Determination of a strong organic ligand dissolved in seawater: Thorium-complexing capacity of oceanic dissolved organic matter

Abstract

A new method has been developed to measure a strong ligand in oceanic dissolved organic matter (DOM) by Th complexation in acidic media and the adsorption of the Th complex onto XAD-2 resin. Th reacts quantitatively with organic binding site of DOM in strong acid media (around 0.1M H⁺ solution), which is equilibrated within 24 hours. According to mass action analysis, Th forms a 1∶1 complex with the binding site in DOM, whose conditional stability constant is 10^6.7M⁻¹. The conditional stability constant of the Th complex in DOM is in good agreement with that determined for oceanic Particulate Matter (PM) under similar experimental conditions. This finding suggests that the chemical properties of the strong ligand in DOM are similar to these in biogenic PM. The Th-complexing capacity in DOM, which corresponds to the total concentration of the strong organic ligand, can be determined (2–3 nM in surface waters) in a small volume (about 200 ml). The method has a detection limit of approximately 0.05 nM for the thorium-complexing capacity of DOM by using²³⁰Th.