Graduate Thesis Or Dissertation
 

Trace metal redistribution in artificial estuarine sediments

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  • The transfer of trace metals within an estuarine system is a complex function of physical and chemical processes unique to that system. Although interactions of metals with individual components has been studied, there has been little investigation of interactions with multiple component systems. Thus to further understand these processes, an artificial sediment was chosen which would contain the major components of a natural estuarine sediment and would be simple enough to allow the analysis of the quantity of metals within each separate component using a relatively simple extraction technique. In this way metals could be initially dosed on individual sediment components and the redistribution of the metals into other components could be followed as a function of time. The stoichiometry of the metal redistribution could then be studied as well as a general determination made of the rate at which the process could occur. An artificial sediment containing sea water, humic acid, montmorillonite clay, ferrous sulfide, and sand was constructed. Metals were dosed only on one component. The metals chosen were copper, cadmium, lead, and zinc at concentrations of 0.2, 0.1, 0.2, and 0.4 mg/kg in the sediment respectively. The metal-dosed sediments were incubated in plastic bottles over a 44 day period under anaerobic conditions. Bottles were removed periodically for chemical analysis using atomic absorption spectroscopy. Metals associated with each sediment component were stripped by a stepwise extraction procedure devised specifically for this sediment mixture. Pore water was removed by centrifugation, humic acid metals by 1% ammonium hydroide, clay metals with 1% nitric acid, and ferrous sulfide metals by hot concentrated nitric acid. Results indicated that metals transfer occurs rapidly and equilibrium is established within five days. Equilibrium was assumed to exist during Days 5 through 14; concentrations of metals were then computed for each metal component and results were correlated with observations found in natural estuarine sediments. Copper was found associated primarily with ferrous sulfide, while cadmium, lead, and zinc were found with the clay fraction. The distribution was discussed in terms of the affinities of the pure sediment components for the individual metals. Anomalous scattering of data at longer incubation times were explained in terms of buffering changes and carryover of particulate matter. The study concluded that metal transfer between sediment components occurs within a period of several days, that the rate of transfer and stoichiometry were the same for each metal regardless of the initial location, and the extraction technique provided a rapid method of determining the partitioning of metals between the sediment components.
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