Adsorption of Pb(II) and benzenecarboxylates onto corundum

An experimental investigation of the adsorption of Pb(II) and three benzenecarboxylate compounds (terephthalate, phthalate and pyromellitate) was conducted using the corundum surface. The experiments were carried out in 0.01 M NaNO3 at 25°C between pH 3 and 11, for separate and mixed Pb and benzenecarboxylate solutions. The experimental data were used to quantify equilibrium surface complexation reactions using the constant capacitance model (CCM) and the extended constant capacitance model (ECCM). The adsorption of Pb was quantified using the complexes >AlOHPb+2–(NO3−1)2 and >AlOPb+1–NO3−1 with Pb as an inner-sphere complex, and with outer-sphere nitrate counterions. The adsorption of the benzenecarboxylates was best described with outer-sphere surface complexes, differing in degrees of protonation to account for adsorption over the entire pH range that was investigated. Pb and phthalate adsorption from mixed Pb–phthalate solutions were modeled using the surface ternary complexes >AlOHPbPht0 with >AlOHPbHPht+1–NO3−1, where the Pb ion serves as a bridge between the mineral surface and a phthalate molecule. Modeling could not be performed for the mixed Pb–terephthalate and Pb–pyromellitate systems because stability constants for aqueous Pb–terephthalate and Pb–pyromellitate complexation have not been determined. Model calculations show that the reduction of surface charge upon benzenecarboxylate adsorption is a function of both the number of functional groups on the adsorbate and on the binding strengths. Results also indicate that the number and the position of functional groups on the benzenecarboxylate compounds have important effects on the adsorption of Pb at low pH. However, at high pH, aqueous Pb–benzenecarboxylate complexation is not strong enough to compete with Pb surface species. The reduction of surface charge, however, was not sufficient to explain Pb adsorption from mixed Pb–phthalate solutions, providing evidence for the formation of the ternary surface complexes.