Lanthanum captured in montmorillonite: Evidence of inner-sphere complexes from X-ray Absorption Spectroscopy investigations

An X-Ray Absorption Spectroscopy (XAS) study of the local structural environment of La ions hosted in montmorillonite is presented. The structural location of La ions in the host phases and whether they are incorporated in the structure or adsorbed at the surface has been assessed. Two clays from the Clay Mineral Society repository (STx and SWy) in pristine form and polymer-modified with penta-ethylen-hexamine (C10H28N6) have been studied. Ca and Na-rich montmorillonite have been contacted with 19 and 200 mM La solutions. XAS results yielded Lasingle bondO distances ranging between 2.57 and 2.61 Å, compatible with literature data for La ions coordinated by water molecules. We provide evidence for the presence of a further La-Si/Al distance suggesting that La may also be bonded to the oxygen atoms on the outer surfaces of the TOT layer. Such a feature may play an important role in limiting the release/efficiency of the process. This paper is a further step in the research for REEs recovery from Electric and Electronic wastes (WEE) via a solid-liquid process, and their re-use as secondary raw materials. The knowledge of the ions-sorbent interaction mechanism, such as its nature and strength, is a fundamental information to tailor sorbents and procedures in practical applications.