Characterization of Metal Ion Binding Sites on Datura Innoxia by Using Lanthanide Ion Probe Spectroscopy

Abstract

The excitation spectra associated with the 7F0↦5D0 transition of Eu+3 has been used to examine the binding sites on cell wall fragments of Datura innoxia. Both native and esterified cell wall fragments were each examined at pH 5 and pH 2 to determine the contributions to metal ion sorption from both the carboxylate and sulfonate functional groups. The excitation spectra have been de-convoluted into the individual groups responsible for metal ion uptake. At least four unique binding sites can be described as being responsible for metal ion uptake. The higher affinity sites involve carboxylates in the binding of Eu+3 in a tridentate (3:1 ligand-to-metal ratio) configuration.

Competitive Metal Ion Binding to Datura innoxia Root and Stem Tissue Materials

“…The magnitude of these associated condition affinity constants (10 4 to 10 9 ) suggests the involvement of surface metal ion complexes [8][9]. This further suggests the potential for greater retention of these metal ions by both root and stem tissues.…”

“…Significant progress continues toward elucidation of chemical processes responsible for phytoextraction interactions [4][5][6][7][8][9][10][11][12][13][14][15]. These include: metal-ion surface complex formation with tissue functional groups [8][9][10][11][12][13][14][15][16]; ion-exchange interactions [10][11][12][13][14][15]; and electrostatic (Coulombic) attraction of metal cations with a net negatively charged plant material [13][14][15]17]. These interactions can be distinguished by decreasing interaction strength and associate affinities [6,15,17].…”

“…These included x-ray [7], infrared [18], and metal luminescence spectroscopy [6,15,19], chromatographic techniques [8,20], and binding isotherm data analysis and simulation [5]. Those studies revealed oxy-anion involvement, typically as carboxylates, in the biosorption of a variety of metal ion species [7,9] on non-viable biomaterials ranging in biocomplexity from single-cell algae to intact plant tissues [5,[6][7][8][9][13][14][15].…”

“…Unfortunately, many of these past studies were restricted to single metal-ion exposure conditions [4][5][6][7][8][9][10][11][12][13][14][15]18]. The potential impact of competitive metal ion binding to specific sites was not addressed in those earlier studies.…”

“…Eu(III) was included because of its chemical similarity to Am(III) [22,23] and its extensive investigation within our laboratory (e.g. [6,9,15]). D. innoxia was selected because of several favorable characteristics pertaining to potential use in phytoextraction applications (e.g.…”