Polychlorinated biphenyl desorption from low organic carbon soils: Measurement of rates in soil-water suspensions

The desorption of 13 PCB congeners from four soils has been investigated using the gas purge technique. The soils from PCB spill sites had been in contact with Aroclor 1242/1254 mixtures for 3 or more years; thus, sorption equilibrium is presumed to have been obtained. Soils were “engineered” ground cover materials used at utility industry substations and consisted of fine rock chips and sandsilt-clay fractions with organic carbon < 0.2%. The congeners contained from three to five chlorine atoms. Agreement of measured Henryʹs Law constants for the 14C-labeled congeners 24′, 22′55′ and 22′44′55′ with literature values established the proper function of the gas purge technique for measurement of congener release rates from soil-water suspensions. For all 13 congeners and all soils: 1) the labile fraction was typically 80 to 90% of the total congener concentration, 2) the majority of the labile fraction was desorbed or released within 48 hours of contact with water, and 3) the release of the remaining nonlabile fraction persisted for over 6 months with complete release estimated to be 1 to 2 years. Release rate constants, kd, decreased with increasing chlorine number with typical values for labile and nonlabile fractions ranging from 1.4 to 0.5 d−1 and 0.008 to 0.0006 d−1, respectively.