Lanthanides for Arsenic Sequestration—Precipitate Structural and Spectroscopic Insights

Managing arsenic remains a major industry challenge due to health, safety, and environmental risks. Recent work from our group shows that low-cost lanthanides (e.g., La and Ce) can quickly form stable arsenates, providing an alternative approach to sequestering arsenic from metallurgical aqueous solutions. This study presents the synthesis and a comprehensive analysis of lanthanide (La, Ce, Nd) arsenates in both hydrated and calcined forms, revealing for the first time, previously unreported hydrated phases (LnAsO 4 · n H 2 O) with semicrystalline structures and high surface areas (100–123 m 2 ·g −1 ). Results show that structural water plays a key role in modifying crystal lattice parameters and diffraction behavior, offering new insight into phase formation. Upon calcination at 800 °C, the precipitates transform into dense, anhydrous rod-like structures with significantly reduced surface area (0.1–0.4 m 2 ·g −1 ) and minor sodium arsenate formation (approximately 0.5 wt.%) is observed. Surface chemistry analysis demonstrates a preferential lanthanide–oxygen bonding environment, with no evidence of lanthanide hydroxides or arsenic adsorption, providing new understanding of bonding characteristics in these systems. The comprehensive characterization presented here fills a key gap in the literature and establishes a reference dataset for future studies on lanthanide arsenates. The study also provides essential baseline information for understanding their solubility and stability behavior.