Adsorption-Dissolution Reactions Affecting the Distribution and Stability of CoII EDTA in Iron Oxide-Coated Sand

Authors

• James E. Szecsody, Pacific Northwest National LaboratoryFollow
• John M. Zachara, Pacific Northwest National LaboratoryFollow
• Patrick Bruckhart, Pacific Northwest National Laboratory

Date of this Version

1994

Citation

Environ. Sci. Technol. 1994, 28, 1706-1716

Abstract

The time-variant chemical behavior of Co^IIEDTA (and other metal-EDTA complexes) was investigated in suspensions of iron oxide-coated sand to identify equilibrium and kinetic reactions that control the mobility of Me^IIEDTA complexes in subsurface environments. Batch experiments were conducted to evaluate the adsorption as a function of pH, concentration, and time and to quantify the rate-controlling step(s) of dissolution of the iron oxide by EDTA complexes. Ionic Co²⁺ exhibited typical cation-like adsorption, whereas Me^IIEDTA adsorption was ligand-like, increasing with decreasing pH. Adsorption isotherms for all reactive species exhibited Langmuir behavior, with site saturation occurring at molar values of <0.5% of Fe_tot. The adsorption of Me^IIEDTA enhanced the apparent solubility of the iron oxide phase, which destabilized the Co^IIEDTA complex, liberating Co²⁺ and Fe^IIIEDTA. The dissolution rate was an order of magnitude slower at pH 6.5 than at pH 4.5 and was influenced by the re-adsorption of solubilized Fe^IIIEDTA. Two multireaction kinetic models were developed that each included Langmuir adsorption for Co²⁺ and metal- EDTA species but differed in their depiction of the dissolution mechanism (i.e., ligand- versus protonpromoted dissolution). Ligand-promoted dissolution was most consistent with the experimental data. It is suggested that Co^IIEDTA will undergo similar reactions in subsurface environments causing complex, distance-variant retardation.