Hydrosulfide oxidation pathways in oxic solutions containing iron(III) chelates

The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-; C(HS-)0 = 50-150 micromol/L) into polysulfides (S(n)2-; n = 2-9), colloidal sulfur, and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta; C(Fe(III)0 = 50-300 micromol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +/- 1 degree C. At higher pH, oxygen was seen to slow down the hydrosulfide conversion rate. For instance, the HS- half-life was 24.8 min in a DO2-saturated iron(III)-cdta solution compared to 11.3 min in the corresponding anoxic solution (pH 10.2, C(HS-)0 = 80 micromol/L, C(Fe(III))0 = 200 micromol/L). In anoxia, HS- oligomerizes into chain-like polysulfides which behave as autocatalysts on the HS- conversion rates. The presence of DO2 disrupts the HS- oligomerization process by generating thiosulfate precursors from polysulfides, a pathway that impedes the HS- uptake. At lower alkaline pH where the hydroxide-free Fe(3+)cdta(4-) is the prevailing iron(III)-cdta species, the "iron(II)-cdta + DO2" oxidative reaction becomes crucial. Oxidative regeneration of iron(III) as Fe(3+)cdta(4-) (being more reactive than Fe(3+)OH(-)cdta(4-)) offsets to some extent the restrictive role of oxygen on the accumulation of polysulfides. Thiosulfate and sulfate were the main end-products for the current experimental conditions to the detriment of colloidal sulfur, which did not form in DO2-saturated solutions.     

Use of spectroscopic techniques for uranium(VI)/montmorillonite interaction modeling

To experimentally identify both clay sorption sites and sorption equilibria and to understand the retention mechanisms at a molecular level, we have characterized the structure of hexavalent uranium surface complexes resulting from the interaction between the uranyl ions and the surface retention groups of a montmorillonite clay. We have performed laser-induced fluorescence spectroscopy (LIFS) and X-ray photoelectron spectroscopy (XPS) on uranyl ion loaded montmorillonite. These structural results were then compared to those obtained from the study of uranyl ions sorbed onto an alumina and also from U(VI) sorbed on an amorphous silica. This experimental approach allowed for a clear determination of the reactive surface sites of montmorillonite for U(VI) sorption. The lifetime values and the U4f XPS spectra of uranium(VI) sorbed on montmorillonite have shown that this ion is sorbed on both exchange and edge sites. The comparison of U(VI)/clay and U(VI)/oxide systems has determined that the interaction between uranyl ions and montmorillonite edge sites occurs via both [triple bond]AlOH and [triple bond]SiOH surface groups and involves three distinct surface complexes. The surface complexation modeling of the U(VI)/montmorillonite sorption edges was determined using the constant capacitance model and the above experimental constraints. The following equilibria were found to account for the uranyl sorption mechanisms onto montmorillonite for metal concentrations ranged from 10(-6) to 10(-3) M and two ionic strengths (0.1 and 0.5 M): 2[triple bond]XNa + UO2(2+) <==> ([triple bond]X)2UO2 + 2Na+, log K0(exch) = 3.0; [triple bond]Al(OH)2 + UO2(2+) <==> [triple bond]Al(OH)2UO2(2+), log K0(Al) = 14.9; [triple bond]Si(OH)2 + UO2(2+) <==> [triple bond]SiO2UO2 + 2H+, log K0(Si1) = -3.8; and [triple bond]Si(OH)2 + 3UO2(2+) + 5H2O <==> [triple bond]SiO2(UO2)3(OH)5- + 7H+, log K0(Si2) = -20.0.     

Adsorption of glyphosate on goethite (alpha-FeOOH): surface complexation modeling combining spectroscopic and adsorption data

N-(phosphonomethyl)glycine (glyphosate, PMG) is the most widely used herbicide, and its adsorption onto soil minerals plays a significant role in its mobility and rate of degradation. In this work, we present the results of the first serious effort to find a realistic surface complexation modelthatfits both adsorption and total proton concentration data for PMG on the common soil mineral, goethite. Special attention was focused on making sure that the final model was in good semiquantitative agreement with previously reported X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopic measurements. Electrostatic effects were accounted for using the Basic Stern model, and the charges of the PMG-containing surface complexes were assumed to be distributed across the 0- and beta-planes. The reactions for the protonation of the goethite surface were described using the 1 pK model. We optimized on the intrinsic formation constants and the charge distributions of the complexes, as well as the initial total proton concentration (I = 0.1 M Na(NO3), 25.0 degrees C), and the following model was obtained. triple bond FeOH(0.5-) + H3L <==> triple bond FeHL(1.5-) + H(+) + H2O Log10beta = 4.70 +/- 0.08, Q0 = -0.18 +/- 0.02 triple bond FeOH(0.5-) + H3L <==> FeL(2.5-) + 2H(+) + H2O Log10beta = -3.9 +/- 0.1, Q0 = -0.7 +/- 0.1 Here, beta is the intrinsic formation constant, Q0 is the charge at the 0-plane, and the errors are reported as one standard deviation. The charge distributions of the complexes are rationalized by considering intramolecular hydrogen bonding between the protons of the amine group and both the phosphonate and carboxylate groups.     

Distribution and early diagenesis of antimony species in sediments and porewaters of freshwater lakes

The study identifies the role played by different components of natural aquatic systems on the poorly known geochemistry of antimony. Different chemical forms of antimony were measured in porewaters and sediments of two Sudbury lakes characterized by contrasting redox conditions at the sediment-water interface. In porewaters, Sb(III) was present under reducing conditions where it could exist as SbS2- according to thermodynamic calculations. Sb(V) was detected mainly under oxic and mildly reducing environments where its presence was attributed to the oxidizing effect of iron and manganese oxyhydroxides or to the slow kinetics of reduction by dissolved sulfide or possible complexation by it. A third form of Sb identified as refractory was obtained after UV irradiation of the water samples, suggesting an association of Sb to low molecular weight natural organic matter. The distribution of Sb in sediments of the two lakes revealed (through the comparison of profiles and statistical correlations) the importance of iron and manganese oxyhydroxides in controlling the behavior of Sb, particularly in the lake where the interface was clearly oxic. Porewater profiles indicate that the dissolution of manganese and iron oxyhydroxides under anoxic conditions leads to the simultaneous release of dissolved Sb previously sorbed onto those compounds. In reducing sediments, the control of the solubility of Sb by iron sulfides is suggested.     

Arsenite retention mechanisms within estuarine sediments of Pescadero, CA

Arsenic, a toxic metalloid, is commonly associated with sulfide minerals in anoxic sediments. Here we characterize arsenic(III) retention on sediments from a sulfidic estuarine marsh using a series of sorption experiments, and probe the structure of retained arsenite with X-ray absorption spectroscopy. Although the extent of sorption varied with sampling locations, several adsorption characteristics were apparent. A fraction of arsenite adsorbed over the entire pH range examined, although it was most extensive at pH greater than 7, and conformed to a Langmuir isotherm. Iron sulfide phases were responsible for As partitioning in these sediments. Initially, an FeAsS-like precipitate formed with a structure similar to those reported for As(III) sorbed on iron sulfides, a complex that is highly reactive. Following reaction for 21 d, much of the FeAsS-like precipitate was converted to As2S3. A drop in the redox potential accompanied this conversion, suggesting that the evolution of sulfide and other reduced species stabilizes bound arsenic. Processes discerned in this study reveal the importance of sulfide minerals in As sequestration within anoxic environments.     

Thioarsenates in sulfidic waters

It has long been recognized that the formation of soluble arsenic sulfur complexes plays a key role for the mobility and toxicity of arsenic in sulfate-reducing environments. Knowledge of the exact arsenic species is essential to understand the behavior of arsenic in sulfidic aquifers and to develop remediation strategies. In the past, monomeric and trimeric thioarsenites were assumed to be the existing species in sulfidic systems. In this study, thioarsenates were identified by IC-ICP/MS in arsenite- and sulfide-containing solutions as well as in a reduced groundwater from a contaminated site. The unexpected finding of an oxidation of As(lll) to As(V) in thioarsenates in strongly reducing systems can be explained by the high affinity between As(Ill) and sulfur. In sulfide-containing solutions without oxidant, As(lll) therefore undergoes disproportionation to thioarsenates (As(V)) and elemental arsenic. It has previously been supposed that mobility as well as toxicity of arsenic increases if the redox state decreases. For sulfidic waters, the opposite is probably the case. Thus, the formation of thioarsenates could be used in connection with remediation strategies. Thioarsenates are highly sensitive to oxygen and pH. This is important for analytical procedures. A loss of soluble arsenic as well as a conversion to arsenite and arsenate may occur if water samples containing thioarsenates are analyzed with conventional methods.     

Nucleophilic substitution of phosphorothionate ester pesticides with bisulfide (HS-) and polysulfides (S(n)2-)

The reactions of five organophosphorus insecticides (OPs) (chlorpyrifos-methyl, parathion-methyl, fenchlorphos, chlorpyrifos, and parathion) with hydrogensulfide/ bisulfide (H2S/HS-) and polysulfides (S(n)2-) were examined in well-defined aqueous solutions over a pH range from 5 to 9. The rates are first-order in the concentration of the different reduced sulfur species. Experiments at 25 degrees C demonstrated that the reaction of the five OPs with the reduced sulfur species follows a SN2 mechanism. The activation parameters of the reaction of OPs with bisulfide were determined from the measured second-order rate constants over a temperature range of 5-60 degrees C. The determined second-order rate constants show that the reaction of an OP with polysulfides is from 15 to 50 times faster than the reaction of the same OP with bisulfide. The dominant transformation products are desalkyl OPs, which indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of the alkoxy groups. And also the results show that these reduced sulfur species are much better nucleophiles, and thus degrade these pesticides faster than the well-studied base hydrolysis by OH-. When the determined second-order rate constants are multiplied with the concentration of HS- and S(n)2- reported in salt marshes and porewater of sediments, predicted half-lives show that abiotic degradation by sulfide species may be of comparable importance to microbially mediated degradation in anoxic environments.     

Nucleophilic substitution reactions of chlorpyrifos-methyl with sulfur species

Chlorpyrifos-methyl is widely used in the control of insects on certain stored grain, including wheat, barley, oats, rice, and sorghum. The reactions of chlorpyrifos-methyl with hydrogensulfide/bisulfide (H2S/HS-), polysulfides (Sn(2-)), thiophenolate (PhS-), and thiosulfate (S2O3(2-)) were examined in well-defined aqueous solutions over a pH range from 5 to 9. The rates are first-order in the concentration of the different reduced sulfur species. The resulting data indicate that chlorpyrifos-methyl undergoes a S(N)2 reaction with the reduced sulfur species. The transformation products indicate that the nucleophilic substitution of reduced sulfur species occurs at the carbon atom of a methoxy group to form the desmethyl chlorpyrifos-methyl. The formation of trichloropyridinol, a minor degradation product, could be attributed entirelyto hydrolysis. The reaction of chlorpyrifos-methyl with thiophenolate leads to the formation of the corresponding methylated sulfur compound. The resulting pseudo-first-order rate constant for chlorpyrifos-methyl with bisulfide yielded a second-order rate constant of 2.2 (+/- 0.1) x 10(-3) M(-1) s(-1). The determined second-order rate constants show that the reaction of chlorpyrifos-methyl with HS- is of the same order of magnitude as the reaction of chlorpyrifos-methyl with S2O3(2-) with a second-order rate constant of 1.0 (+/- 0.1) x 10(-3) M(-1) s(-1). The second-order rate constant for chlorpyrifos-methyl with polysulfides (3.1 (+/- 0.3) x 10(-2) M(-1) s(-1)) is of the same order of magnitude as the one with thiophenolate (2.1 (+/- 0.2) x 10(-2) M(-1) s(-1)). The second-order rate constant for the reaction of polysulfides is approximately 1 order of magnitude greater than that for the reaction with HS-. When the determined second-order rate constants are multiplied by the concentration of HS-, polysulfides and thiosulfate reported in salt marshes and porewaters, predicted half-lives show that the inorganic reduced sulfur species present at environmentally relevant concentrations may represent an important sink for phosphorothionate triesters in coastal marine environments.     

Iron (III) hydrolysis and solubility at 25 degrees C

UV-vis spectrophotometric measurements, potentiometric titrations, and solubility measurements were performed to evaluate the hydrolysis constants for aqueous Fe(III) and the solubility of 2-line ferrihydrite over a wide concentration range (0-3 M NaClO4 and p[H+] 1.54-11.23). From these measurements, Fe3+ was found to hydrolyze to form FeOH2+, Fe2(OH)24+, Fe(OH)2+, Fe(OH)3(0), and Fe(OH)4-. The hydrolysis and solubility constants of these species were determined together with their dependence on ionic strength. The iron (III) hydrolysis constants at infinity dilution were (logbeta(1,1) to logbeta(1,4) and logbeta(2,2))-2.19 +/- 0.02, -5.76 +/- 0.06, -14.30 +/- 0.32, -21.71 +/- 0.24, and -2.92 +/- 0.02, respectively. The solubility product for 2-line ferrihydrite was (logK(s,0)) +3.50 +/- 0.20. The results have been compared with literature values.     

Solid solutions between CrO4- and SO4-ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)(1-x)]3*26 H2O

Chromate is a toxic contaminant of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment. In cementitous systems, CrO4(2?) is thought to be incorporated as a solid solution with SO4(2?) in ettringite. The formation of a solid solution (SS) could lower the soluble CrO4(2?) concentrations. Ettringite containing SO4(2?) or CrO4(2?) and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 °C, a solubility product log K(S0) of ?40.2 ± 0.4 was calculated: log K(CrO4?ettringite) = 6log{Ca2+} + 2log{Al(OH)4(?)} + 3log{CrO4(2?)} + 4log{OH?} + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 ≤ XCrO4 ≤ 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a0) of 2.03. The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO4(2?) in cementitious systems.     

Extended X-ray absorption fine structure analysis of arsenite and arsenate adsorption on maghemite

Arsenic sorption onto maghemite potentially contributes to arsenic retention in magnetite-based arsenic removal processes because maghemite is the most common oxidation product of magnetite and may form a coating on magnetite surfaces. Such a sorption reaction could also favor arsenic immobilization at redox boundaries in groundwaters. The nature of arsenic adsorption complexes on maghemite particles, at near-neutral pH under anoxic conditions, was investigated using X-ray absorption fine structure (XAFS) spectroscopy at the As K-edge. X-ray absorption near edge structure spectra indicate that As(III) does notoxidize after 24 h in any of the sorption experiments, as already observed in previous studies of As(III) sorption on ferric (oxyhydr)oxides under anoxic conditions. The absence of oxygen in our sorption experiments also limited Fenton oxidation of As(III). Extended XAFS (EXAFS) results indicate that both As(III) and As(V) form inner-sphere complexes on the surface of maghemite, under high surface coverage conditions (approximately 0.6 to 1.0 monolayer), with distinctly different sorption complexes for As(III) and As(V). For As(V), the EXAFS-derived As-Fe distance (approximately 3.35 +/- 0.03 A) indicates the predominance of single binuclear bidentate double-corner complexes (2C). For As(III), the distribution of the As-Fe distance suggests a coexistence of various types of surface complexes characterized by As-Fe distances of approximately 2.90 (+/-0.03) A and approximately 3.45 (+/-0.03) A. This distribution can be interpreted as being due to a dominant contribution from bidentate binuclear double-corner complexes (2C), with additional contributions from bidentate mononuclear edge-sharing (2E) complexes and monodentate mononuclear corner-sharing complexes (1V). The present results yield useful constraints on As(V) and As(III) adsorption on high surface-area powdered maghemite, which may help in modeling the behavior of arsenic at the maghemite-water interface.     

Formation quotients of aluminum sulfate complexes in NaCF3SO3 media at 10, 25, and 50 degrees C from potentiometric titrations using a mercury/mercurous sulfate electrode concentration cell

Mercury/mercurous sulfate electrode concentration cells (with liquid junction) are employed in this study to determine the formation constants of aluminum sulfate complexes, with the principal advantage that the change in the free sulfate concentration is measured directly without the need to know the standard potential of the electrode. Potentiometric titrations were conducted at temperatures of 10, 25, and 50 degrees C and ionic strengths of approximately 0.3, 0.5, and 1.0 molal in aqueous solutions of the inert 1:1 electrolyte sodium trifluoromethanesulfonate (NaTr). Stoichiometric molal formation quotients Q1 and Q2, respectively, for the reactions Al3+(aq) + SO4(2-)(aq) AlSO4+(aq) and Al3+(aq) + 2SO4(2-)(aq) <= >Al(SO4)2-(aq) were determined. The values of log Q1 obtained from this work in NaTr media at ionic strengths of 0.3 and 1.0 mol x kg(-1) and 50 degrees C (1.72 +/- 0.08 and 1.35 +/- 0.06, respectively) are in excellent agreement with the values (1.71 +/- 0.2 and 1.32 +/- 0.1) determined in NaCl media from the recent potentiometric study conducted in the same laboratory using a hydrogen electrode concentration cell by Ridley et al. (Ridley, M. K.; Wesolowski, D. J.; Palmer, D. A.; Kettler, R. M. Geochim. Cosmochim. Acta 1999, 62, 459-472). The value of log Q2 (2.05 +/- 0.05) in 1.0 mol x kg(-1) from this work is smaller than the value reported by Ridley et al. (2.6 +/- 0.5) but within the combined experimental error. Empirical isothermal equations are presented to permit calculation of the equilibrium quotients as a function of ionic strength (0-1 mol x kg(-1)), giving log K1 and log K2 values at 25 degrees C and infinite dilution of 3.84 +/- 0.12 and 5.58 + 0.09, respectively. The value for log K1 obtained in this study at 25 degrees C is bracketed within experimental uncertainty by values reported by Kryzhanovskii et al. (Kryzhanovskii, M. M.; Volokhov, Y. A.; Pavlov, L. N.; Eremin, N. I.; Mironov, V. E Zh. Prikl. Khim. 1971, 44,476-479) and Nishide and Tsuchiya (Nishide, T.; Tsuchiya, R. Bull. Chem. Soc. Jpn. 1965, 38, 1398-1400), namely, 3.89 and 3.73, respectively. All other literature values for the first aluminum sulfate association constant are considerably lower than these results, which is also true for the second association constant, although there are few experimental data available for the latter. Empirical equations are also presented for calculating values of log Q1 and log Q2 from 0 to 1 molal ionic strength and from 10 to 125 degrees C, spanning the range of most environmental conditions at which these reactions are important.     

Polyparameter linear free energy relationships for estimating the equilibrium partition of organic compounds between water and the natural organic matter in soils and sediments

Values of the organic-carbon-based partition coefficient (Koc) have often been estimated using one-parameter linear free energy relationships (op-LFERs), which include both correlations between log Koc and log Kow, where Kow is the octanol-water partition coefficient, and op-LFERs that are based on first-order molecular connectivity indices. For chemicals with tendencies toward strong hydrogen-bonding or other specific interactions with the organic phase, however,these methods are notsufficientlyaccurate. Polyparameter LFERs (pp-LFERs) address these shortcomings by explicitly considering contributions toward free energy change from multiple kinds of molecular interactions with both water and bulk organic phases. This paper reviews pp-LFER theory and presents the development of a new pp-LFER for organic chemical partitioning with soil/sediment organic matter (SOM) using a data set of 356 carefully selected experimental values of log Koc for 75 chemicals, including apolar, monopolar, and bipolar compounds. The paradigm of representing SOM by a single pp-LFER is qualitatively supported by our results, and the set of coefficients for the regression log Koc= (1.10+/-0.10)E - (0.72+/-0.14)S + (0.15+/-0.15)A - (1.98+/-0.14)B + (2.28+/-0.14)V + (0.14+/-0.10) represents a proposed set of water-SOM-specific properties for estimating log Koc. The developed correlation outperformed other currently recommended approaches with the given Koc data set and also compared favorably against the use of new multiple class-specific op-LFER regressions. Overall, the pp-LFER approach is recommended over other current methods for the purpose of Koc estimation and especially for polar chemicals.     

EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite

The modes of As(III) sorption onto two-line ferrihydrite (Fh), hematite (Hm), goethite (Gt), and lepidocrocite (Lp) have been investigated under anoxic condition using X-ray absorption spectroscopy (XAS). X-ray absorption near-edge structure spectroscopy (XANES) indicates that the absence of oxygen minimized As(III) oxidation due to Fenton reactions. Extended X-ray absorption fine structure spectroscopy (EXAFS) indicates thatAs(III)forms similar inner-sphere surface complexes on two-line ferrihydrite and hematite that differ from those formed on goethite and lepidocrocite. At high surface coverage, the dominant complex types on Fh and Hm are bidentate mononuclear edge-sharing (2E) and bidentate binuclear corner-sharing (2C), with As-Fe distances of 2.90 +/- 0.05 and 3.35 +/- 0.05 A, respectively. The same surface complexes are observed for ferrihydrite at low surface coverage. In contrast, As(III) forms dominantly bidentate binuclear corner-sharing (2C) sorption complexes on Gt and Lp [d(As-Fe) = 3.3-3.4 A], with a minor amount of monodentate mononuclear corner-sharing (1V) complexes [d(As-Fe) = 3.5-3.6 A]. Bidentate mononuclear edge-sharing (2E) complexes are virtually absent in Gt and Lp at the high surface coverages that were investigated in the present study. These results are compared with available literature data and discussed in terms of the reactivity of iron(III) (oxyhydr)oxide surface sites.     

Solubility and toxicity of antimony trioxide (Sb2O3) in soil

Antimony trioxide (Sb2O3) is a widely used chemical that can be emitted to soil. The fate and toxicity of this poorly soluble compound in soil is insufficiently known. A silt-loam soil (pH 7.0, background 0.005 mmol Sb kg(-1)) was amended with Sb2O3 at various concentrations. More than 70% of Sb in soil solution was present as Sb(V) (antimonate) within 2 days. The soil solution Sb concentrations gradually increased between 2 and 35 days after Sb2O3 amendment but were always below that of soils amended with the more soluble SbCl3 at the lower Sb concentrations. The soil solution Sb concentrations in freshly amended SbCl3 soils (7 days equilibration) were equivalent to those in Sb2O3-amended soils equilibrated for 5 years at equivalent total soil Sb. Our data indicate that the Sb solubility in this soil was controlled by a combination of sorption on the soil surface, Sb precipitation at the higher doses, and slow dissolution of Sb2O3, the latter being modeled with a half-life ranging between 50 and 250 days. Toxicity of Sb to plant growth (root elongation of barley, shoot biomass of lettuce) or to nitrification was found in soil equilibrated with Sb2O3 (up to 82 mmol Sb kg(-1)) for 31 weeks with 10% inhibition values at soil solution Sb concentrations of 110 microM Sb or above. These concentrations are equivalent to 4.2 mmol Sb per kg soil (510 mg Sb kg(-1)) at complete dissolution of Sb2O3 in this soil. No toxicity to plant growth or nitrification was evident in toxicity tests starting one week after soil amendment with Sb2O3, whereas clear toxicity was found in a similar test using SbCl3. However, these effects were confounded by a decrease in pH and an increase in salinity. It is concluded that the Sb(V) toxicity thresholds are over 100-fold larger than background concentrations in soil and that care must be taken to interpret toxicity data of soluble Sb(III) forms due to confounding factors.     

Interaction of uranyl with calcite in the presence of EDTA

Adsorption of uranyl at the surface of calcite was investigated by using batch sorption experiments and synchrotron X-ray standing wave (XSW) measurements. Aqueous solutions containing 236U(VI) (4.5 x 10(-7) to 1.0 x 10(-4) M) and EDTA (5.0 x 10(-7) to 1.1 x 10(-4) M) were reacted for 90 s to 60 min with freshly cleaved calcite (104) surfaces and calcite powders. Surface exchange coefficients, sorption kinetics, and influence of powder surface area/solution volume (SA/V) ratio were investigated by alpha-counting of 236U. Powder sorption results at SA/V = 870 cm2/mL fit a Freundlich isotherm [log [U]surface (in monolayers) = log K + n log [U]aq (in moles/L)], where K = 1.9+/-0.5 and n = 0.9+/-0.1, consistent with uptake of U(VI) by a specific surface reaction where the availability of sorption sites is nonlimiting in the U concentration range measured. Measured U(VI) coverages along this isotherm, based on the calcite (104) surface Ca site density, ranged from 0.04% to 5.4% of a monolayer. Steady state surface coverages were obtained within 90 s. Sorption of U(VI) on calcite (104) single-crystal cleavage surfaces using identical solutions yielded higher coverages, because of increased step density induced by dissolution at the relatively low SA/V ratio (approximately 1) of these measurements. The crystallographic location of the sorbed U(VI) was examined with the synchrotron XSW technique. Measurements were performed at the Advanced Photon Source on fresh calcite (104) cleavage surfaces reacted for 90 s with U(VI) solutions. Coherent fractions for sorbed U ranged from 0.14 to 0.62, and the mean value of the U coherent position was 0.84+/-0.02. This position was independent of dissolved U(VI) concentration and corresponds to a distance between the U atom and the calcite (104) plane of 2.55+/-0.06 A. These results are consistent with U(VI) adsorption atthe calcite surface as an inner-sphere uranyl-carbonate surface complex bonded with the outer oxygen atom(s) of a single surface carbonate group. Steric considerations allow this observed U(VI) surface complex to occur both at step sites ((441)_ and (481)_) and on terrace areas adjacent to Ca vacancies.     

Sb(III) and Sb(V) sorption onto Al-rich phases: hydrous Al oxide and the clay minerals kaolinite KGa-1b and oxidized and reduced nontronite NAu-1

We have studied the immobilization of Sb(III) and Sb(V) by Al-rich phases - hydrous Al oxide (HAO), kaolinite (KGa-1b), and oxidized and reduced nontronite (NAu-1) - using batch experiments to determine the uptake capacity and the kinetics of adsorption and Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy to characterize the molecular environment of adsorbed Sb. Both Sb(III) and Sb(V) are adsorbed in an inner-sphere mode on the surfaces of the studied substrates. The observed adsorption geometry is mostly bidentate corner-sharing, with some monodentate complexes. The kinetics of adsorption is relatively slow (on the order of days), and equilibrium adsorption isotherms are best fit using the Freundlich model. The oxidation state of the structural Fe within nontronite affects the adsorption capacity: if the clay is reduced, the adsorption capacity of Sb(III) is slightly decreased, while Sb(V) uptake is increased significantly. This may be a result of the presence of dissolved Fe(II) in the reduced nontronite suspensions or associated with the structural rearrangements in nontronite due to reduction. These research findings indicate that Sb can be effectively immobilized by Al-rich phases. The increase in Sb(V) uptake in response to reducing structural Fe in clay can be important in natural settings since Fe-rich clays commonly go through oxidation-reduction cycles in response to changing redox conditions.     

Dissolution of technetium(IV) oxide by natural and synthetic organic ligands under both reducing and oxidizing conditions

Technetium-99 (Tc) in nuclear waste is a significant environmental concern due to its long half-life and high mobility in the subsurface. Reductive precipitation of technetium(IV) oxides [TcO(2)(s)] is an effective means of immobilizing Tc, thereby impeding its migration in groundwater. However, technetium(IV) oxides are subject to dissolution by oxidants and/or complexing agents. In this study we ascertain the effects of a synthetic organic ligand, ethylenediaminetetraacetate (EDTA), and two natural humic isolates on the dissolution and solubility of technetium(IV) oxides. Pure synthetic technetium(IV) oxide (0.23 mM) was used in batch experiments to determine dissolution kinetics at pH ～6 under both reducing and oxidizing conditions. All organic ligands were found to enhance the dissolution of technetium(IV) oxides, increasing their solubility from ～10(-8) M (without ligands) to 4 × 10(-7) M under strictly anoxic conditions. Reduced Tc(IV) was also found to reoxidize rapidly under oxic conditions, with an observed oxidative dissolution rate approximately an order of magnitude higher than that of ligand-promoted dissolution under reducing conditions. Significantly, oxidative dissolution was inhibited by EDTA but enhanced by humic acid compared to experiments without any complexing agents. The redox functional properties of humics, capable of facilitating intramolecular electron transfer, may account for this increased oxidation rate under oxic conditions. Our results highlight the importance of complex interactions for the stability and mobility of Tc and thus for the long-term fate of Tc in contaminated environments.