Ferrous sulphate
The cheapest iron on the market, because it is a by-product of titanium dioxide and steel pickling rather than a manufactured coagulant. It comes with a condition attached: ferrous iron must be oxidised to ferric before it coagulates anything.
Ferrous sulphate heptahydrate, FeSO₄·7H₂O, is supplied as a pale green crystal containing 19–20% iron. Fe²⁺ does not form the insoluble hydroxide that coagulation depends on, so it must first be oxidised to Fe³⁺ — by dissolved oxygen at pH above 8.5, by chlorine at 0.64 mg Cl₂ per mg Fe²⁺, or by aeration.
Where that oxidation happens naturally — an aerated stream, a chlorinated stream, or a high-pH lime process — ferrous sulphate is the lowest-cost iron available. Where it does not, use ferric chloride or ferric sulphate instead and accept the higher price.
Specification
| Parameter | Heptahydrate crystal | Monohydrate powder |
|---|---|---|
| FeSO₄ content | ≥ 90% as FeSO₄·7H₂O | ≥ 88% as FeSO₄·H₂O |
| Iron as Fe | 19.0–20.5% | 30.0–32.0% |
| Ferric iron as Fe³⁺ | ≤ 0.5% | ≤ 1.0% |
| Free acid as H₂SO₄ | ≤ 1.0% | ≤ 1.5% |
| Insolubles | ≤ 0.5% | ≤ 1.0% |
| Appearance | Pale green crystal | Off-white to grey powder |
| Bulk density | 1.0–1.2 kg/L | 0.9–1.1 kg/L |
| Solubility at 20 °C | ≈ 265 g/L | ≈ 265 g/L as FeSO₄ |
Where it is genuinely the right product
| Duty | Dose | Why ferrous works here |
|---|---|---|
| Hexavalent chromium reduction, tannery and plating | 3.0 mg Fe²⁺ per mg Cr(VI), pH 2–3 | The reducing power of Fe²⁺ is the point — it reduces Cr(VI) to Cr(III), which then precipitates on pH lift |
| Sulphide precipitation in sewers | 2.0–3.5 mg Fe per mg S²⁻ | FeS precipitates directly; oxidation is not required |
| Phosphorus removal with aeration downstream | 2.0–3.0 mol Fe per mol P | Aeration oxidises the iron in situ at no extra cost |
| Lime softening co-dose | 50–200 mg/L | High pH oxidises Fe²⁺ rapidly with dissolved oxygen |
| Chlorinated potable stream | 20–100 mg/L | Free chlorine oxidises the iron as part of the existing regime |
| Soil and agricultural conditioning | — | An outlet for surplus stock; not a water treatment duty |
Handling
- Dissolve at 10–20% w/v in clean water. Solution is mildly acidic at pH 3–4 and oxidises in the make-up tank within a day or two, so make up daily and keep the tank covered.
- Materials: HDPE, polypropylene, PVC, FRP, rubber-lined steel. Not mild steel, not aluminium, not copper alloys.
- Storage: sealed bags, dry, out of sun, 5–35 °C. Shelf life 6–12 months sealed; the failure mode is surface oxidation and caking rather than loss of iron.
- Staining: same as any iron product — permanent and rust-coloured. Bund and wash down accordingly.
- Protection: goggles, gloves, P2 mask at the bag tip point.
Frequently asked questions
Why does ferrous sulphate need oxidation?
Coagulation depends on forming an insoluble metal hydroxide floc. Ferric hydroxide, Fe(OH)₃, is very insoluble; ferrous hydroxide, Fe(OH)₂, is far more soluble and does not form usable floc at normal pH. So Fe²⁺ has to become Fe³⁺ first — by dissolved oxygen above pH 8.5, by chlorine at 0.64 mg Cl₂ per mg Fe²⁺, or by aeration. Dosed into a neutral, unaerated stream, ferrous sulphate largely passes through and leaves the water tinted.
Is ferrous sulphate cheaper than ferric chloride?
Per kilogram of iron, usually yes, sometimes substantially, because it is a by-product rather than a manufactured product. Whether it is cheaper in use depends entirely on whether oxidation is free. If chlorine has to be bought to oxidise it, the saving disappears — 0.64 mg of chlorine per mg of iron adds up quickly at a 20 mg/L Fe dose.
How is ferrous sulphate used for chromium removal?
At pH 2–3, Fe²⁺ reduces hexavalent chromium to trivalent chromium at about 3 mg of Fe²⁺ per mg of Cr(VI). The pH is then raised to 8–9 and both the chromium hydroxide and the ferric hydroxide precipitate together, and are removed by flotation or settling. This is standard practice in tannery and metal finishing effluent treatment, and here the reducing chemistry is the whole purpose.
Heptahydrate or monohydrate?
Monohydrate carries about 31% iron against 19–20% for heptahydrate, so it ships less water and is more stable in storage. Heptahydrate dissolves faster and is more widely available. On a long freight route, monohydrate is normally the better economics.