A Potentiometric and ~(113)Cd NMR Study of Cadmium Complexation by Natural Organic Matter at Two Different Magnetic Field Strengths

摘要

The binding of cadmium to Suwannee River natural organic matter(NOM)has been investigated across a broad range of Cd/C ratios(0.00056-0.0056)and pH values(3.5-11)by ~(113)Cd NMR spectroscopy at two magnetic field strengths(Bo=9.4 and 11.7 T).Caused by the very peculiar and highly complex nature of the Cd- NOM exchanging system,these ~(113)Cd NMR spectra are characterized by a pH-and concentration-dependent superposition of slow,intermediate,and fast chemical exchange.The complex interplay of solution chemistry and chemical exchange requires a thorough mapping of this Cd-NOM chemically exchanging system through NMR acquisition at two magnetic field strengths and a systematic variation of Cd/C ratios and pH values.The interpretation of ~(113)Cd NMR spectra is greatly facilitated and constrained by simultaneous measurements of pH and pCd,which allows a model-independent calculation of organically bound Cd~(2+)under all experimental conditions.Within the range of chemical conditions applied in this study,~(113)Cd NMR spectrometric evidence is consistent with coordination of cadmium by oxygen,nitrogen,and sulfur ligands in NOM.Under all experimental conditions,cadmium is primarily coordinated to oxygen;however,several lines of evidence point to the participation of nitrogen ligands,even in acidic solutions where nitrogen ligands are primarily bound to protons.Under alkaline conditions,up to one-third of cadmium may be coordinated to nitrogen,and a small,but unquantifiable,percentage of cadmium is coordinated to sulfur ligands,as evidenced by far-low-field ~(113)Cd NMR resonances.