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Frontiers of Environmental Science & Engineering

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    2020, Vol.14  No.5

    This Special Issue, Accounts of Environmental Chemistry and Technology Research, is FESE’s first attempt for account articles. Account articles summarize the achievements of a research group and further extend the discussion with closely related literature. Because a comprehensive review of a large number of publications is not needed, account articles provide a concise, focused, and in-depth discussion of specific research topics. The authors can [Detail] ...

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ISSN 2095-2201 (Print)
ISSN 2095-221X (Online)
CN 10-1013/X
Postal Subscription Code 80-973
Formerly Known as Frontiers of Environmental Science & Engineering in China
2019 Impact Factor: 4.053

, Volume 14 Issue 5

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PREFACE
The First Attempt for Accounts of Environmental Chemistry and Technology Research Collection
Jinyong Liu, Yin Wang, Haoran Wei
Front. Environ. Sci. Eng.. 2020, 14 (5): 91 .   https://doi.org/10.1007/s11783-020-1270-9
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REVIEW ARTICLE
Emerging electrochemical processes for materials recovery from wastewater: Mechanisms and prospects Collection
Lingchen Kong, Xitong Liu
Front. Environ. Sci. Eng.. 2020, 14 (5): 90 .   https://doi.org/10.1007/s11783-020-1269-2
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• Mechanisms for selective recovery of materials in electrochemical processes are discussed.

• Wastewaters that contain recoverable materials are reviewed.

• Application prospects are discussed from both technical and non-technical aspects.

Recovering valuable materials from waste streams is critical to the transition to a circular economy with reduced environmental damages caused by resource extraction activities. Municipal and industrial wastewaters contain a variety of materials, such as nutrients (nitrogen and phosphorus), lithium, and rare earth elements, which can be recovered as value-added products. Owing to their modularity, convenient operation and control, and the non-requirement of chemical dosage, electrochemical technologies offer a great promise for resource recovery in small-scale, decentralized systems. Here, we review three emerging electrochemical technologies for materials recovery applications: electrosorption based on carbonaceous and intercalation electrodes, electrochemical redox processes, and electrochemically induced precipitation. We highlight the mechanisms for achieving selective materials recovery in these processes. We also present an overview of the advantages and limitations of these technologies, as well as the key challenges that need to be overcome for their deployment in real-world systems to achieve cost-effective and sustainable materials recovery.

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Rapid and long-effective removal of broad-spectrum pollutants from aqueous system by ZVI/oxidants Collection
Sana Ullah, Xuejun Guo, Xiaoyan Luo, Xiangyuan Zhang, Siwen Leng, Na Ma, Palwasha Faiz
Front. Environ. Sci. Eng.. 2020, 14 (5): 89 .   https://doi.org/10.1007/s11783-020-1268-3
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• The coupling of oxidants with ZVI overcome the impedance of ZVI passive layer.

• ZVI/oxidants system achieved fast and long-effective removal of contaminants.

• Multiple mechanisms are involved in contaminants removal by ZVI/oxidant system.

• ZVI/Oxidants did not change the reducing property of ORP in the fixed-bed system.

Zero-valent iron (ZVI) technology has recently gained significant interest in the efficient sequestration of a wide variety of contaminants. However, surface passivation of ZVI because of its intrinsic passive layer would lead to the inferior reactivity of ZVI and its lower efficacy in contaminant removal. Therefore, to activate the ZVI surface cheaply, continuously, and efficiently is an important challenge that ZVI technology must overcome before its wide-scale application. To date, several physical and chemical approaches have been extensively applied to increase the reactivity of the ZVI surface toward the elimination of broad-spectrum pollutants. Nevertheless, these techniques have several limitations such as low efficacy, narrow working pH, eco-toxicity, and high installation cost. The objective of this mini-review paper is to identify the critical role of oxygen in determining the reactivity of ZVI toward contaminant removal. Subsequently, the effect of three typical oxidants (H2O2, KMnO4, and NaClO) on broad-spectrum contaminants removal by ZVI has been documented and discussed. The reaction mechanism and sequestration efficacies of the ZVI/oxidant system were evaluated and reviewed. The technical basis of the ZVI/oxidant approach is based on the half-reaction of the cathodic reduction of the oxidants. The oxidants commonly used in the water treatment industry, i.e., NaClO, O3, and H2O2, can be served as an ideal coupling electron receptor. With the combination of these oxidants, the surface corrosion of ZVI can be continuously driven. The ZVI/oxidants technology has been compared with other conventional technologies and conclusions have been drawn.

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Hexavalent chromium in drinking water: Chemistry, challenges and future outlook on Sn(II)- and photocatalyst-based treatment Collection
Haizhou Liu, Xuejun Yu
Front. Environ. Sci. Eng.. 2020, 14 (5): 88 .   https://doi.org/10.1007/s11783-020-1267-4
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• Wide occurrence of Cr(VI) in US source drinking water.

• A strong dependence of occurrence on groundwater sources.

• Elucidate Redox and equilibrium chemistry of Cr(VI).

• Sn(II)-based and TiO2-based reductive treatments hold extreme promise.

• Key challenges include residual waste, Cr(VI) re-generation and socioeconomic drivers.

Chromium (Cr) typically exists in either trivalent and hexavalent oxidation states in drinking water, i.e., Cr(III) and Cr(VI), with Cr(VI) of particular concern in recent years due to its high toxicity and new regulatory standards. This Account presented a critical analysis of the sources and occurrence of Cr(VI) in drinking water in the United States, analyzed the equilibrium chemistry of Cr(VI) species, summarized important redox reaction relevant to the fate of Cr(VI) in drinking water, and critically reviewed emerging Cr(VI) treatment technologies. There is a wide occurrence of Cr(VI) in US source drinking water, with a strong dependence on groundwater sources, mainly due to naturally weathering of chromium-containing aquifers. Challenges regarding traditional Cr(VI) treatment include chemical cost, generation of secondary waste and inadvertent re-generation of Cr(VI) after treatment. To overcome these challenges, reductive Cr(VI) treatment technologies based on the application of stannous tin or electron-releasing titanium dioxide photocatalyst hold extreme promise in the future. To moving forward in the right direction, three key questions need further exploration for the technology implementation, including effective management of residual waste, minimizing the risks of Cr(VI) re-occurrence downstream of drinking water treatment plant, and promote the socioeconomic drivers for Cr(VI) control in the future.

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A review of measurement methods for peracetic acid (PAA) Collection
Chen Cheng, Haodong Li, Jinling Wang, Hualin Wang, Xuejing Yang
Front. Environ. Sci. Eng.. 2020, 14 (5): 87 .   https://doi.org/10.1007/s11783-020-1266-5
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• Physical and chemical properties and application of peracetic acid solution.

• Determination method of high concentration peracetic acid.

• Determination method of residual peracetic acid (low concentration).

Peroxyacetic acid has been widely used in food, medical, and synthetic chemical fields for the past several decades. Recently, peroxyacetic acid has gradually become an effective alternative disinfectant in wastewater disinfection and has strong redox capacity for removing micro-pollutants from drinking water. However, commercial peroxyacetic acid solutions are primarily multi-component mixtures of peroxyacetic acid, acetic acid, hydrogen peroxide, and water. During the process of water treatment, peroxyacetic acid and hydrogen peroxide (H2O2) often coexist, which limits further investigation on the properties of peroxyacetic acid. Therefore, analytical methods need to achieve a certain level of selectivity, particularly when peroxyacetic acid and hydrogen peroxide coexist. This review summarizes the measurement and detection methods of peroxyacetic acid, comparing the principle, adaptability, and relative merits of these methods.

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Reactivity of Pyrogenic Carbonaceous Matter (PCM) in mediating environmental reactions: Current knowledge and future trends Collection
Wenqing Xu, Mark L. Segall, Zhao Li
Front. Environ. Sci. Eng.. 2020, 14 (5): 86 .   https://doi.org/10.1007/s11783-020-1265-6
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• Pyrogenic Carbonaceous Matter (PCM) promote both chemical and microbial synergies.

• Discussion of PCM-enhanced abiotic transformation pathways of organic pollutants.

• Conjugated microporous polymers (CMPs) can mimic the performance of PCM.

• CMPs offer a platform that allows for systematic variation of individual properties.

Pyrogenic Carbonaceous matter (PCM; e.g., black carbon, biochar, and activated carbon) are solid residues from incomplete combustion of fossil fuel or biomass. They are traditionally viewed as inert adsorbents for sequestering contaminants from the aqueous phase or providing surfaces for microbes to grow. In this account, we reviewed the recently discovered reactivity of PCM in promoting both chemical and microbial synergies that are important in pollutant transformation, biogeochemical processes of redox-active elements, and climate change mitigation with respect to the interaction between biochar and nitrous oxide (N2O). Moreover, we focused on our group’s work in the PCM-enhanced abiotic transformation of nitrogenous and halogenated pollutants and conducted in-depth analysis of the reaction pathways. To understand what properties of PCM confer its reactivity, our group pioneered the use of PCM-like polymers, namely conjugated microporous polymers (CMPs), to mimic the performance of PCM. This approach allows for the controlled incorporation of specific surface properties (e.g., quinones) into the polymer network during the polymer synthesis. As a result, the relationship between specific characteristics of PCM and its reactivity in facilitating the decay of a model pollutant was systematically studied in our group’s work. The findings summarized in this account help us to better understand an overlooked environmental process where PCM synergistically interacts with various environmental reagents such as hydrogen sulfide and water. Moreover, the knowledge gained in these studies could inform the design of a new generation of reactive carbonaceous materials with tailored properties that are highly efficient in contaminant removal.

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Recent advances in the electrochemical oxidation water treatment: Spotlight on byproduct control Collection
Yang Yang
Front. Environ. Sci. Eng.. 2020, 14 (5): 85 .   https://doi.org/10.1007/s11783-020-1264-7
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• Byproduct formation mechanisms during electrochemical oxidation water treatment.

• Control byproduct formation by quenchers.

• Process optimization to suppress byproduct formation.

Electrochemical oxidation (EO) is a promising technique for decentralized wastewater treatment, owing to its modular design, high efficiency, and ease of automation and transportation. The catalytic destruction of recalcitrant, non-biodegradable pollutants (per- and poly-fluoroalkyl substances (PFAS), pharmaceuticals, and personal care products (PPCPs), pesticides, etc.) is an appropriate niche for EO. EO can be more effective than homogeneous advanced oxidation processes for the degradation of recalcitrant chemicals inert to radical-mediated oxidation, because the potential of the anode can be made much higher than that of hydroxyl radicals (EOH = 2.7 V vs. NHE), forcing the direct transfer of electrons from pollutants to electrodes. Unfortunately, at such high anodic potential, chloride ions, which are ubiquitous in natural water systems, will be readily oxidized to chlorine and perchlorate. Perchlorate is a to-be-regulated byproduct, and chlorine can react with matrix organics to produce organic halogen compounds. In the past ten years, novel electrode materials and processes have been developed. However, spotlights were rarely focused on the control of byproduct formation during EO processes in a real-world context. When we use EO techniques to eliminate target contaminants with concentrations at μg/L-levels, byproducts at mg/L-levels might be produced. Is it a good trade-off? Is it possible to inhibit byproduct formation without compromising the performance of EO? In this mini-review, we will summarize the recent advances and provide perspectives to address the above questions.

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Remediation of soil and groundwater contaminated with organic chemicals using stabilized nanoparticles: Lessons from the past two decades Collection
Zhengqing Cai, Xiao Zhao, Jun Duan, Dongye Zhao, Zhi Dang, Zhang Lin
Front. Environ. Sci. Eng.. 2020, 14 (5): 84 .   https://doi.org/10.1007/s11783-020-1263-8
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▪ Overviewed evolution and environmental applications of stabilized nanoparticles.

▪ Reviewed theories on particle stabilization for enhanced reactivity/deliverability.

▪ Examined various in situ remediation technologies based on stabilized nanoparticles.

▪ Summarized knowledge on transport of stabilized nanoparticles in porous media.

▪ Identified key knowledge gaps and future research needs on stabilized nanoparticles.

Due to improved soil deliverability and high reactivity, stabilized nanoparticles have been studied for nearly two decades for in situ remediation of soil and groundwater contaminated with organic pollutants. While large amounts of bench- and field-scale experimental data have demonstrated the potential of the innovative technology, extensive research results have also unveiled various merits and constraints associated different soil characteristics, types of nanoparticles and particle stabilization techniques. Overall, this work aims to critically overview the fundamental principles on particle stabilization, and the evolution and some recent developments of stabilized nanoparticles for degradation of organic contaminants in soil and groundwater. The specific objectives are to: 1) overview fundamental mechanisms in nanoparticle stabilization; 2) summarize key applications of stabilized nanoparticles for in situ remediation of soil and groundwater contaminated by legacy and emerging organic chemicals; 3) update the latest knowledge on the transport and fate of stabilized nanoparticles; 4) examine the merits and constraints of stabilized nanoparticles in environmental remediation applications; and 5) identify the knowledge gaps and future research needs pertaining to stabilized nanoparticles for remediation of contaminated soil and groundwater. Per instructions of this invited special issue, this review is focused on contributions from our group (one of the pioneers in the subject field), which, however, is supplemented by important relevant works by others. The knowledge gained is expected to further advance the science and technology in the environmental applications of stabilized nanoparticles.

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Low-cost adsorbents for urban stormwater pollution control Collection
Yang Deng
Front. Environ. Sci. Eng.. 2020, 14 (5): 83 .   https://doi.org/10.1007/s11783-020-1262-9
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• Various low-cost adsorbents are studied for capturing urban stormwater pollutants.

• Adsorbents are selected based on both pollutant adsorption and unexpected leaching.

• Application modes of adsorbents influence their utilization efficacy in practice.

Stormwater represents a major non-point pollution source at an urban environment. To improve the treatment efficacy of stormwater infrastructure, low-cost adsorbents have increasingly gained attention over the past decades. This article aims to briefly discuss several key aspects and principles for utilization of low-cost adsorbents for urban stormwater treatment. To determine whether a low-cost adsorbent is suitable for stormwater treatment, two aspects should be carefully assessed, including: 1) its adsorption mechanisms and behaviors that can influence the binding stre.g.,h, adsorption kinetics, and treatment capacity; and 2) unwanted chemical leaching patterns that can affect the extent of water quality degradation. Furthermore, the application mode of an adsorbent in the system design influences the utilization efficiency. Adsorbents, after dosed to soil media in infrastructure, would eventually become ineffective after oversaturation. In contrast, standalone filters or innovative composite adsorbents (e.g., adsorbent-coated mulch chips) can enable a long-lasting adsorption due to periodic replacement with fresh adsorbents. The aforementioned principles play a key role in the success of urban stormwater treatment with low-cost adsorbents.

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Dual-reaction-center catalytic process continues Fenton’s story Collection
Chao Lu, Kanglan Deng, Chun Hu, Lai Lyu
Front. Environ. Sci. Eng.. 2020, 14 (5): 82 .   https://doi.org/10.1007/s11783-020-1261-x
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• Dual-reaction-center (DRC) system breaks through bottleneck of Fenton reaction.

• Utilization of intrinsic electrons of pollutants is realized in DRC system.

• DRC catalytic process well continues Fenton’s story.

Triggered by global water quality safety issues, the research on wastewater treatment and water purification technology has been greatly developed in recent years. The Fenton technology is particularly powerful due to the rapid attack on pollutants by the generated hydroxyl radicals (•OH). However, both heterogeneous and homogeneous Fenton/Fenton-like technologies follow the classical reaction mechanism, which depends on the oxidation and reduction of the transition metal ions at single sites. So even after a century of development, this reaction still suffers from its inherent bottlenecks in practical application. In recent years, our group has been focusing on studying a novel heterogeneous Fenton catalytic process, and we developed the dual-reaction-center (DRC) system for the first time. In the DRC system, H2O2 and O2 can be efficiently reduced to reactive oxygen species (ROS) in electron-rich centers, while pollutants are captured and oxidized by the electron-deficient centers. The obtained electrons from pollutants are diverted to the electron-rich centers through bonding bridges. This process breaks through the classic Fenton mechanism, and improves the performance and efficiency of pollutant removal in a wide pH range. Here, we provide a brief overview of Fenton’s story and focus on combing the discovery and development of the DRC technology and mechanism in recent years. The construction of the DRC and its performance in the pollutant degradation and interfacial reaction process are described in detail. We look forward to bringing a new perspective to continue Fenton’s story through research and development of DRC technology.

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Interplay of transport processes and interfacial chemistry affecting chromium reduction and reoxidation with iron and manganese Collection
Chao Pan, Daniel Giammar
Front. Environ. Sci. Eng.. 2020, 14 (5): 81 .   https://doi.org//10.1007/s11783-020-1260-y
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• Cr(VI) can be removed by iron-based adsorption, reduction and precipitation.

• Surface-functionalized iron oxide nanoparticles are promising adsorbents for Cr(VI).

• Surface complexation modeling provides quantitative predicts for Cr(VI) adsorption.

• Cr(III) can be remobilized in the presence of Mn(II, III, IV) at certain conditions.

Hexavalent chromium (Cr(VI)) is a water-soluble pollutant in soil and groundwater, the mobility, bioavailability, and toxicity of which can be controlled by transforming to less mobile and more environmentally benign Cr(III) by ways of reduction. This review focused on recent advances in identifying the reaction pathways, kinetics, and products of iron-based techniques for Cr(VI) removal. It also examines new information regarding remobilization of Cr(III) in the existence of complexing ligands and manganese (Mn) of different oxidation states. A range of iron-based techniques can remove Cr(VI) from water by adsorption or reduction-coprecipitation processes. However, the success of a chromium treatment or remediation strategy requires the stability of the Cr(III)-containing solids with respect to solubilization or reoxidation in the settings they are generated. Manganese is ubiquitous in aquatic and terrestrial environments, and the redox cycling of manganese may greatly influence the fate, transport, and distribution of chromium. Coupling of redox reactions of chromium, iron, and manganese involves reaction pathways not only in the aqueous phase but also at solid-aqueous interfaces. To provide a quantitative understanding of these processes, it is essential to develop mechanistically based kinetic and transport models. Continued research should be made on iron-based treatment of Cr(VI)-contaminated water and soils and the stability of the subsequently produced Cr(III)-containing solids at environmentally relevant conditions, which will support improved predictions of chromium’s environmental fate and transport and aid in decision-making for remediation and treatment of Cr contamination.

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Toxicity-oriented water quality engineering Collection
Shengkun Dong, Chenyue Yin, Xiaohong Chen
Front. Environ. Sci. Eng.. 2020, 14 (5): 80 .   https://doi.org/10.1007/s11783-020-1259-4
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• Toxicity-oriented water quality monitoring was proposed.

• Toxicity-oriented water quality engineering control was proposed.

• Future issues to the proposition were discussed.

The fundamental goal of water quality engineering is to ensure water safety to humans and the environment. Traditional water quality engineering consists of monitoring, evaluation, and control of key water quality parameters. This approach provides some vital insights into water quality, however, most of these parameters do not account for pollutant mixtures – a reality that terminal water users face, nor do most of these parameters have a direct connection with the human health safety of waters. This puts the real health-specific effects of targeted water pollutant monitoring and engineering control in question. To focus our attention to one of the original goals of water quality engineering – human health and environmental protection, we advocate here the toxicity-oriented water quality monitoring and control. This article presents some of our efforts toward such goal. Specifically, complementary to traditional water quality parameters, we evaluated the water toxicity using high sensitivity toxicological endpoints, and subsequently investigated the performance of some of the water treatment strategies in modulating the water toxicity. Moreover, we implemented the toxicity concept into existing water treatment design theory to facilitate toxicity-oriented water quality control designs. Suggestions for the next steps are also discussed. We hope our work will intrigue water quality scientists and engineers to improve and embrace the mixture water pollutant and toxicological evaluation and engineering control.

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Ceramic water filter for point-of-use water treatment in developing countries: Principles, challenges and opportunities Collection
Haiyan Yang, Shangping Xu, Derek E. Chitwood, Yin Wang
Front. Environ. Sci. Eng.. 2020, 14 (5): 79 .   https://doi.org/10.1007/s11783-020-1254-9
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• CWF is a sustainable POU water treatment method for developing areas.

• CWF manufacturing process is critical for its filtration performance.

• Simultaneous increase of flow rate and pathogen removal is a challenge.

• Control of pore size distribution holds promises to improve CWF efficiency.

• Novel coatings of CWFs are a promising method to improve contaminant removal.

Drinking water source contamination poses a great threat to human health in developing countries. Point-of-use (POU) water treatment techniques, which improve drinking water quality at the household level, offer an affordable and convenient way to obtain safe drinking water and thus can reduce the outbreaks of waterborne diseases. Ceramic water filters (CWFs), fabricated from locally sourced materials and manufactured by local labor, are one of the most socially acceptable POU water treatment technologies because of their effectiveness, low-cost and ease of use. This review concisely summarizes the critical factors that influence the performance of CWFs, including (1) CWF manufacturing process (raw material selection, firing process, silver impregnation), and (2) source water quality. Then, an in-depth discussion is presented with emphasis on key research efforts to address two major challenges of conventional CWFs, including (1) simultaneous increase of filter flow rate and bacterial removal efficiency, and (2) removal of various concerning pollutants, such as viruses and metal(loid)s. To promote the application of CWFs, future research directions can focus on: (1) investigation of pore size distribution and pore structure to achieve higher flow rates and effective pathogen removal by elucidating pathogen transport in porous ceramic and adjusting manufacture parameters; and (2) exploration of new surface modification approaches with enhanced interaction between a variety of contaminants and ceramic surfaces.

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Locally enhanced electric field treatment (LEEFT) for water disinfection Collection
Jianfeng Zhou, Ting Wang, Cecilia Yu, Xing Xie
Front. Environ. Sci. Eng.. 2020, 14 (5): 78 .   https://doi.org/10.1007/s11783-020-1253-x
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• Nanowire-assisted LEEFT is applied for water disinfection with low voltages.

• LEEFT inactivates bacteria by disrupting cell membrane through electroporation.

• Multiple electrodes and device configurations have been developed for LEEFT.

• The LEEFT is low-cost, highly efficient, and produces no DBPs.

• The LEEFT can potentially be applicable for water disinfection at all scales.

Water disinfection is a critical step in water and wastewater treatment. The most widely used chlorination suffers from the formation of carcinogenic disinfection by-products (DBPs) while alternative methods (e.g., UV, O3, and membrane filtration) are limited by microbial regrowth, no residual disinfectant, and high operation cost. Here, a nanowire-enabled disinfection method, locally enhanced electric field treatment (LEEFT), is introduced with advantages of no chemical addition, no DBP formation, low energy consumption, and efficient microbial inactivation. Attributed to the lightning rod effect, the electric field near the tip area of the nanowires on the electrode is significantly enhanced to inactivate microbes, even though a small external voltage (usually<5 V) is applied. In this review, after emphasizing the significance of water disinfection, the theory of the LEEFT is explained. Subsequently, the recent development of the LEEFT technology on electrode materials and device configurations are summarized. The disinfection performance is analyzed, with respect to the operating parameters, universality against different microorganisms, electrode durability, and energy consumption. The studies on the inactivation mechanisms during the LEEFT are also reviewed. Lastly, the challenges and future research of LEEFT disinfection are discussed.

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(Super)paramagnetic nanoparticles as platform materials for environmental applications: From synthesis to demonstration Collection
Wenlu Li, John D. Fortner
Front. Environ. Sci. Eng.. 2020, 14 (5): 77 .   https://doi.org/10.1007/s11783-020-1256-7
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• The fabrication of monodisperse, (super)paramagnetic nanoparticles is summarized.

• Monolayer and bilayer surface coating structures are described.

• Mono/bilayer coated nanoparticles showed high sorption capacities for U, As, and Cr.

Over the past few decades, engineered, (super)paramagnetic nanoparticles have drawn extensive research attention for a broad range of applications based on their tunable size and shape, surface chemistries, and magnetic properties. This review summaries our recent work on the synthesis, surface modification, and environmental application of (super)paramagnetic nanoparticles. By utilizing high-temperature thermo-decomposition methods, first, we have broadly demonstrated the synthesis of highly monodispersed, (super)paramagnetic nanoparticles, via the pyrolysis of metal carboxylate salts in an organic phase. Highly uniform magnetic nanoparticles with various size, composition, and shape can be precisely tuned by controlled reaction parameters, such as the initial precursors, heating rate, final reaction temperature, reaction time, and the additives. These materials can be further rendered water stable via functionalization with surface mono/bi-layer coating structure using a series of tunable ionic/non-ionic surfactants. Finally, we have demonstrated platform potential of these materials for heavy metal ions sensing, sorption, and separation from the aqueous phase.

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Redox reactions of iron and manganese oxides in complex systems Collection
Jianzhi Huang, Huichun Zhang
Front. Environ. Sci. Eng.. 2020, 14 (5): 76 .   https://doi.org/10.1007/s11783-020-1255-8
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• Mechanisms of redox reactions of Fe- and Mn-oxides were discussed.

• Oxidative reactions of Mn- and Fe-oxides in complex systems were reviewed.

• Reductive reaction of Fe(II)/iron oxides in complex systems was examined.

• Future research on examining the redox reactivity in complex systems was suggested.

Conspectus Redox reactions of Fe- and Mn-oxides play important roles in the fate and transformation of many contaminants in natural environments. Due to experimental and analytical challenges associated with complex environments, there has been a limited understanding of the reaction kinetics and mechanisms in actual environmental systems, and most of the studies so far have only focused on simple model systems. To bridge the gap between simple model systems and complex environmental systems, it is necessary to increase the complexity of model systems and examine both the involved interaction mechanisms and how the interactions affected contaminant transformation. In this Account, we primarily focused on (1) the oxidative reactivity of Mn- and Fe-oxides and (2) the reductive reactivity of Fe(II)/iron oxides in complex model systems toward contaminant degradation. The effects of common metal ions such as Mn2+ , Ca2+, Ni2+, Cr3+ and Cu2+, ligands such as small anionic ligands and natural organic matter (NOM), and second metal oxides such as Al, Si and Ti oxides on the redox reactivity of the systems are briefly summarized.

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