¹¹³Cd NMR binding studies of Cd-fulvic acid complexes: Evidence of fast exchange

The binding of Cd²⁺ to Suwannee River fulvic acid is studied using potentiometric methods and ¹¹³Cd 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 ¹¹³Cd NMR chemical shift changes with constant Cd²⁺ concentration and varying fulvic acid concentration at pH 6.4 are consistent with the fast exchange model. Analysis of the ¹¹³Cd NMR chemical shift data using a Scatchard plot yields a conditional binding constant of 6.7 x 10² 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 x 10² M^-1.