This paper examines a novel method of regenerating saturated activated carbon after adsorption of complex phenolic, polycyclic aromatic hydrocarbons with low energy consumption by using superheated water pretreatment combined with CO2 activation. The effects of the temperature of the superheated water, liquid–solid ratio, soaking time, activation temperature, activation time, and CO2 flow rate of regeneration and adsorption of coal-powdered activated carbon (CPAC) were studied. The results show that the adsorption capacity of iodine values on CPAC recovers to 102.25% of the fresh activated carbon, and the recovery rate is 79.8% under optimal experimental conditions. The adsorption model and adsorption kinetics of methylene blue on regenerated activated carbon (RAC) showed that the adsorption process was in accordance with the Langmuir model and the pseudo-second-order kinetics model. Furthermore, the internal diffusion process was the main controlling step. The surface properties, Brunauer–Emmett–Teller (BET) surface area, and pore size distribution were characterized by Fourier transform infrared spectroscopy (FT-IR) and BET, which show that the RAC possesses more oxygen-containing functional groups with a specific surface area of 763.39 m2 g−1 and a total pore volume of 0.3039 cm3 g−1. Micropores account for 79.8% and mesopores account for 20.2%.

INTRODUCTION

Industrial wastewater refers to the production of waste water, sewage and waste that contains industrial materials, intermediates, products and by-products, some of which can be pollutants (Hu et al. 2015). Industrial wastewater mainly comes from the chemical, textile, paper, steel, and electricity industries as well as others. The types of industrial wastewater are complex. An industrial process can discharge different types of wastewater, which may contain different pollutants with different polluting effects (Reungoat et al. 2012).

Wastewater from a single production plant may contain several contaminants. Similarly, in different industrial enterprises, even if the product, raw materials and processing are different, they may discharge a similar nature of wastewater. For example, refineries, chemical plants, coking plants, etc., are all likely to discharge wastewater containing oil, phenol and other substances (Gadipelly et al. 2014).

Activated carbon (AC) is a porous material that has a high specific surface area and strong adsorption capacity. AC is extensively used in effluent treatment and is especially suitable for domestic water, industrial wastewater, sewage depth treatment, water pollution emergency treatment, and other fields (Stoquart et al. 2012; Kulkarni 2013; Hu et al. 2015). However, exhausted AC absorbs amounts of organic compounds, setting up the adsorption–desorption equilibrium, and therefore cannot be continuously used. The exhausted AC is usually discarded, buried or burned, which results in wasted resources and pollution. Therefore, the regeneration of exhausted AC has important economic and environmental significance.

Exhausted AC used in the treatment of industrial wastewater has mainly adsorbed a large number of complex and volatile oils, and phenolic, polycyclic aromatic hydrocarbon. Complex organic matter was adsorbed in the pores of AC through physical and chemical adsorption; the molecules are closely connected by van der Waals forces, plugging the pores, which makes it difficult to regenerate by the traditional regeneration method.

The conventional regeneration techniques of AC include thermal and chemical reagents as well as biological regeneration (Sabio et al. 2004; Guo et al. 2011; Toh et al. 2013). Thermal regeneration is the most extensively used in industrial applications because of its high efficiency and versatility. However, thermal regeneration typically requires heating to 800–1,100 °C, leading to high energy consumption and carbon loss, the change of the AC chemical surface structure and multiple cycle utilization failure. The chemical reagent regeneration method cannot provide a universal and effective regeneration reagent for all the complex organic compounds in wastewater. In addition, the risk of inducing secondary pollution to the environment due to the utilization of chemical reagents should be considered. The biological regeneration method also has low regeneration efficiency, long regeneration time, and poor versatility. Similarly unconventional techniques including electrochemistry, ultrasound, microwaves, electrodialysis, and plasma discharges have been tried. However, they have proven neither to be technically feasible for large scale continuous operation, nor to be economically viable. As a result, these regenerative technologies are not extensively recognized by the industry.

Water is known to be an effective organic reaction medium, as a reactant, solvent, and a catalyst in organic reactions at certain temperatures and pressures. Superheated water presents properties that are similar to strong acid and alkali (Khanjari et al. 2015, 2016). In the supercritical state, water can rapidly oxidize and decompose macromolecules, but hardly reacts with AC. Charinpanitkul (Charinpanitkul & Tanthapanichakoon 2011) regenerated two commercial ACs saturated with pyridine or phenol by using supercritical water heated to over 400 °C enhanced with hydrogen peroxide oxidation. Francisco (Salvador et al. 2013) used 400–500 °C supercritical water regeneration of phenol-containing AC, in which phenol was cracked to CO2 and H2O; they obtained a good performance of renewable AC. However, the pressure of supercritical water at 400 °C is 260 bar, which is demanding on the experimental equipment and difficult to popularize in practice. Furthermore, few investigations have been reported using superheated water to regenerate saturated AC used for adsorption of complex phenolic, polycyclic aromatic hydrocarbons.

In this paper, coal-exhausted AC (CEAC) first undergoes regenerated pretreatment by using superheated water with a low temperature, then the thermal regeneration method for CO2 activation is applied. Lastly, the effects of the experimental conditions, such as the temperature of the superheated water (Tw), liquid–solid ratio (LSR), soaking time (ts), activation temperature (Ta), activation time (ta), and CO2 flow rate (CFR) on regeneration adsorption performance and recovery rate of regenerated AC (RAC) were studied. The adsorption model, the kinetics of adsorption on methylene blue (MB), and the texture characteristics of RAC were also studied.

MATERIALS AND METHODS

Raw material analysis

CEAC was provided by a domestic industrial wastewater treatment plant in Hunan. The ash element analysis is presented in Table 1, in which the main adsorbent in CEAC is complex phenolic, polycyclic aromatic hydrocarbon and contains trace metal compounds.

Table 1

Trace metal concentrations of CEAC ash

Element Fe Ca Mg Na Co Ti 
Content (%) 0.43 0.22 0.071 0.06 0.056 0.053 0.038 
Element Si Mn Ni Cr Cu As 
Content (%) 0.0031 0.0099 0.0075 0.0037 0.0016 0.00096 0.000085 
Element Fe Ca Mg Na Co Ti 
Content (%) 0.43 0.22 0.071 0.06 0.056 0.053 0.038 
Element Si Mn Ni Cr Cu As 
Content (%) 0.0031 0.0099 0.0075 0.0037 0.0016 0.00096 0.000085 

Experiments

10 g CEAC was washed 2 or 3 times by deionized water to remove acid and partially soluble inorganic substances. Superheated water pretreatment was combined with the thermal regeneration method, wherein the pretreatment method of superheated water was prepared by mixing CEAC with deionized water with a certain LSR, sealing in a polyethylene hydration reactor and heating at the rate of 10 °C/min. Insulation, holding pressure for a certain time, cooling, filtering, and drying were conducted to obtain the AC precursor. The thermal regeneration experiment involves placing the AC precursor in a tubular resistance atmosphere furnace, activating a certain time in the CO2 atmosphere, and then cooling. The regeneration efficiency (RE), is defined as 
formula
1
where Qi, is the adsorption capacity of the regenerated carbon in a given ith re-use cycle, and Q0 the adsorption capacity of the fresh carbon. The recovery rate (Rr) is defined as 
formula
2
where Mi is the quality of the regenerated carbon in a given ith re-use cycle, and M0 the quality of the fresh carbon.

Characterization method

The AC adsorption capacity of the iodine values of the detection method refers to GB/T 7702.7-2008. The absorbance of the MB solution was measured at a wavelength of 665 cm−1 using a 721-visible spectrophotometer (Tianjin Guanze Technology Co., Ltd, China). The elemental contents of AC ash were characterized by X-ray fluorescence (XRF) (Shimadzu 1800, Japan). The infrared absorption Fourier transform infrared spectroscopy (FT-IR) (Nicolet 6700, Thermo Fisher Scientific, USA) was used to measure absorption peaks ranging from 4,000 to 400 cm−1 and the chemical functional groups. The specific surface area and pore size distribution were measured by the Quantachrome (QuadraSorb SI, USA) specific surface area tester. Brunauer–Emmett–Teller (BET) specific surface area and average pore size were calculated. The total pore volume was determined by N2 adsorption and desorption experiments at a relative pressure of 0.99. Density functional theory (DFT) and the Barrett–Joyner–Halenda (BJH) method were used to analyze the pore size distributions from micropores to mesopores.

Adsorption model and kinetics

MB (3,7-bis(dimethylamino)-phenothiazin-5-ium chloride) is a widely used organic dye that can be applied to paper and textile colorants. The MB solution is extensively used as a model compound in the process of adsorption and decoloration of organic matter in wastewater (Hameed et al. 2007). The adsorption capacity of MB is used to study the surface properties of AC and its affinity to adsorbents like complex phenolic, polycyclic aromatic hydrocarbon as well as the adsorption of larger molecules of organic matter to determine the size of the pores. In this paper, MB (Sinopharm Group Chemical Reagent Co. Ltd) was used as the model compound, and the Langmuir and Freundlich adsorption model, pseudo-first-order and pseudo-second-order kinetics, and the intraparticle diffusion model were used to quantitatively describe the adsorption process and adsorption mechanism.

RESULTS AND DISCUSSION

AC regeneration experiment

The CEAC was pretreated by superheated water and then activated by conventional thermal regeneration. Figure 1(a)1(c) show the relationship between Tw, ts and LSR, respectively, and the regeneration efficiency and recovery rate of RAC in the fixed activated conditions. Figure 1(a)1(c) also show that with the increase of Tw, ts, and LSR, the adsorbed organic matter was dissociated from the pores of CEAC by solvent diffusion (Charinpanitkul & Tanthapanichakoon 2011), thus reducing the subsequent activation resistance and eventually increasing the regeneration efficiency of RAC. The regeneration efficiencies were 90.68% (Figure 1(a)), 94.73% (Figure 1(b)), and 94.14% (Figure 1(c)) when Tw = 180°, ts = 3 h, and LSR = 8:1. When the temperature was increased from 180 °C to 200 °C (Figure 1(a)), the regeneration efficiency increased from 90.68% to 92.18%. However, the pressure of superheated water increased from 10 bar to 15 bar, increasing the difficulty of the experimental operation. If ts and LSR values are further increased (Figure 1(b) and 1(c), respectively), the regeneration efficiency hardly changes because the organic matter in the CEAC has a limit of decomposition at a fixed temperature. In addition, the figure shows that changing the pretreatment conditions of the superheated water does not significantly affect the recovery rate of the RAC, as the reaction between superheated water and carbon is minimal.
Figure 1

Experiment of AC regeneration combined with superheated water pretreatment with CO2 activation showing the effects of (a) water temperature, (b) soaking time, (c) LSR, (d) activation temperature, (e) activation time and (f) CO2 flow rate.

Figure 1

Experiment of AC regeneration combined with superheated water pretreatment with CO2 activation showing the effects of (a) water temperature, (b) soaking time, (c) LSR, (d) activation temperature, (e) activation time and (f) CO2 flow rate.

Figure 1(d)1(f) show the results of the fixed thermal pretreatment process (Tw = 180 °C, ts = 3 h, LSR = 8:1), in which the relationships between Ta, ta, CFR, respectively and regeneration efficiency and recovery rate were investigated. The figure shows that the regeneration efficiency increased as the activation temperature and the activation time increased (Figure 1(d) and 1(e)). When Ta = 600 °C and ta = 30 min, the regeneration efficiency reached 101.05%. At this time, Ta and ta continued to increase and the regeneration efficiency did not obviously change; however, the recovery rate significantly decreased. When Ta and ta were further increased, the regeneration efficiency did not improve but the recovery rate significantly decreased because the increase in temperature and time exacerbated the reaction between carbon dioxide and carbon, producing new pores that simultaneously cause the collapse of the original pores. Figure 1(f) shows that an appropriate increase in CFR enhances the regeneration efficiency of AC. However, when the CFR is excessive (more than 0.7 L min−1), the diameter of the primary pores is destroyed, the pores are enlarged, the micropores change into mesopores and even macropores, and the regeneration efficiency and recovery rate decrease further.

The preceding experimental results show that the following are the optimal conditions for AC regeneration from superheated water pretreatment combined with CO2 thermal regeneration: the temperature of the superheated water, LSR, soaking time, activation temperature, activation time, and CO2 flow rate are 180 °C, 3 h, 8:1, 600 °C, 30 min, and 0.7 L min−1, respectively. The regeneration efficiency of AC is 102.25%, and the recovery rate is 79.8%.

The effect of 10 consecutive regeneration cycles on the adsorption behavior and loss of quality of the CEAC was assessed by the regeneration efficiency and recovery rate. The results are presented in Figure 2 and it is obvious to conclude that, under the optimal experimental conditions used, though adsorption performance in the process of multiple recycling can still maintain a high level, the recovery rate decreased steadily. In the tenth regeneration cycle, the regeneration efficiency and recovery rate were 97.5% and 76.6%, respectively. The results show that the regenerative effect of superheated water-assisted regeneration is remarkable, and has almost no effect on the original pores of AC, resulting in high recovery.
Figure 2

Effects of multiple regeneration cycles on regeneration efficiency and recovery of AC.

Figure 2

Effects of multiple regeneration cycles on regeneration efficiency and recovery of AC.

Adsorption model

The Langmuir and Freundlich adsorption models are the most commonly used AC adsorption models (Hameed et al. 2007). The experiments were conducted by adding 200 ml of different concentrations (230, 260, 290, 320, and 350 mg L−1) of standard MB solution and 0.2 g RAC to a 500 ml grinding conical flask and maintaining a constant temperature oscillator for 7 h to reach adsorption equilibrium.

The experimental data were linearly fitted by the Langmuir and Freundlich equations, the plot of Ce/Qe versus Ce and lnCe versus lnQe give a linear graph, respectively, and the fitting curves are shown in Figure 3(a) and 3(b), respectively. The model parameters were evaluated and are presented in Table 2. The adsorption experiment of MB on RAC conforms to the Langmuir and Freundlich equations. By comparing the correlation coefficient R2, the Langmuir equation has a higher fitting degree of more than 0.997, which conforms to the adsorption model of the monolayer. The adsorption of MB by AC is an exothermic process, which is consistent with the adsorption of other ACs (Duan et al. 2012). The adsorption of MB on RAC decreases with the increase of temperature, the maximum value of 330.033 mg g−1 was obtained at 293 K, indicating that the adsorption of MB by RAC is obvious.
Table 2

Isotherm parameters for the removal of MB by RAC at different temperatures

Isotherms Parameters Our study Duan et al. (2012 
Temperature (K)
 
Temperature (K)
 
Hameed et al. (2007) Temperature (K) 
293 313 333 298 303 308 323 
Langmuir Qm/mg g−1 330.03 327.87 324.68 370.37 384.61 395.83 454.20 
KL 2.7297 1.6310 1.316 0.52 1.04 1.20 0.004 
R2 0.998 0.998 0.999 0.99 0.97 0.99 0.999 
Freundlich 1/n 0.096 0.096 0.087 0.16 0.17 0.20 0.268 
KF 254.1 238.38 231.64 200.03 232.66 239.19 171.40 
R2 0.949 0.931 0.975 0.96 0.91 0.93 0.964 
Isotherms Parameters Our study Duan et al. (2012 
Temperature (K)
 
Temperature (K)
 
Hameed et al. (2007) Temperature (K) 
293 313 333 298 303 308 323 
Langmuir Qm/mg g−1 330.03 327.87 324.68 370.37 384.61 395.83 454.20 
KL 2.7297 1.6310 1.316 0.52 1.04 1.20 0.004 
R2 0.998 0.998 0.999 0.99 0.97 0.99 0.999 
Freundlich 1/n 0.096 0.096 0.087 0.16 0.17 0.20 0.268 
KF 254.1 238.38 231.64 200.03 232.66 239.19 171.40 
R2 0.949 0.931 0.975 0.96 0.91 0.93 0.964 
Figure 3

Isotherms for MB dye adsorption onto RAC at different temperatures using (a) the Langmuir and (b) the Freundlich isotherms.

Figure 3

Isotherms for MB dye adsorption onto RAC at different temperatures using (a) the Langmuir and (b) the Freundlich isotherms.

Adsorption kinetics

Adsorption kinetic studies are mainly used to describe the influence of temperature, concentration, adsorbent, and other factors on the adsorption rate. The adsorption process and adsorption results can also be predicted from the adsorption kinetic model (Tan et al. 2008; Vargas et al. 2011). The MB solution of different concentrations was adsorbed by adding 0.2 g RAC at 298 K, and linear fitting was conducted by the pseudo-first-order kinetic and the pseudo-second-order kinetic equation, the plots of lg(Qe-Qt) versus t and t/Qt versus t give linear graphs, respectively. The fitting curve and fitting parameters are shown in Figure 4 and Table 3, respectively. The results are consistent with the pseudo-second-order kinetic model, which has a higher correlation coefficient (R2 > 0.986). The values of Qe,cal are calculated using the pseudo-second-order kinetic model at different concentrations that are close to the experimental results.
Table 3

Kinetics and intraparticle diffusion model parameters for the MB removal of different initial concentrations by regenerated carbon at 298 K

Initial conc. (mg L−1Qe, exp (mg g−1First-order kinetic model
 
Second-order kinetic model
 
Intraparticle diffusion model
 
K1 Qe, cal (mg g−1R2 K2 Qe, cal (mg g−1R2 K3 R2 
260 245 0.00465 199.95 0.9696 0.0000446 256.41 0.9900 10.744 0.96946 10.1987 
280 259 0.00523 194.27 0.9621 0.0000561 264.55 0.9869 10.840 0.96625 25.7157 
300 270 0.00544 181.45 0.9668 0.0000806 277.78 0.9893 10.338 0.97268 60.0828 
320 300 0.00728 187.64 0.9801 0.0000938 311.53 0.9933 10.548 0.96798 94.5570 
Initial conc. (mg L−1Qe, exp (mg g−1First-order kinetic model
 
Second-order kinetic model
 
Intraparticle diffusion model
 
K1 Qe, cal (mg g−1R2 K2 Qe, cal (mg g−1R2 K3 R2 
260 245 0.00465 199.95 0.9696 0.0000446 256.41 0.9900 10.744 0.96946 10.1987 
280 259 0.00523 194.27 0.9621 0.0000561 264.55 0.9869 10.840 0.96625 25.7157 
300 270 0.00544 181.45 0.9668 0.0000806 277.78 0.9893 10.338 0.97268 60.0828 
320 300 0.00728 187.64 0.9801 0.0000938 311.53 0.9933 10.548 0.96798 94.5570 
Figure 4

Pseudo-first-order and pseudo-second-order kinetics for the adsorption of MB by RAC at 298 K.

Figure 4

Pseudo-first-order and pseudo-second-order kinetics for the adsorption of MB by RAC at 298 K.

The pseudo-second-order kinetic equation fits precisely because the model is based on chemical reactions or chemical adsorption via electron sharing, gain and loss. This equation includes all processes of adsorption, such as external liquid film diffusion, surface adsorption, and intra-particle diffusion, which can more realistically and comprehensively reflect the adsorption mechanism of organic matter on AC (Chang & Juang 2004). By contrast, the pseudo-first-order equation is less well fitted to the MB adsorption behavior because of the limitations in the calculation of boundary conditions. The pseudo-first-order model should provide the Qe value before fitting. However, reaching the adsorption equilibrium in the actual adsorption process is time-consuming, and equilibrium is a dynamic process; therefore, accurately measuring the equilibrium adsorption amount is difficult.

The process of adsorbing MB by RAC was further studied by the intraparticle diffusion model. Figure 5 shows that Qt of the adsorbed MB by RAC exhibits a good linear relationship with t0.5; however, Qt did not pass through the origin, which indicates that the internal diffusion process is the main controlling step but is not the only controlling step (Wu et al. 2009). The adsorption was also affected by the external diffusion process of the AC because the MB molecules are smaller than the pores, which is conducive to diffusion in the adsorbent. The high sorption of organic matter is attributed to the high surface energy sites such as defects, edges, and groove areas. Stronger electrostatic attraction also contributes to the higher sorption capacity (Song et al. 2016; Wang et al. 2016). Developed pore structure, internal defects and surface functional groups of RAC contribute to high sorption capacity. The adsorption heterogeneity appeared to be due to the variations and complexity of the adsorptive sites of RAC surfaces. Therefore, different mechanisms may act simultaneously (Yu et al. 2016).
Figure 5

Intraparticle diffusion model for the adsorption of MB by RAC at 298 K.

Figure 5

Intraparticle diffusion model for the adsorption of MB by RAC at 298 K.

Texture characteristics

FT-IR analysis

The AC in the preparation process is attributed to the presence of ash and other heteroatoms. The basic structure has defects and unsaturated valence in which oxygen and other atoms can be attached to these defects in the activation process, form various functional groups, and produce a variety of adsorption properties (Yin et al. 2007). The adsorption capacity of AC strongly influences the oxygen-containing functional groups and nitrogen-containing functional groups (Hulicova-Jurcakova et al. 2009). The FT-IR spectra of the fresh AC, exhausted AC, and RAC are shown in Figure 6. The Sadtler Spectral Handbooks state that the band centered at 3,618 cm−1 for the free-OH stretching vibration contains alcohols and phenols. The band at 2,338 cm−1 has a strong absorption peak for C ≡ C or C ≡ N stretching vibration and various cumulative double bonds. The band at 1,710–1,690 cm−1 is generated by carboxyl or amide. The bands at 1,690–1,670 cm−1 and 1,523 cm−1 are the C = C stretching vibration and the stretching vibration generated by di- or tri-substitution on the C–C chain, respectively. The fresh AC has an absorption peak at 920 cm−1, which is a C–H deformation vibration. After saturated adsorption, the displacement reaches 966 cm−1 and then returns to 929 cm−1 after regeneration. This phenomenon occurs because the functional groups on the surface of AC interact with the adsorbed organic substances, leading to the absorption peak displacement. The absorption band at 670 cm−1 is the C–H deformation vibration of the benzene ring. A weak absorption peak at 445–435 cm−1 for the C–X and S–S stretching vibrations only appeared in exhausted AC and RAC; this result indicates that the exhausted AC adsorbed small amounts of halogen and sulfur compounds during the adsorption process that could not be removed by regeneration. The fresh AC and RAC are more extensive than the exhausted AC with regard to the oxygen-containing functional group, possess more oxygen-containing functional groups, and have a stronger adsorption capacity; these characteristics are consistent with the previous experimental results.
Figure 6

FT-IR spectrum for AC of fresh AC, exhausted AC, and RAC.

Figure 6

FT-IR spectrum for AC of fresh AC, exhausted AC, and RAC.

Pore size analysis

The N2 adsorption–desorption isotherm of RAC is shown in Figure 7. According to the Brunauer isothermal adsorption curve, the adsorption of RAC belongs to the type I isotherm, which is the Langmuir isotherm (Thommes et al. 2015). The type I isotherm is concave to the p/p0 axis, and the amount of adsorption is close to the limiting value. This limiting value of adsorption is determined by the micropore volume rather than the internal surface area. The steep uptake at very low p/p0 is attributed to the enhanced adsorbent–adsorptive interactions in narrow pores, which results in micropores filling at very low p/p0. Isotherms indicate that RAC has a pore size distribution in a broader range, including wider micropores and possibly narrow mesopores (<∼ 2.5 nm).
Figure 7

N2-adsorption/desorption isotherms of RAC.

Figure 7

N2-adsorption/desorption isotherms of RAC.

The pore size distribution and cumulative pore volume of RAC calculated by the BJH method are shown in Figure 8. The pore size distribution of RAC mainly ranges from 0 to 10 nm; the cumulative pore volume rapidly increased in this range. The cumulative pore volume slowly increased when the pore volume was greater than 10 nm, which indicates that the RAC primarily comprised micropores and narrow mesopores, and thus ensured the adsorption capacity of RAC.
Figure 8

BJH and DFT pore size distribution of RAC.

Figure 8

BJH and DFT pore size distribution of RAC.

The BJH method for the characterization of microporous AC is inaccurate and unreliable for an in-depth study (Thommes et al. 2015). Therefore, DFT is required to correctly evaluate RAC micropores and narrow mesopores. The pore size is clearly mainly distributed from 0 to 4 nm with the micropores and the narrower mesopores; this result is consistent with the BJH test result. The peak value at 1.83 nm is 0.10338 cm3 nm−1 g−1. The textural properties of RAC are shown in Table 4, in which the specific surface area, total pore volume, micropores, and mesopores are 763.39 m2 g−1, 0.3039 cm3 g−1, 0.2425 cm3 g−1 and 0.0614 cm3 g−1, respectively, which are all higher than the values of CEAC and close to fresh AC. A certain amount of narrow mesopores ensures the adsorption of larger molecules by RAC.

Table 4

Textural properties of exhausted, regenerated and fresh AC

  RAC Exhausted AC Fresh AC 
BET surface area (m2 g−1763.39 463.28 775.21 
Total pore volume (cm3 g−10.3039 0.2135 0.3086 
Micropores volume (cm3 g−10.2425 0.1807 0.2475 
Mesopores volume (cm3 g−10.0614 0.0328 0.0611 
Micropores volume (%) 79.8 84.6 80.2 
Pore width (Mode) (nm) 0.785 0.534 0.801 
  RAC Exhausted AC Fresh AC 
BET surface area (m2 g−1763.39 463.28 775.21 
Total pore volume (cm3 g−10.3039 0.2135 0.3086 
Micropores volume (cm3 g−10.2425 0.1807 0.2475 
Mesopores volume (cm3 g−10.0614 0.0328 0.0611 
Micropores volume (%) 79.8 84.6 80.2 
Pore width (Mode) (nm) 0.785 0.534 0.801 

CONCLUSION

Superheated water pretreatment combined with traditional CO2 activation can regenerate the CEAC used in the treatment of industrial wastewater. Lower regeneration temperatures reduce energy consumption. The regeneration efficiency and recovery rates are 102.25% and 79.8%, respectively. The repeated regeneration effect is remarkable. The adsorption model of RAC is consistent with the monolayer Langmuir model. The pseudo-second-order kinetics and the intraparticle diffusion model confirm that the RAC strongly absorbs MB. The FT-IR analysis showed that RAC has a wider adsorption peak at the oxygen-containing functional groups than that of CEAC, with more oxygen-containing functional groups and a stronger adsorptive capacity. The RAC mainly comprises micropores and narrow mesopores with a specific surface area of 763.39 m2 g−1.

ACKNOWLEDGEMENT

This work was financially supported by the National Natural Science Foundation of China, Projects (51374253 and 51574289) and Graduate students to explore innovative projects (2017zzts452).

REFERENCES

REFERENCES
Gadipelly
C.
Pérez-González
A.
Yadav
G. D.
Ortiz
I.
Ibáñez
R.
Rathod
V. K.
Marathe
K. V.
2014
Pharmaceutical industry wastewater: review of the technologies for water treatment and reuse
.
Industrial & Engineering Chemistry Research
53
,
11571
11592
.
Khanjari
Y.
Eikani
M. H.
Rowshanzamir
S.
2016
Remediation of polycyclic aromatic hydrocarbons from soil using superheated water extraction
.
The Journal of Supercritical Fluids
111
,
129
134
.
Kulkarni
S. J.
2013
Removal of organic matter from domestic waste water by adsorption
.
International Journal of Science, Engineering and Technology Research (IJSETR)
2
,
1836
1839
.
Reungoat
J.
Escher
B.
Macova
M.
Argaud
F.
Gernjak
W.
Keller
J.
2012
Ozonation and biological activated carbon filtration of wastewater treatment plant effluents
.
Water Research
46
,
863
872
.
Sabio
E.
González
E.
González
J.
González-Garcıa
C.
Ramiro
A.
Ganan
J.
2004
Thermal regeneration of activated carbon saturated with p-nitrophenol
.
Carbon
42
,
2285
2293
.
Salvador
F.
Martin-Sanchez
N.
Sanchez-Montero
M. J.
Montero
J.
Izquierdo
C.
2013
Regeneration of activated carbons contaminated by phenol using supercritical water
.
The Journal of Supercritical Fluids
74
,
1
7
.
Song
W.
Yang
T.
Wang
X.
Sun
Y.
Ai
Y.
Sheng
G.
Hayat
T.
Wang
X.
2016
Experimental and theoretical evidence for competitive interactions of tetracycline and sulfamethazine with reduced graphene oxides
.
Environmental Science: Nano
3
,
1318
1326
.
Stoquart
C.
Servais
P.
Bérubé
P. R.
Barbeau
B.
2012
Hybrid membrane processes using activated carbon treatment for drinking water: a review
.
Journal of Membrane Science
411
,
1
12
.
Thommes
M.
Kaneko
K.
Neimark
A. V.
Olivier
J. P.
Rodriguez-Reinoso
F.
Rouquerol
J.
Sing
K. S.
2015
Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC technical report)
.
Pure and Applied Chemistry
87
,
1051
1069
.
Wang
P.
Wang
X.
Yu
S.
Zou
Y.
Wang
J.
Chen
Z.
Alharbi
N. S.
Alsaedi
A.
Hayat
T.
Chen
Y.
2016
Silica coated fe3o4 magnetic nanospheres for high removal of organic pollutants from wastewater
.
Chemical Engineering Journal
306
,
280
288
.
Yin
C. Y.
Aroua
M. K.
Daud
W. M. A. W.
2007
Review of modifications of activated carbon for enhancing contaminant uptakes from aqueous solutions
.
Separation and Purification Technology
52
,
403
415
.
Yu
S.
Wang
X.
Ai
Y.
Tan
X.
Hayat
T.
Hu
W.
Wang
X.
2016
Experimental and theoretical studies on competitive adsorption of aromatic compounds on reduced graphene oxides
.
Journal of Materials Chemistry A
4
,
5654
5662
.