Wyoming Montmorillonite Research Articles

Adsorption of phenol and aniline on natural and organically modified montmorillonite: experiment and molecular modelling

Natural and intercalated Wyoming montmorillonite (MMT) with the tetramethylammonium (TMA) cations were used for the adsorption of phenol and aniline. Laboratory experiments characterised by adsorption isotherms were compared with the results of molecular modelling simulations. Aniline adsorbed itself strongly on MMT; while using the TMA intercalates (TMA-MMT), its adsorption decreased. On the contrary, the adsorption of phenol on TMA-MMT was moderately higher than on the MMT surface. The MMT surface models were described by empirical force field used in molecular mechanics and dynamics. The Burchart–Universal force field was used in the Cerius2 modelling environment. The modelling results revealed the important role of water forming a moderately concentrated layer on the pure MMT surface. Water molecules enable the adsorption of aniline on MMT and, on the contrary, repel phenol molecules from MMT. In the case of TMA-MMT, lower amount of water near a silicate layer caused decrease in the aniline adsorption and, on the contrary, increase in the phenol adsorption.

Evaluation of redox-active iron sites in smectites using middle and near infrared spectroscopy

Redox processes of structural Fe in clay minerals play an important role in biogeochemical cycles and for the dynamics of contaminant transformation in soils and aquifers. Reactions of Fe(II)/Fe(III) in clay minerals depend on a variety of mineralogical and environmental factors, which make the assessment of Fe redox reactivity challenging. Here, we use middle and near infrared (IR) spectroscopy to identify reactive structural Fe(II) arrangements in four smectites that differ in total Fe content, octahedral cationic composition, location of the negative excess charge, and configuration of octahedral hydroxyl groups. Additionally, we investigated the mineral properties responsible for the reversibility of structural alterations during Fe reduction and re-oxidation. For Wyoming montmorillonite (SWy-2), a smectite of low structural Fe content (2.8 wt%), we identified octahedral AlFe(II)–OH as the only reactive Fe(II) species, while high structural Fe content (>12 wt%) was prerequisite for the formation of multiple Fe(II)-entities (dioctahedral AlFe(II)–OH, MgFe(II)–OH, Fe(II)Fe(II)–OH, and trioctahedral Fe(II)Fe(II)Fe(II)–OH) in iron-rich smectites Ölberg montmorillonite, and ferruginous smectite (SWa-1), as well as in synthetic nontronite. Depending on the overall cationic composition and the location of excess charge, different reactive Fe(II) species formed during Fe reduction in iron-rich smectites, including tetrahedral Fe(II) groups in synthetic nontronite. Trioctahedral Fe(II) domains were found in tetrahedrally charged ferruginous smectite and synthetic nontronite in their reduced state while these Fe(II) entities were absent in Ölberg montmorillonite, which exhibits an octahedral layer charge. Fe(III) reduction in iron-rich smectites was accompanied by intense dehydroxylation and structural rearrangements, which were only partially reversible through re-oxidation. Re-oxidation of Wyoming montmorillonite, in contrast, restored the original mineral structure. Fe(II) oxidation experiments with nitroaromatic compounds as reactive probes were used to link our spectroscopic evidence to the apparent reactivity of structural Fe(II) in a generalized kinetic model, which takes into account the presence of Fe(II) entities of distinctly different reactivity as well as the dynamics of Fe(II) rearrangements.

A theoretical study of swelling and shrinking of hydrated Wyoming montmorillonite

Monte Carlo molecular modelling simulations were performed to investigate the swelling and shrinking behaviour of hydrated Wyoming-type montmorillonite (MMT) including different counterions. MCY, TIP4P, and SPC/E water and potential models were employed to Li- and Na-montmorillonite. The simulation data with the SPC/E water model fitted the experimental swelling curves. The agreement between our results and available experimental and/or preceding theoretical results allow us to predict the swelling and shrinking behaviour of MMT including other cations such as some heavy metal ions (Ni2+, Zn2+ and Pb2+). The hysteresis between swelling and shrinking is due to the different mechanism of adsorption and desorption by breaking and re-formation of hydrogen bonds between the water molecules.

Bioavailability of Organoclay Formulations of Atrazine in Soil

Pesticide formulations based on organoclays have been proposed to prolong the efficacy and reduce the environmental impact of pesticides in soil. This research addressed the question of whether atrazine in organoclay-based formulations is irreversibly sorbed or is bioavailable for bacterial degradation in soil. Different cations of l-carnitine (CAR), tyramine (TYRAM), hexadimethrine (HEXADIM), phenyltrimethylammonium (PTMA), hexadecyltrimethylammonium (HDTMA), and Fe(III) were incorporated into Na-rich Wyoming montmorillonite (SWy-2) and Ca-rich Arizona montmorillonite (SAz-1) at 100% of the cation exchange capacity (CEC) of the clays as a strategy to enhance the affinity of the clay minerals for atrazine. A Buse loam soil from Becker, MN, was treated with three organoclay-based formulations of 14C-atrazine or free herbicide and incubated for 2 weeks. To determine the bioavailability of 14C-atrazine, the soil was inoculated with Pseudomonas sp. strain ADP, which rapidly mineralizes atrazine. At day 0, and after a 2 week incubation, mineralization and the amount of 14C-atrazine residues distributed between the aqueous-extractable, methanol-extractable, and bound fractions in the soil were determined to characterize the availability of nonaged and aged atrazine residues. By the end of the 2 week incubation, the microorganisms had mineralized >80% of the initial readily available (water-extractable) and >70% of the less readily available (methanol-extractable) 14C-atrazine in the soil. Bound residues increased from <4% at day 0 to ∼17% after the 2 week incubation for both the formulated and free forms of atrazine. The results of these incubation experiments show that the bioavailabilities of atrazine were similar in the case of the organoclay formulations and as free atrazine. This indicated that whereas more atrazine was sorbed and less likely to be transported in soil, when formulated as organoclay complexes, it was ultimately accessible to degrading bacteria, so that the herbicide is likely to be naturally attenuated by soil microorganisms.

Organoclays as Soil Amendments to Increase the Efficacy and Reduce the Environmental Impact of the Herbicide Fluometuron in Agricultural Soils

The use of pesticides in agriculture has become a source of pollution of soil and water in the last decades. Extensive pesticide transport losses due to leaching and runoff produce nonpoint source contamination of soils and water. One of the soil processes that reduce pesticide transport losses is adsorption by soil particles; therefore, enhancement of pesticide retention by soil can be used as a strategy to attenuate the environmental impact of pesticides. In this work, organoclays were prepared by treating Wyoming montmorillonite (SWy-2) and Arizona montmorillonite (SAz-1) with different organic cations and were assayed as soil amendments to enhance the retention and reduce the leaching losses of the herbicide fluometuron [N,N-dimethyl-N'-[3-(trifluoromethyl)phenyl] urea] in soils. Two agricultural soils from Southern Spain were selected for being high-risk scenarios of ground and surface water contamination. First, a batch adsorption study was conducted to identify organoclays with high affinity for fluometuron. Among the different organoclays assayed, spermine-treated Wyoming montmorillonite (SW-SPERM) displayed high and reversible adsorption of fluometuron and was selected as an amendment for subsequent persistence, leaching, and herbicidal activity experiments of fluometuron with unamended and amended soils. Amendment of the soils with SW-SPERM at rates of 1%, 2%, and 5% greatly enhanced fluometuron retention by the soils and retarded fluometuron leaching through soil columns. Incubation experiments revealed that the persistence of the herbicide in the amended soils was similar to that in unamended soils and that most of the herbicide was ultimately available for degradation. Bioassays demonstrated that the reduced leaching losses of fluometuron in soils amended with SW-SPERM may result in increased herbicide efficacy if heavy rainfall events occur shortly after herbicide application.

Textural and hydration properties of a synthetic montmorillonite compared with a natural Na-exchanged clay analogue

An aluminous montmorillonite has been synthesized at hydrothermal conditions (623 K, 120 MPa) and compared to the natural Na-exchanged Wyoming montmorillonite (Na-SWy2). The synthetic product is characterized by a high cation exchange capacity (83 meq/100 g) and ethylene glycol specific surface area (764 m 2/g) similar to Na-SWy2 but with a higher purity with one crystalline phase. Compacted smectite samples were percolated with water by using an oedometer cell equipped with a separated injection system, in order to simulate the behaviour of an engineered clay barrier in the context of subsurface waste landfill. Percolation experiments showed very low hydraulic conductivities (10 − 12 m/s) for both synthetic and natural smectites, in addition to the non-macroscopic swelling of the synthetic montmorillonite, in contrast to Na-SWy2. At the nanometric scale, this synthetic clay mineral is characterized by higher hydration states and a better organization in the distribution of water molecules in particles, as proven by X-ray diffraction technique and continuous water vapor gravimetry method. Moreover, adsorption–desorption isotherms of nitrogen revealed the classical feature of swelling clay minerals, and the Brunauer–Emmet–Teller calculation gives a nitrogen specific area of 87 m 2/g for the synthetic smectite, twice higher than for the Wyoming clay, due to its higher mesoporosity. Finally, low-pressure argon adsorption coupled with the Derivative Isotherm Simulation procedure highlighted classical adsorption energy distributions of natural phyllosilicates, and thus the synthetic montmorillonite was characterized by an average of 11 layers per particle in the dry state, with basal and edge surface areas different from Na-SWy2.

Organoclay‐Based Formulations to Reduce the Environmental Impact of the Herbicide Diuron in Olive Groves

Ground and surface water contamination caused by undesirable transport losses of herbicides commonly applied to olive (Olea Europaea) groves in Spain and other Mediterranean countries is demanding strategies to reduce the environmental impact of such herbicides. In this work, organoclay‐based formulations of a herbicide widely applied to olive groves, diuron [3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea], were prepared by preadsorbing the herbicide on three organoclays. Then, the ability of the organoclay‐based formulations to reduce the herbicide release rate and leaching potential in two soils as compared to a standard commercial formulation of the herbicide was assessed. The organoclays were obtained by treating Wyoming montmorillonite (SWy‐2) and Arizona montmorillonite (SAz‐1) with an amount of spermine (SPERM) or hexadecyltrimethylammonium (HDTMA) cations equal to 100% of the cation exchange capacity of the montmorillonites. Batch release and column leaching tests indicated that the organoclay‐based formulations of diuron made the release of the herbicide slower and reduced its leaching potential in soils, as compared with a standard commercial formulation containing the herbicide in a readily available form. Soil incubation experiments revealed that the persistence of the herbicide applied to soil as organoclay‐based formulations was only slightly prolonged compared with the herbicide applied in a readily available form, and that most herbicide in the formulations was ultimately available for degradation. The use of organoclay‐based formulations is proposed as a strategy to reduce extensive transport losses associated with the application of diuron in high‐risk scenarios, such as Mediterranean olive groves.

A Comparative Study of the Use of Organoclay‐Based Formulations and Organic Amendment to Reduce the Leaching of the Herbicide MCPA in Soil

AbstractMCPA (4‐chloro‐2‐methylphenoxyacetic acid) is an acidic herbicide widely used on olive crops in Spain. Due to its anionic form at natural soil pH, there is high risk of leaching and groundwater contamination by the use of this herbicide. The aim of this work was to study the effects of organoclay‐based formulations of MCPA and olive oil waste amendment on MCPA leaching in a sandy loam soil. For this purpose, batch adsorption and column leaching studies were performed. The organoclays used to prepare the clay‐based formulations of MCPA were obtained by treating Wyoming montmorillonite (SWy‐2) and Arizona montmorillonite (SAz‐1) with an amount of hexadecyltrimethylammonium (HDTMA) cation equal to 100% of the CEC of the montmorillonites. The organic residue used in this study was a solid waste from olive oil production (olive oil waste, OOW). The soil was amended with the organic residue at the rate of 10% (w/w). Batch release and column leaching studies indicated that organoclay‐based formulations of MCPA reduced the release rate and the leaching of the herbicide as compared to the use of a conventional formulation containing the herbicide in an immediately available form. The increase in soil organic matter of the soil upon amendment with the organic residue also resulted in greater adsorption and reduced leaching of MCPA in the soil. Accordingly, both the use of organoclay‐based formulations and the amendment of soil with OOW are proposed as efficient strategies to reduce extensive leaching losses associated with the application of MCPA in high‐risk scenarios, such as Mediterranean olive groves.

Synthesis, characterization of mono, di and tri alkyl surfactant intercalated Wyoming montmorillonite for the removal of phenol from aqueous systems

Organoclays were synthesized by the ion exchange of cationic surfactants containing single, double and triple alkyl chains for sodium ions in an aqueous suspension of Wyoming Na-montmorillonite. The characterization of organoclays with and without adsorbed phenol was determined by X-ray diffraction, TEM and thermal analysis. Differences in the surfaces and in the interlayer of the mono, di and tri alkyl chain organoclays resulted in differences in the adsorption efficiency for phenol with tri > di > mono ≫ Na-Mt. The results prove that organoclays can be effective for the removal of phenol from an aqueous solution and this removal is a function of the surfactant molecule and its concentration. In general, the higher the concentration as measured by the CEC value and the greater the number of alkyl chains in the surfactant molecule, the greater the percentage of the phenol that is removed.

Fe3+ intercalation/deposition on vermiculites and interaction with pyrrole: a Mössbauer spectroscopic investigation

AbstractThe Fe3+ uptake from aqueous and methanol solutions of FeCl3·6H2O in two vermiculites from Santa Olalla and Ojén (Spain) and in a montmorillonite from Wyoming has been investigated by Mössbauer spectroscopy at room temperature and 4.2 K. The samples were further characterized by X-ray diffraction. The low temperature Mössbauer spectra reveal the deposition of akaganeite and perhaps some ferrihydrite on the external surface of the clay minerals. The room temperature spectra exhibit two Fe3+ quadrupole doublets in addition to the Fe2+ components of the structural iron in the sheet silicates. On treatment in ⩾0.04 M Fe3+ solutions the Fe3+ quadrupole splittings of 0.90 mm/s and 0.55 mm/s can be assigned to the mineral akaganeite; at smaller Fe3+ concentrations one of these doublets has a quadrupole splitting similar to that of the Fe3+ in the original vermiculites, the other one a quadrupole splitting of 0.6–0.7 mm/s, which can be considered as a hint for an uptake of Fe dimers in the interlayer space irrespective of the solvent used for iron deposition. Preliminary results show that the adsorption of pyrrole in all of our Fe3+-containing samples, regardless of the preparation in aqueous or methanol solution, results in the formation of polypyrrole and a concomitant reduction of some of the Fe3+. An uptake of pyrrole into the interlayer space has been observed only in the Fe3+-containing Wyoming montmorillonite and in the Ojén vermiculite treated with the methanol solution of iron chloride.

Use of modified montmorillonites to reduce herbicide leaching in sports turf surfaces: Laboratory and field experiments

The risk of ground water contamination resulting from rapid leaching of highly mobile pesticides can be reduced through the application of the pesticide adsorbed on a suitable carrier, which limits the amount of pesticide readily available for undesirable leaching losses. The herbicide simazine was used as a model of highly mobile herbicide applied in sports turf surfaces. We investigated the ability of selectively modified montmorillonites to retard the release of the herbicide into the aqueous soil solution and to reduce herbicide leaching from the soil. Fe 3+-Wyoming montmorillonite (Fe-SW), hexadecyl trimethylammonium-Arizona montmorillonite (HDTMA-SA) and octadecylammonium-Arizona montmorillonite (ODA-SA) were mixed with simazine following two different protocols. The resulting complexes were assayed as slow release formulations of the herbicide. In the laboratory, batch release and column leaching tests showed that all montmorillonite-based formulations of simazine released the herbicide slowly in aqueous solution, which resulted in reduced simazine leaching through soil columns compared to the application of the free herbicide. Pretreatment of the soil surface layer with Fe-SW was also effective in retarding the leaching of free simazine through the soil column compared to its leaching in untreated soil columns. In a field experiment, a Fe-SW-based formulation of simazine was applied to 1 m 2 field plots, previously seeded with Princess 77 bermudagrass to simulate a typical sports turfgrass surface. The field experiment revealed that the montmorillonite-based formulation of simazine displayed similar herbicidal efficacy and slower vertical movement of the herbicide compared to a standard commercial formulation. This study shows the usefulness of montmorillonite to reduce ground water contamination by intensive herbicide application in high-risk scenarios such as sports turfgrass surfaces.

Effect of interlayer cation and relative humidity on the hydration properties of a dioctahedral smectite

This paper aims at characterizing the structural evolution of a dioctahedral smec- tite (i.e. Wyoming montmorillonite) saturated by Na + , Cu 2+ or Pb 2+ . Hydration properties of the <2μm size fraction of Wyoming montmorillonite smectite (SWy) source clay (low charge i.e. 0.35) were studied by modelling of X-ray diffraction (XRD) patterns recorded under controlled relative humidity (RH) conditions on Na, Cu or Pb saturated specimens. The Cation Exchange Capacity (CEC) of the starting sample was first Na + saturated to guarantee better dispersion, followed by exchanges with the cations Cu 2+ or Pb 2+ . The resulting com- plexes were respectively labelled Wy- Na, Wy-Cu and Wy-Pb. The qualitative analysis of XRD patterns obtained under room pressure and temperature conditions showed that all complexes have one water layer hydration state. In order to highlight the specific hydration properties of each complex, we studied the XRD patterns under controlled relative humidity (%RH). The quantitative analysis of XRD patterns is achieved using an indirect method based on the comparison of experimental XRD patterns with calculated ones. This quantita- tive study showed that for Wy- Na, Wy-Cu and Wy-Pb at low RH (≈15%) the d001 spacing corresponds to a low hydration state characterized by one water layer, whereas for higher studied rates of relative humidity (75%), differences of hydration state can be distinguished for Wy-Na, Wy-Cu and Wy-Pb samples.

Selective Modification of Clay Minerals for the Adsorption of Herbicides Widely Used in Olive Groves

Ground and surface water contamination by herbicides applied to olive groves in Spain and other Mediterranean countries is demanding strategies to prevent and remediate the environmental problems repeatedly caused by such herbicides. In this study, six different organic cations (L-carnitine, spermine, hexadimethrine, tyramine, phenyltrimethylammonium, and hexadecyltrimethylammonium) were incorporated into Na-rich Wyoming montmorillonite (SWy-2) and Ca-rich Arizona montmorillonite (SAz-1) at two different loadings (50% and 100% of the cation exchange capacity of the clays) as a strategy to enhance the affinity of the clay minerals for three herbicides widely used in olive groves: terbuthylazine, diuron, and MCPA. The modified montmorillonites were characterized and tested as adsorbents of the herbicides through batch adsorption tests. At the experimental conditions used, some of the modified montmorillonites removed more than 95% of the herbicide initially present in aqueous solution, whereas the unmodified clays removed less than 15%. All three herbicides displayed very strong affinities for SAz-1 exchanged with hexadecyltrimethylammonium cations, particularly when these were incorporated at 100% of the cation exchange capacity of the clay mineral. Terbuthylazine and diuron also displayed very strong affinities for SWy-2 exchanged with L-carnitine and spermine, respectively. The chemical characteristics of the organic cation greatly influenced the adsorptive properties of the resultant organoclay. The herbicides were in general reversibly adsorbed by the modified clays. The results indicate that some of the tested modified clays could be suitable for the removal of the assayed herbicides from contaminated water and also as possible supports for the design of slow release formulations of such herbicides to attenuate their environmental impact when used in high-risk scenarios such as olive groves.

Modification of the surfaces of Wyoming montmorillonite by the cationic surfactants alkyl trimethyl, dialkyl dimethyl, and trialkyl methyl ammonium bromides

Surfaces of a Wyoming SWy-2 sodium montmorillonite were modified using microwave radiation through intercalation with the cationic surfactants octadecyltrimethylammonium bromide, dimethyldioctadecylammonium bromide, and methyltrioctadecylammonium bromide by an ion exchange mechanism. Changes in the surfaces and structure were characterized using X-ray diffraction (XRD), thermal analysis (TG) and infrared (IR) spectroscopy. Different configurations of surfactants within montmorillonite interlayer are proposed based on d ( 001 ) basal spacings. A range of surfactant molecular environments within the surface-modified montmorillonite are proposed based upon their thermal decomposition. IR spectroscopy using a smart endurance single bounce diamond attenuated total reflection (ATR) cell has been used to study the changes in the spectra of CH asymmetric and symmetric stretching modes of the surfactants to provide more information of the surfactant molecular configurations.

Incidence of lead uptake on the microstructure of a (Mg, Ca)-bearing bentonite (Prrenjas, Albania)

The article addresses Pb uptake and microstructural/morphological modifications of a (Mg, Ca)-bentonite (Prrenjas, Albania) after interaction with solutions of elevated lead content. Comparison is made with a Wyoming montmorillonite transformed to the Mg-dominated form. Characterised by X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM), the Prrenjas bentonite is predominantly a smectite, containing 10% of interlayered illite, with associated kaolinite, antigorite, halloysite and minor chlorite. Significant magnesium is present in the octahedral layer in association with aluminium and iron. The CEC is rather high (above 80 meq/100 g). Magnesium and calcium are the main cations exchanged during CEC determinations and Pb uptake experiments. From TEM studies, microstructural units are composed by closely packed clay particles with 10 to 15 stacked platelets, in contrast with Wyoming montmorillonite. Exchange experiments with lead nitrate on suspended clays have been performed with concentrations ranging from 10 −5 M to 10 −2 M. A comparable capacity of lead sorption is observed by Atomic Absorption Spectrometry (AAS) for Prrenjas bentonite and Wyoming montmorillonite. At increasing lead uptake, a higher reduction of hydration in interlayer space is observed by XRD on Wyoming montmorillonite. At the highest lead concentrations, X-ray Photoelectron Spectroscopy (XPS) analyses of the Prrenjas Bentonite reveal Pb sorption in a “non interlayer” site. TEM images of the same clay show a significant size decrease of microstructural units in relation to the lead sorption. On the contrary, the Mg-exchanged Wyoming montmorillonite is not modified. Presumably, the “non interlayer lead” uptake results either from surface complexation at an edge site, or from ionic exchange with edge-exposed octahedral magnesium. Both mechanisms could initiate at edge-to-face contacts and result in a breaking process of the microstructural units.

Study of the structural evolution and selectivity of Wyoming montmorillonite in relation with the concentration of Cu2+and Ni2+

Study of the structural evolution and selectivity of Wyoming montmorillonite in relation with the concentration of Cu²⁺and Ni²⁺

Physical Adsorption of H2S Related to the Conservation of Works of Art: The Role of the Pore Structure at Low Relative Pressure

The adsorption isotherms of H2S in selected adsorbents were determined at 298 K, at relative pressures up to about 0.005, aiming the use of these materials in the removal of that pollutant from the museums atmosphere. The Dubinin-Astakhov equation adjusts very well the experimental results, although one cannot interpret the pre-exponential factor w0 as the limiting adsorbed amount. The parameter E, related with the adsorption energy, and the parameter n, that can be associated with the surface heterogeneity of the adsorbents, are correlated and the first is also correlated with the adsorbed amounts. It was not found any expectable relationship between the adsorbed amounts and textural parameters of the adsorbents such as the specific surface area or the microporous volume. This points out that the adsorption of H2S is highly specific. In general, 13X and Y sodium zeolites seem to be the most effective adsorbents, but at lowest tested pressures, near the concentrations found at museums, a pillared clay prepared from a Wyoming montmorillonite seems to be more efficient.

Infrared spectroscopic study of alkyl aldehydes adsorbed on cations supported by layer silicate

The self-supported film specimen of Wyoming montmorillonite as a layer silicate exchanged by cations, Li +, Na +, K +, Ca 2+, Ni 2+, and Al 3+ were allowed to contact acetaldehyde, acrolein and crotonaldehyde within the heatable gas cell. Adsorption mechanism of alkyl aldehydes on cations supported by layer silicate was studied by means of infrared spectroscopy and X-ray. The infrared spectra between 4000 and 1200 cm −1 at different pressures of adsorbates indicated bond formation through carbonyl oxygen. The intensity of the stretching OH was analyzed and resonance form of cationic hydroxyl was proposed as an adsorption site. The carbonyl stretching band, which shifted about 130 cm −1 to lower frequencies was observed only for Ni 2+, Ca 2+, and Al 3+ supported by layer silicate and was attributed to >C O··· M n+ complex formation. A sharp band, which appeared as a shoulder at 1722 for acetaldehyde and 1690 for acrolein and crotonaldehyde, was responsible for the interaction of carbonyl with surface hydroxyl. The second broad band, which appeared at about 1710–1660, was responsible for hydrogen bonding between carbonyl oxygen and cationic hydroxyl group.

Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO 2 2+) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10 −3 M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH) 6] octahedral sites over [Al(O,OH) 6] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO 2. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO 2.

Sorption of 3-amino-1,2,4-triazole and Zn(II) onto montmorillonite

AbstractAcid-base titrations and attenuated total reflectance-infrared (ATR-IR) spectroscopy of solutions containing Zn(NO3)2 and the herbicide 3-amino-1,2,4-triazole suggested that soluble complexes ZnL2+ and Zn(OH)L+ form, where L represents aminotriazole. Sorption experiments and modeling in systems containing K-saturated Wyoming (SWy-K) montmorillonite suggest that at low concentrations the aminotriazole sorbs primarily in cationic form via an ion-exchange mechanism. Sorption isotherms for aminotriazole are ‘s’-shaped, indicating a co-operative sorption mechanism as the concentration of the molecule increases. At higher concentrations, ATR-IR spectroscopy indicated the presence of cationic and neutral triazole molecules on the surface, while X-ray diffraction data suggest interaction with interlayer regions of the clay. When the concentration of the herbicide was high, initial sorption of aminotriazole cations modified the clay to make the partitioning of neutral molecules to the surface more favorable. Experiments conducted in the presence of Zn(II) indicated that below pH 7, Zn(II) and aminotriazole compete for sorption sites, while above pH 7 the presence of Zn(II) enhances the uptake of aminotriazole. The enhancement was attributed to the formation of an inner-sphere ternary surface complex at hydroxyl sites (SOH) on crystal edges, having the form [(SOZn(OH)L)]0.