Gallic acid for the sequestration of Cd2+ and Pb2+ in aqueous solutions

Gallic acid (GA), whose structure is reported in Figure 1, is a naturally occurring polyphenol present in citrus fruit peels, in grapes, hops, oak bark, green tea, displaying antioxidant properties1. This contribution reports the preliminary results of a potentiometric study on the acid-base properties of GA at T = 298.15 K and I/mol dm-3 in NaCl, KCl and (CH3)4NCl ionic media. The gallic acid binding ability towards two natural systems contaminants such as Cd2+ and Pb2+ was also investigated at various ionic strengths in NaCl(aq) by means of potentiometry and UV-Vis spectrophotometry, to evaluate the ligand potential as a sequestering agent for the mentioned cationic pollutants in environmental matrices. The analysis of the acid-base data allowed the determination of the gallic acid protonation constants, which were found to be in good agreement with literature findings2. The weak association constants of Na+/GA and K+/GA species were determined using the ǻpK method3. The dependence of the thermodynamic parameters on ionic strength was modelled using an extended Debye-Hückel type equation. Furthermore, the ligand sequestering ability and the affinity towards the selected cationic contaminants were evaluated by means of the calculation of pL0.54 and pM5 parameters at various conditions including pH ~ 8.1 and I ~ 0.70 mol dm-3, typical of seawater. Lastly, speciation diagrams on the gallic acid behaviour towards Cd2+ and Pb2+ were drawn in the presence of the natural components of seawater as well. References: [1] S. Hussain, S. A. Farooqui, Rahim, Int. J. Emerg. Technol. Appl. Eng. Technol. Sci. 2013, 6-7, 276-279. [2] H. K. J. Powell, M. C, Taylor, Aust. J. Chem. 1982, 35, 739-756. [3] P.G. Daniele, C. Foti, A. Gianguzza, E. Prenesti, S. Sammartano, Coord. Chem. Rev. 2008, 252, 1093±1107. [4] F. Crea, C. De Stefano, C. Foti, D. Milea, S. Sammartano, Curr. Med. Chem. 2014, 21, 3819-3836. [5] K. N. Raymond, C. J. Carrano, Acc. Chem. Res. 1979, 12, 183-190