Black carbon-dominated PCDD/Fs sorption to soils at a former wood impregnation site

The influence of black carbon (BC) on the sorption of 17 native polychlorinated-p-dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) was studied in five soil samples from a sawmill site where wood used to be impregnated with chlorophenol preservatives. The presence of BC caused measured total organic carbon (TOC)–water distribution ratios (KTOC) to be a median factor of 51 (interquartile range 18–68, n = 85) higher than modeled amorphous organic carbon (AOC)–water distribution ratios (KAOC). KTOC was a factor of 73 ± 27 above KAOC for PCDFs (n = 10) and a factor of 20 ± 13 (n = 7) for PCDDs. The reason for this difference is probably that attaining a planar configuration after sorption to BC is less thermodynamically favorable for PCDDs than for PCDFs. BC–water distribution ratios were calculated from KTOC, KAOC and BC contents, and ranged from 109.9 (2,3,7,8-Tetra-CDD) to 1011.5 l kg−1 (Octa-CDF). More than 90% of the PCDD/Fs in the soil was calculated to be BC-sorbed. Dissolved organic carbon (DOC)–water distribution ratios were measured to be in the same order of magnitude as KAOC. This study shows that strong sorption to BC should be included when assessing ecotoxicological risk or modeling transport to groundwater of PCDD/Fs in soil.