Formation of Amorphous Molybdenum Sulfide in Abiotic and Biotic Sulfidic Conditions: A Comparative Study on Molybdenum Sequestration Mechanisms

Earth Sciences Chemistry Geochemistry Inorganic Chemistry Environmental Chemistry
paleoredox proxy molybdenum sequestration sulfate-reducing bacteria molybdenum sulfides molybdate thiomolybdate

Molybdenum (Mo) is one of the most abundant trace metals in the ocean, and its distinct behavior under different redox conditions makes it an effective indicator of paleoceanographic redox conditions. Particularly in anoxic and sulfidic environments, the speciation and behavior of Mo differ significantly from those in oxic environments. However, the mechanisms governing Mo sequestration in sulfidic environments are not yet fully understood. Previous studies have proposed that sulfate-reducing bacteria (SRB) may promote Mo sequestration through active uptake and reduction of Mo or by inducing Fe-independent Mo complexation and reduction at their cell surfaces. Nevertheless, the specific mechanisms of these biological pathways and their relative contributions remain uncertain. Therefore, this study aims to systematically investigate the interactions among Mo(VI) species (e.g., molybdate MoO₄²⁻ or tetrathiomolybdate MoS₄²⁻), ferrous iron (Fe²⁺), and SRB, focusing on the conditions that lead to reductive Mo precipitation, in order to reveal the mechanisms of Mo sequestration in sulfidic environments.

Source of the Paper

This paper was co-authored by Rachel F. Phillips, Weinan Leng, Sheryl A. Singerling, Morgane Desmau, and Jie Xu, affiliated with the School of Molecular Sciences at Arizona State University, the School of Earth, Ocean, and Environment at the University of South Carolina, the National Center for Earth and Environmental Nanotechnology Infrastructure at Virginia Tech, the Schwiete Cosmochemistry Laboratory at Goethe University Frankfurt, and the Canadian Light Source at the University of Saskatchewan, respectively. The paper was published in 2025 in the journal Geo-Bio Interfaces under the title Formation of Amorphous Molybdenum Sulfide in Abiotic and Biotic Sulfidic Conditions: A Comparative Study on Molybdenum Sequestration Mechanisms.

Research Process and Results

1. Experimental Design

This study explored the interactions among Mo(VI) species (MoO₄²⁻ or MoS₄²⁻), Fe²⁺, and SRB under different conditions, with a focus on the processes leading to reductive Mo precipitation. Experiments were divided into biological and abiotic experiments, conducted in solutions with and without SRB, respectively. Two SRB species, Desulfovibrio vulgaris and Desulfotignum balticum, were used to enhance the environmental relevance of the experiments.

2. Experimental Procedures

  • SRB Growth Experiments: SRB were cultured in media containing Mo to observe the inhibitory effects of Mo on SRB growth. Experiments were conducted in media containing either MoO₄²⁻ or MoS₄²⁻, with or without Fe²⁺.
  • Mo Thiolation and Sequestration Experiments: Mo and Fe²⁺ were added at different stages of SRB growth to observe the processes of Mo thiolation and sequestration. The thiolation process was monitored using ultraviolet-visible spectroscopy (UV-Vis), while the composition, structure, and oxidation states of the precipitates were analyzed using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and synchrotron-based X-ray absorption spectroscopy (XAS).
  • Dead SRB Cell Experiments: After SRB growth produced sufficient hydrogen sulfide, the SRB cells were killed by autoclaving, followed by the addition of Mo and Fe²⁺ to observe the effects of dead cells on Mo sequestration.

3. Key Results

  • Inhibition of SRB Growth: The experiments showed that both MoO₄²⁻ and MoS₄²⁻ significantly inhibited SRB growth, with Desulfovibrio vulgaris being more strongly inhibited. This suggests that Mo may inhibit SRB growth by interfering with their sulfate reduction metabolic pathways.
  • Mo Thiolation and Sequestration: In the presence of SRB, the rate of Mo thiolation increased significantly, especially in the presence of Fe²⁺. Fe²⁺ markedly catalyzed the thiolation and sequestration of Mo, indicating its critical role in Mo sequestration.
  • Precipitate Analysis: XPS and XAS analyses revealed that Mo in the precipitates primarily existed as Mo(IV), indicating that Mo was reduced during the thiolation process. Additionally, the composition and structure of the precipitates were nearly indistinguishable under biotic and abiotic conditions, further suggesting that SRB’s role in Mo sequestration is passive, mainly by providing sulfide and potential nucleation sites to promote Mo precipitation.

4. Conclusions

This study demonstrates that Mo sequestration is primarily driven by abiotic processes, with SRB indirectly facilitating this process through sulfide production. Fe²⁺ is a key factor in Mo reduction and sequestration, and it is essential for Mo precipitation even in the presence of SRB. Furthermore, the study reveals the mechanism of Mo sequestration in sulfidic environments: Mo first reacts with FeS complexes, followed by thiolation and reduction, ultimately forming amorphous molybdenum sulfide precipitates.

Significance and Highlights of the Research

1. Scientific Value

This study provides new insights into the mechanisms of Mo sequestration in sulfidic environments, particularly regarding the roles of Fe²⁺ and SRB. The results indicate that Fe²⁺ is a key catalyst for Mo sequestration, while SRB’s role is mainly indirect, promoting Mo precipitation through sulfide production. This finding is significant for interpreting paleoceanographic redox conditions.

2. Application Value

The findings can be used to improve models for reconstructing paleoceanographic redox conditions, especially regarding Mo behavior in sulfidic environments. Additionally, the study provides a reference for understanding the behavior of other trace metals in sulfidic environments.

3. Research Highlights

  • Key Role of Fe²⁺: The study found that Fe²⁺ is essential for Mo sequestration, even in the presence of SRB.
  • Passive Role of SRB: SRB does not directly reduce Mo but indirectly promotes its precipitation by providing sulfide.
  • Amorphous Structure of Precipitates: The composition and structure of Mo precipitates formed under biotic and abiotic conditions are nearly indistinguishable, indicating that SRB’s role is mainly through sulfide production and nucleation site provision.

Additional Valuable Information

The study also explored the differences in Mo thiolation and sequestration under varying pH conditions, finding that thiolation and sequestration are significantly reduced at high pH, suggesting that pH also plays an important role in Mo sequestration. Additionally, the study proposed a sequential mechanism for Mo sequestration: Mo first reacts with FeS complexes, followed by thiolation and reduction, ultimately forming amorphous molybdenum sulfide precipitates.

Through this study, we have gained a deeper understanding of the mechanisms of Mo sequestration in sulfidic environments, providing an important theoretical foundation and experimental basis for future related research.