Competitive reaction modelling in aqueous systems. The case of contemporary reduction of dichromates and nitrates by nZVI

In various Countries, Cr(VI) still represents one of the groundwater pollutant of major concern, mainly due to its high toxicity, furthermore enhanced by the synergic effect in presence of other contaminants. As widely reported in the recent literature, nanoscale zero valent iron particles (nZVI-p) have been proved to be particularly effective in the removal of a wide range of contaminants from polluted waters. In this work, experimental tests of hexavalent chromium reduction in polluted groundwater in the presence of nitrate by nZVI-p are presented and discussed. The effect of different nitrate amounts on Cr(VI) reduction mechanism was investigated and the obtained results were successfully interpreted by the proposed kinetic model. nZVI-p produced by the classical borohydride reduction method were added in to synthetic solutions with the initial concentration of Cr(VI) set at 93, 62 and 31 mg L-1 and different nitrate contents in the range 10-100 mg L-1. According to the experimental results, nitrate showed an adverse effect on Cr(VI) reduction, depending on the nZVI/Cr(VI) and Cr(VI)/NO3 - ratio. The proposed kinetic model soundly grasps the competitive nature of the Cr(VI) reduction process when other chemical species are present in the treated solution

Diffusion-reaction modelling of the degradation of oil-well cement exposed to carbonated brine

The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process.The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process

Diffusion-reaction modelling of the degradation of oil-well cement exposed to carbonated brine

The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process.The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process

Diffusion-reaction model for alkali-silica reaction in concrete

A new diffusion-reaction model for the potentially deleterious Alkali-Silica Reaction (ASR) process in concrete is presented. The model involves three coupled diffusion processes, two in-goingand one out-goingfrom the aggregate viewpoint. Alkali (Na+ and K+) and Calcium (Ca2+) ions diffuse “inwards”, from high molar concentration sites in the pores of the cement paste phase of the concrete specimen or at its boundaries, towards the aggregate-cement paste interfaces or the inner cracks of the aggregates. The OH- ions associated with alkali and calcium ions attack certain forms of silica in the aggregates (the “reactive silica”), dissolving it in the form of silicate ions which in turn diffuse back to the cement paste phase (“outwards”). The final potentially deleterious ASR precipitation process involves those silicate ions, plus calcium and alkalis. It takes place wherever the reactants are available by precipitating silicate hydrates of two kinds (Calcium-Silicate-Hydrates –CSH or Calcium-Alkali-Silicate-Hydrates –CASH) in a proportion depending on concentrations and temperature. The diffusion-reaction equations of this process are discretized in space and time using finite differences. An example of application in 1D is presented to illustrate the capabilities to reproduce realistically the ASR process, including some novel features not usually which are not considered in the available literature, such as the role of calcium in the development of the reaction and the inherent relationship between the reaction product composition and its swelling capacity

Diffusion-reaction modelling of the degradation of oil-well cement exposed to carbonated brine

The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process.The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process

Diffusion-reaction modelling of the degradation of oil-well cement exposed to carbonated brine

The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process.The essential aspects of a diffusion-reaction model in development for the degradation process of oil-well cement exposed to carbonated brine are presented in this paper. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the hardened cement paste pore solution, complemented by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. A sensitivity analysis of some parameters of the model is presented to illustrate the capabilities to reproduce realistically some aspects of the degradation process

Reaction to New Security Threat Class

Each new identified security threat class triggers new research and development efforts by the scientific and professional communities. In this study, we investigate the rate at which the scientific and professional communities react to new identified threat classes as it is reflected in the number of patents, scientific articles and professional publications over a long period of time. The following threat classes were studied: Phishing; SQL Injection; BotNet; Distributed Denial of Service; and Advanced Persistent Threat. Our findings suggest that in most cases it takes a year for the scientific community and more than two years for industry to react to a new threat class with patents. Since new products follow patents, it is reasonable to expect that there will be a window of approximately two to three years in which no effective product is available to cope with the new threat class

Spacecraft Reaction Wheels

This activity is designed to introduce the engineering aspects of precision pointing of spacecraft. Educational levels: Middle school, High school, Middle school, High school

Interfacial Reaction Kinetics

We study irreversible A-B reaction kinetics at a fixed interface separating two immiscible bulk phases, A and B. We consider general dynamical exponent $z$, where $x_t\sim t^{1/z}$ is the rms diffusion distance after time $t$. At short times the number of reactions per unit area, $R_t$, is {\em 2nd order} in the far-field reactant densities $n_A^{\infty},n_B^{\infty}$. For spatial dimensions $d$ above a critical value $d_c=z-1$, simple mean field (MF) kinetics pertain, $R_t\sim Q_b t n_A^{\infty} n_B^{\infty}$ where $Q_b$ is the local reactivity. For low dimensions $d<d_c$, this MF regime is followed by 2nd order diffusion controlled (DC) kinetics, $R_t \approx x_t^{d+1} n_A^{\infty} n_B^{\infty}$, provided $Q_b > Q_b^* \sim (n_B^{\infty})^{[z-(d+1)]/d}$. Logarithmic corrections arise in marginal cases. At long times, a cross-over to {\em 1st order} DC kinetics occurs: $R_t \approx x_t n_A^{\infty}$. A density depletion hole grows on the more dilute A side. In the symmetric case ($n_A^{\infty}=n_B^{\infty}$), when $d<d_c$ the long time decay of the interfacial reactant density, $n_A^s$, is determined by fluctuations in the initial reactant distribution, giving $n_A^s \sim t^{-d/(2z)}$. Correspondingly, A-rich and B-rich regions develop at the interface analogously to the segregation effects established by other authors for the bulk reaction $A+B\to\emptyset$. For $d>d_c$ fluctuations are unimportant: local mean field theory applies at the interface (joint density distribution approximating the product of A and B densities) and $n_A^s \sim t^{(1-z)/(2z)}$. We apply our results to simple molecules (Fickian diffusion, $z=2$) and to several models of short-time polymer diffusion ($z>2$).Comment: 39 pages, 7 figures, uses fund2.sty, submitted to Eur. Phys. J. B, 1 figure added, for short version see cond-mat/980409