Solar radiation as the likely cause of acid-soluble rare-earth elements in sediments of fresh water humic lakes

We studied photochemically induced precipitation of Rare Earth Elements (REEs) in water from a tributary to Plesne Lake, and a tributary to Jiřicka Pond, Czech Republic. Both tributaries had high concentrations of dissolved organic matter (DOM, ~1.8 mmol C L-1). Filtered (0.2 µm) samples were exposed to artificial solar radiation of 350 W m-2 for 48 to 96 hr, corresponding to 3 to 6 days of natural solar radiation in summer at the sampling locations. Experiments were performed with altered and unaltered pH ranging from 3.8 to 6.0. The formation of particulate REEs occurred in all exposed samples with the fastest formation of particulate REEs documented at the original pH. The formation of particulate metals continued in irradiated samples after the end of irradiation, suggesting that photochemically-induced reactions and/or continuing precipitation continue in darkness or in deeper water due to mixing. Results were compared with paleolimnological records in Plesne Lake sediment. At pH 5.0, the photochemic...