Sodium Metabisulfite (SMBS) removes free chlorine through a chemical process known as reduction.
SMBS, which stands for Sodium Metabisulfite (Na₂S₂O₅), is frequently used as a pretreatment method in water systems, particularly for removing free chlorine.
The Chemical Mechanism
When SMBS is added to water, it first reacts with water to form Sodium Bisulfite (NaHSO₃):
Na₂S₂₂O₅ + H₂O → 2 NaHSO₃
The Sodium Bisulfite then reacts with the free chlorine present in the water. Free chlorine typically exists as hypochlorous acid (HOCl) or hypochlorite ion (OCl⁻), depending on the water's pH. The bisulfite reduces the chlorine compounds, converting them into chloride ions (Cl⁻), which are harmless at typical concentrations.
A common simplified reaction showing the removal of hypochlorous acid (a form of free chlorine) by bisulfite is:
NaHSO₃ + HOCl → NaCl + H₂SO₄
In this reaction, the chlorine (Cl) in HOCl is reduced from an oxidation state of +1 to -1 in NaCl (Sodium Chloride), while the sulfur in NaHSO₃ is oxidized. Essentially, the active chlorine is neutralized by being converted into a stable chloride salt.
Why SMBS is Used for Free Chlorine Removal
SMBS is a popular choice for removing free chlorine in various industrial applications. Based on the provided reference, Sodium Metabisulfite (SMBS) is often used as pretreatment to large RO systems for free chlorine removal. Removing chlorine is crucial before water enters sensitive equipment like reverse osmosis (RO) membranes, as chlorine can damage these components.
Beyond just removing chlorine, SMBS also acts as a biostatic. This means it can help inhibit the growth of microorganisms.
When considering alternatives for chlorine removal, Carbon filters are a common alternative for small systems. However, the reference highlights that SMBS has an advantage for larger systems due to a lower risk of bacteria growth that can cause biological fouling. Biological fouling can be a significant issue in water treatment systems, particularly in large installations like those utilizing large RO membranes.
Here's a comparison based on the reference:
| Feature | SMBS | Carbon Filters (for small systems) |
|---|---|---|
| Chlorine Removal | Effective for Free Chlorine Removal | Common Alternative for removal |
| Primary Use Case | Pretreatment for Large RO Systems | Common Alternative for Small Systems |
| Bacteria Growth | Lower risk of bacteria growth (Biostatic) | Higher risk of bacteria growth/biological fouling |
| Mechanism | Chemical Reduction | Adsorption / Catalytic Reduction |
In summary, SMBS removes free chlorine through a chemical reduction process, making it a suitable pretreatment for protecting sensitive systems like large RO membranes, where its biostatic properties and lower risk of bacterial fouling offer advantages over alternatives like carbon filters in those specific applications.
