Resistant rare earth phosphates as possible sources of environmental dissolved rare earth elements: Insights from experimental bio-weathering of xenotime and monazite

Typical resistant REE phosphates xenotime and monazite are important REE carriers in various types of rocks but the supergene mobility of REE in these minerals remains controversial. In this study, we hypothesize that microbes drive the natural weathering of these resistant REE phosphates. We conducted room-temperature bio-weathering experiments of xenotime concentrate with a common soil bacterium (Bacillus thuringiensis, Bt) isolated from a regolith-hosted REE deposit. Our results showed that Bt was able to promote the dissolution of xenotime and monazite in the concentrate, and the release of REE was enhanced by up to two orders of magnitude. In the bio-weathering medium buffered at pH = 6, the apparent release rates of total REE were in the range of 10⁻¹³–10⁻¹² mol·m⁻²·s⁻¹, with Y releasing at the fastest rates of ∼10⁻¹³ mol·m⁻²·s⁻¹. Furthermore, the estimated dissolution rate of monazite (∼10⁻⁹ g·m⁻²·s⁻¹) was one order of magnitude higher than that of xenotime (∼10⁻¹⁰ g·m⁻²·s⁻¹) due to a more refractory nature of xenotime determined by its chemical and mineralogical characteristics. On account of the extremely low solubility of REE phosphates, portions of the released REE could be re-precipitated as meta-stable phosphates during mineral dissolution, resulting in the underestimation of the release of REE from primary minerals. Bt could produce various organic acids and acidify the media, promoting the dissolution of resistant phosphates through proton- and ligand-promoted mechanisms. The results of our study suggest that microbes have a high potential to facilitate REE liberation from resistant xenotime and monazite, posing new insight into the biogeochemical cycling of REE on Earth's surface.