Zimeng Wang

Adsorption of uranium(VI) to manganese oxides: X-ray absorption spectroscopy and surface complexation modeling.

The mobility of hexavalent uranium in soil and groundwater is strongly governed by adsorption to mineral surfaces. As strong naturally occurring adsorbents, manganese oxides may significantly influence the fate and transport of uranium. Models for U(VI) adsorption over a broad range of chemical conditions can improve predictive capabilities for uranium transport in the subsurface. This study integrated batch experiments of U(VI) adsorption to synthetic and biogenic MnO(2), surface complexation modeling, ζ-potential analysis, and molecular-scale characterization of adsorbed U(VI) with extended X-ray absorption fine structure (EXAFS) spectroscopy. The surface complexation model included inner-sphere monodentate and bidentate surface complexes and a ternary uranyl-carbonato surface complex, which was consistent with the EXAFS analysis. The model could successfully simulate adsorption results over a broad range of pH and dissolved inorganic carbon concentrations. U(VI) adsorption to synthetic δ-MnO(2) appears to be stronger than to biogenic MnO(2), and the differences in adsorption affinity and capacity are not associated with any substantial difference in U(VI) coordination.

Mass action expressions for bidentate adsorption in surface complexation modeling: theory and practice.

The inclusion of multidentate adsorption reactions has improved the ability of surface complexation models (SCM) to predict adsorption to mineral surfaces, but variation in the mass action expression for these reactions has caused persistent ambiguity and occasional mishandling. The principal differences are the exponent (α) for the activity of available surface sites and the inclusion of surface site activity on a molar concentration versus fraction basis. Exemplified by bidentate surface complexation, setting α at two within the molar-based framework will cause critical errors in developing a self-consistent model. Despite the publication of several theoretical discussions regarding appropriate approaches, mishandling and confusion has persisted in the model applications involving multidentate surface complexes. This review synthesizes the theory of modeling multidentate surface complexes in a style designed to enable improvements in SCM practice. The implications of selecting an approach for multidentate SCM are illustrated with a previously published data set on U(VI) adsorption to goethite. To improve the translation of theory into improved practice, the review concludes with suggestions for handling multidentate reactions and publishing results that can avoid ambiguity or confusion. Although most discussion is exemplified by the generic bidentate case, the general issues discussed are relevant to higher denticity adsorption.

CFD modeling of a UV-LED photocatalytic odor abatement process in a continuous reactor

This paper presents a model study of a UV light-emitting-diode (UV-LED) based photocatalytic odor abatement process. It integrated computational fluid dynamics (CFD) modeling of the gas flow in the reactor with LED-array radiation field calculation and Langmuir-Hinshelwood reaction kinetics. It was applied to simulate the photocatalytic degradation of dimethyl sulfide (DMS) in a UV-LED reactor based on experimentally determined chemical kinetic parameters. A non-linear power law relating reaction rate to irradiation intensity was adopted. The model could predict the steady state DMS concentration profiles by calculating the advection, diffusion and Langmuir-Hinshelwood reaction kinetics. By affecting the radiation intensity and uniformity, the position of the LED array relative to the catalyst appeared to be a critical parameter determining DMS removal efficiency. Too small distances might yield low quantum efficiency and consequently poor abatement performance. This study provided an example of LED-based photocatalytic process modeling and gave insights into the optimization of light source design for photocatalytic applications.

Comparison of the Effects of Extracellular and Intracellular Organic Matter Extracted From Microcystis aeruginosa on Ultrafiltration Membrane Fouling: Dynamics and Mechanisms

Algae organic matter (AOM), including intracellular organic matter (IOM) and extracellular organic matter (EOM), are major membrane foulants in the treatment of algae-polluted water. In this study, the effects of EOM and IOM (at dissolved organic concentrations of 8 mg/L) on the fouling of a poly(ether sulfone) ultrafiltration (UF) membrane were investigated using a dead-end down-flow UF unit. Changes in the membrane pore geometry and the interaction energy between the membrane and foulants were analyzed based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. The data (relative standard deviation within 10%) showed that UF was able to retain 57% and 46% of IOM and EOM respectively, while the corresponding membrane fluxes rapidly reduced to 28% and 33% of their respective initial values after a specific filtration volume of only 3.75 mL/cm(2). The fouling model implied that cake formation was the major mechanism. Specifically, IOM foulant had a much greater free energy of cohesion (-59.08 mJ/m(2)) than EOM foulant (3.2 mJ/m(2)), leading to the formation of a compacted cake layer on the membrane surface. In contrast, small molecules of hydrophobic EOM tended to be adsorbed into the membrane pores, leading to significant reduction of the pore size and membrane flux. Therefore, the overall fouling rates caused by EOM and IOM were comparable when both of the above-mentioned mechanisms were considered.

Oxidative UO2 dissolution induced by soluble Mn(III)

The stability of UO2 is critical to the success of reductive bioremediation of uranium. When reducing conditions are no longer maintained, Mn redox cycling may catalytically mediate the oxidation of UO2 and remobilization of uranium. Ligand-stabilized soluble Mn(III) was recently recognized as an important redox-active intermediate in Mn biogeochemical cycling. This study evaluated the kinetics of oxidative UO2 dissolution by soluble Mn(III) stabilized by pyrophosphate (PP) and desferrioxamine B (DFOB). The Mn(III)-PP complex was a potent oxidant that induced rapid UO2 dissolution at a rate higher than that by a comparable concentration of dissolved O2. However, the Mn(III)-DFOB complex was not able to induce oxidative dissolution of UO2. The ability of Mn(III) complexes to oxidize UO2 was probably determined by whether the coordination of Mn(III) with ligands allowed the attachment of the complexes to the UO2 surface to facilitate electron transfer. Systematic investigation into the kinetics of UO2 oxidative dissolution by the Mn(III)-PP complex suggested that Mn(III) could directly oxidize UO2 without involving particulate Mn species (e.g., MnO2). The expected 2:1 reaction stoichiometry between Mn(III) and UO2 was observed. The reactivity of soluble Mn(III) in oxidizing UO2 was higher at lower ratios of pyrophosphate to Mn(III) and lower pH, which is probably related to differences in the ligand-to-metal ratio and/or protonation states of the Mn(III)-pyrophosphate complexes. Disproportionation of Mn(III)-PP occurred at pH 9.0, and the oxidation of UO2 was then driven by both MnO2 and soluble Mn(III). Kinetic models were derived that provided excellent fits of the experimental results.

Formation of Lead(IV) Oxides from Lead(II) Compounds

Lead(IV) oxide (PbO(2)) is a corrosion product that can develop on lead pipes used for drinking water supply, and its stability can control lead concentrations in tap water. A set of batch experiments were performed to determine the extent of PbO(2) formation as a function of time, pH, the presence of dissolved inorganic carbon (DIC), and free chlorine concentration. Experiments were conducted with four lead(II) compounds that are precursors of PbO(2) formation: dissolved lead(II) chloride, massicot (β-PbO), cerussite (PbCO(3)), and hydrocerussite (Pb(3)(OH)(2)(CO(3))(2)). While PbO(2) formed in the presence and absence of DIC, the presence of DIC accelerated PbO(2) formation and affected the identity of the PbO(2) (scrutinyite vs plattnerite) product. For some conditions, intermediate solids formed that affected the identity of the PbO(2) produced. When no intermediate solids formed, hydrocerussite led to the formation of pure scrutinyite, and lead(II) chloride and massicot led to mixtures of scrutinyite and plattnerite. Based on the experimental results, a conceptual model of lead(IV) oxide formation pathways was proposed.

X-ray Absorption Spectroscopic Quantification and Speciation Modeling of Sulfate Adsorption on Ferrihydrite Surfaces

Sulfate adsorption on mineral surfaces is an important environmental chemical process, but the structures and respective contribution of different adsorption complexes under various environmental conditions are unclear. By combining sulfur K-edge XANES and EXAFS spectroscopy, quantum chemical calculations, and surface complexation modeling (SCM), we have shown that sulfate forms both outer-sphere complexes and bidentate-binuclear inner-sphere complexes on ferrihydrite surfaces. The relative fractions of the complexes vary with pH, ionic strength (I), and sample hydration degree (wet versus air-dried), but their structures remained the same. The inner-sphere complex adsorption loading decreases with increasing pH while remaining unchanged with I. At both I = 0.02 and 0.1 M, the outer-sphere complex loading reaches maximum at pH ∼5 and then decreases with pH, whereas it monotonically decreases with pH at I = 0.5 M. These observations result from a combination of the ionic-strength effect, the pH dependence of anion adsorption, and the competition between inner- and outer-sphere complexation. Air-drying drastically converts the outer-sphere complexes to the inner-sphere complexes. The respective contributions to the overall adsorption loading of the two complexes were directly modeled with the extended triple layer SCM by implementing the bidentate-binuclear inner-sphere complexation identified in the present study. These findings improve our understanding of sulfate adsorption and its effects on other environmental chemical processes and have important implications for generalizing the adsorption behavior of anions forming both inner- and outer-sphere complexes on mineral surfaces.

Kinetics of lead(IV) oxide (PbO2) reductive dissolution: Role of lead(II) adsorption and surface speciation

Lead(IV) oxide (PbO(2)) is a corrosion product on lead pipes used for drinking water distribution, and its dissolution can control lead release to drinking water. This study evaluated the adsorption of Pb(II) to PbO(2) and its impact on the dissolution rate of PbO(2). The dissolution rate of PbO(2) was determined as a function of pH in the absence and presence of free chlorine using continuously-stirred tank reactors. Pb(II) adsorption was examined as a function of pH and initial Pb(II) concentrations. The dissolution rate of PbO(2) increased with decreasing pH. The presence of free chlorine inhibited PbO(2) dissolution. The dissolution of PbO(2) involves a coupled reduction-detachment process, and a model was developed that accounts for the adsorption of Pb(II) from the reduction. The extent of Pb(II) adsorption to PbO(2) increased with increasing pH and Pb(II) concentrations until reaching a plateau. Adsorption was interpreted with a surface complexation model using the diffuse double-layer model and a single surface complex. The dissolution rate of PbO(2) was directly related to the distribution of the PbO(2) surface species predicted by the surface complexation model. The dissolution rate was predominantly controlled by >Pb(IV)OH(2)(+) for acidic conditions and by>Pb(IV)OH and>Pb(IV)O(-) at neutral to basic conditions.

Influence of Dissolved Metals on N-Nitrosamine Formation under Amine-based CO2 Capture Conditions

As the prime contender for postcombustion CO2 capture technology, amine-based scrubbing has to address the concerns over the formation of potentially carcinogenic N-nitrosamine byproducts from reactions between flue gas NOx and amine solvents. This bench-scale study evaluated the influence of dissolved metals on the potential to form total N-nitrosamines in the solvent within the absorber unit and upon a pressure-cooker treatment that mimics desorber conditions. Among six transition metals tested for the benchmark solvent monoethanolamine (MEA), dissolved Cu promoted total N-nitrosamine formation in the absorber unit at concentrations permitted in drinking water, but not the desorber unit. The Cu effect increased with oxygen concentration. Variation of the amine structural characteristics (amine order, steric hindrance, -OH group substitution and alkyl chain length) indicated that Cu promotes N-nitrosamine formation from primary amines with hydroxyl or carboxyl groups (amino acids), but not from secondary amines, tertiary amines, sterically hindered primary amines, or amines without oxygenated groups. Ethylenediaminetetraacetate (EDTA) suppressed the Cu effect. The results suggested that the catalytic effect of Cu may be associated with the oxidative degradation of primary amines in the absorber unit, a process known to produce a wide spectrum of secondary amine products that are more readily nitrosatable than the pristine primary amines, and that can form stable N-nitrosamines. This study highlighted an intriguing linkage between amine degradation (operational cost) and N-nitrosamine formation (health hazards), all of which are challenges for commercial-scale CO2 capture technology.

Effect of diffusive transport limitations on UO2 dissolution.

The effects of diffusive transport limitations on the dissolution of UO(2) were investigated using an artificial groundwater prepared to simulate the conditions at the Old Rifle aquifer site in Colorado, USA. Controlled batch, continuously-stirred tank (CSTR), and plug flow reactors were used to study UO(2) dissolution in the absence and presence of diffusive limitations exerted by permeable sample cells. The net rate of uranium release following oxidative UO(2) dissolution obtained from diffusion-limited batch experiments was ten times lower than that obtained for UO(2) dissolution with no permeable sample cells. The release rate of uranium to bulk solution from UO(2) contained in permeable sample cells under advective flow conditions was more than 100 times lower than that obtained from CSTR experiments without diffusive limitations. A 1-dimensional transport model was developed that could successfully simulate diffusion-limited release of U following oxidative UO(2) dissolution with the dominant rate-limiting process being the transport of U(VI) out of the cells. Scanning electron microscopy, X-ray diffraction, and extended X-ray absorption fine structure spectroscopy (EXAFS) characterization of the UO(2) solids recovered from batch experiments suggest that oxidative dissolution was more evident in the absence of diffusive limitations. Ca-EXAFS spectra indicate the presence of Ca in the reacted UO(2) solids with a coordination environment similar to that of a Ca-O-Si mineral. The findings from this study advance our overall understanding of the coupling of geochemical and transport processes that can lead to differences in dissolution rates measured in the field and in laboratory experiments.