ADSORPTION OF PB(II) AND BENZENECARBOXYLATES ONTO CORUNDUM

An experimental investigation of the adsorption of Pb(II) and three be nzenecarboxylate compounds (terephthalate, phthalate and pyromellitate ) was conducted using the corundum surface. The experiments were carri ed out in 0.01 M NaNO3 at 25 degrees C between pH 3 and 11, for separa te and mixed Pb and benzenecarboxylate solutions. The experimental dat a were used to quantify equilibrium surface complexation reactions usi ng the constant capacitance model (CCM) and the extended constant capa citance model (ECCM). The adsorption of Pb was quantified using the co mplexes > AlOHPb+2-(NO3-1)(2) and > AlOPb+1-NO3-1 with Pb as an inner- sphere complex, and with outer-sphere nitrate counterions. The adsorpt ion of the benzenecarboxylates was best described with outer-sphere su rface complexes, differing in degrees of protonation to account for ad sorption over the entire pH range that was investigated. Pb and phthal ate adsorption from mixed Pb-phthalate solutions were modeled using th e surface ternary complexes > AlOHPbPht(o) 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 constant s for aqueous Pb-terephthalate and Pb-pyromellitate complexation have not been determined. Model calculations show that the reduction of sur face charge upon benzenecarboxylate adsorption is a function of both t he number of functional groups on the adsorbate and on the binding str engths. Results also indicate that the number and the position of func tional groups on the benzenecarboxylate compounds have important effec ts on the adsorption of Pb at low pH. However, at high pH, aqueous Pb- benzenecarboxylate complexation is not strong enough to compete with P h surface species. The reduction of surface charge, however, was not s ufficient to explain Pb adsorption from mixed Pb-phthalate solutions, providing evidence for the formation of the ternary surface complexes. (C) 1998 Elsevier Science B.V. All rights reserved.