| Summary: | ABSTRACT
Sand is often used as a passive barrier to slow release of metals from the sediment and to separate benthic organisms from the sediment. Materials that effectively adsorb metals have the potential to provide significantly greater effectiveness by further retarding metal release. In this thesis, the effectiveness of apatite and Phosphil®, which contain phosphate in a form that can absorb many metals, is evaluated with a series of sorption and migration column experiments using Cr, Cu, Zn and Pb.
Langmuir shape isotherms were observed suggesting that the effectiveness of these materials decreases at high concentration. The sorption isotherm experiments also revealed that the phosphate cap materials were much more sorbing than sand and therefore would more effectively retard contaminant migration.
Column experiments designed to study metal transport from field contaminated sediment had difficulty achieving measurable migration depths (due to intermixing between sediment and cap materials) within a reasonable time scale. An analytical model of contaminant migration was developed for this situation. Metal migration in the same capping materials was studied utilizing experimental columns in which high concentrations of metals were ponded over a solid layer of capping materials. Migration profiles were measured in two ways: a traditional sectioning method followed by ICP-MS analysis of the section; and non-destructive scanning using synchrotron X-ray Fluorescence. This method allowed determination of metal profiles with sub-mm resolution. In migration experiment Zn migrated fastest among all four metals and sand exhibited the least retardation of any metals. These are consistent with the equilibrium sorption data. Although the experiments with a high concentration metal solution allowed the observation of metal migration in reasonable periods of time, the experimental setup resulted in buoyancy effects which artificially enhanced metal migration.
A finite difference model incorporating Langmuir isotherm and non-equilibrium effects and using the initial observed profile as an initial condition was developed to simulate metal migration in these systems and therefore quantify the effectiveness of the various cap materials. Model inferred retardation factors indicated metal sorption in the order of Phosphil > Florida Phosphate > sand (e.g. Cu in tetra-element system were 100, 43 and 0.46 respectively).
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