Bioelectrochemical reactor to manage anthropogenic sulfate pollution for freshwater ecosystems: Mathematical modeling and experimental validation.

Anthropogenic sulfate loading into otherwise low-sulfate freshwater systems can cause significant ecological consequences as a biogeochemical stressor. To address this challenge, in situ bioremediation technologies have been developed to leverage naturally occurring microorganisms that transform sulfate into sulfide rather than implementing resource-intensive physio-chemical processes. However, bioremediation technologies often require the supply of electron donors to facilitate biological sulfate reduction. Bioelectrochemical systems (BES) can be an alternative approach for supplying molecular hydrogen as an electron donor for sulfate-reducing bacteria through water electrolysis. Although the fundamental mechanisms behind BESs have been studied, limited research has evaluated the design and operational parameters of treatment systems when developing BESs on a scale relevant to environmental systems. This study aimed to develop an application-based mathematical model to evaluate the performance of BESs across a range of reactor configurations and operational modes. The model was based on sulfate transformation by hydrogenotrophic sulfate-reducing bacteria coupled with the recovery of solid iron sulfide species formed by the oxidative dissolution of dissolved ferrous iron from a stainless steel anode. Sulfate removal closely corresponded to the rate of electrolytic hydrogen production and hydraulic residence time but was less sensitive to specific microbial rate constants. The mathematical model results were compared to experimental data from a pilot-scale BES tested with nonacidic mine drainage as a case study. The close agreement between the mathematical model and the pilot-scale BES experiment highlights the efficacy of using a mathematical model as a tool to develop a conceptual design of a scaled-up treatment system.

Exploring the Utility of Compound-Specific Isotope Analysis for Assessing Ferrous Iron-Mediated Reduction of RDX in the Subsurface.

Subsurface contamination with the explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) at ordnance production and testing sites is a problem because of the persistence, mobility, and toxicity of RDX and the formation of toxic products under anoxic conditions. While the utility of compound-specific isotope analysis for inferring natural attenuation pathways from stable isotope ratios has been demonstrated, the stable isotope fractionation for RDX reduction by iron-bearing minerals remains unknown. Here, we evaluated N and C isotope fractionation of RDX during reduction by Fe(II) associated with Fe minerals and natural sediments and applied N isotope ratios to the assessment of mineral-catalyzed RDX reduction in a contaminant plume and in sediment columns treated by in situ chemical reduction. Laboratory studies revealed that RDX was reduced to nitroso compounds without denitration and the concomitant ring cleavage. Fe(II)/iron oxide mineral-catalyzed reactions exhibited N isotope enrichment factors, εN, between -6.3±0.3‰ and -8.2±0.2‰, corresponding to an apparent 15N kinetic isotope effect of 1.04-1.05. The observed variations of the δ15N of ∼15‰ in RDX from groundwater samples suggested an extent of reductive transformation of 85% at an ammunition plant. Conversely, we observed masking of N isotope fractionation after RDX reduction in laboratory flow-through systems, which was presumably due to limited accessibility to reactive Fe(II).

Neonicotinoid Insecticides in Surface Water, Groundwater, and Wastewater Across Land-Use Gradients and Potential Effects.

Neonicotinoid insecticides cause adverse effects on nontarget organisms, but more information about their occurrence in surface and groundwater is needed across a range of land uses. Sixty-five sites in Minnesota, USA, representing rivers, streams, lakes, groundwater, and treated wastewater, were monitored via collection of 157 water samples to determine variability in spatiotemporal neonicotinoid concentrations. The data were used to assess relations to land use, hydrogeologic condition, and potential effects on aquatic life. Total neonicotinoid concentrations were highest in agricultural watersheds (median = 12 ng/L), followed by urban (2.9 ng/L) and undeveloped watersheds (1.9 ng/L). Clothianidin was most frequently detected in agricultural areas (detection frequency = 100%) and imidacloprid most often in urban waters (detection frequency = 97%). The seasonal trend of neonicotinoid concentrations in rivers, streams, and lakes showed that their highest concentrations coincided with spring planting and elevated streamflow. Consistently low neonicotinoid concentrations were found in shallow groundwater in agricultural regions (<1.2-16 ng/L, median = 1.4 ng/L). Treated municipal wastewater had the highest concentrations across all hydrologic compartments (12-48 ng/L, median = 19 ng/L), but neonicotinoid loads from rivers and streams (median = 4100 mg/d) were greater than in treated wastewater (700 mg/d). No samples exceeded acute aquatic-life benchmarks for individual neonicotinoids, whereas 10% of samples exceeded a chronic benchmark for neonicotinoid mixtures. Although 62% of samples contained 2 or more neonicotinoids, the observed concentrations suggest there were low acute and potential chronic risks to aquatic life. This the first study of its size in Minnesota and is critical to better understanding the drivers of wide-scale environmental contamination by neonicotinoids where urban, agricultural, and undeveloped lands are present. Environ Toxicol Chem 2021;40:1017-1033. © 2020 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Assessment of 2,4-Dinitroanisole Transformation Using Compound-Specific Isotope Analysis after In Situ Chemical Reduction of Iron Oxides.

Ferrous iron-bearing minerals are important reductants in the contaminated subsurface, but their availability for the reduction of anthropogenic pollutants is often limited by competition with other electron acceptors including microorganisms and poor accessibility to Fe(II) in complex hydrogeologic settings. The supply of external electron donors through in situ chemical reduction (ISCR) has been proposed as one remediation approach, but the quantification of pollutant transformation is complicated by the perturbations introduced to the subsurface by ISCR. Here, we evaluate the application of compound specific isotope analysis (CSIA) for monitoring the reduction of 2,4-dinitroanisole (DNAN), a component of insensitive munitions formulations, by mineral-bound Fe(II) generated through ISCR of subsurface material from two field sites. Electron balances from laboratory experiments in batch and column reactors showed that 3.6% to 11% of the total Fe in the sediments was available for the reduction of DNAN and its partially reduced intermediates after dithionite treatment. The extent of DNAN reduction was successfully quantified from its N isotope fractionation measured in the column effluent based on the derivation of a N isotope enrichment factor, εN, derived from a comprehensive series of isotope fractionation experiments with numerous Fe(II)-bearing minerals as well as dithionite-reduced subsurface materials. Our observations illustrate the utility of CSIA as a robust approach to evaluate the success of in situ remediation through abiotic contaminant reduction.

Mineral identity, natural organic matter, and repeated contaminant exposures do not affect the carbon and nitrogen isotope fractionation of 2,4-dinitroanisole during abiotic reduction.

The recent development of insensitive munitions, such as 2,4-dinitroanisole (DNAN), as components of military explosives has generated concern for potential subsurface contamination and created a need to fully characterize their transformation processes. Compound specific isotope analysis (CSIA) has proven to be a useful means of assessing transformation pathways according to characteristic stable isotope fractionation patterns. The C and N isotope fractionation of DNAN associated with abiotic and enzymatic hydrolysis was recently assessed. The extent to which DNAN isotope fractionation will be affected by other potentially competing transformation pathways known for nitroaromatic compounds (e.g., reduction) and if previous knowledge can be extrapolated to other environmental matrices remains to be understood. Here, we investigated the C and N isotope fractionation and reaction rate constants of DNAN during abiotic reduction mediated by mineral-associated Fe(ii) species as a function of mineral type, natural organic matter presence, and repeated exposures to DNAN. Though rate constants varied, N and C apparent kinetic isotope effects (AKIEs) remained consistent across all experiments (averaged values of 15N-AKIE = 1.0317 ± 0.0064 and 13C-AKIE = 1.0008 ± 0.0005) and revealed significant 15N- and minimal 13C-enrichment in agreement with previous work on nitroaromatic compounds. Moreover, the observed fractionation was clearly distinct from trends for abiotic and enzymatic hydrolysis. This study provides a strengthened basis for the use of CSIA as a robust tool for monitoring DNAN degradation in complex environmental matrices as a component of future remediation efforts.

Mineralogy and buffer identity effects on RDX kinetics and intermediates during reaction with natural and synthetic magnetite.

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is known to undergo reduction mediated by ferrous iron in the presence of minerals, including magnetite. Idealized laboratory conditions may not provide representative reaction kinetics or pathways compared to field conditions. The effects of magnetite mineral morphology, the aquifer material matrix, the presence of aqueous Fe(II), and the buffer identity on RDX reduction kinetics and intermediate formation are investigated in this work. Reactions in bicarbonate buffer were substantially slower than those performed in 3-(N-morpholino)propanesulfonic acid (MOPS) buffer, and the presence of quartz and clays in magnetite-containing aquifer material resulted in slower reaction kinetics and production of additional iron oxide phases. Buffer identity also changed the rate controlling step and reaction product distribution. Conditions as close to those expected in field systems are necessary to evaluate the reaction rates and pathways of RDX in reduced groundwater systems.