Geochemical Behaviour of Rare Earth Elements Throughout an Acid Mine Drainage Passive Treatment System in the Lousal Mine Area, Portugal

Abstract A passive remediation system was installed between 2010 and 2015 at the Lousal legacy mine site in southwestern Portugal to minimize the impact of AMD. This is the first study of the geochemical behaviour of rare earth elements (REE) in the area after implementation of the passive treatment system. The total and dissolved concentrations of REE is similar throughout the basins/wetlands and acidic ponds in both sampling campaigns, which is mainly attributed to the conservative behaviour of these elements in acidic water. In areas where mixing of acidic and circumneutral water occurs, the REE were partially removed from solution by adsorption onto newly formed Al and Fe precipitates. Sorption onto the precipitates varies between the rare earths, with a preferential fixation of heavy REE (HREE) compared to the light REE (LREE). The normalized distribution patterns exhibit the characteristic convex-up enrichment in middle rare earth elements for AMD and display an enrichment of LREE relative to HREE in areas where mixing of waters occurs, and the pH is close to circumneutral.