Rhenium (Re)

1. Basic Information

2. Physical and Chemical Properties

Rhenium is a silver-gray metal with a very high melting point (3170°C), only below tungsten and carbon. It has a density of 20.5 g/cm3 at room temperature, making it one of the densest elements. Rhenium is resistant to corrosion but will slowly tarnish in moist air. In fine powder form, rhenium can burn in air. Rhenium forms compounds with varying degrees of oxidation, ranging from -1 to +7, with +7 being the most stable.

3. Presence in Water and Health Effects

Rhenium is rarely found in natural water due to its rarity in the Earth's crust. When present, it is usually in very low concentrations. The health effects of rhenium exposure are not yet fully understood due to its rarity and lack of extensive research. However, some studies suggest that rhenium salts may cause irritation to the eyes, skin and respiratory tract. Ingestion of large amounts may cause gastrointestinal tract irritation. The toxicological properties of rhenium have not been fully investigated, and more research is needed to understand its long-term health effects.

4. Water Treatment Applications and Removal Methods

Although rhenium is rarely a major contaminant in water treatment, there are several methods that can be used to remove it if needed:

• Ion Exchange: Specialized ion exchange resins can be used to remove rhenium species from water. Strong anion exchange resins have been shown to be effective for the removal of rhenium oxyanion complexes.
• Adsorption: Activated carbon or other specialized adsorbents can be used to remove rhenium from water.
• Membrane Technology: Reverse osmosis (RO) and nanofiltration (NF) can be effective in removing rhenium ions from water.
• Chemical Precipitation: In some cases, rhenium can be precipitated as insoluble salts and removed through sedimentation or filtration.

5. Industrial Use in Water Treatment

Rhenium itself is rarely used directly in water treatment due to its scarcity and high price. However, some rhenium-containing alloys may be used in corrosion-resistant components for water treatment equipment operating under extreme conditions.

6. Case Studies or Real World Application Examples

Although direct applications of rhenium in water treatment are limited, there are some examples of indirect uses:

• Industrial Wastewater Treatment: In industries that use rhenium catalysts, such as petroleum refining, wastewater may contain traces of rhenium. Specialized water treatment systems have been developed to recover rhenium from these waste streams, for both economic and environmental purposes.
• Desalination: Some components in high-pressure desalination systems may utilize rhenium-containing alloys to improve corrosion resistance in aggressive seawater environments.

7. Regulatory Guidelines and Standards

Currently, there are no regulatory guidelines or standards specific to rhenium in drinking water or wastewater in most countries. This is largely due to its rarity and lack of evidence of significant health effects. However, some countries may have restrictions for the release of rhenium in industrial wastewater, especially in areas with rhenium mining or processing activities.

8. Environmental Impact and Sustainability Considerations

The environmental impact of rhenium is relatively small due to its rarity. However, there are some considerations:

• Mining: Rhenium extraction, which is often a by-product of molybdenum and copper mining, can have localized environmental impacts.
• Recycling: Due to its scarcity and high value, there is a strong push to recycle rhenium from used equipment and catalysts.
• Emissions: Burning fossil fuels can release small amounts of rhenium into the atmosphere, which can eventually enter water systems through atmospheric deposition.

9. Future Trends and Research in Water Treatment Involving Rhenium

Several research areas and emerging trends involve rhenium in the context of water treatment:

• Rhenium Recovery: Development of more efficient methods to recover rhenium from industrial wastewater and processing streams.
• New Catalysts: Research on rhenium-based catalysts for the degradation of organic pollutants in water.
• Sensors: Development of rhenium-based sensors to detect specific contaminants in water.
• Corrosion Resistant Alloys: Research on new rhenium-containing alloys for applications in highly corrosive water environments.

10. Interesting Facts Related to Water Treatment

• Rhenium is one of the rarest elements in the Earth's crust, with an estimated abundance of only about 1 ppb.
• Although rare, rhenium has the third highest melting point of all elements, making it an attractive material for high temperature applications, including in geothermal water treatment.
• The isotopes rhenium-186 and rhenium-188 are used in cancer treatment, and research is underway to ensure that medical waste containing these isotopes does not contaminate water sources.
• Some types of bacteria can use rhenium in their metabolic processes, potentially paving the way for new bioremediation methods to remove rhenium from contaminated water.