Molecular characterization of Ulex europaeus biochar obtained from laboratory heat treatment experiments – A pyrolysis–GC/MS study

Abstract: Gorse species (Ulex sp.) are ubiquitous in the shrublands of NW Spain and have the potential to become key players in an integral biofuel/biochar program in NW Spain. Here we present molecular characterization (using pyrolysis–GC/MS) of a biochar “thermosequence” obtained by laboratory heating of Ulex europaeus wood in a muffle furnace between 200 and 600°C (T CHAR). Low temperature chars (T CHAR ≤350°C) produced significant amounts of pyrolysis products of which the precursor biopolymer could be recognized, while high-temperature chars (T CHAR ≥400°C) produced mainly phenols and monocyclic and polycyclic aromatic hydrocarbons, which are not specific for any biopolymer. Carbohydrate could hardly be recognized at T CHAR ≥350°C. The thermal rearrangement of polyphenols, mainly lignin, was reflected in more detail (1) C3-side chain shortening and probably depolymerization (T CHAR 200–350°C), (2) demethoxylation of syringyl and probably also some guaiacyl lignin (T CHAR 300–400°C), (3) elimination of virtually all remaining methoxyl groups (T CHAR 350–400°C), through dehydroxylation and demethoxylation, (4) almost complete dehydroxylation of lignin and other biopolymers (T CHAR 400–500°C), (5) progressive condensation into polyaromatic structures (T CHAR 300–500°C) and (6) partial elimination of alkyl bridges between (poly)aromatic moieties (T CHAR 450–500°C). These results were supported by Fourier transform infrared spectroscopy (FTIR) of the same samples. We conclude that pyrolysis–GC/MS can be used as a rapid molecular screening method of gorse-derived biochar. Molecular properties elucidation is an essential part of predicting the stability and agronomical behavior of gorse-derived biochar after future implementation in soils. [Copyright &y& Elsevier]