the understanding of MnO₂-involved oxidation in water treatment and natural processes.

ZSM-5-(C@Fe) activated peroxymonosulfate for effectively degrading ciprofloxacin: In-depth analysis of degradation mode and degradation path

In this work, ZSM-5-(C@Fe), as a peroxymonosulfate (PMS) heterogeneous activator, was synthesized, characterized, and evaluated for activating PMS to degrade ciprofloxacin (CIP) in wastewater. Zeolite Socony Mobil-5 (ZSM-5) was utilized to enhance structural stability and provided a scaffold to graft Fe doping C nanocomposites activator. Pyrolytic metal-organic frameworks (MOFs) can use crystal structure to construct stable carbon-encapsulated Fe nanocomposites. The formation of C-O-Si, C-O-Al and C-Fe was the key to the stability of catalysts. Fe doping in ZSM-5-(C@Fe) formed non-radical degradation pathway was the key to improve the degradation efficiency. The experimental data indicated ZSM-5-(C@Fe) could completely remove 20 mg L^-1 CIP within 15 min and achieve good results in the experiments of treating actual wastewater, which could reduce 40% COD of the paper mill aerobic pond effluent. The Fukui function calculation and UHPL C-H RMS/MS analysis elucidated that the ¹O₂-dominated electrophilic reaction and the ZSM-5-(C@Fe) complex PMS-dominated nucleophilic reaction were the main pathways for CIP degradation and the radical degradation pathway (·OH and SO₄^-˙) plays auxiliary role. In addition, two new degradation pathways of the N29 and C27 in quinolone ring and the N22 in piperazine ring were discovered. This finding had important implications for the removal of N from organic pollutants.

Removal of sulfadiazine and ciprofloxacin by clays and manganese oxides: Coupled sorption-oxidation kinetic model

Sorption onto clays (montmorillonite and kaolinite), oxidation and sorption by manganese oxides (synthesized MnO and natural MnO), and coupled sorption-oxidation experiments were conducted for the removal of antibiotics sulfadiazine (SDZ) and ciprofloxacin (CIP) at pH 5 and 8. Individual sorption and oxidation modelling were carried out using the first-order kinetic model. A coupled sorption-oxidation kinetic model was developed to predict the simultaneous sorption and oxidation process. The coupled sorption-oxidation enhanced the antibiotic sorption, with the first-order sorption rate constants in the simultaneous presence of clays and manganese oxides (k_sorp) being higher than those with clays only (k_sorp⁰). In contrast, a depression was observed; the first-order oxidation and sorption combination rate constants in the simultaneous presence of manganese oxides and clays (k_MnO) were lower than those with manganese oxides only (k_MnO⁰). In the coupled sorption-oxidation reaction, 13.5-62.5% of SDZ and CIP removal was attributed to the sorption. The SDZ and CIP species distributions at pH 5 affected the coupled sorption and oxidation systems more than those at pH 8. The best removal efficiency was achieved by the montmorillonite-synthesized MnO combination, mainly due to the higher surface area (A_BET) and pore size of montmorillonite and synthesized MnO combination compared to other clays and manganese oxides combinations.

Carbamazepine toxicity and its co-metabolic removal by the cyanobacteria Spirulina platensis

Bioremediation of pharmaceutical-contaminated wastewater using microalgae has attracted increasing attention. Cyanobacteria, which are important prokaryotic microalgae, are widely distributed in different water environments, and have the advantages of simple culture and a fast growth rate. However, studies on either the toxicity of pharmaceutical contaminants (PhCs) to cyanobacteria or the removal of PhCs by cyanobacteria are scarce. In this study, carbamazepine (CBZ) and Spirulina platensis were selected as model PhCs and cyanobacteria, respectively. CBZ (>1 mg/L) had toxicity effects on S. platensis, showing maximal growth inhibition (34.0%) at 100 mg/L after 10 days of cultivation. At CBZ < 25 mg/L, S. platensis showed a trend similar to that of eukaryotic microalgae in increasing superoxide dismutase and catalase activities and content of chlorophylls, carotenoids, carbohydrates, and lipids. These results indicated that S. platensis had a similar protective mechanism to CBZ toxicity as that of the eukaryotic microalgae. Increasing CBZ concentration (50-100 mg/L) significantly decreased these biochemical characteristics and photosynthetic activity owing to the serious damage of the structure and function of S. platensis. However, with increasing cultivation time, the growth and photosynthetic activity of S. platensis recovered from the toxicity of CBZ. S. platensis showed a maximum of 30.97 ± 1.30% removal of CBZ (1 mg/L), mainly through biodegradation. Addition of 0.3 mg/L glucose enhanced this removal efficiency to 50.13 ± 2.51% via co-metabolism. These findings indicated that S. platensis can be used for the removal of CBZ or other PhCs from wastewater.

Transformation of m-aminophenol by birnessite (δ-MnO2) mediated oxidative processes: Reaction kinetics, pathways and toxicity assessment

The m-aminophenol (m-AP) is a widely used industrial chemical, which enters water, soils, and sediments with waste emissions. A common soil metal oxide, birnessite (δ-MnO₂), was found to mediate the transformation of m-AP with fast rates under acidic conditions. Because of the highly complexity of the m-AP transformation, mechanism-based models were taken to fit the transformation kinetic process of m-AP. The results indicated that the transformation of m-AP with δ-MnO₂ could be described by precursor complex formation rate-limiting model. The oxidative transformation of m-AP on the surface of δ-MnO₂ was highly dependent on reactant concentrations, pH, temperature, and other co-solutes. The UV-VIS absorbance and mass spectra analysis indicated that the pathway leading to m-AP transformation may be the polymerization through the coupling reaction. The m-AP radicals were likely to be coupled by the covalent bonding between unsubstituted C2, C4 or C6 atoms in the m-AP aromatic rings to form oligomers as revealed by the results of activation energy and mass spectra. Furthermore, the toxicity assessment of the transformation productions indicated that the toxicity of m-AP to the E. coli K-12 could be reduced by MnO₂ mediated transformation. The results are helpful for understanding the environmental behavior and potential risk of m-AP in natural environment.

Mechanisms of bisulfite/MnO2-accelerated transformation of methyl parathion

Although bisulfite is able to activate manganese oxides for enhanced oxidation of organic contaminants with donor-electron functions, the removal mechanisms for some esters remain poorly understood. In this study, we investigated the bisulfite/MnO₂-accelerated transformation of methyl parathion (MP), a recalcitrant and toxic organophosphorus pesticide (OPP). The removal rate constants of MP depended on pH, oxygen conditions, and the ratio between [HSO₃^-] and [MnO₂]. MP transformation declined by 36% with the addition of pyrophosphate as a scavenging agent for Mn(III)aq. [Mn(OH)(SO₃H)]⁺, a reactive intermediate, may be involved in enhancing the transformation of MP. The overall reaction can be divided into three distinct processes. The first process comprises two steps: the dissolution of MnO₂ reduced by HSO₃^- and the formation of a Mn-sulfite complex by a relatively fast substitution-controlled process. The second process is much slower and forms a precursor organometallic complex between the MP and Mn(IV/III). The third process involves a series of redox/hydrolysis reactions via aqueous and surface reactions. The mechanisms of each process were interpreted using kinetic observation and product identification data. This study improved the fundamental understanding of the MnO₂/HSO₃^- reaction process, thereby increasing the feasibility for remediating OPP pollution of the soil-water environment.

Electrochemical activation of peroxymonosulfate with ACF cathode: Kinetics, influencing factors, mechanism, and application potential

The combination of peroxymonosulfate (PMS) and electrolysis with an activated carbon fiber (ACF) as cathode (E-ACF-PMS) was systematically investigated. A synergistic effect was observed in the E-ACF-PMS process. Compared with the E-ACF-PDS process, the E-ACF-PMS process spent one-third as much energy for elimination of carbamazepine (CBZ). Increased PMS concentration, current density, and pH value significantly enhanced CBZ elimination. It was also noted that the presence of phosphate (PO₄^3-), bicarbonate (HCO₃^-), and humic acid (HA) inhibited CBZ removal, while the presence of chloride ion (Cl^-) accelerated it. According to radical scavenging experiments and the estimation of relative contribution, reactive oxygen species oxidation (including OH, SO₄^•-, and ¹O₂) played an important role in CBZ degradation, accounting for 75.67%. We systematically explored the production mechanism for ¹O₂ and the results demonstrated that ¹O₂ was mainly generated on the cathode, rather than generated by O₂^•- or O₂ reported by other researchers. Possible degradation pathways for CBZ in E-ACF-PMS process were also proposed. Finally, the potential for practical applications was explored and compared with E-ACF-PDS. The results of SEM images, BET, and nitrogen adsorption isotherm before and after ACF reuse for 50 times suggested that ACF could maintain its adsorption capacity and catalytic ability in the E-ACF-PMS process. Testing also suggested that the protection of ACF in electrochemical oxidation was based on its relatively high current intensity and removal efficiency. The removal efficiencies of other organic pollutants, including nitrobenzene (NB), sulfamethoxazole (SMX), diclofenac (DC), and tetracycline (TC) were also evaluated. In addition, experiments were conducted to study the effects of different water matrices and toxicology implications and results demonstrated that substituting PMS for PDS in an E-ACF system could create a more efficient, sustainable, and with less secondary toxicity process for wastewater treatment.

Effect of environmental factors on the oxidative transformation of cephalosporin antibiotics by manganese dioxides

This study aimed to explore the oxidation and transformation of the cephalosporins cefotaxime (CTX), cephalexin (CFX), cephradine (CFD), cephapirin (CFP) and cefazolin (CFZ) by δ-MnO2. The results showed that the MnO2 oxidation rate was promoted by environmental factors such as higher MnO2 loading, lower initial cephalosporin concentration and lower solution pH. The inhibitory effect occurred in the presence of dissolved organic matter and dissolved cations (inhibitory capacity: Mn2+ > Ca2+ > Mg2+ > Fe3+). Total organic carbon analysis indicated that the transformation byproducts of the cephalosporins are less reactive and persistent under MnO2 oxidation. Twelve transformation byproducts (9 CFP byproducts and 3 CTX byproducts) were identified, and two oxidative transformation pathways were proposed: one occurred in the cephem for CFP, and the other occurred at the substituent at the amine position for CTX. The effect of solar light on the oxidation of the five cephalosporin antibiotics by δ-MnO2 was also investigated, and the results indicated that the initial dissolution rate of δ-MnO2 under sunlight was approximately eight times faster than that in the dark in the presence of CFP.

Visible-light-driven photocatalytic degradation of diclofenac by carbon quantum dots modified porous g-C3N4: Mechanisms, degradation pathway and DFT calculation

Metal-free photocatalysts have attracted growing concern in recent years. In this work, a new class of carbon quantum dots (CQDs) modified porous graphitic carbon nitride (g-C₃N₄) is synthesized via a facile polymerization method. With the optimal CQDs loading, the CQDs modified g-C₃N₄ exhibits ∼15 times higher degradation kinetic towards diclofenac (DCF) than that of pure g-C₃N₄. The enhanced photocatalytic activity can be ascribed to the improved separation of charge carriers as well as the tuned band structure. Moreover, a photosensitation-like mechanism is proposed to elucidate the photo-generated electrons transfer and reactive radicals formation. CQDs are anchored to g-C₃N₄ surface via C-O bond, which provide channels for the preferential transfer of photo-excited electrons on DCF molecule to the conduction band of g-C₃N₄. Superoxide radical (·O₂^-) dominates the degradation of DCF, while holes (h⁺) show a negligible contribution. Density functional theory (DFT) calculation successfully predicts that the sites on DCF molecule with high Fukui index (f⁰) are preferable to be attacked by radicals. DCF degradation pathway mainly includes ring hydroxylation, ring closure and C-N bond cleavage processes. Acute toxicity estimation indicates the formation of less toxic intermediates/products compared to DCF after photocatalysis. Moreover, the hybrid photocatalysts exhibit good reusability in five consecutive cycles. This work not only proposes a deep insight into photosensitation-like mechanism in the photocatalysis system by using C₃N₄-based materials, but also develops new photocatalysts for potential application on removal of emerging organic pollutants from waters and wastewaters.

Visible-light-driven photocatalytic degradation of diclofenac by carbon quantum dots modified porous g-C3N4: Mechanisms, degradation pathway and DFT calculation

Metal-free photocatalysts have attracted growing concern in recent years. In this work, a new class of carbon quantum dots (CQDs) modified porous graphitic carbon nitride (g-C₃N₄) is synthesized via a facile polymerization method. With the optimal CQDs loading, the CQDs modified g-C₃N₄ exhibits ∼15 times higher degradation kinetic towards diclofenac (DCF) than that of pure g-C₃N₄. The enhanced photocatalytic activity can be ascribed to the improved separation of charge carriers as well as the tuned band structure. Moreover, a photosensitation-like mechanism is proposed to elucidate the photo-generated electrons transfer and reactive radicals formation. CQDs are anchored to g-C₃N₄ surface via CO bond, which provide channels for the preferential transfer of photo-excited electrons on DCF molecule to the conduction band of g-C₃N₄. Superoxide radical (·O₂^-) dominates the degradation of DCF, while holes (h⁺) show a negligible contribution. Density functional theory (DFT) calculation successfully predicts that the sites on DCF molecule with high Fukui index (f⁰) are preferable to be attacked by radicals. DCF degradation pathway mainly includes ring hydroxylation, ring closure and CN bond cleavage processes. Acute toxicity estimation indicates the formation of less toxic intermediates/products compared to DCF after photocatalysis. Moreover, the hybrid photocatalysts exhibit good reusability in five consecutive cycles. This work not only proposes a deep insight into photosensitation-like mechanism in the photocatalysis system by using C₃N₄-based materials, but also develops new photocatalysts for potential application on removal of emerging organic pollutants from waters and wastewaters.