Manganese-oxidizing and manganese-reducing bacteria (MOB and MRB) play critical roles in corrosion processes, particularly in water systems and industrial environments where manganese is present. These microorganisms influence corrosion through biofilm formation, redox cycling, and the deposition of manganese oxides, which create local conditions conducive to corrosion. This article examines the mechanisms by which MOB and MRB contribute to material degradation, explores their biochemical pathways, and discusses implications for corrosion management
1. Introduction
Microbiologically influenced corrosion (MIC) is a pervasive issue in industrial settings, especially in environments containing metals like iron and manganese. While sulfate-reducing bacteria (SRB) are traditionally associated with MIC, recent research highlights manganese-oxidizing and -reducing bacteria (MOB and MRB) as significant contributors to corrosion processes. These microorganisms affect material integrity through redox reactions and the formation of manganese oxides that alter the local electrochemical environment, exacerbating corrosion.
Manganese is abundant in natural and industrial systems, particularly in water systems, where it often exists alongside iron. This overlap creates an environment in which manganese-related microbial processes can operate synergistically with iron-related ones, increasing the complexity and severity of corrosion.
2. Manganese-Oxidizing and -Reducing Bacteria
2.1 Manganese-Oxidizing Bacteria (MOB)
Manganese-oxidizing bacteria are known for their ability to oxidize manganese (Mn²⁺) into manganese oxides (MnO₂), a process that occurs primarily under aerobic conditions. Common MOB genera include Leptothrix, Pseudomonas, and Bacillus, which have been identified in biofilms associated with corroded surfaces. MOB contribute to corrosion in the following ways:
- Manganese Oxide Deposition: Manganese oxides are deposited on surfaces as solid particles, creating a biofilm layer with strong oxidizing potential. These oxides act as local cathodes, accelerating the corrosion of the underlying metal by facilitating electron transfer.
- Biofilm Formation and Growth: MOB form biofilms on metal surfaces, which concentrate manganese and oxygen, furthering the redox processes that lead to localized corrosion.
- Catalytic Effect: The manganese oxides produced by MOB can catalyze additional oxidation reactions, leading to enhanced metal dissolution and pitting corrosion. The high surface area of MnO₂ particles enables electron transfer that accelerates corrosion, particularly in steel and iron materials commonly used in water and wastewater systems.
The manganese oxide deposits create microenvironments with variable oxygen concentrations that stimulate the growth of other anaerobic microbes, such as MRB and SRB, which can work in concert with MOB to further promote corrosion.
2.2 Manganese-Reducing Bacteria (MRB)
Manganese-reducing bacteria perform the reverse reaction, reducing manganese oxides back to Mn²⁺ under anaerobic conditions. Common MRB genera include Shewanella, Geobacter, and Desulfuromonas. These bacteria typically inhabit anaerobic niches within biofilms, where they utilize MnO₂ as an electron acceptor, especially in environments with low oxygen levels. The corrosion mechanisms associated with MRB include:
- Dissolution of Manganese Oxides: MRB dissolve manganese oxide deposits, releasing Mn²⁺ ions back into the environment. This dissolution weakens the metal surface by depleting the protective oxide layer and exposing it to further microbial colonization and corrosion.
- Electrochemical Reactions: MRB use MnO₂ as an electron acceptor, a process that generates localized acidity (e.g., via organic acid production), which further corrodes the metal surface. Additionally, Mn²⁺ ions released in the process can migrate and react with oxygen at other sites, re-forming MnO₂ in a cycle that continuously damages the metal.
- Synergistic Effect with MOB and SRB: MRB often coexist with MOB and SRB within the same biofilm, contributing to complex redox cycling that enhances corrosion. For example, MOB create MnO₂, which MRB then reduce, generating localized acid production and further promoting MIC.
3. Mechanisms of Corrosion by MOB and MRB
The corrosion processes mediated by MOB and MRB involve intricate redox interactions that modify the electrochemical properties of metal surfaces, often leading to accelerated pitting and general corrosion. The following are key mechanisms by which MOB and MRB influence corrosion:
3.1 Redox Cycling of Manganese
In environments where oxygen levels fluctuate, manganese undergoes continuous redox cycling between Mn²⁺ and MnO₂. This cycling is driven by MOB under aerobic conditions and MRB under anaerobic conditions, creating zones of varying acidity and oxygen concentration on the metal surface. The alternation between oxidation and reduction weakens the metal over time, particularly when combined with iron and sulfur redox processes in mixed microbial communities.
3.2 Biofilm-Associated Corrosion
MOB and MRB form biofilms on metal surfaces, especially in water and soil environments. These biofilms create microenvironments where manganese oxidation and reduction can proceed independently of the bulk water chemistry, leading to localized corrosion. The biofilm matrix stabilizes MnO₂ deposits, which act as local cathodes, enhancing electron transfer and promoting pitting corrosion. The biofilm also serves as a scaffold that recruits other microbes, such as SRB, creating synergistic interactions that further accelerate corrosion.
3.3 Manganese Oxides as Catalysts
Manganese oxides produced by MOB act as potent catalysts for corrosion reactions. MnO₂ has a high surface area and a strong oxidizing potential, which facilitates the reduction of oxygen and enhances electron transfer from the metal surface. These reactions lead to localized acidification, further driving corrosion. Additionally, MnO₂ particles embedded in biofilms can create galvanic cells, with manganese oxides acting as cathodic sites and the underlying metal as the anode, resulting in an electrochemical imbalance that promotes metal dissolution.
4. Implications for Corrosion Control
The presence of MOB and MRB in environments susceptible to MIC suggests a need for corrosion management strategies that target these specific microorganisms. Strategies for controlling MOB and MRB include:
Biocide Treatment: Biocides that effectively target MOB and MRB can help reduce biofilm formation and manganese cycling. However, the selection of biocides must account for biofilm penetration and long-term efficacy, as biofilms can protect bacteria from chemical treatments.
Environmental Control: Altering environmental parameters such as pH, oxygen levels, and manganese availability can disrupt the redox cycling necessary for MOB and MRB activity. For example, controlling oxygen levels can suppress MOB growth, while limiting organic carbon sources can inhibit MRB activity.
Physical Cleaning: Regular mechanical cleaning of surfaces exposed to manganese-rich waters can remove biofilms and manganese oxide deposits, reducing the surface area available for microbial colonization. This approach is especially effective in water systems where sediment and biofilm accumulation contribute to corrosion.
Manganese Filtration and Removal: Filtration techniques that remove manganese from water can reduce the availability of this metal for microbial processes, thereby limiting the growth of MOB and MRB. This approach is particularly relevant for water systems where manganese content is high.
5. Conclusion
Manganese-oxidizing and -reducing bacteria are critical players in MIC, particularly in environments rich in manganese and other metals. Through their roles in manganese cycling, biofilm formation, and electrochemical modifications, MOB and MRB create microenvironments that accelerate material degradation. Understanding these microbial interactions is essential for developing targeted corrosion control strategies, including biocide treatments, environmental management, and physical cleaning. Future research should focus on advancing detection and monitoring techniques for MOB and MRB, as well as exploring innovative approaches to mitigate their impact on corrosion in industrial systems.
References
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- Little, B., & Lee, J. S. (2009). Microbiologically Influenced Corrosion. Hoboken, NJ: Wiley.
- Hamilton, W. A. (2003). Microbially influenced corrosion as a model system for the study of metal microbe interactions: A unifying electron transfer hypothesis. Biofouling, 19(1), 65-76.
