Trapping and mobilisation of arsenic and lead in former mine tailings –Environmental conditions effects

Abstract

Abstract Tailings of two former mines located in the French Massif Central (La Petite Faye in Creuse Dpt. and Enguialès in Aveyron Dpt.) were studied in order to forecast the mobility of arsenic and others metallic trace elements (MTE) resulting from sulphides oxidation. Arsenic and MTE could potentially be toxic as a contamination in the sediments, soils, waters…The formation of new products resulting from the alteration processes is especially interesting because of the abilities of these products to trap these metals or metalloids. We have focussed our study on the alteration products in order to determine their role in the immobilisation of arsenic and others MTE. The two selected sites present closed Eh-pH conditions (acidic and oxidant). Nevertheless La Petite Faye site display less leaching than in the site of Enguialès due to its layout: the tailings in the former site are disposed in a settling basin whereas in the latter one, they are spread out on a steep slope. After a chemical and mineralogical characterisation of the tailings, we have estimated according to the Eh-pH conditions the solubilities of the various secondary products. In both sites, the hydrochemical study (for la Petite Faye) and laboratory experiments (for Enguialès) highlight the presence of mineralogical products with low solubility in the current geochemical conditions. At la Petite Faye, scorodite and beudantite mainly ensure an efficient trapping of As and Pb. These two minerals are stable for acidic and oxidant conditions, even if scorodite is more sensitive to pH variations and could transform into goethite. The low As (< 0,1 mg.L−1) and Pb (under detection limit) concentrations measured in the water table along the six-months hydrological study confirm this previous result. In Enguialès, high quantities of As are trapped in amorphous products (Fe/As ~ 1.7). Subsequently As is more easily remobilised and is trapped secondarily by goethite, hematite or jarosite but in few quantities in comparison with those observed in amorphous products (1.4 wt.% < As < 3.5 wt.%). This releasing is highlighted by the leaching experiments, which indicate a significant remobilisation of As issued from amorphous products. However, in current conditions, the properties of the iron oxyhydroxides or of the iron sulphate ensure an efficient As trapping. The different behaviours of the trapping of As and Pb in the two sites are mainly explained by the hydrological context, the tailings layout, their granulometry and the cristallinity of the secondary products.