Probe of cadmium(II) binding on soil fulvic acid investigated by ¹¹³Cd NMR spectroscopy

Binding of cadmium(II) on soil fulvic acid (FA) was investigated over a range of fulvate-to-cadmium concentration ratios (8 - 59 equiv. mol^-1) using ¹¹³Cd NMR spectroscopy. The ¹¹³Cd chemical shift of cadmium bound on fulvate was observed in a more downfield region (δ -20.4 to - 15.6) than that bound on synthetic polymers, poly(acrylic acid) (PAA: δ -36.6 to -38.2), poly(methacrylic acid) (PMAA: δ -34.0 to -25.4), and poly(vinyl benzoic acid) (PVBA: δ -34.7 to -31.2). The calculated values of individual chemical shifts for the species CdL⁺ and CdL₂ (L: carboxylate) formed in Cd(II)-carboxylate systems (e.g., acetate, benzoate) are δ -22 to -24 and δ -39 to -40, respectively. The relative downfield shift of cadmium(II)-fulvate suggests that functional groups (e.g., hydroxyl and neutral N donor) other than carboxylates may be involved in cadmium coordination. The chemical shifts of cadmium complexes of hydroxycarboxylates (e.g., glycolate) or carboxylates containing neutral N donor (e.g., picolinate) were generally observed in more downfield regions than their carboxylate counterparts.