The widespread use of nanoparticles (NPs) and other potentially ecotoxic chemical compounds to enhance the quality of consumer products has raised concern about their fate in the environment. Indeed, those products have to be ultimately disposed of and along with them also NPs and contaminants could end up in landfills or in facilities where waste and nano-waste, as well as biomass products, are stored and/or treated. Among the possible pathways involving their release in the environment, this thesis' focus is in particular on gaseous emissions during two main applications, namely waste treatment and bioenergy generation. In this frame, the effect of contaminants is considered not only in terms of their toxicity but also in terms of their effect on energy efficiency or potential damages caused to the facilities along the process chain. For this purpose, new online and offline methods were developed by using hyphenated analytical techniques to achieve complementary and superior power of investigation.
Firstly, the speciation of siloxane compounds were studied during bio-methane production from manure and agricultural waste. The samples were collected using a liquid quench (LQ) sampling system that concentrates condensable and non-condensable compounds in a liquid solvent in continuous contact with the gas probed, for an improved time resolution. The offline investigation required a complete method development for the combination of a separation technique, such as gas chromatography (GC), and detectors able to provide chemical information. The novelty was represented by the simultaneous use of GC with a flame ionization detector (FID) and inductively coupled plasma mass spectrometry (ICP-MS), which could fulfill the requirements of recent regulations for the specific application, with lower detection limits compared to existing methods.
Secondly, the fate of zinc oxide (ZnO) NPs were investigated during the combustion of impregnated wood, as a model of nano-waste. ZnO was specifically considered because of its use in paints and waterproofing agents. The potential release of NPs during nano-waste treatment was investigated with an online method that combines an RDD (rotating disk diluter) used as conditioning and dilution unit, an SMPS (scanning mobility particle sizer) providing the particle size distribution and ICP-MS used as a multi-elemental analytical technique. An initial assessment of the performances of the RDD-SMPS-ICP-MS was required, to correct some major assumptions used in the SMPS data treatment. The technique was successfully applied even to heterogeneous matrices. The particular capability of performing online sampling and simultaneous size-resolved and elemental analysis on the off-gases produced during wood combustion was providing the needed insights to verify the predicted mechanism of reduction and further re-oxidation of ZnO nano-objects. Thanks to the calibration strategy coupling a TGA (thermogravimetric analyzer) with RDD-ICP-MS, a successful determination of their concentration was also possible.
In conclusion, the development of specific hyphenated techniques was proven to be crucial to improve the capabilities of the already existent state-of-the-art methods and to develop new ones that could give more insight on the two main scientific questions that were the driving force of this thesis.
EPFL2019
