Ability of Cistus salviifolius L. to phytostabilize gossan mine wastes amended with ash and organic residues

The São Domingos mine is within the Iberian Pyrite Belt, one of the oldest mining districts in Europe, with one of the largest concentrations of polymetallic massive sulfide deposits. Mine waste heaps are considered extreme environments, since they possess high concentrations of potentially hazardous elements (PHE). When the presence of PHE is combined with adverse physical characteristics and low contents of organic matter and nutrients, the development of most plants is inhibited and the biodiversity of the area is drastically reduced. The transfer of chemical elements among the different solid phases contributes to control their behaviour, mobility and availability. Autochthonous plant species, such as Cistus salvifoliius L., which are well adapted to extreme environments and are able to grow naturally in degraded environments may contribute to minimize the negative chemical impacts and improve the landscape quality. However, the environmental rehabilitation processes associated to the development of these plants (phytostabilization) are very slow and the combined use of materials/wastes that improve some physico-chemical characteristics of the matrix is necessary. This work studied the effectiveness of the phytostabilization with C. salvifoliius of gossan mine wastes from the mine of São Domingos amended with organic/inorganic wastes. The amendments used were (g/kg of gossan): biomass ash (BA, 2.5), a mixture of organic residues (OR, 120) and a mixture of both (BA+OR).

The mine wastes have very acid pH, between 3.44 and 3.54, high total concentrations of PHE, such as As, Cu and Pb, and low concentrations of organic C and nutrients. The available fraction of Al, Ca, Fe, K, Mg and V in the mine wastes was, in general, less than 8% of their total concentration while for As, Cu, Mo, Pb and Zn it was less than 4.3% of the total. Nonetheless Mn availability lay between 21.2 and 24.8%.

The best vegetative development, that occurred without visible signs of toxicity, was obtained in the two treatments with organic wastes (OR and BA+OR). Independently of the treatment, this species does not behave as accumulator of PHE. High levels of Catalase activity were quantified in the soluble fraction of the roots from OR treatment and in the ionically bound fraction of roots from BA+OR treatment. As for Peroxidase, the activity quantified in the soluble fraction was slightly higher in roots than in shoots, while in the ionically bound fraction high values were measured both in shoots and roots.

The application of amendments, especially with organic matter (mixture or single application), together with the phytostabilization allowed the improvement of the plant-soil system namely at level of a better plant cover and improvement of several chemical characteristics of mine wastes (e.g. decrease of the majority of PHE in the available fraction, increase of fertility and water-holding capacity), leading to speed up the environmental rehabilitation.