Simultaneous mercury oxidation and NO reduction in a membrane biofilm reactor

Advances in mechanism for the microbial transformation of heavy metals: implications for bioremediation strategies

Heavy metals are extensively discharged through various anthropogenic activities, resulting in an environmental risk on a global scale. In this case, microorganisms can survive in an extreme heavy metal-contaminated environment via detoxification or resistance, playing a pivotal role in the speciation, bioavailability, and mobility of heavy metals. Therefore, studies on the mechanism for the microbial transformation of heavy metals are of great importance and can provide guidance for heavy metal bioremediation. Current research studies on the microbial transformation of heavy metals mainly focus on the single oxidation, reduction and methylation pathways. However, complex microbial transformation processes and corresponding bioremediation strategies have never been clarified, which may involve the inherent physicochemical properties of heavy metals. To uncover the underlying mechanism, we reclassified heavy metals into three categories based on their biological transformation pathways, namely, metals that can be chelated, reduced or oxidized, and methylated. Firstly, we comprehensively characterized the difference in transmembrane pathways between heavy metal cations and anions. Further, biotransformation based on chelation by low-molecular-weight organic complexes is thoroughly discussed. Moreover, the progress and knowledge gaps in the microbial redox and (de)methylation mechanisms are discussed to establish a connection linking theoretical advancements with solutions to the heavy metal contamination problem. Finally, several efficient bioremediation strategies for heavy metals and the limitations of bioremediation are proposed. This review presents a solid contribution to the design of efficient microbial remediation strategies applied in the real environment.

Mechanistic insights into tris(2-chloroisopropyl) phosphate biomineralization coupled with lead (II) biostabilization driven by denitrifying bacteria

The ubiquity and persistence of organophosphate esters (OPEs) and heavy metal (HMs) pose global environmental risks. This study explored tris(2-chloroisopropyl)phosphate (TCPP) biomineralization coupled to lead (Pb2+) biostabilization driven by denitrifying bacteria (DNB). The domesticated DNB achieved synergistic bioremoval of TCPP and Pb2+ in the batch bioreactor (efficiency: 98 %).TCPP mineralized into PO43- and Cl-, and Pb2+ precipitated with PO43-. The TCPP-degrading/Pb2+-resistant DNB: Achromobacter, Pseudomonas, Citrobacter, and Stenotrophomonas, dominated the bacterial community, and synergized TCPP biomineralization and Pb2+ biostabilization. Metagenomics and metaproteomics revealed TCPP underwent dechlorination, hydrolysis, the TCA cycle-based dissimilation, and assimilation; Pb2+ was detoxified via bioprecipitation, bacterial membrane biosorption, EPS biocomplexation, and efflux out of cells. TCPP, as an initial donor, along with NO3-, as the terminal acceptor, formed a respiratory redox as the primary energy metabolism. Both TCPP and Pb2+ can stimulate phosphatase expression, which established the mutual enhancements between their bioconversions by catalyzing TCPP dephosphorylation and facilitating Pb2+ bioprecipitation. TCPP may alleviate the Pb2+-induced oxidative stress by aiding protein phosphorylation. 80 % of Pb2+ converted into crystalized pyromorphite. These results provide the mechanistic foundations and help develop greener strategies for synergistic bioremediation of OPEs and HMs.

Treatment of volatile organic compounds and other waste gases using membrane biofilm reactors: A review on recent advancements and challenges

This article provides a comprehensive review of membrane biofilm reactors for waste gas (MBRWG) treatment, focusing on studies conducted since 2000. The first section discusses the membrane materials, structure, and mass transfer mechanism employed in MBRWG. The concept of a partial counter-diffusion biofilm in MBRWG is introduced, with identification of the most metabolically active region. Subsequently, the effectiveness of these biofilm reactors in treating single and mixed pollutants is examined. The phenomenon of membrane fouling in MBRWG is characterized, alongside an analysis of contributory factors. Furthermore, a comparison is made between membrane biofilm reactors and conventional biological treatment technologies, highlighting their respective advantages and disadvantages. It is evident that the treatment of hydrophobic gases and their resistance to volatility warrant further investigation. In addition, the emergence of the smart industry and its integration with other processes have opened up new opportunities for the utilization of MBRWG. Overcoming membrane fouling and developing stable and cost-effective membrane materials are essential factors for successful engineering applications of MBRWG. Moreover, it is worth exploring the mechanisms of co-metabolism in MBRWG and the potential for altering biofilm community structures.

Removal of nitric oxide in bioreactors: a review on the pathways, governing factors and mathematical modelling

The constant surge in nitric oxide in the atmosphere results in severe environmental degradation, negatively impacting human health and ecosystems, and is presently a global concern. Widely used physicochemical technologies for nitric oxide (NO) removal comes with high installation and operational costs and the production of secondary pollutants. Thus, biological treatment has been emphasized over the last two decades, but the poor solubility of NO in water makes it a challenging issue. The present article reviews the various technical aspects of biological treatment of nitric oxide, including the removal pathways and reactor configurations involved in the process. The most widely used technologies in this regard are chemical adsorption processes followed by biological reactors like biofilters, biotrickling filters and membrane bioreactors that enhance NO solubility and offer the flexibility and scope of further improvement in process design. The effect of various experimental and operational parameters on NO removal, including pH, carbon source, gas flow rate, gas residence time and presence of inhibitory components in the flue gas, is also discussed along with the developed mathematical models for predicting NO removal in a biological treatment system. There is an extensive scope of investigation regarding the development of an economical system to remove NO, and an exhaustive model that would optimize the process considering maximum practical parameters encountered during such operation. A detailed discussion made in this article gives a proper insight into all these areas.

Liquid-gas phase transition enables microbial electrolysis and H2-based membrane biofilm hybrid system to degrade organic pollution and achieve effective hydrogenotrophic denitrification of groundwater

Electron-donor Lacking was the limiting factor for the denitrification of oligotrophic groundwater and hydrogenotrophic denitrification provided an efficient approach without secondary pollution. In this study, a hybrid system with microbial electrolysis cell (MEC) assisted hydrogen-based membrane biofilm reactor (MBfR) was established for advanced groundwater denitrification. The liquid-gas phase transition prevented the potential pollution from organic wastes in MEC to groundwater, while the bubble-free diffusion of MBfR promoted hydrogen utilization efficiency. The negative-pressure extraction from MEC and the positive pressure for gas supply into MBfR increased the hydrogen proportion and current density of MEC, and improved the kinetic constant K of the denitrification reaction in MBfR. With actual groundwater, the MEC-MBfR hybrid system achieved a nitrate reduction of 97.8% with an effluent NO₃^--N of 2.2 ± 1.0 mg L^-1. The hydrogenotrophic denitrifiers of Thauera, Pannonibacter, and Azonexus, dominated the denitrification biofilm on the membrane and elastic filler in MBfR.

Variation of the toxicity caused by key contaminants in industrial wastewater along the treatment train of Fenton-activated sludge-advanced oxidation processes

Industrial wastewater contains a mixture of refractory and hazardous pollutants that have comprehensive toxic effects. We investigated the treatment of a long-chain industrial wastewater treatment train containing Fenton, biological anoxic/oxic (AO), and heterogeneous ozone catalytic oxidation (HOCO) processes, and evaluated their detoxification effect based on the analysis of the genic toxicity of some key contaminants. The results showed that although the effluent met the discharge standard in terms of traditional quality parameters, the long-chain treatment process could not effectively detoxify the industrial wastewater. The analysis results of summer samples showed that the Fenton process increased the total toxicity and genotoxicity of the organics, concerned metals, and non-volatile pollutants, whereas the A/O process increased the toxicity of the organics and non-volatile pollutants, and the HOCO process led to higher toxicity caused by metals and non-volatile pollutants. The outputs of the winter samples indicated that the Fenton process reduced the total toxicity and genotoxicity caused by non-volatile pollutants but increased that of the organics and concerned metals. The effect of the A/O process on the effluent toxicity in winter was the same as that in summer, whereas the HOCO process increased the total toxicity and genotoxicity of the metals in winter samples. Correlation analysis showed that various toxicity stresses were significantly correlated with the variation of these key pollutants in wastewater. Our results could provide a reference for the optimization of industrial wastewater treatment plants (IWTPs) by selecting more suitable treatment procedures to reduce the toxicity of different contaminants.

Lead removal in flue gas from sludge incineration by denitrification: Insights from metagenomics and metaproteomics

Flue gas lead emission during sludge incineration damages to human health and ecological environment seriously. Therefore, a denitrifying bio-trickling filter (DNBTF) for lead removal in flue gas from sludge incineration was investigated. Lead removal efficiency was up to 90.7% in 60 days' operation. Lead speciation in biofilms of DNBTF consists of 84.27% residue lead, 15.18% organic bound lead, and less than 1% exchangeable and reducible lead. Lead resistant bacteria and lead resistant-denitrifying bacteria accounted for 85.04% and 58.25%, respectively. Lead resistant microorganisms(Pseudomonas, Azoarcus, Stappia, Pararhodobacter, Paracoccus, Azospirillum, Hyphomonas, Rhodobacter, Polymorphum, Brevunimonas, Stenotrophomonas) could resist the toxicity of Pb²⁺ in flue gas by transport protein and binding protein, and detoxify Pb²⁺ in flue gas by extracellular polymeric substances (EPS) adsorption, protein binding and precipitation under the action of resistance genes, such as pbrAB, golT, troABCD, znuABC, czcABCD, pcoB, copA, as shown by integrated metagenomic and metaproteomic analyses. The biofilm was characterized by FTIR, XRD, 3D-EEM, and SEM-EDS. XRD and SEM-EDS spectra indicated the formation of pyromorphite from bioconversion of lead in flue gas. Lead-containing flue gas was bio-stabilized in the form of pyromorphite and HA-Pb via complexation of humic acids in extracellular polymeric substances (EPS), biosorption and biodeposition. This provides a new way of sludge incineration flue gas lead removal using a denitrifying biotricking filter.

The inhibitory effects and underlying mechanism of high ammonia stress on sulfide-driven denitrification process

Sulfide-driven denitrification (SD) process has been widely studied for treating wastewater containing sulfate and ammonia in recent years. But influence of high ammonia stress on the SD process and microbial community remained unclear. In this work, a series of tests were conducted to investigate effects of different ammonia stress (200-3000 mg-total ammonia nitrogen (TAN)/L) on denitrification efficiency, byproduct accumulation and microbial community of the SD process. According to our results, the SD process was severely inhibited, and 32.67 ± 5.15 mg/L NO₂^--N was accumulated when ammonia stress reached 3000 mg TAN/L. But the inhibited SD process could recover in about 40 days when ammonia stress was decreased to 200 mg TAN/L. After analyzing the microbial community, Thiobacillus sp. (Thiobacillus sp. 65-29, Thiobacillus sp. SCN 64-317, Thiobacillus sp. 63-78 and Thiobacillus denitrificans) was confirmed as dominant bacteria responsible for the SD process. Further, expression of narG, napA, nirK and nirS were inhibited under high ammonia stress, thus making the SD process stuck in NO₃^- and NO₂^- reduction step. This study reveals the inhibitory effects of high ammonia stress on the SD process and its possible underlying mechanism with discussion in gene level.

Nitric oxide and nitrite removal by partial denitrifying hollow-fiber membrane biofilm reactor coupled with nitrous oxide generation as energy recovery

Nitrogen oxide (NOx) emissions cause significant impacts on the environment and must therefore be controlled even more stringently. This requires the development of cost-effective removal strategies which simultaneously create value-added by-products or energy from the waste. This study aims to treat gaseous nitric oxide (NO) by hollow-fibre membrane biofilm reactor (HFMBfR) in the presence of nitrite (NO2-) and evaluate nitrous oxide (N₂O) emissions formed as an intermediate product during the denitrification process. Accumulated N₂O can be utilised in methane oxidation as an oxidant to produce energy. In the first stage of the study, the HFMBfR was operated by feeding only gaseous NO as the nitrogen source. During this period, the best performance was achieved with 92% NO removal efficiency (RE). In the second stage, both NO gas and NO2- were supplied to the system, and 91% NO and 99% NO2- reduction were achieved simultaneously with the maximum N₂O generation of 386 ± 31 ppm. Lower influent carbon to nitrogen (C/N) ratios, such as 4.5 and 2.0, and higher NO2--N loading rate of 158 mg N day^-1 favoured N₂O generation. An improved NO removal rate and N₂O accumulation were seen with the increasing amount of PO43- in the medium. The 16S rDNA sequencing analysis revealed that Alicycliphilus denitrificans and Pseudomonas putida were the dominant species. The study shows that an HFMBfR can be successfully used to eliminate both NO2- and gaseous NO and simultaneously generate N₂O by adjusting the system parameters such as C/N ratio, NO2- and PO43- loading.

Biological treatments of mercury and nitrogen oxides in flue gas: biochemical foundations, technological potentials, and recent advances

Nitrogen oxides (NO_x) and mercury (Hg) are commonly found coexistent pollutants in combustion flue gas. Ever-increasing emission of atmospheric Hg and NO_x has caused considerable environmental risks. Traditional flue gas demercuration and denitration techniques have many socioeconomic, technological and environmental drawbacks. Biotechnologies can be a promising and prospective alternative strategy. This article discusses theoretical foundation (biochemistry and genomic basis) and technical potentials (Hg⁰ bio-oxidation coupled to denitrification) of bioremoval of Hg and NO_x in flue gas and summarized recent experimental and technological advances. Finally, several specific technical perspectives have been put forward to better guide future researches.

Study on the oxidative stress and transcriptional level in Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1 isolated from chromium-contaminated soil

The bioreduction of Cr(VI) and Hg(II) has become a hot topic in the field of heavy metals bioremediation. However, the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria is still not clear. In this work, a novel Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1, was isolated from chromium landfill at a chromate factory, which was used to investigate the mechanism of antioxidant stress during the Cr(VI) and Hg(II) reduction process. The results demonstrated that the removal of Cr(VI) and Hg(II) by A. indicus yy-1 from solution was through reduction rather than biosorption. The reduction rates of Cr(VI) and Hg(II) by resting cells reached 59.71% and 31.73% at 24 h with initial concentration of 10 mg L^-1, respectively. X-ray photoelectron spectroscopy (XPS) analysis further showed that Cr(III) and Hg(0) were mainly the Cr(VI)- and Hg(II)-reduced productions, respectively. Results of physiological assays showed Hg(II) was more toxic to A. indicus yy-1 than Cr(VI), and the activities of antioxidant enzymes (SOD and CAT) were significantly increased in A. indicus yy-1 for relieving the oxidative stress. The transcriptional level of genes related to Cr(VI) and Hg(II) reductases and antioxidant enzymes were up-regulated, indicating that the reductases have participated in the reduction of Cr(VI) and Hg(II), and SOD and CAT served as the vital antioxidant enzymes for defending the oxidative stress. This work provides a deep insight into the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria, which helps seek the highly resistant heavy metal reducing bacteria.

Effects of high ammonia loading and in-situ short-cut nitrification in low carbon‑nitrogen ratio wastewater treatment by biocathode microbial electrochemical system

The microbial electrochemical system (MES) has great advantages in wastewater treatment for rapid chemical oxygen demand (COD) removal and low sludge yield rate. Herein, biocathode MES was proposed to remove COD from high-ammonia wastewater with low carbon‑nitrogen ratio and regulate the nitrogen forms in effluent for ANAMMOX process. The biocathode was more sensitive to ammonia nitrogen (NH₄⁺-N) than anode and determined the power generation of MES. With COD of 500-550 mg L^-1 in influent, increasing NH₄⁺-N from 50 to 150 mg L^-1 improved maximum power output (P_max) from 3.0 ± 0.2 to 3.4 ± 0.1 W m^-3, which was then reduced with further increase of NH₄⁺-N from 300 to 600 mg L^-1. However, for the cathodic reductive current, the negative effects of ammonia only revealed with NH₄⁺-N ≥ 450 mg L^-1. The cathodic equilibrium potential drop determined the power degradation, because the increased reductive compounds (NH₄⁺ and COD) in catholyte. The high NH₄⁺-N reduced the abundance of denitrifiers, exoelectrogens and organic-degrading bacteria on electrodes, while that of nitrogen-fixing bacteria increased. External alkalinity addition achieved in-situ short-cut nitrification and nitrite accumulation. With comparable NH₄⁺ and NO₂^-, limited NO₃^- and low COD, the biocathode MES effluent was then suitable for subsequence ANAMMOX process.

Bioconversion of Hg0 into HA-Hg for simultaneous removal of Hg0 and NO in a denitrifying membrane biofilm reactor

Bacterial mercury oxidation coupled to denitrification offers great potential for simultaneous removal of elemental mercury (Hg⁰) and nitric oxide (NO) in a denitrifying membrane biofilm reactor (MBfR). Four potentially contributory mechanisms tested separately, namely, membrane gas separation, medium absorption, biosorption and biotransformation, which contributed 4.9%/7.2%, 8.1%/8.9%, 38.8%/9.5% and 48.2%/84.9% of overall Hg⁰/NO removal in MBfR. Herein, Hg⁰ bio-oxidation, oxidative Hg⁰ biosorption and denitrification played leading roles in simultaneous removal of Hg⁰ and NO. Living microbes performed simultaneous Hg⁰ bio-oxidation and denitrification, in which Hg⁰ as electron donor was biologically oxidized to oxidized mercury (Hg²⁺), while NO as the terminal electron acceptor was denitrified to N₂. The Hg²⁺ further complexed with humic acids in extracellular polymeric substances via functional groups (-SH, -OH, -NH^- and -COO^-) and formed humic acids bound mercury (HA-Hg). Non-living microbial matrix performed oxidative Hg⁰ biosorption, in which Hg⁰ may be physically adsorbed by cellular matrix, then non-metabolically oxidized to Hg²⁺ via oxidative complexation with -SH in humic acids and finally cleavage of S-H bond and surface charge transfer led to formation of HA-Hg. Therefore, bioconversion of Hg⁰ to HA-Hg by Hg⁰ bio-oxidation and oxidative Hg⁰ biosorption coupled with NO denitrification to N₂ dynamically cooperated to accomplish simultaneous removal of Hg⁰ and NO in MBfR.

Reduction of FeII(EDTA)-NO by Mn powder in wet flue gas denitrification technology: stoichiometry, kinetics, and thermodynamics

Conversion of Fe^II(EDTA)-NO or Fe^III(EDTA) into Fe^II(EDTA) is a key process in a wet flue gas denitrification technology with Fe^II(EDTA) solution. In this work, the stoichiometry, kinetics, and thermodynamics of Fe^II(EDTA)-NO reduction by Mn powder were investigated. We first studied the Fe^II(EDTA)-NO reduction and product distribution to speculate a possible stoichiometry of Fe^II(EDTA)-NO reduction by Mn powder. Then, the effects of major influencing factors, such as pH value, temperature, and Mn concentration, were studied. The pseudo-second-order model was established to describe the Fe^II(EDTA)-NO reduction. Simultaneously, according to Arrhenius and Eyring-Polanyi equations, the reaction activation energy (E_a), enthalpy of activation (∆H^‡), and entropy of activation (∆S^‡) were calculated as 23.68 kJ/mol, 21.148 kJ/mol, and - 149.728 J/(k mol), respectively. Additionally, simultaneous reduction of Fe^III(EDTA) and Fe^II(EDTA)-NO was investigated to better study the mechanism of Fe^II(EDTA) regeneration, suggesting that there was a competition between the two reduction processes. Finally, a simple schematic mechanism of NO absorption by Fe^II(EDTA) combined with regeneration of manganese ion and ammonium was proposed. These fundamental researches could offer a valuable guidance for wet flue gas denitrification technology with Fe^II(EDTA) solution.

Bio-oxidation of Elemental Mercury into Mercury Sulfide and Humic Acid-Bound Mercury by Sulfate Reduction for Hg0 Removal in Flue Gas

Bioconversion of elemental mercury (Hg⁰) into immobile, nontoxic, and less bioavailable species is of vital environmental significance. Here, we investigated bioconversion of Hg⁰ in a sulfate-reducing membrane biofilm reactor (MBfR). The MBfR achieved effective Hg⁰ removal by sulfate bioreduction. 16 S rDNA sequencing and metagenomic sequencing revealed that diverse groups of mercury-oxidizing/sulfate-reducing bacteria (Desulfobulbus, Desulfuromonas, Desulfomicrobium, etc.) utilized Hg⁰ as the initial electron donor and sulfate as the terminal electron acceptor to form the overall redox. These microorganisms coupled Hg⁰ bio-oxidation to sulfate bioreduction. Analysis on mercury speciation in biofilm by sequential extraction processes (SEPs) and inductively coupled mass spectrometry (ICP-MS) and by mercury temperature programmed desorption (Hg-TPD) showed that mercury sulfide (HgS) and humic acid-bound mercury (HA-Hg) were two major products of Hg⁰ bio-oxidation. With HgS and HA-Hg comprehensively characterized by X-ray diffraction (XRD), excitation-emission matrix spectra (EEM), scanning electron microscopy-energy disperse spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR), it was proposed that biologically oxidized mercury (Hg²⁺) further reacted with biogenic sulfides to form cubically crystallized metacinnabar (β-HgS) extracellular particles. Hg²⁺ was also complexed with functional groups -SH, -OH, -NH^-, and -COO^- in humic acids from extracellular polymeric substances (EPS) to form HA-Hg. HA-Hg may further react with biogenic sulfides to form HgS. Bioconversion of Hg⁰ into HgS was therefore achieved and can be a feasible biotechnique for flue gas demercuration.