The Science behind the Technology

A buffered sodium dithionite (Na₂S₂O₄) solution is injected into the aquifer using wells.  Sodium dithionite dissolves in water producing two sodium (Na⁺¹) ions and a dithionite ion (S₂O₄^-2), which produces to sulfoxyl (SO₂^-1) ions:

Na₂S₂O₄ => 2Na⁺¹ + S₂O₄^-2 

S₂O₄^-2 => 2SO₂^-1

The sulfoxyl ions react with the iron-bearing minerals in the aquifer to transform Fe⁺³ [Fe(III)] to Fe⁺² [Fe(II)]:

SO₂^-1 + Fe⁺³ + H₂O => SO₃^-2 + Fe⁺² + 2H⁺¹

Dissolved chromium is precipitated as a byproduct of redox chemical reactions with iron-bearing minerals in the aquifer materials. Fe(II) donates one electron to Cr(VI).  In the process, Fe(II) is oxidized to Fe(III) and the Cr(VI) is reduced to Cr(III).  The net chemical reaction is:

3Fe⁺² + CrO₄^-2 + 5H⁺¹ => 3Fe⁺³ + Cr(OH)₃ + H₂O

The ISRM treatment zone also rapidly depletes the dissolved oxygen in the aquifer: 

4Fe⁺² + O₂ + 4H⁺¹ => 4Fe⁺³ + 2H₂O

Click on the References tab to learn more about these chemical reactions under Redox Chemistry. Review the EPA report on In Situ Treatment.