Sequestering ability of some chelating agents towards methylmercury(II)

A study on the interactions between CH3Hg+ and some S, N and O donor ligands (2-mercaptopropanoic acid (thiolactic acid (H2TLA)), 3-mercaptopropanoic acid (H2MPA), 2-mercaptosuccinic acid (thiomalic acid (H3TMA)), D,L-penicillamine (H2PSH), L-cysteine (H2CYS), glutathione (H3GSH), N,N′-bis(3-aminopropyl)-1-4-diaminobutane (spermine (SPER)), 1,2,3,4,5,6-benzenehexacarboxylic acid (mellitic acid (H6MLT)) and ethylenediaminetetraacetic acid (H4EDTA)) is reported. The speciation models in aqueous solution and the possible structures of the complexes formed are discussed on the basis of potentiometric, calorimetric, UV spectrophotometric and electrospray mass spectrometric results. For the CH3Hg+–S donor ligand systems, the formation ofML1–z and MLH2–z complex species is observed, together with a diprotonated MLH23–z species for CYS2−, PSH2− and GSH3− and the mixed hydrolytic one ML(OH)−z for TLA2− and MPA2−. The dependence of the stability on ionic strength and on temperature is also analysed. In the other CH3Hg+-L systems (L0MLT6−, SPER and EDTA4−), ML1–z, MLH2–z and MLH23–z complex species are formed, together with the MLH34–z species for SPER, the mixed hydrolytic ML(OH)–z one for SPER and EDTA, and the M2L2–z for EDTA only. On the basis of the speciation models proposed, the sequestering ability of the ligands towards methylmercury(II) cation is evaluated. All S donor ligands show a good sequestering power (at 10−11molL−1 level, in the pH range 4 to 8) following the trend MPA2−<PSH2−<GSH3−<TLA2−<CYS2−<TMA3−, while significantly lower is the sequestering ability of MLT, SPER and EDTA (at 10−3 10−5 mol L−1 level, in the pH range 4 to 8).