Bacterial Toxicity of Sulfidated Nanoscale Zerovalent Iron in Aerobic and Anaerobic Systems: Implications for Chlorinated Solvent Clean-Up Strategies
Academic Background
The widespread use and improper disposal of chlorinated solvents (such as perchloroethylene and trichloroethylene) have led to severe contamination of soil and groundwater worldwide. These pollutants not only threaten groundwater security but may also affect human health through the food chain. Although traditional microbial reductive dechlorination techniques can degrade these pollutants, the degradation rates are relatively low, and the process often stops at more toxic intermediate products. To improve degradation efficiency, nanoscale zero-valent iron (nZVI) materials have been introduced into pollution remediation due to their ability to rapidly degrade chlorinated solvents through chemical reactions. However, the high reactivity of nZVI may also pose toxicity to microbial communities, especially when combined with microbial remediation technologies, making this issue particularly prominent.
In recent years, sulfidated nanoscale zero-valent iron (S-nZVI) has garnered attention as a novel material due to its enhanced selectivity and reactivity toward chlorinated solvents. However, research on the potential toxicity of S-nZVI to microbial communities remains limited. This study aims to evaluate the toxicity of S-nZVI under aerobic and anaerobic conditions, particularly its impact on chlorinated solvent-degrading microbial communities, to provide a scientific basis for combined biotic and abiotic remediation strategies.
Source of the Paper
This paper was jointly completed by a research team from the Technical University of Denmark, the Capital Region of Denmark’s Groundwater Section, the GFZ German Research Center for Geosciences, the Free University of Berlin, and the University of Copenhagen. The paper was published on April 3, 2024, in the journal Geo-Bio Interfaces, titled Bacterial toxicity of sulfidated nanoscale zerovalent iron in aerobic and anaerobic systems: implications for chlorinated solvent clean-up strategies.
Research Process and Results
1. Material Synthesis and Characterization
The study first synthesized two different sulfidated nanoscale zero-valent iron materials: one using sodium sulfide (Na₂S) and the other using sodium dithionite (Na₂S₂O₄) for sulfidation. The materials were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and high-energy X-ray scattering (HEXRD). The results showed that Na₂S-S-nZVI materials formed a uniform iron sulfide (FeS) layer on the surface, while Na₂S₂O₄-S-nZVI materials exhibited a more complex structure, including a mixed phase of iron sulfide and ferrous hydroxide (Fe(OH)₂).
2. Bacterial Toxicity Experiments
The study selected Shewanella oneidensis MR-1 (S. MR-1) as the experimental subject to evaluate the toxicity of S-nZVI and nZVI materials under aerobic and anaerobic conditions. Cell viability was measured using colony-forming units (CFU) and adenosine triphosphate (ATP) bioluminescence assays. The results showed that under aerobic conditions, the toxicity of S-nZVI materials to S. MR-1 was significantly lower than that of nZVI materials, with Na₂S-S-nZVI exhibiting the lowest toxicity. Under anaerobic conditions, the toxicity differences among the three materials were smaller, but S-nZVI materials still demonstrated lower toxicity.
3. Mixed Microbial Community Toxicity Experiments
The study further assessed the impact of S-nZVI and nZVI materials on the commercial dechlorinating microbial community KB-1® and a laboratory-cultured trichloroethylene (TCE)-degrading microbial community. Community activity was monitored using ATP bioluminescence assays. The results showed that the KB-1® community exhibited significantly higher tolerance to S-nZVI and nZVI compared to the laboratory-cultured community, especially under high-concentration (1000 mg/L) exposure, where the ATP activity of the KB-1® community remained high. In contrast, the laboratory-cultured community rapidly lost viability upon exposure to nZVI.
4. Toxicity Mechanism Analysis
Scanning electron microscopy (SEM) observations revealed that under anaerobic conditions, nZVI and Na₂S₂O₄-S-nZVI materials formed a large amount of mineral precipitates on their surfaces, which closely adhered to the cell surfaces, potentially leading to cell inactivation. In contrast, Na₂S-S-nZVI materials exhibited better stability without significant mineral precipitate formation. Additionally, the study found that toxicity under aerobic conditions was primarily related to the generation of reactive oxygen species (ROS), while toxicity under anaerobic conditions was associated with direct cell membrane damage and mineral precipitate formation.
Conclusions and Significance
This study demonstrates that sulfidated nanoscale zero-valent iron (S-nZVI) exhibits lower bacterial toxicity in chlorinated solvent remediation, particularly under aerobic conditions, where Na₂S-S-nZVI materials show significant advantages. Furthermore, the high tolerance of the KB-1® community to S-nZVI and nZVI indicates its important potential in combined biotic and abiotic remediation strategies. The findings provide a scientific basis for optimizing chlorinated solvent remediation technologies, especially in designing combined biotic and abiotic remediation strategies, where the use of S-nZVI materials can significantly reduce toxicity to microbial communities, thereby improving remediation efficiency.
Research Highlights
- Sulfidation Reduces nZVI Bacterial Toxicity: Under aerobic conditions, sulfidation significantly reduced the toxicity of nZVI to S. MR-1, with Na₂S-S-nZVI materials exhibiting the lowest toxicity.
- Impact of Different Sulfidation Methods: nZVI materials sulfidated with sodium sulfide demonstrated better stability and lower toxicity, while materials sulfidated with sodium dithionite exhibited higher oxidation and toxicity.
- High Tolerance of the KB-1® Community: The KB-1® community showed significantly higher tolerance to S-nZVI and nZVI compared to laboratory-cultured communities, indicating its valuable application in combined remediation strategies.
- Diversity of Toxicity Mechanisms: Toxicity under aerobic conditions was primarily related to ROS generation, while toxicity under anaerobic conditions was associated with mineral precipitation and cell membrane damage.
Other Valuable Information
The study also highlights that the selective reactivity and lower toxicity of sulfidated nanoscale zero-valent iron materials make them promising for environmental remediation. Future research could further explore the impact of different sulfidation methods on material performance and how to optimize sulfidation processes to enhance material remediation efficiency and environmental friendliness.