Kinetic study of time-dependent fixation of UVI on biochar

Biochar, a by-product from the production of biofuel and syngas by gasification, was tested as a material for adsorption and fixation of U^VI from aqueous solutions. A batch experiment was conducted to study the factors that influence the adsorption and time-dependent fixation on biochar at 20°C, including pH, initial concentration of U^VI and contact time. Uranium (U^VI) adsorption was highly dependent on pH but adsorption on biochar was high over a wide range of pH values, from 4.5 to 9.0, and adsorption strength was time-dependent over several days. The experimental data for pH>7 were most effectively modelled using a Freundlich adsorption isotherm coupled to a reversible first order kinetic equation to describe the time-dependent fixation of U^VI within the biochar structure. Desorption experiments showed that U^VI was only sparingly desorbable from the biochar with time and isotopic dilution with ²³³U^VI confirmed the low, or time-dependent, lability of adsorbed ²³⁸U^VI. Below pH 7 the adsorption isotherm trend suggested precipitation, rather than true adsorption, may occur. However, across all pH values (4.5-9) measured saturation indices suggested precipitation was possible: autunite below pH 6.5 and either swartzite, liebigite or bayleyite above pH 6.5.

Adsorption of Phenol and p-Nitrophenol on Activated Carbon: Determination of Effective Diffusion Coefficients

Adsorption of phenol and p-nitrophenol in aqueous solution on granular activated carbon has been studied in a stirred-tank system. Equilibrium isotherms and concentration decay curves were obtained experimentally for each adsorbate in the finite batch adsorber, for three temperatures in the range 274-313 K. Effective internal diffusion coefficients, Di, of the two adsorbates were calculated by means of a mathematical model of diffusion-adsorption that takes into account ordinary liquid phase diffusion and surface diffusion mechanisms. Surface diffusion coefficients, DS, were also calculated and correlated with temperature and surface coverage. Isosteric heat of adsorption and activation energy for the adsorption of p-nitrophenol were calculated; values obtained are between those of characteristic physical and chemical adsorption.

Structure and Transport Properties of Nanostructured Materials

In the present manuscript, we have presented the simulation of nanoporous aluminum oxide using a molecular-dynamics approach with recently developed dynamic charge transfer potential using serial/parallel programming techniques (Streitz and Mintmire Phys. Rev. B 1994, 50, 11996). The structures resembling recently invented ordered nanoporous crystalline material, MCM-41/SBA-15 (Kresge et al. Nature 1992, 359, 710), and inverted porous solids (hollow nanospheres) with up to 10 000 atoms were fabricated and studied in the present work. These materials have been used for separation of gases and catalysis. On several occasions including the design of the reactor, the knowledge of surface diffusion is necessary. In the present work, a new method for estimating surface transport of gases based on a hybrid Monte Carlo method with unbiased random walk of tracer atom on the pore surface has been introduced. The nonoverlapping packings used in the present work were fabricated using an algorithm of very slowly settling rigid spheres from a dilute suspension into a randomly packed bed. The algorithm was modified to obtain unimodal, homogeneous Gaussian and segregated bimodal porous solids. The porosity of these solids was varied by densification using an arbitrary function or by coarsening from a highly densified pellet. The surface tortuosity for the densified solids indicated an inverted bell shape curve consistent with the fact that at very high porosities there is a reduction in the connectivity while at low porosities the pores become inaccessible or dead-end. The first passage time distribution approach was found to be more efficient in terms of computation time (fewer tracer atoms needed for the linearity of Einstein's plot). Results by hybrid discrete-continuum simulations were close to the discrete simulations for a boundary layer thickness of 5lambda.

Mass transfer in VOC adsorption on zeolite: experimental and theoretical breakthrough curves

From experimental results of adsorption of volatile organic compounds (VOCs) on zeolite, we propose simulations of the breakthrough curves based on the Linear Driving Force model. Experiments were run on fixed beds of hydrophobic commercial zeolites. Pollutants chosen are from several chemical classes with different polarities. A good agreement between experimental and numerical results is found when an adjustable value of the internal mass-transfer coefficient is used. A constant value of effective diffusivity is found independent of the nature and the amount of VOCs adsorbed. A relation linking intrapellet mass-transfer coefficient and equilibrium constant is proposed, including the average effective diffusivity, to make predictions of breakthrough curves for any kind of volatile organic pollutant in gaseous effluents.