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Pore structure and adsorption performance of the KOH-activated carbons
prepared from corncob
Ru-Ling Tseng
a,b,∗, Szu-Kung Tseng
aaGraduate Institute of Environmental Engineering, National Taiwan University, Taiwan
bDepartment of Safety, Health and Environmental Engineering, National United University, Miao-Li 360, Taiwan Received 17 November 2004; accepted 16 February 2005
Available online 13 April 2005
Abstract
Carbonaceous adsorbents with controllable surface area were chemically activated with KOH at 780◦C from chars that were carbonized from corncobs at 450◦C. The pore properties, including BET surface area, pore volume, pore size distribution, and mean pore diameter of these activated carbons, were characterized by the t-plot method based on N2adsorption isotherms. Two groups are classified according to
the types of adsorption/desorption isotherms. Group I corncob-derived activated carbons, with KOH/char ratios from 0.5 to 2, exhibited BET surface area ranging from 841 to 1221 m2/g. Group II corncob-derived activated carbons, with KOH/char rations from 3 to 6, showed high BET surface areas, from 1976 to 2595 m2/g. From scanning electron microscopic (SEM) results, the surface morphology of honeycombed holes on corncob-derived activated carbons was significantly influenced by the KOH/char ratios. The adsorption kinetics of methylene blue, basic brown 1, acid blue 74, 2,4-dichlorophenol, 4-chlorophenol, and phenol from water at 30◦C were studied on the two groups of activated carbons, which were suitably described by two simplified kinetic models, pseudo-first-order and pseudo-second-order equations. The effective particle diffusivities of phenols and dyes at the corncob-derived activated carbons of group II are higher than those of ordinary activated carbons. The high-surface-area activated carbons were demonstrated to be promising adsorbents for pollution control and for other applications.
2005 Elsevier Inc. All rights reserved.
Keywords: Activated carbons; KOH activation; Corncob; Pore properties; Adsorption
1. Introduction
Liquid-phase adsorption has been demonstrated to be a highly efficient method for the removal of colors and odors as well as organic and inorganic matter from chemical processes or waste effluents. In general, activated carbons in both granular and powdered forms are the most widely used adsorbents because of their excellent adsorption capa-bility for organic pollutants [1], which are usually related to their specific surface area, pore volume, and porosity[2]. In addition, the adsorption properties of activated carbons were found to depend strongly on the activation process and the nature of raw materials. Moreover, in developing the
*Corresponding author. Fax: +886 37 333187. E-mail address:trl@nuu.edu.tw(R.-L. Tseng).
porosity of carbons by means of both physical and chemi-cal activation processes, an understanding of the influence of preparation variables is very important[3]. Particularly, the development of micropores and mesopores is of great impor-tance because the adsorption of large amounts and various chemicals from gas or liquid streams on activated carbons resulted from the formation of these pores. Recent progress of industrial technologies provides a new application of ac-tivated carbons in supercapacitors, which, at the same time, requires the carbons to have a desired pore structure[4].
Recently, we prepared a series of steam-activated carbons derived from pinewood[5], corncob[6], bagasses[7], plum kernels[8], and bamboo[9]. Based on these studies, intensi-fying the activation, such as at an elevated temperature or a longer activation time, would increase the BET surface area (Sp) and the mesopore ratio (Vmeso/Vpore) simultaneously. In 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.
addition, carbons prepared from eucalyptus wood activated in an atmosphere containing CO2and a small amount of O2
showed very similar results[10]. However, the mesopore ra-tio was not proporra-tionally enhanced by an increase in the chemical dosages when carbons were fabricated by chemical activation with KOH, ZnCl2, H3PO4, etc. Accordingly, their Vmeso/Vpore ratios are usually smaller than 0.2, while they
sometimes increased abruptly up to 0.5[11–15]. Hence, un-derstanding of this special phenomenon and precise control of the pore size distribution of chemically activated carbons are essential for several applications, such as pollution con-trol and supercapacitors.
In fact, relatively few researches on the preparation and characterization of activated carbons derived from corncobs have been reported in the literature, although corncobs are a cheap and abundant agricultural waste of no economic value
[16]. However, corncob-derived activated carbons have been shown to be highly porous and rich in mesopores, exhibit-ing high adsorption capacity for methylene blue and Pb2+ ions[16]. In addition, the chemical activation of corncobs with ZnCl2 was reported to be a very suitable process for
the preparation of corncob-derived activated carbons with an essentially microporous structure[17]. Note that for all previous work on the preparation and characterization of ac-tivated carbons derived from vegetation, chemical activation was performed on the precursors of vegetation. Accordingly, the intrinsic advantages of chemical activation from chars have been omitted.
Based on all the above viewpoints and our previous ex-perience in the preparation and characterization of porous carbons[5–7,18], this work tried to demonstrate the unique structure of carbons chemically activated with KOH from chars that were carbonized from corncobs. The structural properties of these carbons (activated in various KOH/char ratios) include the BET surface area, pore size distribution, and total pore volume. In addition, merits of the structures of these activated carbons in the adsorption of methylene blue, basic brown 1, acid blue 74, 2,4-dichlorophenol, 4-chlorophenol, and phenol in aqueous media are system-atically discussed. Moreover, some typical results shown in the literature are also compared to demonstrate the promis-ing applicability of these corncob-derived activated carbons with various KOH/char ratios.
2. Materials and methods
2.1. Preparation of carbons by KOH activation
Corncobs were dried at 110◦C for 24 h, placed in a sealed ceramic oven, and heated at a rate of 5◦C/min from room temperature to 450◦C. In the meantime, N2was poured into
the oven at a rate of 3 dm3/min for 1.5 h. Under such
oxygen-deficient conditions, corncobs were thermally de-composed to porous carbonaceous materials and hydrocar-bon compounds. This is the carhydrocar-bonization step.
After the carbonization step, a char/corncob ratio of 35.7% was obtained. The chars were removed, crushed, and sieved to a uniform size ranging from 0.833 to 1.65 mm. These powders were well mixed with water and KOH in a stainless steel beaker with the weight ratio of KOH/char equal to 0, 0.5, 1, 2, 3, 4, and 6. Water was evaporated at 130◦C for 24 h, and these dried mixtures consisting of chars and KOH (without KOH lost) was obtained. The dried mix-tures were placed in a sealed ceramic oven, heated at a rate of 10◦C/min to 780◦C, and kept at this temperature for 1 h. In the meantime, nitrogen gas was flowed into the oven at a rate of 3 dm3/min. The activated products were cooled to
room temperature and washed with deionized water. These samples were poured to a beaker containing 0.1 mol/dm3 HCl (250 cm3) and stirred for 1 h. Finally, they were washed with hot water until pH of the washing solution reached ca. 6–7[19]. The dried powders were sieved in the size ranged from 0.177 to 0.42 mm.
2.2. Measurements of physical properties
The yield was defined as the weight ratio of final carbons to the initial dried raw materials. The BET surface area of carbons (Sp) was obtained from the N2adsorption isotherm
at 77 K with a sorptiometer (porous materials, BET-202A). Prior to this measurement, the samples were first dried in an oven at 130◦C overnight and then, quickly placed into the sample tube. After that, the tube was heated to 230◦C and evacuated for 4 h until the pressure was less than 10−4Torr. The total pore volume (Vpore) was reduced from the
adsorp-tion data based on the manufacturer’s software and the pore size distribution was derived from the BJH theory[20]. The micropore volume (Vmicro) and external surface area (Sext)
were deduced using the t -plot method[21,22]. The surface area corresponding to the micropores (Smicro) was obtained
from the difference between Spand Sext[23].
2.3. Procedures for adsorption experiments
Six adsorbates, including acid blue74 (AB74), basic brown 1 (BB1), methylene blue (MB), 2,4-dichlorophenol (2,4-DCP), 4-chlorophenol (4-CP), and phenol, were analyt-ical reagent grade (Merck). Molecular weights are respec-tively 466.4, 419.4, 284.3, 163.0, 128.6, and 94.1 g/mol. The aqueous phase for adsorption was prepared by dissolving MB, BB1, AB74, 2,4-DCP, 4-CP, and phenol in deionized water without pH adjustment. Under the investigated con-ditions, the initial pH was about 6.60 for 200 g/m3of MB, 3.95 for 200 g/m3of BB1, 6.13 for 200 g/m3of AB74, and 6.10 for 1 mol/dm3of phenol, 4-CP, and 2,4-DCP.
Kinetic experiments were carried out in a Pyrex glass ves-sel with inner diameter 100 mm, height 130 mm, and fitted with four glass baffles (10 mm in width). The aqueous so-lution (0.6 dm3) with 0.3 g carbon powders was agitated at 500 rpm using a Cole–Parmer Servodyne agitator hav-ing a six-flat-blade impeller (12 mm high and 40 mm wide).
Fig. 1. Adsorption/desorption isotherms of N2at 77 K on KOH activated carbons derived from corncob (KOH/char ratio is 0 (E), 0.5 (1), 1.0 (e), 2.0 (!), 3.0 (2), 4.0 (a), and 6.0 ("), respectively).
When carbons were added into the vessel, adsorption time was recorded. The vessel was also immersed in a water bath controlled at 30◦C. During the experiment, aqueous samples (5 cm3) were taken from the solution and the concentrations were analyzed, the concentration of solutes in the aqueous phase was determined with a Hitachi UV/vis spectropho-tometer (U-2001). Each experiment was repeated at least three times under identical conditions. The amount of ad-sorption at time t , qt (mol/kg), was similarly calculated by
(1)
qt=
(C0− Ct)V
W ,
where Ct is the liquid concentration at time t in mol/m3.
The experiment error was within 4%.
3. Results and discussion
3.1. Physical properties of activated carbons
Identifying the pore structure of adsorbents is an essen-tial procedure before designing the adsorption processes, which is commonly determined by the adsorption of inert gases[1,24].Fig. 1shows the typical adsorption/desorption isotherms of N2at 77 K for all activated carbons with
var-ious KOH/char values. Two different types of curves are found in Fig. 1: group I activated carbons were prepared with the KOH/char values ranging from 0 to 2 (empty cir-cles in the figure) and group II carbons were fabricated with the KOH/char values from 3 to 6 (real circles in the figure). Note that when P /P0values vary from 0 to 1.0, the adsorbed
volumes of group I activated carbons are nearly unchanged. For group II carbons, the initial adsorbed volumes are almost the same (488 cm3/g) at P /P0= 0, while under specified P /P0values, the adsorbed volume increases with increasing
KOH/char value. When P /P0value is equal to or above 0.2,
the increase in the amount of adsorption becomes very small. All the above results indicate that developing the pore struc-ture of corncob-derived activated carbons strongly depends on the KOH/char value.
Fig. 2. Pore size distribution of the KOH activated carbons derived from corncob.
Fig. 2 shows the typical pore size distribution of all corncob-derived carbons activated from chars with KOH. For group I, one peak is clearly visible, which is located in the mesopore region with pore size between 3 and 5 nm. The above findings are similar to the pore size distribution for the corncob-derived activated carbon prepared by steam activa-tion[6]. On the other hand, most pores of group II carbons are below 3 nm.
Typical SEM photographs of activated carbons with the KOH/char values of 1 and 4 are shown inFigs. 3a–3c and 3d–3f. FromFigs. 3a–3c, many large holes (with diameters about 10 µm) in a honeycomb shape were clearly found on the surface of group I activated carbons. In addition, these irregular cottony holes were surrounded with contracted and twisted walls. Moreover, on the basis of the results shown in
Figs. 1 and 2as well asTable 1, group I activated carbons should be highly porous. On the other hand, for group II, as found inFigs. 3d–3f, the honeycomb-shaped holes are well arranged and regular. In addition, the walls of these holes are thick and smooth without any cottony structure. Note that all the above large holes should originate from the corncob precursors, which are not detectable by the N2 adsorption
technique of the BET method.
Based on the SEM observations, the following hypothe-ses are proposed. When chars were soaked in a large amount of KOH, a thin film of KOH should be coated on their sur-face and the interior of chars should be covered with KOH completely. During the activation in an inert atmosphere, surface pyrolysis does not occur on the surface of chars, re-sulting in the absence of a cottony structure. Accordingly, the final activated carbons maintain the original structure features of chars; i.e., holes with smooth and thick walls are not twisted or deformed. However, the soaking of KOH into the interior of holes favors the development of micropores (chemical activation), resulting in an increase in the amount of micropores[19]. On the contrary, when char is soaked in a relatively small amount of KOH, most of the KOH should be seeped deeply into the interior of chars, while the sur-face coating of KOH on these chars should be incomplete. If this is the case, the surface of chars, should undergo vi-olent pyrolysis creating an irregular cottony feature on the
Fig. 3. Observations with SEM: (a), (b), (c) are KOH/char= 1, (d), (e), (f) are KOH/char = 4. Table 1
Physical properties of carbons derived from corncob under KOH activation conditions
KOH/char ratio Sp(m2/g) Smicro/Sp(–) Vpore(cm3/g) Vmicro/Vpore(–) Dp(nm) Yield (%) ρb(kg/m3)
Char <10 – – – – 35.7 263 0 309 0.851 0.20 0.677 2.6 15.0 285 0.5 841 0.929 0.49 0.821 2.3 21.0 266 1 1071 0.923 0.69 0.805 2.6 22.3 198 2 1221 0.919 0.87 0.768 2.9 19.5 135 3 1976 0.943 1.10 0.871 2.3 19.2 98 4 2402 0.930 1.29 0.844 2.1 18.1 112 6 2595 0.899 1.43 0.780 2.2 15.9 94
surface of holes. Accordingly, contracted, twisted, thin, and deformed walls of honeycomb holes are clearly found. How-ever the soaking of KOH in the char interior still promotes the etching during the activation process, which creates mi-cropores. Therefore, surface areas increase with increasing the KOH/char value. This indicates that both surface py-rolysis and interior etching processes occur simultaneously during the preparation of group I activated carbons.
Table 1shows the pore properties of all activated carbons, including Sp, Smicro/Sp, Vpore, Vmicro/Vpore, Dp, yield, and ρb. Note that Sp of all activated carbons are gradually
in-creased with increasing the KOH/char ratio. These results are in agreement with those in Refs.[25,26]. The Spvalues
of group I are ranged from 841 to 1221 m2/g meanwhile for
group II activated carbons, Spvalues of 1976–2595 m2/g are
higher than the highest Sp value (1806 m2/g) of
corncob-derived activated carbons (activated with 15 wt% KOH) re-ported in Ref.[2]. Since the corncob precursors were directly soaked with KOH in Ref. [2], which is different from the two-step process employed in this study, the very high Sp
values of group II activated carbons are reasonably attributed to the efficient chemical activation of chars in this work.
FromTable 1, the fraction of micropore area, Smicro/Sp,
for the carbons derived from corncob with KOH activation ranges from 0.90 to 0.94, which is larger than that of the steam-activated carbon (0.87)[6]. In addition, the fraction of mircropore volume, Vmicro/Vpore, located between 0.77
and 0.87 for the KOH-activated carbons, which is larger than that of the steam-activated one (0.66)[6]. These results also indicate that KOH activation promotes the formation of mi-cropores, as clearly observed inFig. 2.
FromTable 1, when the KOH/char ratio increased from 0.5 to 6, Spwas gradually increased from 841 to 2595 m2/g.
However, the Vmicro/Vporeratios equal 0.82, 0.81, 0.77, 0.87,
0.84, and 0.78, respectively, indicating an irregular depen-dence on the KOH/char ratios. This is probably due to the interior etching process of KOH activation. Note that for the carbons activated with chemicals (such as KOH, ZnCl2, and
H3PO4), Sp was found to increase with increasing
chem-ical dosages, while the Vmicro/Vpore ratios did not always
increase proportionally. For example, in the case of coconut shells, when ZnCl2/shell ratios were equal to 0.25, 0.5, and
0.75, Sp values of 1017, 1355, and 1510 m2/g were
ob-tained, while the Vmicro/Vporevalue equaled 0.82, 0.86, and
0.86, respectively[11]. In another case, on the other hand, when the ZnCl2/shell ratio was increased from 1 to 3, the Vmicro/Vpore ratio increased rapidly[11]. The former case,
similar to our system, belongs to the interior etching process within chars by chemicals, which produces uniform microp-ores (seeFig. 2andTable 1).
Yield is related with the economics of activated carbon manufacture. Activated carbons prepared from corncobs by steam[6], 50 wt% H3PO4[16], 15 wt% KOH, and 37.5 wt%
K2CO3[2]had a yield value of 10.4, 18.3, 12.8, and 17.4%,
respectively. From Table 1, when the KOH/char value is equal to 0 (i.e., only pyrolysis), the yield is only 15%. As the KOH/char values reach 0.5 and 1.0, yields rise to 21 and 22.3%, respectively. Note that under these conditions, sur-face pyrolysis and micropore formation by KOH activation occur simultaneously. However, when the KOH/char value is increased from 2 to 6, yield decreases from 19.5 to 15.9%, probably due to the increase in the advanced development in micropores since Sprises quickly.
FromTable 1, Vpore increases with increasing KOH/char
ratio; i.e., Vpore increases from 0.49 to 1.43 cm3/g, while
the Vpore values of group II carbons (1.10–1.43 cm3/g) are
much higher than those of group I (0.49–0.87 cm3/g). In
addition, activated carbons prepared from corncobs with ac-tivation by 15 wt% KOH were reported to have the highest
Vpore value (0.87 cm3/g) in Ref.[2]. The above difference
is attributed to the fact that the walls of honeycomb holes in our activated carbons are not contracted or twisted and many micropores (and large Vpore values) are created from
the KOH activation in the char interior when chars are acti-vated at high KOH/char values.
FromTable 1, a larger average pore size (Dpranging from
2.31 to 2.86 nm) is obtained for the carbons of group I in comparison with that of group II (Dpranging from 2.15 to
2.23 nm). In addition, Dp values of group I activated
car-bons are higher than that of other corncob-derived activated carbons in Refs.[2,6,16,17]. On the other hand, Dpvalues
of group II activated carbons are close to the reported values found in these literature.
The last item inTable 1is bulk density (ρb). For the
acti-vated carbons of group I (i.e., KOH/char ratio from 0.5 to 2),
ρbvalues decrease from 266 to 135 kg/m3. These values are
compatible to the data reported in the literature; for example,
ρbwas found to be 450 kg/m3for olive-mill waste activated
by 200 wt% KOH[26]and 262 kg/m3for macadamia nut-shell activated by 100 wt% KOH[27]. On the other hand, for the carbons of group II, ρb values, between 94 and
112 kg/m3, are much lower than that of KOH-activated car-bons prepared from ligneous matter found in Refs.[26,27]. Note the much higher Spand Vpore values of group II
acti-vated carbons in comparison with that of actiacti-vated carbons prepared from corncobs found in Refs. [2,6,16,17]. This unique structure of group II activated carbons is believed to decrease the diffusion resistance of organics within granu-lar activated carbons (GAC), which is studied in Section3.2
to demonstrate the promising application potential in several fields.
3.2. Adsorption kinetics
From Ref. [28], activated carbons prepared through chemical activation of KOH under 500–900◦C still are of many abundant functional groups. The presence of different functional groups on the carbon surface (e.g., carboxylic, carbonyl, hydroxyl, ether, quinone, lactone) implies that there exist many types of solute–adsorbent interactions[29]. In addition, any kinetic or mass transfer representation is likely to be global. Accordingly, in this work, two kinetic models are adopted to examine the mechanism of adsorp-tion processes.
First, the kinetics of adsorption is analyzed through the pseudo-first-order equation shown as[30,31]
(2)
dqt/dt= k1(qe− qt),
where k1is the pseudo-first-order rate constant (1/min) and qe denotes the amount of adsorption at equilibrium. After
integration, by application of the conditions qt= 0 at t = 0
and qt= qt at t= t, Eq.(2)becomes
(3) log(qe− qt)= log qe− k1 2.303 t,
where the value of qemust be obtained independently from
the equilibrium experiments.
The pseudo-second-order equation based on the adsorp-tion capacity is of the form[32]
(4)
dqt/dt= k2(qe− qt)2,
where k2 is the pseudo-second-order rate constant (kg/
con-Table 2
Kinetic parameters and standard deviations for the adsorption of dyes on the group I corncob activated carbon
Solute KOH/char First order Second order
k1× 102(min) qe(mol/kg) q (%) q (%) MB 0.5 6.38 0.349 16.7 26.1 1 6.17 0.600 3.8 11.5 2 6.84 0.840 4.7 15.4 BB1 0.5 4.14 0.242 5.8 16.8 1 5.42 0.456 5.3 16.6 2 6.07 0.557 6.5 17.5 AB74 0.5 5.38 0.023 6.9 11.1 1 6.31 0.130 4.0 16.9 2 5.73 0.193 2.4 14.6 Average 6.2 16.3 Table 3
Kinetic parameters and standard deviations for the adsorption of dyes on the group II corncob activated carbon
Solute KOH/char First order Second order
q (%) k2(kg/(mol min)) qe(mol/kg) q (%)
MB 3 5 0.217 1.08 4.7 4 8.6 0.173 1.27 5.5 6 10.9 0.255 1.28 1.7 BB1 3 12 0.230 0.68 2.2 4 13.3 0.217 0.74 1.9 6 8.5 0.310 0.75 4.3 AB74 3 11.9 0.306 0.31 4.7 4 12.1 0.201 0.35 3.3 6 10.5 0.149 0.38 2.8 Average 10.3 3.5
ditions, an integration equation is obtained:
(5) t qt = 1 k2qe2 + 1 qe t.
It is noticed that k2and qein Eq.(5)can be obtained from
the intercept beforehand.
Evidently, the validity of these models can be checked by each linear plot of log(qe− qt) vs t and (t/qt) vs t . The
ap-plicability of each model is quantitatively determined from the normalized standard deviation qt(%), which is defined
as
(6)
qt(%)= 100
[(qt,exp− qt,cal)/qt,exp]2
(N− 1) ,
where N is the number of data points.
Table 2lists the results for the adsorption of dyes onto group I activated carbons for all models. It is obvious that the adsorption of MB, BB1, and AB74 onto the group I carbons is best described by the pseudo-first-order equation with an average qt of 6.2. The pseudo-second-order
equa-tion has an average qt of 16.3, which is larger than that of
the pseudo-first-order equation. This result is in compliance with those for the adsorptions of dyes BR22 and AB25 onto corncob-derived carbons activated by steam[6].
Table 3lists the results for the adsorption of dyes onto group II activated carbons for all models. It is obvious that
the three dyes’ adsorption onto the group II carbons is best described by the pseudo-second-order equation with an av-erage qt of 3.5. Comparisons of the simulated values from
these two models and the experimental results are shown in Fig. 4. Fig. 4a shows the adsorption of MB, in which curves are the simulated results calculated from the pseudo-first-order and pseudo-second-order equations. These curves show that group I activated carbons (empty symbols) are bet-ter fitted with the pseudo-first-order equation and those of group II (solid symbols) are better described by the pseudo-second-order equation. Figs. 4b and 4c show the adsorp-tion of BB1, and AB74, respectively. The results are very similar to those found in Fig. 4a. For the shapes of two modeled curves, the curves of the pseudo-first-order tion are flatter while those of the pseudo-second-order equa-tion is steeper. For the adsorpequa-tion of larger molecule dyes, group I carbons are adequately fitted by the former models, while group II carbons are reasonably described by the lat-ter model. In addition, group II carbons adsorb dyes more quickly.
Table 4lists the fitting results for the adsorption of nols for all models. It is obvious that the adsorption of phe-nol, 4-CP, and 2,4-DCP on all activated carbons is most suitably described by the pseudo-second-order equation with an average qt of 4.4%.Figs. 5a–5cshow the curves of the
2,4-Table 4
Kinetic parameters and standard deviations for the adsorption of phenols on the all corncob activated carbon
Solute KOH/char First order Second order
q (%) k2(kg/(mol min)) qe(mol/kg) q (%)
Phenol 0.5 16.0 0.105 1.90 11.1 1 14.7 0.062 2.48 2.6 2 12.9 0.060 2.47 2.3 3 13.3 0.070 2.61 2.5 4 6.1 0.070 3.27 5.2 6 5.8 0.081 3.47 5.6 4-CP 0.5 9.0 0.067 2.22 8.1 1 10.9 0.073 3.49 3.2 2 11.0 0.084 3.86 3.5 3 12.1 0.074 4.32 2.6 4 7.0 0.074 4.90 4.0 6 6.2 0.074 5.16 5.7 2,4-DCP 0.5 9.7 0.105 1.54 4.8 1 10.5 0.062 2.99 2.9 2 8.2 0.060 3.55 5.8 3 12.8 0.059 4.17 3.1 4 8.8 0.064 4.56 2.8 6 8.8 0.070 4.80 3.0 Average 10.2 4.4
Fig. 4. Comparison of the pseudo-first-order and pseudo-second-order mod-els for adsorption of (a) MB, (b) BB1, and (c) AB74 on the corncob car-bons activated with different KOH/char (KOH/char ratio 0.5 (1), 1.0 (e), 2.0 (!), 3.0 (2), 4.0 (a), and 6.0 ("), respectively: pseudo-first-order (- - -) and pseudo-second-order (—)).
DCP, 4-CP, and phenol. They show that all activated carbons are excellently described by the pseudo-second-order equa-tions.
Fig. 5. Compare of the pseudo-first-order and pseudo-second-order models for adsorption of (a) MB, (b) BB1, and (c) AB74 on the corncob carbons ac-tivated with different KOH/char (KOH/char ratio 0.5 (1), 1.0 (e), 2.0 (!), 3.0 (2), 4.0 (a), and 6.0 ("), respectively: pseudo-first-order (- - -) and pseudo-second order (—)).
The adsorption of phenols onto the corncob-derived acti-vated carbons prepared by steam activation is better fitted by the pseudo-second-order equation; further, the
adsorp-Table 5
Effective particle diffusivity for the sorption of phenols and dyes from water at 30◦C Activated carbon Ds(m2/s)× 1012
KOH/char Phenol 4-CP 2,4-DCP MB BB1 AB74
0.5 3.98 8.22 7.45 4.67 6.60 4.68 1 8.10 10.40 8.41 6.53 4.33 7.87 2 12.30 12.50 9.66 7.44 4.99 4.90 3 12.80 16.00 10.80 11.00 7.24 4.52 4 27.30 17.40 13.10 9.88 7.44 5.48 6 33.50 19.60 15.30 14.40 10.60 6.53 Table 6
Compare of literature and this work values for effective particle phase diffusivity for the sorption of phenols and dyes
Adsorbent Solute Ds(m2/s)× 1012 Reference
Activated carbon Phenol 0.124 [36]
Activated carbon Chlorophenol 0.629 [36]
Activated carbon Nitrophenol 0.378 [36]
Activated carbon Pentachlorophenol 0.226 [36]
Activated carbon Acid dye, Tectilon red 2B 0.170 [37]
Coal; Cecarbon GAC40 Phenol 1.41 [35]
Coconut shell; 208c Phenol 2.76 [35]
Wood; Pica 103 Phenol 3.80 [35]
Straw type 5; 800/200/12 Phenol 2.21 [35]
Tyre type 6; 900/100/18 Phenol 2.63 [35]
Mixed adsorbent (fly ash and coal) Chrome dye 2.88 [38]
Corncob-activated carbon
Group I (KOH/char= 1) Phenol 8.10 This work
Group II (KOH/char= 4) Phenol 27.30 This work
Group I (KOH/char= 1) Methylene blue 6.53 This work
Group II (KOH/char= 4) Methylene blue 9.88 This work
tion of dyes is better fitted by the pseudo-first-order equation
[8]. This work shows that adsorption kinetics of group I activated carbons is similar to that of the steam-activated carbons. However, group II activated carbons are apt to the pseudo-second-order model equation, which show steep, rapid adsorption rates; they show the characteristics of KOH-activated carbons. In addition, group I KOH-activated carbons have both physical and chemical activation characteristics.
The adsorption kinetics of organics should be functions of the size/structure of organics and the pore structure of acti-vated carbons. From the results shown inFig. 2andTable 1, most pores developed in group I activated carbons should be microporous and their diameters are believed to be sig-nificantly smaller than 2 nm. The larger mean diameter of the group I carbons in comparison with that of group II is attributed to the mesopores (about 4 nm) within the former carbons (seeFig. 2). Accordingly, the size of most pores de-veloped within the activated carbons of group II is very close to 2 nm and uniform, which favors the diffusion of organics in both small (i.e., phenols) and large (i.e., dyes) sizes. Note that for both group I and II carbons, the adsorption kinet-ics for phenols is suitably described by the pseudo-second-order equation (see Fig. 5). This equation is based on the equilibrium chemical adsorption, which predicts the behav-ior over the whole range of studies, strongly supporting the validity, and agrees with chemisorption (chemical reaction) being rate-controlling[33,34]. The reaction mechanism may
mainly result from hydrogen binding between the hydroxyl groups of phenols and the functional groups, such as car-boxylic, on the activated carbon surface. On the other hand, such heterogeneous reactions may be retarded within more hindered pore environments due to structural effects. Hence, the adsorption of dyes within the group I activated carbons, obeying the pseudo-first-order equation, should be ascribed by this phenomenon due to pore structure, since most micro-pores within group I carbons are significantly less than 2 nm. In order to compare the rate of uptake of dyes and phenols from water, an effective diffusion coefficient for the adsorp-tion of adsorbates into activated carbons by the Fick’s second law was proposed and calculated in this work. The fraction approaching to equilibrium, F (t ), depends only on the di-mensionless time parameter, Dt /r02[35]:
(7)
F (t )=1− exp−Dst π2/r02
0.5 .
The half-time for adsorption is given by substituting
F (t )= 0.5; i.e.,
(8)
t50= 0.030r02/Ds,
where r0 is the particle size radius assuming spherical
geometry and Ds is the effective particle phase diffusivity.
The calculated values of Ds for phenol and dye
adsorp-tion on each of the activated carbon samples (assuming
r0= 0.149 mm; T = 30◦C) are given inTable 5. For the
in the range between 3.98× 10−12and 33.5× 10−12m2/s.
In addition, Dsfor the adsorption of dyes for all samples lies
in the range from 4.67× 10−12to 14.4× 10−12m2/s.
Table 6 shows that Ds values of phenols range from
1.24× 10−13to 3.8× 10−12m2/s, which are only about
1/10 of those obtained in this study. In addition, Ds values
of chrome dye and acid dye were found to be 2.82× 10−12 and 1.7× 10−13m2/s, respectively, which are also lower
than those of dye adsorbed by the carbons prepared in this study. All the above adsorption results reveal that the car-bons chemically activated by KOH from chars carbonized from corncobs show a unique pore structure (the uniform size of most pores developed within the activated carbons of group II is very close to 2 nm) that favors the transportation of organics in both small (i.e., phenols) and large (i.e., dyes) sizes to the adsorption sites within the solid interior. There-fore, activated carbons with high Ds values are successfully
developed in this work. These activated carbons should be worthy of being applied to supercapacitors since the capac-itive performance of carbons was shown to be describable from the adsorption behavior of organics in our previous work[39].
4. Conclusions
From the SEM observation, corncob-derived activated carbons of group I with the KOH/char ratio from 0.5 to 2 show a cottony, twisted, and deformed surface of honey-comb holes with contracted walls. For group II activated carbons with their KOH/char ratio between 3 and 6, regu-lar honeycomb holes with thick and smooth walls are clearly visible. The BET surface area for the group I activated car-bons ranges from 841 to 1221 m2/g; meanwhile it is
be-tween 1976 and 2595 m2/g, for group II. Based on the pore
characteristics (Smicro/Sp, Vmicro/Vpore, Dp, ρb, and yield),
surface pyrolysis and micropore developing by KOH acti-vation should undergo simultaneously during the actiacti-vation process of group I carbons while only the KOH interior etch-ing process occurs for group II. For the adsorption kinetics, data from the adsorption of MB, BB1, and AB74 onto the group I activated carbons are more suitably fitted by the pseudo-first-order equation. When phenols (phenols, 4-CP, and 2,4-DCP) are used as the adsorbates, pseudo-second-order equations more adequately describe the adsorption be-havior. The adsorption rate of dyes and phenols onto the group II carbons is fast and the pseudo-second-order equa-tion is a suitable model. Effective particle diffusivity, calcu-lated from the t50method, shows that the carbons chemically
activated by KOH from chars carbonized from corncobs have high Ds, which is 10 times the Dsof ordinary activated
carbons. The group II activated carbons with a uniform size of most micropores (very close to 2 nm) not only have high specific surface areas but also exhibit unique high Dsvalues
for all adsorbates.
Acknowledgment
Financial support of this work by the National Science Council of the Republic of China under Contract NSC 92-2211-E-239-006 is gratefully acknowledged.
Appendix A. Nomenclature
Ct solute concentration in the aqueous phase at time t
(mol/m3)
C0 initial solute concentration in the aqueous phase
(mol/m3)
Dp mean pore diameter (nm)
Ds effective particle diffusivity (m2/s) F (t ) the fractional approach to equilibrium (–)
k1 rate constant of pseudo-first-order equation defined
in Eq.(2)(1/min)
k2 rate constant of pseudo-second-order equation
de-fined in Eq.(4)(kg/(mol min))
qe amount of adsorption at equilibrium (mol/kg) qt amount of adsorption at time t (mol/kg)
qt normalized standard deviation defined in Eq. (6)
(%)
r0 particle size radius (m) Smicro micropore surface area (m2/g) Sp total BET surface area (m2/g)
t adsorption time (min)
V volume of the solution (m3)
Vmicro micropore volume (cm3/g) Vpore total pore volume (cm3/g)
W amount of dry adsorbents used (kg)
ρb bulk density (kg/m3)
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