Microbial Synthesis of Arsenic and Selenium Nanomaterials by Shewanella

Introduction to Selenium and Arsenic: Toxicity and Opportunity. Selenium, which readily substitutes for sulfur in biochemical pathways, becomes toxic at elevated concentrations but is vital in industries like electronics, glass manufacturing, and metallurgy. In the environment, selenium exists in several oxidation states, primarily as soluble selenate (Se(VI)) and selenite (Se(IV)) in oxidizing conditions, and as insoluble elemental selenium (Se(0)) in reducing environments. Similarly, arsenic is a highly toxic metalloid, with As(V) and As(III) being its most common and mobile forms. Its natural presence in aquifers is exacerbated by anthropogenic activities such as mining and industrial manufacturing. Traditional chemical precipitation methods for removing these contaminants from wastewater are inefficient, often generating non-recyclable sludge. However, their recovery as valuable nanomaterials presents a sustainable alternative that can offset remediation costs.

The Value of Biogenic Nanomaterials. The growing demand for rare earth elements in electronics fabrication generates expansive volumes of toxic waste, highlighting the need for innovative resource recovery. Microbially-synthesized electronic biological materials (e-biologics), such as arsenic and selenium nanotubes, represent a promising "green" solution for nanoscale fabrication. These biogenic nanomaterials possess high aspect ratios and unique size-dependent properties, making them valuable building blocks for advanced optical, optoelectronic, and electrochemical devices. Since microorganisms can act as inexpensive catalysts sourced from waste streams, they offer a sustainable route to harvest precursor arsenic and selenium and transform them into high-value e-biologics.

Mechanisms of Arsenic Reduction and Nanotube Synthesis in Shewanella. Shewanella species possess sophisticated genetic machinery to handle arsenic, coupling its reduction to energy generation or using it as a detoxification strategy. These bacteria encode two primary arsenate reductases: the respiratory Arr system, linked to energy conservation, and the resistance Ars system. The arr operon, typically consisting of arrA and arrB genes, includes proteins with iron-sulfur clusters and molybdenum cofactors essential for electron transfer. Under anaerobic conditions, specific Shewanella strains, such as HN-41, produce an extracellular network of filamentous arsenic-sulfide (As-S) nanotubes. These biogenic nanotubes, with diameters of 20-100 nm, exhibit valuable properties including electric conductivity, photoluminescence, and photoactivity.

Composition and Variability in Arsenic Nanotube Formation. The mineralogical composition of these As-S nanotubes is complex, primarily consisting of amorphous As2S3 nanofibers with an indirect optical band gap of 2.37 eV, alongside crystalline As8S9_x minerals previously thought to require high-temperature synthesis. While not all Shewanella strains produce these nanostructures, several, including S. alga BrY and S. putrefaciens CN-32, do so when provided with As(V) and thiosulfate. The rapid formation of nanotubes in these strains is likely due to the presence of highly active ArrA and ArsC arsenate reductases. In contrast, S. oneidensis MR-1, which lacks a full arr operon, exhibits a delayed and mechanistically distinct pathway for nanotube formation.

Selenium Nanoparticle Synthesis and Mercury Sequestration. Parallel to arsenic transformation, Shewanella species perform dissimilatory reduction of soluble Se(VI) and Se(IV) to insoluble elemental selenium (Se(0)), precipitating them as nanoparticles. This microbial synthesis of Se(0) nanospheres is a subject of significant interest for nanotechnology. A notable application is demonstrated by S. putrefaciens 200, which produces reactive amorphous Se nanospheres that can capture bacterially reduced Hg(0). This process results in the formation of stable HgSe nanoparticles, providing a novel strategy for mercury removal from aquatic environments without the risk of secondary pollution through methylation or volatilization.

Structural Characteristics and Challenges in Selenium Recovery. Biogenic Se(0) nanospheres produced by Shewanella are typically 0.2-0.3 µm in diameter, form extracellularly, and possess a compact, uniform monoclinic crystalline structure that is difficult to replicate through chemical synthesis. Furthermore, Shewanella sp. HN-41 can fabricate one-dimensional nanostructures like nanowires and nanoribbons in dimethyl sulfoxide (DMSO) solutions, with crystallinity and shape controlled by the Se/DMSO ratio. However, technological challenges remain, as these biogenic nanospheres contain organic impurities and exhibit colloidal properties that complicate their separation and recovery from treated wastewater, necessitating the development of novel harvesting methods.