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Iron speciation in coastal rainwater : oxidation kinetics and organic complexation

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
Bernard Jason Smith (Creator)
Institution
The University of North Carolina Wilmington (UNCW )
Web Site: http://library.uncw.edu/
Advisor
Joan Willey

Abstract: The redox kinetics and organic complexation of iron were investigated in rainwater collected from a coastal site in Wilmington, NC between September 2001 and September 2003. A series of authentic rain samples was irradiated with simulated sunlight to photoproduce Fe(II) and the kinetics of its reoxidation back to Fe(III) was monitored. The oxidation of Fe(II) by hydrogen peroxide during dark storage followed second order kinetics with an average rate constant of 0.024mM-1h-1. Using sequential regression analysis the rate loss could be predicted accurately by the following equation (given that H2O2 is in mM and Fe(II) is in nM units): rate loss = -16.6 + 1.1[H2O2] + .28[Fe(II)]. In addition to studying the redox kinetics of photoproduced Fe(II) in rainwater the importance of organic complexation on Fe speciation was also evaluated. After approximately 2 h the concentration of photochemically produced Fe(II) decreased until it reached levels at or near pre-irradiation values. A series of experiments demonstrated that the photochemically produced Fe(II) rapidly returned to initial concentrations, suggesting that essentially all the Fe(II) and most of the Fe(III) in rainwater in the absence of sunlight occurs organically complexed. When rainwater was UV oxidized the Fe(II) concentration declined to much lower concentrations than before irradiation, because the organic Fe(II) complex stabilizing Fe(II) in rainwater was destroyed. A second series of experiments demonstrated that Fe(II) present in authentic rainwater was stabilized for up to four hours once added to seawater. When the rain was UV oxidized prior to addition to seawater, the Fe(II) was oxidized almost immediately in the seawater again, suggesting essentially all the Fe(II) in rain is organically complexed. In summary, the results presented in this study suggests that during the daytime when sunlight is present there is a dynamic interconversion between inorganic Fe(II) and Fe(III) species. Once solar irradiation is removed, there is a organically complexed form of Fe(II) which is stabilized against oxidation. Therefore, the concentrations of Fe(II) and Fe(III) measured in rainwater at any given time depends on the photochemical history of the sample.

Additional Information

Publication
Thesis
A Thesis Submitted to the University of North Carolina at Wilmington in Partial Fulfillment Of the Requirements for the Degree of Master of Science
Language: English
Date: 2009
Keywords
Oxidation, Photochemistry, Rain-water (Water-supply)--Iron content--North Carolina--Wilmington
Subjects
Rain-water (Water-supply) -- Iron content -- North Carolina -- Wilmington
Oxidation
Photochemistry