113Cd NMR Binding Studies of Cd−Fulvic Acid Complexes: Evidence of Fast Exchange

The binding of Cd 2+ to Suwannee River fulvic acid is studied using potentiometric methods and 113 Cd NMR spectroscopy. The potentiometric results indicate 6.4 and 1.2 mequiv/g carboxylic and phenolic acid groups for the Suwannee River fulvic acid standard. At pH 6.4, the fraction of of the carboxylic acid groups that are deprotonated is 0.89. The observed 113 Cd NMR chemical shift changes with constant Cd 2+ concentration and varying fulvic acid concentration at pH 6.4 are consistent with the fast exchange model. Analysis of the 113 Cd NMR chemical shift data using a Scatchard plot yields a conditional binding constant of 6.7 × 10 2 mL/mg. A number-average apparent molecular weight of 800 g/mol, gives a value of 0.9 for the fraction of the fulvic acid carboxylate sites that are bound to cadmium and a cadmium−fulvic acid complex formation constant of 5.4 × 10 2 M - 1 .