DOM accumulation in the hyporheic zone promotes geogenic Fe mobility: A laboratory column study

Cd migration in water-level fluctuation zones of Three Gorges reservoir: Interactions of periphytic biofilms and tryptophan

Periphytic biofilms (PBs) and dissolved organic matter (DOM) are key factors affecting the migration of Cd at the "water-sediment" interface. However, the specific effects of PBs and protein-like components of DOM on Cd migration within the "water-biofilm-sediment" system remain poorly understood. This study simulates the dissolution and re-immobilization of Cd at the "water-biofilm-sediment" interface in the water-level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR), aiming to clarify the role of PBs in Cd stability. The results indicate that the concentration of dissolved Cd increased by 0.96-fold (R2 = 0.992) with each doubling of L-tryptophan (L-Trp) concentration, as the functional groups of L-Trp (COOH and NH) formed complexes with Fe-Mn oxide-bound Cd in the sediment. In the presence of PBs and at an L-Trp concentration of 150 mg·L-1, PBs utilized L-Trp, resulting in a 16.7 % reduction in dissolved organic carbon (DOC) (p < 0.05) and an increase in protein content. Additionally, PBs contributed to a 35.12 % reduction in the peak concentration of dissolved Cd, thereby stabilizing the final Cd levels. Cd enrichment within PBs, facilitated by functional groups such as >CC<, OH, >CO, and CO, led to a significant increase in Cd content (0.43 mg·kg-1, p < 0.05). These findings suggest that L-Trp degradation and Cd enrichment by PBs act synergistically to promote the re-immobilization of dissolved Cd. This study offers a novel perspective on Cd migration in WLFZs and provides insights that may be applicable to other aquatic environments where PBs are present.

Effects of BDE-47 injection on vertical redox zonation and microbial community assemblage in capped sediment columns

Polybrominated diphenyl ethers (PBDEs) are of significant interest in ecological risk assessment and bioremediation in sediments. However, their impact on microbial diversity and activity in capped lake sediments remains unclear, despite the widespread use of capping in lake management. In this study, two series of sediment columns were established to examine vertical redox zonation at 2-cm intervals from 2 to 16 cm and evaluate the impact of artificially injected PBDEs on microbial communities during a 60-day capping period. Variations in redox indicators, including nitrate, sulfate, total dissolved iron, and total dissolved manganese in porewater, showed that the capping layer (1 cm, d = 75 μm) increased the redox potential of subsurface sediments. BDE-47 was primarily concentrated in the injection layer (4-6 cm), but over time, it exhibited upward migration (0.4-0.7 cm) and a broader distribution range (0.5-1.0 cm), with no consistent decrease in the total BDE-47 mass. Microbial α-diversity declined, whereas microbial network analysis revealed increased connectivity and enhanced cooperation within communities in the BDE group. Notably, negative correlations between microbial taxa and iron exclusively in the BDE group, suggesting that BDE-47 counteracted capping-induced iron reduction. In contrast, sulfate showed an opposite trend with iron between the BDE and noBDE groups. Methanolinea [Euryarchaeota] and certain co-metabolizing dechlorinating bacteria, such as Flavobacterium dominated in the capping layer, were correlated to BDE-47. These findings provide the first evidence of redox-regulated natural attenuation of PBDEs in capped lake sediments, shedding light on their environmental impact and guiding sediment management strategies.

Changes in chemical speciation and mobility of arsenic during the mixing of arsenic-bearing "snow-melting" system effluent and river water in the Ishikari Plain, Japan

An effective and ingenious method called "snow-melting" system was widely implemented for snow management in the Ishikari Plain, Japan. In this system, groundwater is pumped up, mixed with snow, and discharged into a nearby river. Since the groundwater in the Ishikari Plain is contaminated with arsenic (As), the impacts of directly discharging As-bearing groundwater into the river were assessed and monitored. In-situ monitoring data collected between 2013 and 2015 showed that As concentrations were higher in the groundwater (23-95 μg/L) than in the river water (2-71 μg/L). The major As speciation in the groundwater and river water were dissolved arsenite (As(III)) and As in suspended iron (Fe)-bearing solids, respectively. Precipitation of dissolved Fe when "snow-melting" system effluent mixes with the river water could be attributed to more oxic and oxidizing conditions of the resulting fluid mixture. Dissolved iron (Fe), mainly as ferrous ion (Fe2+), coexisted with dissolved As(III) in groundwater, so after mixing with the river water, Fe2+ was oxidized to ferric ion (Fe3+) and then precipitated as amorphous Fe oxyhydroxide phases that also sequestered dissolved arsenate (As(V)) via adsorption and coprecipitation. A strong correlation between As and Fe contents in river sediments was also observed, suggesting that Fe-bearing phases play an essential role in As immobilization. The results also showed a strong interaction between groundwater and river water that affected the chemical speciation and mobility of As and Fe. In addition, it was found that discharging As-bearing groundwater did not have profound impact on river water quality. Based on the results, dissolved Fe, Fe-bearing solid phases, and geochemical conditions strongly influenced how As speciates and migrates in a system where two fluids are mixed. This study could provide significant insights concerning the impacts of As on surrounding environments where As-bearing groundwaters are used and discharged without treatment.

Effect of biosurfactants on the transport of polyethylene microplastics in saturated porous media

Microplastic (MP) pollution has become a significant global environmental issue, and the potential application of biosurfactants in soil remediation has attracted considerable attention. However, the effects of biosurfactants on the transport and environmental risks of MPs are not fully understood. This study investigated the transport of polyethylene (PE) in the presence of two types of biosurfactants: typical anionic biosurfactant (rhamnolipids) and non-ionic biosurfactant (sophorolipids) using column experiments. We explored the potential mechanisms involving PE surface roughness and the influence of dissolved organic matter (DOM) on PE transport in the column under the action of biosurfactants, utilizing the Wenzel equation and fluorescence analysis. The results revealed that both the concentration of biosurfactants and the surface roughness of PE were advantageous for the adhesion of biosurfactants to the PE surface, thereby enhancing the mobility of PE in the column. The proportion of hydrophobic substances in various DOM sources is a critical factor that enhances PE transport in the column. However, the biosurfactant-mediated enhancement of PE transport was inhibited by the biosurfactant-DOM mixture. This was mainly due to DOM occupying the adhesion sites of biosurfactants on PE surfaces. Moreover, the mobility of PE in the presence of sophorolipids is higher than that in the presence of rhamnolipids because the combined hydrophobic and electrostatic forces between PE and sophorolipids create synergistic effects that improve PE stability. Additionally, the mobility of PE increased with rising pH and decreasing ionic strength. These findings provide a more comprehensive understanding of MP transport when using biosurfactants for soil remediation.

Comparative impact analysis of nitrate reduction by typical components of natural organic compounds in magnetite-bearing riparian zones

As the key interface, the nitrate removal capacity of riparian zones is receiving close attention. Although naturally occurring organic compounds in this environment play a pivotal role in shaping microbial communities and influencing the nitrate removal capacity, the relevant research is inadequate. Given the complexity of riparian environments, in this study, we added representative natural organic matter (fulvic acid, butyric acid, naphthalene, starch, and sodium bicarbonate) as carbon conditions and incorporated magnetite to simulate riparian zone components. The study investigated the nitrate degradation efficiency and microbial responses under different natural carbon conditions in real iron-containing environments. Butyric acid exhibited the most efficient nitrate reduction, followed in descending order by naphthalene, starch, sodium bicarbonate, and humic acid. However, this did not imply that butyric acid efficiently removed nitrogen; instead, the nitrogen would circulate in the environment in the form of ammonium. Denitrification and DNRA were the primary drivers of nitrate reduction in each system, while naphthalene and humic acid systems also exhibited nitrification and mineralization. Nitrogen-fixing bacteria represent a unique microbial community in the butyrate system. Further, the synergistic degradation of naphthalene and nitrate demonstrated significant potential applications. High-throughput sequencing revealed that carbon conditions exerted selective pressure on microorganisms, driving Fe (Ⅱ)/Fe (Ⅲ) transformation by shaping the microbial community structure and influencing the nitrogen cycling process.

Influence mechanisms of dissolved organic matter and iron minerals on naphthalene attenuation during river infiltration

Natural attenuation of naphthalene (NAP) in riverbank filtration zones is vital for maintaining water quality and is affected by dissolved organic matter (DOM) and iron minerals. However, the effects of DOM and iron minerals on the attenuation of NAP remain unclear. In this study, the attenuation mechanisms of NAP under the influence of DOM and iron minerals were explored in a riverside source area. Field dynamic monitoring data revealed that the NAP concentration in groundwater is mainly influenced by DOM, effective bound‑iron, and the intensity of river water infiltration recharge. Column experiments indicated that DOM with α-Fe2O3 or α-FeO(OH) reduced medium permeability by 8.16 % or 6.85 %, respectively, increasing water retention time. However, they had different effects on the attenuation of NAP. The coexistence of α-Fe2O3 and DOM enhanced NAP attenuation capacity by 9.13 %-45.91 %, while α-FeO(OH) and DOM reduced it by -13.25 % to -24.13 %. These effects were attributed to changes in the medium permeability, particle size, secondary mineral formation, and microbial community structure. Specifically, α-Fe2O3 and DOM reduced medium permeability, increasing the adsorption and biodegradation reaction time of NAP, and promoted secondary mineral (FeCO3) formation, increasing the adsorption capacity of medium for NAP, while α-FeO(OH) and DOM underwent cementation, resulting in larger particles and reduced adsorption capacity for NAP. Additionally, α-FeO(OH) and DOM promoted Shewanlla growth, inhibiting NAP attenuation by competing with NAP-degrading bacteria. These findings improve the understanding of the natural attenuation of polycyclic aromatic hydrocarbons (PAHs) in riverbank filtration, offering a basis for evaluating and controlling PAH pollution risks.

Insights into the characteristics of changes in dissolved organic matter fluorescence components on the natural attenuation process of toluene

Natural attenuation (NA) is of great significance for the remediation of contaminated groundwater, and how to identify NA patterns of toluene in aquifers more quickly and effectively poses an urgent challenge. In this study, the NA of toluene in two typical soils was conducted by means of soil column experiment. Based on column experiments, dissolved organic matter (DOM) was rapidly identified using fluorescence spectroscopy, and the relationship between DOM and the NA of toluene was established through structural equation modeling analysis. The adsorption rates of toluene in clay and sandy soil were 39 % and 26 %, respectively. The adsorption capacity and total NA capacity of silty clay were large. The occurrence of fluorescence peaks of protein-like components and specific products indicated the occurrence of biodegradation. Arenimonas, Acidovorax and Brevundimonas were the main degrading bacteria identified in Column A, while Pseudomonas, Azotobacter and Mycobacterium were the main ones identified in Column B. The pH, ORP, and Fe(II) were the most important factors affecting the composition of microbial communities, which in turn affected the NA of toluene. These results provide a new way to quickly identify NA of toluene.

Insight into spatial variations of DOM fractions and its interactions with microbial communities of shallow groundwater in a mesoscale lowland river watershed

Dissolved organic matter (DOM) plays a crucial role in driving biogeochemical processes and determining water quality in shallow groundwater systems, where DOM could be susceptible to dynamic influences of surface water influx. This study employed fluorescence excitation-emission matrix (EEM) spectroscopy combined with principal component coefficients, parallel factor analysis (PARAFAC), co-occurrence network analysis and structural equation modeling (SEM) to examine changes of DOM fractions from surface water to shallow groundwater in a mesoscale lowland river basin. Combining stable isotope and hydrochemical parameters, except for surface water (SW), two groups of groundwater samples were defined, namely, deeply influenced by surface water (IGW) and groundwater nearly non-influenced by surface water (UGW), which were 50.34 % and 19.39 % recharged by surface water, respectively. According to principal component coefficients, reassembled EEM data of these categories highlighted variations of the tyrosine-like peak in DOM. EEMs coupled with PARAFAC extracted five components (C1-C5), i.e. C1, protein-like substances, C2 and C4, humic-like substances, and C3 and C5, microbial-related substances. The abundance of the protein-like was SW > IGW > UGW, while the order of the humic-like was opposite. The bacterial communities exhibited an obvious cluster across three regions, which hinted their sensitivity to variations in environmental conditions. Based on co-occurrence, SW represented the highest connectivity between bacterial OTUs and DOM fractions, followed by IGW and UGW. SEM revealed that microbial activities increased bioavailability of the humic-like in the SW and IGW, whereas microbial compositions promoted the evolution of humic-like substances in the UGW. Generally, these results could be conducive to discern dissimilarity in DOM fractions across surface water and shallow groundwater, and further trace their interactions in the river watershed.

Mechanisms of cadmium release from manganese-rich sediments driven by exogenous DOM and the role of microorganisms

Dissolved organic matter (DOM) is a crucial component of natural sediments that alters Cd sequestration. Nevertheless, how different types of DOM fuel Cd mobilization in Mn-rich sediments has not been elucidated. In the present study, four typical DOM, fluvic acid (FA), bovine serum albumin (BSA), sodium alginate (SA), and sodium dodecyl benzene sulfonate (SDBS), were used to amend Cd-contaminated sediment to study their effects on Cd/Mn biotransformation and microbial community response. The results demonstrated that different DOM drive microbial community shifts and enhance microbially mediated Mn oxide (MnO) reduction and Cd release. The amendment of terrestrial- and anthropogenic-derived DOM (FA and SDBS) mainly contributed to enriching Mn-reducing bacteria phylum Proteobacteria, and its abundance increased by 38.16-74.47 % and 56.41-73.98 %, respectively. Meanwhile, microbial-derived DOM (BSA and SA) mainly stimulated the abundances of metal(loid)-resistant bacteria phylum Firmicutes. Accompanied by microbial community structure, diversity, and co-occurrence network shifts, the DOM concentration and oxidation-reduction potential changed, resulting in enhanced Cd mobilization. Importantly, FA stimulated Cd release most remarkably, probably because of the decreased cooperative interactions between bacterial populations, stronger reduction of MnOs, and higher aromaticity and hydrophobicity of the sediment DOM after amendment. This study linked DOM types to functional microbial communities, and explored the potential roles of different DOM types in Cd biotransformation in lake sediments.

Source apportionment and source specific health risk assessment of HMs and PAHs in soils with an integrated framework in a typical cold agricultural region in China

A new innovative methodology system framework for source apportionment and source-specific risk assessment has been proposed and actively applied to identify the contamination characteristics, oriented sources and health risks associated with contamination levels of Heavy metals (HMs) and Polycyclic Aromatic Hydrocarbons (PAHs) in soils, a typical cold agricultural region in Northeastern China. To achieve this meaningful goal, a large-scale dataset including 1780 top soil samples, 10 HMs and 16 priority PAHs has been organized and collected from a typical study area in China. The total concentrations of the 10 selected HMs in study area range from 0.05 to 2147.40 mg/kg, with an average of 549.25 ± 541.37 mg/kg. The average concentrations of PAHs for (3-6)-rings are 16.60 ± 18.90, 26.40 ± 28.20, 9.51 ± 13.00 and 1.99 ± 5.30 ng/g, respectively. On the base of optimized literature source fingerprints for HM and PAH, a widely used receptor model, positive matrix factorization (PMF) has been applied to apportion the contamination sources HMs and PAHs in soils. Then source-specific health risk of soil HMs and PAHs have been assessed using the probabilistic incremental lifetime cancer risk model incorporated with source apportionment results data. Fertilizer residues/coke oven comprise the primary contamination source contributors of HMs and PAHs with corresponding contributions of 32.23 % and 27.93 % for HMs and 37.94 % for PAHs. Fertilizer/pesticide residues contributes most to the risks of soil HMs (28.8 %), followed by fossil fuel combustion (24.6 %), mining activities (20.2 %), traffic and vehicle emission (16.3 %) and electroplating/dyeing (14.1 %). Meanwhile, the ranking of health risks from the five identified contamination sources of soil PAHs are resident discharge, coal-fired boilers, coke oven emission, gasoline combustion and power plant, with the contribution of 27.1 %, 25.3 %, 17.3 %, 15.5 % and 14.8 %. And relatively, source-specific risk assessment demonstrates fossil fuel and coal combustion contribute the greatest impact to the total risk of HMs and PAHs (61.7 % and 56.1 %), respectively. This study provides a good example of how the source specific health risk assessment can be utilized to reduce the contamination in soils.