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Biogeochemical reactions occur unevenly in space and time, but this heterogeneity is often simplified as a linear average due to sparse data, especially in subsurface environments where access is limited. For example, little is known about the spatial variability of groundwater denitrification, an important process in removing nitrate originating from agriculture and land use conversion. Information about the rate, arrangement, and extent of denitrification is needed to determine sustainable limits of human activity and to predict recovery time frames. Here, we developed and validated a method for inferring the spatial organization of sequential biogeochemical reactions in an aquifer in France. We applied it to five other aquifers in different geological settings located in the United States and compared results among 44 locations across the six aquifers to assess the generality of reactivity trends. Of the sampling locations, 79% showed pronounced increases of reactivity with depth. This suggests that previous estimates of denitrification have underestimated the capacity of deep aquifers to remove nitrate, while overestimating nitrate removal in shallow flow paths. Oxygen and nitrate reduction likely increases with depth because there is relatively little organic carbon in agricultural soils and because excess nitrate input has depleted solid phase electron donors near the surface. Our findings explain the long-standing conundrum of why apparent reaction rates of oxygen in aquifers are typically smaller than those of nitrate, which is energetically less favorable. This stratified reactivity framework is promising for mapping vertical reactivity trends in aquifers, generating new understanding of subsurface ecosystems and their capacity to remove contaminants.

This study uses a membrane‐less reactor to explore the bioelectrochemical remediation of real contaminated groundwater from chlorinated aliphatic hydrocarbons (CAHs) and nitrates. The research focuses on testing a column‐type bioelectrochemical reactor to stimulate in situ degradation of contaminants through the supply of electrons by a graphite granules biocathode. After a preliminary laboratory characterization and operation with a synthetic feeding solution, a field test is conducted in a real contaminated site, where the reactor demonstrates effective degradation of CAHs and inorganic anions. Notably, the cathodic potential promotes the reductive dechlorination of chlorinated species. Simultaneously, nitrate reduction, sulfate reduction, and methanogenesis occurr, influencing the overall coulombic efficiency of the process. The use of real groundwater, compared to the synthetic medium, significantly decreases the coulombic efficiency of reductive dechlorination, dropping from 2.43% to 0.01%. Concentration profiles along the bioelectrochemical reactor allow for a deeper description of the reductive dechlorination rate at different flow rates, as well as increase the knowledge about reduction and oxidation mechanisms. Scaling up the technology presents several challenges, including the optimization of coulombic efficiency and the management of competing microbial metabolisms. The study provides a valuable contribution toward advancing bioelectrochemical technologies for the bioremediation of complex contaminated sites.

Nitrate contamination from agricultural and industrial activities poses significant risks to human health and aquatic ecosystems. Biochar, particularly when chemically modified, has emerged as a sustainable and effective adsorbent; however, the influence of modification sequence and operational conditions on nitrate removal is not fully understood. In this study, rapeseed (Brassica napus L.) -derived biochars were modified with aluminum through three distinct pathways: pre-pyrolysis (MP), post-pyrolysis (PM), and post-pyrolysis with re-pyrolysis (PMP). Comprehensive characterization using BET, SEM, EDS, XRD, and FTIR showed that PM biochar exhibited the highest surface area, uniform mesoporous structure, stable aluminum content, and abundant oxygen-containing functional groups, resulting in superior nitrate adsorption. Batch experiments demonstrated that adsorption efficiency is strongly affected by operational parameters, including initial nitrate concentration, contact time, adsorbent dose, solution pH, and the presence of competing anions, with CO₃ 2 ⁻ and SO₄ 2 ⁻ having the strongest inhibitory effects. Regeneration tests indicated that PM biochar retained ~ 76% of its initial adsorption capacity after five adsorption–desorption cycles, confirming its reusability. Nonlinear machine learning models, including Random Forest (RF), Support Vector Regression (SVR), and Linear Regression (LR), were applied to predict nitrate removal, with RF achieving the highest predictive accuracy (R 2  = 0.892, RMSE = 0.078), demonstrating robustness and generalization. This work highlights the critical role of modification sequence in tailoring biochar structure and functionality and integrates AI-based modeling to provide a data-driven framework for designing high-performance biochars for sustainable nitrate removal from water.

Nitrate (NO ) and cadmium (Cd ) are common water pollutants with distinct chemical behaviors, often requiring different removal strategies. This study presents a low-cost synthesis of carbonated hydroxyapatite nanopowder (cHA), Ca (PO ) (CO ) (OH) ( = 0.13-0.17), using eggshell waste as a calcium precursor, aimed at removing both NO and Cd from wastewater. SEM and TEM analyses revealed a porous nanostructure with an average particle size of 13.53 ± 6.43 nm and a specific surface area of 7.568 m /g. Adsorption experiments were conducted under varying conditions, including contact time (0.3-3 h), dosage (0.3-2 g/L), initial concentrations (10-100 mg/L for NO ; 5-15 mg/L for Cd ), and temperature (22 and 50 ± 2 °C). Cd removal reached up to 99% at pH 2-4.5, while NO removal peaked at 38% in competitive systems, within 30 min. In single-ion systems, maximum nitrate uptake was 19.14 mg/g at 50 °C. Characterization using FT-IR, EDS, and XRD (with Rietveld refinement) confirmed carbonate B-type substitution and structural changes due to ion exchange and chemisorption. The results demonstrate that cHA derived from food waste is an efficient and sustainable sorbent, particularly for cadmium removal in contaminated water.

This study investigates the effectiveness of biochar immobilized with algicidal Bacillus sp. AK3 and denitrifying Alcaligenes sp. M3 in mitigating harmful algal blooms (HABs) and reducing nitrate pollution in aquatic environments. Over a six-day period, we analyzed changes in algal bloom-forming Microcystis density, chlorophyll-a levels (indicative of algal biomass), nitrate concentration, and microbial community composition in water treated with biochar and Bacillus sp. AK3 and Alcaligenes sp. M3-immobilized biochar. In water treatment using the AK3 and M3-immobilized biochar, Microcystis density decreased from 600,000 cells/mL to 80,000 cells/mL, and chlorophyll-a concentrations also substantially reduced, from 85.7 µg/L initially to 42.8 µg/L. Nitrate concentrations in the AK3 and M3-immobilized biochar treatment significantly decreased from approximately 23 mg/L to around 14 mg/L by Day 6, demonstrating the enhanced denitrification capabilities of the immobilized Alcaligenes sp. M3 and associated bacterial communities. The results also showed significant shifts in bacterial communities, with a decrease in Microcystis , highlighting the specific algicidal activity of Bacillus sp. AK3. The study underscores the potential of biochar-based treatments as a sustainable and effective approach for improving water quality and mitigating the environmental impacts of nutrient pollution and HABs.

This research investigated the capture of nitrate by magnesium ions in plasma-activated water (PAW) and its antifungal effect on the cell viability of the newly emerged mushroom pathogen Cryptococcus pseudolongus. Optical emission spectra of the plasma jet exhibited several emission bands attributable to plasma-generated reactive oxygen and nitrogen species. The plasma was injected directly into deionized water (DW) with and without an immersed magnesium block. Plasma treatment of DW produced acidic PAW. However, plasma-activated magnesium water (PA-Mg-W) tended to be neutralized due to the reduction in plasma-generated hydrogen ions by electrons released from the zero-valent magnesium. Optical absorption and Raman spectra confirmed that nitrate ions were the dominant reactive species in the PAW and PA-Mg-W. Nitrate had a concentration-dependent antifungal effect on the tested fungal cells. We observed that the free nitrate content could be controlled to be lower in the PA-Mg-W than in the PAW due to the formation of nitrate salts by the magnesium ions. Although both the PAW and PA-Mg-W had antifungal effects on C. pseudolongus, their effectiveness differed, with cell viability higher in the PA-Mg-W than in the PAW. This study demonstrates that the antifungal effect of PAW could be manipulated using nitrate capture. The wide use of plasma therapy for problematic fungus control is challenging because fungi have rigid cell wall structures in different fungal groups.

Background: Nitrate is a contaminant of drinking water in agricultural areas and is found at high levels in some vegetables. Nitrate competes with uptake of iodide by the thyroid, thus potentially affecting thyroid function. Methods: We investigated the association of nitrate intake from public water supplies and diet with the risk of thyroid cancer and self-reported hypothyroidism and hyperthyroidism in a cohort of 21,977 older women in Iowa who were enrolled in 1986 and who had used the water supply for > 10 years. We estimated nitrate ingestion from drinking water using a public database of nitrate measurements (1955–1988). Dietary nitrate intake was estimated using a food frequency questionnaire and levels from the published literature. Cancer incidence was determined through 2004. Results: We found an increased risk of thyroid cancer with higher average nitrate levels in public water supplies and with longer consumption of water exceeding 5 mg/L nitrate-N (for ≥5 years at >5 mg/L, relative risk [RR] = 2.6 [95% confidence interval (CI) = 1.1–6.2]). We observed no association with prevalence of hypothyroidism or hyperthyroidism. Increasing intake of dietary nitrate was associated with an increased risk of thyroid cancer (highest vs. lowest quartile, RR = 2.9 [1.0–8.1]; P for trend = 0.046) and with the prevalence of hypothyroidism (odds ratio = 1.2 [95% CI = 1.1–1.4]), but not hyperthyroidism. Conclusions: Nitrate may play a role in the etiology of thyroid cancer and warrants further study.

A mixed sulfur–iron particles packed reactor (SFe reactor) was developed to simultaneously remove total nitrogen (TN) and total phosphorus (TP) of the secondary effluent from municipal wastewater treatment plants. Low effluent TN (<1.5 mg/L) and TP (<0.3 mg/L) concentrations were simultaneously obtained, and high TN removal rate [1.03 g N/(L·d)] and TP removal rate [0.29 g P/(L·d)] were achieved at the hydraulic retention time (HRT) of 0.13 h. Kinetic models describing denitrification were experimentally obtained, which predicted a higher denitrification rate [1.98 g N/(L·d)] of SFe reactor than that [1.58 g N/(L·d)] of sulfur alone packed reactor due to the mutual enhancement between sulfur-based autotrophic denitrification and iron-based chemical denitrification. A high TP removal obtained in SFe reactor was attributed to chemical precipitation of iron particles. Microbial community analysis based on 16S rRNA revealed that autotrophic denitrifying bacteria Thiobacillus and Sulfuricella were the dominant genus, indicating that autotrophic denitrification played important role in nitrate removal. These results indicate that sulfur and iron particles can be packed together in a single reactor to effectively remove nitrate and phosphorus.

In this study, zero-valent iron (ZVI), nanoscale zero-valent iron (nZVI), Fe(II), and Mn(II) were investigated for their effects on mixotrophic denitrification coupled with cadmium (Cd(II)) adsorption process by Acinetobacter sp. SZ28. The removal rates of nitrate were 0.228 mg L −1  h −1 (ZVI), 0.133 mg L −1  h −1 (nZVI), 0.309 mg L −1  h −1 (Fe(II)) and 0.234 mg L −1  h −1 (Mn(II)), respectively. The Cd(II) removal efficiencies were 97.23% (ZVI), 95.79% (nZVI), 80.63% (Fe(II)), and 84.58% (Mn(II)), respectively. Meteorological chromatography analysis indicated that the characteristics of gas composition were different under different electron donor conditions. Moreover, characterization of bacterial metabolites produced by strain SZ28 under different conditions was analyzed. Sequence amplification identified the presence of the nitrate reductase gene ( nap A) and Mn(II)-oxide gene ( cum A) in strain SZ28. The results of XRD and SEM indicated that ZVI, nZVI, Fe(II), and Mn(II) were oxidized into corresponding oxides. XPS spectra indicated that the Cd(II) was adsorbed onto biogenic precipitation.

The long-term efficacy of stormwater treatment systems requires continuous pollutant removal without substantial re-release. Hence, the division of incoming pollutants between temporary and permanent removal pathways is fundamental. This is pertinent to nitrogen, a critical water body pollutant, which on a broad level may be assimilated by plants or microbes and temporarily stored, or transformed by bacteria to gaseous forms and permanently lost via denitrification. Biofiltration systems have demonstrated effective removal of nitrogen from urban stormwater runoff, but to date studies have been limited to a 'black-box' approach. The lack of understanding on internal nitrogen processes constrains future design and threatens the reliability of long-term system performance. While nitrogen processes have been thoroughly studied in other environments, including wastewater treatment wetlands, biofiltration systems differ fundamentally in design and the composition and hydrology of stormwater inflows, with intermittent inundation and prolonged dry periods. Two mesocosm experiments were conducted to investigate biofilter nitrogen processes using the stable isotope tracer 15NO3(-) (nitrate) over the course of one inflow event. The immediate partitioning of 15NO3(-) between biotic assimilation and denitrification were investigated for a range of different inflow concentrations and plant species. Assimilation was the primary fate for NO3(-) under typical stormwater concentrations (∼1-2 mg N/L), contributing an average 89-99% of 15NO3(-) processing in biofilter columns containing the most effective plant species, while only 0-3% was denitrified and 0-8% remained in the pore water. Denitrification played a greater role for columns containing less effective species, processing up to 8% of 15NO3(-), and increased further with nitrate loading. This study uniquely applied isotope tracing to biofiltration systems and revealed the dominance of assimilation in stormwater biofilters. The findings raise important questions about nitrogen release upon plant senescence, seasonally and in the long term, which have implications on the management and design of biofiltration systems.