¹¹³Cd NMR and Fluorescence Studies of Multiple Binding Mechanisms of Cd(II) by the Suwannee River Fulvic Acid

Abstract

The binding of Cd (II) by the fulvic acid from a Suwannee River (FA) was investigated at various pH values and reactants ratios by 113Cd NMR and fluorescence spectroscopy. The NMR results provided evidences that the FA-Cd interactions occur through a variety of binding modes and mechanisms. Different kinds of organically bound Cd-species were detected in the 1.8-10.8 pH range depending on the FA/Cd ratios. Labile complexes (amenable to Cd-aminoacidic and Cd-hydroxy interactions or outer-sphere complexes) were observed at low pH and FA/Cd levels while stronger interactions (of carboxylate-type or inner-sphere complexation) took place as the pH and/or the FA concentration were increased. At pH ca. 6 insoluble FA-Cd adducts were primarily produced but, at relatively large FA concentration, only soluble complexes, stable in the whole pH 1.8-10.8 range, were formed. A complementary analysis, by fluorescence spectroscopy, provided clear evidences of FA-Cd association/aggregation phenomena. While no noticeable effects occurred with soluble samples, the formation of insoluble adducts led to significant enhancements of the emission fluorescence spectra. Although other explanations could not be excluded, this result was accounted for by modifications of the optical properties of the ligand itself due to sedimentation of the heavier components. Fluorescence enhancement was also observed on samples before the effective precipitation and interpreted as spectroscopic evidence of the onset of aggregation phenomena.

Application of a constrained regularization method to extraction of affinity distributions: Proton and metal binding to humic substances

“…Otto et al [66,67] studied Cd(II) binding to fulvic acids using 113 Cd chemical shift and relaxation measurements, concluding that there are two clearly differentiated sites: one strong binding site best represented by a polydentate carboxylate model (albeit an hydroxycarboxylate site could also be possible), and another weaker site, attributed to monocarboxylate binding. This last type of site was also considered as the more abundant by Grassi and Daquino [74]. Thus, the results of Fig.…”

“…Cd(II) binding to HS has been quite intensely studied [10,16,[66][67][68][69][70][71][72][73] and, owing to the relative abundance of the 113 Cd isotope, NMR studies have provided important structural information on the binding sites [66,67,74,75]. Otto et al [66,67] studied Cd(II) binding to fulvic acids using 113 Cd chemical shift and relaxation measurements, concluding that there are two clearly differentiated sites: one strong binding site best represented by a polydentate carboxylate model (albeit an hydroxycarboxylate site could also be possible), and another weaker site, attributed to monocarboxylate binding.…”

Nuclear Magnetic Resonance Analysis of Natural Organic Matter

“…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 ).…”