Multispecies contaminant transport in undisturbed columns of weathered fractured saprolite

Author

Melanie A. Mayes

Date of Award

5-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Claudia Mora

Committee Members

Philip Jardine, Larry McKay

Abstract

Laboratory-scale tracer experiments were conducted to investigate the geochemical and hydrological processes that govern the fate and transport of organically-chelated radionuclides in clay-rich residual soils. Undisturbed columns (8.5 cm diameter x 15 cm length) of weathered, fractured shale saprolite were obtained from a proposed waste disposal site at the Oak Ridge National Laboratory. Single and multispecies saturated transport experiments were conducted to determine the rates and mechanisms governing the transport of ^57.58Co(ΙΙ)EDTA^2-, ⁵⁷Co(ΙΙΙ)EDTA^-, ¹⁰⁹CdEDTA^2-, and H⁵¹Cr0₄^-. The transport of reactive contaminants through undisturbed, heterogeneous materials was affected by nonequilibrium conditions, as a result of physical processes, such as multi-region flow, or chemical processes, such as ratelimited sorption or transformation reactions. Geochemical reactions between soil mineral surfaces and chelated contaminants resulted in redox reactions and chelate dissociation, coupled with dissolution of soil minerals. Co(ΙΙ)EDTA^2- was oxidized to Co(ΙΙΙ)EDTA^-, and a pulse of aqueous Mn suggested that surficial Mn(ΙV) oxides may have catalyzed the redox reaction. Subsurface Fe and A1 oxides dissociated the CdEDTA^2- chelate, resulting in the formation of Fe(ΙΙΙ)EDTA^-, Al(ΙΙΙ)EDTA^-, and uncomplexed Cd. The divalent Cd²⁺ was transported as a reactive cation, where stabilization may occur by adsorption to soil surfaces. The transport of Cr(VΙ) was governed by the reduction to Cr(ΙΙΙ) in the presence of soil organic matter, followed by irreversible sorption and/or precipitation of the trivalent cation Cr³⁺. Both Cr(VΙ) and Cr(ΙΙΙ) were irreversibly sorbed, but the proportion of trivalent Cr was greater in a column amended with organic matter, which suggested that oxidation of organics catalyzed the reduction of Cr(VΙ). Nonequilibrium conditions during transport were identified using flow interruption, which allowed ratelimited processes to approach equilibrium. Results were modeled with a version of the convective dispersive equation where flow was segregated into mobile and immobile flow regions. Optimization of the model was completed on the flow interruption, to estimate the rate of diffusive mass transfer between the two regions. Retardation coefficients were obtained by fitting to the breakthrough curves. The results indicated that physical nonequilibrium was significant during column transport studies, and that some geochemical reactions between soil and contaminants were rate-limited, which has implications for predictions of contaminant migration.

Recommended Citation

Mayes, Melanie A., "Multispecies contaminant transport in undisturbed columns of weathered fractured saprolite. " Master's Thesis, University of Tennessee, 1999.
https://trace.tennessee.edu/utk_gradthes/9912