Fact sheet: Chemical Oxidation with Hydrogen Peroxide (Fenton Reaction)—in situ

From: Public Services and Procurement Canada

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Description

In situ hydrogen peroxide oxidation treatment refers to the injection of liquid hydrogen peroxide into a contaminated soil and/or aquifer in order to convert contaminants into non-toxic compounds, mainly water vapour and carbon dioxide.

The hydrogen peroxide oxidation reaction is relatively slow and this technique alone is not enough to thoroughly degrade organic compounds. However, when combined with a catalyst such as ferrous iron (Fe2+), the hydrogen peroxide oxidation potential is increased substantially. Hydroxyl radicals (OH?) produced from the decomposition of hydrogen peroxide due to ferrous iron are highly reactive, non-specific oxidants. The catalyst can be part of the soil matrix or added as a solution.

The hydrogen peroxide oxidation reaction combined with a catalyst ferrous iron at a pH between 2.5 and 3.5 is known as the “Fenton reaction.”  Fenton’s reaction is pH dependent, that is, hydroxyl radical production is more effective under acidic conditions than in alkaline environments. However, stabilizers are currently being developed to improve the effectiveness of oxidation at higher pHs.

Hydrogen peroxide oxidation is suitable for a wide range of contaminants such as petroleum hydrocarbons, phenolic compounds, TCA, PCE, TCE, DCE, VC, BTEX, chlorobenzene, 1,4-dioxane, MTBE and tert-butylalcohol (TBA).

Internet links:

Implementation of the technology

In situ chemical oxidation (ISCO) with hydrogen peroxide consists of injecting a solution of hydrogen peroxide in the soils and/or groundwater using injection wells, infiltration trenches, soil mixing or, is used to bring a strong oxidant in contact with the contaminant. The objective is to bring the oxidant in contact with contaminants. For halogenated compounds intermediate compounds may be formed. Multiple injection events (usually two or three) are often required. The primary issue in chemical oxidation is the distribution of treatment media in the subsurface.

Implementation of hydrogen peroxide oxidation projects may include:

  • bench scale or pilot tests. Laboratory scale tests are generally required to determine optimal parameters such as oxidant quantities and concentrations. On-site pilot tests are used to determine the type of injection wells, the radius of influence and spacing of the injection wells, the concentrations and injection flow rates of oxidant solutions;
  • mobilization, site access and temporary facilities;
  • reagent delivery and storage, which could entail such measures as:
    • groundwater well installation;
    • infiltration trench/drain construction;
    • injection of aqueous treatment solutions;
    • deep soil mixing with solid or slurry reagents.
  • environmental monitoring;
  • decommissioning of installed equipment (including groundwater wells);
  • repeated injections may be required (in two or three phases).       

Hydrogen peroxide may also be added as sludge (calcium peroxide) when a longer reaction time is required, for example in low permeability formations, to allow for more uniform distribution.

Materials and storage

  • The reagents used in conjunction with hydrogen peroxide include:
    • iron-catalyzed for the production of hydroxyl radicals (such as ferrous sulfate ferric sulfate);
    • acid chelants in order to keep the iron in aqueous solution (such as citric acid); Chemical stabilizers in order to slow down the oxidant’s chemical decomposition (such as phosphate).
  • On-site storage may include strong peroxide solution, acids and/or bases, iron catalyst and chemical additives (chelating agents and stabilizers). Proper separation and containment is very important.
  • In some instances, oxidant reactivity with subsurface contaminants, including unexploded munitions and explosives, may be sufficient to result in combustion.
  • Solid and liquid oxidants must be handled and stored safely.

Waste and Discharges

  • Complete mineralization of organic compounds leaves carbon dioxide, water, and inorganic ions (like chloride). In some instances, complete mineralization doesn’t occur.
  • Hydrogen peroxide leaves few residuals other than oxygen. In some instances, there may be concerns with respect to the presence of an oxygen-rich vapour where there are significant quantities of a combustible contaminant (such as petroleum hydrocarbons).
  • Individual treatment amendments may leave individual residuals (detailed descriptions of ingredients or residuals for “proprietary” formulations can sometimes be difficult to obtain; certificates of analysis should be provided by the vendor).
  • System installation typically requires drilling or excavating in contaminated areas, resulting in the handling and disposal of contaminated soils, typically containerized and disposed of off-site. 
  • There is potential for contaminated or oxidant rich groundwater out of the treatment zone. Properly designed injections do not result in uncontrolled flow of oxidants or contaminated groundwater along preferential pathways.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Organic matter content
  • Metals concentrations
  • Concentration of oxidant-consuming substances includes:
    • natural organic matter not considering the contaminants
    • reduced minerals
    • carbonate
    • other free radical scavengers
  • Reaction parameters include:
    • kinetic
    • stoichiometry
    • thermodynamic parameters
  • Contaminant concentrations present in the following phases:
    • adsorbed
    • dissolved
    • free

Physical analysis

  • Soil granulometry
  • Presence of non-aqueous phase liquids (NAPLs)
  • Soil buffering capacity

Recommended trials for detailed characterization

Chemical trials

  • Evaluation of the matrix oxidant demand

Notes:

On-site treatment trials will establish the efficiency of the technology and the parameters that influence the treatment time and cost (e.g. residence time, pump flow rate, requirements for pre-treatment, etc.).

Physical trials

  • Vapour survey
  • Evaluation of the radius of influence
  • Evaluation of operating pressure/vacuum

Hydrogeological trials

  • Tracer tests

Notes:

Tests examining the effect of temperature change on hydraulic conductivity and establishing the zone of freezing with a pilot scale tubing system are recommended to properly design the full-scale containment system.

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of receptors:
    • presence of potential environmental receptors
    • presence of above and below ground infrastructure
    • the risk of off-site migration

Phase III

  • Soil stratigraphy
  • Identification of preferential pathways for contaminant migration
  • Volume of contaminated material to treat
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Applications

  • Allow treatment of saturated zone by injecting hydrogen peroxide into the groundwater.
  • Vadose zone treatment is more complex than the saturated zone, especially of the soil is very permeable as the oxidant requires sufficient contact time with the contaminant for the oxidation to occur.
  • Optimal performance in acidic environments with a pH ranging from 2 to 4.

Applications to sites in northern regions

In situ chemical oxidation (ISCO) is potentially applicable to remote northern sites, however, significant impediments to material transport and injection equipment mobilization must be overcome. Given the high cost of mobilizing reagents and equipment, ISCO may be pursued on a one-time basis with the objective of reducing concentrations to levels amendable to monitor natural attenuation or another alternative. Northern applications require climate-appropriate design, including consideration of permafrost and of seasonal changes in ground conditions.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Applies
Control
Does not exist
Dissolved contamination
Applies
Free Phase
Does not exist
Physical
Does not exist
Residual contamination
Applies
Resorption
Applies
Thermal
Does not exist

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Does not exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
With restrictions
Chlorobenzenes
Applies
Explosives
With restrictions
Metals
Does not apply
Monocyclic aromatic hydrocarbons
With restrictions
Non metalic inorganic compounds
Does not apply
Pesticides
With restrictions
Petroleum hydrocarbons
Applies
Phenolic compounds
Applies
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
With restrictions

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Does not apply

Notes:

Contaminant rebound in groundwater frequently observed following ISCO treatment necessitates multiple treatment campaigns. 

Long-term considerations (following remediation work)

  • Oxidants are consumed relatively quickly and do not persist long enough to penetrate low permeability materials. At many sites, contaminants continue to diffuse out of untreated low permeability structures, such as silt or clay lenses, for many years. 
  • Soil sterilization post oxidation is not a major issue, as repeated investigations have demonstrated that microbial communities quickly recolonize the treatment areas.

Secondary by-products and/or metabolites

The chemical reaction that occurs with the application of this technique releases significant quantities of dissolved oxygen, which can improve biodegradation and natural attenuation.

By-product formation can be a concern if complete reaction cannot be obtained; benchtop and/or pilot testing, as well as strict quality control for injected materials, is typically required. 

Oxidation products are usually (but not always) less toxic, more mobile and more biodegradable than parent compounds. For example, MTBE may degrade into acetone of tert butyl formate. Petroleum hydrocarbons may generate acetone or alcohols. Explosives (RDX and HMX) may create elevated levels of nitrate.

Compared to other oxidants, hydrogen peroxide has a low potential to generate by-products.

Limitations and Undesirable Effects of the Technology

  • Soil permeability and matrix heterogeneity influence the migration of oxidants, hence, the efficiency of the technology.
  • For optimum efficiency pH must usually range from 2 to 5. However, some peroxide oxidation can be performed under neutral conditions with the appropriate catalysts.
  • Heat release from the exothermic reaction of oxidation may increase the volatilization and/or desorption of contaminants.
  • Chelating agents may be needed to increase iron availability.
  • Prior to ISCO treatment, free phase hydrocarbons must be removed.
  • High concentrations of oxidant consumers (such as organic matter) may increase the required quantity of oxidants and subsequently increase costs.
  • This treatment requires storing and manipulating dangerous materials.
  • Chemical oxidation deliberately causes extreme changes in geochemistry and creates a strongly oxidizing environment (to the extent that incompatible conduit, pipe or other underground materials can be damaged).
  • Metals can be mobilized within the treatment zone due to a change in oxidation states and/or pH (such as chromium). Low (acidic) pH is common. Note that metals generally are attenuated by various mechanisms within a short distance of the injection sites.
  • Non-target organic carbon may be mineralized, changing the fate and transport of hydrophobic compounds (contaminants may be de-sorbed as total organic carbon is mineralized; in some cases this increases the effectiveness of the oxidant application by making more contaminants accessible for destruction).
  • Injected fluids may displace (or “push”) contaminated pore water ahead of the injection front, leading to short-lived but dramatic changes in the distribution of groundwater contamination.
  • Oxidation reactions are exothermic. The Fenton process is highly reactive and produces high levels of CO2 and Oxygen gas bubbles and due care must be used to prevent buildup of underground pressures, damage to underground utilities, fire hazard, or the mobilization of contaminated soil vapour along preferential pathways such as utility trenches. 
  • Spills of oxidants and/or improper mixing of incompatible chemicals must be avoided. Oxidation can ignite flammable materials and thermal decomposition can release oxygen and heat to intensify the blaze. Explosions can occur if a gas-evolving reaction is contained or if incompatible materials (like hydrogen peroxide and potassium permanganate) are mixed.
  • Concentrated hydrogen peroxide solutions and high in situ reduced iron concentrations can create dangerous conditions. In isolated early applications of Fenton’s reaction, contaminants were volatilized and expelled into the air in blowback from injection points and/or in blowouts and soil fractures. Occurrences of gas evolution and heat mobilizing flammable hydrocarbon vapours were also observed. If strong acids are employed, additional accident scenarios are of concern.
  • Fenton’s typically requires a low pH and is infeasible in hard, well-buffered solutions. Carbonate rock may preclude application unless chelants are used (“modified Fenton’s”).

Complementary technologies that improve treatment effectiveness

  • The addition of stabilizing agents, such as phosphates, can improve hydrogen peroxide mobility and dispersion, which increases the zone of influence around the injection well
  • Oxidation using hydrogen peroxide or the Fenton’s reagent can be combined with ozone. A combination of oxidizing agents reduces processing time and increases the potential for complete degradation of petroleum hydrocarbons. A number of proprietary formulas are available, and several of them combine oxidants with oxygen-releasing compounds to stimulate aerobic post-oxidation bioremediation.
  • Free phase contaminants should be removed prior to chemical oxidation treatment. The use of heat and / or co-solvents (tertiary butyl alcohol, acetone) to increase the dissolution of LPNA has proved its worth.

Required secondary treatments

  • In situ chemical oxidation can be followed by a polishing biological step to treat residual contamination.
  • The installation of a soil vapour extraction system may be required, especially when the treatment is applied under or near a building due to risks of vapour intrusion.

Application examples

Application examples are available at these websites:

Performance

  • Potential for complete decontamination in a short period of time.
  • Production of volatile organic compounds (VOCs) is limited.
  • Automated injection systems can be used to control the procedure.

Measures to improve sustainability or promote ecological remediation

  • Schedule optimization for resource sharing and fewer days of mobilization.
  • Use of renewable energy and energy-efficient equipment (such as geothermal or solar energy for reagent delivery).
  • Evaluate source of oxidants.
  • Use of gravity injection techniques.
  • Use of groundwater for on-site chemical solution preparation.
  • Evaluate delivery options by rail (for large volume of oxidants) rather than trucks.
  • Use of recyclable bulk solution containers.

Potential impacts of the application of the technology on human health

Unavailable for this fact sheet

References

Author and update

Composed by : Serge Delisle, Eng. M.Sc., National Research Council

Updated by : Karine Drouin, M.Sc., National Research Council

Updated Date : March 1, 2009

Latest update provided by : Marianne Brien, P.Eng., Christian Gosselin, P.Eng., M.Eng., Golder Associés Ltée

Updated Date : March 22, 2019

Version:
1.2.1