A critical evaluation of sample pretreatment for storage of contaminated sediments to be investigated for the potential mobility of their heavy metal load

The identification of the geochemical forms of heavy metals in contaminated sediments gives information on their availability. This requires the use of a geochemical speciation procedure such as the one developed by Tessier et al. (1979). In addition to the imperfections of these protocols, their results can vary depending on the technique used for the preservation of sediments which must be suited to the materials studied and to particularities of the investigation. This study was carried out on superficial river sediments, seriously polluted by Cu, Cd and Pb. Compared to fresh sediment, none of the drying methods studied (freeze-drying, air-drying and oven-drying at 105 degrees C) completely preserve the distribution of Cu, Pb, Zn and Cd in the various geochemical fractions of the sediment. The modifications depend directly on the quantities of metals present in the various fractions of the sediment, the effects being more marked when the quantity is smallest. This results in a decrease in metals in the exchangeable fraction and in those bound to carbonates under the action of atmospheric oxygen and a corresponding increase in the other fractions. To minimize this, freeze-drying and air-drying are satisfactory techniques which enable preservation of sediments representative of the environment.