CATALYZED HYDROGEN PEROXIDE
Catalyzed hydrogen peroxide (CHP) is a solution of hydrogen peroxide and a ferrous iron catalyst, which together generates hydroxyl free radicals that act as the active oxidizing agent (referred to as the Haber-Weiss mechanism, specific to Fenton’s reagent). The basic radical-producing mechanism is characterized as:
(1) H2O2 + Fe+2 → OH• + OH– + Fe+3
where H2O2 is hydrogen peroxide, Fe+2 is ferrous iron, OH• is hydroxyl free radical, OH– is hydroxyl ion, and Fe+3 is ferric iron. CHP chemistry is complex, involving a number of additional reactions producing both oxidants and reductants that contribute to contaminant destruction:
(2) OH• + Fe+2 → OH– + Fe+3
(3) Fe+3 + H2O2 → H+ + HO2• + Fe+2
(4) Fe+2 + HO2• → Fe+3 + HO2–
(5) Fe+3 + HO2• → Fe+2 + O2 + H+
(6) OH· + H2O2 → H2O + HO2•
where HO2• is hydroperoxyl radical, HO2– is hydroperoxyl anion, O2 is molecular oxygen, H+ is hydronium ion, and H2O is water. Additional reactions occur with organic compounds. The suite of reactions associated with CHP is complex but very effective at destroying many organic compounds dissolved in groundwater, sorbed to soil, or existing as non-aqueous phase liquids in the subsurface. CHP is generally most efficient under acidic pH conditions (pH <5) because oxidation of iron (from Fe+2 to Fe+3) by other reactions is minimized, hydrous ferric iron oxides are less likely to precipitate and remove iron from solution, and bicarbonate (which competes with the organic compounds for hydroxyl radicals) is absent.
The hydroxyl radical generated by CHP is the most powerful oxidant utilized for environmental remediation purposes and has an oxidation potential of 2.8 V. Oxidation of an organic compound by CHP is a rapid and exothermic (but controllable) reaction. Rate constants for reactions of hydroxyl radicals with common environmental pollutants are typically in the range of 107 to 1010 M-1s-1, and 100% mineralization is generally complete in minutes. Intermediate compounds are primarily naturally occurring carboxylic acids. The end products of oxidation are primarily carbon dioxide and water. None of the injected reagents pose an environmental hazard. Unconsumed H2O2 naturally degrades to oxygen and water after injection.
Once the radical-producing reaction (equation 1) occurs, the lifetime of the hydroxyl radical is short (nanoseconds). Thus, the key to an effective in-situ treatment with CHP is to stabilize and achieve distribution of the H2O2 through the formation, before the reaction shown in equation 1 occurs. Distribution of H2O2 in the subsurface is achievable under stabilized conditions.
Oxidative or Reductive Systems…
The suite of reactions associated with CHP (reactions 1‐6) is generally considered an oxidative system, and hydroxyl radical production (reaction 1) is the predominant reaction. However, research has demonstrated that compounds such as carbon tetrachloride, which is not reactive towards OH•, are also destroyed. Superoxide radicals, likely formed by reaction 6, have been identified as the reactive reductant species responsible. Superoxide radical is also likely responsible for enhanced desorption of organic compounds from soil‐sorbed phases and from DNAPL phases, making them more susceptible to degradation by either reductive or oxidative processes.
The Geo-Cleanse® Process has been developed with this as a key objective, and after 20 years of success, Geo-Cleanse continues to refine and improve the method. Due to the rate of reaction and oxidizing power, CHP is generally most effectively applied as a source reduction technology. Its advantage is to destroy significantly contaminated groundwater, NAPLs and sorbed-phase source areas and to achieve approximately 85-95% contaminant mass destruction.