ADSORPTION-DISSOLUTION REACTIONS AFFECTING THE DISTRIBUTION AND STABILITY OF CO(II)EDTA IN IRON OXIDE-COATED SAND

The time-variant chemical behavior of Co(II)EDTA (and other metal-EDTA complexes) was investigated in suspensions ofiron oxide-coated sand to identify equilibrium and kinetic reactions that control the mobility of Me(II)DTA 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(II)EDTA 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(II)EDTA enhanced the apparent solubility of the iron oxide phase, which destabilized the Co(II)EDTA complex, liberating Co2+ and Fe(III)EDTA. The dissolution rate was an order of magnitude slower at pH 6.5 than at pH 4.5 and was influenced by there-adsorption of solubilized Fe(III)EDTA. 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 proton-promoted dissolution). Ligand-promoted dissolution was most consistent with the experimental data. It is suggested that Co(II)DTA will undergo similar reactions in subsurface environments causing complex, distance-variant retardation.