High redox potential promotes oxidation of pyrite under neutral conditions: Implications for optimizing pyrite autotrophic denitrification

Hydrogeological and geological partitioning of iron and sulfur cycling bacterial consortia in subsurface coal-based mine waters

Pyrite oxidation drives iron and sulfur availability across Earth's subsurface and is partly microbially mediated. Subsurface microbial communities accelerate this process at circumneutral pH directly by weathering pyritic surfaces and indirectly by causing changes to the surrounding microenvironment, thereby further accelerating pyrite weathering. However, our understanding of community structure dynamics and associated biogeochemistry in Fe- and S-rich lithologies, e.g. pyritic coal, is limited. Here, we present the first comprehensive regional and seasonal genus-level survey of bacterial groundwater communities in a pyritic coal-based aquifer in the South Wales Coalfield (SWC), using 16S rRNA gene amplicon sequencing. Seasonal changes in community structure were limited, suggesting limited influence of surface processes on subsurface communities. Instead, hydrogeologically distinct mine water blocks (MWB) and coal rank largely explained bacterial community structure variation across sites. Fe(II)-oxidizing Betaproteobacteriales genera Gallionella and Sideroxydans dominated the bacterial communities across nine sites and seven MWBs, while three sites within a single MWB, were dominated by S-oxidizing Epsilonbacteraeota genera Sulfuricurvum and Sulfurovum. The cooccurrence of pairs of Fe(II)- and S-oxidizing bacterial genera suggests functional redundancy, which coupled with genus-specific morphologies and life strategies, indicates the importance of distinct environmental and ecological niches within the SWC groundwater at seasonal and regional scales.

Pyrite and PHBV combined as substrates for groundwater denitrification

Nitrate pollution in groundwater has steadily increased globally, posing a potential threat to human health. Introduction of exogenous electron donors can significantly enhance nitrogen removal from nitrate-contaminated groundwater. Yet, conventional individual autotrophic or heterotrophic denitrification approaches have the disadvantage of low efficiency or high cost. This study investigated the performance of a laboratory-scale solid-phase denitrification (SPD) permeable reactive barrier (PRB) using a polyhydroxybutyrate-co-valerate (PHBV)/pyrite mixture as an electron donor for groundwater denitrification. Two different mass ratios (1:1 and 1:2) were established for the experimental setup. The results showed that under influent levels between 20 and 37 mg·L-1, the PHBV/pyrite system at a ratio of 1:1 achieved a maximum nitrate removal efficiency of 97.03%, with a nitrate removal rate of 99.13 mg NO 3 - - N NO 3 - - N ·L-1·d-1. Moreover, the PHBV/pyrite system at 1:2 reached 97.65% and 111.04 mg NO 3 - - N ·L-1·d-1 in terms of the optimum nitrate removal efficiency and rate. Dissolved organic carbon was undetectable in the effluent in both systems. The nitrate removal performance of the PHBV/pyrite system at 1:2 was superior to the one at 1:1, implying appropriate addition of pyrite in mixtrophic systems could enhance denitrification in groundwater. Additionally, the dominant genera identified were respectively Cloacibacterium and Acinetobacter in two systems, indicating that varying PHBV/pyrite ratios can modulate the succession of dominant nitrogenremoving microorganisms. Specifically, the system at 1:2 favoured aerobic microbial growth, thereby enhancing the efficiency of biological nitrogen removal. The findings have provided a valuable alternative for mixtrophic denitrification in in-situ remediation of nitrate-polluted groundwater.

Differences in the efficiency and mechanisms of different iron-based materials driving synchronous nitrogen and phosphorus removal

Iron-dependent denitrification has been substantially investigated worldwide due to the advantages of low cost, high efficiency, and synchronized phosphorous removal. However, differences in nitrogen metabolism processes with different iron-based materials as electron donors have not been systematically studied. This study investigated the efficacy of nitrogen and phosphate removal using various iron-based materials as electron donors. Substantial nitrogen removal was demonstrated, with complete TN removal with Fe/C powder as electron donor, while relatively lower TN removal was achieved in iron scrap, Fe/C granular, and pyrite systems with removal efficiency of 86 ± 3.5%, 78 ± 5.7%, and 61 ± 3.1%, respectively. However, the high efficiency could only be sustained for a short time and further microbial metabolisms were significantly suppressed by microbial encapsulation caused by iron-bearing minerals. The introduction of fresh electron donors revitalized denitrification activity, with TN removal improved back to more than 80%. Similar trends were observed in phosphate removal, with increased efficiencies corresponding to ferrous ion release, reaching the highest level of 94 ± 0.02% with iron scrap as electron donor. Microbial community analysis revealed distinct compositions, with sulfur autotrophic denitrifying bacteria prevailing in pyrite systems, hydrogenotrophic denitrifying bacteria dominating with Fe/C powder, and Fe(II) oxidizing bacteria (FeOB) governing in Fe scrap and Fe/C granular systems for denitrification. Predicted genomic analysis elucidated mechanisms underlying nitrogen and phosphorus removal, emphasizing the importance of direct electron transfer via cytochrome C and ferrous ion transportation. Furthermore, differences in nitrogen metabolism among systems were highlighted, with Fe/C powder facilitating hydrogen oxidation proteins expression, while pyrite systems favored sulfur oxidation process, and iron scrap selected Fe(II) transportation and oxidation. This study provided valuable insights into revealing mechanism for efficient nitrogen and phosphorus removal with iron-based materials as electron donors.

Insight into shortening mechanisms of start-up time for three-dimensional biofilm electrode reactor/pyrite-autotrophic denitrification coupled system

In this study, a three-dimensional biofilm electrode reactor (3D-BER)/pyrite-autotrophic denitrification (PAD) coupled (3D-BER-PAD) system was constructed, aiming at investigating the effect of current on the start-up period of the system. The results showed that increasing current could shorten the system's start-up period and improve nitrate removal efficiency (NRE). When the current was 20 mA, the system could start stabilization after approximately 13 days and maintain a stable NRE (88.2 ± 3.4 %) with low energy consumption (0.05 ± 0.003 kW·h/gNO3--N). Additionally, an appropriate current (10 or 20 mA) promoted the reproduction of denitrifying bacteria (e.g., Thiobacillus and Thermomonas) and the expression of functional genes involved in denitrification and sulfur oxidation. Finally, the denitrification mechanism and electron transfer model in the 3D-BER-PAD system were proposed. This study has reference value for the rapid start-up and the improvement of treatment efficiency in the 3D-BER-PAD system.

Performance and by-product generation in sulfur-siderite/limestone autotrophic denitrification systems: Enhancing nitrogen removal efficiency and operational insights

Sulfur autotrophic denitrification technology is a promising nitrogen removing process and is suitable for the tail water of sewage treatment plants with easy biodegradation and low C/N ratio. Nitrogen removal efficiency and along-path variation of related product concentrations in the sulfur-siderite autotrophic denitrification (SSAD) and sulfur-limestone autotrophic denitrification (SLAD) systems were comprehensively investigated in this work. The optimal denitrification conditions for SSAD and SLAD systems were pH of 7, HRT of 3 h, temperatures of 20-25 °C with NO3--N removal rates of more than 99%. Although a greater capacity for alkalinity was provided by limestone than siderite, siderite can also meet the advanced nitrogen removal of SSAD system. A transient accumulation of NO2--N in the SLAD system eventually decreased to 0.02 mg/L, while S2- concentration gradually increased relative to SSAD. It might be due to the fact that Fe2+ promoted the nitrogen removal efficiency of SSAD system and further reduced the content of intermediates in the nitrogen removal process. The results obtained may provide the scientific basis and technical countermeasures for the application of sulfur autotrophic denitrification in actual low-C/N wastewater.

Calcined pyrite accelerates sulfur metabolic and electron transfer in driving targeted microbial fuel cell denitrification

The sulfur powder as electron donor in driving dual-chamber microbial fuel cell denitrification (S) process has the advantages in economy and pollution-free to treat nitrate-contained groundwater. However, the low efficiency of electron utilization in sulfur oxidation (ACE) is the bottleneck to this method. In this study, the addition of calcined pyrite to the S system (SCP) accelerated electron generation and intra/extracellular transfer efficiency, thereby improving ACE and denitrification performance. The highest nitrate removal rate reached to 3.55 ± 0.01 mg N/L/h in SCP system, and the ACE was 103 % higher than that in S system. More importantly, calcined pyrite enhanced the enrichment of functional bacteria (Burkholderiales, Thiomonas and Sulfurovum) and functional genes which related to sulfur metabolism and electron transfer. This study was more effective in removing nitrate from groundwater without compromising the water quality.

A novel paraffin-based N/P controlled-release material for biostimulation of phenol biodegradation in groundwater

To address the problem of the weak natural restoration ability of oligotrophic groundwater environments, a novel N/P controlled-release material (CRM) for biostimulation, prepared by an improved method, was developed. CRMs can encapsulate N and P (N/P) salts for sustained release in aquifers. Paraffin-based CRMs can be used to control N/P release rates by adjusting the particle size of CRMs and the mass ratio of the paraffin. The developed CRMs had a more remarkable adaptability to groundwater than other materials. Specifically, 0.4-cm CRMs released N/P stably and efficiently over a wide temperature range (7-25 ℃), and the release properties of various CRMs were not affected by pH. The release of N/P followed Fickian diffusion, and a dissolution-diffusion model was established to elucidate the mechanism of the controlled release. In contrast to bare N/P, CRMs obviously enhanced the biodegradation rate of phenol and prolonged the effectiveness of supplying N/P. The degradation rate of phenol in the CRM system increased by 20.8 %. The different supply modes of N/P, CRMs and bare N/P, resulted in differences in salinity. Metagenomic analysis showed that this difference changed the proportion of various phenol-degrading genera and thus changed the abundance of genes associated with the phenol degradation pathway.

Microbial communities and denitrification mechanisms of pyrite autotrophic denitrification coupled with three-dimensional biofilm electrode reactor

Denitrification is of great significance for low C/N wastewater treatment. In this study, pyrite autotrophic denitrification (PAD) was coupled with a three-dimensional biofilm electrode reactor (BER) to enhance denitrification. The effect of current on denitrification was extensively studied. The nitrate removal of the PAD-BER increased by 14.90% and 74.64% compared to the BER and the PAD, respectively. In addition, the electron utilization, extracellular polymeric substances secretion, and denitrification enzyme activity (NaR and NiR) were enhanced in the PAD-BER. The microbial communities study displayed that Dokdonella, Hydrogenophaga, Nitrospira, and Terrimonas became the main genera for denitrification. Compared with the PAD and the BER, the abundance of the key denitrification genes narG, nirK, nirS, and nosZ were all boosted in the PAD-BER. This study indicated that the enhanced autotrophic denitrifiers and denitrification genes were responsible for the improved denitrification in the PAD-BER. PRACTITIONER POINTS: PAD-BER displayed higher nitrate removal, EPS, NAR, and NIR activity. The three types of denitrification (HD, HAD, and PAD) and their contribution percentage in the PAD-BER were analyzed. HAD was dominant among the three denitrification processes in PAD-BER. Microbial community composition and key denitrification genes were tested to reveal the denitrification mechanisms.

Amorphous Fe substrate enhances nitrogen and phosphorus removal in sulfur autotrophic process

The autotrophic denitrification of coupled sulfur and natural iron ore can remove nitrogen and phosphorus from wastewater with low C/N ratios. However, the low solubility of crystalline Fe limits its bioavailability and P absorption capacity. This study investigated the effects of amorphous Fe in drinking water treatment residue (DWTR) and crystalline Fe in red mud (RM) on nitrogen and phosphorus removal during sulfur autotrophic processes. Two types of S-Fe cross-linked filler particles with three-dimensional mesh structures were obtained by combining sulfur with the DWTR/RM using the hydrogel encapsulation method. Two fixed-bed reactors, sulfur-DWTR autotrophic denitrification (SDAD) and sulfur-RM autotrophic denitrification (SRAD), were constructed and stably operated for 236 d Under a 5-8-h hydraulic retention time, the average NO3--N, TN, and phosphate removal rates of SDAD and SRAD were 99.04 %, 96.29 %, 94.03 % (SDAD) and 97.33 %, 69.97 %, 82.26 % (SRAD), respectively. It is important to note that fermentative iron-reducing bacteria, specifically Clostridium_sensu_stricto_1, were present in SDAD at an abundance of 58.17 %, but were absent from SRAD. The presence of these bacteria facilitated the reduction of Fe (III) to Fe (II), which led to the complete denitrification of the S-Fe (II) co-electron donor to produce Fe (III), completing the iron cycle in the system. This study proposes an enhancement method for sulfur autotrophic denitrification using an amorphous Fe substrate, providing a new option for the efficient treatment of low-C/N wastewater.

Deciphering the influencing mechanism of hydraulic retention time on purification performance of a mixotrophic system from the perspective of reaction kinetics

At present, eutrophication is increasingly serious, so it is necessary to effectively reduce nitrogen and phosphorus in water bodies. In this study, a pyrite/polycaprolactone-based mixotrophic denitrification (PPMD) system using pyrite and polycaprolactone (PCL) as electron donors was developed and compared with pyrite-based autotrophic denitrification (PAD) system and PCL-based heterotrophic denitrification (PHD) system through continuous flow experiment. The removal efficiency of NO3--N (NRE) and PO43--P (PRE) and the contribution proportion of PAD in the PPMD system were significantly increased by prolonging hydraulic retention time (HRT, from 1 to 48 h). When HRT was equal to 24 h, the PPMD system conformed to the zero-order kinetic model, so NRE and PRE were mainly limited by the PAD process. When HRT was equal to 48 h, the PPMD system met the first-order kinetic model with NRE and PRE reaching 98.9 ± 1.1% and 91.8 ± 4.5%, respectively. When HRT = 48 h, the NRE and PRE by PAD system were 82.7 ± 9.1% and 88.5 ± 4.7%, respectively, but the effluent SO42- concentration was as high as 152.1 ± 13.7 mg/L (the influent SO42- concentration was 49.2 ± 3.3 mg/L); the NRE by PHD system was 98.5 ± 1.7%, but the PO43--P could not be removed ideally. The concentrations of NO3--N, total nitrogen, PO43--P, and SO42- in the PPMD system also showed distinct changes along the reactor column. In addition, the microbial diversity analysis showed that prolonging HRT (from 24 to 48 h) increased the abundance of autotrophic denitrifying microorganisms in the PPMD system, ultimately increasing the contribution proportion of PAD.

Insights into the transport and bio-degradation of dissolved inorganic nitrogen in the biochar-pyrite amended stormwater biofilter using dynamic modeling

The stormwater biofilter is a prevailing green infrastructure for urban stormwater management, but it is less effective in dissolved nitrogen removal, especially for nitrate. The mechanism that governs the nitrate leaching and performance stability of stormwater biofilters is poorly understood. In this study, a water quality model was developed to predict the ammonium and nitrate dynamics in a biochar-pyrite amended stormwater biofilter. The transport of dissolved nitrogen species was described by advection-dispersion models. The kinetics of adsorption and pyrite-based autotrophic denitrification are included in the model and simulated with a steady-state saturated flow. The model was calibrated and validated using eleven storm events. The modeling results reveal that the contribution of pyrite-based autotrophic denitrification to nitrate leaching alleviation improves with the increased drying duration. The nitrate removal efficiency was affected by a series of design parameters. Pyrite filling rate has a minor effect on nitrate removal promotion. Service area ratio and submerged zone depth are the key parameters to prevent nitrate leaching, as they influence the emergence and discharge time of nitrate breakthrough. The high inflow volume (high service area ratio) and small submerged zone can lead to earlier and increased discharge of peak nitrate otherwise the peak nitrate could be retained in the submerged zone and denitrified during the drying period. The developed mechanistic model provides a useful tool to evaluate the treatment ability of stormwater biofilters under varying conditions and offers a guideline for biofilter design optimization.

Nitrate removal in iron sulfide-driven autotrophic denitrification biofilter: Biochemical and chemical transformation pathways and its underlying microbial mechanism

Iron sulfides-based autotrophic denitrification (IAD) is effective for treating nitrate-contaminated wastewater. However, the complex nitrate transformation pathways coupled with sulfur and iron cycles in IADs are still unclear. In this study, two columns (abiotic vs biotic) with iron sulfides (FeS) as the packing materials were constructed and operated continuously. In the abiotic column, FeS chemically reduced nitrate to ammonium under the ambient condition; this chemical reduction reaction pathway was spontaneous and has been overlooked in IAD reactors. In the biotic column (IAD biofilter), the complex nitrogen-transformation network was composed of chemical reduction, autotrophic denitrification, dissimilatory nitrate reduction to ammonium (DNRA) and sulfate reducing ammonium oxidation (Sulfammox). Metagenomic analysis and XPS characterization of the IAD biofilter further validated the roles of functional microbial communities (e.g., Acidovorax, Diaphorobacter, Desulfuromonas) in nitrate reduction process coupled with iron and sulfur cycles. This study gives an in-depth insight into the nitrogen transformations in IAD system and provides fundamental evidence about the underlying microbial mechanism for its further application in biological nitrogen removal.

Coupled pyrite and sulfur autotrophic denitrification for simultaneous removal of nitrogen and phosphorus from secondary effluent: feasibility, performance and mechanisms

The discharge standards of nitrogen (N) and phosphorus (P) in wastewater treatment plants (WWTPs) have become increasingly strict to reduce water eutrophication. Further reducing N and P in effluent from municipal WWTPs need to be achieved effectively and eco-friendly. In this study, a carbon independent pyrite and sulfur autotrophic denitrification (PSAD) system using pyrite and sulfur as electron donor was developed and compared with pyrite autotrophic denitrification (PAD) and sulfur autotrophic denitrification (SAD) systems through batch and continuous flow biofilter experiments. Compare to PAD and SAD, PSAD was more effective in simultaneous removal in N and P. At hydraulic retention time (HRT) 3 h, average effluent concentrations of total nitrogen (TN) and total phosphate (TP) of 1.40 ± 0.03 and 0.19 ± 0.02 mg/L were achieved when treating real secondary effluent with 20.65 ± 0.24 mg/L TN and 1.00 ± 0.24 mg/L TP. The improvement in simultaneous removal of N and P was attributed to the coupling of PAD and SAD in enhancing the transformation of sulfur and iron and enlarging the reaction zone in the pyrite and sulfur autotrophic denitrification biofilter (PSADB) system. Therefore, more biomass was accumulated and the microbial denitrification functional stability, including electrons transfer and consumption was enhanced on the surface of pyrite and sulfur particles in the PSADB system. Moreover, autotrophic denitrifiers (Thiobacillus and Ferritrophicum), sulfate-reducing bacteria (Desulfocapsa) and iron reducing bacteria (Geothrix), acting as contributors to microbial nitrogen, sulfur and iron cycle, were specially enriched. In addition, the leaching of iron ions was promoted, which facilitated the removal of phosphate in the form of Fe3(PO4)2·8H2O and Fe3PO4. PSADB has proven to be an efficient technology for simultaneous removal of N and P, which could meet increasingly stringent discharge standards effectively and eco-friendly.

Pyrite-based denitrification combined with electrochemical disinfection to remove nitrate and microbial contamination from groundwater

Nitrate and microbial contamination of groundwater can occur in countries that face intense urbanization and inadequate sanitation. When groundwater is the main drinking water source, as is often the case in such countries, the need to remove these contaminants becomes acute. The combination of two technologies is proposed here, a biological step to denitrify and an electrochemical step to disinfect the groundwater, thereby aiming to reduce the chemical input and the footprint of groundwater treatment. As such, a pyrite-based fluidized bed reactor (P-FBR) was constructed to autotrophically denitrify polluted groundwater. The P-FBR effluent was disinfected in an electrochemical cell with electrogenerated Cl2. Nitrate was removed with 79% efficiency from an initial 178 mg NO3- L-1 at an average denitrification rate of 171 mg NO3- L-1 d-1, with 18 h hydraulic retention time (HRT). The electrochemical unit achieved a 3.8-log reduction in total coliforms with a 41.7 A h m-3 charge density.

Pyrite and sulfur-coupled autotrophic denitrification system for efficient nitrate and phosphate removal

The inefficiency of nitrogen removal in pyrite autotrophic denitrification (PAD) and the low efficiency of PO₄^3--P removal in sulfur autotrophic denitrification (SAD) limit their potential for engineering applications. This study examined the use of pyrite and sulfur coupled autotrophic denitrification (PSAD) in batch and column experiments to remove NO₃^--N and PO₄^3--P from sewage. The effluent concentration of NO₃^--N was 0.32 ± 0.11 mg/L, with an average Total nitrogen (TN) removal efficiency of 99.14%. The highest PO₄^3--P removal efficiency was 100% on day 18. There was a significant correlation between pH and the efficiency of PO₄^3--P removal. Thiobacillus, Thiomonas and Thermomonas were found to be dominant at the bacterial genus level in PSAD. Additionally, the abundance of Thermomonas in the PSAD was greater than that observed in the SAD reactor. This result indirectly indicates that the PSAD system has more advantages in reducing N₂O.

Kinetics of Hydroxyl Radical Production from Oxygenation of Reduced Iron Minerals and Their Reactivity with Trichloroethene: Effects of Iron Amounts, Iron Species, and Sulfate Reducing Bacteria

Reactive oxygen species generated during the oxygenation of different ferrous species have been documented at groundwater field sites, but their effect on pollutant destruction remains an open question. To address this knowledge gap, a kinetic model was developed to probe mechanisms of •OH production and reactivity with trichloroethene (TCE) and competing species in the presence of reduced iron minerals (RIM) and oxygen in batch experiments. RIM slurries were formed by combining different amounts of Fe(II) and sulfide (with Fe(II):S ratios from 1:1 to 50:1) or Fe(II) and sulfate with sulfate reducing bacteria (SRB) added. Extents of TCE oxidation and •OH production were both greater with RIM prepared under more reducing conditions (more added Fe(II)) and then amended with O₂. Kinetic rate constants from modeling indicate that •OH production from free Fe(II) dominates •OH production from solid Fe(II) and that TCE competes for •OH with Fe(II) and organic matter (OM). Competition with OM only occurs in experiments with SRB, which include cells and their exudates. Experimental results indicate that cells and/or exudates also provide electron equivalents to reform Fe(II) from oxidized RIM. Our work provides new insights into mechanisms and environmental significance of TCE oxidation by •OH produced from oxygenation of RIM. However, further work is necessary to confirm the relative importance of reaction pathways identified here and to probe potentially unaccounted for mechanisms that affect abiotic TCE oxidation in natural systems.

Corncob-pyrite bioretention system for enhanced dissolved nutrient treatment: Carbon source release and mixotrophic denitrification

Solid biomass waste amendment and substrates modification in bioretention systems have been increasingly used to achieve effective dissolved nutrients pollution control in stormwater runoff. However, the risk of excess chemical oxygen demand (COD) leaching from organic carbon sources is often overlooked on most occasions. Pyrite is an efficient electron donor for autotrophic denitrification, but little is known about the efficacy of autotrophic-heterotrophic synergistic effect between additional carbon source and pyrite in bioretention. Here, four bioretention columns (i.e., corncob column (C), pyrite column (P), the corncob-pyrite layered column (L-CP), and the corncob-pyrite mixed column (M-CP)) were designed and filled with soil, quartz sand, and modified media to reveal the synergistic effects. The results showed that the corncob-pyrite layered bioretention could maintain low COD effluent concentration with high stability and efficiency in treating dissolved nutrients. When the influent nitrogen and phosphorus concentrations were 8.46 mg/L and 0.94 mg/L, the average removal rates of ammonia nitrogen, total inorganic nitrogen, and phosphate were 83.6%, 70.52%, and 76.35%, respectively. The scouring experiment showed that placing the corncob in the mulch layer was beneficial to the sustained release of dissolved organic carbon (DOC). Erosion pits were found in the SEM images of used pyrite, indicating that autotrophic denitrifying bacteria in the bioretention could react with pyrite as an electron donor. The relative abundance of Thiobacillus in the submerged zone of the corncob-pyrite layered bioretention reached 38.39%, indicating that the carbon source in the mulch layer increased the relative abundance of Thiobacillus. Coexisting heterotrophic and autotrophic denitrification in this bioretention created a more abundant microbial community structure in the submerged zone. Overall, the corncob-pyrite layered bioretention is highly promising for stormwater runoff treatment, with effective pollution removal and minimal COD emission.

Effect of electric current intensity on performance of polycaprolactone/FeS2-based mixotrophic biofilm-electrode reactor

In this study, a bioelectrochemical system consisting of pyrite-based autotrophic denitrification (PAD) and heterotrophic denitrification (HD) was established to polish nitrate wastewater. The loading of electric current (EC) could stimulate the dissolution of pyrite. Appropriate EC (I ≤ 30 mA) was conducive to nitrate removal, too high EC (I = 40 mA) would inhibit nitrate removal and lead to an obvious accumulation of NO₂^--N and NH₄⁺-N. Microbial analysis revealed that the increase of EC could inhibit the diversity of heterotrophic microbes, but appropriate EC (I = 10 mA) could increase the diversity of autotrophic microbes. The EC loading was conducive to the enrichment of iron autotrophic denitrifiers (Ferritrophicum), pyrite-oxidizing bacteria (Thiobacillus, Sulfurimonas), and sulfur autotrophic denitrifiers (Dechloromonas, Thiobacillus, and Arenimonas). The EC loading enlarged the contribution of PAD, making PAD a dominant pathway in denitrification.

Reductive Cr(VI) Removal under Different Reducing and Electron Donor Conditions—A Soil Microcosm Study

Increased groundwater and soil contamination by hexavalent chromium have led to the employment of a variety of detoxification methods. Biological remediation of Cr(VI) polluted aquifers is an eco-friendly method that can be performed in situ by stimulating the indigenous microbial population with organic and inorganic electron donors. In order to study the effect of different redox conditions on microbial remediated Cr(VI) reduction to Cr(III), microcosm experiments were conducted under anaerobic, anoxic, and sulfate-reducing conditions and at hexavalent chromium groundwater concentrations in the 0–3000 μg/L range, with groundwater and soil collected from an industrial area (Inofyta region). As electron donors, molasses, emulsified vegetable oil (EVO), and FeSO4 were employed. To quantitatively describe the degradation kinetics of Cr(VI), pseudo-first-order kinetics were adopted. The results indicate that an anaerobic system dosed with simple or complex external organic carbon sources can lead to practically complete Cr(VI) reduction to Cr(III), while the addition of Fe2+ can further increase Cr(VI) removal rate significantly. Furthermore, Cr(VI) microbial reduction is possible in the presence of NO3− at rates comparable to anaerobic Cr(VI) microbial reduction, while high sulfate concentrations have a negative effect on Cr(VI) bioreduction rates in comparison to lower sulfate concentrations.

Simultaneous bio-reduction of nitrate and Cr(VI) by mechanical milling activated corn straw

Abundant lignocellulose waste is an ideal energy source for environmental bioremediation, but its recalcitrance to bioavailability makes this a challenging prospect. We hypothesized that the disruption of straw's recalcitrant structure by mechanochemical ball milling would enhance its availability for the simultaneous bioreduction of nitrate and Cr(VI). The results showed that the ball-milling process increased the quantity of water-soluble organic matter released from corn straw and changed the composition of organic matter by strongly disrupting its lignocellulose structure. The increase in ball-milling time increased the specific surface area of the straw and favored the adhesion of microorganisms on the straw surface, which enhanced the bioavailability of the energy in the straw. Substantially increased removal of NO₃^--N (206.47 ± 0.67 mg/g) and Cr(VI) (37.62 ± 0.09 mg/g) was achieved by using straw that was ball milled for 240 min, which validated that ball milling can improve the utilization efficiency of straw by microorganisms. Cellular and molecular biological analyses showed that ball-milled straw increased microbial energy metabolism and cellular activity related to the electron transport chain. This work offers a potential way to achieve the win-win goal of utilizing agricultural wastes and remediating environmental pollution.

Cooperative denitrification in biocathodes under low carbon to nitrogen ratio conditions coupled with simultaneous degradation of ibuprofen in photoanodes

A two-chamber microbial photoelectrochemical cell (MPEC) with a denitrification biocathode and an α-Fe₂O₃/P3HT photoanode was established aimed to enhance nitrate removal from wastewater of low carbon sources and achieve simultaneous ibuprofen degradation. The results demonstrated that the average removal of NO₃^--N in the biocathode reached 96.56 ± 0.72% when the COD/NO₃^--N was 0.75, and the relative contributions of heterotrophic and autotrophic denitrification were 21.47% and 78.53%, respectively. When there was no organic source in the influent, the maximum removal of NO₃^--N was less than 45%. High-throughput sequencing revealed that both heterotrophic bacteria, such as Bacillus, and autotrophic bacteria (e.g., Thermomonas and Hydrogenophaga) dominated in the cooperative denitrification biocathode. Moreover, the functional gene analysis showed that the abundance of genes related to denitrification was highest in the cooperative denitrification biocathode. In addition, the photocatalytic degradation efficiency of ibuprofen in the anode chamber attained 67.95% ± 0.97%.

Synergistic removal of fluoride, calcium, and nitrate in a biofilm reactor based on anaerobic microbially induced calcium precipitation

Fluoride (F^-) and calcium (Ca²⁺) are primary causes of skeleton fluorosis and scaling, posing a grievous threat to aquatic lives and public health. Therefore, a novel strategy for polluted groundwater in immobilized biofilm reactor based on the anaerobic microbial induced calcium precipitation (MICP) was proposed, in which loofah was used as a multifunctional strain Cupriavidus sp. W12 growth carrier. Effects of different hydraulic retention time (HRT), initial F^-concentration, and pH on the synchronous removal of pollutants were examined. Under stable operation conditions, the highest efficiencies for Ca²⁺, F^-, and nitrate (NO₃^--N) reached 76.73%, 94.92%, and 100%, respectively. Furthermore, gas chromatography (GC), Fluorescence excitation-emission matrix (EEM), X-ray diffraction (XRD), Scanning electron microscope-energy dispersive spectroscope (SEM-EDS), and Fourier transform infrared spectrometer (FTIR) comprehensively clarified the mechanism of pollutants removal. The results elucidated that the removal of various pollutants was achieved through a combination of anaerobic MICP, adsorption, and co-precipitation. Besides, high-throughput sequencing analysis showed that Cupriavidus had a predominant proportion of 42.36% in the reactor and had stability against pH impact. As the first application of a biofilm reactor based on anaerobic MICP, it put forward a new insight for efficient defluorination and decalcification.

Vertical-flow constructed wetland based on pyrite intensification: Mixotrophic denitrification performance and mechanism

Deep nitrogen removal from low-carbon wastewater is a pressing water treatment challenge as of yet. Eight sets of vertical-flow constructed wetland (VFCW) intensified by pyrite were designed and applied to treat with low C/N ratio wastewater in this research. The results showed that the addition of pyrite (100% added) significantly promoted TN removal with an efficiency higher than 27.05% under low C/N ratio conditions, indicating that mixotrophic denitrification was achieved in VFCW. Microbial analysis showed that the community structure and diversity of microorganisms were changed significantly, and the growth of autotrophic (Thiobacillus) and heterotrophic bacteria (Thauera) concomitantly enhanced. It is recommended that the addition amount of pyrite is 75% of the wetland volume, meantime, mixing evenly with 25% high porosity substrate (such as activated carbon, volcanic stone, etc.), which could enhance the effective adhesion of microorganisms and their contact area with pyrite, ultimately improve the denitrification capacity of the VFCW.

High efficient bio-denitrification of nitrate contaminated water with low ammonium and sulfate production by a sulfur/pyrite-based bioreactor

Sulfur-based autotrophic denitrification (SAD) and pyrite-based autotrophic denitrification (PAD) are important technologies that address nitrate pollution, but high sulfate production and low denitrification efficiency, respectively, limit their application in engineering. A bio-denitrification reactor with sulfur and pyrite as filler materials was studied to remove NO₃^--N from nitrate contaminated water. At an influent NO₃^--N concentration of 50 mg/L, NO₃^--N removal efficiency of the sulfur/pyrite-based bioreactor was 99.2%, producing less NH₄⁺-N and SO₄^2- than the sulfur-based bioreactor, even after long-term operation. Denitrification performance was significantly related to environmental variable, especially dissolved oxygen. Proteobacteria and Epsilonbacteraeota were the predominant phyla in the sulfur/pyrite-based bioreactor, and fewer dissimilatory nitrate reductions to ammonia process-related bacteria were enriched compared to those in the sulfur-based bioreactor. Sulfur-pyrite bio-denitrification provides an efficient alternative method for treatment of nitrate contaminated water.