Metal Complexation with Different types of Soluble and Adsorbed Freshwater Ligands Followed by DPASV

Abstract

In order to understand metal speciation in a polluted river (Este River, Northern Portugal) filtrate, freeze dried particles and organics desorbed from surfaces were titrated with Cd(II) and Zn(II), followed by differential pulse anodic stripping voltammetry (DPASV). The obtained results are compared with those previously published for Pb(II) and Cu(II). Due to the heterogeneity of the system, a continuous and a discrete ligand model were used to interpret the titration data. Two types of ligands could be detected and quantified by the discrete ligand model: small molecules with high affinities for cations such as Cd(II), Cu(I), and Zn(II) and macromolecules with higher affinities for Pb(II) and Cu(II). Small ligands were strongly adsorbed onto the particles, as inferred from the desorption of Zn(II) during titration with Pb(II) and Cd(II). The total concentrations of the different ligands and the complex formation constants with the different metals are reported.

Appendix

In labile heterogeneous complexing systems, free metal ions are in equilibrium with different complexing sites. If all complexes have similar sizes, significantly different from that of the metal ion, a mean diffusion coefficient should be defined as:

$$ \overline D =\frac{D_{\rm M} \left[ \hbox{M} \right]+D_{\rm ML} \left[ \hbox{ML} \right]}{\left[ \hbox{M} \right]_{\rm t}} $$

(6)

where D _M and D _ML are the diffusion coefficients for simple and complexed metal ion, respectively. In this case the current intensity peak, measured during voltammetric titrations, will be proportional to total metal concentration in solution:

$$ i_{\rm p}^{\rm M+L} =S\overline D ^{1/2}\left[ \hbox{M} \right]_{\rm t} $$

(7)

where S is the proportionality constant that can be taken from calibration plot. So, from i _p we can determine $\overline D$ and, using Eq. 6, values of mean stability constants $\overline K (i_{\rm p})$ can be determined.

If peak potential shifts to anodic values during titration with metal ion, complexes are labile and, if there is an excess of total ligand concentration at the electrode surface during the stripping step, we have, at 20°C for potentials expressed in mV:

$$ \Delta E_{\rm p}=E_{\rm ps}-E_{\rm pc}=\frac{58.16}{n}\left[ {\log \left( {1+\left[ \hbox{L} \right]_{\rm t} K (E_{\rm p})} \right)+\frac{1}{2}\log \frac{\overline D }{D_{\rm M}}}\right] $$

(8)

where E _ps and E _pc are the peak potentials for free and complexed metal ions, respectively. Using Eq. 8 values of mean stability constants $\overline K (E_{\rm p})$ could be estimated.

Using DPASV technique for heterogeneous systems the lability may be different during deposition and stripping scans, even for similar time scales of both steps. In fact, in this case each step is associated to different affinity parameters due to different ligand to metal ratios in each diffusion layer. Indeed for similar time scales if fully lability is obtained during deposition, the same is true for the stripping since a lower ligand to metal ratio is obtained. The dissociation process lability can be assessed comparing the current intensity values (i _p) for different stirring rates during the deposition step. The system will be labile if the ratio $\phi =\frac{i_{\rm p}^{\rm M+L}}{i_{\rm p}^{\rm M}}$ does not change when increasing the stirring rate. For quasi labile complexes the increase on stirring rate would decrease $\phi$ values because, the time to attain equilibrium decreases when increasing the stirring rate (Botelho et al 2001).