Simultaneous Phenol Removal and Resource Recovery from Phenolic Wastewater by Electrocatalytic Hydrogenation

Synergy of atomic hydrogen reduction and reactive oxygen species oxidation over confined Mn bifunctional site for electrocatalytic deep mineralization

Traditional reduction or oxidation processes generating one-component free radicals face challenges in deep dechlorination and mineralization of chlorophenols from wastewater. Herein, an efficient electrocatalytic process has been developed, which couples atomic H* reduction with reactive oxidation species (•OH and 1O2) oxidation on a bifunctional cathode for 4 -chlorophenol (4 -CP) removal. The N - doped carbon nanotubes encapsulated manganese nanoparticles was fabricated as cathode, which could generate atomic H* , initiating nucleophilic hydrodechlorination in presence of confined MnO sites. Subsequently, electrophilic oxidation by generating mainly 1O2 on confined Mn7C3 sites and •OH on confined MnO sites, facilitating the oxidative processes. Experimental results and theory calculations demonstrated that reductive dechlorination and oxidative mineralization processes could mutually promote each other, resulting in an enhancement factor of 2.90. At pH 7, this process achieved 100 % removal for 4 -CP, 84 % dechlorination, 76 % total organic carbon (TOC) removal and low energy consumption (0.76 kWh g-1TOC) within 120 min. Notably, TOC for chlorophenols containing Cl substituents at different positions and real lake water containing 4 -CP could be almost completely removed. This research establishes confined non-noble bifunctional active sites that synergistically enhance reductive dechlorination and oxidative degradation processes, holding significant treatment potential for application in deep mineralization of organochlorine from water/wastewater.

Macrocyclic porphyrin photocatalysts without metal chelation: A novel pathway for complete degradation of tough halophenols with longwave visible LED light source

Halophenols are toxic and persistent pollutants in water environments which poses harm to various organisms. Due to their high stability and long residence time, ultraviolet radiation, heavy metals and oxidizing agents have been largely adopted on treating these compounds. However, these treatment methods could pose toxicity or hazardous risks to the marine environment and plant operators. In this study, a water-soluble porphyrin photocatalyst was synthesized and introduced for halophenol treatment using UV-free LED white light. The porphyrin catalyst is a macrocyclic ring consisting of pyrroles linked with methine bridges, the highly conjugated ring provided the superior functionality of visible light absorption. Surprisingly, over 99 % degradation of halophenols and over 90 % dehalogenation have been achieved without metal chelation, even higher than those of transition metal porphyrins with inclusion of Fe3+, Zn2+, Cu2+, Co2+, Ni2+, and Mn2+. Ring-opening reactions were confirmed with the formation of carboxylic acids; dicarboxylic acids like acrylic acid, and malonic acid; while fumaric acid was the main product. Total organic carbon results indicated no CO2 produced during the reaction. Triplet absorbance and scavenger studies also indicated that singlet oxygen and conduction band electrons are the main radical species for halophenol degradation. The 100-fold singlet emission quenching over triplet absorption quenching indicated that the excited electrons tend to be transferred via singlet state. This concept brings along new approaches detoxifying halophenol-related wastewater without UV, metals and other additives, which is more environmentally-friendly and sheds light to the conversion of toxic materials into useful chemical precursors.

Energy Recovery from Hexavalent Chromium Reduction for In Situ Electrocatalytic Hydrogen Peroxide Production

Recovering chemical energy embedded in pollutants is significant in achieving carbon-neutral industrial wastewater treatment. Considering that industrial wastewater is usually treated in a decentralized manner, in situ utilization of chemical energy to achieve waste-to-treasure should be given priority. Herein, the chemical energy released by the electroreduction of Cr(VI) was used to enhance on-site H2O2 generation in a stacked flow-through electrochemical system. The driving force of water flow efficiently coupled O2 evolution with 2-e O2 reduction to facilitate H2O2 generation by transporting anode-produced O2 to the cathode. Meanwhile, the chemical energy released by Cr(VI) promoted O2 evolution and impeded H2 evolution by regulating the electrode potentials, accounting for the enhanced H2O2 generation. The system could completely reduce 10-100 ppm of Cr(VI), reaching the maximum H2O2 concentration of 2.41 mM. In particular, the H2O2 concentrations in the Cr(VI)-containing electrolyte were 10.6-88.1% higher than those in the Cr(VI) free electrolyte at 1.8-2.5 V. A 24-day continuous experiment demonstrated the high efficiency and stability of the system, achieving a 100% reduction efficiency for 100 ppm of Cr(VI) and producing ∼1.5 mM H2O2 at 1.8 V. This study presents a feasible strategy for Cr(VI) detoxification and synchronous on-site H2O2 generation, providing a new perspective for innovative Cr(VI) wastewater treatment toward resource utilization.

Recent Progress in Electrocatalytic Conversion of Lignin: From Monomers, Dimers, to Raw Lignin

Lignin, as the second largest renewable biomass resource in nature, has increasingly received significant interest for its potential to be transformed into valuable chemicals, potentially contributing to carbon neutrality. Among different approaches, renewable electricity-driven biomass conversion holds great promise to substitute a petroleum resource-driven one, owing to its characteristics of environmental friendliness, high energy efficiency, and tunable reactivity. The challenges lie on the polymeric structure and complex functional groups in lignin, requiring the development of efficient electrocatalysts for lignin valorization with enhanced activity and selectivity toward targeted chemicals. In this Review, we focus on the advancement of electrocatalytic valorization of lignin, from monomers, to dimers and to raw lignin, toward various value-added chemicals, with emphasis on catalyst design, reaction innovation, and mechanistic study. The general strategies for catalyst design are also summarized, offering insights into enhancing the activity and selectivity. Finally, challenges and perspectives for the electrocatalytic conversion of lignin are proposed.

Two-dimensional metal-organic framework nanozyme-mediated portable paper-based analytical device for dichlorophen assay

The metal-organic frameworks (MOFs) nanozyme-mediated paper-based analytical devices (PADs) have shown great potential in portable visual determination of phenolic compounds in the environment. However, most MOF nanozymes suffer from poor dispersibility and block-like structure, which often prompts deposition and results in diminished enzymatic activity, severely hindering their environmental applications. Here, we proposed colorimetric PADs for the visual detection of dichlorophen (Dcp) based on its significant inhibitory effect on the two-dimensional (2D) MOF nanozyme activity. Specifically, we synthesized a 2D Cu TCPP (Fe) (defined as 2D-CTF) MOF nanozyme exhibiting excellent dispersibility and remarkable peroxidase-like (POD-like) activity, which could catalyze the oxidation and subsequent color change of 3,3',5,5'-tetramethylbenzidine even under neutral conditions. Notably, the POD-like activity of 2D-CTF demonstrated a unique response to Dcp because of the occupation of Fe-N4 active sites on the 2D-CTF. This property enables the use of 2D-CTF as a highly efficient catalyst to develop colorimetric PADs for naked-eye and portable detection of Dcp. We believe that the proposed colorimetric PADs offer an efficient method for Dcp assay and open fresh avenues for the advancement of colorimetric sensors for analyzing of phenolic toxic substances in real samples.

Mechanism of Electrocatalytic H2 Evolution, Carbonyl Hydrogenation, and Carbon-Carbon Coupling on Cu

Aqueous-phase electrocatalytic hydrogenation of benzaldehyde on Cu leads not only to benzyl alcohol (the carbonyl hydrogenation product), but Cu also catalyzes carbon-carbon coupling to hydrobenzoin. In the absence of an organic substrate, H2 evolution proceeds via the Volmer-Tafel mechanism on Cu/C, with the Tafel step being rate-determining. In the presence of benzaldehyde, the catalyst surface is primarily covered with the organic substrate, while H* coverage is low. Mechanistically, the first H addition to the carbonyl O of an adsorbed benzaldehyde molecule leads to a surface-bound hydroxy intermediate. The hydroxy intermediate then undergoes a second and rate-determining H addition to its α-C to form benzyl alcohol. The H additions occur predominantly via the proton-coupled electron transfer mechanism. In a parallel reaction, the radical α-C of the hydroxy intermediate attacks the electrophilic carbonyl C of a physisorbed benzaldehyde molecule to form the C-C bond, which is rate-determining. The C-C coupling is accompanied by the protonation of the formed alkoxy radical intermediate, coupled with electron transfer from the surface of Cu, to form hydrobenzoin.

Competitive mechanism of salt-tolerance/degradation-performance of organic pollutant in bacteria: Na+/H+ antiporters contribute to salt-stress resistance but impact phenol degradation

Phenolic-laden wastewater is typically characterized by its high toxicity and high salinity, imposing serious limits on the application of bioremediation. Although a few halotolerant microorganisms have been reported to degrade phenol, their removal efficiency on high concentrations of phenol remains unsatisfactory. What's more, the deep interaction molecular mechanism of salt-tolerance/phenol-degradation performance has not been clearly revealed. Here, a halotolerant strain Aeribacillus pallidus W-12 employed a meta-pathway to efficiently degrade high concentration of phenol even under high salinity conditions. Investigation of salt-tolerance strategy indicated that four Na+/H+ antiporters, which are widely distributed in bacteria, synergistically endowed the strain with excellent salt adaptability. All these antiporters differentially but positively responded to salinity changes and induction of phenol, forming a synergistic transport effect on salt ions and phenol. In-depth analysis revealed a competitive relationship between salt tolerance and degradation performance, which significantly impaired the degradation efficiency at relatively high salinity. The efficient degradation performance of W-12 under different phenol concentrations and salinity conditions indicated its bioremediation potential for multiple types of phenolic wastewater. Collectively, the competitive mechanism of salt tolerance and degradation performance enlightens a new strategy of introducing or re-constructing Na+/H+ antiporters to further improve bioremediation efficiency of hypersaline organic wastewater.

Amperometry for real-time and on-site monitoring of phenol and H2O2 during the treatments

In conventional wastewater treatment processes, a predetermined quantity of chemicals is introduced at the onset, without ongoing monitoring of the treatment progress. Thus, it is difficult to perform timely intervention in the treatment process. Herein, we develop an amperometry-guided wastewater treatment strategy based on a green oxidation process with H2O2 and an iron-tetraamidomacrocyclic ligand (Fe-TAML) catalyst. During the process, users can monitor both phenol and H2O2 concentrations in real time and then intervene by adding more H2O2 to accelerate the reaction. As a proof of concept, a wastewater sample containing 9.3 ppm of phenol is treated by using the amperometry-guided strategy with 1 dosage of Fe-TAML (0.45 ppm) and 3 dosages of H2O2 (1.86 ppm). After the treatment, phenol concentration in the wastewater decreases to 0 ppm after 21 min. In contrast, with only 1 dosage of Fe-TAML (0.45 ppm) and 1 dosage of H2O2 (1.86 ppm), the reaction slows down after 5 min and stops prematurely. After that, the reaction kinetics of ppb-level phenol are investigated, in which the phenol rate and the rate constant are estimated. Compared to conventional detections, the designed amperometry shows faster response, lower limit of detection (LOD, phenol: 11 ppb, H2O2: 80 ppb) and consumable cost, easier operation, and no pollution generated. This example demonstrates the importance of early intervention during wastewater treatment with the help of real-time information.

Controllable Pyridine N-Oxidation-Nucleophilic Dechlorination Process for Enhanced Dechlorination of Chloropyridines: The Cooperation of HCO4- and HO2. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2024;58:4438-4449. [PMID: 38330552 DOI: 10.1021/acs.est.3c09878] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Dechlorination of chloropyridines can eliminate their detrimental environmental effects. However, traditional dechlorination technology cannot efficiently break the C-Cl bond of chloropyridines, which is restricted by the uncontrollable nonselective species. Hence, we propose the carbonate species-activated hydrogen peroxide (carbonate species/H2O2) process wherein the selective oxidant (peroxymonocarbonate ion, HCO4-) and selective reductant (hydroperoxide anion, HO2-) controllably coexist by manipulation of reaction pH. Taking 2-chloropyridine (Cl-Py) as an example, HCO4- first induces Cl-Py into pyridine N-oxidation intermediates, which then suffer from the nucleophilic dechlorination by HO2-. The obtained dechlorination efficiencies in the carbonate species/H2O2 process (32.5-84.5%) based on the cooperation of HCO4- and HO2- are significantly higher than those in the HO2--mediated sodium hydroxide/hydrogen peroxide process (0-43.8%). Theoretical calculations confirm that pyridine N-oxidation of Cl-Py can effectively lower the energy barrier of the dechlorination process. Moreover, the carbonate species/H2O2 process exhibits superior anti-interference performance and low electric energy consumption. Furthermore, Cl-Py is completely detoxified via the carbonate species/H2O2 process. More importantly, the carbonate species/H2O2 process is applicable for efficient dehalogenation of halogenated pyridines and pyrazines. This work offers a simple and useful strategy to enhance the dehalogenation efficiency of halogenated organics and sheds new insights into the application of the carbonate species/H2O2 process in practical environmental remediation.

Selective electrocatalytic transformation of highly toxic phenols in wastewater to para-benzoquinone at ambient conditions

The selective transformation of organics from wastewater to value-added chemicals is considered an upcycling process beneficial for carbon neutrality. Herein, we present an innovative electrocatalytic oxidation (ECO) system aimed at achieving the selective conversion of phenols in wastewater to para-benzoquinone (p-BQ), a valuable chemical widely utilized in the manufacturing and chemical industries. Notably, 96.4% of phenol abatement and 78.9% of p-BQ yield are synchronously obtained over a preferred carbon cloth-supported ruthenium nanoparticles (Ru/C) anode. Such unprecedented results stem from the weak Ru-O bond between the Ru active sites and generated p-BQ, which facilitates the desorption of p-BQ from the anode surface. This property not only prevents the excessive oxidation of the generated p-BQ but also reinstates the Ru active sites essential for the rapid ECO of phenol. Furthermore, this ECO system operates at ambient conditions and obviates the need for potent chemical oxidants, establishing a sustainable avenue for p-BQ production. Importantly, the system efficacy can be adaptable in actual phenol-containing coking wastewater, highlighting its potential practical application prospect. As a proof of concept, we construct an electrified Ru/C membrane for ECO of phenol, attaining phenol removal of 95.8% coupled with p-BQ selectivity of 73.1%, which demonstrates the feasibility of the ECO system in a scalable flow-through operation mode. This work provides a promising ECO strategy for realizing both phenols removal and valuable organics recovery from phenolic wastewater.

Critical Review of Pd-Catalyzed Reduction Process for Treatment of Waterborne Pollutants

Catalyzed reduction processes have been recognized as important and supplementary technologies for water treatment, with the specific aims of resource recovery, enhancement of bio/chemical-treatability of persistent organic pollutants, and safe handling of oxygenate ions. Palladium (Pd) has been widely used as a catalyst/electrocatalyst in these reduction processes. However, due to the limited reserves and high cost of Pd, it is essential to gain a better understanding of the Pd-catalyzed decontamination process to design affordable and sustainable Pd catalysts. This review provides a systematic summary of recent advances in understanding Pd-catalyzed reductive decontamination processes and designing Pd-based nanocatalysts for the reductive treatment of water-borne pollutants, with special focus on the interactions and transformation mechanisms of pollutant molecules on Pd catalysts at the atomic scale. The discussion begins by examining the adsorption of pollutants onto Pd sites from a thermodynamic viewpoint. This is followed by an explanation of the molecular-level reaction mechanism, demonstrating how electron-donors participate in the reductive transformation of pollutants. Next, the influence of the Pd reactive site structure on catalytic performance is explored. Additionally, the process of Pd-catalyzed reduction in facilitating the oxidation of pollutants is briefly discussed. The longevity of Pd catalysts, a crucial factor in determining their practicality, is also examined. Finally, we argue for increased attention to mechanism study, as well as precise construction of Pd sites under batch synthesis conditions, and the use of Pd-based catalysts/electrocatalysts in the treatment of concentrated pollutants to facilitate resource recovery.

Heterogenous Iron Oxide Assemblages for Use in Catalytic Ozonation: Reactivity, Kinetics, and Reaction Mechanism

Heterogeneous catalytic ozonation (HCO) has gained increasing attention as an effective process to remove refractory organic pollutants from industrial effluents. However, widespread application of HCO is still limited due to the typically low efficacy of catalysts used and matrix passivation effects. To this end, we prepared an Al2O3-supported Fe catalyst with high reactivity via a facile urea-based heterogeneous precipitation method. Due to the nonsintering nature of the preparation method, a heterogeneous catalytic layer comprised of γ-FeOOH and α-Fe2O3 is formed on the Al2O3 support (termed NS-Fe-Al2O3). On treatment of a real industrial effluent by HCO, the presence of NS-Fe-Al2O3 increased the removal of organics by ∼100% compared to that achieved with a control catalyst (i.e., α-Fe2O3/Al2O3 or γ-FeOOH/Al2O3) that was prepared by a conventional impregnation and calcination method. Furthermore, our results confirmed that the novel NS-Fe-Al2O3 catalyst demonstrated resistance to the inhibitory effect of high concentration of chloride and sulfate ions usually present in industrial effluent. A mathematical kinetic model was developed that adequately describes the mechanism of HCO process in the presence of NS-Fe-Al2O3. Overall, the results presented here provide valuable guidance for the synthesis of effective and robust catalysts that will facilitate the wider industrial application of HCO.

Enhanced Hydrosaturation Selectivity and Electron Transfer for Electrocatalytic Chlorophenols Hydrogenation on Ru Sites

Electrocatalytic hydrogenation is acknowledged as a promising strategy for chlorophenol dechlorination. However, the widely used Pd catalysts exhibit drawbacks, such as high costs and low selectivity for phenol hydrosaturation. Herein, we demonstrate the potential and mechanism of Ru in serving as a Pd substitute using 2,4,6-trichlorophenol (TCP) as a model pollutant. Up to 99.8% TCP removal efficiency and 99% selectivity to cyclohexanol, a value-added compound with an extremely low toxicity, were achieved on the Ru electrode. In contrast, only 66% of TCP was removed on the Pd electrode, with almost no hydrosaturation selectivity. The superiority of Ru over Pd was especially noteworthy in alkaline conditions or the presence of interfering species such as S2-. The theoretical simulation demonstrates that Ru possesses a hydrodechlorination energy barrier of 0.72 eV, which is comparable to that on Pd. Meanwhile, hydrosaturation requires an activation energy of 0.69 eV on Ru, which is much lower than that on Pd (0.92 eV). The main reaction mechanism on Ru is direct electron transfer, which is distinct from that on Pd (indirect pathway via atomic hydrogen, H*). This work thereby provides new insights into designing cost-effective electrocatalysts for halogenated phenol detoxification and resource recovery.

Unveiling the spatially confined oxidation processes in reactive electrochemical membranes

Electrocatalytic oxidation offers opportunities for sustainable environmental remediation, but it is often hampered by the slow mass transfer and short lives of electro-generated radicals. Here, we achieve a four times higher kinetic constant (18.9 min-1) for the oxidation of 4-chlorophenol on the reactive electrochemical membrane by reducing the pore size from 105 to 7 μm, with the predominate mechanism shifting from hydroxyl radical oxidation to direct electron transfer. More interestingly, such an enhancement effect is largely dependent on the molecular structure and its sensitivity to the direct electron transfer process. The spatial distributions of reactant and hydroxyl radicals are visualized via multiphysics simulation, revealing the compressed diffusion layer and restricted hydroxyl radical generation in the microchannels. This study demonstrates that both the reaction kinetics and the electron transfer pathway can be effectively regulated by the spatial confinement effect, which sheds light on the design of cost-effective electrochemical platforms for water purification and chemical synthesis.

Bi-functional biochar-g-C3N4-MgO composites for simultaneously minimizing pollution：Photocatalytic degradation of pesticide and phosphorus recovery as slow-release fertilizer

Significant progress has been made in the development of phosphorus recovery adsorbents and photocatalysts for degradation of pesticides. However, the bifunctional materials for phosphorus recovery and photocatalytic degradation of pesticides have not been designed, and the mechanism of the interaction between photocatalysis and P adsorption remains unexplored. Herein, we develop biochar-g-C3N4-MgO composites (BC-g-C3N4-MgO) with bi-function application to minimize water toxicity and eutrophication. The results show phosphorus adsorption capacity of the BC-g-C3N4-MgO composite reaches 111.0 mg·g-1, and its degradation ratio of dinotefuran reaches 80.1% within 260 min. The mechanism studies show that MgO can play variety roles in BC-g-C3N4-MgO composite, in which can improve the adsorption capacity of phosphorus, enhance the utilization efficiency of visible light and the separation efficiency of photoinduced electron-hole pairs. The biochar existed in BC-g-C3N4-MgO serves as charge transporter with a good conductivity, which promotes the fluent transfer of photo-generated charge carriers. The ESR indicates that both •O2- and •OH generated from BC-g-C3N4-MgO are responsible for dinotefuran degradation. Finally, pot experiments reveal that P laden BC-g-C3N4-MgO promotes the growth of pepper seedlings with high P utilization efficiency of 49.27%.

Electrochemical reduction of wastewater by non-noble metal cathodes: From terminal purification to upcycling recovery

A shift beyond conventional environmental remediation to a sustainable pollutant upgrading conversion is extremely desirable due to the rising demand for resources and widespread chemical contamination. Electrochemical reduction processes (ERPs) have drawn considerable attention in recent years in the fields of oxyanion reduction, metal recovery, detoxification and high-value conversion of halogenated organics and benzenes. ERPs also have the potential to address the inherent limitations of conventional chemical reduction technologies in terms of hydrogen and noble metal requirements. Fundamentally, mechanisms of ERPs can be categorized into three main pathways: direct electron transfer, atomic hydrogen mediation, and electrode redox pairs. Furthermore, this review consolidates state-of-the-art non-noble metal cathodes and their performance comparable to noble metals (e.g., Pd, Pt) in electrochemical reduction of inorganic/organic pollutants. To overview the research trends of ERPs, we innovatively sort out the relationship between the electrochemical reduction rate, the charge of the pollutant, and the number of electron transfers based on the statistical analysis. And we propose potential countermeasures of pulsed electrocatalysis and flow mode enhancement for the bottlenecks in electron injection and mass transfer for electronegative pollutant reduction. We conclude by discussing the gaps in the scientific and engineering level of ERPs, and envisage that ERPs can be a low-carbon pathway for industrial wastewater detoxification and valorization.

Targeted Decomplexation of Metal Complexes for Efficient Metal Recovery by Ozone/Percarbonate

Traditional methods cannot efficiently recover Cu from Cu(II)-EDTA wastewater and encounter the formation of secondary contaminants. In this study, an ozone/percarbonate (O₃/SPC) process was proposed to efficiently decomplex Cu(II)-EDTA and simultaneously recover Cu. The results demonstrate that the O₃/SPC process achieves 100% recovery of Cu with the corresponding k_obs value of 0.103 min^-1 compared with the typical ^•OH-based O₃/H₂O₂ process (81.2%, 0.042 min^-1). The carbonate radical anion (CO₃^•-) is generated from the O₃/SPC process and carries out the targeted attack of amino groups of Cu(II)-EDTA for decarboxylation and deamination processes, resulting in successive cleavage of Cu-O and Cu-N bonds. In comparison, the ^•OH-based O₃/H₂O₂ process is predominantly responsible for the breakage of Cu-O bonds via decarboxylation and formic acid removal. Moreover, the released Cu(II) can be transformed into stable copper precipitates by employing an endogenous precipitant (CO₃^2-), accompanied by toxic-free byproducts in the O₃/SPC process. More importantly, the O₃/SPC process exhibits excellent metal recovery in the treatment of real copper electroplating wastewater and other metal-EDTA complexes. This study provides a promising technology and opens a new avenue for the efficient decomplexation of metal-organic complexes with simultaneous recovery of valuable metal resources.

Achieving High-Efficient Photoelectrocatalytic Degradation of 4-Chlorophenol via Functional Reformation of Titanium-Oxo Clusters

Rational design of crystalline catalysts with superior light absorption and charge transfer for efficient photoelectrocatalytic (PEC) reaction coupled with energy recovery remains a great challenge. In this work, we elaborately construct three stable titanium-oxo clusters (TOCs, Ti₁₀Ac₆, Ti₁₀Fc₈, and Ti₁₂Fc₂Ac₄) modified with a monofunctionalized ligand (9-anthracenecarboxylic acid (Ac) or ferrocenecarboxylic acid (Fc)) and bifunctionalized ligands (Ac and Fc). They have tunable light-harvesting and charge transfer capacities and thus can serve as outstanding crystalline catalysts to achieve efficient PEC overall reaction, that is, the integration of anodic organic pollutant 4-chlorophenol (4-CP) degradation and cathodic wastewater-to-H₂ conversion. These TOCs can all exhibit very high PEC activity and degradation efficiency of 4-CP. Especially, Ti₁₂Fc₂Ac₄ decorated with bifunctionalized ligands exhibits better PEC degradation efficiency (over 99%) and H₂ generation than Ti₁₀Ac₆ and Ti₁₀Fc₈ modified with a monofunctionalized ligand. The study of the 4-CP degradation pathway and mechanism revealed that such better PEC performance of Ti₁₂Fc₂Ac₄ is probably due to its stronger interactions with the 4-CP molecule and better ^•OH radical production. This work not only presents the effective combination of organic pollutant degradation and simultaneously H₂ evolution reaction using crystalline coordination clusters as both anodic and cathodic catalyst but also develops a new PEC application for crystalline coordination compounds.

Magnetic reed biochar materials as adsorbents for aqueous copper and phenol removal

Organics and heavy metals are common pollutants in many wastewaters and water bodies. Adsorption processes by magnetic materials can rapidly remove these pollutants from water and effectively recycle adsorbent. In this study, magnetic analyzer, X-ray diffraction, Flourier transform infrared spectroscopy, and granulometry were used to characterize the synthesized magnetic reed biochar materials (ZnFe₂O₄/biochar). Influences of adsorption time, pH, temperature, initial solution concentration, and adsorption equilibrium concentration on adsorption performances were investigated for Cu²⁺ and phenol adsorption by ZnFe₂O₄/biochar. Adsorption kinetic and isotherm models were used to describe the adsorption processes. Adsorption of phenol and Cu²⁺ by ZnFe₂O₄/biochar reached saturation within 45 min and increased slightly with the increase of temperature from 15 to 45 °C. Adsorption of Cu²⁺ increased with the increase of pH, while the adsorption of phenol peaked at pH = 6. The adsorption processes fit the pseudo-second order kinetics model, and both conformed to the Langmuir model. The fitting results show that the maximum single-component adsorption capacity of phenol and Cu²⁺ by ZnFe₂O₄/biochar is 63.29 and 12.20 mg/g, and the maximum bi-component adsorption capacity reaches 40.16 and 9.48 mg/g, respectively. All the findings demonstrate that ZnFe₂O₄/biochar has good adsorption performance for phenol and Cu²⁺.

Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration-Bipolar Membrane Electrodialysis-Based Closed-Loop Process

Traditional technologies such as precipitation and coagulation have been adopted for fluoride-rich and silica-rich wastewater treatment, respectively, but waste solid generation and low wastewater processing efficiency are still the looming concern. Efficient resource recovery technologies for different wastewater treatments are scarce for environment and industry sustainability. Herein, a resource capture ultrafiltration-bipolar membrane electrodialysis (RCUF-BMED) system was designed into a closed-loop process for simultaneous capture and recovery of fluoride and silica as sodium silicofluoride (Na₂SiF₆) from mixed fluoride-rich and silica-rich wastewaters, as well as achieving zero liquid discharge. This RCUF-BMED system comprised two key parts: (1) capture of fluoride and silica from two wastewaters using acid, and recovery of the Na₂SiF₆ using base by UF and (2) UF permeate conversion for acid/base and freshwater generation by BMED. With the optimized RCUF-BMED system, fluoride and silica can be selectively captured from wastewater with removal efficiencies higher than 99%. The Na₂SiF₆ recovery was around 72% with a high purity of 99.1%. The aging and cyclic experiments demonstrated the high stability and recyclability of the RCUF-BMED system. This RCUF-BMED system has successfully achieved the conversion of toxic fluoride and silica into valuable Na₂SiF₆ from mixed wastewaters, which shows great application potential in the industry-resource-environment nexus.

Cooperation of multiple active species generated in hydrogen peroxide activation by iron porphyrin for phenolic pollutants degradation

The narrow acid pH range and the nonselectivity of the dominant •OH limit the Fenton systems to remediate the organic wastewater. Inspired by the role of heme in physiological processes, we employed iron porphyrin as a novel homogeneous catalyst to address this issue. Multiple active species are identified during the activation of H₂O₂, including high-valent iron porphyrin ((por)Fe(IV)) species ((por)Fe(IV)-OH, (por)^+•Fe(IV)=O) and oxygen-centered radicals (•OH, HO₂•/•O₂^-), as well as atomic hydrogen (*H) and carbon-centered radicals. With the cooperation of these active species, the degradation of pollutants could be resistant to the interference of concomitant ions and proceed over a wide pH range. This cooperative behavior is further verified by intermediates identified from bisphenol A degradation. Specifically, the presence of *H could facilitate the cleavage of the C-C bond and the addition of unsaturated or aromatic molecules. (Por)^+•Fe(IV)=O could hydroxylate substrates with an oxygen rebound mechanism. Hydrogen atom abstraction of contaminants could be performed by (por)Fe(IV)-OH to form desaturated products by attacking oxygen-centered radicals. The ecotoxicity of bisphenol A could be significantly decreased through degradation. This study would provide a new approach to wastewater treatment and shed light on the interaction between metalloporphyrin and peroxide in an aqueous solution.

Electrocatalytic Hydrogenation Boosts Reduction of Nitrate to Ammonia over Single-Atom Cu with Cu(I)-N3C1 Sites

The conversion of nitrate to ammonia can serve two important functions: mitigating nitrate pollution and offering a low energy intensity pathway for ammonia synthesis. Conventional ammonia synthesis from electrocatalytic nitrate reduction reactions (NO₃RR) is often impeded by incomplete nitrate conversion, sluggish kinetics, and the competition of hydrogen evolution reactions. Herein, atomic Cu sites anchored on micro-/mesoporous nitrogen-doped carbon (Cu MNC) with fine-tuned hydrophilicity, micro-/mesoporous channels, and abundant Cu(I) sites were synthesized for selective nitrate reduction to ammonia, achieving ambient temperature and pressure hydrogenation of nitrate. Laboratory experiments demonstrated that the catalyst has an ammonia yield rate per active site of 5466 mmol g_Cu^-1 h^-1 and transformed 94.8% nitrate in wastewater containing 100 mg-N L^-1 to near drinking water standard (MCL of 5 mg-N L^-1) at -0.64 V vs RHE. Extended X-ray absorption fine structure (EXAFS) and theoretical calculations showed that the coordination environment of Cu(I) sites (Cu(I)-N₃C₁) localizes the charge around the central Cu atoms and adsorbs *NO₃ and *H onto neighboring Cu and C sites with balanced adsorption energy. The Cu(I)-N₃C₁ moieties reduce the activation energy of rate-limiting steps (*HNO₃ → *NO₂, *NH₂ → *NH₃) compared with conventional Cu(II)-N₄ and lead to a thermodynamically favorable process to NH₃. The as-prepared electrocatalytic cell can run continuously for 84 h (14 cycles) and produce 21.7 mg_NH3 with only 5.64 × 10^-3 kWh energy consumption, suitable for decentralized nitrate removal and ammonia synthesis from nitrate-containing wastewater.