PERSULFATE
Persulfates are strong oxidants that have a wide variety of industrial applications. Dissolution of a persulfate salt such as sodium persulfate (Na2S2O8) in water liberates persulfate anion (S2O8-2), which is an oxidizer:
Na2S2O8 (solid) → 2Na+ + S2O8-2
S2O8-2 + 2H+ + 2e– → 2HSO4–
The persulfate anion has an oxidation potential of 2.1 V. In addition to direct oxidation by the persulfate anion, the persulfate anion can be catalyzed to produce a persulfate radical (SO4•–):
S2O8-2 + initiator → SO4•– + (SO4•– or SO4-2)
The persulfate radical is a more powerful oxidizer than the persulfate anion, and has an oxidation potential of 2.6 V. Commonly used activators to catalyze persulfate are heat, hydrogen peroxide, transition metals, and high pH. For economical and technical reasons the most commonly used activators in the environmental industry for in-situ chemical oxidation are transition metals and high pH. With high pH activation (also referred to as alkaline activation), sodium persulfate can also generate hydroxyl radicals:
S2O8-2 + 2H2O → HO2– + S2O4-2 + 3H+
HO2– + S2O8-2 → SO4•– + SO4-2+ H+ + O2•–
SO4•– + OH– → OH• + SO4-2
Activated sodium persulfate (ASP) chemistry is similar to catalyzed hydrogen peroxide chemistry with regards to the oxidants and reductants being produced, the rapid oxidation of organics, and rate constants for reactions with common environmental pollutants. However, ASP is not as exothermic and the amount of off-gassing produced is negligible. With ASP oxidation, the end products are primarily carbon dioxide and sulfate. Unconsumed sodium persulfate naturally degrades to sulfate and sodium after injection.