Sorption of Yttrium and the Rare Earth Elements on the Marine Macroalga Ulva lactuca

Abstract

Trace metal interactions with organic matter are relatively poorly understood, though organic matter is ubiquitous in aquatic environments and likely instrumental in controlling metal geochemistry. To better understand the mechanisms underlying metal interactions with organic substrates, sorption of Yttrium and the Rare Earth Elements (YREEs) on Ulva lactuca, a marine macroalga, was studied in batch laboratory experiments at different ionic strengths over a large pH range (2.7 - 8.5). At all ionic strengths and experimental pH values, colloid-bound YREEs make up a substantial portion of sorbed metals as described by a two-site Langmuir model, which has implications for bioremediation and metal sorption studies. YREE sorption on U. lactuca can be modeled as a function of pH with a three-site non-electrostatic surface complexation model, and patterns of conditional YREE complexation constants were used to determine possible identities of metal-complexing functional groups.