Transfer of heavy metals to plants in technosols remediated with calcareous residues.

When dealing with restoration and remediation projects of zones contaminated by waste and activities derived from mining operations, all the aspects that affect the neighbouring areas dedicated to the cultivation of vegetables are especially relevant, and should be carefully considered. According to previous studies carried out, the areas of mining influence affect to a very different extent the bordering areas depending not only of the characteristics of the soil but also of the type of mining activity that was developed, and so of the primary mineralogy involved, and even of the particular type of vegetable that is being cultivated. The diversity and complexity of these factors suggest the convenient of studying the process by clarifying the mechanisms of transfer of potentially toxic elements from the soil to the biotic environment, with the soil-plant-biotic chain sequence.

In this work an experimental study was carried out with this purpose making use of experiments carried out at the greenhouse scale in order to find a model that could clarify the processes that could take place in restored soils.

 

Construction and demolition residues (CDRs) as well as other residues containing high concentrations in limestone filler were used to prepare technosols, and the transfer factors (TF) and bioconcentration factors (BCF) of potentially toxic elements were measured for the selected plant species (chard and broccoli). In this way, in addition to the soil remediation, benefit can be obtained of the re-valorization of such type of  easily available, low cost residues. For the realization of the tests in the greenhouse, four technosols were prepared and experiments were planned in duplicate, which constituted eight experimental units, each one containing 21 large pots disposed in such a way that leachates were poured in the same storing tank.  

The technosols were prepared as follows:

• Vegetable soil or reference soil. (T1)
• Contaminated soil: 50% reference soil + 50% mine residue (T2).
• Amended Land 1: 75% contaminated soil + 25% CDR (T3).
• Amended soil 2: 75% contaminated soil + 25% Limestone filler (T4).

 

A statistical study was carried out to evaluate the relationships between the levels of potentially toxic elements (Pb and Cd) found in rhizospheres and root contents and the levels present in the technosols prepared. Conclusions could be obtained as regards  the mobility of  these elements, the characteristics of the technosols and the type of cultivation. The data allows a model to be outlined that could be translated at a higher scale for an effective remediation of large zones.