Modeling the adsorption of free and heavy metal complex-bound EDTA onto red mud by a nonelectrostatic surface complexation model

摘要

The adsorption of free and divalent heavy metal (copper, cadmium, and lead) complex-bound EDTA from meal-EDTA mixture solutions on a composite adsorbent having a heterogeneous surface, i.e., bauxite waste red mud, has been investigated and modeled wtih the aid of a nonelectrostatic surface complexation approach in respect to adsorbate concentration and pH dependency of EDTA adsorption. EDTA was selected as the modeling ligand in view of its wide usage as an anthropogenic chelating agenta nd its abundance in natural waters. The adsorption experiments were conducted for pure EDTA or metal-EDTA complexes alone, or in mixtures containing (EDTA + metal-EDTA). For all studied cases, the solid adsorbent pahse concentrations of the adsorbed species were found by using the derived model equations with acceptable ocmpatibility of experimental and theoretically generated adsorption isotherms. The model basically assumed two modes of metal bonding to the surface hydroxyls: ionic (outer-sphere) binding of the EDTA anion (H_2Y~(2-)) or anionic metal-EDTA complex (MY~(2-)) to the cationic surface site (approx SOH_2~+), and outer-sphere binding of H_2Y~(2-) or MY~(2-) to the neutral approx SOH site, possibly via hydrogen-bonding. The model was useful for EDTA and metal-EDTA mixture solution either at their natural pH of equilibration with the sorbent, or after pH elevation with NaOH titration up to pH <= pzc of red mud. Thus adsorption of every single species (H_2Y~(2-) or MY~(2-)) or of possible mixtures (H_2Y~(2-) + MY~(2-)) at natural pH or after NaOH titration could be calcualted by the use of simple quadratic equations at low metal loadings, once the initial concentrations of the corresponding species, i.e., [H_2Y~(2-)]_0 or [MY~(2-)]_0, wre known. The compatibility of theoretical and experimental data pairs of adsorbed species concentrations was verified by means of nonlinear regression analysis. The findings of this study, together with the previously developed (M~(2+) + MY~(2-)) mixtures adsorption model, can be further developed to serve environmental risk assessent concerning the expansion of a metal-organic (synthetic organic ligand or soil humic acids) contaminant plume with ground water movement in soil basically consisting of hydrated oxide-type minerals.