Characterization of biochar-derived organic matter extracted with solvents of differing polarity via ultrahigh-resolution mass spectrometry

Abstract

In recent years, biochar, a porous carbon-based material, has gained attention for its application prospects in contaminated soil remediation and soil improvement. Biochar-derived organic matter has a key role in influencing the migration and transformation of soil elements and pollutants. However, existing research concerning the molecular characteristics of biochar-derived organic matter is limited. Here, we used four polar solvents — dichloromethane (CH2Cl2), acetone (CH3COCH3), methanol (CH3OH), and distilled water (H2O) — to extract organic matter from soybean straw biochar and wheat straw biochar by accelerated solvent extraction (ASE). We characterized the extracts using Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). We found considerable differences in organic matter according to the extraction solvents; such differences were related to the polarity of the solvent, as well as intermolecular forces between the solvent and organic matter. CH3OH extracted the most biochar-extractable organic matter components because CH3OH can weaken or destroy oxygen bridge bonds in biochar and form hydrogen bonds with small-molecule organic compounds. CH3OH and H2O have strong extraction capacity for compounds containing heteroatoms. CH2Cl2-extractable organic matter is relatively labile and bioavailable, while CH3OH- and H2O-extractable organic matters are relatively stable. In addition, the binding capacity of biochar-derived organic matter for minerals and pollutants differed among fractions, in part because of differences in molecular weight, atomic O/C and H/C ratios, heteroatom distribution, and biomolecular compounds present in biochar-derived organic matter. The findings in this study help to select appropriate extractants to analyze biochar-derived organic matter for various research purposes, and provides a theoretical basis for biochar-based remediation of contaminated soil.