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The reductive dehalogenation of hexachloroethane (CzCLj), carbon tetrachloride (CC14), and bromoform (CHBr3) was examined at 50 "C in aqueous solutions containing either (1) 500 pM of 2,6-anthrahydroquinone disulfonate (AHQDS), (2) 250 pM Fe2+, or (3) 250 pM HS-. The pH ranged from 4.5 to 11.5 for AHQDS solutions and was 7.2 in the Fez+ solutions and 7.8 in the HS-solutions. The observed disappearance of C&16 in the presence of AHQDS was pseudo-first-order and fit k'ccb = ko[A(OH)zI + kdA(0H)O-I + kz[A(O)~~-l whereA(OH)2, A(0H)O-, and A(0)n2-represent the concentrations of the three forms of the AHQDS in solution. The values of ko, kl, and kz were -0,0.031, and 0.24 M-l s-l, respectively. The addition of 25 mg of C/L of humic acid or organic matter extracted from Borden aquifer solids to aqueous solutions containing 250 I~.M HS-or Fe2+ increased the reduction rate by factors of up to 10. The logarithms of the rate constants for the disappearance of CzCl6 and C C 4 in seven different experimental systems were significantly correlated; log k'cc14 = 0.64 log k'czc~ -0.83 with r2 = 0.80. The observed trend in reaction rates of C&l6 > CCl4 > CHBr3 is consistent with a decreasing trend in one-electron reduction potentials.

The transport of adsorbing and complexing metal ions in porous media was investigated with a series of batch and column experiments and with reactive solute transport modeling. Pulses of solutions containing U(VI) were pumped through columns filled with quartz grains, and the breakthrough of U(VI) was studied as a function of variable solution composition (pH, total U(VI) concentration, total fluoride concentration, and p H-buffering capacity). Decreasing p H and the formation of nonadsorbing aqueous complexes with fluoride increased U(VI) mobility. A transport simulation with surface complexation model (SCM) parameters estimated from batch experiments was able to predict U(VI) retardation in the column experiments within 30%. SCM parameters were also estimated directly from transport data, using the results of three column experiments collected at different p H and U(VI) pulse concentrations. SCM formulations of varying complexity (multiple surface types and reaction stoichiometries) were tested to examine the trade-off between model simplicity and goodness of fit to breakthrough. A two-site model (weak-and strong-binding sites) with three surface complexation reactions fit these transport data well. With this reaction set the model was able to predict (1) the effects of fluoride complexation on U(VI) retardation at two different p H values and (2) the effects of temporal variability of p H on U(VI) transport caused by low p H buffering. The results illustrate the utility of the SCM approach in modeling the transport of adsorbing inorganic solutes under variable chemical conditions. coordination of metal ions and decrease the extent of metal adsorption via the formation of weakly or nonadsorbing aqueous complexes. In batch systems the competition can be modeled using geochemical equilibrium models that account for aqueous speciation and surface complex formation, as has been demonstrated for uranium(VI) by Waite et al. [1994]. However, the metal-ligand interactions can be very complex, as increased adsorption of metals in the presence of ligands has also been observed [Davis and Leckie, 1978]. In a study involving Co(II)-EDTA complexes on iron-oxide-containing subsurface sands, Zachara et al. [1995] concluded that Co(II)-EDTA 2-complexes may adsorb as strongly as Co 2+ under certain geochemical conditions. The transport of adsorbing ions has often been modeled with the use of linear isotherms or a distribution coefficient K d to describe the adsorption process [Freeze and Cherry, 1979]. Under constant chemical conditions the adsorption isotherms of transition metal and actinide elements are usually nonlinear and are usually best described by the Freundlich isotherm [Davis and Kent, 1990]. However, under variable chemical conditions the adsorption of these elements is highly dependent on aqueous speciation, and their adsorption is not well described by isotherms or a distribution coefficient Kd. Thus it can be expected that the use of isotherms or Kd will often be inadequate in modeling the transport of these ele...

Assessing the quantity of U(VI) that participates in sorption/desorption processes in a contaminated aquifer is an important task when investigating U migration behavior. U-contaminated aquifer sediments were obtained from 16 different locations at a former U mill tailings site at Naturita, CO (U.S.A.) and were extracted with an artificial groundwater, a high pH sodium bicarbonate solution, hydroxylamine hydrochloride solution, and concentrated nitric acid. With an isotopic exchange method, both a KD value for the specific experimental conditions as well as the total exchangeable mass of U(VI) was determined. Except for one sample, KD values determined by isotopic exchange with U-contaminated sediments that were in equilibrium with atmospheric CO2 agreed within a factor of 2 with KD values predicted from a nonelectrostatic surface complexation model (NEM) developed from U(VI) adsorption experiments with uncontaminated sediments. The labile fraction of U(VI) and U extracted by the bicarbonate solution were highly correlated (r2 = 0.997), with a slope of 0.96 +/- 0.01. The proximity of the slope to one suggests that both methods likely access the same reservoir of U(VI) associated with the sediments. The results indicate that the bicarbonate extraction method is useful for estimating the mass of labile U(VI) in sediments that do not contain U(IV). In-situ KD values calculated from the measured labile U(VI) and the dissolved U(VI) in the Naturita alluvial aquifer agreed within a factor of 3 with in-situ KD values predicted with the NEM and groundwater chemistry at each well.