Kinetic and calorimetric study of the adsorption of dyes on mesoporous activated carbon prepared from coconut coir dust

Kinetic and calorimetric study of the adsorption of dyes on mesoporous activated carbon prepared from coconut coir dust
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  Journal of Colloid and Interface Science 298 (2006) 515– Kinetic and calorimetric study of the adsorption of dyes on mesoporousactivated carbon prepared from coconut coir dust Jeremias de Souza Macedo, Nivan Bezerra da Costa Júnior, Luis Eduardo Almeida,Eunice Fragoso da Silva Vieira, Antonio Reinaldo Cestari, Iara de Fátima Gimenez,Neftali Lênin Villarreal Carreño, Ledjane Silva Barreto ∗  Laboratório de Síntese e Aplicação de Materiais, Departamento de Química—CCET, Universidade Federal de Sergipe,49000-100 São Cristovão, Sergipe, Brazil Received 17 October 2005; accepted 14 January 2006Available online 23 February 2006 Abstract Mesoporous activated carbon has been prepared from coconut coir dust as support for adsorption of some model dye molecules from aqueoussolutions. The methylene blue (MB) and remazol yellow (RY) molecules were chosen for study of the adsorption capacity of cationic and anionicdyes onto prepared activated carbon. The adsorption kinetics was studied with the Lagergren first- and pseudo-second-order kinetic models aswell as the intraparticle diffusion model. The results for both dyes suggested a multimechanism sorption process. The adsorption mechanisms inthe systems dyes/AC follow pseudo-second-order kinetics with a significant contribution of intraparticle diffusion. The samples simultaneouslypresent acidic and basic sites able to act as anchoring sites for basic and acidic dyes, respectively. Calorimetric studies reveal that dyes/ACinteraction forces are correlated with the pH of the solution, which can be related to the charge distribution on the AC surface. These AC samplesalso exhibited very short equilibrium times for the adsorption of both dyes, which is an economically favorable requisite for the activated carbondescribed in this work, in addition to the local abundance of the raw material. © 2006 Elsevier Inc. All rights reserved. Keywords:  Activated carbon; Coir dust; Porosity; Adsorption kinetics; Heat of adsorption 1. Introduction Many economic activities generate a large volume of re-sidues, such as those from agriculture, and in this context co-conut shell is a very common residue in tropical countries.When coconut shell is processed, industrially valuable longfibers are removed, leaving a considerable fraction of dustcomposed of short- to medium-length fibers, known as coirdust [1]. Although the estimated costs of these materials are U.S. $0.80 / 10 kg, the coir remains available as a waste prod-uct for which no important industrial uses have been developedand is normally disposed of or incinerated. To overcome thisproblem, one option is to develop a chemical industry based on * Corresponding author. Fax: +55 7932126651.  E-mail address: (L.S. Barreto). the use of this kind of abundant raw material [2]. Biomass is widely used as an alternative energy source [3–6], but its use as a precursor of several different products has also been describedelsewhere [7].An alternative route for the exploitation of agriculturalwastes relies on preparation of activated carbon (AC) as de-scribed in previous papers [8–10]. Activated carbon is the most commonly used support in processes of water separation andpurification, owing mainly to its highly porous structure andhigh surface area [11]. These physicochemical properties of  activated carbons are essentially dependent on the prepara-tion conditions, which can be based on physical or chemicalprocesses, both occurring at high temperatures [12,13]. The ac- tivation process promotes conversion of already existing micro-to mesopores as well as formation of new micropores [13]. Thisenlargement of pore diameters upon activation makes activatedcarbons effective for adsorption of dyes from aqueous solu- 0021-9797/$ – see front matter  © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.01.021  516  J.S. Macedo et al. / Journal of Colloid and Interface Science 298 (2006) 515–522 Fig. 1. Molecular structures and sizes of (a) methylene blue and (b) remazol yellow calculated with molecular modeling system CAChe. tion,asdescribedinseveralpapers[14–16].Mostcommercially available activated carbons are predominantly microporous, be-ing especially suitable for the adsorption of small species onthe pore surface. As the fraction of mesopores can be increasedby activation treatments, the obtained activated carbons are ex-pected to be very efficient as adsorbents of large molecules. Theadsorption capacity of activated carbons is generally evaluatedunder equilibrium conditions. On the other hand, the adsorp-tion mechanisms are best understood through kinetic models,which describe process dynamics. For instance, some verycommonmodelsaretheLagergrenpseudo-second-orderkineticmodel [17] and the intraparticle diffusion equations [18], which are used in the present study.This work describes the evaluation of a mesoporous acti-vated carbon prepared from coconut coir dust as support foradsorption of some model dye molecules from aqueous solu-tions. The model dye molecules chosen for studying the adsorp-tion capacity of the prepared activated carbon were methyleneblue (MB) and remazol yellow (RY). Fig. 1 shows the molecu-lar structures and sizes calculated with the molecular modelingsystem CAChe. 2. Materials and methods All chemicals were reagent grade and used without furtherpurification. ZnCl 2  was purchased from Vetec (Brazil); the co-conut coir dust employed as AC precursor was kindly providedby DILIMP—a local fiber processing industry. The dyes, com-mercially available as Reactive Yellow GR (reactive yellow 15)and Methylene Blue (C.I. 52015), were free gifts from theSantista Textiles Industries (Sergipe/Brazil), provided by theDystar Dyes Company and used without purification. For ad-sorption studies 10 − 4 molL − 1 stock solutions of MB and RYwere prepared prior to dilution to the required concentrations.Dye solutions were prepared and the initial pH was adjustedwith biphtalate/NaOH to the following pH values: 4.0, 6.0, 7.0,and 8.0. The pH variation was followed during all adsorptionexperiments. 2.1. Preparation of the activated carbons Prior to use, the coconut coir dust was dried in an oven for12 h at 100 ◦ C and sifted through a 100-mesh sieve.Zinc chloride (ZnCl 2 ), used as activating agent, was mixedwith coir dust prior to the activation step in the solid state inthe mass proportion 3 ZnCl 2 :1 coir dust. The activation processwas carried out in the following steps: coir dust impregnatedwith ZnCl 2  was heated in a furnace under N 2  at 10 ◦ C / min upto 800 ◦ C. In the sequence the atmosphere was changed to CO 2 and the conditions were kept for 2 h following by cooling toroom temperature under N 2  [19]. After cooling, the sampleswere stirred for 3 h in 0.1 molL − 1 HCl, filtered, washed withdistilled water until a negative Cl − test result was obtained, anddried for 12 h at 100 ◦ C. Characterization was accomplished bythe techniques described below.   J.S. Macedo et al. / Journal of Colloid and Interface Science 298 (2006) 515–522  517 2.2. Nitrogen isotherms Textural characterization of AC samples was carried outby N 2  adsorption–desorption isotherm measurements at 77 Kusing a Quantachrome Instruments Autosorb-1C. Prior to themeasurements, the samples were outgassed under vacuum at473 K. 2.3. Adsorption studies In a typical adsorption experiment, 5 mg of AC was addedto 20 ml of 10 − 5 molL − 1 methylene blue solution of a givenpH value in 100 ml polyethylene centrifuge tubes. The sam-ples were subjected to rotation at 200 rpm at room temperature(25 ± 3 ◦ C). Increasing dye–support contact times ranging from0 to 120 min were studied. The residual adsorbate solution con-centration was determined by the absorbance at 664 nm formethylene blue and at 420 nm for remazol yellow using UV/viscalibration curves. UV/vis measurements were performed witha FEMTO 800 XI spectrophotometer, using buffer solutionscontaining 5 mg of activated carbon as blank, in each pH value. 2.4. Calorimetric determinations Calorimetric measurements were performed at 298.15 K ina SETARAM C80 mixing calorimeter. The calorimeter’s per-formance and details of operation have been described previ-ously [20]. Samples of approximately 50 mg of AC were put into the lower part of the mixing cell, closed by a thin cir-cular membrane of PTFE. Into the upper part of the mixingcell, a volume of 3.0 ml of a specific dye solution was added.After complete stabilization of the baseline, a movable rod en-ables the dye solution to be pushed into the container withthe AC. Each individual experiment yields a thermal effect,  Q r ,which was corrected by subtracting the corresponding wettingeffect,  Q w , of the activated carbon in the pure solvent, i.e., thebuffer solution. The thermal effect of membrane breaking forthe empty cell was found to be negligible compared to the  Q r and  Q w  values. All obtained results are averages of two parallelexperiments. 3. Results and discussion 3.1. AC textural characterization Fig. 2 shows a N 2  adsorption isotherm onto activated carbonprepared from coconut coir dust, where a progressive incrementin adsorbed nitrogen is observed at all pressure ranges. The hys-teresis loop at high  P/P  0  values is associated with the filling of mesopores by capillary condensation, with the assumption thatthe pores have been filled with the condensed molecules. Thissuggests a type IV isotherm, typical of mesoporous solids [21].The pore-size distribution curve of activated carbon, calcu-lated by the BJH method [22], strongly supports the evidence for mesoporosity proposed on the basis of the shape of N 2  ad-sorption isotherms. A dramatic increase in surface area from4 to 1884 m 2 / g is verified as compared to the raw material. Fig. 2. Nitrogen adsorption (—)/desorption (---) isotherms of coconut coirdust-derived activated carbon.Fig. 3. Pore size distribution of powder of coir dust-derived activated carbon. The pore diameter range of AC was found to lie between 20and 40 Å (see Fig. 3). Those results suggest that the preparedAC sample can be practically effective for removing methyleneblue (MB) and remazol yellow (RY) from water solution, be-cause the molecular sizes are 13.82 and 15.66 Å, respectively,ascalculatedbytheAM1method[23].Thesedataarepresented in Table 1.Changes in the chemical nature of the lignocellulosic pre-cursor are expected to occur upon carbonization and activation.These possible modifications were studied by infrared spec-troscopy, as presented in Fig. 4.Before the carbonization, the material has a typical lignocel-lulosic composition, presenting bands at 3413 cm − 1 , assignedto  ν (O–H), at 2923 cm − 1 , assigned to  ν (C–H) from methyl andmethylenegroups,at1620cm − 1 ,assignedto  ν (C = O)fromcar-boxylate groups and at 1377 cm − 1 , assigned to  δ (C–H) frommethyl groups [12]. After carbonization and activation, which are followed by fixation of carbon and elimination of other el-ements in the form of water and other species, some changescan be observed, such as the presence of a very broad bandcentered at 3388 cm − 1 ( ν C–H) and at 2352 cm − 1 ( ν C ≡ C),along with the disappearance of the band at 1620 cm − 1 andthe presence of a band at 1558 cm − 1 ( ν C = C from aromatics).Terminal aliphatic groups remain, such as CH 3 , as evidenced  518  J.S. Macedo et al. / Journal of Colloid and Interface Science 298 (2006) 515–522 Fig. 4. FTIR spectra (KBr discs) of (a) coir dust and (b) activated carbon.Table 1Surface and physical properties of AC from coconut fiberParameter ValueAdsorption capacity for H + ions (mmol / g carbon) 1 . 0902 ± 0 . 0501Adsorption capacity for OH − ions (mmol / g carbon) 0 . 6774 ± 0 . 0110C (wt%) 77 . 18 ± 4 . 45H (wt%) 1 . 68 ± 0 . 48N (wt%) 1 . 80 ± 0 . 12 S  BET  (m 2 g − 1 ) 1884Pore diameter (nm) 2–4 by the band at 1379 cm − 1 , and by continuous absorption inthe region of 2900 cm − 1 [12]. These changes evidence the for-mation of structures containing multiple carbon–carbon bondssuchasaromaticringsandC ≡ Cunits,aswellastheeliminationof srcinally present oxygen and hydrogen atoms. It is worthmentioning that the material is noncrystalline, as evidenced byabsence of diffraction peaks in the XRD measurements (notshown). 3.2. Effect of contact time Fig. 5 shows the UV/vis absorption spectra before and aftertreatment with the AC for both dye solutions. The decrease inabsorption intensity with increased contact time reveals the ACefficiency in removal of MB and RY from aqueous solutionsand also suggests that a large adsorption extent takes place atshort time intervals, close to 20 min for both dyes.A series of experiments have been performed to evaluate theadsorption capacity ( q t  ) of the activated carbons as a functionof contact time. The adsorption capacity was calculated fromthe difference between the initial and final MB concentrationafter a contact time of 120 min, as follows,(1) q t   = (C i − C f  )V/m, where  q t   is the adsorption capacity (mass MB (mg) / massAC (g)),  C i  and  C f   are initial and final concentrations, respec-tively,  V   is the volume (L) of the dye solution, and  m  is the ACmass (g). Each experimental result was obtained by averagingthe data from two parallel experiments. (a)(b)Fig. 5. UV/vis absorption spectra for dye solutions, at pH 7.0, before and afterthe treatment with the AC. (a) MB, initial concentration 1 . 0 × 10 − 5 molL − 1 ;(b) RY, initial concentration 7 . 0 × 10 − 5 molL − 1 . Forthisstudytheinitialconcentrationsofdyesolutionswere1 . 0 × 10 − 5 molL − 1 for MB and 7 . 0 × 10 − 5 molL − 1 for RY.The plot of adsorption capacity ( q t  ), obtained from Eq. (1), ver-sus contact time  t   can be observed in Fig. 6. The time required for maximum adsorption of the dye is known as the equilibriumtime and was found in this study to be close to 120 min for bothdyes.Thisequilibriumtimecanbeconsideredveryshort,whichis an economically favorable condition for the activated carbondescribed here.For both dyes, the plots show two distinct adsorption pro-files,whichsuggestamultimechanismsorptionprocessasithasbeendiscussedinpreviousworks[24–26].Theregimeobservedin the first region, between 0 and 20 min, can be related to anexternal diffusion process, in which the dye molecules migrateto reach the AC external surface (surface layer diffusion). Dueto the absence of mechanical barriers in this step, the removalof dyes by adsorption on AC is a fast process. The adsorptionprofile observed in the second region of the curves starts from20 min and can be related to an internal diffusion process inwhich the dye molecules penetrate into the porous AC struc-ture (intraparticle diffusion). The equilibrium of the adsorptionprocess is reached thus at contact times above 20 min. Alsothe smooth and continuous feature of the curves is a commonbehavior and it has been related in previous studies with mono-layer coverage of dye on the surface the adsorbent [27].   J.S. Macedo et al. / Journal of Colloid and Interface Science 298 (2006) 515–522  519 To evaluate the mechanism of the adsorption process, a first-order and a pseudo-second-order kinetic model and a diffusionmodel were applied to the experimental data. 3.3. Intraparticle diffusion model The possibility of intraparticle diffusion was examined usingthe intraparticle diffusion model [28], taking into account that during the course of adsorption the adsorbed amount is propor-tional to the square root of the contact time,(2) q t   = K d t  1 / 2 , where  K d  is the intraparticle diffusion constant. (a)(b)Fig. 6. Adsorption capacity curves of AC from aqueous dye solutions: (a) MB,initial concentration 1 . 0 × 10 − 5 molL − 1 ; (b) RY, initial concentration 7 . 0 × 10 − 5 molL − 1 . Experiments were performed at 25 ◦ C. Equation (2) was applied to the adsorption data with contacttimes changing from 20 to 120 min, corresponding to the linearportion of the curves. The rate constant for intraparticle diffu-sion  K d  was calculated from the slope of linear portion. Theresulting plots of   q t   versus  t  1 / 2 can be found in Fig. 7.The curves presented  R -values close to 0.99, indicating thesignificant contribution of the intraparticle diffusion process.The linear regression and  K d  values are presented in Table 2. 3.4. First-order and pseudo-second-order models For the determination of adsorption constants, kinetic dataobtained from batch studies were evaluated using the Lagergren (a)(b)Fig. 7.  q t   versus  t  1 / 2 plots from intraparticle diffusion model: (a) MB, initialconcentration 1 . 0 × 10 − 5 molL − 1 ; (b) RY, initial concentration 7 . 0 × 10 − 5 molL − 1 . Experiments were performed at 25 ◦ C.Table 2Kinetic values calculated agreement with intraparticle diffusion model for methylene blue and remazol yellow on ACMB (cationic)—1 . 0 × 10 − 5 molL − 1 RY (anionic)—7 . 0 × 10 − 5 molL − 1 pH  K d  R K d  R 8.0 3 . 69 × 10 − 2 0.9879 4 . 72 × 10 − 1 0.98527.0 2 . 91 × 10 − 2 0.9396 4 . 28 × 10 − 1 0.99326.0 3 . 52 × 10 − 2 0.9612 6 . 28 × 10 − 1 0.99244.0 3 . 42 × 10 − 2 0.9604 8 . 16 × 10 − 1 0.9912
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