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

Gorse species (Ulexsp.) 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 degrees C (T-CHAR). Low temperature chars (T-CHAR <= 350 degrees C) produced significant amounts of pyrolysis products of which the precursor biopolymer could be recognized, while high-temperature chars (T-CHAR >= 400 degrees 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 degrees C. The thermal rearrangement of polyphenols, mainly lignin, was reflected in more detail (1) C-3-side chain shortening and probably depolymerization (T-CHAR 200-350 degrees C), (2) demethoxylation of syringyl and probably also some guaiacyl lignin (T-CHAR 300-400 degrees C), (3) elimination of virtually all remaining methoxyl groups (T-CHAR 350-400 degrees C), through dehydroxylation and demethoxylation, (4) almost complete dehydroxylation of lignin and other biopolymers (T-CHAR 400-500 degrees C), (5) progressive condensation into polyaromatic structures (T-CHAR 300-500 degrees C) and (6) partial elimination of alkyl bridges between (poly)aromatic moieties (T-CHAR 450-500 degrees 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. (C) 2012 Elsevier B.V. All rights reserved.