In this study, the dynamic adsorption of methylene blue dye onto CuO-acid modified sepiolite was investigated. Meanwhile, the equilibrium and kinetic data of the adsorption process were studied to understand the adsorption mechanism. Furthermore, a high-temperature gas stream was applied to regenerate the adsorbent. The results showed that the Langmuir isotherm model was applied to describe the adsorption process. The positive value of enthalpy change indicated that the adsorption process was endothermic in nature. In the dynamic adsorption process, the best adsorption performance was achieved when the ratio of column height to diameter was 2.56 and the treatment capacity was 6 BV/h. The optimal scenario for regeneration experiments was the regeneration temperature of 550–650 °C, the space velocity of 100 min−1 and the regeneration time of 10 min. The effective adsorption of CuO-acid modified sepiolite was kept for 12 cycles of adsorption and regeneration.

INTRODUCTION

Dyestuffs are extensively used in the textile, leather, printing, plastic, cosmetics, and food industries, etc. (He et al. 2008). Dye wastewater can cause a lot of environmental problems in the receiving media (He et al. 2008); it is difficult to biodegrade due to complex chemical compounds with toxic, carcinogenic and mutagenic properties (Eren et al. 2010; Demirbas & Nasi 2010). Therefore, dye removal from industries wastewater has become a significant environmental issue. Conventional methods for dye removal from wastewater include adsorption, chemical oxidation, electrolysis, chemical coagulation, photocatalysis and membrane technology (Eren et al. 2010; Demirbas & Nasi 2010). Among these methods, adsorption has been considered as an efficient method due to its characteristics of simplicity, high efficiency, regeneration potential and recycling of adsorbing material (Eren et al. 2010; Demirbas & Nasi 2010). However, the economics and feasibility of adsorbent and its regeneration are important factors for the practical application. Currently, many environmentally friendly natural materials, such as agricultural wastes, bentonite, fly ash, and zeolite, which are inexpensive and available in local natural resources, are used as adsorbents for treating industries wastewaters (He et al. 2008; Kocaoba 2009; Demirbas & Nasi 2010). Sepiolite is a clay mineral with an ideal formula Mg8Si12O30(OH)2 (Tabak et al. 2009); its structure of sepiolite is formed by alternate building of blocks and tunnels that grow up in the fiber direction (Sabah & Majdan 2009). This unique fibrous structure gives sepiolite a large specific surface area and high adsorption capacity. Therefore, sepiolite has been widely applied in the fields of dyes, heavy metals, nitrite, ammonia, phosphorus, and pesticide wastewater treatment, among others (Eren et al. 2010). Nevertheless, the adsorption capacity of raw sepiolite is limited. By modification, the adsorption performance of sepiolite could be dramatically improved and could create good conditions for the regeneration of sepiolite (Olmos et al. 2011; Hassan & Hameed 2011). But the present studies of the adsorption process are still focused on the static adsorption process. The dynamic adsorption process of modified sepiolite has been rarely reported.

Meanwhile, adsorption is a separation process; the contaminants are only transferred onto adsorbents and not be decomposed to harmless substances (Zhang et al. 2014). Once an adsorbent reaches saturation after a long time of operation, it cannot be regenerated any more; as such it becomes a contaminant as well. (Zhang et al. 2014; Lee et al. 2015). The popular regeneration methods of adsorbents have been investigated (Zhang et al. 2014; Lee et al. 2015), mainly including thermal calcinations, chemical elution, photocatalysis, and chemical oxidation. By the various regeneration methods, adsorbents can be regenerated and recover their original adsorption capacities (Netskina et al. 2015). Potentially, the high-temperature gas stream catalytic oxidation method could be an alternative way to achieve energy recovery and utilization, since it can effectively utilize the heat exhaust generated by the production process in factories, such as the boiler flue gas. This method can achieve economic, environmentally friendly and sustainable adsorbent regeneration and recycling. However, until now it has been reported rarely.

Therefore, the objective of this study was firstly to investigate the adsorption of methylene blue (MB) dye onto CuO-acid modified sepiolite adsorbent, including both the adsorption isotherm and the dynamic adsorption process. Additionally, the regeneration efficiency of high-temperature gas stream catalytic oxidation methods was estimated. Meanwhile, the characteristics of the adsorbent before and after regeneration were comparatively analyzed by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectra.

MATERIALS AND METHODS

Materials

Sepiolite was purchased from Dongfeng Co., Ltd in Zhengzhou, China. MB was purchased from the Xilong Chemical Co., Ltd in Guangzhou, China. All reagents, such as copper nitrate and H2SO4, were analytical grade and all solutions were prepared with distilled water.

Preparation of the CuO-acid modified sepiolite

Ten grams of raw sepiolite was added into 100 mL H2SO4 (0.4 mol/L), stirring for 4 h at room temperature for the activation. Then the acid modified sepiolite was rinsed to neutral with distilled water, then dried at 105 °C for 2 h. Subsequently, 0.7:10 mass ratio of copper nitrate and acid modified sepiolite was dissolved in deionized water at room temperature. The sample was dried in an 80 °C water bath, and then it was roasted at 350 °C for 4 h. The CuO-acid modified sepiolite adsorbent was thus prepared. The samples were dried at room temperature and sputter-coated with gold. The surface morphologies of adsorbents were visualized using an FEI Quanta 200 FEG SEM. In addition, the MB and adsorbent samples were analyzed by using a Spectrum One FTIR spectrometer. And the scanning conditions of FTIR were determined at a spectral range of 4,000 to 450 cm−1, with a resolution of 4 cm−1.

Adsorption equilibrium experiments

The adsorption equilibrium experiments were carried out in 250 mL flasks, each of which contained 100 mL MB solution. CuO-acid modified sepiolite (0.2 g) was added to a flask and shaken at 170 rpm in a thermostatic shaker to reach equilibrium at different temperature. The MB concentrations before and after adsorption in the supernatant were determined using a Hach DR2800 spectrophotometer at 664 nm.

The amount of adsorption at equilibrium, Qe (mg/g), was calculated by Equation (1) (Cobas et al. 2014): 
formula
1
where C0 and Ce (mg/L) were the liquid-phase concentrations of MB at initial and equilibrium, respectively. V was the volume of the solution (L) and W was the mass of CuO-acid modified sepiolite adsorbent used (g).

Adsorption kinetics experiments

The adsorption kinetics experiments were performed in 250 mL flasks, each of which contained 100 mL of MB solution. 0.2 g CuO-acid modified sepiolite was added to a flask and shaked at 170 rpm in a thermostatic shaker. The samples were taken at preset time intervals and the concentrations of MB were rapidly analyzed.

The adsorption capacity at time t, Qt (mg/g) was calculated by Equation (2) (Cobas et al. 2014): 
formula
2
where C0 and Ct (mg/L) were the liquid-phase concentrations of MB at initial and at any time, respectively. V was the volume of the solution (L) and W was the mass of CuO-acid modified sepiolite adsorbent used (g).

Dynamic adsorption and regeneration experiments

Figure 1 is a schematic diagram of the dynamic adsorption experiment system used in the study. The inner diameter of the adsorption column was 22 mm and the height was 400 mm. The CuO-acid modified sepiolite was added into the adsorption column. MB solution of initial concentration 400 mg/L was passed through the column at different inflow rates by a peristaltic pump. The effluent concentration of MB was analyzed for a period of time.
Figure 1

Installation for the column studies. (1) Influent; (2) peristaltic pump; (3) thermometer; (4) adsorption column; (5) supporting net; (6) insulation sleeve; (7) cover; (8) thermocouple; (9) exhaust gas treatment device; (10) temperature controller; (11) air compressor; (12) filter net; (13) air flow meter; (14) absorbent; (15) air valve; (16) effluent valve.

Figure 1

Installation for the column studies. (1) Influent; (2) peristaltic pump; (3) thermometer; (4) adsorption column; (5) supporting net; (6) insulation sleeve; (7) cover; (8) thermocouple; (9) exhaust gas treatment device; (10) temperature controller; (11) air compressor; (12) filter net; (13) air flow meter; (14) absorbent; (15) air valve; (16) effluent valve.

In the regeneration process, the effluent valve was turned off and the air valve was turned on (Figure 1). The experimental regeneration temperature was controlled at 450–650 °C and the heating control system was opened to heat the adsorption column. Then the air compressor was started up, adjusting the gas flow rate to 3 L/min. The adsorbent was continually regenerated by the high-temperature gas. The exhaust gas could discharge from the outlet through a rotatable cover. Recycled exhaust gas was absorbed by the fluid adsorption.

RESULTS AND DISCUSSION

Adsorption studies of MB onto CuO-acid modified sepiolite

Adsorption isotherms indicate how the adsorption molecules distribute between the liquid phase and solid phase, and can be described the adsorption capacity of an adsorbent (Cobas et al. 2014; Duman et al. 2015). Meanwhile, the adsorption isotherms can help to explore the adsorption mechanism and to optimize the utilization of adsorbents (Cobas et al. 2014; Duman et al. 2015). Figure 2 shows the equilibrium adsorption capacity versus equilibrium concentrations of MB at various temperatures. At the same equilibrium concentration, the equilibrium adsorption capacity of MB was increased significantly with the temperature increasing. Thermodynamic parameters can be determined to understand the nature of the adsorption process. The enthalpy change (ΔH) was calculated using the Clausius–Clapeyron equation (Equation (3)) (Tan et al. 2008; Sebben & Pendleton 2015): 
formula
3
where K was the adsorption equilibrium constant, R was the gas constant, T was the absolute temperature, respectively.
Figure 2

Adsorption isotherm for MB onto CuO-acid modified sepiolite at different temperatures.

Figure 2

Adsorption isotherm for MB onto CuO-acid modified sepiolite at different temperatures.

ΔH values were calculated from the slope values of the plot of lnCe versus 1/T (Figure 3). At Qe of 25, 30 and 35 mg/g, the values of ΔH were 57, 58.5 and 66.2. The positive value of ΔH indicated that the adsorption process of MB was endothermic in nature, which showed that the adsorption process of MB onto the CuO-acid modified sepiolite was an endothermic process. Therefore, the Qm was increased with the increasing temperature.
Figure 3

The linear relationship between lnCe and 1/T.

Figure 3

The linear relationship between lnCe and 1/T.

The equilibrium data are usually analyzed using Langmuir and Freundlich isotherm models (Cobas et al. 2014). The Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of adsorption sites of uniform strategies of adsorption process with no transmigration of adsorbate on the plane of the adsorbent surface (Tan et al. 2008; Cobas et al. 2014; Duman et al. 2015). The linear form of the Langmuir isotherm equation was given as follows (Equation (4)) (Tan et al. 2008): 
formula
4
where Ce was the equilibrium concentration of the adsorbate (mg/L), Qe was the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Qm and KL were Langmuir constants related to adsorption capacity and rate of adsorption, respectively (Demirbas & Nasi 2010).
The Freundlich isotherm model speculated that the adsorption occurs on heterogeneous surfaces and the adsorption process is without creating a molecule–adsorbent association after adsorption (Cobas et al. 2014). The linear form of the Freundlich isotherm equation was given as follows (Equation (5)) (Duman et al. 2015): 
formula
5
where KF was a Freundlich constant representing the adsorption capacity and n was a constant depicting the adsorption intensity (Duman et al. 2015).

The equilibrium data were fitted by the Langmuir isotherm and Freundlich isotherm models, as shown in Tables 1 and 2. The R2 value indicated that the adsorption data of MB onto the CuO-acid modified sepiolite at five different temperatures were well fitted to the Langmuir isotherm model. At the same time, the Qm was increased with increasing temperature. The maximum adsorption capacity and Langmuir constant were calculated from the slope and intercept of the linear plots of Ce/Qe versus Ce, respectively. The results showed that the increasing temperature was favorable for adsorption. Based on the Langmuir isotherms, the maximal absorption capacities were 38.6, 46.9, 48.1, 53.8 and 64.5 mg/g at temperature 293, 298, 303, 313 and 323 K, respectively.

Table 1

Langmuir parameters for the adsorption

T (K) Equation Qm (mg/g) KL (L/mg) R2 
293 Ce/Qe = 0.0259Ce + 0.2484 38.6 0.104 0.996 
298 Ce/Qe = 0.0213Ce + 0.2298 46.9 0.093 0.994 
303 Ce/Qe = 0.0208Ce + 0.101 48.1 0.206 0.999 
313 Ce/Qe = 0.0186Ce + 0.0978 53.8 0.190 0.997 
323 Ce/Qe = 0.0155Ce + 0.0836 64.5 0.185 0.990 
T (K) Equation Qm (mg/g) KL (L/mg) R2 
293 Ce/Qe = 0.0259Ce + 0.2484 38.6 0.104 0.996 
298 Ce/Qe = 0.0213Ce + 0.2298 46.9 0.093 0.994 
303 Ce/Qe = 0.0208Ce + 0.101 48.1 0.206 0.999 
313 Ce/Qe = 0.0186Ce + 0.0978 53.8 0.190 0.997 
323 Ce/Qe = 0.0155Ce + 0.0836 64.5 0.185 0.990 
Table 2

Freundlich parameters for the adsorption

T (K) Equation KF 1/n R2 
293 ln Qe = 0.202 ln Ce + 2.63 13.9 0.202 0.873 
298 ln Qe = 0.209 ln Ce + 2.93 18.7 0.209 0.868 
303 ln Qe = 0.25 ln Ce + 2.62 13.7 0.250 0.897 
313 ln Qe = 0.219 ln Ce + 3 20.1 0.219 0.877 
323 ln Qe = 0.208 ln Ce + 3.21 24.8 0.208 0.854 
T (K) Equation KF 1/n R2 
293 ln Qe = 0.202 ln Ce + 2.63 13.9 0.202 0.873 
298 ln Qe = 0.209 ln Ce + 2.93 18.7 0.209 0.868 
303 ln Qe = 0.25 ln Ce + 2.62 13.7 0.250 0.897 
313 ln Qe = 0.219 ln Ce + 3 20.1 0.219 0.877 
323 ln Qe = 0.208 ln Ce + 3.21 24.8 0.208 0.854 

Figure 4 shows the adsorption kinetics of MB onto CuO-acid modified sepiolite at 298, 308 and 318 K. It was shown that the adsorption rate for MB was rapid at the beginning then increased slowly until the plateau of adsorption equilibrium was achieved. The adsorbate transport from the solution phase to the surface of the adsorbent particles occurs in several steps. For the present investigation, different kinetic models were considered for the interpretation of the experimental data. The selected kinetic models were the Lagergren model equation and intraparticle diffusion model equation.
Figure 4

Adsorption kinetic of MB on CuO-acid modified sepiolite.

Figure 4

Adsorption kinetic of MB on CuO-acid modified sepiolite.

The adsorption kinetics was examined with a first-order rate expression, the so-called Lagergren equation, and it can be expressed as Equation (6) (Dutta et al. 1997): 
formula
6
where F was Qt/Qe, and kad was the adsorption rate constant.
The linear form of the intraparticle diffusion model equation was given as Equation (7) (Liu & Zhang 2015): 
formula
7
where kp (mg/g min1/2) was the intraparticle diffusion rate constant, and C was a constant.

The fitting of experimental data to the linear forms for the Lagergren equation kinetic models is shown in Table 3. Meanwhile, the intraparticle diffusion rate constant was obtained from the slope of the straight line of Qt versus t1/2 (Table 4). The R2 values were higher than 0.96 at 298, 308 and 318 K. This showed that the intraparticle diffusion was the dominant factor for the adsorption. The adsorption rate constants were increased and the displacement spots on CuO-acid modified sepiolite were increased with the increasing temperature.

Table 3

Lagergren model parameters for the adsorption

Kinetic model T (K) kad (min−1C R2 
Lagergren 298 −0.006 −0.185 0.952 
308 −0.007 −0.317 0.949 
318 −0.008 −0.327 0.962 
Kinetic model T (K) kad (min−1C R2 
Lagergren 298 −0.006 −0.185 0.952 
308 −0.007 −0.317 0.949 
318 −0.008 −0.327 0.962 
Table 4

Intraparticle diffusion model parameters for the adsorption

Kinetic model T (K) kp (mg/g min½C R2 
Intraparticle diffusion 298 1.07 5.59 0.983 
308 1.12 8.87 0.979 
318 1.26 11.00 0.967 
Kinetic model T (K) kp (mg/g min½C R2 
Intraparticle diffusion 298 1.07 5.59 0.983 
308 1.12 8.87 0.979 
318 1.26 11.00 0.967 

Dynamic adsorption of MB onto CuO-acid modified sepiolite

Effect of height–diameter ratio on the breakthrough curves of dynamic adsorption

Nowadays, numerous studies are focusing on adsorption of dyes from static systems. However, the adsorption data acquired in static mode are often not applicable for treatment systems where the contact time is not sufficient to attain equilibrium; then, continuous adsorption experiments are needed (Fernandez et al. 2014). Adsorption in a fixed bed is more preferable because of its ability to process large quantities of solutions in the continuous mode (Zhou et al. 2013). Thus, the continuous removal of MB solution was assessed through experiments in a column, and the effect of height–diameter (H/D) ratio of the adsorption equipment on dynamic adsorption was studied, as shown in Figure 5. It was found that the H/D ratio significantly influenced the dynamic adsorption of MB onto the CuO-acid modified sepiolite. The breakthrough profile had different lengths with ratios of 1.28, 1.92, and 2.56. When the H/D ratio of the adsorption equipment was 1.28, the adsorption capacity was lowest. The treatment capacity of the MB solution was increased rapidly and the retention time was also increased with increasing ratio of H/D. Breakthrough point for the MB solution occurred later appeared by increasing the H/D ratio, indicating a better removal performance with H/D ratio of 2.56.
Figure 5

Effect of H/D ratio on the dynamic adsorption.

Figure 5

Effect of H/D ratio on the dynamic adsorption.

Effect of influent flow rate on the breakthrough curves of dynamic adsorption

The effect of the influent flow rate on the breakthrough profiles of dynamic adsorption of MB onto the CuO-acid modified sepiolite is illustrated in Figure 6. BV/h is space velocity, which indicates the unit volume per unit time that flowed through the adsorbent average fluid volume in the column. As can be observed, the variation of the influent flow rate has a negligible influence on the shape of the breakthrough curves (García-Mateos et al. 2015). The higher the influent flow rate, the lower the breakthrough point, while the amount of solutes adsorbed on the CuO-acid modified sepiolite was obviously affected by the influent flow rate (Zhou et al. 2013). By increasing the flow rate, the solid–liquid contact time was reduced and the length of the transfer area was increased. Therefore, it led to reduction of the penetrable treatment capacity. In contrast, the penetrable treatment capacity was increased with decreasing flow rate. But the flow velocity was too slow, which led to operating time increase and prolonging of the processing cycle. Therefore, the best treatment capacity was 6 BV/h.
Figure 6

Effect of influent flow rate on the dynamic adsorption.

Figure 6

Effect of influent flow rate on the dynamic adsorption.

Regeneration conditions of the CuO-acid modified sepiolite

When the adsorption equilibrium is reached, a regeneration step is needed to restore the adsorption capacity (Zhang et al. 2014). High-temperature gas stream catalytic oxidation method was applied in this study to regenerate the CuO-acid modified sepiolite adsorbent. To assess the effect of gas stream temperatures, the selected temperature range was 450–650 °C, the space velocity was kept at 150 min−1 and the regeneration time was set at 10 min in the regeneration experiments (Figure S1, supplementary material, available with the online version of this paper). In the range 450–650 °C, the regeneration rate increasing significantly with increasing regeneration temperatures. The regeneration rate was only 68.6% at 450 °C, while it was almost 100% at 650 °C regeneration temperature. Therefore, it was considered suitable for the regeneration temperature range to be 550–650 °C.

The effect of space velocity on regeneration rate was investigated in the range 0–150 min−1, with the regeneration temperature of 650 °C and the regeneration time of 10 min (Figure S2, supplementary material, available with the online version of this paper). The regeneration rate constantly increased significantly with the space velocity increase, and the regeneration rates were nearly 98.6% and 99.2%, respectively, at velocity of 100 min−1 and 150 min−1. Therefore, the space velocity of 100 min−1 was adopted.

Figure S3 (supplementary material, available with the online version of this paper) shows the regeneration efficiency for the CuO-acid modified sepiolite at different regeneration times. The results clearly indicated that the regeneration rate was higher in pace with increasing regeneration time, and regeneration time had a positive effect on repeated adsorption rate. The results showed that the regeneration rate was 99.2% in 10 min, while only 82.5% in regeneration of 3 min.

SEM and FTIR of the adsorbent samples

The surfaces of CuO-acid modified sepiolite, CuO-acid modified sepiolite after adsorption of MB and the regeneration adsorbent were characterized by SEM. Figure 7 shows that the CuO-acid modified sepiolite was fibrous aggregates and had more fiber filament on the surface. After the adsorption process, the fiber became fuzzy. Because the adsorbent was added into the MB solution for adsorption experiment, it was speculated MB was adsorbed on the surface of CuO-acid modified sepiolite. After regeneration, most of the surface of absorbent appeared fibrous. This could explain why the CuO-acid modified sepiolite had adsorption activity again after the high-temperature gas stream catalytic oxidation.
Figure 7

SEM of the adsorbents. (a) Before adsorption; (b) after adsorption; (c) after regeneration.

Figure 7

SEM of the adsorbents. (a) Before adsorption; (b) after adsorption; (c) after regeneration.

The peaks in the FTIR spectra of pure MB (Figure S4(a)) represented the stretching vibration of C-C in the aromatic ring at 1,601 cm−1 and the stretching vibration of C = N at 1,491 cm−1. In addition, the stretching vibration of C-H in methylene appeared at 1,397 cm−1. It can be seen from Figure S4(b) and S4(c) that there were some changes in the FTIR spectra of CuO-acid modified sepiolite after MB adsorption. The stretching vibration of C-C for MB was observed at 1,601 cm−1 after the adsorption process. As illustrated in Figure S4(d), the disappearance of the C-C stretching at 1,601 cm−1 and C-N stretching at 1,356 cm−1 might be due to high-temperature gas stream regeneration (Figure S4 is available with the online version of this paper). Thus, CuO-acid modified sepiolite recovered its adsorption activity.

Adsorption/regeneration cycles

Recycle number is an important indicator for evaluation of the performance of adsorbent. The CuO-acid modified sepiolite was used in 19 adsorption/regeneration cycles at optimum regeneration conditions (regeneration temperature of 650 °C, space velocity of 100 min−1, and regeneration time of 10 min). The elevated adsorption capacity remained until the 10th adsorption cycle and the regeneration rate was more than 85%. A dropdown of regeneration was found after the 12th adsorption/regeneration cycles, while the regeneration rate was around 75%. This was particularly interesting from the practical point of view due to the possibility of using the CuO-acid modified sepiolite for a longer operation time.

CONCLUSION

The present investigation showed that the CuO-acid modified sepiolite was a promising adsorbent for removal of dyes from aqueous solutions. The adsorption of MB onto CuO-acid modified sepiolite was an endoergic process, and the Langmuir isotherm model equation described the adsorption process. In the dynamic adsorption, the optimal operating conditions were that the ratio of H/D was 2.56 and influent flow rate was 6 BV/h. The adsorption capacity was kept stable for at least 12 cycles for adsorption and regeneration. Therefore, it was highly feasible to use CuO-acid modified sepiolite as an adsorbent to remove MB from water and to apply high-temperature gas stream catalytic oxidation method for the adsorbent regeneration.

ACKNOWLEDGEMENTS

The authors acknowledge the research grant provided by the National Natural Science Foundation of China (Grant No. 21166005) and Natural Science Foundation of Guangxi (Grant No. 2015GXNSFAA139267) for providing financial support.

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Supplementary data