Sequestration of (CH3)Hg+ by amino-polycarboxylic chelating agents

The speciation of mono-methylmercury(II) cation (MeHg+) in the presence of Nitrilotriacetate (NTA), Ethylenediamine-N,N,N′,N′-tetraacetate (EDTA), diethylenetriamine-N,N,N′,N″,N″-pentaacetate (DTPA) and (S,S)- Ethylenediamine-N,N′-disuccinic acid (S,S-EDDS) was investigated at I=0.1 mol L−1 (NaCl) with the aim to assess a trend of sequestering capacity of the amino‐polycarboxylic (APCs) ligand class towards this cation in aqueous solution. The results obtained gave evidence for the formation of a mononuclear [MeHg(APC)] complex species, differently protonated MeHg(HiL) species (i=1 to 3, depending on the APC considered), a mixed hydroxo species [MeHg(APC)(OH)] and a binuclear [(MeHg)2(APC)] species in all the systems investigated. 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,SEDDS, respectively) whereas, as expected, MeHg–NTA complexes are less stable (logK(MeHg)(NTA)=9.04). 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, at the same ionic strength of the experiments. On the basis of the quite high pL0.5 values it can be asserted that APCs can be used as sequestering agents for the remediation of contaminated soils and water. Of particular interest, from the environmental point of view, is the very good sequestering capacity of the S,S,-EDDS which is, among the APCs considered, the highest biodegradable chelating agent.