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Activating natural bentonite as a cost-effective adsorbent for removal of Congo-red in wastewater
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JIEC-1979; No. of Pages 9 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Activating natural bentonite as a cost-effective adsorbent for removal of Congo-red in wastewater Manjot Toor a, Bo Jin a,*, Sheng Dai a, Vipasiri Vimonses b a b
School of Chemical Engineering, The University of Adelaide, Adelaide 5005, SA, Australia Santos GLNG Project, 32 Turbot Street, Brisbane 4455, QLD, Australia
A R T I C L E I N F O
Article history: Received 14 January 2014 Accepted 8 March 2014 Available online xxx Keywords: Activation Adsorption Bentonite Congo red Surface properties
A B S T R A C T
The bentonite is a widely available and abundant natural mineral, and can be a low cost adsorbent for water and wastewater treatment. This study reported here was directed towards identifying a costeffective activation protocol for enhancing the adsorption capacity of Australian bentonite for removal of toxic contaminants in wastewater. We investigated three protocols including thermal activation (TA), acid activation (AA), and combined acid and thermal activation (ATA). The results showed that these activation protocols under designed conditions can enhance the surface area and porosity of the raw bentonite. The best ATA protocol considered here brought a 70% increase in the surface area compared to 65% and 20% for the best AA and TA protocols, respectively. The optimal ATA protocol identified in the study leads to approximately 25% increase in the Congo-red adsorption capacity of the activated bentonite. This activation method could be a cost-effective approach to enhance the adsorption capacity, applicability and selectivity of natural clay materials, making them as promising and low cost adsorbents for wastewater treatment. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Clays are finding growing application in wastewater treatment as adsorbents due to their wide availability, low-cost, and good intrinsic adsorption characteristics. There are a number of natural clays that are used for the removal of chemical pollutants from wastewater, including kaolin, bentonite and zeolite [1–6]. Among these clays, bentonite is one of the most widely used adsorbents as it possesses a net negative surface charge, making it effective for removal of cationic compounds [6]. However, the adsorption capacity and selectivity of the natural bentonite towards anionic compounds appear to be less promising. Thus, a systematic surface modification of the bentonite is, however, necessary to enhance its adsorption capacity and possibly to improve its adsorption selectivity for the removal of anionic compounds from wastewater [7]. The chemical compositions which determine properties of the clays i.e. layer charge, cation exchange capacity, adsorption capacity and morphology, can be varied depending upon their
* Corresponding author. Tel.: +61 8 8313 7056. E-mail address: [email protected] (B. Jin).
origins. These factors play a significant role in the modification of the natural clays. A number of methods for modifying clay minerals have been studied with a view to enhancing their adsorption capacities [8,9], including acid activation [10], treatment with cationic surfactants [11], clay-rubber composite, thermal activation [12], polymer addition by interparticle polymerization [13], binding of inorganic and organic anions, and grafting of organic compounds [14]. Among these, acid activation is one of the most commonly used modification techniques because it is a simple and low-cost process [12,15]. Dyes are used extensively in industry, including in the manufacture of like textile, cosmetics, pulp and paper, paints, pharmaceuticals, and carpet, as well as in printing [3]. Many dyes are toxic and biologically non-degradable due to complex chemical structure and the presence of aromatic ring in their structure [16]. Azo dyes, for example, contain one or more azo groups with aromatic ring and sulfonate group, and can be transformed into more hazardous substances under anaerobic conditions [17]. They can cause allergy, dermatitis, skin irritation and mutation in human bodies [5]. These dyes at even 0.005 ppm in water are highly visible and can reduce light energy transfer efficiency in a photosynthesis system [18]. The removal of the dyes from wastewater before discharging it into the mainstream is, therefore, essential.
http://dx.doi.org/10.1016/j.jiec.2014.03.033 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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The adsorption of Congo red (CR) using three Australian clay minerals namely; sodium bentonite, kaolin and zeolite has been studied in our group [2,6,12]. We found that sodium bentonite was the most effective adsorption material for CR removal. The study reported here was aimed at identifying a technically and economically feasible protocol for the activation of raw Australian bentonite that would lead to a promising enhancement in its adsorption capacity to remove CR from industrial wastewater. The study focused on three commonly used methods: thermal activation (TA), acid activation (AA) and a combination of acid and thermal activation (ATA). Physical and chemical characteristics of the activated bentonites were evaluated by analyzing the surface area and pore size, and were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD).
2. Methods and materials 2.1. Materials Australian sodium bentonite (Active Gel 150) obtained from Unimin Australia Limited contains high montmorillonite and low grit [2,12]. Congo red (obtained from Labchem Ajax Finechem Australia), an anionic dye, is widely used as a surrogate indicator to simulate industrial wastewater when testing for adsorption capacity of porous materials in wastewater. Chemical composition and characteristics of the bentonite and CR were described in detail previously [6,12]. 2.2. Activation of raw bentonite 2.2.1. Thermal activation The thermal activation was performed by heating 10 g of the bentonite to the desired temperature in a range of 50–500 8C in a muffle furnace. The temperature was allowed to rise steadily to the desired temperature in 5 min. The samples were maintained at the desired temperature and were heated for predetermined time varying from 10 to 120 min. The heated samples were then cooled and stored in a desiccator. 2.2.2. Acid activation The acid activation was carried out in a 250 mL breaker with 100 mL working volume. The samples were mixed properly in a rotary shaker (Ratek OM 15 orbital) at 100 rpm for 10 min. The raw bentonite was treated with HCl at a determined concentration of 0.05–0.5 M at 60–100 8C. The acid to clay ratio was 1:10 (mL/g). The acid activation was terminated with addition of large amount of double-ionized water. The acidified bentonite was then washed several times until Cl ion was undetectable in the supernatant using 0.1 M silver acetate solution. The final sample was centrifuged at 3000 rpm and was dried at 55 8C for 12 h and then stored in a desiccator. 2.2.3. Combined acid and thermal activation The combined acid and thermal activation of the bentonite was conducted by a two-step procedure. The bentonite was first activated by HCl over a concentration range from 0.05 to 0.5 M, as described above. The acidified bentonite was then subjected to thermal activation at a low temperature range of 50–150 8C for 20 min in a muffle furnace. The samples were cooled in a desiccator.
Brunauer–Emmett–Teller (BET) Surface Area Analysis and Barrett–Joyner–Halenda (BJH) Pore Size Analyser. BET specific surface area and pore size measurements were performed using a Micromeritics gas adsorption analyser (Gemini Type 2375) at 77 0.5 K in liquid nitrogen. Prior to the surface analysis, the sample vessels loaded with ca. 0.5–1.0 g were vacuum treated 12 h at 105 8C and evacuation pressure of 50 mTorr. Nitrogen adsorption isotherms of the samples were then analysed for the specific surface area using the BET equation. 2.3.2. Fourier transform infra red spectra FTIR spectroscopy was used to understand the effect of the acidification on the surface chemistry of the modified bentonite. The FTIR spectra were recorded in 4000–400 cm1 using Nicolet 6700 FT-IR with Smart orbit Attenuated Total Reflectance (ATR) accessory. The ATR was fitted with Diamond crystal on powders that can be analyzed without the formation of a pellet with potassium bromide required with other standard FT-IR approaches. The infra-red spectra of the raw and modified bentonites were obtained in the powder form by placing the samples on the Diamond crystal to obtain FTIR spectra. 2.3.3. Scanning electron microscopy Morphological characteristics of the bentonite were examined using a Philips XL30 SEM at accelerating voltage of 15 kV, beam size 3.0, working distance 10 and magnification of 8000. The samples were coated with carbon under vacuum before analysis to prevent the accumulation of static electric charge on the surface of the bentonite particles. Several microscope images were taken to compare surface properties of the raw and modified bentonite samples. 2.4. Experimental design and procedure 2.4.1. Adsorption performance Adsorption of CR on bentonites was carried out in a batch system. Initial CR concentration in the experiments was set at 100 mg L1, otherwise as stated in the text. The bentonite-CR suspensions in the flasks were agitated in a rotary shaker (Ratek OM 15 orbital) at 150 rpm and 30 8C for 24 h to ensure that equilibrium was reached. The samples were then centrifuged in Eppendorf Centrifuge 5415R (Germany) at 3200 rpm for 20 min to separate the CR solution from the adsorbent. All sampling and tests were conducted in triplicates. Data were calculated from the average of the triplicates. 2.4.2. Analysis and calculation of adsorption capacity The CR concentration was determined by UV–visible spectrophotometry (model g, Helios, UK) at 496.5 nm [6]. The absorbance concentration profile was obtained by plotting the calibration curve between the absorbance and CR concentration. The absorbance for each sample was converted using the calibration factor obtained from the calibration curve to calculate the final dye concentration. The amount of dye adsorbed on the surface of adsorbent at time t can be calculated from the mass balance equation:
qe ¼ ðC i C e Þ
V m
2.3. Characterization of bentonite 2.3.1. Surface area and pore size The specific surface area and average pore size of the bentonite samples were analyzed using, respectively,
where qe is the amount of dye adsorbed per unit mass of adsorbent (mg g1), Ci is the initial dye concentration (mg L1), Ce is the equilibrium dye concentration (mg L1), V is the volume of dye solution (mL) and m is the mass of bentonite (g).
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3. Results and discussion 3.1. Characteristics of activated bentonite 3.1.1. Surface area Fig. 1 presents the specific surface areas of the activated bentonite by AA, TA and ATA. The surface area of the TA bentonite increased with the temperature increased up to 100 8C and then gradually decreased beyond 100 8C. Thermal activation under a high temperature can remove water molecules and other impurities. The initial increase in the surface area with temperature is due to the removal of adsorbed and hydrated water molecules, and volatile organic compounds attached on the surface of the raw bentonite, whilst the calcination at a higher temperature can alter the chemical and physical properties of the bentonite [19]. However the changes in structure and composition upon heating can be variable, depending on the chemical composition of the clays and the heating regime [1,20]. Excessive heating may lead to irreversible collapse of structure and interlayer spaces of the clays [21]. This collapse of the interlayer spaces brings particles closer to one
Fig. 1. BET specific surface area of bentonite activated by (a) thermal activation (SD 0.12), (b) acid activation (SD 0.17) and (c) combined acid and thermal activation (SD 0.21).
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another, resulting in decreasing surface area [22]. Therefore, TA needs to be carried out in a particular temperature range. Our results confirmed that TA at 100 8C could be favorable for enlarging the surface area of the bentonite, leading to more than 20% increase in surface area (33.15 m2 g1) compared to the raw ¨ nal and Sarıkaya [23] examined the bentonite (25.70 m2 g1). O thermal activation of bentonite in a range of 100 8C to 1300 8C, and found that the surface area increased as the temperature increased and then decreased at a temperature over 400 8C. To determine the possible crystal phase changes of the activated bentonite, we examined the raw and activated bentonite samples with XRD analysis (XRD, Bruker AXS D8 Advance diffractions) using a Huber Guinier Image Plate G670 with Co Ka1 radiation over an angular range of 5–908. The XRD patterns were recorded on a Shimadzu LabX-600 diffractometer with Cu Ka radiation (l = 0.1548 nm). XRD is the basic technique to determine the bulk structure and composition of materials with crystalline structure. Our XRD measurements showed that the original bentonite is mainly composed of amorphous phase and contains less than 10% crystal phase present in the form of quartz [2]. Unlike bentonite, raw kaolin and zeolite had more than 70% crystal phase of total mass of the clays in the form of quartz, aluminite, and diaspora [2]. Our XRD patterns of the activated bentonite showed that a minor change of crystal phase was found in the bentonite activated by either AA using HCI at a concentration lower than 0.2 M, and or TA at a temperature lower than 200 8C (data not shown). We can assume this minor change of crystal phase would have limited impact on surface properties of the bentonite activated at a low temperature and low acid concentration as conducted in this study. Generally, the surface area of the AA bentonite increases with acid concentration until a maximum surface area is reached [24]. Fig. 1b shows that the surface area increased with acid concentration from 0.05 M to 0.1 M. A maximum surface area (72.86 m2 g1) was obtained by the bentonite activated using 0.1 M HCl. This increase is likely attributed to the removal of impurities, replacement of exchangeable cations (K+, Na+, Ca2+) with hydrogen ions, and leaching of Al3+, Fe3+ and Mg2+ from the octahedral and tetrahedral sites in the bentonite which exposes the edges of platelets [25]. Further increase in HCl concentration beyond 0.1 M resulted in a decrease in the surface area. This is due to the deeper penetration of HCl into the voids and excessive leaching of Al3+, Fe3+ and Mg2+, leading to the collapse of layered structure and close packing of particles [26]. In our study the surface area of the AA bentonite at the optimum condition was recorded as 72.86 m2 g1 (0.1 M HCl). SiO2, Al2O3 and CaO, which are major components of the studied bentonite, have zero point charge values of 2.2, 8.3 and 11, respectively [2,19]. At an acidic pH, the negative charge of silica sites of adsorbent is counterbalanced by H+ ions hence reducing hindrance to diffusion of dye ions. Whereas, there is a significant increase in electrostatic attractions between negative charges of anionic dyes and positive charges of alumina and calcia sites, thereby increasing dye adsorption [12,21]. As the system pH increases, the number of positively charged sites raises and the number of negatively charged sites declines [6]. An integration of acid and thermal activations was subsequently tested. We found that a larger surface area of the activated bentonite can be gained through the ATA (Fig. 1c). The surface area increased to 84.12 m2 g1 when AA (0.1 M HCl) was followed by AT at 50–100 8C. The results suggested that the acid activation of the bentonite with 0.1 M HCl at 50 8C can be employed as an economical technique for modifying bentonite to enhance its surface properties and adsorption capacity. The surface properties of the acid activated clays can be enhanced further if the acid activation is followed by the thermal activation [27].
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We summarize the changes of surface area of the raw and AA bentonite from other workers and compared their results with those observed in this study. Data shown in Table 1 indicate that the enhancement of the surface areas obtained in this study appeared to be similar to those obtained by previous studies. However, the concentration of HCI (0.1 M) used in our study was far lower than those cases (>2 M) [25–27]. This suggests that the bentonite adsorbents obtained here may be a particularly suitable for the water treatment due to less acid use, while they provided larger surface area for the adsorption of organic compounds. The AA bentonite using low concentration acid results in a low cost and low chemical use activation protocol, which can be economically and environmentally beneficial. 3.1.2. Pore size The pore size analysis of the bentonite samples was carried out to understand the variation in the pore size from the raw bentonite to the activated ones (Table 2). The thermal activation is the most commonly used physical method to remove the water molecules in the clay particles [28]. The porosity of clay can be affected by the adsorbed and hydrated water molecules in the bentonite. Table 2a shows the variation in average pore sizes of the TA bentonite with respect to temperature changes. An initial decrease in the pore size corresponded to the increase in temperature. This is due to the removal of adsorbed water and formation of mesopores and micropores. During this dehydration, which is a reversible process, the adsorbed and hydrated water attached on the bentonite surface can be removed [23]. The external surface and mesopores are the first to undergo a change on thermal activation, resulting in decreasing pore size. A further temperature increase beyond 100 8C resulted in enlarging in the pore size. The second stage of TA is known as dehydroxylation which is an irreversible process. The water molecules in the interlayer spaces are removed, and the interlayer spaces collapse, where the particles come closer to one another and start forming aggregates. The dehydroxylation leads to the transformation of micro and mesopores into macropores, resulting in the increase in the average pore size [21]. Heating the bentonite over the dehydroxylation temperature can collapse its structure completely due to destruction of the layered structure of the bentonite. Calcination may lead to a movement of octahedral cations within the octahedral sheet. In addition, the change in structure of the bentonite after calcinations is also attributed to the exchange of cations in the interlayers [29]. Bojemueller et al. [30] stated that the edges of the layers are attacked at high temperatures. The destruction of the edges results in the release of aluminum or hydroxyaluminum cations, and consequently forming more mesopores. The collapse of interlayer spaces may result in damaging the multi-layered structure of clay [15,21]. As a result, clay mineral loses it originality, forming new phases, such as mullite, cristobalite and feldspars with different properties than the parent clay [1,31].
Table 2 Average pore size of bentonite activated by (a) TA, and (b) AA and ATA. (a) Temperature (8C)
Pore size (A˚)
Raw bentonite (RB) 50 100 200 300 400 500
50.28 0.27 46.13 0.05 41.12 0.11 48.19 0.42 51.65 0.22 50.95 0.45 56.73 0.66
(b) Acid concentration (M)
0.05 0.075 0.10 0.25 0.50
AA bentonite
ATA bentonite
Pore size (A˚)
Pore size (A˚)
49.36 0.39 48.89 0.27 42.08 0.17 46.17 0.54 53.78 0.31
48.57 0.15 46.63 0.07 38.92 0.09 44.12 0.12 56.13 0.31
The changes in pore sizes of the AA and ATA bentonites are present in Table 2b. The average pore size of the AA bentonites decreased as acid concentration increased up to 0.1 M, and a further increase in acid concentration resulted in increasing the pore size. The initial decrease in pore size can be attributed to the removal of impurities and replacement of exchangeable cations by hydrogen ions which are smaller in size than the exchangeable cations [26]. Also, the decrease in pore size can be also due to unoccupied octahedral and tetrahedral spaces left by the leaching of Mg2+, Fe3+ and Al3+ by acid [28]. The subsequent increase in pore size was associated with HCl concentration beyond 0.1 M. As the interlayer space collapses the particles become closer to one another and combine together, resulting in the decrease in the number of the micropores. The empty spaces lead to an increase in the pore size, and consequently transforming micropores into mesopores. A further increase in the size of empty spaces results in the disappearance of mesopores at some locations due to disintegration of the crystal structure [22]. The disappearance of mesopores can lead to a significant decrease in the surface area. The results obtained here are in line with those reported elsewhere [15,27]. The average pore size of the AA (0.1 M HCl) bentonite is 42.08 A˚ which lies in the mesopore region. The pore size of the ATA bentonite (Table 2b) appeared to decrease with HCl concentration up to 0.10 M and then increased beyond 0.10 M. The mechanism of these changes in pore size is similar to that of the AA bentonite. However, an additional step of thermal activation resulted in removing the adsorbed and hydrated water, and volatile organic compounds. This mechanism creates new pores and provides more adsorption sites facilitating higher adsorption of dyes on the bentonite. The average pore size of the ATA (0.1 M HCl) is 38.916 A˚, indicating mesopore region.
Table 1 Surface area of various acid activated bentonites. Surface area of raw bentonite (m2 g1)
Surface area of activated bentonite (m2 g1)
Increase in surface area (%)
Adsorption capacity (mg g1)
Acid Con. (M)
Reference
29.23 50 50 52 25.7 80 71.95 52
165.8 250 200 184 84.12 240.9 109.8 76
82.37 80 75 71.74 69.45 66.79 34.47 31.58
5.9 6.2 5.8 6.2 7.0 5.3 5.9 4.8
2 M HCl 1 M HCl 2 M HCl 2 M HCl 0.1 M HCl 2 M H2SO4 5 M H2SO4 0.5 M HCl
Tsai et al. [25] Vimonses et al. [6] Vimonses et al. [6] Liu [14] This study Korichi et al. [26] Yildiz et al. [27] Liu [14]
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It is suggested that dehydration at a high temperature can cause the irreversible collapse of the structure. The clay platelets are bonded electrostatically. Therefore, dehydration of cations can result in reducing porosity and adsorption ability of the clay [1,15]. For instance, the study by Heller-Kallai [32] revealed a gradual increase in the strength of the studied kaolin and bentonite, and clay sediment with an increase in heating temperature. Yet, such significant and permanent increase in their strength occurred only after dehydroxylation. By continuing heating at a temperature above dehydroxylation stage, the clays became resistant to disintegration upon immersion in water. Thus, thermo-analytical measurement of the clay minerals is often performed to understand the stability of their structure against the temperature gradient. Our previous studies revealed that the kaolin demonstrated the highest stability due to the strong bond between its aluminosilicate layers [2,6,33]. In case of zeolites, the dehydration at high temperatures (>200 8C) is observed as a result of a strong interaction between the electrostatic charged framework and the water molecules [34]. In addition to this, an improvement in adsorption performance of the TA clays was also reported in many publications [32–34]. The changes of the clay structure after thermal activation at 500 8C for 5 h was evidenced by the formation of an amorphous phase in
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the TA clays. This is due to the loss of hydroxyl groups, which cause a rearrangement of the original structure of the clay. Vieira et al. [35] evaluated the morphological changes of the TA bentonite and its adsorption efficiency toward nickel in a porous bed reactor. The clay was heated at 500 8C for 24 h to increase its mechanical resistance and to eliminate some impurities. They reported that the calcination led to the increase of the clay surface area and the development of micro and mesoporosity due to bond water release and dehydroxylation occurring. However, the calcination can cause the reduction of the clay density compared to the untreated one; whilst the adsorption of nickel in clay pores resulted in a slight increase of actual density. Thus, it may conclude that thermal activation at a high temperature may not be beneficial for enhancing adsorption capacity of the bentonite for CR removal. 3.1.3. Surface morphology The SEM examination was carried out to assess the effect of the activation protocols on the surface morphology of the bentonites. Herein, we present a few representative SEM images of the raw and activated bentonites in Fig. 2. The surface of the raw bentonite (Fig. 2a) appears to be smooth due to closely packed flakes in contrast to the ragged appearance of the TA bentonite (Fig. 2b and c). Thermal activation at 100 8C can make the bentonite more
Fig. 2. SEM images of (a) raw bentonite, TA bentonite at (b) 100 8C and (c) 500 8C; AA bentonite using (d) 0.075 M HCl, (e) 0.1MHCl and (f) 0.5MHCl; and ATA bentonite using (g) 0.075 M HCl, (h) 0.1MHCl and (i) 0.5MHCl at 100 8C for 20 min.
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porous structure compared to raw bentonite (Fig. 2a and b). These results are also in accordance with the specific surface areas of the TA bentonite (Fig. 1a). The SEM image of the TA bentonite indicates that the interlayer spaces may have been collapsed, resulting in more tightly bound structure with reduction in porous structure at 500 8C (Fig. 2c). The surface morphology of the AA bentonite, as shown in Fig. 2d–f, indicates that the AA results in leaching of cations, making the clay surface more porous. Using 0.075 M HCl clumps of uneven surface can be formed along with flat flakes of low porosity (Fig. 2d). As the increase in acid concentration to 0.1 M the surface becomes highly porous, contributing more pores generated (Fig. 2e). Further increase in acid concentration reduced the porosity rendering clay surface rather flat (Fig. 2f). Fig. 2g–i show that the surface morphology of the raw bentonite has undertaken significant changes under the ATA, i.e. the bentonite was activated using HCl at 0.075–0.5 M, and followed by heating 100 8C for 20 min. The edges of the platelets were first to be exposed to acid attack. The formation of smaller pores takes place as the impurities are removed and the exchangeable cations are replaced by H+ ions. These physio-chemical reactions alter the morphology as the pores open up. Thus, the bentonite surface became more porous and homogeneous. The SEM images revealed that after the thermal activation of the AA (0.075 M HCl) bentonite the clay has become more porous (Fig. 2g). The bentonite activated by 0.1 M HCl followed by TA exhibited highly porous structure (Fig. 2h). This was also evidenced from the increase in surface area. Comparing the SEM images in Fig. 2f with Fig. 2i, it is interesting to note that a combination of TA (100 8C) with AA (0.5 M HCl) resulted in forming more pores on bentonite surface than the AA bentonite using by 0.5 M HCl. 3.2. Chemical composition of activated bentonite FTIR studies of the activated bentonites assist the identification of the minerals present in the bentonite. The coupled vibrations are appreciable due to the availability of various constituents. To recognize mineral species and identify characteristic bands of the bentonite, the FTIR spectra were performed in the range from 4000 to 400 cm1 to investigate the effect of activation on the chemical composition of the bentonite. Published collections of spectra of clay minerals as those reported by Palanivel and Velraj [36] are very useful for identification and characterization of the bentonite mineral. Fig. 3 shows that there are three main absorption regions: 3000–3800 cm1, 1300–1800 cm1, and 500–1200 cm1, and sharp differences could be found in each region of the AA and TA bentonite samples. The IR spectrum of raw bentonite indicates that montmorillonite is the dominant mineral phase in this bentonite (Fig. 3). The absorption band at 3620 cm1 is due to stretching vibrations of structural OH groups of montmorillonite. The KBr curve of natural bentonite was characteristic of montmorillonite with a single sharp band at 3620 cm1 and a broad band at 3435 cm1 of the OH stretching vibration of structural hydroxyl groups and water. In the lower frequency region, montmorillonite had a strong band at 1045 cm1 ponding to the Si–O stretching (in-plane) vibration of bentonite mineral. The absorption band at 1635 cm1 was attributed to the OH deformation mode of water. IR peaks at 916 cm1 was attributed to Al–OH–Al. The 692 cm1 band corresponded to coupled Al–O and Si–O out-of-plane vibrations. Quartz was present as indicated by the bands at 791 cm1. Water molecules exhibit three fundamental vibrational modes—an asymmetric stretching (n3), a symmetric (n1) stretching and an H–O–H bending (n2) [37]. In the bentonite–water systems, the positions of the water absorption bands are affected by physicochemical properties of the bentonite matrix. The stretching
Fig. 3. FTIR spectra of raw and activated bentonites by (a) AA at different HCl concentrations, and (b) TA at different temperatures for 20 min.
vibrations for H2O in transmittance spectra of hectolitre were reported at 3620–3640 cm1 for the asymmetric (n3) and 3430– 3580 cm1 for the symmetric (n1) band [36]. Our IR spectrum of raw bentonite shows that the intensity of the n1 vibration (3435 cm1) is weaker than the n3 vibration (3620 cm1). 3.2.1. Effect of acid activation The changes in the functional groups provide an indication of the changes of the chemical composition of the AA bentonite. FTIR spectra for the raw and AA bentonites (Fig. 3a) assist us to understand the effect of AA on the chemical structure of bentonite. During the course of the AA the protons penetrate into the bentonite layers attacking the OH groups, leading to the alteration in the adsorption bands attributed to the OH vibrations and octahedral cations. The intensity of stretching bands observed at 3620 cm1 (Al–OH–Al along with the Al–Mg–OH stretching vibrations) decreased with the acid concentration [4,5]. The increase in the acid concentration resulted in the decrease of the peaks of the bands associated with the adsorbed water at 3435 (H–O–H stretching) and 1635 cm1 (H–O–H bending). It is interesting to note that the band at 3435 cm1 became broader, while the peak at 916 cm1 for Al–OH–Al appeared to be week for the AA bentonite using 0.075 M and 0.1 M HCl. The band at 916 cm1 was disappeared for the AA bentonite using 0.5 M HCl. The peak of Si–O–Si band at 988 cm1 remained unchanged with the intensity of acid concentration. Similar results have been reported by Christidis et al. [7]. The bands at 916 cm1 were disappeared with the increase in acid concentration due to the
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increased substitution of Al2+ ions by Fe3+ ions. The transformation of the tetrahedral sheet was found at 791 cm1. The AA leads to the formation of amorphous silica, indicated by the increased intensity of the peak, which may provide more adsorption sites [38]. The intensity of band at 692 cm1 (Al–OH–Si bending) decreased with the increase in acid concentration, signifying the partial dissolution of aluminum ions present in the octahedral sheet of bentonite. This band was disappeared from the AA bentonite using HCl at a concentration higher than 0.25 M. FTRI is sensitive to the changes of the bentonite mineral structure upon acid activation [37]. During the acid attack protons penetrate into the bentonite mineral layers and attack the structural OH groups. The resulting dehydroxylation is connected with successive release of the central atoms from the octahedral as well as with removal of Al from the tetrahedral sheets. At the same time a gradual transformation of the layered tetrahedral sheet to a treedimensional framework proceeds. The final reaction product is an amorphous partly protonated silica phase. However, for industrial applications, complete decomposition of the parent internal structure is usually not wanted. 3.2.2. Effect of activation temperature Fig. 3b shows the FTIR spectra of the bentonites heated at 60 8C, 80 8C and 100 8C. The increase in activation temperature resulted in the deeper penetration of protons into the clay layers, decreasing the intensity of peaks at 3620 cm1 (Al–OH–Al along with the Al–Mg–OH stretching vibrations), 3435 cm1 (H–O–H stretching) and 1634 cm1 (H–O–H bending) [5,37]. The peak of Si–O–Si band at 1045 cm1 remains unchanged. The bands at 916 cm1 were disappeared when the bentonite was heated at a temperature higher than 80 8C. The intensity of the peak increases at 791 cm1 indicates the alteration of tetrahedral sheet [22,36]. The intensity of band at 692 cm1 (Al–OH–Si bending) decreased as temperature increased, showing the dissolution of Al ions that present in the octahedral sheet of the bentonite. FTIR results revealed that the temperature rise results in the disintegration of structure at a given acid concentration and reaction time. The effect of heating temperature and time on the thermal stability of the bentonite was previously studied in our group [2]. Differential thermal analysis (DTA) of the bentonite was carried out to analyze the effect of heating on the surface and structural properties of the bentonite. The thermogravimetric analysis (TGA) was also performed to estimate the mass of water evolved corresponding to the weight loss [15]. Results of DTA and TGA presented in Fig. 4 indicate that the dehydration stage corresponding to the removal of adsorbed and hydrated water occurred at 85 8C. This is the first endothermic peak on DTA plot and the corresponding derivative weight loss on TGA plot. It could be seen that the maxima of the DTA curve occurs at 127 8C. The removal of adsorbed and hydrated water provides additional adsorption sites, and leading to the increase in surface area. Further heating indicates the transition from dehydration stage to dehydroxylation stage. The second endothermic peak appears at 640 8C, showing the maximum exothermic breaking point at 985 8C. Heating beyond these temperatures results in collapsing the interlayer spaces, and consequently rupturing the clay structure. This structure rupture can cause morphology changes of the bentonite minerals, which is supported by the decrease in surface area at high temperatures, as discussed in the previous section. The DTA plot also suggests that the thermal activation of bentonite should be carefully carried out in the temperature range between 85 8C and 640 8C (Fig. 4a). Heating above the hydroxylation stage may cause irreversible damage to the structure of bentonite. The total mass loss of 15% is observed in the temperature range of 50 8C to 750 8C as shown in Fig. 4b. The weight loss in the dehydration stage was 10% in the temperature
Fig. 4. (a) Differential thermal analysis profile of sodium bentonite and (b) thermogravimetric profile of sodium bentonite [2].
range of 50 to 200 8C and further heating with increase in temperature shows 4% weight loss in the temperature range of 200 to 750 8C. 3.3. Adsorption capacity of the activated bentonite 3.3.1. Thermal activation: Heating temperature and time Here, we investigated how the TA affects the CR adsorption performance and capacity of the bentonite with respect to activation temperature and heating time. The CR adsorption increased with an increase in the temperature (Fig. 5). The increase in CR adsorption is likely attributed to the enlarged specific surface area and the removal of bound water from interlayer spaces. The maximum CR adsorption was given by the bentonite which was activated at 100 8C, corresponding to the maximum specific surface area. Similar results have been reported by Koyuncu [39] for the adsorption of metolachor on thermal activated bentonite. In contrast, the CR adsorption was found to decrease as temperature varied from 200 8C to 500 8C. This decrease in CR adsorption is likely due to the decrease in surface area and the availability of adsorption sites affecting the adsorption capacity of TA bentonite. Similar results have been reported by Aytas et al. [29] who studied the adsorption of Uranium (VI) on TA bentonite. It could be inferred from the results that TA at 100 8C for 20 min can enhance the adsorption capability of bentonite by 15%. To investigate the effect of heating time, the thermal activation was carried out in a reaction time ranging from 20 min to 2 h. Fig. 5 shows that the CR adsorption onto the TA bentonite increased with heating time at all temperature ranges, while the maximum
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Fig. 5. Effect of heating time on CR adsorption on TA bentonite (initial dye concentration: 100 mg L1, bentonite dosage 1 g L1), [data obtained from three replicated at each trial: 50 8C (SD 0.15), 100 8C (SD 0.18), 200 8C (SD 0.21), 500 8C (SD 0.24)].
adsorption was achieved in 1 h 40 min heating. In contrast, a low CR adsorption was observed with the prolonged heating over 1.5 h. The results indicate that apart from temperature, heating time also influences the adsorption capacity of the TA bentonite. If the heating is continued over the applicable range it may result in the destruction of the structure of the bentonite. The increase in heating time from 20 min to 1 h and 40 min contributed only a 5% increase in adsorption (Fig. 5). Therefore, from an economical point of view, 20 min should be considered as a sufficient heating time for the thermal activation of bentonite. Thus, otherwise as stated, 20 min heating time was used in the following experiments. 3.3.2. Acid activation Fig. 6 shows that CR adsorption increased with an increase in acid concentration until 0.1 M HCl. The maximum amount of Congo red was adsorbed on the AA bentonite using 0.1 M HCl. The increase in acid concentration beyond 0.1 M HCl results in excessive leaching of cations and the collapse of interlayer spaces. Bentonite is a rock formed, and highly colloidal and plastic clay, which is composed mainly of a very soft montmorillonite [2,12]. The montmorillonite is a clay mineral of the smectite group, which has a small amount of other contents, such as feldspar, biotite, illite and crystalline quartz [2,22,37]. Special properties of the bentonite are its ability to absorb large quantities of water and form
thixotrophic gels, with an accompanying increase in volume up to several times of its dry bulk. A crystalline structure of the bentonite is known as 2:1 type aluminosilicate, presenting an octahedral alumina between two tetrahedral silica layers [39]. These three layer units are stacked one above another with oxygen molecules in neighbouring layers adjacent to each other [6]. The parallel layers of the clay structure are held together by weak electrostatic forces, allowing water and other polar molecules to enter between layers and induce an expansion of mineral structure [40]. Substitutions of ions of lower valence within the lattice structure, for instance Al3+ for Si4+ in the tetrahedral sheet and Mg2+ for Al3+ in the tetrahedral layer, result in an excess of negative charge in the lattice [1,15]. These imbalanced charges in the interlayer space are equalised by the dominant cations, typically Na+, K+, Mg2+, and Ca2+. These cations can be released into water under acidic condition and are exchangeable due to their loose binding together with broken bonds with bentonite, which are attributed to a high cation exchange capacity of bentonite [20,33]. Adsorption capacity of activated bentonites from different studies is listed in Table 1. Our AA bentonite demonstrated a comparably high adsorption capacity. It is interesting to note that concentration of the HCl used in our study is very low. 3.3.3. Combined acid and thermal activation We experimentally assessed that adsorption capability of the ATA bentonite which was first activated by HCl at 0.05–0.5 M, followed by thermal activation at 50, 100 and 150 8C for 20 min. Adsorption profile data (Fig. 6) show that the CR adsorption increased with the acid concentration regardless the variation in the heating temperature. However, a high activation temperature resulted in a decrease in the CR adsorption (Fig. 6). At a given HCl concentration, the increase in the temperature corresponded to a shorter time required to achieve the maximum surface area. Babaki et al. [28] proposed a relationship between the temperature and the optimum time required. They performed the acid activation of bentonite by sulfuric acid. The results revealed that as the temperature increased, a shorter heating time is required to achieve the maximum surface area. Maximum CR adsorption of approximately 7.0 mg g1 was obtained by the ATA bentonite with 0.1 M HCl at 100 8C. Activated carbon has been used as the most commonly used commercial adsorbent for industrial water and wastewater treatment process for many decades due its high removal efficiency and capacity. To enhance its specific and overall absorption capacity for removal of toxic contaminants by incorporation nanoparticles in the activated carbon has drown an interest in recent studies [41–43]. However, the high costs, carbon source use and carbon emission associated with the production and application of activated carbon has been a challenge. In this study, we explored the cost-effective activation protocol to enhance adsorption capacity of the natural bentonite as adsorbent for removal toxic dyes from industrial wastewater. Our results showed that the ATA bentonite (0.1 M HCl + 100 8C) performed the highest adsorption capacity of approximately 7.0 mg g1, followed by the AA bentonite (0.1 M HCl) with 6.4 mg g1, while the lowest adsorption improvement was given by the TA bentonite. Compared with adsorption capacity of the raw bentonite, the ATA bentonite led to approximately 25% increase in removal of Congo red. 4. Conclusions
Fig. 6. Effect of activation temperature on the adsorption capacity of AA bentonite (initial dye concentration: 100 mg L1, bentonite dosage 1 g L1, [data obtained from three replicated at each trial: 50 8C (SD 0.15), 100 8C (SD 0.18), 150 8C (SD 0.22)].
From this study, we have identified a low cost and efficient activation protocol for enhancing the adsorption capacity of natural bentonite. The activation processes, which were carried out at low acid concentration and low heating temperature, could
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significantly enhance specific surface area and pore size, and alter surface morphology of the bentonite. The results revealed that thermal activation at 100 8C for 20 min exhibited more than 20% increase in surface area of the bentonite. However an increase beyond 100 8C expressed a decrease in surface area due to degeneration of the crystal structure. Acid activation (0.1 M HCl) resulted in enlarging up to 65% surface area of the raw bentonite. Overall, approximately 70% increase in surface area was observed for the bentonite activated by the combined acid and thermal activation. The adsorption capacity of raw bentonite was also enhanced by the activations and followed the order: ATA > AA > TA > RB. The overall adsorption capability of bentonite modified by ATA was augmented by more than 25%. The bentonite is a widely available and abundant natural mineral, and can be a low cost adsorbent for water and wastewater treatment. Our experimental results revealed that bentonite modified by ATA (0.1 M HCl) for 20 min followed by thermal activation at 100 8C can significantly enhance the surface and adsorption properties of raw bentonite. This activation method could be a cost-effective approach for clay bentonite modification to enhance its adsorption capacity and applicability, making it as promising adsorbent for water and wastewater treatment. Acknowledgment The authors thank the support from Bionanotechnology Laboratory: Water, Energy and Materials (BioNanoTech) at The University of Adelaide, Australia. References [1] [2] [3] [4]
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Activating natural bentonite as a cost-effective adsorbent for removal of Congo-red in wastewater Manjot Toor a, Bo Jin a,*, Sheng Dai a, Vipasiri Vimonses b a b
School of Chemical Engineering, The University of Adelaide, Adelaide 5005, SA, Australia Santos GLNG Project, 32 Turbot Street, Brisbane 4455, QLD, Australia
A R T I C L E I N F O
Article history: Received 14 January 2014 Accepted 8 March 2014 Available online xxx Keywords: Activation Adsorption Bentonite Congo red Surface properties
A B S T R A C T
The bentonite is a widely available and abundant natural mineral, and can be a low cost adsorbent for water and wastewater treatment. This study reported here was directed towards identifying a costeffective activation protocol for enhancing the adsorption capacity of Australian bentonite for removal of toxic contaminants in wastewater. We investigated three protocols including thermal activation (TA), acid activation (AA), and combined acid and thermal activation (ATA). The results showed that these activation protocols under designed conditions can enhance the surface area and porosity of the raw bentonite. The best ATA protocol considered here brought a 70% increase in the surface area compared to 65% and 20% for the best AA and TA protocols, respectively. The optimal ATA protocol identified in the study leads to approximately 25% increase in the Congo-red adsorption capacity of the activated bentonite. This activation method could be a cost-effective approach to enhance the adsorption capacity, applicability and selectivity of natural clay materials, making them as promising and low cost adsorbents for wastewater treatment. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Clays are finding growing application in wastewater treatment as adsorbents due to their wide availability, low-cost, and good intrinsic adsorption characteristics. There are a number of natural clays that are used for the removal of chemical pollutants from wastewater, including kaolin, bentonite and zeolite [1–6]. Among these clays, bentonite is one of the most widely used adsorbents as it possesses a net negative surface charge, making it effective for removal of cationic compounds [6]. However, the adsorption capacity and selectivity of the natural bentonite towards anionic compounds appear to be less promising. Thus, a systematic surface modification of the bentonite is, however, necessary to enhance its adsorption capacity and possibly to improve its adsorption selectivity for the removal of anionic compounds from wastewater [7]. The chemical compositions which determine properties of the clays i.e. layer charge, cation exchange capacity, adsorption capacity and morphology, can be varied depending upon their
* Corresponding author. Tel.: +61 8 8313 7056. E-mail address: [email protected] (B. Jin).
origins. These factors play a significant role in the modification of the natural clays. A number of methods for modifying clay minerals have been studied with a view to enhancing their adsorption capacities [8,9], including acid activation [10], treatment with cationic surfactants [11], clay-rubber composite, thermal activation [12], polymer addition by interparticle polymerization [13], binding of inorganic and organic anions, and grafting of organic compounds [14]. Among these, acid activation is one of the most commonly used modification techniques because it is a simple and low-cost process [12,15]. Dyes are used extensively in industry, including in the manufacture of like textile, cosmetics, pulp and paper, paints, pharmaceuticals, and carpet, as well as in printing [3]. Many dyes are toxic and biologically non-degradable due to complex chemical structure and the presence of aromatic ring in their structure [16]. Azo dyes, for example, contain one or more azo groups with aromatic ring and sulfonate group, and can be transformed into more hazardous substances under anaerobic conditions [17]. They can cause allergy, dermatitis, skin irritation and mutation in human bodies [5]. These dyes at even 0.005 ppm in water are highly visible and can reduce light energy transfer efficiency in a photosynthesis system [18]. The removal of the dyes from wastewater before discharging it into the mainstream is, therefore, essential.
http://dx.doi.org/10.1016/j.jiec.2014.03.033 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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The adsorption of Congo red (CR) using three Australian clay minerals namely; sodium bentonite, kaolin and zeolite has been studied in our group [2,6,12]. We found that sodium bentonite was the most effective adsorption material for CR removal. The study reported here was aimed at identifying a technically and economically feasible protocol for the activation of raw Australian bentonite that would lead to a promising enhancement in its adsorption capacity to remove CR from industrial wastewater. The study focused on three commonly used methods: thermal activation (TA), acid activation (AA) and a combination of acid and thermal activation (ATA). Physical and chemical characteristics of the activated bentonites were evaluated by analyzing the surface area and pore size, and were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD).
2. Methods and materials 2.1. Materials Australian sodium bentonite (Active Gel 150) obtained from Unimin Australia Limited contains high montmorillonite and low grit [2,12]. Congo red (obtained from Labchem Ajax Finechem Australia), an anionic dye, is widely used as a surrogate indicator to simulate industrial wastewater when testing for adsorption capacity of porous materials in wastewater. Chemical composition and characteristics of the bentonite and CR were described in detail previously [6,12]. 2.2. Activation of raw bentonite 2.2.1. Thermal activation The thermal activation was performed by heating 10 g of the bentonite to the desired temperature in a range of 50–500 8C in a muffle furnace. The temperature was allowed to rise steadily to the desired temperature in 5 min. The samples were maintained at the desired temperature and were heated for predetermined time varying from 10 to 120 min. The heated samples were then cooled and stored in a desiccator. 2.2.2. Acid activation The acid activation was carried out in a 250 mL breaker with 100 mL working volume. The samples were mixed properly in a rotary shaker (Ratek OM 15 orbital) at 100 rpm for 10 min. The raw bentonite was treated with HCl at a determined concentration of 0.05–0.5 M at 60–100 8C. The acid to clay ratio was 1:10 (mL/g). The acid activation was terminated with addition of large amount of double-ionized water. The acidified bentonite was then washed several times until Cl ion was undetectable in the supernatant using 0.1 M silver acetate solution. The final sample was centrifuged at 3000 rpm and was dried at 55 8C for 12 h and then stored in a desiccator. 2.2.3. Combined acid and thermal activation The combined acid and thermal activation of the bentonite was conducted by a two-step procedure. The bentonite was first activated by HCl over a concentration range from 0.05 to 0.5 M, as described above. The acidified bentonite was then subjected to thermal activation at a low temperature range of 50–150 8C for 20 min in a muffle furnace. The samples were cooled in a desiccator.
Brunauer–Emmett–Teller (BET) Surface Area Analysis and Barrett–Joyner–Halenda (BJH) Pore Size Analyser. BET specific surface area and pore size measurements were performed using a Micromeritics gas adsorption analyser (Gemini Type 2375) at 77 0.5 K in liquid nitrogen. Prior to the surface analysis, the sample vessels loaded with ca. 0.5–1.0 g were vacuum treated 12 h at 105 8C and evacuation pressure of 50 mTorr. Nitrogen adsorption isotherms of the samples were then analysed for the specific surface area using the BET equation. 2.3.2. Fourier transform infra red spectra FTIR spectroscopy was used to understand the effect of the acidification on the surface chemistry of the modified bentonite. The FTIR spectra were recorded in 4000–400 cm1 using Nicolet 6700 FT-IR with Smart orbit Attenuated Total Reflectance (ATR) accessory. The ATR was fitted with Diamond crystal on powders that can be analyzed without the formation of a pellet with potassium bromide required with other standard FT-IR approaches. The infra-red spectra of the raw and modified bentonites were obtained in the powder form by placing the samples on the Diamond crystal to obtain FTIR spectra. 2.3.3. Scanning electron microscopy Morphological characteristics of the bentonite were examined using a Philips XL30 SEM at accelerating voltage of 15 kV, beam size 3.0, working distance 10 and magnification of 8000. The samples were coated with carbon under vacuum before analysis to prevent the accumulation of static electric charge on the surface of the bentonite particles. Several microscope images were taken to compare surface properties of the raw and modified bentonite samples. 2.4. Experimental design and procedure 2.4.1. Adsorption performance Adsorption of CR on bentonites was carried out in a batch system. Initial CR concentration in the experiments was set at 100 mg L1, otherwise as stated in the text. The bentonite-CR suspensions in the flasks were agitated in a rotary shaker (Ratek OM 15 orbital) at 150 rpm and 30 8C for 24 h to ensure that equilibrium was reached. The samples were then centrifuged in Eppendorf Centrifuge 5415R (Germany) at 3200 rpm for 20 min to separate the CR solution from the adsorbent. All sampling and tests were conducted in triplicates. Data were calculated from the average of the triplicates. 2.4.2. Analysis and calculation of adsorption capacity The CR concentration was determined by UV–visible spectrophotometry (model g, Helios, UK) at 496.5 nm [6]. The absorbance concentration profile was obtained by plotting the calibration curve between the absorbance and CR concentration. The absorbance for each sample was converted using the calibration factor obtained from the calibration curve to calculate the final dye concentration. The amount of dye adsorbed on the surface of adsorbent at time t can be calculated from the mass balance equation:
qe ¼ ðC i C e Þ
V m
2.3. Characterization of bentonite 2.3.1. Surface area and pore size The specific surface area and average pore size of the bentonite samples were analyzed using, respectively,
where qe is the amount of dye adsorbed per unit mass of adsorbent (mg g1), Ci is the initial dye concentration (mg L1), Ce is the equilibrium dye concentration (mg L1), V is the volume of dye solution (mL) and m is the mass of bentonite (g).
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3. Results and discussion 3.1. Characteristics of activated bentonite 3.1.1. Surface area Fig. 1 presents the specific surface areas of the activated bentonite by AA, TA and ATA. The surface area of the TA bentonite increased with the temperature increased up to 100 8C and then gradually decreased beyond 100 8C. Thermal activation under a high temperature can remove water molecules and other impurities. The initial increase in the surface area with temperature is due to the removal of adsorbed and hydrated water molecules, and volatile organic compounds attached on the surface of the raw bentonite, whilst the calcination at a higher temperature can alter the chemical and physical properties of the bentonite [19]. However the changes in structure and composition upon heating can be variable, depending on the chemical composition of the clays and the heating regime [1,20]. Excessive heating may lead to irreversible collapse of structure and interlayer spaces of the clays [21]. This collapse of the interlayer spaces brings particles closer to one
Fig. 1. BET specific surface area of bentonite activated by (a) thermal activation (SD 0.12), (b) acid activation (SD 0.17) and (c) combined acid and thermal activation (SD 0.21).
3
another, resulting in decreasing surface area [22]. Therefore, TA needs to be carried out in a particular temperature range. Our results confirmed that TA at 100 8C could be favorable for enlarging the surface area of the bentonite, leading to more than 20% increase in surface area (33.15 m2 g1) compared to the raw ¨ nal and Sarıkaya [23] examined the bentonite (25.70 m2 g1). O thermal activation of bentonite in a range of 100 8C to 1300 8C, and found that the surface area increased as the temperature increased and then decreased at a temperature over 400 8C. To determine the possible crystal phase changes of the activated bentonite, we examined the raw and activated bentonite samples with XRD analysis (XRD, Bruker AXS D8 Advance diffractions) using a Huber Guinier Image Plate G670 with Co Ka1 radiation over an angular range of 5–908. The XRD patterns were recorded on a Shimadzu LabX-600 diffractometer with Cu Ka radiation (l = 0.1548 nm). XRD is the basic technique to determine the bulk structure and composition of materials with crystalline structure. Our XRD measurements showed that the original bentonite is mainly composed of amorphous phase and contains less than 10% crystal phase present in the form of quartz [2]. Unlike bentonite, raw kaolin and zeolite had more than 70% crystal phase of total mass of the clays in the form of quartz, aluminite, and diaspora [2]. Our XRD patterns of the activated bentonite showed that a minor change of crystal phase was found in the bentonite activated by either AA using HCI at a concentration lower than 0.2 M, and or TA at a temperature lower than 200 8C (data not shown). We can assume this minor change of crystal phase would have limited impact on surface properties of the bentonite activated at a low temperature and low acid concentration as conducted in this study. Generally, the surface area of the AA bentonite increases with acid concentration until a maximum surface area is reached [24]. Fig. 1b shows that the surface area increased with acid concentration from 0.05 M to 0.1 M. A maximum surface area (72.86 m2 g1) was obtained by the bentonite activated using 0.1 M HCl. This increase is likely attributed to the removal of impurities, replacement of exchangeable cations (K+, Na+, Ca2+) with hydrogen ions, and leaching of Al3+, Fe3+ and Mg2+ from the octahedral and tetrahedral sites in the bentonite which exposes the edges of platelets [25]. Further increase in HCl concentration beyond 0.1 M resulted in a decrease in the surface area. This is due to the deeper penetration of HCl into the voids and excessive leaching of Al3+, Fe3+ and Mg2+, leading to the collapse of layered structure and close packing of particles [26]. In our study the surface area of the AA bentonite at the optimum condition was recorded as 72.86 m2 g1 (0.1 M HCl). SiO2, Al2O3 and CaO, which are major components of the studied bentonite, have zero point charge values of 2.2, 8.3 and 11, respectively [2,19]. At an acidic pH, the negative charge of silica sites of adsorbent is counterbalanced by H+ ions hence reducing hindrance to diffusion of dye ions. Whereas, there is a significant increase in electrostatic attractions between negative charges of anionic dyes and positive charges of alumina and calcia sites, thereby increasing dye adsorption [12,21]. As the system pH increases, the number of positively charged sites raises and the number of negatively charged sites declines [6]. An integration of acid and thermal activations was subsequently tested. We found that a larger surface area of the activated bentonite can be gained through the ATA (Fig. 1c). The surface area increased to 84.12 m2 g1 when AA (0.1 M HCl) was followed by AT at 50–100 8C. The results suggested that the acid activation of the bentonite with 0.1 M HCl at 50 8C can be employed as an economical technique for modifying bentonite to enhance its surface properties and adsorption capacity. The surface properties of the acid activated clays can be enhanced further if the acid activation is followed by the thermal activation [27].
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We summarize the changes of surface area of the raw and AA bentonite from other workers and compared their results with those observed in this study. Data shown in Table 1 indicate that the enhancement of the surface areas obtained in this study appeared to be similar to those obtained by previous studies. However, the concentration of HCI (0.1 M) used in our study was far lower than those cases (>2 M) [25–27]. This suggests that the bentonite adsorbents obtained here may be a particularly suitable for the water treatment due to less acid use, while they provided larger surface area for the adsorption of organic compounds. The AA bentonite using low concentration acid results in a low cost and low chemical use activation protocol, which can be economically and environmentally beneficial. 3.1.2. Pore size The pore size analysis of the bentonite samples was carried out to understand the variation in the pore size from the raw bentonite to the activated ones (Table 2). The thermal activation is the most commonly used physical method to remove the water molecules in the clay particles [28]. The porosity of clay can be affected by the adsorbed and hydrated water molecules in the bentonite. Table 2a shows the variation in average pore sizes of the TA bentonite with respect to temperature changes. An initial decrease in the pore size corresponded to the increase in temperature. This is due to the removal of adsorbed water and formation of mesopores and micropores. During this dehydration, which is a reversible process, the adsorbed and hydrated water attached on the bentonite surface can be removed [23]. The external surface and mesopores are the first to undergo a change on thermal activation, resulting in decreasing pore size. A further temperature increase beyond 100 8C resulted in enlarging in the pore size. The second stage of TA is known as dehydroxylation which is an irreversible process. The water molecules in the interlayer spaces are removed, and the interlayer spaces collapse, where the particles come closer to one another and start forming aggregates. The dehydroxylation leads to the transformation of micro and mesopores into macropores, resulting in the increase in the average pore size [21]. Heating the bentonite over the dehydroxylation temperature can collapse its structure completely due to destruction of the layered structure of the bentonite. Calcination may lead to a movement of octahedral cations within the octahedral sheet. In addition, the change in structure of the bentonite after calcinations is also attributed to the exchange of cations in the interlayers [29]. Bojemueller et al. [30] stated that the edges of the layers are attacked at high temperatures. The destruction of the edges results in the release of aluminum or hydroxyaluminum cations, and consequently forming more mesopores. The collapse of interlayer spaces may result in damaging the multi-layered structure of clay [15,21]. As a result, clay mineral loses it originality, forming new phases, such as mullite, cristobalite and feldspars with different properties than the parent clay [1,31].
Table 2 Average pore size of bentonite activated by (a) TA, and (b) AA and ATA. (a) Temperature (8C)
Pore size (A˚)
Raw bentonite (RB) 50 100 200 300 400 500
50.28 0.27 46.13 0.05 41.12 0.11 48.19 0.42 51.65 0.22 50.95 0.45 56.73 0.66
(b) Acid concentration (M)
0.05 0.075 0.10 0.25 0.50
AA bentonite
ATA bentonite
Pore size (A˚)
Pore size (A˚)
49.36 0.39 48.89 0.27 42.08 0.17 46.17 0.54 53.78 0.31
48.57 0.15 46.63 0.07 38.92 0.09 44.12 0.12 56.13 0.31
The changes in pore sizes of the AA and ATA bentonites are present in Table 2b. The average pore size of the AA bentonites decreased as acid concentration increased up to 0.1 M, and a further increase in acid concentration resulted in increasing the pore size. The initial decrease in pore size can be attributed to the removal of impurities and replacement of exchangeable cations by hydrogen ions which are smaller in size than the exchangeable cations [26]. Also, the decrease in pore size can be also due to unoccupied octahedral and tetrahedral spaces left by the leaching of Mg2+, Fe3+ and Al3+ by acid [28]. The subsequent increase in pore size was associated with HCl concentration beyond 0.1 M. As the interlayer space collapses the particles become closer to one another and combine together, resulting in the decrease in the number of the micropores. The empty spaces lead to an increase in the pore size, and consequently transforming micropores into mesopores. A further increase in the size of empty spaces results in the disappearance of mesopores at some locations due to disintegration of the crystal structure [22]. The disappearance of mesopores can lead to a significant decrease in the surface area. The results obtained here are in line with those reported elsewhere [15,27]. The average pore size of the AA (0.1 M HCl) bentonite is 42.08 A˚ which lies in the mesopore region. The pore size of the ATA bentonite (Table 2b) appeared to decrease with HCl concentration up to 0.10 M and then increased beyond 0.10 M. The mechanism of these changes in pore size is similar to that of the AA bentonite. However, an additional step of thermal activation resulted in removing the adsorbed and hydrated water, and volatile organic compounds. This mechanism creates new pores and provides more adsorption sites facilitating higher adsorption of dyes on the bentonite. The average pore size of the ATA (0.1 M HCl) is 38.916 A˚, indicating mesopore region.
Table 1 Surface area of various acid activated bentonites. Surface area of raw bentonite (m2 g1)
Surface area of activated bentonite (m2 g1)
Increase in surface area (%)
Adsorption capacity (mg g1)
Acid Con. (M)
Reference
29.23 50 50 52 25.7 80 71.95 52
165.8 250 200 184 84.12 240.9 109.8 76
82.37 80 75 71.74 69.45 66.79 34.47 31.58
5.9 6.2 5.8 6.2 7.0 5.3 5.9 4.8
2 M HCl 1 M HCl 2 M HCl 2 M HCl 0.1 M HCl 2 M H2SO4 5 M H2SO4 0.5 M HCl
Tsai et al. [25] Vimonses et al. [6] Vimonses et al. [6] Liu [14] This study Korichi et al. [26] Yildiz et al. [27] Liu [14]
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It is suggested that dehydration at a high temperature can cause the irreversible collapse of the structure. The clay platelets are bonded electrostatically. Therefore, dehydration of cations can result in reducing porosity and adsorption ability of the clay [1,15]. For instance, the study by Heller-Kallai [32] revealed a gradual increase in the strength of the studied kaolin and bentonite, and clay sediment with an increase in heating temperature. Yet, such significant and permanent increase in their strength occurred only after dehydroxylation. By continuing heating at a temperature above dehydroxylation stage, the clays became resistant to disintegration upon immersion in water. Thus, thermo-analytical measurement of the clay minerals is often performed to understand the stability of their structure against the temperature gradient. Our previous studies revealed that the kaolin demonstrated the highest stability due to the strong bond between its aluminosilicate layers [2,6,33]. In case of zeolites, the dehydration at high temperatures (>200 8C) is observed as a result of a strong interaction between the electrostatic charged framework and the water molecules [34]. In addition to this, an improvement in adsorption performance of the TA clays was also reported in many publications [32–34]. The changes of the clay structure after thermal activation at 500 8C for 5 h was evidenced by the formation of an amorphous phase in
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the TA clays. This is due to the loss of hydroxyl groups, which cause a rearrangement of the original structure of the clay. Vieira et al. [35] evaluated the morphological changes of the TA bentonite and its adsorption efficiency toward nickel in a porous bed reactor. The clay was heated at 500 8C for 24 h to increase its mechanical resistance and to eliminate some impurities. They reported that the calcination led to the increase of the clay surface area and the development of micro and mesoporosity due to bond water release and dehydroxylation occurring. However, the calcination can cause the reduction of the clay density compared to the untreated one; whilst the adsorption of nickel in clay pores resulted in a slight increase of actual density. Thus, it may conclude that thermal activation at a high temperature may not be beneficial for enhancing adsorption capacity of the bentonite for CR removal. 3.1.3. Surface morphology The SEM examination was carried out to assess the effect of the activation protocols on the surface morphology of the bentonites. Herein, we present a few representative SEM images of the raw and activated bentonites in Fig. 2. The surface of the raw bentonite (Fig. 2a) appears to be smooth due to closely packed flakes in contrast to the ragged appearance of the TA bentonite (Fig. 2b and c). Thermal activation at 100 8C can make the bentonite more
Fig. 2. SEM images of (a) raw bentonite, TA bentonite at (b) 100 8C and (c) 500 8C; AA bentonite using (d) 0.075 M HCl, (e) 0.1MHCl and (f) 0.5MHCl; and ATA bentonite using (g) 0.075 M HCl, (h) 0.1MHCl and (i) 0.5MHCl at 100 8C for 20 min.
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porous structure compared to raw bentonite (Fig. 2a and b). These results are also in accordance with the specific surface areas of the TA bentonite (Fig. 1a). The SEM image of the TA bentonite indicates that the interlayer spaces may have been collapsed, resulting in more tightly bound structure with reduction in porous structure at 500 8C (Fig. 2c). The surface morphology of the AA bentonite, as shown in Fig. 2d–f, indicates that the AA results in leaching of cations, making the clay surface more porous. Using 0.075 M HCl clumps of uneven surface can be formed along with flat flakes of low porosity (Fig. 2d). As the increase in acid concentration to 0.1 M the surface becomes highly porous, contributing more pores generated (Fig. 2e). Further increase in acid concentration reduced the porosity rendering clay surface rather flat (Fig. 2f). Fig. 2g–i show that the surface morphology of the raw bentonite has undertaken significant changes under the ATA, i.e. the bentonite was activated using HCl at 0.075–0.5 M, and followed by heating 100 8C for 20 min. The edges of the platelets were first to be exposed to acid attack. The formation of smaller pores takes place as the impurities are removed and the exchangeable cations are replaced by H+ ions. These physio-chemical reactions alter the morphology as the pores open up. Thus, the bentonite surface became more porous and homogeneous. The SEM images revealed that after the thermal activation of the AA (0.075 M HCl) bentonite the clay has become more porous (Fig. 2g). The bentonite activated by 0.1 M HCl followed by TA exhibited highly porous structure (Fig. 2h). This was also evidenced from the increase in surface area. Comparing the SEM images in Fig. 2f with Fig. 2i, it is interesting to note that a combination of TA (100 8C) with AA (0.5 M HCl) resulted in forming more pores on bentonite surface than the AA bentonite using by 0.5 M HCl. 3.2. Chemical composition of activated bentonite FTIR studies of the activated bentonites assist the identification of the minerals present in the bentonite. The coupled vibrations are appreciable due to the availability of various constituents. To recognize mineral species and identify characteristic bands of the bentonite, the FTIR spectra were performed in the range from 4000 to 400 cm1 to investigate the effect of activation on the chemical composition of the bentonite. Published collections of spectra of clay minerals as those reported by Palanivel and Velraj [36] are very useful for identification and characterization of the bentonite mineral. Fig. 3 shows that there are three main absorption regions: 3000–3800 cm1, 1300–1800 cm1, and 500–1200 cm1, and sharp differences could be found in each region of the AA and TA bentonite samples. The IR spectrum of raw bentonite indicates that montmorillonite is the dominant mineral phase in this bentonite (Fig. 3). The absorption band at 3620 cm1 is due to stretching vibrations of structural OH groups of montmorillonite. The KBr curve of natural bentonite was characteristic of montmorillonite with a single sharp band at 3620 cm1 and a broad band at 3435 cm1 of the OH stretching vibration of structural hydroxyl groups and water. In the lower frequency region, montmorillonite had a strong band at 1045 cm1 ponding to the Si–O stretching (in-plane) vibration of bentonite mineral. The absorption band at 1635 cm1 was attributed to the OH deformation mode of water. IR peaks at 916 cm1 was attributed to Al–OH–Al. The 692 cm1 band corresponded to coupled Al–O and Si–O out-of-plane vibrations. Quartz was present as indicated by the bands at 791 cm1. Water molecules exhibit three fundamental vibrational modes—an asymmetric stretching (n3), a symmetric (n1) stretching and an H–O–H bending (n2) [37]. In the bentonite–water systems, the positions of the water absorption bands are affected by physicochemical properties of the bentonite matrix. The stretching
Fig. 3. FTIR spectra of raw and activated bentonites by (a) AA at different HCl concentrations, and (b) TA at different temperatures for 20 min.
vibrations for H2O in transmittance spectra of hectolitre were reported at 3620–3640 cm1 for the asymmetric (n3) and 3430– 3580 cm1 for the symmetric (n1) band [36]. Our IR spectrum of raw bentonite shows that the intensity of the n1 vibration (3435 cm1) is weaker than the n3 vibration (3620 cm1). 3.2.1. Effect of acid activation The changes in the functional groups provide an indication of the changes of the chemical composition of the AA bentonite. FTIR spectra for the raw and AA bentonites (Fig. 3a) assist us to understand the effect of AA on the chemical structure of bentonite. During the course of the AA the protons penetrate into the bentonite layers attacking the OH groups, leading to the alteration in the adsorption bands attributed to the OH vibrations and octahedral cations. The intensity of stretching bands observed at 3620 cm1 (Al–OH–Al along with the Al–Mg–OH stretching vibrations) decreased with the acid concentration [4,5]. The increase in the acid concentration resulted in the decrease of the peaks of the bands associated with the adsorbed water at 3435 (H–O–H stretching) and 1635 cm1 (H–O–H bending). It is interesting to note that the band at 3435 cm1 became broader, while the peak at 916 cm1 for Al–OH–Al appeared to be week for the AA bentonite using 0.075 M and 0.1 M HCl. The band at 916 cm1 was disappeared for the AA bentonite using 0.5 M HCl. The peak of Si–O–Si band at 988 cm1 remained unchanged with the intensity of acid concentration. Similar results have been reported by Christidis et al. [7]. The bands at 916 cm1 were disappeared with the increase in acid concentration due to the
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increased substitution of Al2+ ions by Fe3+ ions. The transformation of the tetrahedral sheet was found at 791 cm1. The AA leads to the formation of amorphous silica, indicated by the increased intensity of the peak, which may provide more adsorption sites [38]. The intensity of band at 692 cm1 (Al–OH–Si bending) decreased with the increase in acid concentration, signifying the partial dissolution of aluminum ions present in the octahedral sheet of bentonite. This band was disappeared from the AA bentonite using HCl at a concentration higher than 0.25 M. FTRI is sensitive to the changes of the bentonite mineral structure upon acid activation [37]. During the acid attack protons penetrate into the bentonite mineral layers and attack the structural OH groups. The resulting dehydroxylation is connected with successive release of the central atoms from the octahedral as well as with removal of Al from the tetrahedral sheets. At the same time a gradual transformation of the layered tetrahedral sheet to a treedimensional framework proceeds. The final reaction product is an amorphous partly protonated silica phase. However, for industrial applications, complete decomposition of the parent internal structure is usually not wanted. 3.2.2. Effect of activation temperature Fig. 3b shows the FTIR spectra of the bentonites heated at 60 8C, 80 8C and 100 8C. The increase in activation temperature resulted in the deeper penetration of protons into the clay layers, decreasing the intensity of peaks at 3620 cm1 (Al–OH–Al along with the Al–Mg–OH stretching vibrations), 3435 cm1 (H–O–H stretching) and 1634 cm1 (H–O–H bending) [5,37]. The peak of Si–O–Si band at 1045 cm1 remains unchanged. The bands at 916 cm1 were disappeared when the bentonite was heated at a temperature higher than 80 8C. The intensity of the peak increases at 791 cm1 indicates the alteration of tetrahedral sheet [22,36]. The intensity of band at 692 cm1 (Al–OH–Si bending) decreased as temperature increased, showing the dissolution of Al ions that present in the octahedral sheet of the bentonite. FTIR results revealed that the temperature rise results in the disintegration of structure at a given acid concentration and reaction time. The effect of heating temperature and time on the thermal stability of the bentonite was previously studied in our group [2]. Differential thermal analysis (DTA) of the bentonite was carried out to analyze the effect of heating on the surface and structural properties of the bentonite. The thermogravimetric analysis (TGA) was also performed to estimate the mass of water evolved corresponding to the weight loss [15]. Results of DTA and TGA presented in Fig. 4 indicate that the dehydration stage corresponding to the removal of adsorbed and hydrated water occurred at 85 8C. This is the first endothermic peak on DTA plot and the corresponding derivative weight loss on TGA plot. It could be seen that the maxima of the DTA curve occurs at 127 8C. The removal of adsorbed and hydrated water provides additional adsorption sites, and leading to the increase in surface area. Further heating indicates the transition from dehydration stage to dehydroxylation stage. The second endothermic peak appears at 640 8C, showing the maximum exothermic breaking point at 985 8C. Heating beyond these temperatures results in collapsing the interlayer spaces, and consequently rupturing the clay structure. This structure rupture can cause morphology changes of the bentonite minerals, which is supported by the decrease in surface area at high temperatures, as discussed in the previous section. The DTA plot also suggests that the thermal activation of bentonite should be carefully carried out in the temperature range between 85 8C and 640 8C (Fig. 4a). Heating above the hydroxylation stage may cause irreversible damage to the structure of bentonite. The total mass loss of 15% is observed in the temperature range of 50 8C to 750 8C as shown in Fig. 4b. The weight loss in the dehydration stage was 10% in the temperature
Fig. 4. (a) Differential thermal analysis profile of sodium bentonite and (b) thermogravimetric profile of sodium bentonite [2].
range of 50 to 200 8C and further heating with increase in temperature shows 4% weight loss in the temperature range of 200 to 750 8C. 3.3. Adsorption capacity of the activated bentonite 3.3.1. Thermal activation: Heating temperature and time Here, we investigated how the TA affects the CR adsorption performance and capacity of the bentonite with respect to activation temperature and heating time. The CR adsorption increased with an increase in the temperature (Fig. 5). The increase in CR adsorption is likely attributed to the enlarged specific surface area and the removal of bound water from interlayer spaces. The maximum CR adsorption was given by the bentonite which was activated at 100 8C, corresponding to the maximum specific surface area. Similar results have been reported by Koyuncu [39] for the adsorption of metolachor on thermal activated bentonite. In contrast, the CR adsorption was found to decrease as temperature varied from 200 8C to 500 8C. This decrease in CR adsorption is likely due to the decrease in surface area and the availability of adsorption sites affecting the adsorption capacity of TA bentonite. Similar results have been reported by Aytas et al. [29] who studied the adsorption of Uranium (VI) on TA bentonite. It could be inferred from the results that TA at 100 8C for 20 min can enhance the adsorption capability of bentonite by 15%. To investigate the effect of heating time, the thermal activation was carried out in a reaction time ranging from 20 min to 2 h. Fig. 5 shows that the CR adsorption onto the TA bentonite increased with heating time at all temperature ranges, while the maximum
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Fig. 5. Effect of heating time on CR adsorption on TA bentonite (initial dye concentration: 100 mg L1, bentonite dosage 1 g L1), [data obtained from three replicated at each trial: 50 8C (SD 0.15), 100 8C (SD 0.18), 200 8C (SD 0.21), 500 8C (SD 0.24)].
adsorption was achieved in 1 h 40 min heating. In contrast, a low CR adsorption was observed with the prolonged heating over 1.5 h. The results indicate that apart from temperature, heating time also influences the adsorption capacity of the TA bentonite. If the heating is continued over the applicable range it may result in the destruction of the structure of the bentonite. The increase in heating time from 20 min to 1 h and 40 min contributed only a 5% increase in adsorption (Fig. 5). Therefore, from an economical point of view, 20 min should be considered as a sufficient heating time for the thermal activation of bentonite. Thus, otherwise as stated, 20 min heating time was used in the following experiments. 3.3.2. Acid activation Fig. 6 shows that CR adsorption increased with an increase in acid concentration until 0.1 M HCl. The maximum amount of Congo red was adsorbed on the AA bentonite using 0.1 M HCl. The increase in acid concentration beyond 0.1 M HCl results in excessive leaching of cations and the collapse of interlayer spaces. Bentonite is a rock formed, and highly colloidal and plastic clay, which is composed mainly of a very soft montmorillonite [2,12]. The montmorillonite is a clay mineral of the smectite group, which has a small amount of other contents, such as feldspar, biotite, illite and crystalline quartz [2,22,37]. Special properties of the bentonite are its ability to absorb large quantities of water and form
thixotrophic gels, with an accompanying increase in volume up to several times of its dry bulk. A crystalline structure of the bentonite is known as 2:1 type aluminosilicate, presenting an octahedral alumina between two tetrahedral silica layers [39]. These three layer units are stacked one above another with oxygen molecules in neighbouring layers adjacent to each other [6]. The parallel layers of the clay structure are held together by weak electrostatic forces, allowing water and other polar molecules to enter between layers and induce an expansion of mineral structure [40]. Substitutions of ions of lower valence within the lattice structure, for instance Al3+ for Si4+ in the tetrahedral sheet and Mg2+ for Al3+ in the tetrahedral layer, result in an excess of negative charge in the lattice [1,15]. These imbalanced charges in the interlayer space are equalised by the dominant cations, typically Na+, K+, Mg2+, and Ca2+. These cations can be released into water under acidic condition and are exchangeable due to their loose binding together with broken bonds with bentonite, which are attributed to a high cation exchange capacity of bentonite [20,33]. Adsorption capacity of activated bentonites from different studies is listed in Table 1. Our AA bentonite demonstrated a comparably high adsorption capacity. It is interesting to note that concentration of the HCl used in our study is very low. 3.3.3. Combined acid and thermal activation We experimentally assessed that adsorption capability of the ATA bentonite which was first activated by HCl at 0.05–0.5 M, followed by thermal activation at 50, 100 and 150 8C for 20 min. Adsorption profile data (Fig. 6) show that the CR adsorption increased with the acid concentration regardless the variation in the heating temperature. However, a high activation temperature resulted in a decrease in the CR adsorption (Fig. 6). At a given HCl concentration, the increase in the temperature corresponded to a shorter time required to achieve the maximum surface area. Babaki et al. [28] proposed a relationship between the temperature and the optimum time required. They performed the acid activation of bentonite by sulfuric acid. The results revealed that as the temperature increased, a shorter heating time is required to achieve the maximum surface area. Maximum CR adsorption of approximately 7.0 mg g1 was obtained by the ATA bentonite with 0.1 M HCl at 100 8C. Activated carbon has been used as the most commonly used commercial adsorbent for industrial water and wastewater treatment process for many decades due its high removal efficiency and capacity. To enhance its specific and overall absorption capacity for removal of toxic contaminants by incorporation nanoparticles in the activated carbon has drown an interest in recent studies [41–43]. However, the high costs, carbon source use and carbon emission associated with the production and application of activated carbon has been a challenge. In this study, we explored the cost-effective activation protocol to enhance adsorption capacity of the natural bentonite as adsorbent for removal toxic dyes from industrial wastewater. Our results showed that the ATA bentonite (0.1 M HCl + 100 8C) performed the highest adsorption capacity of approximately 7.0 mg g1, followed by the AA bentonite (0.1 M HCl) with 6.4 mg g1, while the lowest adsorption improvement was given by the TA bentonite. Compared with adsorption capacity of the raw bentonite, the ATA bentonite led to approximately 25% increase in removal of Congo red. 4. Conclusions
Fig. 6. Effect of activation temperature on the adsorption capacity of AA bentonite (initial dye concentration: 100 mg L1, bentonite dosage 1 g L1, [data obtained from three replicated at each trial: 50 8C (SD 0.15), 100 8C (SD 0.18), 150 8C (SD 0.22)].
From this study, we have identified a low cost and efficient activation protocol for enhancing the adsorption capacity of natural bentonite. The activation processes, which were carried out at low acid concentration and low heating temperature, could
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significantly enhance specific surface area and pore size, and alter surface morphology of the bentonite. The results revealed that thermal activation at 100 8C for 20 min exhibited more than 20% increase in surface area of the bentonite. However an increase beyond 100 8C expressed a decrease in surface area due to degeneration of the crystal structure. Acid activation (0.1 M HCl) resulted in enlarging up to 65% surface area of the raw bentonite. Overall, approximately 70% increase in surface area was observed for the bentonite activated by the combined acid and thermal activation. The adsorption capacity of raw bentonite was also enhanced by the activations and followed the order: ATA > AA > TA > RB. The overall adsorption capability of bentonite modified by ATA was augmented by more than 25%. The bentonite is a widely available and abundant natural mineral, and can be a low cost adsorbent for water and wastewater treatment. Our experimental results revealed that bentonite modified by ATA (0.1 M HCl) for 20 min followed by thermal activation at 100 8C can significantly enhance the surface and adsorption properties of raw bentonite. This activation method could be a cost-effective approach for clay bentonite modification to enhance its adsorption capacity and applicability, making it as promising adsorbent for water and wastewater treatment. Acknowledgment The authors thank the support from Bionanotechnology Laboratory: Water, Energy and Materials (BioNanoTech) at The University of Adelaide, Australia. References [1] [2] [3] [4]
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