Selenium

Selenium is a trace mineral that the human body needs in very small amounts, but it plays a big role in keeping us healthy. It is a non-metallic element found in Earth's crust and is chemically related to sulfur and tellurium. In the Periodic Table, Selenium has the symbol “Se” and atomic number 34.

Even though we only need a little bit of selenium, it’s essential for many biological processes, including metabolism, immunity, and protection from cell damage.

1.0Discovery and History 

Selenium was discovered in 1817 by the Swedish chemist Jöns Jakob Berzelius. He found it while studying impurities in sulfuric acid. Initially, scientists thought it was a form of tellurium, but later realized it was a new element.
Since then, selenium has been studied for its chemical versatility and biological significance in living organisms.

2.0Occurrence and Extraction 

Selenium is not found free in nature. Instead, it occurs in small amounts in sulfide ores such as copper pyrites (CuFeS₂) and galena (PbS). It is often recovered as a by-product during the refining of copper and nickel.

Extraction Process:

1. Selenium is obtained from anode mud formed during the electrolysis of copper.
2. The mud is roasted with sodium carbonate.
3. The resulting compound is treated with water to form selenium dioxide (SeO₂).
4. Finally, SeO₂ is reduced using sulfur dioxide gas to produce pure selenium.

3.0Physical Properties of Selenium

Selenium exists in three main forms: amorphous (red), crystalline gray, and black selenium. The grey crystalline form conducts electricity better when exposed to light, making it photo-conductive. This unique property is used in photocells and solar cells.

4.0Chemical Properties of Selenium

Selenium exhibits multiple oxidation states: −2, +2, +4, and +6. It reacts differently depending on temperature and the reactant.

Reactions:

• With oxygen: Se + O₂ → SeO₂ (Selenium dioxide – a white solid)
• With hydrogen: H₂ + Se → H₂Se (Hydrogen selenide – a toxic gas)
• With metals: Forms metal selenides such as ZnSe, CdSe, etc.

Selenium dioxide dissolves in water to form selenious acid (H₂SeO₃), a weak acid.

5.0Biological Importance of Selenium

Selenium is essential for both humans and animals. Though needed in trace amounts, it has significant roles:

• Antioxidant Defense: Selenium forms part of the enzyme glutathione peroxidase, which protects cells from oxidative damage caused by free radicals.
• Thyroid Function: It helps regulate thyroid hormone production.
• Immunity: Boosts the body’s defense system.
• Reproductive Health: Important for fertility and healthy sperm function.
• Heart Health: Adequate selenium levels may reduce the risk of heart disease.

However, excessive selenium intake can be toxic. The recommended daily intake for humans is approximately 55 micrograms (µg).

Sources of Selenium

Here are some natural food sources rich in selenium:

Selenium content in plant-based foods depends on the selenium concentration in the soil where the plants are grown.

6.0Deficiency of Selenium

A lack of selenium in the diet can cause several health problems:

• Weakened immunity
• Muscle weakness or pain
• Fatigue
• Poor thyroid function
• Keshan disease – a type of heart disease found in selenium-deficient regions of China

7.0Toxicity of Selenium

While selenium is essential, excessive intake can cause selenosis, a condition that leads to:

• Hair and nail loss
• Skin rashes
• Garlic-like breath odour
• Nervous system problems

The safe upper limit for adults is 400 µg/day.

8.0Uses of Selenium

Selenium isn’t just biologically important — it’s also valuable in industry:

• Glass Manufacturing: Used to remove green tint from glass.
• Electronics: Selenium’s photoconducodour
• Pigments: Used in paints, plastics, and ceramics.
• Alloys: Improves the strength and machinability of metals.