Abstract
There is significant iron deposition in the oceans, approximately 14–16 Tg annually from mineral dust aerosols, but only a small percentage (approx. 3%) of it is soluble and, thus, bioavailable. In this work, we examine the effect of mineralogy, particle size, and surface area on iron solubility in pure mineral phases to simulate atmospheric processing of mineral dust aerosols during transport. Pure iron-bearing minerals common to Saharan dust were partitioned into four size fractions (10–2.5, 2.5–1, 1–0.5, and 0.5–0.25 µm) and extracted into moderately acidic (pH 4.3) and acidic (pH 1.7) leaching media to simulate mineral processing during atmospheric transport. Results show that, in general, pure iron-bearing clay materials present an iron solubility (% dissolved Fe/total Fe in the mineral) an order of magnitude higher than pure iron oxide minerals. The relative solubility of iron in clay particles does not depend on particle size for the ranges examined (0.25–10 μm), while iron in hematite and magnetite shows a trend of increasing solubility with decreasing particle size in the acidic leaching medium. Our results indicate that while mineralogy and aerosol pH have an effect on the solubilization of iron from simulated mineral dust particles, surface processes of the aerosol might also have a role in iron solubilization during transport. The surface area of clay minerals does not change significantly as a function of particle size (10–0.25 µm), while the surface area of iron oxides is strongly size dependent. Overall, these results show how mineralogy and particle size can influence iron solubility in atmospheric dust.
Highlights
Oceans are responsible for removing approximately one-third of anthropogenic CO2 from theEarth’s atmosphere [1]
This study focuses on particles ranging from 10 μm to less than 0.25 μm in diameter with a specific focus on particle sizes between
Soluble iron is defined as iron that has passed through a 0.22 μm filter and has been analyzed by inductively coupled plasma mass spectrometry (ICP-MS)
Summary
Oceans are responsible for removing approximately one-third of anthropogenic CO2 from the. Atmospheric mineral dust contains approximately 3% iron by mass and can undergo long range transport, resulting in an estimated 14–16 Tg of iron deposited into the oceans annually [7,12]. Modelling studies suggest that the long-range transport of aluminosilicate minerals which release nanoparticulate ferrihydrite and soluble iron, may provide an important source of bioavailable iron to remote ocean regions [18,19,20]. The factors affecting the solubilization of iron during atmospheric transport of mineral dust need to be considered as aerosols are a large source of soluble iron to the open ocean. Two other factors affecting iron solubilization during transport are related to the composition of the mineral dust: mineralogy, and particle size and surface area. The moderately acidic leaching medium (pH 4.3) was used to simulate cloud water, while the acidic leaching medium (pH 1.7) was used to simulate a marine aerosol solution
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