Charalambos Papelis

Distinguishing between interlayer and external sorption sites of clay minerals using X-ray absorption spectroscopy

Metals dissolved in natural aquatic environments are often sorbed on high surface area, smectite-clay minerals. Using X-ray absorption spectroscopy (XAS), it is shown that at low pH and Na ion concentration Co(II) forms outer-sphere, mononuclear surface complexes with permanent-charge sites in the interlayer region of smectites. XAS analysis indicates that with increasing pH and Na ion concentration Co(II) is increasingly excluded from the interlayer permanent-charge sites and forms polynuclear species with external surface-hydroxyl sites. These studies demonstrate the potential of XAS for distinguishing between the sorption of trace metals on interlayer and external sites of smectites as a function of changing solution conditions.

Modeling the rate of cadmium and selenite adsorption on micro- and mesoporous transition aluminas

The rates of cadmium and selenite uptake by porous aluminas were studied using three porous transition aluminas. The three adsorbents differed in size and pore structure, CP-5 and CP-100 being the smallest and largest particles, respectively, both exhibiting some microporosity, and C-33 being intermediate-size, mesoporous particles. The rate data were interpreted with a diffusion model, assuming solute diffusion in a sphere from limited volume. The diffusion model was in fair agreement with the rate data, suggesting that cadmium and selenite uptake is controlled by intraparticle mass transfer. Solute uptake by the smaller particles (CP-5) was considerably faster than uptake by the larger particles (CP-100). The measured apparent diffusivities for both adsorbates and all adsorbents were orders of magnitude lower than bulk aqueous diffusivities, in accordance with expectations for highly retarded sorption. The measured effective diffusivities were substantially lower than aqueous molecular diffusivities, suggesting the presence of strong hindrance effects in these microporous adsorbents.

Measuring the specific surface area of natural and manmade glasses: effects of formation process, morphology, and particle size

The specific surface area of natural and manmade solid materials is a key parameter controlling important interfacial processes in natural environments and engineered systems, including dissolution reactions and sorption processes at solid/fluid interfaces. To improve our ability to quantify the release of trace elements trapped in natural glasses, the release of hazardous compounds trapped in manmade glasses, or the release of radionuclides from nuclear melt glass, we measured the specific surface area of natural and manmade glasses as a function of particle size, morphology, and composition. Volcanic ash, volcanic tuff, tektites, obsidian glass, and in situ vitrified rock were analyzed. Specific surface area estimates were obtained using krypton as gas adsorbent and the BET model. The range of surface areas measured exceeded three orders of magnitude. A tektite sample had the highest surface area (1.65 m 2 g 1 ), while one of the samples of in situ vitrified rock had the lowest surface area (0.0016 m 2 g 1 ). The specific surface area of the samples was a function of particle size, decreasing with increasing particle size. Different types of materials, however, showed variable dependence on particle size, and could be assigned to one of three distinct groups: (1) Samples with low surface area dependence on particle size and surface areas approximately two orders of magnitude higher than the surface area of smooth spheres of equivalent size. The specific surface area of these materials was attributed mostly to internal porosity and surface roughness. (2) Samples that showed a trend of decreasing surface area dependence on particle size as the particle size increased. The minimum specific surface area of these materials was between 0.1 and 0.01 m 2 g 1 and was also attributed to internal porosity and surface roughness. (3) Samples whose surface area showed a monotonic decrease with increasing particle size, never reaching an ultimate surface area limit within the particle size range examined. The surface area results were consistent with particle morphology, examined by scanning electron microscopy, and have significant implications for the release of radionuclides and toxic metals in the environment. # 2002 Elsevier Science B.V. All rights reserved.

Modeling ground water flow and radioactive transport in a fractured aquifer.

Three-dimensional numerical modeling is used to characterize ground water flow and contaminant transport at the Shoal nuclear test site in north-central Nevada. The fractured rock aquifer at the site is modeled using an equivalent porous medium approach. Field data are used to characterize the fracture system into classes: large, medium, and no/small fracture zones. Hydraulic conductivities are assigned based on discrete interval measurements. Contaminants from the Shoal test are assumed to all be located within the cavity. Several challenging issues are addressed in this study. Radionuclides are apportioned between surface deposits and volume deposits in nuclear melt glass, based on their volatility and previous observations. Surface-deposited radionuclides are released hydraulically after equilibration of the cavity with the surrounding ground water system, and as a function of ground water flow through the higher-porosity cavity into the low-porosity surrounding aquifer. Processes that are modeled include the release functions, retardation, radioactive decay, prompt injection, and ingrowth of daughter products. Prompt injection of radionuclides away from the cavity is found to increase the arrival of mass at the control plane but is not found to significantly impact calculated concentrations due to increased spreading. Behavior of the other radionuclides is affected by the slow chemical release and retardation behavior. The transport calculations are sensitive to many flow and transport parameters. Most important are the heterogeneity of the flow field and effective porosity. The effect of porosity in radioactive decay is crucial and has not been adequately addressed in the literature. For reactive solutes, retardation and the glass dissolution rate are also critical.

X-ray Photoelectron Spectroscopic Studies of Cadmium and Selenite Adsorption on Aluminum Oxides.

The sorption of cadmium(ll) and selenite on two porous, high surface area aluminum oxides and a nonporous crystalline aluminum oxide (α-Al 2 O 3 , corundum) was studied by X-ray photoelectron spectroscopy (XPS). The porous adsorbents used (ALCOA types CP-5 and C-33) had different sizes and pore structure but otherwise had similar characteristics. CP-5 particles were smaller than C-33 and partly microporous ; C-33 particles were mesoporous and more crystalline than the CP-5 particles. The total aqueous concentrations of cadmium(ll) and selenite were 1.0 x 10 -4 and 1.0 x 10 -3 M, respectively. Maximum surface coverages of the porous adsorbents, as estimated by XPS, were 0.8 and 0.5 monolayers for cadmium(ll) and selenite, respectively. XPS estimates of cadmium and selenite surface coverages agreed well with the hypothesis that adsorbate intraparticle diffusion followed by sorption is the predominant mechanism of cadmium and selenite uptake by porous aluminas under these experimental conditions. XPS results of cadmium and selenite sorption on corundum agreed well with expected surface coverages, based on sorption isotherm data. These results have significant implications for the fate and transport of trace elements in the environment and the remediation of wastewaters and contaminated groundwaters.

Cation and anion sorption on granite from the Project Shoal Test Area, near Fallon, Nevada, USA

Abstract The fate and transport of dissolved contaminants are largely determined by the degree of contaminant interaction with mineral surfaces. Compounds with high sorption affinity for the mineral phases present are retarded compared with groundwater flow. To reduce the uncertainty in modeling the transport of radionuclides and other contaminants at the Project Shoal Underground Test Area, near Fallon, Nevada, USA, a parametric sorption study with three cations (lead, strontium, and cesium) and two anions (selenite and chromate) was conducted. The granite from the Project Shoal Test Area was characterized and used for equilibrium sorption experiments in simple electrolyte matrices and synthetic groundwaters from the test area. Lead displayed a typical cation sorption behavior with fractional uptake increasing with increasing pH. In addition, lead sorption was essentially ionic strength independent, suggesting strong binding and substantial retardation under these conditions. Sorption experiments with strontium could not be performed, because of the high strontium content of the rock. Sorption of cesium was weakly pH dependent, suggesting sorption on cation exchange sites. Chromate and selenite displayed typical anion sorption behavior. For both anions, it appeared as if increased ionic strength resulted in increased fractional uptake. Parameters describing linear and Freundlich isotherms were estimated for a variety of conditions. Context abstract : The fate and transport of contaminants dissolved in ground- and surface waters are largely dependent on the degree of contaminant interaction with the mineral surfaces present in the flowpath of the water. The degree of interaction depends on a number of factors including mineral composition, solution composition, pH and temperature. Changes in geochemical conditions may have dramatically different effects on the behavior of different ions. Contaminant transport models using sorption parameters that are outside their range of applicability may, therefore, lead to substantial errors in the prediction of contaminant migration. To minimize the uncertainty in radionuclide migration modeling at the Project Shoal Underground Test Area, near Fallon, Nevada, USA, a parametric sorption study with three cations (lead, strontium and cesium) and two anions (selenite and chromate) was conducted. The results clearly indicate that lead sorption was strongly pH dependent, whereas cesium sorption was almost pH independent. In addition, because of the high strontium content of the granite, strontium, a cation considered reactive in most geochemical environments, appeared to be very mobile. Although the sorption isotherm parameters derived are applicable only to the specific material and ions, this study clearly demonstrates the importance of detailed aquifer characterization studies and experimental studies that quantify the effects of changing geochemical conditions on sorption.

Use of drinking water treatment solids for arsenate removal from desalination concentrate

Desalination of impaired water can be hindered by the limited options for concentrate disposal. Selective removal of specific contaminants using inexpensive adsorbents is an attractive option to address the challenges of concentrate management. In this study, two types of ferric-based drinking water treatment solids (DWTS) were examined for arsenate removal from reverse osmosis concentrate during continuous-flow once-through column experiments. Arsenate sorption was investigated under different operating conditions including pH, arsenate concentration, hydraulic retention time, loading rate, temperature, and moisture content of the DWTS. Arsenate removal by the DWTS was affected primarily by surface complexation, electrostatic interactions, and arsenate speciation. Results indicated that arsenate sorption was highly dependent on initial pH and initial arsenate concentration. Acidic conditions enhanced arsenate sorption as a result of weaker electrostatic repulsion between predominantly monovalent H2AsO4(-) and negatively charged particles in the DWTS. High initial arsenate concentration increased the driving force for arsenate sorption to the DWTS surface. Tests revealed that the potential risks associated with the use of DWTS include the leaching of organic contaminants and ammonia, which can be alleviated by using wet DWTS or discarding the initially treated effluent that contains high organic concentration.

Sorption mechanisms of Sr and Pb on zeolitized tuffs from the Nevada test site as a function of pH and ionic strength

Abstract The sorption of Sr2+ and Pb2+ on zeolitized tuffs from the Nevada Test Site (NTS) was investigated using macroscopic batch sorption experiments and X-ray absorption spectroscopy (XAS) as a function of geochemical parameters, including pH, ionic strength, and type of background electrolyte. The sorption of Sr2+ is dependent on the ionic strength of the medium and independent of pH, suggesting that Sr2+ sorption is controlled by ion exchange at permanent charge sites. At higher ionic strengths, background electrolyte cations compete effectively with Sr2+ for cation-exchange sites and Sr2+ sorption is suppressed. At the two lower ionic strengths (0.01 and 0.1 M), Pb2+ sorption is also consistent with adsorption by cation exchange. At the highest ionic strength (1.0 M), however, exclusion of Pb2+ from cation-exchange sites resulted in pH dependent adsorption, consistent with sorption on amphoteric surface hydroxyl sites or formation of surface precipitates. XAS was used to test these hypotheses. Based on XAS data, Sr2+ formed hydrated surface complexes coordinated with approximately eight O atoms at an average distance of 2.60 (±0.02) Å, regardless of conditions, consistent with the formation of mononuclear, outer-sphere surface complexes at the Ca2 site in the B channel of clinoptilolite. The coordination environment of sorbed Pb2+ was more complex and a function of pH and ionic strength. The first shell consisted of two to three O atoms at an average distance of 2.20 (±0.02) Å. At low pH and ionic strength, XAS data were consistent with Pb2+ adsorption at the Na1 and Ca2 cation exchange sites in channels A and B of clinoptilolite, respectively. At the highest ionic strength (1.0 M) and low pH, XAS provides evidence for formation of Pb2+ monodentate, corner-sharing inner-sphere complexes, whereas at higher pH, XAS analysis is consistent with formation of edge-sharing bidentate inner-sphere complexes. As surface coverage increased, appearance of a second Pb2+ peak suggests the formation of polynuclear, inner-sphere surface complexes. These results have significant implications for the transport of radionuclides and other contaminants at the NTS and other nuclear test sites and for the modeling of these processes.

Improved understanding of bimolecular reactions in deceptively simple homogeneous media: From laboratory experiments to Lagrangian quantification

Medium heterogeneity affects reaction kinetics by controlling the mixing of reactant particles, but the linkage between medium properties and reaction kinetics is difficult to build, even for simple, relatively homogeneous media. This study aims to explore the dynamics of bimolecular reactions, aniline + 1,2-naphthoquinone-4-sulfonic acid → 1,2-naphthoquinone-4-aminobenzene, in relatively homogeneous flow cells. Laboratory experiments were conducted to monitor the transport of both conservative and reactive tracers through columns packed with silica sand of specific diameters. The measured tracer breakthrough curves exhibit subdiffusive behavior with a late-time tail becoming more pronounced with decreasing sand size, probably due to the segregated flow regions formed more easily in columns packed with smaller size sand. Numerical analysis using a novel Lagrangian model shows that subdiffusion has a twofold effect on bimolecular reactions. While subdiffusion enhances the power-law growth rate of product mass by prolonging the exposure of reactant particles in the depletion zone, the global reaction rate is constrained because subdiffusion constrains the mobility of reactant particles. Reactive kinetics in deceptively simple homogeneous media is therefore controlled by subdiffusion, which is sensitive to the dimensions of packed sand.

Evaluation of Cesium, Strontium, and Lead Sorption, Desorption, and Diffusion in Volcanic Tuffs from Frenchman Flat, Nevada Test Site: Macroscopic and Spectroscopic Investigations

The interaction of radionuclides and other contaminants with minerals and other aquifer materials controls the rate of migration of these contaminants in groundwater. The stronger these interactions, the more a radionuclide will be retarded. Processes such as sorption and diffusion often control the migration of inorganic compounds in aquifers. In addition, these processes are often controlled by the nature of ions of interest, the nature of the aquifer materials, and the specific geochemical conditions. Parameters describing sorption and diffusion of radionuclides and other inorganic ions on aquifer materials are used in transport codes to predict the potential for migration of these contaminants into the accessible environment. Sorption and diffusion studies can help reduce the uncertainty of radionuclide transport modeling on the Nevada Test Site (NTS) and other nuclear testing areas. For example, reliable sorption equilibrium constants, obtained under a variety of conditions, can be used to suggest a plausible sorption mechanism and to provide retardation parameters that can be used in transport models. In addition, these experiments, performed under a variety of conditions, can lead to models that can accommodate changing geochemical conditions. Desorption studies can probe the reversibility of reactions and test whether the reversibility assumed by equilibrium models is justified. Kinetic studies can be used to probe the time-dependent limitations of reactions and suggest whether an equilibrium or kinetic model may be more appropriate. Finally, spectroscopic studies can be used to distinguish between different sorption mechanisms, and provide further guidance with respect to model selection.