Hexavalent Chromium Reduction to Cr(III): The Reduction-Precipitation Route

Why can't hexavalent chromium be precipitated directly?

Conventional heavy-metal removal relies on raising pH to precipitate a sparingly soluble metal hydroxide. That route fails for Cr(VI) because chromium in the +6 oxidation state does not exist as a hydrolysable cation. In water it speciates as oxyanions — chromate (CrO₄²⁻), hydrogen chromate (HCrO₄⁻) and, at higher concentration and lower pH, dichromate (Cr₂O₇²⁻):

2 HCrO₄⁻ ⇄ Cr₂O₇²⁻ + H₂O

These anions are highly soluble across the entire practical pH range, are not retained by adding alkali, and Cr(VI) is a confirmed carcinogen with tight discharge limits (often ≤0.1 mg/L for the hexavalent fraction). The distinction matters because the trivalent form, Cr(III), behaves in the exact opposite way: it is a hard, hydrolysing cation that is essentially insoluble as its hydroxide over the mid-pH range. The whole treatment strategy therefore hinges on a single oxidation-state change — move chromium from an anion that alkali cannot touch to a cation that alkali removes almost quantitatively.

The only robust removal path is to change its oxidation state. Cr(VI) must first be reduced to Cr(III), a cation that hydrolyses and precipitates as Cr(OH)₃. Chromium is the classic exception covered alongside cadmium, nickel and zinc in our wider heavy metals removal and compliance guidance.

What is the reduction chemistry and reagent stoichiometry?

The workhorse reductant is sulphur(IV): sodium metabisulphite (SBS, Na₂S₂O₅), sulphur dioxide (SO₂) or sodium bisulphite. In water SBS hydrolyses to bisulphite, which supplies the electrons:

Na₂S₂O₅ + H₂O → 2 NaHSO₃

The half-reactions at low pH are the Cr(VI)/Cr(III) couple (3 electrons per Cr) and the sulphite/sulphate couple (2 electrons per S). Balancing electrons gives 3 SO₃²⁻ per 2 Cr — the governing redox equation below.

The ferrous alternative uses Fe(II) as a 1-electron donor, so 3 mol Fe²⁺ reduce 1 mol Cr(VI):

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

Ferrous sulphate is cheap and effective but generates roughly three moles of Fe(III) hydroxide sludge per mole of Cr, sharply increasing sludge volume. SBS keeps the sludge almost entirely chromic, which is why it dominates in plated-metal and tannery effluent.

Mechanistically, the electron transfer proceeds through a chromium(VI)–sulphite ester intermediate (HCrO₃·OSO₂⁻) that decomposes to give the unstable Cr(V) and Cr(IV) states before the final two-electron step lands at Cr(III). Because those intermediates are short-lived only under strong acidity, the reaction that is near-instantaneous at pH 2 becomes a multi-hour crawl at neutral pH — the chemistry, not just the equilibrium, demands the low-pH window discussed below.

How much sodium metabisulphite is needed per mg of Cr(VI)?

Convert the redox ratio to a mass dose. Iorungwa et al. (2012) confirmed the reaction is first-order in both Cr(VI) and metabisulphite and consumes S(IV) at close to the stoichiometric 3:2 ratio, with a modest excess needed in practice to drive kinetics and cover competing oxygen demand.

Derivation of the constant: 3 mol S(IV) per 2 mol Cr means 1.5 mol S(IV) per mol Cr. One SBS delivers 2 S(IV), so 0.75 mol SBS per mol Cr. In mass terms: 0.75 × 190.1 / 52.0 = 2.74 mg SBS per mg Cr(VI). Compare with the ferrous route: 3 mol Fe per mol Cr × 278.0 g/mol FeSO₄·7H₂O / 52.0 = 16.0 mg copperas per mg Cr(VI) — nearly six times the reagent mass and far more sludge.

Why does the reduction need low pH and ORP control?

The Cr(VI)/Cr(III) reduction potential is strongly pH-dependent — the Nernst expression carries a large proton term, so acidity is both a thermodynamic driver and a kinetic accelerant:

The reduction rate law reported by Iorungwa et al. (2012) is second-order overall — first-order in Cr(VI) and first-order in metabisulphite — with a strong inverse dependence on hydrogen-ion activity:

−d[Cr(VI)]/dt = k[Cr(VI)][S(IV)][H⁺]ⁿ

with the proton order n typically between 1 and 2. Physically this means the reaction half-life scales by roughly an order of magnitude for each pH unit removed: a batch that clears in seconds at pH 2 may take tens of minutes at pH 4 and effectively never finish at pH 6. This is why reactor retention and acid dosing are designed together — skimping on acid forces oversized tanks and reagent excess to compensate.

The practical window is pH 2–3 (usually set with sulphuric acid). Above pH ~4 the reaction slows dramatically and bisulphite is instead lost to autoxidation by dissolved oxygen. Because the reagent is dosed to an oxidation state rather than a concentration, the natural control handle is ORP (redox potential), not a colorimeter. Fully oxidised Cr(VI) liquor sits around +700 to +1000 mV (vs Ag/AgCl); complete reduction to Cr(III) with a slight sulphite excess pulls ORP down to roughly +250 to +350 mV. Yokogawa (2021) and Hach both recommend cascaded control: a pH loop trimming acid to hold ~2.5, and an ORP loop trimming SBS to hold the reduction setpoint. The two loops interact — acid demand rises as reduction proceeds because protons are consumed — so the ORP setpoint is validated against a Cr(VI) grab test during commissioning and re-checked whenever feed strength shifts.

Worked example: SBS dose and ORP setpoint for a plating line

A chrome-plating rinse discharges Q = 12 m³/h at C_Cr = 85 mg/L Cr(VI). Design the reduction stage.

• Cr(VI) mass load: 12 m³/h × 85 g/m³ = 1,020 g/h = 1.02 kg Cr(VI)/h.
• Theoretical SBS: 1,020 g/h × 2.74 = 2,795 g/h ≈ 2.80 kg SBS/h.
• Applied SBS (F = 1.2): 2.80 × 1.2 = 3.36 kg SBS/h to cover kinetics and DO demand.
• Acid demand: 5 mol H⁺ per 2 mol Cr = 2.5 mol H⁺/mol Cr. Cr load = 1,020/52.0 = 19.6 mol/h → 49 mol H⁺/h ≈ 2.4 kg H₂SO₄/h from the redox alone, plus whatever is needed to pull the raw liquor to pH 2.5.
• Control setpoints: pH loop holds 2.5 ± 0.2; ORP loop holds +300 mV (Ag/AgCl). A rising ORP means unreduced Cr(VI) is breaking through — the SBS pump ramps up.

At a 12 m³/h flow the reduction reactor is sized for ≥20 minutes of retention (about 4 m³) to guarantee complete conversion before the liquor overflows to the neutralisation/precipitation tank. Sizing the acid, reagent and buffer volumes for a specific line is exactly the kind of detail we firm up during a chromium treatment design review.

How is Cr(III) precipitated, and what residual is achievable?

Once ORP confirms full reduction, caustic (or lime) raises the pH into the Cr(OH)₃ solubility minimum. Chromic hydroxide is amphoteric, so there is a distinct optimum — too little alkali leaves Cr³⁺ in solution, too much re-dissolves it as chromite (CrO₂⁻). The choice of alkali is not neutral: lime (Ca(OH)₂) is cheap and adds calcium that helps sweep and densify the floc for faster settling, but it produces more sludge and can scale dosing lines; sodium hydroxide gives a cleaner, lower-volume precipitate at higher reagent cost. Whichever is used, the alkali demand includes not only the reduced liquor but the residual acid dosed upstream, so the neutralisation load is often several times the hydroxide needed for chromium alone.

Worked residual at pH 8.5: [OH⁻] = 10^{−(14−8.5)} = 10^−5.5 = 3.16 × 10⁻⁶ M. Then [Cr³⁺] = 6.3 × 10⁻³¹ / (3.16 × 10⁻⁶)³ = 6.3 × 10⁻³¹ / 3.16 × 10⁻¹⁷ = 2.0 × 10⁻¹⁴ M — vanishingly small, <10⁻⁸ mg/L in theory.

The theoretical figure is never met in practice. Three effects lift the real residual: (i) fine Cr(OH)₃ colloids that resist gravity settling and pass a clarifier; (ii) chromium held in soluble organic or ammine complexes that shift the effective K_sp; and (iii) a broad, flat solubility trough that punishes small pH excursions on the alkaline side, where re-dissolution as chromite begins. Real effluents therefore settle at 0.05–0.5 mg/L total Cr, so a sand or membrane polishing filter, or a co-precipitation with ferric coagulant to sweep the colloids, is fitted where the consent is tight. See how these numbers map onto UK permit values in our note on effluent discharge standards in the UK.

How is the resulting chromium sludge classified and handled?

The precipitate is a Cr(III) hydroxide floc. Critically, once reduction is complete and verified, the sludge contains trivalent chromium — far less toxic and mobile than Cr(VI). However, incomplete reduction, or later re-oxidation, can leave leachable Cr(VI), so classification hinges on a leaching test (e.g. WAC / TCLP-type analysis) rather than on the process intent alone.

Because the dewatered cake is usually a hazardous waste on chromium content, minimising sludge mass matters: the SBS route (chromic sludge only) generates a fraction of the volume of the ferrous route (chromic + ferric sludge). A single guard against re-oxidation is worth designing in: hold a small residual reductant demand through the precipitation stage so that any trace of atmospheric or nitrate-driven re-oxidation is immediately mopped up, and confirm with a periodic Cr(VI) leachate test on the cake rather than assuming the reduction held. Where the site aims to recover or recycle water, the reduced, precipitated stream is the point at which chromium is removed from the balance before downstream consent-driven effluent treatment.

Reduction-precipitation sequence for Cr(VI)

1. Acidify to pH 2–3. Dose sulphuric acid under pH control to reach the low-pH window that makes reduction fast and thermodynamically favourable.
2. Dose reductant to an ORP setpoint. Feed sodium metabisulphite (or ferrous sulphate) trimmed by an ORP loop to about +250 to +350 mV, confirming full Cr(VI) to Cr(III) conversion.
3. Provide reaction retention. Hold the liquor 15–30 minutes in a mixed reactor so reduction completes before overflow. Verify with a Cr(VI) grab test.
4. Raise pH to 8–9. Add caustic or lime under pH control to the Cr(OH)3 solubility minimum, avoiding over-dosing that redissolves chromium as chromite.
5. Clarify and dewater. Settle or filter the Cr(III) hydroxide floc; polish if the consent is tight; dewater the sludge and classify by leaching test.