Year

2003

Degree Name

Doctor of Philosophy

Department

Department of Chemistry

Abstract

In order to elucidate major natural control on the diagenetic behaviour of arsenic in the sediments Port Kembla Harbour, a heavily industrialized port, vertical profiles of total and species of arsenic in porewater and total arsenic in different sediment solid phases and grain size fractions were measured. Four sediment solid phases were "operationally defined" as exchangeable, AVS, reducible and residual. Under an oxygen free atmosphere the samples were sliced to 2-cm thick layers and wet sieve to separate the sediment particles of < 63, > 63 and > 250 μm sizes. A four-step sequential extraction protocol was applied to these sieved samples to extract arsenic associated with these operationally defined solid phases.

Arsenic distributions are typically characterized by sub surface maxima in the porewater and in the solid. These sub surface maxima occur at redox transition zones at which total Fe declines. These patterns reflect a strong redox coupling between As and Fe. In the sediment surface arsenic probably associates with Fe-oxyhydroxide that dissolves upon burial at the redox transition zone, leaving high porewater As and low solid Fe in this zone. The dissolved As diffuses upwards, then depending on the prevailing redox condition in surface sediment allows for the As fraction to either diffuses to overlying water (under mildly reducing conditions) or remains in the porewater and precipitates (under mildly oxidizing condition).

[As]porewater and [AS]solid concentrations generally increase with increasing distance from the Darcy Road Drain which parallels with the increase in fine grain content of the sediments. This pattern reflects an important role of the drain effluent in distributing arsenic and material in the Port Kembla Harbour. The fine grain fraction carries further from the shore and is enriched with arsenic and other metals (Fe, Cu, Zn and Pb).

The residual phase accounts for the great portion of total As over whole core and this type of association presumably does not become involved in diagenetic processes. This probably reflects that portion of arsenic entering the harbour and settling in the form of stable solid phases. Residual phase is probably associated with pyrite in deeper layers. Dissolution-precipitation reactions as a result of diagenetic processes involve the arsenic from the exchangeable and AVS fraction only and occur intensively in the upper 6 cm of the sediment cores.

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