Evaluation of the performance of chemical extractants to mobilise metals for remediation of contaminated samples

The main objective was to evaluate the efficiency of chemical extractants to mobilise metals from contaminated samples under different land uses: industrial, agricultural and forest soils, and acidic and basic tailings ponds. Four extractants (citric, oxalic, Diethylenetriamine pentaacetate - DTPA - and Ethylenediaminetetraacetic - EDTA - acids) at different concentrations were tested. Extractants efficiency was dependent on sample type and specific metal as influenced by sample characteristics, mineralogy and speciation. We found that EDTA was the most efficient extractant for all metals in the forest soil; DTPA for Cu, Pb and Zn, and EDTA for Cd in the agricultural soil; EDTA for Pb and Zn, DTPA for Cu and oxalic acid for Cd in the industrial soil; EDTA for Cd and Zn in the acidic tailings with the other extractants showing similar performance; DTPA for Pb, DTPA/EDTA for Zn, citric acid for Cd and Cu in the basic tailing. The optimum extractant concentration across samples was the maximum tested for Pb (40 mM), 20 mM for Zn and 5-10 mM for Cu and Cd. Metal mobilisation varied widely across samples (maximum percentage of extraction shown from total metal amount): 27-81% for Cd, 3-55% for Cu, 1-24% for Pb and 2-16% for Zn. Soil pH was the most influential factor to explain the amount of metals extracted. This study evidenced the effectiveness of organic acids to mobilise metals, the possibility of using natural/nontoxic weak acids for some samples and metals and the convenience of performing preliminary tests with different extractants and concentrations as a previous necessary step before large scale remediation projects.