Measurement of cadmium(II) and calcium(II) complexation by fulvic acids using 113Cd NMR

Aquatic and terrestrial fulvic acids are environmentally important in pollution transport because they affect the bioavailability and transport of metal ions. The complexation of the metal ions, Cd(II) and Ca(II), with several fulvic acids is examined in this study using 113Cd NMR. Our results indicate that Cd(II) predominately binds to the oxygen containing functional groups of the fulvic acids. A single 113Cd NMR resonance is observed in NMR spectra of Cd(II)-fulvic acid solutions indicating fast exchange between free and complexed cadmium species. An average association equilibrium constant, K(Cd), is determined from NMR spectra measured for the titration of fulvic acid with Cd(II). The K(Cd) values determined for the four fulvic acids studied range between 1.2 and 3.5 x 10(3) M(-1). Competitive binding between Ca(II) and Cd(II) is used to indirectly determine an average association equilibrium constant, K(Ca), for Ca(II) with each fulvic acid. Overall K(Ca) values range from 4.6 to 7.8 x 10(2) M(-1).