Submitted to: Science of the Total Environment
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/18/2006
Publication Date: 8/30/2006
Citation: Fan, Z., Casey, F., Larsen, G.L., Hakk, H. 2006. Fate and transport of 1278-TCDD, 1378-TCDD, and 1478-TCDD in soil-water systems. Science of the Total Environment. 371:323-333. Interpretive Summary: Because 2,3,7,8-tetrachlorodibenzo-p-dioxin (2378-TCDD) is toxic, studies of it’s fate and transport in soil are dangerous and difficult. However, several nontoxic forms of this toxic-TCDD exist which allow such studies to be conducted in relative safely and can provide significant experimental information about how this highly toxic dioxin survives and is transported in natural soils. The nontoxic forms used in these studies were 1,2,7,8-tetrachlorodibenzo-p-dioxin (1278-TCDD), 1,3,7,8-tetrachlorodibenzo-p-dioxin (1378-TCDD), and 1, 4, 7, 8-tetrachlorodibenzo-p-dioxin (1478-TCDD). Soils were mixed with solutions containing each of these nontoxic dioxins to determine how they interact with soils. Solutions of each of the nontoxic dioxins were also each passed over a column containing soil to study their fate and transport as they past through soil. In each of these studies the nontoxic dioxins were tagged with radiolabeled [14C] so the dioxins or any compound they may have been changed into by the soil or absorption to the soil could be monitored. The results from the experiments where the dioxins were mixed with the soil showed that they are more tightly bound to the soil with high organic mater content than to the soil with low organic matter content; however, it took a longer contact time to approach adsorption equilibrium in soils with the higher organic matter content. The soil column studies showed that very little dioxins are transported through the soil. Lower amounts of the dioxins were transported through soils with higher organic matter content than soils containing lower organic matter. Transport was thought to result because the dioxins attach to small clay particles (colloidals) which move rapidly through the soil. Most of these dioxins were found to be tightly bound to the top layers of the soil.
Technical Abstract: The most toxic dioxin is 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (2378-TCDD), and obtaining comprehensive experimental data is challenging. However, several nontoxic isomers of 2378-TCDD exist, and can provide significant experimental evidence about this highly toxic dioxin, 2378-TCDD. The goal of this study was to obtain experimental evidence for the fate and transport of 2378-TCDD in natural soils using its nontoxic isomers, 1, 2, 7, 8-tetrachlorodibenzo-p-dioxin (1278-TCDD), 1, 3, 7, 8-tetrachlorodibenzo-p-dioxin (1378-TCDD), and 1, 4, 7, 8-tetrachlorodibenzo-p-dioxin (1478-TCDD). Batch sorption and miscible-displacement experiments, in various soils, were done using [14C]-radiolabeled TCDDs, while metabolism of these compounds was monitored. The results from the batch experiments indicated a high adsorption affinity of all the TCDD isomers to soils and a strong correlation to organic matter (OM) content. 1278-TCDD, 1378-TCDD and 1478-TCDD (TCDDs) were more tightly bound to the soil with high OM than to the soil with low OM; however, it took a longer contact time to approach sorption equilibrium of TCDDs in the soil with high OM. Miscible-displacement breakthrough curves indicated chemical nonequilibrium transport, where there was a rate-limited or kinetic sorption likely caused by OM. Combustion analyses of extracted soil from the soil columns showed that most TCDDs were adsorbed in the top 1–5 cm of the column. These column combustion results also showed that adsorption was correlated to specific surface and soil depth, which suggests possible colloidal transport.