Structural characterization of charcoal size-fractions from a burnt Pinus pinea forest by FT-IR, Raman and surface-enhanced Raman spectroscopies

Charcoal is mainly composed by aromatic C but is characterized by several degrees of aromaticity, which complicate its identification and quantification in natural environments by conventional analyses. Charcoal is an almost ubiquitous component of soil, although often occurring in minor amounts. Hence, there is a great interest in studying the charcoal structure and understanding the behaviour of charcoal in soils. In soil, it has been demonstrated that the chemical structure of charcoal is also dependant on its particle-size. In this paper we have applied infrared, normal Raman, SERS and SEF spectroscopies to study four size-fractions of charcoal (>2 mm, 2-1 mm, 1-0.5 mm and <0.5 mm) and their alkaline extracts from a burnt Pinus pinea forest. Second derivative FT-IR in agreement with the normal Raman spectroscopy have given information about the size of aromatic rings and the nature of substituted groups in size-fractions of charcoal. Carbon cluster size progressively decreased in the smallest fractions suggesting that higher graphitization and crystallization occurred in the coarsest fraction (>2 mm). The presence of organic N was well evidenced by the vibration of iso-cyanate and cyanamide groups in the fractions. This result is of great environmental importance because N may be immobilized in form of stable molecules like N-heterocyclic poliaromatic structures, and persists in soils. The soluble organic C (SOC) extracted from charcoal size-fractions did not differ by IR and Raman spectra of bulk charcoal. Instead SERS and SEF provided further information regarding the ratio between the aromatic component and the aliphatic cross-linking one in studied fractions.