Adding value to natural clays as low-cost adsorbents of methylene blue in polluted water through honeycomb monoliths manufacture
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A natural Moroccan illite–smectite was used as an adsorbent for the removal of methylene blue (MB) from aqueous solutions. The clay was characterized by FTIR spectroscopy, TGA, SEM–EDS, X-ray fluorescence, XRD and N2 physisorption. The influence of pH, temperature and time on the MB adsorption by the clay was investigated. The maximum equilibrium adsorption capacity was 100 mg g−1 at 45 °C. The kinetic behavior and the isotherms better-fitted with the pseudo-second-order and Langmuir models, respectively. Clay honeycomb monoliths (50 cells cm−2) were obtained by means of extrusion from the starting material without any additive except water. The structured filters exhibited better performance under dynamic conditions than the powdered clay, adding value to the application of this low-cost adsorbent.
KeywordsAdsorption Clay Honeycomb monolith Methylene blue
The releases of textile industry are in general loaded with organic micropollutants, in particular different detergents and dyes. The latter are often used in excess and can be classified according to their structure as anionic and cationic [1, 2]. In aqueous solution, anionic dyes carry a net negative charge due to the presence of sulfonate (SO3−) groups [3, 4], while cationic dyes carry a net positive charge due to the presence of protonated amine or sulfur-containing groups . Due to their strong interaction with many surfaces of synthetic and natural fabrics , a broad variety of physicochemical and biological techniques has been developed and tested in the treatment of effluents loaded with these contaminants [7, 8]. These processes include precipitation, ionic exchange, filtration on membrane and irradiation. However, these processes are often expensive and lead to the generation of large quantities of sludges, in addition to the formation of compounds derived from the degradation of the active molecules, which are sometimes even more toxic [9, 10]. The accumulation and the low biodegradability of these compounds make the various treatments difficult to apply . Among the wastewater treatment processes, adsorption remains a widely used approach for being easy to implement. The activated carbon is the adsorbent most largely employed, because of its great capacity of adsorption in agreement with its high specific surface area [12, 13, 14]. However, this adsorbent is expensive and poses the problem of its regeneration for a multiple use. The search for new effective and economic adsorbents thus proves attractive [15, 16]. In this context, the use of other adsorbent materials such as clays or natural zeolites is of great interest [17, 18].
The aim of our work was to optimize the adsorption performance of a natural argillaceous material, without any purification step, through its casting as structured filter for a possible replacement of the expensive adsorbents used in the polluted water treatment. In particular, we were interested in eliminating a cationic dye in aqueous solution by retention onto a local natural clay. Moreover, special attention was paid to the fabrication of honeycomb monoliths from this material and their further use for the same application, taking into account that most previous studies reporting on the use of clays to retain dyes employed powders [19, 20, 21, 22], references dealing with clay structured filters being still comparatively scarce . Let us also consider the advantages of the honeycomb monolithic design for the treatment of a high volume/flow, which are characteristic of environmental applications, in comparison with packed-bed reactors in which pressure drop may become a serious problem .
Here, we specifically report on the ability of an interstratified illite–smectite to eliminate methylene blue (MB) present in water. This clay was selected because in a previous study  it demonstrated to have a high adsorption capacity even of heavy metals such as lead. On the other hand, MB was chosen considering the vast literature available regarding the use of this organic dye in adsorption studies in liquid phase [26, 27, 28, 29, 30, 31] with which we could compare our own results. Moreover, it was also selected according to its significant presence in the wastewaters proceeding from the local Moroccan textile industry. Undoubtedly, there are other colorant molecules that from an environmental viewpoint could be also interesting to adsorb such as methyl orange, malachite green, crystal violet, Congo red, rhodamine B, among others. However, MB is the most studied one as well illustrated by the fact that there are even reviews just focused on this dye [30, 31]. Since its discovery almost a century and half ago , MB is one of the most commonly used thiazine (cationic) dyes, and many different adsorbents have been reported for its removal from aqueous solutions . Although it is not particularly hazardous, it has still some toxicity , and additionally, its quantitative detection is relatively simple. In this study, we first used a conventional batch-contact-time method , investigating the equilibrium of MB adsorption onto the powdered interstratified illite–smectite and fitting the data to the most used models (Langmuir, Freundlich) and also to the less employed Sips equation. The uptake of MB on the clay was examined as a function of adsorbate concentration, pH, adsorption temperature and contact time. Furtherly, honeycomb monoliths were also manufactured from the starting clay powder and tested in the retention of methylene blue under dynamic conditions via recirculated plug-flow adsorption experiments. This study was performed in order to add value to these low-cost adsorbents and to investigate their potential for a real application.
2 Materials and methods
The clay used in this study as dye adsorbent was an interstratified illite–smectite collected from the north region of Morocco known as Chefchaouen. It had a CEC of 63 mEq/100 g as measured by the well-known cobalt (III)-hexamine trichloride method .
The basic dye used in this study was methylene blue (MB), from Fluka, with C16H18ClN3S × 3H2O chemical formula which is of analytical grade. This cationic dye model presents decentralized positive charge on the organic framework, which could play a major role in keeping the species on the surface of the clay, the wavelength of 664 nm corresponding to its maximum absorbance . Literature on the MB adsorption by clays is certainly vast, and it has even been used as a method to evaluate CEC and surface areas of clays [31, 32, 33]. In this research, working solutions of MB were prepared with deionized water and from a stock solution of 500 mg L−1 to give the required initial concentrations (C0 = 20–500 mg L−1) for each experimental run .
2.2 Clay monoliths preparation
2.3 Characterization of the adsorbents
XRF compositional analysis of the starting clay was carried out in a Bruker S4 Pioneer spectrometer.
The textural characterization was performed by means of N2 physisorption at −196 °C using an automatic Autosorb IQ (Quantachrome) analyzer. The clay samples (both in the form of powder and as honeycomb monoliths) were degassed at 150 °C for 2 h. The obtained isotherms were used to calculate their specific surface area (SBET) and micro- and mesoporosity. Total pore volume (Vp) data were calculated from the amount of nitrogen adsorbed at a 0.97 relative pressure value (P/P0). Pore size distribution and the average pore were examined by the BJH method from the desorption branch of the isotherms.
The FTIR spectra in transmission mode were obtained using a Thermo vertex 70 FTIR spectrophotometer. About 1 mg of sample clay was mixed with approximately 200 mg of dried KBr to get pellets. Measurements were taken over the range 4000–400 cm−1 in the absorbance mode, with a spectral resolution of 4 cm−1. This technique was employed to study the clay samples both before and after the MB adsorption.
The thermogravimetric analysis (TGA) was carried out under air in a Shimadzu TGA-50 thermobalance over 50 mg of powdered samples using a heating rate of 10 °C min−1.
SEM images and EDS compositional data of the clay powder were obtained using a QUANTA-200 scanning electron microscope equipped with a Phoenix microanalysis system using a nominal resolution of 3 nm. SEM images of the clay calcined at 450 °C for 4 h and of the clay monolith after the same treatment were acquired in a field emission gun (FEG) scanning electron microscope (Nova NanoSEM 450).
XRD studies were carried out in a Bruker diffractometer, D8 Advance 500 model. Diffractograms were recorded using CuKα radiation, and the 2θ angle ranged from 1.5° to 75°, with a step of 0.017° and a counting time per step of 1 s. Refinement of the analysis was performed by applying a Rietveld method by means of the FullProf program . In addition, semiquantitative analysis of the mineralogical composition was made by means of the PowderCell 2.4 software.
2.4 Batch adsorption experiments
The adsorption of MB over the fresh clay powder was studied in a batch equilibration system. Different parameters including initial concentration of MB, contact time, stirring speed and pH of the MB solutions were adjusted, the latter by adding 0.1 N HNO3 or 0.1 N NaOH solutions. In general, the adsorption experiments were carried out by varying the initial concentrations from 50 to 500 mg L−1. In each case, 0.2 g of sample was added to a conical flask which contained the MB solution (50 mL). Two types of experiments were performed. First, the adsorption kinetic curves for two initial concentrations of MB (200 and 300 mg L−1) were obtained at room temperature in order to study the effect of contact time (1 min–4 h) on the adsorption of MB on the clay. The suspensions were continuously shaken at 150 rpm, and the experimental data were examined by pseudo-first-order and pseudo-second-order kinetics [38, 39]. For the fitting, only data up to saturation (approx. 30 min) were considered. In the second type of experiments, the suspensions were stirred at the same speed as before during 4 h (considering the results from the kinetic study) under variable temperature conditions (20, 25, 35 and 45 °C). The adsorption experiments were carried out in a closed system. The MB solutions were kept in thermal equilibrium using an automatic thermostat system, including a water bath and an integral shaker drive. Three theoretical isotherm models, Langmuir, Freundlich and Sips , were considered to fit the experimental data.
In all cases, the remaining MB in balance was analyzed by UV–visible spectrophotometry. The amounts of MB adsorbed were calculated from the difference between initial and final or equilibrium concentration of the corresponding solutions.
2.5 Dynamic adsorption experiments
The capacity to adsorb MB by the clay honeycomb monoliths was studied at room temperature in a homemade system (Fig. 1), using approx. 6.5 g weighted monoliths (L = 5.2 cm approx.) and a 1200 cm3 min−1 recirculated flow of the MB aqueous solution (1 L). This study allows testing the potential application of the monoliths in the treatment of polluted liquid effluents under more realistic conditions. Experimental conditions (mass of adsorbent and dye concentration) were different to those selected for the batch experiments because the two types of experiments are not comparable. In order to make a comparison with the powder, a packed column containing the same quantity of powdered clay (Ø = 1.5 cm, L = 2.5 cm), with quartz wool at the outlet to avoid powder drag, and previously submitted to the same calcination treatment as the monolith (450 °C, 4 h), was also tested in the same experimental setup of the monolith. Blank experiments confirmed previously a negligible MB adsorption over the quartz wool. In both cases, column and monolith, two different initial concentrations of MB, were studied: 20 and 100 mg L−1. As for the batch experiments, the adsorption capacity was analyzed by measuring the absorbance at 664 nm of the residual liquid phase, after filtration, in a UV–visible Cary 50 spectrophotometer from Varian.
3 Results and discussion
3.1 Sample characterization
X-ray fluorescence analysis of oxide content (wt%) for the clay sample investigated
Textural data of the investigated samples as estimated from N2 physisorption
Total pore volume
3.2 Adsorptive performance of the powdered clay
3.2.1 Effect of pH
3.2.2 Adsorption kinetics
Kinetic parameters for the adsorption of MB on the clay sample
(g mg−1 min−1)
Parameters determined by theoretical models for the adsorption of MB over different adsorbents
(g mg−1 min−1)a
− 4.3/− 8.8
− 7.9/− 12.4
Kaolin and zeolite
Mesoporous silicate material (dolomite)
− 0.3/− 4.5
− 0.3/− 3.0
− 30.8/− 35.3
Natural clay mineral
− 0.07/− 0.03
Natural Moroccan illite–smectite
− 32.7/− 33.7
3.2.3 Adsorption isotherms
The experiments carried out show that the capacity of adsorption increases with the initial concentration. In addition, the shape of the obtained isotherms suggests the saturation of the surface sites and therefore the formation of the monolayer. A similar trend was found by other authors when studying MB adsorption on different natural clays [18, 43, 47]. Moreover, according to Giles classification , our isotherms, with independence of the temperature studied (20–45 °C), are of type H which are the result of the dominance of strong ionic adsorbate–adsorbent interactions . In this sense, a chemical adsorption of positively charged functional groups of MB on the negatively charged surface groups of the clay is proposed.
Maximum adsorption capacity of various adsorbents for MB according to Langmuir model
Kaolin and zeolite
Mesoporous silicate material (dolomite)
Moroccan raw and decanted clays
Natural clay mineral
Raw ball clay
Modified ball clay
Natural Moroccan illite–smectite
Langmuir, Freundlich and Sips parameters for MB adsorption by the clay sample
KF (mg g−1)
Qm (mg g−1)
KL (L mg−1)
Qm (mg g−1)
KS (L mg−1)
3.2.4 Adsorption thermodynamics
KL (L mg−1)
KL (L mol−1)
∆G° (kJ mol−1)
65.78 × 104
65.78 × 104
60.45 × 104
60.45 × 104
45.20 × 104
45.20 × 104
35.54 × 104
35.54 × 104
3.3 Methylene blue adsorption on the clay honeycomb monolith
After studying the retention of MB on our clay as a powder, we conducted an additional study using it in the form of honeycomb monolith but under dynamic conditions (Fig. 1). Evidences for the MB adsorption on the clay monolith were obtained not only from its coloring after the process (Fig. 1) but also through FTIR analysis of crushed pieces (Fig. 2). As it can be observed, new peaks at 1602, 1486, 1393, 1353 and 1333 cm−1 were detected after the dye was adsorbed. The first two can be assigned to C=C and C=N, and C=S+ stretching vibrations of the MB heterocycle, respectively, while those at lower frequency might be associated with the symmetrical and asymmetrical bending vibrations of the CH3 functional groups of MB and the stretching vibrations of the C–N terminal saturated dimethylamino groups [56, 57]. It should be added that the position of the peaks previously attributed to the clay was not shifted as a consequence of the interaction with MB. This is consistent with the electrostatic nature of the interaction between adsorbate and adsorbent above suggested by the isotherms. It is also reasonable considering that our clay was not functionalized to enhance its adsorptive performance as other authors did . The only significant change observed after the treatment with MB is the recovery of intensity of the bands located in the upper region, reasonably due to rehydration of the calcined clay that constitutes the matrix of the monolith under the aqueous medium treatment.
It is also noteworthy that in the case of the packed column the flow measured at the outlet was immediately decreased from 1200 to 400 cm3 min−1, whereas the flow kept constant during the whole experiment in the case of the monolith. This observation has a double significance. First, it demonstrates the advantage of using the honeycomb monolith to avoid pressure drop problems, a virtue that joins that of easy replacement upon saturation. Second, it gives double value to the results obtained with this design because it operated under less favored conditions for adsorption such as a three times lower contact time than that employed for the packed powder . Moreover, the results obtained with the clay monoliths of this study are also more interesting than those we previously observed for similar clay honeycomb monoliths that even had been activated by means of coal templating, measuring under the same experimental conditions .
According to textural, chemical and structural characterization, the studied raw argillaceous material mainly consisted of a thermally stable interstratified illite–smectite with appropriate porosity and surface area for use as adsorbent. Its maximum methylene blue (MB) adsorption capacity was found to be 100 mg g−1 at 45 °C. The adsorption isotherms of MB dye could be satisfactorily described by the equations of Langmuir model. The kinetic study of the MB retention by the clay revealed that the adsorption process is of second order, while the thermodynamic study indicated its exothermic character and spontaneity in the range from 20 to 45 °C.
The results obtained with honeycomb monoliths extruded from this clay without additives were of particular interest. They proved that the application of structured filters under dynamic conditions could be more effective than using packed columns of the same material but in powdered form. For example, the integral clay honeycomb monoliths allowed almost complete removal of MB from 1 L of aqueous solution containing 20 mg L−1 of MB, in recirculating flow experiments and for relatively short times (< 6 h), operating with 6.5 g weighted monoliths at 1200 cm3 min−1.
Financial support from the MINECO-Spain/FEDER (MAT2017-87579-R and MAT2017-84228-R Projects) and the Junta de Andalucía (FQM-110 group). M. Ahrouch is grateful for its AUE-UCA fellowship. We acknowledge the electron microscopy and X-ray diffraction divisions of the SC-ICyT of University of Cádiz, and Dr. G.A. Cifredo for his help in the XRD analysis.
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no competing interests.
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