Freezing-Enhanced
Dissolution of Iron Oxides: Effects
of Inorganic Acid Anions
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Abstract
Dissolution
of iron from mineral dust particles greatly depends
upon the type and amount of copresent inorganic anions. In this study,
we investigated the roles of sulfate, chloride, nitrate, and perchlorate
on the dissolution of maghemite and lepidocrocite in ice under both
dark and UV irradiation and compared the results with those of their
aqueous counterparts. After 96 h of reaction, the total dissolved
iron in ice (pH 3 before freezing) was higher than that in the aqueous
phase (pH 3) by 6–28 times and 10–20 times under dark
and UV irradiation, respectively. Sulfuric acid was the most efficient
in producing labile iron under dark condition, whereas hydrochloric
acid induced the most dissolution of the total and ferrous iron in
the presence of light. This ice-induced dissolution result was also
confirmed with Arizona Test Dust (AZTD). In the freeze–thaw
cycling test, the iron oxide samples containing chloride, nitrate,
or perchlorate showed a similar extent of total dissolved iron after
each cycling while the sulfate-containing sample rapidly lost its
dissolution activity with repeating the cycle. This unique phenomenon
observed in ice might be related to the freeze concentration of protons,
iron oxides, and inorganic anions in the liquid-like ice grain boundary
region. These results suggest that the ice-enhanced dissolution of
iron oxides can be a potential source of bioavailable iron, and the
acid anions critically influence this process