SPME-GC-MS DETERMINATION OF MOBILE COMPOUNDS IN BIOCHAR

摘要

Biochar production from the pyrolysis of residual biomass could be a strategy to turn often unexploited streams of secondary products into a valuable resource. Given its high stability and potential amendment effect, biochar use in agriculture is promising. However, thermochemical process conditions highly influence biochar physical and chemical properties to the extent that low molecular weight compounds could be retained onto its porous complex surface. When not irreversibly adsorbed, these substances could have a certain degree of mobility in water or could be released as vapours (VOCs), potentially generating both beneficial as well as detrimental effect on biota (e.g. plants, microorganisms). The need of a deep understanding of biochar "volatilome" is important when considering large scale applications to soil and handling of this material and represents an emerging field of research. Qualitative assessment of these compounds was performed on biochar and similar matrices, e.g. on hydrochars by headspace (HS) and gas chromatography-mass spectrometry (GC-MS). Solid-phase microextraction (SPME) is a solventless valid technique for the determination of low molecular weight compounds in a variety of matrices. However, its application to biochar is new and few reports have dealt with this approach. Due to its on-fiber pre-concentration capabilities, SPME was proved to be a valuable technique for the characterization of trace compounds in biochars; direct immersion SPME coupled with GC-MS was effective in determining lignin phenols from methanol extracts of lignocellulosic biochars. In this study, a faster HS-SPME-GC-MS method with minimal sample treatment (weighting, heating) was developed and tested on biochar samples obtained from the pyrolysis of different biomass substrates; data were compared with SPME of aqueous extracts to obtain a deep knowledge of mobile trace organic compounds that may occur in biochar. Different classes of molecules were detected after HS-SPME, ranging from non-polar alkyl aromatics, small PAHs, phenols, nitrogen-organic compounds to polar fatty acids, effectively showing that this technique is suitable to determine with minimal sample treatment a wide range of volatile compounds in biochars from various feedstock and the process conditions.