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Activated carbons from sewage sludge
Desalination 277 (2011) 377–382
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Activated carbons from sewage sludge Application to aqueous-phase adsorption of 4-chlorophenol Victor Manuel Monsalvo ⁎, Angel Fernandez Mohedano, Juan Jose Rodriguez Seccion de Ingenieria Quimica, Facultad de Ciencias, Universidad Autonoma de Madrid. C/Francisco Tomas y Valiente 7, Madrid 28049, Spain
a r t i c l e
i n f o
Article history: Received 12 November 2010 Received in revised form 29 March 2011 Accepted 25 April 2011 Available online 18 May 2011 Keywords: Sewage sludge Valorisation Activated carbon Adsorption 4-Chlorophenol
a b s t r a c t Activated carbons of different characteristics have been prepared from dried sewage sludge using CO2, air and KOH as activating agents. The adsorption capacity of the resulting materials has been checked using 4chlorophenol as a target compound in aqueous solution. CO2 and air-activation led to carbons of low BET area which increased with the activation temperature but did not reach 100 m 2/g at the best. The high ash content of the starting material (23%, d.b.) limits the development of porosity since the partial gasification does not affect the inorganic matter. In the case of air-activation, the resulting surface remains fairly low even in ashfree basis, reaching no more than 250 m 2/g. Activation with KOH allows a much higher development of porosity. Using a KOH to solids ratio of 3:1 and a temperature of 750 °C a BET area above 1800 m 2/g with a mesopore volume higher than 0.35 cm 3/g was reached. In spite of those dramatic differences on the textural properties, the air-activated carbons showed an adsorption capacity for 4-chlorophenol of almost 85% that of the KOH-activated carbons. These results warrant in principle air-activation at moderate temperature as a promising way of sewage sludge valorisation to inexpensive adsorbents for the removal of water pollutants. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The increasing generation of sewage sludge derived from wastewater treatment demands the development of new ways of valorisation. Nowadays, management of sewage sludge includes composting for farmland utilization, incineration and landfilling [1]. However, this last will be severely limited in a short-term due to the increasing competition for landfill space, growing costs and more stringent environmental regulations. Incineration of sewage sludge requires strict control of the pollutants generated [2, 3] and in many cases suffers of social and political opposition. Different ways of valorisation are being investigated in the last two decades including the use of sewage sludge as precursor for adsorbent materials with environmental applications. Some authors have investigated the direct application of dried sewage sludge as adsorbent [4] although its low surface area, below 5 m 2/g [5–7], represents a severe limitation in this respect. However, more acceptable adsorbents can be obtained from sewage sludge by different ways. Pyrolysis can produce a carbonaceous solid with around 60 m2/g surface area [8], requiring temperatures above 700 °C to increase such porosity [9]. Although this type of carbons has been applied on the decontamination of several gaseous and aqueous off-streams, many industrial applications demand more efficient adsorbents. Acti⁎ Corresponding author. Tel.: + 34 4978035; fax: + 34 4973516. E-mail addresses: [email protected] (V.M. Monsalvo), [email protected] (A.F. Mohedano), [email protected] (J.J. Rodriguez). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.04.059
vated carbon has been witnessed as an adsorbent with large porous surface area, controllable pore structure, high thermo-stability and low acid/base reactivity [10]. Owning to its low initial cost, simplicity of design, insensitivity to toxic substances, high adsorption capacity and regenerability, activated carbons appeared to be the most versatile and suitable candidate for micropollutants removal from wastewaters [11, 12]. This fact has motivated a noticeable interest for the development of carbon-based adsorbents from many different precursors for industrial applications. Activated carbons can be obtained by both the so-called chemical [13–18] and physical [6, 19, 20] activation of sewage sludge. Recent works have demonstrated the possibility of developing surface areas over 1500 m 2/g through activation with KOH [21, 22]. The resulting materials have been tested in some gas-phase applications like odor control in wastewater treatment plants [23]. The removal of heavy metals and dyes from wastewaters by means of KOH and CO2activated sewage sludge has been also studied [7, 21, 22, 24]. Air-activation of sewage sludge has been scarcely studied in spite of the fact that this appears in principle the easiest way for the valorisation of that type of waste to an inexpensive adsorbent material with potential applications for the removal of gas and water pollutants. Activated carbons of less than 100 m 2/g BET surface area have been obtained upon air-activation of pre-pyrolysed sewage sludge and tested for Fe3+ removal from water [20]. In this work we investigate the direct activation of dry sewage sludge with CO2, air and KOH at different temperatures and holding
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times, since these two variables have been reported to be important for the final characteristics of the resulting materials [18, 25], which are evaluated as adsorbents for the 4-chlorophenol removal from aqueous solution. Chlorophenols are a representative group of harmful compounds specifically included in the lists of priority and hazardous pollutants. They are released into water from a diversity of sources such as oil exploration and production, wood and textile preservation, accidental spills, chlorination of water and wastewaters containing phenol, degradation of herbicides and pesticides and leaching from waste disposal [26–29].
2.3. Adsorption experiments 100 mg of activated carbon were mixed in stoppered bottles with 100 mL of aqueous solutions containing different concentrations of 4CP (0.1–2.0 g/L) at 15 °C and pH 7.0 (buffered solution of phosphate) and stirred at 200 rpm (equivalent) in a thermostatized shaker up to equilibration. The concentration of 4-CP was analyzed by HPLC/UV (Prostar, Varian) using a C18 column as stationary phase (Microsorb MW-100-5) and a mixture of acetonitrile and H2O (40:60, vol.) as mobile phase. The flow rate was maintained at 1 mL/min and a wavelength of 280 nm was used. The experimental data were fitted to the Langmuir (1) and Redlich–Peterson (2) equations:
2. Materials and methods 2.1. Adsorbents preparation The aerobic granular sludge was collected from a sequencing batch reactor treating industrial wastewaters from a cosmetics factory. Primarily, samples were washed with distilled water until constant conductivity, and then they were dried at 105 °C up to constant weight. The resulting solids were ground and sieved to a particle size in the range of 0.1–0.25 mm. Activation was performed in a vertical tube furnace electrically heated. The working temperature was reached at a 10 °C/min heating rate and the gas (CO2, air or N2 in the case of KOH-activation) was continuously passed at a 100 N-mL/min flow rate. Different holding times (0.5, 2.0 and 4.0 h) and temperatures (700 and 800 °C) were tested for the activation with CO2. Air-activated materials were also obtained at different holding times (0.5, 2.0 and 4.0 h) and temperatures (200, 300 and 400 °C). The resulting materials were washed with distilled water before being used in adsorption experiments. Chemical activation with KOH was carried out by physical mixing of the ground hydroxide lentils and dried sewage sludge. KOH to solids ratios of 1:1 and 3:1 (w:w) were employed. Activation was performed at 450 and 750 °C under N2 atmosphere for 30 min and the resulting samples were washed with 5 M HCl, and then with distilled water until constant pH.
2.2. Characterization of the starting material and the activated carbons The C, H, N, and S elemental composition of the solid samples was determined by a CHNS analyzer (LECO CHNS-932). The standard test method ASTM D2866-94 was used to determine the ash content by means of a thermogravimetric analyzer (SDTA851e). The metals content was examined by inductively coupled plasma atomic emission spectroscopy (ICP-MS) using a model Elan 6000 Sciex Perkin Elmer apparatus. The porous structure was characterized from the 77 K N2 adsorption/desorption isotherms (Autosorb-1, Quantachrome). The samples were previously outgassed for 8 h at 10− 3 Torr and 150 °C. The pH slurry value of the carbons was measured upon stirring in distilled water until constant pH. SEM images were obtained using a Hitachi S-3000N apparatus. Table 1 summarizes the characterization of the starting dried sludge.
Table 1 Characterization of the starting sewage sludge (composition in %, d.b.). C H N S Oa Ash content (%) SBET (m2/g) pHslurry a
By difference.
48.7 7.5 9.4 0.6 10.8 23 b3 m2/g 6.9
Na (mg/g) Mg (mg/g) Al (mg/g) P (mg/g) K (mg/g) Ca (mg/g) Ti (μg/g) Fe (μg/g)
10.90 3.16 10.00 3.37 1.84 4.61 64 283
qe =
QL ⋅KL ⋅Ce 1 + KL ⋅Ce
ð1Þ
qe =
A ⋅Ce 1 + B⋅Ceβ
ð2Þ
where qe is the equilibrium adsorbate loading onto the adsorbent (mmol/g), Ce the equilibrium liquid-phase concentration of the adsorbate (mmol/L), QL the monolayer adsorption capacity (mmol/g), KL the Langmuir constant (L/mmol), A the Redlich–Peterson constant (L/g), B a constant having units of (L/mmol)β, and β an exponent that lies between 0 and 1. The parameters of these equations were calculated by using a nonlinear regression fitting method. The results reported were the average values from duplicate runs. In all the cases, the standard errors were lower than 5%. 3. Results and discussions SEM images of the dried granules of aerobic sewage sludge used as precursor and the resulting activated carbons are shown in Fig. 1. The dried sewage sludge is a non-porous material with a surface area lower than 3 m 2/g. The materials resulting from physical activation had irregular shapes. The chemical activation process with KOH gave rise to a fluffy material of low density and high porosity. 3.1. CO2 and air activation Table 2 summarizes the values of the BET surface area and micropore volume of the carbon materials activated with CO2 and air. BET surface areas below 100 m 2/g were developed with both activating agents. With CO2, increasing the holding time up to 2 h gave rise to BET surface areas higher than those reported in the literature [7] at similar temperatures. No significant increase of the BET surface area was observed at higher holding times except with air at the lowest temperature tested. This last variable shows some significant effect on the development of surface area upon activation both with air and CO2 within the ranges investigated. The high ash content of the precursor (23%, d.b.) limits severely the development of porosity on a total weight basis since partial gasification does not affect to the inorganic matter. Nevertheless, the resulting materials from both CO2 and air-activation showed higher BET surface areas than the 65 m 2/g reported in the literature from steam-activation of sewage sludge at 800 °C [6]. The C/H molar ratio increased up to 3.0 and 1.6 in the CO2 and airactivated carbons, respectively, versus 0.5 of the dry sludge consistently with the dehydrogenative polymerisation and dehydrative polycondensation reactions commonly occurring upon thermal treatment, with significant loss of aliphatic hydrogen through olygomerisation. CO2activation was accompanied by a substantial decrease of the S content which reached values of 0.1–0.2% on a dry basis versus the 0.6% of the starting sludge solids (Table 1). The temperature must be a determining
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Fig. 1. SEM images of dried sewage sludge (a), CO2-activated carbon at 700 °C for 4 h (b), air-activated carbon at 400 °C for 4 h (c) and KOH-activated carbon at 450 °C with 3:1 KOH: solids ratio (d).
factor in this case, since air-activation, carried out at much lower temperature did not allow that reduction of the S content. 3.2. KOH activation Activation with KOH gave rise to carbons with a well developed porous structure showing both the temperature and the KOH to solids ratio a beneficial effect on that respect (Table 3). Increasing the temperature from 450 to 750 °C provokes a dramatic increase of the BET surface area affecting more remarkably to the microporosity although with an important contribution of mesoporosity also. At that temperature using a KOH to solids ratio of 3 allows obtaining frankly high values of BET and external or non-microporous surface areas (up to around 1800 and 380 m2/g, respectively). This development of porosity was higher than the obtained under similar activation conditions, from other substrates like coals, fibers, pistachio shells, corncob, pitch and fir
Table 2 Surface areas and micropore volumes of the carbons from CO2 and air-activation. T (°C)
Holding time (h)
CO2 activation 700 0.5 2.0 4.0 800 0.5 2.0 4.0 Air activation 200 0.5 2.0 4.0 300 0.5 2.0 4.0 400 0.5 2.0 4.0
SBET (m2/g)
Vmicropores (cm3/g)
11 75 79 20 94 97
0.01 0.05 0.06 0.02 0.04 0.09
7 34 47 13 51 53 15 92 91
b 0.01 0.03 0.05 0.01 0.05 0.05 0.03 0.06 0.07
wood [30–38]. Also, this activation procedure is much more effective than other based on commonly used chemical activating agents such as ZnCl2 [18, 39, 40], H3PO4 [18] or H2SO4 [5, 18, 39–41] with the dry solids from sewage sludge. Ros et al. [21] used an acid washing step previous to KOH-activation. That increased the surface area of the starting precursor but, according to our results does not produce any significant benefit regarding the final surface area achieved upon KOH activation. With regard to the evolution of the elemental analysis upon KOH-activation, a dramatic decrease of the N content at the highest activation temperature (750 °C) is noteworthy. This has been attributed by previous authors to the conversion of carbon-nitrogen moieties into cyanides caused by the alkali activation through the dehydrogenation of amino groups derived from proteins, nucleic acids and dead microorganisms [24, 42]. An important advantage of the chemical activation process is that it takes a shorter time than the required for physical activation. 3.3. Adsorption of 4-CP Adsorption isotherms of 4-CP by the different activated carbons are shown in Figs. 2, 3 and 4. In general, the experimental data fitted the Redlich–Peterson equation better than the Langmuir one most commonly used for liquid-phase-adsorption. That was particularly true in the case of the CO2-activated carbons. Table 4 summarizes the results of fitting
Table 3 Porous structure of the KOH-activated carbons. T (°C)
KOH:solids ratio
SBET (m2/g)
Vmesopores (cm3/g)
Vmicropores (cm3/g)
Sexternal (m2/g)
450
1:1 3:1 1:1 3:1
131 262 950 1832
0.12 0.16 0.23 0.36
b0.01 0.01 0.40 0.75
110 184 268 379
750
380
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Fig. 2. Adsorption isotherms of 4-CP onto the CO2-activated carbons.
the experimental adsorption data to the aforementioned isotherm equations. As can be seen in Fig. 2, similar adsorption capacities were observed for all the CO2-activated carbons, since the slight increase of the surface area when increasing the activation temperature from 700
Fig. 4. Adsorption isotherms of 4-CP onto the KOH-activated carbons.
to 800 °C (Table 2) did not cause any significant improvement. The analysis of the maximum adsorption capacity (QL) of the air-activated carbons indicates that is related with the BET surface area developed. Table 4 Fitting of the adsorption data of 4-CP onto the activated carbons from sewage sludge. T (°C) Holding time (h)
Redlich–Peterson
KOH:solids A ratio (L/g)
Fig. 3. Adsorption isotherms of 4-CP onto the air-activated carbons.
CO2-activated materials 700 2.0 0.722 4.0 0.646 800 2.0 0.862 4.0 0.903 Air-activated materials 200 0.5 0.060 2.0 0.389 4.0 0.315 300 0.5 0.538 2.0 1.317 4.0 0.291 400 0.5 0.280 2.0 0.254 4.0 0.235 KOH-activated materials 450 1:1 2.095 3:1 10.734 750 1:1 2.912 3:1 3.794
Langmuir
B β (L/mmol)
R2
KL QL R2 (L/mmol) (mmol/g)
0.134 0.033 0.251 0.231
0.978 0.988 0.676 0.873
0.993 0.992 0.995 0.998
0.583 0.404 0.134 0.573
1.999 2.570 3.205 1.970
0.980 0.960 0.972 0.941
0.052 0.927 0.108 0.071 0.752 0.031 0.431 0.010 0.346
1.062 0.811 1.140 1.067 0.943 1.074 0.883 0.747 0.804
0.995 0.996 0.995 0.993 0.997 0.998 0.997 0.997 0.993
0.249 0.382 0.185 0.456 0.598 0.237 0.261 0.201 0.159
0.332 0.697 1.922 1.815 1.993 1.644 1.555 2.046 2.372
0.998 0.991 0.993 0.984 0.995 0.997 0.994 0.997 0.965
1.607 10.844 1.712 1.235
0.935 0.742 0.969 1.044
0.996 0.995 0.997 0.996
1.165 1.910 1.478 1.474
1.498 1.559 1.809 2.820
0.997 0.989 0.996 0.994
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Nevertheless, the disturbances observed in such relation can be associated with differences in the surface chemistry. Despite the numerous studies published on the adsorption of phenolic compounds by activated carbons, the mechanism by which these compounds are adsorbed is not still completely well understood [43]. That mechanism is not only determined by π–π interactions and donor–acceptor complex formation but also by the solvent effect balancing the influence of those two contributions [44]. Additionally, the effect of the surface properties of activated carbons on adsorption has to be considered [45]. A relatively large amount of information describes the behavior of activated carbons with oxygen containing basic functional surface groups that in general cause enhanced sorption of phenolic compounds [46–48]. In addition, elimination of acidic oxygen-containing groups from activated carbons was suggested to be a way to increase adsorbability of phenols [46–50], through both an increased water cluster formation, thus obscuring the basal carbons, and a reduction of the availability of the π electrons [43]. However, Yin et al. [51] also noted that at high phenols concentrations, those phenols could be adsorbed by not only the mechanism of physisorption, but also by surface polymerisation, in which the phenolic compounds undergo oxidative coupling reactions on the carbon surface to produce polymeric compounds [52]. For this process the presence of oxygen is necessary and the high inorganic content of sewage sludge-based adsorbents could be advantageous for its potential use as adsorbent [24]. As mentioned, adsorption experiments were carried out at neutral pH, in which 4-CP is in the nonionic form (pKa = 9.38) and thus the adsorption must be governed by non-electrostatic interactions between the solute molecule and sorbent surface, i.e., the dispersive interactions between the π-electrons of the solute aromatic ring and the adsorbent. The surface oxygen, nitrogen and sulfur complexes influence these dispersive interactions to a large extent thus affecting the adsorption mechanism [50]. Owing to the high thermal stability of N and S hetero-atoms, they remain in the carbon upon air activation due to the low temperatures used. N and S contents of 7.5 ± 1.2% and 2.2 ± 0.8% (d.b.), respectively, were detected in these materials. The presence of N-substituents gives to activated carbons increased ability to adsorb chlorophenols from water [45]. On the opposite, the high temperatures employed when using CO2 or KOH as activating agents gave rise to a significant decrease of S content to less than 1% (d.b.), and a great reduction of N content was observed as well when activating with KOH at 750 °C (0.8%, d.b.). This fact could provoke some loss of adsorption capacity [45]. Nevertheless, this phenomenon is clearly overcome by the profuse porosity development upon KOH activation. Consistently with their much higher surface area the KOHactivated carbons yielded the highest adsorption capacities (Fig. 4). Nevertheless, the differences are significantly lower than the expected from a direct dependence of the porous structure. The maximum value obtained from the Langmuir equation for the monolayer capacity of the air-activated carbon was only 16% lower than that of the KOH-activated one in spite of the fact that the BET surface area of this last was about twenty times higher. It has to be pointed out that the adsorption capacities reported for the KOH-activated carbons were obtained after washing these carbons with 5 M HCl and, after that, with distilled water until constant pH. Values up to almost four times lower were obtained when using only distilled water for washing after the activation process. The positive effect of HCl washing can be due to the liberation of the partial blockage of the pores caused by the remaining potassium by-products from the activating agent, which may be not completely removed upon washing simply with distilled water [18, 24, 50, 53]. Most of the works dealing with the preparation of activated carbons from sewage sludge use the pre-pyrolysed material as precursor. However, in the present work, dried sewage sludge was directly used as starting material and KOH showed to be more effective with this
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precursor itself than with its pyrolytic char where some porosity had been already developed [53]. As can be observed in Fig. 4b, holding times can be reduced from 2 to 0.5 h by using KOH instead of physical activation agents without compromising the maximum 4-CP adsorption capacities obtained. 4. Conclusions Activated carbons of fairly different porous structure can be obtained by physical (CO2, air) and chemical (KOH) activation of dried sewage sludge. Physical activation did not yield activated carbons with more than 100 m 2/g BET surface area whereas around twenty times higher surface area was achieved by chemical activation. The high ash content of the starting material severely limits the development of porosity upon physical activation since CO2 or air partial gasification do not affect to the ash components. Nevertheless, in spite of these dramatic differences fairly close capacities were found for 4-CP liquid-phase adsorption. Thus, air-activation can be viewed as a simple way of valorisation of dry sewage sludge to inexpensive activated carbon. Acknowledgments The authors greatly appreciate financial support from the Spanish MCI through the projects CTM2010-15682 and CDS2006-0044. Victor M. Monsalvo thanks the MCI and the ESF for a research grant. References [1] D. Fytili, A. Zabaniotou, Utilization of sewage sludge in EU application of old and new methods — a review, Renew. Sustain. Energy Rev. 12 (2007) 116–140. [2] J. Werther, T. Ogada, Sewage sludge combustion, Prog. Energy Combust. Sci. 25 (1998) 55–116. [3] M.N. Folgueras, R.M. Díaz, J. Xilberta, I. Prieto, Volatilization of trace elements for coal-sewage sludge blends during their combustion, Fuel 82 (2003) 1939–1948. [4] Z. Aksu, J. Yener, A comparative adsorption/biosorption study of monochlorinated phenols onto various sorbents, Waste Manag. 21 (2001) 695–702. [5] M.J. Martin, A. Artola, M.D. Balaguer, M. Rigola, Activated carbons developed from surplus sewage sludge for the removal of dyes from dilute aqueous solutions, Chem. Eng. J. 94 (2003) 231–239. [6] S. Rio, L. Le Coq, C. Faur, D. Lecomte, P. Le Cloirec, Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation — application to air and water treatments, Process Saf. Environ. Prot. 84 (2006) 258–264. [7] C. Jindarom, V. Meeyoo, B. Kitiyanan, T. Rirksomboom, P. Rangsunvigit, Surface characterization and dye adsorptive capacities of char obtained from pyrolysis/ gasification of sewage sludge, Chem. Eng. J. 133 (2007) 239–246. [8] M. Inguanzo, A. Domínguez, J.A. Menéndez, C.G. Blanco, J.J. Pis, On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions, J. Anal. Appl. Pyrolysis 63 (2002) 209–222. [9] J. Ábrego, J. Arauzo, J.L. Sánchez, A. Gonzalo, T. Cordero, J. Rodriguez-Mirasol, Structural changes of sewage sludge char during fixed-bed pyrolysis, Ind. Eng. Chem. Res. 48 (2009) 3211–3221. [10] P. Chingombe, B. Saha, R.J. Wakeman, Surface modification and characterization of a coal-based activated carbon, Carbon. 43 (2005) 3132–3143. [11] Q. Hamdaoui, Batch study of liquid-phase adsorption of methylene blue using cedar sawdust and crushed brick, J. Hazard. Mater. 135 (2006) 264. [12] K.Y. Foo, B.H. Hameed, Detoxification of pesticide waste via activated carbon adsorption process, J. Hazard. Mater. 175 (2010) 1–11. [13] P.C. Chiang, J.H. You, Use of sewage sludge for manufacturing adsorbents, Can. J. Chem. Eng. 65 (1987) 922–927. [14] G.Q. Lu, Preparation and evaluation of adsorbents from waste carbonaceous materials from SOx and NOx removal, Environ. Prog. Sustain. Energy 15 (1996) 12–18. [15] F. Rozada, L.F. Calvo, A.I. García, J. Martín-Villacorta, M. Otero, Dye adsorption by sewage sludge-based activated carbons in batch and fixed-bed systems, Bioresour. Technol. 87 (2003) 221–230. [16] F. Rozada, M. Otero, A. Morán, A.I. García, Activated carbons from sewage sludge and discarded tyres: production and optimization, J. Hazard. Mater. B124 (2005) 181–191. [17] S. Rio, C. Faur-Brasquet, L. Le Coq, P. Courcoux, P. Le Cloirec, Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation — application to air and water treatments, Chemosphere 58 (2005) 423–437. [18] F.-S. Zhang, J.O. Nriagu, H. Itoh, Mercury removal from water using activated carbons derived from organic sewage sludge, Water Res. 39 (2005) 389.
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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Activated carbons from sewage sludge Application to aqueous-phase adsorption of 4-chlorophenol Victor Manuel Monsalvo ⁎, Angel Fernandez Mohedano, Juan Jose Rodriguez Seccion de Ingenieria Quimica, Facultad de Ciencias, Universidad Autonoma de Madrid. C/Francisco Tomas y Valiente 7, Madrid 28049, Spain
a r t i c l e
i n f o
Article history: Received 12 November 2010 Received in revised form 29 March 2011 Accepted 25 April 2011 Available online 18 May 2011 Keywords: Sewage sludge Valorisation Activated carbon Adsorption 4-Chlorophenol
a b s t r a c t Activated carbons of different characteristics have been prepared from dried sewage sludge using CO2, air and KOH as activating agents. The adsorption capacity of the resulting materials has been checked using 4chlorophenol as a target compound in aqueous solution. CO2 and air-activation led to carbons of low BET area which increased with the activation temperature but did not reach 100 m 2/g at the best. The high ash content of the starting material (23%, d.b.) limits the development of porosity since the partial gasification does not affect the inorganic matter. In the case of air-activation, the resulting surface remains fairly low even in ashfree basis, reaching no more than 250 m 2/g. Activation with KOH allows a much higher development of porosity. Using a KOH to solids ratio of 3:1 and a temperature of 750 °C a BET area above 1800 m 2/g with a mesopore volume higher than 0.35 cm 3/g was reached. In spite of those dramatic differences on the textural properties, the air-activated carbons showed an adsorption capacity for 4-chlorophenol of almost 85% that of the KOH-activated carbons. These results warrant in principle air-activation at moderate temperature as a promising way of sewage sludge valorisation to inexpensive adsorbents for the removal of water pollutants. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The increasing generation of sewage sludge derived from wastewater treatment demands the development of new ways of valorisation. Nowadays, management of sewage sludge includes composting for farmland utilization, incineration and landfilling [1]. However, this last will be severely limited in a short-term due to the increasing competition for landfill space, growing costs and more stringent environmental regulations. Incineration of sewage sludge requires strict control of the pollutants generated [2, 3] and in many cases suffers of social and political opposition. Different ways of valorisation are being investigated in the last two decades including the use of sewage sludge as precursor for adsorbent materials with environmental applications. Some authors have investigated the direct application of dried sewage sludge as adsorbent [4] although its low surface area, below 5 m 2/g [5–7], represents a severe limitation in this respect. However, more acceptable adsorbents can be obtained from sewage sludge by different ways. Pyrolysis can produce a carbonaceous solid with around 60 m2/g surface area [8], requiring temperatures above 700 °C to increase such porosity [9]. Although this type of carbons has been applied on the decontamination of several gaseous and aqueous off-streams, many industrial applications demand more efficient adsorbents. Acti⁎ Corresponding author. Tel.: + 34 4978035; fax: + 34 4973516. E-mail addresses: [email protected] (V.M. Monsalvo), [email protected] (A.F. Mohedano), [email protected] (J.J. Rodriguez). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.04.059
vated carbon has been witnessed as an adsorbent with large porous surface area, controllable pore structure, high thermo-stability and low acid/base reactivity [10]. Owning to its low initial cost, simplicity of design, insensitivity to toxic substances, high adsorption capacity and regenerability, activated carbons appeared to be the most versatile and suitable candidate for micropollutants removal from wastewaters [11, 12]. This fact has motivated a noticeable interest for the development of carbon-based adsorbents from many different precursors for industrial applications. Activated carbons can be obtained by both the so-called chemical [13–18] and physical [6, 19, 20] activation of sewage sludge. Recent works have demonstrated the possibility of developing surface areas over 1500 m 2/g through activation with KOH [21, 22]. The resulting materials have been tested in some gas-phase applications like odor control in wastewater treatment plants [23]. The removal of heavy metals and dyes from wastewaters by means of KOH and CO2activated sewage sludge has been also studied [7, 21, 22, 24]. Air-activation of sewage sludge has been scarcely studied in spite of the fact that this appears in principle the easiest way for the valorisation of that type of waste to an inexpensive adsorbent material with potential applications for the removal of gas and water pollutants. Activated carbons of less than 100 m 2/g BET surface area have been obtained upon air-activation of pre-pyrolysed sewage sludge and tested for Fe3+ removal from water [20]. In this work we investigate the direct activation of dry sewage sludge with CO2, air and KOH at different temperatures and holding
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times, since these two variables have been reported to be important for the final characteristics of the resulting materials [18, 25], which are evaluated as adsorbents for the 4-chlorophenol removal from aqueous solution. Chlorophenols are a representative group of harmful compounds specifically included in the lists of priority and hazardous pollutants. They are released into water from a diversity of sources such as oil exploration and production, wood and textile preservation, accidental spills, chlorination of water and wastewaters containing phenol, degradation of herbicides and pesticides and leaching from waste disposal [26–29].
2.3. Adsorption experiments 100 mg of activated carbon were mixed in stoppered bottles with 100 mL of aqueous solutions containing different concentrations of 4CP (0.1–2.0 g/L) at 15 °C and pH 7.0 (buffered solution of phosphate) and stirred at 200 rpm (equivalent) in a thermostatized shaker up to equilibration. The concentration of 4-CP was analyzed by HPLC/UV (Prostar, Varian) using a C18 column as stationary phase (Microsorb MW-100-5) and a mixture of acetonitrile and H2O (40:60, vol.) as mobile phase. The flow rate was maintained at 1 mL/min and a wavelength of 280 nm was used. The experimental data were fitted to the Langmuir (1) and Redlich–Peterson (2) equations:
2. Materials and methods 2.1. Adsorbents preparation The aerobic granular sludge was collected from a sequencing batch reactor treating industrial wastewaters from a cosmetics factory. Primarily, samples were washed with distilled water until constant conductivity, and then they were dried at 105 °C up to constant weight. The resulting solids were ground and sieved to a particle size in the range of 0.1–0.25 mm. Activation was performed in a vertical tube furnace electrically heated. The working temperature was reached at a 10 °C/min heating rate and the gas (CO2, air or N2 in the case of KOH-activation) was continuously passed at a 100 N-mL/min flow rate. Different holding times (0.5, 2.0 and 4.0 h) and temperatures (700 and 800 °C) were tested for the activation with CO2. Air-activated materials were also obtained at different holding times (0.5, 2.0 and 4.0 h) and temperatures (200, 300 and 400 °C). The resulting materials were washed with distilled water before being used in adsorption experiments. Chemical activation with KOH was carried out by physical mixing of the ground hydroxide lentils and dried sewage sludge. KOH to solids ratios of 1:1 and 3:1 (w:w) were employed. Activation was performed at 450 and 750 °C under N2 atmosphere for 30 min and the resulting samples were washed with 5 M HCl, and then with distilled water until constant pH.
2.2. Characterization of the starting material and the activated carbons The C, H, N, and S elemental composition of the solid samples was determined by a CHNS analyzer (LECO CHNS-932). The standard test method ASTM D2866-94 was used to determine the ash content by means of a thermogravimetric analyzer (SDTA851e). The metals content was examined by inductively coupled plasma atomic emission spectroscopy (ICP-MS) using a model Elan 6000 Sciex Perkin Elmer apparatus. The porous structure was characterized from the 77 K N2 adsorption/desorption isotherms (Autosorb-1, Quantachrome). The samples were previously outgassed for 8 h at 10− 3 Torr and 150 °C. The pH slurry value of the carbons was measured upon stirring in distilled water until constant pH. SEM images were obtained using a Hitachi S-3000N apparatus. Table 1 summarizes the characterization of the starting dried sludge.
Table 1 Characterization of the starting sewage sludge (composition in %, d.b.). C H N S Oa Ash content (%) SBET (m2/g) pHslurry a
By difference.
48.7 7.5 9.4 0.6 10.8 23 b3 m2/g 6.9
Na (mg/g) Mg (mg/g) Al (mg/g) P (mg/g) K (mg/g) Ca (mg/g) Ti (μg/g) Fe (μg/g)
10.90 3.16 10.00 3.37 1.84 4.61 64 283
qe =
QL ⋅KL ⋅Ce 1 + KL ⋅Ce
ð1Þ
qe =
A ⋅Ce 1 + B⋅Ceβ
ð2Þ
where qe is the equilibrium adsorbate loading onto the adsorbent (mmol/g), Ce the equilibrium liquid-phase concentration of the adsorbate (mmol/L), QL the monolayer adsorption capacity (mmol/g), KL the Langmuir constant (L/mmol), A the Redlich–Peterson constant (L/g), B a constant having units of (L/mmol)β, and β an exponent that lies between 0 and 1. The parameters of these equations were calculated by using a nonlinear regression fitting method. The results reported were the average values from duplicate runs. In all the cases, the standard errors were lower than 5%. 3. Results and discussions SEM images of the dried granules of aerobic sewage sludge used as precursor and the resulting activated carbons are shown in Fig. 1. The dried sewage sludge is a non-porous material with a surface area lower than 3 m 2/g. The materials resulting from physical activation had irregular shapes. The chemical activation process with KOH gave rise to a fluffy material of low density and high porosity. 3.1. CO2 and air activation Table 2 summarizes the values of the BET surface area and micropore volume of the carbon materials activated with CO2 and air. BET surface areas below 100 m 2/g were developed with both activating agents. With CO2, increasing the holding time up to 2 h gave rise to BET surface areas higher than those reported in the literature [7] at similar temperatures. No significant increase of the BET surface area was observed at higher holding times except with air at the lowest temperature tested. This last variable shows some significant effect on the development of surface area upon activation both with air and CO2 within the ranges investigated. The high ash content of the precursor (23%, d.b.) limits severely the development of porosity on a total weight basis since partial gasification does not affect to the inorganic matter. Nevertheless, the resulting materials from both CO2 and air-activation showed higher BET surface areas than the 65 m 2/g reported in the literature from steam-activation of sewage sludge at 800 °C [6]. The C/H molar ratio increased up to 3.0 and 1.6 in the CO2 and airactivated carbons, respectively, versus 0.5 of the dry sludge consistently with the dehydrogenative polymerisation and dehydrative polycondensation reactions commonly occurring upon thermal treatment, with significant loss of aliphatic hydrogen through olygomerisation. CO2activation was accompanied by a substantial decrease of the S content which reached values of 0.1–0.2% on a dry basis versus the 0.6% of the starting sludge solids (Table 1). The temperature must be a determining
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Fig. 1. SEM images of dried sewage sludge (a), CO2-activated carbon at 700 °C for 4 h (b), air-activated carbon at 400 °C for 4 h (c) and KOH-activated carbon at 450 °C with 3:1 KOH: solids ratio (d).
factor in this case, since air-activation, carried out at much lower temperature did not allow that reduction of the S content. 3.2. KOH activation Activation with KOH gave rise to carbons with a well developed porous structure showing both the temperature and the KOH to solids ratio a beneficial effect on that respect (Table 3). Increasing the temperature from 450 to 750 °C provokes a dramatic increase of the BET surface area affecting more remarkably to the microporosity although with an important contribution of mesoporosity also. At that temperature using a KOH to solids ratio of 3 allows obtaining frankly high values of BET and external or non-microporous surface areas (up to around 1800 and 380 m2/g, respectively). This development of porosity was higher than the obtained under similar activation conditions, from other substrates like coals, fibers, pistachio shells, corncob, pitch and fir
Table 2 Surface areas and micropore volumes of the carbons from CO2 and air-activation. T (°C)
Holding time (h)
CO2 activation 700 0.5 2.0 4.0 800 0.5 2.0 4.0 Air activation 200 0.5 2.0 4.0 300 0.5 2.0 4.0 400 0.5 2.0 4.0
SBET (m2/g)
Vmicropores (cm3/g)
11 75 79 20 94 97
0.01 0.05 0.06 0.02 0.04 0.09
7 34 47 13 51 53 15 92 91
b 0.01 0.03 0.05 0.01 0.05 0.05 0.03 0.06 0.07
wood [30–38]. Also, this activation procedure is much more effective than other based on commonly used chemical activating agents such as ZnCl2 [18, 39, 40], H3PO4 [18] or H2SO4 [5, 18, 39–41] with the dry solids from sewage sludge. Ros et al. [21] used an acid washing step previous to KOH-activation. That increased the surface area of the starting precursor but, according to our results does not produce any significant benefit regarding the final surface area achieved upon KOH activation. With regard to the evolution of the elemental analysis upon KOH-activation, a dramatic decrease of the N content at the highest activation temperature (750 °C) is noteworthy. This has been attributed by previous authors to the conversion of carbon-nitrogen moieties into cyanides caused by the alkali activation through the dehydrogenation of amino groups derived from proteins, nucleic acids and dead microorganisms [24, 42]. An important advantage of the chemical activation process is that it takes a shorter time than the required for physical activation. 3.3. Adsorption of 4-CP Adsorption isotherms of 4-CP by the different activated carbons are shown in Figs. 2, 3 and 4. In general, the experimental data fitted the Redlich–Peterson equation better than the Langmuir one most commonly used for liquid-phase-adsorption. That was particularly true in the case of the CO2-activated carbons. Table 4 summarizes the results of fitting
Table 3 Porous structure of the KOH-activated carbons. T (°C)
KOH:solids ratio
SBET (m2/g)
Vmesopores (cm3/g)
Vmicropores (cm3/g)
Sexternal (m2/g)
450
1:1 3:1 1:1 3:1
131 262 950 1832
0.12 0.16 0.23 0.36
b0.01 0.01 0.40 0.75
110 184 268 379
750
380
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Fig. 2. Adsorption isotherms of 4-CP onto the CO2-activated carbons.
the experimental adsorption data to the aforementioned isotherm equations. As can be seen in Fig. 2, similar adsorption capacities were observed for all the CO2-activated carbons, since the slight increase of the surface area when increasing the activation temperature from 700
Fig. 4. Adsorption isotherms of 4-CP onto the KOH-activated carbons.
to 800 °C (Table 2) did not cause any significant improvement. The analysis of the maximum adsorption capacity (QL) of the air-activated carbons indicates that is related with the BET surface area developed. Table 4 Fitting of the adsorption data of 4-CP onto the activated carbons from sewage sludge. T (°C) Holding time (h)
Redlich–Peterson
KOH:solids A ratio (L/g)
Fig. 3. Adsorption isotherms of 4-CP onto the air-activated carbons.
CO2-activated materials 700 2.0 0.722 4.0 0.646 800 2.0 0.862 4.0 0.903 Air-activated materials 200 0.5 0.060 2.0 0.389 4.0 0.315 300 0.5 0.538 2.0 1.317 4.0 0.291 400 0.5 0.280 2.0 0.254 4.0 0.235 KOH-activated materials 450 1:1 2.095 3:1 10.734 750 1:1 2.912 3:1 3.794
Langmuir
B β (L/mmol)
R2
KL QL R2 (L/mmol) (mmol/g)
0.134 0.033 0.251 0.231
0.978 0.988 0.676 0.873
0.993 0.992 0.995 0.998
0.583 0.404 0.134 0.573
1.999 2.570 3.205 1.970
0.980 0.960 0.972 0.941
0.052 0.927 0.108 0.071 0.752 0.031 0.431 0.010 0.346
1.062 0.811 1.140 1.067 0.943 1.074 0.883 0.747 0.804
0.995 0.996 0.995 0.993 0.997 0.998 0.997 0.997 0.993
0.249 0.382 0.185 0.456 0.598 0.237 0.261 0.201 0.159
0.332 0.697 1.922 1.815 1.993 1.644 1.555 2.046 2.372
0.998 0.991 0.993 0.984 0.995 0.997 0.994 0.997 0.965
1.607 10.844 1.712 1.235
0.935 0.742 0.969 1.044
0.996 0.995 0.997 0.996
1.165 1.910 1.478 1.474
1.498 1.559 1.809 2.820
0.997 0.989 0.996 0.994
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Nevertheless, the disturbances observed in such relation can be associated with differences in the surface chemistry. Despite the numerous studies published on the adsorption of phenolic compounds by activated carbons, the mechanism by which these compounds are adsorbed is not still completely well understood [43]. That mechanism is not only determined by π–π interactions and donor–acceptor complex formation but also by the solvent effect balancing the influence of those two contributions [44]. Additionally, the effect of the surface properties of activated carbons on adsorption has to be considered [45]. A relatively large amount of information describes the behavior of activated carbons with oxygen containing basic functional surface groups that in general cause enhanced sorption of phenolic compounds [46–48]. In addition, elimination of acidic oxygen-containing groups from activated carbons was suggested to be a way to increase adsorbability of phenols [46–50], through both an increased water cluster formation, thus obscuring the basal carbons, and a reduction of the availability of the π electrons [43]. However, Yin et al. [51] also noted that at high phenols concentrations, those phenols could be adsorbed by not only the mechanism of physisorption, but also by surface polymerisation, in which the phenolic compounds undergo oxidative coupling reactions on the carbon surface to produce polymeric compounds [52]. For this process the presence of oxygen is necessary and the high inorganic content of sewage sludge-based adsorbents could be advantageous for its potential use as adsorbent [24]. As mentioned, adsorption experiments were carried out at neutral pH, in which 4-CP is in the nonionic form (pKa = 9.38) and thus the adsorption must be governed by non-electrostatic interactions between the solute molecule and sorbent surface, i.e., the dispersive interactions between the π-electrons of the solute aromatic ring and the adsorbent. The surface oxygen, nitrogen and sulfur complexes influence these dispersive interactions to a large extent thus affecting the adsorption mechanism [50]. Owing to the high thermal stability of N and S hetero-atoms, they remain in the carbon upon air activation due to the low temperatures used. N and S contents of 7.5 ± 1.2% and 2.2 ± 0.8% (d.b.), respectively, were detected in these materials. The presence of N-substituents gives to activated carbons increased ability to adsorb chlorophenols from water [45]. On the opposite, the high temperatures employed when using CO2 or KOH as activating agents gave rise to a significant decrease of S content to less than 1% (d.b.), and a great reduction of N content was observed as well when activating with KOH at 750 °C (0.8%, d.b.). This fact could provoke some loss of adsorption capacity [45]. Nevertheless, this phenomenon is clearly overcome by the profuse porosity development upon KOH activation. Consistently with their much higher surface area the KOHactivated carbons yielded the highest adsorption capacities (Fig. 4). Nevertheless, the differences are significantly lower than the expected from a direct dependence of the porous structure. The maximum value obtained from the Langmuir equation for the monolayer capacity of the air-activated carbon was only 16% lower than that of the KOH-activated one in spite of the fact that the BET surface area of this last was about twenty times higher. It has to be pointed out that the adsorption capacities reported for the KOH-activated carbons were obtained after washing these carbons with 5 M HCl and, after that, with distilled water until constant pH. Values up to almost four times lower were obtained when using only distilled water for washing after the activation process. The positive effect of HCl washing can be due to the liberation of the partial blockage of the pores caused by the remaining potassium by-products from the activating agent, which may be not completely removed upon washing simply with distilled water [18, 24, 50, 53]. Most of the works dealing with the preparation of activated carbons from sewage sludge use the pre-pyrolysed material as precursor. However, in the present work, dried sewage sludge was directly used as starting material and KOH showed to be more effective with this
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precursor itself than with its pyrolytic char where some porosity had been already developed [53]. As can be observed in Fig. 4b, holding times can be reduced from 2 to 0.5 h by using KOH instead of physical activation agents without compromising the maximum 4-CP adsorption capacities obtained. 4. Conclusions Activated carbons of fairly different porous structure can be obtained by physical (CO2, air) and chemical (KOH) activation of dried sewage sludge. Physical activation did not yield activated carbons with more than 100 m 2/g BET surface area whereas around twenty times higher surface area was achieved by chemical activation. The high ash content of the starting material severely limits the development of porosity upon physical activation since CO2 or air partial gasification do not affect to the ash components. Nevertheless, in spite of these dramatic differences fairly close capacities were found for 4-CP liquid-phase adsorption. Thus, air-activation can be viewed as a simple way of valorisation of dry sewage sludge to inexpensive activated carbon. Acknowledgments The authors greatly appreciate financial support from the Spanish MCI through the projects CTM2010-15682 and CDS2006-0044. Victor M. Monsalvo thanks the MCI and the ESF for a research grant. References [1] D. Fytili, A. Zabaniotou, Utilization of sewage sludge in EU application of old and new methods — a review, Renew. Sustain. Energy Rev. 12 (2007) 116–140. [2] J. Werther, T. Ogada, Sewage sludge combustion, Prog. Energy Combust. Sci. 25 (1998) 55–116. [3] M.N. Folgueras, R.M. Díaz, J. Xilberta, I. Prieto, Volatilization of trace elements for coal-sewage sludge blends during their combustion, Fuel 82 (2003) 1939–1948. [4] Z. Aksu, J. Yener, A comparative adsorption/biosorption study of monochlorinated phenols onto various sorbents, Waste Manag. 21 (2001) 695–702. [5] M.J. Martin, A. Artola, M.D. Balaguer, M. Rigola, Activated carbons developed from surplus sewage sludge for the removal of dyes from dilute aqueous solutions, Chem. Eng. J. 94 (2003) 231–239. [6] S. Rio, L. Le Coq, C. Faur, D. Lecomte, P. Le Cloirec, Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation — application to air and water treatments, Process Saf. Environ. Prot. 84 (2006) 258–264. [7] C. Jindarom, V. Meeyoo, B. Kitiyanan, T. Rirksomboom, P. Rangsunvigit, Surface characterization and dye adsorptive capacities of char obtained from pyrolysis/ gasification of sewage sludge, Chem. Eng. J. 133 (2007) 239–246. [8] M. Inguanzo, A. Domínguez, J.A. Menéndez, C.G. Blanco, J.J. Pis, On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions, J. Anal. Appl. Pyrolysis 63 (2002) 209–222. [9] J. Ábrego, J. Arauzo, J.L. Sánchez, A. Gonzalo, T. Cordero, J. Rodriguez-Mirasol, Structural changes of sewage sludge char during fixed-bed pyrolysis, Ind. Eng. Chem. Res. 48 (2009) 3211–3221. [10] P. Chingombe, B. Saha, R.J. Wakeman, Surface modification and characterization of a coal-based activated carbon, Carbon. 43 (2005) 3132–3143. [11] Q. Hamdaoui, Batch study of liquid-phase adsorption of methylene blue using cedar sawdust and crushed brick, J. Hazard. Mater. 135 (2006) 264. [12] K.Y. Foo, B.H. Hameed, Detoxification of pesticide waste via activated carbon adsorption process, J. Hazard. Mater. 175 (2010) 1–11. [13] P.C. Chiang, J.H. You, Use of sewage sludge for manufacturing adsorbents, Can. J. Chem. Eng. 65 (1987) 922–927. [14] G.Q. Lu, Preparation and evaluation of adsorbents from waste carbonaceous materials from SOx and NOx removal, Environ. Prog. Sustain. Energy 15 (1996) 12–18. [15] F. Rozada, L.F. Calvo, A.I. García, J. Martín-Villacorta, M. Otero, Dye adsorption by sewage sludge-based activated carbons in batch and fixed-bed systems, Bioresour. Technol. 87 (2003) 221–230. [16] F. Rozada, M. Otero, A. Morán, A.I. García, Activated carbons from sewage sludge and discarded tyres: production and optimization, J. Hazard. Mater. B124 (2005) 181–191. [17] S. Rio, C. Faur-Brasquet, L. Le Coq, P. Courcoux, P. Le Cloirec, Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation — application to air and water treatments, Chemosphere 58 (2005) 423–437. [18] F.-S. Zhang, J.O. Nriagu, H. Itoh, Mercury removal from water using activated carbons derived from organic sewage sludge, Water Res. 39 (2005) 389.
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