| 摘要: |
| 利用pH 4.7醋酸铵溶液和pH 2硫酸溶液对四个沙尘总悬浮颗粒物（TSP）样品进行溶解性铁提取，分别模拟沙尘含铁矿物在雨水和大气酸作用下的溶解。结果显示：在pH 4.7醋酸铵溶液中沙尘铁的溶解性介于2.30% — 5.74%，平均值为4.17%；在pH 2硫酸溶液中沙尘铁的溶解性为4.72% — 7.27%，平均值为6.13%。两种溶液中溶解性铁的价态组成存在明显差异，在pH 4.7醋酸铵中，溶解性铁主要以三价铁（FeⅢ）形式存在，占全部溶解性铁的76.7% — 98.3%，二价铁（FeⅡ）仅占溶解性铁的1.7% — 23.3%；在pH 2硫酸溶液中FeⅡ的相对比例明显增加，占全部溶解性铁的17.3% — 50.0%。推测在酸作用下富含二价铁的矿物（如碳酸盐矿物）发生迅速溶解，改变了溶解性铁的组成。忽视这一机制可能不利于准确理解沙尘铁溶解性增强机理。 |
| 关键词: 腾格里沙漠 二价铁 三价铁 含铁矿物 |
| DOI：10.7515/JEE201602009 |
| CSTR：32259.14.JEE201602009 |
| 分类号: |
| 基金项目:中国科学院战略性先导科技专项（XDB05000000）；国家自然科学基金项目（40872211） |
| 英文基金项目:Strategic Priority Research Program of Chinese Academy of Sciences (XDB05000000); National Natural Science
?????????????Foundation of China (40872211) |
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| pH-dependent dissolved iron speciation of dust collected in dust source region |
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pH-dependent dissolved iron speciation of dust collected in dust source region1,2,3,4
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1. Key Laboratory of Aerosol Chemistry and Physics (KLACP), Chinese Academy of Sciences,
Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710049, China;4. College of Science and Technology, Hainan Tropical Ocean University, Sanya 572022, China
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| Abstract: |
| Background, aim, and scope About 30% of the world oceans like the Equatorial Pacific and the Southern Oceans are comprised of high-nutrient low-chlorophyll (HNLC) regions where phytoplankton growth is limited by bioavailable iron(Fe) supply. It is believed that increased bioavailable Fe supply to these regions will stimulate phytoplankton growth, leading to enhanced atmospheric CO2 sequestration and nitrogen fixation by the ocean, which can have profound impacts on climate. Mineral dust originated from the world semi-arid and arid regions is the dominant source of Fe to the remote oceans. Direct field measurements reported Fe solubility of mineral dust ranging from less than 0.1% to as much as 80% with higher solubility values generally observed over remote parts of the oceans, implying that atmospheric processing can increase the Fe solubility during dust transport. To understand how atmospheric process will influence Fe dissolution behavior of mineral dust, Fe solubility and dissolved Fe speciation of mineral dust in typical atmospheric solutions were investigated. Materials and methods In April 2015, four dust Total Suspended Particle (TSP) samples were collected at the Tengger Desert, a well-known Asian dust source region. A quarter of the dust filter sample was extracted with ammonium acetate (pH 4.7) and sulfuric acid (pH 2) to simulate Fe dissolution in the rainwater and acidic aerosol water, respectively. Dissolved Fe and Fe2+ in the extraction solution were measured colorimetrically using the Ferrozine method. Total Fe on the filter was determined with X-Ray Fluorescence. Results Fe solubility was 4.17% (2.30% — 5.74%) in the ammonium acetate and 6.13% (4.72% — 7.27%) in the sulfuric acid. Large difference in soluble iron speciation was observed in the two solutions. In the ammonium acetate, Fe3+ was the dominant species accounting for 76.7% — 98.3% of the total dissolved iron and Fe2+ only constituted a minor part (1.73% — 23.3%). In the sulfuric acid, Fe2+ was significantly mobilized and accounted for a much higher fraction of total dissolved iron (17.3% — 50.0%) than that in the ammonium acetate. Discussion The results show that both Fe solubility and dissolved Fe speciation were strongly dependent on the pH of the extraction solution. Since mineral dissolution in the acidic solution is a proton-promoted process, it is reasonable that Fe solubility was significantly enhanced when the solution become more acidic. However, there exists many factors that may lead to a difference in dissolved Fe speciation in the two extraction solutions. These factors include (1) redox reaction between dissolved Fe and the extractant, (2) Photo-reduction of dissolved Fe, (3) Selective dissolution of Fe-bearing mineral in different extraction solution. On one hand, no redox reaction between dissolved Fe and ammonium acetate or between dissolved Fe and sulfuric acid is expected to occur in the extraction solution. On the other hand, all of the extraction experiments were conducted under dark condition. The only possible reason for the difference in dissolved Fe speciation in the two solutions lies in the selective dissolution of Fe-bearing minerals. Asian dust has a very high content of carbonate minerals and some Fe was trapped in the crystal lattice these carbonate minerals in the form of Fe2+. Moreover, dissolution rate of carbonate minerals is much more sensitive to pH than other Fe-bearing minerals. We proposed the large difference in soluble iron speciation was caused by fast dissolution of Fe2+-bearing minerals like carbonate when the pH of the extraction was lowered. Conclusions We found that both that Fe solubility and dissolved Fe speciation of mineral dust were strongly pH-dependent. Lowered pH of atmospheric solution not only leads to enhanced Fe solubility but also results in significant change in dissolved Fe speciation. This large change in the dissolved Fe speciation was possibly caused by fast dissolution of Fe2+-bearing minerals like carbonate under more acidic solution. When dust is transported in the atmosphere, the atmospheric solution around dust particle may become extremely acidic, leading to mobilization of Fe2+-bearing minerals as well as change in the dissolved Fe speciation. A lack of awareness of this pH-initiated change in dissolved Fe speciation may hamper an accurate understanding of factors that control dust iron solubility. Recommendations and perspectives This study reveals a new mechanism that may lead to a change in dissolved Fe speciation of mineral dust when dust is subject to atmospheric processing. This pH-initiated change in dissolved Fe speciation should not be ignored when we explain the factors that control dust iron solubility. |
| Key words: Tengger Desert Fe2+ Fe3+ Fe-bearing minerals |
