Chromium Geochemistry: Eh-pH diagram-Toxicity-Stability Diagrams

Chromium 0.1 ppm with 150 ppm HCO3-SO4-- Brezinaite Formation Chromium 0.1 ppm with 150 ppm HCO3-SO4-- Brezinaite Formation

Chromium is highly insoluble in water. Chromium can only be in solution at a very low pH below 3.5. In this exercise we will generate Eh-pH diagrams to study stable and  soluble phases. Cr(VI) is much more toxic and mobile than Cr(III). So, speciation of chromium is very important. Hexavalent chromium exists in alkaline, strongly oxidizing environments while Trivalent chromium exists in moderately oxidizing and reduced environments.

Most common oxidation states: 0, +3, +6

  • 0:  Elemental Chromium (Cr)
  • +3: Trivalent Chromium:     Species: Cr+3, Cr2O3
  • +6: Hexavalent Chromium:     Species: CrO42-, Cr2O7-

Drinking Water Standards:

  • MCLG: 0.1 ppm
  • MCL: 0.1 ppm

Chromium content in sea-water varies strongly, and is usually between 0.2 and 0.6 ppb. Rivers contain approximately 1 ppb of chromium. Many chromium compounds are relatively water insoluble. Chromium (III) compounds are water insoluble because these are largely bound to floating particles in water. Hexavalent chromium is very toxic to flora and fauna.

Chromium 0.1 ppm: in pure water
Figure 1: Chromium 0.1 ppm-in pure water

In natural surface and ground water Eh is about 0.5 V. So, at this Eh, chromium is insoluble between 3 to 7. At higher pH chromate and chromite ions would in the solution.

Now, lets add 150 bicarbonate in the solution above. Lime application is common in Acid Mine Drainages where raised alkalinity helps iron to precipitate. Higher pH also makes Manganese insoluble. Ler’s see how alkalinity may affect chromium.

Chromium 0.1 ppm with 150 ppm HCO3-Eh-pH
figure 2: Chromium 0.1 ppm with 150 ppm HCO3-Eh-pH

Figure 2 clearly shows that alkalinity has no effect on the solubility of chromium. As you can tell there are no changes in the solid and dissolved species in the diagram.

Now, let’s add 300 mg/l sulfate in the solution to check effect of SO4– ion.

Chromium 0.1 ppm with 150 ppm HCO3-SO4-- Brezinaite Formation
Figure 3: Chromium 0.1 ppm with 150 ppm HCO3-SO4– Brezinaite Formation

Figure 3 introduces Brezinaite in to the diagram as solid phase. So, under highly reducing condition Cr3S4 (Brezinaite) mineral would precipitate.

Remediation:
Chromium removal from water can be done by  ion exchangers and active carbon can be applied for this purpose.

EPA Fact Sheet on Chromium

Chromium was regarded with great interest because of its high corrosion  resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steel.Chromium is the 21st most abundant element in Earth’s crust with an average concentration of 100 ppm.


About Editor
Ankan Basu is a Certified Professional Geologist (CPG) with 10+ years of experience in the field of geology, hydrogeology and geochemistry.

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