Recognition of Heavy Metals by Using Resorcin[4]arenes Soluble in Water

Abstract

The complexing properties of two water-soluble resorcin[4]arenes (tetrasodium 5,11,17,23-tetrakissulfonatemethylen 2,8,14,20-tetra(butyl)resorcin[4]arene, Na₄BRA, and tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(2-(methylthio)ethyl)resorcin[4]arene, Na₄SRA) with polluting heavy metals such as Cu²⁺, Pb²⁺, Cd²⁺ and Hg²⁺ were studied by conductivity, and the findings were confirmed by using other techniques to try to apply this knowledge to removing them. The results indicate that Na₄BRA is able to complex Cu²⁺ in a 1:1 ratio and Pb²⁺ in a 1:2 ratio, while Na₄SRA complexes Hg²⁺ in a 1:1 ratio. On the contrary, no indications have been observed that either of the resorcin[4]arenes studied complexes the Cd²⁺ ions. The results suggest that the bonds established between the sulfur atoms located at the lower edge of the SRA^4- and the solvent hydrogens could prevent the entry of the guest into the host cavity. However, in the case of Hg²⁺ ions, the entry is favoured by the interactions between the sulfur donor atoms present on the lower edge of Na₄SRA and the Hg²⁺ ions. Therefore, it can be said that Na₄BRA is selective for Cu²⁺ and Pb²⁺ ions and Na₄SRA is selective for Hg²⁺ ions.

Keywords: heavy metals; inorganic pollutants; recognition; resorcin[4]arenes.

Figure 1

Resorcin[4]arene sulfonate structure (Na₄RA).

Figure 2

Resorcin[4]arenes sulfonated, whose complexing properties with Cu²⁺, Pb²⁺, Cd²⁺ and Hg²⁺ were evaluated. (a) Tetrasodium 5,11,17,23-tetrakissulfonate methylen-2,8,14,20-tetra(butyl)resorcin[4]arene (Na₄BRA). (b) Tetrasodium 5,11,17,23-tetrakissulfonatemethylen-2,8,14,20-tetra(2-(methylthio) ethyl)resorcin[4]arene (Na₄SRA).

Figure 3

Specific conductivity versus the [Cu²⁺]/[Na₄BRA] ratio. The solid blue lines (which are represented displaced parallel for better visualization) are shown as a visual aid to indicate the cut-off point that relates to the change in the slope of the specific conductivity with respect to the stoichiometry of the complex.

Figure 4

Potentiometric titration of Cu²⁺ with Na₄BRA, using a copper selective electrode (Cu-ISE). The lines are shown as a visual aid to identify the 1:1 stoichiometry of the complex.

Figure 5

Specific conductivity versus the [Pb²⁺]/[Na₄BRA] ratio. The solid blue lines are shown as a visual aid to indicate the cut-off point that relates the change in slope of the specific conductivity with respect to the stoichiometry of the complex.

Figure 6

Specific conductivity versus the [Hg²⁺]/[Na₄BRA] and [Cd²⁺]/[Na₄BRA] ratios. The solid blue lines are displayed as a visual aid to see that there is no appreciable change in the slope of the plot.

Figure 7

Specific conductivity versus the [Hg²⁺]/[Na₄SRA] ratio. The solid blue lines are shown as a visual aid to indicate the cut-off point that relates the change in slope of the specific conductivity with respect to the stoichiometry of the complex.

Figure 8

Specific conductivity versus the [guest]/[Na₄SRA] ratio; being the guest (Δ) Pb²⁺, (◊) Cd²⁺, (•) Cu²⁺. The solid blue lines are displayed as a visual aid to see that there is no appreciable change in the slope of the plots.