Mineral waste from coal mining for removal of astrazon red dye from aqueous solutions

Desalination 264 (2010) 181–187

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Mineral waste from coal mining for removal of astrazon red dye from aqueous solutions C.A.P. Almeida a, A. dos Santos a, S. Jaerger a, N.A. Debacher b, N.P. Hankins c,⁎ a b c

Departamento de Química, Universidade Estadual do Centro-Oeste, 85040-080 Guarapuava, PR, Brazil Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil Centre for Sustainable Water Engineering, Department of Engineering Science, The University of Oxford, Parks Road, Oxford OX1 3PJ, UK

a r t i c l e

i n f o

Article history: Received 1 July 2010 Received in revised form 9 September 2010 Accepted 17 September 2010 Available online 30 October 2010 Keywords: Langmuir isotherm Kinetics Mineral waste Astrazon red dye Thermodynamic parameters

a b s t r a c t The use of mineral waste from coal mining (MWCM) as an adsorbent for the removal of astrazon red dye (AR) from aqueous solution was studied in detail. Batch adsorption experiments were carried out under varied conditions, such as different initial concentrations of AR, contact time, pH, temperature and calcination of the adsorbent. Investigations revealed that the maximum colour removal was observed for unbuffered solutions. MWCM calcinated at 400 °C (MWCM400) was more efficient for dye removal than samples calcinated at other temperatures. The adsorption isotherm of AR on MWCM400 was determined and correlated with the Langmuir and Freundlich models; the results indicated a better fit for the Langmuir model at all the temperatures studied. Kinetic data were fitted with both pseudo-first-order and pseudo-second-order kinetic models, and the data were found to follow the latter model more adequately. Calculated thermodynamic and kinetic parameters indicate a predominantly physisorption mechanism for the adsorption of AR onto MWCM400. The amount of AR adsorbed by MWCM400 per unit area was found to be two or three times greater than that by several comparable adsorbents. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Many industries use dyes to colour their products. Amongst them are the textile industries, which use many synthetic dyes in their final products. The discharge of dyes into natural waters, even in low concentrations, may cause environmental problems in aquatic life by reducing sunlight penetration and by increasing chemical oxygen demand. Adsorption is a well-known equilibrium separation process. The adsorption process has been shown to be an effective and attractive method for the treatment of industrial wastewaters containing coloured dyes, heavy metals and other inorganic and organic impurities [1,2]. Adsorption has been found to be more advantageous than other techniques for water re-use in terms of initial cost, flexibility and simplicity of design, ease of operation, and insensitivity to toxic pollutants [2]. Activated carbon is the most popular and widely used adsorbent in water treatment. This has been attributed to the extensive surface area, micro-porous structure, high adsorption capacity, and high degree of surface reactivity [3]. However, it is expensive and its regeneration is difficult to perform [4]. This fact has motivated investigations into the use of many other adsorbents available locally.

⁎ Corresponding author. Tel.: + 44 1865 273027; fax: + 44 1865 273010. E-mail address: [email protected] (N.P. Hankins). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.09.023

Amongst them are perlite [5], activated carbons prepared from sawdust [6], bentonite [7], organoclays [8], sewage sludge [9], fly ash [10], soy meal hull [11], sepiolite [12], peat [13], coconut bunch waste [14], chaff [15], mixtures of activated clay and activated carbon [16], barro branco [17], montmorillonite [18], and so on. Mineral waste from coal mining (MWCM) is an abundant byproduct in the southern part of Brazil, available as a raw material source. It is a rock composed of many minerals, and its low cost guarantees an economically successful application in water treatment processes. The aim of the present work was to study, in detail, the utilization of MWCM in the removal of Astrazon red dye (AR) from aqueous solutions. 2. Experimental 2.1. Materials Samples of MWCM were obtained from the Esperança Mine (Carbonífera Metropolitana, Treviso, Santa Catarina state, Brazil). MWCM contains the chemical components shown in Table 1 (as revealed by X-ray fluorescence by an XRF-Philips PW2400), and the physicochemical characteristics summarized in Table 2 [19]. All reagents used were of at least analytical grade. AR (basic red 46) dye was obtained from Quimisa (Brusque, Brazil), with the chemical structure shown in Fig. 1 (357.5 g/mol molecular weight).

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C.A.P. Almeida et al. / Desalination 264 (2010) 181–187 Table 1 Chemical components of MWCM.

+ CH3

Component

Weight (%)a

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O MnO TiO2 P2O5 Loss ignition Total

49.07 23.36 2.92 0.99 0.57 0.16 1.85 0.04 1.05 0.06 19.93 100.00

a

N N

H3C

Cl-

N

N N

N

H3C

Fig. 1. Chemical structure of AR.

2.5. Method

The values are expressed as percent w/w.

AR is a cationic dye with one azo group, and with peak absorbance at λmax = 530 nm.

2.2. Preparation of MWCM MWCM was crushed and sieved to select particles in the 210 to 297 μm size range, using ASTM standard sieves. The sieved material was washed several times with distilled water to remove adhered fine particles and other impurities. The washed material was then dried in an oven at 80 °C for 24 h, and stored in sealed flasks to be used in further experiments. Five MWCM fractions of 5 g were calcinated in a muffle furnace at 80 °C, 200 °C, 400 °C, 600 °C, and 800 °C for 4 h and stored in flasks for further use.

2.3. Preparation of solutions A stock solution of AR (5.60 mmol L−1) was prepared by dissolving accurately weighed quantities of dye into distilled water. The use of the latter was to prevent and minimize possible interferences in the study. From the stock solution, diluted ones were obtained at the desired concentrations (0.56–5.03 mmol L−1).

2.4. Equipment pH measurements were made using a multi-parameter LABCOR Consort C535 instrument and absorbance was measured by a Varian Cary 50 UV–Vis spectrophotometer. For the absorbance measurements, a 1-cm light path cell was used at λmax = 530 nm. Batch experiments were carried out using glass cells connected to a thermostated bath (Nova Ética).

Batch adsorption studies were carried out at different initial dye concentrations, contact times, pH, temperatures and calcination temperatures to obtain the best conditions for the removal of AR. AR solutions (50 mL) of known concentration with 1.0 g of adsorbent were stirred at 450 rpm. Aliquots of the AR solutions were collected at regular time intervals, and then diluted and centrifuged for 10 min at 2000 rpm. The supernatants were then analyzed by a UV–vis spectrophotometer. The amount of AR adsorbed onto the adsorbent during the process, qt (mol g−1), was calculated by the mass balance equation, as follows: qt = ðC0 −Ct Þ

V m

ð1Þ

where C0 and Ct are the initial and final liquid-phase concentrations of dye solution (mol L−1), respectively, V is the volume of dye solution (0.05 L), and m is the mass of adsorbent used (1.0 g). The effect of calcination of MWCM was studied by performing experiments using calcinated samples at 80 °C (MWCM80), 200 °C (MWCM200), 400 °C (MWCM400), 600 °C (MWCM600) and 800 °C (MWCM800) for 4 h. AR solutions of concentration 2.80 mmol L−1 at 45 °C were used. Experiments were performed using unbuffered (pH 5.5–7.5) and buffered solutions of pH 3.0 (KC8H5O4 0.1 mol L−1 + HCl 0.1 mol L−1), pH 6.0 (KC8H5O4 0.1 mol L−1 + NaOH 0.1 mol L−1), pH 8.0 (KH2PO4 0.1 mol L−1 + NaOH 0.1 mol L−1) and pH 11.0 (Na2CO3 0.1 mol L−1 + HCl 0.1 mol L−1) at the same initial dye concentration of 1.68 mmol L−1 and at 25 °C. Before the adsorption experiments, buffered AR solutions were stored for 10 days to verify possible changes in the maximum absorbance. The unbuffered solutions were used in most of the experiments. 3. Results and discussion 3.1. Effect of calcination

Table 2 Physicochemical characteristics of MWCM. Property

Values 2

−1

BET surface area (m g ) MWCM80 MWCM400 MWCM800 Total pore volume (cm3 g−1) (MWCM400) Average pore diameter (Å) (MWCM400, by BJH) Zeta potential (mV) pH 1.73 pH 3.0 pH 5.0 pH 7.0 pH 9.0 pH 11.0

9 19 11 32.9 × 10−3 69.3 0.0 −5.5 −10.5 −12.6 −13.8 −14.3

The effect of calcination of MWCM on AR adsorption follows the series MWCM800 b MWCM600 b MWCM80 ≅ MWCM200 b MWCM400. This effect can be explained by considering the changes of MWCM with increasing temperature. MWCM is a compacted rock, formed mainly by kaolinite and organic matter [19]. Kaolinite loses water molecules and the organic matter is removed with increasing temperature. Similar results have been found for sepiolite which loses water molecules with increasing temperature [20]. The fact that the MWCM has a higher surface area at 400 °C than at 80 °C is a result of the loss of organic matter and adsorbed water [21]; the fact that MWCM has a higher surface area at 400 °C than at 800 °C is a result of loosened structural water molecules. Kaolinite shows a strong endothermic peak at about 560 °C in the DTA curve due to dehydroxylation, when it is transformed into metakaolinite [21]. The decrease in the MWCM surface area when calcinated at 800 °C, as shown in Table 2, may also be a result of the

3.2. Effect of pH The percentage removal of AR in buffered solution of pH 3, 6, 8 and unbuffered (5.5–7.5) solutions were 63.8%, 68.3%, 62.6% and 87.2%, respectively. It can be observed that the AR removal by MWCM400 is higher for the unbuffered solution. The negatively charged sites should favour the adsorption of dye cations, due to electrostatic attraction [23]. However, in the present study there might be interference effects and competition between cations from the buffered solution and the AR cations for the MWCM400 sites, considering that the adsorbent zeta potential is negative throughout the pH range, as shown in Table 2.

qe (µmol g-1)

removal of micropores [22] (i.e. sintering), owing to structural collapse. Since the MWCM400 has the largest surface area, as shown in Table 2, it was used in all experiments.

183

140

100

120

90

100

80

80

70

60

qe

AR Removal (%)

C.A.P. Almeida et al. / Desalination 264 (2010) 181–187

60

% 40

50 1

2

3

4

5

C0 (mmol L-1) Fig. 3. Effect of initial dye concentration on AR removal by MWCM400 at 45 °C.

3.3. Effect of contact time The effect of contact time on AR removal by MWCM400 is shown in Fig. 2, for two different initial AR concentrations. As can be seen in Fig. 2, the rate of AR uptake on MWCM400 is higher during the initial stages, and gradually decreases to an almost constant level as equilibrium is established. Because the AR uptake versus time curve is monotonic, smooth, and continuous towards saturation, it suggests possible monolayer coverage of the dye on the surface of the adsorbent [4]. In adsorption systems where the adsorbent is negatively charged and the adsorbate is positively charged, a monolayer of adsorbate is frequently formed on the surface of the adsorbent, and the rate of removal of adsorbate species from aqueous solution is controlled primarily by the rate of diffusive transport of the adsorbate species from the exterior boundary layer to the interior sites of the adsorbent particles [24]. 3.4. Effect of initial dye concentration The experimental results of AR adsorption at different initial dye concentrations, at 45 °C, are shown in Fig. 3. From Fig. 3, it is observed that the amount of adsorbed AR increases over almost the whole range of initial dye concentrations, until the saturation of the MWCM400 surface is reached at approximately C0 = 3.36 mmol L−1, while the percent adsorption efficiency decreases [25]. At high dye concentrations, AR adsorption becomes limited by the available active sites [24]. 140 120

qt (µmol g-1)

C0 = 0.56 mmol L-1 - qt

60

C0 = 2.24 mmol L-1 - qt C0 = 0.56 mmol L-1 - %

60

C0 = 2.24 mmol L-1 - %

40

40

AR removal (%)

80

80

The effect of temperature on the adsorption of AR is presented in Table 3. As can be seen, the amount of dye adsorbed increases with increasing temperature from 25 °C to 45 °C and decreases from 45 °C to 65 °C [4]. The effect of temperature on the adsorption process can be analysed from two points of view. Increasing temperature increases the rate of diffusion of the adsorbate molecules across the external boundary layer and through the internal pores of the adsorbent particles, and reduces the viscosity of the solution [18]. In addition, the equilibrium between adsorption and desorption is moved towards desorption and the capacity of the adsorbent for a particular adsorbate is reduced by increasing the temperature [18]. 3.6. Adsorption isotherms Adsorption isotherms provide a useful description of how adsorbates will interact with adsorbents, and are critical in optimizing the use of an adsorbent [26]. The most commonly used isotherm models are Langmuir [27] and Freundlich [28]. The Langmuir isotherm can be represented by Eq. (2). qe =

Q KL Ce 1 + KL Ce

ð2Þ

Its linearized form can be written as: Ce 1 C = + e Q KL qe Q

100

100

3.5. Effect of temperature

ð3Þ

In the equations above, qe is the amount of solute adsorbed per unit weight of MWCM400 (mol g−1), Ce is the equilibrium concentration (mol L−1), Q is the monolayer adsorption capacity (mol g−1) and KL is the constant related to the affinity of the binding sites and the energy of adsorption (L mol−1). Q and KL are calculated from the intercepts and slopes of the straight line plots of Ce/qe versus Ce. The Freundlich model is generally given by: 1=n

ð4Þ

qe = KF Ce 20

or in logarithmic form:

20 0

0 0

50

100

150

200

Time (h) Fig. 2. Effect of contact time on AR removal by MWCM400 at 45 °C.

ln qe = ln KF +

1 ln Ce n

ð5Þ

In these equations, the KF constant is a rough indicator of the adsorption capacity and n is the adsorption intensity [29]. Both

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Table 3 Amount and percentage of AR removal by MWCM400 at different temperatures. Initial concentration of AR in mmol L−1 (C0) 0.56

1.12

1.68

2.24

Temperature (°C)

Amount of AR removal by MWCM400 in μmol g

25 45 65

27.78 27.72 27.36

54.80 54.88 54.99

78.41 82.07 78.83

−1

2.80

3.36

3.92

4.48

5.03

5.60

104.50 124.64 113.71

119.86 132.73 134.46

123.47 135.47 145.65

124.53 132.73 132.00

129.43 138.15 112.08

125.01 140.20 115.08

74.64 89.03 81.22

71.35 79.01 80.04

62.99 69.12 74.31

55.59 59.25 58.93

51.46 54.93 44.56

44.65 50.93 41.10

(qe)

92.84 101.87 94.38

Percentage of AR removal by MWCM400 (%) 25 45 65

99.21 99.00 97.71

97.86 98.00 98.20

93.35 97.70 93.85

82.89 90.96 84.27

constants can be determined from a linear fit of experimental data, ln qe versus ln Ce, according to Eq. (5). Parameters for the Langmuir and Freundlich isotherms were obtained using Eqs. (3) and (5), and presented in Table 4. Table 4 shows much higher correlation coefficients for Langmuir than for Freundlich, for all temperatures. The lower correlation coefficient obtained for the Freundlich isotherm confirms the poorer applicability of this model for the AR-MWCM400 system. The fact that the Langmuir isotherm fits the experimental data implies eventual monolayer coverage of AR onto the MWCM400 particles. This may be due to the homogenous distribution of similar active sites on the MWCM400 surface and because the solid surface is negatively charged and the AR is positively charged given, so that electrostatic attraction drives the adsorption process until all adsorbent surface charges are neutralised [29]. The “favourable” nature of the Langmuir equation, related to the effect of isotherm shape, can be expressed in terms of a dimensionless separation factor, or equilibrium parameter, RL [30,31], which is defined by:

RL =

1 1 + KL C0

ð6Þ

The value of RL indicates whether the shape of the isotherm is unfavourable (RL N 1), linear (RL = 1), favourable (0 b RL b 1), or irreversible (R L = 0). R L values for the AR adsorption onto MWCM400 are in the range of 0.011–0.100, 0.006–0.055 and 0.003– 0.028 for 25 °C, 45 °C and 65 °C respectively, showing favourable processes [30,31].

shows that the rate of AR adsorption onto MWCM400 is higher when the solution is more diluted. The first-order rate equation of Lagergren [32] describes adsorption in liquid/solid systems, and it has been called pseudo-first-order. One may distinguish a kinetic equation based on the adsorption capacity of the solid where the activated sites on the adsorbent play the most import role (the ‘solid film’ model) from one based on the concentrations in solution (the ‘liquid film’ model) [33]. For the solid film model, the governing equation is: lnðqe −qt Þ = ln qe −k1 t

ð8Þ

where k1 represents the pseudo-first-order rate constant (h−1). This rate constant is determined from the slope of the plot of ln (qe − qt) versus t. The pseudo-first-order model does not fit adsorption system well [34] for the whole range of contact times and the temperatures studied, and the data will not be shown here. This might be related to the fact that the adsorbent and the adsorbate play a similar role in the adsorption process, which should give a better fit to the second order relation. The kinetic data were further analyzed using a pseudo-secondorder relation, based on the concentration in solution [35–37]. The governing equation is: t 1 1 = + t qt qe k2 q2e

ð10Þ

where k2 is the pseudo-second-order rate constant (g mol−1 h−1). If pseudo-second-order kinetics is applicable, the plot of t/qt versus t should give a linear relationship, and the value of the constants k2 and qe can be determined from the intercept and the slope.

3.7. Kinetics of the adsorption process

Table 4 Langmuir and Freundlich parameters.

C0 = 0.56 mmol L-1

5

C0 = 2.80 mmol L-1 C0 = 4.48 mmol L-1

4

Ct (mmol L-1)

The adsorption rate of a solute onto a solid in aqueous solution is strongly influenced by several parameters related to the solid state and to the physicochemical conditions under which adsorption is carried out. It is a phenomenon whose kinetics are often complex [26], and numerous kinetic models have been proposed to elucidate the mechanism by which dyes are adsorbed [3]. Fig. 4 shows the effect of initial AR concentration on the uptake of AR onto MWCM400 at 45 °C. A comparison of the kinetic curves

C0 = 5.60 mmol L-1

3

2

1

Temperature (°C)

KL (L g−1)

Q (μmol g−1)

KF (L g−1)

1/n

25 45 65

2.053 4.314 8.114

128.37 139.80 131.90

4.704 14.255 4.780

0.214 0.202 0.248

r Langmuir

Freundlich

0.99888 0.99966 0.99862

0.97836 0.91834 0.92977

0 0

50

100

150

200

250

300

350

400

Time (h) Fig. 4. Effect of initial AR concentration on the uptake of AR onto MWCM400 at 45 °C.

C.A.P. Almeida et al. / Desalination 264 (2010) 181–187

For the pseudo-second-order model, the results are summarised in Table 5. The higher r2 values confirm that the adsorption data are better represented by pseudo-second-order kinetics for the entire adsorption process studied. It has been reported that a pseudosecond-order model indicates a rate-controlling step which is governed by a process which is chemisorption in nature. Nevertheless, physisorption and chemisorption are difficult to distinguish in certain situations. Indeed, in some cases, a degree of both types of bonding can be present simultaneously, involving both physical Coulombic forces and chemical valence forces established through the sharing or the exchange of electrons between sorbent and sorbate. This is similar to the way in which covalent bonds may occur between two atoms having some degree of ionic character, and vice-versa [38]. The pseudo-second-order kinetics for the adsorption of AR onto MWCM400 can be based upon a heterogeneous reaction, assuming that the dynamic equilibrium is set up when the rate of adsorption is equal to the rate of desorption. The rate of adsorption depends on the collision of an adsorbate molecule with the vacant adsorbent surface site, and is proportional to adsorbate concentration. The MWCM400 adsorbent is negatively charged and the AR adsorbate is positively charged, so a full monolayer of adsorbate is formed on the surface of the adsorbent when the balance of negative and positive charges on the surface is equal, and at this point the rate of adsorption can be considered equal to the rate of desorption. 3.8. Thermodynamic parameters In an isolated system, where energy cannot be gained or lost, entropy change is often the driving force for spontaneous processes [39]. The fundamental criterion of spontaneity is given by the Gibbs free energy change. Thermodynamic properties such as standard enthalpy change (ΔadsHo), standard entropy change (ΔadsSo), and standard free energy change (ΔadsGo) of adsorption can be estimated by using the following equations [40]: ln KL = −

Δads H o Δ So + ads RT R

ð11Þ

o

Δads G = −RT ln KL

ð12Þ

In these equations, R is the gas constant (R = 8.34 J mol−1 K−1), and T is absolute temperature in K. Eq. (11) provides the enthalpy and the entropy changes by plotting ln KL versus T−1. From the intercept and the slope of the straight, ΔadsSo and ΔadsHo can be estimated. The results obtained yield a correlation coefficient (r) of 0.99857. The ΔadsHo and ΔadsSo values are thus found to be +28.2 kJ mol−1 and +174.8 J K−1, respectively, while the ΔadsGo values are −24.0 kJ mol−1, −27.4 kJ mol−1 and −31.0 kJ mol−1 (at 25 °C, 45 °C and 65 °C, respectively). Positive values of ΔadsHo and ΔadsSo indicate an endother-

185

mic process and increasing randomness, respectively, at the solidsolution interface during the adsorption process. The initially adsorbed solvent molecules which are displaced by the adsorbate species gain more translational entropy than that which the dye ions lose, allowing for prevalence of randomness in the system [10]. The negative values of ΔadsGo demonstrate a spontaneous adsorption process. Generally, adsorption processes with ΔadsGo values in the −20 to 0 kJ mol−1 range correspond to physisorption, while those with values in the −80 to −400 kJ mol−1 range correspond to chemisorption [41]. Since ΔadsGo changed from −24.0 to −31.0 kJ mol−1 when the temperature increased from 25 °C to 65 °C, it can be concluded that the adsorption mechanism is dominated by physisorption. 3.9. Activation parameters Using the Arrhenius equation (Eq. (15)) with the pseudo-secondorder rate constants for the same solution concentrations, but each one at a different temperature, it is possible to gain some insight into the molecular mechanism of the adsorption process. ln k2 = ln A−

Ea RT

ð15Þ

Here Ea is the activation energy (J mol−1) and A the temperatureindependent Arrhenius factor (g mol−1 s−1), called the pre-exponential factor. The slope of the plot of ln k2 versus T−1 can be used to evaluate Ea [36]. The activation energy in the Arrhenius expression can be identified with the extra energy needed to reach the transition state, and the expression exp(−Ea/RT) represents the fraction of collisions among dye molecules and adsorption sites with an energy in excess of Ea [40] for effective adsorption. Low activation energies (5–40 kJ mol−1) are characteristic of physical adsorption, while higher ones (40–800 kJ mol−1) suggest chemisorption [12]. The Arrhenius plot presented r of 0.98265 and 21 kJ mol−1 for Ea. The low activation energy value confirms a physical adsorption mechanism, as previously suggested by the estimate of free energy change. 3.10. Amount of AR adsorbed by MWCM400 compared with several adsorbents In order to examine the relative performance of MWCM400 as an adsorbent for AR, the amount of AR adsorbed from a given solution by MWCM400 per unit surface area was compared with the amount of AR adsorbed by Moroccan clay [42], Sepiolite [43], Bentonite [44] and Activated Carbon developed from surplus sewage sludge [45]. As can be seen clearly in Fig. 5, the amount of AR adsorbed by MWCM400 per unit area was found to be two or three times greater than that by the other adsorbents. The fact that MWCM400 adsorbs

Table 5 Pseudo-second-order kinetic parameters for adsorption of AR on MWCM400 at different temperatures. C0 (mmol L−1)

25 °C

0.56 1.12 1.68 2.24 2.80 3.36 3.92 4.48 5.03 5.60

37.51 0.43 1.80 1.40 1.90 0.41 0.34 0.29 0.27 0.21

3

10 k2 (g μmol

45 °C –1

–1

h )

3

r2

10 k2 (g μmol

1.0000 0.9996 0.9983 0.9958 0.9957 0.9955 0.9955 0.9920 0.9925 0.9901

113.40 12.83 3.07 1.39 1.11 0.46 0.50 0.72 0.64 0.82

65 °C –1

–1

h )

r2

103 k2 (g μmol–1 h–1)

r2

1.0000 1.0000 0.9998 0.9992 0.9982 0.9931 0.9951 0.9979 0.9972 0.9959

273.55 82.61 15.81 8.09 7.34 1.21 0.92 0.73 0.69 0.68

0.9995 0.9999 0.9991 0.9988 0.9984 0.9987 0.9983 0.9960 0.9865 0.9874

Abbreviations: C0 = initial MB concentration, k2 = pseudo-second-order rate constant, and r2 = linear regression correlation coefficient.

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United Kingdom. Andressa dos Santos thanks Coordenação de Aperfeiçoamento de pessoal de Nível Superior (CAPES) for the fellowships provided. References

Fig. 5. Comparison of the amount of AR adsorbed by MWCM400 (45 °C), with Moroccan Clay (25 °C) [42], Sepiolite (40 °C) [43], Bentonite (25 °C) [44] and Activated Carbon developed from surplus sewage sludge (20 °C) [45].

more AR per unit area (and also, in this comparison, per unit mass) than many other adsorbents confers on it an advantage. It should also be noted that MWCM400 is a waste-product of the coal mining industry, whose accumulation steadily increases in locations near the mine, and the use of MWCM400 for treating waste water offers a more sustainable way of operating the mine with reduced environmental impact. 4. Conclusions The adsorption process of AR onto MWCM improved when MWCM was calcinated at 400 °C for 4 h and unbuffered solutions were used. The equilibrium adsorption was well described in terms of the Langmuir isotherm, suggesting monolayer coverage of AR onto MWCM400 particles involving a physisorption process with weak adsorbate–adsorbate interactions. By Langmuir isotherm we found the maximum amount of AR adsorbed of 128.37 μmol g −1 , 139.80 μmol g−1 and 131.90 μmol g−1 for 25 °C, 45 °C and 65 °C, respectively. Kinetic data are fit best by a pseudo-second-order kinetic model, with higher values for the linear regression correlation coefficient than those for a pseudo-first-order model. The values for ΔadsHo and ΔadsSo were found to be + 28.2 kJ mol−1 and +174.8 J K−1, respectively, indicating an endothermic nature and increasing randomness at the solid/solution interface, suggesting that displacement of solvent from surface to solution plays a key role. The values of ΔadsGo, from −24.0 kJ mol−1 to −31.0 kJ mol−1 (from 25 °C to 65 °C, respectively) indicate a spontaneous adsorption process at higher temperature. The activation energy was consistent with a process mainly involving physisorption. From the present study, it can be said that MWCM shows good promise as an effective adsorbent for the removal of AR from wastewaters in an industrial treatment process, especially for textile industry effluents. Application of this material as an adsorbent will bring two benefits to the environment: a) it will make use of a deposited waste from coal mining, and b) it will remove dyes from wastewater in the textile industries. The MWCM when saturated with AR can now be used as a construction material, or it can be calcinated again to be reused as an adsorbent. Acknowledgements Carlos Alberto Policiano Almeida is grateful for the research grant provided by the Brazilian agencies Ministério de Ciência e Tecnologia (MCT) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). He also thanks Universidade Estadual do Centro-Oeste (UNICENTRO) in Brazil for his sabbatical leave, hosted by the Department of Engineering Science at the University of Oxford,

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