Characterization of carbonaceous combustion residues. I. Morphological, elemental and spectroscopic features

Abstract

Scanning electron microscopy, surface area determination, elemental analysis, organic matter extraction and solid-state cross polarization/magic angle spinning and Bloch decay/magic angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy were used to investigate distinctive features among carbonaceous combustion residues. Black carbon (BC) samples included diesel soot, urban dust, carbon black, chimney soot, vegetation fire residues, wood and straw charcoals. Particles varied from small spheres (<50 nm) in fossil BC (>100 m 2/g), to large layered structures in plant-derived BC (generally <8 m 2/g). Chimney soot also included large (>1 μm) liquid-like structures, while spherules >100 nm were unique to urban dust. The ratios of amorphous to soot carbon (SC) (isolated by thermal degradation) were not necessarily correlated with the degree of aromaticity estimated from H/C ratios. In particular, values of SC in diesel soot were clearly overestimated. Solvent-extractable organic matter (SEOM) was <2% for charcoals and carbon black, but >13% for urban dust, chimney and diesel soot. SEOM is thought to clog pores or to form large waxy globules, hence reducing surface areas. The ratio of polar/nonpolar SEOM was generally <7 for fossil BC, but >30 for plant-derived BC. NMR analysis revealed essentially one chemical shift in the aromatic C region of charcoals, while diesel soot also showed important aliphatic contributions. Aliphatic and oxygenated C predominated over aryl C in urban dust and chimney soot. These morphological and chemical characteristics of the BC samples are discussed in terms of their environmental implications.