Giora Rytwo

Adsorption of diquat, paraquat and methyl green on sepiolite: experimental results and model calculations

Abstract Adsorption of the divalent organic cations paraquat (PQ), diquat (DQ) and methyl green (MG) on sepiolite was determined experimentally and investigated with an adsorption model. The largest amounts of DQ, PQ and MG adsorbed were between 100% and 140% of the cation exchange capacity (CEC) of sepiolite. In previous experiments with monovalent organic cations (dyes), the largest amounts of dyes adsorbed were about 400% of the CEC of sepiolite. Consequently, it was proposed that most of this adsorption was to neutral sites of the clay. The large differences between the adsorption of these divalent organic cations and the monovalent dyes may indicate that there is almost no interaction between DQ, PQ and MG and the neutral sites of sepiolite. This assumption was confirmed by infrared (IR) spectroscopy measurements, that did not show changes in the peaks arising from the vibrations of external SiOH groups of the clay when the divalent organic cations were added. Adsorption results were compared with calculations of an adsorption model that combines the Gouy–Chapman solution and specific binding in a closed system. The model considers cation adsorption on neutral sites of the clay, in addition to adsorption to mono- or divalent negatively charged sites, forming neutral or charged complexes. The model could adequately simulate the adsorption of the divalent organic cations DQ and PQ when added alone, and could yield good fit for the competitive adsorption experiment between the monovalent dye methylene blue and DQ. In competitive adsorption experiments, when total cationic charges exceeded the CEC, monovalent organic cations were preferentially adsorbed on the clay at the expense of the divalent cations.

Hybrid materials based on clays for environmental and biomedical applications

Nanostructured hybrids derived from clays are materials of increasing interest based on both structural characteristics and functional applications, including environmental and biomedical uses. This review introduces some recent examples of nanostructured clay derivatives (organoclays) useful as adsorbents or photocatalysts for environmental applications such as the removal of pollutants or development of environmentally oriented pesticide formulations. The second group of nanostructured materials considered here are related to the so-called bio-nanohybrids, formed by combination of an inorganic solid (clay mineral) with organic entities from biological origin at the nanometric scale. Bionanocomposites are an emerging group of nanomaterials resulting from the assembly of different clay minerals and biopolymers. Among the proposed applications, the development of novel hybrid materials for scaffolds and regenerative medicine, as well as new substrates to immobilize biological species from enzymes to viruses, is notable. Hybrid materials based on layered double hydroxides are receiving special attention in view of the possible applications as drug delivery systems.

Use of methylene blue and crystal violet for determination of exchangeable cations in montmorillonite

A procedure for the determination of cation exchange capacity (CEC) and the amounts of exchangeable cations adsorbed to montmorillonite is proposed. The method consists of a single incubation of the clay in a suspension containing a low concentration of an organic dye of large binding affinity, followed by analysis of the displaced inorganic cations by inductively-coupled plasma emission spectrometry (ICPES). The CEC is obtained by taking the largest sum of displaced exchangeable cations. Montmorillonite suspensions were incubated with methylene blue (MB) or crystal violet (CV) at dye concentrations below 4 mM, for one, three or fourteen days. For total dye concentrations up to the CEC, all the dye was adsorbed and equivalent amounts of exchangeable cations were released. Both dyes could adsorb to the clay in excess of the CEC.After one day of incubation in the presence of dye concentrations of about 50% in excess of the CEC, the total amounts of cations released were reduced to below the CEC. This reduction was interpreted as due to massive aggregation of the clay particles induced by the dye. With CV the total amounts of cations released after three or fourteen days of incubation increased and became equal to the CEC.The same CEC was found for Na-, Ca- and SWy1 crude-montmorillonite, by employing either of the dyes.

Adsorption of monovalent organic cations on sepiolite; experimental results and model calculations

Adsorption of neutral organic molecules and the monovalent organic cations methylene blue (MB) and crystal violet (CV) to sepiolite was determined experimentally and investigated by an adsorption model. The largest amounts of MB and CV adsorbed were about 4-fold of the cation exchange capacity (CEC) of sepiolite. Consequently, it was proposed that most of the above described adsorption was to neutral sites of the clay. The adsorption model considered combines the Gouy-Chapman solution and specific binding in a closed system. The model was extended by allowing cation adsorption to neutral sites of the clay, in addition to adsorption to negatively charged sites and adsorption to neutral complexes formed from 1 cation adsorbed to a negative surface site. The amount of available neutral sites was determined from the adsorption of the neutral molecule Triton-X 100 (TX100). The model could adequately simulate the adsorption of the neutral molecules TX100 and crown ether 15-crown-5 (15C5) as well as the organic cations. Due to aggregation of MB molecules in solution, their adsorption was somewhat less than that of CV at the larger added concentrations. A consideration of the molecular dimensions of TX100, MB and CV suggested that their adsorption was mostly to external sites of the clay and that their entry to the sepiolite channels was largely excluded. This interpretation is supported by infrared spectroscopy (IR) measurements, which show large perturbations of the peak corresponding to vibrations of external Si-OH groups of the clay and confirm complete occupancy of external sites by MB and CV.

The Use of Clay-Polymer Nanocomposites in Wastewater Pretreatment

Some agricultural effluents are unsuitable for discharge into standard sewage-treatment plants: their pretreatment is necessary to avoid clogging of the filtering devices by colloidal matter. The colloidal stability of the effluents is mainly due to mutual repulsive forces that keep charged particles in suspension. Pretreatment processes are based on two separate stages: (a) neutralization of the charges (“coagulation”) and (b) bridging between several small particles to form larger aggregates that sink, leaving clarified effluent (“flocculation”). The consequent destabilization of the colloidal suspension lowers total suspended solids (TSSs), turbidity, and other environmental quality parameters, making the treatments that follow more efficient. Clay-based materials have been widely used for effluent pretreatment and pollutant removal. This study presents the use of nanocomposites, comprised of an anchoring particle and a polymer, as “coagoflocculants” for the efficient and rapid reduction of TSS and turbidity in wastewater with a high organic load. The use of such particles combines the advantages of coagulant and flocculant by neutralizing the charge of the suspended particles while bridging between them and anchoring them to a denser particle (the clay mineral), enhancing their precipitation. Very rapid and efficient pretreatment is achieved in one single treatment step.

Clarification of olive mill and winery wastewater by means of clay–polymer nanocomposites

Highly polluted effluents from olive mills and wineries, among others, are unsuitable for discharge into standard sewage-treatment plants due to the large amounts of organic and suspended matter. Efficiency of all management practices for such effluents depends on an effective pretreatment that lowers the amount of suspended solids. Such pretreatments are usually based on three separate stages, taking a total of 2 to 6h: coagulation-neutralizing the colloids, flocculation-aggregating the colloids into larger particles, and separation via filtration or decanting. Previous studies have presented the concept of coagoflocculation based on the use of clay-polymer nanocomposites. This process adds a higher density clay particle to the flocs, accelerating the process to between 15 and 60 min. This study examined suitable nanocomposites based on different clays and polymers. The charge of the compounds increased proportionally to the polymer-to-clay ratio. X-ray diffraction (XRD) measurements indicated that in sepiolite-based nanocomposites there is no change in the structure of the mineral, whereas in smectite-based nanocomposites, the polymer intercalates between the clay layers and increases the spacing depending on the polymer-to-clay ratio. Efficiency of the coagoflocculation process was studied with a dispersion analyzer. Sequential addition of olive mill or winery effluents with a boosting dose of nanocomposites may yield a very efficient and rapid clarification pretreatment.

Metachromasy as an indicator of photostabilization of methylene blue adsorbed to clays and minerals.

The influence of methylene blue adsorption to different clays on its photodegradation was studied. Methylene blue in solution was decomposed by sunlight in a zero-order process. Adsorption to some clay minerals (sepiolite and vermiculite) and a zeolite (clinoptilolite) accelerated the degradation process, and converted it to a first-order reaction. On the other hand, adsorption to other clay minerals (palygorskite and montmorillonite) stabilized the dye and prevented its degradation. Interestingly, in the clay-dye complexes that exhibited stability, clear metachromasy of the adsorbed methylene blue occurred, whereas the effect was not observed in the clay-dye complexes that underwent photodegradation.

Improved efficiency of a divalent herbicide in the presence of clay, by addition of monovalent organocations

A severe decrease in efficacy of contact herbicides such as diquat (DQ) is caused by dust accumulated on the weeds. Such inactivation was explained by the adsorption of herbicides on dust particles. As a result of this inactivation, the rates of pesticides needed for efficient pest control are usually larger than the recommended rates, which increases the expenses, and pose a real danger to the environment and to the ecological balance of watersheds. We proposed the hypothesis that the addition of a monovalent organic cation to the herbicide formulation might improve the efficacy by lowering the amount of herbicide that is inactivated by the sorption to clay minerals. This hypothesis was tested on lettuce and pepper plants dusted with clay, using the commercial formulation of diquat (“Reglone”) and adding the monovalent organic cations acriflavin (AF) and mepiquat (MQ). We observed that a clay mineral with a high cation exchange capacity (CEC), such as montmorillonite, reduced the herbicidal activity of DQ more than a lower CEC clay as sepiolite. However, addition of 5–20 mM concentrations of AF or MQ may overcome the influences of a clay dust on the activity of a cationic herbicide. Such addition allowed the obtainment of good herbicidal results with less than 1/3 of the normal used amount of herbicide. The results of our research point out to a mechanism that may lower the needed doses of contact herbicides, due to a competitive adsorption on the clay–dust particles between the pesticide and monovalent organic cations that leaves more herbicide available to act on the weed.

Adsorption of Surfactants, Dyes and Cationic Herbicides on Sepiolite and Palygorskite: Modifications, Applications and Modelling

Abstract Adsorption of organic molecules, such as surfactants, dyes and cationic herbicides on sepiolite and palygorskite, is reviewed with an emphasis on modelling and applications. The experimental procedures employed in the reviewed studies included adsorption, XRD, SEM, HRTEM, FTIR, zeta potential and thermal analysis measurements. The surfactants whose adsorption and interactions with the clay minerals were studied included anionic, neutral and cationic ones. These two clays are structurally similar, having a similar content of zeolitic and bound water and IR absorption pattern; the differences include a larger b axis and a larger surface area per weight, a larger fraction of MgO and smaller fractions of Al2O3 and Fe2O3 in the case of sepiolite. Modelling of adsorption of organic cations by the clay minerals considered the electrostatic equations of Gouy–Chapman and specific binding in a closed system. In the case of monovalent organic cations, the model considers neutral complexes formed from binding of a cation to a negatively charged surface site, and positively charged ones which are formed by a binding of another cation to the neutral complex. Both clay minerals have an abundance of neutral silanol (Si OH) sites on the external surfaces. The number of these N sites exceeds by several-fold the number of negatively charged sites, that is, the CEC (cation-exchange capacity). The model accounts for the ability of charged organic monovalent cations to adsorb to neutral binding sites on the silicate layer. The adsorbed amounts of two cationic dyes, methylene blue and crystal violet were more than fourfold of the CEC. In contrast, the largest adsorbed amounts of the divalent organic cations paraquat, diquat and methyl green on sepiolite were between 100% and 140% of its CEC. This outcome suggested that these divalent organic cations do not interact with the neutral sites of sepiolite. This assumption was confirmed by IR measurements, which did not show changes of the peaks arising from the vibrations of external Si OH groups of the clay, when the divalent organic cations were added, unlike changes which were clearly observed when monovalent organic cations were added to the mineral. The model succeeded to simulate and yield predictions for adsorption of organic cations by sepiolite and palygorskite.

EVIDENCE OF DEGRADATION OF TRIARYLMETHINE DYES ON TEXAS VERMICULITE

Syntheticdyes in industrial effluents pose a significant risk to human health and the environment, so much effort has been expended to degrade them using various methods, including the use of clay minerals as catalysts. The purpose of this study was to advance understanding of the mechanisms for clay-catalyzed degradation of crystal violet (CV) and other triarylmethine dyes using three different vermiculite clays (Llano, Texas, VTx-1; Ojen, OV; and Russian, RV), a montmorillonite (SWy-1), and a Spanish sepiolite (SEP). While OV, RV, SWy-1, and SEP showed almost no activity with respect to dye degradation, VTx-1 caused complete removal of the dye from solution up to the equivalent of 200% of the cation exchange capacity of the clay. While large amounts of dye were removed from the solution, no change in basal spacing was observed by X-ray diffraction. The kinetics of removal of CV from solution began after a lag period of >10 days in a process that can be described by pseudo-second order kinetics. By comparison, adsorption of CV onto SWy-1 and SEP was immediate, without any lag period. Sonication treatment of the VTx-1 vermiculite suspension caused the CV removal process to begin immediately. Fourier-transform infrared measurements of adsorption of CV on clays revealed that for the OV and RV vermiculites, SEP sepiolite, and SWy-1 montmorillonite the spectra were similar to the original dye; the spectra of the VTx-1-dye differed considerably, however, exhibiting vibrations of methylene groups (—CH2—) which were not present in the CV molecule. The significant changes in the IR spectrum indicated that CV underwent degradation on the surface of the VTx-1 vermiculite. Carbon-content analysis led to the conclusion that degradation products remained bound to the clay. Similar effects were observed for two other triarylmethine dyes (malachite green and methyl green) added to VTx-1, indicated that it may, therefore, be considered suitable as a sorbent to remove and decompose such dyes from industrial effluents. Pretreatment by sonication would remove the need for long incubation times.