- Email: [email protected]
Removal of methylene blue from aqueous solution by adsorption on pyrophyllite
Journal of Molecular Liquids 209 (2015) 267–271
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Removal of methylene blue from aqueous solution by adsorption on pyrophyllite Jian Zhang, Yan Zhou, Meiyan Jiang, Juan Li, Jiawei Sheng ⁎ College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, China
a r t i c l e
i n f o
Article history: Received 31 March 2015 Received in revised form 26 May 2015 Accepted 28 May 2015 Available online xxxx Keywords: Pyrophyllite Methylene blue Adsorption Pickling–grinding Kinetics
a b s t r a c t The effectiveness of adsorption for dye removal from solutions has made it an ideal alternative to other expensive treatment methods. The ability of raw and pickling–grinding modified pyrophyllite powders to adsorb methylene blue has been investigated in this study by looking at the dependence of absorption on pH values, adsorbent mass and the initial methylene blue concentration. The measured D50 of raw particles was 21.42 μm, which was decreased to 7.18 μm after grinding treatment, and to 5.55 μm after pickling–grinding. We showed that the raw and modified pyrophyllite powders have a high adsorptive capacity for dyes. The absorption ability of raw and modified pyrophyllite powders decreased with the increase of the initial methylene blue concentration in solution, while increased with the pH value. Increase in the adsorption with adsorbent mass was attributed to increase adsorbent surface area and availability of more adsorption sites. The adsorption capacity of raw powders reached 3.71 mg/g, and the numbers of pickling, grinding and pickling–grinding powders increased to 3.83 mg/g, 3.94 mg/g and 4.24 mg/g, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to rapid industrial development, pollution of water bodies by industries is an issue of major concern. Many industrial processes in China use different synthetic chemical dyes to color their final product. Most of the used solutions containing dyes are discarded as effluents. Due to the complex chemical structure of these dyes, they are resistant to breakdown by chemical, physical and biological treatments. Color impedes light penetration, retards photosynthetic activity, inhibits the growth of biota and also has a tendency to chelate metal ions which produce micro-toxicity to fish and other organisms [1,2]. Among the various available water treatment techniques adsorption is the most reliable and efficient technique for decoloration, in which the recovery and recycling of the adsorbent materials can be achieved along with the distinct advantages of nonproduction of any toxic sludge and cost effectiveness [3–14]. This has encouraged the development of adsorbents that are abundantly available and economical. The use of clean, cost-efficient, and biodegradable adsorbents could be a good tool to minimize the environmental impact caused by manufacturing and textile byproducts [14–16]. Adsorption is known to be a promising technique, which has great importance due to the ease of operation and comparable low cost of application in the decoloration process. Commercially activated carbon is a remarkably highly adsorbent
⁎ Corresponding author at: College of Materials Science and Engineering, Zhejiang University of Technology, Zhaohui No. 6, Hangzhou, Zhejiang 310032, China. E-mail address: [email protected] (J. Sheng).
http://dx.doi.org/10.1016/j.molliq.2015.05.056 0167-7322/© 2015 Elsevier B.V. All rights reserved.
material with a large number of applications in the remediation of contaminated groundwater and industrial wastes such as colored effluents. Since activated carbon is an expensive adsorbent due to its high costs of manufacturing and regeneration, much attention has been focused on various naturally occurring adsorbents such as chitosan, zeolites, fly ash, coal, papermill sludge, and various clay minerals [17]. Among these new adsorbents, clays have been shown to be the most promising alternatives due to their local availability, technical feasibility, easy engineering applications, highly specific surface area, and cost effectiveness [15,16,18–24]. Nowadays numerous low cost adsorbents are available including products of agricultural origin such as wood dust, sugarcane, fruit peel, wheat straw, and apple pomance [3]. Recently, the application of pyrophyllite on waste water treatment has become of great interest in China because of its abundance in local reserves as well as its inexpensiveness. The characteristics of pyrophyllite are related to its powerful adsorbent properties and its ability to adsorb organic or inorganic ions from aqueous solution. A considerable amount of work has also been reported in the literature regarding the potential use of pyrophyllite in the removal of heavy metals and dye molecules [25–27]; however, the adsorption properties of pyrophyllite on methylene blue (MB) are still scarcely known. The purpose of this study is to focus attention on the adsorption of MB on pyrophyllite from aqueous solutions. The dynamical behaviors of adsorption were measured on the effect of reaction time, MB concentration, pH value and temperature. Furthermore, raw pyrophyllite particles were modified by a pickling–grinding treatment and a comparative study of the removal of MB from aqueous solutions by raw and pickling–grinding particles was carried out as well.
268
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271
2. Materials and methods The pyrophyllite mineral used in this study originated from the Zhejiang province, located in the south-eastern region of China. The pyrophyllite powder composed of (wt.%): 66.08 SiO2, 19.92 Al2O3, 0.70 Fe2O3, 0.64 K2O, 0.21 Na2O, 0.12 CaO, 0.27 Ti2O and 12.33 of weight loss, was used in this study. Particle sizes were analyzed by the LS-POP laser particle size analyzer. Pickling–grinding particles were prepared by pickling raw particles in 0.55 mol/L HCl solution at a temperature of 80 °C for 6 h. The remaining HCl was removed by washing until the pH of the suspension was close to 7. Pickling particles were then milled in water in a Planet Style Ball Mill for 3 h. Finally, the pickling–grinding particles were obtained by filtration and dried at 110 °C for 2 h. Thermogravimetric analysis and differential thermal analysis (TG–DTA) were carried out simultaneously in static air with an automatic thermal analyzer system (Pyris Diamond). To get the TG–DTA curves, powder samples of about 40 mg were packed loosely into a platinum holder and were thermally treated under flow of air at a heating rate of 10 °C/min (range: 20–1000 °C). Calcinated α-alumina was taken as the reference. Methylene blue with a labeled purity of more than 98% was used as model dye without further purification. Deionized water was used to make the dye solutions of desired concentration. The MB solution shows an intense absorption peak in the visible region at 665 nm. In an adsorption process, a change in the intensity of this peak can be used to characterize the removal of dye from the solution. The adsorption of MB on pyrophyllite was calculated by monitoring the changes in absorption value of the solution on a spectrophotometer (Shimadzu UV-2550) at ambient room temperature, using a 1 cm quartz cell. Adsorption tests of MB on prepared pyrophyllite particles were studied using a batch process by mixing 0.1–1.0 g of adsorbent mass in plugged conical flasks with 100 mL of MB solutions of concentration ranging from 10 to 60 mg/L. The contents of the dye solution were then shaken for a given amount of reaction time using a magnetic stirrer operated at a constant speed. A constant temperature bath was used to keep the temperature constant at 20 ± 1 °C. The effect of pH was studied over a pH range of 7–11. The pH value was adjusted by the addition of dilute aqueous solutions of NaOH (0.10 M). Samples were withdrawn at appropriate time intervals and the contents were then centrifuged and the supernatant solution was pipetted out and monitored instantaneously on the spectrophotometer for absorption values. The absorbance values obtained in solutions before and after adsorption were then used to calculate the removal of the MB on pyrophyllite powders. Each experimental point was an average of three independent adsorption tests.
and Ti2 + from the octahedral layer into acid solution and forming more hydrophilic –OH groups on the vacant sites. Adsorption is further influenced by particle sizes [3]. The measured D50 of raw particles was 21.42 μm, while the D50 of pickling particles was decreased to 7.18 μm after grinding treatment, and to 5.55 μm after pickling–grinding. Pickling particles were milled to decrease the particle size, thus caused the destruction of Van der Waals forces [28]. The TG–DTA curves of raw pyrophyllite are shown in Fig. 1. According to directions of the various peaks, the DTA curve can be divided into three regions. In the first region, up to 100 °C, endothermic dehydration of pyrophyllite is the major thermal reaction. The second region is between 380 and 700 °C, and the mass loss for pyrophyllite in this region is 2.7%. The exothermic DTA peaks at 417.9 and 447.6 °C may be expected from the release of structural OH for an ideal pyrophyllite. The third region occurs at temperatures above 700 °C, and the broad endothermic peak centered at 919.1 °C may be due to the dehydroxylation reaction of pyrophyllite and the crystallization of mullite [21,29]. 3.2. Methylene blue adsorption on raw and modified pyrophyllite Experiments were conducted with raw and modified pyrophyllite at constant adsorbent dosage (0.2 g/100 mL), pH (neutral), and temperature (20 °C) for 24 h by varying MB concentrations (10–60 mg/L). It is evident from Fig. 2 that pickling–grinding powder has more adsorption efficiency in comparison to other powders at all initial MB concentrations studied. Decolorization of solution was 70.1%, 71.2%, 75.5% and 79.7% by raw, picking, grinding, and pickling–grinding powders, respectively, at 20 mg/L dye concentration. The absorption ability of four powders decreased with the increase of the initial MB concentration in solution. Four materials had almost similar adsorption efficiency and intendancy if initial MB concentration in solution was up to 60 mg/L. The difference observed here may arise from the morphology and size of the particles and the total surface area of the materials used [21]. 3.3. Effect of solution pH pH is one of the most influencing factors for dye adsorption as it directly affects the dissociative and adsorptive ability of the dye on the adsorbent surface [3]. To study the effect of pH on MB adsorption on raw and modified pyrophyllite, the experiments were carried out at 10 mg/L initial MB concentration with 0.2 g/100 mL adsorbent mass at 20 °C for 24 h equilibrium time. Fig. 3 shows the effect of solution pH on MB adsorptions. All samples showed a similar adsorption efficiency at high pH solution above 11. The adsorption reaction was affected by
3. Results and discussion
TG
417.9
DTA 447.6
Endo 919.1
The pyrophyllite has an Al2[Si4O10](OH)2 unit-cell formula and a hydrous aluminium phyllosilicate with the dioctahedral structure. Since the tetrahedral–octahedral–tetrahedral unit is electrically balanced as neutral on the basal plane, the successive 2:1 layers are held together by Van der Waals forces [28]. Thus, breaks occur easily along the plane between layers and crystal imperfections widely occur in pyrophyllite, for example, Al3+ instead of Si4+ and Fe2+ instead of Al3+. The existence of impurity (K, Fe, Ti, Na) was found, together with the theoretical composition of pure pyrophyllite. A complicated chemical composition plays an important role in ion exchange and adsorption. For instance, since some Al3 + were substituted by Fe2 + or Ti2 +, surface particles would form negative charge sites. Consequently, positive ions must be adsorbed to keep a balanced charge. This is considered to be one of the major factors causing the MB dye molecules to be adsorbed by pyrophyllite particles. Additionally, the chemical analysis of pickling particles showed an obvious decrease of Al3 +, Fe2 + and Ti2 +. Pickling showed that adsorption could be improved by dissolving Al3 +, Fe2 +
Exo
3.1. Characteristics of pyrophyllite
0
200
400
600
800
o
Temperature, C Fig. 1. Thermal analysis of raw pyrophyllite.
1000
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271
100
MB removal, %
concentration (10 mg/L), temperature (20 °C) and pH (7) constant at different contact times. As shown in Fig. 4, the removal was increased with adsorbent mass from 0.1 g/100 mL to 0.3 g/100 mL, while had almost no change from 0.3 g/100 mL to 1.0 g/100 mL. Maximum dye removal was achieved within 5–40 min after which MB concentration in the test solution was almost constant. Increase in the adsorption with adsorbent mass can be attributed to increased adsorbent surface area and availability of more adsorption sites. But unit adsorption decreased with the increase in adsorbent mass. This may be attributed to overlapping or aggregation of adsorption sites resulting in a decrease in total adsorbent surface area available to MB and an increase in diffusion path length. Equilibrium time was lesser at higher adsorbent doses [2].
raw powders pickling powders grinding powders pickling-grinding powders
80
269
60
40
20
3.5. Adsorption kinetics 0 0
10
20
30
40
50
60
MB concentration, mg/L Fig. 2. Effect of adsorbents on MB adsorption (adsorbent mass: 0.2 g/100 mL; pH: 7; equilibrium time: 24 h).
pH as well as the affinities of the electrical charge for the surface. The electrical charge at the oxide surface/aqueous to protonation/deprotonation of the surface hydroxyl can be qualified by [30]: –MOH þ Hþ →MOHþ 2
ð1Þ
–MOH þ OH− →−MO− þ H2 O
ð2Þ
and at the isoelectrical point (IEP) − ½MOHþ 2 ⇋½–MO :
ð3Þ
Since the IEP of pyrophyllite is at about 2.3 [31], the reaction responsible for the surface charge of the solid is mainly due to the reaction of Eq. (2) at pH range of 7 to 11. Consequentially, the pyrophyllite surface in water has a net negative surface charge. In addition, the surface charges become more negative as the pH increases, thus favoring the adsorption of MB cations due to the enhanced electrostatic attractive interactions between the pyrophyllite surface and the MB cations present in the solution. Additionally, it should be noted that all of the adsorption capacities of raw powders increased as the pH increased. 3.4. Effect of adsorbent mass The adsorption of MB was studied by changing the quantity of adsorbent (0.–1.0 g/100 mL) in the test solution while keeping the initial dye
The equilibrium adsorption isotherms are of fundamental importance in the design of adsorption systems. In a batch system, equilibrium is established between the liquid phase (free solution) and the solid phase (adsorbent-attached solute) concentrations. It can be described by adsorption isotherms determined at a fixed temperature. The kinetics of MB adsorption on raw and modified pyrophyllite samples was measured while keeping the initial dye concentration (10 mg/L), temperature (20 °C), adsorbent mass (0.2 g/100 mL), and pH (7). Fig. 5 illustrates the relative adsorption capacity as a function of the reaction time. The adsorption capacity of raw powders arrived 3.71 mg/g, and the numbers of pickling, grinding and pickling–grinding powders increased to 3.83 mg/g, 3.94 mg/g and 4.24 mg/g, respectively. The data suggests that pickling or grinding facilitated the adsorption of pyrophyllite. Among them, pickling–grinding powders appeared to be the best in facilitating the adsorption of pyrophyllite clay. For further evaluation of the adsorption process, this study calculated the kinetic data according to a pseudo-second-order model [17], where the form of the model can be described by following equation: dqt ¼ ks ðqe −qt Þ2 dt
ð4Þ
where ks is the rate constant of the pseudo-second-order model and qe and qt are the amount of the MB adsorbed at equilibrium and at time t, respectively. After integration of Eq. (4) for boundary conditions t = 0 to t = t and qt = 0 to qt = qt, then it can be simplified to t 1 t ¼ þ : qt ks q2e qe
ð5Þ
100
80
95
raw powders pickling powders grinding powders pickling-grinding powders
90
MB removal, %
MB removal, %
100
60
1.0g/100ml 0.3g/100ml 0.2g/100ml 0.1g/100ml
40
85 6
7
8
9
10
11
Initial pH of MB solution
20 0
5
10
15
20
25
30
35
Reaction time, min Fig. 3. Effect of pH on MB adsorption (initial MB concentration: 10 mg/L; adsorbent mass: 0.2 g/100 mL; equilibrium time: 24 h).
Fig. 4. Effect of adsorbent mass on MB absorption.
40
45
270
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271
4.4
qt , mg/g
4.0
3.6
raw particles pickling particles grinding particles pickling-grinding particles
3.2
of raw and modified pyrophyllite powders decreased with the increase of the initial MB concentration in solution. The adsorption reaction was affected by pH as well as the affinities of the electrical charge for the surface. Increase in the adsorption with adsorbent mass can be attributed to increase adsorbent surface area and availability of more adsorption sites. The adsorption capacity of raw powders arrived 3.71 mg/g, and the numbers of pickling, grinding and pickling–grinding powders increased to 3.83 mg/g, 3.94 mg/g and 4.24 mg/g, respectively. A good agreement between the pseudo-second-order function model and the experimental data fits is achieved. Raw and modified pyrophyllite clays are cheap and easily available in the countryside of China. The data may be useful for designing and fabricating an economically cheap treatment process using pyrophyllite clays for the removal of MB from dilute industrial effluents. Acknowledgments
0
5
10
15
20
25
30
Reaction time, min Fig. 5. Adsorption isotherms for MB onto raw and modified pyrophyllite particles.
The authors gratefully acknowledge the financial support for this work from the Zhejiang Provincial Natural Science Foundation of China (2007C11104-2) and Zhejiang Provincial Natural Science Foundation of China (LY14E020004).
Fig. 6 shows the plots of qt against time for MB onto raw and modified t
pyrophyllite powders. A good agreement between the pseudo-secondorder function and the experimental data fits is achieved (R2 N 0.9995). Thus, the calculated rate constant (ks) of raw, pickling, grinding and pickling–grinding samples is obtained as 1.17, 1.01, 0.74, and 0.79, respectively. The process of the removal of MB from aqueous solution by pyrophyllite can be considered as a chemical adsorption or chemisorption involving valence forces through the sharing of electrons between the hydrophilic edge sites of pyrophyllite and polar dye ions.
4. Summary The ability of pyrophyllite to adsorb methylene blue has been investigated in this study. The adsorption of MB was dependent on adsorbent surface characteristics, pH values, adsorbent mass and the initial MB concentration in the solution. Raw pyrophyllite particles were modified by pickling and grinding. Pickling made some Al3+, Fe2+ and Ti2+ to be dissolved from the octahedral layer into acid solution and form more hydrophilic –OH groups on the vacant sites. Grinding caused a sharp reduction in size. It was found that the raw and modified pyrophyllite powders have a high adsorptive capacity for MB. The absorption ability
raw particles pickling particles grinding particles pickling-grinding particles
8
t/qt
6
4
2
0 0
5
10
15
20
25
30
35
Reaction time, min Fig. 6. Plot of qt against time for MB onto raw and modified pyrophyllite particles. t
References [1] G. McKay, M.S. Otterburn, A.G. Sweeney, The removal of color from effluent using various adsorbents. III. Silica: rate process, Water Res. 14 (1) (1980) 15–20. [2] V.K. Garg, M. Amita, R. Kumar, R. Gupta, Basic dye (methylene blue) removal from simulated wastewater by adsorption using Indian Rosewood sawdust: a timber industry waste, Dyes Pigments 63 (2004) 243–250. [3] A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, Adsorption studies on the removal of coloring agent phenol red from wastewater using waste materials as adsorbents, J. Colloid Interface Sci. 337 (2) (2009) 345–354. [4] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, Decoloration treatment of a hazardous triarylmethane dye, Light Green SF (Yellowish) by waste material adsorbents, J. Colloid Interface Sci. 342 (2) (2010) 518–527. [5] V.K. Gupta, S.K. Srivastava, D. Mohan, S. Sharma, Design parameters for fixed bed reactors of activated carbon developed from fertilizer waste for the removal of some heavy metal ions, Waste Manag. 17 (8) (1998) 517–522. [6] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, Adsorptive removal of hazardous anionic dye “Congo red” from wastewater using waste materials and recovery by desorption, J. Colloid Interface Sci. 340 (1) (2009) 16–26. [7] V.K. Gupta, S. Agarwal, T.A. Saleh, Synthesis and characterization of alumina-coated carbon nanotubes and their application for lead removal, J. Hazard. Mater. 185 (1) (2011) 17–23. [8] V.K. Gupta, R. Jain, A. Mittal, T.A. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Photocatalytic degradation of toxic dye amaranth on TiO2/UV in aqueous suspensions, Mater. Sci. Eng. 32 (1) (2012) 12–17. [9] V.K. Gupta, A. Nayak, Cadmium removal and recovery from aqueous solutions by novel adsorbents prepared from orange peel and Fe2O3 nanoparticles, Chem. Eng. J. 180 (2012) 81–90. [10] T.A. Saleh, V.K. Gupta, Photo-catalyzed degradation of hazardous dye methyl orange by use of a composite catalyst consisting of multi-walled carbon nanotubes and titanium dioxide, J. Colloid Interface Sci. 371 (1) (2012) 101–106. [11] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, Removal and recovery of Chrysoidine Y from aqueous solutions by waste materials, J. Colloid Interface Sci. 344 (2) (2010) 497–507. [12] H. Khani, M.K. Rofouei, P. Arab, V.K. Gupta, Z. Vafaei, Multi-walled carbon nanotubes-ionic liquid-carbon paste electrode as a super selectivity sensor: application to potentiometric monitoring of mercury ion(II), J. Hazard. Mater. 183 (1–3) (2010) 402–409. [13] A. Mittal, D. Jhare, J. Mittal, Adsorption of hazardous dye Eosin Yellow from aqueous solution onto waste material De-oiled Soya: isotherm, kinetics and bulk removal, J. Mol. Liq. 179 (2013) 133–140. [14] V.K. Gupta, R. Jain, A. Nayak, S. Agarwal, M. Shrivastava, Removal of the hazardous dye—tartrazine by photodegradation on titanium dioxide surface, Mater. Sci. Eng. C 31 (5) (2011) 1062–1067. [15] M. Stydini, I.K. Dimitris, X.E. Verykios, Visible light-induced photocatalytic degradation of acid orange 7 in aqueous TiO2 suspensions, Appl. Catal. Environ. 47 (2004) 189–201. [16] J. Yi, L. Zhang, Removal of methylene blue dye from aqueous solution by adsorption onto sodium humate/polyacrylamide/clay hybrid hydrogels, Biores. Technol. 99 (2008) 2182–2186. [17] A. Gücek, S. Şener, S. Bilgen, Adsorption and kinetic studies of cationic and anionic dyes on pyrophyllite from aqueous solutions, J. Colloid Interface Sci. 286 (2005) 53–60. [18] M. Doğan, Y. Özdemir, M. Alkan, Adsorption kinetics and mechanism of cationic methyl violet and methylene blue dyes onto sepiolite, Dyes Pigments 75 (2007) 701–713.
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271 [19] B. Bestani, N. Benderdouche, B. Benstaali, M. Belhakem, A. Addou, Methylene blue and iodine adsorption onto an activated desert plant, Biores. Technol. 99 (2008) 8441–8444. [20] T.A. Khan, E.A. Khan, Shahjahan, Removal of basic dyes from aqueous solution by adsorption onto binary iron-manganese oxide coated kaolinite: non-linear isotherm and kinetics modeling, Appl. Clay Sci. 107 (2015) 70–77. [21] E. Eren, B. Afsin, Investigation of a basic dye adsorption from aqueous solution onto raw and pre-treated bentonite surfaces, Dyes Pigments 76 (2008) 220–225. [22] D. Mohan, S. Chander, Removal and recovery of metal ions from acid mine drainage using lignite — a low cost sorbent, J. Hazard. Mater. 137 (2006) 1545–1553. [23] A. Czimerova, L. Jankovic, J. Bujdak, Effect of the exchangeable cations on the spectral properties of methylene blue in clay dispersions, J. Colloid Interface Sci. 274 (2004) 126–132. [24] N. Miyamoto, R. Kawai, K. Kuroda, M. Ogawa, Adsorption and aggregation of a cationic cyanine dye on layered clay minerals, Appl. Clay Sci. 16 (2000) 161–170. [25] A.M. Scheidegger, D.L. Sparks, Kinetics of the formation and the dissolution of nickel surface precipitates on pyrophyllite, Chem. Geol. 132 (1996) 157–164.
271
[26] S. Saxena, M. Prasad, S.S. Amritphale, N. Chandra, Adsorption of cyanide from aqueous solutions at pyrophyllite surface, Sep. Purif. Technol. 24 (2001) 263–270. [27] H. Sayılkan, S. Erdemoğlu, S. Sener, Surface modification of pyrophyllite with amino silane coupling agent for the removal of 4-nitrophenol from aqueous solutions, J. Colloid Interface Sci. 275 (2004) 530–538. [28] M. Erdemoğlu, M. Sarıkaya, The effect of grinding on pyrophyllite flotation, Miner. Eng. 15 (2002) 723–725. [29] L.A. Pérez-Maqueda, O.M. Montes, E.M. González-Macias, F. Franco, J. Poyato, J.L. Pérez-Rodríguez, Thermal transformations of sonicated pyrophyllite, Appl. Clay Sci. 3–4 (2004) 201–207. [30] P. Turan, M. Doğan, M. Alkan, Uptake of trivalent chromium ions from aqueous solutions using kaolinite, J. Hazard. Mater. 148 (2007) 56–63. [31] M. Alkan, Ö. Demirbaş, M. Doğan, Electrokinetic properties of kaolinite in mono- and multivalent electrolyte solutions, Microporous Mesoporous Mater. 1–3 (2005) 51–59.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Removal of methylene blue from aqueous solution by adsorption on pyrophyllite Jian Zhang, Yan Zhou, Meiyan Jiang, Juan Li, Jiawei Sheng ⁎ College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310032, China
a r t i c l e
i n f o
Article history: Received 31 March 2015 Received in revised form 26 May 2015 Accepted 28 May 2015 Available online xxxx Keywords: Pyrophyllite Methylene blue Adsorption Pickling–grinding Kinetics
a b s t r a c t The effectiveness of adsorption for dye removal from solutions has made it an ideal alternative to other expensive treatment methods. The ability of raw and pickling–grinding modified pyrophyllite powders to adsorb methylene blue has been investigated in this study by looking at the dependence of absorption on pH values, adsorbent mass and the initial methylene blue concentration. The measured D50 of raw particles was 21.42 μm, which was decreased to 7.18 μm after grinding treatment, and to 5.55 μm after pickling–grinding. We showed that the raw and modified pyrophyllite powders have a high adsorptive capacity for dyes. The absorption ability of raw and modified pyrophyllite powders decreased with the increase of the initial methylene blue concentration in solution, while increased with the pH value. Increase in the adsorption with adsorbent mass was attributed to increase adsorbent surface area and availability of more adsorption sites. The adsorption capacity of raw powders reached 3.71 mg/g, and the numbers of pickling, grinding and pickling–grinding powders increased to 3.83 mg/g, 3.94 mg/g and 4.24 mg/g, respectively. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to rapid industrial development, pollution of water bodies by industries is an issue of major concern. Many industrial processes in China use different synthetic chemical dyes to color their final product. Most of the used solutions containing dyes are discarded as effluents. Due to the complex chemical structure of these dyes, they are resistant to breakdown by chemical, physical and biological treatments. Color impedes light penetration, retards photosynthetic activity, inhibits the growth of biota and also has a tendency to chelate metal ions which produce micro-toxicity to fish and other organisms [1,2]. Among the various available water treatment techniques adsorption is the most reliable and efficient technique for decoloration, in which the recovery and recycling of the adsorbent materials can be achieved along with the distinct advantages of nonproduction of any toxic sludge and cost effectiveness [3–14]. This has encouraged the development of adsorbents that are abundantly available and economical. The use of clean, cost-efficient, and biodegradable adsorbents could be a good tool to minimize the environmental impact caused by manufacturing and textile byproducts [14–16]. Adsorption is known to be a promising technique, which has great importance due to the ease of operation and comparable low cost of application in the decoloration process. Commercially activated carbon is a remarkably highly adsorbent
⁎ Corresponding author at: College of Materials Science and Engineering, Zhejiang University of Technology, Zhaohui No. 6, Hangzhou, Zhejiang 310032, China. E-mail address: [email protected] (J. Sheng).
http://dx.doi.org/10.1016/j.molliq.2015.05.056 0167-7322/© 2015 Elsevier B.V. All rights reserved.
material with a large number of applications in the remediation of contaminated groundwater and industrial wastes such as colored effluents. Since activated carbon is an expensive adsorbent due to its high costs of manufacturing and regeneration, much attention has been focused on various naturally occurring adsorbents such as chitosan, zeolites, fly ash, coal, papermill sludge, and various clay minerals [17]. Among these new adsorbents, clays have been shown to be the most promising alternatives due to their local availability, technical feasibility, easy engineering applications, highly specific surface area, and cost effectiveness [15,16,18–24]. Nowadays numerous low cost adsorbents are available including products of agricultural origin such as wood dust, sugarcane, fruit peel, wheat straw, and apple pomance [3]. Recently, the application of pyrophyllite on waste water treatment has become of great interest in China because of its abundance in local reserves as well as its inexpensiveness. The characteristics of pyrophyllite are related to its powerful adsorbent properties and its ability to adsorb organic or inorganic ions from aqueous solution. A considerable amount of work has also been reported in the literature regarding the potential use of pyrophyllite in the removal of heavy metals and dye molecules [25–27]; however, the adsorption properties of pyrophyllite on methylene blue (MB) are still scarcely known. The purpose of this study is to focus attention on the adsorption of MB on pyrophyllite from aqueous solutions. The dynamical behaviors of adsorption were measured on the effect of reaction time, MB concentration, pH value and temperature. Furthermore, raw pyrophyllite particles were modified by a pickling–grinding treatment and a comparative study of the removal of MB from aqueous solutions by raw and pickling–grinding particles was carried out as well.
268
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271
2. Materials and methods The pyrophyllite mineral used in this study originated from the Zhejiang province, located in the south-eastern region of China. The pyrophyllite powder composed of (wt.%): 66.08 SiO2, 19.92 Al2O3, 0.70 Fe2O3, 0.64 K2O, 0.21 Na2O, 0.12 CaO, 0.27 Ti2O and 12.33 of weight loss, was used in this study. Particle sizes were analyzed by the LS-POP laser particle size analyzer. Pickling–grinding particles were prepared by pickling raw particles in 0.55 mol/L HCl solution at a temperature of 80 °C for 6 h. The remaining HCl was removed by washing until the pH of the suspension was close to 7. Pickling particles were then milled in water in a Planet Style Ball Mill for 3 h. Finally, the pickling–grinding particles were obtained by filtration and dried at 110 °C for 2 h. Thermogravimetric analysis and differential thermal analysis (TG–DTA) were carried out simultaneously in static air with an automatic thermal analyzer system (Pyris Diamond). To get the TG–DTA curves, powder samples of about 40 mg were packed loosely into a platinum holder and were thermally treated under flow of air at a heating rate of 10 °C/min (range: 20–1000 °C). Calcinated α-alumina was taken as the reference. Methylene blue with a labeled purity of more than 98% was used as model dye without further purification. Deionized water was used to make the dye solutions of desired concentration. The MB solution shows an intense absorption peak in the visible region at 665 nm. In an adsorption process, a change in the intensity of this peak can be used to characterize the removal of dye from the solution. The adsorption of MB on pyrophyllite was calculated by monitoring the changes in absorption value of the solution on a spectrophotometer (Shimadzu UV-2550) at ambient room temperature, using a 1 cm quartz cell. Adsorption tests of MB on prepared pyrophyllite particles were studied using a batch process by mixing 0.1–1.0 g of adsorbent mass in plugged conical flasks with 100 mL of MB solutions of concentration ranging from 10 to 60 mg/L. The contents of the dye solution were then shaken for a given amount of reaction time using a magnetic stirrer operated at a constant speed. A constant temperature bath was used to keep the temperature constant at 20 ± 1 °C. The effect of pH was studied over a pH range of 7–11. The pH value was adjusted by the addition of dilute aqueous solutions of NaOH (0.10 M). Samples were withdrawn at appropriate time intervals and the contents were then centrifuged and the supernatant solution was pipetted out and monitored instantaneously on the spectrophotometer for absorption values. The absorbance values obtained in solutions before and after adsorption were then used to calculate the removal of the MB on pyrophyllite powders. Each experimental point was an average of three independent adsorption tests.
and Ti2 + from the octahedral layer into acid solution and forming more hydrophilic –OH groups on the vacant sites. Adsorption is further influenced by particle sizes [3]. The measured D50 of raw particles was 21.42 μm, while the D50 of pickling particles was decreased to 7.18 μm after grinding treatment, and to 5.55 μm after pickling–grinding. Pickling particles were milled to decrease the particle size, thus caused the destruction of Van der Waals forces [28]. The TG–DTA curves of raw pyrophyllite are shown in Fig. 1. According to directions of the various peaks, the DTA curve can be divided into three regions. In the first region, up to 100 °C, endothermic dehydration of pyrophyllite is the major thermal reaction. The second region is between 380 and 700 °C, and the mass loss for pyrophyllite in this region is 2.7%. The exothermic DTA peaks at 417.9 and 447.6 °C may be expected from the release of structural OH for an ideal pyrophyllite. The third region occurs at temperatures above 700 °C, and the broad endothermic peak centered at 919.1 °C may be due to the dehydroxylation reaction of pyrophyllite and the crystallization of mullite [21,29]. 3.2. Methylene blue adsorption on raw and modified pyrophyllite Experiments were conducted with raw and modified pyrophyllite at constant adsorbent dosage (0.2 g/100 mL), pH (neutral), and temperature (20 °C) for 24 h by varying MB concentrations (10–60 mg/L). It is evident from Fig. 2 that pickling–grinding powder has more adsorption efficiency in comparison to other powders at all initial MB concentrations studied. Decolorization of solution was 70.1%, 71.2%, 75.5% and 79.7% by raw, picking, grinding, and pickling–grinding powders, respectively, at 20 mg/L dye concentration. The absorption ability of four powders decreased with the increase of the initial MB concentration in solution. Four materials had almost similar adsorption efficiency and intendancy if initial MB concentration in solution was up to 60 mg/L. The difference observed here may arise from the morphology and size of the particles and the total surface area of the materials used [21]. 3.3. Effect of solution pH pH is one of the most influencing factors for dye adsorption as it directly affects the dissociative and adsorptive ability of the dye on the adsorbent surface [3]. To study the effect of pH on MB adsorption on raw and modified pyrophyllite, the experiments were carried out at 10 mg/L initial MB concentration with 0.2 g/100 mL adsorbent mass at 20 °C for 24 h equilibrium time. Fig. 3 shows the effect of solution pH on MB adsorptions. All samples showed a similar adsorption efficiency at high pH solution above 11. The adsorption reaction was affected by
3. Results and discussion
TG
417.9
DTA 447.6
Endo 919.1
The pyrophyllite has an Al2[Si4O10](OH)2 unit-cell formula and a hydrous aluminium phyllosilicate with the dioctahedral structure. Since the tetrahedral–octahedral–tetrahedral unit is electrically balanced as neutral on the basal plane, the successive 2:1 layers are held together by Van der Waals forces [28]. Thus, breaks occur easily along the plane between layers and crystal imperfections widely occur in pyrophyllite, for example, Al3+ instead of Si4+ and Fe2+ instead of Al3+. The existence of impurity (K, Fe, Ti, Na) was found, together with the theoretical composition of pure pyrophyllite. A complicated chemical composition plays an important role in ion exchange and adsorption. For instance, since some Al3 + were substituted by Fe2 + or Ti2 +, surface particles would form negative charge sites. Consequently, positive ions must be adsorbed to keep a balanced charge. This is considered to be one of the major factors causing the MB dye molecules to be adsorbed by pyrophyllite particles. Additionally, the chemical analysis of pickling particles showed an obvious decrease of Al3 +, Fe2 + and Ti2 +. Pickling showed that adsorption could be improved by dissolving Al3 +, Fe2 +
Exo
3.1. Characteristics of pyrophyllite
0
200
400
600
800
o
Temperature, C Fig. 1. Thermal analysis of raw pyrophyllite.
1000
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271
100
MB removal, %
concentration (10 mg/L), temperature (20 °C) and pH (7) constant at different contact times. As shown in Fig. 4, the removal was increased with adsorbent mass from 0.1 g/100 mL to 0.3 g/100 mL, while had almost no change from 0.3 g/100 mL to 1.0 g/100 mL. Maximum dye removal was achieved within 5–40 min after which MB concentration in the test solution was almost constant. Increase in the adsorption with adsorbent mass can be attributed to increased adsorbent surface area and availability of more adsorption sites. But unit adsorption decreased with the increase in adsorbent mass. This may be attributed to overlapping or aggregation of adsorption sites resulting in a decrease in total adsorbent surface area available to MB and an increase in diffusion path length. Equilibrium time was lesser at higher adsorbent doses [2].
raw powders pickling powders grinding powders pickling-grinding powders
80
269
60
40
20
3.5. Adsorption kinetics 0 0
10
20
30
40
50
60
MB concentration, mg/L Fig. 2. Effect of adsorbents on MB adsorption (adsorbent mass: 0.2 g/100 mL; pH: 7; equilibrium time: 24 h).
pH as well as the affinities of the electrical charge for the surface. The electrical charge at the oxide surface/aqueous to protonation/deprotonation of the surface hydroxyl can be qualified by [30]: –MOH þ Hþ →MOHþ 2
ð1Þ
–MOH þ OH− →−MO− þ H2 O
ð2Þ
and at the isoelectrical point (IEP) − ½MOHþ 2 ⇋½–MO :
ð3Þ
Since the IEP of pyrophyllite is at about 2.3 [31], the reaction responsible for the surface charge of the solid is mainly due to the reaction of Eq. (2) at pH range of 7 to 11. Consequentially, the pyrophyllite surface in water has a net negative surface charge. In addition, the surface charges become more negative as the pH increases, thus favoring the adsorption of MB cations due to the enhanced electrostatic attractive interactions between the pyrophyllite surface and the MB cations present in the solution. Additionally, it should be noted that all of the adsorption capacities of raw powders increased as the pH increased. 3.4. Effect of adsorbent mass The adsorption of MB was studied by changing the quantity of adsorbent (0.–1.0 g/100 mL) in the test solution while keeping the initial dye
The equilibrium adsorption isotherms are of fundamental importance in the design of adsorption systems. In a batch system, equilibrium is established between the liquid phase (free solution) and the solid phase (adsorbent-attached solute) concentrations. It can be described by adsorption isotherms determined at a fixed temperature. The kinetics of MB adsorption on raw and modified pyrophyllite samples was measured while keeping the initial dye concentration (10 mg/L), temperature (20 °C), adsorbent mass (0.2 g/100 mL), and pH (7). Fig. 5 illustrates the relative adsorption capacity as a function of the reaction time. The adsorption capacity of raw powders arrived 3.71 mg/g, and the numbers of pickling, grinding and pickling–grinding powders increased to 3.83 mg/g, 3.94 mg/g and 4.24 mg/g, respectively. The data suggests that pickling or grinding facilitated the adsorption of pyrophyllite. Among them, pickling–grinding powders appeared to be the best in facilitating the adsorption of pyrophyllite clay. For further evaluation of the adsorption process, this study calculated the kinetic data according to a pseudo-second-order model [17], where the form of the model can be described by following equation: dqt ¼ ks ðqe −qt Þ2 dt
ð4Þ
where ks is the rate constant of the pseudo-second-order model and qe and qt are the amount of the MB adsorbed at equilibrium and at time t, respectively. After integration of Eq. (4) for boundary conditions t = 0 to t = t and qt = 0 to qt = qt, then it can be simplified to t 1 t ¼ þ : qt ks q2e qe
ð5Þ
100
80
95
raw powders pickling powders grinding powders pickling-grinding powders
90
MB removal, %
MB removal, %
100
60
1.0g/100ml 0.3g/100ml 0.2g/100ml 0.1g/100ml
40
85 6
7
8
9
10
11
Initial pH of MB solution
20 0
5
10
15
20
25
30
35
Reaction time, min Fig. 3. Effect of pH on MB adsorption (initial MB concentration: 10 mg/L; adsorbent mass: 0.2 g/100 mL; equilibrium time: 24 h).
Fig. 4. Effect of adsorbent mass on MB absorption.
40
45
270
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271
4.4
qt , mg/g
4.0
3.6
raw particles pickling particles grinding particles pickling-grinding particles
3.2
of raw and modified pyrophyllite powders decreased with the increase of the initial MB concentration in solution. The adsorption reaction was affected by pH as well as the affinities of the electrical charge for the surface. Increase in the adsorption with adsorbent mass can be attributed to increase adsorbent surface area and availability of more adsorption sites. The adsorption capacity of raw powders arrived 3.71 mg/g, and the numbers of pickling, grinding and pickling–grinding powders increased to 3.83 mg/g, 3.94 mg/g and 4.24 mg/g, respectively. A good agreement between the pseudo-second-order function model and the experimental data fits is achieved. Raw and modified pyrophyllite clays are cheap and easily available in the countryside of China. The data may be useful for designing and fabricating an economically cheap treatment process using pyrophyllite clays for the removal of MB from dilute industrial effluents. Acknowledgments
0
5
10
15
20
25
30
Reaction time, min Fig. 5. Adsorption isotherms for MB onto raw and modified pyrophyllite particles.
The authors gratefully acknowledge the financial support for this work from the Zhejiang Provincial Natural Science Foundation of China (2007C11104-2) and Zhejiang Provincial Natural Science Foundation of China (LY14E020004).
Fig. 6 shows the plots of qt against time for MB onto raw and modified t
pyrophyllite powders. A good agreement between the pseudo-secondorder function and the experimental data fits is achieved (R2 N 0.9995). Thus, the calculated rate constant (ks) of raw, pickling, grinding and pickling–grinding samples is obtained as 1.17, 1.01, 0.74, and 0.79, respectively. The process of the removal of MB from aqueous solution by pyrophyllite can be considered as a chemical adsorption or chemisorption involving valence forces through the sharing of electrons between the hydrophilic edge sites of pyrophyllite and polar dye ions.
4. Summary The ability of pyrophyllite to adsorb methylene blue has been investigated in this study. The adsorption of MB was dependent on adsorbent surface characteristics, pH values, adsorbent mass and the initial MB concentration in the solution. Raw pyrophyllite particles were modified by pickling and grinding. Pickling made some Al3+, Fe2+ and Ti2+ to be dissolved from the octahedral layer into acid solution and form more hydrophilic –OH groups on the vacant sites. Grinding caused a sharp reduction in size. It was found that the raw and modified pyrophyllite powders have a high adsorptive capacity for MB. The absorption ability
raw particles pickling particles grinding particles pickling-grinding particles
8
t/qt
6
4
2
0 0
5
10
15
20
25
30
35
Reaction time, min Fig. 6. Plot of qt against time for MB onto raw and modified pyrophyllite particles. t
References [1] G. McKay, M.S. Otterburn, A.G. Sweeney, The removal of color from effluent using various adsorbents. III. Silica: rate process, Water Res. 14 (1) (1980) 15–20. [2] V.K. Garg, M. Amita, R. Kumar, R. Gupta, Basic dye (methylene blue) removal from simulated wastewater by adsorption using Indian Rosewood sawdust: a timber industry waste, Dyes Pigments 63 (2004) 243–250. [3] A. Mittal, D. Kaur, A. Malviya, J. Mittal, V.K. Gupta, Adsorption studies on the removal of coloring agent phenol red from wastewater using waste materials as adsorbents, J. Colloid Interface Sci. 337 (2) (2009) 345–354. [4] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, Decoloration treatment of a hazardous triarylmethane dye, Light Green SF (Yellowish) by waste material adsorbents, J. Colloid Interface Sci. 342 (2) (2010) 518–527. [5] V.K. Gupta, S.K. Srivastava, D. Mohan, S. Sharma, Design parameters for fixed bed reactors of activated carbon developed from fertilizer waste for the removal of some heavy metal ions, Waste Manag. 17 (8) (1998) 517–522. [6] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, Adsorptive removal of hazardous anionic dye “Congo red” from wastewater using waste materials and recovery by desorption, J. Colloid Interface Sci. 340 (1) (2009) 16–26. [7] V.K. Gupta, S. Agarwal, T.A. Saleh, Synthesis and characterization of alumina-coated carbon nanotubes and their application for lead removal, J. Hazard. Mater. 185 (1) (2011) 17–23. [8] V.K. Gupta, R. Jain, A. Mittal, T.A. Saleh, A. Nayak, S. Agarwal, S. Sikarwar, Photocatalytic degradation of toxic dye amaranth on TiO2/UV in aqueous suspensions, Mater. Sci. Eng. 32 (1) (2012) 12–17. [9] V.K. Gupta, A. Nayak, Cadmium removal and recovery from aqueous solutions by novel adsorbents prepared from orange peel and Fe2O3 nanoparticles, Chem. Eng. J. 180 (2012) 81–90. [10] T.A. Saleh, V.K. Gupta, Photo-catalyzed degradation of hazardous dye methyl orange by use of a composite catalyst consisting of multi-walled carbon nanotubes and titanium dioxide, J. Colloid Interface Sci. 371 (1) (2012) 101–106. [11] A. Mittal, J. Mittal, A. Malviya, V.K. Gupta, Removal and recovery of Chrysoidine Y from aqueous solutions by waste materials, J. Colloid Interface Sci. 344 (2) (2010) 497–507. [12] H. Khani, M.K. Rofouei, P. Arab, V.K. Gupta, Z. Vafaei, Multi-walled carbon nanotubes-ionic liquid-carbon paste electrode as a super selectivity sensor: application to potentiometric monitoring of mercury ion(II), J. Hazard. Mater. 183 (1–3) (2010) 402–409. [13] A. Mittal, D. Jhare, J. Mittal, Adsorption of hazardous dye Eosin Yellow from aqueous solution onto waste material De-oiled Soya: isotherm, kinetics and bulk removal, J. Mol. Liq. 179 (2013) 133–140. [14] V.K. Gupta, R. Jain, A. Nayak, S. Agarwal, M. Shrivastava, Removal of the hazardous dye—tartrazine by photodegradation on titanium dioxide surface, Mater. Sci. Eng. C 31 (5) (2011) 1062–1067. [15] M. Stydini, I.K. Dimitris, X.E. Verykios, Visible light-induced photocatalytic degradation of acid orange 7 in aqueous TiO2 suspensions, Appl. Catal. Environ. 47 (2004) 189–201. [16] J. Yi, L. Zhang, Removal of methylene blue dye from aqueous solution by adsorption onto sodium humate/polyacrylamide/clay hybrid hydrogels, Biores. Technol. 99 (2008) 2182–2186. [17] A. Gücek, S. Şener, S. Bilgen, Adsorption and kinetic studies of cationic and anionic dyes on pyrophyllite from aqueous solutions, J. Colloid Interface Sci. 286 (2005) 53–60. [18] M. Doğan, Y. Özdemir, M. Alkan, Adsorption kinetics and mechanism of cationic methyl violet and methylene blue dyes onto sepiolite, Dyes Pigments 75 (2007) 701–713.
J. Zhang et al. / Journal of Molecular Liquids 209 (2015) 267–271 [19] B. Bestani, N. Benderdouche, B. Benstaali, M. Belhakem, A. Addou, Methylene blue and iodine adsorption onto an activated desert plant, Biores. Technol. 99 (2008) 8441–8444. [20] T.A. Khan, E.A. Khan, Shahjahan, Removal of basic dyes from aqueous solution by adsorption onto binary iron-manganese oxide coated kaolinite: non-linear isotherm and kinetics modeling, Appl. Clay Sci. 107 (2015) 70–77. [21] E. Eren, B. Afsin, Investigation of a basic dye adsorption from aqueous solution onto raw and pre-treated bentonite surfaces, Dyes Pigments 76 (2008) 220–225. [22] D. Mohan, S. Chander, Removal and recovery of metal ions from acid mine drainage using lignite — a low cost sorbent, J. Hazard. Mater. 137 (2006) 1545–1553. [23] A. Czimerova, L. Jankovic, J. Bujdak, Effect of the exchangeable cations on the spectral properties of methylene blue in clay dispersions, J. Colloid Interface Sci. 274 (2004) 126–132. [24] N. Miyamoto, R. Kawai, K. Kuroda, M. Ogawa, Adsorption and aggregation of a cationic cyanine dye on layered clay minerals, Appl. Clay Sci. 16 (2000) 161–170. [25] A.M. Scheidegger, D.L. Sparks, Kinetics of the formation and the dissolution of nickel surface precipitates on pyrophyllite, Chem. Geol. 132 (1996) 157–164.
271
[26] S. Saxena, M. Prasad, S.S. Amritphale, N. Chandra, Adsorption of cyanide from aqueous solutions at pyrophyllite surface, Sep. Purif. Technol. 24 (2001) 263–270. [27] H. Sayılkan, S. Erdemoğlu, S. Sener, Surface modification of pyrophyllite with amino silane coupling agent for the removal of 4-nitrophenol from aqueous solutions, J. Colloid Interface Sci. 275 (2004) 530–538. [28] M. Erdemoğlu, M. Sarıkaya, The effect of grinding on pyrophyllite flotation, Miner. Eng. 15 (2002) 723–725. [29] L.A. Pérez-Maqueda, O.M. Montes, E.M. González-Macias, F. Franco, J. Poyato, J.L. Pérez-Rodríguez, Thermal transformations of sonicated pyrophyllite, Appl. Clay Sci. 3–4 (2004) 201–207. [30] P. Turan, M. Doğan, M. Alkan, Uptake of trivalent chromium ions from aqueous solutions using kaolinite, J. Hazard. Mater. 148 (2007) 56–63. [31] M. Alkan, Ö. Demirbaş, M. Doğan, Electrokinetic properties of kaolinite in mono- and multivalent electrolyte solutions, Microporous Mesoporous Mater. 1–3 (2005) 51–59.