The currently available processes for soil remediation can be divided into five generalised categories:
Biological methods involve the transformation or mineralisation of contaminants to less toxic, more mobile, or more toxic but less mobile, forms. This can include fixation or accumulation in harvestable biomass crops, though this approach is discussed more fully later.
The main advantages of these methods are their ability to destroy a wide range of organic compounds, their potential benefit to soil structure and fertility and their generally nontoxic, ‘green’ image. On the other hand, the process end-point can be uncertain and difficult to gauge, the treatment itself may be slow and not all contaminants are conducive to treatment by biological means.
Toxic compounds are destroyed, fixed or neutralised by chemical reaction. The principal advantages are that under this approach, the destruction of biologically recalcitrant chemicals is possible and toxic substances can be chemically converted to either more or less biologically available ones, whichever is required. On the downside, it is possible for contaminants to be incompletely treated, the reagents necessary may themselves cause damage to the soil and often there is a need for some form of additional secondary treatment.
This involves the physical removal of contaminated materials, often by concen-tration and excavation, for further treatment or disposal. As such, it is not truly remediation, though the net result is still effectively a clean-up of the affected site. Landfill tax and escalating costs of special waste disposal have made remedi-ation an increasingly cost-effective option, reversing earlier trends which tended to favour this method. The fact that it is purely physical with no reagent addition may be viewed as an advantage for some applications and the concentration of contaminants significantly reduces the risk of secondary contamination. However, the contaminants are not destroyed, the concentration achieved inevitably requires containment measures and further treatment of some kind is typically required.
Solidification is the encapsulation of contaminants within a monolithic solid of high structural integrity, with or without associated chemical fixation, when it is then termed ‘stabilisation’. Vitrification uses high temperatures to fuse contami-nated materials.
One major advantage is that toxic elements and/or compounds which cannot be destroyed, are rendered unavailable to the environment. As a secondary benefit, solidified soils can stabilise sites for future construction work. Nevertheless, the contaminants are not actually destroyed and the soil structure is irrevocably dam-aged. Moreover, significant amounts of reagents are required and it is generally not suitable for organic contaminants.
Contaminants are destroyed by a heat treatment, using incineration, gasifica-tion, pyrolysis or volatisation processes. Clearly, the principal advantage of this approach is that the contaminants are most effectively destroyed. On the nega-tive side, however, this is achieved at typically very high energy cost, and the approach is unsuitable for most toxic elements, not least because of the strong potential for the generation of new pollutants. In addition, soil organic matter, and, thus, at least some of the soil structure itself, is destroyed.