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

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. Mossbauer 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.