Anion Dependence of Transport of Mercury Ion through Nafion-117 Membrane

Abstract

Studies on isotopic and ion-exchange kinetics of mercury ions in Nafion-117 membrane have been carried out with (203)Hg radiotracer in the presence of Cl(-) and NO(3)(-) in solution. The results of isotopic-exchange kinetics indicate that mercury ions diffuse into the membrane as monovalent cation from HgCl(2) solution while as divalent ion from Hg(NO(3))(2) solution. The studies on the kinetics of ion exchange of Hg(2+) with Na(+) follow the prediction of the Nernst-Planck equation when NaNO(3) is used as an external salt solution. The Nernst-Planck equation fails to predict the kinetics when NaCl is used as an external salt solution, indicating that the complexation of Cl(-) with Hg(2+) in the membrane influences the kinetics. Permeation studies using (203)Hg and (36)Cl radiotracer between two HgCl(2) solutions show that the permeability coefficients of mercury and chloride ions are the same, indicating the cotransport of mercury and chloride ions through the membrane. Ion-exchange equilibrium studies using a mixture of HgCl(2) and HNO(3) solution were carried out to ascertain the species transporting through the membrane. The equilibrium sorption of mercury in the membrane shows the uptake of an ionic species, presumably HgCl(+), not a neutral salt. The speciation diagrams, calculated as a function of pH, show wide divergence of species present in HgCl(2) and Hg(NO(3))(2) solution and explain the difference in membrane transport behavior for HgCl(2) and Hg(NO(3))(2) solution. The results show that any ion-exchange-membrane-based separation of Hg(2+) needs careful consideration regarding the anions present in the solution, as it influences the speciation of mercury and hence its transport behavior through the membrane.