Biochar Meets Single-Atom: A Catalyst for Efficient Utilization in Environmental Protection Applications and Energy Conversion.

Single-atom catalysts (SACs), combining the advantages of multiphase and homogeneous catalysis, have been increasingly investigated in various catalytic applications. Carbon-based SACs have attracted much attention due to their large specific surface area, high porosity, particular electronic structure, and excellent stability. As a cheap and readily available carbon material, biochar has begun to be used as an alternative to carbon nanotubes, graphene, and other such expensive carbon matrices to prepare SACs. However, a review of biochar-based SACs for environmental pollutant removal and energy conversion and storage is lacking. This review focuses on strategies for synthesizing biochar-based SACs, such as pre-treatment of organisms with metal salts, insertion of metal elements into biochar, or pyrolysis of metal-rich biomass, which are more simplistic ways of synthesizing SACs. Meanwhile, this paper attempts to 1) demonstrate their applications in environmental remediation based on advanced oxidation technology and energy conversion and storage based on electrocatalysis; 2) reveal the catalytic oxidation mechanism in different catalytic systems; 3) discuss the stability of biochar-based SACs; and 4) present the future developments and challenges regarding biochar-based SACs.

Mediated Peroxymonosulfate Activation at the Single Atom Fe-N3 O1 Sites: Synergistic Degradation of Antibiotics by Two Non-Radical Pathways.

The activation of persulfates to degrade refractory organic pollutants is a hot issue in advanced oxidation right now. Here, it is reported that single-atom Fe-incorporated carbon nitride (Fe-CN-650) can effectively activate peroxymonosulfate (PMS) for sulfamethoxazole (SMX) removal. Through some characterization techniques and DFT calculation, it is proved that Fe single atoms in Fe-CN-650 exist mainly in the form of Fe-N3 O1 coordination, and Fe-N3 O1 exhibited better affinity for PMS than the traditional Fe-N4 structure. The degradation rate constant of SMX in the Fe-CN-650/PMS system reached 0.472 min-1 , and 90.80% of SMX can still be effectively degraded within 10 min after five consecutive recovery cycles. The radical quenching experiment and electrochemical analysis confirm that the pollutants are mainly degraded by two non-radical pathways through 1 O2 and Fe(IV)═O induced at the Fe-N3 O1 sites. In addition, the intermediate products of SMX degradation in the Fe-CN-650/PMS system show toxicity attenuation or non-toxicity. This study offers valuable insights into the design of carbon-based single-atom catalysts and provides a potential remediation technology for the optimum activation of PMS to disintegrate organic pollutants.

Tunable schiff-based networks with different bonding sites for enhanced photocatalytic activity under visible-light irradiation: The effects of steric hindrance.

Organic polymers hold great potential in photocatalysis considering their low cost, structural tailorability, and well-controlled degree of conjugation for efficient electron transfer. Among the polymers, Schiff base networks (SNWs) with high nitrogen content have been noticed. Herein, a series of SNWs is synthesized based on the melamine units and dialdehydes with different bonding sites. The chemical and structural variation caused by steric hindrance as well as the related photoelectric properties of the SNW samples are investigated, along with the application exploration on photocatalytic degradation and energy production. The results demonstrate that only SNW-o based on o-phthalaldehyde responds to visible light, which extends to over 550 nm. SNW-o shows the highest tetracycline degradation rate of 0.02516 min-1, under 60-min visible light irradiation. Moreover, the H2O2 production of SNW-o is 2.14 times higher than that of g-C3N4. The enhanced photocatalytic activity could be ascribed to the enlarged visible light adsorption and intramolecular electron transfer. This study indicates the possibility to regulate the optical and electrical properties of organic photocatalysts on a molecular level, providing an effective strategy for rational supramolecular engineering to the applications of organic materials in photocatalysis.

Transition Metal Single-Atom Catalysts for the Electrocatalytic Nitrate Reduction: Mechanism, Synthesis, Characterization, Application, and Prospects.

Excessive accumulation of nitrate in the environment will affect human health. To combat nitrate pollution, chemical, biological, and physical technologies have been developed recently. The researcher favors electrocatalytic reduction nitrate reaction (NO3 RR) because of the low post-treatment cost and simple treatment conditions. Single-atom catalysts (SACs) offer great activity, exceptional selectivity, and enhanced stability in the field of NO3 RR because of their high atomic usage and distinctive structural characteristics. Recently, efficient transition metal-based SACs (TM-SACs) have emerged as promising candidates for NO3 RR. However, the real active sites of TM-SACs applied to NO3 RR and the key factors controlling catalytic performance in the reaction process remain ambiguous. Further understanding of the catalytic mechanism of TM-SACs applied to NO3 RR is of practical significance for exploring the design of stable and efficient SACs. In this review, from experimental and theoretical studies, the reaction mechanism, rate-determining steps, and essential variables affecting activity and selectivity are examined. The performance of SACs in terms of NO3 RR, characterization, and synthesis is then discussed. In order to promote and comprehend NO3 RR on TM-SACs, the design of TM-SACs is finally highlighted, together with the current problems, their remedies, and the way forward.

When biochar is involved in rhizosphere dissipation and plant absorption of pesticides: A meta-analysis.

Clarifying the influences of biochar input on the rhizosphere dissipation and plant absorption of pesticides is a crucial prerequisite for utilizing biochar in the restoration of pesticide-contaminated soils. Nevertheless, the application of biochar to pesticide-contaminated soils does not always achieve consistent results on the rhizosphere dissipation and plant absorption of pesticides. Under the new situation of vigorously promoting the application of biochar in soil management and carbon sequestration, a timely review is needed to further understand the key factors affecting biochar remediation of pesticide-contaminated soil. In this study, a meta-analysis was conducted utilizing variables from three dimensions of biochar, remediation treatment, and pesticide/plant type. The pesticide residues in soil and the pesticide uptake by plant were used as response variables. Biochar with high adsorption capacity can impede the dissipation of pesticides in soil and mitigate their absorption by plants. The specific surface area of biochar and the type of pesticide are critical factors that affect pesticide residues in soil and plant uptake, respectively. Applying biochar with high adsorption capacity, based on specific dosages and soil characteristics, is recommended for the remediation of continuously cultivated soil contaminated with pesticides. This article aims to provide a valuable reference and understanding for the application of biochar-based soil remediation technology and the treatment of pesticide pollution in soil.

An overview of recent advances and future prospects of three-dimensional biofilm electrode reactors (3D-BERs).

Three-dimensional biofilm electrode reactors (3D-BERs) have attracted extensive attention in recent years due to their wide application range, high efficiency and energy saving. On the basis of traditional bio-electrochemical reactor, 3D-BERs are filled with particle electrodes, also known as the third electrodes, which can not only be used as a carrier for microbial growth, but also improve the electron transfer rate of the whole system. This paper reviews the constitution, advantages and basic principles of 3D-BERs as well as current research status and progress of 3D-BERs in recent years. The selection of electrode materials, including cathode, anode and particle electrode are listed and analyzed. Different constructions of reactors, like 3D-unipolar extended reactor and coupled 3D-BERs are introduced and discussed. Various contaminants degraded by 3D-BERs including nitrogen, azo dyes, antibiotics and the others are calculated and the corresponding degradation effects are described. The influencing factors and mechanisms are also introduced. At the same time, according to the research advances of 3D-BERs, the shortcomings and weakness of this technology in the current research process are analyzed, and the future research direction of this technology is prospected. This review aims to summarize recent studies of 3D-BERs in bio-electrochemical reaction and open a bright window to this booming research theme.

Catalyst-Free Photochemical Activation of Peroxymonosulfate in Xanthene-Rich Systems for Fenton-Like Synergistic Decontamination: Efficacy of Proton Transfer Process.

Catalyst-free visible light assisted Fenton-like catalysis offers opportunities to achieve the sustainable water decontamination, but the synergistic decontamination mechanisms are still unclear, especially the effect of proton transfer process (PTP). The conversion of peroxymonosulfate (PMS) in photosensitive dye-enriched system was detailed. The photo-electron transfer between excited dye and PMS triggered the efficient activation of PMS and enhanced the production of reactive species. Photochemistry behavior analysis and DFT calculations revealed that PTP was the crucial factor to determine the decontamination performance, leading to the transformation of dye molecules. The excitation process inducing activation of whole system was composed of low energy excitations, and the electrons and holes were almost contributed by LUMO and HOMO. This work provided new ideas for the design of catalyst-free sustainable system for efficient decontamination.

Insight into disinfection byproduct formation potential of aged biochar and its effects during chlorination.

Biochar can achieve multiple benefits including solid waste management, polluted water remediation, carbon sequestration, and emission reduction. However, various environmental factors (such as temperature variations and dry-wet alternation) and microbial activity may lead to the fragmentation, dissolution, and oxidation of biochar. These accelerate the dissolution of biochar-derived dissolved organic matter (DOM) and then influence disinfection byproducts formation potential (DBPFP) throughout the water treatment process. In this paper, biochars from six biomass feedstocks with five aging processes were prepared, and the DBPFP of biochar and its derived DOM were first studied systematically. Different aging processes might increase the DBPFP of biochar by increasing DOM content and changing the fraction distribution of DOM derived from biochar. Especially, the DBPFP of biochar increased apparently with the chemical aging process. Coexisting with the environmental concentration of humic acid, even aged biochar showed the potential to reduce DBPFP and integrated toxic risk value of the mixed system. In this study, the DBPFP of biochar-derived DOM during the disinfection process is confirmed, and the results can give information to the selection of biomass feedstocks of biochar and its service life in the water treatment process.

Managing Fenton-treated sediment with biochar and sheep manure compost: Effects on the evolutionary characteristics of bacterial community.

Fenton oxidation is a widely used method for the fast and efficient treatment of contaminated sediment, but few studies have investigated the management of Fenton-treated sediment for resource utilization. In this study, the evolutionary characteristics of bacterial community composition in Fenton-treated riverine sediment were investigated using 16S rRNA gene sequencing after the incorporation of rice straw biochar and sheep manure compost. The Fenton treatment caused a decline in the relative abundance of Bacteroidetes from 39% to 8% on the 7th day, and using biochar and compost rapidly increased the relative abundance of Firmicutes from 13% to 61% and 57%, respectively. Applying 1.25 wt% biochar after the Fenton treatment contributed to high Shannon diversity indices of 4.80, 4.69, and 4.76 on the 7th, 28th, and 56th day, respectively. The reduced differences of Shannon indexes on the 56th day indicated that the bacterial diversity among different treatments tended to be similar over time. The genera Flavisolibacter and Bacillus were representatively detected on the 7th day in the untreated sediment and Fenton/biochar-treated sediment, respectively. The number of feature bacteria decreased significantly from 88 on the 7th day to 29 on the 56th day. The community functions for the carbon, nitrogen, and sulfur cycles were sensitive to the Fenton-treatment and the subsequent treatment with biochar and compost. This study may provide a useful reference for follow-up work on the remediation of contaminated sediment using advanced oxidation processes, and promote the development of resource utilization of amended sediment.

Covalent organic framework photocatalysts: structures and applications.

In the light of increasing energy demand and environmental pollution, it is urgently required to find a clean and renewable energy source. In these years, photocatalysis that uses solar energy for either fuel production, such as hydrogen evolution and hydrocarbon production, or environmental pollutant degradation, has shown great potential to achieve this goal. Among the various photocatalysts, covalent organic frameworks (COFs) are very attractive due to their excellent structural regularity, robust framework, inherent porosity and good activity. Thus, many studies have been carried out to investigate the photocatalytic performance of COFs and COF-based photocatalysts. In this critical review, the recent progress and advances of COF photocatalysts are thoroughly presented. Furthermore, diverse linkers between COF building blocks such as boron-containing connections and nitrogen-containing connections are summarised and compared. The morphologies of COFs and several commonly used strategies pertaining to photocatalytic activity are also discussed. Following this, the applications of COF-based photocatalysts are detailed including photocatalytic hydrogen evolution, CO2 conversion and degradation of environmental contaminants. Finally, a summary and perspective on the opportunities and challenges for the future development of COF and COF-based photocatalysts are given.

In Situ Grown Single-Atom Cobalt on Polymeric Carbon Nitride with Bidentate Ligand for Efficient Photocatalytic Degradation of Refractory Antibiotics.

Semiconductor photocatalysis is a promising technology to tackle refractory antibiotics contamination in water. Herein, a facile in situ growth strategy is developed to implant single-atom cobalt in polymeric carbon nitride (pCN) via the bidentate ligand for efficient photocatalytic degradation of oxytetracycline (OTC). The atomic characterizations indicate that single-atom cobalt is successfully anchored on pCN by covalently forming the CoO bond and CoN bond, which will strengthen the interaction between single-atom cobalt and pCN. This single-atom cobalt can efficiently expand optical absorption, increase electron density, facilitate charge separation and transfer, and promote OTC degradation. As the optimal sample, Co(1.28%)pCN presents an outstanding apparent rate constant for OTC degradation (0.038 min-1 ) under visible light irradiation, which is about 3.7 times than that of the pristine pCN. The electron spin resonance (ESR) tests and reactive species trapping experiments demonstrate that the 1 O2 , h+ , •O2- , and •OH are responsible for OTC degradation. This work develops a new way to construct single-atom-modified pCN and provides a green and highly efficient strategy for refractory antibiotics removal.

Black Phosphorus, a Rising Star 2D Nanomaterial in the Post-Graphene Era: Synthesis, Properties, Modifications, and Photocatalysis Applications.

Semiconductor photocatalysis, a sustainable and renewable technology, is deemed to be a new path to resolve environmental pollution and energy shortage. The development of effective photocatalysts, especially the metal-free photocatalysts, is a critical determinant of this technique. The recently emerged 2D material of black phosphorus with distinctive properties of tunable direct bandgap, ultrahigh charge mobility, fortified optical absorption, large specific surface area, and anisotropic structure has captured enormous attention since the first exfoliation of bulk black phosphorus into mono- or few layered phosphorene in 2014. In this article, the state-of-the-art preparation methods are first summarized for bulk black phosphorus, phosphorene, and black phosphorus quantum dot and then the fundamental structure and electronic and optical properties are analyzed to evaluate its feasibility as a metal-free photocatalyst. Various modifications on black phosphorus are also summarized to enhance its photocatalytic performance. Furthermore, the multifarious applications such as solar to energy conversion, organic removal, disinfection, nitrogen fixation, and photodynamic therapy are discussed and some of the future challenges and opportunities for black phosphorus research are proposed. This review reveals that the rising star of black phosphorus will be a multifunctional material in the postgraphene era.

Water-compatible halloysite-imprinted polymer by Pickering emulsion polymerization for the selective recognition of herbicides.

A water-compatible molecularly imprinted polymer was prepared by Pickering emulsion polymerization using halloysite nanotubes as stabilized solid particles. During polymerization, we used 4-vinylpyridine as monomer, divinylbenzene as cross-linking agent, toluene as porogen, 2,2-azobisisobutyronitrile as initiator, 2,4-dichlorophenoxyacetic acid as template to form the oil phase, and Triton X-100 aqueous solution to form the water phase. The halloysite nanotubes molecularly imprinted polymer was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. Kinetic and equilibrium bindings were also employed to evaluate the adsorption properties of the imprinted polymer. The imprinted polymer showed better selectivity, more rapid kinetic binding (60 min) for 2,4-dichlorophenoxyacetic acid in pure water compared with rebinding in toluene. The imprinted polymer was used as a sorbent to enrich and separate 2,4-dichlorophenoxyacetic acid from water, and was detected by high-performance liquid chromatography with UV detection.

Separation and purification of both tea seed polysaccharide and saponin from camellia cake extract using macroporous resin.

A novel method to separate and purify tea seed polysaccharide and tea seed saponin from camellia cake extract by macroporous resin was developed. Among four kinds of resins (AB-8, NKA-9, XDA-6, and D4020) tested, AB-8 macroporous resin possessed optimal separating capacity for the two substances and thus was selected for the separation, in which deionized water was used to elute tea seed polysaccharide, 0.25% NaOH solution to remove the undesired pigments, and 90% ethanol to elute tea seed saponin. Further dynamic adsorption/desorption experiments on AB-8 resin-based column chromatography were conducted to obtain the optimal parameters. Under optimal dynamic adsorption and desorption conditions, 18.7 and 11.8% yield of tea seed polysaccharide and tea seed saponin were obtained with purities of 89.2 and 96.0%, respectively. The developed method provides a potential approach for the large-scale production of tea seed polysaccharide and tea seed saponin from camellia cake.

Preparation and Application of Single-Atom Cobalt Catalysts in Organic Synthesis and Environmental Remediation.

The development of high-performance catalysts plays a crucial role in facilitating chemical production and reducing environmental contamination. Single-atom catalysts (SACs), a class of catalysts that bridge the gap between homogeneous and heterogeneous catalysis, have garnered increasing attention because of their unique activity, selectivity, and stability in many pivotal reactions. Meanwhile, the scarcity of precious metal SACs calls for the arrival of cost-effective SACs. Cobalt, as a common non-noble metal, possesses tremendous potential in the field of single-atom catalysis. Despite their potential, reviews about single-atom Co catalysts (Co-SACs) are lacking. Accordingly, this review thoroughly summarized various preparation methodologies of Co-SACs, particularly pyrolysis; its application in the specific domain of organic synthesis and environmental remediation is discussed as well. The structure-activity relationship and potential catalytic mechanism of Co-SACs are elucidated through some representative reactions. The imminent challenges and development prospects of Co-SACs are discussed in detail. The findings and insights provided herein can guide further exploration and development in this charming area of catalyst design, leading to the realization of efficient and sustainable catalytic processes.

Oxygen Doping Cooperated with Co-N-Fe Dual-Catalytic Sites: Synergistic Mechanism for Catalytic Water Purification within Nanoconfined Membrane.

Atom-site catalysts, especially for graphitic carbon nitride-based catalysts, represents one of the most promising candidates in catalysis membrane for water decontamination. However, unravelling the intricate relationships between synthesis-structure-properties remains a great challenge. This study addresses the impacts of coordination environment and structure units of metal central sites based on Mantel test, correlation analysis, and evolution of metal central sites. An optimized unconventional oxygen doping cooperated with Co-N-Fe dual-sites (OCN Co/Fe) exhibits synergistic mechanism for efficient peroxymonosulfate activation, which benefits from a significant increase in charge density at the active sites and the regulation in the natural population of orbitals, leading to selective generation of SO4 •-. Building upon these findings, the OCN-Co/Fe/PVDF composite membrane demonstrates a 33 min-1 ciprofloxacin (CIP) rejection efficiency and maintains over 96% CIP removal efficiency (over 24 h) with an average permeance of 130.95 L m-2 h-1. This work offers a fundamental guide for elucidating the definitive origin of catalytic performance in advance oxidation process to facilitate the rational design of separation catalysis membrane with improved performance and enhanced stability.

Spatial confinement: A green pathway to promote the oxidation processes for organic pollutants removal from water.

Organic pollutants removal from water is pressing owing to the great demand for clean water. Oxidation processes (OPs) are the commonly used method. However, the efficiency of most OPs is limited owing to the poor mass transfer process. Spatial confinement is a burgeoning way to solve this limitation by use of nanoreactor. Spatial confinement in OPs would (i) alter the transport characteristics of protons and charges; (ii) bring about molecular orientation and rearrangement; (iii) cause the dynamic redistribution of active sites in catalyst and reduce the entropic barrier that is high in unconfined space. So far, spatial confinement has been utilized for various OPs, such as Fenton, persulfate, and photocatalytic oxidation. A comprehensive summary and discussion on the fundamental mechanisms of spatial confinement mediated OPs is needed. Herein, the application, performance and mechanisms of spatial confinement mediated OPs are overviewed firstly. Subsequently, the features of spatial confinement and their effects on OPs are discussed in detail. Furthermore, environmental influences (including environmental pH, organic matter and inorganic ions) are studied with analyzing their intrinsic connection with the features of spatial confinement in OPs. Lastly, challenges and future development direction of spatial confinement mediated OPs are proposed.

Rational design of cobalt sulfide anchored on nitrogen-doped carbon derived from cyanobacteria waste enables efficient activation of peroxymonosulfate for organic pollutants oxidation.

With the increasing of eutrophication in water body, algae blooms have become one of the global environmental problems. The cyanobacteria waste has placed a severe burden on the environment and transforming cyanobacteria into functional materials may be a wise approach. Herein, cobaltous sulfide/nitrogen-doped biochar (N-BC/CoSx) composite was synthesized by pyrolysis of cyanobacteria waste. The N-BC/CoSx showed excellent performance in peroxymonosulfate (PMS) activation for enrofloxacin (ENR) degradation, which could remove more than 90% ENR within 60 min. The influencing factors of pH and catalyst dosage on ENR removal efficiency were studied. The N-BC/CoSx showed good recyclability in the cycle runs. The radicals (O2•-, OH andSO4•-) and the non-radical species (charge transfer and 1O2) were generated in the ENR degradation. The cycle of Co(II)/Co(III) m ay contribute to the radical generation process. This work proved that metal sulfide modified cyanobacteria biochar has a specific application value in water pollution control and provides a new method for resource utilization of cyanobacteria.

Structure-performance correlation guided cerium-based metal-organic frameworks: Superior adsorbents for fluoride removal in water.

Fluoride in the hydrosphere exceeds the standard, which could be critically hazardous to human health and the natural environment. The adsorption method is a mature and effective way to remove pollutants in water, including fluoride. In this study, we synthesized three kinds of cerium-based metal-organic frameworks (Ce-MOFs) with different structures and properties by modulating the organic ligands (i.e., trimesic acid (BTC), 1,2,4,5-benzenetetracarboxylic acid (PMA), and terephthalic acid (BDC)) via the solvothermal method. The adsorption kinetics of Ce-MOFs on fluoride well fit the pseudo second order model, and their adsorption isotherms also conform to Langmuir isothermal model. The thermodynamic study reveals that the adsorption process is a spontaneous endothermic reaction. The maximum saturated adsorption capacities of Ce-BTC, Ce-PMA, and Ce-BDC are 70.7, 159.6, and 139.5 mg g-1, respectively. Ce-MOFs have stable and excellent adsorption capacity at pH = 3-9. Coexisting anions (Cl-, SO42-, and NO3-) do not affect the performance of Ce-MOFs for fluoride removal. Moreover, Ce-MOFs also show their broad prospect as superior fluoride adsorbents because of their excellent performance and reusability in real water samples. Organic ligands have a remarkable influence on the defluoridation performance of Ce-MOFs. This work will provide a feasible idea for designing MOFs as superiors adsorbents for defluoridation.

Single atom Mn anchored on N-doped porous carbon derived from spirulina for catalyzed peroxymonosulfate to degradation of emerging organic pollutants.

Highly efficient single atom catalysts are critical to substantially promote for peroxymonosulfate (PMS) activation to organic pollutant degradation, but it remains a challenge at present. Herein, single atom Mn anchored on N-doped porous carbon (SA-Mn-NSC) was synthesized by ball milling of Mn-doped carbon nitride and spirulina biochar to dominantly activate PMS. The precursor of carbon nitride and spirulina possessed a strong coordinating capability for Mn(II), facilitating the formation of highly dispersed nitrogen-coordinated Mn sites (Mn-N4). The SA-Mn-NSC catalyst exhibited high activity and stability in the heterogeneous activation of PMS to degrade a wide range of pollutants within 10 min, showing an outstanding degradation rate constant of 0.31 min-1 in enrofloxacin (ENR) degradation. The high surface density of Mn-N4 sites and abundant interconnected meso-macro pores were highly favorable for activating PMS to produce 1O2 and high-valent manganese (Mn(IV)) for pollutant degradation. This work offers a new pathway of using a low-cost and easily accessible single-atom catalysts (SACs) and could inspire more catalytic oxidation strategies.