Using Cadmium-113 NMR Spectrometry To Study Metal Complexation by Natural Organic Matter

• Published in: Environmental science & technology 1998-02, Vol.32 (4), p.483-487
• Main Authors: Li, Jian, Perdue, E. Michael, Gelbaum, Leslie T
• Format: Article
• Language: English
• Subjects: Applied sciences; Biological and physicochemical phenomena; Cadmium; Chemical elements; Chemistry; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Isotopes; Metals; Natural water pollution; NMR; Nuclear magnetic resonance; Pollution; Pollution, environment geology; Scientific imaging; Water treatment and pollution
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es9704848

Because of its relatively high natural abundance and the broad range of chemical shifts (900 ppm), 113Cd is a potentially excellent istope for NMR studies of metal binding sites of natural organic matter (NOM). This paper presents a 113Cd NMR study of the effects of pH and, to a lesser extent, Cd/C molar ratio on the complexation of cadmium ion by Suwannee River NOM. Spectra are presented for solutions ranging in pH from 3.6 to 9.0 and ranging in Cd/C ratio from 0.0013 to 0.0068. At acidic pH, a single somewhat sharp peak is observed, but more complex spectra are obtained at alkaline pH. These changes indicate that Cd−NOM exchange rates are much faster at acidic pH, where the concentration of free Cd2+ is greatest. Under nearly all the conditions of Cd/C ratio and pH that were used, Cd2+ is primarily coordinated by O donor atoms, probably in carboxyl groups. At high pH, some evidence of Cd2+ coordination by N donor atoms is observed. No evidence of Cd2+ ions coordinated by S donor atoms was observed in the NMR spectra, even at Cd/C ratios that were significantly lower than the S/C ratio of the NOM.