Microwave regeneration of phenol-loaded activated carbons obtained from Arundo donax and waste fiberboard

Abstract

A study was performed for the microwave regeneration of Arundo donax activated carbon (ADAC) and waste fiberboard activated carbon (WFAC) loaded with phenol. The regeneration of spent activated carbon (AC) under microwave irradiation was investigated. The optimum conditions of regeneration were microwave power 800 W, microwave irradiation time 7 min for 0.1 g AC. When phenol solution was treated under the optimal conditions, two activated carbons (ACs) behaved differently during ten successive adsorption–regeneration cycles. The phenol adsorption for regenerated WFAC was not as significantly reduced after nine cycles of adsorption–regeneration in comparison to regenerated ADAC. The structural properties of fresh and regenerated ADAC and WFAC were characterized by scanning electronic microscopy (SEM), Raman spectra and N₂ adsorption isotherms. After one adsorption–regeneration cycle, the surface area of the regenerated ADAC and WFAC were 1554 and 1833 m² g⁻¹, respectively, corresponding to 91.47% of original ADAC (1688 m² g⁻¹) and 95.07% of original WFAC (1928 m² g⁻¹), respectively. After ten adsorption–regeneration cycles, the surface area of regenerated ADAC and WFAC were equivalent to 34.74% of fresh ADAC and 47.80% of the fresh WFAC, respectively.

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1. Introduction

Nowadays phenol wastewater pollution has received significantly increased attention. Phenol in the environment primarily comes from the commercial production of a wide variety of resins such as epoxy resins and adhesives, phenolic resins being used as construction materials, as well as from the production of polyamides for various applications.¹ Untreated wastewater containing phenol is harmful to organisms since phenol is typically toxic and resistant to biological degradation.^1,2 Thus, it is important to remove the phenol pollutant from wastewater before discharge.

The treatment methods of wastewater containing phenol include chemical oxidation, biological methods, ion exchange and adsorption.^2,3 Of these methods adsorption is most practical and versatile. Activated carbon (AC) is deemed to an excellent adsorbent due to it being efficient, promising and a well-established technology in wastewater treatment.^4–6 It is of high interest for research institutions to use activated carbon as a means of removing phenol from wastewater. AC contains a large number of pores and a large specific surface,^7,8 so it can effectively remove coloration, bad smell and most organic pollutants in secondary effluents. However, the exhausted carbon is usually discarded, buried or burned, which causes a waste of resources and environmental pollution.⁹ Hence, the regeneration of spent AC has important significance.

Conventional regeneration techniques in industrial applications include chemical, biological and thermal methods. These methods are time and energy consuming so resulting in higher cost.^6,10–12 Specifically, thermal regeneration is time and energy consuming,⁸ and also prone to cause hot spots in the fixed bed.⁶ Chemical regeneration requires specific extracting agents to elute the adsorbed compounds or oxidizing chemical agents to decompose the adsorbed compounds. However, the risk of ignition caused from such agents should be considered before adopting this technique. In addition, an extra step for recycling the extraction agent is usually involved after the desorption process, resulting in higher investment cost.⁶ Biological regeneration has been proposed as an economical process, but this technique requires long time reaction and can only be applied to biodegradable substances.^6,10,12 Among various kinds of regeneration methods, microwave irradiation has very broad application prospects due to its heating at a molecular level and the quick thermal reaction.^11,13,14 Gu et al.¹⁵ used a facile high temperature (750–800 °C) microwave assisted method to fabricate core–shell carbon nanostructures from polyaniline (PANI)–magnetite (Fe₃O₄) nanocomposites, which had potential applications in environmental remediation application, especially in the highly efficient removal of heavy metals from wastewater; Zhu et al.¹⁶ had demonstrated that magnetic carbon nanocomposite fabrics prepared by microwave assisted heating had higher removal capacity of Cr(VI) from polluted water, which was attributed to the highly porous structure of the nanocomposites making the removal process faster. Although based on thermal principles to develop the AC regeneration technology, the microwave irradiation regeneration method is different from conventional thermal regeneration and can be regarded as superior to conventional regeneration techniques.¹⁰ The principle is to use AC adsorption of polar material molecules in the microwave field which induces dipole turning-direction polarization, converting electromagnetic energy into heat energy, by which water and organic material are heated and vaporised.¹¹ The regeneration technology is more efficient, energy saving, time saving and allows precise temperature control during intermittent use.^13,17–22 In addition, microwave regeneration has little effect on the pore texture of activated carbon on removal of phenol.²²

AC can be obtained from many raw materials such as durian shells, jackfruit peel, orange peel, mangosteen peel and so on. However, their surface areas are reduced obviously after five regeneration cycles.^6,14 It is important to find better raw materials maintaining their adsorption properties during regeneration. In the present work, microwave regeneration of Arundo donax and waste fiberboard activated carbons loaded with phenol were studied. The effects of microwave power and irradiation time on the regeneration were investigated, and the characterization of activated carbons was performed by scanning electronic microscopy, Raman spectra and N₂ adsorption isotherms.

2. Materials and methods

The Arundo donax (AD) and waste fiberboard (WF) activated carbons were kindly provided by Beijing Forestry University wood factory and Beijing Jiahekailai Furniture and Design Company, respectively. WF was obtained in the furniture manufacturing process and contained 10% ureaformaldehyde resin adhesive by mass. Other chemicals were analytical grade and were purchased from Beijing Lanyi Chemical reagent.

2.1. Adsorbate

Phenol, a pollutant which is hard to degrade in the natural environment was chosen as the adsorbate in the present study. A standard stock solution of 5000 mg L⁻¹ was prepared by dissolving 2.5 g phenol in 500 mL of double distilled water. Working solutions were prepared by diluting the phenol stock solution to a concentration of 1000 mg L⁻¹.

2.2. Adsorbent

The preparation process of AC was described in our previous work.²³ Briefly, the preparation process was divided into carbonization and activation. In the carbonization step, AD and WF were heated to a final temperature of 500 °C, maintained for 60 min. The AD and WF carbonized samples were then ground and screened to the desired size of 40–60 meshes. In the activation process, the oven-dried samples were mixed with KOH solution with an impregnation ratio (KOH : char) of 3 : 1 (wt%). The samples were then activated at 850 °C for 50 min in a nitrogen atmosphere. After cooling, these activated carbons (ACs) were washed with 0.05 M HCl solution and deionized water until the pH of the solution reached neutral.

2.3. Adsorption equilibrium studies

Arundo donax activated carbon (ADAC) and waste fiberboard activated carbon (WFAC) adsorption studies were carried out in a 250 mL Erlenmeyer flask containing 0.1 g adsorbent and 25 mL of phenol solution with a concentration of 1000 mg L⁻¹. The flasks were agitated in a constant-temperature shaker at 30 °C and shaking speed of 115 rpm for 100 min. After adsorption, the phenol of the supernatant solution was measured and the reacted AC was filtered off and dried at 100 °C for 4 h before regeneration. Phenol uptake at equilibrium, q_e (mg g⁻¹), was calculated by:

q_e = (C₀ − C_e)V/W (1)

where C₀ and C_e (mg L⁻¹) are the liquid-phase concentrations of phenol initially and at equilibrium, respectively, V (L) is the volume of the solution and W (g) is the amount of adsorbent in use.

2.4. Microwave regeneration procedure and experimental design

A microwave tube furnace (MKG-M2HB, MAKEWAVE) with constant power output was employed for the AC regeneration. Microwave regeneration was carried out at different irradiation times and power, aiming to obtain the optimal conditions. For the effect of microwave power on the regeneration performance experiment, the irradiation time was 7 min and the microwave power was set at 500, 600, 700, 800 or 900 W, respectively. For the effect of irradiation time on the regeneration performance experiment, the microwave power was 800 W and irradiation time was set for 2, 3, 5, 7 or 9 min, respectively. In each experiment, the mass of AC was 0.1 g. Regenerated AC (0.1 g) was used to adsorb 25 mL of phenol solution with a concentration of 1000 mg L⁻¹. Under the optimal conditions, ten successive adsorption–regeneration cycles were studied.

2.5. Characterization of activated carbons

The effects on the pore texture of AC were analyzed by scanning electronic microscopy (SEM), Raman spectra and N₂ adsorption–desorption isotherms. SEM investigations were carried out on S-3400N (Hitachi). Raman spectra were recorded with LABRAM HR EVOLUTION (HORIBA) at 473 nm with a charge-coupled device detector. The surface physical properties of AC were characterized by nitrogen adsorption at −196 °C with an accelerated surface area in a porosimetry system (ASAP 2010, Micromeritics). Before the measurements, all samples were outgassed in vacuum at 573 K for at least 2 h. The Brunauer–Emmett–Teller (BET) surface area (S_BET) was calculated by the BET equation. The total pore volume (V_tot) was evaluated by the N₂ adsorption isotherm at a relative pressure of 0.995. The micropore volume and micropore surface area were obtained from the t-plot method. Pore size distribution in the micropore range was determined by the Barrett–Joyner–Halenda (BJH) method.

3. Results and discussion

3.1. Effects of regeneration on microwave power and irradiation time

The saturated ADAC and WFAC were regenerated by setting different microwave powers and irradiation times, respectively. The adsorption performances of the regenerated AC are shown in Fig. 1 and 2. Fig. 1 illustrates that the adsorption performance improves with time. However, only a slight rise is observed from 7–9 min. Fig. 2 demonstrates that the adsorption performance increases with power at 500–800 W but a slight fall is observed at 900 W, which might due to that the higher temperature could destroy the pore structure. Considering the energy saving, we chose microwave power and irradiation times of 800 W and 7 min, respectively, for 0.1 g AC as the optimum regeneration conditions. The results show that the microwave regeneration method is more time-saving than thermal and biological methods, more economic than chemical and thermal methods, and has greater flexibility than bio-regeneration that can be only applied to the biodegradable substances.^6,10,14

Fig. 1 Regeneration of ADAC and WFAC at different irradiation times. Error bars represent standard deviations.

Fig. 2 Regeneration of ADAC and WFAC at different microwave power. Error bars represent standard deviations.

3.2. Variation of adsorption uptake with the regeneration cycles

The adsorption capacity of fresh ADAC and WFAC were 229 and 231 mg g⁻¹, respectively. Fig. 3 illustrates the variation of adsorption uptake over ten adsorption–regeneration cycles. The phenol adsorption for WFAC was reduced in successive adsorption–regeneration cycles but to a lesser degree than for ADAC. After ten cycles of adsorption–regeneration, ADAC and WFAC adsorption uptakes were 94 and 117 mg g⁻¹, respectively, corresponding to 41.01% of fresh ADAC and 50.65% of fresh WFAC, respectively. Toh et al.²⁴ regenerated AC loaded phenol for over 8 h by bio-regeneration and the regeneration efficiency was less than 80% after one cycle of adsorption–regeneration. You et al.²⁵ used electrochemical method to regenerate AC adsorbed EDTA and the electrolysis time was 60 min. Román et al.²⁶ regenerated AC loaded p-nitrophenol and the regeneration efficiency was less than 79% after one cycle of adsorption–regeneration. He et al.²⁷ used yeast and a chemical method to regenerate AC loaded MB and the regeneration efficiency was over 100%, but this method required yeast initially to treat saturated AC for 8 h. Clearly microwave regeneration is efficient in terms of regeneration time and efficiency and regenerated WFAC has better adsorption capacity than regenerated ADAC.

Fig. 3 The variation of adsorption uptake as a function of regeneration cycles for (a) ADAC and (b) WFAC. Error bars represent standard deviations.

It can be shown that the regeneration degrees for two ACs were different, probably attributed to the difference of chemical composition, dielectric characteristics, physio-chemical properties, initial adsorptive capacities and the fraction of microwave energy absorbed by the carbon samples.²⁸ Especially, due to WF being bonded with urea formaldehyde resin adhesives, the surface of WFAC contained nitrogen groups such as pyridinic nitrogen, pyrrolic nitrogen, quaternary nitrogen, oxidized nitrogen, making WFAC more alkaline and increasing the adsorption of phenol.^29,30

3.3. Proposed mechanisms

The mechanisms of microwave regeneration involve the following. Due to microwave heating being internal and volumetric, the interior of the AC granules can be heated directly and the temperature gradient decreases from the center to the surface. AC adsorption for phenols is widely assumed to be both physical and chemical. The physisorption of phenols on AC includes dispersion interactions, hydrophobic interactions and hydrogen bonding, mainly through dispersion interactions between the π-electrons of the aromatic ring and those of the carbon basal planes.^31,32 The interactions with the carbon surface are weak, so physically adsorbed compounds would be likely to leave the carbon surface without undergoing oxidation. Moreno-Castilla³¹ demonstrated that chemisorbed phenolic compounds binding through electrostatic interactions were more strongly bound than those of dispersion interactions. The chemically adsorbed compounds would likely be oxidized by ambient oxygen, and phenols with good thermal stability produce CO or CO₂ at high temperature.³³ Phenol could be removed as much as possible with increasing microwave power and irradiation time, though the use of too high a temperature possibly destroying the pore structure should be considered. After microwave regeneration, acidic groups are reduced in number and alkaline groups increase.¹⁰

The mechanisms of decline in adsorption uptake with increasing regeneration cycles include pore blockage resulting from carbonaceous residue by the decomposition of adsorbate molecules, the annealing effect in the carbonaceous skeleton when the saturated ACs were exposed to microwave radiation and carbon loss during the microwave regeneration operations.²²

3.4. Textural characterization of ACs

The SEM images of different AC samples are shown in Fig. 4. As can be observed, original ADAC and WFAC showed surfaces with a large amount of pores (Fig. 4a and d). After the regeneration process, the pores became rougher and a small number of pores were damaged (Fig. 4b, c, e and f). We can see the pores were similar after ten cycles of adsorption–regeneration compared with that after one cycle of adsorption–regeneration. This shows that microwave regeneration can greatly maintain the pore shape.

Fig. 4 SEM images of different AC samples for (a) ADAC-original, (b) ADAC-one cycle, (c) ADAC-ten cycles, (d) WFAC-original, (e) WFAC-one cycle and (f) WFAC-ten cycles.

Raman spectroscopy further demonstrated whether the structure varied after regenerating ADAC and WFAC, comparing with the original ADAC and WFAC, respectively. Two conspicuous peaks are presented in Fig. 5, where the “D-band” and “G-band” are centered at ∼1350 and ∼1590 cm⁻¹. The “G-band” is ascribed to “E_2g2 C=C stretching vibrations” in the graphite crystallites, and the “D-band” is attributed to turbostratic or disordered carbonaceous structures. The relative intensity ratio of the D to G bands (I_D/I_G) shows the degree of disorder or defects in the carbon structure.^34,35 From Fig. 5, the I_D/I_G of ADAC slightly increased from 0.90 (original) to 0.91 (ten cycles), and I_D/I_G of WFAC decreased from 0.91 (original) to 0.86 (ten cycles). The results mean that the disorder or defects of regenerated ADAC are basically unchanged, while that of regenerated WFAC decreased. The phenomenon might contribute to the observation that regenerated WFAC can maintain greater adsorption capacity than regenerated ADAC.

Fig. 5 Raman spectra of ADAC and WFAC.

Nitrogen adsorption–desorption procedure provides qualitative information on the porosity and adsorption mechanisms of the carbonaceous materials.³⁶ The analysis of nitrogen adsorption (Fig. 6) shows that the adsorbed volume increased significantly at lower relative pressures indicating the micropores were present. Under the IUPAC classification, the isotherm shows a combination of type I and type II isotherms. Fig. 6 shows an apparent hysteresis hoop (H4 type) in the desorption branch at relative pressures above 0.8, which implied the presence of mesopores.³⁷ Thus, this adsorption behavior indicates that the two ACs were a combination of microporous–mesoporous structures. The nitrogen isotherms after one regeneration cycle were similar to the initial activated carbons. After ten successive regeneration cycles, the shape of the nitrogen isotherms moves gradually downward, as compared to the original ACs. The phenomenon is probably due to the pores of AC being damaged or blocked with increasing regeneration cycles.²²

Fig. 6 N₂ adsorption isotherms at 77 K for (a) ADAC and (b) WFAC.

Pore size distribution was evaluated using the BJH method as illustrated in Fig. 7. It is known that the pores of adsorbents are grouped into micropores (d < 2 nm), mesopores (d = 2–50 nm) and macropores (d > 50 nm).³⁸ From Fig. 7, it can be seen that the sharpest peak occurs at above 1 nm of pore diameters, indicating that a majority of the pores fall into the range of micropores. Meanwhile, Fig. 7 shows that there were a small amount of mesopores corresponding to the analysis results of the N₂ adsorption–desorption isotherms. Whether comparing original, one regeneration cycle or ten regeneration cycles of ACs, the nitrogen isotherms of WFAC were higher than those of ADAC, probably due to that WFAC contained nitrogen groups increasing the adsorption capacity as described previously, and the pore structure of ADAC could be more likely to be damaged resulting in micropores of ADAC probably more likely transforming into mesopores compared to WFAC with increasing regeneration cycles.

Fig. 7 Pore size distribution for (a) ADAC and (b) WFAC.

Table 1 summarizes the key surface physical parameters obtained from the N₂ adsorption isotherms. As can be seen in Table 1, all textural parameters decrease after ten regeneration cycles. Other than the specific surface area and the micropore surface area, textural parameters increase slightly after one regeneration cycle. After one adsorption–regeneration cycle, S_BET of the regenerated ADAC and WFAC were 1554 and 1833 m² g⁻¹, corresponding to 91.47% of original ADAC (1688 m² g⁻¹) and 95.07% of original WFAC (1928 m² g⁻¹), respectively. After ten adsorption–regeneration cycles, the S_BET of regenerated ADAC was 586.4 m² g⁻¹ and WFAC was 921.6 m² g⁻¹, equivalent to 34.74% of fresh ADAC and 47.80% of the fresh WFAC, respectively. In comparison, the S_BET of regenerated ACs obtained from durian shells, jackfruit peel, orange peel, mangosteen peel and oil palm shells were 621.51, 567.75, 543.89, 485.89 and 563.14 m² g⁻¹, respectively, after five regeneration cycles.^6,14,37 The results show that ADAC and WFAC are good adsorbents, and WFAC is better than ADAC.

Table 1 Key surface physical parameters obtained from the N₂ adsorption isotherms

Sample	SBET (m^2 g^−1)	Smi (m^2 g^−1)	Vtot (cm^3 g^−1)	Vtot (cm^3 g^−1)	Vme (cm^3 g^−1)	WP (nm)
Fresh (ADAC)	1688	1431	1.023	0.735	0.616	2.423
One cycle (ADAC)	1554	1185	1.051	0.653	0.727	2.705
Ten cycles (ADAC)	586.4	401.4	0.419	0.220	0.331	2.860
Fresh (WFAC)	1928	1738	1.094	0.886	0.609	2.270
One cycle (WFAC)	1833	1651	1.128	0.920	0.655	2.461
Ten cycles (WFAC)	921.6	864.0	0.483	0.408	0.298	2.098

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4. Conclusion

This present study highlighted the potential of microwave techniques for regeneration of phenol-loaded activated carbons obtained from Arundo donax and waste fiberboard. When phenol solution was treated under the optimal conditions of microwave power of 800 W and irradiation time of 7 min, WFAC could maintain greater adsorption capacity than ADAC during ten successive adsorption–regeneration cycles. After ten cycles of adsorption–regeneration, regenerated ADAC and WFAC adsorption uptakes were 41.01% of fresh ADAC and 50.65% of fresh WFAC, and the S_BET of regenerated ADAC was 586.4 m² g⁻¹ and WFAC was 921.6 m² g⁻¹, equivalent to 34.74% of fresh ADAC and 47.80% of the fresh WFAC, respectively. Microwave irradiation is more efficient to regenerate ACs than conventional regeneration methods, and WFAC is a better adsorbent than ADAC.

Acknowledgements

This study was funded by state Forestry Administration through project 201204807: the study on the technology and mechanism of the activated carbon electrode preparation from the waste hard board.

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