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

Abstract

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 113 Cd NMR spectroscopy. The 113 Cd 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 CdL 2 (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. Key words: fulvic acid, polyfunctionality, binding sites, chemical shift, 113 Cd NMR.