The relationship between particulate uranium and thorium-complexing capacity of oceanic particulate matter

Thorium-complexing capacity (ThCC), which is defined as the amount of thorium adsorption onto particulate matter (PM) in 0.1 mol l−1 HCl solution by complexation, has been introduced as a new oceanographic parameter. The ThCC implies the concentration of a strong organic ligand in PM. To specify chemically the strong ligand in PM, we compared the ThCC in PM with particulate uranium, which exists as an organic complex in seawater. The ThCC in PM was fairly correlated with particulate U, whose relationship allows the estimation of the conditional stability constant of the organic uranium complex in PM based on the mass action law. The estimated conditional stability constant of the organic uranium complex under seawater conditions (1014.5 l mol−1) is greater than that determined for organic copper complexes, whose order of magnitude coincides with the result of the metal adsorption on microorganisms. These findings suggest that the strong ligand corresponding to the ThCC in PM, which is directly related to the complexation of metals in PM, is originating from marine organisms.