Thallium in environmental compartments affected by acid mine drainage (AMD) from the Iberian Pyrite Belt (IPB): From rocks to the ocean

Abstract

This works investigates the origin, behavior and fate of Tl in acid mine drainage (AMD) affected areas at catchment scale, following the path from rocks to the ocean. To address this issue, comprehensive data set of Tl in rocks, waters, secondary minerals, plants and other environmental compartments is presented, using the Iberian Pyrite Belt (SW Spain) as representative example. The content of Tl in host rocks (mean of 0.51 mg/kg) exhibits moderate positive correlations with elements such as K and Rb, whereas no correlations were observed in sulfides (27 mg/kg) between Tl, Fe and other metal/loids commonly found in these minerals, such as As, Pb, Cd, Cu or Zn. During sulfide oxidation processes, Tl is mobilized from sulfides, as evidenced by the depletion of Tl in gossans (1.8 mg/kg), and host rock minerals, leading to a Tl enrichment in AMD leachates (mean of 242 μg/L), with concentrations of up to 8.3 mg/L, several orders of magnitude higher than those reported in natural waters. The precipitation of secondary minerals, with large surface areas, may be a sink for Tl, especially in jarosite minerals (8.4 mg/kg). Thallium can be also removed during the treatment of AMD in alkaline passive treatment systems due mainly to sorption processes onto Fe and Al secondary minerals (i.e., schwertmannite and basaluminite, respectively).Mean Tl contents of 13 mg/kg have been observed in wastes dumped in abandoned mines of the IPB, mainly spoil heaps, slags, roasted pyrite, heap leaching wastes and tailings. However, there is no clear relationship between Tl content and the type of mining wastes. These wastes can suffer weathering, leading to an enrichment of Tl in soils. Previous studies reported that <25% of total Tl is easily extractable, being mainly adsorbed to crystalline Fe oxides in acidic soils and Al oxides in neutral-alkaline soils. Despite this, Tl may be translocated by plants. The translocation of Tl in plants of the IPB has not been properly addressed, however previous studies in other areas showed a low phytoavailability of Tl compared to Cd and Zn, although Tl translocation appears to be strongly controlled by plant species or by differences in Tl speciation. The weathering of rocks, mine wastes and soils may lead to the release of notable amounts of both dissolved and particulate Tl to the hydrosphere. In acidic conditions, Tl seems to be mainly transported by the dissolved phase in AMD-affected streams and rivers, with <15% being transported by the particulate matter. This latter Tl transport may be associated to its incorporation into diatoms and Fe minerals such as jarosite after replacement of Tl+ by H+ in their structure. Subsequent release by desorption processes from jarosite and diatoms in acidic conditions can occur. This process has also been observed in estuaries affected by AMD, thus, Tl transported by jarosite minerals in the particulate matter and diatoms are released back to the estuarine waters across the salinity gradient due to the increasing proportion of unreactive TlCl0 and K+ ions, which compete for adsorption sites in jarosite with Tl+. Thus, enhanced transport of Tl to the oceans is observed in AMD-affected systems.