Thermodynamic data for Pb2+ and Zn2+ sequestration by biologically important S-donor ligands, at different temperatures and ionic strengths

Abstract

Thermodynamic parameters for the interactions of cysteine (cys) and penicillamine (psh) with Pb(II) and glutathione (gsh) with Pb(II) and Zn(II) were determined in NaNO₃ or NaCl aqueous solution by potentiometry, at different ionic strengths (0 < I ≤ 1 mol L⁻¹) and temperatures (15 ≤ t ≤ 45 °C). For Pb²⁺ systems, the formation of PbL, PbLH, PbLH₂ and PbL₂ species was found, together with PbLOH for cys and psh, and PbL₂H and PbL₂H₂ for gsh. The speciation models for Pb²⁺–psh and –gsh systems were confirmed by UV spectrophotometric measurements. For the Zn²⁺–gsh system, a more complex speciation model was obtained with the formation of ZnL, ZnLH, ZnLH₂, ZnL₂, ZnL₂H, ZnL₂H₂, ZnLOH and ZnL₂OH species. From the dependence of formation constants on temperature, rough ΔH values were evaluated: the main contribution to the complexation free energy is entropic in nature, with small enthalpic values. Moreover, from the dependence on ionic strength, formation constants extrapolated at I = 0 mol L⁻¹ were obtained. The sequestering ability of the ligands towards Pb²⁺ and Zn²⁺ was evaluated by determining the pL_0.5, i.e. the −log of the concentration of the ligand able to complex half of the metal ion fraction. All these ligands show good sequestering ability. For Pb²⁺–cys, –psh and –gsh systems, pL_0.5 reaches the value of 8.2, 9.0 and 5.9, respectively, at pH = 7, I = 0.1 mol L⁻¹ and t = 25 °C. Under the same experimental conditions, the sequestering ability of gsh towards Zn²⁺ is lower, with pL_0.5 = 4.1.