PhD Defense | Dileesha Kotte Hewa | Speciation and transport of rare earth elements in passive treatment systems of the Iberian Pyrite Belt

Speciation and transport of rare earth elements in passive treatment systems of the Iberian Pyrite Belt

Rare Earth Elements (REE) are a group of elements that are not only essential for the clean energy transition but may also pose an environmental challenge. One example of such a challenge is the REE emission from acid mine drainage (AMD). This occurs when sulfide minerals are exposed to air and water. The AMD waters are very acidic, leading to the dissolution of associated metals, metalloids, and REE. The REE concentrations in AMD are several orders of magnitude higher than environmental standards.

Passive treatment systems are typically used to remediate AMD from abandoned mines. Alkaline materials, such as CaCO₃, increase pH and precipitate metals. It is unclear if these systems can adequately remove REE from AMD. This study aimed to unravel the fate of REE along the entire passive treatment system, paying due attention to the form by which the REE are scavenged and to the rate and the extent of the REE removal.

This study showed that trivalent light REE are incorporated into gypsum (CaSO₄) structure at low pH due to the similarity of the ionic radii of REE³⁺ and Ca²⁺. This was evidenced by synchrotron-based analysis and by LA-ICP-MS, along with equilibrium and kinetic modeling. At higher pH, REE are scavenged in the passive treatment system by sorption on, and co-precipitation with aluminum and iron-containing amorphous secondary mineral phases. The removal of REE in the system to concentrations below acceptable environmental standards requires a sufficiently large pH, which, in turn, depends on the residence time of AMD within the treatment system. This study also showed that alkaline waste products such as biomass ash or blast furnace slags are effective alkalinization materials, albeit less efficient than pure lime. This study proved acceptable for REE removal from AMD in a passive alkaline treatment system due to their association with precipitated gypsum or iron- and aluminum minerals.

Promoter:

• Erik Smolders (KU LEUVEN)

SCK CEN mentors:

• Delphine Durce
• Sonia Salah