Emissions of Secondary Formed ZnO Nano-Objects from the Combustion of Impregnated Wood. An Online Size-Resolved Elemental Investigation.

The release of secondary nano-objects formed during waste combustion processes is becoming a matter of concern, considering their known toxicity and the fact that the 100% efficiency of filtering systems is not always ensured. An increased cytotoxicity and genotoxicity on human peripheral blood lymphocytes is known particularly in the case of ZnO, which is often contained in paints and waterproof agents, heading to a relevant quantity present in the waste wood material. In this study, the behavior of ZnO nanoparticles during wood combustion and the effect of the reduction potential of generated carbon species on the release of secondarily formed ZnO-containing nano-objects were investigated. By hyphenating a modified scanning mobility particle sizer (SMPS) and inductively coupled mass spectrometry (ICP-MS), it was possible to obtain simultaneously size-resolved and chemical information on the emitted nanoparticles. Through the established correlation between SMPS and ICP-MS signals, Zn-containing particles were efficiently resolved from the combustion generated particles. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) on size-selected particles confirmed the SMPS and ICP-MS data. The use of electron diffraction allowed determining the structure of the crystalline materials as hexagonal ZnO. A possible mechanism of reduction of ZnO to Zn and further reformation as secondary nano-objects is proposed.