REE behavior during evaporative precipitation in a severely affected-AMD creek (SW Spain)

The Iberian Pyrite Belt (IPB) covers a vast part of the SW of Spain and Portugal. This region is rich in sulfide deposits which have been exploited intensively. As a consequence, there is a large number of sources of acid mine drainages, characterized by very extreme pH conditions and high pollution load. Waters affected by acid mine drainage (AMD) can contain high concentrations of rare earth elements (REE) which have a great interest from the scientific and economic point of view. This work is focused on the REE behavior during evaporative concentration during the summer along the Agrio River, a small stream deeply affected by AMD coming from the Río Tinto mines. The pH varied from around 2.8 to 1.5 and the electrical conductivity from 5.8 to 15 mS/cm. Concentration of ∑REE varied from around 1.3 mg/L to around 5.3 mg/L at the end of the summer. In September, with the lowest discharges, most elements showed an increase along the reach (Al 12%, Zn 8%, Cu 4%) due to evaporation. REE also showed an enrichment, which varied from 7% (La to Nd) up to 10-11% (Ho to Lu), which seems to indicate a slightly higher affinity of the evaporative salts for LREE in relation to HREE. Although REE concentrations varied widely between the different samplings, the NASCnormalized patterns are very constant, showing a negative Eu anomaly together with the enrichment of MREE typical of AMD-affected waters. The amount of dissolved ∑REE transported by this small river was surprising, varying between 9 kg/day in September and 26 kg/day in May.