Under acidic atmospheric conditions, iron leached from atmospheric mineral dust may influence the distribution of bioavailable iron at a global scale. However, the effects of non-Fe-containing minerals on iron dissolution remain unknown. This work describes metal-metal synergistic and antagonistic effects on iron dissolution that go beyond aggregation and ionic strength effects in mineral dust mixtures. In this study, we investigated iron mobilization by proton-promoted dissolution in natural mineral dust samples from the Kalahari Desert (SZ1) and Australian Red Dawn event (RO), along with one iron oxide proxy, hematite. The total iron dissolution in 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 dissolution data for hematite artificially mixed with metal oxides. Total iron dissolution in hematite, mixed with TiO2, is 1.5- and 2-fold higher compared to that of just hematite under dark and light conditions, respectively. However, dissolution of hematite is suppressed when mixed with Al2O3 and CaO. Under dark conditions, furthermore, dissolved Fe(II) fraction is enhanced for hematite when mixed with TiO2 compared to that of other mixtures or hematite alone. Yet, dissolved Fe(II) is lower in hematite mixed with TiO2 under light conditions compared to that of hematite alone, suggesting photo-oxidation of Fe(II) by reactive oxygen species, such as OH radicals.
Bibliographical noteFunding Information:
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. The authors thank the Department of Chemistry, New Mexico Institute of Mining and Technology. The Institute for Rock Magnetism (IRM) is supported by the Instruments and Facilities Program of the NSF Division of Earth Science. This is IRM contribution 1801.
The authors are grateful to Paolo D'Odorico and Abi Bhattachan for providing the Kalhari sample, to Stephen Cattle for providing the Australian sample, and to Ray Kokaly for determining iron phases from reflectance spectroscopy. This research was supported in part by the Climate and Land Use Change Program of the U.S. Geological Survey. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The authors also thank George Breit for his thorough and insightful review which greatly improved this manuscript.
- Bioavailable iron
- Iron dissolution
- Mineralogy effects
- Redox cycling
- Titanium dioxide