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TECHNICAL REPORTS

Organic Compounds in the Environment

Sorption of High Explosives to Water-Dispersible Clay: Influence of Organic Carbon, Aluminosilicate Clay, and Extractable Iron

^a SpecPro, Inc., U.S. Army Engineer Research and Development Center, 3909 Halls Ferry Rd., Vicksburg, MS 39180
^b Environmental Processes Branch, Environmental Lab., U.S. Army Engineer Research and Development Center, Vicksburg, MS. SpecPro, Inc., 3530 Manor Drive, Suite 4, Vicksburg, Mississippi 39180
^c current address, Biosphere 2, Univ. of Arizona, Tucson, AZ 85721-0158

^* Corresponding author (Kateryna.Dontsova{at}gmail.com).

Received for publication April 21, 2008. Explosives in soils can present environmental problems for military installations. Fine, mobile particles represent the most reactive fraction of the soil and, therefore, are expected to adsorb explosives and potentially facilitate their transport. The objective of this study was to determine the relative significance of phyllosilicate clay, organic matter, and two forms of extractable iron in adsorption of 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by the colloidal water-dispersible clay (WDC) fraction of the soil. The WDC fraction of two mineral and one organic soil was separated and then treated to remove organic carbon (OC) and several forms of iron (Fe_o, oxalate extractable, and Fe_d, dithionite-citrate extractable). Adsorption coefficients were determined for whole soils, untreated, and treated WDC. For mineral soils, adsorption of TNT and RDX on the WDC was greater than on the whole soil. The presence of OC increased explosives sorption by WDC. When OC was removed, iron interfered with TNT sorption. In the presence of OC, removal of Fe_o decreased RDX adsorption and increased TNT adsorption indicating different adsorption mechanisms. Organic carbon was a more significant indicator of explosives adsorption by WDC than clays or iron oxides and hydroxides. Therefore, OC is the most likely medium for facilitated transport of TNT and RDX.

Abbreviations: AOE, acid oxalate extraction • CEC, cation exchange capacity • DCE, dithionite-citrate extraction • DOC, dissolved organic carbon • Fe_d, dithionite-citrate extractable iron • Fe_o, acid oxalate extractable iron • HE, high explosives • HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine • HPLC, high performance liquid chromatography • K_d, adsorption coefficient • K_OC, adsorption coefficient normalized to fraction of organic carbon • OC, organic carbon • POC, particulate organic carbon • RDX, hexahydro-1,3,5- trinitro-1,3,5-triazine • TNT, 2,4,6-trinitrotoluene • SA, surface area • WDC, water-dispersible clay