Reactive Blue 5G Adsorption onto Activated Carbon: Kinetics and

Article pubs.acs.org/jced

Reactive Blue 5G Adsorption onto Activated Carbon: Kinetics and Equilibrium Tiago D. Martins,† Daiana Schimmel, Joaõ B. O. dos Santos,‡ and Edson A. da Silva* School of Chemical Engineering, Western State University of Parana, Rua da Faculdade 645, Jardim La Salle, 85903-000, Toledo, PR Brazil ABSTRACT: Adsorption studies are essential before implementation in an industrial plant. We studied the Reactive Blue 5G removal by activated carbon from Pinus sp. Physical and chemical characteristics of the adsorbent were evaluated with N 2 adsorption/desorption experiments at 77 K, and equilibrium and kinetics studies were performed in a batch reactor at several temperatures and pH values. Our results showed that the activated carbon had a high surface area, effective diffusion coefficients were of the order of 10−12 cm2·s−1, and mass transfer coefficients were present in both fluid and solid phases. Equilibrium and kinetics experiments confirmed that pH and temperature play an important role: the dye uptake became higher as the temperature and pH decreased, and the optimum removal was at 303 K and pH 2. Finally, thermodynamics analysis suggested that the process is exothermic and chemical in nature. In summary, we showed that activated carbon can be more effective than other adsorbents already used to remove the Reactive Blue 5G from wastewater.

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INTRODUCTION Estimates show that there are more than 100 000 types of dyes available for industrial application. Many of them are toxic and carcinogenic. If released directly in nature, they can reduce light penetration and gas solubility, affecting all of the living animals and plants on that habitat.1 Several processes could be used to remove dyes from industrial wastewater, such as coagulation, ozonation, membrane filtration, ion exchange, decolourization, and so forth. Each one has advantages and drawbacks. Among these, adsorption has been shown to be a viable and low-cost option.2 In adsorption processes a porous solid (adsorbent) is used to capture soluble substances present in aqueous solution (dyes for instance). Adsorption studies are generally focused on adsorbent selection, and papers which evaluate its performance for dye removal can be commonly found on the literature: agroindustrial byproducts,3−5 chitosan,6−8 bacterial and fungal biomass,9−11 industrial waste,12,13 and minerals,14−16 among others has already been considered. In addition, special attention had been given to activated carbon.17−28 The studies involving this material showed that it could be a more efficient and low-cost material for dye uptake. One of the most used dyes used by industrial laundries is Reactive Blue 5G. It is often used for the dyeing of jeans fabric. Chemically it is classified as an azo-dye, due the presence of N N bonds, as shown in Table 1.29 Besides its importance, only a few studies concerning its removal using adsorption can be found in the literature.3,19,30−33 The main objective of this work was to perform Reactive Blue 5G adsorption by a commercial activated carbon and to obtain several information about the process, such as carbon characteristics and equilibrium and kinetics aspects. © XXXX American Chemical Society

Table 1. Properties of the Adsorbate Used in This Work

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EXPERIMENTAL SECTION Materials. The adsorbate used was a synthetic effluent prepared with the Reactive Blue 5G dye, produced by Texpal Chemical S.A. and provided by the company. Table 1 shows the properties of the selected adsorbate. The adsorbent was a commercial activated carbon (the material was manufactured from Pinus sp. by AlphaCarbo Industrial S.A. (Guarapuava, PR/ Brazil) and supplied by the company). Adsorbent Characterization. The properties of activated carbons are a result of their preparation. In fact, all of the aspects of an adsorption process depend upon these physical and chemical characteristics. These information can indicate if the process of carbon activation is adequate for such propose or appoint another (and better) ways to prepare them to avoid low efficiency and higher costs. Received: August 30, 2012 Accepted: December 3, 2012

A

dx.doi.org/10.1021/je300946j | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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determined by ultraviolet visible spectroscopy (Spectro Vision model UV−vis SB-1810S) at 610 nm. Finally, the dye equilibrium amount in the adsorbent (qeq, mg·g−1) was calculated using the equation:

Adsorption is a molecular level process. Therefore, it is important to know the adsorbent used at microscopic level. We determined experimentally the main adsorbent physical properties: mean particle diameter, specific superficial area, total pore and average pore diameter. The point of zero charge pH (pHPZC) was the only chemical property experimentally determined. Optical microscopy (Leica-Q500IW, State University of Unicamp) was used to evaluate particle size distribution. The results were statistically analyzed using histograms to calculate the mean particle diameter. N2 adsorption/desorption experiments were performed to determine adsorbent specific superficial areas and volumes. The essays were conducted at the Chemical Engineering Department (State University of Maringá) using the Nova 1200 Series (QuantaChrome) and the software Autosorb Automated Gas Sorption System Report, Version 1.19. The adsorption and desorption of N2 were carried out at a temperature of 77 K. Before the analysis, all of the samples were dried at 573 K, under vacuum, during 3 h. Then, the specific surface area was determined by the Brunauer−Emmett−Teller (BET) method,34 and total pore volumes were calculated from the amount of N2 adsorbed at P/Po = 0.975.35 The method “trial of the 11 points”, previously described by Regalbuto and Robles,36 was used to determine the pHPZC. Mixtures with 50 mg of adsorbent and 50 mL of aqueous solution, at 11 different initial pH values (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12adjusted with solutions of HCl or NaOH 0.1 mol·L−1) were prepared. After 24 h, the pH of each solution was measured. The pHPZC was estimated from the plot of final pH versus initial pH. The test were carried out in duplicate. Adsorption Experiments. Adsorption studies are usually divided into two categories: kinetics and equilibrium. Kinetic experiments provide knowledge about the speed in which the solute molecules are captured and about the diffusion mechanisms. The amount of solute removed, composition of the phases at end of the process, the adsorbent selectivity, and some knowledge about the energy interaction between the adsorbent and the adsorbate are obtained from equilibrium experiments. These variables depend on several parameters related to physicochemical characteristics of the solute, adsorbent, and solution. Knowing this information is important to design efficient effluent treatment systems. For kinetic studies, several Erlenmeyer flasks containing 2 L of the Reactive Blue 5G dye solution at 500 mg·L−1 were put in contact with 6 g of the activated carbon particles. The system was maintained at pH 2 and 100 rpm. To evaluate the temperature effect on the adsorption kinetics, this study was performed at three different temperatures: 303 K, 318 K, and 333 K. Equilibrium studies were divided into two parts: temperature and pH influence. All of the equilibrium essays were carried out during 48 h in Erlenmeyer flasks containing 50 mL of the dye solution and 0.3 g of activated carbon. The initial concentration of the solutions ranged from 150 to 1000 mg·L−1, and agitation was maintained at 100 rpm. The effect of three different temperatures on adsorption equilibrium was evaluated: 303 K, 318 K, and 333 K. In equilibrium studies, solution of the pH were kept constant at 2. To study the pH effect on the dye uptake, solutions at four different fixed pHs were studied: 2.0, 4.0, 6.0, and 8.0. In this case, the system temperature was kept constant at 303 K. In all adsorption experiments, pH was controlled using a solution of 0.1 mol·L−1 HCl. The samples taken were filtered using a Milipore 47 μm paper filter. The dye concentration was

qeq =

V (C0 − Ceq)

(1) M where V is the volume of the solution, C0 is the initial concentration of the dye in solution, Ceq is the concentration of the dye in solution at equilibrium, and M is the adsorbent mass in dry base. All kinetic and equilibrium experiments were carried out in duplicate.

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RESULTS AND DISCUSSION Adsorbent Characterization. The values of the adsorbent physical properties are presented in Table 2. The results showed Table 2. Physical Properties of the Activated Carbon property

value

BET surface area (m2·g−1) external surface area (m2·g−1) particle diameter (μm) total pore volume (cm3·g−1)

618.7 65.74 41.53 0.34

that the BET surface area is considerably large (618.7 m2·g−1) and that only about 10 % of its area represents the external surface. These values are in agreement with recent works found in the literature which shows that they are are typical for activated carbons.17,18,20,21,23 The pHPZC is defined as the pH at which the adsorbent surface presents a neutral charge. When pH < pHPZC, the surface has a net positive charge, while at pH > pHPZC it is negative. The pHPZC can be experimentally determined as the point where the line pHfinal = pHinitial crosses the plot of final pH versus initial pH, obtained from the procedure described in Adsorption Characterization section. In Figure 1 the plot we obtained is shown. These data are shown in Table 3. One can see that pHPZC for the activated carbon is around pH 8.10. This value is due the combination of the influence of hydroxyl, aldehyde, ketone, N− O, and carboxylic acid functional groups present at the carbon surface.22 These groups can be protonated or deprotonated depending on pH value. By comparing to the pHPZC value, the

Figure 1. pH at the point of zero charge for the activated carbon. ■, relation of final pH vs initial pH. The line represents the equation: final pH = initial pH. Statistical error bars are smaller than symbol sizes. B



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Table 3. pH Data Obtained in the pHPZC Experimenta

a

initial pH

final pH

3.00 4.28 5.21 6.54 7.20 8.12 9.40 10.10 11.09 11.87

3.10 5.67 7.15 7.58 7.97 8.10 8.15 8.22 8.87 10.44

Table 4. Kinetic Data Obtained from Different Temperaturesa 303 K t/s

C/mol·kg

0 60 180 300 600 900 1200 1800 3600 7200 10800 21600 28800 43200 86400 108000 129600 172800

The standard uncertainty u is u(pH) = 0.01.

adsorbent used in this work presented a positive net charge in all essays. Adsorption Kinetics. Figure 2 shows the kinetic curves obtained in the three temperatures tested. These data are also

a

318 K −1

320.33 320.33 261.66 215.31 183.71 183.71 181.61 142.00 103.44 97.12 85.54 75.00 67.63 60.31 56.15 50.93 49.92 52.09

C/mol·kg

333 K −1

398.81 324.72 302.80 298.42 307.19 276.51 197.63 142.86 112.18 96.84 85.89 81.51 68.43 48.74 41.54 38.72 36.55 36.59

C/mol·kg−1 399.81 284.25 254.64 245.60 227.50 218.12 214.91 205.04 180.37 153.23 138.42 134.72 117.45 84.22 67.00 64.59 64.66 65.96

The standard uncertainty u is u(C) = 0.01 mol·kg−1.

The pseudo-first-order model is mathematically expressed by the differential equation: dqt dt

= k1(qeq − qt )

(2)

where: qt and qeq are the amount adsorbed at the time t and at equilibrium, respectively, k1 is the pseudo-first-order kinetic rate parameter, and t is the time. Integrating eq 2 between 0 and t and 0 and q, it becomes:

Figure 2. Kinetic results for dye adsorption at different temperatures. □, 303 K; ○, 318 K; △, 333 K. Statistical error bars are smaller than symbol sizes.

qt = qeq (1 − e−k 2t )

(3)

The pseudo-second-order model is expressed as:

presented in Table 4. The form of these curves is very familiar. Adsorption occurred rapidly at the beginning of the process and became slower at the final stages. This behavior occurs because initially all active sites are available for adsorption. Its removal becomes difficult due to the decrease of these sites and the repulsive forces that are present. Thus, the sorption velocity decreases, and finally it reaches the equilibrium state. To ensure that the process reaches the equilibrium, the experiments were performed until 48 h, but the results pointed that, at 25 h, all runs had reached this condition. The effect of the temperature on dye removal is very significant: the equilibrium was reached 50 % faster at 303 K, if compared to the other temperatures. Nowadays, mathematical modeling is a powerful tool that, besides mathematical equations, can provide several information about a given process. In the case of adsorption processes, the kinetic parameters and mechanism, as well as the maximum capacity of adsorption, can be determined using modeling techniques. Over the years, several models were proposed in an attempt to understand the fundamental aspects of adsorption kinetics. Pseudo-first-order (or Lagergren model),37 pseudosecond-order (or Ho’s model),38 and effective diffusion models (ED)39 are known as the models that best describe this aspect. In this work, all kinetic data obtained were modeled by these three models.

dqt dt

= k 2(qeq − qt )2

(4)

where: k2 is the pseudo-second-order kinetic rate parameter. Similarly, an explicit equation for qt can be obtained integrating eq 4 between 0 and t and 0 and q: qt =

k 2qeq 2t 1 + k 2qeq t

(5)

The effective diffusion (ED) model was previously described by Ruthven,39 and it is based on the mass balance for the solid phase. It has the following considerations: the particle is spherical, equilibrium concentration in the fluid phase does not differ much from the initial concentration on that phase, and heat transfer between the particle and the fluid surrounding it is rapid, so temperature gradients can be considered negligible. Mathematically, this model is expressed by: ∂q ∂q ⎞ 1 ∂⎛ = 2 ⎜r 2Def ⎟ ∂t ∂r ⎠ r ∂r ⎝

(6)

with the following initial and boundary conditions:

q(r , 0) = 0 C

(7) dx.doi.org/10.1021/je300946j | J. Chem. Eng. Data XXXX, XXX, XXX−XXX


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q(rp , t ) = q0

(8)

∂q ∂r

(9)

adsorbents showed that pseudo-second-order kinetics is the main mechanism for dye adsorption.4−6,8−11,13−16,18,20,22,24−26 The values of the effective diffusion parameters are inversely proportional to the temperature, which is in perfect agreement with our experimental data. The coefficient estimations are in agreement with several studies found in the literature: Alkan et al.14 and Dogan et al.15 reported that the ED coefficient for maxilon blue GRL on septiolite are lies in the 10−10 cm2·s−1 to 10−8 cm2·s−1 range. Also, Cotoruelo et al.25 reported diffusion coefficients between 10−12 cm2·s−1 and 10−11 cm2·s−1 for the adsorption of crystal violet using lignin-based activated carbons, and Sahmoune and Ouazene,41 whom studied the removal of Astrazon Yellow using Aleppo pine-tree sawdust, determined diffusion coefficients to be in the order of 10−11 cm2·s−1. Dyes can self-associate and aggregate in aqueous solutions. Dimerization occurs even at concentrations near 10−4 mol·L−1 to 10−6 mol·L−1, and aggregates occur between the dimers previously formed.42 According to Coates,43 temperature also influence these two reactions and usually for higher temperatures the number of aggregates increases. The observed decrease in adsorption kinetics at 303 K and 318 K could be due to the reduction of the diffusivity induced by aggregate formation. Modeling and interpretation of adsorption data can be more accurate if its mechanism at molecular level is well-established. Generally, sorption process involves three main steps: (i) diffusion at the liquid film, (ii) pore diffusion, and (iii) adsorption at the sites available. The one which occurs slowly is said to be the rate-limiting step and corresponds to the speed of the process. The last step (iii) is assumed to occur rapidly; therefore it is considered to be negligible. One way to evaluate whether steps (i) or (ii) are the rate-controlling factor is analyzing the plot of the equation:

=0 r=0

An analytical solution for eqs 6 to 9 can be derived, and it is given by the equation:39 q̅ 6 =1− 2 q0 π

∞

∑ n=1

⎛ n2π 2D t ⎞ 1 ef ⎟ exp⎜⎜ − 2 2 n rp ⎟⎠ ⎝

(10)

In eqs 6 to 10, rp is the particle radius, Def is the constant effective diffusion parameter, n is the number of the terms of the series, q0 is the amount adsorbed at the particle surface, in equilibrium with the fluid phase, and q̅ is the average concentration through the particle: rp 3 q̅ = 3 q r 2 dr rp 0 t (11)

∫

Each model has only one parameter that needs to be estimated using optimization procedures (k1, k2, and Def). We calculated them using the experimental data for each fixed temperature. To perform this task, the downhill Simplex method40 was used to minimize the objective function expressed by eq 12. ⎛ q exp − q mod ⎞2 t ⎟ FObj = ⎜⎜ t exp ⎟ q ⎝ ⎠ t

(12)

The results obtained by the kinetic modeling are presented in Table 5, as well the absolute average deviation (AAD), expressed Table 5. Parameters of the Kinetic Models Fitted to the Experimental Data T/K

model pseudo-first-order

303 318 333 pseudo-second-order 303 318 333 effective diffusion 303 318 333

parameter k1/s−1 1.83·10−3 6.88·10−3 1.67·10−3 k2/g·mg−1·s−1 2.90·10−5 7.68·10−6 3.33·10−5 Def/cm2·s−1 3.16·10−12 1.68·10−12 1.37·10−12

qt = kD t + C

statistical parameters AAD/% 16.74 19.36 21.82 AAD/% 13.19 15.87 18.73 AAD/% 12.26 12.95 15.08

where kD is the intraparticle diffusion constant and C is a parameter that provides information about the layer boundary effect. This analysis was first described by Weber and Morris,44 and according to the authors, if this plot behaves linearly, intraparticle diffusion is the rate-limiting step. We estimated the number of linear segments (and the boundaries between them) by using the PLR technique.45 Details for calculation procedures can be found in their original paper. From Figure 3 it is clear that there are three diffusion steps present in the removal of Reactive Blue 5G. In fact, for our results, the adsorption process occurs in two main different regions: external surface and macro- and mesopores. The first curve portion is related to the external surface adsorption. It is generally the fastest part of the process, where the adsorbate diffuses through the solution bulk to the particle surface. When the surface reaches saturation, intraparticle diffusion takes place, and the second curve portion forms. This stage is related to the macro- and meso-pore diffusion, where the intraparticle diffusion is the rate-controlling step on the adsorbent. The third curve portion is the equilibrium stage, where the process slows down and the maximum adsorption is attained.3,14 The parameters of each linear segment shown in Figure 3, as well its correlation coefficients, are presented in Table 6. According to this table, the following sequence: kD,303K > kD,318K > kD,333K confirms our previous findings, which indicated that adsorption kinetics is slower at higher temperatures. If the plot crosses through the origin, intraparticle diffusion is the only resistance factor. For our experimental data, this may not hold

r2 0.86 0.89 0.62 r2 0.94 0.94 0.76 r2 0.98 0.98 0.96

by eq 13, and the correlation coefficients (r2). The superscripts exp and mod mean experimental and model, respectively, and Ndata are the number of samples collected. 1 AAD = Ndata

Ndata

∑ 1

exp qeq − qeqmod exp qeq

(14)

(13)

From Table 5, one can see that the ED model fitted better the kinetic data, if compared to pseudo-order kinetic models. Our result disagrees with those obtained by Fiorentin et al.3 and Fagundes-Klen et al.,33 which showed that pseudo-second-order kinetics is the main mechanism for Reactive Blue 5G removal. Also, from several studies of dye adsorption using different D



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Figure 4. Experimental adsorption isotherms obtained for the Reactive Blue 5G removal at different fixed pH. □, pH 2; ●, pH 4; △, pH 6. ∗, pH 8. Statistical error bars are smaller than symbol sizes. Figure 3. Intraparticle diffusion plots. □, 303 K; ○, 318 K; △, 333 K. The solid lines correspond to the equations presented in Table 6. Statistical error bars are smaller than symbol sizes.

Therefore, the affinity between the adsorbent surface and the Reactive Blue 5G molecules decreased. The equilibrium amount of the dye was 140.48 mg·g−1, 108.46 mg·g−1, 111.63 mg·g−1, and 115.85 mg·g−1 for pH 2, 4, 6, and 8, respectively. Other authors also reported higher dye adsorption capacities at acidic pH using several adsorbents.3−10,20,22,27,28,33 The behavior of Reactive Blue 5G adsorption can be explained by means of the competitive adsorption of the mixture solute/H+ (or OH−) present in the solution. pH directly affects the protonation/deprotonation process of the functional groups present at the adsorbent surface. In a previous work Schiemmel et al.,22 who used the same activated carbon used in this work, determined that at its surface carboxyl, hydroxyl, aldehyde, and ketone functional groups are presented. Some of the functional groups present at the adsorbent surface receive H+ ions and the net surface charge becomes positive. pH also affects the charge distribution in the dye molecule. In acidic conditions, the Reactive Blue 5G molecules are deprotonated, and negative groups such R-SO3− and R-O− become part of its structure (Table 1). The combination of those factors may probably be the cause of the high adsorption capacity at lower pH.3,5 All of the equilibrium data obtained in this work were used to fit the parameters of four different adsorption isotherms commonly used: Langmuir, Freundlich, Sips, and Tóth. The equations of each isotherm are presented in Table 8, as well as the parameters estimated. To fit the models, the downhill Simplex method40 was used, and the objective function minimized was the one expressed by eq 12, in which the subscript t corresponds to the equilibrium concentration at the adsorbent. In Table 9 all of the estimated adsorption isotherm parameters and the calculated AAD and r2 are presented. All of the models fitted the experimental data well, once the AADs were low and the correlation coefficients were relatively high. The model that best suited the experimental data for each pH was not the same for all of the tested pH. Data obtained at pH 2 were best fitted by the Tóth isotherm, at pH 4 and 8 by the Sips isotherm, and at pH 6 by the Freundlich isotherm. This result suggests that, at each pH level, different adsorption mechanisms are involved and different energy interactions between the adsorbing sites and solute are present. Temperature Effect. Figure 5 and Table 10 show the results for equilibrium experiments performed at the three different selected temperatures. The behavior of the curve is similar for experiments at 318 K and 333 K. However, as concentration increases, significant deviations suggest that 303 K was the best

Table 6. Kinetic Parameters of the Intraparticle Model curve portion

T/K

kD/mg·g−1·s−0.5

C/mg·g−1

r2

303 318 333

1.68 1.57 0.46

23.24 21.15 67.04

0.92 0.97 0.90

303 318 333

0.10 0.11 0.13

87.50 108.06 94.13

0.97 0.94 0.97

first

second

true. Since C ≠ 0 for all segments obtained, there may be some boundary layer resistance. Multiple-step behavior was also reported in several studies using different adsorbents for dye adsorption,14,18,23 including other azo-dyes.6,7,15 Fiorentin et al.3 reported that, when using orange bagasse to remove Reactive Blue 5G from aqueous solution, the adsorption kinetics present two-step behavior. The authors discussed that the two stages involved consist in surface and macropore diffusion, respectively, and that intraparticle diffusion was not the only resistance factor. Adsorption Equilibrium. The adsorbent performance is often evaluated by using equilibrium isotherms adjusted from experimental equilibrium data. From these results, several conclusions may be appointed about the maximum uptake capacity, the nature of the adsorbent, and about the solute removal. Two of the main parameters that influence adsorption are temperature and pH. The former influences adsorption capacity due the effect in molecular agitation and the organization of the particles in the system. pH affects dissociation of the chemical groups present in the solute molecule and in the adsorbent. To study the influence of each of these parameters, experimental equilibrium data and modeling are important. pH effect. Figure 4 shows the equilibrium profiles obtained from the experiments at the different fixed pH values tested. These data are also shown in Table 7. From Figure 4, the isotherm curvature indicates that the dye molecules are favorably adsorbed on the activated carbon.39 At same Ceq (< 50 meq·L−1), qeq decreased as pH increased. E



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Table 7. Equilibrium Data Obtained at Different pH and 303 Ka pH 2

a

pH 4

pH 6

pH 8

Ceq/mol·kg−1

qeq/mg·g−1

Ceq/mol·kg−1

qeq/mg·g−1

Ceq/mol·kg−1

qeq/mg·g−1

Ceq/mol·kg−1

qeq/mg·g−1

0.00 0.04 0.11 0.08 1.06 2.93 20.85 42.18 74.13 127.99 167.80

0.00 56.13 71.61 88.87 95.19 111.96 121.04 122.80 124.65 137.13 140.48

0.00 0.88 0.57 3.24 17.53 83.26 131.83 184.78

0.00 56.76 71.56 86.52 98.05 100.50 102.71 108.85

0.00 10.25 7.16 12.66 37.61 71.39 85.09 122.70 157.21

0.00 60.39 76.63 86.04 102.55 102.34 103.37 106.39 111.76

0.00 0.00 41.26 44.92 85.09 91.48 111.56 117.95 149.91 181.13

0.00 50.99 59.99 73.08 84.75 93.34 100.09 107.66 115.40 115.40

The standard uncertainty u is u(Ceq) = 0.01 mol·kg−1, and the expanded uncertainty Uc is Uc(qeq) = 0.01 mg·g−1 (k = 2).

Table 8. Adsorption Isotherm Equations Used isotherm

equation

1. Langmuir

qeq =

2. Freundlich

n qeq = kCeq

3. Sips

qeq =

4. Tóth

qeq =

adjustable parameters

qmbCeq

qm, b

1 + bCeq

k, n

n qmbCeq n 1 + bCeq

qm, b, n

n qmCeq

qm, b, n

n 1/ n (b + Ceq )

Table 9. Adsorption Isotherm Parameters Obtained for Different pH and 303 K statistical parameters

parametersa model

pH

qm

b

2 4 6 8

119.50 105.33 117.78 162.62

17.06 1.59 0.13 0.01

K

Figure 5. Experimental adsorption isotherms obtained for the Reactive Blue 5G removal at different temperatures. □, 303 K; ∗, 318 K; ●, 333 K. Statistical error bars are smaller than symbol sizes.

AAD/%

r2

9.88 7.60 8.81 4.37

0.87 0.82 0.78 0.96

0.09 0.09 0.17 0.46

7.28 6.00 6.66 6.63

0.90 0.87 0.88 0.91

n

Lower temperatures may affect the mobility of the dye molecule and should provide access to the surface and the activated carbon pores, thus changing the adsorption capacity of the carbon. When the process temperature is decreased from 333 K to 303 K, the adsorption capacity increased from 120 mg·g−1 to 140 mg·g−1. Similar behavior was also reported by other authors;5−7,9,22,46 however it is not a rule for all systems.3,10,11,14−17,21,27,28 The aggregate formation also plays a role for the temperature effect on adsorption equilibrium. These substances are larger molecules and can suffer from hysteric effects. Thus, the amount adsorbed in the meso- and micropores is reduced. These data were also fitted to the adsorption models presented in Table 8. The adsorption isotherm parameters, AAD, and r2 obtained from this data fitting are presented in Table 11. As before, all isotherms fitted the data well, which is confirmed by the high r2 calculated. The Tóth isotherm was the one with the highest r2 and lowest AAD in all cases. This result, as well as the result for pH influence, indicates that the activated carbon used in this work interacts heterogeneously with the dye molecule. Thermodynamics is another important parameter to be evaluated. It can clarify whether adsorption is endo- or exothermic, as well as the binding mechanism of the removal. The Henry constant (KH) can be calculated by the following equation: qeq KH = lim Ceq → 0 Ceq (15)

Langmuir

Freundlich 2 4 6 8

87.51 66.82 48.12 9.37

Sips 2 4 6 8

144.59 121.45 260.47 162.61

2.00 1.16 0.19 0.01

0.33 0.36 0.26 0.99

6.01 5.63 6.73 4.36

0.92 0.90 0.87 0.97

2 4 6 8

149.13 129.25 140.74 172.05

0.11 0.19 0.13 0.02

0.26 0.26 0.09 0.90

5.99 5.66 6.68 4.56

0.92 0.90 0.87 0.96

Tóthb

qm = [mg·g−1]; b = L·mg−1; n = [adim]; k = [mg1−n·Ln·g−1]. bb = (mg·L−1)n.

a

temperature for the dye removal. For all temperatures the form of the isotherms indicates that the carbon is a good adsorbent to perform such a removal.39 F



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Table 10. Equilibrium Data Obtained at Different Temperatures and pH 2a 303 K

a

318 K

333 K

Ceq/mol·kg−1

qeq/mg·g−1

Ceq/mol·kg−1

qeq/mg·g−1

Ceq/mol·kg−1

qeq/mg·g−1

0.00 0.04 0.11 0.08 1.06 2.93 20.85 42.18 74.13 127.99 167.80

0.00 56.13 71.67 88.86 95.19 111.96 121.04 122.80 124.65 137.13 140.48

0.00 0.68 0.28 0.55 0.97 2.74 34.73 48.06 82.53 130.00

0.00 20.80 33.16 51.93 70.66 80.08 99.10 111.67 121.13 130.13

0.00 0.28 0.31 0.50 0.35 0.68 0.97 6.25 59.52 68.65

0.00 36.44 31.52 47.85 68.34 78.09 98.48 115.31 122.19 132.42

The standard uncertainty u is u(Ceq) = 0.01 mol·kg−1, and the expanded uncertainty Uc is Uc(qeq) = 0.01 mg·g−1 (k = 2).

Table 11. Adsorption Isotherm Parameters Obtained for Different Temperatures at pH 2 parametersa model

T/K

qm

b

303 318 333

119.50 112.31 132.24

17.06 1.23 1.11

statistical parameters k

AAD/%

r2

9.88 6.29 16.03

0.87 0.92 0.80

0.09 0.19 0.22

7.28 11.35 26.00

0.90 0.91 0.46

n

Langmuir

Freundlich 303 318 333

87.51 50.94 53.78

Sips 303 318 333

144.59 116.18 132.26

2.03 1.05 1.11

0.33 0.83 0.99

6.07 7.16 16.00

0.92 0.93 0.81

303 318 333

149.13 118.12 132.92

0.12 0.65 1.14

0.26 0.73 0.96

6.00 6.96 15.83

0.92 0.93 0.82

Tóthb

a

qm = [mg·g−1]; b = L·mg−1; n = [adim]; k = [mg1−n·Ln·g−1]. bb = (mg·L−1)n.

According to Moreira et al.,46 the apparent enthalpy of adsorption ΔH can be calculated by the equation: ⎛ ΔH ⎞ ⎟ KH = K 0 exp⎜ − ⎝ RT ⎠

crucial factor. Studying a proper manner of preparing the activated carbon, raising the amount of functional groups used for adsorption, as well as its surface area, could increase this process efficiency even more.

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(16)

where R is the universal gas constant and T is the absolute temperature. For the Tóth isotherm model the Henry constant is given by KH = qmb−1/n. We determined the apparent enthalpy of adsorption was using the plot of KH versus T for the temperature range of 303 K to 333 K and fitting the parameters of the exponential model using Microsoft Excel. The value obtained was −238.04 kJ·mol−1. The negative sign indicates that the removal is exothermic, which explains why the removal was fastest and higher at the lower temperature studied in this work. The magnitude of the enthalpy of adsorption can give an idea if the mechanism of removal is physical or chemical. In this case, it suggests a chemisorption mechanism. Only energies between 0 kJ·mol−1 and −40 kJ·mol−1 would indicate physical adsorption. Several authors also reported a chemisorption mechanism for dyes adsorption using different adsorbents.5,12,13 Chemisorption implies that the amount of functional groups, such as carboxyl, hydroxyl, aldehyde, and ketone, at the adsorbent surface is a

CONCLUSION

Adsorption of Reactive Blue 5G dye by activated carbon was studied through equilibrium and kinetics experiments. Our results revealed that the activated carbon had a high surface area. Kinetics experiments showed that the removal process is faster for lower temperatures, the mass transfer rate-limiting step was not only intraparticle diffusion, and diffusion coefficients are of the order of 10−12 cm2·s−1which is comparable with previous results. Equilibrium isotherms suggested that the activated carbon could be an excellent adsorbent for Reactive Blue 5G dye removal. The dye removal was higher at pH 2 and 303 K: the maximum capacity was up to 10 times those published previously by Fiorentin et al.3 and Fagundes-Klen et al.33 The value of the apparent enthalpy of adsorption suggested that the process was exothermic and chemical in nature. In summary, we presented a viable route for Reactive 5G dye removal using adsorption processes. G



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Article

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses †

Chemical Engineering School, University of Campinas UNICAMP, P.O. Box 6066, 13083-970, Campinas SP, Brazil. ‡ Department of Chemical Engineering, Federal University of São Carlos, Rod. Washington Luiz, km 235, 13565-905, São Carlos SP, Brazil. Notes

The authors declare no competing financial interest.

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Reactive Blue 5G Adsorption onto Activated Carbon: Kinetics and

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