Sustained production of superoxide radicals by manganese oxides under ambient dark conditions

Synthesis of Terminal Bifunctional Aliphatic Compounds via Catalytic Oxidation of 1,6-Hexanediol Over Pt-Loaded BiVO4

Terminal bifunctional aliphatic compounds are important intermediates for synthesis of drugs, food additives, polymers, while oxidative conversion from diols in mild conditions and the insight mechanism are rarely investigated. In this work, Pt-loaded BiVO4 (4%Pt/S-BVO and 4%Pt/H-BVO) is proposed to be utilized for the oxidation of 1,6-hexanediol (1,6-HDO). In dark conditions, the electronic metal-support interactions between Pt and BiVO4 and abundant oxygen vacancies (OVs) activated C-H and O2, leading to the primary oxidation of terminal hydroxyl group of 1,6-HDO. The generated·O2 - radicals enable further oxidation of aldehyde group to carboxyl group. With illumination, the photo-generated holes promoted oxidation of hydroxyl group to aldehyde group while the increased ·O2 - radicals promoted the oxidation of aldehyde group to carboxyl group. The introduced heat originated from the photothermal effect and an additional heat source is supposed to boost the mass transfer of molecules and ·O2 - radicals. In particular, with more abundant OVs and exposed {040} facets, more ·O2 - radicals, and improved charge separation, 4%Pt/S-BVO exhibit 90% conversion of 1,6-HDO with selectivity of 98.6% to TBACs in 6 h. Those findings highlight the great promise of catalytic organics transformation by integrating solar energy to enhance the reaction efficiency.

The pH-sensitive polymerization process of phenol on hexagonal birnessite

Birnessite, a common manganese oxide present in soil, interacts with organic compounds through various mechanisms, including adsorption, oxidative decomposition, and oxidative polymerization. However, this polymerization pathway has been relatively underreported. This study focuses on the reaction pathways of phenols on hexagonal birnessite and the contribution of polymerization process. The results indicate that about 94 % of phenol can be effectively removed by birnessite from solution via polymerization and degradation processes under acidic conditions. When solution initial pH is 2.0, approximately 80 % of phenol undergoes the polymerization pathway, which is predominantly influenced by solution pH, while the other 20 % undergoes degradation pathway. The polymeric products and intermediates were further analyzed. Moreover, the polymerization mechanism was elucidated. Specifically, the zero-point charge (pHPZC) of the hexagonal birnessite is 2.44. When the solution pH is below this pHPZC, phenol is adsorbed to the surface of birnessite, subsequently undergoing a two-electron transfer process to generate phenoxonium ion intermediates, which subsequently condense into multimers. These findings provide new insights into the influence of birnessite on the fate of phenolic pollutants through the polymerization mechanism.

Innovative asymmetric CoSA-N-Ti3C2Tx catalysis: unleashing superoxide radicals for rapid self-coupling removal of phenolic pollutant

The polymerization pathway of contaminants rivals the traditional mineralization pathway in water purification technologies. However, designing suitable oxidative environments to steer contaminants toward polymerization remains challenging. This study introduces a nitrogen-oxygen double coordination strategy to create an asymmetrical microenvironment for Co atoms on Ti3C2Tx MXenes, resulting in a novel Co-N2O3 microcellular structure that efficiently activates peroxymonosulfate. This unique activation capability led to the complete removal of various phenolic pollutants within 3 min, outperforming the representative Co single-atom catalysts reported in the past three years. Identifying and recognizing reactive oxygen species highlight the crucial role of ⋅O2 -. The efficient pollutant removal occurs through a ⋅O2 --mediated radical pathway, functioning as a self-coupling reaction rather than deep oxidation. Theoretical calculations demonstrate that the electron-rich pollutants transfer more electrons to the catalyst surface, inducing the reduction of dissolved oxygen to ⋅O2 - in the Co-N2O3 microregion. In a practical continuous flow-through application, the system achieved 100 % acetaminophen removal efficiency in 6.5 h, with a hydraulic retention time of just 0.98 s. This study provides new insights into the previously underappreciated role of ⋅O2 - in pollutant purification, offering a simple strategy for advancing aggregation removal technology in the field of wastewater treatment.

Sustained Tl(I) removal by α-MnO2: Dual role of tunnel structure incorporation and surface catalytic oxidation

Manganese oxide-based filtration technologies are considered cost-effective for thallium (Tl) removal in engineered systems. However, current gaps in understanding the heterogeneous adsorption and oxidation mechanisms of typical tunneled α-MnO2 may lead to a serious underestimation of its long-term Tl removal potential. In this study, α-MnO2 could continuously remove Tl(I) during the 584-h reaction, with its irreversible removal eventually increasing to 81 %-95 % in different anionic environments. The adsorbed low-loaded Tl(I) is preferentially oxidized, whereas the high-loaded Tl tends to be adsorbed in a nonoxidative pathway by α-MnO2. The nonoxidized Tl(I) was gradually immobilized in the stable thalliomelane-like tunnel structure. More importantly, the synergism of surface Mn(III)-oxygen vacancies (Ov) on α-MnO2 could catalyze the oxidation of Tl(I). Furthermore, the oxidized Tl(III) was bound to the tunnel surface via double edge-sharing and double corner-sharing. In addition, the phosphate anion occupied the surface active site and inhibited the oxidation of Tl(I), thereby reducing the binding strength of Tl. This study provides a new perspective on the effectiveness and stability of Tl(I) removal by MnO2 and highlights the neglected mechanism of Mn(III)-Ov mediating Tl(I) oxidation, which expands our understanding of the removal and transformation fate of Tl in MnO2-engineered systems.

Manganese Dioxides Induce the Transformation and Protection of Dissolved Organic Matter Simultaneously: A Significance of Crystallinity

Interactions between manganese dioxides (MnO2) and dissolved organic matter (DOM) have long been the subject of scientific inquiry. However, the effect of MnO2 crystallinity on the DOM fate remains unclear. Herein, we comprehensively investigate the adsorption, protection, and mineralization of DOM by MnO2 with various crystallinities (order of crystallinity: γ-30 < γ-90 < γ-120). The results show that DOM adsorption is positively correlated with the specific surface area (SSA) of MnO2; γ-30 with the largest SSA adsorbs the highest amount of DOM, resulting in DOM protection. However, γ-90 and γ-120 with a smaller SSA could induce the Maillard reaction and thereby promote the formation of geopolymerized organic matter, leading to reduced bioavailability of DOM. Furthermore, the capability of MnO2 to mineralize DOM decreases in the order γ-120 > γ-90 > γ-30, and it is determined by both Mn4+ and hydroxyl radical (·OH) content. In particular, the contribution of radical-based oxidation of ·OH to DOM mineralization is 64.8, 47.4, and 23.7% for γ-30, γ-90, and γ-120, respectively. We propose that crystallinity of MnO2 may have a significant but hitherto unexplored influence on the global carbon cycle over geological time.

The effect of redox fluctuation on carbon mineralization in riparian soil: An analysis of the hotspot zone of reactive oxygen species production

Riparian zones are important depositional environments at the catchment scale and provide environmental services such as carbon sequestration. This zone is a highly dynamic interface for oxygen and electron exchange, which confers the basis for reactive oxygen species (ROS) production. However, the differences in soil ROS production and their impact on carbon turnover across various redox locations within the riparian zone remain to be fully elucidated. In this study, we investigated the distribution characteristics and generation mechanism of ROS in riparian soil based on soil samples collected in a three-month field monitoring experiment, with additional incubation experiments conducted to examine the effect of hydroxyl radical (•OH) on soil organic carbon (SOC) mineralization. The obtained results demonstrated that the riverine wetland was the hotspot zone for •OH production, with the production flux of 13.05 μmol kg-1 d-1, which was significantly higher than that in floodplain (7.29 μmol kg-1 d-1) and riverbank soils (8.61 μmol kg-1 d-1). Moreover, •OH levels displayed distinct rhythmic fluctuations, with significantly higher concentrations at low water levels compared to those at high water levels, and remained essentially flat over three cycles. The statistic analysis revealed that the ROS production was highly dependent on reduced species and microbial community structure, which function as biogeochemical batteries and electron shuttles under redox fluctuations. Furthermore, the generated •OH involved in the abiotic mineralization of SOC, contributing to 13.1‒21.8 % of total CO2 efflux. Compared to particulate organic carbon (POC), mineral-associated organic carbon (MAOC) fractions of SOC were more susceptible to •OH attacks. The findings provide a novel insight to comprehensively assess the redox process on riparian carbon turnover.

Plasmonic Pd-Sb nanosheets for photothermal CH4 conversion to HCHO and therapy

Photothermal catalysis effectively increases catalytic activity by using the photothermal effect of metal nanomaterials; however, the combination of strong light absorption and high catalytic performance remains a challenge. Here, we demonstrate hexagonal ~5-nanometer-thick palladium antimony (chemical formula as Pd8Sb3) nanosheets (NSs) that exhibit strong light absorption within full spectral and localized surface plasmon resonance (LSPR) effects in the visible region. Such LSPR features lead to strong photothermal effects, and Pd8Sb3 NSs aqueous dispersion enables enhanced photothermal methane (CH4) conversion to formaldehyde (HCHO) under full-spectrum light irradiation at 1.7 watts per square centimeter, leading to selectivity of ~98.7%, productivity of ~665 millimoles per gram of catalyst, ~700 times higher than that of Pd NSs. Mechanism investigations suggest that different radicals were generated on Pd8Sb3 (·OH) and Pd NSs (·O2-), where Pd8Sb3 NSs displays stronger adsorption strength to CH4 and facilitates CH4 oxidation to HCHO. Besides, the strong light absorption ability of Pd8Sb3 NSs enables photothermal therapy for breast cancer.

Rice straw returning enhances cadmium activation by accelerating iron cycling thus hydroxyl radical production in paddy soils during drainage

Straw returning is widely found elevating the bioavailability of cadmium (Cd) in paddy soils with unclear biogeochemical mechanisms. Here, a series of microcosm incubation experiments were conducted and spectroscopic and microscopic analyses were employed. The results showed that returning rice straw (RS) efficiently increased amorphous Fe and low crystalline Fe (II) to promote the production of hydroxyl radicals (OH) thus Cd availability in paddy soils during drainage. On the whole, RS increased OH and extractable Cd by 0.2-1.4 and 0.1-3.3 times, respectively. While the addition of RS effectively improved the oxidation rate of structural Fe (II) mineral (i.e., FeS) to enhance soil Cd activation (up to 38.5 %) induced by the increased OH (up to 69.2 %). Additionally, the existence of CO32- significantly increased the efficiency level on OH production and Cd activation, which was attributed to the improved reactivity of Fe (II) by CO32- in paddy soils. Conclusively, this study emphasizes risks of activating soil Cd induced by RS returning-derived OH, providing a new insight into evaluating the safety of straw recycling.

Seasonal and Spatial Fluctuations of Reactive Oxygen Species in Riparian Soils and Their Contributions on Organic Carbon Mineralization

Reactive oxygen species (ROS) are ubiquitous in the natural environment and play a pivotal role in biogeochemical processes. However, the spatiotemporal distribution and production mechanisms of ROS in riparian soil remain unknown. Herein, we performed uninterrupted monitoring to investigate the variation of ROS at different soil sites of the Weihe River riparian zone throughout the year. Fluorescence imaging and quantitative analysis clearly showed the production and spatiotemporal variation of ROS in riparian soils. The concentration of superoxide (O2•-) was 300% higher in summer and autumn compared to that in other seasons, while the highest concentrations of 539.7 and 20.12 μmol kg-1 were observed in winter for hydrogen peroxide (H2O2) and hydroxyl radicals (•OH), respectively. Spatially, ROS production in riparian soils gradually decreased along with the stream. The results of the structural equation and random forest model indicated that meteorological conditions and soil physicochemical properties were primary drivers mediating the seasonal and spatial variations in ROS production, respectively. The generated •OH significantly induced the abiotic mineralization of organic carbon, contributing to 17.5-26.4% of CO2 efflux. The obtained information highlighted riparian zones as pervasive yet previously underestimated hotspots for ROS production, which may have non-negligible implications for carbon turnover and other elemental cycles in riparian soils.

Structural Fe(II)-induced generation of reactive oxygen species on magnetite surface for aqueous As(III) oxidation during oxygen activation

Magnetite is a reductive Fe(II)-bearing mineral, and its reduction property is considered important for degradation of contaminants in groundwater and anaerobic subsurface environments. However, the redox condition of subsurface environments frequently changes from anaerobic to aerobic owing to natural and anthropogenic disturbances, generating reactive oxygen species (ROS) from the interaction between Fe(II)-bearing minerals and O2. Despite this, the mechanism of ROS generation induced by magnetite under aerobic conditions is poorly understood, which may play a crucial role in As(III) oxidation. Herein, we found that magnetite could activate O2 and induce the oxidative transformation of As(III) under aerobic conditions. As(III) oxidation was attributed to the ROS generated via structural Fe(II) within the magnetite octahedra oxygenation. The electron paramagnetic resonance and quenching tests confirmed that O2•-, H2O2, and •OH were produced by magnetite. Moreover, density function theory calculations combined with experiments demonstrated that O2•- was initially formed via single electron transfer from the structural Fe(II) to the adsorbed O2; O2•- was then converted to •OH and H2O2 via a series of free radical reactions. Among them, O2•-and H2O2 were the primary ROS responsible for As(III) oxidation, accounting for approximately 52 % and 19 % of As(III) oxidation. Notably, As(III) oxidation mainly occurred on the magnetite surface, and As was immobilized further within the magnetite structure. This study provides solid evidence regarding the role of magnetite in determining the fate and transformation of As in redox-fluctuating subsurface environments.

Self-motivated photoaging of microplastics by biochar-dissolved organic matter under different pyrolysis temperatures

Dissolved organic matter (DOM) released from biochar (BDOM) can interact with microplastics (MPs) in the environment, inevitably affecting their environmental behaviour. Information regarding the influence of BDOM on MPs during photoaging and associated variations in the MP aging mechanism remains unclear. This study evaluated the effect of BDOM on the aging of polystyrene (PS) MPs. The results showed that among three pyrolysis temperatures, low-temperature BDOM significantly enhanced the photoaging process of PS MPs, with the smallest average particle size and highest carbonyl index value after 15 days of aging under light conditions. The DOM level decreased after 5 days, increased after 5-10 days, and stabilised after 15 d. BDOM accelerates PS MPs aging, leading to more DOM released from PS, which can be transformed into 1O2 via triplet-excited state (3DOM⁎ and 3PS⁎) to further enhance PS MPs aging, resulting in the realisation of the self-accelerated aging process of PS MPs. 1O2 plays a crucial role in the self-motivated accelerated aging process of PS MPs. These findings provide new insights into the effects of the DOM structure and composition on reactive oxygen species generation during MPs aging.

Mineral-mediated stability of organic carbon in soil and relevant interaction mechanisms

Soil, the largest terrestrial carbon reservoir, is central to climate change and relevant feedback to environmental health. Minerals are the essential components that contribute to over 60% of soil carbon storage. However, how the interactions between minerals and organic carbon shape the carbon transformation and stability remains poorly understood. Herein, we critically review the primary interactions between organic carbon and soil minerals and the relevant mechanisms, including sorption, redox reaction, co-precipitation, dissolution, polymerization, and catalytic reaction. These interactions, highly complex with the combination of multiple processes, greatly affect the stability of organic carbon through the following processes: (1) formation or deconstruction of the mineral-organic carbon association; (2) oxidative transformation of the organic carbon with minerals; (3) catalytic polymerization of organic carbon with minerals; and (4) varying association stability of organic carbon according to the mineral transformation. Several pieces of evidence related to the carbon turnover and stability during the interaction with soil minerals in the real eco-environment are then demonstrated. We also highlight the current research gaps and outline research priorities, which may map future directions for a deeper mechanisms-based understanding of the soil carbon storage capacity considering its interactions with minerals.

Shewanella oneidensis MR-1 dissimilatory reduction of ferrihydrite to highly enhance mineral transformation and reactive oxygen species production in redox-fluctuating environments

Diverse paths generated by reactive oxygen species (ROS) can mediate contaminant transformation and fate in the soil/aquatic environments. However, the pathways for ROS production upon the oxygenation of redox-active ferrous iron minerals are underappreciated. Ferrihydrite (Fh) can be reduced to produce Fe(II) by Shewanella oneidensis MR-1, a representative strain of dissimilatory iron-reducing bacteria (DIRB). The microbial reaction formed a spent Fh product named mr-Fh that contained Fe(II). Material properties of mr-Fh were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Magnetite could be observed in all mr-Fh samples produced over 1-day incubation, which might greatly favor the Fe(II) oxygenation process to produce hydroxyl radical (•OH). The maximum amount of dissolved Fe(II) can reach 1.1 mM derived from added 1 g/L Fh together with glucose as a carbon source, much higher than the 0.5 mM generated in the case of the Luria-Bertani carbon source. This may confirm that MR-1 can effectively reduce Fh and produce biogenetic Fe(II). Furthermore, the oxygenation of Fe(II) on the mr-Fh surface can produce abundant ROS, wherein the maximum cumulative •OH content is raised to about 120 μM within 48 h at pH 5, but it is decreased to about 100 μM at pH 7 for the case of MR-1/Fh system after a 7-day incubation. Thus, MR-1-mediated Fh reduction is a critical link to enhance ROS production, and the •OH species is among them the predominant form. XPS analysis proves that a conservable amount of Fe(II) species is subject to adsorption onto mr-Fh. Here, MR-1-mediated ROS production is highly dependent on the redox activity of the form Fe(II), which should be the counterpart presented as the adsorbed Fe(II) on surfaces. Hence, our study provides new insights into understanding the mechanisms that can significantly govern ROS generation in the redox-oscillation environment.

Biomineral deposits and coatings on stone monuments as biodeterioration fingerprints

Biominerals deposition processes, also called biomineralisation, are intimately related to biodeterioration on stone surfaces. They include complex processes not always completely well understood. The study of biominerals implies the identification of organisms, their molecular mechanisms, and organism/rock/atmosphere interactions. Sampling restrictions of monument stones difficult the biominerals study and the in situ demonstrating of biodeterioration processes. Multidisciplinary works are required to understand the whole process. Thus, studies in heritage buildings have taken advantage of previous knowledge acquired thanks to laboratory experiments, investigations carried out on rock outcrops and within caves from some years ago. With the extrapolation of such knowledge to heritage buildings and the advances in laboratory techniques, there has been a huge increase of knowledge regarding biomineralisation and biodeterioration processes in stone monuments during the last 20 years. These advances have opened new debates about the implications on conservation interventions, and the organism's role in stone conservation and decay. This is a review of the existing studies of biominerals formation, biodeterioration on laboratory experiments, rocks, caves, and their application to building stones of monuments.

Electrocatalytic degradation of p-nitrophenol on metal-free cathode: Superoxide radical (O2•-) production via molecular oxygen activation

Although metal-free electrodes in molecular oxygen-activated Fenton-like wastewater treatment technologies have been developed, the reactive oxygen species (ROS) generation mechanisms are still not sufficiently clear. As a typical example of refractory phenolic wastewater, p-nitrophenol (PNP) has been widely studied. This study demonstrated the critical role of superoxide radicals (O2•-) in PNP degradation by metal-free electrodes through electron spin resonance (ESR), ROS quenching, and density functional theory (DFT) tests. The most superior metal-free electrode exhibited a mass activity of approximately 133.5 h-1 gcatalyst-1. Experimental and theoretical studies revealed the mechanism of O2•- generation via oxygen activation, including one- and three-electron transfer pathways, and found that O2•- mainly attacked the nitro group of PNP to degrade and transform the pollutant. This study enhances the mechanistic understanding of metal-free materials in the electrochemical degradation of refractory pollutants.

The pH-sensitive transformation of birnessite and its effect on the fate of norfloxacin

Birnessite plays a crucial role in regulating the fate of contaminants in soil, which is affected by the crystal structure of birnessite. In this study, the transformation of triclinic birnessite to hexagonal birnessite was examined at various pH values, and their reactivity towards norfloxacin was investigated. The findings indicate that the conversion from triclinic birnessite to hexagonal birnessite occurs under pH conditions lower than 7. The lower of the solution pH where the birnessite formed, the higher the surface reactivity. Throughout the transformation process, the migration of Mn3+ and the increased interlayer protons generated more reactive oxygen species, which enhanced the surface reactivity towards norfloxacin. Specifically, at a conversion pH of 1, the norfloxacin removal rate significantly increases from 14% to 97% compared to triclinic birnessite. The mechanism of norfloxacin removal by triclinic and hexagonal birnessite is illustrated. These findings provide valuable insights into the dynamic transformation of birnessites in aqueous environments with varying pH values and their impact on norfloxacin removal.

Antimony(III) removal by biogenic manganese oxides formed by Pseudomonas aeruginosa PA-1: kinetics and mechanisms

In this study, Pseudomonas aeruginosa PA-1, a manganese-oxidizing bacterium screened from the soil at a manganese mining area, was found to be tolerated to Sb(III) stress during the Mn(II) oxidation, and the generated biological manganese oxide (BMO) outperformed the identical type of Abiotic-MnOX in terms of oxidation and adsorption of Sb(III). Adsorption kinetics and isotherm experiments indicated that Sb(III) was primarily adsorbed through chemisorption and multilayer adsorption on BMO; the maximum adsorption capacity of BMO was 143.15 mg·g-1. Removal kinetic studies showed that the Sb(III) removal efficiency by BMO was 72.38-95.71% after 15 min, and it could be up to 96.32-98.31% after 480 min. The removal procedure could be divided into two stages, fast (within 15 min) and slow (15 ~ 480 min), both of which exhibited first-order kinetic behavior. Dynamic fitting in two steps revealed that the removal speed correlated to the level of dissolved Sb(III) with low Sb(III) concentrations, but with the initial concentration being high, the removal speed rate was independent of dissolved Sb(III). During the whole process, the Sb(III) removal speed by BMO was also higher than that by the Abiotic-MnOX. Combining multiple spectroscopic techniques revealed that Sb(V) was generated through the Sb(III) oxidation by BMO and replacing surface metal hydroxyl groups to form the complex internal Mn-O(H)-Sb(V) or generating stable Mn(II)-antimonate precipitates on the surface. In addition, microbial metabolites, including tryptophan and humus, in BMO may be complex with Sb(III) and Sb(V) to achieve the treatment of Sb(III). This research investigates the factors and mechanisms influencing the adsorption and removal of Sb(III) by BMO, which could aid in its future engineering applications for the BMO.

Effect of KMnO4/pH adjustment of extracellular polymeric substances under waste activated sludge on sludge dewatering

In this study, we examine the dewaterability of sludge after treatment by KMnO4 at various pH levels, with the goal of understanding the dewaterability of strong oxidizers to waste activated sludge. Good dewatering performance is observed, with capillary suction times (CST) reduced from 263.4 to 30.1 s, and specific resistance to filtration (SRF) falling by 19.6%. Proteins and polysaccharides in tightly bound extracellular polymeric substances (EPS) were also significantly reduced. Based on spectroscopic and electrochemical analysis, we propose mechanisms for the improved dewatering in terms of changes to the sludge's physicochemical properties and EPS. Under strong oxidation, the structure surrounding the bound water is oxidized and bound water is released, so the dewaterability of the sludge is improved.Weiliang Pan and Jiaoni Li contributed equally to this work.

Impact of aqueous environments on hydrogen peroxide activation by manganese oxides: Kinetics and the critical role of bicarbonate

MnO₂ activating H₂O₂ is a promising way in the field of advanced oxidation processes (AOPs) and in situ chemical oxidation (ISCO) to remove contaminants. However, few studies have focused on the influence of various environmental conditions on the performance of MnO₂-H₂O₂ process, which restricts the application in real world. In this study, the effect of essential environmental factors (ionic strength, pH, specific anions and cations, dissolved organic matter (DOM), SiO₂) on the decomposition of H₂O₂ by MnO₂ (ε-MnO₂ and β-MnO₂) were investigated. The results suggested that H₂O₂ degradation was negatively correlated with ionic strength and strongly inhibited under low pH conditions and with phosphate existence. DOM had a slight inhibitory effect while Br^-, Ca²⁺, Mn²⁺ and SiO₂ placed negligible impact on this process. Interestingly, HCO₃^- inhibited the reaction at low concentrations but promoted H₂O₂ decomposition at high concentrations, possibly due to the formation of peroxymonocarbonate. This study may provide a more comprehensive reference for potential application of H₂O₂ activation by MnO₂ in different water systems.

Arsenopyrite dissolution in circumneutral oxic environments: The effect of pyrophosphate and dissolved Mn(III)

The oxidative dissolution of As from arsenopyrite, one important arsenic mineral in reducing conditions, poses an environmental hazard to natural aquatic systems. The dissolution of arsenopyrite occurs slowly due to the surface precipitates of iron oxides in circumneutral oxic environments. However, the presence of natural ligands and coexisting metals may change the release of Fe species, which would be of critical importance to the dissolution of arsenopyrite. Here, we investigated the oxidative dissolution of arsenopyrite induced by pyrophosphate (PP) and dissolved Mn(III) species as a natural occurring Mn species with strong complexation affinity to PP. With the presence of PP, the formation of Fe(II)-PP complexes and its rapid oxidation to dissolved Fe(III)-PP species resulted in a substantial increase in the generation of hydroxyl radicals (^•OH) under ambient dark conditions, contributing to faster dissolution of arsenopyrite and higher percentage of As(V) in the dissolved products. Dissolved Mn(III), though considered as an extra oxidant besides oxygen, unexpectedly acted as a radical scavenger for ^•OH and inhibited the production of As(V). Moreover, the oxidation of sulfur species differed in the two systems as significant formation of thiosulfate was observed with the presence of PP, which did not occur in the system with dissolved Mn(III). Overall, the effects of dissolved Mn(III) and PP on the dissolution of arsenopyrite and the subsequent transformation of Fe, As and S species have important implications for disentangling the interactions among these metastable elements, and for assessing their transport and environmental impacts in aquatic systems.

Manganese redox cycling in immobilized bioreactors for simultaneous removal of nitrate and 17β-estradiol: Performance, mechanisms and community assembly potential

The application of bio-manganese (Mn) redox cycling for continuous removal of contaminants provides promise for addressing coexisting contaminants in groundwater, however, the feasibility of constructing Mn redox cycling system (MCS) through community assembly remains to be elucidated. In this study, Mn-reducing strain MFG10 and Mn-oxidizing strain MFQ7 synergistically removed 94.67 % of 17β-estradiol (E2) within 12 h. Analysis of potential variations in Mn oxides suggested that MCS accelerated the production of reactive oxygen species (ROS) and Mn(III), which interacted to promote E2 removal. After continuous operation of the Mn ore-based immobilized bioreactor for 270 days, the experimental group (EG) achieved average removal efficiencies of 89.63 % and 97.57 % for NO₃^--N and E2, respectively. High-throughput sequencing results revealed complex symbiotic relationships in EG. Community assembly significantly enhanced the metabolic and physiological activity of the bioreactor, which promoting the expression of core functions including nitrogen metabolism, Mn cycling and organic matter resistance.

Simultaneous oxidation of trace organic contaminant and Mn(II) by Mn(VII): Accelerating role of dissolved oxygen

Permanganate (Mn(VII)) is a widely used oxidant in water treatment, which can oxidize trace organic contaminants (TrOCs) and Mn(II). Interestingly, this study found that presence of Mn(II) could accelerate the abatement of bisphenol A by Mn(VII) only under oxic condition. Herein, the effects of Mn(II) and dissolved oxygen (DO) on the abatement of TrOCs by Mn(VII) oxidation and the related mechanism were investigated. Results indicate that DO was involved in the Mn(VII)/Mn(II) reaction, with the reaction stoichiometry of Δ[Mn(VII)]:Δ[Mn(II)] determined to be 1:2 and 1:1.5 in the presence and absence of DO, respectively. Quenching and electron paramagnetic resonance tests verified that both superoxide radicals (O₂^•-) and reactive Mn species contributed to the accelerated abatement of TrOCs (bisphenol A, methyl phenyl sulfoxide, and methyl phenyl sulfone) in the Mn(VII)/Mn(II) process. Specifically, O₂^•- was produced through the one-electron reduction of DO and made an important contribution (32.4%-100%) to the abatement of selected TrOCs. This study reveals that Mn(II) could enhance TrOC abatement by Mn(VII) oxidation, and DO played a pivotal role in the Mn(VII)/Mn(II) process.

Impact of manganese sulfide (MnS) oxygenation-induced oxidization on aqueous organic contaminants: Insight into the role of the hydroxyl radical (HO·)

Manganese sulfide (MnS) has unique reactive abilities and can affect the fate and toxicity of contaminants in the natural environment, specifically sulfidic sediments that undergo biogeochemical changes due to natural and artificial processes. However, the effect of oxidization induced by the oxygenation of MnS on organic contaminants remains poorly understood. Herein, we revealed that the hydroxyl radical (HO·) was the dominant reactive oxidant for the rapid degradation of the assessed hydrophobic organic contaminants (including azo dye, nitroaromatic compounds, pesticide, and an endocrine disrupt chemical) during the oxygenation of MnS based on the competitive dynamic experiments, quenching experiments and electron spin resonance (ESR) methods. The removal rates of the assessed organic contaminants were significantly dependent on MnS dosage and co-solutes, including sediment humic acid, metal ions (Mn²⁺and Fe³⁺), and inorganic anions (PO₄^3-and Cl^-). HO· scavenging by sulfide and its oxidation products (e.g., elemental sulfur), rather than dissolved Mn²⁺, was responsible for the low utilization efficiency of HO· for the assessed contaminants. The contribution of the manganese oxide (MnO₂) generated by the oxygenation of MnS to the examined degradation of contaminants could be neglected. Considered collectively, the reaction between H₂O₂ and MnO₂ generated superoxide radicals (O₂^-·) which dominated the generation of HO· in an oxic MnS suspension. The results suggest that the impact of oxidization induced by the oxygenation of MnS on environmental contaminants should be of concern in both natural and engineered systems.

Oxygen Limitation Accelerates Regeneration of Active Sites on a MnO2 Surface: Promoting Transformation of Organic Matter and Carbon Preservation

Birnessite (δ-MnO₂) is a layered manganese oxide widely present in the environment and actively participates in the transformation of natural organic matter (NOM) in biogeochemical processes. However, the effect of oxygen on the dynamic interface processes of NOM and δ-MnO₂ remains unclear. This study systematically investigated the interactions between δ-MnO₂ and fulvic acid (FA) under both aerobic and anaerobic conditions. FA was transformed by δ-MnO₂ via direct electron transfer and the generated reactive oxygen species (ROS). During the 32-day reaction, 79.8% of total organic carbon (TOC) in solution was removed under anaerobic conditions, unexpectedly higher than that under aerobic conditions (69.8%), suggesting that oxygen limitation was more conducive to the oxidative transformation of FA by δ-MnO₂. The oxygen vacancies (O_V) on the surface of δ-MnO₂ were more exposed under anaerobic conditions, thus promoting the adsorption and transformation of FA as well as regeneration of the active sites. Additionally, the reaction of FA with δ-MnO₂ weakened the strongly bonded lattice oxygen (O_latt), and the released O_latt was an important source of ROS. Interestingly, a part of organic carbon (OC) was preserved by forming MnCO₃, which might be a novel mechanism for carbon preservation. These findings contribute to an improved understanding of the dynamic interface processes between MnO₂ and NOM and provide new insights into the effects of oxygen limitation on the cycling and preservation of OC.

Enhanced transformation capability towards benzo(a)pyrene by Fe(III)-modified manganese oxides

Manganese oxides (Mn oxides) are ubiquitous and may coexist with Fe(III) ions in soil environments. In this study, acid birnessite, alkaline birnessite, cryptomelane, pyrolusite, manganite, and their Fe(III)-modified analogues were synthesized and used for benzo(a)pyrene transformation. Fe-modified Mn oxides show a markedly enhanced transformation capability towards benzo(a)pyrene. Specifically, the benzo(a)pyrene transformation rate constants k for Bir-H, Bir-OH, Cry, Pyr, and Man were 0.49, 0.080, 0.0071, 0.0055, and 0.0022 h^-1, respectively. After Fe(III) modification, the transformation rate constants were increased to 22, 2.7, 0.25, 0.0072 and 0.0098 h^-1, respectively. Fe(III)-modified layered birnessites exhibited better activity than Fe(III)-modified tunnel Mn oxides, which was attributed to their high Fe(III) contents and abundant active free radicals. Fe(III) was found to accept electrons from benzo(a)pyrene, thereby accelerating the benzo(a)pyrene transformation. Moreover, modification with Fe(III) increased the surface adsorbed water and oxygen, and promoted the generation of active free radicals. Finally, the physicochemical and biochemical properties of transformation products showed the environmental benefits of this process. Overall, the results indicate that the occurrence of Fe(III) ions could promote the removal of PAHs in Mn oxides-rich soils, and this study provides a credible understanding of PAH fates in natural soils.

Hydroxylamine promoted hydroxyl radical production and organic contaminants degradation in oxygenation of pyrite

The heterogeneous Fenton-like process using pyrite (FeS₂) is increasingly recognized as a promising advanced oxidation process for removal of organic contaminants. However, the slow regeneration of Fe(II) limits the generation of reactive oxygen species for environment implication. To overcome this drawback, hydroxylamine was applied to enhance the reactivity of FeS₂ to degrade organic contaminants under oxic conditions. Results showed that hydroxylamine facilitated the regeneration of Fe(II) on FeS₂ surface to promote reactive oxygen species generation, thereby efficiently degrading different organic contaminants. The underlying mechanism was further elucidated that the presence of hydroxylamine enhanced electron transfer from FeS₂ to O₂ to produce superoxide radicals (O₂^•-), hydrogen peroxide (H₂O₂) and hydroxyl radical (HO^•) via Fenton-like pathways, which induced the rapid degradation of organic contaminants (e.g., sulfamethoxazole (SMX)). The reactivity of FeS₂ for organic contaminant degradation changed negligibly after seven cycles in the presence of hydroxylamine. The effects of pH and inorganic anions on SMX degradation were also clarified in details. The finding of this study would provide a novel strategy to enhance the contaminants degradation by FeS₂-based advanced oxidation technologies for environmental remediation.

The removal performance and mechanisms of tetracycline over Mn-rich limonite

Naturally occurring Mn-rich limonite mainly composed of goethite and manganese oxides was used to remove tetracycline (TC) from the aqueous solution. The effects of dosage, initial solution pH, temperature, and coexisting anions on TC removal were investigated. Results showed that 95% of TC (30.0 mg·L^-1) was removed in a wide pH range of 3.0-9.0 by limonite with high specific surface area (145.0 m²·g^-1) and mesoporous structure. The presence of Cl^-, NO₃^-, and SO₄^2- in the studied concentration range did not influence TC removal efficiency significantly, while PO₄^3- inhibited the adsorption of TC over limonite due to the competition with TC for active sites. Integrated with the FT-IR analysis, electrostatic interaction and complexation were proved to be the adsorption mechanisms of TC by limonite. The quenching experiments and ESR analysis revealed that singlet oxygen (¹O₂) also was involved in TC degradation. In addition, limonite displayed an efficient recycling performance and stability after four cycles. This study revealed that the Mn-rich limonite was a promising adsorbent for TC removal from aqueous solutions and promoted the application of natural mineral material in the environmental field.

Spontaneous nutrient recovery and disinfection of aquaculture wastewater via Mg-coconut shell carbon composites

Aquaculture wastewater contained large amounts of pathogenic microorganisms, nitrogen (N) and phosphorus (P). In this study, the nutrient recoveries and wastewater disinfection were simultaneously achieved using Mg-coconut shell carbon (Mg-CSC). The composites were prepared by a ball milling method. The hydrogen peroxide (H₂O₂) was in-situ generated by the dissolved oxygen reduction driven by Mg corrosion on the CSC surface, which inactivated the microorganisms. Besides that, Mg corrosion provided sufficient Mg ions and appropriate pH conditions for struvite formation. The results show that 5.4-log E.coli removal was achieved under different conditions. Improving the Mg/CSC ratio and composite dosage could shorten the time required for disinfection. In addition to H₂O₂, singlet oxygen played a critical role. Reactive oxygen species destroyed the cellular structure and killed the bacteria. The recoveries of NH₄⁺-Nand P under certain conditions were about 60% and 91%, respectively. An increased composite dosage could improve the recovery ratio of P. Excessive dosages were not beneficial for removing NH₄⁺-N. The characterization result revealed that struvite crystals were the main precipitates on the CSC surface. The Mg-CSC composites also revealed satisfied nutrient recovery and disinfection performances in the real aquaculture wastewater treatment process.

Iron-based metal-organic framework derived pyrolytic materials for effective Fenton-like catalysis: Performance, mechanisms and practicability

In this study, a new catalyst was fabricated by pyrolysis under nitrogen atmosphere with MIL-53(Fe) as the precursor, and was applied to catalyze Fenton-like process. Effects of calcination temperature and pH on decontamination performance, and stability of materials were investigated. Under optimal conditions (calcination temperature of 500 °C and pH of 5.0), the new Fenton-like system remained low iron leaching, and achieved high pseudo-first-order rate constant of 0.0251 min^-1 for bisphenol S (BPS) removal, which is much higher than those in MIL-53(Fe), and nano-Fe₃O₄ catalyzed Fenton-like systems. The superiority of the new catalyst for Fenton-like catalysis was attributed to high specific surface area, as well as formed Fe(II), coordinatively unsaturated iron center and the Fe-O/Fe-C compounds based on the analyses of characterizations. Furthermore, main active species for BPS degradation was identified as hydroxyl radicals, and total hydroxyl radical generation was determined by trapping experiments. The degradation pathways of BPS were also proposed by intermediates monitoring. Moreover, this catalyst showed good potential for practical application, according to the evaluation of reuse, different pollutants degradation, and BPS removal in real wastewater. We believe this study developed a new catalyst with high catalytic activity, high stability and wide application scope, and also sheds light on further development of metal-organic frameworks for Fenton-like catalysis.