Silicon (Si)

1. Basic Information

2. Physical and Chemical Properties

Silicon is a metalloid with a strong metallic luster and is very brittle. It is usually tetravalent in its compounds, although it is sometimes bivalent. Pure silicon is an intrinsic semiconductor. Chemically, silicon is similar to metals and tends to be electropositive. It forms tetrapositive ions and various covalent compounds. Silicon can also form various series of hydrides, halides, and oxygen-containing compounds.

3. Presence in Water and Health Effects

Silicon is found in various natural dioxide and silicate forms in water. In drinking water, dissolved silica is usually present in the form of silicic acid. Silicon is non-toxic in its natural form and does not accumulate in certain organs of the body. However, chronic exposure to crystalline silica dust can cause silicosis and increase the risk of lung cancer. Some studies have also linked silica exposure to autoimmune disorders.

4. Water Treatment Applications and Removal Methods

Silica removal from water is important to prevent scaling of membrane systems and industrial equipment. Removal methods include:

• Ion exchange using specialized strong base anion resins
• Reverse osmosis
• Electrodeionization
• Precipitation with magnesium or aluminum at high pH
• Adsorption using activated alumina or metal hydroxide modified resins

For water with a high organic content, special fouling-resistant anion exchange resins can be used. However, the capacity of the resin for silica is limited due to its weak ionic nature.

5. Industrial Use in Water Treatment

Colloidal silica is used as a coagulant in water treatment. Silicon compounds are also used in anti-scaling and anti-corrosion formulations for industrial water systems. In the production of ultrapure water, silica removal to ppb levels is essential to prevent fouling of membrane systems and sensitive equipment.

6. Real World Application Case Study

A power generation plant in Europe experienced silica fouling problems in their reverse osmosis system. They implemented a silica selective ion exchange system using metal hydroxide loaded resin, which successfully reduced the silica concentration from 20 ppm to less than 1 ppm. This significantly improved the efficiency of the RO system and reduced the frequency of membrane cleaning.

In the Japanese semiconductor industry, silica removal down to ppb levels was achieved using a combination of reverse osmosis and electrodeionization. This enables the production of ultrapure water required for advanced chip manufacturing.

7. Regulatory Guidelines and Standards

There are no health standards for silica in drinking water, as it is considered non-toxic. However, some countries set maximum limits for silica in boiler feed water and cooling systems to prevent scale. For example:

• ASME: <150 ppb for high pressure boilers
• US EPA: <50 mg/L for drinking water (aesthetic guidelines)

8. Environmental Impact and Sustainability Considerations

Natural silica in water has no significant negative environmental impact. However, discharge of high silica-containing effluents from industrial processes can cause scaling in water bodies. Silica removal methods such as ion exchange generate waste salts that require proper management. More sustainable silica removal technologies, such as regenerable adsorbents, are being developed.

9. Future Trends and Research

Current research in silica removal includes:

• Development of silica-selective membranes
• Utilization of nanomaterials for more efficient silica adsorption
• New electrochemical methods for silica removal
• Biotechnological approaches using microorganisms or enzymes
• Optimization of hybrid processes that combine different technologies

The main focus is on improving the efficiency and reducing the cost of silica removal, especially for ultrapure water and seawater treatment applications.

10. Interesting Facts Related to Water Treatment

• Silica can help prevent lead pipe corrosion by forming a protective layer.
• Some studies suggest that silica in drinking water may provide health benefits, including better bone health.
• Diatoms, microscopic organisms with silica shells, are used as natural filtration media in water treatment.
• Silicon-based membrane technology is being developed for more efficient desalination.
• Silica gel, a synthetic form of amorphous silica, is used as an adsorbent in water treatment and air purification.