Phosphate effects on rare earth element sorption onto kaolinite: molecular-scale insights at circumneutral pH

Rare earth elements (REEs) are critical minerals that are indispensable for the clean energy transition. Understanding their occurrence and behavior in highly weathered environments provides valuable insights into the identification and prediction of potential REE resources. Phosphate (P) plays an important role in controlling the geochemical behaviors of REE during weathering and secondary deposition. REE-phosphate minerals, such as monazite and xenotime, are among the important natural sources of REE. However, the molecular-scale reaction mechanisms underlying phosphate-REE interactions in weathering environments remain unclear. This study investigates the interaction between phosphate and yttrium on the surface of kaolinite, a representative and abundant clay mineral in highly weathered environments. At circumneutral pH 6, phosphate inhibits yttrium mobilization by forming ternary kaolinite-yttrium-phosphate complexes and surface precipitation. The reaction mechanisms identified in this study are distinct from previously reported adsorption-dominated processes on clay minerals that govern REE immobilization during chemical weathering. Solid-state nuclear magnetic resonance (NMR) spectroscopy, including³¹P direct polarization magic angle spinning (DP/MAS) and¹H –³¹P cross polarization (CP) rotational echo adiabatic passage double resonance (REAPDOR) analyses, provides direct molecular-scale evidence for the yttrium-phosphate surface complexation on kaolinite. Quantification of atomic distances by numerical simulations further substantiates the formation of ternary surface complexes. These mechanistic findings enhance our understanding of phosphate-mediated REE mobilization, transport, and redeposition in natural environments.