Reaction Mechanism Of Airborne Iron Containing Aerosol On Catalytic Oxidation Of Sulfur Dioxide

Atmospheric particles or aerosols play an important role in global climate, human health and many aspects of earth environment. Sulfur dioxide, the principal sulfur-containing anthropogenic pollutant, acts as a precursor of sulfuric acid which contributes to acid rain and particulate sulfate. It can cause the adverse health effects and destroy the ecosystem. The oxidation of SO₂ in the atmosphere may occur in the gas phase, in the aqueous phase of clouds and fogs, and on the surface of aerosol particles, but the latter reaction mechanisms has not been elucidated in detail yet. Iron in aerosols is the major source of phytoplankton's micronutrient in surface seawater. Moreover, it has been demonstrated to be the limiting nutrient factor for primary productivity especially in high nutrient, low chlorophyll (HNLC) regions.Based on above reasons, a better understanding of the heterogeneous reaction between SO₂ and aerosols, especially iron-containing dust, and the source of soluble iron is of interest.In this thesis, the heterogeneous reaction between SO₂ and Fe₂O₃ (or: Fe₂O₃ + NaCl) was investigated to explore the role of iron in oxidation of SO₂ and the reaction mechanism. In the first section, the determination method of Fe(II) by HPLC was selected , modified and the optimal condition was obtained. Fe(II) was extracted using succinic acid-disodium tetraborate buffer solution (pH5.6). It can complex with the reagent ferrize to form [Fe(FZ)₃]²⁺, then determinate at 254 nm. RSD was 3.48% and the recovery was 90-102%. The concentration of Fe(II), one of the heterogeneous reaction products, had been determined by this method.The second part is to study the heterogeneous reaction mechanism between SO₂ and Fe₂O₃. A combined technique of DRIFTS, XPS, HPLC, IC and pH meter is employed to demonstrate the formation of Fe(II)(aq) and SO₄^2- in the heterogeneous reaction between SO₂ and Fe₂O₃. The results revealed that absorbed water is the most crucial factor in the formation of Fe(II)(aq) and SO₄^2- Under ambient condition (T = 291 K, RH = 68 %) within 30 min, for 1 mg of Fe₂O₃ with the exhaustion of 53.6 μg SO₂, it could produce 12.6 ng Fe(II)(aq) and 56.2 μg SO₄^2- The concentration of the product [SO₄^2-], was three orders of magnitude higher than that of Fe(II)(aq), indicating that large amount of SO₄^2- production was formed via catalysis by the transitional metal Fe. The reaction mechanism of SO₂ oxidation on Fe₂O₃ is proposed. It could be concluded clearly that Fe(II)(aq) is not only a catalyst involved in thisheterogeneous SO₂ oxidation , but also one of the end products of this heterogeneous reaction. This important finding provides the strong evidence to support the hypothesis of the coupling and feedback of iron with sulfur in aerosols during their long-range transport.The final part is a pilot study on the reaction mechanism between SO₂ and mixed particles of Fe₂O₃ and NaCl. The products were analyzed by HPLC,IC and pH meter. The results showed that the higher relative humility and the addition of NaCl were favorable for the formation of Fe(II)(aq). A buffering role that prevented the pH from decreasing was found when NaCl presented. With the reaction time increasing, pH decreased and then increased; Fe(II) increased; SO₄^2- was formed rapidly, subsequently decreased and finally reached a plateau. A possible mechanism was proposed. The presence of NaCl complicates the coupling and the feedback of iron with sulfur in marine atmosphere. So Fe-S coupling and the possible positive feedback in marine atmosphere is an important mechanism which may affect the global climate change and is worthy to be further studied.