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Factors controlling the solubility of aerosol trace metals in the atmosphere and on mixing into seawater
Previous work has shown that the type and pH history of an aerosol governs trace metal solubility in rainwater. This study concentrates on the crustal elements Al, Fe and Mn and identifies additional processes which affect dissolution not only in the atmosphere but also on mixing into seawater. Aero...
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| Published in: | Aquatic geochemistry 1996, Vol.1 (4), p.355-374 |
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| Main Authors: | , |
| Format: | Article |
| Language: | English |
| Citations: | Items that this one cites Items that cite this one |
| Online Access: | Get full text |
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| Summary: | Previous work has shown that the type and pH history of an aerosol governs trace metal solubility in rainwater. This study concentrates on the crustal elements Al, Fe and Mn and identifies additional processes which affect dissolution not only in the atmosphere but also on mixing into seawater. Aerosol dissolution experiments (at aerosol concentrations of about 30 mg l super(-1)) show manganese exhibiting high solubility at the low pH values typical of clouds (54 plus or minus 2.5% at pH 2, with results expressed in mole percent units) with 85% of this increase occurring within 6 hours of acidification. The percentage dissolution decreases to 50% at pH values representative of rainwater (pH 5.5) and to 26 plus or minus 4% at pH 8, typical of seawater. No such dramatic solution phase removal occurs at pH 8 in the presence of inorganic anions (to a final solubility of 44 plus or minus 2%). Thus the extent of manganese dissolution depends strongly on whether aerosols are cycled through acidic environments and on subsequent inorganic complexation once rainwater mixes into sea. Aluminium shows highest dissolution (7.1 plus or minus 0.6%) at low pH with 78% of this increase occurring within 6 hours of acidification. Rapid solution phase removal occurs on increasing the pH to that representative of rainwater (to 0.9 plus or minus 0.4% with 87% of this decrease occurring within 15 min). As a consequence of acid cycling and aluminium's amphoteric nature, solubility is enhanced at seawater pH (2.3 plus or minus 0.3%) over that in rain. Iron shows a strong pH-solubility relationship with highest solubility at low pH (4.7 plus or minus 0.2%), 70% of this value being reached within 6 hours of acidification, and decreasing rapidly to 0.17% as pH is raised to 8. Addition of inorganic anions at pH 8 to simulate mixing into seawater causes a further decrease in solubility, perhaps due to anion induced colloid destabilisation. Photochemical reduction also effects solubility under low pH conditions with Fe(II) comprising 1% of the total iron in the Saharan Aerosol used and 8.4% in an Urban material at a pH of approximately 2. This element shows rapid solution phase removal with increasing particulate load which is tentatively rationalised in terms of a simple Kd approach. |
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| ISSN: | 1380-6165 1573-1421 |
| DOI: | 10.1007/BF00702739 |