Series part 3 – Chromium Reduction and Removal Methods

In Part 2 of this series, we discussed how metals can be removed from wastewater by altering their solubility and forcing precipitation via the addition of carbonate, hydroxide or sulfur-based compounds.  While this method works well for most, certain metals, like chromium require specialized treatment.   

Chromium is a naturally occurring transition metal found in the earth’s crust and most commonly present in the form of Trivalent Chromium “Cr(III)”.  Cr(III) is considered non-toxic and typically present in food and dietary supplements.  However, in hexavalent form, often referred to as Chromium VI (Cr(VI)), it is highly toxic and considered a carcinogen.   

 While Cr(VI) can occur naturally in trace amounts, its production and use is common within a variety of industrial process applications.  Some of which include electroplating, stainless-steel production, leather tanning, pigment production and textile processing.  Because Cr(VI) is so widely used, “in-house” and municipal wastewater treatment must focus on its conversion to a non-toxic form and remove it before environmental discharge.   

 In terms of treatment, if the desire is to recover and re-use Cr(VI), ion exchange or reverse osmosis can be used.  If precipitation and removal via filtration or sedimentation is the goal, Cr(VI) must be chemically reduced to Cr(III).     

 To reduce Cr(VI), it must receive electrons to lower its oxidation state from +6 to +3.  This electron transfer is achieved via the addition of a reducing agent.  Within wastewater treatment, reducing agents that are commonly used include the following: 

• Sodium metabisulfite or other derivatives (e.g., sodium bisulfite, sulfur dioxide, sodium thiosulfate) 

• Ferrous Sulfate  

 The decision to use one reducing agent over another often depends on several factors.     

• The volume of water to be treated 

• The concentration of Cr(VI) that needs to be removed 

• Product cost and availability 

• A plant’s ability to segregate a specific waste stream and treat it independently.    

In cases where high concentrations of Cr(VI) are present (e.g., electroplating or pigment production), waste stream isolation and treatment with sodium metabisulfite is preferred.  For streams that cannot be isolated or subject to lower overall, but variable concentrations (e.g., municipal or raw water), ferrous sulfate is a practical alternative. The following sections help describe the treatment process that is followed when each are applied.   

 Cr(VI) Reduction and Precipitation via the addition of Sodium-Metabisulfite 

 When using sodium metabisulfite to reduce Cr(VI) in wastewater, the following reactions occur: 

1. Na2S2O5 + H2O (l) → 2 NaHSO3 (aq) 
2. H2CrO4 (s) + NaHSO3 (s) + H2SO4 (s) → Cr2(SO4)3 (s) + 3 NaHSO4 (s) + 4H2O(s) 

This reduction process is most effective under acidic conditions, typically at pH 2 to 2.5. Sulfuric acid is used to lower the pH, after which metabisulfite is added to reach an Oxidation Reduction Potential (ORP) of around 200 mV. Once this ORP is achieved, the assumption is that over time, all Cr(VI) will have been reduced. The pH is then raised using sodium hydroxide or lime to precipitate Cr(III). 

Figure 1: Cr(VI) Reduction & Precipitation using Meta-Bisulfite

Cr(VI) Reduction and Precipitation via the addition of Ferrous Sulphate 

Using ferrous sulfate also involves a two-step process: the reduction of Cr(VI) to Cr(III) followed by a precipitation step to enhance its removal.   

When applied, ferrous sulfate introduces Fe(II) which oxidizes to Fe(III) and donates electrons to reduce Cr(VI) to Cr(III).  The simplified redox reaction(s) that takes place is as follows: 

The stoichiometric demand placed on Fe(II) and the reduction of Cr(VI) is 3:1. Due to inefficiencies that are a natural part of any treatment process (competing reactions, etc.), a safety factor is applied to that molar ratio.  Typically, a factor of 10 is used which results in the application of chemistry at a molar ratio of 30:1. From there, field testing can be used to better define the true demand that is present within the water.   

Ideally, these reactions work best in a pH environment that ranges between 7 and 9. Both for the reduction of Cr(VI) and precipitation of Cr(III).  If the removal of Cr(III) is insufficient and not meeting targeted needs, consider manipulating the pH or supplementally adding a precipitation aid.    

The diagram below provides an overview of how this treatment approach is typically laid out.    

Figure 2: Reduction and precipitation of Cr(VI) by ferrous sulfate 

If you’re dealing with chromium in wastewater, Aquasan’s team of experts are ready to help. From chemical sourcing to technical support, we provide reliable solutions to ensure effective metal removal and peace of mind. 

At Aquasan, your success is how we measure ours!