Formation of cyanogen chloride from amino acids and its stability with free chlorine and chloramine.

Cyanogen chloride (CNCl) is a disinfection by-product found in chlorinated and chloraminated drinking water. Although its chronic health effects are not well established, CNCl has been used as a chemical warfare agent and thus its presence in drinking water is of concern. CNCl is not currently regulated in the United States; however, it was on USEPA's 1991 Drinking Water Priority List and many facilities were required to report CNCl concentration under the Information Collection Rule. Uncertainty about the sources, formation mechanism, and stability of CNCl under water treatment conditions has been a factor limiting the establishment of regulatory standards. This research sought to improve our understanding of these issues. The findings of this research will help drinking water authorities to assess the necessity to regulate CNCl and determine the regulatory details, such as precursor, disinfection practice, temperature, and pH. The findings will also help water treatment utilities employ possible control strategies. Based on experimental results, this research has concluded: (1) amino acids are selectively important as CNCl precursor with glycine being the only important precursor; (2) CNCl formation from glycine agrees with a complex formation mechanism, in which glycine is completely converted to CNCl at pH 6--8 by pseudo first order kinetics; (3) once formed, CNCl decomposes with free chlorine through hypochlorite-catalyzed hydrolysis by second order kinetics with respect to hypochlorite and CNCl concentrations. CNCl, however, remains stable with chloramine. The different stability of CNCl with free chlorine and chloramine may, in part, explain the higher CNCl concentration observed in pre-chlorination post-chloramination systems than in chlorination systems; and (4) compared to many other amino acids, glycine is less reactive for chlorine, so when chlorine is not in excess such as drinking chlorinated water and during food preparation, most of glycine may not have the chance to react with chlorine and produce CNCl. The major difficulty in the study of CNCl formation and decay was that the traditional methods of CNCl analysis are not real-time measurements. A relatively new technique, in-line membrane introduction mass spectrometry (MIMS), was applied to overcome the analytical difficulty.Ph.D.Applied SciencesEnvironmental engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125170/2/3186712.pd

Interfacial Forces are Modified by the Growth of Surface Nanostructures

Nanostructures formed by chemical reaction can modify the interfacial forces present in aqueous solution near a surface. This study uses force-volume microscopy to explore this phenomenon for the growth of manganese oxide nanostructures on rhodochrosite. The interfacial forces above the oxide nanostructures are dominated by electrostatic repulsion for probe−surface separations greater than ca. 2 nm but are overtaken by van der Waals attraction for shorter distances. Across the investigated pH range 5.0−9.7, the maximum repulsive force occurs 2.4 (±1.1) nm above the oxide nanostructures. The magnitude of the repulsive force decreases from pH 5.0 to 6.5, reaches its minimum at 6.5, and then increases steadily up to pH 9.7. Specifically, fmax(pN) = 23(±4)[6.8(±2.1) pH] for pH < 6.5 and fmax(pN) = 19(±2)[pH 6.1(±1.0)] for pH ≥ 6.5. This dependence indicates that oxide nanostructures have a point of zero charge in the pH range 6−7. In comparison to the nanostructures, the rhodochrosite substrate induces only small interfacial forces in the same pH range, suggesting a neutral or weakly charged surface. The quantitative mapping of interfacial forces, along with the associated influencing factors such as pH or growth of nanostructures, provides a basis for more sophisticated and accurate modeling of processes affecting contaminant immobilization and bacterial attachment on mineral surfaces under natural conditions.Earth and Planetary SciencesEngineering and Applied Science

Chemical Bath Deposition of Aluminum Oxide Buffer on Curved Surfaces for Growing Aligned Carbon Nanotube Arrays

Direct growth of vertically aligned carbon nanotube (CNT) arrays on substrates requires the deposition of an aluminum oxide buffer (AOB) layer to prevent the diffusion and coalescence of catalyst nanoparticles. Although AOB layers can be readily created on flat substrates using a variety of physical and chemical methods, the preparation of AOB layers on substrates with highly curved surfaces remains challenging. Here, we report a new solution-based method for preparing uniform layers of AOB on highly curved surfaces by the chemical bath deposition of basic aluminum sulfate and annealing. We show that the thickness of AOB layer can be increased by extending the immersion time of a substrate in the chemical bath, following the classical Johnson–Mehl–Avrami–Kolmogorov crystallization kinetics. The increase of AOB thickness in turn leads to the increase of CNT length and the reduction of CNT curviness. Using this method, we have successfully synthesized dense aligned CNT arrays of micrometers in length on substrates with highly curved surfaces including glass fibers, stainless steel mesh, and porous ceramic foam

Photoinduced Crystallization and Activation of Amorphous Titanium Dioxide

Titanium dioxide (TiO₂) is one of the most common photosensitive materials used in photocatalysis, solar cells, self-cleaning coatings, and sunscreens. Although the crystalline TiO₂ phases such as anatase and rutile are well-known to be photoactive, whether amorphous TiO₂ is active in photocatalytic reactions is still controversial. Here we show that amorphous TiO₂ prepared by the commonly used sol–gel method of tetrabutyl titanate hydrolysis is active in photocatalytic water reduction and methylene blue oxidation under the irradiation of a xenon lamp. The amorphous TiO₂ gains photoactivity after an induction period of approximately an hour, suggesting that phase transition is involved. Using an extensive series of microscopic and spectroscopic analyses, we further show that the photoinduced crystallization by amorphous TiO₂ forms a nanometer-thin layer of rutile nanocrystallites under the irradiation in the middle ultraviolet range. The resulting core–shell nanoparticles have a bandgap of 3.3 eV and are enriched with surface-active sites including reduced titanium and oxygen vacancies. The revelation of photoinduced crystallization raises the possibility of preparing photosensitive TiO₂ using low-temperature radiation techniques that can not only save energy but also incorporate heat-sensitive components into manufacturing

Binder-Free Carbon Nanotube Electrode for Electrochemical Removal of Chromium

Electrochemical treatment of chromium-containing wastewater has the advantage of simultaneously reducing hexavalent chromium (Cr^VI) and reversibly adsorbing the trivalent product (Cr^III), thereby minimizing the generation of waste for disposal and providing an opportunity for resource reuse. The application of electrochemical treatment of chromium is often limited by the available electrochemical surface area (ESA) of conventional electrodes with flat surfaces. Here, we report the preparation and evaluation of carbon nanotube (CNT) electrodes consisting of vertically aligned CNT arrays directly grown on stainless steel mesh (SSM). We show that the 3-D organization of CNT arrays increases ESA up to 13 times compared to SSM. The increase of ESA is correlated with the length of CNTs, consistent with a mechanism of roughness-induced ESA enhancement. The increase of ESA directly benefits Cr^VI reduction by proportionally accelerating reduction without compromising the electrode’s ability to adsorb Cr^III. Our results suggest that the rational design of electrodes with hierarchical structures represents a feasible approach to improve the performance of electrochemical treatment of contaminated water

Opposing Effects of Humidity on Rhodochrosite Surface Oxidation

Rhodochrosite (MnCO₃) is a model mineral representing carbonate aerosol particles containing redox-active elements that can influence particle surface reconstruction in humid air, thereby affecting the heterogeneous transformation of important atmospheric constituents such as nitric oxides, sulfur dioxides, and organic acids. Using in situ atomic force microscopy, we show that the surface reconstruction of rhodochrosite in humid oxygen leads to the formation and growth of oxide nanostructures. The oxidative reconstruction consists of two consecutive processes with distinctive time scales, including a long waiting period corresponding to slow nucleation and a rapid expansion phase corresponding to fast growth. By varying the relative humidity from 55 to 78%, we further show that increasing humidity has opposing effects on the two processes, accelerating nucleation from 2.8(±0.2) × 10^–3 to 3.0(±0.2) × 10^–2 h^–1 but decelerating growth from 7.5(±0.3) × 10^–3 to 3.1(±0.1) × 10^–3 μm² h^–1. Through quantitative analysis, we propose that nanostructure nucleation is controlled by rhodochrosite surface dissolution, similar to the dissolution–precipitation mechanism proposed for carbonate mineral surface reconstruction in aqueous solution. To explain nanostructure growth in humid oxygen, a new Cabrera–Mott mechanism involving electron tunneling and solid-state diffusion is proposed