Monitoring of the Aggregation Process of Dense Colloidal Silica Suspensions in a Stirred Tank by Acoustic Spectroscopy

The aim of this study is to analyze the behaviour of dense colloidal suspensions in flow by acoustic spectroscopy. The destabilization and the aggregation of stable colloidal silica dispersions in a stirred tank are obtained by addition of salt. Experiments were made in order to observe the influence of different operating parameters, like silica concentration, temperature and stirring speed, on the behaviour of the suspended particles. The use of online acoustic spectroscopy to analyse the process enables us to evaluate the evolution of the silica suspension properties during the aggregation processes. For example, the influence of physicochemical and hydrodynamics parameters on the aggregation process can be simply explained on the basis of the acoustic attenuation spectra. Thus the direct analysis of the spectra can give information on the evolution of the aggregation process and a fast comparison of the effects of the various operational parameters

Experimental Characterization and Population Balance Modelling of the Dense Silica Suspensions Aggregation Process.

Concentrated suspensions of nanoparticles subjected to transport or shear forces are commonly encountered in many processes where particles are likely to undergo processes of aggregation and fragmentation under physico-chemical interactions and hydrodynamic forces. This study is focused on the analysis of the behavior of colloidal silica in dense suspensions subjected to hydrodynamic forces in conditions of destabilization. A colloidal silica suspension of particles with an initial size of about 80 nm was used. The silica suspension concentration was varied between 3% and 20% of weight. The phenomenon of aggregation was observed in the absence of any other process such as precipitation and the destabilization of the colloidal suspensions was obtained by adding sodium chloride salt. The experiments were performed in a batch agitated vessel. The evolution of the particle size distributions versus time during the process of aggregation was particularly followed on-line by acoustic spectroscopy in dense conditions. Samples were also analyzed after an appropriate dilution by laser diffraction. The results show the different stages of the silica aggregation process whose kinetic rates depend either on physico-chemical parameters or on hydrodynamic conditions. Then, the study is completed by a numerical study based on the population balance approach. By the fixed pivot technique of Kumar and Ramkrishna [1996. On the solution of population balance equations by discretization—I. A fixed pivot technique. Chemical Engineering Science 51 (8), 1311–1332], the hypothesis on the mechanisms of the aggregation and breakage processes were justified. Finally, it allows a better understanding of the mechanisms of the aggregation process under flowing conditions

Analyse du comportement de suspensions concentrées de silice colloïdale lors de leur destabilisation électrostatique en cuve agitée

L’objectif de cette étude est d’analyser par spectroscopie acoustique le comportement, dans des conditions de déstabilisation, de la silice colloïdale en suspension dense soumise à des forces hydrodynamiques. Le phénomène d’agrégation des particules, obtenu par déstabilisation de dispersions stables, est provoqué par ajout de sel et est mis en oeuvre dans une cuve agitée. Des expériences ont été menées dans le but d’observer l’influence de différents paramètres opératoires (concentration en silice, température et vitesse d’agitation) sur le comportement des particules en suspension. L’analyse par la spectroscopie acoustique en ligne du procédé a permis d’appréhender l’évolution de certaines propriétés des suspensions de silice au cours des processus d’agrégation mis en jeu

Caractérisation et comportement de suspensions concentrées de nanoparticules sous écoulement : Application aux processus d'agrégation et de rupture

L'objectif est d'améliorer la connaissance des méthodes de caractérisation des propriétés granulométriques et structurales de suspensions denses de nanoparticules placées sous des conditions hydrodynamiques comparables à celles de procédés réels. Ce travail porte sur l'agrégation de silice colloïdale en cuve agitée afin de parvenir, par l'exploitation de méthodes de caractérisation en milieu dense en ligne du procédé (spectroscopie acoustique) ainsi qu'en milieu dilué hors ligne, à une meilleure compréhension des mécanismes d'agrégation de nanoparticules. Le suivi des distributions de taille et de la structuration de la suspension a révélé l'existence de deux étapes d'agrégation de cinétiques différentes influencées par la physico-chimie du procédé et par l'hydrodynamique du système. Enfin la modélisation par les bilans de population des processus d'agrégation et de rupture des particules en suspension a permis de justifier les hypothèses émises sur les mécanismes d'agrégation observés. ABSTRACT : The aim of this study is to get a better knowledge of the different characterization techniques of particle size and particle structure for dense colloidal suspensions submitted to hydrodynamic conditions similar to those found in real processes. The aggregation process of colloidal silica in a stirred tank is investigated in this work. The aggregation mechanisms are better understood thanks to the use of on-line characterization method in dense conditions (acoustic spectroscopy) and off-line characterization in diluted conditions. The existence of two aggregation steps with different kinetics was highlighted by the following of the particle size distributions and the suspension structure. The evolution is influenced by the process physico-chemical parameters and the hydrodynamics of the system. Finally, the assumptions formulated for the experimental aggregation mechanisms were justified by the results of a population balance numerical model for aggregation and breakag

Organic pollution selective degradation by adsorption on high silica zeolites and regeneration by ozone

This study concerns the treatment of industrial airstreams polluted with volatile organic compounds (VOCs), and focuses on highlighting the experimental feasibility of a new hybrid process combining VOC adsorption on hydrophobic zeolites and regeneration by an ozonated airstream in a single reactor. A preliminary study of high-silica zeolites has shown the different kinds of interactions between gaseous ozone and two mineral zeolitic adsorbents, a faujasite Y (FauY) and a ZSM-5 type silicalite (SilZ), and the conservation of the solids adsorption properties (capacities and selectivity) after their contact with ozone. Here, results concerning the contact of gaseous ozone and zeolites saturated with adsorbed VOC are presented. Using temperature measurements inside the fixed bed, oxidation of two models VOC, methyl ethyl ketone (MEK) and toluene (TOL), was observed on both zeolites. Adsorbents were fully regenerated, and the adsorption capacities of the zeolites after their contact with ozone were not affected when used in a discontinuous adsorption / oxidation mode. In addition, the adsorbed VOC were totally mineralized. Detection and identification of oxidation sub-products traces using mass spectrometer - gas chromatography (formic, oxalic, acetic acids, and acetaldehyde for TOL and 2,3-butanedione and acetic acid for MEK) makes possible the indentification of oxidative species. Moreover, a mass balance on oxygen showed that all the produced ozone was used for organic compounds oxidation. There was no or very little ozone lost via side-effect decomposition/deactivation mechanisms. At last, using the zeolites selectivity determined in a previous study toward a binary MEK-TOL mixture, the adsorption / ozonation cyclic process enables the recovery of the minor compound and the selective oxidation of the other component. The great interest of this new process is clearly highlighted, and the technology transfer to industrial operating plants is now studied

Aggregation of silica nanoparticles in concentrated suspensions under turbulent, shear and extensional flows.

The production of nanoparticles in concentrated suspensions requires strict control of the stability of the systems which are strongly influenced by the physico-chemical properties and the hydrodynamic conditions they are placed in. This study deals with the analysis of the aggregation processes of a colloidal silica suspension destabilized by addition of salt under different flows: a turbulent flow performed in a stirred tank, a pure shear flow created thanks to a Couette geometry and an extensional flow obtained in a four-roll mill (Taylor cell). During the aggregation process, the silica suspensions behave as shear-thinning fluids and the variation of their apparent viscosity can be related to the evolution of the size distribution of the aggregates in the suspension. Pure shear and turbulent flows at an equivalent strain rate exhibit almost the same behaviour. The viscosity and the aggregate size decrease with the shear rate. On the contrary, the apparent viscosity and the aggregate size distributions were not very sensitive to a change of an extensional constraint within the considered range. Indeed, although aggregates obtained in the Taylor cell were bigger than in the Couette cell, the apparent viscosity was higher in the latter case. Different aggregate structures, characterized by their fractal dimension, were finally predicted depending on the hydrodynamic nature of the main flow under which they were produced

Study on the stability of suspensions based on biomimetic apatites aimed at biomedical applications

Nanobiotechnologies have lately attracted much attention, both from therapeutic and diagnosis perspectives. In this view, the development of colloidal formulations of biocompatible nanoparticles capable of interacting with selected cells or tissues raises a particular interest, especially in linkwith cell-based pathologies such as cancer. In this context, the follow-up of colloidal stability and other physico-chemical features is of foremost relevance. In this contribution, we have focused our study on hybrid colloids based on biomimetic nanocrystalline apatites (analogous to those found in bone mineral) stabilized by adsorption of 2 aminoethylphosphate (AEP) molecules; these nanoparticles being intended to interact with cancer cells either for medical imaging (by conferring luminescence features to the apatite phase) or for therapeutic purposes. We show that various physico-chemical characteristics of the suspensions vary with time, including viscosity and mean particle size, suggesting a progressive structuration of the suspensions. Similar modifications were also noticed during the purification by dialysis. Finally,we report on preliminary experiments aimed at drying the colloids while retaining their capacity to recover their initial state after re-immersion in aqueous medium; so as to enable extended storage periods for the nanoparticles in their dry state while allowing their re-suspension at the time of use. On this matter, the addition of glucose prior to freeze-drying was found to be an effective way to avoid the formation of aggregates during drying. This contribution thus confers additional information relative to the stability of AEP-stabilized biomimetic

Agrégation de dispersions concentrées de silice colloïdale sous écoulement

L’objectif de cette étude est d’analyser le comportement de suspensions denses de nanoparticules soumises à des forces hydrodynamiques lors de procédés d’agrégation. Le phénomène d’agrégation des particules, obtenu par déstabilisation de dispersions stables de silice colloïdale a été observé ici en l’absence de tout autre phénomène d’apparition ou de croissance des particules. La déstabilisation des suspensions colloïdales, mise en oeuvre dans une cuve agitée discontinue, est provoquée par ajout de sel, modifiant ainsi la force ionique du système. L’utilisation de différentes techniques de caractérisation de la granulométrie des particules (spectroscopie acoustique, spectroscopie par corrélation de photons) et de leurs propriétés physico-chimiques (stabilité, pH, potentiel zêta) a permis d’appréhender les propriétés des suspensions initiales. La technique de spectroscopie acoustique a ensuite été utilisée lors d’expériences d’agrégation réalisées sous différentes conditions de concentration en solide pour analyser l’évolution au cours du temps de la distribution de taille des agrégats en milieu concentré et définir les mécanismes d’agrégation de la silice colloïdale sous agitation

Nanoparticles in wastewaters: hazards, fate and remediation

The increasing use of nanoparticleswill inevitably result in their release into the aquatic environment and thereby cause the exposure of living organisms. Due to their larger surface area, high ratio of particle number tomass, enhanced chemical reactivity, and potential for easier penetration of cells, nanoparticles may be more toxic than larger particles of the same substance. Some researchers have been showing some relations between nanoparticles and certain diseases. However, the doses, surface shapes, material toxicity and persistence of nanoparticles may all be factors in determining harmful biological effects. In order to better evaluate their risks, potential exposure route of nanoparticles has to be taken into consideration aswell. Finally, a brief summary of techniques for nanoparticle removal inwaters andwastewaters is presented, but it seems that no treatment can absolutely protect the public from exposure to a large-scale dissemination of nanomaterials

Ibuprofen loading into mesoporous silica nanoparticles using Co-Spray drying: A multi-scale study

Mesoporous Silica Nanoparticles (MSN) are used in an increasing number of applications in nanomedicine. Their synthesis and external/internal functionalization have been extensively studied as well as their biological properties. Nevertheless, the conventional drug loading processes of MSN (such as impregnation), do not enable sufficient efficiency and are difficult to consider on an industrial scale. To overcome these limitations, we implemented an innovative co-spray-drying process, using a nano spray-dryer, to load MSN with ibuprofen molecules. In this contribution, complementary techniques were used to perform a multi-scale characterization of the loaded particles. Spray-dried powders have been analysed from aggregates size and morphology to pore loading and ibuprofen conformation. This study demonstrates that ibuprofen/silica weight ratio in the initial suspension strongly affects the location (into mesopores or external) and the conformation (crystallized, amorphous or liquid-like) of ibuprofen. The quantification of each phase has allowed calculating precise loading rates and demonstrate tunable pore filling