Abstract

Colored effluent and a large amount of sludge are major pollutant sources derived from textile industry activity. In this research, the idea for converting textile sludge into a potential adsorbent was conducted through a carbonization process in order to solve the colored effluent problem. Textile sludge was carbonized at a temperature ranging from 400 to 800 °C in the absence of oxygen. Maximum adsorption capacity of carbonized sludge for methylene blue removal reached 60.30 mg/g when the sludge was carbonized at 600 °C with specific surface area of 138.9 m2/g and no significant alteration was observed until 800 °C. Experimental research by using a real wastewater also showed that there was almost no disruption during adsorption of methylene blue into surface of carbonized sludge. While reactivation process revealed that the regeneration of carbonized sludge was applicable by secondary heating at the same carbonization temperature. Furthermore, the application of this research demonstrated that the carbonized textile sludge was a good adsorbent for methylene blue removal and had a capability to be reactivated.

INTRODUCTION

Textile sludge, a by-product from textile industry activity causes a serious problem for environment due to its toxic characteristic with hazardous contaminants. A large amount of the sludge makes another problem for disposal (European Commission Directorate General for Environment 2001; Balasubramanian et al. 2006). Landfilling is one disposal method for sludge. However, it has the disadvantage of hazardous leachate production, which causes groundwater pollution by penetration through a soil layer (Schultz & Kjeldsen 1986). In addition to the textile sludge problem, the colored effluent as a residue from a dyeing process also causes a water pollution problem. The conversion of a waste into a valuable product receives much attention in recent years. Various recycling technologies have been developed along with the propagation of ‘zero emission’ concept. The utilization of textile sludge to solve the colored effluent problem is an idea in this research. Previous researches were conducted to investigate the applicability of various types of sludge as a potential adsorbent. For instance, activated carbon derived from aerobically digested sludge from a wastewater treatment plant was produced through physical and/or chemical activation (Martin et al. 2003). Sludge from a biological wastewater treatment of petrochemical industry in Taiwan had the potential to be an adsorbent for removing Orange II and Chrysophenin dyes (Hsiu-Mei et al. 2009). Sewage sludge-based activated carbon obtained from an urban wastewater treatment plant has been studied for methylene blue and saphranin dyes removal (Rozada et al. 2003). The dried activated sludge was hydrolyzed with sodium hydroxide (NaOH) in order to produce an adsorbent for reactive dyes removal (Gulnaz et al. 2006). Earlier research indicated that the production of adsorbent from sludge is possible. The conversion of textile sludge into potential adsorbent is interesting to discuss. Because textile sludge-based adsorbent will be applicable to uptake color pollutants from textile effluents, which will realize a scenario of closed-loop waste treatment solution in textile industry.

Various activation processes, either physical or chemical activation, were discussed to get the porous material of sludge. Smith et al. (2009) mentioned that chemical activation was a good method to create porosity of adsorbent. However, the chemical residue after activation may cause a further pollution. In this research a process of carbonization without further activation was applied to producing adsorbent from a textile sludge for avoiding a further pollution by chemical residues of activation, simplifying the production procedure, and cutting down the energy cost. The objective of this research was to demonstrate the availability of carbonized textile sludge as a potential adsorbent through the evaluation of its adsorption capacity for methylene blue removal. If the carbonized textile sludge can be applicable as an adsorbent for dyes removal, the carbonized sludge production from textile sludge will contribute to establish a low emission system for textile waste management in the future.

MATERIALS AND METHODS

Textile sludge and dye

Textile sludge was obtained from a textile company in Wakayama Prefecture, Japan. The physical characteristics of sludge are mushy, black color and bad odor. Sludge was received as dewatered before. Dewatered sludge contains 77% water content, 17% organic matter (ignition loss) and 6% ash content. The carbon content of raw dried sludge was 31.4% while the nitrogen content was 6.3%. Both of carbon and nitrogen contents were measured with an automatic high sensitive NC analyzer (NCH-22F, Sumigraph, Japan). The chemical composition of ash in the sludge was analyzed with an energy dispersive X-Ray spectrometer type EDX-800HS (Shimadzu, Japan) is shown in Table 1.

Table 1

Chemical composition of textile sludge (ash form)

Component (%) 
SiO2 10.6 
TiO2 3.2 
Al2O3 45.6 
Fe2O3 11.0 
MnO 0.1 
MgO 1.9 
CaO 2.8 
Na212.4 
K21.0 
P2O5 3.0 
SO3 2.3 
Component (%) 
SiO2 10.6 
TiO2 3.2 
Al2O3 45.6 
Fe2O3 11.0 
MnO 0.1 
MgO 1.9 
CaO 2.8 
Na212.4 
K21.0 
P2O5 3.0 
SO3 2.3 
Methylene blue trihydrate with the color index of C.I. Basic Blue 9 trihydrate was chosen as an adsorbate (dye) to investigate the dye removal performance of carbonized textile sludge-based adsorbent. It was obtained from Nacalai Tesque Japan with the chemical formula of C16H18N3·S·Cl·3H2O and has 373.90 molecular weight. The chemical structure of methylene blue is shown in Figure 1 (Molbase 2013). Methylene blue powder was dried in a drying oven at 105 °C overnight to get a constant weight. Then 1.00 g of methylene blue was dissolved into deionized water to get 1,000 mg/L of stock solution. The stock solution was then diluted with deionized water to get the methylene blue solution with a certain concentration for adsorption experiment. All of deionized water used has 0.05 × 10−4 S/m of electrical conductivity. In order to investigate the concentration of dye during the experiment, the following calibration curve was developed and used for transforming absorbance to methylene blue concentration. A linear regression equation is following below.  
formula
(1)
where y is the absorbance at 665 nm (cm−1), x is the dye concentration (mg/L), and R2 is the coefficient of determination.
Figure 1

Chemical structure of methylene blue.

Figure 1

Chemical structure of methylene blue.

Adsorbent production by carbonization

Carbonization is a process to produce carbonaceous material by heating under an inert atmosphere (Smith et al. 2009). The purpose of carbonization is to remove the non-carbon element through thermal decomposition (Wigmans 1986). In this research, textile sludge was carbonized under 400, 500, 600, 700, and 800 °C in the absence of oxygen with a muffle furnace (FO410, Yamato, Japan) for 2 h (Figure 2). The carbonized sludge was crushed with a cyclone mill (1033-300A model, Yoshida Seisakusho, Japan) and then sieved using a testing sieve (IIDA Manufacturing, Japan) to get the powder form at the particle diameter below 250 μm.

Figure 2

Experimental setup of carbonization process.

Figure 2

Experimental setup of carbonization process.

Batch adsorption experiment

A batch system was used for adsorption experiments. A series of 300 mL of conical flask containing 0.5 g of carbonized sludge and 100 mL methylene blue solution was shaken with a bio-shaker (TB-9R-3F, Takasaki Scientific Instrument, Japan) at 150 rpm at 25 °C. In order to determine the equilibrium time, the shaking time was firstly examined in the range of 5 to 2,560 min. After the shaking was finished, the solution was filtrated using a polyethersulfone (PES) syringe filter with 0.1-μm pores (Membrane Solutions, Dallas, TX, USA). The absorbance of filtrate was measured with a UV-Visible Spectrophotometer (UV-2550, Shimadzu, Japan) at the peak wavelength of 665 nm to identify the residual concentration of dye after adsorption. Finally, the adsorption capacity was calculated using the following equation (Metcalf & Eddy 2003).  
formula
(2)
where qe is the adsorption capacity, namely the amount of methylene blue adsorbed per unit weight of carbonized sludge (mg/g), Co is the initial concentration of methylene blue (mg/L), Ce is the final concentration of methylene blue after adsorption (mg/L), V is the volume of solution (L), and m is the weight of carbonized sludge (g). All of adsorption experiments were triplicated.

RESULTS AND DISCUSSION

Characterization of carbonized textile sludge-based adsorbent

Figure 3 shows the carbonized sludge at 600 °C on powder form and surface morphology analysis on the 5,000 of magnificent with a scanning electron microscope (SEM) (JSM-6510 LV series, JEOL, Japan). The properties of carbonized sludge are listed in Table 2. The carbon content was analyzed with an automatic high sensitive NC analyzer (NCH-22F, Sumigraph, Japan).

Table 2

Properties of carbonized textile sludge-based adsorbent

No. Carbonization
 
C (%) N (%) BET surface area (m2/g) pH 
Temperature (°C) Time (h) 
400 31.05 3.88 76.3 5.8 
500 27.24 2.86 97.6 9.4 
600 29.65 2.50 138.9 9.8 
700 29.64 1.78 137.7 9.8 
800 27.44 1.16 136.1 9.9 
No. Carbonization
 
C (%) N (%) BET surface area (m2/g) pH 
Temperature (°C) Time (h) 
400 31.05 3.88 76.3 5.8 
500 27.24 2.86 97.6 9.4 
600 29.65 2.50 138.9 9.8 
700 29.64 1.78 137.7 9.8 
800 27.44 1.16 136.1 9.9 
Figure 3

Carbonized sludge at 600 °C on powder form (a) with a SEM image (b).

Figure 3

Carbonized sludge at 600 °C on powder form (a) with a SEM image (b).

The specific surface area is one of important factors to determine the quality of porous material. Specific surface area was measured according to the Brunauer–Emmett–Teller (BET) N2 adsorption method. As shown in Table 2, the specific surface area increased with increasing carbonized temperature below 600 °C. However, no significant difference was observed at the carbonization temperature from 600 to 800 °C.

Thermogravimetric differential thermal analysis (TG-DTA) was conducted with a thermogravimetric differential thermal analyzer (TG-DTA 2000 SA, Bruker AXS, Japan) in order to investigate the thermal behavior of the sludge. In the TG-DTA 10 ± 0.3 mg of oven-dried sludge was placed on an Al2O3 crucible with a standard balance of 10 ± 0.3 mg of α-Al2O3. Then, the dried sludge sample was heated at the heating rate of 10 °C/min until the temperature reached 1,200 °C. Oxygen-free nitrogen (N2) gas was flowed at 0.3 L/min. As shown in Figure 4, three peaks were detected by DTA at 364 °C followed by 454 and 595 °C. The TG-DTA diagrams reach a plateau when the organic matter in the sample has been vaporized. Accordingly, Figure 4 suggests that the constant specific surface area of carbonized sludge at the carbonization temperature from 600 to 800 °C was caused by the complete vaporization of organic matter in the sludge.

Figure 4

Thermogravimetric Differential Thermal Analysis (TG-DTA) of textile sludge (dried at 105 °C).

Figure 4

Thermogravimetric Differential Thermal Analysis (TG-DTA) of textile sludge (dried at 105 °C).

The pH of carbonized sludge was investigated based on the Japan Industrial Standard Test Methods for Activated Carbon (Japanese Standard Association 2007). In the test a 100-mL glass beaker containing 1.0 g of adsorbent with 100 mL of pure water was gently boiled at 100 °C for 5 min. The solution was then cooled, and the solution pH was measured with a pH meter (B-212, Horiba, Japan). As shown in Table 2, the pH increased with the rise in carbonization temperature. Mendez & Gasco (2005) mentioned that the pH increase was related with a decrease in carboxyl groups on the sludge as the rise in temperature. In this research, the pH reached a plateau around 9.8 over the carbonization temperature of 600 °C, at which organic matter in the sludge was completely vaporized as aforementioned. The complete vaporization resulted in the complete consumption of carboxyl groups on the sludge. Accordingly, our results were supported the opinion by Mendez & Gasco (2005).

Batch contact time required for adsorption equilibrium

The effect of batch contact time on the adsorption capacity was investigated to determine the equilibrium time of adsorption process. The initial dye concentration for the adsorption test was set in the range of 50 to 250 mg/L, which was equivalent to the mass ratio of dye to adsorbent of 10 to 50 mg/g. The residual dye concentration after adsorption did not reach to 0 mg/L in all tests.

As shown in Figure 5, adsorption capacities of all adsorbents tested reached a plateau after a batch contact time of 1,280 min (21 h 20 min). No significant adsorption occurred hereafter. Equilibrium time plays an important role in exact estimation of the adsorption capacity. Therefore, the batch contact time of 1,280 min was used for the following adsorption isotherm experiment.

Figure 5

Changes in adsorption capacity with the extension of batch contact time.

Figure 5

Changes in adsorption capacity with the extension of batch contact time.

Adsorption isotherm

The initial dye concentration for the determination of adsorption isotherm ranged from 30 to 130 mg/L for the adsorbent prepared at the carbonization temperature of 400 °C, from 160 to 260 mg/L for 500 °C, and from 250 to 350 mg/L for 600, 700 and 800 °C. The adsorption isotherm test was performed at 150 rpm at 25 °C and at 1,280 min of batch contact time. The approximation of Freundlich and Langmuir adsorption isotherms was applied to Figure 6 to identify the adsorption mechanism of methylene blue onto the carbonized sludge. The Langmuir adsorption isotherm is expressed by the following equation (Langmuir 1918).  
formula
(3)
where qmax is the maximum monolayer adsorption capacity of carbonized sludge (mg/g); KL is the Langmuir capacity factor (L/mg). On the other hand, the Freundlich adsorption isotherm is as follows (McKay 1982):  
formula
(4)
where Kf is the Freundlich capacity factor (mg/g)(L/mg)1/n, and 1/n is the Freundlich intensity parameter.
Figure 6

Relationship between adsorption capacity and equilibrium dye concentration.

Figure 6

Relationship between adsorption capacity and equilibrium dye concentration.

As shown in the coefficient of determination R2 value in Table 3, all qe and Ce data observed were fitted better with Langmuir adsorption isotherm than with Freundlich adsorption isotherm. Langmuir isotherm assumes a homogenous surface bounding, which indicates that methylene blue molecules covered the carbonized sludge surface by monolayer adsorption (Karadag et al. 2007). The maximum capacity (qmax) in Table 3 shows that there was no significant difference at 600 °C and higher carbonization temperatures. The dependency of qmax on the carbonization temperature accorded with that of BET surface area shown in Table 2. In addition, it was inferred that methylene blue removal was also influenced by the pH condition. Figure 7 shows the solution pH before and after adsorption and adsorbent pH. The solution pH after adsorption was related to the adsorbent pH and increased with the carbonization temperature. This alkaline condition was thought to be suitable for methylene blue removal because of its cationic characteristic. Initial pH influences on the surface charge of adsorbent. Positively charged surface will be achieved easily at a lower pH due to the existence of hydrogen ion (H+); on the contrary, negatively charged one will be formed at a higher pH (Ncibi et al. 2007). Accordingly, a higher pH will be favorable to cationic dye adsorption.

Table 3

Freundlich and Langmuir adsorption isotherm parameters

Adsorbent Freundlich isotherm parameter
 
Langmuir isotherm parameter
 
1/n Kf (mg/g)(L/mg)1/n R2 qmax (mg/g) KL (L/mg) R2 
CS 400 °C 0.23 ± 0.024 4.33 ± 0.27 0.82 11.26 ± 1.36 0.26 ± 0.10 1.00 
CS 500 °C 0.10 ± 0.015 32.33 ± 1.54 0.95 45.25 ± 0.24 1.32 ± 0.29 1.00 
CS 600 °C 0.07 ± 0.003 44.97 ± 0.54 0.94 60.30 ± 0.50 0.85 ± 0.14 1.00 
CS 700 °C 0.06 ± 0.002 43.09 ± 0.75 0.92 57.91 ± 0.61 0.57 ± 0.06 1.00 
CS 800 °C 0.07 ± 0.004 43.57 ± 0.16 0.97 59.57 ± 0.75 0.62 ± 0.02 1.00 
Adsorbent Freundlich isotherm parameter
 
Langmuir isotherm parameter
 
1/n Kf (mg/g)(L/mg)1/n R2 qmax (mg/g) KL (L/mg) R2 
CS 400 °C 0.23 ± 0.024 4.33 ± 0.27 0.82 11.26 ± 1.36 0.26 ± 0.10 1.00 
CS 500 °C 0.10 ± 0.015 32.33 ± 1.54 0.95 45.25 ± 0.24 1.32 ± 0.29 1.00 
CS 600 °C 0.07 ± 0.003 44.97 ± 0.54 0.94 60.30 ± 0.50 0.85 ± 0.14 1.00 
CS 700 °C 0.06 ± 0.002 43.09 ± 0.75 0.92 57.91 ± 0.61 0.57 ± 0.06 1.00 
CS 800 °C 0.07 ± 0.004 43.57 ± 0.16 0.97 59.57 ± 0.75 0.62 ± 0.02 1.00 

CS stands for carbonized sludge, while the plus-minus sign (±) means sample standard deviation of triplicated samples.

Figure 7

Effect on the carbonization temperature with pH.

Figure 7

Effect on the carbonization temperature with pH.

Adsorption of methylene blue in a real wastewater

In order to simulate the performance of carbonized sludge in the real system, a raw wastewater was prepared by mixing 500 mL of the municipal wastewater in Ryukoku University and 100 mL of a methylene blue solution with the concentration of 500 mg/L. The mixing of the municipal wastewater dropped the methylene blue concentration from 83.3 to 43.8 mg/L due to adsorption onto suspended solids. The average pH of wastewater is 6.5. Then, the raw wastewater was mixed with 400-mL of activated sludge, which decreased methylene blue concentration to 9.7 mg/L because of adsorption onto activated sludge. Furthermore, aeration was performed for 8 h followed by sedimentation for 3 h to simulate a typical activated sludge process. The following aeration–sedimentation further decreased methylene blue concentration with the removal efficiency of 76.6%. After the activated sludge process, a series of adsorption test using carbonized sludge prepared at 600 °C was performed at the various adsorbent doses of 0.05–0.40 g/L, that is, conical flasks of 300 mL containing 0.005–0.040 g of the adsorbent and 100 mL of the treated water were shaken with a bio-shaking machine (TB-9R-3F, Takasaki Scientific Instrument, Japan) at 150 rpm at 25 °C for 1,280 min (21 h 20 min). As a result, the adsorbent dose of not less than 0.20 g/L (inputted adsorption mass of 0.020 g) was required for the complete removal of methylene blue judging from spectrophotometry. However, as shown in Figure 8, the color seemed to slightly remain in the solution. The detection of methylene blue with a spectrophotometer is not necessarily reliable when its concentration is much lower than the lowest concentration of standard samples for the calibration curve. Thus, we decided that 0.03-g of adsorbent mass was suitable amount of adsorbent to reach complete removal of colored effluent, which was equivalent to the 0.3 g/L-wastewater of adsorbent dose.

Figure 8

Photographs of wastewater treated by an activated sludge process followed by the adsorption using carbonized sludge prepared at 600 °C. The inputted mass of the carbonized sludge adsorbent to 100 mL of wastewater was described in the label.

Figure 8

Photographs of wastewater treated by an activated sludge process followed by the adsorption using carbonized sludge prepared at 600 °C. The inputted mass of the carbonized sludge adsorbent to 100 mL of wastewater was described in the label.

The objective of the adsorption test using real wastewater was not only to determine the dose of adsorbent for complete removal but also to investigate the influence of other contaminant, which potentially disrupted the adsorption process. In order to investigate the influence of other contaminants to adsorption treatment, the observed residual concentration of methylene blue was compared with that predicted one using the Langmuir adsorption isotherm shown in Table 3. The predicted residual concentration could be determined by substituting the qe in Equation (2) for the qe in Equation (3). Consequently, a new Equation (5) could be obtained as follows:  
formula
(5)
where ObsCo is observed initial concentration (mg/L), ObsCe is the observed residual concentration (mg/L) and PredCe is the predicted residual concentration (mg/L). As a result of the activated sludge process, the initial dye concentration (ObsCo) was 2.3 mg/L for further adsorption treatment. The adsorption test was then performed with the adsorbent dose of 0.05–0.40 g/L. The result showed that the ObsCo was zero (complete removal) at the adsorbent dose not less than 0.20 g/L. When PredCe is equal to or higher than ObsCe, it means that there is no disruption during adsorption of dye molecule onto the carbonized sludge adsorbent. On the other hand, when PredCe is lower than ObsCe, it means that the disruption of the adsorption process is detected. Table 4 summarizes PredCe and ObsCe. The PredCe was lower than the ObsCe at 0.05 g/L of the adsorbent dose. On the other hand, when the adsorbent dose increased to 0.10 g/L or higher, the PredCe exceeded the ObsCe. The small adsorbent dose may describe that other contaminants interfere in the adsorption of dye. In practical systems, textile wastewater contains not only dyes but also other pollutants that can disrupt the dye adsorption. Therefore, when the adsorbent dose is too small to adsorb all adsorptive pollutants, the association of adsorption sites with pollutants is severely competitive. As a result, the inhibition effect of coexisting pollutants on dye adsorption is more clearly observed. This is true of the carbonized sludge dose of 0.05 g/L in this research. However, the inhibition effect is weakened when the adsorbent dose is high enough for adsorptive removal of pollutants, because of an abundance of adsorption sites supplied. This is true of the carbonized sludge dose of 0.20 g/L and more. Thus, the completely removal was reached at 0.30 g/L means that the adsorption process of carbonized sludge at 600 °C was almost no disruption during methylene blue removal in the real wastewater.
Table 4

Comparison between ObsCe and PredCe for methylene blue removal on the real wastewater

Dose of carbonized sludge at 600 °C (g/L) PredCe (mg/L) ObsCe (mg/L) 
0.05 0.79 1.07 
0.10 0.50 0.48 
0.20 0.27 
0.30 0.17 
0.40 0.12 
Dose of carbonized sludge at 600 °C (g/L) PredCe (mg/L) ObsCe (mg/L) 
0.05 0.79 1.07 
0.10 0.50 0.48 
0.20 0.27 
0.30 0.17 
0.40 0.12 

Possibility of reactivation of carbonized sludge at 600 °C

The reusability of adsorbent for further adsorption treatment was investigated through the following reactivation procedure. The used carbonized sludge prepared at 600 °C was dried in a drying oven at 105 °C overnight. Then, the aforementioned carbonization procedure was applied again for thermal reactivation. The adsorption capacity of reactivated adsorbent was measured again according to the same procedure as described in the previous batch adsorption experiment. Table 5 summarizes the obtained results. The reactivation resulted in the weight loss of 13.8% and the adsorption capacity loss of 13.8%. When m0 [g] and qe0 [mg/g] symbolize the initial mass of adsorbent and initial adsorption capacity, respectively, the initial adsorption potential of adsorbent can be calculated to be m0qe0 [mg]. In comparison with the initial adsorption potential, the adsorption potential recovered by the reactivation can be estimated by multiplying 0.862m0 with 0.862qe0, which resulted in 0.743m0qe0. Accordingly, 74.3% of initial adsorption potential could be used again by the reactivation. Thus, it was indicated that the carbonized sludge was reusable one or two times through the reactivation, though a certain loss of adsorption potential was unavoidable.

Table 5

Reactivation performance of secondary carbonization for carbonized sludge at 600 °C

Replication test Adsorbent weight loss (%) Adsorption capacity loss (%) 
12.3 15.2 
14.7 15 
14.3 11.1 
Average 13.8 13.8 
Replication test Adsorbent weight loss (%) Adsorption capacity loss (%) 
12.3 15.2 
14.7 15 
14.3 11.1 
Average 13.8 13.8 

CONCLUSION

The successful conversion of textile sludge into an adsorbent by carbonization was demonstrated in this research. The methylene blue adsorption capacity of carbonized sludge produced was investigated by a series of adsorption tests. The adsorption capacity increased with a rise in carbonization temperature until 600 °C and reached a plateau over 600 °C. The TG-DTA revealed that organic matter of the sludge was vaporized at 595 °C and lower. As a result of the vaporization, the specific surface area of adsorbent also reached a plateau over 600 °C of carbonization. In this research, carbonization at 600 °C was thought to be an optimum temperature for adsorbent production from the textile sludge, because the specific surface area of 138.9 m2/g and the maximum adsorption capacity of 60.30 mg/g were obtained at 600 °C. In addition, the application of carbonized sludge adsorbent on the real wastewater treated by the activated sludge process showed a good performance, namely the complete removal of methylene blue under the adsorbent dose of 0.30 g/L-wastewater or higher. It indicated that there was almost no inhibition during adsorption of methylene blue by other impurities. The carbonized sludge adsorbent could be reusable one or two times through the re-carbonization, though about 26% of adsorption potential was lost during the re-carbonization.

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