Dissolution of Fe from Fe-bearing minerals during the brown-carbonization processes in atmosphere

Previous studies found Fe dissolution in atmosphere correlates to biomass burning, while the underlying mechanisms need to be further investigated. In this study, we reported a laboratory investigation about Fe dissolution behavior of two model Fe-bearing clay minerals of montmorillonite (SWy-2) and illite (IMt-2), and one standard mineral dust of Arizona test dust (AZTD) in atmospheric condition (pH = 2), after the minerals engaging into the brown-carbonization reaction with guaiacol, which is a commonly detected volatile phenol substance in biomass burning. The results show that the pre-brown-carbonization reaction promoted Fe dissolution from all the three minerals, attributing to the reduction of Fe(III) by gaseous guaiacol. The Fe dissolution from SWy-2, IMt-2 and AZTD were also compared under both light and dark conditions to simulate the daytime and nighttime atmospheric processes. As a result, model solar irradiation further promoted Fe dissolution from IMt-2 and AZTD, since both minerals contain moderate photo-reducible Fe(III) oxide or/and Fe(III) oxyhydroxide. The promotive effect of solar irradiation on Fe dissolution from AZTD would be gradually diminished because the photo-reactive Fe(III) is also guaiacol-reducible. Whereas, it was on the contrary for SWy-2 which does not contain the Fe(III) (oxyhydr-)oxide phase. And more dependently, the photo-induced hydroxyl radical (OH) on SWy-2 would re-oxidize the formed Fe(II), unless sufficient amount of guaiacol or brown-carbonization products on SWy-2 consumed the OH and complexed with surface coordinated Fe(III) forming photo-reducible Fe(III). The results of this study suggested the brown carbonization process on minerals would greatly mediate the Fe dissolution behavior from the Fe-bearing mineral dusts in atmosphere. Similar processes might need to be taken into consideration to accurately evaluate the input of Fe from atmosphere to open oceans.