Complexation of Eu (III) By Fulvic Acid

摘要

It has been determined that fulvic acid (FA) at concentration levels as low as 10 exp -4 molar (approx.20ppm), on a monomer basis, exists in aqueous media as a well-defined molecular aggregate which exhibits microgel properties. The unambiguous deduction that FA exists as a separate phase (microgel) when dispersed in aqueous media is based on the following observations: (1) In studying the distribution of Eu exp 154 between 8 percent crosslinked Dowex 50 resin in the Na exp + -ion form and fulvic acid at several different NaCl concentration levels and over and extended pH range from 4 to as high as 10 it has been shown that only one FA-complexed species of Eu exp +3 , (Eu(COOR)) exp ++ n, is formed, the magnitude of its apparent constant of formation, being roughly proportional to the third power of the NaCl concentration employed in the study and (2) although the apparent pK of the fulvic acid, because of significant aberration of the pH data by CO sub 2 sorption has been quantitatively measured in only a few instances, an inverse relationship between apparent pK and ionic strength has been conclusively demonstrated. Observations (1) and (2) can only correlate with the fact that the concentrations of free mobile ion counterions in solution are a reflection of the equilibrium between H exp + , COOR exp - Eu exp +++ , HCOOR and Eu(COOR) exp ++ in the microgel defined by the FA aggregate which controls the concentration ratio of H exp + , Eu exp +++ and Na exp + in the aqueous phase. Thus if Na exp + , the potential determining ion, is reduced or raised in solution by a factor of 10 in parallel sets of experiments, the quantity of H exp + and Eu exp +++ in solution is reduced or raised, about a factor of 10 exp 1 and 10 exp 3 , respectively, so that the Donnan equilibria between these diffusible components in the microgel and aqueous phases (Eu Cl sub 3 and NaCl, and HCl adn NaCl) can be fulfilled; the exact proportionality is effected by microgel flexibility. (Atomindex citation 15:010635)