Nitrate, as the most stable form of nitrogen pollution, widely exists in aquatic environment, which has great potential threat to ecological environment and human health. Heterotrophic ...denitrification, as the most economical and effective method to treat nitrate wastewater, has been widely and deeply studied. From the perspective of heterotrophic denitrification, this review discusses nitrate removal in the aquatic environment, and the behaviors of different carbon source types were classified and summarized to explain the cyclical evolution of carbon and nitrogen in global biochemical processes. In addition, the denitrification process, electron transfer as well as denitrifying and hydrolyzing microorganisms among different carbon sources were analyzed and compared, and the commonness and characteristics of the denitrification process with various carbon sources were revealed. This study provides theoretical support and technical guidance for further improvement of denitrification technologies.
Display omitted
•The commonality and characteristics of the denitrification process are elaborated.•Methods to avoid the negative impacts of different carbon sources are introduced.•Strengths and weaknesses of different carbon sources during the reaction are studied.•The molecular biology mechanism and future research needs are emphasized.
Display omitted
•The threshold of K+ concentration for nitrate reduction was 229.78 ± 25.80 mg-K/L.•1.15–1.88 fold denitrification rate was improved under different K+ concentration.•The evolution ...pathway and utilization sequence were revealed.•Pseudomonas and Thiobacillus are the unique species in 229.78 ± 25.80 mg-K/L.
Heterotrophic denitrification based on solid carbon sources has been widely investigated for nitrogen removal in recent years. In this study, the response of the heterotrophic denitrification process under different K+ concentrations was clarified. Additionally, the denitrification enhancement mechanism was revealed and resource utilization of agricultural waste was achieved. A series of batch tests were conducted to study the effect of different K+ concentrations on the denitrification performance, dissolved organic matter (DOM) dissolution and microbial community structure. Results demonstrate that the threshold of K+ concentration for the NO3−-N and NO2−-N reduction rates were 229.78 ± 25.80 and 159.10 ± 24.60 mg-K/L, respectively. Excitation-emission matrix (EEM) analysis identified the main DOM components associated with tyrosine-like, tryptophan-like and humic-like substances, as well as illustrated the evolutionary behavior and utilization of DOM. High throughput 16S rRNA gene sequencing indicates that a K+ concentration of 229.78 ± 25.80 mg-K/L exhibited the highest diversity of functional species associated with fermentation and denitrification. The genera Pseudomonas and Thiobacillus were the unique denitrifiers at this K+ concentration. The correlation of K+ concentration, DOM dissolution of different regions and microorganism structure were analyzed using correlation matrix and PCA, and the appropriate K+ concentration of different functional microorganisms survival was optimized by this analysis method.
Nitrate pollution presents a serious threat to the environment and public health. As an excellent heterotrophic denitrification carbon source, banana peel (a kind of agricultural waste) provides a ...feasible alternative to deal with the persistent high concentrations of nitrate pollution. Although the feasibility and economy of banana peel for denitrification have already been reported, the long-term stability and mechanism were still unclear. The coupling mechanism of organic matters and microorganism in the denitrification process was systematically investigated through a 17-cycle experiment. The results showed that significant NO3−-N removal load and rate of 164.42 mg/g and 4.69 mg/(L·h) after long-term tests could be obtained. Organic matter analysis and 16S rRNA sequencing showed that the evolution of organic matter was dominated by Anaerolineaceae (fermenting bacteria), and, in the final step, the humification of organic matter was realized. Moreover, the presence of Lentimicrobium (denitrifying bacteria) was indispensable for the continuous removal of high concentrations of nitrate. The main functional gene of nitrogen transformation in this reaction system was NirS (haem-containing). This lab-scale heterotrophic denitrification process could contribute to a better understanding of the carbon and nitrogen cycles in the biogeochemical cycles to some extent, and it also provides a reference for the construction of highly efficient nitrate degradation reactors, based on agricultural wastes.
•Banana peel as a carbon source was verified to be efficient and sustainable.•The transfer pathways of C, N and electron in banana peel system were illustrated for the first time.•Lentimicrobium and Aquabacterium were the dominant denitrification bacteria in banana peel system.•A new approach based on banana peel for nitrate pollution remediation has been proposed.
Landfill leachate is a type of complex organic wastewater, which can easily cause serious negative impacts on the human health and ecological environment if disposed improperly. Electrochemical ...technology provides an efficient approach to effectively reduce the pollutants in landfill leachate. In this review, the electrochemical standalone processes (electrochemical oxidation, electrochemical reduction, electro-coagulation, electro-Fenton process, three-dimensional electrode process, and ion exchange membrane electrochemical process) and the electrochemical integrated processes (electrochemical-advanced oxidation process (AOP) and biological electrochemical process) for landfill leachate treatment are summarized, which include the performance, mechanism, application, existing problems, and improvement schemes such as cost-effectiveness. The main objective of this review is to help researchers understand the characteristics of electrochemical treatment of landfill leachate and to provide a useful reference for the design of the process and reactor for the harmless treatment of landfill leachate.
Display omitted
•Electrochemical processes for landfill leachate treatment•Advantages and shortcomings of electrochemical processes•Improvement schemes of electrochemical processes•Future research needs on landfill leachate electrochemical treatment
The problem of co-occurring nitrate and vanadium contamination is posing a serious risk to groundwater ecosystem. Although heterotrophic microbial remediation strategies for NO3−-N or V(V) have been ...reported, little is known regarding the selection of PRB materials and environmental risk assessment during co-contamination remediation. Herein, this study compared the pollutant removal load and removal performance of four fillers over 120-d to screen for the best fillers (CB: nitrate (36.85 mg/g), V(V) (2.07 mg/g)). UV–vis and fluorescence spectra were used to analyze the distribution of dissolved organic matters (DOM) in effluent and to assess the possible environmental risk. The distribution of microbial community structure revealed that the type of carbon source was one of the important factors to shape the community structure. Based on metagenomic techniques, the advantage of co-pollution removal in the CB system were energy storage and preferential electron utilization of NH4+-N and organic nitrogen production. Moreover, the detoxification of vanadium in CB was mainly mediated by receiving electrons from the electron transfer chain, while vanadium in GL was mainly mediated by NarG and NapA. The phenomenon of V(V) re-release during the experiment completes our understanding of the biogeochemical cycling of vanadium, nitrogen and carbon. Overall, the findings of this study provide new insights into remediation strategies for NO3−-N and V(V) co-pollutants, and can provide guiding recommendations for the selection and optimization of technologies and fillers in the remediation of co-pollutants in actual sites.
Display omitted
•CB showed the most superior ability to remove nitrate and V(V).•The re-release behavior of V(V) was observed for the first time in PRB.•NH4+ and organic nitrogen production were beneficial for NO3−-N and V(V) reduction.•Energy storage mechanism was important for the persistence and stability of CB reactor.•Electron transport chain in CB was the main mechanism of V(V) reduction.
The coexistence and distribution of nitrate and vanadate co-pollutants in groundwater have become an environmental problem, which needs to be urgently addressed. Although recent advances in the ...combination of activated sludge and agro-industrial waste have improved co-pollution removal, conventional activated sludge methods focused primarily on enhancing abiotic priming effects, neglecting the biological priming effects of colonizing microorganisms in agro-industrial waste, which limited the insights into the treatment mechanism. The removal of nitrate and vanadate was microbially mediated and highlighted the abiotic enhancement mechanism of the solid phase carbon source in the presence of microorganisms. Microorganisms in conventional activated sludge methods should be continuously incubated with the co-contaminants for at least 30 days to demonstrate stable and effective removal of the co-contaminants. This study demonstrated that the coupled system of unsterilized agro-industrial waste and untamed sludge achieved competitive nitrate (0.1319 h−1) and vanadate reduction rates (0.0359 h−1) without sludge acclimation. Compared with the control experiments, the nitrate and vanadate reduction rate increased 1.95–7.62 times and 2.85–5.36 times, respectively. The colonized functional microbes carried by agro-industrial waste played an important role in the microbial priming effect of co-pollutant removal. This work contributes to the efficient and economical treatment of the co-pollution of inorganic salts and heavy metals.
Display omitted
•Indigenous microbes attached to natural biomass had the ability to degrade nitrate or V(V).•Interspecies cooperation among microbes eliminated the sludge cultivation period.•The key metabolic pathways related to energy metabolism were significantly upregulated in the coupled system.•Priming effects of indigenous microbes of natural biomass played an essential role.
Sulfur-based autotrophic bioremediation is recognized as an environmentally-friendly and effective method for the treatment of Cr(VI) in groundwater. However, inorganic carbon (IC), especially ...IC-rich solid kitchen waste, has rarely been reported as an important factor in the autotrophic process. In China, kitchen waste containing IC is generated in large quantities, and in combination with Cr(VI) autotrophic treatment technology in groundwater can achieve a win-win situation. Herein, the efficiency of Cr(VI)-bioreduction coupling solid inorganic carbon (SIC) (e.g. marble, egg shell, oyster shell, and NSAD synthetic material) and liquid inorganic carbon (LIC) was compared for the first time. After 18 d incubation, there were significant differences in Cr(VI) reduction efficiency and microbial community between SIC-bioreactors and LIC-bioreactors. Higher electron transfer activity, greater bioavailability of organics, and multiple Cr(VI) reductases were detected in SIC-biosystems, which effectively promoted Cr(VI) energy metabolism and enzyme-mediated biological reduction. High-throughput 16S rRNA gene sequencing reveled multiple cooperative mechanism in different substrate biosystems. This study not only advances the understanding of SIC coupled with Cr(VI) autotrophic bioreduction, but also provides new insights for the treatment of solid kitchen waste and groundwater bioremediation.
Display omitted
•Application of solid inorganic carbon materials enhances Cr(VI)-bioreduction.•Cr(VI) reduction was weakened by sulfate accumulation.•Increasing cytochrome c and NADH accelerated the electron transfer process.•The microbial synergy mechanism was discriminated in different biosystems.
Display omitted
•PECS can further improve COD removal under the similar treatment cost.•PECS can short separation time of precipitates by regulating precipitates evolution.•PECS was beneficial to the ...formation of Fe3O4 during electrolysis.•PECS was an alternative to improve the existing electro-coagulation process.
Electro-coagulation process is an alternative method for the treatment of old landfill leachate. However, limited removal of chemical oxygen demand (COD) and long solid–liquid separation times are the key technical problems hindering its practical application. In this study, a persulfate-enhanced electro-coagulation system (PECS) was constructed to improve the removal of COD and shorten the settling time of the flocculation precipitates. Compared with the traditional electro-coagulation process, the PECS can improve the COD removal from 41.1% to 72.6% (60 min reaction and current density of 50 mA/cm2) using activated persulfate oxidation process. The PECS process involves the formation of sulfate and hydroxyl radicals as the main oxidants. The dissolved organic matter in the old landfill leachate mainly consisted of macromolecular humus-like substances. PECS can reduce the aromaticity and the complexity of organic matter, and generated small molecular-weight organic matter, which was helpful in further alleviating the pressure of subsequent treatment of the leachate. The evolution path of precipitates in PECS followed the process of α- FeOOH/Fe2O3·H2O → Fe3O4, which promoted the formation of Fe3O4, improved the precipitation performance, and reduced the volume of precipitates. PECS could improve the degree of degradation of organic matter, increase the removal of COD, and shorten the separation time of precipitates under similar treatment costs to conventional processes. The results of this study provide an important theoretical basis for improving the performance of existing electro-coagulation equipment in practical engineering applications.
