Surface complexation modeling of Pb(II), Cd(II) and Se(IV) onto iron hydroxides in single and bisolute systems
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Many environmental problems require reliable quantitative prediction of the fate and transport of metal ion contaminants in surface and groundwaters. Surface Complexation Models (SCMs) have emerged as the most promising tools for predicting contaminant ion sorption to iron (hydr)oxides. The reliability of SCMs is highly dependent on the ability to select appropriate surface complexation reactions and model parameters. This research focused on developing a strategy for developing a selfconsistent database for the Triple Layer Model (TLM) parameters for two iron (hydr)oxides, ferrihydrite (HFO) and goethite (α-FeOOH). To this end, spectroscopic data were used to guide the selection of surface complexation model reactions, and to the extent possible, theoretically based parameter estimation techniques were used to characterize the mineral surfaces throughout this work. The adsorption of two divalent metals, Cd(II) and Pb(II), and one oxyanion, Se(IV) onto α-FeOOH and ferrihydrite was modeled in single and bi-solute systems using the TLM. Selection of all surface complexes was supported by evidence from extended x-ray absorption fine structure spectroscopy. The fixed surface site density values based on tritium exchange experiments were used (Ns = 10.2 and 16.4 sites /nm2 for ferrihydrite and goethite, respectively). Ferrihydrite was modeled using one surface site while goethite was modeled with strong and weak sites, with the following distribution: Nwk = 90% and Nst =10%. Cd(II) and Pb(II) adsorption were modeled using bidentate and monodentate surface complexes onto ferrihydrite. The model was able to predict the competitive adsorption behavior of Cd(II) and Pb(II) in bisolute systems. For the goethite system, Cd(II) and Pb(II) adsorption was modeled using bidentate surface complexes on both weak and strong goethite surface sites. The TLM was able to predict the competitive adsorption behavior of Cd(II) and Pb(II) in bisolute systems. Se(IV) adsorption onto goethite was investigated in the presence of Cd(II) or Pb(II) and was modeled using bidentate surface complexes. The TLM predicted data reasonably well in low and medium surface coverage single-solute systems; however, it did not predict adsorption behavior for a high surface coverage adsorption edge. In bisolute systems, the TLM overestimated adsorption enhancement for low and medium surface coverage adsorption edges. TLM predictions using lower surface site densities improved the oxyanion predictions but diminished the quality of the divalent metal ion predictions.