GALLIC ACID FOR THE SEQUESTRATION OF ORGANOMETALS IN AQUEOUS SOLUTIONS

Gallic acid is a naturally occurring polyphenol present in citrus fruit peels, in grapes, hops, oak bark, green tea, displaying antioxidant properties 1. This contribution reports preliminary results from an investigation on the acid-base properties of gallic acid and its interactions with two seawater pollutants, namely CH3Hg+ and (CH3)2Sn2+, also with the purpose of exploring its potential as a sequestering agent for organometals in environmental matrices. Potentiometric, UV-Vis spectrophotometric and spectrofluorimetric titrations were carried out at different temperatures (288.15 ≤ T/K ≤ 318.15) and ionic strengths (0.10 ≤ I/mol dm-3 ≤ 1.00) in NaCl ionic medium, the principal inorganic constituent of many natural fluids 2. The analysis of the acid-base data allowed the determination of the ligand protonation constants, which were found to be in good agreement with literature findings 3. The enthalpy change values were also determined. For each organometal/gallic acid system, speciation schemes featured by protonated species, 1:1 stoichiometry complexes and hydrolytic mixed species were determined. Gallic acid showed a higher complexing ability towards (CH3)2Sn2+ with respect to CH3Hg+. The dependence of the thermodynamic parameters on ionic strength and temperature was modelled using an extended Debye-Hückel type and the Van’t Hoff equations, respectively. Furthermore, the ligand sequestering ability and the affinity towards the selected cationic contaminants were investigated by means of the calculation of pL0.5 4 and pM 5 parameters at various conditions including pH ~ 8.1 and I ~ 0.70 mol dm-3, which are characteristic of seawater. References: 1. Hussain, S., Farooqui, & Rahim, S. A., Int. J. Emerg. Technol. Appl. Eng. Technol. Sci. 6-7 (2013) 276-279. 2. Buffle, J, 1988. Complexation Reactions in Aquatic Systems: an Analytical Approach. Ellis Horwood: Chichester. 3. Powell, H. K. J., & Taylor, M. C,. Aust. J. Chem. 35 (1982) 739-756. 4. Crea, F., De Stefano, C., Foti, C., Milea, D., & Sammartano, S., Curr. Med. Chem. 21 (2014) 3819-3836. 5. Raymond, K. N., & Carrano, C. J., Acc. Chem. Res. 12 (1979) 183-190.