Examination of Cadmium(II) Complexation by the Suwannee River Fulvic Acid Using 113Cd NMR Relaxation Measurements

Aquatic and terrestrial fulvic acids are environmentally important because they affect the bioavailability and transport of metal ions. Prior studies demonstrated that Cd(II) binds to the oxygen containing functional groups of fulvic acids. The complexation of Cd(II) is further investigated in this study using 113Cd NMR relaxation measurements for solutions of the Suwannee River fulvic acid (SRFA). Spin−lattice (T 1) and spin−spin (T 2) relaxation times are measured over a range of Cd(II):SRFA ratios. The results clearly indicate two types of Cd(II) binding sites for the SRFA. A series of model ligands was also examined to gain further understanding of the two types of binding motifs present in the fulvic acid. The results for a model compound containing several carboxylate functionalities in near proximity correspond very closely to the results obtained for the strong binding sites of the Cd(II)−SRFA complexes.