Interaction of Inorganic Mercury(II) with Polyamines, Polycarboxylates, and Amino Acids

A study on chemical equilibria of different Hg2+-organic ligand systems is reported to establish the sequestration power of N/O donor groups versus metal. Ligand classes considered are: O-donors (succinate, 1,2,3,4-butanetetracarboxylate, 1,2,3,4,5,6-benzenehexacarboxylate, polymethacrylate, and polyacrylates); N-donors (ethylenediamine, diethylenetriamine, spermidine, triethylentetramine, tetraethylenpentamine, poly(allylamine), and poly(ethylenimine)); and amino acids (glycine, histidine, and aspartic acid). Equilibria were studied potentiometrically in NaNO3 at I ) 0.1 mol ·L-1 and t ) 25 °C. For some systems, the dependence on ionic strength in the range (0 e I e 1) mol ·L-1 (NaNO3) and the overall enthalpy changes, by calorimetric measurements, were also determined. The sequestering ability of ligands toward Hg2+ was evaluated, defining the amount of ligand necessary to sequester 50 % of the total metal cation concentration, pL50. The dependence on temperature and on ionic strength of pL50 was determined. General trends for the formation parameters for Hg2+-O-donor and -N-donor species depending on the binding sites were found. The free energy of binding for both unprotonated and monoprotonated species of carboxylates (ΔGb ) -21.7 ( 3 kJ· mol-1) and for unprotonated species of amines (ΔGb ) -35.4 ( 5 kJ· mol-1) was estimated.