Organic Acid and Dissolved Oxygen Effects on Rare Earth Element and Yttrium Release from Phosphate Minerals

Elements in the rare earth element series plus yttrium (REEY) have received increased attention of recent, as studies have investigated their utility as tracers in rock/water interaction processes, groundwater flow and mixing, soil genesis, and as proxies for evaluating paleoceanographic and paleoclimatic change. A more thorough understanding of how redox status and low molecular weight organic acids (LMWOAs) affect selective REEY dissolution is particularly important for developing and evaluating these elements as proxies. For example, previous studies noted strong positive correlations between aqueous REEY and dissolved organic carbon concentrations. Therefore, the objective of this research was to investigate the effects of LMWOAs and molecular oxygen concentration on REEY release from REEY-containing phosphate minerals. Apatite and monazite were reacted under batch conditions in presence and absence of two aliphatic and aromatic organic acids under oxic and anoxic conditions. Organic acids were found to enhance REEY release from apatite and monazite, and increasing organic acid concentrations from 1 to 10 mM resulted in greater dissolution. Citrate enhanced mineral dissolution to the greatest extent, whereas dissolution in the presence of salicylate or absence of ligand is lowest. Middle rare earth elements (MREE) were released from monazite to a greater extent than light rare earth elements (LREE) in the presence of LMWOAs, relative to ligand-free samples. Similar trends were not observed for apatite. Additionally, release of REEY from the minerals was not impacted by PO₂. These data support the hypothesis that release of REEY from the minerals studied is enhanced by organic acids. Thus, it seems plausible that geochemical and mineralogical signatures of REEY may have utility for distinguishing the presence of terrestrial organisms during soil weathering processes on early earth. However, the development of such signatures may be mitigated, in part, by soil mineral composition and the types and concentration of LMWOAs present.