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Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for the pharmaceutical industry
Journal of Cleaner Production xxx (2014) 1e8
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Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry Qimeng Li a, Yanshan Qi b, Canzhu Gao a, * a b
School of Environmental Science and Engineering, Shandong University, Jinan, Shandong 250100, People's Republic of China Shandong Sanrun Environmental Protection and Technology Co. Ltd, Jinan, Shandong 250100, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2014 Received in revised form 29 July 2014 Accepted 1 August 2014 Available online xxx
The purpose of this work was to regenerate spent powered activated carbon (PAC) which was exhausted in the decolorization of sodium salicylate (NaSA) liquor. In this research, a facile procedure of chemical regeneration was performed and a simple and accurate spectrophotometric method was applied. In order to obtain the optimal operation conditions, influences of following parameters were verified: soaking time, heating temperature, acid concentration and reaction time. PAC was also characterized by thermogravimetric analysis (TGA) to investigate the adsorbateeadsorbent interactions and their dependences on temperature. The optimal conditions for PAC regeneration were as follows: NaOH (1 M) soaking time of 1 h, H2SO4 concentration of 0.31 mol L1 and agitating at 95 C for 1 h. The peaks of derivative thermogravimetric (DTG) pyrolysis profiles of exhausted PAC appeared at low temperatures (~180 C and ~260 C), which demonstrated the adsorption of colored contaminants was a weak physisorption. In pilot plant experiments, it was found that after four consecutive adsorption-regeneration cycles, the adsorption capacity of PAC was maintained at high levels, even higher than virgin PAC. Therefore, this method was very simple, economical and had successfully applied in industrial scale. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Activated carbon Chemical regeneration Spectrophotometry Decolorization Sodium salicylate
1. Introduction Currently, activated carbon is recognized as a universal adsorbent to remove various contaminants from different matrixes (Zhang et al., 2012, 2013, 2014). Due to the advantages of high surface areas, well-developed micropores and fast adsorption kinetics, activated carbon is considered as an excellent and promising material used in gas separation (Gurrath et al., 2000), food pro denas cessing (Mahapatra et al., 2012), emissions control (Lillo-Ro et al., 2006), catalysis (Nabais et al., 2008) and pollutant and odor removal processes (Moreno-Piraj an and Giraldo, 2011). In common practice, when activated carbon reaches its saturation limit, it is simply incinerated or discarded, which gives rise to a secondary source of pollution (Dias et al., 2007; Wan and Wang, 2013). Regeneration of such material is of major importance if this process is considered economically attractive. On the other hand, the factories have been driven by increasingly stringent emission norms to regenerate and reuse saturated activated carbon (Liu et al., 2012; Ning et al., 2011; Sathishkumar et al., 2012).
* Corresponding author. Tel.: þ86 53188365296. E-mail addresses: [email protected], [email protected] (C. Gao).
Color removal is one of the most difficult requirements faced by pharmaceutical factories. Since color is a key parameter, PAC is often used to purify pharmaceuticals and their intermediates (Hernando et al., 2006). Dosage used in decoloring process is as much as 5e20 g/L of PAC/solution mixture. As a preferred adsorbent used for color removal of pharmaceuticals, the widespread utilization of PAC is restricted due to its high cost (Wei et al., 2011). Furthermore, according to the notice of Chinese Ministry of Environmental Protection for pharmaceutical industry pollution control technology (No.18 [2012]), reuse and recycle of spent activated carbon should be given priority. Over the years, a wide variety of methods used for activated carbon regeneration have been reported. Common regeneration techniques in industrial applications are based on thermal (steam, carbon dioxide or inert atmosphere) and chemical methods (pHswing or extraction with solvents). Being the most widely used regeneration technology, thermal regeneration will undergo sequential steps of drying, pyrolysis and gasification in a multiple hearth furnace (Alvarez et al., 2004; Maroto-Valer et al., 2006; Sheintuch and Matatov-Meytal, 1999). However, this process has some drawbacks of high cost, high-energy consumption and significant deterioration of pore structure (Ania et al., 2005a,b). Also, a considerable amount of carbon (5%e15%) is generally lost by
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Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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Q. Li et al. / Journal of Cleaner Production xxx (2014) 1e8
attrition, burn-off and washout. It is also possible to heat the adsorbent electrically, but this approach would cause hot spots in the fixed bed (Ania et al., 2004). Although bio-regeneration may be considered as an economical process, the regeneration is generally slow and requires adsorbed species being totally biodegradable, which is not the case in many pollutants (Purkait et al., 2007; Yaghmaeian et al., 2014). Moreover, bio-regeneration is difficult to be industrialized. For these reasons, chemical regeneration of PAC is attractive and should be revisited. Chemical regeneration is performed through desorption or decomposition of adsorbates using specific chemical reagents (Cazetta et al., 2011). The process of chemical regeneration is performed relatively rapidly with no carbon attrition (Lu et al., 2011). Acid and alkali are traditionally used to dissolve adsorbed substances and restore the adsorption capacity. The regeneration efficiency strongly depends on the solubility of adsorbates in solvents and adsorbateecarbon surface interactions (Karanfil and Dastgheib, 2004). The target purified product (NaSA) in the present study is a valuable intermediate chemical in the fabrication of a common acetylsalicylic acid, which is always used as a wart-removing medicine (Hernando et al., 2006). The goal of this work was to find an economic method for the regeneration of PAC which was exhausted in the decolorization of NaSA mother liquor for pharmaceutical industry. In our research, PAC pre-saturated with industrial NaSA mother liquor was regenerated with NaOH, H2SO4 and various redox agents by batch experiments. To this end, chemical regeneration by alkali-acid method had been successfully applied in industrial scale. Thus, this paper provides a theoretical and practical guidance for recycling waste medicinal PAC. 2. Experimental 2.1. Materials A commercial powered activated carbon (#302) was obtained from Hangmu Timer Industrial Co., Ltd. Activated Carbon Branch, Zhejiang, China. Prior to use, PAC was washed repeatedly by distilled water to remove soluble impurities and minimize the interference of small particles. Afterwards, it was oven-dried at 105 C for 16 h prior to storage in a desiccator. The characteristics of PAC according to the manufactor are presented in Table 1. Sulfuric acid (H2SO4), sodium hydroxide (NaOH), hydrogen peroxide (H2O2, 30 wt%), potassium permanganate (KMnO4), ferrous sulfate (FeSO4) and methylene blue (MB) were purchased from Tianjin Chemical & Reagent Co., China. All chemicals and solvents used in this study were analytical grade. All of the solutions were prepared by deionized water. The NaSA mother liquor supplied by Xinhua Pharmaceutical Co., Ltd (Shandong, China) was selected as the test solution. The NaSA liquor contains plenty of colored impurities and pigments. All adsorption experiments were carried out with raw NaSA liquor
Table 1 Characteristics of the powdered activated carbon. Characteristics
Specifications
Particle size (mm) Specific surface area (m2/g) Total intrusion volume (cm3/g) Methylene blue adsorption (mL/0.1 g) Ash (%) Acid soluble (%) Chloride (%) Heavy metal (%) Moisture content (%) pHpzc
45 1024 1.05 13 2.5 1.5 0.2 0.005 5 6.6
without dilution for industrial applicability. The absorbances of fresh and treated NaSA liquor were analyzed through a UV spectrophotometer (Shimadzu, UV2450). 2.2. Adsorption procedure According to industrial application, 0.1 g PAC is enough for 20 mL NaSA mother liquor in decoloring process. In this study, adsorption experiments were carried out with NaSA mother liquor in a temperature controlled shaker at 30 C for 8 h, with the shaking speed of 120 rpm. When adsorptive equilibrium was reached (the equilibrium state was considered to be attained after 2 h in practical application), the solution was separated from the PAC by filtration prior to analysis. Thereafter, the absorbance of NaSA solution was determined by a UVevis spectrophotometer. In each adsorption process, the procedure was replicated under identical conditions. Blank runs without PAC were conducted simultaneously. For each experimental point, three parallel runs were taken and only the average value was given. 2.3. Regeneration of saturated PAC After reaching the saturation limit, spent PAC was filtered and washed with large amounts of deionized water to eliminate any excess of pollutants. Following this, the PAC was dried in an oven at 105 C for 24 h, and then impregnated PAC was obtained. In none of the cases, and for all PAC samples assayed, did any losses of carbon mass due to careful operation. The exhausted PAC was pretreated with 1 M NaOH solution. Firstly, adding 50 g of exhausted PAC in a 100 mL glass conical flask with 50 mL NaOH solution. Then the PAC/solution mixture was bathed and shaken in a thermal oscillator tank at 30 C for 8 h. The resulting PAC slurry was washed with deionized water until a constant pH was reached and dried in an oven at 105 C for 24 h to remove the moisture. The water loss rate was about 54.5% (Wwet: Wdried ¼ 2.22:1). Afterwards, part of the regenerated PAC was used in adsorption procedure again, as mentioned in section 2.2. The performance of regenerated PAC was evaluated by the absorbance of NaSA solution. The lower the absorbance of NaSA solution, the more colored impurities were absorbed, which meant the regeneration efficiency was better. Note that the notation of EPAC represents the exhausted PAC which was addressed by 1 M NaOH solution. 2.3.1. Oxidation regeneration Batch experiments were carried out with the addition of H2O2 in 50 mL flasks. The experiments were repeated with variations of parameters such as H2O2 dosage (increasing from 0.5 mL to 2.5 mL with deionized water decreasing from 9.5 mL to 7.5 mL) and temperature (75, 85 and 95 C). Additional parameters were as follows: 5 g of dried alkali-treated PAC, 0.5 g of FeSO4$7H2O and 5 mL of 20 wt% H2SO4 solution. Afterwards, these flasks were bathed and shaken in a thermal oscillator tank for 4 h. Then, the PAC/solution mixture was filtrated by a Buchner funnel. Finally, the regenerated PAC was washed with large amounts of deionized water and ovendried at 105 C for 8 h. Oxidation experiments using KMnO4 as the oxidizing agent were performed in accordance with the procedure of H2O2 oxidation regeneration described above. Experimental parameters were as follows: 5 g of dried alkali-treated PAC, 10 mL of 20 wt% H2SO4 solution, 5 mL of deionized water and KMnO4 with different doses (0.5 g and 1 g). Afterwards, these flasks were bathed and shaken in a thermal oscillator tank at 95 C for 4 h. Then, the PAC/solution mixture was filtrated by a Buchner funnel. Finally, the regenerated PAC was washed with large amounts of deionized water and oven-
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
Q. Li et al. / Journal of Cleaner Production xxx (2014) 1e8
dried at 105 C for 8 h. The notations of OPAC-1 and OPAC-2 represent the PAC was oxidized with 0.5 g and 1 g of KMnO4 in 15 mL solution, respectively. Control experiments in the absence of oxidant were conducted in parallel.
2.3.2. Alkali-acid-based chemical regeneration The experiments were repeated with variations in parameters of soaking time (in 1 M NaOH), heating temperature and H2SO4 concentration. Batch experiments were performed by measuring several units of dried alkali-treated PAC of 5 g in some flasks separately. Then pouring in various volumes of 20 wt% H2SO4 (0.5, 1.5 and 2.5 mL) with desired volume of deionized water (14.5, 13.5 and 12.5 mL). Afterwards, the mixtures were shaken at 120 rpm and 95 C for different time intervals. At each predetermined time, part of the PAC/solution mixture was withdrawn from the flasks and filtrated. The remaining procedure was consistent with the procedures as mentioned above.
3
3. Results and discussion The main purpose of using PAC to refine NaSA mother liquor is to remove colored impurities (pigments and colored pollutants) during the purification process. In practice, sodium hydrosulfite (Na2S2O4) is also used along with the PAC in decolorization process as the antioxidant. The Na2S2O4 has a good decolorizing effect on NaSA liquor. Thus, absorbance of NaSA liquor in this study is much higher than that in practical application with the use of Na2S2O4. 3.1. Pretreatment by NaOH solution
2.3.4. Calculation of regeneration efficiency The efficacy of regeneration procedure was evaluated by the adsorption capacity of PAC at equilibrium. Regeneration efficiency (RE) was selected to assess the extent of recovered adsorption capacity of spent PAC. Following Equation (1) was employed to quantify the RE.
Desorption is a phenomenon whereby adsorbed substances were released from or through the surface (Cazetta et al., 2011). NaOH can weaken the Van der Waals force between adsorbate and micropore surface of PAC and undermine the chemical bonds between adsorbate and surface functional groups, thereby colored impurities could be eluted (Berenguer et al., 2010). In our experiments, all exhausted PAC was pretreated with 1 M NaOH solution. Effect of soaking time on the regeneration of PAC was evaluated from 1 h to 12 h. As can be observed in Fig. 1, after being pretreated by 1 M NaOH, part of the adsorption capacity of PAC was restored. In addition, the absorbance of NaSA liquor treated by EPAC from 1 h to 12 h did not differ noticeable, which indicated the restored adsorption capacities were basically the same. Taken together, 1 h was selected as the reasonable soaking time for the follow-up experiments. While treated with NaOH solution, most of organic matters, including pigments, became soluble salts and were easily brought into aqueous phase. Thus, the porous structure of PAC got cleaned and part of adsorption capacity of PAC was restored. In addition, with the increase doses of EPAC, the absorbance of supernate decreased significantly. This being the case, we can also use double doses of EPAC which was treated only by NaOH solution in practical application.
RE% ¼ ðAr =A0 Þ 100%
3.2. H2O2 oxidation regeneration
2.3.3. Thermal analysis The samples of virgin PAC, regenerated PAC and saturated PAC were characterized by thermal analysis. TGA/DSC analyses were carried out using SDT-Q600 V8.3 Build 101 Simultaneous DSC-TGA Instrument. For each measurement, about 25 mg carbon sample was heated under N2 atmosphere (100 mL/min) from 50 C to 800 C, at the rate of 10 C/min.
(1)
where A0 was the absorbance of NaSA liquor treated by original PAC; Ar was the absorbance of NaSA liquor treated by regenerated PAC.
2.3.5. MB adsorption experiments Adsorption experiments with methylene blue (MB) were also conducted in batch mode to evaluate the adsorption uptake of regenerated PAC. For each adsorption test, 0.1 g of PAC and 10 mL of 1.5 g/L MB solution were mixed in a 50 mL flask. The flask was then shaken in a thermostatic shaker at 30 C with shaking speed of 120 rpm for 20 min. Thereafter, the mixture was filtrated by a Buchner funnel. The absorbance of filtrated solution was determined by a UVevis spectrophotometer.
It is reported that H2O2 has strong oxidization ability to open up blocked micropores and remove surface functional groups of PAC (Arslan-Alaton et al., 2008; Georgi and Kopinke, 2005). The reaction between H2O2 and the Fe2þ in the PAC/solution mixture resulted in the formation of highly reactive, nonselective hydroxyl radicals ($OH, E0 ¼ 2.7 V) that could oxidize colored impurities (Kan and
2.4. Pilot plant experiments The equilibrium adsorption capacity was considered to be attained with an adsorption period of 2 h, which was determined according to practical parameters. Regeneration experiments were conducted after each adsorption process determine the loss in adsorptive capacity, and alternatively to know the extent of regeneration. Together, adsorption and regeneration were referred to as one cycle. Repetitive adsorption-regeneration experiments were performed by the identical stock of PAC. The fresh NaSA liquor was renewed for each re-adsorption test. Samples were denoted as PACRi (Ri is the number of regeneration cycle).
Fig. 1. Effect of NaOH solution on the regeneration of exhausted PAC.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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Huling, 2009). Fe2þ can serve as the active species. Thus, H2O2/PAC system may effectively destroy organics and regenerate spent PAC. The effects of temperature and H2O2 concentration on the H2O2 oxidation regeneration of spent PAC were investigated. For the sake of comparison, parallel tests with original PAC were conducted. 3.2.1. Effect of temperature Heating temperature is an important factor for regeneration. From the thermodynamics point of view, since adsorption is an exothermic process, higher temperature is unfavorable for adsorption and thus helps desorption. In our initial experiments, desorption capability at 30 C and 60 C was limited. Thus, we tried to improve desorption efficiency by increasing the temperature. To observe the effect of temperature on the restoration of adsorption capacity, experiments were carried out at three different temperature (75, 85 and 95 C), using 5 g of dried PAC which had been soaked by 1 M NaOH, along with 0.5 g of FeSO4$7H2O, 5 mL of 20 wt% H2SO4, 1.5 mL of 30 wt% H2O2 and 8.5 mL of deionized water. As shown in Fig. 2, with the temperature increasing from 75 C to 95 C, the adsorption capacity kept stable at 75 C and 85 C and increased considerably at 95 C, suggesting that higher temperature contributed to the regeneration of PAC. Therefore, higher temperature can fasten the pervasion rate of regeneration reagent into the micropores so that RE was enhanced. Thus, 95 C was selected as the optimal temperature for subsequent experiments.
Fig. 3. Effect of H2O2 doses on the regeneration of exhausted PAC.
3.3. Chemical regeneration As reported, pH value is an essential parameter in most solid/ liquid adsorption processes (Berenguer et al., 2010). This is because it influences not only the surface charge of carbon but also the ionization state of dissolved materials in PAC/solution mixture. Therefore, traditional acid-alkali regeneration of exhausted PAC was revisited in our study. For the sake of comparison, parallel tests with original PAC were performed.
3.2.2. Effect of H2O2 dosage As mentioned above, H2O2 oxidation system can open up blocked micropores and remove surface functional groups of PAC, thus can restore the adsorption capacity. The experiments were conducted at 95 C, using 5 g of dried PAC which had been soaked by 1 M NaOH, along with 0.5 g of FeSO4$7H2O and 5 mL of 20 wt% H2SO4. The H2O2 doses were varied from 0.5 mL to 2.5 mL, while the total volume of H2O2 and deionized water was maintained at 10 mL. However, as illustrated in Fig. 3, the absorbances were nearly identical as H2O2 doses increased from 0.5 mL to 2.5 mL. This might be due to the minimum dose of H2O2 was also excessive. On the other hand, the adsorption capacity of regenerated PAC was partially restored. On account of the short life-span of $OH which originated mainly near the carbon surface, it would be difficult for $OH to reach deeper adsorption sites. Therefore, oxidation of adsorbed pollutants was constrained.
3.3.1. Effect of H2SO4 concentration After being pretreated by 1 M NaOH, there were still some hardly departed adsorbates left inside of micropores. H2SO4 can act as a reactive reagent to change adsorbate polarity, thus the RE was raised. Experiments were carried out by measuring four units of 5 g of dried PAC which has been soaked by 1 M NaOH solution, and then pouring the PAC into four 50 mL conical flasks, respectively. Adding 1, 2, 3 and 5 mL H2SO4 solution (20 wt%) separately into above flasks, along with desired volume of deionized water to total volume of 15 mL. Afterwards, these flasks were bathed and shaken at 95 C for 4 h. Finally, the filtrated PAC was washed with large amounts of deionized water and oven-dried. Regenerated PAC was then used for adsorption procedure.
Fig. 2. Effect of temperature on the regeneration of exhausted PAC by H2O2 oxidation.
Fig. 4. Effect of 20 wt% H2SO4 doses on the regeneration of exhausted PAC.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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Fig. 4 shows the effect of H2SO4 doses on the regeneration of PAC. Compared with the PAC which was treated only by NaOH solution, further treatment by 20 wt% H2SO4 solution appeared excellent regeneration effect. Thus, 20 wt% H2SO4 solution could effectively remove the adsorbates which adhered hardly to inner micropores. Comparative experiments were also conducted by doubling the amount of PAC. When the dosage of H2SO4 solution (20 wt%) was 2 mL, the best adsorption capacity of regenerated PAC was obtained. Hence, the optimal dosage of H2SO4 solution (20 wt%) was 2 mL, which was equal to the H2SO4 concentration of 0.31 mol L1. 3.3.2. Effect of heating time Since the interphase mass-transfer coefficient is a hydrodynamic property, the regeneration of PAC may be influenced by the transfer rate at fixed temperature. Hence, heating time is an important factor that can affect the RE. The experiments were conducted by measuring 5 g of dried PAC which had been soaked by 1 M NaOH and pouring it into a conical flask, then adding 2 mL of 20 wt% H2SO4 solution and 13 mL of deionized water into the flask. After that, the flask was bathed and shaken in a thermal oscillator tank at 95 C. For different time intervals (1, 3 and 5 h), part of the PAC/solution mixture was withdrawn and filtered. The filtrated PAC was washed with large amounts of deionized water and ovendried. Regenerated PAC was then used for adsorption procedure. Fig. 5 shows the effect of heating time on the regeneration of PAC with time increasing from 1 h to 5 h. As can be observed, variations of absorbance of NaSA solution were negligible. It can be assumed that further increase in heating time did not make sense. Thereby 1 h was enough for PAC regeneration and chosen as the optimal heating time. 3.3.3. DSC analysis As reported in previous studies, the adsorbates adsorbed by PAC were divided into two categories: physically and chemically (Ania et al., 2007, 2005b). Fig. 6 shows the DTG pyrolysis profiles of virgin PAC, regenerated PAC and exhausted PAC. Obviously, pyrolysis profiles of adsorbates show tremendous differences. The high-intensity peaks centered at around 180 C and 260 C can only be assigned to the adsorbed contaminants trapped in the micropores of PAC, since it was absent before adsorption. The peaks in the curve of saturated PAC indicate the desorption behavior proceeds at low temperatures. Therefore, adsorption of colored impurities exhibits a
Fig. 5. Effect of heating time with H2SO4 solution on the regeneration of exhausted PAC.
Fig. 6. DTG curves for virgin PAC, regenerated PAC and exhausted PAC.
physisorption feature, which suggests that the forces involved in this process are of a weak nature. As a result, desorption could occur to a greater extent and proceed readily, while fewer residues were deposited on the internal structure of PAC after regeneration. On the other hand, the wide peak at temperature above 650 C may be due to the decomposition of surface functionalities. 3.3.4. MB adsorption experiments MB was also chosen as the adsorbate to investigate the adsorption properties of regenerated PAC. The experiments were performed according to GB/T12496.10-1999. The adsorption isotherms of MB on the virgin, regenerated and modified activated carbons were demonstrated in Fig. 7. Apparently, the capabilities of MB adsorption of three kind of PAC varied substantially. The modified virgin PAC adsorbed more MB than regenerated PAC, and the latter adsorbed more than virgin PAC. Thus, their adsorption capacity decreased in the order: modified virgin PAC > regenerated PAC > virgin PAC, which demonstrated that chemical treatment had a positive effect on the adsorption capacity of PAC. 3.3.5. Optimal condition for regenerating spent PAC Based on above experiments, confirmation tests were carried out by measuring 5 g of exhausted PAC in a conical flask. Next, 5 mL
Fig. 7. Adsorption of MB by virgin PAC, regenerated PAC and modified PAC.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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Under these conditions, colored pollutants can be easily transferred into liquid phase, thus the adsorptive capacity of PAC was restored. It can be observed in Fig. 8 that the adsorption capacity of PAC could be completely restored with no adverse effects. Thereby, this method can greatly reduce the attaching force between colored impurities and surface of micropores. Consequently, chemical regeneration by alkali-acid treatment can be utilized as an efficient technique to regenerate PAC which was exhausted in NaSA liquor decolorization. 3.4. KMnO4 oxidation regeneration
Fig. 8. Comparison of decolorization efficiency of vrigin PAC and regenerated PAC.
In addition to H2O2, KMnO4 was also used as a redox agent to regenerate exhausted PAC. KMnO4 solution was expected to penetrate internal pores within the PAC and oxidized adsorbed contaminants. As mentioned in section 2.3.1, the notations of OPAC1 and OPAC-2 represent the PAC was oxidized with 0.5 g and 1 g of KMnO4 in 15 mL solution, respectively. As can be seen in Fig. 9, the adsorptive capacity of PAC treated by KMnO4 solution was unsatisfactory. Further increase in KMnO4 dose even did worse in adsorptive capacity. The very low RE indicated that the oxidation reaction had a negative influence on the regeneration of PAC. It was found that the RE of KMnO4 oxidation regeneration was even lower than that of NaOH regeneration. This might be due to the oxidation reaction triggered losses of electrons and produced acidic functional groups, which led to the formation of water clusters. As a consequence, some micropores became blocked and inaccessible for adsorption so that the adsorption capacity was decreased. 3.5. Assessment of regeneration efficacy by pilot plant experiments
Fig. 9. Regeneration of PAC with different KMnO4 doses.
of NaOH solution (1 M) was added into the flask and shaken in a thermal oscillator at 30 C for 1 h. After that, the PAC was filtrated by Buchner funnel. The resulting PAC slurry was washed with deionized water until a constant pH was reached. Then the filtrated PAC was carefully transferred into the flask, and pouring in 2 mL of 20 wt% H2SO4 solution and 13 mL of deionized water. Afterwards, the PAC/solution mixture was shaken at 95 C for 1 h. Finally, the PAC was separated from the solution by filtration. The filtrated PAC was washed with large amounts of deionized water again and oven-dried at 105 C for 8 h.
The alkali-acid method was applied in pilot tests to regenerate the exhausted PAC. The plant tests were conducted at Xinhua Pharmaceutical Co., Ltd. in Shandong Province, China. In summary, chemical regeneration by alkali-acid method had excellent regeneration efficiencies over four successive adsorption and regeneration cycles. The schematic flow chart of regeneration experiments is presented in Fig. 10. Firstly, 500 L of deionized water and 20 kg of NaOH were added into a stainlessesteel reaction cauldron (1 m3). The mixture was stirred for 5 min, and adding in 500 kg of exhausted PAC. After stirring for 1 h, the PAC/solution mixture was filtered through a plate and frame filter press. Next, the filter cake was transferred into the stainlessesteel reaction cauldron again and washed with large amounts of water for several times. The cleaned filter cake was then added into a glass-lined reaction cauldron (1 m3), along with 500 L deionized water and 16 kg of 98% H2SO4. The mixture was stirred for 1 h with temperature maintained at 95 C. Afterwards, the carbon slurry was also filtered and washed for several times. Finally, the regenerated PAC was oven-dried at
Fig. 10. Schematic flow chart of pilot plant experiments.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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confirmed that the adsorbed pollutants were retained by forces with weaker nature and physisorption was the main adsorption mechanism of colored impurities. This technique has advantages of less investment in equipment (minimal cost) and simple operation, which make it possible to industrialize the technics with economic and environmental benefits.
References
Fig. 11. Variation of adsorption uptake with different regeneration cycle (by alkali-acid regeneration).
105 C for 8 h. These conditions were all defined based on optimal experimental conditions at laboratory. Variations of adsorption uptake of regenerated PAC in different regeneration cycles are illustrated in Fig. 11. The number of the curve indicates adsorptioneregeneration cycles undergone by PAC. It appeared that the RE was unexpectedly high, even after four repetitive regeneration cycles. Furthermore, although the porosity of PAC was gradually altered during the successive adsorptionregeneration cycles, the induced changes were less pronounced. The adsorptive capacity of modified virgin PAC was the highest. This might be due to the inner structural changes of PAC. For this reason, the enhancement of RE may not be exclusively attributed to the complete desorption of the adsorbate, but also to an increase in the overall uptake linked to the alterations of adsorbent. For another, the RE of PAC regenerated in the factory seemed higher than that in the laboratory. It was on account of the adsorption period used in the factory was 2 h, while in laboratory was 8 h. As a conclusion, chemical regeneration by alkali-acid method could maintain the adsorptive capacity and extend the life span of PAC. On the other hand, although the damage of granularity of regenerated PAC after each adsorption-regeneration cycle was slight, the crushing strength of PAC should also be considered in the decolorization process. Thus, adsorption-regeneration cycles of PAC were taken for four times. Thereafter, the PAC was completely exhausted and sent to Qingdao New World hazardous waste disposal center for ultimate disposal. Meanwhile, the regeneration method provided in this study, which could achieve considerable economic and environmental benefits, had been successfully applied in an industrial scale. Analysis on economic benefits showed that the cost could be reduced by￥0.88 million Yuan per year. At the same time, spent carbon emissions were reduced by 240 tons per year.
4. Conclusions This study highlighted the effectiveness of chemical regeneration by alkali-acid method to regenerate spent medicinal PAC used in decolorization of NaSA mother liquor. The adsorption capacity of PAC was unexpectedly high during four adsorption-regeneration cycles. Modified virgin PAC was also demonstrated with increased adsorption capacity. Furthermore, this technique could be effectively applied through a simple spectrophotometry method, which is suitable for industrial applications. The DTG evolution profile
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Arslan-Alaton, I., Gursoy, B.H., Schmidt, J.-E., 2008. Advanced oxidation of acid and reactive dyes: effect of Fenton treatment on aerobic, anoxic and anaerobic processes. Dyes Pigments 78, 117e130. s, D., Morallon, E., 2010. Berenguer, R., Marco-Lozar, J., Quijada, C., Cazorla-Amoro Electrochemical regeneration and porosity recovery of phenol-saturated granular activated carbon in an alkaline medium. Carbon 48, 2734e2745. Cazetta, A.L., Vargas, A.M., Nogami, E.M., Kunita, M.H., Guilherme, M.R., Martins, A.C., Silva, T.L., Moraes, J.C., Almeida, V.C., 2011. NaOH-activated carbon of high surface area produced from coconut shell: kinetics and equilibrium studies from the methylene blue adsorption. Chem. Eng. J. 174, 117e125. nchez-Polo, M., 2007. Dias, J.M., Alvim-Ferraz, M., Almeida, M.F., Rivera-Utrilla, J., Sa Waste materials for activated carbon preparation and its use in aqueous-phase treatment: a review. J. Environ. Manage. 85, 833e846. Georgi, A., Kopinke, F.-D., 2005. 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Trichloroethylene adsorption by fibrous and granular activated carbons: aqueous phase, gas phase, and water vapor adsorption studies. Environ. Sci. Technol. 38, 5834e5841. denas, M., Fletcher, A., Thomas, K., Cazorla-Amoro s, D., Linares-Solano, A., Lillo-Ro 2006. Competitive adsorption of a benzeneetoluene mixture on activated carbons at low concentration. Carbon 44, 1455e1463. Liu, Y., Guo, Y., Gao, W., Wang, Z., Ma, Y., Wang, Z., 2012. Simultaneous preparation of silica and activated carbon from rice husk ash. J. Clean. Prod. 32, 204e209. Lu, P.-J., Lin, H.-C., Yu, W.-T., Chern, J.-M., 2011. Chemical regeneration of activated carbon used for dye adsorption. J. Taiwan Inst. Chem. E 42, 305e311. Mahapatra, K., Ramteke, D., Paliwal, L., 2012. Production of activated carbon from sludge of food processing industry under controlled pyrolysis and its application for methylene blue removal. J. Anal. Appl. Pyrol. 95, 79e86. Maroto-Valer, M.M., Dranca, I., Clifford, D., Lupascu, T., Nastas, R., Leon y Leon, C.A., 2006. Thermal regeneration of activated carbons saturated with ortho- and meta-chlorophenols. Thermochim. Acta 444, 148e156. n, J., Giraldo, L., 2011. Activated carbon obtained by pyrolysis of poMoreno-Piraja tato peel for the removal of heavy metal copper (II) from aqueous solutions. J. Anal. Appl. Pyrol. 90, 42e47. Nabais, J.V., Carrott, P., Ribeiro Carrott, M., Luz, V., Ortiz, A.L., 2008. Influence of preparation conditions in the textural and chemical properties of activated carbons from a novel biomass precursor: the coffee endocarp. Bioresour. Technol. 99, 7224e7231. Ning, P., Wang, X., Bart, H.-J., Tian, S., Zhang, Y., Wang, X.-Q., 2011. Removal of phosphorus and sulfur from yellow phosphorus off-gas by metal-modified activated carbon. J. Clean. Prod. 19, 1547e1552. Purkait, M., Maiti, A., DasGupta, S., De, S., 2007. Removal of congo red using activated carbon and its regeneration. J. 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Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry Qimeng Li a, Yanshan Qi b, Canzhu Gao a, * a b
School of Environmental Science and Engineering, Shandong University, Jinan, Shandong 250100, People's Republic of China Shandong Sanrun Environmental Protection and Technology Co. Ltd, Jinan, Shandong 250100, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 June 2014 Received in revised form 29 July 2014 Accepted 1 August 2014 Available online xxx
The purpose of this work was to regenerate spent powered activated carbon (PAC) which was exhausted in the decolorization of sodium salicylate (NaSA) liquor. In this research, a facile procedure of chemical regeneration was performed and a simple and accurate spectrophotometric method was applied. In order to obtain the optimal operation conditions, influences of following parameters were verified: soaking time, heating temperature, acid concentration and reaction time. PAC was also characterized by thermogravimetric analysis (TGA) to investigate the adsorbateeadsorbent interactions and their dependences on temperature. The optimal conditions for PAC regeneration were as follows: NaOH (1 M) soaking time of 1 h, H2SO4 concentration of 0.31 mol L1 and agitating at 95 C for 1 h. The peaks of derivative thermogravimetric (DTG) pyrolysis profiles of exhausted PAC appeared at low temperatures (~180 C and ~260 C), which demonstrated the adsorption of colored contaminants was a weak physisorption. In pilot plant experiments, it was found that after four consecutive adsorption-regeneration cycles, the adsorption capacity of PAC was maintained at high levels, even higher than virgin PAC. Therefore, this method was very simple, economical and had successfully applied in industrial scale. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Activated carbon Chemical regeneration Spectrophotometry Decolorization Sodium salicylate
1. Introduction Currently, activated carbon is recognized as a universal adsorbent to remove various contaminants from different matrixes (Zhang et al., 2012, 2013, 2014). Due to the advantages of high surface areas, well-developed micropores and fast adsorption kinetics, activated carbon is considered as an excellent and promising material used in gas separation (Gurrath et al., 2000), food pro denas cessing (Mahapatra et al., 2012), emissions control (Lillo-Ro et al., 2006), catalysis (Nabais et al., 2008) and pollutant and odor removal processes (Moreno-Piraj an and Giraldo, 2011). In common practice, when activated carbon reaches its saturation limit, it is simply incinerated or discarded, which gives rise to a secondary source of pollution (Dias et al., 2007; Wan and Wang, 2013). Regeneration of such material is of major importance if this process is considered economically attractive. On the other hand, the factories have been driven by increasingly stringent emission norms to regenerate and reuse saturated activated carbon (Liu et al., 2012; Ning et al., 2011; Sathishkumar et al., 2012).
* Corresponding author. Tel.: þ86 53188365296. E-mail addresses: [email protected], [email protected] (C. Gao).
Color removal is one of the most difficult requirements faced by pharmaceutical factories. Since color is a key parameter, PAC is often used to purify pharmaceuticals and their intermediates (Hernando et al., 2006). Dosage used in decoloring process is as much as 5e20 g/L of PAC/solution mixture. As a preferred adsorbent used for color removal of pharmaceuticals, the widespread utilization of PAC is restricted due to its high cost (Wei et al., 2011). Furthermore, according to the notice of Chinese Ministry of Environmental Protection for pharmaceutical industry pollution control technology (No.18 [2012]), reuse and recycle of spent activated carbon should be given priority. Over the years, a wide variety of methods used for activated carbon regeneration have been reported. Common regeneration techniques in industrial applications are based on thermal (steam, carbon dioxide or inert atmosphere) and chemical methods (pHswing or extraction with solvents). Being the most widely used regeneration technology, thermal regeneration will undergo sequential steps of drying, pyrolysis and gasification in a multiple hearth furnace (Alvarez et al., 2004; Maroto-Valer et al., 2006; Sheintuch and Matatov-Meytal, 1999). However, this process has some drawbacks of high cost, high-energy consumption and significant deterioration of pore structure (Ania et al., 2005a,b). Also, a considerable amount of carbon (5%e15%) is generally lost by
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Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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attrition, burn-off and washout. It is also possible to heat the adsorbent electrically, but this approach would cause hot spots in the fixed bed (Ania et al., 2004). Although bio-regeneration may be considered as an economical process, the regeneration is generally slow and requires adsorbed species being totally biodegradable, which is not the case in many pollutants (Purkait et al., 2007; Yaghmaeian et al., 2014). Moreover, bio-regeneration is difficult to be industrialized. For these reasons, chemical regeneration of PAC is attractive and should be revisited. Chemical regeneration is performed through desorption or decomposition of adsorbates using specific chemical reagents (Cazetta et al., 2011). The process of chemical regeneration is performed relatively rapidly with no carbon attrition (Lu et al., 2011). Acid and alkali are traditionally used to dissolve adsorbed substances and restore the adsorption capacity. The regeneration efficiency strongly depends on the solubility of adsorbates in solvents and adsorbateecarbon surface interactions (Karanfil and Dastgheib, 2004). The target purified product (NaSA) in the present study is a valuable intermediate chemical in the fabrication of a common acetylsalicylic acid, which is always used as a wart-removing medicine (Hernando et al., 2006). The goal of this work was to find an economic method for the regeneration of PAC which was exhausted in the decolorization of NaSA mother liquor for pharmaceutical industry. In our research, PAC pre-saturated with industrial NaSA mother liquor was regenerated with NaOH, H2SO4 and various redox agents by batch experiments. To this end, chemical regeneration by alkali-acid method had been successfully applied in industrial scale. Thus, this paper provides a theoretical and practical guidance for recycling waste medicinal PAC. 2. Experimental 2.1. Materials A commercial powered activated carbon (#302) was obtained from Hangmu Timer Industrial Co., Ltd. Activated Carbon Branch, Zhejiang, China. Prior to use, PAC was washed repeatedly by distilled water to remove soluble impurities and minimize the interference of small particles. Afterwards, it was oven-dried at 105 C for 16 h prior to storage in a desiccator. The characteristics of PAC according to the manufactor are presented in Table 1. Sulfuric acid (H2SO4), sodium hydroxide (NaOH), hydrogen peroxide (H2O2, 30 wt%), potassium permanganate (KMnO4), ferrous sulfate (FeSO4) and methylene blue (MB) were purchased from Tianjin Chemical & Reagent Co., China. All chemicals and solvents used in this study were analytical grade. All of the solutions were prepared by deionized water. The NaSA mother liquor supplied by Xinhua Pharmaceutical Co., Ltd (Shandong, China) was selected as the test solution. The NaSA liquor contains plenty of colored impurities and pigments. All adsorption experiments were carried out with raw NaSA liquor
Table 1 Characteristics of the powdered activated carbon. Characteristics
Specifications
Particle size (mm) Specific surface area (m2/g) Total intrusion volume (cm3/g) Methylene blue adsorption (mL/0.1 g) Ash (%) Acid soluble (%) Chloride (%) Heavy metal (%) Moisture content (%) pHpzc
45 1024 1.05 13 2.5 1.5 0.2 0.005 5 6.6
without dilution for industrial applicability. The absorbances of fresh and treated NaSA liquor were analyzed through a UV spectrophotometer (Shimadzu, UV2450). 2.2. Adsorption procedure According to industrial application, 0.1 g PAC is enough for 20 mL NaSA mother liquor in decoloring process. In this study, adsorption experiments were carried out with NaSA mother liquor in a temperature controlled shaker at 30 C for 8 h, with the shaking speed of 120 rpm. When adsorptive equilibrium was reached (the equilibrium state was considered to be attained after 2 h in practical application), the solution was separated from the PAC by filtration prior to analysis. Thereafter, the absorbance of NaSA solution was determined by a UVevis spectrophotometer. In each adsorption process, the procedure was replicated under identical conditions. Blank runs without PAC were conducted simultaneously. For each experimental point, three parallel runs were taken and only the average value was given. 2.3. Regeneration of saturated PAC After reaching the saturation limit, spent PAC was filtered and washed with large amounts of deionized water to eliminate any excess of pollutants. Following this, the PAC was dried in an oven at 105 C for 24 h, and then impregnated PAC was obtained. In none of the cases, and for all PAC samples assayed, did any losses of carbon mass due to careful operation. The exhausted PAC was pretreated with 1 M NaOH solution. Firstly, adding 50 g of exhausted PAC in a 100 mL glass conical flask with 50 mL NaOH solution. Then the PAC/solution mixture was bathed and shaken in a thermal oscillator tank at 30 C for 8 h. The resulting PAC slurry was washed with deionized water until a constant pH was reached and dried in an oven at 105 C for 24 h to remove the moisture. The water loss rate was about 54.5% (Wwet: Wdried ¼ 2.22:1). Afterwards, part of the regenerated PAC was used in adsorption procedure again, as mentioned in section 2.2. The performance of regenerated PAC was evaluated by the absorbance of NaSA solution. The lower the absorbance of NaSA solution, the more colored impurities were absorbed, which meant the regeneration efficiency was better. Note that the notation of EPAC represents the exhausted PAC which was addressed by 1 M NaOH solution. 2.3.1. Oxidation regeneration Batch experiments were carried out with the addition of H2O2 in 50 mL flasks. The experiments were repeated with variations of parameters such as H2O2 dosage (increasing from 0.5 mL to 2.5 mL with deionized water decreasing from 9.5 mL to 7.5 mL) and temperature (75, 85 and 95 C). Additional parameters were as follows: 5 g of dried alkali-treated PAC, 0.5 g of FeSO4$7H2O and 5 mL of 20 wt% H2SO4 solution. Afterwards, these flasks were bathed and shaken in a thermal oscillator tank for 4 h. Then, the PAC/solution mixture was filtrated by a Buchner funnel. Finally, the regenerated PAC was washed with large amounts of deionized water and ovendried at 105 C for 8 h. Oxidation experiments using KMnO4 as the oxidizing agent were performed in accordance with the procedure of H2O2 oxidation regeneration described above. Experimental parameters were as follows: 5 g of dried alkali-treated PAC, 10 mL of 20 wt% H2SO4 solution, 5 mL of deionized water and KMnO4 with different doses (0.5 g and 1 g). Afterwards, these flasks were bathed and shaken in a thermal oscillator tank at 95 C for 4 h. Then, the PAC/solution mixture was filtrated by a Buchner funnel. Finally, the regenerated PAC was washed with large amounts of deionized water and oven-
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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dried at 105 C for 8 h. The notations of OPAC-1 and OPAC-2 represent the PAC was oxidized with 0.5 g and 1 g of KMnO4 in 15 mL solution, respectively. Control experiments in the absence of oxidant were conducted in parallel.
2.3.2. Alkali-acid-based chemical regeneration The experiments were repeated with variations in parameters of soaking time (in 1 M NaOH), heating temperature and H2SO4 concentration. Batch experiments were performed by measuring several units of dried alkali-treated PAC of 5 g in some flasks separately. Then pouring in various volumes of 20 wt% H2SO4 (0.5, 1.5 and 2.5 mL) with desired volume of deionized water (14.5, 13.5 and 12.5 mL). Afterwards, the mixtures were shaken at 120 rpm and 95 C for different time intervals. At each predetermined time, part of the PAC/solution mixture was withdrawn from the flasks and filtrated. The remaining procedure was consistent with the procedures as mentioned above.
3
3. Results and discussion The main purpose of using PAC to refine NaSA mother liquor is to remove colored impurities (pigments and colored pollutants) during the purification process. In practice, sodium hydrosulfite (Na2S2O4) is also used along with the PAC in decolorization process as the antioxidant. The Na2S2O4 has a good decolorizing effect on NaSA liquor. Thus, absorbance of NaSA liquor in this study is much higher than that in practical application with the use of Na2S2O4. 3.1. Pretreatment by NaOH solution
2.3.4. Calculation of regeneration efficiency The efficacy of regeneration procedure was evaluated by the adsorption capacity of PAC at equilibrium. Regeneration efficiency (RE) was selected to assess the extent of recovered adsorption capacity of spent PAC. Following Equation (1) was employed to quantify the RE.
Desorption is a phenomenon whereby adsorbed substances were released from or through the surface (Cazetta et al., 2011). NaOH can weaken the Van der Waals force between adsorbate and micropore surface of PAC and undermine the chemical bonds between adsorbate and surface functional groups, thereby colored impurities could be eluted (Berenguer et al., 2010). In our experiments, all exhausted PAC was pretreated with 1 M NaOH solution. Effect of soaking time on the regeneration of PAC was evaluated from 1 h to 12 h. As can be observed in Fig. 1, after being pretreated by 1 M NaOH, part of the adsorption capacity of PAC was restored. In addition, the absorbance of NaSA liquor treated by EPAC from 1 h to 12 h did not differ noticeable, which indicated the restored adsorption capacities were basically the same. Taken together, 1 h was selected as the reasonable soaking time for the follow-up experiments. While treated with NaOH solution, most of organic matters, including pigments, became soluble salts and were easily brought into aqueous phase. Thus, the porous structure of PAC got cleaned and part of adsorption capacity of PAC was restored. In addition, with the increase doses of EPAC, the absorbance of supernate decreased significantly. This being the case, we can also use double doses of EPAC which was treated only by NaOH solution in practical application.
RE% ¼ ðAr =A0 Þ 100%
3.2. H2O2 oxidation regeneration
2.3.3. Thermal analysis The samples of virgin PAC, regenerated PAC and saturated PAC were characterized by thermal analysis. TGA/DSC analyses were carried out using SDT-Q600 V8.3 Build 101 Simultaneous DSC-TGA Instrument. For each measurement, about 25 mg carbon sample was heated under N2 atmosphere (100 mL/min) from 50 C to 800 C, at the rate of 10 C/min.
(1)
where A0 was the absorbance of NaSA liquor treated by original PAC; Ar was the absorbance of NaSA liquor treated by regenerated PAC.
2.3.5. MB adsorption experiments Adsorption experiments with methylene blue (MB) were also conducted in batch mode to evaluate the adsorption uptake of regenerated PAC. For each adsorption test, 0.1 g of PAC and 10 mL of 1.5 g/L MB solution were mixed in a 50 mL flask. The flask was then shaken in a thermostatic shaker at 30 C with shaking speed of 120 rpm for 20 min. Thereafter, the mixture was filtrated by a Buchner funnel. The absorbance of filtrated solution was determined by a UVevis spectrophotometer.
It is reported that H2O2 has strong oxidization ability to open up blocked micropores and remove surface functional groups of PAC (Arslan-Alaton et al., 2008; Georgi and Kopinke, 2005). The reaction between H2O2 and the Fe2þ in the PAC/solution mixture resulted in the formation of highly reactive, nonselective hydroxyl radicals ($OH, E0 ¼ 2.7 V) that could oxidize colored impurities (Kan and
2.4. Pilot plant experiments The equilibrium adsorption capacity was considered to be attained with an adsorption period of 2 h, which was determined according to practical parameters. Regeneration experiments were conducted after each adsorption process determine the loss in adsorptive capacity, and alternatively to know the extent of regeneration. Together, adsorption and regeneration were referred to as one cycle. Repetitive adsorption-regeneration experiments were performed by the identical stock of PAC. The fresh NaSA liquor was renewed for each re-adsorption test. Samples were denoted as PACRi (Ri is the number of regeneration cycle).
Fig. 1. Effect of NaOH solution on the regeneration of exhausted PAC.
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Huling, 2009). Fe2þ can serve as the active species. Thus, H2O2/PAC system may effectively destroy organics and regenerate spent PAC. The effects of temperature and H2O2 concentration on the H2O2 oxidation regeneration of spent PAC were investigated. For the sake of comparison, parallel tests with original PAC were conducted. 3.2.1. Effect of temperature Heating temperature is an important factor for regeneration. From the thermodynamics point of view, since adsorption is an exothermic process, higher temperature is unfavorable for adsorption and thus helps desorption. In our initial experiments, desorption capability at 30 C and 60 C was limited. Thus, we tried to improve desorption efficiency by increasing the temperature. To observe the effect of temperature on the restoration of adsorption capacity, experiments were carried out at three different temperature (75, 85 and 95 C), using 5 g of dried PAC which had been soaked by 1 M NaOH, along with 0.5 g of FeSO4$7H2O, 5 mL of 20 wt% H2SO4, 1.5 mL of 30 wt% H2O2 and 8.5 mL of deionized water. As shown in Fig. 2, with the temperature increasing from 75 C to 95 C, the adsorption capacity kept stable at 75 C and 85 C and increased considerably at 95 C, suggesting that higher temperature contributed to the regeneration of PAC. Therefore, higher temperature can fasten the pervasion rate of regeneration reagent into the micropores so that RE was enhanced. Thus, 95 C was selected as the optimal temperature for subsequent experiments.
Fig. 3. Effect of H2O2 doses on the regeneration of exhausted PAC.
3.3. Chemical regeneration As reported, pH value is an essential parameter in most solid/ liquid adsorption processes (Berenguer et al., 2010). This is because it influences not only the surface charge of carbon but also the ionization state of dissolved materials in PAC/solution mixture. Therefore, traditional acid-alkali regeneration of exhausted PAC was revisited in our study. For the sake of comparison, parallel tests with original PAC were performed.
3.2.2. Effect of H2O2 dosage As mentioned above, H2O2 oxidation system can open up blocked micropores and remove surface functional groups of PAC, thus can restore the adsorption capacity. The experiments were conducted at 95 C, using 5 g of dried PAC which had been soaked by 1 M NaOH, along with 0.5 g of FeSO4$7H2O and 5 mL of 20 wt% H2SO4. The H2O2 doses were varied from 0.5 mL to 2.5 mL, while the total volume of H2O2 and deionized water was maintained at 10 mL. However, as illustrated in Fig. 3, the absorbances were nearly identical as H2O2 doses increased from 0.5 mL to 2.5 mL. This might be due to the minimum dose of H2O2 was also excessive. On the other hand, the adsorption capacity of regenerated PAC was partially restored. On account of the short life-span of $OH which originated mainly near the carbon surface, it would be difficult for $OH to reach deeper adsorption sites. Therefore, oxidation of adsorbed pollutants was constrained.
3.3.1. Effect of H2SO4 concentration After being pretreated by 1 M NaOH, there were still some hardly departed adsorbates left inside of micropores. H2SO4 can act as a reactive reagent to change adsorbate polarity, thus the RE was raised. Experiments were carried out by measuring four units of 5 g of dried PAC which has been soaked by 1 M NaOH solution, and then pouring the PAC into four 50 mL conical flasks, respectively. Adding 1, 2, 3 and 5 mL H2SO4 solution (20 wt%) separately into above flasks, along with desired volume of deionized water to total volume of 15 mL. Afterwards, these flasks were bathed and shaken at 95 C for 4 h. Finally, the filtrated PAC was washed with large amounts of deionized water and oven-dried. Regenerated PAC was then used for adsorption procedure.
Fig. 2. Effect of temperature on the regeneration of exhausted PAC by H2O2 oxidation.
Fig. 4. Effect of 20 wt% H2SO4 doses on the regeneration of exhausted PAC.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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5
Fig. 4 shows the effect of H2SO4 doses on the regeneration of PAC. Compared with the PAC which was treated only by NaOH solution, further treatment by 20 wt% H2SO4 solution appeared excellent regeneration effect. Thus, 20 wt% H2SO4 solution could effectively remove the adsorbates which adhered hardly to inner micropores. Comparative experiments were also conducted by doubling the amount of PAC. When the dosage of H2SO4 solution (20 wt%) was 2 mL, the best adsorption capacity of regenerated PAC was obtained. Hence, the optimal dosage of H2SO4 solution (20 wt%) was 2 mL, which was equal to the H2SO4 concentration of 0.31 mol L1. 3.3.2. Effect of heating time Since the interphase mass-transfer coefficient is a hydrodynamic property, the regeneration of PAC may be influenced by the transfer rate at fixed temperature. Hence, heating time is an important factor that can affect the RE. The experiments were conducted by measuring 5 g of dried PAC which had been soaked by 1 M NaOH and pouring it into a conical flask, then adding 2 mL of 20 wt% H2SO4 solution and 13 mL of deionized water into the flask. After that, the flask was bathed and shaken in a thermal oscillator tank at 95 C. For different time intervals (1, 3 and 5 h), part of the PAC/solution mixture was withdrawn and filtered. The filtrated PAC was washed with large amounts of deionized water and ovendried. Regenerated PAC was then used for adsorption procedure. Fig. 5 shows the effect of heating time on the regeneration of PAC with time increasing from 1 h to 5 h. As can be observed, variations of absorbance of NaSA solution were negligible. It can be assumed that further increase in heating time did not make sense. Thereby 1 h was enough for PAC regeneration and chosen as the optimal heating time. 3.3.3. DSC analysis As reported in previous studies, the adsorbates adsorbed by PAC were divided into two categories: physically and chemically (Ania et al., 2007, 2005b). Fig. 6 shows the DTG pyrolysis profiles of virgin PAC, regenerated PAC and exhausted PAC. Obviously, pyrolysis profiles of adsorbates show tremendous differences. The high-intensity peaks centered at around 180 C and 260 C can only be assigned to the adsorbed contaminants trapped in the micropores of PAC, since it was absent before adsorption. The peaks in the curve of saturated PAC indicate the desorption behavior proceeds at low temperatures. Therefore, adsorption of colored impurities exhibits a
Fig. 5. Effect of heating time with H2SO4 solution on the regeneration of exhausted PAC.
Fig. 6. DTG curves for virgin PAC, regenerated PAC and exhausted PAC.
physisorption feature, which suggests that the forces involved in this process are of a weak nature. As a result, desorption could occur to a greater extent and proceed readily, while fewer residues were deposited on the internal structure of PAC after regeneration. On the other hand, the wide peak at temperature above 650 C may be due to the decomposition of surface functionalities. 3.3.4. MB adsorption experiments MB was also chosen as the adsorbate to investigate the adsorption properties of regenerated PAC. The experiments were performed according to GB/T12496.10-1999. The adsorption isotherms of MB on the virgin, regenerated and modified activated carbons were demonstrated in Fig. 7. Apparently, the capabilities of MB adsorption of three kind of PAC varied substantially. The modified virgin PAC adsorbed more MB than regenerated PAC, and the latter adsorbed more than virgin PAC. Thus, their adsorption capacity decreased in the order: modified virgin PAC > regenerated PAC > virgin PAC, which demonstrated that chemical treatment had a positive effect on the adsorption capacity of PAC. 3.3.5. Optimal condition for regenerating spent PAC Based on above experiments, confirmation tests were carried out by measuring 5 g of exhausted PAC in a conical flask. Next, 5 mL
Fig. 7. Adsorption of MB by virgin PAC, regenerated PAC and modified PAC.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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Under these conditions, colored pollutants can be easily transferred into liquid phase, thus the adsorptive capacity of PAC was restored. It can be observed in Fig. 8 that the adsorption capacity of PAC could be completely restored with no adverse effects. Thereby, this method can greatly reduce the attaching force between colored impurities and surface of micropores. Consequently, chemical regeneration by alkali-acid treatment can be utilized as an efficient technique to regenerate PAC which was exhausted in NaSA liquor decolorization. 3.4. KMnO4 oxidation regeneration
Fig. 8. Comparison of decolorization efficiency of vrigin PAC and regenerated PAC.
In addition to H2O2, KMnO4 was also used as a redox agent to regenerate exhausted PAC. KMnO4 solution was expected to penetrate internal pores within the PAC and oxidized adsorbed contaminants. As mentioned in section 2.3.1, the notations of OPAC1 and OPAC-2 represent the PAC was oxidized with 0.5 g and 1 g of KMnO4 in 15 mL solution, respectively. As can be seen in Fig. 9, the adsorptive capacity of PAC treated by KMnO4 solution was unsatisfactory. Further increase in KMnO4 dose even did worse in adsorptive capacity. The very low RE indicated that the oxidation reaction had a negative influence on the regeneration of PAC. It was found that the RE of KMnO4 oxidation regeneration was even lower than that of NaOH regeneration. This might be due to the oxidation reaction triggered losses of electrons and produced acidic functional groups, which led to the formation of water clusters. As a consequence, some micropores became blocked and inaccessible for adsorption so that the adsorption capacity was decreased. 3.5. Assessment of regeneration efficacy by pilot plant experiments
Fig. 9. Regeneration of PAC with different KMnO4 doses.
of NaOH solution (1 M) was added into the flask and shaken in a thermal oscillator at 30 C for 1 h. After that, the PAC was filtrated by Buchner funnel. The resulting PAC slurry was washed with deionized water until a constant pH was reached. Then the filtrated PAC was carefully transferred into the flask, and pouring in 2 mL of 20 wt% H2SO4 solution and 13 mL of deionized water. Afterwards, the PAC/solution mixture was shaken at 95 C for 1 h. Finally, the PAC was separated from the solution by filtration. The filtrated PAC was washed with large amounts of deionized water again and oven-dried at 105 C for 8 h.
The alkali-acid method was applied in pilot tests to regenerate the exhausted PAC. The plant tests were conducted at Xinhua Pharmaceutical Co., Ltd. in Shandong Province, China. In summary, chemical regeneration by alkali-acid method had excellent regeneration efficiencies over four successive adsorption and regeneration cycles. The schematic flow chart of regeneration experiments is presented in Fig. 10. Firstly, 500 L of deionized water and 20 kg of NaOH were added into a stainlessesteel reaction cauldron (1 m3). The mixture was stirred for 5 min, and adding in 500 kg of exhausted PAC. After stirring for 1 h, the PAC/solution mixture was filtered through a plate and frame filter press. Next, the filter cake was transferred into the stainlessesteel reaction cauldron again and washed with large amounts of water for several times. The cleaned filter cake was then added into a glass-lined reaction cauldron (1 m3), along with 500 L deionized water and 16 kg of 98% H2SO4. The mixture was stirred for 1 h with temperature maintained at 95 C. Afterwards, the carbon slurry was also filtered and washed for several times. Finally, the regenerated PAC was oven-dried at
Fig. 10. Schematic flow chart of pilot plant experiments.
Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008
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confirmed that the adsorbed pollutants were retained by forces with weaker nature and physisorption was the main adsorption mechanism of colored impurities. This technique has advantages of less investment in equipment (minimal cost) and simple operation, which make it possible to industrialize the technics with economic and environmental benefits.
References
Fig. 11. Variation of adsorption uptake with different regeneration cycle (by alkali-acid regeneration).
105 C for 8 h. These conditions were all defined based on optimal experimental conditions at laboratory. Variations of adsorption uptake of regenerated PAC in different regeneration cycles are illustrated in Fig. 11. The number of the curve indicates adsorptioneregeneration cycles undergone by PAC. It appeared that the RE was unexpectedly high, even after four repetitive regeneration cycles. Furthermore, although the porosity of PAC was gradually altered during the successive adsorptionregeneration cycles, the induced changes were less pronounced. The adsorptive capacity of modified virgin PAC was the highest. This might be due to the inner structural changes of PAC. For this reason, the enhancement of RE may not be exclusively attributed to the complete desorption of the adsorbate, but also to an increase in the overall uptake linked to the alterations of adsorbent. For another, the RE of PAC regenerated in the factory seemed higher than that in the laboratory. It was on account of the adsorption period used in the factory was 2 h, while in laboratory was 8 h. As a conclusion, chemical regeneration by alkali-acid method could maintain the adsorptive capacity and extend the life span of PAC. On the other hand, although the damage of granularity of regenerated PAC after each adsorption-regeneration cycle was slight, the crushing strength of PAC should also be considered in the decolorization process. Thus, adsorption-regeneration cycles of PAC were taken for four times. Thereafter, the PAC was completely exhausted and sent to Qingdao New World hazardous waste disposal center for ultimate disposal. Meanwhile, the regeneration method provided in this study, which could achieve considerable economic and environmental benefits, had been successfully applied in an industrial scale. Analysis on economic benefits showed that the cost could be reduced by￥0.88 million Yuan per year. At the same time, spent carbon emissions were reduced by 240 tons per year.
4. Conclusions This study highlighted the effectiveness of chemical regeneration by alkali-acid method to regenerate spent medicinal PAC used in decolorization of NaSA mother liquor. The adsorption capacity of PAC was unexpectedly high during four adsorption-regeneration cycles. Modified virgin PAC was also demonstrated with increased adsorption capacity. Furthermore, this technique could be effectively applied through a simple spectrophotometry method, which is suitable for industrial applications. The DTG evolution profile
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Please cite this article in press as: Li, Q., et al., Chemical regeneration of spent powdered activated carbon used in decolorization of sodium salicylate for pharmaceutical industry, Journal of Cleaner Production (2014), http://dx.doi.org/10.1016/j.jclepro.2014.08.008