Direct Determination of Absolute Radical Quantum Yields in Hydroxyl and Sulfate Radical-Based Treatment Processes.

The absolute radical quantum yield (Φ) is a critical parameter to evaluate the efficiency of radical-based processes in engineered water treatment. However, measuring Φ is fraught with challenges, as current quantification methods lack selectivity, specificity, and anti-interference capabilities, resulting in significant error propagation. Herein, we report a direct and reliable time-resolved technique to determine Φ at pH 7.0 for commonly used radical precursors in advanced oxidation processes. For H2O2 and peroxydisulfate (PDS), the values of Φ•OH and ΦSO4•- at 266 nm were measured to be 1.10 ± 0.01 and 1.46 ± 0.05, respectively. For peroxymonosulfate (PMS), we developed a new approach to determine Φ•OHPMS with terephthalic acid as a trap-and-trigger probe in the nonsteady state system. For the first time, the Φ•OHPMS value was measured to be 0.56 by the direct method, which is stoichiometrically equal to ΦSO4•-PMS (0.57 ± 0.02). Additionally, radical formation mechanisms were elucidated by density functional theory (DFT) calculations. The theoretical results showed that the highest occupied molecular orbitals of the radical precursors are O-O antibonding orbitals, facilitating the destabilization of the peroxy bond for radical formation. Electronic structures of these precursors were compared, aiming to rationalize the tendency of the Φ values we observed. Overall, this time-resolved technique with specific probes can be used as a reliable tool to determine Φ, serving as a scientific basis for the accurate performance evaluation of diverse radical-based treatment processes.

Driving factors for distribution and transformation of heavy metals speciation in a zinc smelting site.

Heavy metal pollution at an abandoned smelter pose a significant risk to environmental health. However, remediation strategies are constrained by inadequate knowledge of the polymetallic distribution, speciation patterns, and transformation factors at these sites. This study investigates the influence of soil minerals, heavy metal occurrence forms, and environmental factors on heavy metal migration behaviors and speciation transformations. X-ray diffraction analysis revealed that the minerals associated with heavy metals are mainly hematite, franklinite, sphalerite, and galena. Sequential extraction results suggest that lead and zinc are primarily present in the organic-sulfide fractions (F4) and residual form (F5) in the soil, accounting for over 70% of the total heavy metal content. Zinc displayed greater instability in carbonate-bound (16%) and exchangeable (2%) forms. The migration and diffusion patterns of heavy metals in the subsurface environment were visualized through the simulation of labile state heavy metals, demonstrating high congruence with groundwater pollution distribution patterns. The key environmental factors influencing heavy metal stable states (F4 and F5) were assessed by integrating random forest models and redundancy analysis. Primary factors facilitating Pb transformation into stable states were available phosphorus, clay content, depth, and soil organic matter. For Zn, the principal drivers were Mn oxides, soil organic matter, clay content, and inorganic sulfur ions. These findings enhance understanding of the distribution and transformation of heavy metal speciation and can provide valuable insights into controlling heavy metal pollution at non-ferrous smelting sites.

The formation of multi-metal(loid)s contaminated groundwater at smelting site: Critical role of natural colloids.

The occurrence and migration of colloids at smelting sites are crucial for the formation of multi-metal(loid)s pollution in groundwater. In this study, the behavior of natural colloids (1 nm-0.45 µm) at an abandoned smelting site was investigated by analyzing groundwater samples filtered through progressively decreasing pore sizes. Smelting activities in this site had negatively impacted the groundwater quality, leading to elevated concentrations of zinc (Zn), lead (Pb), arsenic (As), and cadmium (Cd). The results showed that heavy metal(loid)-bearing colloids were ubiquitous in the groundwater with the larger colloidal fractions (∼75 -450 nm) containing higher abundances of pollutants. It was also observed that the predominant colloids consisted of Zn-Al layered double hydroxide (LDH), sphalerite, kaolinite, and hematite. By employing multiple analytical techniques, including leaching experiments, soil colloid characterization, and Pb stable isotope measurements, the origin of groundwater colloids was successfully traced to the topsoil colloids. Most notably, our findings highlighted the increased risk of heavy metal(loid)s migration from polluted soils into adjacent sites through the groundwater because of colloid-mediated transport of contaminants. This field-scale investigation provides valuable insights into the geochemical processes governing heavy metal(loid) behavior as well as offering pollution remediation strategies specifically tailored for contaminated groundwater.

Unveiling the role of pore characteristics in sludge dewatering: Visualization by Nano-CT and micromodel study.

The solid-liquid separation is an indispensable and primary link in the process of sludge treatment and disposal. The past research was focused primarily on the technique explorations of sludge dewatering and always disregarded the internal pore structure and water migration behavior in sludge. In this work, the real three-dimensional pore structure of sludge was obtained by Nano-CT. Based on this, a pore-scale heterogeneous sludge micromodel was firstly presented, and the water flooding experiment was carried out to visualize the water migration behavior. The results showed that the sludge structure transformed from sheet-like floc to microsphere particles, and then agglomerated into large globular granules during anaerobic ammonia oxidation. And the equivalent pore size increases from 342 µm to 617 µm, improving the sludge dewaterability characterized by capillary suction time (CST). The most significant implication of this work was revealing the critical role of invalid connected pore in sludge dewatering. Such pore was not contributed to fluid flow but the circulating vortex in it even induced energy dissipation, thus deteriorated the sludge dewaterability. This work may be helpful to understand the critical role of pore characteristic in water migration and shed light on the new dewatering techniques from the perspective of regulating sludge structure.

Electrochemically selective ammonium recovery from wastewater via coupling hydrogen bonding and charge storage.

Electrochemical ammonium (NH4+) storage (EAS) has been established as an efficient technology for NH4+ recovery from wastewater. However, there are scientific difficulties unsolved regarding low storage capacity and selectivity, restricting its extensive engineering applications. In this work, electrochemically selective NH4+ recovery from wastewater was achieved by coupling hydrogen bonding and charge storage with self-assembled bi-layer composite electrode (GO/V2O5). The NH4+ storage was as high as 234.7 mg N g-1 (> 102 times higher than conventional activated carbon). Three chains of proof were furnished to elucidate the intrinsic mechanisms for such superior performance. Density functional theory (DFT) showed that an excellent electron-donating ability for NH4+ (0.08) and decrease of diffusion barrier (22.3 %) facilitated NH4+ diffusion onto electrode interface. Physio- and electro-chemical results indicated that an increase of interlamellar spacing (14.3 %) and electrochemical active surface area (ECSA, 388.9 %) after the introduction of GO were responsible for providing greater channels and sites toward NH4+ insertion. Both non-ionic chemical-bonding (V5+=O‧‧‧H, hydrogen-bonding) and charge storage were contributed to the higher capacity and selectivity for NH4+. This work offers underlying guideline for exploitation a storage manner for NH4+ recovery from wastewater.

Characterization of the microplastic photoaging under the action of typical salt ions of biological nitrogen removal processes.

Microplastics (MPs) are one of the most prevalent and diverse contaminants, and wastewater treatment plants are significant MP aggregators. Controlling the pollution caused by microplastics requires an understanding of how they age. The properties of the MPs photoaging process under the influence of salt ions typical of biological nitrogen elimination processes were disclosed in this work. The aging process of polyvinyl chloride microplastics (PVC-MPs) was greatly slowed down by greater HCO3- and NO2- concentrations, according to a comparison of the carbonyl index changes that occurred during photoaging. The carbonyl index had a negative correlation with the thermal stability of the photo-aged PVC-MPs, and aging accelerated the elimination of chlorine from the water. The samples were aged by UV radiation after 36 h at 40 °C, and the amount of chlorine eliminated was 10.13 times greater than that of the original MPs samples. It was discovered that the leachate concentration of aged MPs dramatically increased with decreasing particle size and was positively connected with the level of aging by comparing the concentration of leachate for two particle sizes (1 mm and 100 m). Photoaging caused MPs to become rougher, which in turn improved the NO3--N, NH4+-N, and NO2--N adsorption by PVC-MPs.

Dosing with pyrite significantly increases anammox performance: Its role in the electron transfer enhancement and the functions of the Fe-N-S cycle.

Anaerobic ammonium oxidation (anammox) represents an energy-efficient process for biological nitrogen removal from ammonium-rich wastewater. However, there are mechanistic issues unsolved regarding the low microbial electron transfer and undesired accumulation of nitrate in treated water, limiting its widespread engineering applications. We found that the addition of pyrite (1 g L-1 reactor), an earth-abundant iron-bearing sulfide mineral, to the anammox system significantly improved the nitrogen removal rate by 52% in long-term operation at a high substrate shock loading (3.86 kg N m-3 d-1). Two lines of evidence were presented to unravel the underlying mechanisms of the pyrite-induced enhancement. Physiochemical evidence indicated that an increase of cytochromes c and Fe-S protein was responsible for the accelerated electron transfer among metabolic enzymes. Multi-omics evidence showed that the depletion of nitrate was attributed to the Fe-N-S cycle driven by nitrate-dependent Fe(II) oxidation and S-based denitrification. This study deepens our understanding of the roles of electron transfer and the Fe-N-S cycle in anammox systems, providing a fundamental basis for the development of mediators in the anammox process for practical implications.

Mapping forest and site quality of planted Chinese fir forest using sentinel images.

Normally, forest quality (FQ) and site quality (SQ) play an important role in evaluating actual and potential forest productivity. Traditionally, these assessment indices (FQ and SQ) are mainly based on forest parameters extracted from ground measurement (forest height, age, density, forest stem volume (FSV), and DBH), which is labor-intensive and difficult to access in certain remote forest areas. Recently, remote sensing images combined with a small number of samples were gradually applied to map forest parameters because of the various advantages of remote sensing technology, such as low cost, spatial coverage, and high efficiency. However, FQ and SQ related to forest parameters are rarely estimated using remote sensing images and machine learning models. In this study, the Sentinel images and ground samples of planted Chinese fir forest located in the ecological "green-core" area of Changzhutan urban cluster, were initially employed to explore the feasibility of mapping the FQ and SQ. And then, four types of alternative variables (backscattering coefficients (VV and VH), multi-spectral bands, vegetation indices, and texture characteristics) were extracted from Sentinel-1A and Sentinel-2A images, respectively. After selecting variables using a stepwise regression model, three machine learning models (SVR, RF, and KNN) were employed to estimate various forest parameters. Finally, the FQ of the study region was directly mapped by the weights sum of related factors extracted by the factor analysis method, and the SQ was also extracted using mapped forest height and age. The results illustrated that the accuracy of estimated forest parameters (DBH, H, and Age) was significantly higher than FSV, FCC, and Age and the largest and smallest rRMSEs were observed from FSV (0.38~0.40) and forest height (0.20~0.21), respectively. Using mapped forest parameters, it also resulted that the rRMSEs of estimated FQ and SQ were 0.19 and 0.15, respectively. Furthermore, after normalization and grading, the grades of forest quality were mainly concentrated in grades I, II, and III in the study region. Though the accuracy of mapping FQ and SQ is limited by the saturation phenomenon, it is significantly proved that using machine learning models and Sentinel images has great potential to indirectly map FQ and SQ.

[Removal Characteristics of Four Typical Antibiotics in Denitrification System].

Wastewater treatment plants are an important gathering place for antibiotics, where denitrification plays a vital role in biological nitrogen removal. In order to explore the removal characteristics of antibiotics in a denitrifying sludge system, norfloxacin (NOR), oxytetracycline (OTC), sulfamethoxazole (SMX), and trimethoprim (TMP) were selected to investigate their transformation under different carbon source conditions in a denitrification system. The contribution of adsorption and biodegradation for antibiotic removal was also evaluated in this study. The results showed that a certain proportion of NOR, OTC, and TMP could be removed by denitrification, whereas NOR and OTC could act as the sole carbon source for denitrification. The removal of NOR and OTC showed a rapid adsorption and then slow biodegradation trend in the denitrification system. The contributions of adsorption were recorded as 83.5% and 58.9% for NOR and OTC removal, respectively. More than 40% were adsorbed by extracellular polymer substances (EPS), whereas the P450 enzyme played an important role in the OTC biodegradation process, with a contribution of 20%.

Understanding the slight inhibition of high As(III) stress on nitritation process: Insights from arsenic speciation and microbial community analyses.

Nitritation process with ammonia-oxidizing bacteria frequently suffers inhibition from heavy metals in industrial wastewater treatment. However, As(III), one of the most toxic metalloids, showed slight inhibition though the arsenic accumulation content in the sludge reached 91.8 mg L-1 in this study. Here, we combined long-term reactor operation with microbiological analyses to explore the slight inhibition mechanisms of As(III) on nitritation consortia. The results showed that no obvious changes induced by As(III) occurred in apparent characteristics and morphology of the nitritation consortia, whereas dosing As(III) induced shifts in the arsenic speciation and microbial community. 84.1% of As(III) was oxidized to As(V) in the acclimated sludge, decreasing the toxicity of As(III) to nitritation consortia. Insight to the microbial community, the relative abundances of Thermaceae and Phycisphaeraceae responsible for As(III) oxidation were increased to 7.4% and 6.6% under the stress of high-concentration As(III), respectively. Further, these increased arsenite-oxidizing bacteria probably accepted electron acceptor NO2- from ammonia-oxidizing bacteria to oxidize As(III). Our results indicated that microbial As(III) oxidation was the dominant detoxification pathway, providing new insights into nitritation characteristics under long-term As(III) stress.

Current progress on plastic/microplastic degradation: Fact influences and mechanism.

Plastic pollution, particularly non-degradable residual plastic films and microplastics (MPs), is a serious environmental problem that continues to worsen each year. Numerous studies have characterized the degradation of plastic fragments; however, there is known a lack of about the state of current physicochemical biodegradation methods used for plastics treatment and their degradation efficiency. Therefore, this review explores the effects of different physicochemical factors on plastics/MPs degradation, including mechanical comminution, ultraviolet radiation, high temperature, and pH value. Further, this review discusses different mechanisms of physicochemical degradation and summarizes the degradation efficiency of these factors under various conditions. Additionally, the important role of enzymes in the biodegradation mechanism of plastics/MPs is also discussed. Collectively, the topics discussed in this review provide a solid basis for future research on plastics/MPs degradation methods and their effects.

Distinct membrane fouling characteristics of anammox MBR with low NO2--N/NH4+-N ratio.

The bacterial growth and death, and extracellular polymeric substances (EPS) and soluble microbial products (SMP) in aerobic membrane bioreactor (MBR) cause severe membrane fouling. Anammox bacteria grow slowly but produce much EPS and SMP. Therefore, the membrane fouling characteristic of anammox MBR is still indistinct. A NO2--N/NH4+-N < 1.0 into in the influent of an anammox MBR applies to investigate: 1) the slowest growing anammox bacteria (Candidatus Jettenia) could be enriched or not; 2) its membrane fouling characteristic. Results showed that Candidatus Jettenia successfully accumulated from 0.01% to 26.19%. The fouling characteristic of anammox MBR was entirely different from other MBRs. Firstly, obvious low transmembrane pressure (<4 KPa, 125 days) and low amount of foulants (0.22 gVSS/m2) might result from N2 production and the slow-growing Candidatus Jettenia. Secondly, the analysis of the components of membrane foulants indicated that polysaccharides of SMP in the gel layer and pore foulants were the key factors affecting membrane fouling. Finally, the large particle size of foulants (200 µm) might be caused by anammox bacteria living inside the foulants under anaerobic conditions. This study provides systematic insights into membrane characteristics of anammox MBR and a basis for the enrichment of anammox bacteria by MBR.

Spinel LiMn2O4 as a Capacitive Deionization Electrode Material with High Desalination Capacity: Experiment and Simulation.

Capacitive deionization (CDI) is a newly developed desalination technology with low energy consumption and environmental friendliness. The surface area restricts the desalination capacities of traditional carbon-based CDI electrodes while battery materials emerge as CDI electrodes with high performances due to the larger electrochemical capacities, but suffer limited production of materials. LiMn2O4 is a massively-produced lithium-ion battery material with a stable spinel structure and a high theoretical specific capacity of 148 mAh·g-1, revealing a promising candidate for CDI electrode. Herein, we employed spinel LiMn2O4 as the cathode and activated carbon as the anode in the CDI cell with an anion exchange membrane to limit the movement of cations, thus, the lithium ions released from LiMn2O4 would attract the chloride ions and trigger the desalination process of the other side of the membrane. An ultrahigh deionization capacity of 159.49 mg·g-1 was obtained at 1.0 V with an initial salinity of 20 mM. The desalination capacity of the CDI cell at 1.0 V with 10 mM initial NaCl concentration was 91.04 mg·g-1, higher than that of the system with only carbon electrodes with and without the ion exchange membrane (39.88 mg·g-1 and 7.84 mg·g-1, respectively). In addition, the desalination results and mechanisms were further verified with the simulation of COMSOL Multiphysics.

Hydroxylamine addition enhances fast recovery of anammox activity suffering Cr(VI) inhibition.

Hydroxylamine (NH2OH), one of the most important intermediates of anammox was employed to test the recovery performance because of its stimulation to anammox bacteria. Batch test indicated simultaneous addition of 1.83 ~ 9.17 mg N /L NH2OH relieved Cr(VI) inhibition because of extracellular reduction to Cr(III). The recovery efficiency (RE) was over 166%, with the effluent Cr(VI) and Cr(III) below 0.25 and 0.12 mg/L, respectively. Anammox activity after Cr(VI) inhibition was effectively recovered by 8 mg N/L NH2OH with RE at 218%. The long-term operation showed 1 ~ 2 mg N/L NH2OH accelerated the recover speed of nitrogen removal rate with 2.84 folds, as well as improving NH4+ conversion ratio and reducing NO3- production. After 55 days recovery, extracellular polymeric substance concentration, anammox activity and heme content recovered better with NH2OH addition. This study will provide the theoretical basis for rapid recovery of anammox activity by NH2OH when suffering Cr(VI) inhibition.

Development and simulation of a struvite crystallization fluidized bed reactor with enhanced external recirculation for phosphorous and ammonium recovery.

Recovering nitrogen and phosphorus from waste water in the form of struvite is an effective way to recycle resources. The insufficient purity of the resulting struvite and the large loss of nitrogen and phosphorus are the challenges at present. Therefore, it is urgent to develop innovative method in struvite crystallization process for efficient nitrogen and phosphorus recovery. This study proposed a crystallization method to reduce the loss of nitrogen and phosphorus by a struvite fluidized bed reactor (FBR) with optimized structure and operation conditions. The properties of struvite obtained under various conditions in the reactor were studied, and the internal operating conditions of the reactor were simulated with COMSOL Multiphysics to verify the effectiveness of the reactor optimization. This reactor achieved stable operation under the conditions of N/P = 1:1 and pH = 9.0. The purity of struvite obtained reached 98.5%, the conversion rate of ammonia nitrogen reached 97.2%, and struvite crystals could grow to 84 µm within 24 h. The simulation results showed that the Venturi tubes installed at multiple locations increased the turbulent energy to 4 × 10-4 m2/s2, which greatly improved the mass transfer efficiency. The trajectory of the crystal particles was consistent with the fluid flow field, which promoted the purification and growth of the crystal. In general, the new FBR with enhanced external recirculation would be a very feasible way to improve crystal growth and crystal purification of struvite, and it could enhance the recovery efficiency of nitrogen and phosphorus with reduced cost.

Spatial Distribution and Source of Inorganic Elements in PM2.5 During a Typical Winter Haze Episode in Guilin, China.

Guilin, a famous tourist city, is located in northeast Guangxi Province of Southwest China. However, recently, abnormal haze events occurred frequently in the winter. To characterize inorganic elements in PM2.5 and associated sources during a winter haze episode, 30 samples were collected from 6 sites in Guilin from December 16 to 20, 2016, and 24 inorganic elements were measured using an inductively coupled plasma mass spectrometer. The results showed that the sum of 24 inorganic elements varied from 5.47 ± 0.45 to 9.26 ± 0.73 µg m-3, and accounting for 6.81% ± 13.35% to 8.63% ± 15.05% of PM2.5 at all sites. Among them, crustal elements, including K, Ca, Na, Mg, Al, Fe, and Ti contributed approximately 82% ± 6%-90% ± 3%. Cluster results combined the coefficient of divergence and hierarchical cluster for inorganic elements and the sites showed that YS designated as the background site had obvious spatial heterogeneity, specially, mass concentration, and Igeo (index of geoaccumulation) values of Ni, Cr, Mo, and Ba were higher than those at the other five sites, which indicating that PM2.5 in Guilin was significantly affected by interregional transport. The results of source apportionment showed that Al, Ti, B, Fe, Ca, Mg, and Cr were derived from road and building dust, whereas Sb, As, and Hg originated from coal combustion, Co and V from vehicle emission (such as diesel and gasoline combustion), and other metals (Zn, Pb, Mn, Ba, Cu, Ni, Se, Cd, Mo, Tl, K, and Na) from coal combustion and industrial processes.

Performance, microbial community and inhibition kinetics of long-term Cu2+ stress on an air-lift nitritation reactor with self-recirculation.

Biological nitrogen removal process could be affected due to the presence of heavy metals owing to their toxicity and accumulation in the sludge. In this study, the impact of Cu2+ shock on a long-term nitritation operation was investigated in an air-lift reactor with self-recirculation. Both the dynamics of microbial community and inhibition kinetics under Cu2+ stress were ascertained. The results showed that Cu2+ exerted severe inhibition on nitritation performance of an air-lift reactor (ALR) at 25 mg/L. The corresponding NH4+-N removal efficiency decreased to below 50%, which was mainly due to the variation of microbial community structure, especially the inhibition of nitrifiers like Nitrosomonas (the relative abundance decreased from 30% to 1% after Cu2+ inhibition). Kinetic parameters were obtained and compared after fitting the Haldane model. The long-term Cu2+ stress on the ALR aggravated the ammonium affinity and the resistance to substrate self-inhibition of the nitritation sludge, but reduced the resistance to Cu2+ inhibition. Furthermore, Cu2+ acted as uncompetitive inhibitor on nitritation process. Our results provide new insights into the nitritation characteristics under long-term Cu2+ stress.

Long-term domestication to Mn stresses alleviates the inhibition on anammox process.

Heavy metals such as Mn2+ are common contaminants in ammonium-rich wastewater. The information of Mn2+ effect on anammox process needs further investigation. The short- and long-term effects of Mn2+ on anammox were explored by anammox granular sludge. Batch tests showed that the half inhibition value (IC50 ) of Mn2+ was 4.83 mg/L. The anammox activity was severely inhibited in 0.5 hr under 15 mg/L Mn2+ . However, after long-term domestication by increasing the concentration of Mn2+ , both the low-load reactor (R1) and the high-load reactor (R2) performed well, achieving volumetric nitrogen removal rate of 6.36 kg/(m3 ·d) and 13.99 kg/(m3 ·d), respectively. The average ammonium and nitrite removal efficiency of both reactors under 200 mg/L Mn still maintained above 90%. The results from long-term reactors' operation showed that the serious inhibition effect indicated by the batch test was significantly exaggerated. The granules became dispersed after long-term operation in the high-load reactor (R2) which might be correlated to the high osmotic pressure caused by high Mn2+ load, and the mechanism needs to be investigated further. PRACTITIONER POINTS: The half inhibition value of Mn2+ on anammox sludge was 4.83 mg/L in batch experiment. 200 mg/L Mn2+ did not cause any inhibition on anammox process during long-term operation. Granular sludge is finer under high nitrogen loads with 200 mg/L Mn stress.

Quantitative determination of cavitation formation and sludge flotation in Anammox granules by using a new diffusion-reaction integrated mathematical model.

The granulation of anaerobic ammonium oxidation (Anammox) biomass plays a key role in high rate performance of upflow-type Anammox reactors. However, the formation of cavitation inside granules may result in sludge flotation problem, which negatively affects the operation stability. For quantitative evaluation of the Anammox granules flotation in upflow reactors, an integrated mathematical model was formulated based on the principles that the limitation of substrate diffusion would result in bacterial starvation, lysis and subsequently aiding the formation of cavitation in the inner zone of granules. The proposed model is used to investigate the possible mechanism of cavitation formation and granules flotation. The combined modelling and experimental results showed that the model predictions matched well with the actual floating behavior of granules (R2 = 0.83 for settled sludge and 0.76 for floating sludge). Based on the model results, the granule flotation could be divided into three zones namely (i) no-flotation zone (no flotation occurrence), (ii) transition zone (flotation with a part of granules), and (iii) flotation zone (inevitable flotation occurrence). The floating behavior of granules was mainly influenced by granule diameter (2.5-4.5 mm) and substrate concentration (NO2-N, 50-250 mg/L) in the transition zone. The optimum granule diameter to avoid flotation but with excellent settling performance was identified around 2.5 mm. Additionally, the granule size is more sensitivity to flotation than substrate concentration. Hence, controlling the size of granules is more important to alleviate granule flotation in Anammox reactors' operation.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction.

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.