Regeneration of granular activated carbon by an electrochemical process

Separation and Purification Technology 64 (2008) 227–236

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Regeneration of granular activated carbon by an electrochemical process Chih-Huang Weng ∗ , Ming-Chien Hsu Department of Civil and Ecological Engineering, I-Shou University, Da-Hsu Township, Kaohsiung 84008, Taiwan

a r t i c l e

i n f o

Article history: Received 26 November 2007 Received in revised form 16 March 2008 Accepted 1 October 2008 Keywords: Activated carbon Adsorption Electrochemical regeneration Methylene blue

a b s t r a c t An electrochemical (EC) process was used to investigate the effectiveness of regeneration of field-spent granular activated carbon (SGAC) collected from a wastewater treatment plant. The influences of regeneration parameters such as processing time, voltage gradient, and processing fluid, were performed. The performance of EC regeneration was evaluated via batch methylene blue (MB) adsorption tests. Increasing voltage and prolonging the process would increase the regeneration efficiency (RE). By considering the energy demand, the optimum conditions for a 91.1% RE with an energy cost of 39 US$/ton were: 0.1 M NaCl, 24 h, and 5 V/cm. Compared to thermal, ultrasonic, and base washing regenerations, this EC process is effective and economic viable for GAC regeneration. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Due to its high adsorption capacity, activated carbon (AC) has long been extensively used in water treatment for contaminants separation. Because AC is very expensive, regeneration is a necessary follow-up step after break through. To lower the operation cost, effective regeneration methods need to be applied to the exhausted AC. AC regeneration can be accomplished by the technologies, including thermo, wet oxidation, ultrasound, and Fenton oxidation. Thermo and wet oxidation are the two regeneration methods widely used in industrial and water treatment plants. The main concerns over thermal regeneration are expensive in terms of energy consumption, time consume, and suffering 5–15% carbon loss due to oxidation and attrition [1]. Regeneration by wet air oxidation is also costly in terms of operation and equipments because it needs to be operated at high temperature and elevated pressure [2]. In Taiwan, presently, three wet air oxidation regeneration (WAR) facilities are operating in industrial wastewater treatment plant for regeneration of powder AC. The main obstacles encountered of operating WAR are not only concerning carbon loss, but it also requires high electricity and maintenance costs. An alternative regeneration approach, ultrasound, has been showed that when the ultrasound process controlled under 20 kHz at 300–850 ◦ C for 3–5 h, up to 60–70% regeneration efficiency (RE) could be reached for GAC saturated with trichloroethylene [3]. Fenton oxidation has been reported to be a promising technique for AC regeneration, by which up to 91% RE for AC loaded with methyl tert-butyl ether

∗ Corresponding author. Tel.: +886 7 6578957; fax: +886 7 6577461. E-mail address: [email protected] (C.-H. Weng). 1383-5866/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2008.10.006

could be achieved by applying 1.7–2.0% H2 O2 and 3 g/L of FeSO4 [4]. Though ultrasonic and Fenton oxidation could be served as a way for removal of toxic organic compounds from AC, the in-depth study of the two techniques still need to be carried to improve their regeneration effectiveness and to establish operation conditions for a variety of contaminants. Most recently, an alternative, electrochemical (EC), method was shown to be an effective approach to regenerate AC [5–13], by which the RE could achieve as high as 70–100%. This method can be operated in situ and has not only the advantage of oxidize organic contaminants in the anode, but it also allows recovering activated adsorbing site of exhausted AC. EC regeneration by submerging GAC in an EC batch reactor filled with an electrolyte has been tested [12–14]. The RE of EC regeneration was affected much by the electrolyte type and concentration [14]. Increasing the period of regeneration and the current usually resulted in an increase of RE [11,12,14]. The electrolyte, necessary for electron transfer was normally NaCl, however, others such as Na2 SO4 , NaHCO3 , Na2 CO3 , CH3 COONa, could also be applied. Testing results [13] of AC preloaded with phenol showed that the use of 1% NaCl achieved a better RE than that of Na2 SO4 , NaHCO3 , Na2 CO3 . The benefits of using NaCl as electrolyte were further demonstrated by Brown et al. [11]; when under the conditions of 2% NaCl and current density of 5–62 mA/cm2 , a maximum of 100% RE of a carbon-based adsorbent (Nyex 100) which loaded with a crystal violet dye was achieved. A 99% RE was found for AC saturated with toluene using an EC filter-press cell [9] whereupon the electrolytic solution was continuously stirred and forced to pass through the cell containing AC; and the toluene stripping efficiency higher than 90% was obtained. Zhou and Lei [8] reported a 90% RE of AC loaded with p-nitrophenol using a fluidized cylindrical EC reac-

228

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

Table 1 Type of electrode and supporting electrolyte used in electrochemical regeneration of activated carbon. Electrode, anode

Electrode, cathode

Supporting electrolytes

Reference

Platinum Iridium-based mixed metal oxide coated titanium Ti/SnO2 -Sb Nickel ␤-PbO2

Platinum Stainless steel

2% (w/w) NaCl 1–2% (w/w) NaCl

Zhang et al. [12] Brown et al. [6]

Graphite Nickel Stainless steel

0.5 M NaOH NaOH, Na2 CO3 , Na2 SO4 1–5 g/L NaCl, 10 g/L Na2 SO4

Garcia-Oton et al. [9] Garcia-Oton et al. [9] Zhou and Lei [8], Cong and Wu [15]

tion for 1.5 h, and the energy consumption per unit kilogram of activated carbon treated was 2.78 kWh/kg. By using a recirculation flow reactor with 0.1 M NaCl, Karimi-Jashni et al. [10] found that RE of AC loaded with phenol decreased with increasing the degree of electrolyte mixing. In view of the past studies of EC regeneration, the lay out of the EC reactor used is somewhat different (Table 1), and the electrode can be as follows: Ti/SnO2 -Sb (anode), graphite (cathode); ␤PbO2 (anode), stainless steel (cathode); iridium-based mixed metal oxide coated titanium (anode), stainless steel (cathode); platinum (anode, cathode). As can be found in the literature, there are commons, such as the use of NaCl as a supporting electrolyte (0.5–2%, w/w) and application of constant DC electric current (1–12 mA/cm2 ) to the system. However, practices of constant voltage to the EC system and the use of field-spent AC in the EC regeneration have not yet being tried. Information pertaining to power consumption of EC regeneration and comparison of other regeneration methods using the same AC were hardly found in the literature. Beside, the use of graphite electrode for both cathode and anode has not been found in the relevant literature. In the present study, experiments were conducted to look into the effectiveness of using an EC process to regenerate a field-spent GAC. Other regeneration method, including ultrasound, steam, and base washing, were also conducted and compared to the EC method. To evaluate the performance of regeneration, the adsorption isotherms of methylene blue (MB) onto regenerated GACs and virgin GAC (VGAC) were established. The effects of electric potential gradient, processing time, and processing fluid on the RE and the corresponding energy cost of the present EC process were also evaluated. As such, optimum process conditions were obtained. There is a hope that the results of this study would provide useful information for design GAC regeneration. 2. Materials and methods 2.1. Granular activated carbon

of three compartments: anode reservoir (10 cm length), AC bed (8 cm length), and cathode reservoir (5 cm length filled with glass bead of 0.1 mm diameter). A 3-cm free board was maintained and was used for venting the gas produced during the regeneration. An amount of 15 g SGAC bed was fixed above the glass bead. Two sets of graphite rod electrodes (0.64 cm in diameter, Union Carbon, USA) were installed at each side of the reservoirs. The electrodes were connected to a direct current power supply capable of operating current 0–10 A and voltage 0–100 V, providing a constant electric gradient of 1–5 V/cm. Electrode compartments were saturated with processing fluid for half day before the EC regeneration started. During the EC operation, current density across the carbon bed, the effluent total organic carbon (TOC) collected from the bottom of the column, and reservoir pH values were monitored. One blank test with fresh GAC was run in parallel to monitor the possible leaching of TOC from GAC itself. TOC was measured by a TOC analyzer (Appolo 900, USA). At the end of the test, the GAC was removed from the compartment and equally mixed. A scanning electron microscope (SEM) (Hitachi S2700) was used to characterize the photomicrography of GAC (virgin, spent, and regenerated). All GAC samples were gold plated and an electron acceleration voltage of 15 kV was applied for SEM observation. Experiments were all operated at room temperature (27 ◦ C). Table 2 lists the detailed experimental conditions for EC regeneration. Alternatively, other generation methods, including base washing, ultrasound, and steam were also conducted and compared to the EC process. Experimental conditions for these methods are shown in Table 2. In the base washing, 0.1 M NaOH was used as washing solution. A peripatetic pump was connected to the SGAC packed column (Fig. 1) creating a steady flow rate of 10 mL/min and continuously circulated for 48 h. While washing the SGAC, the base solution was replaced every 4 h and leaching of TOC was also monitored throughout the process. Ultrasound experiment was conducted by preparing a beaker containing 200 mL distilled water and 15 g of SGAC. The beaker were placed in an ultrasonic reactor (Brason 2210 R-MT, USA) with a frequency of

A field-spent granular activated carbon (SGAC) previously loaded with leachate was collected from a wastewater treatment plant. The SGAC was not yet being considered regeneration and was directly landfilled. Upon collection, SGAC sample was ovendried at 105 ◦ C for 1 day. Particles less than 16 mesh (1.19 m/m) were stored in a sealed glass ware and used for the experiments. The SGAC sample has a pH of 6.8, Brunauer, Emmett, Teller (BET) N2 specific surface area (SSA) of 454.8 m2 /g with pore volume of 0.301 mL/g determined by a surface area analyzer (Beckman Coulter SA3100). For the purposes of determining the percentage regeneration, VGAC sample was also collected. The BET-N2 SSA and pore volume of VGAC are 720 m2 /g and 0.432 mL/g, respectively. 2.2. Experimental setup for electrochemical regeneration The EC regeneration experiments were conducted by a laboratory-scale acrylic extruded cylindrical column shown in Fig. 1. The column has an inner diameter of 1.9 cm and consists

Fig. 1. (a) Schematic diagram of the EC regeneration. (b) Layout of electrode.

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

229

Table 2 Experimental conditions for EC regeneration. Test no.

GAC

Regeneration method

Processing fluid/electric gradient

Processing time (h)

1 2 3 4 5 6 7 8 9

Spent Virgin Spent Spent Spent Spent Spent Spent Spent

None None Electrochemical Electrochemical Electrochemical Electrochemical Electrochemical Electrochemical Electrochemical

0 0 12 12 6 12 24 48 48

10

Spent

11 12

Spent Spent

Base washing Ultrasound Pressure steam

None None 0.1 M NaCl/1 V/cm 0.1 M NaCl/3 V/cm 0.1 M NaCl/5 V/cm 0.1 M NaCl/5 V/cm 0.1 M NaCl/5 V/cm 0.1 M NaCl/5 V/cm Tap-water/5 V/cm Experimental condition 0.1 M NaOH, 10 mL/min circulation 47 kHz 10 psi, 100 ◦ C

47 kHz and lasted for 2 h. Steam regeneration was performed by placing the solution in a high pressure cooker (S-328, First Lady, Taiwan) under a pressure of 10 psi at 100 ◦ C for 0.5 h cooking. At the end of regeneration, the AC samples were rinsed with distilled water several times and dried for 1 day at 105 ◦ C. MB adsorption was used to evaluate the performance of each regeneration process. The mass of TOC from ultrasonic and steam regeneration was measured.

2.3. Evaluation of regeneration efficiency The cationic MB dye or basic blue 9, used in this study was purchased from Riedel-de Haën Co., Germany. Fig. 2 shows the structural formula of MB, which has a chemical formula of C16 H18 ClN3 S with a molecular weight of 373.9 g/mol. Concentration of MB was determined using a spectrophotometer (HACH DR-2010, USA) at a wavelength of 665 nm. As the adsorption isotherms could be used for determining the RE of AC [16], batch adsorption experiments were conducted. The maximum adsorption capacity was used to evaluate the performance of regeneration. The experimental procedures were described as follows: (1) prepare of 125-mL polyethylene (PE) bottles containing a constant concentration of NaNO3 (2.5 × 10−2 M) and various MB concentrations ranging from 5 × 10−5 to 1 × 10−3 M. (2) Adjust solution pH to 9.0 with either HNO3 or NaOH. (3) Add a given amount of regenerated GAC (0.4 g/L) into the solution. (4) Shake these bottles on a reciprocal shaker at 150 excursions/min for 20 days at room temperature. It was confirmed that this contact time was adequate for reaching equilibrium adsorption based on the results of kinetic study. (5) At the end of shaking, record the final pH of the mixed liquor. (6) Filter the liquor through the 0.45-␮m Gelman filter paper to collect the supernatant. (7) Analyze the residual MB concentration in the supernatant. Control samples without present adsorbent in the mixed suspension were also performed. To determine the effectiveness of the regeneration of each process, virgin and field-spend GAC without regeneration were run in parallel.

Fig. 2. Molecular structure of MB.

48 2 0.5

3. Results and discussion 3.1. Current variation within compartment The EC process can be controlled under either constant voltage or current. Most of the past EC studies were conducted under constant current. Normally the resistance across the conductive media would increase under a constant voltage condition. Such phenomenon would result in a decrease of current passed. Because the power consumption is directly proportional to the current across the conductive medium, the power expenditure of a constant current operation would be greater than the process under a constant voltage. In this study, the EC system was kept at constant electrical (voltage) gradient across the conductive specimen, allowing the current to change over processing time. Current density was calculated by dividing the current passing through the carbon bed to the cross-sectional area of the column. The variations of current densities throughout the duration for the seven EC regeneration experiments are depicted in Fig. 3. It appears that the current was affected much by the applied electrical gradient (Fig. 3(a)), processing time (Fig. 3(b)), and the processing fluid (Fig. 3(c)). A general trend was found in all tests, the current density initially increased to a certain value, and then it decreased thereafter within 4 h. The decrease of current with time reflects the total electric resistance increased in the system, which may attribute to the build up of clogging in the flow path near the cathode zone. The decrease of current would result in a change in effluent conductivity. As shown in Fig. 3(a), after 4 h elapsed, the current remains almost the same for the test of low electric gradient of 1 V/cm. When the electric gradient increased to 3 V/cm, the current increased greatly, showing the process has overcome the resistance in the EC system. It was evidenced that the current increase markedly when a higher electric gradient of 5 V/cm was applied. It appears that the minimum electric gradient to conquer the system resistance is 3 V/cm. Such finding has not yet been seen in the relevant EC studies. Therefore, electrical gradient of 5 V/cm was used to study the effects of processing time and fluids on the EC regeneration. Fig. 3(b) shows current density variation at different periods of operation. Similar variation trend was observed for each test. When the process was lasted for either 24 h or 48 h, the current did gradually increase to a relatively stable value of about 9 mA/cm2 . The current became stable which suggests that the bed resistance becomes constant. As comparing the current density variation in Fig. 3(c), a higher current density was observed for the processing fluid of 0.1 M NaCl (Test 8) than that of tap water (Test 9). The conductivity of tap water is only 5 × 103 mS/cm, which did not provide enough

230

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

Fig. 4. Reservoir pH affected by EC regeneration. (a) Effects of electric gradient; (b) effects of EC processing time; (c) effects of processing fluid. Fig. 3. Current density affected by EC condition during regeneration. (a) Effects of electric gradient; (b) effects of EC processing time; (c) effects of processing fluid.

conductive ions for this process. Thereby a much lower current was resulted using tap water as process fluid.

theless, as a higher concentration of electrolyte was applied to the system, it would create a greater impact on the pH changes (Fig. 4(c)). The buildup of such strong pH gradient is attributed to the water electrolysis reaction:

3.2. pH variation within reservoirs

Anode : 2H2 O → O2 (g) + 4H+ + 4e−

(1)

Cathode : 2H2 O + 2e− → H2 (g) + 2OH−

(2)

The pHs measured in the reservoirs at various time periods are given in Fig. 4. When the process with 0.1 M NaCl, the pH of anode fluid decreased rapidly from neutral to around 1.8–2.0 while the cathode fluid increased drastically to about 11.5–12.5 for the entire EC process period. The pH variation was not affected much by the electric gradient and the time elapsed (Fig. 4(a) and (b)). Never-

In the test of tap water, the pH of cathodic fluid initially increased to 11.2 within the first 4 h, and then it gradually decreased to neutral pH for the entire EC process period (Fig. 4(c)). At the anode side of solution, because the alkaline solution produced at cathode would neutralize the anodic solution even though the H+ ion was contin-

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

231

voltage and processing time. Despite a portion of TOC leached was derived from GAC itself, increasing the processing time and electric gradient would enhance TOC leaching (Fig. 5(a) and (b)). A near linear relationship between the total mass of TOC leached and process time was observed for a time span of 48 h. From the results presented herewith, during the EC treatment, the oxidation of organic substance can be neglected. Results also indicated that TOC leaching amount is due to the charge, e.g., H+ and OH− , passed through SGAC. A much less amount of TOC leached was observed for the EC process fluid of tap water (Fig. 5(c)) under an electrical gradient of 5 V/cm. Such low TOC leached can be attributed in part to the low current across the EC system for the Test of 9 (Fig. 3(c)) since the tap water could not provide sufficient conductive ions in the process operation. It appears that the EC regeneration is largely attributed to the desorption of the organic substances from SGAC, rather than any oxidation of these components. Since the waste processing fluid contains high concentration of nonbio-degradable organic substances, additional follow up process for the treatment of this waste should be considered. 3.4. Efficiency of regeneration Adsorption of MB onto regenerated GAC was used to evaluate the effectiveness of EC regeneration. Basically, pH and temperature play an important role in affecting the adsorption. In this study, all adsorption experiments were conducted at pH 9.0 and room temperature of 27 ◦ C. Langmuir isotherm [17] was used to model the MB adsorption. This isotherm is applicable to describe monolayer adsorption onto a surface having a finite number of identical sites and assumes no immigration of adsorbate on the surface plane. The form of Langmuir isotherm is as follow: qe =

Fig. 5. Leaching of TOC from SGAC affected by EC condition during regeneration. (a) Effects of electric gradient; (b) effects of EC processing time; (c) effects of processing fluid.

uously generating in this region. This would lead the pH of anodic fluid hardly increases (Fig. 4(c)) and the pH only slightly decreased to only 5.5 throughout the entire of EC operation period. The pH variation in Test 9 (tap water) is corresponding to a less amount of current density occurred (Fig. 3(c)). Obviously, these pH changes are greatly associated with the conductivity of processing fluid. 3.3. Effluent TOC TOC was used to evaluate the amount of organic substances desorbed from SGAC sample during the EC operation. Fig. 5 shows cumulative TOC leached from SGAC under different EC conditions. The leaching patterns of TOC for all EC experiments were similar. As shown, the mass of TOC leached was governed by the applied

KL Ce Qm 1 + KL Ce

(3)

where Qm is the maximum adsorption capacity (mol/g) and KL is adsorption constant (L/mol). The MB adsorption isotherms for the EC regenerated GAC affected by electric gradient, time elapsed, and processing fluids were depicted in Fig. 6. The best fits of the isotherm model were presented as smooth lines in the isotherm plots. All solid lines in Fig. 6 shows a good-fit of calculated value to the measured data for the Langmuir model. The corresponding maximum monolayer adsorption capacity (Qm ) and Langmuir constants (KL ) are listed in Table 3. The high value of r2 (>0.90) indicates that the experimental data was well correlated to the Langmuir model. VGAC and SGAC without any EC treatment were also tested for their adsorption capacity, and the results were shown in Fig. 6(a) and Table 3. As illustrated in Table 3, field-spent carbons investigated in this study was still presented high adsorption capacity for MB. However, the adsorption capacity of original SGAC (Test 2) is much lower than that of VGAC (Test 1). A Qm value of 6.78 × 10−4 mol/g for SGAC was determined with respect to that of 1.23 × 10−3 mol/g for VGAC. It appears that the field-spent GAC still presented largely available sites for MB adsorption (55.1% of VGAC) even though it was regarded as being exhausted. Albeit this SGAC has been thrown away by the user, the high adsorption MB capacity may inspire the user rethinking its adsorption value for dye removal. Prior to be used as adsorbent, regeneration and clearing the pore space of SGAC being occupied by the leachate constituents is deem necessary to recover more activate site, preventing the possible contamination from desorbing contaminants. Results show that the amount of MB adsorbed on EC regenerated GAC increased with increasing electric gradient (Fig. 6(a)). When the electric gradient increasing from 1 to 5 V/cm, the values of Langmuir constant (KL ) remain almost constant while the values

232

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

of Qm increase from 8.12 × 10−4 to 9.15 × 10−4 mol/g. When these Qm values were compared to that of VGAC, the RE reached only 66.0–74.4%. Therefore, prolonging the processing time was tried to further improve the regeneration. Fig. 6(b) shows the influence of processing time on adsorption. Table 3 shows that the maximum adsorption capacities for the EC regenerated GAC are 8.38 × 10−4 , 9.15 × 10−4 , 1.12 × 10−3 , and 1.01 × 10−3 mol/g, respectively for the processing time of 6, 12, 24, and 48 h. The corresponding regeneration efficiencies ranged from 68.1 to 91.1% when the EC process elapsed from 6 to 48 h, respectively. Results showed the EC optimum condition for a RE of 91.1% is 24 h processed by 0.1 M NaCl at electric gradient of 5 V/cm. Prolonging the EC treatment would increase the charge across the specimen and thereby increase the efficiency of regeneration. As shown in Fig. 5(a) and (b), more TOC was leached from SGAC when the process was operated under higher electric gradient or longer processing time. Although additional treatment for more than 24 h (Test 8) resulted in increase in TOC, the results shown herein did not exhibit an increase in RE. Such phenomenon can also attribute to the fact that the strength of charge passed (current density) that responsible for regenerating did not increase with treatment time (Fig. 3(b)) after 24 h. As can be seen (Fig. 5(b)), the increase of additional TOC might release either from the GAC itself or from the accumulated materials on the SGAC that was not adsorbed on the active carbon sites; therefore the regeneration efficiency was not increase. During the experiment, since the carbonaceous materials (GAC) experienced in strong acidic (pH 1.8–2.0) and basic (pH 11.5–12.5) environment, it could enhance the release of soluble organic substances in the carbonaceous deposits. Fig. 6(c) shows the result of MB adsorption onto GAC regenerated by EC process with different processing fluids. Under EC condition of 5 V/cm and 48 h operation, the capacity of SGAC treated by 0.1 M NaCl (Test 8) and tape water (Test 9) to adsorb MB are 1.01 × 10−3 and 9.16 × 10−4 mol/g, respectively. The corresponding RE is 82.1 and 75.1%, respectively, for Tests 8 and 9. The adsorption affinity constant, KL , for Test 8 (36,205 L/mol) is also much higher than that of Test 9 (20,023 L/mol). The reason for a better RE is attributed to greater charge passed due to the higher current that results from a more conductivity electrolyte. Results clear indicated that the EC process with high concentration of supporting electrolyte, i.e. 0.1 M NaCl, would recover more activated site of SGAC. 3.5. SEM observation

Fig. 6. Isotherms for adsorption of MB onto EC regenerated GAC. (a) Effects of electric gradient; (b) effects of EC processing time; (c) effects of processing fluid.

Fig. 7 shows the SEM micrographs of the GACs at 10,000 times magnification. The image in Fig. 7(a) shows that the VGAC particle is mostly irregular in shape and has identical porous surface. In Fig. 7(b), the blurred image showing the surface of raw SGAC particles is less porous and covers mostly by adsorbing materials and less porous. After EC treatment (5 V/cm, 24 h, 0.1 M NaCl), the

Table 3 The Langmuir constants for adsorption of MB onto EC regenerated GAC. Test

GAC type

Regeneration conditions

Qm (mol/g)

KL (L/mol)

r2

SSA (m2 /g)

RE (%)

Power (kWh/ton)

Energy cost (US$/ton)

1 2 3 4 5 6 7 8 9 10 11 12

Virgin Spent EC-GAC EC-GAC EC-GAC EC-GAC EC-GAC EC-GAC EC-GAC Base-GAC Ultrasonic-GAC Steam-GAC

None None 0.1 M NaCl/1 V/cm/12 h 0.1 M NaCl/3 V/cm/12 h 0.1 M NaCl/5 V/cm/6 h 0.1 M NaCl/5 V/cm/12 h 0.1 M NaCl/5 V/cm/24 h 0.1 M NaCl/5 V/cm/48 h Tap-water/5 V/cm/48 h 0.1 M NaOH/48 h 47 kHz/2 h 10 psi/100 ◦ C/0.5 h

1.23 × 10−3 6.78 × 10−4 8.12 × 10−4 8.62 × 10−4 8.38 × 10−4 9.15 × 10−4 1.12 × 10−3 1.01 × 10−3 9.16 × 10−4 9.24 × 10−4 8.28 × 10−4 8.98 × 10−4

84,230 17,021 24,098 22,928 24,638 24,793 22,450 36,205 20,023 23,834 33,067 21,064

0.918 0.939 0.911 0.908 0.906 0.923 0.955 0.933 0.933 0.931 0.904 0.939

926 510 611 649 631 689 843 760 689 695 623 676

– 55.1 66.0 70.1 68.1 74.4 91.1 82.1 74.6 75.1 67.3 73.0

– – 5.5 80.8 73.6 241.0 649.2 1,362.7 72.4 – – –

– – 0.3 4.8 4.4 14.5 39.0 81.8 4.3 – – –

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

233

image (Fig. 7(c)) shows the treated SGAC particle has porous and clear surface similar to the VGAC. Table 4 lists some BET-N2 surface area and pore volume values of various types of GAC. The BET-N2 SSA and average pore radius for the SGAC were measured to be 403 m2 /g and 0.246 mL/g, respectively, using a Brunauer, Emmett, Teller (BET) N2 area analyzer (Coulter SA3100, Beckman). After EC treatment, the N2 -SSA and pore volume have increased to 612 m2 /g and 0.246 mL/g, respectively. The recovered N2 -SSA and pore volume of EC regenerated are 85% and 87%, respectively, with respect to those of VGAC. Based on the SEM observations, N2 -SSA and pore volume measurements, it appears that the EC regeneration process has rendered SGAC less adsorbing species, thereby increasing pore size and recovering more activated site surface area allowable for adsorption. Some adsorbing sites still were not recovered by EC, which may associate with the accumulation of recalcitrant organic and inorganic substances in carbon during the application.

3.6. Comparison of other regeneration method

Fig. 7. SEM images (10,000×) of (a) Virgin GAC, (b) Spent GAC, and (c) EC treated SGAC.

For the comparison purposes, other methods for GAC regeneration were conducted. The mass of TOC collected from steam and ultrasonic regeneration were only 2.6 and 2.8 mg, respectively, which were way below the values of EC regeneration (Fig. 5) and could be attribute to the fact that the desorbing organic substances were destroyed during the experiment. An amount of 12.8 mg TOC was measured when a concentration of 0.1 M NaOH solution was continuously flushed into SGAC bed. This clear indicated that basic solution could enhance the leaching of organic substances from SGAC. The leached organic substances likely consist of humic acid and/or fulvic acid as they are soluble in strong alkaline solution. The high amount of organic substance desorbed from SGAC would result in recovering more adsorbing site. Results of MB adsorption onto SGAC regenerated from methods of ultrasonic, steam, and base washing are shown in Fig. 8. Parameters for Langmuir model fitting are listed in Table 3. By comparing the magnitude of Qm , the RE order is as follow: EC (1.12 × 10−3 ) > base washing (9.24 × 10−4 ) > steam (8.98 × 10−4 ) > ultrasonic (8.28 × 10−4 mol/g). Since optimization of the methods other than EC regeneration has not been made in this work, more research on the other method may be needed as to give more precise comparison and to confirm the preference of EC process over others. A comparison of MB adsorption capacities of various types of AC is given in Table 4. It is seen that the adsorption capacity of EC regenerated GAC (1.12 × 10−3 mol/g) is quite similar to the investigations of steam regenerated GAC (1.06 × 10−3 mol/g) and is higher than the commercial GAC (Nuchar C-190) and other types of AC

Table 4 Comparison of Lagmuir parameters for the adsorption of MB onto GAC. Types Commercial powder AC GAC Virgin GAC Steam regenerated GAC EC regenerated GAC Steam regenerated GAC GAC Virgin F400 GAC Jute fiber carbon GAC Groundnut shell carbon Waste apricot AC GAC (Diosgenin) Granular AC (Nuchar C-190) Coconut shell fibers carbon Coir pith carbon

N2 -SSA (m2 /g) NA NA 719.4 734 612.0 857.1 1,135 1,038 NA 425 NA 1060 702.2 NA 978 167

pH

Qm (mol/g)

7.4 NA 9 NA 9 9 NA 7 4 NA 7.4 NA 7 5.5 6.0 6.9

−3

2.62 × 10 1.39 × 10−3 1.23 × 10−3 1.06 × 10−3 1.12 × 10−3 1.03 × 10−3 8.38 × 10−4 7.73 × 10−4 6.85 × 10−4 4.79 × 10−4 4.41 × 10−4 3.21 × 10−4 3.13 × 10−4 2.79 × 10−4 5.24 × 10−5 1.57 × 10−5

K (L/mol)

Sources

1.79 × 10 5.99 × 104 8.42 × 104 8.03 × 102 2.24 × 104 1.78 × 105 NA NA NA NA 4.79 × 104 2.69 × 105 7.84 × 104 1.20 × 104 2.57 × 104 3.48 × 105

Kannan and Sundaram [18] Kumar and Sivanesan [19] This study Warhurst et al. [20] This study Qada et al. [21] Castro et al. [22] Miguel et al. [23] Senthilkumaar et al. [24] Azargohar et al. [25] Kannan and Sundaram [18] Basar [26] Zhang et al. [27] Dhodapkar et al. [28] Singh et al. [29] Kavitha and Namasivayam [30]

5

234

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236 Table 6 Comparison of specific surface area and pore volume of various types of GAC. GAC type

Pore volume (mL/g)

N2 -SSA (m2 /g)

MB-SSA (m2 /g)

Virgin EC regenerated Field-spent

0.432 0.376 0.246

720 612 403

926 843 510

been used for SSA estimation for decades. Most recently, Weng and Pan [38] reported that MB could be used for SSA determination of sludge ash particles and the results were close to the value of BET method. By assuming the GAC surface was homogeneous and completely covered by MB molecules as the adsorption isotherm is established, the SSA (m2 /g) of the GAC can be estimated by the adsorption capacity of MB: Fig. 8. Isotherms for adsorption of MB onto regenerated GAC by different methods.

SSA = Qm × N × A

derived from groundnut shell, coir pith, synthetic crude coke, waste apricot, jute fiber, and coconut shell fiber. As shown in Table 4, the MB adsorption affinity constant of EC regenerated GAC are high compared with the listed carbon. A comparison of RE of various types of AC regeneration is illustrated in Table 5. As shown, the researchers were beginning to pay attention to the regeneration of AC loaded with organic contaminants. Phenol is the most common target contaminant often used in the AC regeneration study. Other more toxic compounds such as trichloroethylene, toluene, and MTBE were also selected. As shown, values of RE from EC (70–99%) are generally higher than other methods (60–95%). The EC process used in this work achieved a 91% RE, confirming the effectiveness this process.

where N is Avogadro’s number (6.023 × 1023 molecules/mol) and A is the surface area occupied by a monolayer of MB molecules. The dimensions of MB molecule are 16.9 Å × 7.4 Å × 3.8 Å [34]. If the MB molecules were of tilted orientation with their largest face inclined roughly at 65–70◦ on the surface of clay (mica), the covered area is 69.6 Å2 [34]. However, the most common assumption of the covered area is the MB molecular lies flat on its largest surface [37,39], and thus the area covered by one MB molecular is 125 Å2 , which was used for SSA estimation. Table 3 lists the calculated SSA of GAC based on Qm values and Eq. (4). The SSA values for regenerated GAC determined by MB adsorption range from 340 to 470 m2 /g depending upon the regeneration condition conducted. As shown in Table 3, VGAC, SGAC, and EC treated GAC (Test 7) samples have MB-SSA of 926, 510, and 843 m2 /g, respectively, which are approximately 20–25% higher than the value of BET-SSA (Table 6). Studies have also indicated that the BET-N2 method underestimates the SSA of soil rich in swelling clay as comparing the SSA value determined from MB adsorption [37,39]. It must be noted that the equilibrium time for MB adsorption conducted in this study was 20 days with respected to the BET-N2 method of few minutes for gas absorption. It is speculated that the hydrated GAC would allow more activated site for MB adsorbed within such long period. As such, it would be likely that more MB molecular could sufficiently reach the micropore of the GAC.

3.7. Specific surface area determination The most frequent methods being used for determination of the SSA of adsorbents is BET-N2 gas absorption method. The main concern of using this method is that the obtained SSA value may not properly represent the true adsorption site of a hydrated adsorbent in aqueous solution, because the adsorbent is exposed to gas–solid system in the determination procedure. An alternative to the BET method, the adsorption of dyes from aqueous solution has been used to determine the SSA of layered silicates [34–37]. MB has long

(4)

Table 5 Comparison of GAC regeneration efficiency. Method

Process time (h)

Temp. (◦ C)

Carbon type

Contaminant

RE (%)

Sources

EC EC EC EC EC EC EC EC EC Ultrasonic Ultrasonic Steam Base washing Wet oxidation Thermo Thermo (steam) Microwave Chemical -Fenton Photocatalytic Photocatalytic

12 5 10 5 5 0.4 1.5 2 3 3–4 2 0.5 48 3 3–5 1 4 2 3 72

Room Room Room Room Room Room Room Room 30 NA Room 100 Room 150–250 300–850 800 1000 NA 25 50

Field-spent GAC Filtrasorb 400 F-400 and WV-B Filtrasorb F-400 Coconut shell GAC Nyex 100 carbon Woody GAC Activated carbon fiber Westvaco (WV-A1100) GAC Field-spent GAC Field-spent GAC Field-spent GAC GAC GAC Field-spent GAC GAC GAC GAC GAC

Leachate Phenol Phenol Phenol Phenol Crystal violet dye p-Nitrophenol Acid orange dye Toluene Trichloroethylene Leachate Leachate Leachate Reactive dye p-Nitrophenol Water treatment plant Phenol MTBE Methanol Phenol

91.1 80–95 70 95 85.2 98 90 90 99 64 67.3 73 75.1 95 60–70 60–82 70–80 91 80 78.4

This study Zhang et al. [12] Karimi-Jashni and Narbaitz [10] Narbaitz and Cen [14] Zhang [13] Brown et al. [11] Zhou and Lei [8] Zhou and Lei [8] Han et al. [5] Lim and Okada. [3] This study This study This study Shende and Mahajani [2] Sabio et al. [1] Miguel et al. [23] Xitao et al. [31] Huling et al. [4] Tao et al. [32] Liu et al. [33]

Note: EC stands for electrochemical regeneration.

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

235

fee in Taiwan (0.06 US$/kWh). With 5 V/cm of electrical gradient, the RE of GAC shown herewith did not increase even though the process was prolonged from 24 to 48 h and the energy cost increases from 39.0 to 81.8 US$/ton. RE and energy cost affected by electric gradient and time elapsed were shown in Fig. 9. By accounting the energy cost without compromising the RE, the optimum conditions for the present process are: electrical gradient 5 V/cm, processing time 24 h, processing fluid 0.1 M NaCl. The cost of VGAC is about 1000 US$/ton with respect to the EC regeneration cost of 39.0 US$/ton, depicting the cost-effective of this process. The EC process can be operated in situ. Findings revealed that the EC process is economic viable and can effectively recover more activate adsorbing site. 4. Conclusions This paper investigated the effectiveness of regeneration spent GAC by an EC process under constant electric gradient. MB adsorption was used to evaluate the process performance. Findings are summarized as follows:

Fig. 9. GAC regeneration and energy cost affected by (a) electric gradient and (b) time elapsed.

3.8. Cost analysis for EC process For a constant-voltage EC process, the energy expenditures, P (Wh), was related to the time integral of the current across the GAC compartment. The Electric power expenditure per ton of GAC treated, E (Wh/ton), is calculated as follows: E=

P 1 = W W



VIdt

(5)

where W = mass of GAC (ton); V = the applied voltage (V); I = the current (A); t = time elapsed (h). Table 3 lists the energy consumption as determined from Eq. (5). The values of energy expenditure were directly proportional to the electrical gradient and processing time. As shown in Table 3, aside of Test 8, a higher RE was observed in the test that consumed more electrical energy under identical experimental conditions. In comparison with the energy expenditure of Tests 7 and 8, Test 8 did not show a substantial increase in RE while the processing time increased from 24 to 48 h and the E value doubled from 649.2 to 1362.7 Wh/ton. The drawback of using EC process is the consumption of energy. Table 3 shows the energy cost of the EC process. Noted that the analysis is preliminary. When it comes to the field, capital costs, chemicals fees, maintenances and running costs of EC apparatus should not be excluded. Energy requirement per unit mass of GAC treated range from 5.5 to 1362.7 kWh/ton, equivalent to the energy cost of about 0.3–81.8 USD/ton based on the industrial rate of current electricity

1. EC has rendered more activated site being recovered than those of other regeneration method such as base washing, ultrasound, and steam under the condition conducted in this study. Regeneration efficiency of EC process was depended on the charge passed through the spent GAC. The processing fluid with tap water exhibited a much worse performance than that of 0.1 M NaCl due to the lack of supporting electrolyte present in the solution. Increasing treatment time and electrical gradient would also result in a better regeneration because of greater charge passed due to the higher current in the EC system. 2. Findings revealed that a higher regeneration efficiency occurred in the test that may consume more electrical energy under identical conditions. However, when the process of 5 V/cm was extended from 24 to 48 h, despite of an increase of additional TOC, results did not show a substantial increase in regeneration efficiency while the electrical energy boosted from 649.2 to 1362.7 Wh/ton. The increase of additional TOC might release from the accumulated materials on the SGAC that was not occupied on the active carbon site; therefore the regeneration efficiency was not increase. 3. Based on the relationship between regeneration efficiency and energy cost, the optimum conditions for the present EC process are electrical gradient 5 V/cm, processing time 24 h, processing fluid 0.1 M NaCl. A 91.1% activated adsorbing site was recovered with an energy cost of 39 US$/ton under this optimum condition. 4. Findings revealed that the EC process is economic viable and can be as an effective process for regeneration for GAC, particular for GAC loaded with leachate. Future study may focus on the scale-up of this EC process and the possible implication of this process in the treatment plant. The in-depth performance and more precise cost analysis may need to be carried on further, and as such testing results would be served as a base line data for designing commercial EC process. Acknowledgements This study was partially supported by the National Science Council of Taiwan (NSC 96-2221-E-214-013-MY3). The authors thank the fellows of Y.C. Landfill Plant, Taiwan for providing the GAC samples. The authors express their thanks to the fellows of MANALAB, I-Shou U. for supporting the SEM analysis.

236

C.-H. Weng, M.-C. Hsu / Separation and Purification Technology 64 (2008) 227–236

References ˜ [1] E. Sabio, E. González, J.F. González, C.M. González-García, A. Ramiro, J. Ganan, Thermal regeneration of activated carbon saturated with p-nitrophenol, Carbon 42 (2004) 2285–2293. [2] R.V. Shende, V.V. Mahajani, Wet oxidative regeneration of activated carbon loaded with reactive dye, Waste Manage. 22 (2002) 73–83. [3] J. Lim, M. Okada, Regeneration of granular activated carbon using ultrasound, Ultrason. Sonochem. 12 (2005) 277–282. [4] S.G. Huling, P.K. Jones, W.P. Ela, R.G. Arnold, Fenton-driven chemical regeneration of MTBE-spent GAC, Water Res. 39 (2005) 2145–2153. [5] Y. Han, X. Quan, S. Ruan, W. Zhang, Integrated electrochemically enhanced adsorption with electrochemical regeneration for removal of acid orange 7 using activated carbon fibers, Sep. Purif. Technol. 59 (2008) 43– 49. [6] N.W. Brown, E.P.L. Roberts, A. Chasiotis, T. Cherdron, N. Sanghrajka, Atrszine removal using adsorption and electrochemical regeneration, Water Res. 38 (2004) 3067–3074. [7] J.D. Snyder, J.G. Leesch, Methyl bromide recovery on activated carbon with repeated adsorption and electrothermal regeneration, Ind. Eng. Chem. Res. 40 (2001) 2925–2933. [8] M.H. Zhou, L.C. Lei, Electrochemical regeneration of activated carbon loaded with p-nitrophenol in a fluidized electrochemical reactor, Electrochim. Acta 51 (2006) 4489–4496. [9] M. Garcia-Oton, F. Montilla, M.N. Lillo-Rodenas, E. Morallon, J.L. Vazquez, Electrochemical regeneration of activated carbon saturated with Toluene, J. Appl. Electrochem. 35 (2005) 319–325. [10] A. Karimi-Jashni, M. Roberto, R.M. Narbaitz, Electrochemical reactivation of granular activated carbon: effect of electrolyte mixing, J. Environ. Eng.: ASCE 131 (2005) 443–449. [11] N.W. Brown, E.P.L. Roberts, A.A. Garforth, R.A.W. Dryfe, Electrochemical regeneration of a carbon-based adsorbent loaded with crystal violet dye, Electrochim. Acta 49 (2004) 3269–3281. [12] H. Zhang, L. Ye, H. Zong, Regeneration of phenol-saturated activated carbon in an electrochemical reactor, J. Chem. Technol. Biotechnol. 77 (2002) 1246–1250. [13] H. Zhang, Regeneration of exhausted activated carbon by electrochemical method, Chem. Eng. J. 83 (2002) 81–85. [14] R.M. Narbaitz, J. Cen, Electrochemical regeneration of granular activated carbon, Water Res. 28 (1994) 1771–1778. [15] Y.Q. Cong, Z.C. Wu, Self-regeneration of activated carbon modified with palladium catalyst for electrochemical dechlorination, Chin. Chem. Lett. 18 (2007) 1013–1016. [16] R.M. Narbaitz, J. Cen, Alternative methods for determining the percentage regeneration of activated carbon, Water Res. 31 (1997) 2532–2542. [17] I. Langmuir, The adsorption of gases on plane surfaces of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361–1403. [18] N. Kannan, M.M. Sundaram, Kinetics and mechanism of removal of methylene blue by adsorption on various carbons—a comparative study, Dyes Pigm. 51 (1) (2001) 25–40. [19] K.V. Kumar, S. Sivanesan, Equilibrium data, isotherm parameters and process design for partial and complete isotherm of methylene blue onto activated carbon, J. Hazard. Mater. B 134 (2006) 237–244. [20] A.M. Warhurst, G.L. Mcconnachie, S.J.T. Pollard, Characterisation and applications of activated carbon produced from moringa oleifera seed husks by single-step steam pyrolysis, Water Res. 31 (1997) 759–766.

[21] E.N.E. Qada, S.J. Allen, G.M. Walker, Adsorption of methylene blue onto activated carbon produced from steam activated bituminous coal: a study of equilibrium adsorption isotherm, Chem. Eng. J. 124 (2006) 103–110. [22] J.B. Castro, P.R. Bonelli, E.G. Cerrella, A.L. Cukierman, Phosphoric acid activation of agricultural residues and bagasse from sugar cane: influence of the experimental conditions on adsorption characteristics of activated carbons, Ind. Eng. Chem. Res. 39 (2000) 4166–4172. [23] G.S. Miguel, S.D. Lambert, N.J.D. Graham, The regeneration of field-spent granular activated carbons, Water Res. 35 (2001) 2740–2748. [24] S. Senthilkumaar, P.R. Varadarajan, K. Porkodi, C.V. Subbhuraam, Adsorption of methylene blue onto jute fiber carbon: kinetics and equilibrium studies, J. Colloid Interface Sci. 284 (2005) 78–82. [25] R. Azargohar, A.K. Dalai, Production of activated carbon from luscar char: experimental and modeling studies, Micropor. Mesopor. Mater. 85 (2005) 219– 225. [26] C.A. Basar, Applicability of the various adsorption models of three dyes adsorption onto activated carbon prepared waste apricot, J. Hazard. Mater. B 135 (2006) 232–241. [27] C. Zhang, Y. Wang, X. Yan, Liquid-phase adsorption: characterization and use of activated carbon prepared from diosgenin production residue, Colloids Surfaces A: Physicochem. Eng. Aspects 280 (2006) 9–16. [28] R. Dhodapkar, N.N. Rao, S.P. Pande, S.N. Kaul, Removal of basic dyes from aqueous medium using a novel polymer: Jalshakti, Bioresour. Technol. 97 (2006) 877–885. [29] K.P. Singh, D. Mohan, S. Sinha, G.S. Tondon, D. Gosh, Color removal from wastewater using low cost activated carbon derived from agricultural waste material, Ind. Eng. Chem. Res. 42 (2003) 1965–1976. [30] D. Kavitha, C. Namasivayam, Experimental and kinetic studies on methylene blue adsorption by coir pith carbon, Bioresour. Technol. 98 (2007) 14–21. [31] L. Xitao, Q. Xie, B. Longli, C. Shuo, Z. Yazhi, Simultaneous pentachlorophenol decomposition and granular activated carbon regeneration assisted by microwave irradiation, Carbon 42 (2004) 415–422. [32] Y. Tao, C.Y. Wu, D.W. Mazyck, Removal of methanol from pulp and paper mills using combined activated carbon adsorption and photocatalytic regeneration, Chemosphere 65 (2006) 35–42. [33] S.X. Liu, C.L. Sun, S.R. Zhang, Photocatalytic regeneration of exhausted activated carbon saturated with phenol, Bull. Environ. Contam. Toxicol. 73 (2004) 1017–1024. [34] G. Hahner, A. Marti, N.D. Spencer, W.R. Caseri, Orientation and electronic structure of methylene blue on mica: a near edge X-ray adsorption fine structure spectroscopy study, J. Chem. Phys. 104 (1996) 7749–7757. [35] R.J. Wakeman, B.L. Sorensen, Filtration characterization and specific surface area measurement of activated sludge by rhodamine B adsorption, Water Res. 30 (1996) 115–121. [36] C.H. Weng, E.E. Chang, P.C. Chiang, Characteristics of new coccine dye adsorption onto digested sludge particulates, Water Sci. Technol. 44 (2001) 279– 284. [37] J.C. Santamarina, K.A. Klein, Y.H. Wang, E. Prencke, Specific surface: determination and relevance, Can. Geotech. J. 39 (2002) 233–241. [38] C.H. Weng, Y.F. Pan, Adsorption characteristics of methylene blue from aqueous solution by sludge ash, Colloids Surfaces A: Physicochem. Eng. Aspects 274 (2006) 154–162. [39] Y. Yukselen, A. Kaya, Comparison of methods for determining specific surface area of soils, J. Geotech. Geoenviron. Eng. 132 (2006) 931–936.