Principle
This concept can be execuded by using the following techniques:
Figure: Diagram of in-situ chemical oxidation ((http://www.clu-in.org/download/remed/chemox.pdf)
In-situ chemical oxidation is a technique aimed at the in-situ destruction of (organic) pollutants via the injection of an oxidation agent into the soil; normally into the water-saturated zone. The described oxidation agents that are used are permanganate, peroxide (Fenton’s reagents or modified Fenton’s), ozone, mixture of ozone and peroxide, per-carbonate and per-sulphate.
An example of in-situ oxidation is the destruction of perchloroethylene (PCE) with permanganate:
4KMnO4 + 3C2Cl4+ 4H2O ® 6CO2 + 4MnO2 + 4K+ + 12Cl- + 8H+
Implementation area and implementation conditions
In-situ chemical oxidation is suitable for treating core zones in most organic pollutants, such as chlorinated ethylenes, PAH's and oil components. It is not, or is less suited to compounds that are more difficult to oxidise, such as chloroalkanes and saturated aliphates (old, weathered oils) or debris-like pollution (agglomerates like tar fragments, paint residue etc.) (http://www.clu-in.org/download/remed/chemox.pdf).
This technique is namely suited to well or reasonably permeable soils with a ground-water level of >1 m-mv. If the soil is difficult to permeate, special dosage methods are possible, like ‘deep soil mixing” and/or “fracturing” techniques. Natural organic substances (OS) and/or reduced inorganic compounds such as Fe2+ can cause the required quantity of oxidising agents to fall sharply - influencing the matrix demand in the soil. The matrix demand should be determined in advance in a laboratory setting. An example of a ‘low’ matrix demand for permanganate is 0.5 kg potassium permanganate per m³ of wet aquifer material; a ‘high’ value is 10 kg per m³.
Other important design factors/boundary conditions are:
- Total waste content;
- Organic matter in soil and ground-water;
- Chemical oxygen demand (COD);
- pH of soil and ground-water;
- Permeability of the soil and the soil texture and structure;
- The Redox conditions of the soil and the ground-water (Eh, dissolved oxygen);
- Iron(II) content of the soil and/or the ground-water;
- Alkalinity of the soil and ground-water (carbonates and bicarbonates are collectors of free-radicals);
- Permanganate impurities (commercially available permanganate can contain heavy metals like chromium):
- In-situ oxidation of a pollution core can have an inhibitive effect on in-situ reductive bio-remediation in the plume.
- If oxidisable metals are present that become more mobile or toxic on being oxidised (like chromium), chemical oxidation may be detrimental;
- Fenton’s reagents and ozone are not very stable in soil/ground-water.
- Fenton’s reagents work optimally at pH 2-4 and can be implemented in a soil with a pH up to 7; permanganate works optimally at pH 7-8 (applicable within 5-9); ozone works optimally at a neutral pH;
- The negative effects can be: Gas production, considerabe heating (namely when peroxide is implemented), toxic by-products, reduction of all bio-mass (“bleaching” of the soil). It is more difficult to implement permanganate in a NAPL core due to the forming of strong manganate-oxide precipitation with possible “clogging”;
- It is difficult, or impossible to clean peat layers with in-situ oxidation
For further technical information about in-situ chemical treatments, we refer you to the following link: http://www.clu-in.org/download/toolkit/inchem.pdf.
Costs
An American reference states a cost price that varies between ca. 40 and 240 euros per m³ of to-be-cleaned soil volume (http://www.gwrtac.org/pdf/Atlanta/Totalv3_ppt_%5BRead-Only%5D.pdf).
According to a Dutch supplier, the costs amount to 30 to 80 euros/m³, with an average of 40-50 euros/m³. The costs of a prior laboratory-scale investigation, to determine the attainability of this technique, are around 2000 – 5000 euros; pilot tests cost between €30.000 and 40.000.
Table 3‑1: Comparison of cost price per gram oxidation
|
Oxidant |
Cost €/kg |
|
Fenton’s |
2,1 |
|
Calcium-peroxide |
2,1 |
|
Potassium permanganate |
4,1 |
|
Sodium permanganate |
16,4 |
|
Sodium-persulphate |
2,8 |
Environmental burden and measures to be implemented
With correct dimensioning (full use of the applied oxidant in the polluted soil zone) there is little environmental burden; permanganate is changed to the non-harmful manganate oxide: An oxide that is also naturally present in soils.
Fenton’s reagents can lead to gas development and heating, whereby pollutants may disperse uncontrollably. Thus, in-situ oxidation with Fenton’s reagents could be combined with soil air extraction to limit the risk.
If ozone is used, an energy consuming ozone generator is necessary. Also attention must be payed to prevent uncontrolled releas of ozon to the atmosphere., as this is toxic.
In general the necessary personal and environment protective measures must be taken when using strong oxidising chemicals.