Removal of Methylene Blue from Aqueous Solutions by Sewage

Article pubs.acs.org/jced

Removal of Methylene Blue from Aqueous Solutions by Sewage Sludge Based Granular Activated Carbon: Adsorption Equilibrium, Kinetics, and Thermodynamics Liheng Liu,† Yan Lin,‡ Yuanyuan Liu,† Hui Zhu,† and Qiang He*,† †

Key Laboratory of Three Gorges Reservoir Region’s Eco-environment, Ministry of Education, Chongqing University, Chongqing 400045, China ‡ College of Resources and Environmental Science, Chongqing University, Chongqing 400044, China ABSTRACT: Sewage sludge based granular activated carbon (SSGAC) was prepared using calcium sulfate as binder. The porous structure and surface chemical properties of SSGAC were characterized by N2 adsorption isotherm, FTIR, PZC, and Boehm titration. The results showed that the SSGAC was a typical mesoporous adsorbent of which the percentages of mesoporous surface area and volume are 72.81 % and 88.89 %, and the acid groups were dominant on its surface. The adsorption mechanism of MB adsorption onto SSGAC was also investigated by different adsorption models. The results showed that the maximum MB adsorption amount could be 131.8 mg g−1, and the MB adsorption onto SSGAC was a heterogeneous, spontaneous, and endothermic process while physisorption was dominant. The MB adsorption process was well described by the pseudosecondorder kinetic mode, and the MB diffusion in micropores was the potential ratecontrolling step. isotherms, kinetics, and thermodynamics were used to fit the MB adsorption data.

1. INTRODUCTION Due to the extensive application of dyes in industries, a large number of dye wastewater emissions pose a serious threat to the environment and human health.1 Therefore, various technologies, such as biological treatment, coagulants, oxidizing agents, photocatalysis, ultrafiltration, electrochemical, and adsorption,2 have been developed to treat the dye wastewater. Among these methods, the adsorption is considered to be more effective and simple than others.3 However, the frequent replacement of adsorbent makes the processing cost of adsorption rather high. To solve this problem, many cheap raw materials have been used to prepare activated carbons, such as agricultural wastes, woods, bamboo and nutshells.4 In recent years, researchers have paid much attention to prepare powdered activated carbons from sewage sludge by chemical or physical activation,5,6 and the samples prepared have been used for the removal of heavy metals7,8 and dyes9,10 in liquid. Until now, there is no study on production of sewage sludge based granular activated carbons using calcium sulfate as binder reported in the literature. Methylene blue (MB) is a dye commonly used in printing and dying industry. It can cause a variety of diseases of the respiratory system, digestive system, and heart, such as dyspnea, diarrhea, and tachycardia.11 It is often used as a model for studying dyes removal from aqueous solutions.12 In this work, granular activated carbon for MB removal was prepared from sewage sludge using calcium sulfate as binder. The porous structure and surface functional groups of the products were also characterized. To study the dye removal of product prepared, many adsorption models of adsorption © 2013 American Chemical Society

2. EXPERIMENTAL AND METHODS 2.1. Preparation and Characterization of Adsorbent. Dewatered sewage sludge from Dadukou wastewater treatment plant, Chongqing, P. R. China, was chosen as the raw material to prepare activated carbon, while calcium sulfate supplied by Kelong chemical agent corporation, Chengdu, P. R. China, was chosen as the binder. The proximate analysis of the raw material was shown in Table 1. Table 1. Proximate Analysis of the Raw Mmaterial sample

moisture (%)

volatile matter (%)

ash (%)

fixed carbon (%)

sludge

71.2

45.7

47.45

6.85

The sewage sludge was first dried at 105 °C for 48 h and then crushed into particles with a size of less than 2 mm. Briefly, sludge powder and calcium sulfate were mixed according to an optimized mass proportion of 10:3. After this, 20 mL of water was added to obtain a paste. The paste was extruded into cylindrical samples (4 mm diameter and 9 mm long) by a extrusion equipment from South China University of Technology. Cylindrical samples were dried for 12 h at 105 °C. Subsequently, the temperature was raised (5 °C/min) to 300 Received: April 3, 2013 Accepted: July 9, 2013 Published: July 18, 2013 2248

dx.doi.org/10.1021/je4003543 | J. Chem. Eng. Data 2013, 58, 2248−2253

Journal of Chemical & Engineering Data

Article

and 700 °C and kept for 1 h under nitrogen flow (0.6 L/min). Last, the samples were cooled in the nitrogen atmosphere, and the adsorbent prepared was named as SSGAC. The characterization of pore structure of the sample was carried out by the N2 adsorption isotherm at 77K (Micromeritics ASAP 2020M). The total surface area, SBET, was determined from isotherm using the BET equation. The total pore volume (Vtotal) was calculated to be the liquid volume of N2 at a relative pressure (p/p0) of 0.995. The specific microporous surface area (Smicro), microporous volume (Vmicro), mesoporous surface area (Smeso), and mesoporous volume (Vmeso) were estimated from the t-plot method and BJH model, respectively. Fourier transform infrared spectroscopy (FTIR, PerkinElmer Spectrum GX) was used to characterize the surface functional groups of sample. Potassium bromide (KBr) pellets pressed were spanned and recorded between (4000 and 450) cm−1. The point of zero charge (PZC) and amounts of surface functional groups of the sample were measured by weight method13 and Boehm titration,14 respectively. 2.2. Batch Adsorption Experiment. The effects of operating conditions (pH, adsorbent dosages, adsorption time, initial dye concentration, and adsorption temperature) on MB removal were studied. In all experiments, 100.0 mL of MB solution in 250.0 mL Erlenmeyer flasks were stirred at 150.0 rpm. The concentrations of MB were determined by a UV−vis spectrophotometer (Hach DR5000) at its maximum wavelength of 665 nm. The MB adsorption onto SSGAC at equilibrium and time t, qe (mg g−1), and qt (mg g−1), were computed by eqs 1 and 2, respectively. C − Ce qe = 0 V W

pseudo‐first‐order kinetic equation log(qe − qt ) = log qe − k1t pseudo‐second‐order kinetic model

intraparticle diffusion model

liquid film diffusion model −1

+C

⎛ q ⎞ ln⎜⎜1 − t ⎟⎟ = −k fdt qe ⎠ ⎝

(8)

(9)

−1

ΔG = ΔH − T ΔS

(10)

ΔG = −RT ln Kd

(11)

Kd =

qe(W /V ) Ce

(12) −1

where ΔG (kJ mol ) is the Gibbs free energy change, ΔH (kJ mol−1) is the change in enthalpy change, ΔS (J (mol K)−1) is the change in entropy, Kd is the distribution coefficient of adsorption, R (8.314 J (mol K)−1) is the universal gas constant, and T (K) is Kelvin temperature.

(1)

Ce 1 1 = + Ce qe Q LKL QL

3. RESULTS AND DISCUSSION 3.1. Characterization of Adsorbent Prepared. Figure 1 shows nitrogen adsorption/desorption isotherm curve onto the

(3)

1 log Ce nF

(4)

RT RT ln KT + ln Ce bT bT −1

qt = k idt

0.5

where qe (mg g ) and qt (mg g ) are the MB adsorption amounts at equilibrium and at time t, respectively, k1 (d‑1) is the pseudofirst-order rate constant, k2 (g mg‑1 d‑1) is the pseudosecond-order rate constant, kid (mg g‑1 d‑0.5) is the intraparticle diffusion rate constant, C (mg g−1) is a constant related to the thickness of the boundary layer, and kfd (h‑1) is liquid film diffusion rate constant. 2.5. Adsorption Thermodynamic. Thermodynamic analysis of MB removal by SSGAC was assessed by energy change of adsorption. These energy change of adsorption were calculated by the following equations:

C0 − C t V (2) W −1 where C0, Ct, and Ce (mg L ) are MB concentration of initialization, time t, and equilibrium and V (L) and W (g) are solution volume and adsorbent dosage, respectively. 2.3. Adsorption Equilibrium. Three isotherm equations, Langmuir, Freundlich,1 and Temkin,15 were used to fit the MB adsorption isotherm data. The three equations were shown as eqs 3−5, repectively.

qe =

t 1 t = + qt qe k 2qe 2 (7)

qt =

log qe = log KF +

(6)

(5)

Figure 1. N2 adsorption/desorption isotherm at 77 K.

−1

where QL (mg g ) and KL (L g ) are Langmuir constants, KF (L mg−1) and 1/nF are the Freundlich adsorption constants, R is the ideal gas constant (8.314 J mol−1 K−1), T (K) is Kelvin temperature, KT (L mg−1) is the equilibrium binding constant, bT is Tempkin isotherm constant, and RT/bT (J mol−1) is related to the heat of adsorption. 2.4. Adsorption Kinetics. Pseudofirst-order kinetic model, pseudosecond-order kinetic model,16 intraparticle diffusion model, and liquid film diffusion model17 were used to study adsorption kinetics and mechanisms of MB removal by SSGAC. These models were expressed as

SSGAC from sewage sludge at 77 K. According to International Union of Pure and Applied Chemistry (IUPAC) classification, the isotherm suggests a type II adsorption/desorption, in which the most of the porosity of SSGAC is in mesoporous dimensions. The results obtained from analyzing the isotherm are as follows: SBET = 14.27 m2 g−1, Smicro = 1.9354 m2 g−1, Smeso = 10.39 m2 g−1, Vtotal = 0.0288 cm3 g−1, Vmicro = 0.0008 cm3 g−1, and Vmeso = 0.0256 cm3 g−1. The percentages of mesoporous surface area and volume are 72.81 % and 88.89 %. 2249



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The bands at 1637.3 cm−1 and 1420.3 cm−1 are attributed to CO stretching vibration of carbonyl groups and stretch vibration of C−O from carboxyl group, respectively. The bands at 1119.7 cm−1 and 1155 cm−1 are ascribed to C−O stretching in alcohol and asymmetric stretching vibration (−C−O−C− ring). The bands around 1020 cm−1 may be assigned to C−OH stretching. A broad bands between (400 and 800) cm−1 are because of bending vibration of −OH groups, stretching vibration of C−O groups, −NH2 group, or C−H (benzene derivatives) functional group. The results of PZC measurement are shown in Figure 3, which shows that the PZC of SSGAC is about 5.53. This means the surface of SSGAC is positively charged in solutions with a pH below the PZC, and negatively charged in solutions with a pH above the PZC. The amounts of surface functional groups for SSGAC are as follows: basic groups 3.48 mmol g−1, acidic groups 3.96 mmol g−1, phenolic hydroxyl groups 0.55 mmol g−1, and carboxy groups 2.49 mmol g−1. 3.2. Effect of Operational Parameters on MB Adsorption. 3.2.1. Effect of Solution pH. pH is an important factor affecting the adsorbate onto adsorbent, which changes chemical properties of solution and adsorbent. Figure 4a shows the variation of amounts of MB adsorbed onto SSGAC at equilibrium (qe) with solution pH. It is obvious that the MB adsorption amounts increase with pH change of dye solution from 1 to 12. The increase of cationic dye (MB) adsorption results from that as increase of pH, acidic substances on adsorbents surface are gradually neutralized, and negative surface charge of adsorbents are continuously increased.18 A similar result of MB adsorption was also reported by refs 19 and 20, respectively. 3.2.2. Effect of Initial MB Concentration. The influence of initial MB concentration in the range of (100 to 400) mg L−1 on MB adsorption capacity of SSGAC was investigated and the results are shown in Figure 4b. As the MB concentration of aqueous solution increases, the qe of SSGAC for MB enhances from (27.68 to 75.88) mg g−1. The result may be due to that MB concentration gradient between solution and adsorbent surface and mass transfer driving force are higher at high MB concentration.21,22 3.2.3. Effect of Adsorbent Dosages. Adsorbent dosage is another important factor to affect the capacity of adsorbent. The dependence of adsorbent dosages on the MB adsorption was studied and the results are shown as Figure 4c. As seen from the figure, MB adsorption amount of SSGAC decrease with an increasing amount of SSGAC. This may be attributed to the increase of unsaturated surfaces.23 3.2.4. Effect of adsorption temperature. The variation of MB adsorption with adsorption temperature is shown in Figure 4d. It is obvious that the MB adsorptive capacity at higher

Figure 2. FT-IR spectra of SSGAC.

Figure 3. Point of zero charge for SSGAC.

Figure 4. Effect of operational parameters on adsorption of MB on SSGAC: (a) effect of solution pH; (b) effect of initial MB concentration; (c) effect of adsorbent dosages; (d) effect of adsorption temperature.

According to the FT-IR spectrum of the SSGAC produced at optimized conditions (shown in Figure.2), the band at 3448.1 cm−1 could be due to O−H stretching vibration in alcohols.

Figure 5. Linear fits of the isotherm models. 2250



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Table 2. Adsorption Isotherm Constants for MB onto SSGAC Langmuir isotherm temp

QL

KL

K

mg g−1

L mg−1

288 298 308

84.75 123.8 131.8

0.0064 0.0048 0.0052

Freundlich isotherm R2

KF

Temkin isotherm R2

1/nF

bT

L mg−1 0.9947 0.9903 0.9906

R2

KT L mg−1

1.638 1.495 1.656

0.6332 0.6991 0.7022

0.9739 0.9739 0.9790

130.2 102.1 97.77

0.0661 0.0603 0.0642

0.9920 0.9760 0.9865

Table 3. Values of Four Different Errors about Isotherm Equations SSE

SAE

ARE

ARSE

isotherms

288 K

298 K

308 K

288 K

298 K

308 K

288 K

298 K

308 K

288 K

298 K

308 K

Langmuir Freundlich Temkin

2.90 49.42 7.68

6.93 52.27 39.83

7.42 96.79 26.25

2.94 13.10 5.57

4.09 12.96 13.59

5.07 17.40 10.44

0.02 0.07 0.05

0.02 0.06 0.10

0.02 0.07 0.08

0.02 0.09 0.07

0.02 0.07 0.14

0.03 0.09 0.12

the adsorption increase with increasing temperature could be caused by a surface chemical reaction in line with the view of other authors.20 3.3. Adsorption Isotherms. The linear fitting results of MB adsorption equilibrium data by Langmuir, Freundlich and Temkin isotherms are presented in Figure 5. The constants of the three isotherm equations and R2 values are listed in Table 2. All R2 values are greater than 0.97, suggesting the adsorption process of MB onto SSGAC could be well described by different isotherms. However, the values of R2 from Langmuir isotherm are greater than those from other isotherms, indicating the adsorption process of MB onto SSGAC is better described by Langmuir isotherm. Moreover, Table 3 shows the results of the sum of the squares of the errors (SSE), sum of the absolute errors (SAE), average relative error (ARE), and average relative standard error (ARSE). It can be seen that the values of SSE, SAE, ARE, and ARSE about Langmuir model were the smallest among the three isotherms, respectively. So it could be said that the Langmuir model is best to fit the experimental data according to R2. Langmuir model is a theoretical model which assumes that adsorbate molcules adsorbed on adsorbent surface is monolayer. The separation factor (RL) of Langmuir model defined as RL = 1/(1 + KLC0,max)25 suggests the adsorption to be reversible (RL = 0), favorable (0 < RL < 1), linear (RL = 1), or unfavorable (RL > 1). At 288 K, 298 K, and 308 K, the RL values are 0.2809, 0.3425, and 0.3247, respectively, suggesting that MB adsorption onto SSGAC is favorable. Freundlich’s isotherm is an empirical equation which considers the adsorbent surface energetically heterogeneous. If the value of n is in the range from 1 to 10, the adsorption is favorable. The values of nF listed in Table2 are 1.579, 1.430, and 1.424, which also suggest the MB adsorption onto SSGAC is favorable. The Temkin isotherm model assumes that the fall of adsorption heat is linear rather than logarithmic. According to the values of bT presented in Table 2, the heat of adsorption (RT/bT) at 288 K, 298 K, and 308 K are 18.39, 24.26, and 26.19 J mol−1, respectively, indicating that the contribution of chemisorption to MB adsorption amount gradually increases with temperature increase. 3.4. Adsorption Kinetics. Figure 6 shows the fitting results of experimental kinetic data for MB adsorption on SSGAC by pseudofirst-order kinetic model, pseudosecond-order kinetic model, intraparticle diffusion model and liquid film diffusion

Figure 6. Kinetic fits for MB adsorption on SSGAC: (a) initial MB concentration; (b) adsorbent dosages; (1) pseudofirst-order kinetic model; (2) pseudosecond-order kinetic model; (3) intraparticle diffusion model; (4) liquid film diffusion model.

temperature is bigger in the temperature range studied, indicating that MB adsorption onto SSGAC may be endothermic.24 In addition to activated diffusion in micropores, 2251



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Table 4. Kinetic Parameters for MB Adsorption on SSGAC pseudo-first-order C0 mg g−1 100 200 400

qe,exp

qe1,cal

mg g−1

mg g−1

27.68 48.02 75.88

14.21 24.94 31.51

qe1,cal mg g−1

106.4 69.22 46.74 kid,1

g L−1 1 2 3

mg g−1

mg g−1

0.7307 0.8084 0.7974

27.68 48.02 75.88

−0.8606 5.155 20.05

R2

k2

27.79 0.0162 48.10 0.0105 74.02 0.0110 pseudo-second-order

0.9949 0.9983 0.9995

qe,exp

qe2,cal

R2

mg g−1

mg g−1

0.8290 0.8223 0.7341

106.4 69.22 46.74

96.62 63.05 44.07

0.0088 0.0243 0.0419 liquid film diffusion

0.9994 0.9997 0.9997

C2

R22

kfd

Cfd

R2

22.47 40.94 65.30

0.9339 0.8292 0.8794

0.0861 0.0920 0.0860

k1

R12

C1

100 8.954 200 12.21 400 16.15 adsorbent dosage

R2

53.97 0.0301 26.15 0.0276 15.32 0.0335 intraparticle diffusion

C0 mg g−1

0.0374 0.0399 0.0374 pseudo-first-order

qe,exp

1 2 3

qe2,cal

k1

mg g−1

adsorbent dosage g L−1

pseudo-second-order qe,exp

kid,2

0.9638 0.5057 0.9552 0.5912 0.9513 0.9469 intraparticle diffusion

k2

R2

−0.6668 −0.6564 −0.8789 liquid film diffusion

0.7756 0.8404 0.8310

kid,1

C1

R12

kid,2

C2

R22

kfd

Cfd

R2

22.63 10.81 8.631

22.50 28.72 18.95

0.9489 0.9379 0.9506

1.774 1.157 0.0466

83.34 55.91 42.62

0.8292 0.8444 0.9887

0.0693 0.0636 0.0772

−0.6787 −0.9742 −1.115

0.8290 0.8223 0.7341

−1303.6) J mol−1 for MB adsorption on SSGAC at (288, 298, and 308) K, respectively, suggesting the MB adsorption onto SSGAC is spontaneous, and the higher temperature is favorable for MB adsorption.24 The positive ΔH indicates MB adsorption onto SSGAC is endothermic. And because the ΔH value of 6.63 KJ/mol is in the range of 2.1−20.9 kJ/mol which corresponds to a physical sorption,3 the adsorption of MB on SSGAC was mainly taken place by physisorption. The positive ΔS of 25.84 J/mol suggests an increase in randomness at the solid/solution interface during the MB adsorption.

Table 5. Thermodynamic Parameters of MB Adsorption on SSGAC ΔG/(J mol−1) ΔH/(KJ mol−1)

ΔS/(J mol−1)

288 K

298 K

308 K

6.63

25.84

−786.8

−1130.0

−1303.6

model. The constants calculated of the four kinetic models and R2 values are listed in Table 4. As shown in Figure 6(a-1, 2, and b-1, 2), the results of the pseudosecond-order kinetic model fitting for experimental data are better than the pseudofirst-order kinetic model. It can be seen that R2 values of the pseudosecond-order kinetic model listed in Table 4 are all greater than 0.99. Moreover, the qe2,cal values calculated by pseudosecond-order equation are almost consistent with the experimental results (qe,exp). These evidence indicate the adsorption of MB onto SSGAC follows pseudosecond-order kinetic model. Figure 6(a-3) and (b-3) show that there are two linear portions in the time ranges of (0 to 8) h and (12 to 24) h, respectively. The initial linear portion with higher slope indicates the instantaneous adsorption or external surface adsorption, suggesting the mass transfer at an early stage of adsorption is to be achieved through macropore diffusion.22 The second portion with higher slope is the gradual adsorption stage in micropores,17 which is the potential rate-controlling step. The plots of ln(1 − qt/qe) vs t, for liquid film diffusion (Figure 6(a-4) and (b-4)), are nonlinear, and the R2 values are only in the range of 0.70−0.85 (Table 3). These suggest that liquid film diffusion model is not suitable for describing the MB adsorption onto SSGAC. 3.5. Adsorption Thermodynamics. TheΔG, ΔS, and ΔH for the adsorption of MB onto SSGAC in temperature range studied are calculated and shown in Table 5. The values of ΔG obtained from eqs 11 and 12 are (−768.8, −1130.0, and

4. CONCLUSION In this study, granular activated carbon for MB adsorption has been successfully from sewage sludge with calcium sulfate as binder. The pore structure characterization of SSGAC showed that the sample was typical mesoporous activated carbon. And the percentages of mesoporous surface area and volume are 72.81 % and 88.89 %. The results of FT-IR, PZC, and Boehm titration indicated that the acid groups were dominant. Batch MB adsorption experiments indicated the operating conditions, pH, adsorbent dosages, adsorption time, initial dye concentration, and adsorption temperature, had important influences on MB adsorption amount. The maximum MB adsorption capacities calculated by Langmuir mode could be 131.8 mg g−1 under the conditions studied. Moreover, the adsorption mechanism of MB adsorption onto SSGAC was also investigated by different adsorption models. The results suggested that MB adsorption onto SSGAC was a heterogeneous adsorption in which physisorption was dominant. However, the contribution of chemisorption to MB adsorption amount gradually increased with temperature increase. The adsorption process was well described by the pseudosecondorder kinetic mode, and the MB diffusion in micropores was the potential rate-controlling step. The thermodynamic analysis showed that MB adsorption onto SSGAC was spontaneous and endothermic. 2252



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 23 65120980. Fax: +86 23 65120980. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major National Science and Technology Program for Water Pollution Control and Treatment (No. 2009ZX07318-008-003).

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REFERENCES

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Removal of Methylene Blue from Aqueous Solutions by Sewage

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