Progress in layered double hydroxides (LDHs): Synthesis and application in adsorption, catalysis and photoreduction

Layered double hydroxides (LDHs), also known as anionic clays, have attracted significant attention in energy and environmental applications due to their exceptional physicochemical properties. These materials possess a unique structure with surface hydroxyl groups, tunable properties, and high stability, making them highly desirable. In this review, the synthesis and functionalization of LDHs have been explored including co-precipitation and hydrothermal methods. Furthermore, extensive research on LDH application in toxic pollutant removal has shown that modifying or functionalizing LDHs using materials such as activated carbon, polymers, and inorganics is crucial for achieving efficient pollutant adsorption, improved cyclic performance, as well as effective catalytic oxidation of organics and photoreduction. This study offers a comprehensive overview of the progress made in the field of LDHs and LDH-based composites for water and wastewater treatment. It critically discusses and explains both direct and indirect synthesis and modification techniques, highlighting their advantages and disadvantages. Additionally, this review critically discusses and explains the potential of LDH-based composites as absorbents. Importantly, it focuses on the capability of LDH and LDH-based composites in heterogeneous catalysis, including the Fenton reaction, Fenton-like reactions, photocatalysis, and photoreduction, for the removal of organic dyes, organic micropollutants, and heavy metals. The mechanisms involved in pollutant removal, such as adsorption, electrostatic interaction, complexation, and degradation, are thoroughly explained. Finally, this study outlines future research directions in the field.

Redox mediators boost NOx reduction via trade-off electron charges using a cube-shaped (cMn@rGO) catalyst; mechanism and electrochemical study

Gaseous pollutants like sulfur dioxide and nitrogen oxide(s) (SO2, NOx) have been increasing exponentially for the last two decades, which have had adverse effects on human health, aquatic life, and the environment. Recently, for air pollution taming, manganese/oxide (Mn/MnO) has become a very promising heterogeneous catalyst due to its environment-friendly, low-price, and remarkable catalytic abilities for toxic gases. In this work, cube-shaped Mn nanoparticles (cMn NPs) were decorated on the surface of reduced graphene oxide (rGO) by the solvothermal method. The resulting cMn@rGO composite was employed for electrochemical NOx reduction. However, the microscopic (TEM/HRTEM) and structural analysis were utilised to investigate the morphology and characteristics of the cMn@rGO composite. This electrochemical-based treatment for NOx reduction is employed by using electron shuttle or redox mediators. Here, four distinct redox mediators are used to address electrochemical obstacles, which effectively facilitate electron transportation and promoted NOx reduction on the electrode surface. These mediators not only significantly enhanced the NOx conversion into valuable products, i.e., N2 and N2O, but also made the process smooth with high performance. Among these mediators, neutral red (N.R) exhibited extraordinary potential in enhancing NOx reduction. The obtained results indicated that the remarkable catalytic performance (∼93%) of the cMn@rGO can be attributed to several factors, including the catalyst's three-dimensional architecture structure and abundant active sites. The designed catalyst (cMn@rGO) is not only cost-effective and sustainable but also exhibits excellent potential in effectively reducing NOx, which could be beneficial for large-scale NOx abatement.

Heavy metals in five commonly consumed fish species from River Swat, Pakistan, and their implications for human health using multiple risk assessment approaches

This study analyzed the levels of heavy metals bioaccumulation in commonly consumed riverine fish species, including G. cavia, T. macrolepis, G. gotyla, S. plagiostomus, and M. armatus from River Swat in Pakistan, and quantify their potential risk to children and adults in general and fisherfolk communities using multiple pollution and risk assessment approaches. The highest metal detected by inductive coupled plasma mass spectrometry (ICP-MS) was Zn, which ranged from 49.61 to 116.83 mg/kg, followed by Fe (19.25-101.33 mg/kg) > Mn (5.25-40.35 mg/kg) > Cr (3.05-14.59 mg/kg) > Ni (4.26-11.80 mg/kg) > Al (1.59-12.25 mg/kg) > Cu (1.24-8.59 mg/kg) > Pb (0.29-1.95 mg/kg) > Co (0.08-0.46 mg/kg) > Cd (0.01-0.29 mg/kg), demonstrating consistent fluctuation with the safe recommendations of global regulatory bodies. The average bioaccumulation factor (BAF) values in the examined fish species were high (BAF > 5000) for Pb, Zn, Mn, Cu, Cr, Ni, and Cd, bioaccumulate (1000 > BAF < 5000) for Co, and probable accumulative (BAF <1000) for Fe, and Al, while the overall ∑heavy metals pollution index (MPI) values were greater than one (MPI > 1) indicating sever heavy metals toxicity in G. cavia, followed by S. plagiostomus, M. armatus, G. gotyla, and T. macrolepis. The multivariate Pearson's correlation analysis identified the correlation coefficients between heavy metal pairs (NiCr, CuCr, PbCr, AlCo, CuNi, and PbNi), the hierarchical cluster analysis (CA) determined the origin by categorizing heavy metal accumulation into Cluster-A, Cluster-B, and Cluster-C, and the principal component analysis (PCA) discerned nearby weathering, mining, industrial, municipal, and agricultural activities as the potential sources of heavy metals bioaccumulation in riverine fish. As per human risk perspective, S.plagiostomus contributed significantly to the estimated daily intake (EDI) of heavy metals, followed by G.cavia > M.armatus > G.gotyla > T.macrolepis in dependent children and adults of the fisherfolk followed by the general population. The non-carcinogenic target hazard quotient (THQ) and hazard index (HI) values for heavy metal intake through fish exposure were < 1, while the carcinogenic risk (CR) for individual metal intake and the total carcinogenic risk (TCR) for cumulative Cr, Cd, and Pb intake were within the risk threshold of 10-6-10-4, suggesting an acceptable to high non-carcinogenic and carcinogenic risk for both children and adults in the fisherfolk, followed by the general population.

Strategic analysis on development of simultaneous adsorption and catalytic biodegradation over advanced bio-carriers for zero-liquid discharge of industrial wastewater

With rapid industrial development, millions of tons of industrial wastewater are produced that contain highly toxic, carcinogenic, mutagenic compounds. These compounds may consist of high concentration of refractory organics with plentiful carbon and nitrogen. To date, a substantial proportion of industrial wastewater is discharged directly to precious water bodies due to the high operational costs associated with selective treatment methods. For example, many existing treatment processes rely on activated sludge-based treatments that only target readily available carbon using conventional microbes, with limited capacity for nitrogen and other nutrient removal. Therefore, an additional set-up is often required in the treatment chain to address residual nitrogen, but even after treatment, refractory organics persist in the effluents due to their low biodegradability. With the advancements in nanotechnology and biotechnology, novel processes such as adsorption and biodegradation have been developed, and one promising approach is integration of adsorption and biodegradation over porous substrates (bio-carriers). Regardless of recent focus in a few applied researches, the process assessment and critical analysis of this approach is still missing, and it highlights the urgency and importance of this review. This review paper discussed the development of the simultaneous adsorption and catalytic biodegradation (SACB) over a bio-carrier for the sustainable treatment of refractory organics. It provides insights into the physico-chemical characteristics of the bio-carrier, the development mechanism of SACB, stabilization techniques, and process optimization strategies. Furthermore, the most efficient treatment chain is proposed, and its technical aspects are critically analysed based on updated research. It is anticipated that this review will contribute to the knowledge of academia and industrialist for sustainable upgradation of existing industrial wastewater treatment plants.

Spontaneous intra-electron transfer within rGO@Fe2O3-MnO catalyst promotes long-term NOx reduction at ambient conditions

Iron (Fe)-based catalysts are widely used for taming nitrogen oxides (NO_x) containing flue gas, but the regeneration and long-term reusability remains a concern. The reusability can be acquired by external additives, and resultantly can not only increase the cost but can also add to process complexity as well as secondary pollutants. Herein, a self-sustainable material is designed to regenerate the catalyst for long-term reusability without adding to process complexity. The catalyst is based on reduced graphene-oxide impregnated by Fe₂O₃-MnO (rGO@Fe₂O₃-MnO; G-F-M) for spontaneous intra electron (e^-)-transfer from Mn to Fe. The developed catalyst; G-M-F exhibited 93.7% NO_x reduction, which suggests its high catalytic activity. The morphological and structure characterizations confirmed the Fe/Mn loading, contributing to e^--transfer between Mn and Fe due to its conductivity. The synthesized G-F-M showed higher NO_x reduction about 2.5 folds, than rGO@Fe₂O₃ (G-FeO) and rGO@MnO_x (G-MnO_x). The performance of G-M-F without and with an electrochemical system was also compared, and the difference was only 5%, which is an evidence of the spontaneous e^- transfer between the Mn and Fe-NO_x complex. The designed catalyst can be used for a long time without external assistance, and its efficiency was not affected significantly (<3.7%) in the presence of high oxygen contents (8%). The as-prepared G-M-F catalyst has great potential for executing a dual role NOx removal and self-regeneration of catalyst (SRC), promoting a sustainable remediation approach for large-scale applications.

Separation of Fe from wastewater and its use for NOx reduction; a sustainable approach for environmental remediation

The nitrogen and sulphur oxide (NO_x and SO₂) emissions are causing a serious threat to the existence of life on earth, requiring their effective removal for a sustainable future. Among various approaches, catalytic or electrochemical reduction of air pollutants (NO_x) has gained much attention due to its high efficiency and the possibility of converting these gases into valuable products. However, the required catalysts are generally synthesized from lab-grade chemicals, which may not be a sustainable approach. Herein, a sustainable approach is presented to synthesize an efficient iron-based catalyst directly from industrial/lake wastewater (WW) for NO_x-reduction. According to the theoretical calculations and experimental results, Fe-ions could be readily recovered from wastewater because it has the best adsorption efficiency among all other co-existing metals (Ni²⁺, Cd²⁺, Co²⁺, Cu²⁺, and Cr⁶⁺). The subsequent experimental investigations confirmed the preferential Fe adsorption from different WW streams to develop Fe₃O₄@EDTA-Fe composite, whereby Fe₃O₄ could be used due to its high recycling ability, and ethylenediaminetetraacetic acid (EDTA) acted as a chelating agent to adsorb Fe-metal from effluents. The Fe₃O₄@EDTA-Fe exhibited high efficiency (≥87%) for NO_x reduction even in the presence of high-degree oxygen contents (10-12%). Moreover, Fe₃O₄-EDTA-Fe showed excellent long-term stability for 24 h and maintained more than 80% NO_x reduction. The fabricated catalyst has a great potential for executing a dual role simultaneously for Fe-recovery and NO_x removal, promoting the circular economy concept and providing a potentially sustainable remediation approach for large-scale applications.

First report of perfluoroalkyl acids (PFAAs) in the Indus Drainage System: Occurrence, source and environmental risk

Perfluoroalkyl acids (PFAAs) are of global interest due to their persistence in the aquatic environment. This study assessed the occurrence of PFAAs in the Indus Drainage System and discerned their potential sources and environmental risks for the first time in Pakistan. 13 perfluoroalkyl carboxylic acids (PFCAs) and 4 perfluoroalkyl sulfonates (PFSAs) were analyzed to verify the dominant prevalence of short-chain PFAAs in the environment since the phase-out of long-chain perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). A significant variation (p ≤ 0.05) of individual PFAAs between the monitoring sites was confirmed by data normality tests Kolmogorov-Smirnov and Shapiro-Wilk, suggesting that different locations contribute differently to individual PFAAs concentrations. ΣPFAAs concentrations in riverine water and sediments ranged from 2.28 to 221.75 ng/L and 0.78-29.19 ng/g dw, respectively. PFBA, PFPeA, and PFHxA were the most abundant PFAAs, and on average accounted for 14.64, 13.75, and 12.97 ng/L of ∑PFAAs in riverine water and 0.34, 0.64, and 0.79 ng/g dw of ∑PFAAs in sediments. ΣPFAAs mean contamination in the drainage was significantly (p < 0.05) high in River Chenab followed by River Indus > Soan > Ravi > Kabul > Swat with more prevalence of short-chain (C4-C7) PFCAs followed by PFOA, PFBS, PFOS, PFNA, PFDA, PFHxS, PFUnDA, and PFDoDA. The correlation analysis determined the PFAAs' fate and distribution along the drainage, indicating that PFAAs with carbon chains C4-C12, except for PFSAs with carbon chains C6-C8, were most likely contaminated by the same source, the values of Kd and Koc increased linearly with the length of the perfluoroalkyl carbon chain, better understand the transport and partitioning of individual PFAAs between riverine water and sediments, where the HCA and PCA discerned industrial/municipal wastewater discharge, agricultural and surface runoff from nearby fields, and urban localities as potential sources of PFAAs contamination. The collective mass flux of short-chain (C4-C7) PFCAs was 5x higher than that of PFOS + PFOA, suggesting a continuous shift in the production and usage of fluorinated replacements for long-chain PFAAs with short-chain homologs. In terms of risk, individual PFAAs pollution in the drainage was within the world's risk thresholds for human health, with the exception of PFBA, PFPeA, PFHpA, PFHxA, PFOA, PFNA, and PFBS, whereas for ecology, the concentrations of individual PFAAs did not exceed the ecological risk thresholds of the United States of America, Canada, European Union (EU), Italy, Australia, and New Zealand, with the exception of PFSAs, whose detected individual concentrations were significantly higher than the EU, Australian and New Zealander PFSAs guidelines of 0.002 μg/L, 0.00047 μg/L, 0.00065 μg/L, 0.00013 μg/L, and 0.00023 μg/L, respectively, which may pose chronic risks to the regional ecosystem and population.

Heavy metal pollution in the soil of a riverine basin: distribution, source, and potential hazards

Soil pollution with heavy metals (HMs) has become a world environmental problem. This study focuses on surface soil contamination with Cr, Mn, Co, Ni, Cu, Zn, Cd, Hg, Pb, Fe, and Al, their sources, and potential hazards along the basin of River Swat, Pakistan. The average concentrations (mg/kg) of HMs were the most abundant for Al (24,730.19) followed by Fe (22,419.41) > Mn (386.78) > Zn (57.75) > Cr (38.07) > Ni (32.46) > Cu (23.43) > Pb (19.59) > Co (10.77) > Cd (3.18) > Hg (0.12). The concentrations of Cr and Mn in 5.45% each, Co in 10.90%, Zn in 27.27%, Cu in 36.36%, Ni in 41.81%, and Hg in 92.72% of the total soil samples exceeded their respective background values. The geostatistical approaches determined the distribution patterns of HM pollution along the basin, whereas the statistics of principal component analysis exposed the likely sources of HM contamination in the area. Pollution indices evaluated the overall HM distribution and pollution status in the area. Contamination factor showed a high degree of HM contamination in 82% of the total sampling sites, while the geo-accumulation index designated low to moderate contamination with Cr, Mn, Co, Ni, Cu, Zn, Hg, and Pb, and moderate to extreme contamination with Cd, Fe, and Al. The trend of ecological toxicity showed potential ups and downs along with the sites from low to considerable hazard (< 95 < PEHI < 190), whereas the human carcinogenic hazard was within the USEPA acceptable limits (1 × 10^-7-1 × 10^-4), but the non-carcinogenic hazard was higher than the threshold (HI > 1) for children because they are more exposed than adults.

Is HFPO-DA (GenX) a suitable substitute for PFOA? A comprehensive degradation comparison of PFOA and GenX via electrooxidation

Due to the potential hazard of perfluorooctanoic acid (PFOA), hexafluoropropylene oxide dimer acid (HFPO-DA, GenX) has become a typical alternative since 2009. However, GenX has recently been reported to have equal or even greater toxicity and bioaccumulation than PFOA. Considering the suitability of alternatives, it is quite essential to study and compare the degradation degree between PFOA and GenX in water. Therefore, in the present study, a comprehensive degradation comparison between them via electrooxidation with a titanium suboxide membrane anode was conducted. The degradation rate decreased throughout for PFOA, while it first increased and then decreased for GenX when the permeate flux increased from 17.3 L to 100.3 L m^-2·h^-1. The different responses of PFOA and GenX to flux might be attributed to their different solubilities. In addition, the higher k_obs of PFOA demonstrated that it had a better degradability than GenX by 2.4-fold in a mixed solution. The fluorinated byproduct perfluoropropanoic acid (PFPrA) was detected as a GenX intermediate, suggesting that ether bridge splitting was needed for GenX electrooxidation. This study provides a reference for assessing the degradability of GenX and PFOA and indicates that it is worth reconsidering whether GenX is a suitable alternative for PFOA from the point of view of environmental protection.

Rational design of biogenic PdxAuy nanoparticles with enhanced catalytic performance for electrocatalysis and azo dyes degradation

The green biogenic PdAu nanoparticles (bio-PdAu NPs) exhibits remarkable catalytic performance in hydrogenation, which is highly desired. However, the catalytic principles and effectiveness of bio-Pd^xAu^y NPs in response to various catalytic systems (electrocatalysis and suspension-catalysis) are unclear. Herein, a facile synthetic strategy for bio-Pd^xAu^y NPs synthesis with controlled size and the catalytic principles for hydrogen evolution reaction (HER) and azo dye degradation is reported. In the biosynthetic process, the size and composition of the bio-Pd^xAu^y NPs could be precisely controlled by predesigning the precursor mass ratio of Pd/Au, and the Au proportion showed a linear relationship with the size of NPs (R² = 0.92). The obtained bio-Pd^xAu^y NPs exhibit variable activity in electrocatalysis (HER) and suspension-catalysis (azo dye degradation). For electrocatalysis, the formation of conductive networks that facilitates the extracellular electron transfer is crucial. It was revealed that the bio-Pd²Au⁸ exhibited superior electrocatalytic performance in HER/toward hydrogen evolution, with a maximum current density of 1.65 mA cm^-2, which was 1.54 times higher than that commercial Pd/C (1.07 mA cm^-2). The high electrocatalytic activity was attributed to its appropriate size (81.38 ± 6.14 nm) and uniform distribution on the cell surface, which promoted the extracellular electron transfer by constructing a conductive network between catalyst and electrode. However, for suspension-catalysis, the size effect and synergistic effect of bimetallic NPs have a more prominent effect on the degradation of azo dyes. As the increase of Au proportion the particle size decreases, and the catalytic activity of bio-Pd^xAu^y improved significantly. The response principles of bio-Pd^xAu^y proposed in this study provide a reliable reference for the rational design of bio-based bimetallic catalysts with enhanced catalytic performance.

Heavy metals contamination, potential pathways and risks along the Indus Drainage System of Pakistan

Riverine water exposed to heavy metals (HMs) pollution is a major concern in the world because of its serious effects on ecosystem and human health. This study assessed the pollution status, sources, diffusion and potential risks of Mn, Co, Cu, Zn, Cr, Ni, Cd, Hg and Pb for the first time along the entire Indus Drainage System of Pakistan. The concentrations of nine HMs in the riverine water ranged from 5.05-101.59 μg/L with a mean value of 41.51 μg/L. The overall metals quantification along the drainage was significantly high (27% of the total) in River Chenab followed by River Indus (26%) > Soan (20%) > Ravi (19%) > Kabul (5%) > Swat (3%). The potential sources of contamination were identified to be the surrounding geogenic activities, industrial/municipal wastewater discharges, agricultural and surface runoffs by using multivariate statistics including metals correlation analysis, hierarchical cluster analysis and principal component analysis. The average mass flux of ∑HMs in the entire drainage was approximately 10.24 tons/year, to which the River Indus contributed 84% of the total, Chenab 11%, Ravi 3%, Kabul 1%, and Soan 1% with more prevalence of biological essential (Zn&Mn) and non-essential (Ni&Cr) metals. In terms of ecological risk, the riverine water metals contamination (1.59 to 57.06) was within the risk threshold (ERI < 110), while the risks of ∑carcinogenic metals for exposed children and adults along the basin were significantly influenced between acceptable to high cancer risk by Cd, Co, Ni, Cr and Pb.

Perturbation of clopyralid on bio-denitrification and nitrite accumulation: Long-term performance and biological mechanism

The contaminant of herbicide clopyralid (3,6-dichloro-2- pyridine-carboxylic acid, CLP) poses a potential threat to the ecological system. However, there is a general lack of research devoted to the perturbation of CLP to the bio-denitrification process, and its biological response mechanism remains unclear. Herein, long-term exposure to CLP was systematically investigated to explore its influences on denitrification performance and dynamic microbial responses. Results showed that low-concentration of CLP (<15 mg/L) caused severe nitrite accumulation initially, while higher concentrations (35-60 mg/L) of CLP had no further effect after long-term acclimation. The mechanistic study demonstrated that CLP reduced nitrite reductase (NIR) activity and inhibited metabolic activity (carbon metabolism and nitrogen metabolism) by causing oxidative stress and membrane damage, resulting in nitrite accumulation. However, after more than 80 days of acclimation, almost no nitrite accumulation was found at 60 mg/L CLP. It was proposed that the secretion of extracellular polymeric substances (EPS) increased from 75.03 mg/g VSS at 15 mg/L CLP to 109.97 mg/g VSS at 60 mg/L CLP, which strengthened the protection of microbial cells and improved NIR activity and metabolic activities. Additionally, the biodiversity and richness of the microbial community experienced a U-shaped process. The relative abundance of denitrification- and carbon metabolism-associated microorganisms decreased initially and then recovered with the enrichment of microorganisms related to the secretion of EPS and N-acyl-homoserine lactones (AHLs). These microorganisms protected microbe from toxic substances and regulated their interactions among inter- and intra-species. This study revealed the biological response mechanism of denitrification after successive exposure to CLP and provided proper guidance for analyzing and treating herbicide-containing wastewater.

Accelerated bioremediation of a complexly contaminated river sediment through ZVI-electrode combined stimulation

Complexly contaminated river sediment caused by reducible and oxidizable organic pollutants is a growing global concern due to the adverse influence on ecosystem safety and planetary health. How to strengthen indigenous microbial metabolic activity to enhance biodegradation and mineralization efficiency of refractory composite pollutants is critical but poorly understood in environmental biotechnology. Here, a synergetic biostimulation coupling electrode with zero-valent iron (ZVI) was investigated for the bioremediation of river sediments contaminated by 2,4,6-tribromophenol (TBP, reducible pollutant) and hydrocarbons (oxidizable pollutants). The bioremediation efficiency of ZVI based biostimulation coupling electrode against TBP debromination and hydrocarbons degradation were 1.1-3 times higher than the electrode used solely, which was attributed to the shape of distinctive microbial communities and the enrichment of potential dehalogenators (like Desulfovibrio, Desulfomicrobium etc.). The sediment microbial communities were significantly positively correlated with the enhanced degradation efficiencies of TBP and hydrocarbons (P < 0.05). Moreover, the coupled system predominately increased positive microbial interactions in the ecological networks. The possible mutual relationship between microbes i.e., Thiobacillus (iron-oxidizing bacteria) and Desulfovibrio (dehalogenator) as well as Pseudomonas (electroactive bacteria) and Clostridium (hydrocarbons degraders) were revealed. This study proposed a promising approach for efficient bioremediation of complexly contaminated river sediments.

Recent advances in hybrid wet scrubbing techniques for NOx and SO2 removal: State of the art and future research

Recently, the discharge of flue gas has become a global issue due to the rapid development in industrial and anthropogenic activities. Various dry and wet treatment approaches including conventional and hybrid hybrid wet scrubbing have been employing to combat against these toxic exhaust emissions. However, certain issues i.e., large energy consumption, generation of secondary pollutants, low regeneration of scrubbing liquid and high efficieny are hindering their practical applications on industrial level. Despite this, the hybrid wet scrubbing technique (advanced oxidation, ionic-liquids and solid engineered interface hybrid materials based techniques) is gaining great attention because of its low installation costs, simultaneous removal of multi-air pollutants and low energy requirements. However, the lack of understanding about the basic principles and fundamental requirements are great hurdles for its commercial scale application, which is aim of this review article. This review article highlights the recent developments, minimization of GHG, sustainable improvements for the regeneration of used catalyst via green and electron rich donors. It explains, various hybrid wet scrubbing techniques can perform well under mild condition with possible improvements such as development of stable, heterogeneous catalysts, fast and in-situ regeneration for large scale applications. Finally, it discussed recovery of resources i.e., N₂O, NH₃ and N₂, the key challenges about several competitive side products and loss of catalytic activity over time to treat toxic gases via feasible solutions by hybrid wet scrubbing techniques.

Total nitrogen removal in biochar amended non-aerated vertical flow constructed wetlands for secondary wastewater effluent with low C/N ratio: Microbial community structure and dissolved organic carbon release conditions

Biochar was utilized to intensify constructed wetland (CW) for further organic and nitrogen removal from secondary wastewater. Four sets of non-aerated biochar amended vertical flow CW (VFCW) were developed to investigate the synergistic effects of biochar and microbes on pollutant removal. Results showed that the average COD and nitrogen removal efficiencies of VFCW1 (with 1% w/w biochar with microbe and plants) achieved 89.1 ± 5.6% and 90.2 ± 3.1% respectively, and their corresponding removal rates of 10.2 ± 0.8 mg-COD/(m³.d) and 3.57 ± 0.3 mg-TN/(m³.d) which were 35 and 52.3% higher than control. The biochar's dissolved organic carbon release in VFCWs indicated that water and acidic media portray the optimum conditions for nitrogen removal. The 16S RNA gene sequencing analysis indicated that in the biochar-amended VFCWs, bacterial phylum Proteobacteria (24.13-51.95%) followed by Chloroflexi (5.64-25.01%), Planctomycetes (8.48-14.43%), Acidobacteria (2.29-11.65%) were abundantly enhanced. Conclusively, incorporating biochar in non-aerated VFCWs is an efficient technique for enhancing nitrogen removal from secondary effluent.

Utilization of electrochemical treatment and surface reconstruction to achieve long lasting catalyst for NOx removal

The development of catalysts has seen tremendous growth recently but most strategies only report utilization of catalysts for a few initial cycles without taking into account the influence of oxygen poisoning. Here, the magnetic Fe₃O₄@EDTA-Fe (MEFe, having a core Fe₃O₄ particle with EDTA-Fe coating) was investigated as a model catalyst for long-term recycling for the removal of nitrogen oxide (NO_x) from NO/O₂ mixture, followed by N₂O recovery. The concentration of oxygen in the flue gas was found to have a strong impact on NO_x absorption and catalytic response. To circumvent the oxygen poisoning, the MEFe was subjected to electrochemical treatment in the presence of neutral red (N.R.) and NO removal efficiency was ∼95 % noted. Furthermore, the surface of the catalyst degraded significantly (p < 0.05) after 6-7 repetitive cycling due to surface catalytic reactions, surface poisoning, oxidation of metallic species as well as residual stresses. The MEFe surface was reconstructed after 7 cycles using EDTA solution and Fe source to achieve similar surface coating as the fresh MEFe catalyst. The reconstructed MEFe exhibited similar NO_x absorption capability as the fresh MEFe and the reconstruction loop was repeated several times to achieve long term cycling, which make the catalyst cost-effective. Hence, it is proposed that a successful regeneration process can be employed for promising, sustainable and long-lasting catalytic treatment of air pollutants.

Biosynthesis of silver nanoparticles using Sida acuta extract for antimicrobial actions and corrosion inhibition potential

Nanotechnology exhibits a multidisciplinary area and gained interests for researchers. Nanoparticles produced via physical and chemical methods affects ecosystem drastically. Green synthesis is the charming technique that is inexpensive and safe for the environment. This study aimed to explore the antibacterial actions of as-synthesized silver nanoparticles (Ag-NPs) against Escherichia coli, Staphylococcus aureus and Streptococcus faecalis. Also, the anti-corrosion actions confirmed that the Ag-NPs proved as good inhibitors. In this way, Ag-NPs were prepared via biosynthesis technique by consuming the ground leaves and stem of 'Sida acuta' as a capping agent. The Ag-NPs were formed by irradiation of the aqueous solution of silver nitrate (AgNO₃) with extract of S. acuta stem and leaves. The as-synthesized reaction mixture of Ag-NPs was found to exhibit an absorbance band at 446-447 nm, by an UV/VIS spectrophotometer, which is a characteristic of Ag-NPs due to the surface plasmon resonance absorption band. The X-ray diffraction and transmission electron microscopy (TEM) were used for the confirmation of Ag-NPs' variety dimension, morphology and dispersion. The infrared spectra confirmed the bio-fabrication of the Ag-NPs displayed the existence of conceivable functional groups responsible for the bio-reduction and capping. The antimicrobial actions were measured and the zone of inhibition was compared with standard antibiotics.

NO Removal with Efficient Recovery of N2O by Using Recyclable Fe3O4@EDTA@Fe(II) Complex: A Novel Approach toward Resource Recovery from Flue Gas

Traditional technologies for handling nitrogen oxides (NO _x) from flue gas commonly entail the formation of harmless nitrogen gas (N₂), while less effort has been made to recover the N-containing chemicals produced. In this work, we developed a novel nanomagnetic adsorbent, Fe₃O₄@EDTA@Fe(II) (MEFe(II)), for NO removal. The NO adsorbed by MEFe(II) was then selectively converted to N₂O, a valuable compound in many industries, by using sulfite (a product from desulfurization in flue gas treatment) as the reductant for the regeneration of MEFe(II). Because of the magnetic and solid properties of MEFe(II), the processes of NO adsorption and N₂O recovery can be readily carried out under their optimal pH conditions in separate systems. In addition, the produced N₂O is easily handled without unwanted release to the atmosphere. At the optimal pH (7.5 and 8.0 for NO adsorption and N₂O recovery, respectively), the maximum NO adsorption capacity of MEFe(II) was measured as 0.303 ± 0.037 mmol·g^-1, over 90% of which was converted to N₂O during the recovery process. Moreover, MEFe(II) exhibited good five consecutive cycles. All of above reactions were performed at room temperature. These findings indicate MEFe(II) may hold great potential for application to NO removal from flue gas with the benefits of resource recovery, decreased chemical use, and low energy consumption.

Microbial Photoelectrotrophic Denitrification as a Sustainable and Efficient Way for Reducing Nitrate to Nitrogen

Biological removal of nitrate, a highly concerning contaminant, is limited when the aqueous environment lacks bioavailable electron donors. In this study, we demonstrated, for the first time, that bacteria can directly use the electrons originated from the photoelectrochemical process to carry out the denitrification. In such photoelectrotrophic denitrification (PEDeN) systems (denitrification biocathode coupling with TiO₂ photoanode), nitrogen removal was verified solely relying on the illumination dosing without consuming additional chemical reductant or electric power. Under the UV illumination (30 mW·cm^-2, wavelength at 380 ± 20 nm), nitrate reduction in PEDeN apparently followed the first-order kinetics with a constant of 0.13 ± 0.023 h^-1. Nitrate was found to be almost completely converted to nitrogen gas at the end of batch test. Compared to the electrotrophic denitrification systems driven by organics (OEDeN, biocathode/acetate consuming bioanode) or electricity (EEDeN, biocathode/abiotic anode), the denitrification rate in PEDeN equaled that in OEDeN with a COD/N ratio of 9.0 or that in EEDeN with an applied voltage at 2.0 V. This study provides a sustainable technical approach for eliminating nitrate from water. PEDeN as a novel microbial metabolism may shed further light onto the role of sunlight played in the nitrogen cycling in certain semiconductive and conductive minerals-enriched aqueous environment.

2-[(E)-(Naphthalen-2-yl)imino-meth-yl]phenol

In the title compound, C(17)H(13)NO, the azomethine double bond adopts an E conformation. The naphthyl ring system and the benzene ring form a dihedral angle of 8.09 (10)°. The near-planar conformation of the molecule is consolidated by an intra-molecular O-H⋯N hydrogen bond, which forms an S(6) ring. In the crystal, mol-ecules are arranged in a zigzag fashion parallel to the c axis.

4-Benzene-sulfonamido-benzoic acid

In the mol­ecule of the title sulfonamide compound, C₁₃H₁₁NO₄S, the dihedral angle between the planes of the benzene ring and the carboxyl substituent group is 6.7 (4)°. The two aromatic rings are inclined at 45.36 (15)° to one another. In the crystal, adjacent mol­ecules are linked via classical inter­molecular N—H⋯O and O—H⋯O, and non-classical C—H⋯O hydrogen bonds, which stabilize the crystal structure.

1-Ammonionaphthalene-2-sulfonate

In the mol­ecule of the zwitterionic title compound, C₁₀H₉NO₃S, an intra­molecular N—H⋯O hydrogen bond results in the formation of an almost planar six-membered ring (r.m.s daviation = 0.0150 Å), which is oriented at a dihedral angle of 1.63 (3)° with respect to the naphthalene ring system. In the crystal structure, inter­molecular N—H⋯O hydrogen bonds link the mol­ecules into a two-dimensional network.

2-Benzenesulfonamidobenzoic acid

In the title compound, C₁₃H₁₁NO₄S, the dihedral angle between the planes of the benzene ring and the carboxyl group is 13.7 (1)°. The mol­ecular structure contains intra­molecular N—H⋯O and C—H⋯O hydrogen-bonding inter­actions, while the crystal packing is stabilized by C—H⋯O and O—H⋯O hydrogen bonds and C—H⋯π inter­actions. The O—H⋯O hydrogen bonds form a cyclic dimer, with graph-set motif R²₂(8), about a centre of symmetry.

5-Benzene-sulfonamido-2-chloro-benzoic acid

In the title compound, C(13)H(10)ClNO(4)S, the dihedral angle between the aromatic ring planes is 87.07 (6)° and an intra-molecular C-H⋯O inter-action occurs. In the crystal, inversion dimers linked by two O-H⋯O hydrogen bonds arise from the carboxyl groups. N-H⋯O hydrogen bonds link the dimers into chains and short C-Cl⋯π and S-O⋯π contacts are also seen.