Effects of a revegetation on heavy metals leaching from slag heaps

In many parts of the world, non-ferrous metallurgy has produced large quantities of wastes which were piled in slag heaps. Due to the high residual heavy metals content in slag, such heaps may represent a threat for the surrounding environment and human health. Therefore, natural or assisted revegetation of slag heaps is usually considered as a commonsense mitigation measure since it may reduce the dispersion of contaminants by wind or water erosion, runoff and water percolation through the substrate. However, the effect of plants on the leaching of heavy metals is still unclear. This study aimed to determine to what extent the plant colonization could affect the leaching of heavy metals (Cd, Pb and Zn) from two contrasted slag. Columns made of PVC were filled in with 2 kg of a mixture consisting in 1900 g of quartz (500-1000 µm) and 100 g of slag (100-500 µm). Two plants (Agrostis tenuis and Lupinus albus) were separately grown in the columns for seven weeks. A control treatment, i.e. without plants, was also carried out. All columns were continuously irrigated at a slow rate with a macronutrient solution. Nitrogen was supplied as nitrate form in order to avoid exacerbating acidification of the rhizosphere, which could mask other plant effects such as organic molecules exudation. Concentrations of heavy metals in leachates and in plants were measured every week and at the end of the experiment, respectively. For Cd, Pb and Zn, the metal amounts exported by both plants were negligible compared to the metal amounts lost by leaching. However, the presence of A. tenuis largely increased the concentrations of Cd, Pb and Zn in leachates from both substrates while L. albus only induced higher metal concentrations in leachates from one of them. Moreover, when substrates were covered by A. tenuis, the concentrations of metals in leachates were sometimes so high that the reduction of the percolated solution volume due to plant uptake was not sufficient to reduce the total metal mass exported. When plants were present, acidification of leachates was not observed. This suggested that the increase of heavy metals concentrations in leachates in presence of plants was related to other factors than pH. Results of this study demonstrated the potential of growing plants to mobilize heavy metals from slag and stressed the need to carefully select the plant species for revegetation strategies.