Desalination 250 (2010) 335–338
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Equilibrium modeling for the adsorption of methylene blue from aqueous solutions on activated clay minerals☆ Y. El Mouzdahir a, A. Elmchaouri a, R. Mahboub a, A. Gil b,⁎, S.A. Korili b a b
Université Hassan II Mohammedia, Faculté des Sciences et Techniques, Laboratoire d'Electrochimie et Chimie Physique, BP 146, 20650 Mohammedia, Morocco Department of Applied Chemistry, Los Acebos Building, Public University of Navarra, Campus of Arrosadia, E-31006 Pamplona, Spain
a r t i c l e
i n f o
Article history: Received 8 February 2008 Accepted 15 September 2008 Available online 9 October 2009 Keywords: Acid activation Adsorption Chemical activation Clay Methylene blue
a b s t r a c t This work reports the application of an activated clay mineral as adsorbent for the removal of a basic dye, methylene blue (MB), from aqueous solutions. The thermal treatment at 300 °C for 2 h and the acid activation with nitric acid of 0.5 mol/dm3 under reﬂux conditions improve the adsorption capacity of the raw clay mineral. A maximum of 500 mg/g of MB at equilibrium is achieved. Equilibrium data are mathematically modelled using the Freundlich, Langmuir and Toth isotherm adsorption models. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Principal contaminants found in wastewater include organic compounds, toxic metals and microbial pathogens and parasites that are a great danger to the environment and human health . The degradation of dyes and their derived products can be toxic and carcinogenic, even at low concentrations . There is a need to treat the efﬂuents that contain dyes prior to their discharge into receiving waters. Among the methods that have been applied to remove dye from wastewater , the adsorption processes have been reported as low-cost techniques for the treatment of textile industry efﬂuents and pigments. Activated carbon is a well-known adsorbent for dye removal from wastewater , but its high cost has restricted its use, particularly in developing countries. Therefore, many researchers have investigated the feasibility of using cheap available materials as removal agents for dyes. Results obtained from the literature on methylene blue adsorption by several adsorbents are summarized in Table 1. In this work, the removal behaviour of a natural Moroccan clay is evaluated by studying the adsorption of methylene blue from aqueous solutions. The starting material was thermally treated and acid leached by an inorganic acid, to obtain samples with modiﬁed adsorption properties. The effect of the experimental conditions on the clay adsorption behaviour was studied by varying the temperature of treatment from 100 °C to 800 °C and the concentration of the nitric acid used for the chemical modiﬁcation of the clay. The ☆ Presented at the 1st Conference on Environmental Management, Engineering, Planning and Economics (CEMEPE), Skiathos, Greece, 24-28 June, 2007. ⁎ Corresponding author. E-mail address: [email protected]
(A. Gil). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.09.052
adsorption data were analysed according to Freundlich, Langmuir and Toth equation isotherms. The adsorption capacity of the clay suggests that the material could be used as a low-cost alternative in wastewater treatment for the removal of colour and dyes. 2. Experimental 2.1. Starting material The material used in the present study was a puriﬁed Moroccan clay mineral from Middle Atlas (Morocco). Chemical analysis was carried out by X-ray ﬂuorescence spectrometry using a Philips PW 1400 spectrometer, and gave the following composition: 43.73 wt.% SiO2, 4.51 wt.% Al2O3, 25.39 wt.% MgO, 0.84 wt.% Fe2O3, 0.06 wt.% MnO, 0.04 wt.% Li2O, 7.91 wt.% CaO, 0.69 wt.% Na2O, 3.54 wt.% SO3, 0.07 wt.% P2O5 and 0.92 wt.% K2O. X-ray diffraction studies were carried out on a Philips PX 1820 instrument using Cu Kα radiation. The XRD pattern of the raw sample revealed the presence of contaminants such as quartz, feldspar, calcite and aragonite. Montmorillonite can be identiﬁed by the peaks appearing at 2θ 6.92°, 60.74° and 59.83°. The cation exchange capacity (CEC) was measured using the ammonium acetate method. The samples were ﬁrst ammonium exchanged, then the ammonium ions in the supernatant were deprotonated into ammonia with a sodium hydroxide solution. Ammonia content was determined by distillation into a known amount of acid and back titrated by Kjeldahl method. The CEC of the clay mineral obtained this way was 0.87 meq/g. Nitrogen adsorption experiments were performed at −196 °C using a static volumetric apparatus, Micromeritics ASAP 2010 adsorption
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Table 1 Adsorption capacity (qe) of methylene blue on various adsorbents. Adsorbent
Kaolinite Zeolite Montmorillonite Natural bentonite Activated carbons
17 25 110 300 380–528
    
3.1.1. Freundlich isotherm The Freundlich model assumes that the sorption takes place on heterogeneous surfaces. The model gives a representation of the equilibrium between the amount of adsorbate in solution and that on the surface of the adsorbent, 1 = mF
qe = kF ·Ce
analyser. Prior to the adsorption measurements, the samples were outgassed at 300 °C for 2 h, under a vacuum better than 0.1 Pa. The BET speciﬁc surface area  was found to be 125 m2/g and the micropore volume, calculated according to the t-method , was 0.04 cm3/g.
where qe (mg/g) is the equilibrium amount of adsorbate adsorbed by the solid, Ce (mg/dm3) is the equilibrium concentration of adsorbate
2.2. Thermal treatment In order to study the effect of the thermal treatment of the clay mineral on the removal of MB, samples of the puriﬁed clay were heated in a furnace between 100 °C and 800 °C for 2 h. The hot samples were cooled down to room temperature, dispersed in distilled water and dried overnight at 50 °C. The thermally treated samples are designated hereafter as S-T, where T indicates the temperature of the treatment. 2.3. Acid activation In order to study the effect of the acid treatment of the clay mineral, the samples were immersed in 0.5 dm3 of boiling HNO3 solution of varying concentration, without stirring for an optimised time of 2 h. The resulting solids were thoroughly washed with distilled water and then dried at room temperature. The acid treated samples are designated hereafter as S-M, where M indicates the concentration in mol/dm3 of the HNO3 solution. 2.4. Adsorption experiments procedure An aqueous solution of MB was prepared from its chloride salt (methylene blue chloride, Merck) and suitably diluted to the required initial concentrations, between 100 and 1200 mg/dm3. In order to determine the equilibrium adsorption capacity of the clay mineral samples, 50 cm3 of adsorbate solution were put in an 100 cm3 stoppered conical ﬂask placed in a temperature controlled bath shaker and 0.1 g of clay was added. The ﬂasks were sealed with stopper and the shaken for 2 h at 350 rpm. The pH was maintained at a value of 5.6, by addition of solutions of HCl or NaOH. Then the ﬂasks were taken out and the supernatant was centrifuged for 10 min at 9500 rpm. The concentrations of MB were measured using a UV–vis spectrophotometer (Standard UNICAM) at a wavelength corresponding to the maximum absorbance, 663 nm. Calibration experiments were carried out in order to exclude the experimental error due to sorption of MB on the walls of the glass vessels. The adsorbed amount of MB on clays was calculated from the initial and ﬁnal concentrations according to the equation: qe = V·ðC0 Ce Þ = m
where C0 is the initial concentration (mg/dm3), Ce is the equilibrium concentration (mg/dm3), V is the volume of the solution (dm3) and m is the amount of clay (mg). 3. Theoretical approach 3.1. Adsorption isotherms The adsorption equilibrium data of MB on modiﬁed clay minerals were ﬁtted applying Freundlich, Langmuir and Toth isotherm models .
Fig. 1. Equilibrium adsorption data of methylene blue on clay minerals: A. raw clay mineral; B. thermal activated clay mineral; C. acid-activated clay mineral.
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and, kF and mF are empirical constants, indicative of the extent of adsorption and the adsorption effectiveness, respectively.
delamination of the aluminosilicate layers, which may enhance the available space between the silicate layers to hold the clay species at the proper reaction sites. This leads to a cross-linking structure and thus develops mesoporosity to the material resulting in high surface area, which improves the MB adsorption capacity. The treatment at temperatures higher than 300 °C caused a decrease of adsorption capacity to less than 250 mg/g. A possible reason of this behaviour could be the dehydroxylation of the clay layers, which can be accompanied by movements of octahedral cations within the octahedral sheets . Besides these structural changes, the calcination can also modify the structural properties and inﬂuence the morphology of crystalline building. The MB adsorption capacity of the acid-leached samples was found to be favoured by relatively mild treatment conditions (HNO3, 0.5 mol/dm3) reaching values as high as 500 mg/g (see Fig. 1C). This may be due to the creation of porosity together with the disaggregation and delamination of clay platelets being responsible for the increase in the structural properties . A more severe acid treament condition produces an important decrease of the adsorption capacity up to 100 mg/g, which can be related to a partial dissolution of the clay mineral.
3.1.2. Langmuir isotherm The Langmuir theory assumes that a saturation point is reached beyond which no further adsorption can occur and the saturation monolayer can be then represented by the following expression, qe =
qL ·kL ·Ce 1 + kL ·Ce
where qe (mg/g) is the equilibrium amount of adsorbate adsorbed by the solid, Ce (mg/dm3) is the equilibrium concentration of adsorbate, qL (mg/g) and kL (dm3/mg) are the Langmuir constants, representing the monolayer adsorption capacity and the energy of adsorption, respectively. 3.1.3. Toth isotherm The Toth model is an improvement of the Freundlich and Langmuir equations. The equation describes well the adsorption behaviour under sub-critical conditions of several adsorbates. qe =
qT ·kT ·Ce ½1 + ðkT ·Ce ÞmT 1 = mT
4.1.1. Modeling of MB adsorption isotherms The adsorption equilibrium data were ﬁtted to the Freundlich, Langmuir and Toth isotherm models. Each model constants are used for predicting the adsorption capacities and also for incorporating into mass transfer relationship to predict the design of contacting experiments . The experimental data and the three modelled isotherms are presented in Fig. 1A. The best ﬁtting results were obtained with the Toth isotherm model, therefore this isotherm is the one selected to be presented together with the experimental data in Fig. 1B and C. The parameters were estimated by non-linear regression and the values are summarized in Table 2. The estimated monolayer capacities, qL and qT, are in accordance with the experimental results. The product of the Toth parameters (qT⋅kT), which is related to the interaction of the adsorbate (MB) with the surface , is higher for the acid-activated samples than for the thermally activated ones.
where qe (mg/g) is the equilibrium amount of adsorbate adsorbed by the solid t, Ce (mg/dm3) is the equilibrium concentration of adsorbate, qT (mg/g) and kT (dm3/mg) are the Toth constants, representing the monolayer adsorption capacity and the energy of adsorption, respectively, and mT is an empirical constant. 4. Results and discussions 4.1. Adsorption capacity of the clay minerals The amount of MB adsorbed per unit of mass of clay and the equilibrium concentration in the aqueous solution at room temperature are presented in Fig. 1. The adsorption efﬁciency and effectiveness of the clays increase with increasing initial MB concentration. The adsorptive capacity of the raw clay mineral was found to be 350 mg/g (see Fig. 1A). In the case of the thermally modiﬁed clay minerals, the adsorption capacities were found to vary depending on the temperature of treatment. The treatment between 50 and 300 °C improved the adsorption capacity from about 350 mg/g to 500 mg/g. The thermal treatment in this range must have resulted in the removal of non-clay matter and the generation of basic or anionic sites for binding MB cations [5,12]. A further important mechanism is the particle delamination of the clay . The calcination leads to a
5. Conclusions The adsorption of a pure organic dye, methylene blue, on a raw clay mineral was undertaken in the static mode at room temperature and atmospheric pressure. The adsorbed quantity at equilibrium reached 350 mg/g. The thermal and acid activation of this material improved the adsorption capacity up to 500 mg/g. The adsorption equilibrium of the MB/clay system is most suitably described by the Toth model.
Table 2 Freundlich, Langmuir and Toth equation parameters for the methylene blue adsorption. Thermal activation
Freundlich kF mF R2 Langmuir qL (mg/g) kL (dm3/mg) R2 Toth qT (mg/g) kT (dm3/mg) mT R2
140 6.8 0.97
93 4.6 0.97
153 6.7 0.995
101 4.1 0.98
87 3.8 0.98
83 3.8 0.98
87 4.3 0.98
91 5.4 0.992
24 2.7 0.994
140 6.8 0.97
91 3.6 0.96
55 3.7 0.96
42 3.8 0.98
35 3.7 0.96
30 5.3 0.96
384 0.021 0.98
423 0.013 0.990
421 0.021 0.9990
539 0.011 0.992
531 0.010 0.993
499 0.010 0.993
437 0.012 0.994
331 0.016 0.998
311 0.006 0.998
384 0.021 0.98
558 0.016 0.995
337 0.015 0.98
237 0.017 0.98
205 0.017 0.98
99 0.041 0.997
345 0.006 17 0.998
365 0.006 4 0.9994
400 0.012 1.5 0.9993
491 0.008 1.6 0.993
475 0.007 1.7 0.994
447 0.007 1.7 0.994
377 0.006 3.0 0.998
303 0.008 2.0 0.9994
322 0.006 0.9 0.998
345 0.006 17 0.998
545 0.015 1.1 0.996
358 0.020 0.8 0.98
308 0.052 0.5 0.993
215 0.021 0.8 0.990
101 0.047 0.9 0.997
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Acknowledgements This work was supported by the CNRST-Morocco and the Spanish Agency of International Cooperation-AECI (A/2825/05 and A/6525/ 06). S.A.K. acknowledges the ﬁnancial support from the Spanish Ministry of Education and Science through the Ramon-y-Cajal program.
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