Efficiency of Cd, Zn and Pb immobilization by soil amendments immediately after their application

Abstract

The objective of this work was to evaluate the effectiveness of amendments in a contaminated soil to be restored using phytostabilization, immediately after the application of the additives. Metal concentration in pore-water was determined as metal bioavailability parameter. Fractionation on carbonate and exchangeable phases were also performed as an additional parameter to evalúate metal bioavailability in soil. The studied soil originated from an abandoned open-cast mine in Toledo (Spain). Total Pb, Cd and Zn concentrations were higher than the threshold valúes of the Spanish legislation. Different additives were evaluated in a greenhouse experiment: two types of cyclonic ashl modified alumninosilicates originating from two different coal burning electricity piants using fluidised bed technology and steel shots, mainly consisting of iron oxides. Two weeks before sampling, pots were wetted at 50 % the water retention capacity with distilled water. Samples were taken after 15, 60, 100 and 205 days. Pore water extracts (Rhizon sampler) were analysed for determine Zn, Cd and Pb (ICP-AES). Soil samples were taken to evalúate pH, electrical conductivity total, exchangeable and carbonate adsorbed metáis. At the end of the experiment, piants that spontaneously colonised the soil were harvested to assess Zn, Cd and Pb (ICP-AES) in the biomass. Immediately after the application of the additives, metal concentration ¡n pore water and exchangeable phase decreased was confirmed by Zn and Cd absorption by the piants. All these parameters seem to support that the presence of cyclonic ash improves the association of Zn and Cd to the less mobile phases. Although the Pb in the pore water solution decreased and the exchangeable fraction and the fraction specifically adsorbed to carbonates did not increase in soils treated with additives, the valúes found in plant questioned that this metal was immobilised in soils.