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Frontiers of Environmental Science & Engineering

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    2020, Vol.14  No.4

    Front Cover Story (see: Xinjie Wang, Yang Li, Jian Zhao, Hong Yao, Siqi Chu, Zimu Song, Zongxian He, Wen Zhang, 2020, 14: 56)
    Magnetotactic bacteria (MTB) are a group of Gram-negative prokaryotes that respond to the geomagnetic field. This unique property is attributed to the biosynthesis of magnetic nanoparticles within MTB’s magnetosomes, an intracellular organelle with magnetic iron-bearing nanocrystals. This review summarizes the most recent studi [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 4

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ERRATUM
Erratum to: One-step ball milling-prepared nano Fe2O3 and nitrogen-doped graphene with high oxygen reduction activity and its application in microbial fuel cells
Xingguo Guo, Qiuying Wang, Ting Xu, Kajia Wei, Mengxi Yin, Peng Liang, Xia Huang, Xiaoyuan Zhang
Front. Environ. Sci. Eng.. 2020, 14 (4): 75 .   https://doi.org/10.1007/s11783-020-1258-5
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NEWS & FOCUS
Chinese Academy of Engineering released Global Engineering Fronts
Xu Wang, Jiao Zhang, Xi Lu, Xia Huang
Front. Environ. Sci. Eng.. 2020, 14 (4): 74 .   https://doi.org/10.1007/s11783-020-1257-6
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RESEARCH ARTICLE
Persistent free radicals in humin under redox conditions and their impact in transforming polycyclic aromatic hydrocarbons
Hanzhong Jia, Yafang Shi, Xiaofeng Nie, Song Zhao, Tiecheng Wang, Virender K. Sharma
Front. Environ. Sci. Eng.. 2020, 14 (4): 73 .   https://doi.org/10.1007/s11783-020-1252-y
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• Regulation of redox conditions promotes the generation of free radicals on HM.

• HM-PFRs can be fractionated into active and inactive types depending on stability.

• The newly produced PFRs readily release electrons to oxygen and generate ROS.

• PFR-induced ROS mediate the transformation of organic contaminants adsorbed on HM.

The role of humic substance-associated persistent free radicals (PFRs) in the fate of organic contaminants under various redox conditions remains unknown. This study examined the characterization of original metal-free peat humin (HM), and HM treated with varying concentrations of H2O2 and L-ascorbic acid (VC) (assigned as H2O2-HM and VC-HM). The concentration of PFRs in HM increased with the addition of VC/H2O2 at concentrations less than 0.08 M. The evolution of PFRs in HM under different environmental conditions (e.g., oxic/anoxic and humidity) was investigated. Two types of PFRs were detected in HM: a relatively stable radical existed in the original sample, and the other type, which was generated by redox treatments, was relatively unstable. The spin densities of VC/H2O2-HM readily returned to the original value under relatively high humidity and oxic conditions. During this process, the HM-associated “unstable” free radicals released an electron to O2, inducing the formation of reactive oxygen species (ROS, i.e., OH and O2). The generated ROS promoted the degradation of polycyclic aromatic hydrocarbons based on the radical quenching measurements. The transformation rates followed the order naphthalene>phenanthrene>anthracene>benzo[a]pyrene. Our results provide valuable insight into the HM-induced transformation of organic contaminants under natural conditions.

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Utilizing transparent and conductive SnO2 as electron mediator to enhance the photocatalytic performance of Z-scheme Si-SnO2-TiOx
Jing Gu, Hongtao Yu, Xie Quan, Shuo Chen, Junfeng Niu
Front. Environ. Sci. Eng.. 2020, 14 (4): 72 .   https://doi.org/10.1007/s11783-020-1251-z
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• A novel Z-scheme Si-SnO2-TiOx with SnO2 as electron mediator is first constructed.

• Transparent and conductive SnO2 can pass light through and promote charge transport.

• VO from SnO2 and TiOx improve photoelectrochemical performances.

• Efficient photocatalytic degradations originate from the Z scheme construction.

Z-scheme photocatalysts, with strong redox ability, have a great potential for pollutants degradation. However, it is challenging to construct efficient Z-scheme photocatalysts because of their poor interfacial charge separation. Herein, by employing transparent and conductive SnO2 as electron mediator to pass light through and promote interfacial charge transportation, a novel Z-scheme photocatalyst Si-SnO2-TiOx (1<x<2) was constructed. The Z-scheme photocatalyst displayed an order of magnitude higher photocurrent density and a 4-fold increase in open-circuit potential compared to those of Si. Moreover, the onset potential shifted negatively for approximately 2.2 V. Benefiting from these advantages, this Z-scheme Si-SnO2-TiOx exhibited efficient photocatalytic performance toward phenol degradation and mineralization. 75% of the phenol was degraded without bias potential and 70% of the TOC was removed during phenol degradation. Other typical pollutants such as bisphenol A and atrazine could also be degraded without bias potential. Introducing a transparent and conductive electron mediator to construct Z-scheme photocatalyst gives a new sight to the improvement of photocatalytic performance in Z scheme.

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A study of peroxyacetyl nitrate at a rural site in Beijing based on continuous observations from 2015 to 2019 and the WRF-Chem model
Yulu Qiu, Zhiqiang Ma, Weili Lin, Weijun Quan, Weiwei Pu, Yingruo Li, Liyan Zhou, Qingfeng Shi
Front. Environ. Sci. Eng.. 2020, 14 (4): 71 .   https://doi.org/10.1007/s11783-020-1250-0
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•PAN concentrations at a rural site near Beijing were monitored from 2015 to 2019.

•PAN concentrations exhibited high values in spring and low values in winter.

•Anomalously southerlies induced extreme high PAN concentration in spring 2018.

Peroxyacetyl nitrate (PAN) is one of the most important photochemical pollutants and has aroused much concern in China in recent decades. However, few studies described the long-term variations in PAN in China. In this study, we continuously monitored the PAN, O3 and NOx concentrations at a regional background site near Beijing from August 2015 to February 2019. Based on the observed concentrations and climate data, we analyzed the seasonal PAN variations. The results revealed that the monthly mean PAN concentration ranged from 0.33–2.41 ppb, with an average value of 0.94 ppb. The PAN concentration exhibited a distinct seasonal variation, with high values in spring and low values in winter. After analyzing the corresponding meteorological data, we found that stronger ultraviolet (UV) radiation, a relatively longer lifetime and a higher background PAN concentration contributed to the high PAN concentrations in spring. In addition, with the utilization of the WRF-Chem (Weather Research and Forecasting with Chemistry) model, the cause of the extremely high PAN concentration in spring 2018 was determined. The model results demonstrated that an anomalously low pressure and the southwesterly winds in northern China might be the main causes of the increased PAN concentration in Beijing and its surrounding area in spring 2018.

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Synthesis of vinasse-dolomite nanocomposite biochar via a novel developed functionalization method to recover phosphate as a potential fertilizer substitute
Nima Kamali, Abdollah Rashidi Mehrabadi, Maryam Mirabi, Mohammad Ali Zahed
Front. Environ. Sci. Eng.. 2020, 14 (4): 70 .   https://doi.org/10.1007/s11783-020-1249-6
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• Nanocomposites were prepared by adding dolomite to vinasse at different ratio.

• Textural and morphological features of adsorbents were studied in detail.

• CCD based RSM was used for investigation of P ion removal by nanocomposite.

• The qm based on Langmuir model for modified vinasse biochar was 178.57 mg/g.

• P loaded nanocomposite improved plant growth and could be utilized as P-fertilizer.

The effectiveness of phosphate (P) removal from aqueous solutions was investigated by novel low-cost biochars synthesized from vinasse and functionalized with calcined dolomite. The vinasse-derived biochar, synthesized via pyrolysis at different temperatures, showed easy preparation and a large surface area. The novel vinasse biochar nanocomposites were prepared by adding dolomite to the vinasse biochars with different weight percentages (10, 20 and 30%). The characteristics of the prepared materials were identified for further understanding of the inherent adsorption mechanism between P ions and vinasse biochars. Vinasse-dolomite nanocomposite was very effective in the adsorption of P species from aqueous media. The effect of the operational factors on Vinasse-dolomite nanocomposite was explored by applying response surface methodology (RSM). According to RSM results, the optimum condition was achieved to be contact time 90 (min), 250 (mg/L) of P concentration and pH 7. Thermodynamic isotherm and kinetic studies were applied on experimental data to understand the adsorption behavior. The Vinasse-dolomite nanocomposite revealed preferential P species adsorption in the presence of co-existing anions. The P species could be recovered by 1.0 M HCl where the efficiency was not affected up to the fifth cycle. The P-loaded Vinasse-dolomite nanocomposite was successfully tested on a plant; it significantly improved its growth and proved its potency as a P-based fertilizer substitute.

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Photocatalytic water splitting of ternary graphene-like photocatalyst for the photocatalytic hydrogen production
Yan Zhang, Yuyan Zhang, Xue Li, Xiaohan Zhao, Cosmos Anning, John Crittenden, Xianjun Lyu
Front. Environ. Sci. Eng.. 2020, 14 (4): 69 .   https://doi.org/10.1007/s11783-020-1248-7
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•The MoS2/SiC/GO composite has a strong photocatalytic activity than SiC.

•The optimal catalyst yielded the highest quantum of 21.69%.

•GO acts as a bridge for electron passage in photocatalytic reaction.

In recent times, therehas been an increasing demand for energy which has resulted in an increased consumption of fossil fuels thereby posing a number of challenges to the environment. In the course finding possible solutions to this environmental canker, solar photocatalytic water splitting to produce hydrogengas has been identified as one of the most promising methods for generating renewable energy. To retard the recombination of photogenerated carriers and improve the efficiencyof photocatalysis, the present paper reports a facile method called the hydrothermal method, which was used to prepare ternary graphene-like photocatalyst. A “Design Expert” was used to investigate the influence of the loading weight of Mo and GO as well as the temperature of hydrothermal reaction and their interactions on the evolution of hydrogen (H2) in 4 h. The experimental results showed that the ternary graphene-like photocatalyst has a strong photocatalytic hydrogen production activity compared to that of pure SiC. In particular, the catalyst added 2.5 wt% of GO weight yielded the highest quantum of 21.69 % at 400–700 nm of wavelength. The optimal evolution H2 in 4 h conditions wasobtained as follows: The loading weight of Mo was 8.19 wt%, the loading weight of GO was 2.02 wt%, the temperature of the hydrothermal reaction was 200.93°C. Under the optimum conditions, the evolution of H2 in 4 h could reach 4.2030 mL.

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In situ synthesis of FeS/Carbon fibers for the effective removal of Cr(VI) in aqueous solution
Rongrong Zhang, Daohao Li, Jin Sun, Yuqian Cui, Yuanyuan Sun
Front. Environ. Sci. Eng.. 2020, 14 (4): 68 .   https://doi.org/10.1007/s11783-020-1247-8
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• FeS/carbon fibers were in situ synthesized with Fe-carrageenan hydrogel fiber.

• The double helix structure of carrageenan is used to load and disperse Fe.

• Pyrolyzing sulfate groups enriched carrageenan-Fe could easily generate FeS.

• The adsorption mechanisms include reduction and complexation reaction.

Iron sulfide (FeS) nanoparticles (termed FSNs) have attracted much attention for the removal of pollutants due to their high efficiency and low cost, and because they are environmentally friendly. However, issues of agglomeration, transformation, and the loss of active components limit their application. Therefore, this study investigates in situ synthesized FeS/carbon fibers with an Fe-carrageenan biomass as a precursor and nontoxic sulfur source to ascertain the removal efficiency of the fibers. The enrichment of sulfate groups as well as the double-helix structure in ι-carrageenan-Fe could effectively avoid the aggregation and loss of FSNs in practical applications. The obtained FeS/carbon fibers were used to control a Cr(VI) polluted solution, and exhibited a relatively high removal capacity (81.62 mg/g). The main mechanisms included the reduction of FeS, electrostatic adsorption of carbon fibers, and Cr(III)-Fe(III) complexation reaction. The pseudo-second-order kinetic model and Langmuir adsorption model both provided a good fit of the reaction process; hence, the removal process was mainly controlled by chemical adsorption, specifically monolayer adsorption on a uniform surface. Furthermore, co-existing anions, column, and regeneration experiments indicated that the FeS/carbon fibers are a promising remediation material for practical application.

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Validation of Bacteroidales-based microbial source tracking markers for pig fecal pollution and their application in two rivers of North China
Youfen Xu, Zong Li, Ruyin Liu, Hongxia Liang, Zhisheng Yu, Hongxun Zhang
Front. Environ. Sci. Eng.. 2020, 14 (4): 67 .   https://doi.org/10.1007/s11783-020-1246-9
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• Pig feces is the predominant excrement produced by animal husbandry in China.

• The PF, Pig-1-BacTaqMan, and Pig-2-BacTaqMan MST assays showed better performance.

• The pig-specific MST assays can contribute to managing the pig fecal pollution.

In China, pig feces is the predominant source of excrement produced by animal husbandry. Improper use or direct discharge of pig feces can result in contamination of natural water systems. Microbial source tracking (MST) technology can identify the sources of fecal pollution in environmental water, and contribute to the management of pig fecal pollution by local environmental protection agencies. However, the accuracy of such assays can be context-dependent, and they have not been comprehensively evaluated under Chinese conditions. We aimed to compare the performance of five previously reported pig-specific MST assays (PF, Pig-Bac1SYBR, Pig-Bac2SYBR, Pig-1-BacTaqMan, and Pig-2-BacTaqMan, which are based on Bacteroidales 16S rRNA gene markers) and apply them in two rivers of North China. We collected a total of 173 fecal samples from pigs, cows, goats, chickens, humans, and horses across China. The PF assay optimized in this study showed outstanding qualitative performance and achieved 100% specificity and sensitivity. However, the two SYBR green qPCR assays (Pig-Bac1SYBR and Pig-Bac2SYBR) cross-reacted with most non-pig fecal samples. In contrast, both the Pig-1-BacTaqMan and Pig-2-BacTaqMan assays gave 100% specificity and sensitivity. Of these, the Pig-2-BacTaqMan assay showed higher reproducibility. Our results regarding the specificity of these pig-specific MST assays differ from those reported in Thailand, Japan, and America. Using the PF and Pig-2-BacTaqMan assays, a field test comparing the levels of pig fecal pollution in rivers near a pig farm before and after comprehensive environmental pollution governance indicated that pig fecal pollution was effectively controlled at this location.

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Distribution of aromatic amines, phenols, chlorobenzenes, and naphthalenes in the surface sediment of the Dianchi Lake, China
Xinyi Hu, Ting Yang, Chen Liu, Jun Jin, Bingli Gao, Xuejun Wang, Min Qi, Baokai Wei, Yuyu Zhan, Tan Chen, Hongtao Wang, Yanting Liu, Dongrui Bai, Zhu Rao, Nan Zhan
Front. Environ. Sci. Eng.. 2020, 14 (4): 66 .   https://doi.org/10.1007/s11783-020-1245-x
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• The total organic pollutant concentrations in sediment were 27.4-1620 ng/g.

• The phenol concentrations were relatively high in the sediment of the Dianchi Lake.

• Average total concentrations decreased as follows: Caohai>Waihai>Haigeng Dam.

• 1,4-dichlorobenzene, 3- or 4-methylphenol, 1,2,4-trichlorobenzene might be risks.

Organic pollutants are widespread environmental pollutants with high toxicity, persistence, and bioaccumulation. Our aim was to investigate the distribution of aromatic amines, phenols, chlorobenzenes, and naphthalenes in the surface sediment of the Dianchi Lake, China. Nineteen surface sediment samples were collected from the Dianchi Lake, and 40 types of organic pollutants were analyzed via gas chromatography–mass spectrometry. The total organic pollutant concentrations in the surface sediment of the Dianchi Lake varied from 27.4 to 1.62 × 103 ng/g. The concentrations of phenols were much higher than those in other water bodies but still within a controllable range, whereas the concentrations of the other organic pollutant classes were similar or even lower. The detection ratio of 3- or 4-methylphenol was the highest (100.00%) among the pollutants. The average total organic pollutant concentrations decreased in the following order: Caohai (540 ng/g)>the middle of Waihai (488 ng/g)>the edge of Waihai (351 ng/g)>Haigeng Dam (90.4 ng/g). Pearson analysis showed a strong correlation among 1-methylnaphthalene, 2-methylnaphthalene, 1,3-dinitronaphthalene, and 1,4-dinitronaphthalene (p<0.01). Caohai, the north lakeshore of Waihai and the south of Waihai showed higher risk because of high concentration; meanwhile, 1,4-dichlorobenzene, 3- or 4-methylphenol and 1,2,4-trichlorobenzene were more likely to cause risks.

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Catalytic oxidation of CO over Pt/Fe3O4 catalysts: Tuning O2 activation and CO adsorption
Zihao Li, Yang Geng, Lei Ma, Xiaoyin Chen, Junhua Li, Huazhen Chang, Johannes W. Schwank
Front. Environ. Sci. Eng.. 2020, 14 (4): 65 .   https://doi.org/10.1007/s11783-020-1244-y
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• Strong metal-support interaction exists on Pt/Fe3O4 catalysts.

• Pt metal particles facilitate the formation of oxygen vacancies on Fe3O4.

• Fe3O4 supports enhance the strength of CO adsorption on Pt metal particles.

The self-inhibition behavior due to CO poisoning on Pt metal particles strongly impairs the performance of CO oxidation. It is an effective method to use reducible metal oxides for supporting Pt metal particles to avoid self-inhibition and to improve catalytic performance. In this work, we used in situ reductions of chloroplatinic acid on commercial Fe3O4 powder to prepare heterogeneous-structured Pt/Fe3O4 catalysts in the solution of ethylene glycol. The heterogeneous Pt/Fe3O4 catalysts achieved a better catalytic performance of CO oxidation compared with the Fe3O4 powder. The temperatures of 50% and 90% CO conversion were achieved above 260°C and 290°C at Pt/Fe3O4, respectively. However, they are accomplished on Fe3O4 at temperatures higher than 310°C. XRD, XPS, and H2-TPR results confirmed that the metallic Pt atoms have a strong synergistic interaction with the Fe3O4 supports. TGA results and transient DRIFTS results proved that the Pt metal particles facilitate the release of lattice oxygen and the formation of oxygen vacancies on Fe3O4. The combined results of O2-TPD and DRIFTS indicated that the activation step of oxygen molecules at surface oxygen vacancies could potentially be the rate-determining step of the catalytic CO oxidation at Pt/Fe3O4 catalysts. The reaction pathway involves a Pt-assisted Mars-van Krevelen (MvK) mechanism.

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Enhanced triallyl isocyanurate (TAIC) degradation through application of an O3/UV process: Performance optimization and degradation pathways
Yapeng Song, Hui Gong, Jianbing Wang, Fengmin Chang, Kaijun Wang
Front. Environ. Sci. Eng.. 2020, 14 (4): 64 .   https://doi.org/10.1007/s11783-020-1243-z
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• UV/O3 process had higher TAIC mineralization rate than O3 process.

• Four possible degradation pathways were proposed during TAIC degradation.

• pH impacted oxidation processes with pH of 9 achieving maximum efficiency.

• CO32– negatively impacted TAIC degradation while HCO3 not.

• Cl can be radicals scavenger only at high concentration (over 500 mg/L Cl).

Triallyl isocyanurate (TAIC, C12H15N3O3) has featured in wastewater treatment as a refractory organic compound due to the significant production capability and negative environmental impact. TAIC degradation was enhanced when an ozone(O3)/ultraviolet(UV) process was applied compared with the application of an independent O3 process. Although 99% of TAIC could be degraded in 5 min during both processes, the O3/UV process had a 70%mineralization rate that was much higher than that of the independent O3 process (9%) in 30 min. Four possible degradation pathways were proposed based on the organic compounds of intermediate products identified during TAIC degradation through the application of independent O3 and O3/UV processes. pH impacted both the direct and indirect oxidation processes. Acidic and alkaline conditions preferred direct and indirect reactions respectively, with a pH of 9 achieving maximum Total Organic Carbon (TOC) removal. Both CO32– and HCO3 decreased TOC removal, however only CO32– negatively impacted TAIC degradation. Effects of Cl as a radical scavenger became more marked only at high concentrations (over 500 mg/L Cl). Particulate and suspended matter could hinder the transmission of ultraviolet light and reduce the production of HO· accordingly.

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Transport of bacterial cell (E. coli) from different recharge water resources in porous media during simulated artificial groundwater recharge
Wei Fan, Qi Li, Mingxin Huo, Xiaoyu Wang, Shanshan Lin
Front. Environ. Sci. Eng.. 2020, 14 (4): 63 .   https://doi.org/10.1007/s11783-020-1242-0
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• The recharge pond dwelling process induced changes in cell properties.

• Cell properties and solution chemistry exerted confounding effect on cell transport.

E. coli cells within different recharge water displayed different spreading risks.

Commonly used recharge water resources for artificial groundwater recharge (AGR) such as secondary effluent (SE), river water and rainfall, are all oligotrophic, with low ionic strengths and different cationic compositions. The dwelling process in recharge pond imposed physiologic stress on Escherichia coli (E. coli) cells, in all three types of investigated recharge water resources and the cultivation of E. coli under varying recharge water conditions, induced changes in cell properties. During adaptation to the recharge water environment, the zeta potential of cells became more negative, the hydrodynamic diameters, extracellular polymeric substances content and surface hydrophobicity decreased, while the cellular outer membrane protein profiles became more diverse. The mobility of cells altered in accordance with changes in these cell properties. The E. coli cells in rainfall recharge water displayed the highest mobility (least retention), followed by cells in river water and finally SE cells, which had the lowest mobility. Simulated column experiments and quantitative modeling confirmed that the cellular properties, driven by the physiologic state of cells in different recharge water matrices and the solution chemistry, exerted synergistic effects on cell transport behavior. The findings of this study contribute to an improved understanding of E. coli transport in actual AGR scenarios and prediction of spreading risk in different recharge water sources.

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A novel approach to preparing ultra-lightweight ceramsite with a large amount of fly ash
Sen Liu, Congren Yang, Wei Liu, Longsheng Yi, Wenqing Qin
Front. Environ. Sci. Eng.. 2020, 14 (4): 62 .   https://doi.org/10.1007/s11783-020-1241-1
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•Ultra-lightweight ceramsite is prepared with 80% fly ash.

•SiO2, Al2O3, and flux contents significantly influence the performance of ceramsite.

•The expansion of ceramsite is caused by the formation of a dense glaze and gas.

•The bulk density of ultra-lightweight ceramsite is only 340 kg/m3.

The disposal of fly ash has become a serious problem in China due to its rapid increase in volume in recent years. The most common method of fly ash disposal is solidification-stabilization-landfill, and the most common reuse is low-value-added building materials. A novel processing method for preparing ultra-lightweight ceramsite with fly ash was developed. The results show that the optimal parameters for preparation of ultra-lightweight ceramsite are as follows: mass ratio of fly ash:kaolin:diatomite= 80:15:5, preheating temperature of 800°C, preheating time of 5 min, sintering temperature of 1220°C, and sintering time of 10 min. The expansion agent is perlite, at 10 wt.% addition. Finally, a ceramsite with bulk density of 340 kg/m3, particle density of 0.68 g/cm3, and cylinder compressive strength of 1.02 MPa was obtained. Because of its low density and high porosity, ultra-lightweight ceramsite has excellent thermal insulation performance, and its strength is generally low, so it is usually used in the production of thermal insulation concrete and its products. The formation of a liquid-phase component on the surface, and generation of a gas phase inside ceramsite during the sintering process, make it possible to control the production of the suitable liquid phase and gas in this system, resulting in an optimization of the expansion behavior and microstructure of ceramsite. These characteristics show the feasibility of industrial applications of fly ash for the production of ultra-lightweight ceramsite, which could not only produce economic benefits, but also conserve land resources and protect the environment.

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Sugarcane bagasse amendment improves the quality of green waste vermicompost and the growth of Eisenia fetida
Linlin Cai, Xiangyang Sun, Dan Hao, Suyan Li, Xiaoqiang Gong, Hao Ding, Kefei Yu
Front. Environ. Sci. Eng.. 2020, 14 (4): 61 .   https://doi.org/10.1007/s11783-020-1240-2
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•Earthworms were able to convert green waste into more plant-available nutrients.

•The part of heavy metals content increased in the compost added by earthworm.

•The addition of SCB to GW did enhance earthworm biomass and humic acid content.

•The resulting vermicomposts were characterized by neutral pH and lower EC value.

Vermicomposting is a feasible method for disposing of lignocellulosic waste while generating a useful product. The current study assessed the potential of vermicomposting green waste mixed with sugarcane bagasse in proportions of 25%, 50%, and 75% (v:v, based on dry weight). The suitability was evaluated based on the agrochemical properties, earthworm biomass, and phytotoxicity. The final vermicomposts exhibited near-neutral pH values (7.1–7.6), and lower EC values (0.43–0.72 mS/cm) and C:N ratios (14.1–19.9).The content of available nutrients and CEC for all the vermicomposts exceeded those of the control compost (without earthworms). For vermicomposts, the average values of NO3-N, AP, AK, and CEC were 53, 517, 1362 mg/kg, and 158 cmol/kg, respectively. The total contents of heavy metals increased in all vermicompost treatments compared to control composts with the following average final percentages: Zn (2.0%), Cr (15.5%), Pb (23.4%), and Cu (44.3%), but these amounts were safe for application in agroforestry. The addition of sugarcane bagasse to green waste significantly increased the content of total humic substance, humic acid and urease activity, acid and alkaline phosphatase activity, and Eiseniafetida reproduction. The addition of 25% sugarcane bagasse to green waste decreased the toxicity to germinating seeds. These results revealed that vermicomposting is a feasible way to degrade green waste into a value-added chemical product.

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Insights from metagenomic, metatranscriptomic, and molecular ecological network analyses into the effects of chromium nanoparticles on activated sludge system
Quan Zheng, Minglu Zhang, Tingting Zhang, Xinhui Li, Minghan Zhu, Xiaohui Wang
Front. Environ. Sci. Eng.. 2020, 14 (4): 60 .   https://doi.org/10.1007/s11783-020-1239-8
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• Cr NPs significantly promoted the denitrifying process and phosphorus removal.

• The quantities of denitrifying and phosphorus genes greatly increased.

• The transcription of denitrifying and phosphorus genes greatly increased.

• The networks associated with nitrogen and phosphorus removal became complicated.

• Cr NPs decreased some genera related to GAOs.

The objectives of this study were to investigate the influence of chromium nanoparticles (Cr NPs) on the nitrogen and phosphorus removal performance and the bacterial structures of an activated sludge (AS) system. Also, we through molecular ecological networks (MENs) discussed the bacterial interactions. At last we researched the change of the functional genes and their expression patterns related to nitrogen and phosphorus removal in an AS system. The results showed that long-term exposure to 1 mg/L Cr NPs significantly promoted the denitrifying process and phosphorus removal in the AS system. The relative abundance of denitrifying and phosphorus removal microorganisms, such as Denitratisoma, Thauera, Dechloromonas, and Defluviicoccus, increased significantly. Candidatus Accumulibacter, well-known as polyphosphate-accumulating organisms (PAOs), increased significantly; the relative abundance of Candidatus Competibacter, known as glycogen-accumulating organisms (GAOs), decreased significantly. Furthermore, metagenomic and metatranscriptomic analysis revealed that most of the genera related to denitrifying and phosphorus removal had greatly increased, according to the quantities of denitrifying and phosphorus genes, and the corresponding transcription likewise greatly increased. Lastly, MENs analysis showed that although the overall network became smaller and looser in the presence of Cr NPs, the microbial connections among members related to nitrogen and phosphorus removal were enhanced. The abundance increases of denitrifiers and PAOs, and their increased transcription of functional genes, together with the enhanced interactions may be associated with the promotion of the denitrifying process and phosphorus removal.

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Co-application of energy uncoupling and ultrafiltration in sludge treatment: Evaluations of sludge reduction, supernatant recovery and membrane fouling control
An Ding, Yingxue Zhao, Zhongsen Yan, Langming Bai, Haiyang Yang, Heng Liang, Guibai Li, Nanqi Ren
Front. Environ. Sci. Eng.. 2020, 14 (4): 59 .   https://doi.org/10.1007/s11783-020-1238-9
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• Effects of metabolic uncouplers addition on sludge reduction were carried out.

• TCS addition effectively inhibited ATP synthesis and reduced sludge yield.

• The effluent quality such as TOC and ammonia deteriorated but not significantly.

• Suitable dosage retarded biofouling during sludge water recovery by UF membrane.

Energy uncoupling is often used for sludge reduction because it is easy to operate and does not require a significant amount of extra equipments (i.e. no additional tank required). However, over time the supernatant extracted using this method can deteriorate, ultimately requiring further treatment. The purpose of this study was to determine the effect of using a low-pressure ultrafiltration membrane process for sludge water recovery after the sludge had undergone an energy uncoupling treatment (using 3,3′,4′,5-tetrachlorosalicylanilide (TCS)). Energy uncoupling was found to break apart sludge floc by reducing extracellular polymeric substances (EPS) and adenosine triphosphate (ATP) content. Analysis of supernatant indicated that when energy uncoupling and membrane filtration were co-applied and the TCS dosage was below 30 mg/L, there was no significant deterioration in organic component removal. However, ammonia and phosphate concentrations were found to increase as the concentration of TCS added increased. Additionally, due to low sludge concentrations and EPS contents, addition of 30–60 mg/L TCS during sludge reduction increased the permeate flux (two times higher than the control) and decreased the hydraulic reversible and cake layer resistances. In contrast, high dosage of TCS aggravated membrane fouling by forming compact fouling layers. In general, this study found that the co-application of energy uncoupling and membrane filtration processes represents an effective alternative method for simultaneous sludge reduction and sludge supernatant recovery.

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Diphenylarsinic acid sorption mechanisms in soils using batch experiments and EXAFS spectroscopy
Meng Zhu, Yongming Luo, Ruyi Yang, Shoubiao Zhou, Juqin Zhang, Mengyun Zhang, Peter Christie, Elizabeth L. Rylott
Front. Environ. Sci. Eng.. 2020, 14 (4): 58 .   https://doi.org/10.1007/s11783-020-1237-x
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• DPAA sorption data was found to fit the Freundlich equation.

Kf was significantly positive correlated with oxalate-extractable Fe2O3.

• Ligand exchange was the main mechanism for DPAA sorption on soils.

• Bidentate binuclear and monodentate mononuclear DPAA bonds were identified.

Diphenylarsinic acid (DPAA) is a phenyl arsenic compound derived from chemical warfare weapons. Macroscopic and microscopic work on DPAA sorption will provide useful information in predicting the partitioning and mobility of DPAA in the soil-water environment. Here, batch experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy were used to investigate the sorption mechanisms of DPAA. The DPAA sorption data from 11 soil types was found to fit the Freundlich equation, and the sorption capacity, Kf, was significantly and positively correlated with oxalate-extractable Fe2O3. The Kf values of eight of the 11 untreated soils (1.51–113.04) significantly decreased upon removal of amorphous metal (hydr)oxides (0.51–13.37). When both amorphous and crystalline metal (hydr)oxides were removed from the untreated soils, the Kf values either decreased or slightly increased (0.65–3.09). Subsequent removal of soil organic matter from these amorphous and crystalline metal (hydr)oxide-depleted samples led to further decreases in Kf to 0.02–1.38, with only one exception (Sulfic Aquic-Orthic Halosols). These findings strongly suggest that ligand exchange reactions with amorphous metal (hydr)oxides contribute most to DPAA sorption on soils. EXAFS data provide further evidence that DPAA primarily formed bidentate binuclear (2C) and monodentate mononuclear (1V) coring-sharing complexes with As-Fe distances of 3.34 and 3.66 Å, respectively, on Fe (hydr)oxides. Comparison of these results with earlier studies suggests that 2C and 1V complexes of DPAA may be favored under low and high surface coverages, respectively, with the formation of 1V bonds possibly conserving the sorption sites or decreasing the steric hindrance derived from phenyl substituents.

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Sulfur cycle as an electron mediator between carbon and nitrate in a constructed wetland microcosm
Wenrui Guo, Yue Wen, Yi Chen, Qi Zhou
Front. Environ. Sci. Eng.. 2020, 14 (4): 57 .   https://doi.org/10.1007/s11783-020-1236-y
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• Fe(III) accepted the most electrons from organics, followed by NO3, SO42‒, and O2.

• The electrons accepted by SO42‒ could be stored in the solid AVS, FeS2-S, and S0.

• The autotrophic denitrification driven by solid S had two-phase characteristics.

• A conceptual model involving electron acceptance, storage, and donation was built.

• S cycle transferred electrons between organics and NO3 with an efficiency of 15%.

A constructed wetland microcosm was employed to investigate the sulfur cycle-mediated electron transfer between carbon and nitrate. Sulfate accepted electrons from organics at the average rate of 0.84 mol/(m3·d) through sulfate reduction, which accounted for 20.0% of the electron input rate. The remainder of the electrons derived from organics were accepted by dissolved oxygen (2.6%), nitrate (26.8%), and iron(III) (39.9%). The sulfide produced from sulfate reduction was transformed into acid-volatile sulfide, pyrite, and elemental sulfur, which were deposited in the substratum, storing electrons in the microcosm at the average rate of 0.52 mol/(m3·d). In the presence of nitrate, the acid-volatile and elemental sulfur were oxidized to sulfate, donating electrons at the average rate of 0.14 mol/(m3·d) and driving autotrophic denitrification at the average rate of 0.30 g N/(m3·d). The overall electron transfer efficiency of the sulfur cycle for autotrophic denitrification was 15.3%. A mass balance assessment indicated that approximately 50% of the input sulfur was discharged from the microcosm, and the remainder was removed through deposition (49%) and plant uptake (1%). Dominant sulfate-reducing (i.e., Desulfovirga, Desulforhopalus, Desulfatitalea, and Desulfatirhabdium) and sulfur-oxidizing bacteria (i.e., Thiohalobacter, Thiobacillus, Sulfuritalea, and Sulfurisoma), which jointly fulfilled a sustainable sulfur cycle, were identified. These results improved understanding of electron transfers among carbon, nitrogen, and sulfur cycles in constructed wetlands, and are of engineering significance.

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REVIEW ARTICLE
Magnetotactic bacteria: Characteristics and environmental applications
Xinjie Wang, Yang Li, Jian Zhao, Hong Yao, Siqi Chu, Zimu Song, Zongxian He, Wen Zhang
Front. Environ. Sci. Eng.. 2020, 14 (4): 56 .   https://doi.org/10.1007/s11783-020-1235-z
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• Magnetotactic bacteria (MTB) synthesize magnetic nanoparticle within magnetosomes.

• The morphologic and phylogenetic diversity of MTB were summarized.

• Isolation and mass cultivation of MTB deserve extensive research for applications.

• MTB can remove heavy metals, radionuclides, and organic pollutants from wastewater.

Magnetotactic bacteria (MTB) are a group of Gram-negative prokaryotes that respond to the geomagnetic field. This unique property is attributed to the intracellular magnetosomes, which contains membrane-bound nanocrystals of magnetic iron minerals. This review summarizes the most recent advances in MTB, magnetosomes, and their potential applications especially the environmental pollutant control or remediation. The morphologic and phylogenetic diversity of MTB were first introduced, followed by a critical review of isolation and cultivation methods. Past research has devoted to optimize the factors, such as oxygen, carbon source, nitrogen source, nutrient broth, iron source, and mineral elements for the growth of MTB. Besides the applications of MTB in modern biological and medical fields, little attention was made on the environmental applications of MTB for wastewater treatment, which has been summarized in this review. For example, applications of MTB as adsorbents have resulted in a novel magnetic separation technology for removal of heavy metals or organic pollutants in wastewater. In addition, we summarized the current advance on pathogen removal and detection of endocrine disruptor which can inspire new insights toward sustainable engineering and practices. Finally, the new perspectives and possible directions for future studies are recommended, such as isolation of MTB, genetic modification of MTB for mass production and new environmental applications. The ultimate objective of this review is to promote the applications of MTB and magnetosomes in the environmental fields.

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