Analytical Methods for the Characterisation of Volatile and Water-Soluble Organic Compounds in Biochar. Relationships with Thermal Stability and Seed Germination

The thesis aimed at investigating the source, identity, and biological effects of organic compounds released from biochar into air and water. Biochar is the carbonaceous product of biomass pyrolysis for agro/environmental purposes. However, its application to soil could be hindered by the presence of contaminants. Apart from priority regulated compounds, a wide range of organic species can be adsorbed onto biochar porous structure during its production, and their mobility in the environment could generate beneficial or negative impacts. Techniques based on solid-phase microextraction (SPME) were developed to determine volatile (VOCs) and water-soluble organic compounds (WSOCs) released from several biochars from different feedstocks and pyrolysis conditions. Head space (HS) and direct immersion (DI)-SPME coupled with GC-MS suitably detected a variety of thermal degradation products of biomass biopolymers (lignin, hemi/cellulose, lipids and proteins). The patterns of VOCs/WSOCs were directly connected with the biochar carbonisation degree measured by H/C molar ratios. Biochars with H/C <0.7 presented distinctively different profiles and reduced intensities compared to less carbonised ones. SPME-GC-MS, ultra-high resolution mass spectrometry (Negative Electrospray Ionisation Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, ESI-FT-ICR-MS) and fluorescence spectroscopy-Parallel Factor Analysis (PARAFAC) were employed to compare the complex composition of WSOCs with that of the vapours condensed into bio-oil during pyrolysis. Biochar preferentially released aromatic species with acid functionalities resembling those of natural organic matter (fulvic acids), while lignin-like structures characterised the bio-oils. Germination tests revealed that high levels of VOCs/WSOCs can induce phytotoxicity, but when biochar contamination is low, organic species were associated to stimulating effects on seedlings growth. The green analytical methods based on SPME were suitable for screening the effect of pyrolysis process conditions on the presence of mobile compounds in the resulting biochar and its quality for soil applications.