In chemical oxidation, chemicals are added to the plume area to chemically oxidise contaminants into less hazardous or less toxic compounds which are also more stable and less mobile. In the case of organic compounds such as petroleum, they are converted into carbon dioxide and water. It is also possible, under the correct conditions and controls, to undertake oxidation as part of an above ground water treatment system.
The most common oxidising agents are hydrogen peroxide (H2O2), potassium permanganate (KMnO4), ozone (O3), and sodium persulfate.
Chemical oxidation can be conducted in-situ or ex-situ; chemical oxidants can be injected into the contamination plume beneath the capillary fringe or mixed into excavated soils and pumped groundwater.
Crystaline potassium permanganate is mixed onsite with abstracted groundwater to an appropriate concentration or is delivered in liquid form. The average life span in the plume is 0 to 3 months (low reactivity compared to Fenton's Reagent but has a longer life span); this oxidant is particularly suited to chlorinated hydrocarbons.
Fentons Reagent (Hydrogen Peroxide + Iron catalyst)
Hydrogen peroxide with iron catalyst, generally injected at concentrations between 5-50% peroxide, average life span <2days in plume (very reactive). Best results in acidic environments (plume may require pH adjustment).
A proprietary mixture supplied by Regenesis (a solid oxidising agent and a liquid activator), both parts are mixed onsite together with abstracted groundwater to a concentration of 3-8% and then injected. Average life span 1-4 weeks in plume (moderate reactivity compared to Fenton's Reagents). Fewer precautions required for storage and handling on site.
To reduce contaminant concentrations below the site specific target levels (SSTLs), multiple injections may be required and oxidation injections are often combined with multi-phase extraction to ensure efficient distribution of the oxidant. The technique relies upon ensuring physical contact between the oxidant and the contaminant, for these reasons efficient injection and distribution is essential.
Chemical oxidation is generally not suitable for low permeability soils such as clay, due to the limited injection capability and depending on the chemical oxidants used, there can be reactions between oxidant and the clay which can cause serious detrimental effects on the geophysical properties of the clay. Other environmental effects can be the oxidation of formation ions such as metals which may have detrimental effects on aquifer properties. This should be given thorough consideration.
Chemical oxidation is only applicable to contaminants that can be oxidised but is not generally suitable for free product or highly elevated concentrations. Suitable contaminants include but are not limited to:
- Petroleum range hydrocarbons (PRO)
- Diesel range hydrocarbons (DRO)
- Poly aromatic hydrocarbons (PAHs)
- Chlorinated solvents
- Some pesticides/herbicides
Chemical oxidation can be conducted in-situ or ex-situ. Ex-situ methods follow excavation, soil processing and mixing. In-situ chemical oxidation systems follow two general methods; direct push (single application), and abstraction-injection recirculation systems (permanent wells). In-situ process equipment includes:
- Chemical oxidant storage tanks
- Secure storage facility of chemicals
- Mixing tank
- Injection pump
- Abstraction pump
- Multi-phase extraction systems
- Direct push drill rig (direct push method)
- Pipework (recirculation)
- Control systems (recirculation)
- Abstraction-injection wells (recirculation)
- Groundwater storage tanks (recirculation)
- Dosing pump (recirculation)
Direct push has the advantage of simplicity and is cheaper than recirculation systems assuming only one injection of oxidant is required (often multiple injections required).
VertaseFLI conducted an ex-situ chemical oxidation of medium chain hydrocarbons that were not amenable to bio-remediation, with hydrogen peroxide. Contaminated material was mixed with a specialised excavator while hydrogen peroxide was injected into the mixing chamber. Approximately 1,500m 3 of contaminated soils was successfully remediated to below the site specific target levels (SSTLs).
Advantages and Limitations
- Proven track record
- In-situ (only requires a small area for unit setup)
- Fast reduction in contaminant concentrations
- Remediated wide range of contaminants
- Multiple applications can be required
- May require large volumes of chemical oxidant (cost)
- Low clay content preferred
- Potentially hazardous chemicals used (hydrogen peroxide, etc)
- Can affect aquifer properties
- Contamination rebound in treated areas can be a problem if plume is not fully delineated