Iron dissolution and speciation in atmospheric mineral dust: Metal-metal synergistic and antagonistic effects

Abstract Under acidic atmospheric conditions, iron leached from atmospheric mineral dustmay influence the distribution of bioavailable iron at a global scale. However, the effects of non-Fe-containing minerals on iron dissolutionremain unknown. This work describes metal-metal synergistic and antagonistic effects on iron dissolutionthat go beyond aggregation and ionic strength effects in mineral dustmixtures. In this study, we investigated iron mobilization by proton-promoted dissolutionin natural mineral dustsamples from the Kalahari Desert (SZ1) and Australian Red Dawn event (RO), along with one iron oxide proxy, hematite. The total iron dissolutionin natural dust samples highly corresponds with the respective amount of Ti, rather than their particle sizes or Fe contents. The dust sample with high Ti content, SZ1, also showed a higher fraction of dissolved Fe(II), under dark conditions. These observations are in good agreement with the dissolutiondata for hematiteartificially mixed with metal oxides. Total iron dissolutionin hematite, mixed with TiO 2 , is 1.5- and 2-fold higher compared to that of just hematiteunder dark and light conditions, respectively. However, dissolutionof hematiteis suppressed when mixed with Al 2 O 3 and CaO. Under dark conditions, furthermore, dissolved Fe(II) fraction is enhanced for hematitewhen mixed with TiO 2 compared to that of other mixtures or hematitealone. Yet, dissolved Fe(II) is lower in hematitemixed with TiO 2 under light conditions compared to that of hematitealone, suggesting photo-oxidation of Fe(II) by reactive oxygen species, such as OH radicals.