Binding of Pb and Zn to Aluminium Oxide and Proton Stoichiometry

The interaction of Pb and Zn with Al20 3 in aqueous solution was studied as a function of pH and metal ion concentration. Results indicated the complexation of metal ions by oxide surfaces is strongly pH dependent; the extent of adsorption is a function of pH with an abrupt change within approximately 2 pH units. The adsorption of Pb and Zn on aluminium oxide can be interpreted in terms of a smface complexation formed by association with one surface AlOH foup, thus releasing one bound hydrogen ion. -AIOH + M+ = -AIOM + H+. It is suggested that the major surface reaction is the formation of a monodentate inner-sphere complex. INTRODUCTION Metallic oxides and silicas are abundant components of the earth's crust. Adsorption of metals from aqueous solution onto oxide surfaces is considered to be an important process in natural environments and in many industrial systems. Examples include the interactions between sediments and the water column, the mobility and transportation of trace metals in natural waters, leaching of metals from landfills, and the use of adsorption for removal or recovery of trace metals in wastewater and water treatment operations. It is therefore of practical and theoretical interest to obtain a detailed understanding of the sorption process at the oxide-water interface. This understanding hinges to a great extent upon elucidation of the nature of particulate surface, and the effects of pH, ionic strength, and metal concentrations. pH is probably the single most important factor influencing metal behavior in aqueous systems. Typically adsorption of metals on oxides increases from near zero to nearly 100% as pH increases through a critical range of 1-2 units wide (Lion et al., 1982; Benyahya and Gamier, 1999). The mobility of metal ions in aquatic environment is often characterized by a distribution coefficient Kd defined as the ratio of the concentration of metal adsotbed on the solid phase (r) to that in solution at equilibrium (Cr). High values of Kd indicate that the metal has been retained by the solid through sorption reactions, while low values ofKd indicate that most of the metal remain in solution where it is available for transportation and geochemical reactions. Trace metal ion adsorptions at oxide/natural waters interfaces are often better described by a distribution coefficient, since their concentrations in natural aquatic systems are usually low. Several mechanisms and models have been developed for metal ion adsorption reactions at the oxide-water interface. (1) James and Healy (1972) proposed an ion-solvent interaction model, which considers electrostatic, solvation, and specific chemical energy interactions as the ion approaches the interface and which implies that a lowering of the ionic charge of the metal species ( e.g., by hydrolysis) decreases the ion-solvent interaction which presents a barrier to close approach of multiple 40 VIRGINIA JOURNALOF SCIENCE charged ions to the surface. The pH at which adsorption of metal ions becomes significant is the pH at which the dissolved cations undergo hydrolysis to hydroxy complexes. It is because the hydrolyzed species do not have a strongly held hydration shell to prevent adsorption. (2) The ion exchange model suggested by Dugger ( 1964 ), according to which metallic cations upon adsorption on the hydrous oxide surface replace protons from hydroxyl groups -AlOH. And (3) Schindler-Stumm surface complex formation model (Stmmn et al., 1970; Huang et al. , 1973; Schindler et al., 1987~ Hohl et al. , 1976; Schindler, 1981) hypothesizes that complexes are formed at the surface of the oxide, which is composed of a hydroxyl species bound to a central cation. In this model the hydrous oxide surface groups, -AlOH, -Alff, or -Al-OH2+ are treated similar to amphoteric functional groups in polyelectrolytes, as complex forming species. The fundamental concept is that adsorption takes place at defined coordination sites (the surface hydrox)'l groups are present in finite number). The surface complexation model permits us to handle adsorption equilibria in the same way as equilibria in solution~ adsorption is thus closely analogous to complex formation in solution, and can be described by similar equations. · Accordingly, we interpret our results of adsorption of metal ions (M2+) on Ati03 in tenns of this latter model and characterize the amphoteric and complex forming properties by the acid-base reactions: +H+ +H+ Alff <===> AlOH <===> AlQH2+ and the following coordination reactions, (thus leading to the formation of either monodentate or bidentate surface complexes.) Al-OH+ M2+ = Al-OM++ H+ or 2Al-OH + M+ = (Al-0)2M + 2H+ where M+ is either Pb2+ or Zn2+ in this study. Competitive complex formation equilibria (metal ion versus H\ or anion versus Off) explain the strong dependence of metal ion (as well as anion) binding on pH. Hence uptake and release of H+ ions in solution can be described by the acidity constants. Similarly, adsorption equilibria involving metal ion are conveniently characterized by stability constants for the formation of surface complexes. According to the above equations, we assume that among other species in solution at the pH studied only free cations (Pb2+ or Zn+, and not another species such as PbOH+ or ZnOH+) form surface complexes (i.e. adsotbed) and that the pH-dependence of the association of metal ions can be explained by the pH-dependence of the surface concentration of the Alff group and the affinity of this group to the free metal ion. Although experimental studies on adsorption conducted to date represents a considerable increase in knowledge, a quantitative application oftlris knowledge to natural aqueous environments is still inadequate. The binding stoichiometry of metal ions to the solid and suspended phase is not yet resolved satisfactorily. The focus of this study was (1) to study the effect of pH on the adsorption of metal ions (lead and zinc) on aluminium oxide, and (2) evaluate quantitatively the interaction of these cations with the surface hydroxy 1 group on aluminium oxide in tenns of proton stoichiometry. BINDING OF Pb A~