Abiotic transformation of perchloroethylene in homogeneous dithionite solution and in suspensions of dithionite-treated clay minerals

The reductive dechlorination of perchloroethylene (PCE) in homogeneous solu tions of dithionite and at the surfaces of, dithionite-citrate-bicarbonate (DCB) treated ferruginous smectite and Na-montmorillonite was studied. Tran sformation products of PCE identified in dosed dithionite-treated samples i ncluded TCE, DCE, 1,1;2-trichloroethane (TCA), 1,1 dichloroethane (DCP;), c hloroacetylene,acetylene, ethene, and ethane. The decomposition of dithioni te to sulfate yielded both protons and electrons necessary far hydrodechlor ination (hydrogenolysis) of PCE. Dithionite treatment of the Fe-poor Na-mon tmorillonite enhanced reductive dechlorination of PCE relative to dithionit e-treated Fe-rich ferruginous smectite, within the range of 11.5-137.8 mM d ithionite. For the same dithionite concentration, the kinetics of the heter ogeneous reactions of PCE was generally faster than that of the homogeneous reaction, and higher concentrations of TCE were measured in the heterogene ous reactions. interestingly, increases in the mass of the clay minerals us ed, the Fe2+ content in the clay mineral structure, or the dithionite conce ntration used did not necessarily enhance the abiotic transformation of PCE , as would otherwise be predicted. The most efficient reductive dechlorinat ion of PCE was observed with 0.5% clay (m/v) treated with 34.5 mM dithionit e buffered at pH 8.5. The solid-state transfer of electrons to surfaces and edges, rather than the redox capacity, limited the dechlorination of PCE b y reduced: ferruginous smectite and/or suspensions containing a higher clay mass. The greater reactivity of ditirionite-reduced montmorillonite than s imilarly treated ferruginous smectite is attributed to (il the well-documen ted layer collapse and aggregation of chemically reduced clays that increas es with the clay's iron content, (ii) the location of solid-phase Fe2+ in t he reduced clay mineral and whether it is accessible or inaccessible for re action with PCE at the mineral edges and surfaces where the reactions are t hought to occur, and (iii) the greater swellability of montmorillonite vers us ferruginous smectite. The faster dechlorination rate of PCE observed wit h dithionite-reduced Fe-poor montmorillonite than similarly reduced iron-ri ch ferruginous smectite suggests that the use of dithionite barriers for in -situ treatment of chlorinated solvent plumes should not be limited to aqui fers with Fe-rich sediments.