- Email: [email protected]
Enhancing removal efficiency of anionic dye by combination and calcination of clay materials and calcium hydroxide
Journal of Hazardous Materials 171 (2009) 941–947
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Enhancing removal efficiency of anionic dye by combination and calcination of clay materials and calcium hydroxide Vipasiri Vimonses a,b , Bo Jin a,b,c,∗ , Christopher W.K. Chow c , Chris Saint b,c a b c
School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia Australian Water Quality Centre, SA Water Corporation, Bolivar, SA 5108, Australia
a r t i c l e
i n f o
Article history: Received 10 March 2009 Received in revised form 17 June 2009 Accepted 17 June 2009 Available online 23 June 2009 Keywords: Dye removal Adsorption Calcination Clay mixtures Congo red
a b s t r a c t We explored a feasible approach to enhance removal capacity of three natural clays for removing anionic dye from aqueous solution. Optimal mixing proportions of the clay materials and temperature range for the calcination were investigated. We found that the removal efficiency can be improved significantly when the clay materials were mixed at certain ratio with the addition of lime and the mixed clay materials were calcined 100–300 ◦ C. Batch experiments were conducted to study the effects of initial concentration, material dosage, contact time and pH on dye elimination. Kinetic study showed that more than 80% dye removal took place in 5 min. A high removal capacity (>575 mg g−1 ) of the mixed clay materials can be achieved at a low adsorbent dose. The mixed clay materials can be easily recovered by thermal treatment. The recovered mixtures demonstrated an enhanced removal capability after a few cycles of removal and regeneration. The results revealed that use of these clay materials could develop a low-cost treatment process for industrial wastewater. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The wastewater from textile industry is known to contain large amount of suspended solids, intense colour, high fluctuating pH, high temperature and Chemical Oxygen Demand (COD) concentration [1]. Direct discharge of dye effluent will create a serious environmental problem. Besides, the presence of dye colour can cause aesthetic pollution and impede penetration of light into water streams, thereby disturbing the ecological aquatic system [2]. Decolourisation of dye containing wastewater can be very intricate by conventional treatment methods since most of the commercially used dyes are resistant to biodegradation, photodegradation and oxidation by oxidising agents [3]. Traditional biological process tends to be inefficient as many dyes are poorly biodegradable and toxic to microorganisms, fluctuating in dye effluent characteristics may lead to direct destruction or inhibition of their catalytic capabilities [4]. Application of other treatment processes such as chemical oxidation, coagulation, and reverse osmosis is limited due to complicated implementation and economic feasibility in large scale operation [5].
∗ Corresponding author at: School of Earth and Environmental Sciences, The University of Adelaide, North Terrace Campus, Adelaide, SA 5005, Australia. Tel.: +61 8 8303 7056; fax: +61 8 8303 6222. E-mail address: [email protected] (B. Jin). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.06.094
Adsorption is one of the most effective and attractive techniques for removal of non-biodegradable contaminants from wastewater due to its simplicity in operation and availability of a wide range of adsorbents. The success of this application not only depends upon adsorption performance of the adsorbents, but also on the availability (constant supply) of materials for the process [6]. Activated carbon is the most common adsorbents being employed in water and wastewater treatment processes; however, even with high removal efficiency this material becomes less attractive due to its high price and regeneration difficulty. To develop adsorption technique into an economically viable process, a low cost adsorbent with high removal efficiency is essential. Extensive investigations have been carried out to identify suitable and relatively cheap adsorbents with good removal of significant quantities of dyes. These low-cost adsorbents including red mud [7], fly ash [8,9] clay mineral [10,11], biosorbents [6,12,13], slag [3], etc. have been reported in the literature. Nonetheless, the adsorption capacity of the low-cost materials was found to be much less effective than the commercially available activated carbon. As a result, large amount of adsorbent was required for dye removal, and thus producing significant volume of sludge waste [14]. The use of natural clays as alternative adsorbents in wastewater treatment would provide several advantages due to their low-cost, abundant availability, non-toxicity and potential of ion exchange for charged pollutants. Our previous study [10] investigated the application potential of natural clay minerals, i.e. sodium bentonite, kaolin, and zeolite, for removal of anionic di-azo dye Congo red
942
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
(CR), as a surrogate indicator, from aqueous solution. The obtained results revealed that the dye removal by these clays can be achieved over broad pH and temperature ranges. The CR removal capacity of 19.9, 5.6 and 4.3 mg g−1 was attained by bentonite, kaolin and zeolite, respectively [10]. However, these adsorption capabilities are still not as good as the commercial adsorbents. Hence, in this study we focused on developing a feasible technical approach of combination and calcination of these natural clay materials to improve dye removal efficiency. The application of a mixture of clay minerals would be able to compromise the indigenous constrains of the individual clays. An optimisation study using calcium hydroxide or slaked lime, which has long been used in wastewater treatment for removal of metal ions, nutrients or as a coagulant/flocculant due to its high porosity, cheap and abundant availability, as an additional calcium source of clay mixture was included. It is anticipated that by formulating a correct mixture and optimising pre-activated condition of these clay mixtures, such an alternative low-cost adsorbent for concentrated dye wastewater can be employed. 2. Materials and methods 2.1. Materials Three Australian clay minerals, sodium bentonite, kaolin and zeolite, were selected to study their dye removal capacity. Sodium bentonite with high montmorillonite content, and dry milled white kaolin clay were obtained from Unimin Australia Ltd., and Escott zeolite was provided by Zeolite Australia Ltd. This zeolite contains clinoptilolite as the main crystalline component. Physico-chemical properties of each material and their key elements which contributed to dye removal were provided by the supplier and present in Table 1 [10]. Calcium hydroxide (Ca(OH)2 ) powder was purchased from Unilab (Ajax Finechem). This slaked lime is a strong base material with high pH (12.5) and slightly soluble in water (solubility of 1.2 g L−1 at 25 ◦ C). All materials were used as received without further purification. Congo red (C32 H22 N6 Na2 O6 S2 , Labchem Ajax Finechem Australia) was prepared to desired concentration by the addition of double-deionised water obtained from Barnstead nanopure Diamond Water ion exchange system with 18.2 M resistivity. 2.2. Characterisation of clay material properties The specific surface area of the materials was measured using Brunauer–Emmett–Teller (BET) method and given in Table 1 [10]. Results were obtained by means of pure liquid N2 adsorption at
77 ± 0.5 K using a Gemini V2.00 surface analyser (Micromeritics, USA). Prior to analysis, all samples were degassed under vacuum at 105 ◦ C for 12 h. The differential temperature analysis (DTA) coupled with thermogravimetric analyser (TG) (TA Instruments) was used to analyse the weight loss of natural clays as a function of elevated temperature. The samples were treated up to 1200 ◦ C at a heating rate of 10 ◦ C/min under high purity nitrogen gas to avoid any possible oxidation by atmospheric oxygen or air. 2.3. Experimental procedure Batch experiments were conducted to optimise the clay mixture content and pre-activated condition, and to investigate the effects of different parameters, i.e. dye concentration, material dosage, kinetic, pH and recyclability of the mixtures on the CR removal efficiency, in order to verify their potential use in wastewater treatment. The CR removal capacity study of clay mixtures was carried out in a batch system. 0.05 g of the mixture was added to 50 ml of the 150 mg L−1 CR concentration, unless otherwise stated. To investigate the effect of pH, the dye solution pH was adjusted with 0.1 M HCl, and 0.1 M NaOH. The mixture suspension was shaken at 150 rpm in a rotary shaker (Ratek OM 15 orbital mixer, Australia) at 30 ± 1 ◦ C. Experiments were carried out over 24 h to ensure that adsorption equilibrium was obtained, except for kinetic study where the samples were withdrawn from the experimental flask at pre-determined time intervals. At the end of the equilibrium, the clay mixture was separated by centrifugation (Eppendorf Centrifuge 5415R, Germany) at 13,200 rpm for 20 min. The supernatants were then filtered using Millex VX filter (Millipore 0.45 m) to ensure the solutions were free from adsorbent particles before measuring the residual dye concentration. To determine the calcination pre-treatment condition, the clay mixtures were initially treated at different temperature ranging from 50 to 600 ◦ C for 0–2.5 h prior to the dye removal test. The obtained optimal condition was subsequently used to pre-treat all the mixtures for further experiments. To simulate a recycling/regeneration process of the used materials, the clay mixtures were pre-loaded with CR. These mixtures were treated at different temperature (300–600 ◦ C) for a required period of time (30–60 min) according to the studied condition. Then, the recycled mixture was re-tested for CR removal to investigate the optimal thermal desorption condition and its maximum regeneration cycle. All experiments were carried out in triplicate, and the average values were taken to minimise random error. 2.4. Analytical method
Table 1 Physicochemical properties of natural adsorbents. Chemical compound (%)
SiO2 Al2 O3 Fe2 O3 CaO K2 O TiO2 MnO MgO Na2 O P2 O5 H2 O LOI CEC Particle size Density Specific surface area
Natural adsorbents Sodium bentonite
Kaolin
Zeolite
56.0 16 4.6 0.9 0.4
48.7 34.6 0.9 0.1 1.2 1.3
3.3 2.9
0.4 0.2
68.26 12.99 1.37 2.09 4.11 0.23 0.06 0.83 0.64 0.06
10.0 5.7 95 mequiv./100 g D85 75 m 1.0 g/cm3 25.70 m2 /g
12.1 D99 38 m 0.6 g/cm3 20.28 m2 /g
8.87 120 mequiv./100 g D100 70 m 1.1–1.6 g/cm3 8.31 m2 /g
2.4.1. Measurement methods Dye concentration was determined colorimetrically by measuring at maximum absorbance of 496.5 nm using a UV–visible spectrophotometer (model ␥, Helios, UK). A calibration curve was plotted between absorbance and concentration of the dye solution to obtain an absorbance–concentration profile. The percentage removal (%) of CR and the adsorption capacity of clay mixture were calculated using the following equations: % = (Ci − Ce ) ×
100 Ci
(1)
q = (Ci − Ce ) ×
V m
(2)
where Ci is the initial CR concentration (mg L−1 ), Ce is the CR concentration at adsorption equilibrium (mg L−1 ), q is the amount of dye uptake per unit of adsorbent, V is the volume of dye solution (ml), and m is the weight of the adsorbents (g).
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
943
Table 2 Clay mineral constituents of the material mixtures. Mineral constituent (%)
Mixture 1
Ca(OH)2 NaBentonite Kaolin Zeolite Total
2
3
4
5
6
7
8
9
10
50 – 30 20
65 15 15 5
60 25 10 5
30 10 10 30
40 5 40 10
70 15 10 5
55 25 10 10
45 50 5 –
70 10 10 10
85 5 5 5
100
100
100
100
100
100
100
100
100
100
3. Results and discussion 3.1. Optimisation of mixing ratio and pre-treatment condition of clay mixtures 3.1.1. Optimisation of clay mixture composition One feasible approach of improving the removal efficiency of clay minerals is by alteration of the clay compositions. From Table 1, it can be observed that SiO2 Al2 O3 , Fe2 O3 and CaO are the key components of the studied clays, depending upon the nature and geographic source of origin. Silica and alumina constituents are long-known to be responsible for dye removal either by adsorption or ion exchange. On the other hand, the role of calcium content on dye removal has been addressed in several studies [8,15]. Zhu et al. [14] reported that the presence of calcium ions can promote anionic dye removal through precipitation process, especially at high dye concentration. Thus, in the present work, the concept of applying a mixture of different natural clays to compromise the indigenous weakness of natural clays, together with additional calcium hydroxide as calcium source were optimised for removal of high concentration of CR from aqueous medium. Based on the results obtained from the previous removal capacity study of individual clays, 10 clay mixtures made up of different clay and lime proportion were prepared. The selected study range of lime was between 20 and 90% of the mixture. Bentonite was chosen as the major clay component with the range between 0 and 50% of the mixture, followed by kaolin (5–40%) and zeolite (0–30%). The actual components of the prepared mixtures are present in Table 2. All mixtures were then tested for dye removal, and the average CR elimination capability of each mixture, compared to those of natural clays is illustrated in Fig. 1. It was observed that all the clay mixtures showed significant improvement in dye removal capability. Out of the ten selected mixtures, mixture 2, 6 and 9 performed very well in terms of removal capacity, 133, 131 and 134 mg g−1 , respectively, as compared to the others. Thus, these three clay mixtures were selected as optimal mixtures for further studies.
Fig. 1. Congo red removal efficiency by different clay mixtures.
3.1.2. Optimisation of pre-thermal activated condition Evidences have shown that the surface properties of aluminosilicate materials can be altered through physio-chemical or thermal treatment [16]. In many cases, the pre-activated thermal treatment such as calcination is often required to develop desirable properties and prevent undesirable properties from developing of the clay minerals [17,18]. Bergaya et al. [16] explained that the heat treatment can cause concomitant changes in clay structure and composition exploited for practical purposes. Due to the distinctive structure among clay minerals, their structural stability upon heat treatment can be diverged from one to another. Therefore, prior to determine the pre-thermal activation ranges for the mixtures, thermoanalytical measurement of bentonite, kaolin and zeolite was performed to obtain the stability profile against temperature gradient. The obtained DTA–TG results were shown in Fig. 2. Upon the heating process, the foreign molecules adsorbed on the surface, i.e. water and other impurities were lost initially. This dehydration stage can be seen from the first endothermic peak of DTA curve at 85, 153 and 55 ◦ C for bentonite (Fig. 2a), kaolin (Fig. 2b) and zeolite (Fig. 2c), respectively, and a significant derivative weight loss of TG data. This would result in more active sites being available for adsorption. As the heating continued, the second endothermic peak (at 640◦ , 503◦ with maximum exothermic breaking point at 985◦ and 170◦ of those bentonite, kaolin and zeolite, respectively) appeared corresponding to the dehydroxylation, which brings about changes of the clay structure and surface function groups, and the bond within the clay begins to break down, causing the collapse of clay structure and decrease in specific surface area. Thus, to enhance removal capability without serious damage to their structure, the calcination of the clay mixture should be carefully operated below these threshold temperatures. By considering the overall thermal strength of the clays, the pre-thermal activation ranges for the clay mixtures was chosen to conduct between 50 and 600 ◦ C. All the three mixtures were calcined at different temperatures (50–600 ◦ C) for a set period of time (1.5 h). The attained results shown in Fig. 3a revealed that CR elimination capacity of most mixtures fluctuated over the treatment ranges. However, the condition for the highest and stable CR removal for all mixtures was observed between 200 and 400 ◦ C of treatment, indicative of an appropriate temperature range for calcination. The mixtures were then subjected for heat treatment at 300 ◦ C, as the representative optimal calcination temperature, at different time span (0–2.5 h) to determine the suitable pre-treatment condition (Fig. 3b). Throughout the calcination treatment, calcium hydroxide was dehydrated and converted into calcium oxide, and consequently formed a porous Ca-based material, while heating of the clay minerals can lead to a decrease of their surface hydration and hydroxylation, in which it in turn relates to the isoelectric point describing the surface acidity of oxides [19]. Such changes in surface properties results in the changes in removal capability of the mixtures. The experimental results indicated that calcination at 300 ◦ C for 1.5 h was the optimal condition for pre-treatment, in which
944
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
Fig. 2. Differential thermal analysis (DTA—heat flow against temperature profile) and thermogravimetric (TG—weight loss against temperature profile) profiles of (a) sodium bentonite, (b) kaolin, and (c) zeolite.
the highest dye removal efficiency of all mixtures can be attained. From Fig. 3b, almost 149 mg g−1 of dye removal capacity can be obtained by all mixtures when pre-activated under this condition. This was approximately 15–20% higher than of those achieved by untreated mixtures. Therefore, this pre-thermal activated condition was applied to the mixtures for the subsequent studies.
Fig. 3. Optimisation of pre-thermal treatment condition of the material mixtures for Congo red removal. Effect of calcination (a) temperature for 1.5 h, and (b) time at 300 ◦ C.
3.2. Evaluation of dye removal performance by the clay mixtures In order to determine the applicability and optimum conditions of the clay mixtures for wastewater treatment, several key parameters were investigated. 3.2.1. Effect of adsorbent dosage The effect of the adsorbent dose on removal of CR is shown in Fig. 4. Similar profiles were observed for all mixtures. It was found that the removal efficiency increased with an increase in mixture dosage. This removal enhancement was ascribed to an increase in adsorption surface area and the availability of active sites [9]. From the results, more than 99% CR removal can be achieved with loading of 1 g L−1 of the adsorbent mixtures. This dosage was considered to be very small compared to the other low-cost adsorbents for similar removal efficiency, which was reported in a range of 8–23 g L−1 [20,21]. From a practical point of view, use of a low dosage of the adsorbent is a crucial issue in an adsorption treatment process, resulting in reducing the operation cost, and generating less sludge. This certainly provides economic benefit for scaling up the process as an option for wastewater treatment.
Fig. 4. Effect of adsorbent dosage on removal of Congo red on the adsorbent mixtures (Congo red concentration 150 mg L−1 ).
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
3.2.2. Effect of initial dye concentration Eight CR concentrations, i.e., 40, 100, 150, 200, 300, 400, 500 and 600 mg L−1 , were selected to investigate the effect of initial dye concentration on the removal efficiency. This concentration range was much greater than these studies reported in the literature [9,13,22]. The results illustrated that the percentage of dye removal tends to decrease when the initial dye concentration increases. From Fig. 5a, mixture 6 showed the highest removal efficiency, followed by mixture 9, 2 and pure Ca(OH)2 , respectively. It was found that the dye removal efficiency of Ca(OH)2 alone was still significant, although it was lower than that of the three clay mixtures. Taking into account the role of clay constituents on colour removal, it can be seen that the removal efficiency of these three mixtures was comparable at low initial concentration (<150 mg L−1 ), in which more than 99% of dye can be eliminated. However, as the initial concentration increased beyond this point, a difference in removal efficiency among mixtures was observed. A superior dye uptake of mixtures 6 and 9 was likely due to the larger fraction of Ca(OH)2 in the mixture. While, a better removal efficiency of mixture 6 over mixture 9 can be attributed to higher amount of sodium bentonite component, providing a greater surface area and adsorption potential than other clays. The overall results indicated a synergetic effect of individual clay and Ca(OH)2 on CR elimination. On the other hand, by considering the amount of dye adsorbed per unit weight of clay minerals (mg g−1 ), it was found that removal capacity of all clay mixtures increases as the initial dye con-
945
Table 3 Congo red removal capacity of various low-cost adsorbents. Adsorbent
Removal capacity (mg g−1 )
Reference
Clay mixture Coal-based mesoporous activated carbons Bentonite Chitosan hydrobeads Ca-bentonite Anilinepropylsilica xerogel Orange peel Bagasse fly ash Activated red mud Activated coir pitch carbon Calcium-rich fly ash Waste red mud
>575 52–189
Present study [24]
158.7 92.59 85.29 22.62 22.44 11.88 7.08 6.72 2.33–9.61 4.05
[11] [25] [26] [27] [13] [9] [22] [21] [23] [28]
centration increases. The amount of dye removal increased from 39 mg g−1 to over 575 mg g−1 when the initial CR concentration increased from 40 to 600 mg L−1 for all mixtures. The positive relation of removal capacity of the mixtures and initial dye concentration can be derived through a linear correlation with r2 > 0.9999 as shown in Fig. 5b. Such increase in the proportional dye removal can be explained by the equilibrium shift during the removal process [23], in which an increase of CR concentration can lead to an increase of mass gradient pressure between the solution and adsorbent and acts as a driving force to transfer dye molecules from bulk solution to the particle surface. This information would be very useful for predicting the performance and design of a singlestage batch adsorption system. The high dye removal efficiency of the clay mixtures is an indication of the feasibility of applying it in wastewater treatment. A comprehensive overview of their CR removal capacity with other low-cost adsorbents is shown in Table 3. 3.2.3. Kinetic study The kinetic study of CR removal by clay mixtures was conducted to investigate dye removal capacity against contact time. The experiment was carried out for 6 h. Samples were taken for dye measurement at different time intervals. A similar kinetic profile for all three mixtures was observed (Fig. 6). The results revealed that colour elimination of all mixtures took place very rapidly, in which 84.3%, 87.1% and 86.3% of dye were removed within the initial 5 min, or accounting for 127, 131 and 130 mg g−1 dye removal capacity by mixtures 2, 6, and 9, respectively. Then, the removal progressively increased with time, in which more than 90% removal can be achieved in 150 min of contact time. Such effective dye elimination in short period of time of these mixtures would be privileged over other low-cost adsorbents when exploiting in wastewater industry.
Fig. 5. Effect of initial dye concentration on removal of Congo red on the adsorbent mixtures (adsorbent dosage 1 g L−1 ).
Fig. 6. Kinetic reaction of Congo red removal by the adsorbent mixtures.
946
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
3.2.4. Effect of pH In this study, the effect of pH on dye removal was investigated for the initial pH solution. It was observed that the clay mixtures showed a strong alkaline property in the solution with pH ranging from 11.5 to 12.1. An increase in dye solution pH was observed instantly after addition of the mixture and subsequently remained stable toward the end of the experiment. The change of removal efficiency of the mixture was found to be insignificant with changing of initial dye solution pH. It is because the alkaline character of clay mixture resulting from high calcium content neutralized the acidic pH of the dye solution. Such pH insensitive character was also reported by Acemio˘glu [23] on removal of CR anionic dye by high Ca-rich fly ash and Zhu et al. [14] on removal of anionic dye by high Ca content red mud. This indicated that pH is not a parameter of concern for CR removal, suggesting the advantages of applying clay mixtures in wastewater treatment where pH of influent is often fluctuated. 3.3. Clay material recovery and life cycle Recycle and regeneration of spent adsorbent is considered as an important economical aspect to minimise the cost of material. Porous materials that are thermally stable are often in great demand as adsorbents. The recovery of clay mixture through thermal treatment showed that as the heating temperature and time increased the higher recovery capacity was obtained. The maximum recovery of all mixtures was achieved at 550 ◦ C for 30 min calcination, beyond these conditions recovery began to decrease (results not shown). The heat generated during calcination decomposed the organic contaminants on the surfaces or in pores of the adsorbent to carbon and then oxidise to carbon oxides in air, leaving the bared surface available for re-adsorption. Meanwhile, heat treatment at a very high temperature can also break down the bond structure of the material and cause pore collapse. This circumstance could reduce its specific surface area and pore volume, and result in lower adsorption capacity of the adsorbent [29]. In this study, the life span of the clay mixtures was also investigated. The spent mixtures were recovered and re-tested (recycled) for CR removal for several times. It is interesting to note that the removal efficiency was improved as the number of recovery increased. It was proposed that the repetitive heat treatment during the recycle process likely promotes the decomposition of small foreign molecules attached subterraneous inside the micropores, providing more available active sites. Once, such hidden molecules is completely eliminated, the maximum removal efficiency can
Fig. 7. Recovery and life span of the mixture adsorbents (calcination at 550 ◦ C, 30 min).
possibly be achieved and becomes independent to the number of recycle runs. This was verified by the insignificant change in removal capacity of the mixtures after the third recycled run (Fig. 7). The CR removal efficiency for all mixture was greater than 99%, even after the fifth run. This result indicated that the mixture has high stability and potentially can be considered as a reusable adsorbent. 4. Conclusion Clay mixtures of different clay minerals, bentonite, kaolin and zeolite, with additional of lime were studied for removal of the toxic anionic dye, CR, from aqueous solution. The mixtures showed substantial enhancement of their thermal stability and dye removal efficiency, in comparison to the individual clays, in which more than 95% dye removal can be achieved with the initial concentration of 600 mg L−1 of CR. Kinetic study revealed the rapid removal reaction, in which large portion of dye was removed within the first few minutes of reaction. Initial pH of the solution showed insignificant effect on CR removal due to strong alkalinity of the mixtures. Recycling study confirmed that the removal capacity of spent-clay mixtures improved after the fifth run, indicating the potential of regeneration of the material. Hence, it is suggested that these clay mixtures have high potential to be employed as an alternative low-cost adsorbent for anionic dye wastewater. Acknowledgements The authors are grateful to Unimin Australia Ltd. and Zeolite Australia Ltd. for kindly supplying clay minerals. This work was supported by the Australian Research Council Linkage Grant through the Water Environmental Biotechnology Laboratory (WEBL) at the University of Adelaide. References [1] C.F. Gurnham (Ed.), Industrial Waste Control, Academic Press, New York, 1965. [2] W.T. Tsai, C.Y. Chang, M.C. Lin, S.F. Chien, H.F. Sun, M.F. Hsieh, Adsorption of acid dye onto activated carbons prepared from agricultural waste bagasse by ZnCl2 activation, Chemosphere 45 (2001) 51–58. [3] K.R. Ramakrishna, T. Viraraghavan, Use of slag for dye removal, Waste Manage. 17 (1997) 483–488. [4] D. Georgiou, P. Melidis, A. Aivasidis, Use of a microbial sensor: inhibition effect of azo-reactive dyes on activated sludge, Bioprocess. Biosyst. Eng. 25 (2002) 79–83. [5] Y.M. Slokar, A. Majcen-Le Marechal, Methods of decoloration of textile wastewaters, Dyes Pigments 37 (1998) 335–356. [6] R. Han, D. Ding, Y. Xu, W. Zou, Y. Wang, Y. Li, L. Zou, Use of rice husk for the adsorption of Congo red from aqueous solution in column mode, Bioresour. Technol. 99 (2008) 2938–2946. [7] S. Wang, H.M. Ang, M.O. Tadé, Novel applications of red mud as coagulant, adsorbent and catalyst for environmentally benign processes, Chemosphere 72 (2008) 1621–1635. [8] S. Wang, H. Wu, A review: environmental-benign utilisation of fly ash as lowcost adsorbents, J. Hazard. Mater. B136 (2006) 482–501. [9] I.D. Mall, V.C. Srivastava, N.K. Agarwal, I.M. Mishra, Removal of Congo red from aqueous solution by bagasse fly ash and activated carbon: kinetic study and equilibrium isotherm analyses, Chemosphere 61 (2005) 492–501. [10] V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint, Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials, Chem. Eng. J. 148 (2009) 354–364. [11] E. Bulut, M. Özacar, I.A. S¸engil, Equilibrium and kinetic data and process design for adsorption of Congo Red onto bentonite, J. Hazard. Mater. 154 (2008) 613–622. [12] Z. Aksu, S¸.S¸. C¸a˘gatay, F. Gönen, Continuous fixed bed biosorption of reactive dyes by dried Rhizopus arrhizus: determination of column capacity, J. Hazard. Mater. 143 (2007) 362–371. [13] C. Namasivayam, N. Muniasamy, K. Gayatri, M. Rani, K. Ranganathan, Removal of dyes from aqueous solutions by cellulosic waste orange peel, Bioresour. Technol. 57 (1996) 37–43. [14] M.X. Zhu, L. Lee, H.H. Wang, Z. Wang, Removal of an anionic dye by adsorption/precipitation processes using alkaline white mud, J. Hazard. Mater. 149 (2007) 735–741. [15] K.R. Ramakrishna, T. Viraraghavan, Dye removal using low cost adsorbents, Water Sci. Technol. 36 (1997) 189–196.
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947 [16] F. Bergaya, B.K.G. Theng, G. Lagaly, Modified clays and clay minerals, in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds.), Handbook of Clay Science, Development in Clay Science, vol. 1, Elsevier, The Netherlands, 2006, pp. 261–422. [17] M.H. Karao˘glu, M. Do˘gan, M. Alkan, Removal of cationic dyes by kaolinite, Micropor. Mesopor. Mater. 122 (2009) 20–27. [18] F. Gan, J. Zhou, H. Wang, C. Du, X. Chen, Removal of phosphate from aqueous solution by thermally treated natural palygorskite, Water Res. 43 (2009) 2907–2915. [19] T. Hanzlíˇcek, M. Steinerová-Vondráková, Investigation of dissolution of aluminosilicates in aqueous alkaline solution under laboratory conditions, Ceram. Silik. 46 (2002) 97–103. [20] G. Annadurai, R.-S. Juang, D.-J. Lee, Use of cellulose-based wastes for adsorption of dyes from aqueous solutions, J. Hazard. Mater. 92 (2002) 63–274. [21] C. Namasivayam, D. Kavitha, Removal of Congo Red from water by adsorption onto activated carbon prepared from coir pith, an agricultural solid waste, Dyes Pigments 54 (2002) 47–58. [22] A. Tor, Y. Cengeloglu, Removal of Congo red from aqueous solution by adsorption onto acid activated red mud, J. Hazard. Mater. B138 (2006) 409–415.
947
[23] B. Acemio˘glu, Adsorption of Congo red from aqueous solution onto calcium-rich fly ash, J. Colloid Interface Sci. 274 (2004) 371–379. [24] E. Lorenc-Grabowska, G. Gryglewicz, Adsorption characteristics of Congo Red on coal-based mesoporous activated carbon, Dyes Pigments 74 (2007) 34–40. [25] S. Chatterjee, S. Chatterjee, B.P. Chatterjee, A.K. Guha, Adsorptive removal of Congo red, a carcinogenic textile dye by chitosan hydrobeads: binding mechanism, equilibrium and kinetics, Colloids Surf. A 299 (2007) 146–152. [26] L. Lian, L. Guo, C. Guo, Adsorption of Congo red from aqueous solutions onto Ca-bentonite, J. Hazard. Mater. 161 (2009) 126–131. [27] F.A. Pavan, S.L.P. Dias, E.C. Lima, E.V. Benvenutti, Removal of Congo red from aqueous solution by anilinepropylsilica xerogel, Dyes Pigments 76 (2008) 64–69. [28] C. Namasivayam, D.J.S.E. Arasi, Removal of Congo Red from wastewater by adsorption onto waste red mud, Chemosphere 34 (1997) 401–417. [29] S. Wang, Z.H. Zhu, Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution, J. Hazard. Mater. B136 (2006) 946–952.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Enhancing removal efficiency of anionic dye by combination and calcination of clay materials and calcium hydroxide Vipasiri Vimonses a,b , Bo Jin a,b,c,∗ , Christopher W.K. Chow c , Chris Saint b,c a b c
School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia Australian Water Quality Centre, SA Water Corporation, Bolivar, SA 5108, Australia
a r t i c l e
i n f o
Article history: Received 10 March 2009 Received in revised form 17 June 2009 Accepted 17 June 2009 Available online 23 June 2009 Keywords: Dye removal Adsorption Calcination Clay mixtures Congo red
a b s t r a c t We explored a feasible approach to enhance removal capacity of three natural clays for removing anionic dye from aqueous solution. Optimal mixing proportions of the clay materials and temperature range for the calcination were investigated. We found that the removal efficiency can be improved significantly when the clay materials were mixed at certain ratio with the addition of lime and the mixed clay materials were calcined 100–300 ◦ C. Batch experiments were conducted to study the effects of initial concentration, material dosage, contact time and pH on dye elimination. Kinetic study showed that more than 80% dye removal took place in 5 min. A high removal capacity (>575 mg g−1 ) of the mixed clay materials can be achieved at a low adsorbent dose. The mixed clay materials can be easily recovered by thermal treatment. The recovered mixtures demonstrated an enhanced removal capability after a few cycles of removal and regeneration. The results revealed that use of these clay materials could develop a low-cost treatment process for industrial wastewater. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The wastewater from textile industry is known to contain large amount of suspended solids, intense colour, high fluctuating pH, high temperature and Chemical Oxygen Demand (COD) concentration [1]. Direct discharge of dye effluent will create a serious environmental problem. Besides, the presence of dye colour can cause aesthetic pollution and impede penetration of light into water streams, thereby disturbing the ecological aquatic system [2]. Decolourisation of dye containing wastewater can be very intricate by conventional treatment methods since most of the commercially used dyes are resistant to biodegradation, photodegradation and oxidation by oxidising agents [3]. Traditional biological process tends to be inefficient as many dyes are poorly biodegradable and toxic to microorganisms, fluctuating in dye effluent characteristics may lead to direct destruction or inhibition of their catalytic capabilities [4]. Application of other treatment processes such as chemical oxidation, coagulation, and reverse osmosis is limited due to complicated implementation and economic feasibility in large scale operation [5].
∗ Corresponding author at: School of Earth and Environmental Sciences, The University of Adelaide, North Terrace Campus, Adelaide, SA 5005, Australia. Tel.: +61 8 8303 7056; fax: +61 8 8303 6222. E-mail address: [email protected] (B. Jin). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.06.094
Adsorption is one of the most effective and attractive techniques for removal of non-biodegradable contaminants from wastewater due to its simplicity in operation and availability of a wide range of adsorbents. The success of this application not only depends upon adsorption performance of the adsorbents, but also on the availability (constant supply) of materials for the process [6]. Activated carbon is the most common adsorbents being employed in water and wastewater treatment processes; however, even with high removal efficiency this material becomes less attractive due to its high price and regeneration difficulty. To develop adsorption technique into an economically viable process, a low cost adsorbent with high removal efficiency is essential. Extensive investigations have been carried out to identify suitable and relatively cheap adsorbents with good removal of significant quantities of dyes. These low-cost adsorbents including red mud [7], fly ash [8,9] clay mineral [10,11], biosorbents [6,12,13], slag [3], etc. have been reported in the literature. Nonetheless, the adsorption capacity of the low-cost materials was found to be much less effective than the commercially available activated carbon. As a result, large amount of adsorbent was required for dye removal, and thus producing significant volume of sludge waste [14]. The use of natural clays as alternative adsorbents in wastewater treatment would provide several advantages due to their low-cost, abundant availability, non-toxicity and potential of ion exchange for charged pollutants. Our previous study [10] investigated the application potential of natural clay minerals, i.e. sodium bentonite, kaolin, and zeolite, for removal of anionic di-azo dye Congo red
942
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
(CR), as a surrogate indicator, from aqueous solution. The obtained results revealed that the dye removal by these clays can be achieved over broad pH and temperature ranges. The CR removal capacity of 19.9, 5.6 and 4.3 mg g−1 was attained by bentonite, kaolin and zeolite, respectively [10]. However, these adsorption capabilities are still not as good as the commercial adsorbents. Hence, in this study we focused on developing a feasible technical approach of combination and calcination of these natural clay materials to improve dye removal efficiency. The application of a mixture of clay minerals would be able to compromise the indigenous constrains of the individual clays. An optimisation study using calcium hydroxide or slaked lime, which has long been used in wastewater treatment for removal of metal ions, nutrients or as a coagulant/flocculant due to its high porosity, cheap and abundant availability, as an additional calcium source of clay mixture was included. It is anticipated that by formulating a correct mixture and optimising pre-activated condition of these clay mixtures, such an alternative low-cost adsorbent for concentrated dye wastewater can be employed. 2. Materials and methods 2.1. Materials Three Australian clay minerals, sodium bentonite, kaolin and zeolite, were selected to study their dye removal capacity. Sodium bentonite with high montmorillonite content, and dry milled white kaolin clay were obtained from Unimin Australia Ltd., and Escott zeolite was provided by Zeolite Australia Ltd. This zeolite contains clinoptilolite as the main crystalline component. Physico-chemical properties of each material and their key elements which contributed to dye removal were provided by the supplier and present in Table 1 [10]. Calcium hydroxide (Ca(OH)2 ) powder was purchased from Unilab (Ajax Finechem). This slaked lime is a strong base material with high pH (12.5) and slightly soluble in water (solubility of 1.2 g L−1 at 25 ◦ C). All materials were used as received without further purification. Congo red (C32 H22 N6 Na2 O6 S2 , Labchem Ajax Finechem Australia) was prepared to desired concentration by the addition of double-deionised water obtained from Barnstead nanopure Diamond Water ion exchange system with 18.2 M resistivity. 2.2. Characterisation of clay material properties The specific surface area of the materials was measured using Brunauer–Emmett–Teller (BET) method and given in Table 1 [10]. Results were obtained by means of pure liquid N2 adsorption at
77 ± 0.5 K using a Gemini V2.00 surface analyser (Micromeritics, USA). Prior to analysis, all samples were degassed under vacuum at 105 ◦ C for 12 h. The differential temperature analysis (DTA) coupled with thermogravimetric analyser (TG) (TA Instruments) was used to analyse the weight loss of natural clays as a function of elevated temperature. The samples were treated up to 1200 ◦ C at a heating rate of 10 ◦ C/min under high purity nitrogen gas to avoid any possible oxidation by atmospheric oxygen or air. 2.3. Experimental procedure Batch experiments were conducted to optimise the clay mixture content and pre-activated condition, and to investigate the effects of different parameters, i.e. dye concentration, material dosage, kinetic, pH and recyclability of the mixtures on the CR removal efficiency, in order to verify their potential use in wastewater treatment. The CR removal capacity study of clay mixtures was carried out in a batch system. 0.05 g of the mixture was added to 50 ml of the 150 mg L−1 CR concentration, unless otherwise stated. To investigate the effect of pH, the dye solution pH was adjusted with 0.1 M HCl, and 0.1 M NaOH. The mixture suspension was shaken at 150 rpm in a rotary shaker (Ratek OM 15 orbital mixer, Australia) at 30 ± 1 ◦ C. Experiments were carried out over 24 h to ensure that adsorption equilibrium was obtained, except for kinetic study where the samples were withdrawn from the experimental flask at pre-determined time intervals. At the end of the equilibrium, the clay mixture was separated by centrifugation (Eppendorf Centrifuge 5415R, Germany) at 13,200 rpm for 20 min. The supernatants were then filtered using Millex VX filter (Millipore 0.45 m) to ensure the solutions were free from adsorbent particles before measuring the residual dye concentration. To determine the calcination pre-treatment condition, the clay mixtures were initially treated at different temperature ranging from 50 to 600 ◦ C for 0–2.5 h prior to the dye removal test. The obtained optimal condition was subsequently used to pre-treat all the mixtures for further experiments. To simulate a recycling/regeneration process of the used materials, the clay mixtures were pre-loaded with CR. These mixtures were treated at different temperature (300–600 ◦ C) for a required period of time (30–60 min) according to the studied condition. Then, the recycled mixture was re-tested for CR removal to investigate the optimal thermal desorption condition and its maximum regeneration cycle. All experiments were carried out in triplicate, and the average values were taken to minimise random error. 2.4. Analytical method
Table 1 Physicochemical properties of natural adsorbents. Chemical compound (%)
SiO2 Al2 O3 Fe2 O3 CaO K2 O TiO2 MnO MgO Na2 O P2 O5 H2 O LOI CEC Particle size Density Specific surface area
Natural adsorbents Sodium bentonite
Kaolin
Zeolite
56.0 16 4.6 0.9 0.4
48.7 34.6 0.9 0.1 1.2 1.3
3.3 2.9
0.4 0.2
68.26 12.99 1.37 2.09 4.11 0.23 0.06 0.83 0.64 0.06
10.0 5.7 95 mequiv./100 g D85 75 m 1.0 g/cm3 25.70 m2 /g
12.1 D99 38 m 0.6 g/cm3 20.28 m2 /g
8.87 120 mequiv./100 g D100 70 m 1.1–1.6 g/cm3 8.31 m2 /g
2.4.1. Measurement methods Dye concentration was determined colorimetrically by measuring at maximum absorbance of 496.5 nm using a UV–visible spectrophotometer (model ␥, Helios, UK). A calibration curve was plotted between absorbance and concentration of the dye solution to obtain an absorbance–concentration profile. The percentage removal (%) of CR and the adsorption capacity of clay mixture were calculated using the following equations: % = (Ci − Ce ) ×
100 Ci
(1)
q = (Ci − Ce ) ×
V m
(2)
where Ci is the initial CR concentration (mg L−1 ), Ce is the CR concentration at adsorption equilibrium (mg L−1 ), q is the amount of dye uptake per unit of adsorbent, V is the volume of dye solution (ml), and m is the weight of the adsorbents (g).
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
943
Table 2 Clay mineral constituents of the material mixtures. Mineral constituent (%)
Mixture 1
Ca(OH)2 NaBentonite Kaolin Zeolite Total
2
3
4
5
6
7
8
9
10
50 – 30 20
65 15 15 5
60 25 10 5
30 10 10 30
40 5 40 10
70 15 10 5
55 25 10 10
45 50 5 –
70 10 10 10
85 5 5 5
100
100
100
100
100
100
100
100
100
100
3. Results and discussion 3.1. Optimisation of mixing ratio and pre-treatment condition of clay mixtures 3.1.1. Optimisation of clay mixture composition One feasible approach of improving the removal efficiency of clay minerals is by alteration of the clay compositions. From Table 1, it can be observed that SiO2 Al2 O3 , Fe2 O3 and CaO are the key components of the studied clays, depending upon the nature and geographic source of origin. Silica and alumina constituents are long-known to be responsible for dye removal either by adsorption or ion exchange. On the other hand, the role of calcium content on dye removal has been addressed in several studies [8,15]. Zhu et al. [14] reported that the presence of calcium ions can promote anionic dye removal through precipitation process, especially at high dye concentration. Thus, in the present work, the concept of applying a mixture of different natural clays to compromise the indigenous weakness of natural clays, together with additional calcium hydroxide as calcium source were optimised for removal of high concentration of CR from aqueous medium. Based on the results obtained from the previous removal capacity study of individual clays, 10 clay mixtures made up of different clay and lime proportion were prepared. The selected study range of lime was between 20 and 90% of the mixture. Bentonite was chosen as the major clay component with the range between 0 and 50% of the mixture, followed by kaolin (5–40%) and zeolite (0–30%). The actual components of the prepared mixtures are present in Table 2. All mixtures were then tested for dye removal, and the average CR elimination capability of each mixture, compared to those of natural clays is illustrated in Fig. 1. It was observed that all the clay mixtures showed significant improvement in dye removal capability. Out of the ten selected mixtures, mixture 2, 6 and 9 performed very well in terms of removal capacity, 133, 131 and 134 mg g−1 , respectively, as compared to the others. Thus, these three clay mixtures were selected as optimal mixtures for further studies.
Fig. 1. Congo red removal efficiency by different clay mixtures.
3.1.2. Optimisation of pre-thermal activated condition Evidences have shown that the surface properties of aluminosilicate materials can be altered through physio-chemical or thermal treatment [16]. In many cases, the pre-activated thermal treatment such as calcination is often required to develop desirable properties and prevent undesirable properties from developing of the clay minerals [17,18]. Bergaya et al. [16] explained that the heat treatment can cause concomitant changes in clay structure and composition exploited for practical purposes. Due to the distinctive structure among clay minerals, their structural stability upon heat treatment can be diverged from one to another. Therefore, prior to determine the pre-thermal activation ranges for the mixtures, thermoanalytical measurement of bentonite, kaolin and zeolite was performed to obtain the stability profile against temperature gradient. The obtained DTA–TG results were shown in Fig. 2. Upon the heating process, the foreign molecules adsorbed on the surface, i.e. water and other impurities were lost initially. This dehydration stage can be seen from the first endothermic peak of DTA curve at 85, 153 and 55 ◦ C for bentonite (Fig. 2a), kaolin (Fig. 2b) and zeolite (Fig. 2c), respectively, and a significant derivative weight loss of TG data. This would result in more active sites being available for adsorption. As the heating continued, the second endothermic peak (at 640◦ , 503◦ with maximum exothermic breaking point at 985◦ and 170◦ of those bentonite, kaolin and zeolite, respectively) appeared corresponding to the dehydroxylation, which brings about changes of the clay structure and surface function groups, and the bond within the clay begins to break down, causing the collapse of clay structure and decrease in specific surface area. Thus, to enhance removal capability without serious damage to their structure, the calcination of the clay mixture should be carefully operated below these threshold temperatures. By considering the overall thermal strength of the clays, the pre-thermal activation ranges for the clay mixtures was chosen to conduct between 50 and 600 ◦ C. All the three mixtures were calcined at different temperatures (50–600 ◦ C) for a set period of time (1.5 h). The attained results shown in Fig. 3a revealed that CR elimination capacity of most mixtures fluctuated over the treatment ranges. However, the condition for the highest and stable CR removal for all mixtures was observed between 200 and 400 ◦ C of treatment, indicative of an appropriate temperature range for calcination. The mixtures were then subjected for heat treatment at 300 ◦ C, as the representative optimal calcination temperature, at different time span (0–2.5 h) to determine the suitable pre-treatment condition (Fig. 3b). Throughout the calcination treatment, calcium hydroxide was dehydrated and converted into calcium oxide, and consequently formed a porous Ca-based material, while heating of the clay minerals can lead to a decrease of their surface hydration and hydroxylation, in which it in turn relates to the isoelectric point describing the surface acidity of oxides [19]. Such changes in surface properties results in the changes in removal capability of the mixtures. The experimental results indicated that calcination at 300 ◦ C for 1.5 h was the optimal condition for pre-treatment, in which
944
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
Fig. 2. Differential thermal analysis (DTA—heat flow against temperature profile) and thermogravimetric (TG—weight loss against temperature profile) profiles of (a) sodium bentonite, (b) kaolin, and (c) zeolite.
the highest dye removal efficiency of all mixtures can be attained. From Fig. 3b, almost 149 mg g−1 of dye removal capacity can be obtained by all mixtures when pre-activated under this condition. This was approximately 15–20% higher than of those achieved by untreated mixtures. Therefore, this pre-thermal activated condition was applied to the mixtures for the subsequent studies.
Fig. 3. Optimisation of pre-thermal treatment condition of the material mixtures for Congo red removal. Effect of calcination (a) temperature for 1.5 h, and (b) time at 300 ◦ C.
3.2. Evaluation of dye removal performance by the clay mixtures In order to determine the applicability and optimum conditions of the clay mixtures for wastewater treatment, several key parameters were investigated. 3.2.1. Effect of adsorbent dosage The effect of the adsorbent dose on removal of CR is shown in Fig. 4. Similar profiles were observed for all mixtures. It was found that the removal efficiency increased with an increase in mixture dosage. This removal enhancement was ascribed to an increase in adsorption surface area and the availability of active sites [9]. From the results, more than 99% CR removal can be achieved with loading of 1 g L−1 of the adsorbent mixtures. This dosage was considered to be very small compared to the other low-cost adsorbents for similar removal efficiency, which was reported in a range of 8–23 g L−1 [20,21]. From a practical point of view, use of a low dosage of the adsorbent is a crucial issue in an adsorption treatment process, resulting in reducing the operation cost, and generating less sludge. This certainly provides economic benefit for scaling up the process as an option for wastewater treatment.
Fig. 4. Effect of adsorbent dosage on removal of Congo red on the adsorbent mixtures (Congo red concentration 150 mg L−1 ).
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
3.2.2. Effect of initial dye concentration Eight CR concentrations, i.e., 40, 100, 150, 200, 300, 400, 500 and 600 mg L−1 , were selected to investigate the effect of initial dye concentration on the removal efficiency. This concentration range was much greater than these studies reported in the literature [9,13,22]. The results illustrated that the percentage of dye removal tends to decrease when the initial dye concentration increases. From Fig. 5a, mixture 6 showed the highest removal efficiency, followed by mixture 9, 2 and pure Ca(OH)2 , respectively. It was found that the dye removal efficiency of Ca(OH)2 alone was still significant, although it was lower than that of the three clay mixtures. Taking into account the role of clay constituents on colour removal, it can be seen that the removal efficiency of these three mixtures was comparable at low initial concentration (<150 mg L−1 ), in which more than 99% of dye can be eliminated. However, as the initial concentration increased beyond this point, a difference in removal efficiency among mixtures was observed. A superior dye uptake of mixtures 6 and 9 was likely due to the larger fraction of Ca(OH)2 in the mixture. While, a better removal efficiency of mixture 6 over mixture 9 can be attributed to higher amount of sodium bentonite component, providing a greater surface area and adsorption potential than other clays. The overall results indicated a synergetic effect of individual clay and Ca(OH)2 on CR elimination. On the other hand, by considering the amount of dye adsorbed per unit weight of clay minerals (mg g−1 ), it was found that removal capacity of all clay mixtures increases as the initial dye con-
945
Table 3 Congo red removal capacity of various low-cost adsorbents. Adsorbent
Removal capacity (mg g−1 )
Reference
Clay mixture Coal-based mesoporous activated carbons Bentonite Chitosan hydrobeads Ca-bentonite Anilinepropylsilica xerogel Orange peel Bagasse fly ash Activated red mud Activated coir pitch carbon Calcium-rich fly ash Waste red mud
>575 52–189
Present study [24]
158.7 92.59 85.29 22.62 22.44 11.88 7.08 6.72 2.33–9.61 4.05
[11] [25] [26] [27] [13] [9] [22] [21] [23] [28]
centration increases. The amount of dye removal increased from 39 mg g−1 to over 575 mg g−1 when the initial CR concentration increased from 40 to 600 mg L−1 for all mixtures. The positive relation of removal capacity of the mixtures and initial dye concentration can be derived through a linear correlation with r2 > 0.9999 as shown in Fig. 5b. Such increase in the proportional dye removal can be explained by the equilibrium shift during the removal process [23], in which an increase of CR concentration can lead to an increase of mass gradient pressure between the solution and adsorbent and acts as a driving force to transfer dye molecules from bulk solution to the particle surface. This information would be very useful for predicting the performance and design of a singlestage batch adsorption system. The high dye removal efficiency of the clay mixtures is an indication of the feasibility of applying it in wastewater treatment. A comprehensive overview of their CR removal capacity with other low-cost adsorbents is shown in Table 3. 3.2.3. Kinetic study The kinetic study of CR removal by clay mixtures was conducted to investigate dye removal capacity against contact time. The experiment was carried out for 6 h. Samples were taken for dye measurement at different time intervals. A similar kinetic profile for all three mixtures was observed (Fig. 6). The results revealed that colour elimination of all mixtures took place very rapidly, in which 84.3%, 87.1% and 86.3% of dye were removed within the initial 5 min, or accounting for 127, 131 and 130 mg g−1 dye removal capacity by mixtures 2, 6, and 9, respectively. Then, the removal progressively increased with time, in which more than 90% removal can be achieved in 150 min of contact time. Such effective dye elimination in short period of time of these mixtures would be privileged over other low-cost adsorbents when exploiting in wastewater industry.
Fig. 5. Effect of initial dye concentration on removal of Congo red on the adsorbent mixtures (adsorbent dosage 1 g L−1 ).
Fig. 6. Kinetic reaction of Congo red removal by the adsorbent mixtures.
946
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947
3.2.4. Effect of pH In this study, the effect of pH on dye removal was investigated for the initial pH solution. It was observed that the clay mixtures showed a strong alkaline property in the solution with pH ranging from 11.5 to 12.1. An increase in dye solution pH was observed instantly after addition of the mixture and subsequently remained stable toward the end of the experiment. The change of removal efficiency of the mixture was found to be insignificant with changing of initial dye solution pH. It is because the alkaline character of clay mixture resulting from high calcium content neutralized the acidic pH of the dye solution. Such pH insensitive character was also reported by Acemio˘glu [23] on removal of CR anionic dye by high Ca-rich fly ash and Zhu et al. [14] on removal of anionic dye by high Ca content red mud. This indicated that pH is not a parameter of concern for CR removal, suggesting the advantages of applying clay mixtures in wastewater treatment where pH of influent is often fluctuated. 3.3. Clay material recovery and life cycle Recycle and regeneration of spent adsorbent is considered as an important economical aspect to minimise the cost of material. Porous materials that are thermally stable are often in great demand as adsorbents. The recovery of clay mixture through thermal treatment showed that as the heating temperature and time increased the higher recovery capacity was obtained. The maximum recovery of all mixtures was achieved at 550 ◦ C for 30 min calcination, beyond these conditions recovery began to decrease (results not shown). The heat generated during calcination decomposed the organic contaminants on the surfaces or in pores of the adsorbent to carbon and then oxidise to carbon oxides in air, leaving the bared surface available for re-adsorption. Meanwhile, heat treatment at a very high temperature can also break down the bond structure of the material and cause pore collapse. This circumstance could reduce its specific surface area and pore volume, and result in lower adsorption capacity of the adsorbent [29]. In this study, the life span of the clay mixtures was also investigated. The spent mixtures were recovered and re-tested (recycled) for CR removal for several times. It is interesting to note that the removal efficiency was improved as the number of recovery increased. It was proposed that the repetitive heat treatment during the recycle process likely promotes the decomposition of small foreign molecules attached subterraneous inside the micropores, providing more available active sites. Once, such hidden molecules is completely eliminated, the maximum removal efficiency can
Fig. 7. Recovery and life span of the mixture adsorbents (calcination at 550 ◦ C, 30 min).
possibly be achieved and becomes independent to the number of recycle runs. This was verified by the insignificant change in removal capacity of the mixtures after the third recycled run (Fig. 7). The CR removal efficiency for all mixture was greater than 99%, even after the fifth run. This result indicated that the mixture has high stability and potentially can be considered as a reusable adsorbent. 4. Conclusion Clay mixtures of different clay minerals, bentonite, kaolin and zeolite, with additional of lime were studied for removal of the toxic anionic dye, CR, from aqueous solution. The mixtures showed substantial enhancement of their thermal stability and dye removal efficiency, in comparison to the individual clays, in which more than 95% dye removal can be achieved with the initial concentration of 600 mg L−1 of CR. Kinetic study revealed the rapid removal reaction, in which large portion of dye was removed within the first few minutes of reaction. Initial pH of the solution showed insignificant effect on CR removal due to strong alkalinity of the mixtures. Recycling study confirmed that the removal capacity of spent-clay mixtures improved after the fifth run, indicating the potential of regeneration of the material. Hence, it is suggested that these clay mixtures have high potential to be employed as an alternative low-cost adsorbent for anionic dye wastewater. Acknowledgements The authors are grateful to Unimin Australia Ltd. and Zeolite Australia Ltd. for kindly supplying clay minerals. This work was supported by the Australian Research Council Linkage Grant through the Water Environmental Biotechnology Laboratory (WEBL) at the University of Adelaide. References [1] C.F. Gurnham (Ed.), Industrial Waste Control, Academic Press, New York, 1965. [2] W.T. Tsai, C.Y. Chang, M.C. Lin, S.F. Chien, H.F. Sun, M.F. Hsieh, Adsorption of acid dye onto activated carbons prepared from agricultural waste bagasse by ZnCl2 activation, Chemosphere 45 (2001) 51–58. [3] K.R. Ramakrishna, T. Viraraghavan, Use of slag for dye removal, Waste Manage. 17 (1997) 483–488. [4] D. Georgiou, P. Melidis, A. Aivasidis, Use of a microbial sensor: inhibition effect of azo-reactive dyes on activated sludge, Bioprocess. Biosyst. Eng. 25 (2002) 79–83. [5] Y.M. Slokar, A. Majcen-Le Marechal, Methods of decoloration of textile wastewaters, Dyes Pigments 37 (1998) 335–356. [6] R. Han, D. Ding, Y. Xu, W. Zou, Y. Wang, Y. Li, L. Zou, Use of rice husk for the adsorption of Congo red from aqueous solution in column mode, Bioresour. Technol. 99 (2008) 2938–2946. [7] S. Wang, H.M. Ang, M.O. Tadé, Novel applications of red mud as coagulant, adsorbent and catalyst for environmentally benign processes, Chemosphere 72 (2008) 1621–1635. [8] S. Wang, H. Wu, A review: environmental-benign utilisation of fly ash as lowcost adsorbents, J. Hazard. Mater. B136 (2006) 482–501. [9] I.D. Mall, V.C. Srivastava, N.K. Agarwal, I.M. Mishra, Removal of Congo red from aqueous solution by bagasse fly ash and activated carbon: kinetic study and equilibrium isotherm analyses, Chemosphere 61 (2005) 492–501. [10] V. Vimonses, S. Lei, B. Jin, C.W.K. Chow, C. Saint, Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials, Chem. Eng. J. 148 (2009) 354–364. [11] E. Bulut, M. Özacar, I.A. S¸engil, Equilibrium and kinetic data and process design for adsorption of Congo Red onto bentonite, J. Hazard. Mater. 154 (2008) 613–622. [12] Z. Aksu, S¸.S¸. C¸a˘gatay, F. Gönen, Continuous fixed bed biosorption of reactive dyes by dried Rhizopus arrhizus: determination of column capacity, J. Hazard. Mater. 143 (2007) 362–371. [13] C. Namasivayam, N. Muniasamy, K. Gayatri, M. Rani, K. Ranganathan, Removal of dyes from aqueous solutions by cellulosic waste orange peel, Bioresour. Technol. 57 (1996) 37–43. [14] M.X. Zhu, L. Lee, H.H. Wang, Z. Wang, Removal of an anionic dye by adsorption/precipitation processes using alkaline white mud, J. Hazard. Mater. 149 (2007) 735–741. [15] K.R. Ramakrishna, T. Viraraghavan, Dye removal using low cost adsorbents, Water Sci. Technol. 36 (1997) 189–196.
V. Vimonses et al. / Journal of Hazardous Materials 171 (2009) 941–947 [16] F. Bergaya, B.K.G. Theng, G. Lagaly, Modified clays and clay minerals, in: F. Bergaya, B.K.G. Theng, G. Lagaly (Eds.), Handbook of Clay Science, Development in Clay Science, vol. 1, Elsevier, The Netherlands, 2006, pp. 261–422. [17] M.H. Karao˘glu, M. Do˘gan, M. Alkan, Removal of cationic dyes by kaolinite, Micropor. Mesopor. Mater. 122 (2009) 20–27. [18] F. Gan, J. Zhou, H. Wang, C. Du, X. Chen, Removal of phosphate from aqueous solution by thermally treated natural palygorskite, Water Res. 43 (2009) 2907–2915. [19] T. Hanzlíˇcek, M. Steinerová-Vondráková, Investigation of dissolution of aluminosilicates in aqueous alkaline solution under laboratory conditions, Ceram. Silik. 46 (2002) 97–103. [20] G. Annadurai, R.-S. Juang, D.-J. Lee, Use of cellulose-based wastes for adsorption of dyes from aqueous solutions, J. Hazard. Mater. 92 (2002) 63–274. [21] C. Namasivayam, D. Kavitha, Removal of Congo Red from water by adsorption onto activated carbon prepared from coir pith, an agricultural solid waste, Dyes Pigments 54 (2002) 47–58. [22] A. Tor, Y. Cengeloglu, Removal of Congo red from aqueous solution by adsorption onto acid activated red mud, J. Hazard. Mater. B138 (2006) 409–415.
947
[23] B. Acemio˘glu, Adsorption of Congo red from aqueous solution onto calcium-rich fly ash, J. Colloid Interface Sci. 274 (2004) 371–379. [24] E. Lorenc-Grabowska, G. Gryglewicz, Adsorption characteristics of Congo Red on coal-based mesoporous activated carbon, Dyes Pigments 74 (2007) 34–40. [25] S. Chatterjee, S. Chatterjee, B.P. Chatterjee, A.K. Guha, Adsorptive removal of Congo red, a carcinogenic textile dye by chitosan hydrobeads: binding mechanism, equilibrium and kinetics, Colloids Surf. A 299 (2007) 146–152. [26] L. Lian, L. Guo, C. Guo, Adsorption of Congo red from aqueous solutions onto Ca-bentonite, J. Hazard. Mater. 161 (2009) 126–131. [27] F.A. Pavan, S.L.P. Dias, E.C. Lima, E.V. Benvenutti, Removal of Congo red from aqueous solution by anilinepropylsilica xerogel, Dyes Pigments 76 (2008) 64–69. [28] C. Namasivayam, D.J.S.E. Arasi, Removal of Congo Red from wastewater by adsorption onto waste red mud, Chemosphere 34 (1997) 401–417. [29] S. Wang, Z.H. Zhu, Characterisation and environmental application of an Australian natural zeolite for basic dye removal from aqueous solution, J. Hazard. Mater. B136 (2006) 946–952.