Solar Radiation and Rare-Earth Elements
Solar Radiation as the Likely Cause of Acid-Soluble Rare-Earth Elements in Sediments of Fresh Water Humic Lakes
It is known that the exposure of natural organic matter to solar radiation results in degradation and oxidation of organic matter. However, it is not only organic matter what is affected by solar radiation. Organic matter is great complexing agent binding in its inner structure other elements such as metals, phosphorus, nitrogen. The photochemically induced changes in the structure of organic matter reduces complexing capacity and previously organically bound elements are released in the water. Depending on oxygen concentration in water, metal element can undergo hydrolysis and depending on pH can result in the formation of insoluble oxyhydroxides of aluminum and iron. Freshly formed oxyhydroxides aggregate and can coagulate with dissolved organic matter and other dissolved species. The formed particles can sediment.
The effect of photochemically induced precipitation of rare-earth elements (REEs) in water from a tributary to Plešné Lake and a tributary to Jiřická Pond, Czech Republic was studied. The formation of particulate REEs occurred in all exposed samples and continued even 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 of this study were compared with paleolimnological records in the Plešné Lake sediment, the photochemically induced formation of particulate REEs explained 10−44% of the REE concentrations in the upper sediment layers. Observed photochemically induced sequestration of REEs into sediments can explain a significant part of the REEs’ history in the Holocene sediment.
Porcal, P., Amirbahman, A., Kopáček, J., Norton, S.A. 2020. Solar radiation as the likely cause of acid-soluble rare-earth elements in sediments of fresh water humic lakes. Environmental Science and Technology 54, 1545 - 1553, DOI: 10.1021/acs.est.9b0514