MODELING THE IMPACT OF PH AND DISSOLVED SALT CONCENTRATIONS ON ZN, NI AND PB SORPTION AND TRANSPORT USING COMBINED ION EXCHANGE AND SURFACE COMPLEXATION MODELS

Sorption of Zn, Ni and Pb and each of the major cations was determined in two different artificial groundwaters (AGW) varying in Na, K, Mg and Ca concentrations over the pH range 4.8 to 6.0. The coarse sand (<2mm, CEC appr. 0.1meq/100g; A_sp = 0.49m²/g) consisted mainly of quartz and feldspars, with traces of goethite, and micrometer size clays (chlorite and illite) in mineral coatings. Because carbonate minerals are absent in this sand, pH was controlled by varying partial pressures of CO₂. To investigate proton sorption and the effect of protons on the release of major metal ions potentiometric titrations over a narrow pH range of 4.5 to 6.0 (to minimize mineral dissolution) were conducted in a parallel series of experiments in AGW without Zn, Ni or Pb.

Metal uptake was complete within 48 hours. Adsorption edges (% metal uptake vs. pH) showed strong dependence on ionic strength for all metals whereas only Zn and Pb sorption was concentration dependent. Dependence of Zn, Ni, and Pb sorption on pH and dissolved salt concentration was modeled by allowing the metal ions to compete with major cations on one type of site but only with protons on a second type of site. For the major cations, exchange constants for type one and surface protonation constant on the second site type were estimated (FITEQL4.0) from the independently obtained potentiometric data set. Dependence of Zn and Pb sorption on concentrations required strong and weak sites of the second type. Three different stoichiometries were investigated, including a bidentate exchange reaction for bivalent cations.

Sorption models (reaction stoichiometries) are being tested for their ability to predict cation concentration time histories during field-scale transport experiments conducted at pH 6 and two different dissolved salt concentrations. Models differ in the magnitude and timing of perturbations in pH values and major cation concentrations observed during the field experiments.