Acid-catalysed dissolution of iron in brake wear aerosol

Abstract

Identifying sources of water-soluble iron (WS-Fe) in the environment is important in understanding the biogeochemical iron cycle and because of human health impacts associated with exposure to iron-containing aerosol. A main mechanism of iron solubilisation is through acid-catalysed reactions in atmospheric aerosol and cloud droplets. In this study the acid-catalysed dissolution of iron in brake wear aerosol was investigated to determine whether brake wear could be an anthropogenic source of WS-Fe. It was found that the iron in brake wear particles becomes soluble when suspended in solutions similar to atmospheric aerosol pH conditions and that the dissolution happens quickly enough to be atmospherically relevant. In solutions of nitric acid and sulphuric acid at pH 1, 2, and 3, 50-100% of available iron was dissolved within 10 hours. The dissolution was found to be very pH dependent, with an approximately 90% decrease in dissolved iron when pH is increased from 1 to 4, suggesting the reaction is highly proton driven. The dissolved iron was speciated and it was found that 75 ± 2.5% was Fe(III) and 25 ± 2.5% was Fe(II) for solutions of sulphuric acid at all pH levels and nitric acid at pH 2-4. However, in nitric acid solutions at pH 1, Fe(II) was completely oxidised into Fe(III). It was found that this oxidation increased with increasing nitrate concentrations at pH 1, suggesting nitrate has a role in the dissolution of iron. These results show that brake wear aerosol could be an important anthropogenic source of WS-Fe in the atmosphere.