Coordination chemistry in atmospheric aerosols: effects on metal solubility, dissolution kinetics, and health

Part of the Atmospheric Chemistry Series, hosted by Beth Nelson (beth.nelson@york.ac.uk).

About 0.2% of the Earth’s water is in the atmosphere. Rain, fog, clouds and deliquescent aerosols create a water phase that can shift the reactivity of volatile and non-volatile species to form new products that would not otherwise form from gas phase reactions. A notable example of processes promoted by aqueous environments is coordination chemistry. Such process leads to the formation of metal-ligand complexes between different species in the aerosols and it can increase the solubility of particle-bound metals, therefore their bioavailability, and their capability to generate reactive oxygen species.

We investigated the formation of metal-organic ligand complexes, especially those involving small dicarboxylic acids, in urban aerosol collected in the city centre of Padua, in the Po Valley (Italy), in marine aerosol from a remote coastal environment in Henties Bay (Namibia), and in volcanic aerosol collected from the summit of Mt Etna (Italy).

Aerosol samples were then characterized for quantification of metals, using inductively coupled plasma mass spectrometry (ICP-MS) and inorganic and organic ligands, using ion chromatography (IC) with conductimetric detection. The model E-AIM was used to calculate the aerosol liquid water content and the pH of the aerosols. Thermodynamic modelling, using Visual MINTEQ, was used to gain the speciation picture of the equilibria in solution in the deliquescent aerosol phase. Synchrotron based X-ray absorption spectroscopy (XAS) was used to directly determine the oxidation state and chemical coordination sphere of iron.

For urban samples, we assessed the effects of metal-ligand complexes formation on the solubility and solubilisation kinetic of metals from the particles to both fog waters and a surrogate lung fluid. We found contrasting results in terms of solubilities and dissolution kinetics in the two aqueous environments studied. Finally, we exposed pulmonary cells to metals in different chemical forms to assess their toxicity.

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