Gadolinium Behavior in Seawater: a Molecular Basis for Gadolinium Anomalies

Gadolinium enrichments in solution relative to europium and terbium, have been investigated by examining europium, gadolinium, and terbium uptake on the macroalgae, Ulva lactuca L. Our laboratory observations indicate that anomalies in the behavior of gadolinium relative to neighboring elements are strongly influenced by carbonate complexation. Our experiments using Ulva lactuca L. in seawater produced positive gadolinium anomalies at total carbonate ion concentrations less than about 2 × 10 ⁻⁵ mol per kg-solution, and dominantly negative gadolinium anomalies at total CO ₃ ²⁻ concentrations greater than about 1 × 10 ⁻⁴ mol per kg-solution. A first order numerical model for oceanic rare earth element (REE) scavenging indicates that gadolinium anomalies in seawater are not simply due to enhanced gadolinium complexation in the solution phase. Gadolinium anomalies in seawater can be explained in terms of differences in europium, gadolinium and terbium solution complexation constants, on one hand, and differences in complexation of europium, gadolinium, and terbium by organic surface ligands on the other. Compared with europium and terbium, gadolinium generally tends to form weaker complexes with both solution ligands and surface ligands.