Ferrous Sulphate for Hexavalent Chromium Reduction in Industrial Effluent

In Kenya's specialized industrial clusters—such as the leather tanning estates along the Athi River industrial corridor and metal electroplating facilities in Nairobi and Mombasa—discharging untreated heavy metal effluent is strictly regulated under NEMA's Environmental Management and Co-ordination (Water Quality) Regulations. Hexavalent chromium (Cr VI, present as chromate CrO₄²⁻ and dichromate Cr₂O₇²⁻) is a potent carcinogen, cellular mutagen, and toxic water contaminant. Because Cr(VI) is completely soluble across standard pH ranges, it cannot be removed by simple alkaline precipitation; it must first be chemically reduced to trivalent chromium (Cr III).

Stage 1: Acidic Chemical Reduction

The reduction of Cr(VI) by bivalent iron is intensely pH-dependent. At neutral or alkaline pH, the reaction is sluggish and incomplete. Industrial effluent treatment plants (ETPs) operate a continuous two-stage reaction sequence:

In the reduction tank, effluent pH is depressed to 2.0–3.0 using concentrated sulphuric acid (H₂SO₄). A 10% solution of Ferrous Sulphate Heptahydrate is introduced with high-shear mechanical agitation:

3Fe²⁺ + HCrO₄⁻ + 7H⁺ → 3Fe³⁺ + Cr³⁺ + 4H₂O

Bivalent iron donates three electrons per chromium atom, converting soluble yellow Cr(VI) into green trivalent Cr³⁺ while oxidizing itself to ferric iron (Fe³⁺). In automated plants, an Oxidation-Reduction Potential (ORP) probe controls chemical feed: as Cr(VI) is consumed, the ORP reading drops from >500 mV down to +250 to +300 mV, signaling complete reduction.

Stage 2: Alkaline Precipitation and Flocculation

The reduced effluent overflows into the neutralization and precipitation tank, where caustic soda (NaOH) or hydrated lime (Ca(OH)₂) is dosed to elevate pH to 8.5–9.2:

Cr³⁺ + 3OH⁻ → Cr(OH)₃↓
Fe³⁺ + 3OH⁻ → Fe(OH)₃↓

At pH 8.8, chromium hydroxide achieves minimum solubility (< 0.05 mg/L). Crucially, the co-precipitating ferric hydroxide flocs act as a natural coagulant aid, entrapping the fine chromium hydroxide particles and forming dense, rapidly settling flocs that drop cleanly in secondary clarifiers.

Stoichiometry, Dosing Ratios, and NEMA Compliance

Stoichiometrically, reducing 1 kg of Cr(VI) requires 3.22 kg of elemental iron, corresponding to approximately 16.0 kg of commercial 98% Ferrous Sulphate Heptahydrate. In industrial wastewater containing background organics, a safety factor of 1.2 to 1.4 is applied, requiring 18 to 22 kg of FeSO₄·7H₂O per kg of hexavalent chromium.

With proper clarifier retention and polymer dosing (0.5–1.0 ppm anionic polyacrylamide), total chromium in treated discharge drops below 0.5 mg/L, comfortably exceeding the NEMA statutory ceiling of 1.0 mg/L total chromium.

KIKI'S Industrial Chemicals supplies bulk Ferrous Sulphate Heptahydrate 98% Min in 50 kg heavy-duty woven polypropylene bags with heat-sealed polyethylene inner moisture liners (Net Wt 50.000 kg, Kaiser Exports India, Lot VP/KE/26/27/05, marked NBO) with same-day Nairobi delivery to Athi River and industrial zones.