There are two primary forms of silica commonly found in water: reactive silica and colloidal silica.
Understanding Silica in Water
Silica (silicon dioxide, SiO₂) is a ubiquitous compound found in rocks, sand, and soil. As water flows over and through the earth's crust, it dissolves and picks up silica, transporting it into various water sources. The form silica takes in water significantly impacts its behavior and how it interacts with systems that use the water.
According to the provided information, the two different forms of silica that end up in water are:
Reactive Silica
- Description: Reactive silica is a form of silica that is dissolved in water. It typically exists as silicic acid (H₄SiO₄) or its ionized forms, particularly at higher pH levels.
- Characteristics: It is described as a very weak acid that readily dissolves in water. Because it is dissolved at a molecular level, it passes through standard filters and is not visible as particles.
- Behavior: It behaves like dissolved salts and ions in water. Its concentration in water is often measured spectrophotometrically after reacting with molybdic acid.
- Implications: While dissolved, high concentrations of reactive silica can still cause issues, particularly in industrial processes like boiler feedwater or reverse osmosis, where it can precipitate out under certain conditions (e.g., concentration, temperature changes) forming scale.
Colloidal Silica
- Description: Colloidal silica consists of extremely small, non-dissolved silica particles suspended in water. These particles are larger than dissolved molecules but too small to settle out easily or be removed by simple filtration.
- Characteristics: It has a glass-like structure. These particles are discrete, solid spheres of silica.
- Behavior: Unlike reactive silica, colloidal silica remains suspended as stable particles due to their size and surface charge. They do not readily dissolve.
- Implications: Due to its abrasive and hard nature, colloidal silica can damage surfaces, glassware and fixtures. It is particularly problematic in sensitive industrial processes like semiconductor manufacturing or high-pressure boiler systems, as it can deposit and form very hard, difficult-to-remove scale. It requires specialized separation techniques like ultrafiltration or specific coagulants for removal.
Comparing Reactive and Colloidal Silica
Understanding the distinction between these two types is crucial for water treatment and industrial applications, as they require different analytical methods and removal techniques.
| Feature | Reactive Silica | Colloidal Silica |
|---|---|---|
| Form | Dissolved (molecular/ionic) | Suspended Particles |
| Structure | Monomeric/oligomeric silicic acid | Solid, glass-like spheres |
| Size | Very small (molecular level) | Small, sub-micron particles (nanometer scale) |
| Behavior | Acts as a weak acid, dissolves | Suspended, does not readily dissolve |
| Visibility | Invisible | Can make water appear turbid (at high concentrations) |
| Removal | Ion exchange, Reverse Osmosis (effective for removal of dissolved ions) | Ultrafiltration, specialized coagulation/flocculation, sedimentation |
| Impact | Forms scale upon concentration/temp changes | Abrasive, hard scale, can damage surfaces/fixtures |
In summary, while both contribute to the total silica content in water, reactive silica is the dissolved, ionic form, and colloidal silica is the suspended, particulate form, each presenting unique challenges and requiring distinct approaches for analysis and treatment.
