Sequestration of organometallic compounds by natural organic matter. Binding of trimethyltin(IV) by fulvic and alginic acids

The binding capacity of fulvic and alginic acids towards trimethyl tin(IV) cation was quantitativelydetermined in order to evaluate the sequestering ability of toxic organometallic compounds bynatural organic matter. Investigations were performed in the pH range of natural waters (5–8.5)where the carboxylate groups, largely present in both sequestering agents, are the main binding sites.A chemical interaction model, according to which both the protonation of polyelectrolyte ligandsand the hydrolysis of the organotin cation in NaCl aqueous solution were considered, was used todefine the speciation of the systems under investigation. Measurements performed at different ionicstrength values (0.1, 0.25, 0.5 and 0.7 mol L−1,NaCl) allowed us to consider the dependence of stabilityconstants on the ionic strength, and to calculate the formation constants at infinite dilution. Resultsobtained show the formation of the complex species TMT(L), TMT(L)2 and TMT(L)(OH) for L=fulvicacid and TMT(L) for L = alginic acid, respectively. In order to compare the strength of interaction ofthese natural poly electrolytes with other analogous synthetic polyelectrolytes, measurements werealso carried out on the trimethyltin(IV)–polyacrylate (5.1 kDa) system, and in this case the formationof TMT(L), TMT(L)2 and TMT(L)(OH) species was found. Results show the following trend ofstability for the species TMT(L) in the systems investigated: TMT–fulvate ≈ TMT–polyacrylate >TMT–alginate. On the basis of the stability data obtained, the lowest concentration of fulvic andalginic acids, able to act as sequestering agents towards triorganotin(IV) cation in the conditions ofnatural waters, was also calculated.