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 (ε15NNO3/N2 and ε18ONO3/N2) might be applied in the future to quantify the efficiency of the bioremediation treatments by whey application at field-scale

Tracing the role of endogenous carbon in denitrification using wine industry by-product as an external electron donor: coupling isotopic tools with mathematical modeling

Nitrate removal through enhanced biological denitrification (EBD), consisting of the inoculation of an external electron donor, is a feasible solution for the recovery of groundwater quality. In this context, liquid waste from wine industries (wine industry by-products, WIB) may be feasible for use as a reactant to enhance heterotrophic denitrification. To address the feasibility of WIB as electron donor to promote denitrification, as well as to evaluate the role of biomass as a secondary organic C source, a flow-through experiment was carried out. Chemical and isotopic characterization was performed and coupled with mathematical modeling. Complete nitrate attenuation with no nitrite accumulation was successfully achieved after 10 days. Four different C/N molar ratios (7.0, 2.0, 1.0 and 0) were tested. Progressive decrease of the C/N ratio reduced the remaining C in the outflow and favored biomass migration, producing significant changes in dispersivity in the reactor, which favored efficient nitrate degradation. The applied mathematical model described the general trends for nitrate, ethanol, dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations. This model shows how the biomass present in the system is degraded to dissolved organic C (DOCen) and becomes the main source of DOC for a C/N ratio between 1.0 and 0. The isotopic model developed for organic and inorganic carbon also describes the general trends of δ13C of ethanol, DOC and DIC in the outflow water. The study of the evolution of the isotopic fractionation of organic C using a Rayleigh distillation model shows the shift in the organic carbon source from the WIB to the biomass and is in agreement with the isotopic fractionation values used to calibrate the model. Isotopic fractionations (ε) of C-ethanol and C-DOCen were −1¿ and −5¿ (model) and −3.3¿ and −4.8¿ (Rayleigh), respectively. In addition, an inverse isotopic fractionation of +10¿ was observed for biomass degradation to DOCen. Overall, WIB can efficiently promote nitrate reduction in EBD treatments. The conceptual model of the organic C cycle and the developed mathematical model accurately described the chemical and isotopic transformations that occur during this induced denitrification

Numerical modeling of enhanced biodenitrification in a laboratory flow-through experiment

High concentration of nitrate (NO3) in water resources has become a widespread and important environmental contaminant, being anthropogenic nitrogen input the principal source of NO3− pollution (Arauzo, 2017). Underanaerobic conditions, microbial reduction of NO3 to N2(g) to oxidize dissolved organic carbon (DOC) is the principal NO3 attenuation process in groundwater aquifers (Matchett et al., 2019)

Isotopic evidence of nitrate degradation by a zero-valent iron permeable reactive barrier: Batch experiments and a field scale study

Permeable reactive barriers (PRBs) filled with zero-valent iron (ZVI) are a well-known remediation approach to treat groundwater plumes of chlorinated volatile organic compounds as well as other contaminants. In field implementations of ZVI-PRBs designed to treat these contaminants, nitrate consumption has been reported and has been attributed to direct abiotic nitrate reduction by ZVI or to denitrification by autochthonous microorganisms using the dissolved hydrogen produced from ZVI corrosion. Isotope tools have proven to be useful for monitoring the performance of nitrate remediation actions. In this study, we evaluate the use of isotope tools to assess the effect of ZVI-PRBs on the nitrate fate for the further optimization of full-scale applications. Laboratory batch experiments were performed using granular cast ZVI and synthetic nitrate solutions at pH 4-5.5 or nitrate-containing groundwater (pH = 7.0) from a field site where a ZVI-PRB was installed. The experimental results revealed nitrate attenuation and ammonium production for both types of experiments. In the field site, the chemical and isotopic data demonstrated the occurrence of ZVI-induced abiotic nitrate reduction and denitrification in wells located close to the ZVI-PRB. The isotopic characterization of the laboratory experiments allowed us to monitor the efficiency of the ZVI-PRB at removing nitrate. The results show the limited effect of the barrier (nitrate reduction of less than 15-20%), probably related to its non-optimal design. Isotope tools were therefore proven to be useful tools for determining the efficacy of nitrate removal by ZVI-PRBs at the field scale

Nitrate and Nitrite Attenuation by Fe(II) Minerals: Biotic and Abiotic Reactions

Nitrate (NO3-) pollution of groundwaterhas become a relevant issue and anenvironmental priority as it is related toecological and human health problems(Rivett et al. 2008) and its concentration is still above the threshold limit of 50mg/L in many areas (Nitrate Directive, 91/676/EEC). Contamination sources of NO3 - are linked to extensive use of fertilizers, inappropriate placement of animal waste and spills from septic system effluents

Feasibility of using rural waste products to increase the denitrification efficiency in a surface flow constructed wetland

A surface flow constructed wetland (CW) was set in the Lerma gully to decrease nitrate (NO3−) pollution from agricultural runoff water. The water flow rate and NO3− concentration were monitored at the inlet and the outlet, and sampling campaigns were performed which consisted of collecting six water samples along the CW flow line. After two years of operation, the NO3− attenuation was limited at a flow rate of ~2.5 L/s and became negligible at ~5.5 L/s. The present work aimed to assess the feasibility of using rural waste products (wheat hay, corn stubble, and animal compost) to induce denitrification in the CW, to assess the effect of temperature on this process, and to trace the efficiency of the treatment by using isotopic tools. In the first stage, microcosm experiments were performed. Afterwards, the selected waste material was applied in the CW, and the treatment efficiency was evaluated by means of a chemical and isotopic characterization and using the isotopic fractionation (ε) values calculated from laboratory experiments to avoid field-scale interference. The microcosms results showed that the stubble was the most appropriate material for application in the CW, but the denitrification rate was found to decrease with temperature. In the CW, biostimulation in autumn-winter promoted NO3− attenuation between two weeks and one month (a reduction in NO3− between 1.2 and 1.5 mM was achieved). After the biostimulation in spring-summer, the attenuation was maintained for approximately three months (NO3− reduction between 0.1 and 1.5 mM). The ε15NNO3/N2 and ε18ONO3/N2 values obtained from the laboratory experiments allowed to estimate the induced denitrification percentage. At an approximate average flow rate of 16 L/s, at least 60% of NO3− attenuation was achieved in the CW. The field samples exhibited a slope of 1.0 for δ18O-NO3− versus δ15N-NO3−, similar to those of the laboratory experiments (0.9-1.2). Plant uptake seemed to play a minor role in NO3− attenuation in the CW. Hence, the application of stubble in the CW allowed the removal of large amounts of NO3− from the Lerma gully, especially when applied during the warm months, but its efficacy was limited to a short time period (up to three months)

The role of siderite on abiotic nitrite reduction by dissolved Fe(II)

Iron redox reactions affect the fate and transformation of groundwater NO3-. Fe(II) present in groundwater as dissolved Fe(II) or Fe(II) sorbed onto mineral surfaces is oxidised into Fe(III) (oxyhydr)oxides using NO3- as an electron acceptor in anoxic conditions by biotic or abiotic means (Bryce et al., 2018). N2O is produced as an end product during abiotic nitrate-reducing Fe(II) oxidation (NRFO) (Wang et al., 2019). NO2-, an intermediate product during NO3- reduction by biotic or abiotic means, is chemically very reactive and readily reduced to N2O by abiotic means (Wankel et al., 2017). Studies have shown that Fe(II) minerals such as iron-rich smectites, green rust and siderite are reactive and can enhance abiotic NO2- reduction (Grabb et al., 2017). The occurrence of abiotic NO2- reduction leads to the relative segregation of the lighter and heavy isotopes of N and O (kinetic isotope fractionation, ε) (Chen & MacQuarrie, 2005) providing an effective tool to quantify abiotic NO2- reduction processes. In the light of this, batch experiments were performed to assess the potential of micro-sized siderite to enhance abiotic NO2- reduction in laboratory batch experiments

Characterisation of the natural attenuation of chromium contamination in the presence of nitrate using isotopic methods. A case study from the Matanza-Riachuelo river basin, Argentina

The groundwater contamination by hexavalent chromium (Cr(VI)) in a site of the Matanza-Riachuelo River basin (MRB), Argentina, has been evaluated by determining the processes that control the natural mobility and attenuation of Cr(VI) in the presence of high nitrate (NO3−) contents. The groundwater Cr(VI) concentrations ranged between 1.9E-5 mM and 0.04 mM, while the NO3− concentrations ranged between 0.5 mM and 3.9 mM

Use of nitrogen and oxygen isotopes of dissolved nitrate to trace field-scale induced denitrification efficiency throughout an in-situ groundwater remediation strategy

In the framework of the Life+ InSiTrate project, a pilot-plant was established to demonstrate the viability of inducing in-situ heterotrophic denitrification to remediate nitrate (NO3−)-polluted groundwater. Two injection wells supplied acetic acid by pulses to an alluvial aquifer for 22 months. The monitoring was performed by regular sampling at three piezometers and two wells located downstream. In the present work, the pilot-plant monitoring samples were used to test the usefulness of the isotopic tools to evaluate the efficiency of the treatment. The laboratory microcosm experiments determined an isotopic fractionation (ε) for N-NO3− of −12.6 and for O-NO3− of −13.3 . These ε15NNO3/N2 and ε18ONO3/N2 values were modelled by using a Rayleigh distillation equation to estimate the percentage of the induced denitrification at the pilot-plant while avoiding a possible interference from dilution due to non-polluted water inputs. In some of the field samples, the induced NO3− reduction was higher than 50% with respect to the background concentration. The field samples showed a reduced slope between δ18O-NO3− and δ15N-NO3− (0.7) compared to the laboratory experiments (1.1). This finding was attributed to the reoxidation of NO2− to NO3− during the treatment. The NO3− isotopic characterization also permitted the recognition of a mixture between the denitrified and partially or non-denitrified groundwater in one of the sampling points. Therefore, the isotopic tools demonstrated usefulness in assessing the implementation of the field-scale induced denitrification strategy

Hydrogeological and multi-isotopic approach to define nitrate pollution and denitrification processes in a coastal aquifer (Sardinia, Italy)

Agricultural coastal areas are frequently affected by the superimposition of various processes, with a combination of anthropogenic and natural sources, which degrade groundwater quality. In the coastal multi-aquifer system of Arborea (Italy)¿a reclaimed morass area identified as a nitrate vulnerable zone, according to Nitrate Directive 91/676/EEC¿intensive agricultural and livestock activities contribute to substantial nitrate contamination. For this reason, the area can be considered a bench test for tuning an appropriate methodology aiming to trace the nitrate contamination in different conditions. An approach combining environmental isotopes, water quality and hydrogeological indicators was therefore used to understand the origins and attenuation mechanisms of nitrate pollution and to define the relationship between contaminant and groundwater flow dynamics through the multi-aquifer characterized by sandy (SHU), alluvial (AHU), and volcanic hydrogeological (VHU) units. Various groundwater chemical pathways were consistent with both different nitrogen sources and groundwater dynamics. Isotope composition suggests a mixed source for nitrate (organic and synthetic fertilizer), especially for the AHU and SHU groundwater. Moreover, marked heterotrophic denitrification and sulfate reduction processes were detected; although, for the contamination related to synthetic fertilizer, the attenuation was inefficient at removing NO3− to less than the human consumption threshold of 50 mg/L. Various factors contributed to control the distribution of the redox processes, such as the availability of carbon sources (organic fertilizer and the presence of lagoon-deposited aquitards), well depth, and groundwater flow paths. The characterization of these processes supports water-resource management plans, future actions, and regulations, particularly in nitrate vulnerable zones