Donald Langmuir

Adsorption of uranyl onto ferric oxyhydroxides: Application of the surface complexation site-binding model

Uranyl adsorption was measured from aqueous electrolyte solutions onto well-characterized goethite, amorphous ferric oxyhydroxide, and hematite sols at 25°C. Adsorption was studied at a total uranyl concentration of 10−5 M, (dissolved uranyl 10−5 to 10−8 M) as a function of solution pH, ionic strength and electrolyte concentrations, and of competing cations and carbonate complexing. Solution pHs ranged from 3 to 10 in 0.1 M NaNO3 solutions containing up to 0.01 M NaHCO3. All the iron oxide materials strongly adsorbed dissolved uranyl species at pHs above 5 to 6 with adsorption greatest onto amorphous ferric oxyhydroxide and least onto well crystallized specular hematite. The presence of Ca or Mg at the 10−3 M level did not significantly affect uranyl adsorption. However, uranyl carbonate and hydroxy-carbonate complexing severely inhibited adsorption. The uranyl adsorption data measured in carbonate-free solutions was accurately modeled with the surface complexation-site binding model of Davis et al. (1978), assuming adsorption was chiefly of the UO2OH+ and (UO2)3(OH)+5, aqueous complexes. In modeling it was assumed that these complexes formed a monodentate UO2OH+ surface complex, and a monodentate, bidentate or tridentate (UO2)3(OH)+5surface complex. Of the latter, the bidentate surface complex is the most likely, based on crystallographic arguments. Modeling was less successful predicting uranyl adsorption in the presence of significant uranyl carbonate and hydroxy-carbonate complexing. It was necessary to slightly vary the intrinsic constants for adsorption of the di- and tricarbonate complexes in order to fit the uranyl adsorption data at total carbonate concentrations of 10−2 and 10−3 M.

The mobility of thorium in natural waters at low temperatures

Thermodynamic properties of 32 dissolved thorium species and 9 thorium-bearing solid phases have been collected from the literature, critically evaluated and estimated where necessary for 25°C and 1 atm pressure. Although the data are incomplete, especially for thorium minerals and organic complexes, some tentative conclusions can be drawn. Dissolved thorium is almost invariably complexed in natural waters. For example, based on ligand concentrations typical of ground water (ΣCl = 10 ppm, ΣF = 0.3 ppm, ΣSO4 = 100 ppm, andΣPO4 = 0.1 ppm), the predominant thorium species are Th(SO4)02, ThF2+2, and Th(HPO4)20below pH ≈ 4.5; Th(HPO4)2−3 from about pH 4.5 to 7.5; and Th(OH)04 above pH 7.5. Based on stability constants for thorium citrate, oxalate and EDTA complexes, it seems likely that organic complexes predominate over inorganic complexes of thorium in organic-rich stream waters, swamp waters, soil horizons, and waterlogged recent sediments. The thorium dissolved in seawater is probably present in organic complexes and as Th(OH)04. The tendency for thorium to form strong complexes enhances its potential for transport in natural waters by many orders of magnitude below pH 7 in the case of inorganic complexing, and below about pH 8 when organic complexing is important. The existence of complexes in addition to those formed with hydroxyl, is apparent from the fact that measured dissolved thorium in fresh surface waters (pH values generally 5–8) usually ranges from about 0.01 to 1 ppb and in surface seawater (pH = 8.1) is about 0.00064 ppb. This may be contrasted with the computed solubility of thorianite in pure water which is only 0.00001 ppb Th as Th(OH)04 above pH 5. Although complexing increases the solubility of thorium-bearing heavy minerals below pH 8, maximum thorium concentrations in natural waters are probably limited in general by the paucity and slow solution rate of these minerals and by sorption processes, rather than by mineral-solution equilibria.

The thermodynamic properties of radium

Abstract The enthalpy, Gibbs free energy, and entropies of aqueous radium species and radium solids have been evaluated from empirical data, or estimated when necessary for 25°C and 1 bar. Estimates were based on such approaches as extrapolation of the thermodynamic properties of Ca, Sr, and Ba complexes and solids plotted against cationic radii and charge to radius functions, and the use of the Fuoss or electrostatic mathematical models of ion pair formation (Langmuir, 1979). Resultant log K (assoc) and ΔH0 (assoc) (kcal/mol) values are: for RaOH+ 0.5 and 1.1; RaCl+ −0.10 and 0.50; RaCO03 2.5 and 1.07; and RaSO04 2.75 and 1.3. Log Ksp and ΔH0 (dissoc) (kcal/mol) values for RaCO3(c) and RaSO4(c) are −8.3 and −2.8, and −10.26 and −9.4, respectively. Trace Ra solid solution in salts of Pb and of the lighter alkaline earths, has been appraised based on published distribution coefficient (D) data, where D ∼- (mM2+)(NRaX)/(mRa2+)(NMX) (m and N are the aqueous molality and mole fraction of Ra and cation M in salt X, respectively. The empirical solid solution data have been used to derive both enthalpies and Gibbs free energies of solid solution of trace Ra in sulfate and carbonate minerals up to 100°C. Results show that in every case D values decrease with increasing temperature. Among the sulfate and carbonate minerals, D values decrease for the following minerals in the order: anhydrite > celestite > anglesite > barite > aragonite > strontianite > witherite > cerussite.

Adsorption of Cu, Pb and Zn by δMnO2: applicability of the site binding-surface complexation model

Abstract A synthetic δMnO 2 was characterized with respect to intrinsic equilibrium constants and surface charge density as a function of pH. Site binding-surface complexation theory was used to model adsorption of Cu, Pb, and Zn onto the δMnO 2 surface. The model response was tested with respect to changes in solution ionic strength, total trace metal concentration, δMnO 2 in suspension, substrate surface area, and the presence of competitive trace metals. Results indicate that both freshly precipitated and aged δMnO 2 act as heterogeneous surfaces with a spectrum of binding site energies. The distribution of site energies was examined by determination of apparent equilibrium binding constants. Highest energy sites were saturated once less than 0.1% of all surface sites had been occupied. Adsorption experiments were performed for solutions which simulate conditions in the channel of a mountainous stream. The natural system is actively precipitating δMnO 2 in a watershed containing base metal sulfide deposits.

The geochemistry of Ca, Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin, Texas

The geochemistry of Ca, Sr, Ba and Ra sulfates in some deep brines from the Palo Duro Basin of north Texas, was studied to define geochemical controls on radionuclides such as 90Sr and 226Ra. Published solubility data for gypsum, anhydrite, celestite, barite and RaSO4 were first reevaluated, in most cases using the ion interaction approach of Pitzer, to determine solubility products of the sulfates as a function of temperature and pressure. Ionic strengths of the brines were from 2.9 to 4.8 m, their temperatures and pressures up to 40°C and 130 bars. Saturation indices of the sulfates were computed with the ion-interaction approach in one brine from the arkosic granite wash fades and four from the carbonate Wolfcamp Formation. All five brines are saturated with respect to gypsum, anhydrite and celestite, and three of the five with respect to barite. All are undersaturated by from 5 to 6 orders of magnitude with respect to pure RaSO4. 226Ra concentrations in the brines, which ranged from 10−11.3 to 10−12.7 m, are not controlled by RaSO4 solubility or adsorption, but possibly by the solubility of trace Ra solid solutions in sulfates including celestite and barite.

Predicting arsenic concentrations in the porewaters of buried uranium mill tailings

Abstract The proposed JEB Tailings Management Facility (TMF) to be emplaced below the groundwater table in northern Saskatchewan, Canada, will contain uranium mill tailings from McClean Lake, Midwest and Cigar Lake ore bodies, which are high in arsenic (up to 10%) and nickel (up to 5%). A serious concern is the possibility that high arsenic and nickel concentrations may be released from the buried tailings, contaminating adjacent groundwaters and a nearby lake. Laboratory tests and geochemical modeling were performed to examine ways to reduce the arsenic and nickel concentrations in TMF porewaters so as to minimize such contamination from tailings buried for 50 years and longer. The tests were designed to mimic conditions in the mill neutralization circuit (3 hr tests at 25°C), and in the TMF after burial (5–49 day aging tests). The aging tests were run at, 50, 25 and 4°C (the temperature in the TMF). In order to optimize the removal of arsenic by adsorption and precipitation, ferric sulfate was added to tailings raffinates 1 having Fe/As ratios of less that 3–5. The acid raffinates were then neutralized by addition of slaked lime to nominal pH values of 7, 8, or 9. Analysis and modeling of the test results showed that with slaked lime addition to acid tailings raffinates, relatively amorphous scorodite (ferric arsenate) precipitates near pH 1, and is the dominant form of arsenate in slake limed tailings solids except those high in Ni and As and low in Fe, in which cabrerite-annabergite (Ni, Mg, Fe(II) arsenate) may also precipitate near pH 5–6. In addition to the arsenate precipitates, smaller amounts of arsenate are also adsorbed onto tailings solids. The aging tests showed that after burial of the tailings, arsenic concentrations may increase with time from the breakdown of the arsenate phases (chiefly scorodite). However, the tests indicate that the rate of change decreases and approaches zero after 72 hrs at 25°C, and may equal zero at all times in the TMF at 4°C. Consistent with a kinetic model that describes the rate of breakdown of scorodite to form hydrous ferric oxide, the rate of release of dissolved arsenate to tailings porewaters from slake limed tailings: (1) is proportional to pH above pH 6–7; (2) decreases exponentially as the total molar Fe/As ratio of tailings raffinates is increased from 1/1 to greater than 5/1; and (3) is proportional to temperature with an average Arrhenius activation energy of 13.4 ± 4.2 kcal/mol. Study results suggest that if ferric sulfate and slaked lime are added in the tailings neutralization circuit to give a raffinate Fe/As molar ratio of at least 3–5 and a nominal (initial) pH of 8 (final pH of 7–8), arsenic and nickel concentrations of 2 mg/L or less, are probable in porewaters of individual tailings in the TMF for 50 to 10,000 yrs after tailings disposal. However, the tailings will be mixed in the TMF, which will contain about 35% tailings with Fe/As = 3.0, and 65% tailings with Fe/As = 5.0–7.7. Thus, it seems likely that average arsenic pore water concentrations in the TMF may not exceed 1 mg/L.

Adsorption of Sr on Kaolinite, Illite and Montmorillonite at High Ionic Strengths

Experimental measurements of Sr adsorption onto kaolinite, illite and montmorillonite in up to 4.0 mol/kg NaCl solutions, were modelled with the surface ionization and complexation triple-layer (SIC) model (Davis et al. [1]) to determine if model adjustments were required for high ionic strengths. Improved model fits to the adsorption data were obtained at high ionic strengths, reflecting a lowered sensitivity of the model. A general reduction in Sr adsorption with increasing ionic strength was caused by an increase in the outer layer surface charge, rather than by a drop in the number of available adsorption sites. Sensitivity analysis showed that the range of values of model constants yielding acceptable fits was as large as variations reported in the literature for these constants. The study demonstrates that adsorption will not retard Sr migration in brines, and that it is unnecessary to introduce a Pitzer ion interaction subroutine (Pitzer [2]) in the SIC model when considering adsorption at high ionic strengths.

The Power Exchange Function: A General Model for Metal Adsorption onto Geological Materials

The empirical data on adsorption of metal cations by naturally occurring soil materials can be systematized in terms of power exchange functions. For example, given the reaction PbX + Ca2+ = CaX + Pb2+, the exchange function is: Kex = ([Pb2+]/[Ca2+]) (CaX/PbX)n where Kex and n are constants, the brackets denote activities of the ions, and CaX and PbX are their mole fractions on sorbent X. This approach is mathematically equivalent to regular solution exchange when the mole fractions of two competing cations sorbed lie between 0.25 and 0.75, and to Freundlich isotherm-type behavior when the mole fraction of the minor cation is < 0.05. Combined literature review and laboratory study show that the exchange behavior of H+, Na+, K+, Ca2+, Mg2+, Cd2+, Co2+, Ni2+, Pb2+, UO 2 2+ and Zn2+ and their hydroxy-complexes on a variety of adsorbents (montmorillonites, beidellite, illite, ferric oxyhydroxides, zeolites, soils and humic materials) can be accurately described for a wide range of competing sorbate concentrations and ratios using from one to three power exchange expressions. The adsorption of the alkali metal and alkaline earth cations on pure clays at about 10−2 to 10−4 M often follows the power exchange function with n = 1, corresponding to simple ion exchange. Adsorption of heavy metals (between 10−3 and 10−7 M) is usually more complex, and fits power exchange functions with n = 0.8 to 2.0. Log-linearization of power exchange expressions yields lines with correlation coefficients usually in the range from 0.98 to 1.00. The power exchange model is a potentially useful predictor of heavy metal levels in the subsurface controlled by adsorption processes.

The stability of UO2OH+ and UO2[HPO4]2−2 complexes at 25°C

Abstract The cumulative association constant (β 2 ) for the geochemically important aqueous complex UO 2 [HPO 4 ] 2− 2 has been determined by potentiometric titration in Na 2 HPO 4 -UO 2 (NO 3 ) 2 solutions in the pH range 3.9–4.7, at ionic strengths below 0.024 molal with the Newton-Raphson method used to compute β 2 from the chemical analytical data. Based on 25 measurements we obtain log β 2 = 18.3 ± 0.2 at 25°C. From the same experiments we compute that the association constant of UO 2 OH + is 8.9 ± 0.1, in disagreement with the value of 8.3 ± 0.3 for this constant given by Baes and Mesmer (1976).

Prediction of Uranium Adsorption by Crystalline Rocks: The Key Role of Reactive Surface Area

Adsorption processes are important in controlling U concentrations in ground water. Quantifying such processes is extremely difficult in that in situ conditions cannot be directly measured. One rock characteristic that must be known to quantify adsorption is the specific surface area of reactive minerals exposed to the ground water. We evaluate here three methods for estimating specific surface area in situ . The first is based on the dissolution kinetics of sodium feldspars, the second on emanation of radon-222 and the third on adsorption of naturally-occurring U. The radon-222 method yields estimates 5 to 8 orders of magnitude greater than those obtained via the other two methods; too large probably because of effects related to fracture geometry. Estimates of specific surface area based on modelling adsorption of natural U by aquifer materials are of comparable magnitude to those from the feldspar-dissolution kinetics approach. These conclusions are based on analyses of water from 145 wells in crystalline-rock aquifers from Pennsylvania, New Jersey, Maryland, and Colorado. Computer modelling of the chemical data using PHREEQE [1] showed that uraninite or coffinite approach saturation in reducing water, limiting total U to −9 m. Generally, U minerals are below saturation in oxidizing ground water, where uranyl-carbonate complexes are the dominant dissolved U species. Autoradioluxographs of thin sections show areas of concentration of radioactivity in the rocks and establish that U is concentrated along fracture boundaries and on ferric oxyhydroxide grain coatings. Because U minerals generally are undersaturated, U mobility is limited by adsorption onto ferric oxyhydroxides and other mineral surfaces. Calculations of uranyl adsorption from the ground water onto goethite using the program M1NTEQ [2] show that adsorption decreases with increased carbonate concentrations due to the formation of uranyl-carbonate complexes. Results of this paper improve our understanding of the mobility of U that might be released into oxidized ground water in crystalline rock from a breached radioactive-waste repository.