Predicting the mobility of heavy metals in soils requires models that accurately describe metal adsorption in the presence of competing cations. They should also be easily adjustable to specific soil materials and applicable in reactive transport codes. In this study, Cd adsorption to an acidic soil material was investigated over a wide concentration range (10 -8 to 10 -2 M CdCl 2 ) in the presence of different background electrolytes (10 -4 to 10 -2 M CaCl 2 or MgCl 2 or 0.05 to 0.5 M NaCl). The adsorption experiments were conducted at pH values between 4.6 and 6.5. A reaction-based sorption model was developed using a combination of nonspecific cation exchange reactions and competitive sorption reactions to sites with high affinity for heavy metals. This combined cation exchange/ specific sorption (CESS) model accurately described the entire Cd sorption data set. Coupled to a solute transport code, the model accurately predicted Cd breakthrough curves obtained in column transport experiments. The model was further extended to describe competitive sorption and transport of Cd, Zn, and Ni. At pH 4.6, both Zn and Ni exhibited similar sorption and transport behavior as observed for Cd. In all transport experiments conducted under acidic conditions, heavy metal adsorption was shown to be reversible and kinetic effects were negligible within time periods ranging from hours up to four weeks.

Batch experiments were conducted to evaluate the ability of various forms of phytate, the hexaphosphoric form of myo-inositol (IP6), to immobilize U, Ni, and other inorganic contaminants in soils and sediments. A Ca-phytate precipitate (Ca(n)-IP6), dodeca sodium-phytate (Na12-IP6), and hydroxyapatite (HA) were added to contaminated soil at rates of 0, 10, 25, and 50 g kg(-1) and equilibrated in 0.001 M CaCl2. The samples were then centrifuged, the solution pH was measured, and the supernatants were filtered prior to analysis for dissolved organic carbon (DOC), U, Ni, P, and other inorganic contaminants, such as As, Cr, Se, and Pb. The residual sediments were air-dried prior to characterization by analytical electron microscopy and extraction with the Toxicity Characteristic Leaching Procedure (TCLP). The solubility of several metals (e.g., U, Pb, Cu) increased with increasing Na12-IP6 when compared with the nonamended control. In some cases immobilization was observed at the lowest Na12-IP6 application rate (10 g kg(-1)) with an increase in solubility observed at the higher rates, demonstrating the importance of metal to ligand ratio. In contrast, Ca(n)-IP6 and HA decreased the solubility of U, Ni, Al, Pb, Ba, Co, Mn, and Zn. For example, soluble U decreased from 2242 to 76 microg kg(-1) and Ni from 58 to 9.6 mg kg with the Ca(n)-IP6 addition, similar to the results observed for HA. Arsenic and Se solubility increased for HA and both forms of IP6, but to a much greater degree for Na12-IP6, suggesting that the increase in pH observed for HA and Na12-IP6, combined with added competition from PO4 and IP6 for sorption sites, resulted in the release of sorbed oxyanion contaminants. The analytical electron microscopy results indicated that metals such as U and Ni were closely associated with secondary Al-rich precipitates in the HA-treated soils, rather than unreacted HA. The analytical electron microscopy results were less definitive for the Ca(n)-IP6-treated soil, although the residual P-containing material was enriched in Al, with lesser amounts of U and Ni.

In this study, dissolved Kr and SF6 gases were used to determine various hydrogeological parameters of laboratory columns under water-saturated and partially saturated conditions as a function of the flow velocity. The dissolved gases behaved conservatively in saturated columns but were significantly retarded in unsaturated conditions as a direct function of the Henry's law constant (KH) and the ratio of column pore spaces filled with air and water (Vg/Vw). Lower aqueous diffusion coefficients for SF6 compared to that for Kr also resulted in significant rate-limited mass transport across gas-water interface. This effect was exacerbated at higher flow velocities as was indicated by the asymmetric shape of breakthrough curves, more so in the case of SF6. A nonequilibrium advective-dispersive transport model accurately described tracer breakthrough and was used to estimate parameters such as final Vg/Vw under partially saturated conditions and partitioning rates. Internally consistent model results were obtained for both dissolved gases despite the wide range in physical properties (e.g., KH and aqueous diffusion coefficients), suggesting that dissolved Kr and SF6 may be used in conjunction to delineate and validate aquifer characteristics simultaneously from a single pulse injection of the tracer.