Formation of non-bioavailable organic residues in soil: Perspectives for site remediation

Abstract

Whenever possible, total clean-up of soils and sediments should have priority over methods to contain the pollutants in the soil environment in a way which reduces their potential eco-toxicological effects. Nevertheless, often a very important fraction of the pollutant remains non-available to the cleaning process, either physico-chemical or biological. This constitutes a major obstacle for both environmental technologists and legislators. Yet, the concept of non-extractable organic residues is well accepted in the EU-legislation for pesticides. In this context, an assessment is made to bind organic pollutants to soil. Physical sorption (comprising surface adsorption, absorption and migration in micro- and nanopores) and chemical binding are examined in terms of quantities and kinetics. Chemical binding offers at present no direct possibilities for practice. Making toxic pollutants less bioavailable by increasing physical sorption represents a pragmatic approach to contractors and regulators. For organic pollutants with acceptable concentration in the soil solution of the order of 1 mg/l, a sorptive loading of the order of 10 000 mg pollutant per kg activated carbon respectively organic matter appears a workable assumption. In case of toxic substances such as pesticides which have a 1000 times lower acceptable level, a sorptive loading of up to 10 mg organic pollutant per kg sorbent can be used. Non-bioavailable pollutants can be considered as representing no direct harm to the environment. In practice, the application of up to 100–200 kg dry weight quality compost per ton dry weight soil or alternatively the supplementation of other sorbents such as powdered activated carbon (up to 100 kg per ton soil) offer possibilities to cost-effective remediation of organic pollutants. Yet, aspects of worst-case ecotoxicology as e.g. excessive leaching with dissolved humic substances or ingestion of soil containing substantial amounts of poorly extractable contaminants by human and soil organisms, need to be examined.