Correlation between eletrokinetic mobility and ionic dyes adsorption of Moroccan stevensite

This study aims at establishing a correlation between the electrical charge of Moroccan stevensite particles and ionic dyes adsorption. The electrophoretic mobility, (Ue), of the stevensite particles in water, was measured at pH 2.5–12 by microelectrophoresis. At pH between 2.5 and 8, Ue remained constant (Ue = − 1.6 10− 8 m2/(V s)), as resulting from the permanent charge of the clay mineral planar surfaces. At pH > 8, the magnitude of electrophoretic mobility increased (Ue = − 2.7 10− 8 m2/(V s)) due to the deprotonation of silanol groups on the surfaces. The anionic Orange G adsorption at the clay mineral–water interface was negligible whereas the methylene blue cations were strongly adsorbed due to the electrostatic attraction

Fonctionnalisation des minéraux argileux d'origine marocaine par TiO2 en vue de l'élimination par photocatalyse de micropolluants organiques des milieux aqueux

Ce travail est consacré à l'élaboration par voie humide (pontage, solvothermale et colloïdale) de photocatalyseur TiO2 supporté sur trois types de minéraux argileux d'origine marocaine: la stévensite, la beidellite et la palygorskite, à leurs caractérisations et finalement à l'évaluation de leurs activités photocatalytiques vis-à-vis de l'élimination en milieu aqueux du colorant anionique l'Orange G (OG) très utilisé en industrie textile. La stévensite et la beidellite sont toutes les deux des smectites de types magnésien trioctaédrique et aluminifère dioctaédrique respectivement. En revanche, la palygorskite est un minéral fibreux riche en Al doté d'un caractère dioctaédrique très marqué. Les matériaux photocatalyseurs supportés développés par le pontage de la stévensite ou de la beidellite ne révèlent pas de formation de piliers interlamellaires de TiO2, mais plutôt l'obtention dans les deux cas de matrice de TiO2 amorphe dans laquelle sont éparpillées quelques rares particules indemnes de phyllosilicates. De même, ceux à base de stévensite élaborés par la méthode solvothermale révèlent des particules du phyllosilicate désintégrées au sein d'une matrice de TiO2 toutefois cristallisée sous forme d'anatase. Néanmoins, la fonctionnalisation selon la voie colloïdale a permis d'immobiliser avec succès des nanoparticules d'anatase (10 nm) sur aussi bien des feuillets plus ou moins exfoliés de stévensite ou de beidellite que sur des fibres de palygorskite. L'anatase attachée aux particules de ces minéraux argileux demeure extraordinairement stable jusqu'à 900 C alors que celle formée en absence de ces phyllosilicates se convertit complètement en rutile vers 650 C. Cette stabilité remarquable de l'anatase supportée est due à l'empêchement de la croissance, par coalescence à haute température, de la taille de ses particules au-delà de la taille critique (30 nm) requise pour sa conversion en rutile relativement moins photoactive. Les essais de photocatalyse révèlent que l'activité catalytique des différents matériaux élaborés croit selon la méthode de fonctionnalisation: pontage - méthode solvothermale - voie colloïdale et aussi selon la nature du minéral argileux : stévensite - beidellite - palygorskite. En outre, les matériaux photocatalyseurs supportés, à base de beidellite ou de palygorskite, développés par la voie colloïdale, manifestent une activité deux fois supérieure à celle de la poudre commerciale TiO2 Degussa P25. Leurs particules floculent aisément, ce qui facilite leur élimination du milieu aqueux sans recourir à la microfilitration requise dans le cas de la Degussa P25.This work was devoted to the elaboration by wet route (pillaring, solvothermal and colloidal) of TiO2 supported photocatalysts on three kinds of clay minerals (stevensite, beidellite and palygorskite) from Morocco, to their characterizations and finally to the evaluation of their photocatalytic activities towards the removal from aqueous media of anionic Orange G dye, widely used in textile industry. Stevensite and beidellite were magnesian trioctahedral and aluminiferous dioctahedral smectites respectively. Nevertheless, palygorskite was a fibrous Al-rich clay mineral with a predominant dioctahedral character. The photocatalyst materials elaborated by the pillaring of stevensite or beidellite did not reveal the formation of TiO2 interlayer pillars, but the observation of an amorphous matrix of Ti-rich phase within which were distributed some rare unaltered particles of phyllosilicates. Those based on stevensite elaborated according to solvothermal method also showed dissolved phyllosilicates particles, but within crystalline TiO2 anatase matrix. Nevertheless, the functionalized materials developed according to colloidal route exhibited successful immobilization of anatase nanoparticles (10 nm) onto as well as more or less exfoliated layers of stevensite or beidellite than on palygorskite fibers. Anatase remained remarkably stable up to 900 C when attached to particles of clay minerals in comparison with that developed in their absence which underwent a complete transformation into rutile at around 650C. This remarkable stability at high temperature of anatase supported on clay minerals particles was due to the hindrance of particles growth by sintering whose the sizes remained below the nucleus critical sizes (30 nm) required for its transition into less photoactive rutile. The photocatalysis tests revealed that the catalytic activity of different elaborated materials increased according to the synthesis route: pillaring process - solvothermal method - colloidal route and according to the nature of clay mineral: stevensite - beidellite - palygorskite. In addition, the supported photocatalysts based on beidellite or palygorskite prepared by colloidal route were found to be twice more active than the commercial TiO2 powder Degussa P25. Furthermore, their particles easily floculated so that they are readily removable from treated solutions without resorting to expensive microfiltration required upon the use of Degussa P25.TOULOUSE-ENSIACET (315552325) / SudocSudocFranceF

Synthesis, characterization and photocatalytic activity of TiO2 supported natural palygorskite microfibers

This study deals with the synthesis of TiO2 supported Moroccan palygorskite fibers and their use as photocatalyst for the removal of Orange G pollutant from wastewater. The TiO2-palygorskite nanocomposite synthesis was accomplished according to a colloidal route involving a cationic surfactant as template (hexadecyltrimethylammonium bromide) assuring hence organophilic environment for the formation of TiO2 nanoparticles. The clay minerals samples were characterized before and after functionalization with TiO2. Anatase crystallizes above ca. 450 °C and remarkably remains stable up to 900 °C. In contrast, pure TiO2 xerogel obtained from titanium tetraisopropoxide (TTIP) showed before calcination a nanocrystalline structure of anatase. By increasing the temperature, anatase readily transforms into rutile beyond 600 °C. The remarkable stability at high temperature of anatase particles immobilized onto palygorskite microfibers was due to the hindrance of particles growth by sintering. Homogeneous monodisperse distribution of anatase particles with an average size of 8 nm was found by TEM and XRD onto palygorskite fibers. This anatase particle size remains below the nucleus critical size (ca. 11 nm) required for anatase–rutile transition. The TiO2 supported palygorskite sample annealed in air at 600 °C for 1 h exhibits the highest photocatalytic activity towards the degradation of Orange G compared to nanocomposite samples prepared under different conditions as well as pure TiO2 powders obtained from the xerogel route or commercially available as Degussa P25

Supported Photocatalyst Based on CuO–TiO2/Palygorskite Nanocomposite Material for Wastewater Treatment

This study deals with the immobilization of mixtures of two semiconductor oxides CuO and TiO2 on fibrous palygorskite clay mineral in order to evaluate their photocatalytic activity for removing Orange G dye as model pollutant from aqueous solutions. The elaboration of CuO–TiO2/Palygorskite nanocomposites was carried out by impregnation with Cu2+ of before hand synthesized TiO2 supported palygorskite (Pal) followed by air annealing for 5 h at 550 °C. In the composite materials, different CuO to CuO + TiO2 molar ratios were used in order to obtain a CuO content in the range 13–30 mol.%. XRD, SEM and TEM equipped with elemental EDS analysis are concordant for showing the crystallization of anatase TiO2 along with CuO whose average size of nanoparticles (NPs) are in the range 6 to 20 nm as determined by TEM. By increasing the CuO content the average size of this oxide remains constant at about 10 nm while that of TiO2 NPs is slightly decreasing from 8.4 to 5.1 nm. Both oxide NPs were successfully attached on palygorskite fibers where they form CuO–TiO2 heterojunctions (grain boundaries like). The CuO–TiO2/Pal supported photocatalyst containing 23% of CuO was found to be the most photoactive material but itremained less active than TiO2/Palygorskite supported photocatalyst. The photocatalytic activity of the mixed nanocomposites is not readily correlated with only one of their main features as CuO content or the average crystallite size of functional oxides indicating that if there are synergistic effects there are also antagonistic effects in particular for high CuO contents

Electrokinetic and ionic dyes adsorption studies of Moroccan stevensite Jbel Rhassoul clay

This study aims at establishing correlation between the nature of electrical charge of morocan stevensite Jbel Rhassoul clay particles and ionic dyes adsorption phenomenon. The electrical charges of the clay mineral particles were assessed by measuring zeta potential (z) using micro electrophoresis technique. Measurements were carried out onto aqueous clay dispersions by varying the pH in the range 2.5 – 12 with additions of HCl or NaOH. On the whole, the electrical charges of stevensite particles remain negative in the whole pH range. Nevertheless, depending on the pH values, two behaviours were observed. For pH ranging from 2 to around 8, the negative electrical charge remains constant (z = - 20 mV) and corresponds to permanent charges of basal surfaces formed of –O-Si-O- groups insensitive to pH variation. Beyond pH = 8, the electrical charge increases (z = - 35 mV) as a result of deprotonation of silanol edge groups. Ionic dyes adsorption experiments show that anionic orange G adsorption is negligible whereas that of cationic methylene blue is greater due to columbic interactions. In accordance with eletrokinetic measurements, the methylene blue adsorption capacity onto stevensite particles significantly increases beyond pH » 8 due to the increase of the whole electrical charge of stevensite particles evolved by silanol edge sites deprotonation

Photocatalytic activity of TiO2/stevensite nanocomposites for the removal of Orange G from aqueous solutions

TiO2/stevensite nanocomposite photocatalysts were synthesized by a solvothermal method using TiCl3/HCl as reactants and the stevensite clay mineral extract as support. The prepared photocatalyst samples were then characterized using various techniques such as X-ray diffraction (XRD), Infrared spectroscopy (IR) and Transmission Electron Microscopy (TEM). The Points of Zero Charge (PZC) of the various samples were evaluated by titration of the non-modified and the Ti-modified clay aqueous dispersions, with cationic surfactant solutions. The photocatalytic activity of the resulting nanocomposites samples were evaluated for the removal of Orange G (OG) from aqueous solution as a model dye pollutant. The data indicate that the formation of Na+-stevensite by the TiO2 particles leads to TiO2/stevensite nanocomposites having higher specific surface areas and mesopore volumes, and lower PZC values. Further, the photocatalytic activity was found to be greater for the TiO2/stevensite nanocomposites having the greatest Ti amount, as compared to a pure TiO2 sample, and increased with the increase of the TiO2 amount in the TiO2/stevensite nanocomposites

Mineralogical and physico-chemical characterizations of clay from Keur Saër (Senegal)

There is interest in exploiting and developing natural resources, particularly deposits of natural clays. Senegal has several clay mineral deposits for which chemical and mineralogical compositions have been little studied. Some of these natural materials are nowadays used in pottery and ceramics. To extend applications, a better basic knowledge is required and, for this objective, the raw clay and separated <2 μm clay fraction from Keur Saër (Senegal) were subjected to chemical and mineralogical studies. Several techniques including X-ray diffraction (XRD), thermal analysis (TG-DTA), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, cation exchange capacity (CEC) measurements and solid state nuclear magnetic resonance (NMR) have been used to characterize the material. It was found that the raw clay and the separated clay fraction consist of a mineral mixture in which kaolinite is the main component. 29Si and 27Al MAS-NMR spectra show the presence of silicon atoms linked to three other silicon atoms via an oxygen atom and six coordinated Al atoms. Significant increases in the specific surface area and cation exchange capacity were observed on purification, reaching a maximum of about 73.2 m2g–1 and 9.5 meq/100 g for the separated fine clay fraction while the values for the raw material were around 28.9 m2g–1 and 7.3 meq/100 g

Influence of the crystal structure of Ag2CO3 on the photocatalytic activity under visible light of Ag2CO3-Palygorskite nanocomposite material

In a companion paper, it has been demonstrated the remarkably beneficial effect of palygorskite clay (Pal) fibers as support material coupled to appropriate thermal treatments and aging under CO2 atmosphere in monitoring the phase composition of Ag2CO3-Pal composite. In this new nanocomposite material, the structure of the functional component Ag2CO3 can be controlled from 100% stable monoclinic (m) to 100% metastable hexagonal β through an adjusted mixture of m- and β-Ag2CO3. The present study deals with the assessment of the visible photocatalytic properties of these various nanocomposite materials towards the removal of Orange G dye from aqueous solutions. It was found that the Ag2CO3-Pal nanocomposite in which Ag2CO3 was single-phased and crystallized with the stable monoclinic structure was more active than the one crystallizing entirely with the metastable β-Ag2CO3 structure. Nevertheless, the composite material containing a mixture of both Ag2CO3 phases with a relative content of 32% of β- and 68% of m-phase was found to be the most photoactive compound of the series. This behavior reveals likely a synergetic effect between both phases in the photocatalytic degradation of the dye under visible light

On the key role of the surface of palygorskite nanofibers in the stabilization of hexagonal metastable β-Ag2CO3 phase in palygorskite-based nanocomposites

This study reports an original remarkable effect of fibrous palygorskite clay mineral in the stabilization at the ambient temperature of the metastable hexagonal β-phase of Ag2CO3 along with the stable monoclinic one (m-Ag2CO3) and the refinement of their particles size (≈5–10 nm). These structural and microstructural features likely arise owing to heterogeneous nucleation induced by the surface of palygorskite fibers between CO32– anions in solution and Ag+ exchanged palygorskite (Ag+-Pal) as reactants kept maturing for short periods, which should not exceed 1 h. Besides, the phase composition of Ag2CO3 supported on palygorskite, namely β- and m-structures can be monitored by carrying on appropriate low temperature treatments under CO2 atmosphere coupled with aging during several months in such a way that either pure β or m single-phases as well as biphased mixtures with controlled composition can be obtained. Taking into account experimental results and literature data, a growth mechanism is discussed

Mineralogical and Physico-Chemical Characterizations of Ferruginous Beidellite-Rich Clay from Agadir Basin (Morocco)

The mechanism of formation of detrital, beidellite-rich clay occurring in the Agadir basin (Morocco) is well documented, but its detailed characterization is incomplete which limits its application. The aim of the present study was to provide further details of the mineralogical and physico-chemical characteristics of this clay. Bulk raw clay and its Na+-saturated, <2 mm fraction were characterized using chemical, structural, and thermal techniques. Measurements of induced streaming potential (e.g. particle charge) and of specific surface area and porous volume are reported. The raw clay contained carbonate and quartz as associated minerals along with phyllosilicates (<2 mm particle size). X-ray diffraction and scanning electron microscopy analyses showed that the <2 mm fraction was dominated by a dioctahedral smectite. Because dehydroxylation of this mineral occurred at 510ºC, and because it re-expanded in ethylene glycol after Li+-saturation followed by heating at 240ºC for 24 h, the mineral was shown to be a beidellite rather than montmorillonite. This assertion was further supported by 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectra showing predominantly negative charges in the tetrahedral sheets due to notable Al-for-Si substitutions. The chemical composition of the <2 mm fraction showed an Fe2O3 content which was ~7.52 wt.% greater than those of other beidellite occurrences but not so much that it would be identified as a nontronite. The absence of stretching and bending absorption bands corresponding to characteristic (Fe2OH) units in mid-infrared spectra and their corresponding fundamental overtones or combination bands in near-infrared spectra supported this notion. The structural formula of the beidellite in the present study was determined to be (Si7.51Al0.49)(Al2.99Fe0.68Mg0.33) (Ca0.03Na0.54Mg0.11)O20(OH)4, having dioctahedral ferruginous characteristics with almost 60% of the negative charge found in tetrahedral sheets. The cation exchange capacity determined from the structural formula was ~108 meq/100 g. The specific surface area and total pore volume were ~82.2 m2/g and 0.136 cm3/g, respectively. Interestingly, a detrital rather than a hydrothermal-alteration origin, as reported for other beidellite occurrences, explains its natural abundance and emphasizes the great interest in it