Factors Influencing the Efficacy of Microbial Remediation of Selenium in Groundwater Near a Coal-Fired Power Plant
Selenium (Selenium) is an essential trace element widely found in nature and involved in various biological metabolic processes. However, when selenium concentrations are too high, they can cause severe toxic effects on humans, animals, and the environment. Industrial activities, especially coal combustion in coal-fired power plants, are one of the main sources of selenium contamination in groundwater. During coal combustion, selenium from fly ash can leach into groundwater through disposal processes, leading to water pollution. The toxic forms of selenium are primarily its oxidized states, such as selenate (Se(VI)) and selenite (Se(IV)), which are highly soluble in water and easily absorbed by organisms, posing threats to ecosystems and human health.
To address this issue, researchers have explored various remediation technologies, among which microbial remediation has become a research hotspot in recent years due to its cost-effectiveness and environmental friendliness. Microbial remediation utilizes the metabolic capabilities of natural microbial communities to reduce toxic selenate and selenite into insoluble elemental selenium (Se(0)), thereby reducing the bioavailability and toxicity of selenium. However, the effectiveness of microbial remediation is influenced by multiple factors, including groundwater chemistry, geological structure, carbon source selection, and microbial community composition. Therefore, a deeper understanding of how these factors affect the selenium reduction process is crucial for optimizing remediation strategies.
Source of the Paper
This paper was authored by a research team from Montana State University, with primary authors including Hannah R. Koepnick, Brent M. Peyton, and Ellen G. Lauchnor. The paper was published in 2025 in the journal Geo-Bio Interfaces, titled Factors Influencing the Efficacy of Microbial Remediation of Selenium in Groundwater Near a Coal-Fired Power Plant. The study was supported by the Montana Water Center and aimed to investigate the efficacy and influencing factors of microbial remediation of selenium in groundwater near a coal-fired power plant.
Research Process and Results
Research Process
1. Experimental Design and Sample Collection
The research team selected four monitoring wells near a coal-fired power plant in southeastern Montana, located in different geological units (alluvial, interburden, coal, and background). By collecting groundwater and biofilm from these wells, they constructed laboratory microcosm systems to simulate the microbial remediation process in groundwater.
2. Carbon Source Selection and Microcosm Construction
The researchers tested the effects of three carbon sources (glycerol, methanol, and molasses) on selenium reduction. Each microcosm system consisted of 100 mL of groundwater amended with 1000 ppb selenium and 3.2-3.4 g of biofilm-coated sand. Carbon sources were added at a concentration of 0.5 mM, and the experiments were conducted at a constant temperature of 10°C to simulate the low-temperature conditions of groundwater.
3. Nitrate and Selenium Monitoring
During the experiment, water samples were collected regularly, and nitrate and selenium concentrations were measured using ion chromatography and inductively coupled plasma mass spectrometry (ICP-MS), respectively. Additionally, selenium speciation was analyzed using hydride generation fluorescence detection to determine the conversion of selenate and selenite.
4. Microbial Community Analysis
After the experiment, the microbial communities in the microcosms were analyzed using 16S rRNA gene sequencing to investigate the effects of different carbon sources and geological units on microbial community composition. Non-metric multidimensional scaling (NMDS) and indicator species analysis were used to identify key microbial taxa associated with selenium reduction.
Key Results
1. Nitrate Removal
Nitrate was effectively removed in all microcosm systems with carbon source amendments, with molasses showing the highest removal rate. The nitrate removal rate was significantly correlated with the type of carbon source, and molasses had a significantly higher removal rate than glycerol and methanol. Additionally, the nitrate removal rate was not related to the geological unit, indicating that the carbon source was the primary factor influencing nitrate removal.
2. Selenium Reduction
Selenium reduction began only after complete nitrate removal, and significant differences were observed among microcosms from different geological units. Microcosms from the alluvial unit showed the best selenium removal, while those from the coal and interburden units performed poorly. Molasses as a carbon source resulted in the fastest selenium reduction, but methanol and glycerol also effectively promoted selenium reduction.
3. Microbial Community Composition
Microbial community composition was significantly correlated with carbon source and geological unit. In microcosm systems with molasses and glycerol as carbon sources, microbial taxa capable of selenium reduction, such as Pseudomonas and Hydrogenophaga, were significantly enriched. Additionally, the researchers found that Desulfosporosinus and Gracilibacter from the order Clostridiales might be key indicator species for selenium reduction.
Conclusions and Significance
This study demonstrates that carbon source and geological unit are key factors influencing the efficacy of microbial remediation of selenium in groundwater. Molasses as a carbon source significantly enhances the removal rates of nitrate and selenium, while the alluvial geological conditions are more favorable for selenium reduction. Furthermore, microbial community composition is closely related to selenium reduction efficacy, and certain microbial taxa may serve as biomarkers for selenium reduction.
This research provides important theoretical and practical guidance for microbial remediation of selenium contamination in groundwater near coal-fired power plants. By optimizing carbon source selection and leveraging microbial communities from specific geological units, the efficiency of selenium removal can be effectively improved, reducing its hazards to the environment and human health.
Research Highlights
- Microbial Remediation in Low-Temperature Environments: This study is the first to investigate selenium microbial reduction at 10°C, providing important insights for groundwater remediation in cold climates.
- Impact of Carbon Source on Remediation Efficacy: Molasses as a carbon source demonstrated significant advantages in nitrate and selenium removal, offering guidance for carbon source selection in practical remediation projects.
- Indicator Role of Microbial Communities: The study identified certain microbial taxa (e.g., Desulfosporosinus and Gracilibacter) closely associated with selenium reduction, which may serve as biomarkers for future remediation projects.
Other Valuable Information
A limitation of this study is that it did not consider the potential effects of other inhibitors (e.g., heavy metal ions) in groundwater on selenium reduction. Future research could further explore the impact of these factors on microbial remediation efficacy to optimize remediation strategies.