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Regeneration of PAC used for reverse osmosis concentrate treatment by wet oxidation
Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
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Regeneration of PAC used for reverse osmosis concentrate treatment by wet oxidation Yanlin Yuan, Ping Gu, Yanling Yang, Guanghui Zhang * School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
Article history: Received 24 March 2015 Received in revised form 29 October 2015 Accepted 31 October 2015 Available online 5 November 2015 Keywords: Reverse osmosis concentrate Powdered activated carbon Regeneration Wet oxidation Adsorption capacity
A B S T R A C T
The wet oxidation (WO) regeneration of powdered activated carbon (PAC) used for reverse osmosis concentrate (ROC) treatment was studied. The exhausted PAC was regenerated at different temperatures and times. The experimental results indicated that the optimum regeneration condition could be achieved at a temperature of 200 8C for 60 min. The mass of carbon in the solid, gas and liquid phases before and after the PAC regeneration was balanced in different regeneration conditions. A molecular weight (MW) distribution of organic matter (OM) in the effluent treated by the PAC regenerated at optimum conditions was performed. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction With the development of the reverse osmosis (RO) membrane, RO technology has found an increasingly wide utilization in water reclamation. While approximately 75% of the influent can be reclaimed, a RO concentrate (ROC) containing all of the constituents rejected by the RO membrane is also generated [1–4]. To reduce the effect of ROC discharge on the environment and/or make effective use of the ROC, the treatment of the ROC from municipal and industrial wastewater treatment plants has become a hot topic in the water industry [5–7]. Different treatment methods for ROC have been intensively studied, such as oxidation and adsorption processes [6,8–16]. Among these technologies, activated carbon (AC) adsorption is a well-established technology that is supported by high depuration efficiency for organic matter (OM) and the possibility of regeneration [17]. Compared to granular activated carbon, powdered activated carbon (PAC) has better thermodynamic and kinetic characteristics [16,18,19]. The OM adsorbed onto AC is only transferred to the solid phase and still need to be properly disposed, or otherwise they still remain a threat to environmental safety. Waste AC is therefore considered to be a hazardous material, and disposal of waste AC could be very expensive. In addition, the cost-effectiveness of using
* Corresponding author. Tel.: +86 22 27405059; fax: +86 22 27405059. E-mail address: [email protected] (G. Zhang).
AC in an adsorption process depends not only on its adsorption capacity but also on the possibility of reusing it repeatedly. Therefore, the research and development of appropriate regeneration methods are needed [20]. Various regeneration processes have been proposed, such as thermal regeneration by means of steam, inert gas and hot water [21,22], solvent extraction [23], supercritical fluid regeneration [24], microwave regeneration [25–30], electrochemical regeneration [31] and wet oxidation (WO) regeneration [32]. Of these regeneration methods, WO uses milder conditions and can achieve good regeneration results. In addition, the WO process has received much attention because it can not only reuse the AC but also destroy most of the adsorbed OM or convert it to biodegradable materials that remain in the liquid phase in small amounts, thereby reducing the environmental impact [33–35]. The purpose of this study was to investigate the regeneration of the PAC used in ROC treatment. To our best knowledge, there are few reports on regeneration PAC adsorbing complex OM from ROC by means of WO currently. The re-adsorption capacity of PAC under different regeneration conditions was evaluated, and the changes in the surface porosity were discussed, with the aim of finding the optimum regeneration conditions. Successive regeneration cycles were also conducted and analysed. To analyse the OM transfer and changes after the PAC regeneration, the mass of carbon in the solid, gas and liquid phases was balanced after the regeneration, the method could be used to evaluate regeneration conditions. In the end, the MW distributions of OM in the raw ROC
http://dx.doi.org/10.1016/j.jiec.2015.10.043 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
and the ROC treated by PAC regenerated under optimum conditions were investigated and compared to that of fresh PAC. In practice, the waste superheated steam could be used to regenerate the exhausted PAC in the factory because the regeneration doesn’t need high temperature. Experimental Materials The ROC was taken from an ultrafiltration membrane bioreactor-RO process in a refinery wastewater treatment plant which was the same as that reported by Wei et al. [19]. The characteristics of the ROC are presented in Table 1. PAC (200 mesh) supplied by Luda Co. Ltd. in Zunhua, China was used for this study. The PAC was baked for 2 h at 105 8C to remove moisture prior to use. The detailed characteristics of the PAC were described by Zhao et al. [16]. The exhausted PAC was prepared by adding 5.00 g fresh PAC and 10.0 L ROC into a cylinder container and then stirring (200 rpm, 25 8C) for 30 min. The PAC was filtered out by a 0.45 mm membrane filter (mixed cellulose ester, Mili, China), and the filtrate was analysed for DOC. Experimental procedure Regeneration of exhausted PAC The regeneration tests were carried out in an autoclave made of stainless steel 316 L with a total volume of 2.0 L (Dalian TONG CHAN high-pressure reactors manufacture Co. Ltd., China). The reactor mainly comprised a gas inlet, an electromagnetic stirrer and a cooling water inlet. A schematic of the autoclave is shown in Fig. 1. The experimental procedures for the regeneration of PAC were as follows. (1) 5.00 g of exhausted PAC and 300 mL of distilled water were put into the autoclave. (2) Oxygen was sparged into the autoclave with the gas port open for a while to remove air. (3) The gas port was closed, and the oxygen gas supply was continued until a pressure of 0.1 MPa was reached. The quantity of the oxygen was calculated roughly to be approximately 0.07 mol. And the amount
99
Table 1 Characteristics of the ROC. Parameter
Unit
Value (average value)
Chemical oxygen demand (COD) Dissolved organic carbon (DOC) pH Conductivity TDS Total hardness (as CaCO3) Cl SO42 NO3
mg/L mg/L – mS/cm mg/L mg/L mg/L mg/L mg/L
82.63–100.1 (91.36) 22.55–32.43 (27.49) 8.36 3.21 1640 710 676.8 61.38 722.0
of oxygen in the autoclave was two times more than the amount of organics that was in the ROC sample treated by the PAC and measured by COD. (4) The slurry was heated to a set temperature and maintained at different times for regeneration. Before opening the cover, the gas sample in the autoclave was taken for analysis, and the temperature and pressure were recorded. (5) The contents in the reactor were then cooled down to room temperature. The carbon slurry was filtered, and the filtrate was analysed for DOC. The carbon was washed with distilled water and dried in an oven at 105 8C for analysis. The exhausted PAC was regenerated at the three temperatures of 150, 200 and 250 8C for the three durations of 30, 60, and 90 min. A process of adsorption and regeneration was termed as a regeneration cycle. Successive regeneration cycles were studied and performed at the optimum regeneration conditions, and four regeneration cycles were carried out. To study the effect of the WO process on the fresh PAC under optimum conditions, it was regenerated and expressed as C-Blank. The fresh PAC and the exhausted PAC were expressed as C-Fresh and C-Exhausted, respectively. To express the experimental conditions concisely, a test of PAC regeneration at a temperature for certain time was written as ‘‘temperature-time’’. For instance, 150-30 corresponds to exhausted PAC that was regenerated at 150 8C for 30 min. PAC adsorption isotherm PAC adsorption experiments were carried out by separately adding ROC (100 mL) and pre-weighed PAC into 250-mL conical flasks that were placed on a shaker (200 rpm, 25 8C) for 30 min. The
Fig. 1. Experimental setup for the regeneration of PAC. (1) Oxygen bottle, (2) pressure valve, (3) cooling water inlet, (4) sample port, (5) electromagnetic stirrer, (6) cooling water outlet, (7) thermocouple, (8) gas port, (9) liquid port, (10) cooling coil, (11) electrical heated wire, (12) insulation layer and (13) temperature and stirring controllers.
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Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
PAC was then filtered out using a 0.45 mm membrane filter (mixed cellulose ester, Mili, China), and the filtrate was analysed for DOC. The experiments were carried out in duplicate, and the average values were reported.
filtrates were collected for DOC analysis. The mass balance principle was then used to calculate the DOC concentration within various fractions. Results and discussion
Analytical methods The adsorption isotherms of the regenerated PAC The porosity of the PAC was determined by measuring the N2 adsorption isotherms at 196 8C in an automatic apparatus (NOVA2200e, Quantachrome Instruments, USA). The isotherms were used to calculate the specific surface area (SBET), total pore volume (VT), and pore size distribution using the density functional theory method [36]. The DOC of the filtrate was determined using a TOC analyser (TOC-VCPH, Shimadzu, Japan). The gas in the reactor after regeneration was analysed by gas chromatography (GC 6890N, Agilent, USA). The column pressure was set at 0.1 MPa, and the carrier gases used were nitrogen and hydrogen. The FID was heated at 250 8C. Air, hydrogen and nitrogen flows for the FID were 400, 30 and 25 mL/min, respectively. The molecular weight (MW) distribution of the OM in the raw and treated ROC samples were analysed by the UF filtration method[37] using a pre-washed UF membrane with MW cut-offs (MWCO) of 1, 3, 10 and 30 kDa (regenerated cellulose with a diameter of 15 cm, Amicon1, Millipore Corporation, USA). A UF cup (Shanghai Institute of Nuclear Research of the Chinese Academy of Sciences, China) was used, and nitrogen gas from a pressurized bottle provided the driving force for filtration with a constant pressure of 0.1 MPa. Prior to the MW distribution measurement, the raw ROC sample was filtered through a 0.45 mm membrane (mixed cellulose ester, Mili, China) to remove large particles. A raw ROC sample with a volume of 300 mL was sequentially fractionated into several sizes from the largest size to the smallest, and the
The Freundlich and Langmuir adsorption models are commonly used to describe the adsorption isotherm. Our previous research found that the PAC adsorption used in the ROC treatment more closely followed the Freundlich adsorption model [16], which is expressed in the following equation: qe ¼ kF C e 1=n
(1)
where Ce is the equilibrium concentration of the adsorbate, mg/L; qe is the amount of adsorbate on the PAC at equilibrium, mg/g; and kF and n are constants. The experimental results show that the determination coefficients of the Freundlich adsorption isotherms for fresh PAC and all regenerated PACs were more than 0.96, so this adsorption model was used in the research. The effect of regeneration temperature The regeneration temperature and duration both had a significant effect on the efficiency of the WO regeneration [34]. Therefore, the effect of temperature on the PAC regeneration was initially studied with a regeneration time of 30 min, and the regeneration temperatures were set to 150, 200 and 250 8C. Fig. 2a is the adsorption isotherm of PAC regenerated under different temperatures. It can be seen from the figure that with the increase in the regeneration temperature, the adsorption amount
Fig. 2. The adsorption isotherms of DOC for regenerated PAC. (a) Effect of the regeneration temperature. (b) Effect of the regeneration time. (c) Effect of successive regeneration cycles.
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104 Table 2 Adsorption characteristics of fresh PAC and regenerated PAC. Sample
Ce (mg/L)
qe(mg/g)
Relative adsorption amount
C-Fresh 150-30 200-30 250-30 200-60 200-90
11.1 17.8 16.3 16.2 15.1 15.6
42.6 23.4 26.9 26.8 27.7 27.8
1.00 0.549 0.631 0.629 0.650 0.653
into the regenerated PAC initially increased and then decreased slightly under the same equilibrium concentration of adsorbate when the regeneration temperature was set to 150, 200 and 250 8C. The experimental conditions illustrate that the optimal regeneration temperature is between 200 8C and 250 8C. According to the adsorption isotherm of the fresh PAC and those in Fig. 2a and b, when the dosage of PAC was 0.5 g/L, the adsorption characteristics of the fresh and regenerated PAC could be calculated by Eq. (1), and the results are shown in Table 2. It could be observed from the table that the relative adsorption amount into the regenerated PAC at 150 8C was 0.451 lower than that of the fresh PAC, suggesting that regeneration at a lower temperature was not advantageous to recovering the adsorption capacity of the PAC. In contrast, when the regeneration temperature was increased to 200 8C and 250 8C, the relative adsorption amount increased and reached 0.629–0.653, and the values under the same regeneration time were almost the same. The higher temperature has both its pros and cons, as on the one hand it favoured the oxidation of OM on the PAC, but excessive temperatures could destroy the structure of the PAC, as discussed in Section 3.2. Therefore, the optimum regeneration result would be achieved when the regeneration temperature was 200 8C. The effect of regeneration time The effect of regeneration time on the PAC regeneration efficiency was investigated after the optimum regeneration temperature was determined. The experiments were conducted when the regeneration temperature was 200 8C, and the regeneration time was 30, 60 and 90 min. The adsorption isotherms of the PAC regenerated for different times are shown in Fig. 2b. As seen from Fig. 2b, the adsorption amount of the PAC regenerated for 30 min was low, but it increased when the regeneration time was 60 and 90 min, with a very small difference between the two times. Therefore, the longer regeneration time could efficiently not promote the recovery of PAC, so the regeneration time of 60 min was the optimum.
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Comparing Fig. 2b with Fig. 2a, it could be observed that the regeneration time had less influence on PAC regeneration than the temperature. With the increase of temperature, the adsorption amount of the regenerated PAC increased at the same equilibrium concentration, but when the regeneration temperature reached a certain value, the increased temperature did not further improve the recovery of PAC but rather damaged the PAC, leading to a lower adsorption capacity. Therefore, the optimum condition for the regeneration of the PAC in this study was a regeneration temperature of 200 8C and a regeneration time of 60 min. Successive adsorption-regeneration cycles at optimum regeneration conditions The effect of the regeneration cycle on PAC adsorption under the optimal conditions was also studied. The adsorption isotherms are shown in Fig. 2c. From the figure, it can be observed that as the number of regeneration cycles increased, the adsorption capacity of the PAC decreased. When the equilibrium concentration Ce was set at 10.00 mg/L, the adsorption amount qe decreased from 20.75 mg/g after the first regeneration cycle to 18.46 mg/g after the second regeneration cycle, eventually falling to 10.67 mg/g after the fourth regeneration cycle. The reason could be because the OM adsorbed on the PAC was not completely removed. When the adsorption regeneration cycle continued, this matter blocked the pores, as discussed in the next section, resulting in a gradual reduction of the adsorption capacity. This conclusion was previously suggested by others [20]. Analysis of PAC surface properties To study the reasons for incomplete regeneration, an analysis of the PAC surface characteristics was carried out. Fig. 3 is the N2 adsorption isotherm of different PACs. The regeneration of the blank carbon was carried out at a temperature of 200 8C for 60 min. As seen from the type of curves in Fig. 3a, the fresh PAC and regenerated PAC are of type I [38]. The adsorption and desorption lines at relatively high pressures do not appear to overlap, which suggests that an increase in the mesoporosity occurred [25,29]. In Fig. 3a, it could be clearly observed that the volume of exhausted PAC decreased sharply, which indicates that OM blocked the PAC pores. However, the volumes of blank carbon and fresh PAC almost coincided. These results differ from the conclusion reported by Shende et al., which is probably due to their use of higher temperature and oxygen pressure [33]. From Fig. 3b, it could be observed that the volume of the regenerated PAC
Fig. 3. N2 adsorption isotherms of different PAC. (a) C-Fresh, C-Exhausted, C-Blank and the PAC regenerated under optimum conditions. (b) Regenerated PAC at different successive regeneration cycles.
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
102 Table 3 Textural characteristics of the series PACs.
following equation:
Sample
SBET (m2/g)
Vtotala (cm3/g)
Vmicrob (cm3/g)
Vmesob (cm3/g)
C-Fresh C-Blank C-Exhausted 200-30 200-60 200-90 200-60 cycle 200-60 cycle 200-60 cycle 200-60 cycle
850.8 841.1 556.8 733.3 750.3 746.0 752.2 749.3 726.6 721.5
0.425 0.416 0.260 0.360 0.380 0.367 0.375 0.366 0.347 0.326
0.218 0.204 0.106 0.173 0.190 0.178 0.186 0.177 0.159 0.146
0.151 0.152 0.104 0.132 0.137 0.135 0.135 0.134 0.131 0.124
a b
1 2 3 4
Evaluated at a relative pressure of 0.99. Evaluated from DFT applied to N2 at 196 8C.
declined gradually with an increasing number of regeneration cycles. Table 3 shows the textural characteristics calculated from the N2 adsorption isotherms of the different PACs. As seen from the table, the specific surface area, total pore volume and micropore volume of blank carbon were slightly less than those of the fresh PAC. However, the mesopore volume of blank carbon was slightly larger than that of fresh PAC. For the fresh PAC having adsorbed the OM in the ROC, i.e., C-Exhausted, its specific surface area and total, micropore and mesopore volumes reduced to approximately 65%, 61%, 48% and 68%, respectively, of their corresponding fresh PAC values. When the regeneration temperature was 200 8C and the regeneration times were 30, 60 and 90 min, the specific surface area of the regenerated PAC first increased and the slightly decreased with an increase of regeneration time, which suggests that the specific surface area of PAC recovery increased with the increased regeneration time. However, when the regeneration time was too long, it could destroy the structure of the PAC [17,21,25,29,39], resulting in a decreased micropore volume, indicating that increasing the regeneration time was not beneficial to the regeneration of the PAC. In addition, with an increase in the number of regeneration cycles, the pore volume size for the PAC decreased gradually, and after the fourth adsorption-regeneration cycle, the micropore volume decreased to 78.5% of its first adsorption. This result may be due to the pore structure clogging and deposition of PAC, as previously reported by Ania et al. [25]. Mass balance analysis of carbon in the solid, liquid and gas phases at different regenerated conditions There are several steps in the WO process, including the desorption of the adsorbate from the AC into water and the oxidation of the adsorbate in water by dissolved oxygen [35]. The OM on PAC should follow the law of mass conservation in the WO process. To analyse the OM transfer and changes after the regeneration, the mass of carbon in solid, gas and liquid phases after PAC regeneration was balanced. In the adsorption process, some of OM in the ROC was transferred to the surface of the PAC, and this was the target source of carbon in the solid phase that could be calculated by the
C s ¼ ðDOC0 DOC1 ÞV 1
(2)
where Cs is the mass of organic carbon adsorbed from the ROC, mg; DOC0 is the DOC value of the ROC before adsorption, mg/L; DOC1 is the DOC value of the ROC after adsorption, mg/L; and V1 is the total volume of ROC treated, L. In this case, as described in Section 2.1, V1 = 10.0 L. During the regeneration process, carbon in both the gaseous and liquid phases could be from the OM that was desorbed from the PAC and/or oxidized, and it could also be from the PAC itself if the oxidation conditions were too extensive. Some of the OM was oxidized completely to form carbon dioxide, and some was oxidized partially to form carbon monoxide and other intermediate products. All oxidation products could exist in both the gaseous and liquid phases. The results of gas analysis are listed in Table 4. The analysis shows that the main components in the gas were carbon dioxide and carbon monoxide, with small amounts of H2, CH4, C2H4, and C2H2. Because the main components in the gaseous phase were carbon monoxide and carbon dioxide, the total mass of carbon in the gaseous phase only included them, and can be calculated by Eq. (3), in which the ideal gas equation was used. 2 X PiV Cg ¼ 12 RT i¼1
(3)
where Cg is the mass of carbon in the gas after regeneration, mg; Pi is the gas partial pressure for component i when the sample was taken, Pa; V is the gas volume above the liquid in the autoclave, m3; R is the universal gas constant; T is the gas temperature when the sample was taken, K; and i denotes carbon monoxide or carbon dioxide in the gas. Because the gas sample was taken under a certain pressure, some gas was dissolved in the regeneration liquid. As this part of the dissolved gas would escape in the following experiment, it should be calculated independently. The mass of carbon originating from the gas and dissolved in the liquid when the gas sample was taken is calculated by Eq. (4), in which Henry’s law was used. 2 X nwPi Cgl ¼ 12 Ki ðT Þ i¼1
(4)
where Cgl is the mass of carbon after regeneration dissolved in the regeneration liquid, mg; Pi is the partial pressure of gas component i above the liquid when the sample was taken, Ki (T) is the Henry’s constant for component i when the temperature is T [40]; nW is the number of moles of the liquid, mol; and i represents carbon monoxide or carbon dioxide in the gas. The mass of carbon in the liquid phase, i.e., the liquid in the autoclave, is calculated by the following equation: Cl ¼ C3V 2
(5)
where Cl is the mass of carbon in the liquid after regeneration, mg; C3 is the DOC value in the liquid, mg/L; and V2 is the liquid volume, L. In this case, V2 = 0.300 L. Table 5 lists the mass of carbon in the different phases before and after the PAC regeneration. The table shows that with an
Table 4 Gas analytical results and sampling condition. Sample
H2 (mL/L)
CO (mL/L)
CO2 (mL/L)
CH4 (mL/L)
C2H4 (mL/L)
C2H2 (mL/L)
Pressure (MPa)
Temperature (K)
150-30 200-30 250-30
19.00 164.2 518.5
1.49 4.28 11.99
19.84 69.24 150.4
10.23 10.14 10.05
0.52 0.43 0.00
0.21 1.08 3.44
0.20 0.20 0.16
308 343 343
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104 Table 5 Mass of carbon in the solid, gas and liquid phases before and after PAC regeneration. Sample
Cs (mg)
Cg (mg)
Cgl (mg)
Cl (mg)
Ct = Cg + Cgl + Cl (mg)
150-30 200-30 250-30
197 199 202
33.9 105 185
4.16 7.69 13.3
35.6 35.5 31.6
73.7 148 230
Table 6 Mass of carbon in the gas and liquid phases for blank carbon at different WO conditions. Temperature(8C)
Cg (mg)
Cgl (mg)
Cl (mg)
Ct = Cg + Cgl + Cl (mg)
150 200 250
7.49 33.6 41.3
0.28 0.90 0.72
0.31 0.81 1.21
8.08 35.3 43.2
increase in the regeneration temperature, the mass of carbon in the gas phase, i.e., Cg, significantly increased, and that dissolved in the liquid, i.e., Cgl, also increased. However, the mass of carbon remaining in the liquid, i.e., Cl, decreased slightly, which could be due to the OM in the liquid being increasingly oxidized and transferred into the gas with increased temperature. When the regeneration temperature increased from 150 8C to 200 8C, the total mass of carbon in the gas and liquid, i.e., Ct, gradually reached that in the solid phase, but the latter was larger than the former. However, when the regeneration temperature was 250 8C, the total mass of carbon in the gas and liquid was slightly larger than that in the solid phase, which was contrary to the mass balance. The result might be attributed to carbonation of the PAC at high temperature, which would produce more carbon oxide and other carbonaceous materials. To analyse the effect of the WO process on fresh PAC, it was also regenerated at different temperatures. Table 6 shows the mass of carbon in blank carbon after the WO process was performed for 30 min at different temperatures. From the table, it can be observed that the mass of carbon in the forms of Cg, Cgl and Cl gradually increased with temperature, but far less than that in the case of the exhausted PAC regeneration. The result suggests that under the experimental conditions, the loss of blank carbon was small, and this was confirmed by the previous blank carbon performance test (e.g., the specific surface area, micropore, and mesopore). Molecular weight (MW) analysis of the fresh and regenerated PAC Table 7 presents the MW distribution of the DOC in the raw ROC. It was found that the original OM of the ROC mainly occurred
103
Table 7 MW distribution in the raw ROC. Fraction (kDa)
<1 1–3 3–10 10–30 >30
DOC (mg/L)
(%)
24.2 0.31 0.40 0.90 0.85
90.8 1.16 1.50 3.38 3.19
in the MW range of <1 kDa. This portion accounted for 90.8%, which suggests that the pollutants in the raw ROC were mainly small organic molecules. Bagastyo et al. also found that the largest fraction of DOC in ROCs from both coastal and inland water reclamation plants was <1 kDa molecules [9]. The total and fractional DOC removal from the ROC by fresh PAC and the PAC regenerated under optimum conditions at different dosages are plotted on Fig. 4. Fig. 4a shows that with the increase of the dosage, the DOC removal rate of fresh PAC increased from approximately 64% to 78%, while that of the regenerated PAC only increased from approximately 50% to 65%. The results illustrate that the PAC was not fully restored to its original adsorption performance after regeneration, which is in agreement with the conclusion of Section 3.1. Fig. 4b shows that the PAC regenerated under optimum condition and fresh PAC behaved similar, with both primarily removing OM with MW of <1 kDa. This might be due to the original OM of the ROC mainly consisting of low MW molecule. The functional parts of the PAC are the micropores and mesopores, which have sizes close to that of the OM. The results of Karanfil et al. also show that better efficiency was obtained when the dimensions of the adsorbates were close to those of the PAC pores [41]. Our previous work also found that PAC adsorption could remove all fractions of the OM in the ROC but with different efficiencies [18]. The dosage of fresh PAC had little effect on OM with MW < 1 kDa, as the removal rate only increased from 93.1% to 93.9% at dosages of 0.5 and 0.9 g/L, respectively. One can deduce that the adsorbed OM was already close to saturation. However, the removal of small DOC molecules from the regenerated PAC increased as the PAC dosage increased from 89.3% to 93.3%. This explains that the adsorption capacity of the regenerated PAC to small molecular OM was lower than that of the fresh PAC at low dosages, but the adsorption ability of the regenerated PAC increased as the dosage increased. Therefore, the dosage of the regenerated PAC can be adjusted to meet the effluent requirements in practical applications, although this needs further research.
Fig. 4. The total (a) and fractional (b) DOC removal rate from the ROC by fresh and regenerated PAC with different dosages.
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Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
Conclusions (1) The PAC used for ROC treatment can be regenerated using the WO process. The regeneration conditions are mild, and the regeneration results are acceptable. (2) The optimum regeneration conditions for the PAC are a temperature of 200 8C and a duration of 60 min. The relative adsorption amount of the regenerated PAC under optimum conditions could reach 0.65, but when the number of regeneration cycles increases, the adsorption capacity gradually decreased. (3) The mass balance principle was first used to evaluate the regeneration conditions. In different regeneration conditions, the main components of the gas were carbon monoxide and carbon dioxide. As the regeneration temperature increases, the oxidation products content also increased. The principle could also be used to evaluate the weight loss after regeneration by WO process. (4) The PAC regenerated under optimum conditions and the fresh PAC behaved similarly, with both primarily removing OM with MW of <1 kDa. OM in other MW ranges were more difficult to remove in this experiment. Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (50908158) and the research fund for the Doctoral Program of Higher Education of China (20090032120040). References [1] C.H. Koo, A.W. Mohammad, F. Suja`, Desalination 271 (2011) 178. [2] X.F. Huang, J. Ling, J.C. Xu, Y. Feng, G.M. Li, Desalination 269 (2011) 41. [3] A.P. Echavarrı´a, V. Falguera, C. Torras, C. Berdu´n, J. Paga´n, A. Ibarz, Food Sci. Technol.—Brazil 46 (2012) 189. [4] S. Shanmuganathan, T.V. Nguyen, W.G. Shim, J. Kandasmy, A. Listowski, S. Vigneswaran, J. Ind. Eng. Chem. 20 (2014) 4499. [5] X.S. Ji, E. Curcio, S.A. Obaidani, G.D. Profio, E. Fontananova, E. Drioli, Sep. Purif. Technol. 71 (2010) 76. [6] P. Westerhoff, H. Moon, D. Minakata, J. Crittenden, Water Res. 43 (2009) 3992.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Regeneration of PAC used for reverse osmosis concentrate treatment by wet oxidation Yanlin Yuan, Ping Gu, Yanling Yang, Guanghui Zhang * School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
A R T I C L E I N F O
Article history: Received 24 March 2015 Received in revised form 29 October 2015 Accepted 31 October 2015 Available online 5 November 2015 Keywords: Reverse osmosis concentrate Powdered activated carbon Regeneration Wet oxidation Adsorption capacity
A B S T R A C T
The wet oxidation (WO) regeneration of powdered activated carbon (PAC) used for reverse osmosis concentrate (ROC) treatment was studied. The exhausted PAC was regenerated at different temperatures and times. The experimental results indicated that the optimum regeneration condition could be achieved at a temperature of 200 8C for 60 min. The mass of carbon in the solid, gas and liquid phases before and after the PAC regeneration was balanced in different regeneration conditions. A molecular weight (MW) distribution of organic matter (OM) in the effluent treated by the PAC regenerated at optimum conditions was performed. ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction With the development of the reverse osmosis (RO) membrane, RO technology has found an increasingly wide utilization in water reclamation. While approximately 75% of the influent can be reclaimed, a RO concentrate (ROC) containing all of the constituents rejected by the RO membrane is also generated [1–4]. To reduce the effect of ROC discharge on the environment and/or make effective use of the ROC, the treatment of the ROC from municipal and industrial wastewater treatment plants has become a hot topic in the water industry [5–7]. Different treatment methods for ROC have been intensively studied, such as oxidation and adsorption processes [6,8–16]. Among these technologies, activated carbon (AC) adsorption is a well-established technology that is supported by high depuration efficiency for organic matter (OM) and the possibility of regeneration [17]. Compared to granular activated carbon, powdered activated carbon (PAC) has better thermodynamic and kinetic characteristics [16,18,19]. The OM adsorbed onto AC is only transferred to the solid phase and still need to be properly disposed, or otherwise they still remain a threat to environmental safety. Waste AC is therefore considered to be a hazardous material, and disposal of waste AC could be very expensive. In addition, the cost-effectiveness of using
* Corresponding author. Tel.: +86 22 27405059; fax: +86 22 27405059. E-mail address: [email protected] (G. Zhang).
AC in an adsorption process depends not only on its adsorption capacity but also on the possibility of reusing it repeatedly. Therefore, the research and development of appropriate regeneration methods are needed [20]. Various regeneration processes have been proposed, such as thermal regeneration by means of steam, inert gas and hot water [21,22], solvent extraction [23], supercritical fluid regeneration [24], microwave regeneration [25–30], electrochemical regeneration [31] and wet oxidation (WO) regeneration [32]. Of these regeneration methods, WO uses milder conditions and can achieve good regeneration results. In addition, the WO process has received much attention because it can not only reuse the AC but also destroy most of the adsorbed OM or convert it to biodegradable materials that remain in the liquid phase in small amounts, thereby reducing the environmental impact [33–35]. The purpose of this study was to investigate the regeneration of the PAC used in ROC treatment. To our best knowledge, there are few reports on regeneration PAC adsorbing complex OM from ROC by means of WO currently. The re-adsorption capacity of PAC under different regeneration conditions was evaluated, and the changes in the surface porosity were discussed, with the aim of finding the optimum regeneration conditions. Successive regeneration cycles were also conducted and analysed. To analyse the OM transfer and changes after the PAC regeneration, the mass of carbon in the solid, gas and liquid phases was balanced after the regeneration, the method could be used to evaluate regeneration conditions. In the end, the MW distributions of OM in the raw ROC
http://dx.doi.org/10.1016/j.jiec.2015.10.043 1226-086X/ß 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
and the ROC treated by PAC regenerated under optimum conditions were investigated and compared to that of fresh PAC. In practice, the waste superheated steam could be used to regenerate the exhausted PAC in the factory because the regeneration doesn’t need high temperature. Experimental Materials The ROC was taken from an ultrafiltration membrane bioreactor-RO process in a refinery wastewater treatment plant which was the same as that reported by Wei et al. [19]. The characteristics of the ROC are presented in Table 1. PAC (200 mesh) supplied by Luda Co. Ltd. in Zunhua, China was used for this study. The PAC was baked for 2 h at 105 8C to remove moisture prior to use. The detailed characteristics of the PAC were described by Zhao et al. [16]. The exhausted PAC was prepared by adding 5.00 g fresh PAC and 10.0 L ROC into a cylinder container and then stirring (200 rpm, 25 8C) for 30 min. The PAC was filtered out by a 0.45 mm membrane filter (mixed cellulose ester, Mili, China), and the filtrate was analysed for DOC. Experimental procedure Regeneration of exhausted PAC The regeneration tests were carried out in an autoclave made of stainless steel 316 L with a total volume of 2.0 L (Dalian TONG CHAN high-pressure reactors manufacture Co. Ltd., China). The reactor mainly comprised a gas inlet, an electromagnetic stirrer and a cooling water inlet. A schematic of the autoclave is shown in Fig. 1. The experimental procedures for the regeneration of PAC were as follows. (1) 5.00 g of exhausted PAC and 300 mL of distilled water were put into the autoclave. (2) Oxygen was sparged into the autoclave with the gas port open for a while to remove air. (3) The gas port was closed, and the oxygen gas supply was continued until a pressure of 0.1 MPa was reached. The quantity of the oxygen was calculated roughly to be approximately 0.07 mol. And the amount
99
Table 1 Characteristics of the ROC. Parameter
Unit
Value (average value)
Chemical oxygen demand (COD) Dissolved organic carbon (DOC) pH Conductivity TDS Total hardness (as CaCO3) Cl SO42 NO3
mg/L mg/L – mS/cm mg/L mg/L mg/L mg/L mg/L
82.63–100.1 (91.36) 22.55–32.43 (27.49) 8.36 3.21 1640 710 676.8 61.38 722.0
of oxygen in the autoclave was two times more than the amount of organics that was in the ROC sample treated by the PAC and measured by COD. (4) The slurry was heated to a set temperature and maintained at different times for regeneration. Before opening the cover, the gas sample in the autoclave was taken for analysis, and the temperature and pressure were recorded. (5) The contents in the reactor were then cooled down to room temperature. The carbon slurry was filtered, and the filtrate was analysed for DOC. The carbon was washed with distilled water and dried in an oven at 105 8C for analysis. The exhausted PAC was regenerated at the three temperatures of 150, 200 and 250 8C for the three durations of 30, 60, and 90 min. A process of adsorption and regeneration was termed as a regeneration cycle. Successive regeneration cycles were studied and performed at the optimum regeneration conditions, and four regeneration cycles were carried out. To study the effect of the WO process on the fresh PAC under optimum conditions, it was regenerated and expressed as C-Blank. The fresh PAC and the exhausted PAC were expressed as C-Fresh and C-Exhausted, respectively. To express the experimental conditions concisely, a test of PAC regeneration at a temperature for certain time was written as ‘‘temperature-time’’. For instance, 150-30 corresponds to exhausted PAC that was regenerated at 150 8C for 30 min. PAC adsorption isotherm PAC adsorption experiments were carried out by separately adding ROC (100 mL) and pre-weighed PAC into 250-mL conical flasks that were placed on a shaker (200 rpm, 25 8C) for 30 min. The
Fig. 1. Experimental setup for the regeneration of PAC. (1) Oxygen bottle, (2) pressure valve, (3) cooling water inlet, (4) sample port, (5) electromagnetic stirrer, (6) cooling water outlet, (7) thermocouple, (8) gas port, (9) liquid port, (10) cooling coil, (11) electrical heated wire, (12) insulation layer and (13) temperature and stirring controllers.
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Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
PAC was then filtered out using a 0.45 mm membrane filter (mixed cellulose ester, Mili, China), and the filtrate was analysed for DOC. The experiments were carried out in duplicate, and the average values were reported.
filtrates were collected for DOC analysis. The mass balance principle was then used to calculate the DOC concentration within various fractions. Results and discussion
Analytical methods The adsorption isotherms of the regenerated PAC The porosity of the PAC was determined by measuring the N2 adsorption isotherms at 196 8C in an automatic apparatus (NOVA2200e, Quantachrome Instruments, USA). The isotherms were used to calculate the specific surface area (SBET), total pore volume (VT), and pore size distribution using the density functional theory method [36]. The DOC of the filtrate was determined using a TOC analyser (TOC-VCPH, Shimadzu, Japan). The gas in the reactor after regeneration was analysed by gas chromatography (GC 6890N, Agilent, USA). The column pressure was set at 0.1 MPa, and the carrier gases used were nitrogen and hydrogen. The FID was heated at 250 8C. Air, hydrogen and nitrogen flows for the FID were 400, 30 and 25 mL/min, respectively. The molecular weight (MW) distribution of the OM in the raw and treated ROC samples were analysed by the UF filtration method[37] using a pre-washed UF membrane with MW cut-offs (MWCO) of 1, 3, 10 and 30 kDa (regenerated cellulose with a diameter of 15 cm, Amicon1, Millipore Corporation, USA). A UF cup (Shanghai Institute of Nuclear Research of the Chinese Academy of Sciences, China) was used, and nitrogen gas from a pressurized bottle provided the driving force for filtration with a constant pressure of 0.1 MPa. Prior to the MW distribution measurement, the raw ROC sample was filtered through a 0.45 mm membrane (mixed cellulose ester, Mili, China) to remove large particles. A raw ROC sample with a volume of 300 mL was sequentially fractionated into several sizes from the largest size to the smallest, and the
The Freundlich and Langmuir adsorption models are commonly used to describe the adsorption isotherm. Our previous research found that the PAC adsorption used in the ROC treatment more closely followed the Freundlich adsorption model [16], which is expressed in the following equation: qe ¼ kF C e 1=n
(1)
where Ce is the equilibrium concentration of the adsorbate, mg/L; qe is the amount of adsorbate on the PAC at equilibrium, mg/g; and kF and n are constants. The experimental results show that the determination coefficients of the Freundlich adsorption isotherms for fresh PAC and all regenerated PACs were more than 0.96, so this adsorption model was used in the research. The effect of regeneration temperature The regeneration temperature and duration both had a significant effect on the efficiency of the WO regeneration [34]. Therefore, the effect of temperature on the PAC regeneration was initially studied with a regeneration time of 30 min, and the regeneration temperatures were set to 150, 200 and 250 8C. Fig. 2a is the adsorption isotherm of PAC regenerated under different temperatures. It can be seen from the figure that with the increase in the regeneration temperature, the adsorption amount
Fig. 2. The adsorption isotherms of DOC for regenerated PAC. (a) Effect of the regeneration temperature. (b) Effect of the regeneration time. (c) Effect of successive regeneration cycles.
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104 Table 2 Adsorption characteristics of fresh PAC and regenerated PAC. Sample
Ce (mg/L)
qe(mg/g)
Relative adsorption amount
C-Fresh 150-30 200-30 250-30 200-60 200-90
11.1 17.8 16.3 16.2 15.1 15.6
42.6 23.4 26.9 26.8 27.7 27.8
1.00 0.549 0.631 0.629 0.650 0.653
into the regenerated PAC initially increased and then decreased slightly under the same equilibrium concentration of adsorbate when the regeneration temperature was set to 150, 200 and 250 8C. The experimental conditions illustrate that the optimal regeneration temperature is between 200 8C and 250 8C. According to the adsorption isotherm of the fresh PAC and those in Fig. 2a and b, when the dosage of PAC was 0.5 g/L, the adsorption characteristics of the fresh and regenerated PAC could be calculated by Eq. (1), and the results are shown in Table 2. It could be observed from the table that the relative adsorption amount into the regenerated PAC at 150 8C was 0.451 lower than that of the fresh PAC, suggesting that regeneration at a lower temperature was not advantageous to recovering the adsorption capacity of the PAC. In contrast, when the regeneration temperature was increased to 200 8C and 250 8C, the relative adsorption amount increased and reached 0.629–0.653, and the values under the same regeneration time were almost the same. The higher temperature has both its pros and cons, as on the one hand it favoured the oxidation of OM on the PAC, but excessive temperatures could destroy the structure of the PAC, as discussed in Section 3.2. Therefore, the optimum regeneration result would be achieved when the regeneration temperature was 200 8C. The effect of regeneration time The effect of regeneration time on the PAC regeneration efficiency was investigated after the optimum regeneration temperature was determined. The experiments were conducted when the regeneration temperature was 200 8C, and the regeneration time was 30, 60 and 90 min. The adsorption isotherms of the PAC regenerated for different times are shown in Fig. 2b. As seen from Fig. 2b, the adsorption amount of the PAC regenerated for 30 min was low, but it increased when the regeneration time was 60 and 90 min, with a very small difference between the two times. Therefore, the longer regeneration time could efficiently not promote the recovery of PAC, so the regeneration time of 60 min was the optimum.
101
Comparing Fig. 2b with Fig. 2a, it could be observed that the regeneration time had less influence on PAC regeneration than the temperature. With the increase of temperature, the adsorption amount of the regenerated PAC increased at the same equilibrium concentration, but when the regeneration temperature reached a certain value, the increased temperature did not further improve the recovery of PAC but rather damaged the PAC, leading to a lower adsorption capacity. Therefore, the optimum condition for the regeneration of the PAC in this study was a regeneration temperature of 200 8C and a regeneration time of 60 min. Successive adsorption-regeneration cycles at optimum regeneration conditions The effect of the regeneration cycle on PAC adsorption under the optimal conditions was also studied. The adsorption isotherms are shown in Fig. 2c. From the figure, it can be observed that as the number of regeneration cycles increased, the adsorption capacity of the PAC decreased. When the equilibrium concentration Ce was set at 10.00 mg/L, the adsorption amount qe decreased from 20.75 mg/g after the first regeneration cycle to 18.46 mg/g after the second regeneration cycle, eventually falling to 10.67 mg/g after the fourth regeneration cycle. The reason could be because the OM adsorbed on the PAC was not completely removed. When the adsorption regeneration cycle continued, this matter blocked the pores, as discussed in the next section, resulting in a gradual reduction of the adsorption capacity. This conclusion was previously suggested by others [20]. Analysis of PAC surface properties To study the reasons for incomplete regeneration, an analysis of the PAC surface characteristics was carried out. Fig. 3 is the N2 adsorption isotherm of different PACs. The regeneration of the blank carbon was carried out at a temperature of 200 8C for 60 min. As seen from the type of curves in Fig. 3a, the fresh PAC and regenerated PAC are of type I [38]. The adsorption and desorption lines at relatively high pressures do not appear to overlap, which suggests that an increase in the mesoporosity occurred [25,29]. In Fig. 3a, it could be clearly observed that the volume of exhausted PAC decreased sharply, which indicates that OM blocked the PAC pores. However, the volumes of blank carbon and fresh PAC almost coincided. These results differ from the conclusion reported by Shende et al., which is probably due to their use of higher temperature and oxygen pressure [33]. From Fig. 3b, it could be observed that the volume of the regenerated PAC
Fig. 3. N2 adsorption isotherms of different PAC. (a) C-Fresh, C-Exhausted, C-Blank and the PAC regenerated under optimum conditions. (b) Regenerated PAC at different successive regeneration cycles.
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104
102 Table 3 Textural characteristics of the series PACs.
following equation:
Sample
SBET (m2/g)
Vtotala (cm3/g)
Vmicrob (cm3/g)
Vmesob (cm3/g)
C-Fresh C-Blank C-Exhausted 200-30 200-60 200-90 200-60 cycle 200-60 cycle 200-60 cycle 200-60 cycle
850.8 841.1 556.8 733.3 750.3 746.0 752.2 749.3 726.6 721.5
0.425 0.416 0.260 0.360 0.380 0.367 0.375 0.366 0.347 0.326
0.218 0.204 0.106 0.173 0.190 0.178 0.186 0.177 0.159 0.146
0.151 0.152 0.104 0.132 0.137 0.135 0.135 0.134 0.131 0.124
a b
1 2 3 4
Evaluated at a relative pressure of 0.99. Evaluated from DFT applied to N2 at 196 8C.
declined gradually with an increasing number of regeneration cycles. Table 3 shows the textural characteristics calculated from the N2 adsorption isotherms of the different PACs. As seen from the table, the specific surface area, total pore volume and micropore volume of blank carbon were slightly less than those of the fresh PAC. However, the mesopore volume of blank carbon was slightly larger than that of fresh PAC. For the fresh PAC having adsorbed the OM in the ROC, i.e., C-Exhausted, its specific surface area and total, micropore and mesopore volumes reduced to approximately 65%, 61%, 48% and 68%, respectively, of their corresponding fresh PAC values. When the regeneration temperature was 200 8C and the regeneration times were 30, 60 and 90 min, the specific surface area of the regenerated PAC first increased and the slightly decreased with an increase of regeneration time, which suggests that the specific surface area of PAC recovery increased with the increased regeneration time. However, when the regeneration time was too long, it could destroy the structure of the PAC [17,21,25,29,39], resulting in a decreased micropore volume, indicating that increasing the regeneration time was not beneficial to the regeneration of the PAC. In addition, with an increase in the number of regeneration cycles, the pore volume size for the PAC decreased gradually, and after the fourth adsorption-regeneration cycle, the micropore volume decreased to 78.5% of its first adsorption. This result may be due to the pore structure clogging and deposition of PAC, as previously reported by Ania et al. [25]. Mass balance analysis of carbon in the solid, liquid and gas phases at different regenerated conditions There are several steps in the WO process, including the desorption of the adsorbate from the AC into water and the oxidation of the adsorbate in water by dissolved oxygen [35]. The OM on PAC should follow the law of mass conservation in the WO process. To analyse the OM transfer and changes after the regeneration, the mass of carbon in solid, gas and liquid phases after PAC regeneration was balanced. In the adsorption process, some of OM in the ROC was transferred to the surface of the PAC, and this was the target source of carbon in the solid phase that could be calculated by the
C s ¼ ðDOC0 DOC1 ÞV 1
(2)
where Cs is the mass of organic carbon adsorbed from the ROC, mg; DOC0 is the DOC value of the ROC before adsorption, mg/L; DOC1 is the DOC value of the ROC after adsorption, mg/L; and V1 is the total volume of ROC treated, L. In this case, as described in Section 2.1, V1 = 10.0 L. During the regeneration process, carbon in both the gaseous and liquid phases could be from the OM that was desorbed from the PAC and/or oxidized, and it could also be from the PAC itself if the oxidation conditions were too extensive. Some of the OM was oxidized completely to form carbon dioxide, and some was oxidized partially to form carbon monoxide and other intermediate products. All oxidation products could exist in both the gaseous and liquid phases. The results of gas analysis are listed in Table 4. The analysis shows that the main components in the gas were carbon dioxide and carbon monoxide, with small amounts of H2, CH4, C2H4, and C2H2. Because the main components in the gaseous phase were carbon monoxide and carbon dioxide, the total mass of carbon in the gaseous phase only included them, and can be calculated by Eq. (3), in which the ideal gas equation was used. 2 X PiV Cg ¼ 12 RT i¼1
(3)
where Cg is the mass of carbon in the gas after regeneration, mg; Pi is the gas partial pressure for component i when the sample was taken, Pa; V is the gas volume above the liquid in the autoclave, m3; R is the universal gas constant; T is the gas temperature when the sample was taken, K; and i denotes carbon monoxide or carbon dioxide in the gas. Because the gas sample was taken under a certain pressure, some gas was dissolved in the regeneration liquid. As this part of the dissolved gas would escape in the following experiment, it should be calculated independently. The mass of carbon originating from the gas and dissolved in the liquid when the gas sample was taken is calculated by Eq. (4), in which Henry’s law was used. 2 X nwPi Cgl ¼ 12 Ki ðT Þ i¼1
(4)
where Cgl is the mass of carbon after regeneration dissolved in the regeneration liquid, mg; Pi is the partial pressure of gas component i above the liquid when the sample was taken, Ki (T) is the Henry’s constant for component i when the temperature is T [40]; nW is the number of moles of the liquid, mol; and i represents carbon monoxide or carbon dioxide in the gas. The mass of carbon in the liquid phase, i.e., the liquid in the autoclave, is calculated by the following equation: Cl ¼ C3V 2
(5)
where Cl is the mass of carbon in the liquid after regeneration, mg; C3 is the DOC value in the liquid, mg/L; and V2 is the liquid volume, L. In this case, V2 = 0.300 L. Table 5 lists the mass of carbon in the different phases before and after the PAC regeneration. The table shows that with an
Table 4 Gas analytical results and sampling condition. Sample
H2 (mL/L)
CO (mL/L)
CO2 (mL/L)
CH4 (mL/L)
C2H4 (mL/L)
C2H2 (mL/L)
Pressure (MPa)
Temperature (K)
150-30 200-30 250-30
19.00 164.2 518.5
1.49 4.28 11.99
19.84 69.24 150.4
10.23 10.14 10.05
0.52 0.43 0.00
0.21 1.08 3.44
0.20 0.20 0.16
308 343 343
Y. Yuan et al. / Journal of Industrial and Engineering Chemistry 34 (2016) 98–104 Table 5 Mass of carbon in the solid, gas and liquid phases before and after PAC regeneration. Sample
Cs (mg)
Cg (mg)
Cgl (mg)
Cl (mg)
Ct = Cg + Cgl + Cl (mg)
150-30 200-30 250-30
197 199 202
33.9 105 185
4.16 7.69 13.3
35.6 35.5 31.6
73.7 148 230
Table 6 Mass of carbon in the gas and liquid phases for blank carbon at different WO conditions. Temperature(8C)
Cg (mg)
Cgl (mg)
Cl (mg)
Ct = Cg + Cgl + Cl (mg)
150 200 250
7.49 33.6 41.3
0.28 0.90 0.72
0.31 0.81 1.21
8.08 35.3 43.2
increase in the regeneration temperature, the mass of carbon in the gas phase, i.e., Cg, significantly increased, and that dissolved in the liquid, i.e., Cgl, also increased. However, the mass of carbon remaining in the liquid, i.e., Cl, decreased slightly, which could be due to the OM in the liquid being increasingly oxidized and transferred into the gas with increased temperature. When the regeneration temperature increased from 150 8C to 200 8C, the total mass of carbon in the gas and liquid, i.e., Ct, gradually reached that in the solid phase, but the latter was larger than the former. However, when the regeneration temperature was 250 8C, the total mass of carbon in the gas and liquid was slightly larger than that in the solid phase, which was contrary to the mass balance. The result might be attributed to carbonation of the PAC at high temperature, which would produce more carbon oxide and other carbonaceous materials. To analyse the effect of the WO process on fresh PAC, it was also regenerated at different temperatures. Table 6 shows the mass of carbon in blank carbon after the WO process was performed for 30 min at different temperatures. From the table, it can be observed that the mass of carbon in the forms of Cg, Cgl and Cl gradually increased with temperature, but far less than that in the case of the exhausted PAC regeneration. The result suggests that under the experimental conditions, the loss of blank carbon was small, and this was confirmed by the previous blank carbon performance test (e.g., the specific surface area, micropore, and mesopore). Molecular weight (MW) analysis of the fresh and regenerated PAC Table 7 presents the MW distribution of the DOC in the raw ROC. It was found that the original OM of the ROC mainly occurred
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Table 7 MW distribution in the raw ROC. Fraction (kDa)
<1 1–3 3–10 10–30 >30
DOC (mg/L)
(%)
24.2 0.31 0.40 0.90 0.85
90.8 1.16 1.50 3.38 3.19
in the MW range of <1 kDa. This portion accounted for 90.8%, which suggests that the pollutants in the raw ROC were mainly small organic molecules. Bagastyo et al. also found that the largest fraction of DOC in ROCs from both coastal and inland water reclamation plants was <1 kDa molecules [9]. The total and fractional DOC removal from the ROC by fresh PAC and the PAC regenerated under optimum conditions at different dosages are plotted on Fig. 4. Fig. 4a shows that with the increase of the dosage, the DOC removal rate of fresh PAC increased from approximately 64% to 78%, while that of the regenerated PAC only increased from approximately 50% to 65%. The results illustrate that the PAC was not fully restored to its original adsorption performance after regeneration, which is in agreement with the conclusion of Section 3.1. Fig. 4b shows that the PAC regenerated under optimum condition and fresh PAC behaved similar, with both primarily removing OM with MW of <1 kDa. This might be due to the original OM of the ROC mainly consisting of low MW molecule. The functional parts of the PAC are the micropores and mesopores, which have sizes close to that of the OM. The results of Karanfil et al. also show that better efficiency was obtained when the dimensions of the adsorbates were close to those of the PAC pores [41]. Our previous work also found that PAC adsorption could remove all fractions of the OM in the ROC but with different efficiencies [18]. The dosage of fresh PAC had little effect on OM with MW < 1 kDa, as the removal rate only increased from 93.1% to 93.9% at dosages of 0.5 and 0.9 g/L, respectively. One can deduce that the adsorbed OM was already close to saturation. However, the removal of small DOC molecules from the regenerated PAC increased as the PAC dosage increased from 89.3% to 93.3%. This explains that the adsorption capacity of the regenerated PAC to small molecular OM was lower than that of the fresh PAC at low dosages, but the adsorption ability of the regenerated PAC increased as the dosage increased. Therefore, the dosage of the regenerated PAC can be adjusted to meet the effluent requirements in practical applications, although this needs further research.
Fig. 4. The total (a) and fractional (b) DOC removal rate from the ROC by fresh and regenerated PAC with different dosages.
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Conclusions (1) The PAC used for ROC treatment can be regenerated using the WO process. The regeneration conditions are mild, and the regeneration results are acceptable. (2) The optimum regeneration conditions for the PAC are a temperature of 200 8C and a duration of 60 min. The relative adsorption amount of the regenerated PAC under optimum conditions could reach 0.65, but when the number of regeneration cycles increases, the adsorption capacity gradually decreased. (3) The mass balance principle was first used to evaluate the regeneration conditions. In different regeneration conditions, the main components of the gas were carbon monoxide and carbon dioxide. As the regeneration temperature increases, the oxidation products content also increased. The principle could also be used to evaluate the weight loss after regeneration by WO process. (4) The PAC regenerated under optimum conditions and the fresh PAC behaved similarly, with both primarily removing OM with MW of <1 kDa. OM in other MW ranges were more difficult to remove in this experiment. Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (50908158) and the research fund for the Doctoral Program of Higher Education of China (20090032120040). References [1] C.H. Koo, A.W. Mohammad, F. Suja`, Desalination 271 (2011) 178. [2] X.F. Huang, J. Ling, J.C. Xu, Y. Feng, G.M. Li, Desalination 269 (2011) 41. [3] A.P. Echavarrı´a, V. Falguera, C. Torras, C. Berdu´n, J. Paga´n, A. Ibarz, Food Sci. Technol.—Brazil 46 (2012) 189. [4] S. Shanmuganathan, T.V. Nguyen, W.G. Shim, J. Kandasmy, A. Listowski, S. Vigneswaran, J. Ind. Eng. Chem. 20 (2014) 4499. [5] X.S. Ji, E. Curcio, S.A. Obaidani, G.D. Profio, E. Fontananova, E. Drioli, Sep. Purif. Technol. 71 (2010) 76. [6] P. Westerhoff, H. Moon, D. Minakata, J. Crittenden, Water Res. 43 (2009) 3992.
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