The application of 113Cd NMR spectrometry to the study of cadmium complexation by natural organic matter

The results of a comprehensive 113Cd NMR study of complexation of Cd2+ by Suwannee River natural organic matter (NOM) are presented and compared with the more familiar behavior of simple systems containing Cd2+ and a single ligand (e.g., ethylenediamine). pH-dependent trends in the chemical speciation of Cd(II), chemical shift of 113Cd, and line width of resonance peaks lead to the conclusion that the contribution of N donor atoms to the primary coordination sphere of Cd2+ increases steadily with increasing pH, so much so that relatively strong downfield resonances can be observed at alkaline pH. Those observations also indicate that there is no accessible, unique combination of magnetic field strength and solution chemistry for which the exchange rates of all forms of Cd are simultaneously fast, intermediate, or slow.