Evaluation of Amd production under experimental conditions

In order to predict the future behaviour of technosols designed for the remediation of sites contaminated with mining waste, a pilot experiment was carried out in containers under an enclosed building.

One-metre cubic polyvinyl chloride (PVC) containers were used and filled with soil of the different types that are abundant on the site. Four soil types were selected with different textures (sandy to silty-clay), different mineralogy and different PTE content. Soil mixtures were also prepared with limestone filler at different proportions (30, 40 and 50 %).

These containers were subjected to a humidity and drying process, simulating the rainfall conditions that the site receives as an average annual value, in 10 successive cycles, maintaining the experience for 4 years. The drains obtained were analysed and their corresponding crystallised phases were obtained and characterised by XRD and electron microscopy. The metals determined were Fe, Cd, Cu, Pb, As and Zn.

This experiment allows us to differentiate the behaviour of the selected soils with respect to humidity, since the untreated soils with a clayey loam texture are AMD producers, with very low pH and a high mobilisation of metals. In contrast, sandy textured soils and soils containing 30 % or more limestone filler have a pH close to 7 and small metal contents close to the limit of quantification.

Efflorescences are more abundant in those leachates rich in Zn and Fe, being copiapite, hexahydrite, bianchite and gypsum the main soluble phases associated with these leachates.  Finally, the percolates with a higher salt load correspond to the third percolation carried out approximately nine months after the start of the experiment.