How does chromium end up in groundwater?
Chromium, evenly distributed in the earth’s crust, is extracted from chromite ore. Chromium with a valence of 6 is the most toxic species. Whilst Crˡˡˡ⁺ is an essential nutriment, Cr˅ˡ⁺ is reported to be carcinogenic.
Chromium has been widely used for many years in various industrial processes as a corrosion inhibitor, pigment and colour fixing agent. (For example, in cooling towers, electroplating and the textile industry).
Despite chromium-based compounds, practice tends to decrease drastically because of health and environmental concern. Groundwater of contaminated areas may still exhibit concentrations above the WHO guideline of 50 µg/l as total chromium.
How is chromium removed from water?
When heavy metals are present as anionic complexes like chromate (CrO₄²-) or dichromate (Cr₂O₇²¯), then Resinex™ A-4, a strong base anion resin can be used with a comparable efficiency as chelating resins. Chromate and dichromate are oxyanions of chromate (Cr˅ˡ⁺) and can be captured by a strong base anion (SBA). The quaternary-amino group of Resinex™ A-4 exhibits a very high affinity for chromate and dichromate ions. The removal mechanism is for one-way-use and is described by the reaction below:
2 R-N+(CH₃)₃ Cl- + CrO₄²¯ → 2 [R-N+(CH₃)₃] CrO₄²- + 2 Cl¯
2 R-N+(CH₃)₃ Cl- + Cr₂O₇²¯ → 2 [R-N+(CH₃)₃] Cr₂O₇²+ 2 Cl¯
With a sulphate inlet concentration below 200 mg/l, the operating capacity of Resinex™ A-4 is expected to be +/- 12 g as Cr per litre of resin.
Recommended chromium removal operating conditions
One AquaFlow™ can be used for the removal of chromium at a superficial contact time of a minimum of 6 minutes, but usually two AquaFlow™ in series, merry-go-round systems are used with a typical flow rate per vessel: 10-30 BV/h.
How do I size a chromium removal system?
Assume a contaminated groundwater with 0.5 mg/l as Cr of chromate (Cr˅ˡ⁺) with an average flow of 30m³/h operating 24/24 -7/7 then we have 0,5 mg/l x 30m³/h x 8600 hpa /(1000 mg/g) /(1000 g/kg) = 129kg. This gives at an average loading of 12kg Cr/m³ Resinex™ A-4 a consumption of approximately 11m³ Resinex™ A-4 per annum. An AquaFlow™ can then be selected from our industrial mobile filter range in order to obtain the desired exchange frequency.
We always recommend you contact our technical expert for sizing of a heavy metal removal system as other metals can interfere in the estimated consumption