troscopic data which suggest that sorbing metal ions form Surface precipitation is an important process in many areas of hydroxo-bridged clusters at high surface coverages (1). In science and technology, including modeling contaminant segregathe present study, the details of this transition are investition from groundwater to solid phases and dispersion of active gated for the Co(II)/Al 2 O 3 system using several probes of phases on catalyst supports. XAFS, TEM, and XPS measurements molecular structural and composition, and a review is made of Co(II) sorbed on Al 2 O 3 demonstrate that surface precipitates of the various processes that may cause surface precipitation.
have formed from solutions that are undersaturated with respect
In past work, the term ''surface precipitation'' has been to any known bulk solid phase. The precipitates have a structure used to describe various precipitation phenomena that take similar to that of Co(OH) 2 (s), but are disordered and have a high concentration of Co vacancies. The data plus thermodynamic place at a solid-water interface, such as heterocoagulation reasoning have been used to analyze the plausibility of various and heterogeneous nucleation of a precipitate phase. In this models for surface precipitation and to show that for Co(II)/ paper, however, we use it in a strict sense of ''surface-Al 2 O 3 it occurs by forming a double-hydroxide phase containing induced precipitation,'' i.e., the formation of a precipitate substrate-derived Al(III) ions. This idea was corroborated by mixunder solution conditions that would, in the absence of the ing aqueous solutions of Al(III) and Co(II) at the pH and concensubstrate solid, be undersaturated with respect to any known tration of the sorption samples, forming a stable colloidal precipisolid phase. The term ''adsorption'' is used to refer to the tate that is less soluble than either Al(OH) 3 or Co(OH) 2 . The Co formation of metal ion surface complexes, while ''uptake'' XAFS of this material was similar to that of the sorption samples.
refers to the partitioning of a dissolved species to a solid Successful quantitative models of metal ion transport in groundphase, irrespective of the sorption mechanism.
water need to include the possibility of forming ternary and higher order precipitates that include ions derived from sparingly soluble An understanding of when and how precipitates form solids. For catalyst impregnation, surface coprecipitation can preat surfaces is important in a number of areas that are of vent production of a well-dispersed precursor material. α§ 1997 scientific and technological importance. For example, an Academic Press understanding of the process of surface precipitation is Key Words: surface precipitation; coprecipitation; double hyrequired for accurate quantitative modeling of metal ion droxide; cobalt(II); alumina; sorption; polynuclear complex; uptake, especially at the highest concentrations ( 5, 6 ) .