Use of amino–polycarboxylic chelating agents for the sequestration of (CH3)Hg+ ion in aqueous solution

Abstract

Among the mercury organic derivatives, mono-methylmercury (MMHg) is by far the most toxic form of mercury because of its high capacity to enter biological membranes and accumulate in living organisms [1], as shown by the biomagnification factors: ~104 from water to edible shell-fish (mussels), and 106 –108 from water to big fishes (e.g. tuna fish) at the top of food chain [2]. The presence of complexing agents in the environment, which form soluble complex species with MMHg, favours the mobility of the ion in the aquatic ecosystems. Among anthropogenic complexing molecules, the most used ones are the aminopolycarboxylic chelating agents (usually called with the acronym APC) which are employed as metal ions sequestering agents in several application fields [3]. With the aim to assess the sequestering capacity of APCs towards mono-methylmercury(II) ion, we report here the results of a systematic study on the interaction of (CH3)Hg+ with nitrilotriacetate (NTA), ethylenediamine-N,N,N’,N’-tetraacetate (EDTA) and diethylenetriamine-N,N,N’,N’’,N’’- pentaacetate (DTPA). Moreover, since most of aminopolycarboxylic ligands are little biodegradable [3, 4], including those above mentioned, and their widespread use could be of environmental concern, we investigated also the interaction of MMHg with (S,S)- Ethylenediamine-N,N′-disuccinic acid (S,S-EDDS) which is, among the APCs, the most biodegradable one. The results obtained in this work show that the stability of MeHg - APC complex species is very similar for DTPA, EDTA and S,S-EDDS (e.g., logKMeHg(APC) = 10.14, 10.03, 10.14 for DTPA, EDTA and S,S-EDDS, respectively) whereas, as expected, MeHg - NTA complexes are less stable (logK(MeHg)(NTA) = 9.04). Stability of (CH3)Hg-S,S-EDDS species show that this ligand can be used successfully as environmental friendly chelating agent alternatively to other less degradable APCs in all their application fields. The sequestering ability of APCs towards MeHg+ ion was studied by calculating the sequestering parameter pL0.5 in the pH range (3 to 11) investigated, in NaCl medium and also by simulating the presence of a non interacting medium (NaX), at the same ionic strength of the experiments. As an example, in Figure are reported the sequestration curves of the complexones towards the MeHg+ ion at pH = 7, in interacting and non interacting ionic media, at I = 0.1 mol L-1 and T = 25°C.