Characterization and phenanthrene sorption of organic matter fractions isolated from organic and mineral soils

Abstract

Sorption of phenanthrene (PHE) to humic acid (HA) and humin (HM) fractions isolated from organic and mineral soils was investigated to better understand sorption processes in varying soil types. Samples were characterized by elemental analysis, X-ray photoelectron spectroscopy, ¹³C nuclear magnetic resonance, and CO₂ adsorption. No clear correlation was found between the distribution coefficient (K_d) and the bulk polarity of the soil organic matters (SOMs). By contrast, PHE K_d values generally increased with increasing surface polarity of the tested SOMs, implying that surface polarity may play a more important role in PHE sorption than the bulk one. The organic carbon (OC)-normalized K_d values (K_oc) of HMs were higher than those of HAs as a result of the higher aliphatic C contents of HMs. For SOMs isolated from mineral soil (MI-SOMs), part of the aliphatic domains may be tightly associated with minerals and were not accessible to PHE molecules, resulting in lower PHE K_oc values of MI-SOMs than the corresponding fractions extracted from the organic soil. This study implies that both chemical characteristics and physical conformation of SOMs are paramount considerations when investigating sorption process of hydrophobic organic compounds in soils.