Ind. Eng. Chem. Res. 2010, 49, 3207–3216
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Removal of Dyes from Wastewaters by Adsorption on Sepiolite and Pansil Araceli Rodrı´guez,* Gabriel Ovejero, Marı´a Mestanza, and Juan Garcı´a* Grupo de Cata´lisis y Procesos de Separacio´n (CyPS), Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, UniVersidad Complutense de Madrid, AVda. Complutense s/n, 28040 Madrid, Spain
In this work, experiments to examine the liquid-phase adsorption features of sepiolite and pansil have been conducted with synthetic dye wastewaters prepared from commercial grade dye, methylene blue (MB). The adsorption experimental results were analyzed in terms of the equilibrium adsorption capacity and equilibrium time. The Langmuir, Freundlich, and Sips adsorption models are applied to describe the isotherm equilibrium and to determine some parameters. The Sips model agrees well with the experimental data, and the pseudosecond-order kinetic model reproduces properly the kinetic experimental data of the system MB-sepiolite. The highest MB adsorption was obtained at acid pH for sepiolite and at basic pH for pansil. Besides, several kinetics models were employed to study the adsorption mechanism of MB on sepiolite. 1. Introduction Nowadays, one of the most severe environmental problems is water pollution. The presence of colored compounds in wastewater is not only aesthetically displeasing, but also hinders light penetration in water, increasing the biological oxygen demand and causing lack of dissolved oxygen to sustain aquatic life.1 Conventional methods for the removal of dyes in effluents include physical, chemical, and biological processes. Adsorption is generally considered to be an effective method for dye removal, and the most widely used adsorbent is activated carbon. However, it suffers from high cost production and regeneration.2 Due to this, a search for cheap and effective adsorbents is needed. Unlike activated carbons, clays are relatively cheap due to their accessibility and abundance. Therefore, there is an increasing demand for porous materials as adsorbents and catalysts supports. Palygorskite and sepiolite, because of their hollow-brick structure, have great potential for the retention of micropollutants such as heavy metals cations and dyes.3 Sepiolite, which has a (Si12)(Mg8)O30(OH)4(OH2)4 · H2O unit-cell formula, is a magnesium hydrosilicate with a microfibrous structure and has a theoretical high surface area and high chemical and mechanical stability.4 The general structure of sepiolite is formed by alternation of blocks and tunnels that grow up in the microfibre direction. Each block consists of two tetrahedrical silica sheets discontinued and inversion of these sheets that give rise to structural tunnels. This mineral structure results in the adsorption sites with high surface irregularities.5 Mass transfer has to be considered for the removal of organic pollutants from aqueous solution. Diffusion of the adsorbate in the liquid phase is not as fast as in the gas phase, and so, the adsorbent particle size may influence adsorbate mass transfer. Adsorbents with a small particle size are preferably used for adsorption from solution phase because they present a large surface area and a small diffusion distance. However, considerable effort may be required to achieve small particle sizes.6 The present work is aimed to study the adsorption capacity of two clays, pansil and sepiolite, for methylene blue removal from aqueous solutions. Batch study was conducted on a * To whom correspondence should be addressed. Tel.: +34-91-3944182. Fax: +34-91-394-4114. E-mail address:
[email protected] (A.R.);
[email protected] (J.G.).
laboratory scale using synthetic dye wastewaters prepared from commercial dye. The scope included the study of surface modification and particle size of pansil and sepiolite, as well as, pH, contact time, and isotherm studies. The Langmuir, Freundlich, and Sips isotherm models were tested for their applicability. Finally, adsorption mechanisms and kinetic studies were developed. 2. Materials and Methods 2.1. Materials. Two kinds of sepiolite supplied by TOLSA S.A. (Spain) were used in this study: sepiolite and pansil. A sepiolite sample was treated before being used in the experiments as follows4 in order to remove impurities and the finest fraction: the suspension containing 10 g · L-1 of sepiolite was mechanically stirred for 24 h at atmosphere temperature. After waiting 2 min, the supernatant was removed and the solid was washed again with distilled water. The solid samples were dried at 105 °C for at least 24 h before their use. Pansil, a high purity sepiolite, was used as received without further purification. 2.2. Dye. Methylene blue (MB) was chosen in this study because of its known strong adsorption onto solids.7 The main characteristics and structure of the dye are shown in Table 1. 2.3. Analytical Techniques. Stock dye solutions of concentration 300 mg · L-1 were prepared. The dye concentration was determined by means of a UV/vis spectrophotometer (Shimadzu spectrophotometer UV-2401 PC).Calibration was done at 631 nm, since this λ enables us to obtain a linear calibration in a wider wavelength range than using λmax. 2.4. Experimental Section. Batch equilibrium experiments with both adsorbents were conducted using 25 mL conical flasks immersed in a thermostatic bath at 30, 40, or 65 °C when corresponding. The samples were stirred until the equilibrium time was reached, when a known volume of the solution was removed and centrifuged for analysis of the supernatant. The effect of pH over dye adsorption was investigated by varying the initial pH solution from 3.0 to 9.0. The pH solution was adjusted with strong acid (HCl) and/or strong base (NaOH) and recorded with a pH meter (Crison 2002). 2.5. Characterization of Adsorbents. Textural characterization of sepiolites was done by using N2 adsorption-desorption at 77 K in a Micromeritics ASAP 2010 apparatus. Scanning electron microscopy (SEM) was conducted at 15 kV using a JSM-6700F field emission scanning microscope to analyze the
10.1021/ie9017435 2010 American Chemical Society Published on Web 03/01/2010
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Table 1. Main Characteristics of Methylene Blue
Table 2. Textural Characterization of Adsorbents mercury porosimetry density (g · cm-3)
N2 porosimetry SBET (m · g )
Sext (m · g )
Vmicropores (cm · g )
ε (%)
bulk
grain
Vp (cm3 · g-1)
157.6 139.1
146.0 116.7
0.0025 0.0091
0.4436 0.2745
0.9063 0.2058
1.6289 0.2836
0.4895
2
sepiolite pansil
-1
2
-1
3
morphology of the solid samples; sample preparation involved dispersing them onto a carbon film supported by copper grids and sputtered with gold. Thermogravimetric analysis (TGA) experiments were performed with a heating rate of 10 °C/min on a Seiko EXSTAR 6000 TGA Instrument from 20 to 900 °C at an helium flow rate of 30 mL · min-1. Fourier transform infrared (FTIR) spectra were collected using a Nicolet Nexus670 FTIR spectrophotometer at 4 cm-1 resolution in KBr tablets (2 mg of sepiolite/98 mg KBr). Mercury porosimetry data were obtained using a Thermo Finnigan PASCAL porosimeter. X-ray Fluorescence (XRF) measurements were performed using a BRUKER S4 EXPLORER system, with software for data acquisition and analysis. 3. Results and Discussion 3.1. Characterization of Adsorbents. Figure 1 shows the nitrogen adsorption and desorption isotherms at 77 K. The textural properties of both materials determined in this study were listed in Table 2. The micropore volume was calculated applying the t-plot method to the experimental N2 adsorption data (Table 2). Both samples show low microporosity and large amounts of N2 adsorbed at high values of p/p0 indicating mesoand macropores, slightly larger for sepiolite than for pansil. Mercury porosimetry results are listed in the same table. It is important to note the very low density, either bulk or particle, presented by pansil. Its appearance is similar to flakes, and it was difficult to manipulate. Figure 2 shows the thermogravimetric (TG) and differential thermal analysis (DTA) curves obtained from sepiolite and
Figure 1. Adsorption/desorption isotherms of N2 for the sepiolite and pansil.
-1
pansil under inert conditions. Both solids possess a good thermostability and low humidity, despite the fact that they were not stored in a heater. However, pansil is slightly more stable, due to its higher purity than sepiolite (it can be seen in the DTA curves). The results of X-ray fluorescence (XRF) measurements are presented in Table 3. The analyses were done for different particle size fractions of sepiolite, to discriminate if the particle size had any effect on the composition. Pansil was also measured. As it can be seen, the pureness of the sepiolite slightly decreases with decreasing particle sizes. The main traces found are Al and Ca. However, pansil is the purest material, due to the fact that it is composed of more than 96% Mg, Si, and O (H cannot be detected in this kind of analysis). The FTIR spectra of sepiolite and pansil were shown in Figure 3. According to Vicente-Rodrı´guez,8 the peaks at 3689, 3631, 3566, 3420, and 3250 cm-1 correspond to the presence of different kinds of water (adsorbed, coordinated, and zeolitic) and to the presence of hydroxyl groups coordinated to metallic
Figure 2. TG and DTA curves of the adsorbents at 10 °C/min under 30 sccm helium.
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Table 3. X-ray Fluorescence Results (percent) sepiolite
Al Ca Fe K Mg Mn O P Ru Si Sr Ti Zn Zr
pansil
dp ) 500-0.417
dp ) 0.294-0.250
dp < 0.125
1.61 0.45 0.88 0.69 13.34 0.05 49.28 0.01 0.00 33.59 0.00 0.12 0.00 0.00
2.98 1.63 1.56 1.51 10.77 0.05 48.67 0.01 0.01 32.61 0.01 0.18 0.01 0.01
3.77 1.49 1.72 2.10 9.82 0.04 48.53 0.01 0.02 32.30 0.01 0.18 0.00 0.01
4.91 3.05 1.83 2.71 7.71 0.09 48.00 0.05 0.02 31.37 0.01 0.23 0.00 0.01
cations, especially Mg-OH bonds. The band at 1660 cm-1 is due to the bending vibrational mode of water. Characteristic bands of silicate structure can be seen between 1400 and 400 cm-1. The peaks at 1020 and 470 cm-1 were produced by the Si-O-Si in-plane vibrations, and the bands at 1212, 1080, and 980 cm-1, by the Si-O bonds. The peak at 646 cm-1 is due to the OH bending vibrations. The peak at 440 cm-1 may be attributed to the bonds Si-O-Mg. Finally, the peak at 783 cm-1 may be produced by tetrahedric aluminum. The morphology of the samples was studied by SEM. Figure 4 shows different views of the materials. The morphologies of pansil (Figure 4a and b) and sepiolite (Figure 4c and d) are significantly different. In pansil microphotographs, the fibres may be clearly observed, but in sepiolite, the fibres form planar aggregates, with filamentous edges. Pores of different size and shape could be observed in this figure. 3.2. Equilibrium Adsorption Experiments. The equilibrium time between is reached with a rate which depends not only on adsorbent diffusion components but also on the adsorbent/ adsorbate interaction. In order to determine equilibrium time, the adsorption of both dyes on adsorbents was studied at a fixed concentration as
Figure 4. SEM micrograph showing the general appearance of both adsorbents.
Figure 5. Adsorption equilibrium time of methylene blue on sepiolite and pansil (C0 ) 300 ppm, V ) 25 mL, T ) 30 °C, pH ) 7.0, W ) 0.05 g).
Figure 6. Effect of initial pH on adsorption capacity on both adsorbents (C0 ) 300 ppm, T ) 30 °C, V ) 25 mL, W ) 0.05 g).
Figure 3. FTIR of sepiolite and pansil.
function of contact time. The results for adsorbate concentration of 300 mg · L-1, adsorbent dose/volume of solution 2 mg · mL-1, and pH 7.0 are shown in Figure 5. As it can be expected, solution dye concentration decreases with time until a constant value is reached at 200 min for Pansil and at 24 h for sepiolite. 3.2.1. Effect of pH on MB Adsorption. The amount of dye adsorbed from aqueous solution was found to be affected by initial pH solution (Figure 6). The adsorption capacity of sepiolite decreased 6.7%, to increase pH from 3 to 9. A small
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Figure 7. Experimental isotherms at several temperatures: (a) sepiolite and (b) pansil (C0 ) 300 ppm, V ) 25 mL).
increase in adsorption capacity (6%) for pansil was observed when the pH was increased from 3 to 7, while at pH 9, this capacity decreased sharply. The dependence of adsorption capacities with pH solution has been widely described in the literature.2 3.2.2. Effect of Temperature on Adsorption Capacity. Temperature has a very important effect on adsorption. Usually, and above all, in gas phase, adsorption is exothermic; this means that the adsorption capacity decreases with increasing temperatures. However, in the liquid phase, the contrary tendency may be found. Increasing the temperature is known to increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particle, due to the decrease in the viscosity of the solution. In addition, it has been observed that the equilibrium capacity of the adsorbent for a particular adsorbate grows as the temperature increases.4 Figure 7a and b show the results of adsorption capacity experiments carried out at three temperatures (30, 40, and 65 °C) for methylene blue adsorption on sepiolite and pansil. As mentioned above, it can be observed that the equilibrium capacity of the sepiolite grows as the temperature increases. This might be caused by a deeper penetration of the dye into the sepiolite because of its larger diffusion coefficient, indicating a possible mechanism of interaction through a reaction between the hydroxyl end groups of the sepiolite and the cationic group in the dye molecule. These interactions may be favored with increasing temperatures.9 3.2.3. Effect of Surface Modifications. Modification of sepiolite to obtain specific properties can be achieved by several treatments that make the resulting solid possess the best performance for dye removal. A mild oxidation treatment is often used, and this helps to create oxygen-containing surface groups. The presence of the polar functional groups enhances the wettability of the surfaces for polar solvents (water). However, it is essential to carry out the oxidation in a controlled manner so as not to destroy the material. In general, oxidation with concentrated acids results in the formation of hydroxyl and carbonyl groups. Acid activation was carried out using HNO3 1 N with a solid to liquid ratio of 1:100 (w/w) for 1 h at 30 °C. An analogous treatment was done using NaOH instead of HNO3.10 The structure of sepiolite, with channels of molecular dimensions, determines that it may be considered as an adsorbent with a uniform microporosity and external porosity. It has been stated that the structural changes of sepiolite with temperature affect the specific surface area and adsorption capacity of this clay. It has been proved11 that specific surface area increases
Figure 8. Effect of surface modification adsorption capacity on sepiolite (C0 ) 300 ppm, V ) 25 mL, T ) 30 °C, W ) 0.05 g).
until 200 °C calcination temperature and, then, continuously decreased with increasing temperature. Due to this maximum, sepiolite was calcinated at 200 °C for 2 h. Despite of all these treatments, only an increase in adsorption capacity of sepiolite has been observed with the treatment with NaOH (Figure 8). 3.2.4. Adsorption Isotherms. The correlation of equilibrium adsorption data by either theoretical or empirical equations is important for the design and operation of adsorption systems. The adsorption isothermal equations usually utilized are Langmuir and Freundlich isotherms for the liquid-solid system. Langmuir Isotherm. Ce Ce 1 ) + qe bqmax qmax
(1)
where qmax (mg · g-1) and b (L · mg-1) are Langmuir constants which are indicators of the maximum adsorption capacity and the affinity of the binding sites, respectively. Another important parameter RL, called the separation factor or equilibrium parameter, may be calculated as follows: RL )
1 1 + K L C0
(2)
This parameter indicates if the adsorption is favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL ) 0), or linear (RL ) 1). As it can be seen in Table 4, RL shows values between 0 and 1, for MB-sepiolite and MB-pansil systems, indicating favorable adsorption.
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Table 4. Isotherm Models Fits Langmuir T (°C)
qsat (mg · g-1)
b (L · mg-1)
RL
r2
SD
SSE
ARE
HYBRID
30 40 65 30 40 65
80.9 80.7 91.9 68.7 70.6 78.1
0.5 0.6 0.2 2.0 1.8 1.1
0.8567 0.8451 0.9327 0.6215 0.6529 0.7489
0.9733 0.7549 0.9680 0.9908 0.9869 0.8157
6.1 6.2 15.8 3.4 4.7 9.7
160.7 285.1 896.2 33.6 67.2 312.5
4.2 6.6 7.3 2.6 4.0 7.5
6.1 6.2 15.8 8.4 16.5 66.6
sepiolite pansil
Freundlich T (°C)
n (-)
KF
r2
SD
SSE
ARE
HYBRID
30 40 65 30 40 65
8.8 10.5 8.9 16.9 15.5 9.9
48.0 51.1 52.3 52.9 53.3 50.9
0.9733 0.7549 0.9679 0.9873 0.9823 0.9428
4.5 4.2 6.7 4.4 7.1 3.8
117.6 116.8 225.5 59.3 196.2 74.3
3.7 3.7 4.8 3.7 5.7 3.4
15.6 20.0 29.6 14.3 42.6 12.7
sepiolite pansil
Sips -1
-1
T (°C)
qsat (mg · g )
b (L.mg )
n (-)
r2
SD
SSE
ARE
HYBRID
30 40 65 30 40 65
92.6 101.0 110.2 71.4 70.6 104.0
0.5 0.5 0.3 2.9 2.9 0.6
2.1 3.0 2.4 1.6 1.4 3.3
0.9905 0.9851 0.9891 0.9982 0.9900 0.9927
3.2 3.9 3.6 1.6 3.3 3.2
57.1 88.5 82.7 7.3 40.9 42.0
2.7 3.6 2.6 1.3 3.1 2.7
8.6 18.6 10.6 2.1 10.5 9.2
sepiolite pansil
Freundlich Isotherm. ln q ) ln KF +
1 ln Ce n
(3)
where KF (mg · g-1) and n (dimensionless) are the Freundlich’s constants, indicating adsorption capacity and adsorption intensity, respectively. Sips Equation (Langmuir-Freundlich). In order to correct the question of the unlimited increase of the adsorbed amount as concentration increases predicted by Freundlich’s equation, Sips proposed an equation with a finite limit when the concentration is sufficiently high: qe ) qs
(bCe)
1/n
where qs (mg · g ) is the saturation capacity and can be either taken as a constant or temperature dependent, b (L · mg-1) is the affinity constant, n (-) is the parameter characterizing the system heterogeneity. If n ) 1, the Langmuir equation would be recovered. As it can be seen in Table 4, the parameter n is greater than unity, suggesting some type of heterogeneity of these MBsepiolite and MB-pansil systems. The larger this parameter, the higher the degree of heterogeneity. In the system MBsepiolite the variation of n with temperature is not clear. The validity of the models was determined by calculating several parameters. One of them is the standard deviation (SD, %), calculated using the following equation:
SD )
- qcal)/qexp]2
n-1
× 100
(5)
where the subscripts exp and cal refer to the experimental and the calculated data, and n is the number of data points.12 The sum of the squares of the differences between the experimental and the calculated data is a common error function
∑ (q
- qexp)2
cal
(6)
i)1
The average relative error (ARE) function minimizes the fractional error distribution across the entire concentration range:
(4)
1 + (bCe)1/n
exp
n
SSE )
ARE (%) )
-1
∑ [(q
but it has a major drawback. The calculated isotherm parameters obtained from this error function will provide the best fit at the highest Ce values. This is due to the square of the errors will increase as concentration does.
100 n
n
∑ i)1
|qcal - qexp | qexp
(7)
With the aim to compare models with different number of parameters, the hybrid error function (HYBRID) was studied: HYBRID (%) )
100 n-p
n
∑ i)1
[
(qexp - qcal)2 qexp
]
(8)
where n is the number of data points and p is the number within the equation. As it was expected, the pansil experimental data fit better to Sips equation than to other equations, due to the fact that this model has three adjusting parameters and the Langmuir and Freundlich models only have two parameters (Table 5). For pansil isotherms, the Sips equation is the best for the three temperatures studied. However, for sepiolite experimental data, Sips did not present significant advantages against the Langmuir model, considering the number of parameters (HYBRID), but it did against the Freundlich one. 3.3. Kinetics Adsorption Experiments. Kinetics adsorption experiments were carried out in the same installation as the equilibrium ones. For each experimental data point, 25 mL of the dye solution at pH ) 3 was continuously stirred. Samples were withdrawn at appropriate time intervals and measured.
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Table 5. Kinetics Models Fits Bangham
stirring speed (rpm) dp (mm) adsorbent mass (g) T (°C)
500 600 700 0.5-0.417 0.294-0.250
