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

Abstract

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 (B(0) = 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.

Dissolved Organic Matter in Freshwaters

“…In several studies (e.g., Larive et al, 1996;Li et al, 1998;Otto et al, 2001a,b;Hertkorn et al, 2004;Perdue et al, 2007), 113 Cd NMR spectrometry has been shown to provide far greater insight into the nature of the ligands to which Cd(II) is bound in Cd-DOM complexes and the rates of exchange reactions between the free and various complexed forms of Cd(II). It has been shown clearly that chemical exchange is quite fast at acidic-to-neutral pH and that it becomes much slower at alkaline pH.…”

Nuclear Magnetic Resonance Analysis of Natural Organic Matter

“…The most obvious choices for studies of NOM are DMSO -d 6 or D 2 O (or D 2 O/NaOD, depending on the solubility of the NOM). D 2 O and D 2 O/NaOD have been employed in numerous NOM studies (Buddrus et al, 1989 ;Larive et al, 1996 ;Dixon et al, 1999 ;Fan et al, 2000 ;Simpson et al, 2001c ;Wang et al, 2003 ;Hertkorn et al, 2004 ;Peuravuori, 2005 ) while others utilize DMSO -d 6 (Wais et al, 1996 ;Simpson, 2001 ;Simpson et al, 2003a ;Uyguner et al, 2004 ;Hertkorn et al, 2006 ;. D 2 O and D 2 O/NaOD have advantages in that the solution conformation adopted by the solute are likely to be more predictable than those adopted in DMSO.…”

“…A number of studies of metallic nuclei have examined: aluminum (Howe et al, 1997 ;Lookman et al, 1997 ;Matthias et al, 2003 ), cesium (Xu et al, 2006 ), vanadium (Lu et al, 1998 ), europium , and lithium (Nielsen et al, 2005 ). However, the most widely applied NMR studies of heavy metals have been the study of cadmium (Chung et al, 1996 ;Larive et al, 1996Larive et al, , 1997Li et al, 1998 ;Otto et al, 2001a,b ;Simpson, 2002 ;Hertkorn et al, 2004 ;Grassi and Daquino, 2005 ;Perdue et al, 2005 ). Cadmium NMR studies have been used to quantify 113 Cd binding (Chung et al, 1996 ;Larive et al, 1996 ;Otto et al, 2001b ), evaluate chemical exchange (Larive et al, 1996 ), identify the groups (mainly oxygen, but also nitrogen and sulfur at certain pH values) responsible for binding (Hertkorn et al, 2004 ), and identify specifi c (inner/outer sphere) complexes (Grassi and Daquino, 2005 ).…”

“…However, the most widely applied NMR studies of heavy metals have been the study of cadmium (Chung et al, 1996 ;Larive et al, 1996Larive et al, , 1997Li et al, 1998 ;Otto et al, 2001a,b ;Simpson, 2002 ;Hertkorn et al, 2004 ;Grassi and Daquino, 2005 ;Perdue et al, 2005 ). Cadmium NMR studies have been used to quantify 113 Cd binding (Chung et al, 1996 ;Larive et al, 1996 ;Otto et al, 2001b ), evaluate chemical exchange (Larive et al, 1996 ), identify the groups (mainly oxygen, but also nitrogen and sulfur at certain pH values) responsible for binding (Hertkorn et al, 2004 ), and identify specifi c (inner/outer sphere) complexes (Grassi and Daquino, 2005 ). Similarly, in the area of organic contaminant interactions, a wide array of studies have been performed (Thorn et al, 1996a(Thorn et al, ,b, 1997Bortiatynski et al, 1997 ;Hinedi et al, 1997 ;Jayasundera et al, 1997 ;Nanny et al, 1997 ;Achtnich et al, 1999 ;Dixon et al, 1999 ;Green et al, 1999 ;Knicker et al, 1999 ;Nanny, 1999 ;Xiong et al, 1999 ;Bruns -Nagel et al, 2000 ;Kohl et al, 2000 ;Emery et al, 2001 ;Smernik and Oades, 2001 ;Kacker et al, 2002 ;Thorn and Kennedy, 2002 ;Strynar et al, 2004 ;Smernik et al, 2006 ).…”

“…The most accessible information in relation to contaminant interactions can be simply inferred from a change in chemical shift of the contaminant species. In the case of metal NMR studies, the observed nucleus is most commonly the metal itself -for example, 113 Cd studies (Dehorter et al, 1992 ;Larive et al, 1996 ;Li et al, 1998 ;Otto et al, 2001a,b ;Hertkorn et al, 2004 ;Grassi and Daquino, 2005 ). In the case of an organic species, many different contaminant nuclei have been studied, including: 1 H Simpson et al, 2004c ), 2 H (Nanny, 1999 ;Nanny and Maza, 2001 ), 13 C (Hatcher et al, 1993 ;Wais et al, 1996 ), 15 N ( Thorn et al, 1996aThorn et al, ,b, 1997Weber et al, 1996 ), and 19 F (Kohl et al, 2000 ;Khalaf et al, 2003 ;Strynar et al, 2004 ).…”