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Kinetics and equilibrium studies from the methylene blue adsorption on diatomite treated with sodium hydroxide
Applied Clay Science 83–84 (2013) 12–16
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Kinetics and equilibrium studies from the methylene blue adsorption on diatomite treated with sodium hydroxide Zhang Jian ⁎, Ping Qingwei, Niu Meihong, Shi Haiqiang, Li Na Liaoning Key Laboratory of Pulp and Paper Engineering, School of Light industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 22 May 2013 Accepted 6 August 2013 Available online 3 September 2013 Keywords: Diatomite Adsorption Kinetics Equilibrium Thermodynamic Methylene blue
a b s t r a c t Diatomite was treated with sodium hydroxide to remove impurity in order to improve its performance as an adsorbent. The raw diatomite and purified diatomite were characterized by scanning electron microscopy, energy dispersive X-ray analysis and Brunauer–Emmett–Teller adsorption. It was found that the surface area was in order of 15.87 m2 g−1 for raw diatomite and 31.35 m2 g−1 for purified diatomite. Scanning electron microscopy images showed the well-developed porous structure of purified diatomite. Purified diatomite improved a more than tenfold increase in adsorption amount from 1.72 mg g−1 to 18.15 mg g−1 and removal efficiency from 8.60% to 90.75% for methyelen blue initial concentration 100 ppm respectively. The kinetics studies showed that experiment data followed pseudo-second-order model better. The equilibrium data was fitted to Langmuir and Freundlich adsorption isotherms and was found that Langmuir model presented the best fit, showing maximum monolayer adsorption capacity of 27.86 mg g−1. The thermodynamic parameters such as the standard enthalpy, standard entropy and standard free energy were evaluated. The obtained results indicated the adsorption of methylene blue onto diatomite treated with sodium hydroxide is endothermic and spontaneous process and confirmed the applicability of this purified inorganic material as an efficient adsorbent for basic dyes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It is estimated that the annual dye production in the world exceeds 7 × 105 tons and more than 100,000 commercially available dyes of different chemical and physical properties are being used (Lee et al., 2006). The complex aromatic structures and xenobiotic properties of dyes make them more difficult to degrade (Asfour et al., 1985; Parida et al., 2011). Therefore it is necessary to reduce dye concentration in the wastewater before their biological treatment processes. Methylene blue (MB), an organic dye, has wide applications (Sheng et al., 2009). Due to its known strong adsorption onto solids, MB often serves as a model compound for removing dyes and organic contaminants from aqueous solutions (Hameed et al., 2007). Although not strongly poisonous, MB can have some harmful effects on human beings (Hajjaji and El Arfaoui, 2009). Owing to these harmful effects on humans, it is necessary to remove MB from aqueous solution. To remove the dyes from aqueous media, different adsorbents including activated carbon (Dabrowski et al., 2005), zeolites (Ozdemir et al., 2004), bentonite (Hong et al., 2009), kaolinite (Ghosh and Bhattacharyya, 2002), garlic peel (Hameed and Ahmad, 2009),
⁎ Corresponding author. Tel.: +86 15942847053; fax: +86 41186323736. E-mail address: [email protected] (J. Zhang). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.08.008
halloysite nanotube (Kiani et al., 2011) and carbon nanotube (Yao et al., 2010) have been studied for adsorption of dye from aqueous solutions and some of them expressed good dye adsorption property. However investigations related to development of novel effective and economical adsorbents are still in progress. Diatomite is fine-grained, low-density biogenic sediment, which consists essentially of amorphous silica (SiO2 · nH2O) derived from opalescent frustules of diatoms (Sljivic et al., 2009). It consists of a wide variety of shapes and characterized by a high porosity up to 80%, low density and high surface area (Al-Qodah et al., 2007). These properties suggest that diatomite is a potential adsorbent for pollutants found in industrial wastewater including dyes (Al-Degs et al., 2001; Al-Ghouti et al., 2004; Khraisheh et al., 2004). Previous studies indicated that after acid treatment, diatomite pore texture can be obtained and has been used successfully to remove dyes. However the acid concentration was usually too high (5 M) (Al-Qodah et al., 2007) and that would hard to operate and easy polluted. Much attention has been focused on the diatomite purification process of low-cost, easy operation and environment friendly. The focus of this research was to purify diatomite by low concentration sodium hydroxide to enhance the adsorptive capabilities of diatomite for basic dye and evaluate its potential for adsorption in removal of methylene blue from the aqueous solution. The characterizations of the diatomite were also performed. In order to establish the removal capacity of purified diatomite, different models of isotherms and adsorption kinetics were fitted to the experiment data.
J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16 1=2
Qt ¼ Ki t
2. Experiment
þC
13
ð5Þ
2.1. Preparation of purified diatomite The raw diatomite samples were bought from Sigma-Aldrich Co. LLC. The elements composition of there sample obtained by energy dispersive X-ray analysis is silica, aluminum, calcium, magnesium, iron and sodium. The particles were passed through a 150 μm mesh metal sieve. The fraction of the particles between 63–250 μm was used for the purification experiments. The purified diatomite was obtained by sodium hydroxide (Sigma-Aldrich, Germany) to enhance the adsorption capacity. The raw diatomite samples were immersed in sufficient amount of 5% (w/w) sodium hydroxide solution at 100°C for 2 h to remove impurities and organics. The digested diatomite was washed several times by deionized water, filtered, dried at 105°C, sieved and stored in closed containers for further tests. The surface area the raw diatomite and purified diatomite was determined from the linear part of the BET plot (P/P0 = 0.05 ~ 0.20) at 77 K using a Quantachrome Autosorb NOVA 2200e volumetric analyzer. Scamming electron microscopy analysis (JEOL, JSM-6460LV) was carried out for raw diatomite and purified diatomite to study the development of morphology and elements. 2.2. Adsorption studies The cationic dye, methylene blue (MB) (Sigma-Aldrich, Germany) was used as an adsorbate. A stock of solution of methylene blue (500 mg L− 1) was prepared and further diluted to the required concentration before used. Batch adsorption was performed in a set of 100 mL flasks containing 50 mL of MB solution with various initial concentrations. The amount of 0.25 g of diatomite was added and equilibrated at different temperature in a temperaturecontrolled water bath shaker (WS-300). After adsorption equilibrium, the concentration of MB in the solution was measured using a UV– visible spectrophotometer (PerkinElmer, LAMBDA35) at 664 nm. The adsorbed capacity (Qe) and removal efficiency (R) of MB adsorbed onto diatomite were calculated according to the following equations: Qe ¼ ðCo−CeÞV=M
ð1Þ
R ¼ ðCo−CeÞ 100=Co
ð2Þ
where Qe is the adsorption capacity at equilibrium, mg g− 1; R is the removal efficiency, %; Co and Ce are the initial and equilibrium concentration of MB in solution, mg L− 1; V is the volume of solution, L; M is the mass of diatomite, g.(Liua et al., 2012). 2.3. Adsorption kinetics Kinetic studies were carried out in flask where a fixed mass of diatomite of 0.25 g was introduced into 50 mL 20 ppm of methylene blue solution at 25 °C. In order to determine the best kinetic model which fits the adsorption experimental data, the pseudo-first-order (Malash and El-Khaiary, 2010), pseudo-second-order (Han et al., 2009) and intraparticle diffusion (Mahmoodi et al., 2012) models were used to understand the adsorption dynamics in relation to time for the methylene blue and diatomite system. These models can be described as follows: LnðQe−QtÞ ¼ lnQe−K1 t
ð3Þ
t 1 1 ¼ t þ Qt K 2 Qe2 Qe
ð4Þ
Where Qt is the amount of adsorbed MB onto diatomite at t moment, mg g−1; K1 (min−1), K2 (g mg−1 min−1), Ki (mg g−1 min−1/2) are the rate constants of the adsorption in pseudo-first-order (Eq. (3)), pseudoseconde-order (Eq. (4)) and intraparticle diffusion (Eq. (5)) respectively. 2.4. Adsorption isotherm Isotherm studies were carried out in flask where a fixed mass of diatomite of 0.25 g was introduced into 50 mL of methylene blue solution with different concentration at 25 °C, 35 °C and 45 °C. The application of adsorption isotherms is very useful to describe the interaction between the adsorbate and the adsorbent of any system. Adsorption coverage over the surface of diatomite was studies using the two well-known isotherm models, Langmuir and Freundlich (Shawabkeh and Tutunji, 2003). The parameters obtained from the different models provide important information on the sorption mechanisms and the surface properties and affinities of the adsorbents. The Langmuir model (Langmuir, 1918) and Freundlish model (Freundlich, 1932) were as follows: Ce Ce 1 ¼ þ Qe Q max Q maxK L
ð7Þ
1 lnCe n
ð8Þ
ln Qe ¼ ln K F þ
Where Qmax is the maximum adsorption capacity, mgg−1; KL is a Langmuir constant relate to the affinity of the binding sites and energy of adsorption, L g−1; KF is a Freundlish constant related to adsorption capacity, L g−1; 1/n is an empirical parameter related to adsorption intensity. 2.5. Adsorption thermodynamic The thermodynamic parameters for the adsorption process, the standard free energy, standard enthalpy and standard entropy were calculated using the following equations (Hsu et al., 2008): ⊿G ¼ −RTlnKL
ð9Þ
⊿G ¼ ⊿H−T⊿S
ð10Þ
lnKL ¼ ⊿S=R−⊿H=RT
ð11Þ
where ⊿G is the standard free energy, kJ mol−1; R is the universal gas constant, 8.314 J mol−1 K−1; T is the absolute solution temperature, K; ⊿H is the standard enthalpy, kJ mol−1; ⊿S is the standard entropy, J K−1. The standard enthalpy and standard entropy values can be calculated from the slope and intercept of the plot of lnKL versus 1/T. 3. Results and discussion 3.1. Characterization of diatomite The specific surface area of raw diatomite was 15.87 m2 g−1. A surface area of 4.21 m2 g−1 for diatomaceous earth was previously reported (Tsai et al., 2006), the different between these two kind of diatomite may caused by the different manufacturing place and measuring method. As SiO2 is the main component of diatomite, treatment with alkalis leads to the formation of soluble silicates SiO2− 3 resulting in the creation of larger pores, flaws, cracks, crevices and in a larger surface area. The total surface area of purified diatomite treated with 5% sodium hydroxide was 31.35 m2 g−1. The sodium hydroxide treatment increased diatomite surface area two times.
14
J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16
Fig. 1. SEM image of datomite.
The morphological structures of raw diatomite and purified diatomite were characterized by SEM and shown in Fig. 1 (Fig. 1a was the SEM image of raw diatomite and Fig. 1b was the SEM image of purified diatomite). The SEM image of raw diatomite in Fig. 1a showed that the structure of diatomite was integral and liked a circle screen. There were many small pores on the surface of sieve tray. However these structures were usually responsible for impurities adhesion, which made the pores smaller or even blocked. The general appearance of the circle screen structure was preserved in the purified diatomite treated in 5% sodium hydroxide and there was no complete collapse of the structure (Fig. 1b). After treatment with sodium hydroxide, the diatomite still showed its multi-pore structure and the pores on the surface became larger. The impurity adhesion was almost gone, which made the pores on the surface of sieve tray larger than that of raw diatomite. In order to verify the removal of the impurity in diatomite, the elements analysis of diatomite was performed by EDX and shown in Fig. 2 (Fig. 2a was EDX spectra of raw diatomite and Fig. 2b was EDX spectra of purified diatomite). Fig. 2a showed that there were some impurity in raw diatomite, such as sodium, magnesium, aluminum, calcium and iron. After treated with sodium hydroxide, the main element silica was preserved and the impurity was removed to a certain extent (Fig. 2b). The EDX spectra of sodium, magnesium and aluminum decreased somewhat. However the EDX spectra of calcium and iron were almost all disappeared. The results were coincident with the experiment data of surface area and Fig. 1. The efficiency of methylene blue removal for raw diatomite and purified diatomite at different initial methylene blue concentration was investigated (Fig. 3).
Fig. 3. Adsorption effect of diatomite.
The amount of adsorbed methylene blue onto purified diatomite increased significantly, no matter for adsorption capacity or for removal efficiency. The Qe value of purified diatomite reached 18.15 mg g−1 while that of raw diatomite was just 1.72 mg g−1 for methylene blue initial concentration 100 ppm. Whereas the R values of purified diatomite and raw diatomite were 8.60% and 90.75% respectively. Therefore the purified diatomite seemed to be a more efficient material than raw diatomite in the methylene blue removal. 3.2. Adsorption kinetics Kinetic studies are important to understand the dynamic of the reaction in terms of order of the rate constant (Cazetta et al., 2011). Since the kinetics parameters provide information for designing and modeling the adsorption process. Three kinetic models, pseudo-first-order model, pseudo-second-order model and diffusion intraparticle diffusion model, were used to examine the mechanism of the adsorption process. The study of the kinetics of adsorption results were shown in Table 1. It is evident from these figures that the experimental points for methylene blue adsorption onto diatomite treated with sodium hydroxide and raw diatomite were best described by the pseudosecond order model. This fact confirmed by the values of R2 shown
Fig. 2. EDX spectra of datomite.
J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16 Table 1 Adsorption kinetic parameters of methylene blue on diatomite. Diatomite sample Qe exp./mg g−1 Pseudo-first-order
Pseudo-second-order
Intraparticle diffusion
−1
Qe cal./mg g K1/min−1 R2 Qe cal./mg g−1 K2/g mg−1 min−1 R2 Ki/mg g−1 min1/2 C R2
Table 2 Adsorption isotherm parameters of methylene blue onto diatomite.
Raw
Purified
0.569
3.844
0.854 0.0136 0.8678 0.755 0.0110 0.9901 0.0317 0.0060 0.9750
2.496 0.0247 0.8612 4.177 0.0096 0.9977 0.1941 0.8088 0.8381
in Table 1 which were 0.9977 and 0.9901 respectively. While the R2 for pseudo-first-order model and intraparticle diffusion model were all less than 0.99 for different diatomite. This suggested that the rate of the adsorption process was preferably controlled by chemisorption. Similar results have been reported by Gholamereza Kiani et al. (Kiani et al., 2011). 3.3. Adsorption isotherm The significance of the adsorption isotherms is that they show how the adsorbate molecules are distributed between the solution and the adsorbent at the equilibrium conditions and the effect of equilibrium concentration on the loading capacity at different temperatures. (Hameed et al., 2008) The effect of methylene blue equilibrium concentration on the adsorption capacity of purified diatomite was carried out at 25 °C, 35 °C and 45 °C as shown in Fig. 4. It is clear that the adsorption capacity of purified diatomite increased as the temperature increased. However the increase was small as the temperature increased from 25 °C to 45 °C, especially from 25 °C to 35 °C. This small increase could be referred to the reversible nature of the adsorption process. This may be caused by the increase of the intraparticle diffusion of methylene blue as the temperature increased. Moreover the desorption phenomena impeded totally as the temperature increased (Al-Ghouti et al., 2003). The adsorption capacity of methylene blue onto purified diatomite increased with increase in temperature, suggesting that the adsorption was endothermic. Two different isotherm models were used to fit the experimental data. Based on the Eqs. (7) and (8), the values of the models constants
Fig. 4. Adsorption isotherm for MB onto purified diatomite.
15
T/°C
25 35 45
Langmuir
Freundlich
Qmax/mg g−1
KL
R2
KF
n
R2
27.86 27.24 26.32
0.2159 0.2493 0.4439
0.9902 0.9902 0.9904
4.5476 4.9268 7.0364
1.40 1.42 1.49
0.9756 0.9730 0.9704
in addition to the values of the correlation coefficient were calculated by plotting lnQe/Ce against Qe and lnQe against lnCe from the slope and intercept at different temperatures (Table 2). According to the coefficients of determination, the Langmuir model fitted better than Freundlich model. It was evident from Table 2 the Langmuir isotherm gave the best fit as indicated by the relative high value of R2 (R2 N 0.99). The experiment data indicated that the adsorption of methylene blue onto purified diatomite was characterized by monolayer coverage of the methylene blue at the outer suface and with maximum adsorption capacity of 27.86mgg-1 at 25°C. It was clear that the values of KF and n increased as the temperature increased, indicating that the adsorption was favorable at high temperature. (Sharma, 2010)
3.4. Adsorption thermodynamic The thermodynamic parameters of the adsorption process of methylene blue onto diatomite treated with sodium hydroxide are the changes in standard enthalpy, standard entropy and standard free energy. The standard entropy and standard enthalpy were obtained from the slope of the plot lnKL versus 1/T as shown in Fig. 5. The values of these parameters were calculated using Eqs. (9), (10) and (11). at various temperature and were shown in Table 3. Table 3 showed that the value of H was positive indicating the endothermic nature of the adsorption of methylene blue onto purified diatomite at different temperature. The positive value of S inferred the affinity of the purified diatomite adsorbent for methylene blue as a result of the increased randomness at the interface between the solid-liquid phases. The values of G were all negative at difference temperature, indicating that the adsorption was spontaneous. The increase in G value with increasing temperature revealed that adsorption of methylene blue onto purified diatomite became more favorable at higher temperature. This fact was previously confirmed by the isotherm experiments at different temperatures.
Fig. 5. Plot of lnKL versus 1/T.
16
J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16
Table 3 Adsorption thermodynamics data of methylene blue onto purified diatomite. T/°C
H/KJmol−1
S/KJmol−1
G/KJmol−1
25 35 45
28.24
0.1388
−13.15 −14.54 −15.93
4. Conclusion It was shown in the present investigation that the treatment of diatomite by sodium hydroxide improved its performance as adsorbent for methylene blue. Improvement in diatomite performance following sodium hydroxide treatment was attributed to a two times increase in surface area from 15.87 m2 g− 1 to 31.35 m2 g− 1, a more tenfold increase in Qe value from 1.72 mg g− 1 to 18.15 mg g− 1 and R value from 8.60% to 90.75% for methyelen blue initial concentaraion 100 ppm. The well-develop porous structure was confirmed by SEM analysis. The adsorption potential of the purified diatomite for the removal of methylene blue was investigated. The kinetic data was best described by the pseudo-second-order kinetic model, which suggested that the process was controlled by chemisorption. The equilibrium data were fitted to two adsorption models and Langmuir isotherm model was the best model to describe the data, showing maximum monolayer adsorption capacity of 27.86 mg g− 1. The thermodynamic parameters indicated that the adsorption of methylene blue onto purified diatomite was an endothermic and spontaneous process. The above results confirmed the potential of the purified diatomite treated with sodium hydroxide as an efficient adsorbent in the methylene blue removal from contaminated solutions. References Al-Degs, Y., et al., 2001. Sorption of lead ions on diatomite and manganese oxides modified diatomite. Water Research 35 (15), 3724–3728. Al-Ghouti, M.A., et al., 2003. The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. Journal of Environmental Management 69 (3), 229–238. Al-Ghouti, M.A., et al., 2004. Flow injection potentiometric stripping analysis for study of adsorption of heavy metal ions onto modified diatomite. Chemical Engineering Journal 104 (1-3), 83–91. Al-Qodah, Z., et al., 2007. Adsorption of methylene blue by acid and heat treated diatomaceous silica. Desalination 217 (1-3), 212–224. Asfour, H.M., et al., 1985. Color removal from textile effluents using hardwood sawdust as an absorbent. Journal of Chemical Technology and Biotechnology A Chemical Technology 35 (1), 28–35.
Cazetta, A.L., et al., 2011. NaOH-activated carbon of high surface area produced from coconut shell: Kinetics and equilibrium studies from the methylene blue adsorption. Chemical Engineering Journal 174 (1), 117–125. Dabrowski, A., et al., 2005. Adsorption of phenolic compounds by activated carbon — a critical review. Chemosphere 58 (8), 1049–1070. Freundlich, H., 1932. Of the adsorption of gases. Section II. Kinetics and energetics of gas adsorption. Introductory paper to section II. Transactions of the Faraday Society 28, 0195–0201. Ghosh, D., Bhattacharyya, K.G., 2002. Adsorption of methylene blue on kaolinite. Applied Clay Science 20 (6), 295–300. Hajjaji, M., El Arfaoui, H., 2009. Adsorption of methylene blue and zinc ions on raw and acid-activated bentonite from Morocco. Applied Clay Science 46 (4), 418–421. Hameed, B.H., Ahmad, A.A., 2009. Batch adsorption of methylene blue from aqueous solution by garlic peel, an agricultural waste biomass. Journal of Hazardous Materials 164 (2–3), 870–875. Hameed, B.H., et al., 2007. Adsorption of basic dye (methylene blue) onto activated carbon prepared from rattan sawdust. Dyes and Pigments 75 (1), 143–149. Hameed, B.H., et al., 2008. Adsorption isotherm, kinetic modeling and mechanism of 2,4,6-trichlorophenol on coconut husk-based activated carbon. Chemical Engineering Journal 144 (2), 235–244. Han, R.P., et al., 2009. Study of equilibrium, kinetic and thermodynamic parameters about methylene blue adsorption onto natural zeolite. Chemical Engineering Journal 145 (3), 496–504. Hong, S., et al., 2009. Adsorption thermodynamics of methylene blue onto bentonite. Journal of Hazardous Materials 167 (1-3), 630–633. Hsu, T.C., et al., 2008. Adsorption of Cu2+ from water using raw and modified coal fly ashes. Fuel 87 (7), 1355–1359. Khraisheh, M.A.M., et al., 2004. Remediation of wastewater containing heavy metals using raw and modified diatomite. Chemical Engineering Journal 99 (2), 177–184. Kiani, G., et al., 2011. Adsorption studies on the removal of Malachite Green from aqueous solutions onto halloysite nanotubes. Applied Clay Science 54 (1), 34–39. Langmuir, I., 1918. The Adsorption of Gases on Plane Surfaces of Glass, Mica and Platinum. Journal of the American Chemical Society 40, 1361–1403. Lee, J.W., et al., 2006. Evaluation of the performance of adsorption and coagulation processes for the maximum removal of reactive dyes. Dyes and Pigments 69 (3), 196–203. Liua, Tonghao, et al., 2012. Adsorption of methylene blue from aqueous solution by graphene. Colloids and Surfaces B: Biointerfaces 90, 197–203. Mahmoodi, N.M., et al., 2012. Kinetic, equilibrium and thermodynamic studies of ternary system dye removal using a biopolymer. Industrial Crops and Products 35 (1), 295–301. Malash, G.F., El-Khaiary, M.I., 2010. Methylene blue adsorption by the waste of AbuTartour phosphate rock. Journal of Colloid and Interface Science 348 (2), 537–545. Ozdemir, O., et al., 2004. Comparison of the adsorption characteristics of azo-reactive dyes on mezoporous minerals. Dyes and Pigments 62 (1), 49–60. Parida, K.M., et al., 2011. A kinetic, thermodynamic, and mechanistic approach toward adsorption of methylene blue over water-washed manganese nodule leached residues. Industrial and Engineering Chemistry Research 50 (2), 843–848. Sharma, Y.C., 2010. Optimization of parameters for adsorption of methylene blue on a lowcost activated carbon. Journal of Chemical and Engineering Data 55 (1), 435–439. Shawabkeh, R.A., Tutunji, M.F., 2003. Experimental study and modeling of basic dye sorption by diatomaceous clay. Applied Clay Science 24 (1–2), 111–120. Sheng, J.W., et al., 2009. Adsorption of methylene blue from aqueous solution on pyrophyllite. Applied Clay Science 46 (4), 422–424. Sljivic, M., et al., 2009. Comparative study of cu(2+) adsorption on a zeolite, a clay and a diatomite from Serbia. Applied Clay Science 43 (1), 33–40. Tsai, W.T., et al., 2006. Characterization and adsorption properties of diatomaceous earth modified by hydrofluoric acid etching. Journal of Colloid and Interface Science 297 (2), 749–754. Yao, Y.J., et al., 2010. Adsorption of methyl violet onto multi-walled carbon nanotubes: equilibrium, kinetics and modeling. Fresenius Environmental Bulletin 19 (5), 854–861.
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Kinetics and equilibrium studies from the methylene blue adsorption on diatomite treated with sodium hydroxide Zhang Jian ⁎, Ping Qingwei, Niu Meihong, Shi Haiqiang, Li Na Liaoning Key Laboratory of Pulp and Paper Engineering, School of Light industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, China
a r t i c l e
i n f o
Article history: Received 6 July 2012 Received in revised form 22 May 2013 Accepted 6 August 2013 Available online 3 September 2013 Keywords: Diatomite Adsorption Kinetics Equilibrium Thermodynamic Methylene blue
a b s t r a c t Diatomite was treated with sodium hydroxide to remove impurity in order to improve its performance as an adsorbent. The raw diatomite and purified diatomite were characterized by scanning electron microscopy, energy dispersive X-ray analysis and Brunauer–Emmett–Teller adsorption. It was found that the surface area was in order of 15.87 m2 g−1 for raw diatomite and 31.35 m2 g−1 for purified diatomite. Scanning electron microscopy images showed the well-developed porous structure of purified diatomite. Purified diatomite improved a more than tenfold increase in adsorption amount from 1.72 mg g−1 to 18.15 mg g−1 and removal efficiency from 8.60% to 90.75% for methyelen blue initial concentration 100 ppm respectively. The kinetics studies showed that experiment data followed pseudo-second-order model better. The equilibrium data was fitted to Langmuir and Freundlich adsorption isotherms and was found that Langmuir model presented the best fit, showing maximum monolayer adsorption capacity of 27.86 mg g−1. The thermodynamic parameters such as the standard enthalpy, standard entropy and standard free energy were evaluated. The obtained results indicated the adsorption of methylene blue onto diatomite treated with sodium hydroxide is endothermic and spontaneous process and confirmed the applicability of this purified inorganic material as an efficient adsorbent for basic dyes. © 2013 Elsevier B.V. All rights reserved.
1. Introduction It is estimated that the annual dye production in the world exceeds 7 × 105 tons and more than 100,000 commercially available dyes of different chemical and physical properties are being used (Lee et al., 2006). The complex aromatic structures and xenobiotic properties of dyes make them more difficult to degrade (Asfour et al., 1985; Parida et al., 2011). Therefore it is necessary to reduce dye concentration in the wastewater before their biological treatment processes. Methylene blue (MB), an organic dye, has wide applications (Sheng et al., 2009). Due to its known strong adsorption onto solids, MB often serves as a model compound for removing dyes and organic contaminants from aqueous solutions (Hameed et al., 2007). Although not strongly poisonous, MB can have some harmful effects on human beings (Hajjaji and El Arfaoui, 2009). Owing to these harmful effects on humans, it is necessary to remove MB from aqueous solution. To remove the dyes from aqueous media, different adsorbents including activated carbon (Dabrowski et al., 2005), zeolites (Ozdemir et al., 2004), bentonite (Hong et al., 2009), kaolinite (Ghosh and Bhattacharyya, 2002), garlic peel (Hameed and Ahmad, 2009),
⁎ Corresponding author. Tel.: +86 15942847053; fax: +86 41186323736. E-mail address: [email protected] (J. Zhang). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.08.008
halloysite nanotube (Kiani et al., 2011) and carbon nanotube (Yao et al., 2010) have been studied for adsorption of dye from aqueous solutions and some of them expressed good dye adsorption property. However investigations related to development of novel effective and economical adsorbents are still in progress. Diatomite is fine-grained, low-density biogenic sediment, which consists essentially of amorphous silica (SiO2 · nH2O) derived from opalescent frustules of diatoms (Sljivic et al., 2009). It consists of a wide variety of shapes and characterized by a high porosity up to 80%, low density and high surface area (Al-Qodah et al., 2007). These properties suggest that diatomite is a potential adsorbent for pollutants found in industrial wastewater including dyes (Al-Degs et al., 2001; Al-Ghouti et al., 2004; Khraisheh et al., 2004). Previous studies indicated that after acid treatment, diatomite pore texture can be obtained and has been used successfully to remove dyes. However the acid concentration was usually too high (5 M) (Al-Qodah et al., 2007) and that would hard to operate and easy polluted. Much attention has been focused on the diatomite purification process of low-cost, easy operation and environment friendly. The focus of this research was to purify diatomite by low concentration sodium hydroxide to enhance the adsorptive capabilities of diatomite for basic dye and evaluate its potential for adsorption in removal of methylene blue from the aqueous solution. The characterizations of the diatomite were also performed. In order to establish the removal capacity of purified diatomite, different models of isotherms and adsorption kinetics were fitted to the experiment data.
J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16 1=2
Qt ¼ Ki t
2. Experiment
þC
13
ð5Þ
2.1. Preparation of purified diatomite The raw diatomite samples were bought from Sigma-Aldrich Co. LLC. The elements composition of there sample obtained by energy dispersive X-ray analysis is silica, aluminum, calcium, magnesium, iron and sodium. The particles were passed through a 150 μm mesh metal sieve. The fraction of the particles between 63–250 μm was used for the purification experiments. The purified diatomite was obtained by sodium hydroxide (Sigma-Aldrich, Germany) to enhance the adsorption capacity. The raw diatomite samples were immersed in sufficient amount of 5% (w/w) sodium hydroxide solution at 100°C for 2 h to remove impurities and organics. The digested diatomite was washed several times by deionized water, filtered, dried at 105°C, sieved and stored in closed containers for further tests. The surface area the raw diatomite and purified diatomite was determined from the linear part of the BET plot (P/P0 = 0.05 ~ 0.20) at 77 K using a Quantachrome Autosorb NOVA 2200e volumetric analyzer. Scamming electron microscopy analysis (JEOL, JSM-6460LV) was carried out for raw diatomite and purified diatomite to study the development of morphology and elements. 2.2. Adsorption studies The cationic dye, methylene blue (MB) (Sigma-Aldrich, Germany) was used as an adsorbate. A stock of solution of methylene blue (500 mg L− 1) was prepared and further diluted to the required concentration before used. Batch adsorption was performed in a set of 100 mL flasks containing 50 mL of MB solution with various initial concentrations. The amount of 0.25 g of diatomite was added and equilibrated at different temperature in a temperaturecontrolled water bath shaker (WS-300). After adsorption equilibrium, the concentration of MB in the solution was measured using a UV– visible spectrophotometer (PerkinElmer, LAMBDA35) at 664 nm. The adsorbed capacity (Qe) and removal efficiency (R) of MB adsorbed onto diatomite were calculated according to the following equations: Qe ¼ ðCo−CeÞV=M
ð1Þ
R ¼ ðCo−CeÞ 100=Co
ð2Þ
where Qe is the adsorption capacity at equilibrium, mg g− 1; R is the removal efficiency, %; Co and Ce are the initial and equilibrium concentration of MB in solution, mg L− 1; V is the volume of solution, L; M is the mass of diatomite, g.(Liua et al., 2012). 2.3. Adsorption kinetics Kinetic studies were carried out in flask where a fixed mass of diatomite of 0.25 g was introduced into 50 mL 20 ppm of methylene blue solution at 25 °C. In order to determine the best kinetic model which fits the adsorption experimental data, the pseudo-first-order (Malash and El-Khaiary, 2010), pseudo-second-order (Han et al., 2009) and intraparticle diffusion (Mahmoodi et al., 2012) models were used to understand the adsorption dynamics in relation to time for the methylene blue and diatomite system. These models can be described as follows: LnðQe−QtÞ ¼ lnQe−K1 t
ð3Þ
t 1 1 ¼ t þ Qt K 2 Qe2 Qe
ð4Þ
Where Qt is the amount of adsorbed MB onto diatomite at t moment, mg g−1; K1 (min−1), K2 (g mg−1 min−1), Ki (mg g−1 min−1/2) are the rate constants of the adsorption in pseudo-first-order (Eq. (3)), pseudoseconde-order (Eq. (4)) and intraparticle diffusion (Eq. (5)) respectively. 2.4. Adsorption isotherm Isotherm studies were carried out in flask where a fixed mass of diatomite of 0.25 g was introduced into 50 mL of methylene blue solution with different concentration at 25 °C, 35 °C and 45 °C. The application of adsorption isotherms is very useful to describe the interaction between the adsorbate and the adsorbent of any system. Adsorption coverage over the surface of diatomite was studies using the two well-known isotherm models, Langmuir and Freundlich (Shawabkeh and Tutunji, 2003). The parameters obtained from the different models provide important information on the sorption mechanisms and the surface properties and affinities of the adsorbents. The Langmuir model (Langmuir, 1918) and Freundlish model (Freundlich, 1932) were as follows: Ce Ce 1 ¼ þ Qe Q max Q maxK L
ð7Þ
1 lnCe n
ð8Þ
ln Qe ¼ ln K F þ
Where Qmax is the maximum adsorption capacity, mgg−1; KL is a Langmuir constant relate to the affinity of the binding sites and energy of adsorption, L g−1; KF is a Freundlish constant related to adsorption capacity, L g−1; 1/n is an empirical parameter related to adsorption intensity. 2.5. Adsorption thermodynamic The thermodynamic parameters for the adsorption process, the standard free energy, standard enthalpy and standard entropy were calculated using the following equations (Hsu et al., 2008): ⊿G ¼ −RTlnKL
ð9Þ
⊿G ¼ ⊿H−T⊿S
ð10Þ
lnKL ¼ ⊿S=R−⊿H=RT
ð11Þ
where ⊿G is the standard free energy, kJ mol−1; R is the universal gas constant, 8.314 J mol−1 K−1; T is the absolute solution temperature, K; ⊿H is the standard enthalpy, kJ mol−1; ⊿S is the standard entropy, J K−1. The standard enthalpy and standard entropy values can be calculated from the slope and intercept of the plot of lnKL versus 1/T. 3. Results and discussion 3.1. Characterization of diatomite The specific surface area of raw diatomite was 15.87 m2 g−1. A surface area of 4.21 m2 g−1 for diatomaceous earth was previously reported (Tsai et al., 2006), the different between these two kind of diatomite may caused by the different manufacturing place and measuring method. As SiO2 is the main component of diatomite, treatment with alkalis leads to the formation of soluble silicates SiO2− 3 resulting in the creation of larger pores, flaws, cracks, crevices and in a larger surface area. The total surface area of purified diatomite treated with 5% sodium hydroxide was 31.35 m2 g−1. The sodium hydroxide treatment increased diatomite surface area two times.
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J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16
Fig. 1. SEM image of datomite.
The morphological structures of raw diatomite and purified diatomite were characterized by SEM and shown in Fig. 1 (Fig. 1a was the SEM image of raw diatomite and Fig. 1b was the SEM image of purified diatomite). The SEM image of raw diatomite in Fig. 1a showed that the structure of diatomite was integral and liked a circle screen. There were many small pores on the surface of sieve tray. However these structures were usually responsible for impurities adhesion, which made the pores smaller or even blocked. The general appearance of the circle screen structure was preserved in the purified diatomite treated in 5% sodium hydroxide and there was no complete collapse of the structure (Fig. 1b). After treatment with sodium hydroxide, the diatomite still showed its multi-pore structure and the pores on the surface became larger. The impurity adhesion was almost gone, which made the pores on the surface of sieve tray larger than that of raw diatomite. In order to verify the removal of the impurity in diatomite, the elements analysis of diatomite was performed by EDX and shown in Fig. 2 (Fig. 2a was EDX spectra of raw diatomite and Fig. 2b was EDX spectra of purified diatomite). Fig. 2a showed that there were some impurity in raw diatomite, such as sodium, magnesium, aluminum, calcium and iron. After treated with sodium hydroxide, the main element silica was preserved and the impurity was removed to a certain extent (Fig. 2b). The EDX spectra of sodium, magnesium and aluminum decreased somewhat. However the EDX spectra of calcium and iron were almost all disappeared. The results were coincident with the experiment data of surface area and Fig. 1. The efficiency of methylene blue removal for raw diatomite and purified diatomite at different initial methylene blue concentration was investigated (Fig. 3).
Fig. 3. Adsorption effect of diatomite.
The amount of adsorbed methylene blue onto purified diatomite increased significantly, no matter for adsorption capacity or for removal efficiency. The Qe value of purified diatomite reached 18.15 mg g−1 while that of raw diatomite was just 1.72 mg g−1 for methylene blue initial concentration 100 ppm. Whereas the R values of purified diatomite and raw diatomite were 8.60% and 90.75% respectively. Therefore the purified diatomite seemed to be a more efficient material than raw diatomite in the methylene blue removal. 3.2. Adsorption kinetics Kinetic studies are important to understand the dynamic of the reaction in terms of order of the rate constant (Cazetta et al., 2011). Since the kinetics parameters provide information for designing and modeling the adsorption process. Three kinetic models, pseudo-first-order model, pseudo-second-order model and diffusion intraparticle diffusion model, were used to examine the mechanism of the adsorption process. The study of the kinetics of adsorption results were shown in Table 1. It is evident from these figures that the experimental points for methylene blue adsorption onto diatomite treated with sodium hydroxide and raw diatomite were best described by the pseudosecond order model. This fact confirmed by the values of R2 shown
Fig. 2. EDX spectra of datomite.
J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16 Table 1 Adsorption kinetic parameters of methylene blue on diatomite. Diatomite sample Qe exp./mg g−1 Pseudo-first-order
Pseudo-second-order
Intraparticle diffusion
−1
Qe cal./mg g K1/min−1 R2 Qe cal./mg g−1 K2/g mg−1 min−1 R2 Ki/mg g−1 min1/2 C R2
Table 2 Adsorption isotherm parameters of methylene blue onto diatomite.
Raw
Purified
0.569
3.844
0.854 0.0136 0.8678 0.755 0.0110 0.9901 0.0317 0.0060 0.9750
2.496 0.0247 0.8612 4.177 0.0096 0.9977 0.1941 0.8088 0.8381
in Table 1 which were 0.9977 and 0.9901 respectively. While the R2 for pseudo-first-order model and intraparticle diffusion model were all less than 0.99 for different diatomite. This suggested that the rate of the adsorption process was preferably controlled by chemisorption. Similar results have been reported by Gholamereza Kiani et al. (Kiani et al., 2011). 3.3. Adsorption isotherm The significance of the adsorption isotherms is that they show how the adsorbate molecules are distributed between the solution and the adsorbent at the equilibrium conditions and the effect of equilibrium concentration on the loading capacity at different temperatures. (Hameed et al., 2008) The effect of methylene blue equilibrium concentration on the adsorption capacity of purified diatomite was carried out at 25 °C, 35 °C and 45 °C as shown in Fig. 4. It is clear that the adsorption capacity of purified diatomite increased as the temperature increased. However the increase was small as the temperature increased from 25 °C to 45 °C, especially from 25 °C to 35 °C. This small increase could be referred to the reversible nature of the adsorption process. This may be caused by the increase of the intraparticle diffusion of methylene blue as the temperature increased. Moreover the desorption phenomena impeded totally as the temperature increased (Al-Ghouti et al., 2003). The adsorption capacity of methylene blue onto purified diatomite increased with increase in temperature, suggesting that the adsorption was endothermic. Two different isotherm models were used to fit the experimental data. Based on the Eqs. (7) and (8), the values of the models constants
Fig. 4. Adsorption isotherm for MB onto purified diatomite.
15
T/°C
25 35 45
Langmuir
Freundlich
Qmax/mg g−1
KL
R2
KF
n
R2
27.86 27.24 26.32
0.2159 0.2493 0.4439
0.9902 0.9902 0.9904
4.5476 4.9268 7.0364
1.40 1.42 1.49
0.9756 0.9730 0.9704
in addition to the values of the correlation coefficient were calculated by plotting lnQe/Ce against Qe and lnQe against lnCe from the slope and intercept at different temperatures (Table 2). According to the coefficients of determination, the Langmuir model fitted better than Freundlich model. It was evident from Table 2 the Langmuir isotherm gave the best fit as indicated by the relative high value of R2 (R2 N 0.99). The experiment data indicated that the adsorption of methylene blue onto purified diatomite was characterized by monolayer coverage of the methylene blue at the outer suface and with maximum adsorption capacity of 27.86mgg-1 at 25°C. It was clear that the values of KF and n increased as the temperature increased, indicating that the adsorption was favorable at high temperature. (Sharma, 2010)
3.4. Adsorption thermodynamic The thermodynamic parameters of the adsorption process of methylene blue onto diatomite treated with sodium hydroxide are the changes in standard enthalpy, standard entropy and standard free energy. The standard entropy and standard enthalpy were obtained from the slope of the plot lnKL versus 1/T as shown in Fig. 5. The values of these parameters were calculated using Eqs. (9), (10) and (11). at various temperature and were shown in Table 3. Table 3 showed that the value of H was positive indicating the endothermic nature of the adsorption of methylene blue onto purified diatomite at different temperature. The positive value of S inferred the affinity of the purified diatomite adsorbent for methylene blue as a result of the increased randomness at the interface between the solid-liquid phases. The values of G were all negative at difference temperature, indicating that the adsorption was spontaneous. The increase in G value with increasing temperature revealed that adsorption of methylene blue onto purified diatomite became more favorable at higher temperature. This fact was previously confirmed by the isotherm experiments at different temperatures.
Fig. 5. Plot of lnKL versus 1/T.
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J. Zhang et al. / Applied Clay Science 83–84 (2013) 12–16
Table 3 Adsorption thermodynamics data of methylene blue onto purified diatomite. T/°C
H/KJmol−1
S/KJmol−1
G/KJmol−1
25 35 45
28.24
0.1388
−13.15 −14.54 −15.93
4. Conclusion It was shown in the present investigation that the treatment of diatomite by sodium hydroxide improved its performance as adsorbent for methylene blue. Improvement in diatomite performance following sodium hydroxide treatment was attributed to a two times increase in surface area from 15.87 m2 g− 1 to 31.35 m2 g− 1, a more tenfold increase in Qe value from 1.72 mg g− 1 to 18.15 mg g− 1 and R value from 8.60% to 90.75% for methyelen blue initial concentaraion 100 ppm. The well-develop porous structure was confirmed by SEM analysis. The adsorption potential of the purified diatomite for the removal of methylene blue was investigated. The kinetic data was best described by the pseudo-second-order kinetic model, which suggested that the process was controlled by chemisorption. The equilibrium data were fitted to two adsorption models and Langmuir isotherm model was the best model to describe the data, showing maximum monolayer adsorption capacity of 27.86 mg g− 1. The thermodynamic parameters indicated that the adsorption of methylene blue onto purified diatomite was an endothermic and spontaneous process. The above results confirmed the potential of the purified diatomite treated with sodium hydroxide as an efficient adsorbent in the methylene blue removal from contaminated solutions. References Al-Degs, Y., et al., 2001. Sorption of lead ions on diatomite and manganese oxides modified diatomite. Water Research 35 (15), 3724–3728. Al-Ghouti, M.A., et al., 2003. The removal of dyes from textile wastewater: a study of the physical characteristics and adsorption mechanisms of diatomaceous earth. Journal of Environmental Management 69 (3), 229–238. Al-Ghouti, M.A., et al., 2004. Flow injection potentiometric stripping analysis for study of adsorption of heavy metal ions onto modified diatomite. Chemical Engineering Journal 104 (1-3), 83–91. Al-Qodah, Z., et al., 2007. Adsorption of methylene blue by acid and heat treated diatomaceous silica. Desalination 217 (1-3), 212–224. Asfour, H.M., et al., 1985. Color removal from textile effluents using hardwood sawdust as an absorbent. Journal of Chemical Technology and Biotechnology A Chemical Technology 35 (1), 28–35.
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