Industrial applications of acid-treated clays as catalysts

Tetracycline adsorption/desorption by raw and activated Tunisian clays

Clay-based adsorbents have applications in environmental remediation, particularly in the removal of emerging pollutants such as antibiotics. Taking that into account, we studied the adsorption/desorption process of tetracycline (TC) using three raw and acid- or base-activated clays (AM, HJ1 and HJ2) collected, respectively, from Aleg (Mazzouna), El Haria (Jebess, Maknessy), and Chouabine (Jebess, Maknessy) formations, located in the Maknessy-Mazzouna basin, center-western of Tunisia. The main physicochemical properties of the clays were determined using standard procedures, where the studied clays presented a basic pH (8.39-9.08) and a high electrical conductivity (446-495 dS m-1). Their organic matter contents were also high (14-20%), as well as the values of the effective cation exchange capacity (80.65-97.45 cmolckg-1). In the exchange complex, the predominant cations were Na and Ca, in the case of clays HJ1 and AM, while Mg and Ca were dominant in the HJ2 clay. The sorption experimental setup consisted in performing batch tests, using 0.5 g of each clay sample, adding the selected TC concentrations, then carrying out quantification of the antibiotic by means of HPL-UV equipment. Raw clays showed high adsorption potential for TC (close to 100%) and very low desorption (generally less than 5%). This high adsorption capacity was also present in the clays after being activated with acid or base, allowing them to adsorb TC in a rather irreversible way for a wide range of pH (3.3-10) and electrical conductivity values (3.03-495 dS m-1). Adsorption experimental data were studied as regards their fitting to the Freundlich, Langmuir, Linear and Sips isotherms, being the Sips model the most appropriate to explain the adsorption of TC in these clays (natural or activated). These results could help to improve the overall knowledge on the application of new low-cost methods, using clay based adsorbents, to reduce risks due to emerging pollutants (and specifically TC) affecting the environment.

Rehydration Driven Acid Impregnation of Thermally Pretreated Ca-Bentonite—Evolution of the Clay Structure

A new approach to acid activation of raw Ca-bentonite was explored. The method consisted in dehydration of clay by thermal pretreatment at 200 °C, followed by immediate impregnation with H₂SO₄ solution. The acid concentration was 1.5 × or 2.0 × cation exchange capacity (CEC) of clay. The volume of the liquid was adjusted so as to leave the material in the apparently dry state. Structural evolution of the activated solids after 1, 2, 3, and 4 weeks of storage was monitored with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR), and chemical analysis. In the macroscopically dry solids, the rehydrated interlayer Ca²⁺ underwent rapid exchange with H₃O⁺ and formed extra-framework gypsum. Acid attack on montmorillonite structure resulted in continuous removal of layer forming Mg, Al, and Fe cations, with Mg²⁺ being eliminated most efficiently. No significant damage to the montmorillonite lattice was observed. Al was extracted both from the tetrahedral and the octahedral sheets. Under less acidic conditions, the monohydrated H-montmorillonite changed upon storage to bi-hydrated form, as a result of clay auto-transformation. Higher concentrations of acid in the pore network of clay stabilized the H-form of montmorillonite. The data indicate that compositional transformation of acid impregnated bentonite extended beyond the one month of aging investigated in the present work.

Comparison of HCl and H2SO4 for the acid activation of a cameroonian smectite soil clay: palm oil discolouration and landfill leachate treatment

Vertisols occupy approximately 1,200,000 ha in Northern Cameroon. Their richness in smectites allows for the production of “bleaching earths” necessary for refining palm oil, and their effluent is used for leachate treatment. In the present work, two mineral acids (HCl and H₂SO₄) were compared, and the most efficient acid with the lowest cost was determined for use in industrial applications. Under similar experimental conditions (ratio of acid solution/clay mass = 5/1, temperature = 97 °C, stirring time = 4 h), the quantity of cations (Fe²⁺, Fe³⁺, Al³⁺) solubilised during acid activation, palm oil discolouration rate by each activated sample and the financial cost of 5 L of acid solution that is required for the acid activation of one kilogram of smectite clay were compared. It was found that 2N H₂SO₄ was more efficient than 1N HCl and 1N H₂SO_4, considering palm oil bleaching efficiency and cost. The filtrate collected after the acid activation of vertisols was rich in H⁺ (2.04.10⁻¹M), Fe²⁺ (2.8.10⁻³M), Fe³⁺ (4.2.10⁻²M) and Al³⁺ (9.2.10⁻²M) ions. One gram of smectite clay material produced 9 mL of this filtrate that was used for the treatment of leachate from a controlled landfill. The leachate colour decreased from 4262 to 285 PtCo units, while the corresponding chemical oxygen demand (COD) decreased from 802 to 128 mg/L. Thus, the most effective acid for industrial bleaching earth production from vertisol is 2N H₂SO₄ acid.

Natural and Modified Montmorillonite Clays as Catalysts for Synthesis of Biolubricants

In this study, natural and modified clays were evaluated as catalysts in an esterification reaction to obtain bio-based lubricants. The biolubricants are environmentally preferred to petroleum-based lubricants because they are biodegradable and non-toxic. Other advantages include very low volatility due to the high molecular weight and excellent viscosity properties with temperature variations. Modifications in natural clay were performed intending to obtain materials with different textural properties that could improve the reaction under study. The modified clays were obtained in two ways: by pillarization using Al₁₃ Keggin polyoxocations or by acid treatments with H₂SO₄, HCl and HNO₃. All samples were evaluated for the esterification reaction of fatty acids from castor oil (FACO) using 2-ethyl-hexanol. During the reaction step, a zeolite-based adsorbent was used for water removal to increase the reaction equilibrium conversion. Gas chromatography and nuclear magnetic resonance were performed to ensure the formation of the products. The highest conversion of fatty acids to esters was obtained using pillared clays. Adding adsorbent in the reaction medium (10 g of 3A zeolite to 100 g of FACO), the conversion improved from 74⁻88 wt % after 6 h at 50 °C.

Quorum Sensing Disruption in Vibrio harveyi Bacteria by Clay Materials

This work describes the use of clay minerals as catalysts for the degradation of quorum sensing molecule N-(3-oxooctanoyl)-dl-homoserine lactone. Certain clay minerals as a result of their surface properties and porosity can catalytically degrade the quorum sensing molecule into smaller fragments. The disruption of quorum sensing by clay in a growing Gram-negative Vibrio harveyi bacteria culture was also studied by monitoring luminescence and population density of the bacteria, wherein quenching of bacterial quorum sensing activity was observed by means of luminescence reduction. The results of this study show that food-grade clays can be used as biocatalysts in disrupting bacterial activity in various media.

Boron enrichment in martian clay

We have detected a concentration of boron in martian clay far in excess of that in any previously reported extra-terrestrial object. This enrichment indicates that the chemistry necessary for the formation of ribose, a key component of RNA, could have existed on Mars since the formation of early clay deposits, contemporary to the emergence of life on Earth. Given the greater similarity of Earth and Mars early in their geological history, and the extensive disruption of Earth's earliest mineralogy by plate tectonics, we suggest that the conditions for prebiotic ribose synthesis may be better understood by further Mars exploration.

Preparation of trimethylchlorosilane-modified acid vermiculites for removing diethyl phthalate from water

A hybrid organic-inorganic material based on vermiculite was prepared to remove diethyl phthalate (DEP) from aqueous solution. Natural vermiculite was activated with HCl to improve the specific surface area and was then modified by silanization using trimethylchlorosilane. Organovermiculite prepared by ion exchange with hexadecyl trimethylammonium bromide (HDTMAB) was also tested for comparison. The leaching of 2 mol L(-1) HCl at 80°C increased the specific surface area of vermiculite from 14.4 to 500.0m(2)g(-1), and the average pore-diameter was decreased from 7.90 nm to 2.75 nm. Fourier transform infrared spectroscopy (FTIR) spectra indicated that trimethysilyl groups were grafted covalently on the surface of acid vermiculites. The specific surface area of trimethylchlorosilane-modified acid vermiculites (TMAVs) (355.4 m(2) g(-1)) was much larger than that of organovermiculite (6.0 m(2) g(-1)). The isotherm adsorption experiments of DEP showed that TMAVs exhibited linear isotherms, suggesting that the uptake of DEP was controlled by partitioning mechanism. The maximal partition coefficient (K(d)) of TMAVs was 3.1 times higher than that of organovermiculite, implying that TMAVs had stronger organic affinity than organovermiculite. The results demonstrate that the adsorption capacity and mechanism of organoclays were controlled by the specific surface area and organic loading, whereas the length of alkyl chain of organic modifier was not the key factor.

Kinetics and thermodynamics of beta-carotene and chlorophyll adsorption onto acid-activated bentonite from Xinjiang in xylene solution

The kinetics and thermodynamics of beta-carotene and chlorophyll adsorption from xylene solution onto acid-activated bentonite (AAB) within the temperature range 65-95 degrees C were investigated. Adsorption of beta-carotene was described well with the Langmuir isotherm, whereas chlorophyll adsorption was determined well with the Freundlich isotherm, and the experimental data on chlorophyll adsorption were also fitted by the Langmuir isotherm to a certain extent, as reflected by correlation coefficients (R(2)) over 0.9865. In addition, the adsorption of beta-carotene and chlorophyll onto AAB are favorable. The pseudo-second-order model was found to explain the kinetics of adsorption of both pigments more effectively. Increase of temperature enhanced the adsorption rate and equilibrium adsorption capacity of beta-carotene and chlorophyll on AAB. The activation energy for the sorption of beta-carotene and chlorophyll on AAB was 19.808 kJ/mol and 16.475 kJ/mol, respectively. The thermodynamic parameters DeltaH(theta), DeltaS(theta) and DeltaG(theta), computed from K(F) of the adsorption isotherm constant, were 21.766 kJ/mol, 92.244 J/ K mol and -9.554 kJ/mol respectively for the adsorption of beta-carotene on AAB at 65 degrees C, and for adsorption of chlorophyll on AAB at 65 degrees C were 31.051 kJ/mol, 93.549 J/K mol and -0.729 kJ/mol, respectively. The adsorption of beta-carotene and chlorophyll in xylene solution on AAB was a spontaneous and endothermic process with increasing in the randomness at the solid-solution interface.

Bandery str., 79013 Lviv, Ukraine, Centre for Clean Technology, Heslington, York, YO10 5DD, UK. Mechanism and Mathematical Model of Н2S Chemisorption on Modified Bentonite

New H2S adsorbent was synthesized on the basis of bentonite. It was analyzed using elemental and BET surface area analysis. Models and mathematical representations of mechanisms that govern the chemisorption of hydrogen sulfide on the chemically treated bentonite were presented. The models adequacy was assessed by means of statistic t-criterion.

Solid acid catalysts from clays: Preparation of mesoporous catalysts by chemical activation of metakaolin under acid conditions

Natural kaolin was treated at 850 or 950 degrees C in air flow to give respectively the metakaolin samples MK8 and MK9. The obtained materials were successively treated at 90 degrees C with a 1 M solution of H(2)SO(4), for various time lengths. The acid treatment of MK8 was found to give a high surface area microporous material with good catalytic properties related to the high density of acid sites, while MK9 gave an ordered mesoporous material with a low density of acid sites. The materials were characterized by several techniques, X-ray powder diffraction, thermogravimetric analysis, N(2) physisorption, scanning electron microscopy, and temperature-programmed desorption of ammonia. The 1-butene isomerization was used as test reaction to evaluate the acidity of the samples.