Remediation of nitrate-nitrogen contaminated groundwater using a pilot-scale two-layer heterotrophic-autotrophic denitrification permeable reactive barrier with spongy iron/pine bark

Organic carbon source excites extracellular polymeric substances to boost Fe0-mediated autotrophic denitrification in mixotrophic system

Fe⁰-mediated autotrophic denitrification (ADN) can be suppressed by iron oxide coverage resulting from Fe⁰ corrosion. The mixotrophic denitrification (MDN) coupling Fe⁰-mediated ADN with heterotrophic denitrification (HDN) can circumvent the weakening of Fe⁰-mediated ADN over operation time. But the interaction between HDN and Fe⁰-mediated ADN for nitrogen removal of secondary effluent with deficient bioavailable organics remains unclear. When the influent COD/NO₃^--N ratio increased from 0.0 to 1.8-2.1, the TN removal efficiency was promoted significantly. The increased carbon source did not inhibit ADN, but promoted ADN and HDN synchronously. The formation of extracellular polymeric substances (EPS) was also facilitated concomitantly. Protein (PN) and humic acid (HA) in EPS increased significantly, which capable of accelerating electron transfer of denitrification. Due to that the electron transfer of HDN occurs intracellularly, the EPS with the capacity of accelerating electron transfer had a negligible influence on HDN. But for Fe⁰-mediated ADN, the increased EPS as well as corresponding PN and HA facilitated TN and NO₃^--N removal significantly, while accelerated the electron release originating from Fe⁰ corrosion. The bioorganic-Fe complexes were generated on Fe⁰ surface after used, meaning that the soluble EPS and soluble microbial products (SMP) participated in the electron transfer of Fe⁰-mediated ADN. The coexistence of HDN and ADN denitrifiers demonstrated the synchronous enhancement of HDN and ADN by the external carbon source. From the perspective of EPS and related SMP, the insight of enhancing Fe⁰-mediated ADN by external carbon source is beneficial to implement high-efficiency MDN for organics-deficient secondary wastewater.

A comprehensive review on permeable reactive barrier for the remediation of groundwater contamination

Groundwater quality is deteriorating due to contamination from both natural and anthropogenic sources. Traditional "Pump and Treat" techniques of treating the groundwater suffer from the disadvantages of a small-scale and energy-intensive approach. Permeable reactive barriers (PRBs), owing to their passive operation, offer a more sustainable strategy for remediation. This promising technique focuses on eliminating heavy metal pollutants and hazardous aromatic compounds by physisorption, chemisorption, precipitation, denitrification, and/or biodegradation. Researchers have utilized ZVI, activated carbon, natural and manufactured zeolites, and other by-products as reactive media barriers. Environmental parameters, i.e., pH, initial pollutant concentration, organic substance, dissolved oxygen, and reactive media by-products, all influence a PRB's performance. Although their long-term impact and performance are uncertain, PRBs are still evolving as viable alternatives to pump-and-treat techniques. The use of PRBs to remove anionic contaminants (e.g., Fluoride, Nitrate, etc.) has received less attention since precipitates can clog the reactive barrier and hinder groundwater flow. In this paper, we present an insight into this approach and the tremendous implications for future scientific study that integrates this strategy using sustainability and explores the viability of PRBs for anionic pollutants.

Removal of nitrate and pesticides from groundwater by nano zero-valent iron injection pulses under biostimulation and bioaugmentation scenarios in continuous-flow packed soil columns

This study evaluates the NO₃^- removal from groundwater through Heterotrophic Denitrification (HDN) (promoted by the addition of acetate and/or an inoculum rich in denitrifiers) and Abiotic Chemical Nitrate Reduction (ACNR) (promoted by pulse injection of zerovalent iron nanoparticles (nZVI)). HDN and ACNR were applied, separately or combined, in packed soil column experiments to complement the scarce research on pulse-injected nZVI in continuous-flow systems mimicking a Well-based Denitrification Barrier. Together with NO₃^-, the removal of two common pesticides (dieldrin and lindane) was evaluated. Results showed that total NO₃^- removal (>97%) could be achieved by either bioestimulation with acetate (converting NO₃^- to N₂(g) via HDN) or by injecting nZVI (removing NO₃^- via ACNR). In the presence of nZVI, NO₃^- was partially converted to N₂(g) and to a lower extent NO₂^-, with unreacted NO₃^- being likely adsorbed onto Fe-(oxy)hydroxides. Combination of both HDN and ACNR resulted in even a higher NO₃^- removal (>99%). Interestingly, nZVI did not seem to pose any toxic effect on denitrifiers. These results showed that both processes can be alterned or combined to take advantage of the benefits of each individual process while overcoming their disadvantages if applied alone. With regard to the target pesticides, the removal was high for dieldrin (>93%) and moderate for lindane (38%), and it was not due to biodegradation but to adsorption onto soil. When nZVI was applied, the removal increased (generally >91%) due to chemical degradation by nZVI and/or adsorption onto formed Fe-(oxy)hydroxides.

Dissimilatory and Cytoplasmic Antimonate Reductions in a Hydrogen-Based Membrane Biofilm Reactor

A hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated to investigate the bioreduction of antimonate [Sb(V)] in terms of Sb(V) removal, the fate of Sb, and the pathways of reduction metabolism. The MBfR achieved up to 80% Sb(V) removal and an Sb(V) removal flux of 0.55 g/m²·day. Sb(V) was reduced to Sb(III), which mainly formed Sb₂O₃ precipitates in the biofilm matrix, although some Sb(III) was retained intracellularly. High Sb(V) loading caused stress that deteriorated performance that was not recovered when the high Sb(V) loading was removed. The biofilm community consisted of DSbRB (dissimilatory Sb-reduction bacteria), SbRB (Sb-resistant bacteria), and DIRB (dissimilatory iron-reducing bacteria). Dissimilatory antimonate reduction, mediated by the respiratory arsenate reductase ArrAB, was the main reduction route, but respiratory reduction coexisted with cytoplasmic Sb(V)-reduction mediated by arsenate reductase ArsC. Increasing Sb(V) loading caused stress that led to increases in the expression of arsC gene and intracellular accumulation of Sb(III). By illuminating the roles of the dissimilatory and cytoplasmic Sb(V) reduction mechanism in the biofilms of the H₂-MBfR, this study reveals that the Sb(V) loading should be controlled to avoid stress that deteriorates Sb(V) reduction.

Removal of nitrate from groundwater by nano-scale zero-valent iron injection pulses in continuous-flow packed soil columns

Injection of zero-valent iron nanoparticles (nZVI) into aquifers has gained increasing attention of researchers for in-situ treatment of NO₃^--contaminated groundwater. nZVI has proved efficient in chemically reducing NO₃^- and, according to recent research efforts, in supporting biological denitrification under favoured conditions. Given the scarce research on nZVI pulsed injection in continuous-flow systems, the objective of this study was to evaluate the effect of nZVI pulses on the removal of NO₃^- from groundwater in packed soil columns and, more particularly, to elucidate whether or not biotic NO₃^- removal processes were promoted by nZVI. Three identical columns were filled with aquifer soil samples and fed with the same nitrate polluted groundwater but operated under different conditions: (A) with application of nZVI pulses and biocide spiked in groundwater, (B) without application of nZVI pulses and (C) with application of nZVI pulses. Results showed that the application of nZVI (at 30 mg/L and 78 mg/L doses) resulted in an immediate and sharp removal of NO₃^- (88-94%), accompanied by an increase in pH (from 7.0 to 9.0-10.0), a drop in redox potential (Eh) (from +420 mV to <100 mV) and a release of Fe(II) and Total Organic Carbon (TOC) in the effluent (to 200 mg/L and 150-200 mg/L, respectively). The released TOC came from the organic polymer used as stabilizer of the nZVI particles. Comparison against the sterilized control column revealed that, under the experimental conditions, no biological denitrification developed and that the removal of NO₃^- was due to chemical reduction by nZVI. The main by-product of the NO₃^- removal was NH₄⁺, which at the prevailing pH was partially converted to NH₃, which dissipated from the aqueous solution resulting in a net removal of total dissolved N. A mass balance of Fe permitted to quantify the percentage of injected nZVI trapped in the column (>98%) and the NO₃^- retention capacity of the nZVI particles (13.2-85.5 mg NO₃^-/g nZVI).

An overview of in-situ remediation for nitrate in groundwater

Faced with the increasing nitrate pollution in groundwater, in-situ remediation has been widely studied and applied on field-scale as an efficient, economical and less disturbing remediation technology. In this review, we discussed various in-situ remediation for nitrate in groundwater and elaborate on biostimulation, phytoremediation, electrokinetic remediation, permeable reactive barrier and combined remediation. This review described principles of each in-situ remediation, application, the latest progress, problems and challenges on field-scale. Factors affecting the efficiency of in-situ remediation for nitrate in groundwater are also summarized. Finally, this review presented the prospect of in-situ remediation for nitrate pollution in groundwater. The objective of this review is to examine the state of knowledge on in-situ remediation for nitrate in groundwater and critically evaluate factors which affect the up-scaling of laboratory and bench-scale research to field-scale application. This helps to better understand the control mechanisms of various in-situ remediation for nitrate pollution in groundwater and the design options available for application to the field-scale.

Integrating sulfur, iron(II), and fixed organic carbon for mixotrophic denitrification in a composite filter bed reactor for decentralized wastewater treatment: Performance and microbial community

Decentralized wastewater treatment in rural areas is an imperative challenge around the world, particularly in developing countries. The composite filter bed reactor is viable for decentralized wastewater treatment, but its performance on nitrogen removal often fluctuates with the unstable influent characteristics and loadings. Here, a composite filter bed reactor integrating sulfur, iron(II), and fixed organic carbon (shaddock peel) was developed and continuously operated under different conditions. The fixed organic carbon source promoted nitrogen removal with an efficiency higher than 90% and reduced effluent sulfate level by 40%, indicating that the integrated electron donors could improve the resistance and stability of the reactor. Moreover, sulfur-oxidizing bacteria (Thiomonas, Sulfuriferula, and Acidithiobacillus), iron-oxidizing bacteria (Ferritrophicum), and denitrifiers (Simplicispira and Hydrogenophaga) were identified in the anoxic/anaerobic layer of the reactor, suggesting that mixotrophic denitrification was stimulated by sulfur, iron(II), and fixed organic carbon. The findings of this study indicate that the developed reactor with the integrated electron donors could be reliable for carbon, nitrogen, and phosphorus removal and promising for the application of decentralized wastewater treatment.

Improved immobilization of soil cadmium by regulating soil characteristics and microbial community through reductive soil disinfestation

Cadmium (Cd) contamination arising from industrialization has attracted increasing attention in recent years. Reductive soil disinfestation (RSD) as an effective agricultural practice has been widely applied for soil sterilization. However, there is little research regarding RSD affecting Cd immobilization. Here, five treatments, namely untreated soil (CK), flooding-treated soil (FL), RSD with 2% ethyl alcohol (EA), 2% sugarcane bagasse (SB), and 2% bean dregs (BD) were designed to detect their performance for Cd immobilization in contaminated soils, and the change of soil properties and microbial communities were monitored. The results revealed that pH significantly increased in FL and RSD-treated soils, but was negatively correlated with the exchangeable fraction of Cd (EX-Cd), while Oxidation-Reduction Potential (Eh) significantly decreased in FL and RSD-treated soils, and was positively correlated with EX-Cd. BD treatment might contribute to the increase of CaCO₃ as shown by X-Ray Diffractomer analysis and strongly decreased the EX-Cd in the soil, but increased the relative abundances of Firmicutes, Planctomycetes, Acidobacteria, and Gemmatimonadetes, which may promote Fe (III) reduction or induce resistance to Cd. Bacterial communities at the phylum and genus levels were closely related to Cd fraction. The FL and RSD treatments moderately altered bacterial specific functions, including iron respiration, which may contribute to remediation of Cd-polluted soil by Fe (III) reduction. Field experiments were conducted to confirm that BD treatment resulted in a significant increase in pH whereas the content of total available Cd was reduced in soils. Compared to the control, concentration of total available Cd of red amaranth, sweet potato, towel gourd, and cowpeas were reduced by approximately 46%, 74%, 72%, and 76% in a BD-treated field, respectively. Our study highlights the potential of RSD as an effective method for Cd immobilization in contaminated soils by improving soil characteristics and altering the composition of the microbial community.

Denitrification using permeable reactive barriers with organic substrate or zero-valent iron fillers: controlling mechanisms, challenges, and future perspectives

Nitrate as a diffusive agricultural contaminant has been causing substantial groundwater quality deterioration worldwide. In situ groundwater remediation techniques using permeable reactive barriers (PRBs) have attracted increasing interest. Particularly, PRBs based on biological denitrification, using the organic substrate as a biostimulator, and chemical nitrate reduction, using zero-valent iron (ZVI) as a reductant, are two major PRB approaches for groundwater denitrification. This review paper analyzed the published studies over the past 10 years (2010-2020) using laboratory, modeling, and field-scale approaches to explore the performance and mechanisms of these two types of PRBs. Important factors affecting the denitrification efficiencies as well as the influential mechanisms were discussed. Several research gaps have been identified and further research needs are discussed in the end.

Nitrate removal uncertainty in stormwater control measures: Is the design or climate a culprit?

Eutrophication is caused by excess nitrate and other nutrient exported via stormwater runoff to surface waters, which is projected to increase as a result of climate change. Despite recent increases in the implementation of stormwater control measures (SCM), nutrient export has not abated, indicating poor or inconsistent removal capacities of SCM for nitrate. However, the cause of the variability is unclear. We show that both design and local climate can explain nitrate removal variability by critically analyzing data reported on the international BMP database for nitrate removal by four common types of SCM: bioretention cells, grass swales, media filters, and retention ponds. The relative importance of climate or design on nitrate removal depends on the SCM type. Nitrate removal in grass swales and bioretention systems is more sensitive to local climate than design specifications, whereas nitrate removal in the retention ponds is less sensitive to climate and more sensitive to design features such as vegetation and pond volume. Media filters without amendment have the least capacity compared to other SCM types surveyed, and their removal capacity was independent of the local climate. Adding amendments made up of carbon biomass, iron-based media, or a mixture of these amendments can significantly improve nitrate removal. The type of carbon biomass is also a factor since biochar does not appear to affect nitrate removal. This analysis can help inform the selection of SCM and modification of their design based on local and projected climate to maximize nitrate removal and minimize eutrophication.

Effects of norfloxacin on nitrate reduction and dynamic denitrifying enzymes activities in groundwater

The impact of antibiotics on denitrification has attracted widespread attention recently. Norfloxacin, as a representative of fluoroquinolone antibiotics, is extensively detected in groundwater. However, whether the release of norfloxacin into the groundwater poses potential risks to denitrification remains unclear. In this study, effect of norfloxacin on denitrification was investigated. The results showed that increasing norfloxacin from 0 to 100 μg/L decreased nitrate removal rate from 0.68 to 0.44 mg/L/h, but enhanced N2O emission by 177 folds. Additionally, 100 μg/L of norfloxacin decreased nitrite accumulation by 50.6%. Corresponding inhibition of norfloxacin on bacterial growth, carbon source utilization, electron transport system activity and genes expression was revealed. Furthermore, denitrifying enzyme dynamic monitoring results showed that norfloxacin inhibited nitrate reductase activity, and enhanced nitrite reductase activity to some extent in denitrification process, which was consistent with the variations of nitrate and nitrite. Meanwhile, sensitivity analysis demonstrated that nitrate reductase was more easily affected by norfloxacin than nitrite reductase. Overall, this study suggests that multiple regulation of denitrifying enzyme activity contributes to evaluating the comprehensive effects of antibiotics on groundwater denitrification.

Geospatial Distribution and Potential Noncarcinogenic Health Risk Assessment of Nitrate Contaminated Groundwater in Southern India: A Case Study

Groundwater nitrate pollution is a serious threat to human health in many regions of the world. The present study was performed to assess the nitrate contamination in groundwater in the region of Nirmal province, South India, where people purely depend on groundwater for drinking purposes. The associated human health risks for different age groups (male, female, and children) also were evaluated based on the United States Environmental Protection Agency model. Results indicate that nitrate concentration in groundwater is in the range of 0.8-130 mg/L with a mean of 36.51 mg/L. Furthermore, 26.47% of groundwater samples exceeded the WHO drinking water guidelines for NO₃^- in the study region. The contribution of oral ingestion is very higher than the dermal contact in the total hazard quotient or noncarcinogenic health risk. The total hazard quotient values ranged from 0.02 to 3.13 for adult males, 0.02 to 3.70 for adult females, and 0.03 to 4.32 for children. The health risk assessment highlights that children are more exposed to the noncarcinogenic health risks of nitrate than adult females and males in the study region. Therefore, specific groundwater quality measures should be formulated to address the health risk problems for children in the study region.

Reductive soil disinfestation incorporated with organic residue combination significantly improves soil microbial activity and functional diversity than sole residue incorporation

Reductive soil disinfestation (RSD) is an effective agricultural practice to eliminate soil-borne pathogens that heavily relies on the organic substrate used. However, the influences of combined application of organic residues on disinfestation efficiency, soil microbiomes, and their associated functional characteristics are still not well-characterized. In this work, four treatments, i.e., untreated soil (CK), RSD with 15 t ha^-1 sugarcane bagasse (SB), bean dregs (BD), and their combinations (1:1, SB+BD), were conducted to investigate their influence on disinfestation efficiency, microbial functional diversity, community diversity, and composition using Biolog analysis, real-time PCR, and high-throughput sequencing. The SB+BD treatment had synergetic effects on soil microbial activity, metabolic activity, and functional diversity with similar efficacy in pathogen elimination and soil salinization alleviation, as compared to the SB and BD treatments. Moreover, the SB+BD treatment distinctly altered the structure and composition of bacterial and fungal communities, especially enriched the core microbiomes associated with soil general functions such as organic decomposition and nitrate removal. The SB+BD treatment also strengthened the soil specific functions including disease suppression through the regulation of unique microbiomes. In addition, the microbial richness, diversity, and evenness were significantly higher in the SB+BD-treated soil as compared to the SB- and BD-treated soils. Taken together, RSD incorporated with organic residue combination not only efficiently restore the degraded soils, but also considerably improve soil functions, which may benefit to the health for the future plant generations. KEY POINTS: • Organic residue combination effectively declines pathogen density. • Organic residue combination improves soil microbial activity and functional diversity. • The enriched core microbiome is responsible for soil general functions. • The induced unique microbiome is important for soil specific functions.

Untangling the nitrate removal pathways for a constructed wetland- sponge iron coupled system and the impacts of sponge iron on a wetland ecosystem

Sponge iron (s-Fe⁰) is a potential alternative electron donor for nitrate reduction. To gain insight into the mechanism of denitrification in a constructed wetland- sponge iron coupled system (CW-Fe⁰ system), the removal performance and reduction characteristics of nitrate in constructed wetlands (CWs) with and without s-Fe⁰ application were compared. Results indicated that s-Fe⁰ intensified the removal of nitrate with a 6h-HRT. The nitrate removal efficiency was improved by 16-76 % with various influent NO₃^--N concentrations (10-30 mg L^-1) and at a chemical oxygen demand(COD)/N ratio of 5. The rates of chemical denitrification were positively correlated with the dosage of s-Fe⁰ and negatively correlated with the influent COD concentration. 16S rDNA sequencing revealed that hydrogen-utilizing autotrophic denitrifier of Hydrogenophaga was highly enriched (accounting for 10 % of the total OTUs) only in CW-Fe⁰ system. The micro-environment created by s-Fe⁰ was suitable for heterotrophic denitrifiers of Thauera, Tessaracoccus and Simplicispira. The determination of physiological indicators for plants showed that the application of s-Fe⁰ causes abiotic stress to wetland plants (Canna indica L.). Nevertheless, s-Fe⁰ can be used as a substrate for CWs, since it allows a high-efficiency removal of nitrate by mediating chemical denitrification and hydrogen-driven autotrophic denitrification.

Hydrilla verticillata-Sulfur-Based Heterotrophic and Autotrophic Denitrification Process for Nitrate-Rich Agricultural Runoff Treatment

Hydrilla verticillata-sulfur-based heterotrophic and autotrophic denitrification (HSHAD) process was developed in free water surface constructed wetland mesocosms for the treatment of nitrate-rich agricultural runoff with low chemical oxygen demand/total nitrogen (C/N) ratio, whose feasibility and mechanism were extensively studied and compared with those of H. verticillata heterotrophic denitrification (HHD) mesocosms through a 273-day operation. The results showed that the heterotrophic and autotrophic denitrification can be combined successfully in HSHAD mesocosms, and achieve satisfactory nitrate removal performance. The average NO₃^--N removal efficiency and denitrification rate of HSHAD were 94.4% and 1.3 g NO₃^--N m^-3·d^-1 in steady phase II (7-118 d). Most nitrate was reduced by heterotrophic denitrification with sufficient organic carbon in phase I (0-6 d) and II, i.e., the C/N ratio exceeded 4.0, and no significant difference of nitrate removal capacity was observed between HSHAD and HHD mesocosms. During phase III (119-273 d), sulfur autotrophic denitrification gradually dominated the HSHAD process with the C/N ratio less than 4.0, and HSHAD mesocosms obtained higher NO₃^--N removal efficiency and denitrification rate (79.1% and 1.1 g NO₃^--N m^-3·d^-1) than HHD mesocosms (65.3% and 1.0 g NO₃^--N m^-3·d^-1). As a whole, HSHAD mesocosms removed 58.8 mg NO₃^--N more than HHD mesocosms. pH fluctuated between 6.9-9.0 without any pH buffer. In general, HSHAD mesocosms were more stable and efficient than HHD mesocosms for NO₃^--N removal from agricultural runoff during long-term operation. The denitrificans containing narG (1.67 × 10⁸ ± 1.28 × 10⁷ copies g^-1 mixture-soil^-1), nirS (8.25 × 10⁷ ± 8.95 × 10⁶ copies g^-1 mixture-soil^-1), and nosZ (1.56 × 10⁶ ± 1.60 × 10⁵ copies g^-1 mixture-soil^-1) of litter bags and bottoms in HSHAD were higher than those in HHD, which indicated that the combined heterotrophic and autotrophic denitrification can increase the abundance of denitrificans containing narG, nirS, and nosZ, thus leading to better denitrification performance.

Microbial community structure associated with submarine groundwater discharge in northern Java (Indonesia)

Submarine groundwater discharge (SGD) can be an important pathway for chemical or biological pollutants from land to the ocean around the world. However, studies on the microbial communities associated with SGD in Southeast Asia, which has been hypothesized as SGD hotspot, remain scarce. In this study, we examined the microbial community composition with 16S rRNA gene sequencing along the hydrological continuum of an SGD site in a tropical urban area of Indonesia. Of the observed parameters in this study, salinity and temperature were the most determinant variables explaining patterns in microbial community composition. The bacterial taxon Burkholderiaceae was predominantly found in low salinity samples, including those from terrestrial groundwater and brackish pore water, while cyanobacteria of the genus Synechococcus sp. CC9902 were indicative of saline SGD and seawater samples. The composition of microbial taxa in each sample pointed to the influence of shallow terrestrial groundwater in the beach pore water, while seawater recirculation dominated the SGD sampling points situated further offshore. We identified taxa containing fecal indicators and potential pathogens at the SGD compartments; however, while a likely explanation, we could not conclude with certainty that SGD was a conduit for these bacteria. Overall, the results from this study show that microbial community analysis can highlight hydrological processes and water quality at the SGD site; thus, they could be useful for environmental policymakers to formulate water management strategies in coastal areas.

Role of heterotrophic aerobic denitrifying bacteria in nitrate removal from wastewater

With the increase in industrial and agricultural activities, a large amount of nitrogenous compounds are released into the environment, leading to nitrate pollution. The perilous effects of nitrate present in the environment pose a major threat to human and animal health. Bioremediation provides a cost-effective and environmental friendly method to deal with this problem. The process of aerobic denitrification can reduce nitrate compounds to harmless dinitrogen gas. This review provides a brief view of the exhaustive role played by aerobic denitrifiers for tackling nitrate pollution under different ecological niches and their dependency on various environmental parameters. It also provides an understanding of the enzymes involved in aerobic denitrification. The role of aerobic denitrification to solve the issues faced by the conventional method (aerobic nitrification-anaerobic denitrification) in treating nitrogen-polluted wastewaters is elaborated.

The performance of pyrite-based autotrophic denitrification column for permeable reactive barrier under natural environment

The effect of temperature on pyrite-based autotrophic denitrification performance and conversion between N species under natural conditions was investigated by using dynamic-flow column experiment. Phosphate and bicarbonate were added trying to enhance denitrification performance when the temperature decreased to 20 °C. However, the temperature had a much more sensitive influence on the denitrification process than substances addition. NO₃^--N removal efficiency decreased with the decrease of temperature. When the temperature was higher than 20 °C, the NO₂^--N reduction process was more sensitive to the temperature drop, while the process of NO₃^--N to NO₂^--N was more sensitive to temperature drop when the temperature was lower than 20 °C. The different influence of temperature drop on the two processes led to changes of the distribution of NO₃^--N, NO₂^--N, and SO₄^2--S along the column. However, the electron contribution of pyrite among the electron donors only changed slightly.

Evaluating the potential use of a dairy industry residue to induce denitrification in polluted water bodies: A flow-through experiment

Improving the effectiveness and economics of strategies to remediate groundwater nitrate pollution is a matter of concern. In this context, the addition of whey into aquifers could provide a feasible solution to attenuate nitrate contamination by inducing heterotrophic denitrification, while recycling an industry residue. Before its application, the efficacy of the treatment must be studied at laboratory-scale to optimize the application strategy in order to avoid the generation of harmful intermediate compounds. To do this, a flow-through denitrification experiment using whey as organic C source was performed, and different C/N ratios and injection periodicities were tested. The collected samples were analyzed to determine the chemical and isotopic composition of N and C compounds. The results proved that whey could promote denitrification. Nitrate was completely removed when using either a 3.0 or 2.0 C/N ratio. However, daily injection with C/N ratios from 1.25 to 1.5 seemed advantageous, since this strategy decreased nitrate concentration to values below the threshold for water consumption while avoiding nitrite accumulation and whey release with the outflow. The isotopic results confirmed that nitrate attenuation was due to denitrification and that the production of DIC was related to bacterial whey oxidation. Furthermore, the isotopic data suggested that when denitrification was not complete, the outflow could present a mix of denitrified and nondenitrified water. The calculated isotopic fractionation values (ε¹⁵N_NO3/N2 and ε¹⁸O_NO3/N2) might be applied in the future to quantify the efficiency of the bioremediation treatments by whey application at field-scale.

Development of effective sequence multi-barrier reactive media for nitrate remediation in groundwater systems

Early depletion of carbon-based electron donors and pollution swapping are among the recalcitrant challenges for in situ remediation technology. There is a pressing need to provide a slow but steady dose of materials for a prolonged period in order to sustain in situ treatments. In an effort to develop effective sequence multi-barrier reactive media that can address these challenges, the present study reports laboratory column experiments using organic (date seed, Moringa oleifera and wood chips) and inorganic (limestone; CaCO3) carbonaceous materials in varying proportions. The substances are supported by paleo sandy beach soil and are sequentially packed in line with the geometric pattern of a hypothetical funnel and gate for nitrate (NO3 -) remediation. The optimized reactive media show remarkable efficiency, remediating 94.4% of NO3 - as a target pollutant within the system. They also remediate 99.20% of the NO3 - down the gradient and 96.4% of its lethal by-product - NO2 -. However, with the generation of an 11-fold biomass build-up, the system is only able to eliminate 53% of the total organic carbon generated. Hence, the advective dispersive features of aquifers are required for the complete remediation of the generated secondary biomass. The results also reveal that the efficiency of the sequential media depends on the composition/ratio of the constituents, flow rate and the initial concentration of NO3 - in the influent. Surface characterization studies (FESEM-EDX and FTIR) show that compared to wood chips, date seed is less prone to degradation, suggesting a longer life span. A degradation study affirms the findings of the surface studies as it reveals that the half-life of date seed is three times longer than that of wood chips. In addition, the study reveals that the double exponential model is more suitable than the single exponential model in determining the rate of decay of both wood chips and date seed.

Performance of a field-scale biological permeable reactive barrier for in-situ remediation of nitrate-contaminated groundwater

We report the performance of a field-scale permeable reactive barrier (PRB) for the biological treatment of nitrate-contaminated groundwater. The reactive material of the PRB consisted of a mixture of gravel and mulch as a carbon source for denitrifying bacteria. The PRB was equipped with a delivery system that allowed injecting NO₃^- at controlled rates from the surface directly into the up-gradient layer of the PRB. This way, NO₃^- concentration entering the PRB was varied (from 1 to 530 mg/L) with the purpose of evaluating the ultimate efficiency of the PRB under different NO₃^- loadings. The PRB was successful at removing NO₃^- from groundwater at inlet concentrations up to 280 mg/L (with NO₃^- removal percentages ≥97%). Monitoring of groundwater at different depths within the PRB provided evidence that NO₃^- underwent denitrification preferably at the deepest part of the PRB, where more favourable reducing conditions were achieved. Among the shortcomings of the PRB were the fluctuations of groundwater fluxes caused by intense rainfalls during the study period, although they generally did not pose concern for the denitrification capacity of the PRB. Emission fluxes of gases (CO₂, CH₄ and N₂O) from the PRB to the atmosphere were also measured. The results are finally compared with the few others reported existing PRBs for nitrate-contaminated groundwater worldwide.

Bacteria-supported iron scraps for the removal of nitrate from low carbon-to-nitrogen ratio wastewater

A novel bacteria-supported iron scraps (BSIS) system was developed for nitrate removal from low carbon-to-nitrogen ratio (C/N) wastewater. The system consisted of low-cost iron scraps and the accumulated denitrifying-related bacteria enriched from an Fe-wastewater environment when the system was operating. After operating for 39 d, the nitrate removal rate of the system increased to 73.55% within 24 h. The extraction of bacteria from the system revealed that iron scraps and bacteria had a synergistic effect on nitrate removal and bacteria only took effect when cooperating with iron. Microbial analysis using high-throughput sequencing showed that Hydrogenophaga, which is closely related to hydrogenotrophic denitrification, became the dominant genus in the system. The system provides a promising approach to the treatment of nitrate in low C/N wastewater and it has the potential for large-scale application due to the low cost, simple operation and relatively high removal rate.

Research on the nitrogen transformation in rhizosphere of winter wheat (Triticum aestivum) under molybdenum addition

Molybdenum (Mo), an essential microelement for plants, animals, and microorganisms, is reported can reduce soil nitrogen (N) residues and regulate plant root growth, but little is known about its effect on soil N transformation in plant-root region. A specially designed rhizobox was used in the present study to investigate the N processes in rhizosphere and non-rhizosphere soils of winter wheat applied with different rates of Mo fertilizer. (1) In the rhizosphere soil, pH values increased with increasing rates of Mo application, nitrate (NO₃^--N) accumulated at the rates of 0.15 and 0.3 mg Mo kg^-1, potential denitrification activity (PDA) was significantly reduced by application of 0.15-1 mg Mo kg^-1, and the copy numbers of narG and nosZ genes were increased by application of 0.15-1 mg Mo kg^-1. (2) In the non-rhizosphere soil, NO₃^--N content decreased by application of 0.15-0.3 mg Mo kg^-1, and narG gene abundance increased obviously by application of 0.3-1 mg Mo kg^-1. (3) Soil pH, NO₃^--N, apparent nitrification rate (ANR), and nosZ gene abundance were significantly higher in rhizosphere than in non-rhizosphere soil. On the contrary, NH₄⁺-N and total N, PDA, the abundance of AOB, and nirK and nirS genes were significantly higher in non-rhizosphere soil. The results indicated that the N transformations in rhizosphere and non-rhizosphere soils were differently affected by soil application of Mo fertilizer, and rhizosphere played a more important role in soil N cycle processes. The regulatory effects of Mo on these processes were to increase plant biomass and N uptake, promote the NO₃^--N accumulation in rhizosphere soil, and weaken the denitrification in both rhizosphere and non-rhizosphere soils.

Microbial reduction of nitrate in the presence of zero-valent iron

Microbial reduction of nitrate in the presence of zero-valent iron (ZVI) was evaluated in anoxic shake flasks to assess the feasibility of ZVI-facilitated biological nitrate removal. Nitrate was completely reduced within 3days in the presence of both ZVI and microorganisms (ZVI-M). In contrast, only 75% of the nitrate was reduced in the presence of ZVI but without microbial inoculum. Nitrate removal was affected by ZVI-M flasks initial pH, nitrate concentration and ZVI dosage. Nitrate removal in the inoculated ZVI flasks system could be divided into two phases: adaptation phase and log phase which could be described by first-order kinetic equations. The analysis of bacterial communities in the inoculated flasks in the absence and presence of ZVI, indicated that the addition of ZVI increased the relative abundance of Methylotenera spp., Alcaligenes eutrophus, Pseudomonas spp. which might play an important role in nitrogen removal. The presence of ZVI could enhance biological denitrification through four mechanisms: the biological reduction of nitrate with 1) electrons derived directly from ZVI; 2) with hydrogen released from ZVI; 3) with Fe²⁺ released from ZVI; and 4) with acetate generated by homoacetogens which utilize H₂ released from ZVI.

Autohydrogenotrophic Denitrification Using the Membrane Biofilm Reactor for Removing Nitrate from High Sulfate Concentration of Water

This study investigated the performance of an autohydrogenotrophic membrane biofilm reactor (MBfR) to remove nitrate from water with high sulfate concentrations. The results of simulated running showed that TN removal could be over than 98.8% with the maximum denitrification rate of 134.6 g N/m3 d under the conditions of the influent sulfate concentrations of 300 mg SO42-/l. The distribution ratio of H2 electron donor for nitrate and sulfate was 70.0 : 26.9 at the high influent loading ratio of sulfate/nitrate of 853.3 g SO42-/m3 d : 140.5 g N/m3 d, which indicated that denitrification bacteria (DB) were normally dominated to complete H2 electron with sulfate bacteria (SRB). The results of molecular microbiology analysis showed that the dominated DB were Rhodocyclus and Hydrogenophaga, and the dominated SRB was Desulfohalobium, under the high influent sulfate concentrations.

The characteristics and performance of sustainable-releasing compound carbon source material applied on groundwater nitrate in-situ remediation

Recently, reductant and carbon source were frequently used in groundwater nitrate remediation worldwide. Agricultural waste as a promising organic carbon source, has been paid much attention but the problem of sustainability, bioavailability and secondary pollution remained unsolved. This study was conducted to depict the characteristic and performance of developed sustainable-releasing compound carbon source material (SCCM) applied on the in-situ remediation of nitrate in shallow groundwater. Results showed the SCCM based on agricultural waste and zero valent-iron (ZVI) has a stable carbon releasing rate, which is suitable for stimulating the low microbial active environment in groundwater continuously, and capable of avoiding rapid TOC releasing in the early stage. The released carbon sources in SCCM leachate were mainly small molecular alcohols and acids with high microbial availability. As in-situ permeable reactive barrier (PRB) filling material, SCCM can form an optimal carbon source radiation range of 20 cm, with a maximum efficient carbon source radius of 1 m, which can reach an extended active zone. A positive correlation between the ZVI content and nitrate removal rate was found. The chemical and microbiological evidence both indicated that the expected chemical reduction and biological denitrification was gradually established. Additionally, the absorption of ammonia and chroma by attapulgite effectively avoided the secondary pollution. In conclusion, the application of SCCM in groundwater nitrate in-situ remediation optimized the nitrate removal efficiency and provided theoretical basis for engineer carbon sources development from straw-type agricultural waste.

Autotrophic Vanadium(V) Bioreduction in Groundwater by Elemental Sulfur and Zerovalent Iron

Vanadium (V) is an emerging contaminant in groundwater that can adversely affect human health. Although bioremediation has been shown effective, little is known on autotrophic V(V) bioreduction in the context of oligotrophic characteristics of groundwater. In this study, we demonstrate that efficient V(V) bioreductions can be coupled with bio-oxidation of elemental sulfur (S(0)) or zerovalent iron (Fe(0)), and the V(V) removal efficiencies reached 97.5 ± 1.2% and 86.6 ± 2.5% within 120 h using S(0) and Fe(0), respectively. V(IV) is the main reduction product and precipitates naturally in near-neutral conditions. Microbial community, functional gene, and metabolites analyses reveal that synthetic metabolisms among autotrophs and heterotrophs played major roles in V(V) reduction using S(0) and Fe(0). These results demonstrate a new approach for V(V) contaminated groundwater remediation.

Potential electron donor for nanoiron supported hydrogenotrophic denitrification: H2 gas, Fe0, ferrous oxides, Fe2+(aq), or Fe2+(ad)?

The mechanism of electron transmission in combined nanoiron-bacteria denitrification cannot be explained by the classic model, in which an Fe⁰H₂-nitrate transferring chain is proposed. In this study, we used characteristic techniques and electrochemical analysis to investigate the necessity of molecular hydrogen for the combined denitrifying system using commercial nanoiron with Alcaligenes eutrophus, and to analyze its potential electron donor. Based on our results, nitrate removal and its by-products (NO₂^- and NH₄⁺) generation was not significantly affected by residual H₂ gas, indicating that H₂ was not necessary for hydrogenotrophic denitrification. As to the potential electron donor analysis, nanoscale zero-valent iron did not appear to be the electron donor due to its high level of toxicity (83% mortality using nanoiron versus 36% in the control cells). In addition, when iron oxides (Fe₃O₄, Fe₂O₃ and FeOOH on the nanoiron surface) and free ferrous ions [Fe²⁺(aq)] were present alone, they were not utilized by the bacteria to degrade nitrate. According to the results of electrochemical analysis, adsorbed ferrous iron [Fe²⁺(ad)] on ferric oxides might be the electron donor in this kind of nitrate removal.

Performance of nanoscale zero-valent iron in nitrate reduction from water using a laboratory-scale continuous-flow system

Nanoscale zero-valent iron (nZVI) is a versatile treatment reagent that should be utilized in an effective application for nitrate remediation in water. For this purpose, a laboratory-scale continuous-flow system (LSCFS) was developed to evaluate nZVI performance in removal of nitrate in different contaminated-water bodies. The equipment design (reactor, settler, and polisher) and operational parameters of the LSCFS were determined based on nZVI characterization and nitrate reduction kinetics. Ten experimental runs were conducted at different dosages (6, 10 and 20 g) of nZVI-based reagents (nZVI, bimetallic nZVI-Cu, CuCl₂-added nZVI). Effluent concentrations of nitrogen and iron compounds were measured, and pH and ORP values were monitored. The major role exhibited by the recirculation process of unreacted nZVI from the settler to the reactor succeeded in achieving overall nitrate removal efficiency (RE) of >90%. The similar performance of both nZVI and copper-ions-modified nZVI in contaminated distilled water was an indication of LSCFS reliability in completely utilizing iron nanoparticles. In case of treating contaminated river water and simulated groundwater, the nitrate reduction process was sensitive towards the presence of interfering substances that dropped the overall RE drastically. However, the addition of copper ions during the treatment counteracted the retardation effect and greatly enhanced the nitrate RE.

Nitrate removal by combined heterotrophic and autotrophic denitrification processes: Impact of coexistent ions

In this current study, sawdust and zero-valent iron (Fe⁰) were used as co-electron donors to evaluate the effects of coexistent ions on the combined heterotrophic and autotrophic denitrification (HAD) processes. The results showed that HCO₃^- and SO₄^2- drastically enhanced nitrate removal. The promotion effect derived from both biological and chemical process by HCO₃^- and chemical process by SO₄^2-. However, Ca²⁺ ions would remarkably increase nitrate removal due to promoting the electron transfer and the metabolic activities of bacteria, whereas the Cu²⁺ ions inhibited the biological process due to the deleterious effect on bacteria. Meanwhile, Fe²⁺ and Fe³⁺ ions exhibited inhibition effect firstly because of their toxicity to bacteria and promotion subsequently due to their enhancement on Fe⁰ chemical denitrification. Moreover, byproducts such as nitrite, ammonium, dissolved organic carbon (DOC), etc. were also influenced by common ions.

Removal of nitrate from water by acid-washed zero-valent iron/ferrous ion/hydrogen peroxide: influencing factors and reaction mechanism

In this paper, a system consisting of acid-washed zero-valent iron (ZVI), ferrous ion (Fe²⁺), and hydrogen peroxide (H₂O₂) was employed for the removal of nitrate (NO₃^-) from water, and the reaction mechanism for this is discussed. The effects of acid-washed ZVI, Fe²⁺, H₂O₂, and initial NO₃^- concentration on nitrate removal were investigated. Acid-washed ZVI before and after reaction with nitrate were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Results reveal that the combined system can enhance the corrosion of ZVI and facilitate aqueous nitrate reduction. The products of nitrate reduction are mainly ammonium, with some N₂. The ZVI particles after reaction may have a core of ZVI with an oxidation layer mainly consisting of Fe₃O₄.

Zero-valent iron-facilitated reduction of nitrate: Chemical kinetics and reaction pathways

The kinetics and mechanisms of the reduction of NO₃^- in solution to NH₃ by 1.5μm diameter zero-valent iron (ZVI_1.5) particles has been examined. The effects of initial pH, ZVI_1.5 particle concentration and initial NO₃^- concentration were also investigated. Results indicate that denitrification by ZVI_1.5 is primarily a pH-dependent, surface-mediated process. At an initial ZVI_1.5 concentrations of 0.832g/L, and an optimal initial pH of 1.62, the NO₃^- concentration was reduced by 95% from 12.50mg/L-N to 0.65mg/L-N, in 120min. Several kinetic models were used to describe the denitrification process based on the ZVI_1.5:NO₃^- ratio. Based on mineralogical and surface analysis of the reacted ZVI_1.5, and detailed solution chemical analysis, the denitrification reaction pathway involves oxidation and partial dissolution of the ZVI_1.5 with the generation of Fe²⁺ and NO₂^- intermediates prior to formation of Fe³⁺ oxyhydroxide (goethite) and NH₃.

Nitrate-rich agricultural runoff treatment by Vallisneria-sulfur based mixotrophic denitrification process

Vallisneria-sulfur based mixotrophic denitrification (VSMD) process was put forward for the treatment of nitrate-rich agricultural runoff with low COD/TN (C/N) ratio in free water surface constructed wetland mesocosms, whose feasibility and mechanism were thoroughly studied through 273-day operation. The results showed that the average NO₃^--N removal efficiency and denitrification rate of VSMD mesocosms were 97.7% and 1.5gNO₃^--Nm^-3d^-1 under 5.0 or higher C/N ratio conditions in phase II (7-117d), which were similar with those of Vallisneria packed heterotrophic denitrification (VHD) mesocosms. However, VSMD mesocosms with 2.0 average C/N ratio in phase III (118-273d) were more stable and efficient than VHD mesocosms. More than 49.4mg NO₃^--N was reduced by VSMD mesocosms than that by VHD mesocosms throughout the operation. NO₂^--N accumulation in phase I (0-6d) had no influence on denitrification performance of VSMD mesocosms. In phase II and III, effluent COD, NH₄⁺-N and NO₂^--N could meet the Class II standard of Environmental quality for surface water (GB3838-2002) if the experiment was carried out in batch mode. pH in VSMD mesocosms fluctuated between 7.0 and 8.9 throughout the operation without any pH buffer. The abundance of three denitrifying genes coding for nitrate (narG), nitrite (nirS), and nitrous oxide (nosZ) reductases in bottom soil and mixture from litter bags was quantified. VSMD could supply more favorable circumstances for the growth of denitrificans containing narG (3.1×10⁸±7.9×10⁷copiesg^-1mixture^-1) and nirS (2.1×10⁸±2.0×10⁶copiesg^-1mixture^-1) in litter bags than VHD, i.e., 8.7×10⁷±1.4×10⁷ and 1.4×10⁸±1.5×10⁷copiesg^-1mixture^-1 for narG and nirS respectively. Sulfur addition in VSMD mesocosms might increase the abundance of denitrificans containing narG and nirS, thus led to better denitrification performance.

Remediation of arsenic-contaminated groundwater using media-injected permeable reactive barriers with a modified montmorillonite: sand tank studies

A modified montmorillonite (MMT) was prepared using an acid activation-sodium activation-iron oxide coating method to improve the adsorption capacities of natural MMTs. For MMT, its interlamellar distance increased from 12.29 to 13.36 Å, and goethite (α-FeOOH) was intercalated into its clay layers. Two novel media-injected permeable reactive barrier (MI-PRB) configurations were proposed for removing arsenic from groundwater. Sand tank experiments were conducted to investigate the performance of the two MI-PRBs: Tank A was filled with quartz sand. Tank B was packed with quartz sand and zero-valent iron (ZVI) in series, and the MMT slurry was respectively injected into them to form reactive zones. The results showed that for tank A, total arsenic (TA) removal of 98.57% was attained within the first 60 mm and subsequently descended slowly to 88.84% at the outlet. For tank B, a similar spatial variation trend was observed in the quartz sand layer, and subsequently, TA removal increased to ≥99.80% in the ZVI layer. TA removal by MMT mainly depended on both surface adsorption and electrostatic adhesion. TA removal by ZVI mainly relied on coagulation/precipitation and adsorption during the iron corrosion. The two MI-PRBs are feasible alternatives for in situ remediation of groundwater with elevated As levels.

Remediation of nitrate-contaminated wastewater using denitrification biofilters with straws of ornamental flowers added as carbon source

Straws of four ornamental flowers (carnation, rose, lily, and violet) were added into denitrification biofilters using gravel as matrix through vertically installed perforated polyvinylchloride pipes to provide organic carbon for the treatment of nitrate-contaminated wastewater operating in batch mode. Removal efficiencies of nitrate and phosphate, as well as temporal variations of nitrogen and carbon during batches 10 and 19, were investigated and assessed. Nitrate removal was efficiently enhanced by the addition of flower straws, but decreased gradually as the organic substances were consumed. Phosphate removal was also improved, although this very limited. High nitrate removal rates were achieved during the initial 12 h in the two batches each lasting for 3 days, along with the depletion of influent dissolved oxygen due to aerobic degradation of the organic compounds. NO2(-)-N of 0.01-2.83 mg/L and NH4(+)-N of 0.02-1.69 mg/L were formed and both positively correlated to the nitrate reduced. Inorganic carbon (IC) concentrations increased during the batches and varied conversely with the nitrate contents, and could be indicative of nitrate removal due to the highly significant positive correlation between NO3(-)-N removed and IC concentration (r(2) = 0.881, p < 0.0001). It is feasible and economical to use the denitrification biofilter to treat nitrate-contaminated wastewater, although further optimization of carbon source addition is still required.

Electro-assisted groundwater bioremediation: fundamentals, challenges and future perspectives

Bioremediation is envisaged as an important way to abate groundwater contamination, but the need for chemical addition and limited bioavailability of electron donors/acceptors or contaminants hamper its application. As a promising means to enhance such processes, electrochemical system has drawn considerable attention, as it offers distinct advantages in terms of environmental benignity, controllability and treatment efficiency. Meanwhile, there are also potential risks and considerable engineering challenges for its practical application. This review provides a first comprehensive introduction of this emerging technology, discusses its potential applications and current challenges, identifies the knowledge gaps, and outlooks the future opportunities to bring it to field application. The need for a better understanding on the microbiology under electrochemical stimulation and the future requirements on process monitoring, modeling and evaluation protocols and field investigations are highlighted.