The effect of the organic matter composition on POPaccumulation in soil

The effect of different humic fractions on polychlorinated biphenyl (PCB) contamination in soils was tested in the field by means of 53 soil samples from a high altitude grassland plateau in the Italian Alps. The three humic fractions (humin, humic acids and fulvic acids) were characterised in parallel by quantifying 12 PCB congeners to establish a direct relationship between PCB levels and humic fraction concentrations. Humin (the most hydrophobic fraction) appears to be the most closely correlated with the amount of PCBs in soil (R2 = 0.83), while fulvic acid shows the lowest correlation (R2 = 0.49). The idea of preferential sorption of hydrophobic compounds in the humin fraction is discussed, and the humin carbon content (fhuminC) is proposed as an improved parameter for evaluating the potential for POP accumulation in soils, replacing total organic carbon (foc). Congener studies revealed that penta- and hexa-substituted-CBs show the optimal combination of physicochemical properties for equilibrating with the humin content in soil. Moreover, fhuminC/foc is conceptually equivalent to the empirical coefficients used in predictive Ksa equations. In our samples, the fhuminC/foc was 0.55, a value in between the empirical coefficients proposed in the literature. In predictive equations, the use of fhuminC instead foc avoids the necessity of using an empirical parameter for a ‘generic’ condition by introducing an experimental parameter (fhuminC) that takes into account a local conditions (organic matter composition).