$^{113}Cd-NMR$ investigation of the interaction between Cd(Ⅱ) and naturally occurring and synthetic polyelectrolyte ligands

Abstract

The interaction of $CdCl_2$ with humic acid (HA) and water soluble synthetic polymer PMAA in aqueous media was investigated using Cd-NMR technique. When progressively increasing amounts of HA(Aldrich-HA:3.68-22.10g/1 ; $T_3$-HA : 4.56-26.14g/1) and PMAA(1.94-11.65 g/1) was added to the $CdCl_2$ solutions ($5.25\times10^{-2}$ mol/1), the intensity of the free Cd ion decreased steadily. Applying suitable NMR conditions (the pulse width of 30$^\circ$, the total delay of 25s), the concentration of free aquo Cd species was determined by integration of the Cd peak intensities. The changes in the chemical shift of the Cd peak under rapid exchanging conditions suggest that $Cd^{2+}$ is predominantly bound to HA and PMAA. Although numerous attempts were carried out (MAS-NMR, Solution-NMR with varying the temperature and concentrations), the Cd-HA complex peak were not observed, probably due to slow tumbling motion of the large sized HA molecules. However, the Cd-PMAA complex peak is observed near-8 ppm, showing that $Cd^{2+}$ ions are readily binding to carboxylate group of the macromolecules. The IR spectra data also provide evidence of the carboxylic group participation in the $Cd^{2+}$ complexatind process. The maximum binding ability of HA and PMAA for $Cd^{2+}$ ions in aquifer systems is determined directly to be 1.66$\pm$0.13, 0.98$\pm$0.13, and 2.70$\pm$0.08 mmol/g for Aldrich-HA, T3-HA, and PMAA respectively. The conditional stability constant ditermined using ultrafiltration method coupled to AAS (Atomic Absorption Spectroscopy) at high ligand metal ion concentrations were log$\beta$= 3.42$\pm$0.04, 3.28$\pm$0.04, and 4.09$\pm$0.28 1 mol$^{-1}$ for Aldrich-HA, T3-HA, and PMAA respectively. This shows that cation binding sites of natrually occurring polyelectrolyte are more diversified.