Soil organic matter as a nanoporous sorbent of organic pollutants

The organic matter fraction of soils is the principal sorbent of sparingly soluble organic compounds and thus plays a fundamental role in the transport, reactivity and bioavailability of organic pollutants in the soil column. The prevailing physical concept of SOM is that of a flexible polymer phase which acts as a dissolution medium for hydrophobic molecules expelled from the polar aqueous environment. This article summarizes recent research concluding that SOM also has an internal nanopore structure that provides specific sorption sites for organic compounds, regardless of their polarity. Thus, a new concept of SOM as a dual-mode (dissolution–hole-filling) sorbent has emerged. The nanoporous structure of SOM is revealed by carbon dioxide adsorption at 273 K. The nanoporosity determined by the Dubinin–Radushkevich transform of the CO2 isotherm correlates with the degree of non-linearity in single-solute organic isotherms and the magnitude of the competitive effect in organic bisolute systems. Evidence links sorption in the hole-filling domain with slow desorption rates, a principal cause of reduced bioavailability of contaminants and a major factor limiting the remediation of contaminated sites.