In-situ chemical oxidation or (ISCO) using hydrogen peroxide has emerged as a proven treatment technology for soil and groundwater. This technology is effective across a breadth of contaminants including: BTEX, MTBE, Chlorinated Solvents, Polyaromatic Hydrocarbons (PAHs), and Polychlorinated Biphenyls (PCBs). Hydrogen peroxide is a versatile chemical that can safely be used as part of a cost-effective approach utilizing either Fenton’s Reagent or Modified Fenton’s Reagent.
FMC has over 50 years of hydrogen peroxide experience and the most extensive network of plants and transloaders in North America. This means that product is always available regionally to support your needs.
FMC is dedicated to supplying the remediation sector and provides an array of services to assist you in utilizing hydrogen peroxide safely and efficiently in the field. Some of the services FMC provides:
- 8% and 17.5% hydrogen peroxide which eliminates the need for on-site dilution.
- A dedicated fleet of drop trailers, temporary tanks, and pumps that can quickly be mobilized for your site.
- Technical assistance with treatability and soil oxidant demand studies.
- Focused technical resources dedicated the environmental sector. For technical questions about chemical oxidation, please contact frank_sessa@fmc.com.
- A strong commitment to safety and Responsible Care®
- Availability of new one-way totes to eliminate drum handling
Fenton’s Reagent and Modified Fenton’s Reagent
Hydrogen peroxide is a powerful oxidant, but at low concentrations (<0.1%) its reaction kinetics are too slow to degrade many contaminants of concern. However, the addition of a ferrous (II) or ferric (III) iron dramatically increases the oxidative strength of peroxide. This increase is attributed to the production of hydroxyl radicals (OH•). In addition, a chain reaction is initiated, causing the formation of new radicals. The reaction of iron catalyzed peroxide oxidation at pH 3-5 is called a “Fenton’s Chemistry” after its discoverer H.J.H. Fenton. The iron/peroxide combination is known as “Fenton’s Reagent.” Fenton’s Chemistry was initially developed at peroxide concentrations of about 300 ppm (0.03%), when Fenton found that adding iron to a tartaric acid solution decomposed the acid. If the pH is less than 5, the iron (III) is reconverted to iron (II), via a side cyclic reaction, and the iron remains in solution to sustain the initiation of hydroxyl radical production.
The basic reaction for the application of Fenton’s Reagent is:
Radical initiation
H2O2 + Fe +2 à Fe +3 + OH- + OH*
Radical Propagation
OH* + RH à R* + OH-
R* + H2O2 à ROH + OH*
Radical Termination
OH* + CO3= (or soil) à HCO3- + OH-
Contaminant of Concern + OH* à CO2 + H2O
A classical Fenton’s system cannot be readily created in-situ, as it is generally too difficult to maintain a well-mixed, low-peroxide concentration in the subsurface. In practice, more concentrated solutions of hydrogen peroxide are injected, ranging 4%–20%, and either followed or preceded by an injection of acidic iron solution. Any deviation from the traditional low-concentration hydrogen peroxide/iron mixture is known as a “Modified Fenton’s System.” This includes the use of higher concentrations of H2O2 or calcium peroxide (CaO2), with or without chelating agents. This type of system is more complicated than traditional Fenton’s Chemistry, and the generation of other radical species has been proposed. Hydroxyl radicals are very strong oxidizing agents. When hydrogen peroxide is present in excess, radical-initiation and propagating reactions are supported, and more radicals are available to react with the contamination. In almost all cases, the intermediates that are produced in these reactions are more biodegradable than and less toxic than the parent compound.
An important side reaction also occurs resulting in the formation of precipitates—it involves the reaction of two end products of this chain reaction, hydroxide ions and Fe (III): Fe3+ + nOH- → amorphous iron oxides (precipitate). Therefore it is necessary to either lower the pH or use chelating agents to maximize the available iron (II). The optimal pH for non-chelated iron is acidic, ranging 3.5–5. Typical acids used to alter the subsurface pH include HCl, H2SO4, and acetic acid. However, organic acids have a tendency to increase side reactions that are undesirable in high-organic soils. Alternatively iron solubility can be increased by the use of a chelating agent. It should be noted that inorganic metal compounds present in the subsurface, manganese, for example, may exert a demand on the hydrogen peroxide and decrease the efficacy of radical initiation.
The exothermic characteristic of hydrogen peroxide, when controlled, can be very beneficial, enhancing desorption and dissolution of sorbed and nonaqueous-phase liquid (NAPL) mass. Thus making –the NAPL available for effective treatment by oxidation or mass transfer systems. However, when decomposition is not controlled, these same characteristics can cause migration of these contaminants. ?)
There are several special considerations that need to be addressed prior to using either Fenton’s Chemistry or Modified Fenton’s Chemistry in the field:
*A low pH can cause dissolved metal concentrations to temporarily increase in the groundwater.
*Excessive heat may be generated if strong solutions of hydrogen peroxide (>10%) are used.
*There is potential gas generation/volatilization of contaminants.
*Carbonate ions capture hydroxyl radicals and exert a strong demand on acids (H+ ions).
*As with all oxidants, the optimal oxidant loading, including both target and non-target compounds, should be determined before application. FMC can help determine this through a treatability study.