Environmental and Green Chemistry of the Rare Earth Elements

"The rare earth elements (REE) encompass the Group 3 transition metals in the Periodic Table, namely scandium (Sc), yttrium (Y), and the lanthanide series elements (lanthanum (La) through lutetium (Lu). The lanthanide series exhibits unique, chemically coherent properties owing to their 4f electron shell. For example, the progressive filling of the 4f shell that accompanies the increase in atomic number across the series is responsible for the “lanthanide contraction”, in which their ionic radii decrease with increasing atomic number. This property leads to slightly different chemical behavior across the series such that the lanthanides are used in geochemistry as if they are 14 stable isotopes of the same element. The partial filling of the 4f electron shell also explains their strong magnetic properties, whereby unpaired electrons in partially filled orbitals spin in the same direction leading to these magnetic characteristics. Their magnetic properties are exploited in the manufacture of strong permanent magnets that are critical to modern high-tech and green-energy industries, including the lightweight, low-maintenance, and sustainable neodymium (Nd) – boron (B) – iron (Fe) magnets that make modern wind turbines possible. The 4f electrons are also responsible for the luminescent properties of REEs because when excited 4f electrons return to their ground state, they emit electromagnetic energy in the visible light spectrum. Consequently, REEs are crucial for manufacturing phosphors for modern digital displays (e.g., computer screens, smart phones, flat screen televisions), fluorescent, and LED lights, among others. Here, I review the important chemical properties of the REEs, their criticality to high-tech and green-energy technologies, geologic sources of REEs, and their environmental geochemistry, which will become increasingly important as anthropogenic-sourced REEs are emitted into the environment via 21st century E-waste streams.