Characterization and acid‐mobilization study of iron‐containing mineral dust source materials

Abstract

Processes that solubilize iron in mineral dust aerosol may increase the amount of iron supplied to ocean surface waters, and thereby stimulate phytoplankton productivity. In particular, the uptake of acids such as H2SO4 and HNO3 on mineral dust surfaces can produce extremely acidic environments that promote iron dissolution. Here, four samples that represent source materials for mineral dust (Saudi Beach sand (SB), Inland Saudi sand (IS), Saharan Sand (SS) and China Loess (CL)) and one commercial reference material (Arizona Test Dust (AZTD)) were characterized, and examined in dissolution studies in solutions of sulfuric, nitric and hydrochloric acid ranging from pH 1 to 3. Mössbauer spectroscopy revealed Fe(III) in all samples, whereas SB, CL and AZTD also contained appreciable Fe(II). Spectra suggest that both Fe(II) and Fe(III) were substituted into aluminosilicates, although CL, AZTD and IS also contained Fe(III) oxide phases. Total iron solubility measured after 24 h ranged between 4–16% of the initial iron content for each material, but did not scale with either the specific surface area or the total iron content of the samples. Instead, we show that Fe(II)‐containing solid phases such as Fe(II)‐substituted aluminosilicates represent a significant, and sometimes dominant, source of soluble Fe in acidic environments. Results of dissolution studies also show that the nature of the acid influences iron solubilization, as elevated concentrations of nitrate encountered from nitric acid at pH 1 suppressed Fe(II) formation. We propose a surface‐mediated, non‐photochemical redox reaction between nitrate and Fe(II), which may contribute to Fe(II)/Fe(III) cycling in the atmosphere.

Simulated atmospheric processing of iron oxyhydroxide minerals at low pH: Roles of particle size and acid anion in iron dissolution

“…α-FeOOH nanoparticles were synthesized according to the method of Anschutz and Penn (35) as previously described (11). Dropwise The spectra for adsorbed sulfate on nanorods and microrods are shown in B and C, respectively.…”

“…For example, Journet et al showed that Fe-containing clays accounted for more than 90% of soluble iron (10). Cwiertny et al proposed that iron in the form of Fe 2þ in clay minerals is a highly labile source of iron (11). Other studies highlight the contribution from anthropogenic sources of soluble iron, mainly from combustion sources (3,12,13).…”

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).…”

“…Dust particles could become very acidic at the early stage of the transport before being neutralized by alkaline gases (Sullivan et al, 2007). Therefore, iron in aluminosilicate minerals potentially represents a significant source of soluble iron in the acidic environment (Cwiertny et al, 2008;Journet et al, 2008;Schroth et al, 2009). …”

“…In addition, cloud processing, which may involve radical reactions in liquid phase, has been suggested to increase the soluble iron particles in the fine mode (Zhu et al, 1992;Shi et al, 2009). The mineralogy of iron also influences the particulate iron solubility and may contribute to the size dependence of the soluble ironrich dust (Claquin et al, 1999;Cwiertny et al, 2008;Journet et al, 2008;Schroth et al, 2009). Compared to mineral dust aerosols, iron from combustion sources is suggested to be more soluble, and found more frequently in smaller particles (Chuang et al, 2005;Guieu et al, 2005;Sedwick et al, 2007;Sholkovitz et al, 2009).…”