Redox chemistry of iron in multiphase atmosphere

Citation

Pehkonen, Simo Olavi (1995) Redox chemistry of iron in multiphase atmosphere. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ezne-6339. https://resolver.caltech.edu/CaltechETD:etd-10182007-133403

Abstract

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Iron redox chemistry was investigated in fog and stratus clouds in urban and remote locations in California, Delaware and New York. It was observed that iron(II) contributed from 20 to 70 % of the total iron in the samples and that iron(III) was bound mostly as oxalato complexes in most samples. The iron(II) oxidation state seemed to correlate best with organic compounds, i.e., carboxylic acids and TOC (total organic carbon), indicating the important role of organic compounds to the redox state of iron.

The applicability of a new spectrophotometric technique for measuring simultaneously iron(II) and iron(III) in atmospheric water samples real time in the field was studied. DPKBH (Di-2-pyridyl ketone benzoylhydrazone) forms complexes with both iron(II) and iron(III) with an absorption maximum at 375 nm for both iron(II)-DPKBH and iron(III)-DPKBH and an absorption maximum at 660 nm for iron(II)-DPKBH. The detection limit of this method is 4 nM of iron with chloroform-water extraction and 0.1 [...] without the extraction. DPKBH forms bis complexes with iron and binds via the oxygen and two nitrogen atoms of the enol form of DPKBH as indicated by a FTIR study of the iron(III)-DPKBH complex.

In addition to field observations, complementary laboratory photoreduction experiments were carried out with a variety of iron oxides and a variety of important atmospheric organic compounds such as oxalate, formate, acetate and formaldehyde. Photoreduction of [...] with formate yielded the highest rates of photoreduction. Stability of the iron oxide and the strength of Fe-O bonds in the lattice played a more important role in the rate of iron photoreduction than the reactive surface area. Hydrogen peroxide was produced in the case of oxalate as the electron donor. Additional iron photoreduction experiments were carried out with halogenated acetic acids (the end products of tropospheric HCFC degradation) as electron donors and it was observed that monohalo acetic acids reduce iron oxides faster and get photooxidized faster compared to acetic acid.

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