their slow decomposition kinetics under general soil condi-The adsorption behavior of ferrocyanide and ferricyanide totions (12). When these complexes are exposed to light, free ward aluminum oxide with a 0.01 M NaClO 4 background electrocyanide is released from the molecules by light irradiation lyte at 25ЊC was investigated. Results obtained from different pH (13). Therefore, in this study, two complex ions, ferrocyavalues demonstrated that the adsorption of these complex ions nide and ferricyanide, have been investigated with respect could be described by a modified Langmuir isotherm in the concento their adsorption on the g-Al 2 O 3 /water interface. tration range 0.5 1 10 04 to 4 1 10 04 M. This modified Langmuir For the oxide/solution interface, the surface charge and isotherm developed here could accurately predict the equilibrium potential gradient decrease with increasing distance from partition between solid and liquid phases while taking proton comthe interface to the solution phase. The concept of solution petition into account. Also, the apparent equilibrium constant coordination chemistry is generally applied in describing (K app ) derived from the Langmuir equation for ferrocyanide or ferricyanide adsorption on aluminum oxide was compared with adsorption of ions at the oxide/solution interface. The triplethe intrinsic constant (K int ) calculated by triple-layer model simulayer model (TLM) developed by Hayes and Leckie ( 14) lation. This comparison revealed a strong correlation between the allows metal ions and anions to form an innersphere comapparent equilibrium constant and the intrinsic constant. Moreplexation at the surface layer (o-plane) or an outer-sphere over, in comparing K app with K int , we can infer that the adsorption complexation at the compact layer (b-plane) (15)(16)(17). This of either ferrocyanide or ferricyanide complexes onto aluminum model also allows counterions to accumulate in a diffuse oxide is achieved through outer-sphere complexation. ᭧ 1996 layer (d-plane) and a compact layer (b-plane). Conse-Academic Press, Inc.