Rare earth element release from phosphate minerals in the presence of organic acids

Abstract

The primary objective of this research was to investigate the effects of aliphatic and aromatic low molecular weight organic acids (LMWOAs) on rare earth element and yttrium (REY) release from the phosphate minerals apatite and monazite. Since prior studies have shown that redox status can affect REY partitioning during incongruent dissolution, a secondary objective was to assess the influence of dissolved O 2 concentration. Increasing LMWOA concentrations from 0 to 10 mM resulted in enhanced REY release. In general, REY release increased in the order: no ligand ≈ salicylate < phthalate ≈ oxalate < citrate. REY–ligand stability constants were only useful for predicting REY release for oxalate reacted with apatite and phthalate reacted with monazite. The role of dissolved oxygen in dissolution of the phosphate minerals was mixed and inconsistent. Mineral type was observed to significantly affect REY pattern development. REY release patterns for apatite range from nearly flat to those exhibiting the lanthanide contraction effect (radius-dependent fractionation); whereas, monazite REY release patterns are best described as exhibiting an M-type lanthanide tetrad effect (radius-independent fractionation). Weathering of apatite in the presence of aliphatic LMWOAs resulted in development of the lanthanide contraction effect fractionation pattern, and the aliphatic LMWOAs further developed MREE and radius-independent fractionation during monazite dissolution. Geochemical and mineral-specific REY signatures may, therefore, have utility for distinguishing the impacts of biota on soil weathering processes on early Earth. The development of such signatures may be mitigated, in part, by accessory mineral composition, the types and concentration of LMWOAs present, and precipitation of secondary minerals.