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Atrazine removal using adsorption and electrochemical regeneration
ARTICLE IN PRESS
Water Research 38 (2004) 3067–3074
Atrazine removal using adsorption and electrochemical regeneration N.W. Brown*, E.P.L. Roberts, A. Chasiotis, T. Cherdron, N. Sanghrajka Department of Chemical Engineering, Environmental Technology Centre, UMIST, P.O. Box 88, ManchesterM60 1QD, United Kingdom Received 28 July 2003; received in revised form 27 February 2004; accepted 15 April 2004
Abstract This paper demonstrates the removal of atrazine using a novel carbon-based adsorbent to below 1.0 mg l1 and its subsequent electrochemical regeneration in a simple electrochemical cell. Effective electrochemical regeneration can be achieved with a treatment time as low as 20 min over a number of adsorption/regeneration cycles using laboratoryprepared solutions. The results suggest that electrochemical modification of the particulate surface on electrochemical regeneration can result in adsorptive capacities three times greater than originally achieved. r 2004 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Atrazine; Carbon; Electrochemical regeneration
1. Introduction Contamination of water is a widespread problem in the UK, Europe and the US as a result of agricultural, non-agricultural and industrial pollution. The extent of the problem is highlighted by the US Geological Survey National Water Quality Assessment program (1992– 1996) where over 95% of surface water and 50% of groundwater samples (sample size 8200) contained at least one pesticide [1]. In the European Union some 700 synthetic substances, 300 of them pesticides, have been detected in water for human consumption [2]. These pesticides are amongst the most refractory organic chemicals and they have been long suspected of causing severe diseases such as leukaemia [2,3]. The European Union 1998 Drinking Water Directive specifies a limit of 0.1 mg l1 for individual pesticides with a maximum pesticide total of 0.5 mg l1. Atrazine is the commonest pollutant in US ground and surface waters [4,5], as well as the most common pesticide pollutant of UK groundwaters [6]. A maximum atrazine concentration 5.42 mg l1 was found in UK *Corresponding author. E-mail address: [email protected] (N.W. Brown).
waters during Environment Agency sampling in 1997 [7]. In a Portuguese water quality program, atrazine was detected in 64% of water samples at levels of up to 0.63 and 30 mg l1 for surface and ground waters, respectively [8]. It is one of the most difficult pesticides to remove from drinking water supplies [7]. Due to its hydrophilic and highly refractory nature, conventional treatment techniques such as coagulation, clarification, filtration and chlorination have been found to be incapable of achieving significant removal of atrazine [9]. Adsorption on activated carbon, either in its granular (GAC) or in its powdered (PAC) form, is the most commonly employed process for treatment of raw drinking water supplies contaminated with atrazine and pesticides in general [10]. The high-internal surface area, the microporous structure and the appearance of certain functional groups on the carbon surface enable the activated carbon to exhibit high-adsorption capacities for dissolved organics. Once these carbons have been exhausted they can be disposed of by landfill or incineration. Alternatively they can be regenerated. Regeneration is widely used as it represents the most commercially viable and environmentally acceptable option [11]. Industrially the most widely used regeneration process is thermal regeneration. However, this is a
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.04.043
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high energy and cost process which results in material losses of around 10% and, except for large users, off-site regeneration. Therefore, research into the development of alternative adsorbents and/or regeneration processes has been carried out. Alternative adsorbents that have been investigated for the removal of atrazine include hypercrosslinked polymers [7] and zeolites and organoclays [12]. Although these materials exhibited lower adsorption capacity for atrazine (compared to activated carbon) the feasibility of any adsorption process depends greatly on the cost of regeneration of spent adsorbents [13]. Adsorbent regeneration techniques that have been investigated include chemical and solvent [7,14,15], microbial [16], ultrasonic [17], wet air oxidation [18] and electrochemical regeneration. Electrochemical regeneration refers to the regeneration of a used adsorbent inside an electrolytic cell. The regeneration involves desorption and/or destruction of the adsorbed organic matter restoring the adsorptive capacity. Electrochemical processes have been widely investigated as methods of treating dissolved organics in wastewater [19–21], but electrochemical regeneration of adsorbents has not been widely investigated. The first report on electrochemical regeneration of activated carbon was by Owen and Barry [22] who achieved regeneration efficiencies of up to 61% and suggested that the process merited further attention. While a number of other researchers have undertaken some research into electrochemical regeneration of activated carbon, Narbaitz and Cen [23] and Zhang [24] have undertaken the most detailed investigations using GACs loaded with phenol, with regeneration efficiencies of up to 95%. Regeneration was found to be greatest with the loaded GAC being placed on the cathode. Narbaitz and Cen [23] suggested that the electrochemical effects are restricted to the external surface of the carbon and for regeneration at the cathode surface the process consisted of initial phenol desorption followed by phenol destruction. To date no research appears to have been conducted on the electrochemical oxidation of atrazine or on the electrochemical regeneration of an adsorbent loaded with atrazine. This paper reports the use of a novel carbon-based adsorbent material, Nyex 100 for the removal of atrazine from aqueous solutions. The paper also reports the electrochemical regeneration of this non-porous and highly conducting material.
2. Materials and methods The Nyex 100 was supplied by Nykin Developments in the form of a wet powder with an average dry solids content of 50%. Nyex 100 was chosen as it contains non-porous particles with no internal surface area. This
was thought likely to result in quick adsorption and electrochemical regeneration rates as intra-particle diffusion would be eliminated (often the rate limiting step). Nyex 100 is a low-cost carbon powder (carbon content over 95% with a mean particle diameter of 124 mm and a range of 10–600 mm) that has been treated by a proprietary process. The BET surface area of the particles was determined by nitrogen adsorption and was found to be 2.75 m2 g1. This is very low compared with an activated carbon with surface areas of up to 2000 m2 g1 [25]. Mercury porosimetry indicated that there were virtually no internal pores in the material explaining the low surface area obtained. Atrazine was supplied by Ehrenstorfer GmbH with a purity of 98.4%. This was prepared as a 10 mg l1 stock solution in deionised water, with 5 days mixing being required for dissolution. All other chemicals were supplied as analytical grade by Aldrich. 2.1. Adsorption kinetics A batch adsorption experiment was conducted to determine the time required to achieve equilibrium. An accurately weighed dose of Nyex 100 was added to a 250 ml of 5 mg l1 Atrazine solution in a 250 ml flask. The flask was sealed and shaken for 18 h (Unimax 1010, Heidolph Instruments). At regular intervals for the first 2 h, 15 ml samples were taken, filtered (Whatman GF/C filter paper) and analysed for unadsorbed atrazine. The flask was then mixed for a further 16 h to ensure equilibrium had been achieved. This demonstrated that equilibrium was achieved within 60 min, with 80% of the equilibrium value being achieved within 10 min. Subsequent adsorption trials used contact times of 60 min. 2.2. Adsorption studies Adsorption isotherms were prepared by adding various known weights of adsorbent to 100 ml of 6– 8 mg l1 atrazine solution in a 250 ml flask. These flasks were sealed and shaken for 1 h. After adsorption the solution was filtered (Whatman GF/C) and analysed. A control experiment to determine the effect of Nyex 100 on the UV adsorbancy of the filtrate was achieved by mixing known quantities of adsorbent with 100 ml of deionised water and mixing for 1 h. The effect of acid conditions on atrazine adsorption onto Nyex 100 was investigated by adjusting the pH of the initial solution to 3 using hydrochloric acid, prior to adding the adsorbent. The ability of the Nyex 100 to achieve low discharge concentrations was investigated by adding known amounts of adsorbent to 500 ml of 10 mg l1 atrazine solution. After 1 h shaking the filtered water samples were analysed for atrazine residues.
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2.3. Electrochemical regeneration Regeneration of the adsorbent was achieved by mixing the filtered, wet adsorbent with solid NaCl and placing the mixture in the anode compartment of a batch electrochemical cell (Fig. 1). It comprised a parallel divided cell with a perforated 316 stainless-steel cathode separated from a mixed metal oxide-coated titanium anode (supplied by Electrode Products Technology Ltd.) by a microporus Daramic 350 membrane. The anode was placed 10 mm from the membrane, with the active area of the anode being dependent on the mass of adsorbent being regenerated, typically 5 cm2. The electrolyte in the cathode compartment was a 2% w/w NaCl solution. The regeneration procedure was (i) Initial adsorption—A known weight of Nyex 100 was added to 200 ml of 6 mg l1 atrazine solution in a 250 ml flask and mixed for 1 h. After adsorption the flask contents were filtered (Whatman GF/C) and the atrazine concentration of the filtrate determined. (ii) Electrochemical regeneration—The loaded, filtered (but still wet approx. 50% dry solids) adsorbent was mixed with NaCl crystals to give a 2% w/w concentration in the liquid phase of the mix. The mixture was pressed into the anode compartment. A DC current of 400 mA was applied for regeneration times in the range 10–60 min. There was no flow in the cell and the only mixing was due to the gas bubbles produced at the electrodes. (iii) Re-adsorption—The contents of the anodic compartment (with no additional treatment) were transferred to a 250 ml volumetric flask to which 200 ml of 6 mg l1 atrazine solution were added and mixed for 1 h. After adsorption the flask contents were filtered (Whatman GF/C) and the atrazine concentration of the filtrate determined.
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For the adsorption/electrochemical regeneration over a number of cycles, steps B and C were repeated. 2.4. Atrazine analysis Atrazine concentrations (0.1–10 mg l1) were determined using UV/VIS spectrometry at a wavelength of 220 nm. However the addition of Nyex 100 to deionised water resulted in a slight increase in the UV adsorbancy of the filtered water at this wavelength, probably due to the leaching of an unknown compound from the Nyex 100. A correcting curve was determined and the data for the atrazine concentrations has been corrected for this leaching effect. Calibration curves were prepared at pH 7 and 3. For lower concentrations, solid-phase extraction was used to pre-concentrate the atrazine prior to analysis using a Varian HPLC system (Prostar with a Hypersil C18 column and a photodiode array detector) with an eluent flow rate of 1.2 ml min1. Eluent was an 80:20 H2O:CH3CN mix with 1 ml l1 H3PO4. This method could determine atrazine concentrations down to 0.5 mg l1 although the accuracy was estimated as no better than 70.5 mg l1. The solid-phase extraction procedure was to pass 10 ml CH3CN, 10 ml CH3OH and 10 ml deionised water consecutively through a C18 cartridge (Varian). Five hundred millilitre of effluent sample was then passed through the cartridge at 6 ml min1, followed by 10 ml of NaCl solution (6 g l1). The cartridge was then dried under vacuum. The atrazine was eluted with 8 ml of CH3CN:CH2Cl (50:50) and dried with a stream of nitrogen. The remaining solids were reconstituted using 1 ml of CH3CN:H2O (20:80) with 1 ml l1 H3PO4.
3. Results and discussion 3.1. Adsorption studies
Fig. 1. Electrochemical cell schematic.
Adsorption studies using both high concentrations (up to 8 mg l1) and low concentrations (less than 10 mg l1) atrazine solutions were undertaken. The high concentrations were used to determine an adsorption isotherm for the atrazine/Nyex system and the low concentrations were used to assess the capability of the Nyex 100 to achieve the low discharge concentrations required. The Freundlich Eq. (1) was found to fit the data effectively. The Freundlich constants were obtained from a log–log plot of the solid-phase concentration (q) versus the liquid-phase concentration (Ce ) at equilibrium (Fig. 2). The values of the Freundlich constants were Kf ¼ 0:279 and 1=n ¼ 0:550; found from
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pH 7 (Fig. 4).
the least-squares fit of the data. q ¼ Kf Ce1=n :
ð1Þ
A least-squares fit of the Langmuir equation was also obtained, but Fig. 3 clearly shows that the Freundlich equation gives a superior fit of the data, particularly at higher concentrations. There is considerable scatter of the points around the adsorption isotherms shown in Fig. 3. This scattering is due to two causes: (1) batch to batch variation of the Nyex 100 and (2) adsorption studies being conducted at room temperatures (this resulted in samples on different days being subjected to different temperatures, 17–26 C). Studies have shown that adsorption is temperature dependent. Since the regeneration of the adsorbent occurs within the anodic compartment of the electrochemical cell, the electrolysis of water is likely to be a significant side reaction generating hydrogen ions (Eq. (2)). As the Nyex is to be reused after regeneration without any additional treatment, the adsorption of atrazine onto Nyex 100 under acid conditions was investigated. This showed that adsorption at pH 3 gave similar results to that at
2H2 O 4e -O2 þ 4Hþ :
The ability of Nyex 100 to achieve low discharge concentrations was investigated by adding different amounts of adsorbent to a solution containing 10 mg l1 of atrazine. This showed that Nyex 100 is capable of achieving low discharge concentrations (o1 mg l1), with a dose rate of around 6 g l1 (Fig. 5). The adsorption isotherm in Fig. 3 and the atrazine removals in Fig. 5 show that the adsorptive capacity (defined as the mass of atrazine adsorbed per gram of adsorbent in mg l1) of Nyex 100 is very much less than activated carbon. Table 1 compares the Freundlich constants calculated in this work with those determined by Streat and Horner [7] for activated carbon F-400 and Speth and Miltner [26] for pulverised F-400. This low capacity is due to the very low surface area of the Nyex 100 particles (2.75 m2 g1) compared with activated carbon (up to 2000 m2 per g1) [25]. However, the cost of treatment is dependent on the cost of regeneration and replacement of adsorbent.
0
-0.6 -0.8 -1 -1.2 -1
-0.5
0
0.5 Log Ce
1
1.5
1 0.8 0.6 0.4 0.2 0 0
Fig. 2. Adsorption isotherm for atrazine/Nyex 100, where Ce and q are in mg l1 and mg g1 respectively. The straight line shows the least-squares fit for the data; log q¼ log Kf þ 1=n log Ce :
1
2
3
4
5
6
7
8
-1 Liquid phase equilibrium concentration Ce (mgl )
Fig. 4. Adsorption capacity at pH 3. The solid line gives the Freudlich equation for fresh Nyex 100 at pH 7.
1.2
100
4.5
90
4
1 80
3.5
70 % removal
0.8
0.6
0.4
3
60 2.5 50
% removal Concentration
2
40
1.5
30
0.2
0
20
1
10
0.5
0 0
2
4 6 8 10 -1 Liquid phase equilibrium concentration Ce (mgl )
12
Fig. 3. Adsorption Isotherms for atrazine adsorption on Nyex 100. The upper thin line shows the Freundlich isotherm and the lower thick line shows the Langmuir isotherm.
Final concentration (µg l-1)
Log q
-0.4
Solid phase equilibrium concentration q (mg g-1)
1.2
-0.2
Solid phase equilibrium concentration q (mg g-1)
ð2Þ
0 0
2
4
6
8
10
12
14
16
18
20
Dose rate g/l
Fig. 5. Removal of atrazine from low concentration solution using adsorption by Nyex 100. Initial atrazine concentration in mg l1.
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Electrochemical regeneration of the Nyex adsorbent is achieved by electrochemical oxidation of the adsorbed atrazine with the adsorbent acting as the anode within the electrochemical cell. Since the total contents of the anodic compartment are transferred (without any treatment after regeneration) to the next adsorption cycle, any desorbed but unoxidised atrazine would be present in the filtrate of the next adsorption cycle. The fact that there is no washing of the adsorbent prior to adsorption means that no secondary effluent is generated, but it does mean that any partially oxidisied organic compounds could contaminate the filtrate after subsequent adsorption cycles. Hence, the fate of the adsorbed organics are important considerations and are the subject of on-going research (which will be the subject of a future paper). Samples of regenerated Nyex 100 (previously loaded using high concentration, 6 mg l1, atrazine solution) were prepared by passing a current of 400 mA for 20 min. This material was then used to prepare an adsorption isotherm to investigate the effect of regeneration. This isotherm (Fig. 6) shows that the regenerated material gives similar results to that obtained by fresh material. The low-adsorptive capacity of the adsorbent means that the process will only be commercially viable for removing atrazine from waters if the adsorbent is capable of being cheaply regenerated many times. Fig. 7 shows the adsorptive load of the adsorbent over a Table 1 Comparison of Freundlich constants for activated carbon (F400) and Nyex 100 Freundlich constant
Nyex 100
F-400 [7]
F-400 Pulverised [26]
K 1=n
0.279 0.550
2488.2 0.486
858 0.291
number of cycles. As can be seen from this figure, there is an increase in the adsorptive capacity of the adsorbent after initial regeneration and this is maintained during subsequent regenerations. In order to investigate the effect of electrochemical regeneration on the adsorptive capacity, a series of electrochemical regenerations were undertaken varying the treatment time of loaded Nyex 100 in the electrochemical cell. These are plotted, Fig. 8, as regeneration time against percentage regeneration (PR). The PR is based on the adsorptive capacity of the regenerated Nyex 100, qr ; compared with the calculated adsorptive capacity, qcal ; of fresh Nyex 100 at the same liquid-phase concentration [27]. PR ¼
qr 100: qcal
ð3Þ
The adsorptive capacity of fresh Nyex 100 is calculated from the Freundlich Eq. (1) using the Adsorptive capacity (mg atrazine/g Nyex 100)
3.2. Electrochemical regeneration
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0.6 0.5 0.4 0.3 0.2 0.1 0 1
2
3
4 Adsorption cycle
5
6
7
Fig. 7. Adsorption capacity of Nyex for a number of adsorption/regeneration cycles. Regeneration conditions; Current 400 mA, Treatment time 20 min, Electrolyte concentration 2% sodium chloride. Cycle 1 uses fresh adsorbent.
350
Percentage Regeneration (%)
300 Solid phase equilibrium concentration q (mg g-1)
0.80 0.70 0.60 0.50 0.40 0.30
250 200 150 100 50
0.20 0 0
0.10 0.00 0.00
1.00
2.00
3.00
4.00
5.00
6.00
-1 Liquid phase equilibrium concentration Ceq (mgl )
Fig. 6. Adsorption isotherm for regenerated material. The solid line gives the Freundlich equation for fresh Nyex 100.
10
20
30 40 Treatment time (mins)
50
60
70
Fig. 8. Regenerated capacity at various treatment times. Regeneration conditions; Current 400 mA, Electrolyte concentration 2% sodium chloride (in cathode and anode). PR (Eq. (3)) is the adsorptive capacity of regenerated compared with fresh Nyex 100.
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experimentally obtained values of the Freundlich constants. The PR was calculated at a number of liquid phase equilibrium concentration values and averaged. As can be seen from Fig. 8, there is a pronounced increase in capacity with increased regeneration time. Two points of interest can be noted from this figure. A regenerated adsorptive capacity in excess of the original adsorptive capacity and an apparent regenerated capacity without electrochemical treatment. This apparent capacity can be explained by the second adsorption onto the loaded Nyex resulting in an increased liquid-phase equilibrium concentration. This is shown schematically in Fig. 9. During the initial loading of the Nyex 100 a liquid-phase equilibrium concentration, Ce1 ; is achieved giving a corresponding solid-phase equilibrium concentration, qe1 : If this loaded Nyex is then used under the same conditions with no regeneration, then, after the system has reached equilibrium, the liquid-phase equilibrium concentration will be Ce2 : This gives a corresponding solid-phase concentration of qe2 : Since there was already a quantity of adsorbate on the adsorbent, qe1 ; the additional adsorbate taken up, Dq ¼ qe2 qe1 . Hence, the apparent PR without treatment. Four possibility causes of the increase in adsorptive capacity above the initial capacity have been identified. (i) The transfer of oxidising species from the electrochemical cell to the atrazine mixture resulting in the removal of atrazine by chemical oxidation rather than adsorption. (ii) Increased atrazine adsorption in acid conditions. (iii) The generation of internal pores increasing the surface area available for adsorption. (iv) Modification of the surface chemistry.
Nyex 100 before a second adsorption (to remove these species) actually increased the adsorptive capacity, the opposite of what would be expected if oxidising species were being transferred [28]. The second option of a pH effect is also possible as the regeneration of the Nyex 100 in the anodic compartment of the electrochemical cell results in a significant decrease in the pH of the atrazine solution. Increasing the treatment time results in a greater reduction in the solution pH. However, as shown in Fig. 5, acid conditions have no significant effect on the adsorptive capacity of fresh Nyex 100. The surface area of fresh (2.75 m2 g1) and regenerated Nyex 100 (2.8 m2 g1) suggest that the formation of internal pores on electrochemical treatment is unlikely. However, SEM photographs of the surface (Fig. 10) do show a surface roughening after treatment. This could be due to an exfoliation of some of the graphene layers within the carbon matrix. The modification of the surface chemistry of activated carbon by anodic electrochemical treatment has been demonstrated [29], although this resulted in a reduction in adsorbate uptake. Hence, a possible explanation for the increase in atrazine adsorption onto Nyex 100, based on a modification of the particle surface and chemistry, is proposed below. This explanation is based on suggested mechanisms for the adsorption of aromatic organics on activated carbon, an area that has been the subject of much research.
Earlier test work on the removal of dyes from water investigated the transfer of oxidising species, for example chlorine or hypochlorite, from the electrochemical cell after electrolysis. The presence of these species was believed to result in chemical oxidation of the pollutants. However that work showed that washing the
qe2
q qe1
q
Ce1
Ce2
Ce
Fig. 9. Schematic of loading onto fresh and loaded Nyex 100 without electrochemical regeneration. The curve gives the adsorption isotherm for fresh Nyex 100.
Fig. 10. SEM pictures of fresh (left) and regenerated (right) Nyex 100 at 1000 magnification.
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The interactions of aromatic organics, specifically phenol, with activated carbon were listed by Laszlo and Szucs [30] as (a) a dispersion effect between the adsorbate aromatic ring and the p electrons of the graphitic structure, (b) electron donor–acceptor interaction between the aromatic ring and the basic surface oxygens (functional groups), (c) electrostatic interactions when ions are present. Functional groups containing oxygen in activated carbons are found primarily at the edges of broken graphitic planes, with basal planes consisting of largefused aromatic structures [31]. Hence the relative contribution of functional groups versus the graphene layers to the development of surface charge are a key issue, but such information is not readily available in the literature [32]. Since electrostatic repulsion of the adsorbate from the graphene layer is thought to be much more detrimental for adsorbent effectiveness than repulsion from the basic oxygen functional groups [32], changing this ratio will change the adsorbate uptake. Mattson et al. [31] proposed that the aromatic compounds, phenol and nitrobenzene, adsorb on activated carbon by a donor–acceptor complex mechanism involving a carbonyl oxygen of the carbon surface acting as the electron donor and the aromatic ring of the solute acting as the acceptor. The oxidation of the activated carbon surface either chemically [33] or electrochemically [29] would result in the oxidation of the carbonyl oxygen to carboxylic acid oxygen. Since carboxylic acid oxygen has a smaller dipole moment than carbonyl oxygen, it would be expected to act as a weaker donor, reducing the adsorption rate, as observed by Coughlin [33], Mehta and Flora [29] and Mattson et al. [31]. Hence, electrochemical regeneration of Nyex 100 would be expected to increase the concentration of carboxylic acid oxygen reducing the adsorbate adsorption rate. However, in the case of Nyex 100, the photographs suggest that the untreated Nyex 100 has a strong graphitic structure and that on electrochemical regeneration the flat graphene layers are being broken up, increasing the surface roughness. Hence, the relative contribution of the edges (oxygen containing functional groups) will increase. Since there is an increase in adsorbate uptake, it suggests that any loss in adsorbate uptake due to the conversion of carbonyl oxygen to carboxylic acid oxygen is less significant than the increase in edge/plane effects due to electrochemical treatment. Further work is required to confirm this proposed mechanism.
4. Conclusions This work has demonstrated that the carbon based adsorbent material, Nyex 100, can be used to remove
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atrazine to low levels, below 1.0 mg l1. One-hundred per cent electrochemical regeneration of the adsorbent is possible by passing a current of 400 mA for 20 min, giving a regeneration charge of 96 C g1 (adsorbent). However increasing the charge passed results in an increase in adsorbtive capacity, believed to be due to an increase in the edge/planar ratio of the Nyex 100. The use of the Nyex 100 over a number of adsorption/ electrochemical regeneration cycles with no loss in capacity has been demonstrated. Whilst this initial work suggests that the process may have merit, significant further work must be undertaken. This investigation has looked at the removal of a single compound from deionised water, a condition never likely to be achieved in practice. The competing effects of other organics, particularly natural organic matter, will need further experiments, both in natural waters and for treating industrial effluents. The reuse of the regenerated adsorbent with no washing gives a significant benefit to the process as no secondary effluent stream is produced. However, the fate of the adsorbed organics becomes of greater significance as there is the potential for desorption of partially desorbed (and potentially more toxic) organics. This is an area of ongoing research and the electrochemical breakdown products are being determined. There is also the need to determine the stability of the adsorbent over long term adsorption/electrochemical regeneration trials as the increase in surface roughness may result in particle attrition. This will have a major impact on the process costings. Significantly further work on optimising the various process parameters and reducing the regeneration charge passed per gram of adsorbent is required.
Acknowledgements This was undertaken with financial assistance from the EPSRC. Further material and financial assistance for this project has been received from Severn Trent Water Ltd., Nykin Developments and Electrode Products Technology Ltd. The authors also wish to express their thanks to Dick Plaisted of the Department of Chemistry, UMIST, who carried out the surface area analysis and pore size determination and Ken Eccleston of Nykin Developments for the use of their facilities.
References [1] Gilliom RJ, Barbash JE, Gobert YM, Kolpin DW, Larson SJ, Nakagaki N, Wilber WG. Pesticides in streams and ground after of the United States, 1993–1995. In: Ninth International Congress of Pesticide Chemistry, 1998. [2] Hurst P, Hay A, Dudley N. The pesticide handbook. London: Journeyman Press; 1991.
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[3] Calderon RL. The epidemiology of chemical contaminants in drinking water. Food Chem Toxicol 2000;38:S13–20. [4] Kolpin DW, Martin JD, Pesticides in ground water: summary statistics: preliminary results from cycle 1 of the National Assessment Program (NAWQA) 1992–2001. Provisional data, US Geological Survey; http://ca.water. usge.gov/pnsp, 2003. [5] Martin JD, Crawford CG, Larson SJ, Pesticides in streams: summary statistics: preliminary results from cycle 1 of the National Assessment Program (NAWQA) 1992–2001. Provisional data, US Geological Survey; http://ca.water.usge.gov/pnsp, 2003. [6] Environment Agency. Pesticides 2001, 2002. Environment Agency, ISBN 1 844320472. [7] Streat M, Horner DJ. Adsorption of highly soluble herbicides from water using activated carbon and hypercrosslinked polymers. Process Saf Environ Prot 2000;78(B5):363–82. [8] Cerejeira MJ, Viana P, Batista S, Pereira T, Silva E, Valerio MJ, Silva A, Ferreira M, Silva Fernandes AM. Pesticides in Portuguese surface and ground waters. Water Res 2003;37:1055–63. [9] Miltner RJ, Baker DB, Speth TF, Fronk CA. Treatment of seasonal pesticides in surface waters. J Am Water Works Assoc 1989;81:43–52. [10] Kouras A, Zouboulis A, Samara C, Kouimtzis Th. Removal of pesticides from aqueous solutions by combined physicochemical processes—the behaviour of lindane. Environ Pollut 1998;103:193–202. [11] San Miguel G, et al. The regeneration of field-spent granular-activated carbons. Water Res 2001;35(11): 2740–8. [12] Bottero JY, Khatib K, Thomas F, Jucker K, Bersillon JL, Mallevialle J. Adsorption of atrazine onto zeolites and organoclays, in the presence of background organics. Water Res 1994;28(2):483–90. [13] Banat FA, Al-Bashir B, Al-Asheh S, Hayajneh O. Adsorption of phenol by bentonite. Environ pollut 2000;107(3):391–8. [14] Leng CC, Pinto NG. An investigation of the mechanisms of chemical regeneration of activated carbon. Ind Eng Chem Res 1996;35(6):2024–31. [15] Cooney DO, et al. Solvent regeneration of activated carbon. Water Res 1983;17(4):403–10. [16] Hutchinson DH, Robinson CW. A microbial regeneration process for antigranulocytes activated carbon. 2. Regeneration studies. Water Res 1990;24(10):1217–23. [17] Zhang HP, Wang S, Liu Z. Ultrasonic regeneration of granular activated carbon. Environ Eng Sci 2003;20(1): 57–64.
[18] Mundale VD, et al. Regeneration of spent activated carbon by wet air oxidation. Can J Chem Eng 1991;69(5):1149–59. [19] Murphy OJ, et al. Direct electrochemical oxidation of organics for waste-water treatment. Water Res 1992;26(4):443–51. [20] Johnson DC, et al. Direct electrochemical degradation of organic wastes in aqueous media. Electrochim Acta 2000;46(2–3):323–30. [21] Panizza M, et al. Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res 2000;34(9):2601–5. [22] Owen PH, Barry JP, Electrochemical carbon regeneration. Environics Inc., Huntington Beach, California. Report number PB 239156, 1972. [23] Narbaitz RM, Cen JQ. Electrochemical regeneration of granular activated carbon. Water Res 1994;28(8):1771–8. [24] Zhang HP, et al. Regeneration of phenol-saturated activated carbon in an electrochemical reactor. J Chem Technol Biotechnol 2002;77(11):1246–50. [25] Streat M, et al. , Sorption of phenol and para-chlorophenol from water using conventional and novel activated carbons. Water Res 1995;29(2):467–72. [26] Speth TF, Miltner RJ. Technical note—adsorption capacity of gac for synthetic organics. J Am Water Works Assoc 1990;82(2):72–5. [27] Narbaitz RM, Cen J. Alternative methods for determining the percentage regeneration of activated carbon. Water Res 1997;31(10):2532–42. [28] Brown NW. PhD Thesis (in preparation). Dept. of Chemical Engineering, UMIST, 2004. [29] Mehta MP, Flora JRV. Effects of electrochemical treatment of granular activated carbon on surface acid groups and the adsorptive capacity for phenol. Water Res 1997;31(9):2171–6. [30] Laszlo K, Szucs A. Surface characterization of polyethyleneterephthalate (PET) based activated carbon and the effect of pH on its adsorption capacity from aqueous phenol and 2,3,4-trichlorophenol solutions. Carbon 2001;39(13):1945–53. [31] Mattson JS, Mark Jr HB, Malbin MD, Weber Jr WJ, Crittenden JC. Surface chemistry of active carbon: specific adsorption of phenols. J Colloid Interface Sci 1969;31(1):116–30. [32] Radovic LR, Moreno-Castilla C, Rivera-Utrilla J. Carbon materials as adsorbents in aqueous solutions. Chemistry and Physics of Carbon 2001;27:227–405. [33] Coughlin RW, Ezra FS, Tan RN. Influence of chemisorbed oxygen in adsorption onto carbon from aqueous solution. J Colloid Interface Sci 1968;28(3/4):386–96.
Water Research 38 (2004) 3067–3074
Atrazine removal using adsorption and electrochemical regeneration N.W. Brown*, E.P.L. Roberts, A. Chasiotis, T. Cherdron, N. Sanghrajka Department of Chemical Engineering, Environmental Technology Centre, UMIST, P.O. Box 88, ManchesterM60 1QD, United Kingdom Received 28 July 2003; received in revised form 27 February 2004; accepted 15 April 2004
Abstract This paper demonstrates the removal of atrazine using a novel carbon-based adsorbent to below 1.0 mg l1 and its subsequent electrochemical regeneration in a simple electrochemical cell. Effective electrochemical regeneration can be achieved with a treatment time as low as 20 min over a number of adsorption/regeneration cycles using laboratoryprepared solutions. The results suggest that electrochemical modification of the particulate surface on electrochemical regeneration can result in adsorptive capacities three times greater than originally achieved. r 2004 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Atrazine; Carbon; Electrochemical regeneration
1. Introduction Contamination of water is a widespread problem in the UK, Europe and the US as a result of agricultural, non-agricultural and industrial pollution. The extent of the problem is highlighted by the US Geological Survey National Water Quality Assessment program (1992– 1996) where over 95% of surface water and 50% of groundwater samples (sample size 8200) contained at least one pesticide [1]. In the European Union some 700 synthetic substances, 300 of them pesticides, have been detected in water for human consumption [2]. These pesticides are amongst the most refractory organic chemicals and they have been long suspected of causing severe diseases such as leukaemia [2,3]. The European Union 1998 Drinking Water Directive specifies a limit of 0.1 mg l1 for individual pesticides with a maximum pesticide total of 0.5 mg l1. Atrazine is the commonest pollutant in US ground and surface waters [4,5], as well as the most common pesticide pollutant of UK groundwaters [6]. A maximum atrazine concentration 5.42 mg l1 was found in UK *Corresponding author. E-mail address: [email protected] (N.W. Brown).
waters during Environment Agency sampling in 1997 [7]. In a Portuguese water quality program, atrazine was detected in 64% of water samples at levels of up to 0.63 and 30 mg l1 for surface and ground waters, respectively [8]. It is one of the most difficult pesticides to remove from drinking water supplies [7]. Due to its hydrophilic and highly refractory nature, conventional treatment techniques such as coagulation, clarification, filtration and chlorination have been found to be incapable of achieving significant removal of atrazine [9]. Adsorption on activated carbon, either in its granular (GAC) or in its powdered (PAC) form, is the most commonly employed process for treatment of raw drinking water supplies contaminated with atrazine and pesticides in general [10]. The high-internal surface area, the microporous structure and the appearance of certain functional groups on the carbon surface enable the activated carbon to exhibit high-adsorption capacities for dissolved organics. Once these carbons have been exhausted they can be disposed of by landfill or incineration. Alternatively they can be regenerated. Regeneration is widely used as it represents the most commercially viable and environmentally acceptable option [11]. Industrially the most widely used regeneration process is thermal regeneration. However, this is a
0043-1354/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2004.04.043
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high energy and cost process which results in material losses of around 10% and, except for large users, off-site regeneration. Therefore, research into the development of alternative adsorbents and/or regeneration processes has been carried out. Alternative adsorbents that have been investigated for the removal of atrazine include hypercrosslinked polymers [7] and zeolites and organoclays [12]. Although these materials exhibited lower adsorption capacity for atrazine (compared to activated carbon) the feasibility of any adsorption process depends greatly on the cost of regeneration of spent adsorbents [13]. Adsorbent regeneration techniques that have been investigated include chemical and solvent [7,14,15], microbial [16], ultrasonic [17], wet air oxidation [18] and electrochemical regeneration. Electrochemical regeneration refers to the regeneration of a used adsorbent inside an electrolytic cell. The regeneration involves desorption and/or destruction of the adsorbed organic matter restoring the adsorptive capacity. Electrochemical processes have been widely investigated as methods of treating dissolved organics in wastewater [19–21], but electrochemical regeneration of adsorbents has not been widely investigated. The first report on electrochemical regeneration of activated carbon was by Owen and Barry [22] who achieved regeneration efficiencies of up to 61% and suggested that the process merited further attention. While a number of other researchers have undertaken some research into electrochemical regeneration of activated carbon, Narbaitz and Cen [23] and Zhang [24] have undertaken the most detailed investigations using GACs loaded with phenol, with regeneration efficiencies of up to 95%. Regeneration was found to be greatest with the loaded GAC being placed on the cathode. Narbaitz and Cen [23] suggested that the electrochemical effects are restricted to the external surface of the carbon and for regeneration at the cathode surface the process consisted of initial phenol desorption followed by phenol destruction. To date no research appears to have been conducted on the electrochemical oxidation of atrazine or on the electrochemical regeneration of an adsorbent loaded with atrazine. This paper reports the use of a novel carbon-based adsorbent material, Nyex 100 for the removal of atrazine from aqueous solutions. The paper also reports the electrochemical regeneration of this non-porous and highly conducting material.
2. Materials and methods The Nyex 100 was supplied by Nykin Developments in the form of a wet powder with an average dry solids content of 50%. Nyex 100 was chosen as it contains non-porous particles with no internal surface area. This
was thought likely to result in quick adsorption and electrochemical regeneration rates as intra-particle diffusion would be eliminated (often the rate limiting step). Nyex 100 is a low-cost carbon powder (carbon content over 95% with a mean particle diameter of 124 mm and a range of 10–600 mm) that has been treated by a proprietary process. The BET surface area of the particles was determined by nitrogen adsorption and was found to be 2.75 m2 g1. This is very low compared with an activated carbon with surface areas of up to 2000 m2 g1 [25]. Mercury porosimetry indicated that there were virtually no internal pores in the material explaining the low surface area obtained. Atrazine was supplied by Ehrenstorfer GmbH with a purity of 98.4%. This was prepared as a 10 mg l1 stock solution in deionised water, with 5 days mixing being required for dissolution. All other chemicals were supplied as analytical grade by Aldrich. 2.1. Adsorption kinetics A batch adsorption experiment was conducted to determine the time required to achieve equilibrium. An accurately weighed dose of Nyex 100 was added to a 250 ml of 5 mg l1 Atrazine solution in a 250 ml flask. The flask was sealed and shaken for 18 h (Unimax 1010, Heidolph Instruments). At regular intervals for the first 2 h, 15 ml samples were taken, filtered (Whatman GF/C filter paper) and analysed for unadsorbed atrazine. The flask was then mixed for a further 16 h to ensure equilibrium had been achieved. This demonstrated that equilibrium was achieved within 60 min, with 80% of the equilibrium value being achieved within 10 min. Subsequent adsorption trials used contact times of 60 min. 2.2. Adsorption studies Adsorption isotherms were prepared by adding various known weights of adsorbent to 100 ml of 6– 8 mg l1 atrazine solution in a 250 ml flask. These flasks were sealed and shaken for 1 h. After adsorption the solution was filtered (Whatman GF/C) and analysed. A control experiment to determine the effect of Nyex 100 on the UV adsorbancy of the filtrate was achieved by mixing known quantities of adsorbent with 100 ml of deionised water and mixing for 1 h. The effect of acid conditions on atrazine adsorption onto Nyex 100 was investigated by adjusting the pH of the initial solution to 3 using hydrochloric acid, prior to adding the adsorbent. The ability of the Nyex 100 to achieve low discharge concentrations was investigated by adding known amounts of adsorbent to 500 ml of 10 mg l1 atrazine solution. After 1 h shaking the filtered water samples were analysed for atrazine residues.
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2.3. Electrochemical regeneration Regeneration of the adsorbent was achieved by mixing the filtered, wet adsorbent with solid NaCl and placing the mixture in the anode compartment of a batch electrochemical cell (Fig. 1). It comprised a parallel divided cell with a perforated 316 stainless-steel cathode separated from a mixed metal oxide-coated titanium anode (supplied by Electrode Products Technology Ltd.) by a microporus Daramic 350 membrane. The anode was placed 10 mm from the membrane, with the active area of the anode being dependent on the mass of adsorbent being regenerated, typically 5 cm2. The electrolyte in the cathode compartment was a 2% w/w NaCl solution. The regeneration procedure was (i) Initial adsorption—A known weight of Nyex 100 was added to 200 ml of 6 mg l1 atrazine solution in a 250 ml flask and mixed for 1 h. After adsorption the flask contents were filtered (Whatman GF/C) and the atrazine concentration of the filtrate determined. (ii) Electrochemical regeneration—The loaded, filtered (but still wet approx. 50% dry solids) adsorbent was mixed with NaCl crystals to give a 2% w/w concentration in the liquid phase of the mix. The mixture was pressed into the anode compartment. A DC current of 400 mA was applied for regeneration times in the range 10–60 min. There was no flow in the cell and the only mixing was due to the gas bubbles produced at the electrodes. (iii) Re-adsorption—The contents of the anodic compartment (with no additional treatment) were transferred to a 250 ml volumetric flask to which 200 ml of 6 mg l1 atrazine solution were added and mixed for 1 h. After adsorption the flask contents were filtered (Whatman GF/C) and the atrazine concentration of the filtrate determined.
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For the adsorption/electrochemical regeneration over a number of cycles, steps B and C were repeated. 2.4. Atrazine analysis Atrazine concentrations (0.1–10 mg l1) were determined using UV/VIS spectrometry at a wavelength of 220 nm. However the addition of Nyex 100 to deionised water resulted in a slight increase in the UV adsorbancy of the filtered water at this wavelength, probably due to the leaching of an unknown compound from the Nyex 100. A correcting curve was determined and the data for the atrazine concentrations has been corrected for this leaching effect. Calibration curves were prepared at pH 7 and 3. For lower concentrations, solid-phase extraction was used to pre-concentrate the atrazine prior to analysis using a Varian HPLC system (Prostar with a Hypersil C18 column and a photodiode array detector) with an eluent flow rate of 1.2 ml min1. Eluent was an 80:20 H2O:CH3CN mix with 1 ml l1 H3PO4. This method could determine atrazine concentrations down to 0.5 mg l1 although the accuracy was estimated as no better than 70.5 mg l1. The solid-phase extraction procedure was to pass 10 ml CH3CN, 10 ml CH3OH and 10 ml deionised water consecutively through a C18 cartridge (Varian). Five hundred millilitre of effluent sample was then passed through the cartridge at 6 ml min1, followed by 10 ml of NaCl solution (6 g l1). The cartridge was then dried under vacuum. The atrazine was eluted with 8 ml of CH3CN:CH2Cl (50:50) and dried with a stream of nitrogen. The remaining solids were reconstituted using 1 ml of CH3CN:H2O (20:80) with 1 ml l1 H3PO4.
3. Results and discussion 3.1. Adsorption studies
Fig. 1. Electrochemical cell schematic.
Adsorption studies using both high concentrations (up to 8 mg l1) and low concentrations (less than 10 mg l1) atrazine solutions were undertaken. The high concentrations were used to determine an adsorption isotherm for the atrazine/Nyex system and the low concentrations were used to assess the capability of the Nyex 100 to achieve the low discharge concentrations required. The Freundlich Eq. (1) was found to fit the data effectively. The Freundlich constants were obtained from a log–log plot of the solid-phase concentration (q) versus the liquid-phase concentration (Ce ) at equilibrium (Fig. 2). The values of the Freundlich constants were Kf ¼ 0:279 and 1=n ¼ 0:550; found from
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pH 7 (Fig. 4).
the least-squares fit of the data. q ¼ Kf Ce1=n :
ð1Þ
A least-squares fit of the Langmuir equation was also obtained, but Fig. 3 clearly shows that the Freundlich equation gives a superior fit of the data, particularly at higher concentrations. There is considerable scatter of the points around the adsorption isotherms shown in Fig. 3. This scattering is due to two causes: (1) batch to batch variation of the Nyex 100 and (2) adsorption studies being conducted at room temperatures (this resulted in samples on different days being subjected to different temperatures, 17–26 C). Studies have shown that adsorption is temperature dependent. Since the regeneration of the adsorbent occurs within the anodic compartment of the electrochemical cell, the electrolysis of water is likely to be a significant side reaction generating hydrogen ions (Eq. (2)). As the Nyex is to be reused after regeneration without any additional treatment, the adsorption of atrazine onto Nyex 100 under acid conditions was investigated. This showed that adsorption at pH 3 gave similar results to that at
2H2 O 4e -O2 þ 4Hþ :
The ability of Nyex 100 to achieve low discharge concentrations was investigated by adding different amounts of adsorbent to a solution containing 10 mg l1 of atrazine. This showed that Nyex 100 is capable of achieving low discharge concentrations (o1 mg l1), with a dose rate of around 6 g l1 (Fig. 5). The adsorption isotherm in Fig. 3 and the atrazine removals in Fig. 5 show that the adsorptive capacity (defined as the mass of atrazine adsorbed per gram of adsorbent in mg l1) of Nyex 100 is very much less than activated carbon. Table 1 compares the Freundlich constants calculated in this work with those determined by Streat and Horner [7] for activated carbon F-400 and Speth and Miltner [26] for pulverised F-400. This low capacity is due to the very low surface area of the Nyex 100 particles (2.75 m2 g1) compared with activated carbon (up to 2000 m2 per g1) [25]. However, the cost of treatment is dependent on the cost of regeneration and replacement of adsorbent.
0
-0.6 -0.8 -1 -1.2 -1
-0.5
0
0.5 Log Ce
1
1.5
1 0.8 0.6 0.4 0.2 0 0
Fig. 2. Adsorption isotherm for atrazine/Nyex 100, where Ce and q are in mg l1 and mg g1 respectively. The straight line shows the least-squares fit for the data; log q¼ log Kf þ 1=n log Ce :
1
2
3
4
5
6
7
8
-1 Liquid phase equilibrium concentration Ce (mgl )
Fig. 4. Adsorption capacity at pH 3. The solid line gives the Freudlich equation for fresh Nyex 100 at pH 7.
1.2
100
4.5
90
4
1 80
3.5
70 % removal
0.8
0.6
0.4
3
60 2.5 50
% removal Concentration
2
40
1.5
30
0.2
0
20
1
10
0.5
0 0
2
4 6 8 10 -1 Liquid phase equilibrium concentration Ce (mgl )
12
Fig. 3. Adsorption Isotherms for atrazine adsorption on Nyex 100. The upper thin line shows the Freundlich isotherm and the lower thick line shows the Langmuir isotherm.
Final concentration (µg l-1)
Log q
-0.4
Solid phase equilibrium concentration q (mg g-1)
1.2
-0.2
Solid phase equilibrium concentration q (mg g-1)
ð2Þ
0 0
2
4
6
8
10
12
14
16
18
20
Dose rate g/l
Fig. 5. Removal of atrazine from low concentration solution using adsorption by Nyex 100. Initial atrazine concentration in mg l1.
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Electrochemical regeneration of the Nyex adsorbent is achieved by electrochemical oxidation of the adsorbed atrazine with the adsorbent acting as the anode within the electrochemical cell. Since the total contents of the anodic compartment are transferred (without any treatment after regeneration) to the next adsorption cycle, any desorbed but unoxidised atrazine would be present in the filtrate of the next adsorption cycle. The fact that there is no washing of the adsorbent prior to adsorption means that no secondary effluent is generated, but it does mean that any partially oxidisied organic compounds could contaminate the filtrate after subsequent adsorption cycles. Hence, the fate of the adsorbed organics are important considerations and are the subject of on-going research (which will be the subject of a future paper). Samples of regenerated Nyex 100 (previously loaded using high concentration, 6 mg l1, atrazine solution) were prepared by passing a current of 400 mA for 20 min. This material was then used to prepare an adsorption isotherm to investigate the effect of regeneration. This isotherm (Fig. 6) shows that the regenerated material gives similar results to that obtained by fresh material. The low-adsorptive capacity of the adsorbent means that the process will only be commercially viable for removing atrazine from waters if the adsorbent is capable of being cheaply regenerated many times. Fig. 7 shows the adsorptive load of the adsorbent over a Table 1 Comparison of Freundlich constants for activated carbon (F400) and Nyex 100 Freundlich constant
Nyex 100
F-400 [7]
F-400 Pulverised [26]
K 1=n
0.279 0.550
2488.2 0.486
858 0.291
number of cycles. As can be seen from this figure, there is an increase in the adsorptive capacity of the adsorbent after initial regeneration and this is maintained during subsequent regenerations. In order to investigate the effect of electrochemical regeneration on the adsorptive capacity, a series of electrochemical regenerations were undertaken varying the treatment time of loaded Nyex 100 in the electrochemical cell. These are plotted, Fig. 8, as regeneration time against percentage regeneration (PR). The PR is based on the adsorptive capacity of the regenerated Nyex 100, qr ; compared with the calculated adsorptive capacity, qcal ; of fresh Nyex 100 at the same liquid-phase concentration [27]. PR ¼
qr 100: qcal
ð3Þ
The adsorptive capacity of fresh Nyex 100 is calculated from the Freundlich Eq. (1) using the Adsorptive capacity (mg atrazine/g Nyex 100)
3.2. Electrochemical regeneration
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0.6 0.5 0.4 0.3 0.2 0.1 0 1
2
3
4 Adsorption cycle
5
6
7
Fig. 7. Adsorption capacity of Nyex for a number of adsorption/regeneration cycles. Regeneration conditions; Current 400 mA, Treatment time 20 min, Electrolyte concentration 2% sodium chloride. Cycle 1 uses fresh adsorbent.
350
Percentage Regeneration (%)
300 Solid phase equilibrium concentration q (mg g-1)
0.80 0.70 0.60 0.50 0.40 0.30
250 200 150 100 50
0.20 0 0
0.10 0.00 0.00
1.00
2.00
3.00
4.00
5.00
6.00
-1 Liquid phase equilibrium concentration Ceq (mgl )
Fig. 6. Adsorption isotherm for regenerated material. The solid line gives the Freundlich equation for fresh Nyex 100.
10
20
30 40 Treatment time (mins)
50
60
70
Fig. 8. Regenerated capacity at various treatment times. Regeneration conditions; Current 400 mA, Electrolyte concentration 2% sodium chloride (in cathode and anode). PR (Eq. (3)) is the adsorptive capacity of regenerated compared with fresh Nyex 100.
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experimentally obtained values of the Freundlich constants. The PR was calculated at a number of liquid phase equilibrium concentration values and averaged. As can be seen from Fig. 8, there is a pronounced increase in capacity with increased regeneration time. Two points of interest can be noted from this figure. A regenerated adsorptive capacity in excess of the original adsorptive capacity and an apparent regenerated capacity without electrochemical treatment. This apparent capacity can be explained by the second adsorption onto the loaded Nyex resulting in an increased liquid-phase equilibrium concentration. This is shown schematically in Fig. 9. During the initial loading of the Nyex 100 a liquid-phase equilibrium concentration, Ce1 ; is achieved giving a corresponding solid-phase equilibrium concentration, qe1 : If this loaded Nyex is then used under the same conditions with no regeneration, then, after the system has reached equilibrium, the liquid-phase equilibrium concentration will be Ce2 : This gives a corresponding solid-phase concentration of qe2 : Since there was already a quantity of adsorbate on the adsorbent, qe1 ; the additional adsorbate taken up, Dq ¼ qe2 qe1 . Hence, the apparent PR without treatment. Four possibility causes of the increase in adsorptive capacity above the initial capacity have been identified. (i) The transfer of oxidising species from the electrochemical cell to the atrazine mixture resulting in the removal of atrazine by chemical oxidation rather than adsorption. (ii) Increased atrazine adsorption in acid conditions. (iii) The generation of internal pores increasing the surface area available for adsorption. (iv) Modification of the surface chemistry.
Nyex 100 before a second adsorption (to remove these species) actually increased the adsorptive capacity, the opposite of what would be expected if oxidising species were being transferred [28]. The second option of a pH effect is also possible as the regeneration of the Nyex 100 in the anodic compartment of the electrochemical cell results in a significant decrease in the pH of the atrazine solution. Increasing the treatment time results in a greater reduction in the solution pH. However, as shown in Fig. 5, acid conditions have no significant effect on the adsorptive capacity of fresh Nyex 100. The surface area of fresh (2.75 m2 g1) and regenerated Nyex 100 (2.8 m2 g1) suggest that the formation of internal pores on electrochemical treatment is unlikely. However, SEM photographs of the surface (Fig. 10) do show a surface roughening after treatment. This could be due to an exfoliation of some of the graphene layers within the carbon matrix. The modification of the surface chemistry of activated carbon by anodic electrochemical treatment has been demonstrated [29], although this resulted in a reduction in adsorbate uptake. Hence, a possible explanation for the increase in atrazine adsorption onto Nyex 100, based on a modification of the particle surface and chemistry, is proposed below. This explanation is based on suggested mechanisms for the adsorption of aromatic organics on activated carbon, an area that has been the subject of much research.
Earlier test work on the removal of dyes from water investigated the transfer of oxidising species, for example chlorine or hypochlorite, from the electrochemical cell after electrolysis. The presence of these species was believed to result in chemical oxidation of the pollutants. However that work showed that washing the
qe2
q qe1
q
Ce1
Ce2
Ce
Fig. 9. Schematic of loading onto fresh and loaded Nyex 100 without electrochemical regeneration. The curve gives the adsorption isotherm for fresh Nyex 100.
Fig. 10. SEM pictures of fresh (left) and regenerated (right) Nyex 100 at 1000 magnification.
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The interactions of aromatic organics, specifically phenol, with activated carbon were listed by Laszlo and Szucs [30] as (a) a dispersion effect between the adsorbate aromatic ring and the p electrons of the graphitic structure, (b) electron donor–acceptor interaction between the aromatic ring and the basic surface oxygens (functional groups), (c) electrostatic interactions when ions are present. Functional groups containing oxygen in activated carbons are found primarily at the edges of broken graphitic planes, with basal planes consisting of largefused aromatic structures [31]. Hence the relative contribution of functional groups versus the graphene layers to the development of surface charge are a key issue, but such information is not readily available in the literature [32]. Since electrostatic repulsion of the adsorbate from the graphene layer is thought to be much more detrimental for adsorbent effectiveness than repulsion from the basic oxygen functional groups [32], changing this ratio will change the adsorbate uptake. Mattson et al. [31] proposed that the aromatic compounds, phenol and nitrobenzene, adsorb on activated carbon by a donor–acceptor complex mechanism involving a carbonyl oxygen of the carbon surface acting as the electron donor and the aromatic ring of the solute acting as the acceptor. The oxidation of the activated carbon surface either chemically [33] or electrochemically [29] would result in the oxidation of the carbonyl oxygen to carboxylic acid oxygen. Since carboxylic acid oxygen has a smaller dipole moment than carbonyl oxygen, it would be expected to act as a weaker donor, reducing the adsorption rate, as observed by Coughlin [33], Mehta and Flora [29] and Mattson et al. [31]. Hence, electrochemical regeneration of Nyex 100 would be expected to increase the concentration of carboxylic acid oxygen reducing the adsorbate adsorption rate. However, in the case of Nyex 100, the photographs suggest that the untreated Nyex 100 has a strong graphitic structure and that on electrochemical regeneration the flat graphene layers are being broken up, increasing the surface roughness. Hence, the relative contribution of the edges (oxygen containing functional groups) will increase. Since there is an increase in adsorbate uptake, it suggests that any loss in adsorbate uptake due to the conversion of carbonyl oxygen to carboxylic acid oxygen is less significant than the increase in edge/plane effects due to electrochemical treatment. Further work is required to confirm this proposed mechanism.
4. Conclusions This work has demonstrated that the carbon based adsorbent material, Nyex 100, can be used to remove
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atrazine to low levels, below 1.0 mg l1. One-hundred per cent electrochemical regeneration of the adsorbent is possible by passing a current of 400 mA for 20 min, giving a regeneration charge of 96 C g1 (adsorbent). However increasing the charge passed results in an increase in adsorbtive capacity, believed to be due to an increase in the edge/planar ratio of the Nyex 100. The use of the Nyex 100 over a number of adsorption/ electrochemical regeneration cycles with no loss in capacity has been demonstrated. Whilst this initial work suggests that the process may have merit, significant further work must be undertaken. This investigation has looked at the removal of a single compound from deionised water, a condition never likely to be achieved in practice. The competing effects of other organics, particularly natural organic matter, will need further experiments, both in natural waters and for treating industrial effluents. The reuse of the regenerated adsorbent with no washing gives a significant benefit to the process as no secondary effluent stream is produced. However, the fate of the adsorbed organics becomes of greater significance as there is the potential for desorption of partially desorbed (and potentially more toxic) organics. This is an area of ongoing research and the electrochemical breakdown products are being determined. There is also the need to determine the stability of the adsorbent over long term adsorption/electrochemical regeneration trials as the increase in surface roughness may result in particle attrition. This will have a major impact on the process costings. Significantly further work on optimising the various process parameters and reducing the regeneration charge passed per gram of adsorbent is required.
Acknowledgements This was undertaken with financial assistance from the EPSRC. Further material and financial assistance for this project has been received from Severn Trent Water Ltd., Nykin Developments and Electrode Products Technology Ltd. The authors also wish to express their thanks to Dick Plaisted of the Department of Chemistry, UMIST, who carried out the surface area analysis and pore size determination and Ken Eccleston of Nykin Developments for the use of their facilities.
References [1] Gilliom RJ, Barbash JE, Gobert YM, Kolpin DW, Larson SJ, Nakagaki N, Wilber WG. Pesticides in streams and ground after of the United States, 1993–1995. In: Ninth International Congress of Pesticide Chemistry, 1998. [2] Hurst P, Hay A, Dudley N. The pesticide handbook. London: Journeyman Press; 1991.
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[3] Calderon RL. The epidemiology of chemical contaminants in drinking water. Food Chem Toxicol 2000;38:S13–20. [4] Kolpin DW, Martin JD, Pesticides in ground water: summary statistics: preliminary results from cycle 1 of the National Assessment Program (NAWQA) 1992–2001. Provisional data, US Geological Survey; http://ca.water. usge.gov/pnsp, 2003. [5] Martin JD, Crawford CG, Larson SJ, Pesticides in streams: summary statistics: preliminary results from cycle 1 of the National Assessment Program (NAWQA) 1992–2001. Provisional data, US Geological Survey; http://ca.water.usge.gov/pnsp, 2003. [6] Environment Agency. Pesticides 2001, 2002. Environment Agency, ISBN 1 844320472. [7] Streat M, Horner DJ. Adsorption of highly soluble herbicides from water using activated carbon and hypercrosslinked polymers. Process Saf Environ Prot 2000;78(B5):363–82. [8] Cerejeira MJ, Viana P, Batista S, Pereira T, Silva E, Valerio MJ, Silva A, Ferreira M, Silva Fernandes AM. Pesticides in Portuguese surface and ground waters. Water Res 2003;37:1055–63. [9] Miltner RJ, Baker DB, Speth TF, Fronk CA. Treatment of seasonal pesticides in surface waters. J Am Water Works Assoc 1989;81:43–52. [10] Kouras A, Zouboulis A, Samara C, Kouimtzis Th. Removal of pesticides from aqueous solutions by combined physicochemical processes—the behaviour of lindane. Environ Pollut 1998;103:193–202. [11] San Miguel G, et al. The regeneration of field-spent granular-activated carbons. Water Res 2001;35(11): 2740–8. [12] Bottero JY, Khatib K, Thomas F, Jucker K, Bersillon JL, Mallevialle J. Adsorption of atrazine onto zeolites and organoclays, in the presence of background organics. Water Res 1994;28(2):483–90. [13] Banat FA, Al-Bashir B, Al-Asheh S, Hayajneh O. Adsorption of phenol by bentonite. Environ pollut 2000;107(3):391–8. [14] Leng CC, Pinto NG. An investigation of the mechanisms of chemical regeneration of activated carbon. Ind Eng Chem Res 1996;35(6):2024–31. [15] Cooney DO, et al. Solvent regeneration of activated carbon. Water Res 1983;17(4):403–10. [16] Hutchinson DH, Robinson CW. A microbial regeneration process for antigranulocytes activated carbon. 2. Regeneration studies. Water Res 1990;24(10):1217–23. [17] Zhang HP, Wang S, Liu Z. Ultrasonic regeneration of granular activated carbon. Environ Eng Sci 2003;20(1): 57–64.
[18] Mundale VD, et al. Regeneration of spent activated carbon by wet air oxidation. Can J Chem Eng 1991;69(5):1149–59. [19] Murphy OJ, et al. Direct electrochemical oxidation of organics for waste-water treatment. Water Res 1992;26(4):443–51. [20] Johnson DC, et al. Direct electrochemical degradation of organic wastes in aqueous media. Electrochim Acta 2000;46(2–3):323–30. [21] Panizza M, et al. Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res 2000;34(9):2601–5. [22] Owen PH, Barry JP, Electrochemical carbon regeneration. Environics Inc., Huntington Beach, California. Report number PB 239156, 1972. [23] Narbaitz RM, Cen JQ. Electrochemical regeneration of granular activated carbon. Water Res 1994;28(8):1771–8. [24] Zhang HP, et al. Regeneration of phenol-saturated activated carbon in an electrochemical reactor. J Chem Technol Biotechnol 2002;77(11):1246–50. [25] Streat M, et al. , Sorption of phenol and para-chlorophenol from water using conventional and novel activated carbons. Water Res 1995;29(2):467–72. [26] Speth TF, Miltner RJ. Technical note—adsorption capacity of gac for synthetic organics. J Am Water Works Assoc 1990;82(2):72–5. [27] Narbaitz RM, Cen J. Alternative methods for determining the percentage regeneration of activated carbon. Water Res 1997;31(10):2532–42. [28] Brown NW. PhD Thesis (in preparation). Dept. of Chemical Engineering, UMIST, 2004. [29] Mehta MP, Flora JRV. Effects of electrochemical treatment of granular activated carbon on surface acid groups and the adsorptive capacity for phenol. Water Res 1997;31(9):2171–6. [30] Laszlo K, Szucs A. Surface characterization of polyethyleneterephthalate (PET) based activated carbon and the effect of pH on its adsorption capacity from aqueous phenol and 2,3,4-trichlorophenol solutions. Carbon 2001;39(13):1945–53. [31] Mattson JS, Mark Jr HB, Malbin MD, Weber Jr WJ, Crittenden JC. Surface chemistry of active carbon: specific adsorption of phenols. J Colloid Interface Sci 1969;31(1):116–30. [32] Radovic LR, Moreno-Castilla C, Rivera-Utrilla J. Carbon materials as adsorbents in aqueous solutions. Chemistry and Physics of Carbon 2001;27:227–405. [33] Coughlin RW, Ezra FS, Tan RN. Influence of chemisorbed oxygen in adsorption onto carbon from aqueous solution. J Colloid Interface Sci 1968;28(3/4):386–96.