Oprensning af tungmetalforurenet jord

English Summary

Based on the reports from the counties to the Danish EPA's ROKA-database, contamination with heavy metals is registered on 21 % of the contaminated sites, which makes contamination with heavy metals the second most common type of contamination after oil and gasoline (30 %). Looking at the reported frequency in groundwater, heavy metals are registered on 6 % of the sites. In groundwater heavy metals are thus the fifth most common contamination (or the next to least most frequent). Soil contaminated with heavy metals is therefore not a common source of groundwater contamination. On the other hand heavy metals are often found in elevated concentrations on extended areas, including areas not registered as contaminated sites.

As a basis for the assessment of the need for support of remedial methods for soil contaminated with heavy metals, available information concerning the extent of heavy metal contamination in Denmark has been evaluated as a part of the present project. The evaluation showed that in the order of 65 mill. ton heavy metal contaminated soil may be in need of some sort of remediation. Of this amount app. 10,5 mill. ton is located on contaminated sites, the rest being made up of diffuse contamination. The main part hereof (app. 80 %) consists of lead contaminated soil connected to the traffic infrastructure. The evaluation also shows, that at present app. 1/4 mill. ton of heavy metal contaminated soil is handled per year.

For the time being there a no Danish facilities or commonly used methods for the remediation of soil contaminated with heavy metals. The soil is therefore often disposed of in landfills. This type of handling of the soil causes adverse impacts on the environment, since it requires excavation and transport of the soil without improving the quality of the soil. lt also takes up disposal capacity at the landfills.

Soil contaminated with heavy metals is one of the focal points of the »Programme for Technology Development« set up by the Danish EPA to support development and technology transfer in connection with the remediation of contaminated soil and groundwater. In the programme it is stated that to be able to minimise the amount of soil contaminated with heavy metals that is disposed of in landfillls, methods for remediation of this type of soil should be developed, e.g. particle separation, extraction and soil washing or stabilisation techniques such as solidification or vitrification.

The present project has been started with the purpose of a systematical evaluation of possible methods for remediation of soil contaminated with heavy metals. A more detailed description of the evaluation method used, is given in chapter 2.

Remediation of soil contaminated with heavy metals is primarily based on an alteration of the mobility of the metals. Either the mobility is enhanced by the increased removal of the metal fraction dissolved in the pore water, thus shifting the equilibrium between the dissolved and the sorbed/precipitated phase, or by chemical manipulation of the sol environment (e.g. by biological means). Or else chemical manipulation of the speciation of the metals is used to increase the binding or the availability of the metals, thus rendering the metals less environmentally hazardous.

The remedial methods studied in the project are grouped in to 4 main groups: electroreclamation, phytoremediation, stabilisation plus soil washing and extraction. Both in situ methods and methods used on excavated soils are described. The methods studied are chosen because they are or are nearly commercially available, if not in Denmark than in Europeo Canada or the US. The 4 main methods are described in chapter 3 through 6.

Each of these chapters begins with a technical description of the method, the principle processes and the main types, the method can be divided into. Then a description of the state-of-the art is given. The third section gives an overview of obtained remediation results and the conditions influencing the efficiency of the clean-up. Quantitative information is given wherever possible. The clean-up is given as a percentage, since it is found to be the best way to illustrate the influence of different relevant factors (including the initial concentration) on the efficiency of the method. As far as information is available, data on costs is given in the fourth section, including economy, time demand and energy consumption. In the fifth section the evaluation is summarised, and the environmental impact, the technical state and capability of the method is commented. All references are listed in section 6 of each chapter.

An overall summary is given in chapter 7 together with an evaluation of the relevance of each method under Danish conditions. Common references are given in Chapter 8.

Based on the estimate of amount and types of soil contaminated with heavy metals in Denmark, relevant remedial methods should be able to:

Methods focusing on high concentrations of single metals (or metal mixtures) utilising harsh chemical (e.g. acid extraction) or physical (e.g. vitrification) manipulation, thus seem to be less relevant.

Of the studied methods both electroreclamation and phytoremediation seem to have a potential. Soil washing will especially be relevant for sandy soils, maybe utilising electroreclamation to further clean up the resulting fraction of fines. A further development of more environmental friendly extractants would be a great improvement of the possibilities to clean up lead contaminated soil.

Similarly a development of specific stabilisation techniques could contribute to the reduction of the environmental impact from possible remaining and difficult removable concentrations of single metals in an otherwise treated mixture. The 2 last mentioned methods are in an early development phase, while the other relevant methods are closer to a practical implementation.