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

Dried root and stem tissue samples from the plant, Datura innoxia, were exposed to solutions containing six metal ions, Zn2+, Cu2+, Cd2+, Ni2+, Pb2+, and Eu2+. The resulting binding isotherms were generated using multiline atomic emission spectra of supernatant solutions. Application of regularized regression analysis to these isotherms revealed binding sites exhibiting both low and high value conditional affinities. Low affinity sites (1.5×10-2 to 2.5×10-1) were postulated to involve electrostatic physisorption processes with large potential capacities (0.11 to 3.2 moles gram-1). These involved Cd2+, Cu2+, Ni2+, Pb2+ and Zn2+ ions. High affinity sites revealed conditional constants varying from 2.5×102 to 1.3×109 with potential capacities of 12 μmoles g-1to 0.15 moleg-1. These were attributed to surface complex formation and included each metal ion studied. This is the first reporting of relative affinities and capacities for metal ion biosorption under competitive conditions involving seven metals. Similarities in measured affinity constants for each metal when comparing root and stem tissue materials suggest similar predominate chemical interactions involving either of these binding mechanisms. These results support mechanisms involving carboxylates for both root and stems materials. Site densities clearly show the vast majority of binding interactions to be governed by more general electrostatic forces. Such would be advantageous for the application of this plant for phytoextraction purposes.