Technical report POSIDON Project

EDTA, which is one of the most commonly applied chelants, can directly promote the Pb extraction by forming strong complexes with Pb and indirectly by enhancing the reductive dissolution of the oxides. The results of the batch experiments showed that the combination of reducing agents with EDTA increased the overall Pb extraction efficiency from the S2.3 Trieste soil sample. The extent of the increase in extraction efficiency was dependent on the type of soil sample and reducing agents. In fact, EDTA-Pb extraction from the S2.3 Trieste soil was slightly increased by the addition of oxalate, but it was greatly increased by the addition of sodium dithionite. The type of reducing agent seemed to greatly affect the Pb extraction. Sodium dithionite is a strong reducing agent, which has a higher reduction potential with respect to oxalate and ascorbic acid, which are mild reducing agents. In particular, EDTA 0.1M and sodium dithionite 0.05M extracted 100% of the total Pb from the S2.3 Trieste soil within 24 hours of incubation. The germination index GI % on the S2.3 soil water extract after the exposition of the soil to leaching solutions EDTA 0,1 M + sodium dithionite at different concentrations (0.05M, 0.025M, 0,01M and 0,005M) showed that the higher concentration of sodium dithionite (0.05M) was toxic (GI% lower than 40%), while the sodium dithionite at lower concentrations was not toxic for seed germination and growth. Starting from the results obtained on the S 2.3 sample, other batch experiments were carried out on other four soil samples collected in the Trieste site. This was in order to demonstrate the effectiveness of the selected leaching solution on Pb leaching in other soil samples representative of the Trieste site. The extraction with EDTA 0.1 M and sodium dithionite 0.05M, resulted less effective in the Pb soil leaching in the four soil samples with respect to the S 2.3 soil. In fact, considering the four soil samples, the maximum Pb concentration in leaching solution was not higher than 40%. The Pb fractionation of the four soil samples showed a prevalence of Pb in the residual fraction (between 60 and 80%), while in sample S2.3 the Pb in this fraction was less than 30%. In addition, the remaining percentage of Pb in the four soil samples was almost exclusively in the oxidizable fraction, while in the sample S2.3, 60% of Pb was in the reducible fraction. In view of this, other batch tests with different oxidant solutions in association with EDTA were carried out on the four soil samples, showing a Pb extraction higher than 50% when EDTA was used in association with H2O2. On the basis of the batch test and germination index results, some preliminary column experiments were set up by passing EDTA 0.1M and sodium dithionite at the 0.025M and 0.01M concentrations, resulting not phytotoxic in germination tests, through the S2.3 Trieste soil sample. The columns were oriented vertically and were packed with 40 grams of soil sieved at 2 mm. The leaching solutions were introduced into the column and allowed to saturate for 24 hours before conducting the leaching tests. At each eluate collection moment, the volume of each eluate fraction was measured and the Pb cumulatively released was calculated. Lead concentration in leaching solutions was performed by ICP instrument. Considering that in 40 grams of S2.3 soil there were 27,5 mg of Pb, in ten days, 97 ml of EDTA 0.1M and sodium dithionite 0.025M leached the 50% of the total Pb (Pb amount in the leaching solution=13,9 mg). The column experiments are ongoing, and we are continuing the monitoring of Pb in the leaching solution in order to establish times and leaching solution volumes able to decontaminate the soil.