Sodium Metabisulfite for Chromium(VI) Reduction and Metal Treatment

Hexavalent chromium (Cr(VI)) is toxic and tightly regulated, and the standard way to remove it from wastewater is to reduce it to the far less harmful trivalent form, Cr(III), and then precipitate it as the hydroxide. Sodium metabisulfite (CAS 7681-57-4) is one of the reagents used for that reduction. This article covers the chemistry, the process, and how metabisulfite fits in metal-finishing, leather and mine-water treatment.

1. The problem with Cr(VI)

Cr(VI) – as chromate and dichromate – is a carcinogen and a serious aquatic pollutant, so industrial effluents from electroplating, metal finishing, leather tanning and some pigment and mine operations must have it reduced and removed before discharge. Cr(III), by contrast, is far less harmful and can be precipitated as insoluble chromium hydroxide.

2. The reduction chemistry

Under acid conditions, sulfur dioxide (generated when metabisulfite dissolves) reduces Cr(VI) to Cr(III):

Metabisulfite dissolves to give bisulfite and SO₂ in solution.
In acid (typically about pH 2 – 3), the SO₂/bisulfite reduces Cr(VI) to Cr(III).
The pH is then raised (typically to about 8 – 9) so the Cr(III) precipitates as chromium hydroxide, which is separated as sludge.

Stage	What happens	Typical pH
Reduction	Metabisulfite (as SO₂/bisulfite) reduces Cr(VI) to Cr(III)	About 2 – 3 (acid)
Neutralisation / precipitation	Cr(III) precipitates as hydroxide	About 8 – 9
Separation	Sludge is settled, filtered or floated off	–

3. Dosage

The dose is set by the stoichiometry of the reduction – a fixed number of moles of SO₂ per mole of Cr(VI) – plus an excess to drive the reaction to completion. In practice:

Measure the Cr(VI) first (for example by titration or a colorimetric method) so the dose is based on the actual load.
Add a controlled excess (a small margin) to ensure complete reduction; monitor the residual Cr(VI) and the oxidation-reduction potential (ORP).
Control pH at the reduction stage (acid) and at the precipitation stage (alkaline).

Because the exact dose depends on the Cr(VI) concentration, the acid used and the target discharge limit, it must be set for your waste stream – not copied from a book value.

4. Where metabisulfite is used for Cr(VI)

Industry	Use
Electroplating and metal finishing	Reduction of chromate in rinse and process wastewater
Leather tanning	Treatment of chrome-bearing effluent
Pigments and surface treatment	Reduction of chromate-bearing waste
Mine water and smelter effluent	Reduction of Cr(VI) before discharge or reuse
Wood treatment	Reduction of chromium in treated-wood effluent

5. Other metal and reducing uses

Dechlorination – the same reagent reduces free chlorine in water and wastewater (see the dechlorination guide).
Cyanide destruction – in gold plants (see mining use).
Metal-ion reduction – metabisulfite is a general reducing agent for other oxidised metal species in effluent treatment.

6. Process control and safety

Parameter	Why it matters
Cr(VI) measurement	Sets the dose; the key process variable
ORP	Confirms the reduction is complete
pH	Acid for reduction, alkaline for precipitation
Sludge handling	The chromium hydroxide sludge is a regulated waste

Acid + metabisulfite releases SO₂. Do not mix metabisulfite with acids outside a controlled, ventilated system – sulphur dioxide is toxic.
Corrosive solid. Handle with dust control and PPE; keep away from acids.
Very toxic to aquatic life in the raw form. Store and handle to prevent release.

Frequently Asked Questions

Is metabisulfite used to reduce Cr(VI)?

Yes – under acid conditions it reduces hexavalent chromium to trivalent chromium, which is then precipitated as the hydroxide and separated.

What pH is used?

Reduction is typically run at about pH 2-3; the pH is then raised to about 8-9 to precipitate the Cr(III).

How much metabisulfite is needed?

The stoichiometric requirement plus an excess, set by the measured Cr(VI) load. Confirm it for your waste stream rather than using a fixed figure.

Is the same reagent used for dechlorination?

Yes – sodium metabisulfite is also the standard dechlorination reagent and is used for cyanide destruction in gold plants.