The occurrence and sources of Ni in ambient air particulates using synchrotron radiation based X-ray fluorescence and X-ray absorption near edge structure†

Nickel (Ni) is a transition metal widespread in the environment, and derives from both natural sources and industrial activities. In turn, environmental nickel pollution may be linked to these natural and anthropogenic sources including sea spray, industrial processes, commercial products (fuels), and agriculture waste burning. High concentrations of Ni in the ambient air might cause acute, chronic, and noxious effects on human health, and are known to induce in particular carcinogenic effects. The present work aims at investigating the presence and related concentration of Ni and its species in ambient air particulates using a combination of laboratory and advanced X-ray synchrotron radiation-based techniques. For this purpose, suspended particulate matter (SPM) and fine particulate matter with an aerodynamic diameter equal to or less than 2.5 μm (PM_2.5) were collected from residential and industrial areas of Cairo, Egypt using an established air sampler setup. The quantitative elemental results indicate remarkably high concentrations of Ni in the ambient air particulates, ranging from 20 to 30 ng m⁻³, which, for most samples, are higher than the annual allowance thresholds as indicated by the World Health Organization (WHO) and in the air quality standard of the European Union. Elemental maps of nickel were acquired to unravel its spatial distribution on the filters carrying the ambient air particulates. Complementary X-ray absorption near edge structure (XANES) spectroscopy at the Ni K-edge (8333 eV) was used to determine Ni speciation. Together with linear combination fitting, our results demonstrate that Ni species at the industrial area is predominantly found under its divalent oxidation state in the studied ambient air particulates.