6533b7d4fe1ef96bd126354f

RESEARCH PRODUCT

Sequestering ability of some chelating agents towards methylmercury(II).

subject

description

A study on the interactions between CH3Hg+ and some S, N and O donor ligands (2-mercaptopropanoic acid (thiolactic acid (H2 TLA)), 3-mercaptopropanoic acid (H2 MPA), 2-mercaptosuccinic acid (thiomalic acid (H3 TMA)), d,l-penicillamine (H2 PSH), l-cysteine (H2 CYS), glutathione (H3 GSH), N,N′-bis(3-aminopropyl)-1-4-diaminobutane (spermine (SPER)), 1,2,3,4,5,6-benzenehexacarboxylic acid (mellitic acid (H6 MLT)) and ethylenediaminetetraacetic acid (H4 EDTA)) 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 of ML1–z and MLH2–z complex species is observed, together with a diprotonated MLH 2 3–z species for CYS 2−, PSH 2− and GSH 3− and the mixed hydrolytic one ML(OH)−z for TLA 2− and MPA 2−. The dependence of the stability on ionic strength and on temperature is also analysed. In the other CH3Hg+-L systems (L = MLT 6−, SPER and EDTA 4−), ML1–z , MLH2–z and MLH2 3–z complex species are formed, together with the MLH3 4–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 10−11 mol L−1 level, in the pH range 4 to 8) following the trend MPA 2− < PSH 2− < GSH 3− < TLA 2− < CYS 2− < TMA 3−, 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).