Insights into the mechanism of light-induced dissolution of goethite in the presence of atmospheric dicarboxylic acids

Biogeochemistry cycle of iron is of great important to the oceanic primary productivity. Increased evidences suggested that organic ligands play an important role in the mobilization of iron from mineral dust aerosol. The... [ view full abstract ]

Biogeochemistry cycle of iron is of great important to the oceanic primary productivity. Increased evidences suggested that organic ligands play an important role in the mobilization of iron from mineral dust aerosol. The purpose of this study is to better understand the iron dissolution processes in the presence of low-molecular organic acid. The dissolution of goethite was investigated in the presence of three low-molecular dicarboxylates. The ligand-promoted dissolution rates of iron in the dark were in the following order: oxalate > malonate > succinate > MSA. Irradiation and deaerated conditions were beneficial to the improvement of iron solubility. The dissolution rate under the irradiation were higher by 5~10 orders of magnitude than that in the dark. The Fe(II)/Fe(t) ratio approached 83%-93%, indicated that the iron mobilization was mostly contributed to the photoreductive dissolution. The dissolution rates of the three dicarboxylate ligands correlated with carbon chain length: n = 2 > 3 > 4, because the space barrier action of carbon chains affected structure and stability of the complexes, thereby inhibit inner-sphere ligand-to-metal electron transfer. The production of active species (H2O2, •OH and O2•−) determined by Acridinium Ester Chemiluminescence detection and Electron spin resonance measurement suggested that the photoreductive dissolution of Fe is very sensitive to dissolved oxygen. Transmission electron microscopy (TEM) gave the direct evidence that dissolution was preferential detached at the reactive sites such as surface defects and/or sharp edges. This study deepens our understanding on the ligand promoted-dissolution mechanisms, and contributes to atmosphere models in the atmosphere.