Photoreductive dissolution of Fe‐containing mineral dust particles in acidic media

Abstract

[1] In this study, the photoreductive dissolution of Fe-containing mineral dust particles in acidic media is investigated. Photolysis experiments were performed using a solar simulator to irradiate acidic mineral dust suspensions prepared from source materials of Inland Saudi sand (IS), Saharan sand (SS) and one commercial sample, Arizona test dust (AZTD). The results show that Fe dissolution is a pH-dependent process, and total Fe solubility decreases with the solution pH increasing. Comparing iron dissolution from different source materials, Fe(II)-containing AZTD is more soluble than IS and SS samples, irrespective of the presence or absence of light. Experiments performed at three different temperatures of 278, 293, and 308 K show that both total dissolved Fe and dissolved Fe(II) increase with temperature and upon light exposure. Results of dissolution studies with AZTD performed at low pH also illustrate that the nature of the acid strongly influences iron solubilization. Fe solubility decreases in the suspensions of H 2 SO 4 and HNO 3 in the presence of light, whereas Fe solubility increases in the HCl solution under the irradiation compared to the dark reaction. Finally, our results confirm earlier work showing that photoreductive dissolution of Fe is very sensitive to dissolved oxygen. Results from this laboratory study show that mineralogy and speciation of iron, as well as environmentally relevant factors including pH, light, O 2 , and nature of the inorganic anion, NO 3 − , SO 4 2− , and Cl − , must be considered when modeling the input of iron to the oceans.

Iron dissolution kinetics of mineral dust at low pH during simulated atmospheric processing

“…The results reported here are based on long-term Fe dissolution experiments in dusts at low pHs. These are consistent with previous Fe dissolution experiments at similar pH conditions on dust and/or soils at shorter time scale (12 h to 120 h) which showed that Fe dissolution rates in dust or soils are strongly pH dependent and that Fe dissolves faster initially (Spokes et al, 1994;Spokes and Jickells, 1996;Mackie et al, 2005;Cwiertny et al, 2008;Fu et al, 2010).…”

“…1) as well as others (Spokes et al, 1994(Spokes et al, , 1996Desboeufs et al, 1999Desboeufs et al, , 2005Mackie et al, 2005;Cwiertny et al, 2008;Fu et al, 2010;Deguillaume et al, 2010). We attribute this as the dissolution of highly reactive first Fe pool.…”

“…We have clearly shown that laboratory-made or commercial ferric oxides are fundamentally different to those in natural dusts and that assuming that all Fe in dust samples is present as a single mineral phase is incorrect and that such assumptions can lead to large errors (∼500%) in predicting the Fe solubility in real samples (Shi et al, 2011). In addition, data in Cwiertny et al (2008), Journet et al (2008), Schroth et al (2009) andFu et al (2010) suggested that speciation and mineralogy of Fe in the dust/soil affects the rate and amount of Fe dissolution. A recent modelling study also suggested that the predicted Fe solubility is sensitive to the type of Fe-containing minerals (i.e., hematite or illite) in the dust that was chosen for modelling (Ito and Feng, 2010).…”

“…Spokes and Jickells (1996) and Mackie et al (2005) also showed that Fe dissolution rate is not dependent on dust/liquid ratio at diluted conditions (i.e., <20 mg L −1 ) at low pH (e.g., ∼2). Cwiertny et al (2008) and Fu et al (2010) investigated the Fe dissolution behaviour of a series of soil or loess samples from pH 1 to 3 at a dust/liquid ratio >2 g L −1 for up to 30 h. They demonstrated that temperature, type of acids, photo-radiation and the nature of the dust all affect Fe dissolution rates. In all of these studies Fe dissolves very fast initially and then slower and Fe dissolution rates are strongly pH dependent.…”

Role of dust alkalinity in acid mobilization of iron

“…Mezkhidze et al (2003) have assumed that hematite is an important source for the dissolved iron over the North Pacific, because most of the iron in surface soil of the Gobi deserts is found in the form of hematite (a-Fe 2 O 3 ) (Hseung and Jackson, 1952;Claquin et al, 1999). However, the assumption of hematite as the solely important source for dissolved iron may need to be revisited, as more comprehensive experimental data for chemical specificity of iron-rich dust become available (Cwiertny et al, 2008;Journet et al, 2008;Schroth et al, 2009;Fu et al, 2010). Here, the illite dissolution is also considered for specification of "structural iron", which is trapped in the crystal lattice of aluminosilicate minerals (RS4 in Table 1).…”