Analytical Procedures for the Analysis of Heavy Metals in Contaminated Soils

Abstract

Heavy metals are held in soils by a number of mechanisms, the measurement of which is usually defined by the extraction procedure. The system is even more complex in contaminated soils, where the metal may be present as relatively insoluble and immobile inorganic or organic forms. The setting of threshold or trigger values for heavy metals in contaminated soils has usually been based on a measurement of total metal. There are three aspects of such measurements which are important to consider. 1. A measure of total amount of metal in a soil gives no indication of its mobility within the soil-plant-water system. It is usually justified on the basis of the risks from dust inhalation or the ingestion of soil (pica). 2. There is a variety of digestion methods in use which determine variable amounts of metal depending on the forms of metal present and the soil type. 3. In addition to a measure of total metal content, some measurement of plant availability / mobility / solubility is required. Contaminated soil is a term used to cover a wide range of materials, including soil which has developed naturally, but which has become contaminated by atmospheric deposition or from a specific source such as an industrial process or sewage sludge application, and a solid waste product which has been dumped and is being considered as a soil, such as mining spoil or metal processing waste. In the first case, metals added to the soil may often be in solution, or readily soluble, and will be taken up by a range of sorption sites in a soil. These may vary from those sites from which the metal ions may be easily removed e.g. cation exchange sites to those where reversibility is difficult e.g. sorption onto oxide surfaces or chelation by hwnified organic matter. In some cases, soil conditions may cause the metal to be precipitated as a new solid phase compound. The chemistry of these soil reactions is reasonably well understood and the behaviour of different soil types predictable. Where the heavy metal is present as a solid waste product which has been dumped and is being treated as a soil, the behaviour will be very different and will depend on the solubility of the various components of the waste. Prediction of the chemical behaviour in such material may be more uncertain and solubility less related to the total metal content. The ICRCL guidelines used in u.K. (lCRCL, 1987) provide a list of contaminant threshold values for total toxic metals (As, Cd, Cr, Hg, Ph, and Se) and total phytotoxic metals (Cu, Ni, Zn) which if exceeded mean that the site should be considered contaminated. These guidelines do not, however, derme the analytical methods by which these metals are to be measured. Measurement of the total metal content of a soil requires that the solid matrix be dissolved completely. This is usually achieved by heating with a strong oxidising acid or more commonly a mixture of acids. As soils are made up largely of silicate minerals, this acid mixture should include hydrofluoric acid if the silicate component is to be fully digested and brought into solution. This is not done in many procedures because of the safety hazards associated with using hydrofluoric acid. Standard methods 731 W. J. van den Brink, R. Bosman andF. Arendt (eds.), Contaminated Soil '95, 731-732.