Wastewater treatment by multi-stage batch adsorption and electrochemical regeneration doi: 10.5599/jese.2012.0019 223 J. Electrochem. Sci. Eng. 2 (2012) 223-236; doi: 10.5599/jese.2012.0019 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Wastewater treatment by multi-stage batch adsorption and electrochemical regeneration FADHIL M. MOHAMMED‡, EDWARD P. L. ROBERTS, ANDREW K. CAMPEN* and NIGEL W. BROWN* School of Chemical Engineering and Analytical Science, University of Manchester. The Mill, Oxford Road, Manchester M13 9PL, UK; ‡ Current address: Ministry of Science and Technology, Baghdad, Iraq *Arvia Technology Ltd, Daresbury Innovation Centre, Keckwick Lane, Daresbury, Cheshire WA4 4FS, UK. Corresponding Author: E-mail: edward.roberts@manchester.ac.uk; Tel.: +44-161-306-8849; Fax: +44-161-306-9321 Received: May 23, 2012; Revised: August 18, 2012; Published: November 10, 2012 Abstract The removal and destruction of a tri-phenyl methane dye, Acid Violet 17 (AV17), from aqueous solution by adsorption and electrochemical regeneration was studied using a graphite intercalation compound (GIC) adsorbent. It was demonstrated that the adsor- bent could be regenerated by anodic oxidation of the adsorbed dye in a simple electro- chemical cell. The GIC adsorbent recovered its initial adsorption capacity after 40 to 60 min of treatment at a current density of 10 mA cm−2, corresponding to a charge of 12 to 18 C g−1 of adsorbent. The charge passed is consistent with that expected for minerali- sation of the dye, suggesting that the dye was removed and destroyed with high charge efficiency. The energy cost of the regeneration was found to be around 120 J per g of adsorbent regenerated or 115 J per mg of the AV17 dye removed and destroyed. A model describing the process of wastewater treatment by multiple cycles of adsorption and electrochemical regeneration, based on adsorption isotherm data, has been deve- loped and validated. It was found that relatively modest improvements in the adsorption capacity of the adsorbent material could significantly improve the process performance. Keywords Adsorption; electrochemical regeneration; graphite intercalation compound; tri-phenyl methane dye; acid violet. Introduction Dyestuff removal has been one of the most persistent problems in wastewater treatment in the last thirty years. Recent methods which have been investigated for dye removal include adsorp- J. Electrochem. Sci. Eng. 2(4) (2012) 223-236 WASTEWATER TREATMENT ADSORPTION AND REGENERATION 224 tion, ion exchange, chemical oxidation, precipitation and biological treatment [1,2]. There are ever more dyes available commercially, and due to their complex structure and synthetic origin, most are difficult to decolorize [3]. Therefore, it is necessary to remove them from liquid wastes at least to a limit accepted by national and international regulatory agencies before discharge. Adsorption is a widely used technique for the removal of dyes from wastewater and can be effective for overall treatment, particularly if the sorbent is cheap, does not require a pre-treatment step before application and is easy to regenerate [4]. Adsorption on activated carbon is a technology which has been widely studied to remove dyes from wastewater but the high capital and regene- ration cost of activated carbon has led to the search for low cost materials. In the last twenty years, many investigators have studied the feasibility of inexpensive, commercially available mate- rials, that are easy to regenerate and re-utilized as many times as possible [5]. In recent years many inexpensive, widely available materials have been investigated as adsorbents to remove dyes from contaminated water. Wood, fly ash, coal, zeolite, silica, and agricultural wastes have all been tried with varying degrees of success [6-14]. The adsorption of dyes from aqueous solutions onto these inexpensive adsorbent materials focused on the determination of the capacity, kinetics, equilibrium isotherms and the effect of different parameters such as thermodynamics, pH, bonding mechanisms, and desorption [14-16]. However, only limited application of such data has been directed to the modelling of the batch adsorption / regeneration process for wastewater treatment. Electrochemical regeneration of activated carbon adsorbents was demonstrated by Narbaitz and Cen in 1994 [17] and there have been several recent studies of this process [18-20]. However, the process is not economic as regeneration is slow and the energy cost is high [21], due to the poor conductivity of the activated carbon bed. Recent work has shown the potential of graphite intercalation compound (GIC) adsorbent materials that can be electrochemically regenerated very rapidly and cheaply [21-24]. Conventional adsorption processes are designed based on the single use of a batch of adsorbent. The rapid electrochemical regeneration process discussed above opens up the possibi- lity of using a batch of adsorbent for multiple cycles of adsorption with regeneration carried out on-site. In this way the adsorbent can be reused many times. In addition, a treatment process can be envisaged where the water is treated by the same batch of adsorbent through several cycles of adsorption and regeneration. This approach has been used to treat radioactive organic waste in the nuclear industry [25]. In this study we aim to carry out the first detailed study of this multi- stage adsorption and electrochemical regeneration process. Experimental Materials The adsorbate used in this study was Acid Violet 17, a powdered, anionic tri-phenyl methane dye, which has three substituent groups and is a mono sodium salt. It was supplied by KEMTEX Educatio- nal Supplies Ltd under the trade name Kenanthrol Violet 2B at a dye content of about 22 %. Analysis is by UV/Vis spectroscopy at a λmax of 542 nm, as described in our earlier paper [26]. The adsorbent used was a graphite intercalated compound (GIC), NyexTM 1000, supplied by Arvia Technology Ltd., which contains around 94 wt. % carbon and has a particle size range of 100 – 700 µm [26]. F. M. Mohammed et al. J. Electrochem. Sci. Eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 225 Adsorption/electrochemical regeneration methodology A laboratory scale sequential batch rig (see Fig. 1) was used for both adsorption and electrochemical regeneration, which were carried out at the ambient laboratory temperature of 20°C. In each experiment 100 g of GIC adsorbent was mixed with 1 L of water in the adsorption zone by sparging air into the bottom of the rig. After adsorption, the adsorbent was allowed to settle into the anodic compartment of the electrochemical regeneration zone. The adsorbent bed was in contact with a graphite anode and was separated from the perforated stainless steel cathode (316L with open area 33%, 3 mm holes) by a microporous polyethylene separator (Daramic 350, Grace GmbH). The separator acts as a barrier to minimize the transport of electrolyte through the membrane and to ensure that the GIC bed does not contact the cathode, as this would short circuit the electrochemical cell. The cathode compartment was filled with 0.3 wt% NaCl solution to provide good conductivity, and the pH of this solution was adjusted to below 2 using hydrochloric acid to ensure that the membrane was stable. The anode, cathode and membrane of the electrochemical cell had dimensions of 10 cm by 7 cm, and the gap between the anode current feeder and the membrane was 2.2 cm. The 100 g of adsorbent used formed a bed of depth 5 cm in the anode compartment. A DC power supply was used to apply a current of 0.5 A to the cell, corresponding to a current density (from studies by Brown, 2005) of 10 mA cm−2 (based on the membrane area). Ideally electrochemical regeneration should occur throughout the bed depth in the anode compartment. Although it is difficult to measure the current distribution in the packed bed electrode, previous studies with GIC adsorbents have examined the effect of bed thickness [22] or separated and tested layers of adsorbent in the regeneration bed [24]. These studies have indicated that complete regeneration can be achieved with bed depths of up to around 20 mm. Prior to each experiment 100 g of fresh adsorbent was mixed with 1 litre of clean water for 30 min before being allowed to settle into the electrochemical regeneration cell. The water was drained off and a current of 0.5 A was applied for 30 min in order to oxidise any organic impurities present on the surface of the adsorbent. A volume of 1 L of solution containing 120 mg L−1 of AV17 was then added to the cell. This concentration was selected in order to saturate the adsorbent and to give a significant equilibrium concentration. This was necessary to ensure that the equilibrium concentration achieved with the regenerated adsorbent can be accurately measured. The adsorbent and AV17 solution were mixed by sparging air into the cell for 120 min. This adsorption time was found to be sufficient to ensure that equilibrium was reached at these conditions. Once the adsorption stage was complete, the air was switched off and the adsorbent particles settled into the anodic compartment of the electrochemical cell. The treated liquid was drained off and a sample was taken and analysed by UV/visible spectroscopy in order to determine the loading of AV17 dye on the adsorbent. The bed was regenerated for a period of between 10 and 120 min. After regeneration the supernatant was drained from above the bed of GIC adsorbent and the adsorption stage was repeated using a fresh AV17 solution (120 mg L−1) which was added to the cell. The adsorption stage was repeated by sparging with air for a period of 120 min. A sample of the solution obtained after adsorption was analysed by UV/vis spectroscopy, as described above. The performance of the regeneration was characterised using the ‘regeneration efficiency’, obtained by comparing the equilibrium adsorbent loading achieved before and after regeneration: / %  100 (1) J. Electrochem. Sci. Eng. 2(4) (2012) 223-236 WASTEWATER TREATMENT ADSORPTION AND REGENERATION 226 where qi and qr are the equilibrium loading (qe) of AV17 on the adsorbent (mg g −1) obtained before and after regeneration, respectively, calculated using Equation (2). (2) where C0 and Ce are the initial and equilibrium concentration of AV17 in solution respectively (mg L−1), V is the volume of solution (L), and W is the mass of adsorbent used (g). Figure 1. Laboratory scale sequential batch rig for electrochemical regeneration of the GIC adsorbent: schematic diagram of the side (a) and front (b) elevation of the rig, and (c) schematic diagram showing a cross section of the electrochemical regeneration zone. (c) Electrochemical regeneration zone Catholyte Adsorbent bed Anode Membrane Cathode Hydrogen Air in Regeneration Zone Adsorption Zone (b) Front View 10 cm 37 cm 30 cm 12.3 cm 2.2 cm 11.5 cm Air in (a) Side View F. M. Mohammed et al. J. Electrochem. Sci. Eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 227 A similar technique was used for multi-stage batch adsorption and electrochemical regeneration, but in this case the solution was not drained off after the adsorption cycle. In this case, the initial concentration of AV 17 was much higher, 668 mg L−1 as this solution was treated by a series of adsorption/regeneration cycles. For these experiments, the amount of GIC adsorbent was increased to 125 g, which formed a bed of 7 cm depth in the anode compartment. In addition, the adsorption time was reduced to 60 min and the regeneration time was 30 min. Results and Discussion Adsorption isotherm The design of a batch adsorber system requires knowledge of the equilibrium isotherm to understand the adsorption process behaviour and provide fundamental physiochemical data for evaluating the maximum capacity of the adsorbent [27]. Batch sorption studies have been perfor- med previously to investigate the kinetic and equilibrium isotherm of the adsorption of the Acid violet 17 [26]. The experimental data were found to fit the Langmuir isotherm model, Equation (3). L e e e1 bk C q bC = + (3) where kL and b are the Langmuir constants related to the capacity of the adsorbent (mg g −1) and the intensity of adsorption, (L mg−1), respectively. There is little data available in the literature on the adsorption of AV17 by activated carbon, but there is some data on other dyes such as methylene blue [28]. The comparison of Langmuir isotherm constants for adsorption of organic dye calculated in this work with those determined by [28] for activated carbon are shown in Table 1. The low surface area of the adsorbent leads to a relatively low adsorption capacity, as indicated by the relatively low value of kL. However, if the adsorptive capacity (kL) is normalised with the specific surface area, it is found that the GIC adsorbent is able adsorb a higher mass of dye per unit area than the activated carbons studied (albeit for a different adsorbate). This suggests that either the GIC has more adsorption sites per unit area or (more likely) that some of the activated carbon surface area in the micropores and is not accessible to the dye molecules. Table 1. Comparison of Langmuir constants and surface area for adsorption of methylene blue (MB), and AV17 onto activated carbons [28] and the GIC adsorbent NyexTM 1000 [29], respectively, at room temperature (23°C) and normal pH. Adsorbent Adsorbate Langmuir constant Surface area (BET), m2 g−1 Adsorptive capacity, mg m−2 kL / mg g −1 b / L mg−1 GIC (NyexTM 1000) AV17 0.987 0.31 1.0 0.987 Activated carbon (PAC1) MB 307 0.12 863.50 0.36 Activated carbon (PAC2) MB 345 0.15 857.14 0.4 Activated carbon (F400) MB 455 0.2 1216.4 0.37 The essential characteristic of the Langmuir isotherm shape on whether adsorption is ‘favourable’ or ‘unfavourable’ can be classified in terms of dimensionless separation factor or equilibrium parameter, RL , Equation (4) [30]. J. Electrochem. Sci. Eng. 2(4) (2012) 223-236 WASTEWATER TREATMENT ADSORPTION AND REGENERATION 228 L mx 1 1 R bC = + (4) where Cmx is the maximum solution concentration studied. The separation factor RL indicates the isotherm shape according to Table 2. Table 2. Effect of separation factor on isotherm shape [30]. 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