Date of Award

Summer 1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean/Earth/Atmos Sciences

Program/Concentration

Oceanography

Committee Director

George T. F. Wong

Committee Member

William M. Dunstan

Committee Member

John R. Donat

Abstract

Iodate and iodide are the two major species of iodine in seawater. In comparison to iodate, iodide is thermodynamically unstable in oxic seawater. The conversion (or reduction) of iodate to iodide may be mediated via biological activities. Since biological activities vary with the season, the conversion of iodate to iodide may also vary seasonally. The conversion (or oxidation) of iodide to iodate is thermodynamically feasible, but the mechanism is poorly known. Hydrogen peroxide, which is ubiquitous in surface seawater, may oxidize iodide to iodate.

Water samples were collected in the middle and lower Chesapeake Bay in different seasons between 1990 and 1992. The concentrations of iodate, iodide, and total iodine were determined by polarography or voltammetry. In the surface water iodate was the dominant form of inorganic iodine in the winter while iodide was the dominant species most of the rest of the year. Dissolved organic iodine was mainly found in the spring. In the deep water iodate was present in significant concentrations in the winter. Iodide was the dominant species in the other three seasons; organic iodine was very minor. In the deep water iodate might be removed to the sediment under oxidizing conditions in the winter. The sediment might act as a source of iodide under reducing conditions in the summer. This evidence indicated that the speciation of iodine varied in different seasons in a coastal marine environment such as the Chesapeake Bay. Furthermore, the Chesapeake Bay might act as a geochemical reactor which reduces iodate in the Bay and transports iodide to the oceans.

The reaction between H202 and iodide was investigated by adding them in artificial seawater and monitoring the variations in the concentrations of H202 and iodide. At neutral conditions (pH 7 to 9), H202 oxidized iodide, probably to molecular iodine or hypoiodite. The disappearance of H202 was first order with respect to iodide and H202, respectively. The reaction rate increased with increasing temperature and increasing salinity, but it was independent of pH within pH 7 to 9. At millimole levels of H202, molecular iodine (or hypoiodite) was immediately reduced back to iodide. The net result was a quasi-catalytic decomposition of H202 in the presence of iodide. The direct oxidation of iodide to iodate was not observed. By extrapolating the results to oceanic conditions (i.e. at sub-μM levels of H202) , molecular iodine (or hypoiodite) may be reduced to iodide by H202, form organic iodine and be converted back to iodide, or further disproportionate to form iodate and iodide. The last possibility may explain the oxidation of iodide to iodate in seawater.

DOI

10.25777/4f3g-hw71

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