Slow desorption of PCBs and chlorobenzenes from soils and sediments: relations with sorbent and sorbate characteristics

The kinetics of slow desorption were studied for four soils and four sedime nts with widely varying characteristics [organic carbon (OC) content 0.5-50 %, organic matter (OM) aromatic content (7-37%)] for three chlorobenzenes a nd five polychlorinated biphenyls (PCBs). Slowly and very slowly desorbing fractions ranged from 1 to 50% (slow) and 3 to 40% (very slow) of the total amount sorbed, and were observed for all compounds and all soils and sedim ents. In spite of the wide variations in sorbate K-OW (factor 1000) and sor bent characteristics, the rate constants of slow (k(slow), around 10(-3) h( -1)) and very slow (k(very slow) 10(-5)-10(-4) h(-1)) desorption appeared t o be rather constant among the sorbates and sorbents (both within a factor of 5). There was a good correlation (r(2) above 0.9) between the distributi on over the slow, very slow and rapid sediment fractions and log K-OC, indi cating that sorbate hydrophobicity may be important for this distribution. No correlation could be found between sorbent characteristics [OC, N, and O in the organic matter, polarity index C/(N + O), OC aromaticity as determi ned by CP-MAS C-13-NMR] and slow desorption parameters (slowly/very slowly desorbing fractions + corresponding rate constants). The absence of (I) a c orrelation between k(slow) and k(very slow), respectively, and OC content, and (2) the narrow range of k(slow) and k(very slow) values, indicates that intra-OM diffusion is not the mechanism of slow or very slow desorption, b ecause on the basis of this mechanism it would be expected that increasing OC content would lead to longer diffusion pathlengths and, consequently, to smaller rate constants. In addition, it was tested whether differential sc anning calorimetry would reveal a glass transition in the soils/sediments. In spite of the sensitivity of the equipment used (changes in heat flow in the micro-Watt range were measurable), a glass transition was not observed. This means that activation enthalpies of slow desorption can be calculated from desorption measurements at various temperatures. In the present study these values ranged from 60 to 100 kJ/mol among the various soils and sedi ments studied. (C) 2000 Elsevier Science Ltd. All rights reserved.