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

Abstract

The time-variant chemical behavior of CoIIEDTA (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 MeII-EDTA 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@) of dissolution of the iron oxide by EDTA complexes. Ionic Co2+ exhibited typical cation-like adsorption, whereas MeIIEDTA 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 FetOt. The adsorption of MeIIEDTA enhanced the apparent solubility of the iron oxide phase, which destabilized the CoIIEDTA complex, liberating Co2+ and FeIIIEDTA. 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 FeIIIEDTA. 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 CoIIEDTA will undergo similar reactions in subsurface environments causing complex, distance-variant retardation.

Radionuclide-Chelating Agent Complexes in Low-Level Radioactive Decontamination Waste; Stability, Adsorption and Transport Potential

“…In this experiment the average effluent pH was nearly the same as the influent (7.5). Based on the quantity of iron oxide coated sand in the column, EDTA concentration in the influent, an EDTA adsorption maximum of 0.0049 moles/mole Fe at pH 4.5 (Szecsody et al 1994) and an estimated fraction of available sites of 0.18 at pH 7.5 (Dzombak and Morel 1989), it was calculated that the adsorption capacity of this soil column for EDTA should be reached at 75 pore volumes, instead of the -32 pore volumes observed.…”

“…Initially the Ni-EDTA complex is rapidly adsorbed to the iron oxides on the sand. After adsorption of the complex, dissociation of Ni 2 + from some of the Ni-EDTA complex occurs, followed by ligand promoted dissolution (Szecsody et al 1994;Stumm and Wieland 1990) of some of the iron oxide. This results in liberation of free Ni 2 + and Fe(llI)-EDTA complexes.…”

Final technical report: The effect of physical and chemical heterogeneities in a porous medium on the transport of bacteria

“…The results indicate that adsorption may occur at specific, more reactive portions of aquifer solids, whereas surface area measurements employing chemically unreactive N2 quantify the entire area of the 23 sample. In these studies, the amounts of Al, Fe, or Mn were not correlated with surface area, measured by N2 adsorption (Szecsody et al, 1994). This result would again suggest a layering on the quartz surface of the Fe oxides coatings.…”

Modification of the finite element heat and mass transfer code (FEHM) to model multicomponent reactive transport

“…Specifically, complexed cobalt does not adsorb as much as free metal cobalt thereby allowing for greater migration of the toxic metal. The subsurface chemistry of these DOE sites is complex making prediction of contaminant migration difficult (Szecsody et al, 1994). ; kfor6 = 1.3125 mole/(kg*s)…”