Kinetics and mechanisms of Zn complexation on metal oxides using EXAFS spectroscopy

Abstract

Zn(II) sorption onto Al and Si oxides was studied as a function of pH (5.1–7.52), sorption density, and ionic strength. This study was carried out to determine the role of the various reaction conditions and sorbent phases in Zn complexation at oxide surfaces. Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to probe the Zn atomic environment at the metal oxide/aqueous interface. For both amorphous silica and high-surface-area gibbsite, Zn sorption kinetics were rapid and reached completion within 24 h. In contrast, Zn sorption on low-surface-area-gibbsite was much slower, taking nearly 800 h for a sorption plateau to be reached. In the case of silica, EXAFS revealed that Zn was in octahedral coordination with first-shell oxygen atoms up to a surface loading of approximately 1 μmol m −2, changing to tetrahedral coordination as surface loading and pH increased. For the high-surface-area gibbsite system, the ZnO first-shell distance was intermediate between values for tetrahedral and octahedral coordination over all loading levels. Zn formed inner-sphere adsorption complexes on both silica and high-surface-area gibbsite over all reaction conditions. For Zn sorption on low-surface-area gibbsite, formation of ZnAl layered double hydroxide (LDH) occurred and was the cause for the observed slow Zn sorption kinetics. The highest pH sample (7.51) in the Zn–amorphous silica system resulted in the formation of an amorphous Zn(OH) 2 precipitate with tetrahedral coordination between Zn and O. Aging the reaction samples did not alter the Zn complex in any of the systems. The results of this study indicate the variability of Zn complexation at surfaces prevalent in soil and aquatic systems and the importance of combining macroscopic observations with methods capable of determining metal complex formation mechanisms.