Date of Award
Doctor of Philosophy in Chemistry
Dana R. Kester
The different chemical forms of iron are an important factor in the marine biochemistry and geochemistry of this element. In this study, the forms and distribution of iron were investigated along a horizontal salinity gradient in the Connecticut River estuary in July 1981, and at a vertical profile in the Slope Water region of the Northwest Atlantic Ocean in September 1981. The redox state of iron was investigated in the offshore water of Peru in January 1984. Since the particulate phase is an important form of iron and other trace metals, the binding of iron, copper, manganese and aluminum with the particles collected in a microcosm were investigated in the summer of 1983. Chemical models, including a dissolved iron solubility model, an iron speciation model, and an adsorption model, were examined using the experimental data obtained in the field to gain a better understanding of the factors that control the behavior of iron in the marine environment.
In the Connecticut River estuary, 80-90% of iron was in the particulate form and followed closely the transport of the total suspended matter. The decrease of the 0.1 to 0.4 μm fraction iron with salinity corresponded to the increase of the particulate iron fraction. Total iron in the Connecticut River estuary appeared to be conservative due to the short residence time of the water not allowing the particles to be removed from the water column. A substantial amount of the "dissolved" iron (0.1 μm filter) in the low salinity region was actually fine colloidal particles. The "dissolved" iron approached the true dissolved value calculated from a solubility model as the salinity increased.
In a Slope Water station, the concentration of total iron was 1-3 nmol/kg near the surface, increasing to 6-9 nmol/kg in the 02 minimum zone and then increasing to 12 nmol/kg above the seafloor. Three different pore size filters retained almost the same amounts of particulate iron (50-60% of the total) except near the bottom nepheloid layer (80% of the total). Results showed that particulate iron was primarily controlled by the physical and biological processes that influence the suspended particulate matter. Dissolved iron was found to be nearly uniform throughout the water column and agreed most closely with the value estimated by a solubility controlled model.
In the offshore waters of Peru, 40 nmol/kg of Fe(II) was found to be present in the low oxygen bottom water at 5-10 km offshore and decreased upward in the water column and with distance offshore. A good corelation between Fe(II) and nitrite indicated that one of the common sources of these reduced species was from the shelf sediments. Total iron levels in excess of 300-500 nmol/kg (80-90% in the particulate form) were found in the surface and bottom water at 5-6 km offshore due to the continental dust input and the resuspension of bottom sediments; most of this iron appeared to be trapped on the shelf.
The binding of iron, copper, manganese and aluminum with the particles collected in the microcosm was examined using a four-step sequential chemical extraction procedure. Iron, copper, manganese and aluminum became progressively enriched and more strongly bound as one compares particles suspended in the water column, falling into a sediment trap and accumulating on the bottom surface. Considerable amounts of iron (60% of the total) were associated with the organic phases of the biogenic particles, but generally, iron was mostly associated with the residual fraction of the natural particles.
This study indicated the importance of not only examining the distribution of total iron, but also the different forms of iron including different size fractions of particulate and colloidal iron, different redox states of the dissolved iron, and different binding phases of the particulate iron in order to gain a better understanding about the chemistry of iron. The behavior of iron is closely related to the physical, biological, and chemical processes in the marine environment.
Hong, Huasheng, "Chemistry of Iron in Different Marine Environments and the Binding of Iron, Copper, Manganese and Aluminum with Particles in a Microcosm System" (1984). Open Access Dissertations. Paper 1468.