Principle
In-situ immobilisation is a technical intervention with which the chemical and/or physical properties of polluted materials or the soil matrix are changed in-situ, in order to immobilise pollutants in the soil. It can be done by injecting hardening substances or substances which cause the pollutant to form a stable chemical precipitation.
Technique 1: Cementation, chemical immobilisation
Techniques based on adding organic or inorganic bonding agents: fixation takes place via the injection or in-situ infiltration of organic or inorganic bonding agents which drastically reduce the permeability of the soil, whereby leaching to groundwater drops sharply or the pollutant’s dispersion speed decreases. Possible inorganic bonding agents or permeability-reducing substances include cement, bentonite, quicklime, gypsum… Pollution–absorbing substances can also be implemented (e.g. clay, zeolites, oxides…) in order to realise additional physical/chemical bonding.
Field of application and application conditions
Infiltration of hardening substances is primarily suitable for the immobilisation of inorganic compounds. Applicability first needs to be tested on a small scale (lab/pilot), whereby leaching tests (column) on mobilised testing material can help to determine the activity of additives as well as the required doses. Some additives, such as cement, can sharply increase pH, which in-turn increases the mobility of lead, arsenic and complex cyanides instead of lowering it.
Cementation c.q. lowering of the permeability, must also be investigated and organised in the long-term (e.g. ageing tests in the lab). The expected influence on hydrology must be carefully investigated in advance (modelling; long-term monitoring in pilot zone…).
A specific type of application is the in-situ bio-precipitation of heavy metals by injecting an organic substrate (see process 7 and 9).
Costs
The costs for in-situ immobilisation are greatly determined by location and pollutant-specific conditions. As an indication, one should consider a price similar to that of ex-situ immobilisation (€ 90/tonne).
Environmental damage and to-be-implemented measures
This is comparable with other injection techniques.