A New Concept of Promoting Nitrate Reduction in Surface Waters: Simultaneous Supplement of Denitrifiers, Electron Donor Pool, and Electron Mediators

Diet dependent trophic transfer of nanoparticles (ZnO and TiO2) along the "photic biofilm-snail" food chain

Multispecies biofilm exhibited high resistance to nanotoxicity by secreting extracellular polymeric substances (EPS) and undergoing alterations in the community composition. Scarce information was available to assess how these changes could further influence the transfer of nanoparticles (NPs) through the biofilm-based food chain. Photic biofilm was exposed to two distinct NPs (ZnO and TiO2) and subsequently grazed by snails. Exposure to different NPs led to variations in biomass, chlorophyll content, EPS productivity, alpha diversity, and community composition of the photic biofilm. The presence of ZnO NPs facilitated the growth of phylum Cyanobacteria while TiO2 promoted EPS productivity of photic biofilm. EPS were capable of embedding NPs (TiO2 and ZnO) within its matrix, thereby mitigating their aggregation within the biofilm matrix. These alterations were subsequently confirmed to have an impact on the trophic transfer factors (TTF) of NPs through the constructed biofilm-snail food chain. The TTF of ZnO was lower than that of TiO2 in feeding scenario 1 (only fed on TiO2 or ZnO biofilm) but higher than that of TiO2 in feeding scenario 2 (fed on TiO2 and ZnO biofilm simultaneously), which was attributed to the shifts in the algae composition and a smaller size of ZnO NPs in EPS. This study demonstrated that the response of photic biofilm to NPs further affected the TTFs of NPs through the food chain.

Integrated evaluation for advanced removal of nitrate using novel solid carbon biochar/corncob/PHBV composite: Insight into electron transfer and metabolic pathways

This study developed a novel Biochar/Corncob/PHBV (BCP) composite material, integrating the electron transfer capability of biochar, the cost-effectiveness of corncob, and the sustained carbon release performance of PHBV. The BCP system achieved a maximum nitrate removal efficiency of 97.3 %, significantly outperforming the single PHBV system (91.05 %), while effectively reducing nitrous oxide and other greenhouse gas emissions. It also demonstrated stable carbon release and enhanced electron transfer capabilities, contributing to a more sustainable denitrification process. The physical and chemical characterization of BCP confirmed that its superior performance is attributed to the uniformly distributed functional groups (e.g., CO and -COOH) on the surface and its porous structure, which facilitated electron transfer and microbial adhesion. Metagenomic and microbial analyses further revealed that BCP enriched functional genera such as Cellulomonas and Chryseobacterium and significantly increased the abundance of key functional genes related to nitrate reduction (e.g., NaR and NiR), enhancing organic matter decomposition and microbial nitrogen transformation. Beyond improving nitrate removal efficiency compared to PHBV, the BCP material offers practical engineering value by addressing carbon source limitations in long-term wastewater treatment applications. Its enhanced electron transfer and microbial enrichment suggest strong potential for application in constructed wetlands, biofilters, and other decentralized wastewater treatment systems. The study demonstrates that the BCP composite is not only a viable alternative to traditional PHBV but also a cost-effective and environmentally friendly material with broad applicability in nitrogen pollution control.

Photocatalytic material-microorganism hybrid systems in water decontamination

Biological processes are widely used technologies for water decontamination, but they are often limited by insufficient bioavailable carbon sources or biorecalcitrant contaminants. The recently developed photocatalytic material-microorganism hybrid (PMH) system combines the light-harvesting capacities of photocatalytic materials with specific enzymatic activities of whole cells, efficiently achieving solar-to-chemical conversion. By integrating the benefits of both photocatalysis and biological processes, the PMH system shows great potential for water decontamination. While recent reviews have focused primarily on its application in green energy development, this review emphasizes the latest advancements in PMH systems for water decontamination, covering various applications, key considerations, and synergistic mechanisms. This review aims to provide a fundamental understanding of the PMH system and explore its broader potential in environmental remediation.

Unlocking the potential of simultaneous organics oxidation and nitrate reduction over an S-scheme magnetic CoFe2O4@g-C3N4 heterojunction under visible light irradiation: Performance and mechanisms

In this study, a novel S-scheme CoFe2O4@g-C3N4 (CF@GCN) heterojunction was prepared via a simple hydrothermal method and its visible-light-driven catalytic potential was evaluated in terms of tetracycline (TC) oxidation and nitrate reduction simultaneously. A series of characterizations validated the effective fabrication of CF@GCN, exhibiting remarkable photocatalytic potentials. In the separated system of TC and nitrate, a nearly 100% removal of TC (5.0 mg/L) was observed within 60 min under the optimal conditions (pH = 5.0, Catalyst dosage = 0.4 g/L). The quenching experiments emphasized the concurrent participation of non-radical (h+) and radical species (O2•- and HO•) in the TC degradation process. Meanwhile, more than 96% of nitrate (100 mg/L) removal was observed in 90 min under the optimal conditions (pH = 3.0, Catalyst dosage = 0.4 g/L, in the presence of formic acid). In the combined system, both TC oxidation and nitrate reduction occurred simultaneously and the efficiency of TC oxidation was lower than the individual system. The novel S-scheme composite has demonstrated exceptional potential in enhancing the reusability and stability of the catalyst, even after five cycles of trials. A detailed mechanistic pathway for TC oxidation and nitrate reduction was proposed, relying on the identification of reactive species and intermediates. In general, our findings propose a promising method with synergistic properties for the simultaneous oxidation of organics and reduction of nitrate in wastewater over an S-scheme heterojunction photocatalytic process.

Distinct effects of flow intermittency on the benthic microbial diversity and their denitrification on different substrates

Global climate change has significantly increased the duration of droughts in intermittent rivers, impacting benthic microbial-mediated biogeochemical processes. However, the response mechanisms of biofilms on different substrate types to alternating dry and wet conditions and their related ecosystem functions remain poorly understood. This study uses high-throughput sequencing and enzyme assays to investigate the impact of gradient drought stress on microbial diversity and functional changes of biofilm communities inhabiting on gravel, cobblestone, and sediment. Results showed that the duration of drought significantly affects microbial diversity, with algal and bacterial α-diversity declining under extended drought across gravel, cobblestone, and sediment substrates. At the same time, fungal diversity was less affected, likely due to their distinct ecological niches and reproductive strategies. β-diversity analysis revealed significant changes in community heterogeneity, with algae and bacteria showing increased Bray-Curtis dissimilarities, indicating distinct adaptation strategies that may affect ecosystem functioning. Fungal communities, however, were less impacted by drought-induced heterogeneity changes. Network analysis showed that drought altered microbial network connectivity, with algal networks displaying decreased path distances, while bacterial networks remained stable, suggesting greater resilience to drought stress. Functional enzyme assays revealed significantly reduced denitrification rates across all substrates post-drought, with distinct denitrifying enzyme activity responses depending on substrate type. Partial least squares path modeling revealed that algal biodiversity were closely linked to the maintaining of enzyme activities, particularly denitrification rates of biofilms on cobblestone and gravel. These findings indicated the critical role of substrate types in shaping microbial responses to drought stress, with distinct microbial groups and diversity indices playing key roles in maintaining ecosystem functions. This study highlights the importance of understanding the interactions between microbial community dynamics and ecosystem functions under varying environmental stressors in river ecosystems.

Harnessing microbes to pioneer environmental biophotoelectrochemistry

In seeking sustainable environmental strategies, microbial biophotoelectrochemistry (BPEC) systems represent a significant advancement. In this review, we underscore the shift from conventional bioenergy systems to sophisticated BPEC applications, emphasizing their utility in leveraging solar energy for essential biochemical conversions. Recent progress in BPEC technology has facilitated improved photoelectron transfer and system stability, resulting in substantial advancements in carbon and nitrogen fixation, degradation of pollutants, and energy recovery from wastewater. Advances in system design and synthetic biology have expanded the potential of BPEC for environmental clean-up and sustainable energy generation. We also highlight the challenges of environmental BPEC systems, ranging from performance improvement to future applications.

Comparative impact analysis of nitrate reduction by typical components of natural organic compounds in magnetite-bearing riparian zones

As the key interface, the nitrate removal capacity of riparian zones is receiving close attention. Although naturally occurring organic compounds in this environment play a pivotal role in shaping microbial communities and influencing the nitrate removal capacity, the relevant research is inadequate. Given the complexity of riparian environments, in this study, we added representative natural organic matter (fulvic acid, butyric acid, naphthalene, starch, and sodium bicarbonate) as carbon conditions and incorporated magnetite to simulate riparian zone components. The study investigated the nitrate degradation efficiency and microbial responses under different natural carbon conditions in real iron-containing environments. Butyric acid exhibited the most efficient nitrate reduction, followed in descending order by naphthalene, starch, sodium bicarbonate, and humic acid. However, this did not imply that butyric acid efficiently removed nitrogen; instead, the nitrogen would circulate in the environment in the form of ammonium. Denitrification and DNRA were the primary drivers of nitrate reduction in each system, while naphthalene and humic acid systems also exhibited nitrification and mineralization. Nitrogen-fixing bacteria represent a unique microbial community in the butyrate system. Further, the synergistic degradation of naphthalene and nitrate demonstrated significant potential applications. High-throughput sequencing revealed that carbon conditions exerted selective pressure on microorganisms, driving Fe (Ⅱ)/Fe (Ⅲ) transformation by shaping the microbial community structure and influencing the nitrogen cycling process.

Antifouling characteristics and mechanisms in visible-light photocatalytic membrane bioreactor based on g-C3N4 modified membrane

A novel visible-light photocatalytic membrane bioreactor (R3) was constructed for membrane fouling control and effluent quality improvement. Specially, g-C3N4 modified membrane was evaluated for the performance of synergistic separation and photocatalysis. Another two parallel reactors, MBRs with ceramic membrane (R1) and g-C3N4 membrane in dark condition (R2), were operated synchronously for comparison. A satisfactory effluent quality was obtained in R3 with COD and NH4+-N around 22.0 mg/L and 1.02 mg/L during 60-day operation, which was superior to R1 (27.8, 1.42 mg/L) and R2 (29.9, 2.26 mg/L). The thickness of cake layer on membranes in R3 (2.46 μm) was thinner than R1 (3.52 μm) and R2 (4.97 μm) after operation, indicating the introduction of visible light could effectively mitigate membranes fouling. Moreover, microorganism community analysis revealed that visible light increased the relative abundance of Bacteroidetes and Chryseolinea, which not only enhanced the activity of microorganisms in metabolizing organic nutrients, but also improved the transfer and utilization of photogenerated electrons on the semiconductor-microorganism interface. The active aromatic protein metabolism and the upregulated related enzymes further demonstrated the synergistic effect of photocatalysis and microbial communities on the membrane fouling mitigation. This work provides a novel application of photocatalysis into antibiofouling effect in MBRs, and opens a strategy for bacteria inactivation and foulants removal with eco-friendly solar energy.

Enhanced nitrate reduction in hypotrophic waters with integrated photocatalysis and biodegradation

Addressing nitrate contamination in water bodies is a critical environmental challenge, and Intimately Coupling Photocatalysis and Biodegradation (ICPB) presents a promising solution. However, there is still debate about the effectiveness of ICPB in reducing nitrate under hypotrophic conditions. Further research is needed to understand its microbial metabolic mechanism and the functional changes in bacterial structure. Here we explored microbial metabolic mechanisms and changes in bacterial structure in ICPB reactors integrating a meticulously screened TiO2/g-C3N4 photocatalyst with biofilm. We achieved a 26.3% increase in nitrate reduction using 12.2% less organic carbon compared to traditional biodegradation methods. Metagenomic analysis of the microbial communities in ICPB reactors revealed evolving metabolic pathways conducive to nitrate reduction. This research not only elucidates the photocatalytic mechanism behind nitrate reduction in hypotrophic conditions but also provides genomic insights that pave the way for alternative approaches in water remediation technologies.

Influence of crop residue-induced Fe-DOC complexation on nitrate reduction in paddy soil

Although complexation between dissolved organic matter (DOM) and ubiquitous Fe is known to have a major influence on electron transferring ability in redoximorphic soil, it was unclear whether and how this complexation affected nitrate reduction and N2O productivity. The nitrate reduction of paddy soil in the presence of crop residues returning under flooding conditions was explored in this study. The rate of nitrate reduction in control soil was 0.0677 d-1, while it improved 1.99 times in treatment soil with Chinese milk vetch (CMV) straw returning. During a 28-day incubation period, N2O productivity decreased 0.08-0.91 ppb in CMV soil and 0.43-0.50 ppb in rice straw soil compared with control. The presence of crop residue increased DOC content and Fe (III) reduction rate, which aided in the formation of Fe (II)-DOC complexation. Meanwhile, the addition of CMV increased the content of DOC by 5.14-78.77 mg/kg and HCl extractable Fe (II) by 35.12-1221.03 mg/kg. Crop residues returning to soil increased the relative abundance of iron reductive and electroactive genera, as well as denitrifying genera with more copies of denitrification genes (Archangiaceae, Gemmatimonadaceae, and Burkholderiaceae). The synergistic effect of Fe-DOC complexation, electroactive genera, and denitrifying genera contributed to up-regulated expression of napA and narG (5.84 × 106 and 3.39 × 107 copies increased in the CMV soil compared to the control) numbers and equally accelerated reduction of nitrate to nitrite, while further nitrite reduction was primarily attributed to the abiotic reaction by Fe (II). From a bio-electrochemical point of view, this work provided new insight into the nitrate reduction of paddy soil impacted by Fe-DOC complexation.

The interplay of hematite and photic biofilm triggers the acceleration of biotic nitrate removal

The soil-water interface is replete with photic biofilm and iron minerals; however, the potential of how iron minerals promote biotic nitrate removal is still unknown. This study investigates the physiological and ecological responses of photic biofilm to hematite (Fe2O3), in order to explore a practically feasible approach for in-situ nitrate removal. The nitrate removal by photic biofilm was significantly higher in the presence of Fe2O3 (92.5%) compared to the control (82.8%). Results show that the presence of Fe2O3 changed the microbial community composition of the photic biofilm, facilitates the thriving of Magnetospirillum and Pseudomonas, and promotes the growth of photic biofilm represented by the extracellular polymeric substance (EPS) and the content of chlorophyll. The presence of Fe2O3 also induces oxidative stress (•O2-) in the photic biofilm, which was demonstrated by electron spin resonance spectrometry. However, the photic biofilm could improve the EPS productivity to prevent the entrance of Fe2O3 to cells in the biofilm matrix and mitigate oxidative stress. The Fe2O3 then promoted the relative abundance of Magnetospirillum and Pseudomonas and the activity of nitrate reductase, which accelerates nitrate reduction by the photic biofilm. This study provides an insight into the interaction between iron minerals and photic biofilm and demonstrates the possibility of combining biotic and abiotic methods to improve the in-situ nitrate removal rate.

Improvement of biotic nitrate reduction in constructed photoautotrophic biofilm-soil microbial fuel cells

The biotic nitrate reduction rate in freshwater ecosystems is typically constrained by the scarcity of carbon sources. In this study, 'two-chambers' - 'two-electrodes' photoautotrophic biofilm-soil microbial fuel cells (P-SMFC) was developed to accelerate nitrate reduction by activating in situ electron donors that originated from the soil organic carbon (SOC). The nitrate reduction rate of P-SMFC (0.1341 d-1) improved by ∼ 1.6 times on the 28th day compared to the control photoautotrophic biofilm. The relative abundance of electroactive bacterium increased in the P-SMFC and this bacterium contributed to obtain electrons from SOC. Biochar amendment decreased the resistivity of P-SMFC, increased the electron transferring efficiency, and mitigated anodic acidification, which continuously facilitated the thriving of putative electroactive bacterium and promoted current generation. The results from physiological and ecological tests revealed that the cathodic photoautotrophic biofilm produced more extracellular protein, increased the relative abundance of Lachnospiraceae, Magnetospirillaceae, Pseudomonadaceae, and Sphingomonadaceae, and improved the activity of nitrate reductase and ATPase. Correspondingly, P-SMFC in the presence of biochar achieved the highest reaction rate constant for nitrate reduction (kobs) (0.2092 d-1) which was 2.4 times higher than the control photoautotrophic biofilm. This study provided a new strategy to vitalize in situ carbon sources in paddy soil for nitrate reduction by the construction of P-SMFC.

Long-term effects of Ag NPs on denitrification in sediment: Importance of Ag NPs exposure ways in aquatic ecosystems

The widespread use of silver nanoparticles (Ag NPs) inevitably leads to their increasing release into aquatic systems, with studies indicating that the mode of Ag NPs entry into water significantly affects their toxicity and ecological risks. However, there is a lack of research on the impact of different exposure ways of Ag NPs on functional bacteria in sediment. This study investigates the long-term influence of Ag NPs on denitrification process in sediments by comparing denitrifies responses to single (pulse injection of 10 mg/L) and repetitive (1 mg/L × 10 times) Ag NPs treatments over 60-day incubation. Results showed that a single exposure of 10 mg/L Ag NPs caused an obvious toxicity on activity and abundance of denitrifying bacteria on the first 30 days, reflecting by the decreased NADH amount, ETS activity, NIR and NOS activity, and nirK gene copy number, which resulted in a significant decline of denitrification rate in sediments (from 0.59 to 0.64 to 0.41-0.47 μmol15N L-1 h-1). While inhibition was mitigated with time and denitrification process recovered to the normal at the end of the experiment, the accumulated nitrate generated in the system showed that the recovery of microbial function did not mean the restoration of aquatic ecosystem after pollution. Differently, the repetitive exposure of 1 mg/L Ag NPs exhibited the evident inhibition on metabolism, abundance, and function of denitrifiers on Day 60, due to the accumulated amount of Ag NPs with the increased dosing number, indicating that the accumulated toxicity on functional microorganic community of repetitive exposure in less toxic concentration. Our study highlights the importance of Ag NPs entry pathways into aquatic ecosystem on their ecological risks, which affected dynamic responses of microbial function to Ag NPs.

Emerging high-ammonia‑nitrogen wastewater remediation by biological treatment and photocatalysis techniques

The bacterial and photocatalysis techniques have been widely applied into the remediation of ammonia nitrogen wastewater. Although traditional microbial methods had been verified useful; more efficient, energy-saving and controllable candidate treatment methods are still urgently needed to cover the increasingly diverse ammonia nitrogen pollution cases. The bacterial treatment technique for ammonia nitrogen mainly depends on the ammonia nitrogen oxidation-reduction (e.g. nitrification, denitrification) by nitrifying bacteria and denitrifying bacteria, but these reactions suffer from slow denitrifying kinetic process and uncontrolled disproportionation reaction. In comparison, the photocatalysis technique based on photoelectrons is more efficient and has some advantages, such as low temperature reaction and long life, while the photocatalysis technique can not perform multiple complex biochemical reactions. Despite much scientific knowledge obtained about this issue recently, such research has yet not been widely adopted in the industry because of many concerns about subsequent catalyst stability and economic feasibility. This review summarized and discussed the very recent achievements and key problems on remediation of high-ammonia‑nitrogen wastewater and oxidation driven by bacterial treatment and photocatalysis techniques, as well as the most promising future directions for these two techniques, especially the potential of jointly bacterial-photocatalysis techniques.

A MXene-based multiple catalyst for highly efficient photocatalytic removal of nitrate

Photocatalytic removal of nitrate in wastewater has attracted wide attention because of its simple operation and environmental protection. However, the preparation of photocatalysts with high efficiency and high nitrogen selectivity is still a challenge. In this paper, TiO₂ is grown in situ on Ti₃C₂ MXene by a simple calcination method and modified with silver particles. The presence of Ti₃C₂ reduces the recombination rate of photogenerated electrons and generates more photogenerated electrons. At the same time, the silver particles also increase the photoelectron density and further improve the carrier separation of the catalyst. Due to its unique structure and optical properties, the prepared photocatalyst shows an excellent nitrate removal rate under a high-pressure mercury lamp. At 500 mg_N/L, the nitrate removal rate reaches 96.1%, and the nitrogen selectivity reaches 92.6%. Even after 5 cycles, the prepared photocatalyst still maintains a high nitrate photocatalytic removal efficiency (89%). The electron transfer path is verified by density functional theory calculations.

A review on the generation, discharge, distribution, environmental behavior, and toxicity (especially to microbial aggregates) of nano-TiO2 in sewage and surface-water and related research prospects

This article reviews the nano-effects and applications of different crystalline nano‑titanium dioxide (nano-TiO₂), identifies their discharge, distribution, behavior, and toxicity to aquatic organisms (focusing on microbial aggregates) in sewage and surface-water, summarizes related toxicity mechanisms, and critically proposes future perspectives. The results show that: 1) based on crystal type, application boundaries of nano-TiO₂ have become clear, extending from traditional manufacturing to high-tech fields; 2) concentration of nano-TiO₂ in water is as high as hundreds of thousands of μg/L (sewage) or several to dozens of μg/L (surface-water) due to direct application or indirect release; 3) water environmental behaviors of nano-TiO₂ are mainly controlled by hydration conditions and particle characteristics; 4) aquatic toxicities of nano-TiO₂ are closely related to their water environmental behavior, in which crystal type and tested species (such as single species and microbial aggregates) also play the key role. Going forward, the exploration of the toxicity mechanism will surely become a hot topic in the aquatic-toxicology of nano-TiO₂, because most of the research so far has focused on the responses of biological indicators (such as metabolism and damage), while little is known about the stress imprint caused by the crystal structures of nano-TiO₂ in water environments. Additionally, the aging of nano-TiO₂ in a water environment should be heeded to because the continuously changing surface structure is bound to have a significant impact on its behavior and toxicity. Moreover, for microbial aggregates, comprehensive response analysis should be conducted in terms of the functional activity, surface features, composition structure, internal microenvironment, cellular and molecular level changes, etc., to find the key point of the interaction between nano-TiO₂ and microbial aggregates, and to take mitigation or beneficial measures to deal with the aquatic-toxicity of nano-TiO₂. In short, this article contributes by 1) reviewing the research status of nano-TiO₂ in all aspects: application and discharge, distribution and behavior, and its aquatic toxicity; 2) suggesting the response mechanism of microbial aggregates and putting forward the toxigenic mechanism of nanomaterial structure; 3) pointing out the future research direction of nano-TiO₂ in water environment.

Multifaceted synergistic electron transfer mechanism for enhancing denitrification by clay minerals

The performance and mechanism of denitrification enhanced by three clay minerals, montmorillonite (Mmt), illite and kaolinite, were first studied. Batch experiments indicated that clay minerals significantly enhanced denitrification at certain concentrations (0.1-1 g/L). The denitrification rate with 1 g/L Mmt was increased by 5.0-fold. The mechanism of clay minerals promoting denitrification was analyzed from three aspects: electron transfer characteristics, interfacial interaction and metabolism activity. Electrochemical tests showed that the clay minerals promoted electron transfer rate by improving current efficiency and electronic accommodation capacity. The biofilm formation on the clay minerals interface indicated that micro-domain catalytic phases were formed, which was beneficial to improve the nitrate reduction rate. In addition, nicotinamide adenine dinucleotide, nitrate reductase and nitrite reductase activities in Mmt-supplemented system were increased by 283.3%, 128.1% and 126.2%, respectively; and extracellular polymeric substance secretion was enhanced, indicating that the addition of clay minerals promoted microbial metabolic activity. Higher microbial diversity and enrichment of electroactive bacteria were observed in the Mmt-supplemented system. Based on the above exploration, the multifaceted synergistic mechanism was proposed to account for the enhanced denitrification performance on clay minerals. Overall, this study expanded understanding of the roles of clay minerals on denitrification and provided strategies for accelerating the biological transformation process.

Aromatic compounds releases aroused by sediment resuspension alter nitrate transformation rates and pathways during aerobic-anoxic transition

Aromatic compounds (ACs) releases aroused by sediment resuspension would certainly change the concentrations of suspended sediment (SPS) and organic carbon, which may alter nitrate-N transformation during aerobic-anoxic transition. To prove this, three typical ACs (aniline, nitrobenzene, and methylbenzene) with different octanol-water partition coefficients (K_ow) were selected to investigate the effects of ACs releases aroused by sediment resuspension on nitrate-N transformation during aerobic-anoxic transition. ACs releases aroused by sediment resuspension accelerated nitrate-N transformation and enhanced the potential for dissimilatory nitrate reduction to ammonium (DNRA), compared to that without sediment resuspension. With sediment resuspension, methylbenzene releases affected nitrate-N transformation rates and pathways more significantly than aniline and nitrobenzene releases. Microbial analysis indicated that sediment resuspension created complicated microbial co-occurrence networks and changed the associations among bacteria; dominant bacteria abundance varied with different ACs releases. Further analysis revealed that ACs distributed in SPS, which increased with logK_ow, indirectly affected nitrate-N transformation rates and pathways via altering dominant bacteria abundance and electron transport system activity (ETSA). Especially, ETSA, which was positively associated with ACs distributed in SPS, affected nitrate-N transformation most directly. Overall, ACs release fate played important roles in nitrate-N transformation, causing ammonia-N retention and alterations in nitrogen cycle during aerobic-anoxic transition.

Contribution of periphytic biofilm of paddy soils to carbon dioxide fixation and methane emissions

Rice paddies are major contributors to anthropogenic greenhouse gas emissions via methane (CH₄) flux. The accurate quantification of CH₄ emissions from rice paddies remains problematic, in part due to uncertainties and omissions in the contribution of microbial aggregates on the soil surface to carbon fluxes. Herein, we comprehensively evaluated the contribution of one form of microbial aggregates, periphytic biofilm (PB), to carbon dioxide (CO₂) and CH₄ emissions from paddies distributed across three climatic zones, and quantified the pathways that drive net CH₄ production as well as CO₂ fixation. We found that PB accounted for 7.1%-38.5% of CH₄ emissions and 7.2%-12.7% of CO₂ fixation in the rice paddies. During their growth phase, PB fixed CO₂ and increased the redox potential, which promoted aerobic CH₄ oxidation. During the decay phase, PB degradation reduced redox potential and increased soil organic carbon availability, which promoted methanogenic microbial community growth and metabolism and increased CH₄ emissions. Overall, PB acted as a biotic converter of atmospheric CO₂ to CH₄, and aggravated carbon emissions by up to 2,318 kg CO₂ equiv ha^-1 season^-1. Our results provide proof-of-concept evidence for the discrimination of the contributions of surface microbial aggregates (i.e., PB) from soil microbes, and a profound foundation for the estimation and simulation of carbon fluxes in a potential novel approach to the mitigation of CH₄ emissions by manipulating PB growth.

Recent advances in metal-mediated nitrogen oxyanion reduction using reductively borylated and silylated N-heterocycles

The reduction of nitrogen oxyanions is critical for the remediation of eutrophication caused by anthropogenic perturbations to the natural nitrogen cycle. There are many approaches to nitrogen oxyanion reduction, and here we report our advances in reductive deoxygenation using pre-reduced N-heterocycles. We show examples of nitrogen oxyanion reduction using Cr, Fe, Co, Ni, and Zn, and we evaluate the role of metal choice, number of coordinated oxyanions, and ancillary ligands on the reductive transformations. We report the experimental challenges faced and provide an outlook on new directions to repurpose nitrogen oxyanions into value-added products.

Comprehensive evaluation of the long-term effect of Cu2+ on denitrifying granular sludge and feasibility of in situ recovery by phosphate

With increased industrial development, vast heavy metals are inevitably discharged into wastewater. Cu²⁺ is one of the most hazardous heavy metals in biotreatment. However, the potential effect of Cu²⁺ on denitrifying granular sludge is still unknown. This work assesses the response of denitrifying granular sludge to Cu²⁺ stress from multiple aspects. The denitrifying granular sludge could tolerate 5 mg L^-1 Cu²⁺, while the nitrogen removal efficiency decreased to 48.5% under 10 mg L^-1 Cu²⁺. Enzyme activity and carbohydrate metabolism were inhibited, and the denitrifying bacteria were washed out under Cu²⁺ stress. The resulting deteriorated state was reversed by phosphate. The nitrogen removal efficiency recovered to 99% after 10 days, and the enzyme activity also recovered to the original level. Membrane transport, transcription and cellular processes were promoted. Overall, the results of this work provide a feasible strategy to rapidly restore the metabolic activity of denitrifying granular sludge under Cu²⁺ stress.

Algal blooms modulate organic matter remineralization in freshwater sediments: A new insight on priming effect

This study provides a novel insight into the degradation of sediment organic matter (SOM) regulated by algae-derived organic matter (AOM) based on priming effect. We tracked the dynamics of SOM mineralization products and pathways, together with priming effects (PE) using the compound-specific stable isotope (δ¹³C) technique following addition of low- and high-density algal debris in sediments. We found that algal debris increased the total carbon oxidation rate, and resulted in denitrification and methanogenesis-dominated SOM mineralization. While iron reduction and sulphate reduction played important roles in the early period of algal accumulation. Total carbon oxidation rate and anaerobic rates (R_anaerobic) were higher in the amended treatments compared with that in the control. Analysis indicated that algal debris had a positive PE on SOM mineralization, which caused an intensified mineralization in the initial phase with over 80% of dissolved inorganic carbon deriving from SOM degradation. Total carbon oxidation rate of SOM deduced from priming effect (R_TCOR-PE) was similar to R_anaerobic, further indicating SOM mineralization was a critical source of the end products. These findings deviate the causal focus from the decomposition of AOM, and confirm the accumulation of AOM as the facilitator of SOM mineralization. Our study offers empirical evidences to advance the traditional view on the effect of AOM on SOM mineralization.

Biophotoelectrochemistry for renewable energy and environmental applications

Biophotoelectrochemistry (BPEC) is an interdisciplinary research field and combines bioelectrochemistry and photoelectrochemistry through the utilization of the catalytic abilities of biomachineries and light harvesters to accomplish the production of energy or chemicals driven by solar energy. The BPEC process may act as a new approach for sustainable green chemistry and waste minimization. This review provides the state-of-the-art introduction of BPEC basics and systems, with a focus on light harvesters and biocatalysts, configurations, photoelectron transfer mechanisms, and the potential applications in energy and environment. Several examples of BPEC applications are discussed including H2 production, CO2 reduction, chemical synthesis, pollution control, and biogeochemical cycle of elements. The challenges about BPEC systems are identified and potential solutions are proposed. The review aims to encourage further research of BPEC toward development of practical BPEC systems for energy and environmental applications.

Feasibility of partial denitrification and anammox for removing nitrate and ammonia simultaneously in situ through synergetic interactions

In this study, the single-stage partial denitrification-anammox (PD-A) process was started-up in 22 days in a lab-scale up-flow sludge blanket (UASB) reactor to treat wastewater containing NH₄⁺-N and NO₃^--N simultaneously. The TN removal rate reached 97.08% with a low effluent TN of 10 mg/L. High-throughput sequencing results revealed the dominant bacterial strains were related to the genus of Thauera and Candidatus Kuenenia. The PD-A system was started-up based on the optimized PD process via inoculated exogenous anammox sludge attributing to the improvement of bacterial adaptation and co-existence by EPS. The PD process was realized in 18 days with the abundance of PD functional bacterium Thauera through fluctuated C/NO-3-N conditions. Moreover, the detrimental effects of starvation on anammox was weaker than that on PD bacteria. The PD-A process was expected to open a new possible perspective in designing NO₃^--N and NH₄⁺-N wastewater treatment plants.

Effect and mechanism of cyclodextrins on nitrate reduction and bio-activity by S.oneidensis.MR-1

Cyclodextrins (CDs) have been widely used due to the excellent solubilization of hydrophobic organics. However, their effect on the biotransformation process of hydrophilic pollutants is unclear. This study first evaluated the effect and mechanism of CDs on nitrate reduction by S.oneidensis.MR-1. The three CDs (α-CD, β-CD and γ-CD) all accelerated nitrate reduction, among which β-CD had the best effect. The nitrate reduction rate was increased by 21.8% with 0.5 mM β-CD. As for the mechanism, β-CD increased the biomass, membrane permeability and EPS of S.oneidensis.MR-1. The nitrate reductase activity was also increased by 1.34-fold with 0.5 mM β-CD. The current exchange density and the electron transfer system activity were increased by 11.4% and 99.5% in the β-CD-supply system, respectively. It confirmed that β-CD enhanced the biological and electrochemical characteristics and then enhanced bio-activity. This study provides a new understanding of CDs in microbial remediation and broadens the practical application.

Effect and ameliorative mechanisms of polyoxometalates on the denitrification under sulfonamide antibiotics stress

The environmental risks of the sulfonamide antibiotics have attracted much attention recently. In this study, the inhibition effects of sulfadiazine (SDZ) on denitrification electron transfer system (ETS) and ameliorative mechanisms of phosphomolybdic acid (PMo12) were first explored. When denitrification was under 2 mg/L SDZ stress, experiments indicated that PMo12 enhanced NO3--N reduction efficiency and rate from 68.30% to 100.00% and 124.22 to 184.59 N/g VSS/h, respectively. Electron transfer rate and consumption efficiency in denitrification ETS were enhanced to ameliorate SDZ inhibition, which was due to the more secreted riboflavin and cytochrome c and the increased denitrifying enzymes activity with PMo12 mediation. In addition, the microbial growth inhibition and cell membrane damage were ameliorated due to the more EPS surrounding microbe with PMo12 mediation. Higher diversity of denitrifying microbe with PMo12 mediation also promoted denitrification under SDZ stress. This work provided promising strategy to ameliorate antibiotics inhibition in the wastewater bio-treatment.

Susceptibility of atrazine photo-degradation in the presence of nitrate: Impact of wavelengths and significant role of reactive nitrogen species

The role of reactive nitrogen species (RNS) formed from nitrate photolysis has aroused interests in transformation of contaminants of emerging concern. This study investigated the influence of UV wavelengths (255, 285 and 365 nm) on photolysis of nitrate for degradation of atrazine (ATZ). The UV₂₈₅/nitrate system showed the fastest rate constant for degradation of ATZ with k_obs of 0.0022 cm² mJ^-1. UV photolysis, RNS, and hydroxyl radical (HO) were identified as main contributors to ATZ degradation in UV/nitrate system. Among the contributors, RNS made the major contribution to degradation of ATZ in UV₂₈₅/nitrate system, while HO is the predominant specie in UV₂₅₅/nitrate system. Variance decomposition analysis showed that degradation of ATZ was slightly impacted by natural organic matter and carbonate/bicarbonate in UV₂₈₅/nitrate system but was dramatically affected in UV₂₅₅/nitrate system. Main transformation products of ATZ in UV₂₈₅/nitrate system were identified and possible pathways were proposed. RNS were confirmed to be favorable for acceleration of ATZ photolysis through further reaction of RNS with hydroxyatrazine (with electron-rich moieties). Our study provides deep insights on the influence of UV wavelength on nitrate photolysis and ATZ degradation, and provides a novel method for remediation of water co-contaminated by nitrate and organic contaminants.

Intensified nitrogen removal by heterotrophic nitrification aerobic denitrification bacteria in two pilot-scale tidal flow constructed wetlands: Influence of influent C/N ratios and tidal strategies

This study investigated the influence of C/N ratios and tidal strategies on nitrogen removal and bacterial communities in two pilot-scale tidal flow constructed wetlands (TFCWs) with simultaneous nitrification-denitrification process. Heterotrophic nitrification aerobic denitrification (HNAD) was the main nitrogen transformation pathway in both TFCWs. High C/N ratios and effluent circulation at low temperature promoted HNAD in TFCWs with high nitrogen removal efficiencies (72.6%-95.5% for NH₄⁺-N and 70.9%~91.8% for TN). Effluent circulation had more influence on bacterial community structure and diversity than C/N ratios. Among 16 detected genera related to nitrogen removal, HNAD bacteria (HNADB) were abundant. Especially, some dominant HNADB (e.g. Aeromonas, Hydrogenophage and Gemmobacter) were core genera, showing positive interactions with other genera related to nitrogen removal. Tidal strategies had more contribution to the shifts in these genera than C/N ratios. This study highlights the importance of HNADB in pilot-scale TFCWs and their responses to C/N ratios and tidal strategies.

Interactions between elevated CO2 levels and floating aquatic plants on the alteration of bacterial function in carbon assimilation and decomposition in eutrophic waters

Elevated atmospheric CO₂ concentration (eCO₂) may have different effects on the bacterial community with regard to C assimilation and decomposition in eutrophic waters compared to that in fresh waters with intermediate levels of nutrients and oceans. Aquatic plant growth under eCO₂ could further modify microbial activities associated with the C cycle in eutrophic waters. Therefore, there is an urgent need to further study how eCO₂ and its interactions with the growth of aquatic plants affect the composition and function of the bacterial community involved in mediating the C cycle in eutrophic waters. Accordingly, we designed a microcosm experiment to investigate the effects of ambient and high CO₂ concentrations on bacterial community composition and function in eutrophic waters with and without the growth of Eichhornia crassipes (Mart.) Solms. The results from 16S rRNA gene sequencing, function prediction, and q-PCR showed that eCO₂ significantly increased the abundance of bacterial and functional genes involved in CO₂ assimilation (photosynthetic bacteria; cbbL IA & IC, cbbL ID, cbbM, pufM) and C decomposition (Acidimicrobiia, Thermoleophilia, Gaiellales; ChiA), illustrating the functional enrichment with photoautotrophy, hydrocarbon degradation, cellulolysis, and aromatic hydrocarbon degradation. However, eCO₂ decreased the abundance of some chemoautotrophic bacteria, including nitrifying bacteria (Nitrospirae, Nitrosomonadaceae). In contrast, the cultivation of E. crassipes decreased the abundance of photosynthetic bacteria but increased the abundance of bacteria involved in complex C decomposition associated with root exudates and degradation, e.g. Fibrobacteres, Sphingobacteriales, Sphingomonadales, and Rhizobiales. eCO₂ and growth of E. crassipes had opposite effects on algal density in eutrophic waters, creating interactive effects that further decreased the diversity of the bacterial community and abundance of some CO₂-assimilating bacteria with nitrifying characteristics (Nitrosomonadaceae) and some C-degrading bacteria (Fibrobacteres) with denitrifying properties (Flavobacteriaceae, Sphingomonadaceae, and Gemmobacter). Therefore, the interactions between aquatic plants and the bacterial community in eutrophic waters under eCO₂ would be beneficial to the environment and help alleviate the greenhouse effect.

TiO2-based catalysts for photocatalytic reduction of aqueous oxyanions: State-of-the-art and future prospects

Nowadays, an increasing discharge of oxyanions to the natural environment has been attracting worldwide attention. TiO₂-based photocatalysis is regarded as one of the most promising technologies for the conversion of toxic oxyanions (such as chromate, nitrate, nitrite, bromate, perchlorate and selenate) to harmless and/or less toxic substances in contaminated waters. Various types of TiO₂-based catalysts have been developed, and each of them exhibits its own advantages in catalytic reduction of oxyanions. However, the application of these nanostructured TiO₂ in real water bodies remains a challenge, with limitations associated with sunlight harvesting abilities, production costs, reuse stability and exposure risks. Herein, we aim to present a critical review on reported TiO₂-based photocatalytic reduction of aqueous oxyanions, provide a comprehensive understanding of the possible reaction pathways of formed active species, and evaluate the reduction performance of different types of TiO₂-based catalysts. In addition, the impact of operating parameters (such as solution pH, temperature, dissolved oxygen and coexisting substances) on catalytic reduction performance is discussed. Furthermore, the perspectives of TiO₂-based photocatalytic reduction of oxyanions are also proposed.

Pretreatment-promoted sludge fermentation liquor improves biological nitrogen removal: Molecular insight into the role of dissolved organic matter

Waste activated sludge (WAS) can be used as carbon sources to support biological nutrient removal (BNR). In this study, thermal-alkaline (THALK), ozonation (OZN), electrolysis (EC) and NaClO-promoted electrolysis (EC-AOP) were investigated to facilitate WAS solubilization and production of volatile fatty acids (VFAs). EEMF-PARAFAC and FT-ICR-MS were employed to characterize the transformation of dissolved organic matter (DOM) in WAS fermentation liquors at molecular level. THALK achieved the highest fluorescence intensity of C1 protein after pretreatment. Proteins and lipids were the dominant DOM in the pretreated WAS, while the DOM shifted towards substances with higher H/C and lower O/C after fermentation. The BNR results showed that THALK (100%) and EC-AOP (96.9%) outperformed other groups (78.9-90.3%) in terms of NO₃-N removal, indicating the significant impact of DOM compositions. Overall, these results demonstrated that THALK and EC-AOP effectively enhanced release of VFAs and DOM, which subsequently improved NO₃-N removal efficiency.

Dissolved organic matter mediates in the anaerobic degradation of 17α-ethinylestradiol in a coupled electrochemical and biological system

Dissolved organic matter (DOM) can act as an electron shuttle in biogeochemical redox reactions to affect the fate of contaminants. Herein DOMs were tested for their ability to mediate in the degradation of 17α-ethinylestradiol (EE2) in a coupled electrochemical and biological system. Fulvic acid (FA) and Sigma humic acid (SHA) were found to promote degradation by the electro-domesticated micro-organisms in the coupled system. Analyses of superoxide dismutase levels, microbial community and clusters of orthologous groups of proteins showed that electrical stimulation promoted their growth and metabolism. It was confirmed that electron transfer in the coupled system was promoted in the presence of DOM as their protein-like components were converted into aromatic substances. The electrical stimulation improved the microorganisms' effectiveness in subsequent biodegradation under anaerobic condition. Stimulated micro-organisms seemed to increase their environmental tolerance and degrade EE2 effectively. These findings provide evidence about the fate of estrogens in bioelectrochemical water treatment.

Effects of titanium dioxide nanoparticles on algal and bacterial communities in periphytic biofilms

The widespread application of commercial TiO₂ NPs inevitably leads to their release into environmental waters through various ways. TiO₂ NPs released into water might be absorbed by and react with periphytic biofilms, which are a kind of aquatic environmental media of important ecological significance, and influence the physiological activity and ecological function of periphytic biofilms. This study investigated the effects of exposure to 1 mg/L and 5 mg/L of TiO₂ NPs on periphytic biofilms cultured indoors. After a 10-day exposure to TiO₂ NPs, the growth (measured by chlorophyll-a content) of microalgal community was inhibited greatly (more than 60%); however, the primary production (indicated by quantum yield) of periphytic biofilms maintained changeless. As for bacteria, TiO₂ NP-exposure increased the bacterial diversity and altered the composition structure. Significant changes were observed in the bacterial communities at the class level, mainly including Alphaproteobacteria, Gammaproteobacteria, Cytophagia, Flavobacteriia, Sphingobacteriia, Synechococcophycideae and Oscillatoriophycideae. The enhancement of metabolic activities (the production of extracellular polymeric substances, especially proteins content increased by 48.51%) of periphytic biofilms was a resistance mechanism to toxicity of NPs. As for extracellular enzyme activities of periphytic biofilms, alkaline phosphatase activity was inhibited (22.43%) after exposed to 5 mg/L of TiO₂ NPs, which posed a threat to phosphorus metabolism of periphytic biofilms. Overall, this study demonstrated that 1 mg/L and 5 mg/L of TiO₂ NPs negatively influenced physiological activities and ecological functions of periphytic biofilms, highlighting that the ecological risks of TiO₂ NPs should be paid attention to.

Light-driven nitrous oxide production via autotrophic denitrification by self-photosensitized Thiobacillus denitrificans

N₂O (Nitrous oxide, a booster oxidant in rockets) has attracted increasing interest as a means of enhancing energy production, and it can be produced by nitrate (NO₃^-) reduction in NO₃^--loading wastewater. However, conventional denitrification processes are often limited by the lack of bioavailable electron donors. In this study, we innovatively propose a self-photosensitized nonphototrophic Thiobacillus denitrificans (T. denitrificans-CdS) that is capable of NO₃^- reduction and N₂O production driven by light. The system converted >72.1 ± 1.1% of the NO₃^--N input to N₂ON, and the ratio of N₂O-N in gaseous products was >96.4 ± 0.4%. The relative transcript abundance of the genes encoding the denitrifying proteins in T. denitrificans-CdS after irradiation was significantly upregulated. The photoexcited electrons acted as the dominant electron sources for NO₃^- reduction by T. denitrificans-CdS. This study provides the first proof of concept for sustainable and low-cost autotrophic denitrification to generate N₂O driven by light. The findings also have strong implications for sustainable environmental management because the sunlight-triggered denitrification reaction driven by nonphototrophic microorganisms may widely occur in nature, particularly in a semiconductive mineral-enriched aqueous environment.

Protection Mechanisms of Periphytic Biofilm to Photocatalytic Nanoparticle Exposure

Researchers are devoting great effort to combine photocatalytic nanoparticles (PNPs) with biological processes to create efficient environmental purification technologies (i.e., intimately coupled photobiocatalysis). However, little information is available to illuminate the responses of multispecies microbial aggregates against PNP exposure. Periphytic biofilm, as a model multispecies microbial aggregate, was exposed to three different PNPs (CdS, TiO₂, and Fe₂O₃) under xenon lamp irradiation. There were no obvious toxic effects of PNP exposure on periphytic biofilm as biomass, chlorophyll content, and ATPase activity were not negatively impacted. Enhanced production of extracellular polymetric substances (EPS) is the most important protection mechanism of periphytic biofilm against PNPs exposure. Although PNP exposure produced extracellular superoxide radicals and caused intracellular reactive oxygen species (ROS) accumulation in periphytic biofilm, the interaction between EPS and PNPs could mitigate production of ROS while superoxide dismutase could alleviate biotic ROS accumulation in periphytic biofilm. The periphytic biofilms changed their community composition in the presence of PNPs by increasing the relative abundance of phototrophic and high nutrient metabolic microorganisms (families Chlamydomonadaceae, Cyanobacteriacea, Sphingobacteriales, and Xanthomonadaceae). This study provides insight into the protection mechanisms of microbial aggregates against simultaneous photogenerated and nanoparticle toxicity from PNPs.