Recent studies show that the sorption of hydrophobic organic contaminants (HOCs) to soil is not adequately described by simple partitioning of HOCs between soil solution and a homogeneous organic matter phase. Partitioning coefficients (K_d or K_OC) are often found to be concentration dependent, with higher values at low HOC concentrations, suggesting the presence of high-affinity, low-capacity sorbent phases. Slow sorption kinetics and sorption-desorption hysteresis are also frequently reported and are incompatible with the partitioning model.

Alternatives to the simple partitioning model have been proposed, many of which invoke the existence of different types of sorption sites – weak partitioning sites and strong non-partitioning sites. The nature of the non-partitioning sites has been of particular interest, with candidates including black carbon, kerogen and less well defined “condensed” or “glassy” phases. So far, the lack of suitable analytical techniques has precluded a definitive identification of these highly sorptive phases.

We have shown that the solid-state ¹³C NMR technique proton spin relaxation editing (PSRE) can identify, characterize and quantify domains within organic matter matrices of the type proposed as highly sorptive phases. For example, we have used PSRE to identify and quantify charcoal in soil. Here, we show that PSRE can be used to determine sorption properties of these domains, whilst avoiding physical fractionation. This is achieved by conducting PSRE on samples sorbed with ¹³C labeled HOCs. The ¹³C label enables easy identification of sorbate molecules at low concentrations.