Graduation Year

2014

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Robert H. Byrne, Ph.D.

Committee Member

Peter R. Betzer, Ph.D.

Committee Member

Edward S. Van Vleet, Ph.D.

Committee Member

David J. Hollander, Ph.D.

Committee Member

Bruce D. Johnson, Ph.D.

Keywords

Ferric Iron, Silicate, Solubility, M-SEAS, Complexation

Abstract

Although iron occurs at extremely low concentrations in the world’s oceans, it is essential for all living organisms. It is the limiting nutrient in High Nutrient Low Chlorophyll (HNLC) areas of the ocean, and exerts critically important influences on levels of atmospheric CO2 and the global carbon cycle. Understanding the chemical processes that govern the fluxes and biogeochemistry of oceanic iron requires thorough assessment of the aqueous physical chemistry of iron and analytical techniques capable of measuring iron at sub-nanomolar concentration measurements. This dissertation extends prior work on the physical and analytical chemistry of iron through (a) investigation of the complexation of iron by silicate in aqueous solutions, (b) investigation the solubility of ferric hydroxide using spectrophotometric procedures over a wide range of pH (c) utilization of novel in-situ instrumentation for iron measurements in seawater.

Previous investigations of ferric iron complexation by silicate ions (SiO(OH)-3) included no measurements at ionic strengths greater than 0.15 molal and produced formation constant estimates at zero ionic strength that differed by more than a factor of two. In this work ferric silicate formation constants were measured at ionic strengths of 0.1, 0.3 and 0.7 molal by ultraviolet absorbance spectroscopy. The dependence of the ferric silicate formation constant on ionic strength at 25° C, summarized using the Bronsted-Guggenheim-Scatchard specific ion interaction (SIT) model, indicated that the ionic strength dependence of the ferric silicate formation constant, (written as Si ∗β1 = [FeSiO(OH)23+][H+][Fe3+]-1[Si(OH)04]-1) can be expressed as: log Si *β1 = (-0.125 ± 0.042) - (2.036 I0.5)/(1+ 1.5I0.5) + (0.588 ± 0.094) I. The result obtained at zero ionic strength is in good agreement with the average result obtained in four previous studies, but with a substantially reduced level of uncertainty.

The solubility of ferric iron in aqueous sodium perchlorate solutions at the ionic strength of seawater was determined by use of novel automated spectrophotometric procedures. Two colorimetric measurement chemistries were utilized to measure dissolved ferric iron concentrations in equilibrium with precipitated amorphous ferric hydroxide over a range of pH between 4.0 and 12.0. Soluble iron concentrations decreased from approximately 3.2 micromolar at pH 4.0 to subnanomolar levels between pH 7.5 and 9.5, and rose to approximately 0.1 micromolar at pH 12. The results of this investigation were in good agreement with solubility results obtained in previous investigations of iron solubility in seawater at circumneutral pH, and previous results obtained in sodium chloride at high pH, but differed from previous results obtained in sodium chloride between pH 7 and pH 9. In view of the agreement between solubility results obtained in seawater and sodium perchlorate (this work) and, in contrast, results in sodium chloride that were more than an order of magnitude lower than were obtained in seawater and sodium perchlorate, it is advisable that further solubility investigations are performed in sodium chloride solutions.

The iron measurement procedures developed for the investigation of ferric iron solubility were incorporated in an in situ spectrophotometric instrument. The Spectrophometric Elemental Analysis System (SEAS) utilizes long pathlength absorbance spectrometry (LPAS) combined with colorimetric protocols to achieve the sensitivity required to measure analytes at nanomolar concentration levels. The M-SEAS was initially tested on cruises in the Eastern Gulf of Mexico in June 2013 and November 2013. Due to limited opportunity for deployments of M-SEAS during these cruises, iron concentration data was obtained from only three casts. During these casts the heater pressure vessel flooded due to a compromised seal, causing the temperature of both channels to be strongly affected by ambient seawater. Further measurements of iron with the M-SEAS instrument in profiling mode will require an engineering analysis and redesign of the faulty seal. The international GEOTRACES program has stated that an improved understanding of the biogeochemical cycles and largescale distributions of trace-elements and isotopes will inform many areas of environmental research, from climate science to planning for future global change. As the only instrument currently capable of continuous in situ measurements of iron, the M-SEAS instrument should greatly enhance capabilities for investigation of iron biogeochemistry.

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