Adsorption characteristics of activated carbons obtained from corncobs

Adsorption characteristics of activated carbons obtained from corncobs

Colloids and Surfaces A: Physicochemical and Engineering Aspects 180 (2001) 209– 221 Adsorption characteristics of ac...

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Colloids and Surfaces A: Physicochemical and Engineering Aspects 180 (2001) 209– 221

Adsorption characteristics of activated carbons obtained from corncobs Abdel-Nasser A. El-Hendawy a,*, S.E. Samra b, B.S. Girgis a a

Physical Chemistry Department, National Research Centre, 12622 Dokki, Cairo, Egypt b Chemistry Department, Faculty of Science, Mansoura Uni6ersity, Mansoura, Egypt Received 3 February 1999; accepted 13 June 2000

Abstract Dried, crushed, corncobs were carbonized at 500°C and steam activated (in one- or two-step schemes), or activated with H3PO4. The products were characterized by N2 adsorption at 77 K, using the BET, as and DR methods. Adsorption capacity was demonstrated by the iodine and phenol numbers, and the isotherms of methylene blue and Pb2 + ions, from aqueous solutions. A distribution of porosity in the carbons was estimated within the various ranges (ultra-, super-, meso- and macropores). Simple carbonization yields a poor adsorbing carbon; only its uptake for iodine was high and proposed to be due to an addition reaction on residual unsaturation of the parent lignocellulosic structures. Enhanced porosity was best associated with chemical activation and/or steam pyrolysis at 700°C. These activated carbons proved highly porous and rich in mesopores, and showed high adsorption capacity for methylene blue and Pb2 + ions. Phenol uptake was found to depend on surface chemical nature of the carbon rather than its porous properties. Corncobs were postulated to be feasible as feedstock to produce good adsorbing carbons, under the one-step activation schemes outlined here. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Activated carbon; Corncobs; H3PO4; Steam activation; Pb2 + removal

1. Introduction At the present time, activated carbons occupy a prominent position among current adsorbents as versatile and universal materials, due to their distinguished properties. Activated carbons are of

* Corresponding author. E-mail address: [email protected] (A.A. El-Hendawy).

interest in many economic sectors and concern many industries as diverse as food processing, pharmaceuticals, chemical, petroleum, mining, nuclear, automobile and vacuum manufacturing. Some of these applications are very demanding with regard to the surface chemistry and the surface characteristics of these adsorbent carbons [1]. They derive their adsorptive properties from the extensive internal pore structure which presents a high surface area available for the adsorp-

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tion of molecular species. Such high porosity is a function of both the precursor as well as the scheme of activation. Precursors to activated carbons are either of botanical origin (e.g. wood, coconut shells and nut shells) or of degraded and coalified plant matter (e.g. peat, lignite and all ranks of coal). Agricultural wastes are considered as very important feedstocks in virtue of two facts: they are renewable sources and low cost materials. In recent years, condensed research on activated carbons from agricultural residues has been reported, e.g. apricot stones [2 – 10], peach stones [4,11,12], cherry stones [5,8,13], date pits [14,15], wheat straw [16], pecan shells, hulls of soybean and rice [17,18], walnut shells [19] and many others (grape seeds, plum stones, almond shells). Very little has been mentioned about corncobs [20,21] although this is a cheap and abundant agricultural waste of no economical value. Schemes for the production of activated carbons are either thermal procedures (physical), or chemical routes. The former involves primary carbonization of the raw material (below 700°C) followed by controlled gasification at higher temperatures (\ 850°C) in a stream of an oxidizing gas (steam, CO2, air, or a mixture). The chemical route is by impregnation of the precursor with a chemical (H2SO4, H3PO4, ZnCl2, alkali metal hydroxides) then heat-treatment at moderate temperatures (400 – 600°C) in a one-step process [22]. An additional one-step treatment route was developed and reported [7,8] where the raw agriculture residue is heated at moderate temperatures (500 –700°C) under a flow of pure steam (denoted as steam-pyrolysis). The quantification of the porosity of activated carbons is often carried out by physical adsorption of gases, N2 at 77 K being the most common adsorbate. Adsorption in micropores does not take place by successive build-up of molecular layers as admitted by the BET theory. Rather, the enhanced interaction potential in micropores induces an adsorption process described as micropore filling. Nevertheless, the BET method has been, and will continue to be, used for microporous adsorbents owing to its simplicity and reasonable predictions [23]. More infor-

mation can be obtained from different procedures, besides the BET method. The asmethod is particularly useful, provided a standard isotherm obtained on a suitable non-porous material is available. This method is referred to as an empirical procedure for isotherm analysis, where as (reduced adsorption) is defined as (n a/ n as)ref where n as is the amount adsorbed by the reference solid at a fixed relative pressure P/ P o = 0.4. It was strongly recommended that the standard isotherm should be one obtained for the particular adsorption system, and not by choosing a type II isotherm which happens to have the same C value as the isotherm on a particular microporous solid [24]. A different approach for isotherm analysis is based on the volume filling of micropores, described by Dubinin and his co-workers. This occurs at lower relative pressures than monolayer and multilayer adsorption because of the dispersion interaction resulting from the overlap of potential energy wells which result as a consequence of the close proximity of the pore walls. The adsorption of a nonpolar vapor in micropores gives rise to type I isotherm which can be interpreted by using the Dubinin –Radushkevich (DR) equation [25] V=Vo exp[− (A/iEo)]2


By convention, V is the volume of adsorbate within the pore structure at relative pressure P/P o; Eo is the characteristic energy of adsorption; Vo is the micropore volume of the adsorbent, the adsorption potential A= RT ln P o/P; and i is the affinity coefficient. The value of i, the shifting factor, is usually taken as equal to 0.35 for N2 at 77 K [25]. In the present study, corncobs were subjected either to simple carbonization at 500°C, followed by steam activation at 850°C, steam-pyrolyzed at 600 –700°C, or activated by phosphoric acid. The products were characterized by N2/77 K, so as to determine their texture characteristics. Adsorption from solution was investigated to assess their capacity to remove different water contaminants: iodine and methylene blue corresponding to small- and medium-sized molecules, phenol to represent this class of pollutants, and Pb2 + ions as an example of metallic pollutants.

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2. Materials and methods

2.1. Acti6ated carbons and char Dried whole corncobs were crushed to attain the size of 0.5 – 2 mm then carbonized in a stainless steel tube, open at both sides, with the help of a tube furnace. Slow heating was followed, about 5 K min − 1, up to 500°C, then the pyrolyzed mass was soaked for 2 h. A portion of this char was steam-activated at 850°C, in a flow of steam/N2, for 1 h. Two carbons were obtained by the steampyrolysis scheme, where the raw material was pre-heated to 350°C, then pure steam (generated in a side setup) admitted and heating continued up to 600 or 700°C and held for 2 h. A fifth carbon was prepared by H3PO4-activation, in light of the previous experience with other agricultural wastes (apricot stones, date pits, rice husks, sugar cane bagasse and cotton stalks) [10,15,18,26,27]. Thus the dried material was mixed with a 50% solution of H3PO4 (at a ratio of 1:1 by weight), thoroughly mixed, left overnight at 80°C, then next day introduced into the same S.S. tube and heated slowly up to 500°C, in the absence of any gas stream, and held for 2 h. The cooled product was subjected to thorough washing with hot water until pH of washings attained ]6.5, and finally dried at 110°C. A description of these carbons, as well as their yield and ash content is given in Table 1. The pH value was estimated for a suspension of 1 g of the finely powdered carbon in distilled water after boiling for 5 min and cooling.


2.2. Procedures of characterization 2.2.1. Gas adsorption Adsorption of N2 gas at 77 K was determined with the help of a vacuum apparatus of conventional type. Analysis of the isotherms was carried out by applying various methods to obtain many porous parameters. (1) By applying the BETequation to get SBET, from volume adsorbed at P/P o = 0.95 to evaluate the total pore volume (Vp) and from both values to calculate the mean pore radius r¯ =2Vp/SBET. (2) Applying the asmethod, making use of the standard values provided by Selles-Perez and Martin-Martinez for non-porous carbon [28]. Three essential parameters can be evaluated: the total surface area (S a), from the slope of the initial linear section extrapolating to zero, the non-microporous surface area (S an) from the slope of the high pressure straight line, and the micropore-volume (V ao) from the intercept of the latter line. (3) Applying the D-R method by plotting ln na (mmol g − 1) versus


RT ln(P o/P) i


to get the micropore volume (V DR o ) from the intercept and the characteristic energy of adsorption (Eo) from the slope of the rectilinear relationship. The mean pore width (L) was obtained from the relationship [25] L=

17.25 Eo


An estimate for the apparent surface area (S KDR) was calculated from the value of V DR o

Table 1 Some characteristics of char and activated carbons Notation

Conditions of preparation

% Yield (global)

Ash (%)


C500 CS600 CS700 CS850 CP-55

Carbonization at 500°C Steam pyrolysis (600°C) Steam pyrolysis (700°C) Steam activation (850°C) 50% H3PO4 (500°C)

21.7 20.8 20.1 8.7 18.3

6.0 5.5 7.7 8.2 9.5

7.3 8.8 9.2 8.8 3.8


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timated by titration against standard Na2S2O3. The adsorption capacity of Pb2 + , from an unbuffered solution of Pb(NO3)2, was carried out by mixing 50 mg of the powdered carbon with 100 ml of Pb2 + solution of varying concentrations and shaking for 24 h. The residual lead was estimated in the filtered solution using a Perkin-Elmer model 2380 atomic absorption spectrometer. Analysis of the adsorption isotherms of MB and Pb2 + was carried out by applying the linear Langmuir equation Ce 1 1 = + ·Ce Xe XmKL Xm


and the Freundlich equation 1 log Xe = log KF + log Ce n

Fig. 1. Adsorption isotherms of N2/77K for the investigated carbons (for notation see Table 1).

(considering it to represent Vm, as postulated by Kaganer [29]) and the micropore surface area (S DR mic ) was determined by S DR mic =

2× 103 ×V DR o L (nm)


2.2.2. Adsorption from solution Adsorption isotherms of the standard dye, methylene blue, were determined for the five carbons. In each experiment, 50 mg of the powdered dried carbon was added to 50 ml of MB solution of varying concentrations and the mixture shaken for 24 h. The residual concentration was determined by measuring its absorbance at 670 nm using a Shimadzu VIS-UV spectrophotometer. The uptake of phenol from aqueous solution was determined with the same spectrophotometer at u = 270.5 nm. Iodine number was estimated by mixing 200 mg of the finely powdered sample with 0.02 N iodine solution, shaken occasionally, left overnight, and then es-


where Ce and Xe are the amounts of substrate in solution and on adsorbent; XL, KF and n are equation constants. The monolayer capacity, Xm, was estimated for both solutes from the respective slopes of the Langmuir isotherms.

3. Results and discussion

3.1. General characteristics of obtained carbons From Table 1, it appears that the considered carbons differ in many respects. Their yield consequent to carbonization and/or activation amounts to 209 2%. Only the two-step conventional steam activation results in a much lower yield of 8.7%. Steam-activated carbons exhibit a basic surface effect (pH= 8.8 –9.2), whereas the H3PO4-activated carbon shows surface acidity. The 500°C-char is neutral with respect to its water extracted solution. Generally, low ash carbons are obtained (5.5 –9.5%) that increase with either burn-off or acid activation. In case of phosphoric acid, it is probable that entrapped dehydrated phosphates lead to such high ash content, and gives its acidic character (pH= 3.8). This diversity might be illustrated in determining their adsorption capacity especially from solution as will be outlined later.

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3.2. Adsorption isotherms of N2 at 77 K Fig. 1 presents these isotherms with different features which could be correlated with porosity development. Analysis of the N2-isotherms wascarried out by applying the BET-equation, the as-method and the Dubinin – Radushkevichmethod (Figs. 2 and 3). Although the BET equation is the most widely used for evaluating the surface area, there is a degree of uncertainty in determining the range of relative pressure over which the equation can be applied. This problem is further complicated in the case of microporous carbons where the range of applicability is narrower [30]. As recommended by these authors [30], application of the BET equation was limited to the low pressure range (P/P o =0.01 – 0.2). The range of linear plots was found to be satisfactory within 0.01B P/P o B0.12 – 0.20 for the five carbons. Table 2 summarizes the evaluated BET-surface areas, total pore volumes and the average pore radii.

Fig. 3. D-R plots for N2/77K-adsorption isotherms of the tested carbons (na, amount adsorbed in mmol g − 1).

Fig. 2. as-Plots for N2/77K-adsorption isotherms of the investigated carbons.

3.2.1. Porous parameters from the hs plots In the original as method, three hypothetical plots were mentioned, a single linear relationship followed by either an upward or downward deviation at higher pressures. Yet, a five type classification of as-plots was distinguished by Selles-Perez and Martin-Martinez [31] for adsorption isotherms of N2 on activated carbons. Their classification implied normal extrapolation of the linear relationship to the origin. A special shape of as-plot has been reported and analyzed by Gonzalves da Silva et al. [23]; this is the case of positive deviation from the origin where the as-plot extrapolates above zero. This positive intercept with the adsorption axis was considered to indicate the presence of ultramicropores (width between 0.3 and 0.72 nm) [23,32]. The capacity of ultramicropores has been estimated by back extrapolation of the first linear section to P/P o = 0. From

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the slope of this line, monolayer equivalent capacity of pores greater than 0.72 nm (supermicropores, meso- and macropores) are calculated. The estimated total surface in this case is the sum of those corresponding to ultramicropores and others (Table 2). However, the area within non-micropores (S ao) and volume of total microporosity (V ao) were estimated from the slope and intercept of the second linear section as usual for the as-plots. From Fig. 2, it is clear that two sets of as-plots appear: one group exhibiting positive deviation (CS600, CS700, CS850) and the second showing extrapolation to the origin (C500 and CP-55). The char (C500) exhibits a type 1-d [31] with extrapolation from h :1, and in the mean time indicates the presence of capillary condensation. The H3PO4-activated (CP-55) shows a linear straight portion with significant slope starting from an as value near 0.8 (type 1b), indicating developed mesoporosity.

3.2.2. Texture parameters from the D-R plots The D-R equation has been shown to linearize type I adsorption isotherm data over very wide

ranges of relative pressures, however deviations from linearity are often observed. Five types of ln na versus (A/i)2 relationships were collected by Byrne and Marsh [33], which appear as either: linear throughout the whole pressure range, negative or positive deviation at high pressures, deviation at low pressures, or total deviation from linearity. For the present carbons, the D-R plots (Fig. 3) appear either as linear throughout with slight deviations at high pressures (C500, CS600, CS700, CS850) or with distinct upward deviation only for carbon CP-55. The behavior of the first case indicates that pore filling progresses from smallest to largest micropores, until all porosity is filled around P/P o = 1.0. As for the H3PO4-activated carbon (CP-55), it exhibits appreciable upward deviation resulting from adsorption in pores wider than micropores, and due to filling of supermicrob, or mesopores, or multilayer adsorption inside wide pores [33]. Many texture characteristics are evaluated by considering the intercept and slope of the respective D-R plots, and with the help of Eqs. (2) and (3). These are listed in Table 3.

Table 2 Texture characteristics of the different carbons Sample

SBET (m2 g−1)

Vp (ml g−1)

r¯ (nm)

S a (m2 g−1)

S an (m2 g−1)

V ao (ml g−1)

Vmeso (ml g−1)b

C500 CS600 CS700 CS850 CP-55

39 618 786 607 960

0.075 0.321 0.430 0.296 0.629

4.88 1.04 1.09 0.97 1.31

40 (664)a (850)a (700)a 923

– 65 108 51 368

0.009 0.215 0.252 0.218 0.249

0.064 0.081 0.121 0.061 0.324

a b

S a obtained by a sum of Su+Sother. Vmeso =V0.95−V0.1.

Table 3 Characteristic parameters from the D-R equation Sample

V DR (ml g−1) o

Eo (kJ mol−1)

L (nm)

C500 CS600 CS700 CS850 CP-55

0.018 0.257 0.385 0.248 0.387

10.7 15.6 13.6 (30.0) 12.6

1.61 1.11 1.27 (0.57) 1.36

2 −1 S DR ) mic (m g

22 463 606 (862) 569

S KDR (m2 g−1)

Vo(s) (ml g−1)

50 749 1080 695 1084

0.009 0.042 0.133 0.030 0.138

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Fig. 4. Distribution of pore volumes for the obtained activated carbons within different pore sizes (w = width).

3.2.3. Comparison between the texture parameters and description of porosity Both the as- and D-R plots demonstrate clearly the distinctive character of the H3PO4-activated carbon. Steam-activated carbons (CS600, CS700 and CS850), as well as the char, exhibit porosity mostly limited to micropores, whereas H3PO4 enhances porosity within the different pore sizes. This is also demonstrated from the values of surface areas, and pore volumes (Tables 2 and 3) estimated within different limits. The evaluated BET-surface areas are mostly lower than the corresponding values estimated by other methods, although they are much nearer to the as-ones. On the other hand, the surface areas evaluated from the D-R plots are much higher, and seem to be unreasonable. Upon comparing their Vm values, these appear at high relative pressures on the adsorption isotherms, for the Vm(DR) at P/P o =0.15 – 0.65. These values are definitely unreasonable and far from those evaluated by either the BET or as-methods (P/P o 5 0.10). For activated carbons, most recent investigators employ the ‘pore volume’ concept as a more reliable parameter for their comparison and char-


acterization. Inside narrow micropores of dimensions 3–7.2 A, , the surface area loses its meaning and the pore volume criterion becomes more adequate for describing such adsorbents. In Table 2, the pore volumes contained in two size ranges (micro- and mesopores) as well as the total pore volumes, are given. Corncob char (500°C) shows low porosity, that is enhanced by steam activation at 850°C. The one-step activation, by steam or H3PO4, generates more structurally developed activated carbons. Fig. 4 displays diagrammatically the pore volume evolution and distribution, where the char is essentially mesoporous and activation generates porosity mostly in the microporous range. This accounts for the tremendous increase in the ‘apparent surface area’ (up to 25-fold) whereas the accompanying increase in total porosity amounts to ca. 12-fold. The H3PO4-activated carbon exhibits the highest total porosity due to its high content of mesoporosity (Table 2) as appears from its nonmicroporous area (S oa ) and mesopore volume (Vmeso). It should be also mentioned that the micropore volumes estimated by the DR-method are always higher than the corresponding values estimated from the as-plots. This has been postulated as indicating wide and very heterogeneous microporosity, and that the DR-method evaluates the total porosity (ultra- and supermicropores) whereas the as-values present only the narrow (ultra) microporosity [34,35]. This is confirmed upon noticing the P/P o values at which these micropore values appear on the adsorption isotherms (for as-method at P/P o = 0.01 –0.03 whereas from the D-R plots at P/P o = 0.15 – 0.65). Consequently, estimated values for the volume within supermicropores (] 7.2 –20.0 A, ) were a estimated from Vo(s) = V DR o − V o and are given in Table 3. This describes more clearly the process of activation, as the char, low temperature singlestep steam pyrolysis (CS600) and steam-activated char (at 850°C) produce essentially ultramicropores. Raising the steam pyrolysis temperature to 700°C, or chemical activation with H3PO4 leads to considerable generation in wide micropores (the super-micropores) in addition to mesopores, i.e. widening of the original narrow micropores.


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3.3. Adsorption from solution One of the important features of activated carbons used in wastewater treatment is their capacity to adsorb various pollutants. Iodine and methylene blue measurements are generally applied as fast industrial test methods for assessing the quality of the adsorbents. Iodine number informs about the internal surface of the activated carbon; 1 mg of iodine adsorbed is considered to represent 1 m2 internal surface. Adsorption of methylene blue from aqueous solution is used to characterize both the mesopore capacity (pore diameters greater than 13 A, ), and also serves as a model compound for adsorption of medium size organic compounds from aqueous solutions [6,8,13]. In addition, the capacity to adsorb phenol is very important since it is one of the most frequent contaminants in industrial wastewater. Besides being carcinogenic, a very uncomfortable feature of phenol is that it reacts with the chlorine producing carcinogenic chlorinated compounds [36]. The iodine, phenol and methylene blue numbers are usually reported in the units of mg g − 1 or mmol g − 1, however, they are additionally expressed here in the form mmol m − 2. This aims to normalize the data with respect to surface area, which meanwhile demonstrates the possible surface density of the substrate molecules per unit surface area.

3.3.1. Iodine and phenol numbers The data in Table 4 indicate that the tested activated carbons adsorb significant amounts of Table 4 Capacity of the prepared carbons in the adsorption of iodine and phenol Notation

Iodine adsorption

Phenol adsorption

mmol g−1 mmol m−2 mmol g−1 mmol m−2 C500 CS600 CS700 CS850 CP-55

1.079 1.921 2.598 2.559 2.209

26.90 2.89 3.05 3.66 2.38

– 1.76 1.89 1.74 1.01

– 2.65 2.22 2.48 1.09

iodine which are strongly related to the degree of activation. As mentioned by Warhurst et al. [37], the iodine number suggested as a minimum by the AWWA [38] for a carbon to be used in removal of low molecular weight compounds is 500 mg g − 1 (corresponding to 1.97 mmol g − 1). Steam-activated carbon prepared at 600°C, with relatively less-developed porosity, adsorbs lower amounts of iodine which is enhanced by activation at higher temperature. Phosphoric acidactivated carbon, CP-55, adsorbs lower amounts of iodine, as mmol g − 1 or mmol m − 2. This cannot be attributed to the presence of inaccessible ultramicropores (57.2 A, ), but is probably due to the highly acidic surface contaminated with abundant phosphate which was reported to reach up to 2.8% of the carbon product [39]. Upon converting the iodine numbers into corresponding surface areas (1 mg$ 1 m2), we get the following SI/SBET ratios: 0.79, 0.84, 0.93 and 0.58 for CS600, CS700, CS850 and CP-55, respectively. These values also confirm the apparently low affinity of CP-55 for the adsorption of iodine, which is believed to be due to its distinctly different chemical surface structure. Corncobs char C500, presents an anomalous adsorbent for iodine, which cannot be related to its poorly-developed porosity. Its surface ratio of SI/SBET = 7.0 and the high surface density of iodine molecules, 26.9 mmol m − 2 demonstrate an abnormal phenomenon which had been observed for many chars of other precursors [40 –43]. Excess uptake of iodine is believed to be probably due to an addition reaction on residual unsaturation in the incompletely charred lignocellulosic material. Iodine consumption in this case would become a result of both chemical as well as physical processes. From Table 4, it also appears that the four activated carbons adsorb appreciable amounts of phenol (1.01 –1.89 mmol g − 1), which are comparable to various reports on activated carbons [37,44 –46]. In these previous reports, all carbons were mostly steam or CO2 activated and adsorption of phenol was postulated to be predominantly controlled by the porosity of the carbon. Juang et al. [47] pointed to the possible formation of hydrogen bonds between the electron-acceptor

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Fig. 5. Adsorption isotherms of methylene blue and predicted isotherms (Lang., Langmuir; Fr, Freundlich models, and exp., experimental).

groups on the carbon surface (e.g. carbonyl and carboxyl) and the hydrogen atoms on phenolic substrates. This might explain the relatively lower uptake of phenol by the H3PO4-activated carbon (CP-55) due to its highly acidic surface (as well as the presence of phosphates) that reduce the adsorption of the very weakly acidic phenol (ca. 50% lower per unit weight or area). Such an effect was previously observed in the uptake of phenol by H3PO4-activated carbons obtained from apricot stone shells [48]. The surface-chemical nature here, as well as in the case of iodine uptake, will affect the capacity of adsorption of both substrates.

3.3.2. Adsorption characteristics of methylene blue (MB) Corncobs char (C500) exhibits no detected affinity for the uptake of MB, as in the case with phenol observed above. The adsorption isotherms of MB, shown in Fig. 5, exhibit ‘H’ type indicating high affinity towards the solute with a steep

rise at low concentrations. Such high affinity appears evident from the values of the high Freundlich constants, KF and n. The Langmuir model equation seems more suitable to fit the experimental data as indicated by the relatively higher linear correlation coefficients except carbon CS700 (Table 5). Using the evaluated model parameters of the Langmuir and Freundlich equations, predicted isotherms were calculated, where the Langmuir model shows relatively better fit to the experimental values (Fig. 5). The prepared activated carbons exhibit good capacity to remove the bulky dye molecule, and depend essentially on the degree of activation and porosity characteristics. The lowest uptake, shown by CS600 is attributed to the poor development of porosity, at such low temperature, with high content of narrow microporosity (accessible area: SMB/SN2  0.34). H3PO4-activated carbon shows the highest capacity for MB, as well as the highest density per unit area (Table 5). This means that H3PO4 devel-


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ops an activated carbon with a well-developed porosity within pore sizes ] 13 A, , available to the uptake of the medium size dye molecules. Adsorption of methylene blue by the activated carbons is thus demonstrated to be purely physical and mostly independent of the surface-chemical nature.

3.3.3. Uptake of Pb 2 + ions from aqueous non-buffered solution In the removal of heavy metals by adsorption, solution pH has been established to be the most important factor. Most metals become less soluble and form hydroxides and oxides as the solution pH is increased [49]. Dimitrova and Mehandgiev [50] have restated that sorption of heavy metals is most effective in alkaline solution, while at pH below 5.0 sorption is negligible due to the competitive effect of hydrogen ions. For lead, the formation of Pb(OH)2(s) was determined and found to begin at above pH 6.25. However, in the presence of activated carbon its removal at pH] 6.5 was not solely due to surface precipitation but to an additional attraction between the carbon surface and aqueous lead/Pb(OH)2(s) [51]. The present experiments aimed to demonstrate the influence of carbon porosity as well as surface pH on the uptake of Pb2 + ions from a standard Pb(NO3)2 solution of pH= 5.5. Fig. 6 shows that adsorption of Pb2 + ions exhibits typical L-type (Langmuir) isotherm, with satisfactory linearization in both the Langmuir and Freundlich models

(Table 6). Only CS600 indicates a very poor fit to either models, probably due to diffusion problems leading to inadequate equilibrium. Lead uptake is less favorable, in comparison to MB, as evident from its much lower equation parameters in both models. Taking into account the high hydration number of Pb2 + ions in aqueous solution, between 4 and 7.5 [52], the surface areas covered by Pb2 + were calculated and are given in Table 6. It appears that varying amounts of lead ions are sequestered by the four activated carbons (0.59 –0.75 mmol g − 1 or 0.70 –0.88 mmol m − 2). However, this variation diminishes upon considering the ratio of surface covered by Pb2 + ions which becomes 0.2590.2, i.e. only a quarter of the active carbon surface is available to lead ions. This means that lead ions are bound to specific sites present to the same extent per unit area on the carbon surface irrespective of the scheme of preparation. Meanwhile, the solution pH here seems to be the decisive factor in determining the uptake of Pb2 + ions; an insignificant role is played by the widely varying carbon surface or slurry pH (3.8 up to 9.2). Corncobs char (C500) exhibits an anomalous affinity to Pb2 + ions shown by a high surface site density of 6.9 mmol m − 2 as well as an estimated area twice that determined by N2 adsorption. Such behavior might be related to the chemical nature of the char; it probably possesses a more hydrophilic character that enhances diffusion and uptake of lead ions free from most of their water of hydration.

Table 5 Adsorption characteristics of methylene blue from aqueous solution Model



r2 KL Xm (mmol g−1) X sm (mmol m−2) SMB (m2 g−1) SMB/SaN


r2 KF n

Adsorbent CS600




0.957 0.108 0.314 0.473 227 0.342

0.797 0.091 0.917 1.079 662 0.779

0.918 0.092 0.722 1.031 521 0.744

0.997 0.044 1.215 1.316 877 0.950

0.934 40 6.8

0.811 75 3.2

0.822 69 4.7

0.913 72 3.3

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Fig. 6. Adsorption isotherms of Pb2 + from aqueous solution and predicted isotherms. Table 6 Adsorption characteristics of Pb2+ ions from aqueous non-buffered solution Model



r2 KL Xm (mmol g−1) X sm (mmol m−2) SPb2+ (m2 g−1) SPb2+/SaN


r2 KF n

Absorbent C500




0.935 0.008 0.275 6.875 83 2.07

0.617 0.012 0.589 0.887 178 0.27

0.982 0.059 0.632 0.743 197 0.23

0.995 0.039 0.493 0.704 148 0.21

0.962 0.082 0.745 0.811 226 0.24

0.977 3.5 2.3

0.908 6.1 1.9

0.967 34.0 4.2

0.992 35.5 5.4

0.993 4.4 1.8

Adsorption of Pb2 + ions had been postulated to be the essential process at solution pH5 5.5, and therefore it should depend on the carbon porosity. This might explain the highest amount of Pb2 + uptake (0.75 mmol g − 1) by the H3PO4activated carbon (CP-55) in spite of its distinctly acidic surface (pH= 3.8).


4. Conclusions Three grades of adsorbing carbons were obtained from an agricultural byproduct, viz corncobs. A char by carbonization at 500°C which yields a poorly developed wide-pored carbon with high capacity for iodine and Pb2 + ions. This is


A.A. El-Hendawy et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 180 (2001) 209–221

ascribed to the presence of an incompletely carbonized residue with unsaturated bonds and oxygen complexes. Steam-activated carbons (in oneor two-step schemes) develop porosity with temperature, and are essentially microporous. These exhibit good adsorbing affinity from solution (iodine, phenol and methylene blue) that depends on their porosity characteristics. Thermal activation under the single-step steam pyrolysis route, at 700°C, provides better adsorbing carbon with much developed porosity and enhanced carbon product. Chemical activation by H3PO4 at 500°C proved very effective in producing high quality activated carbon with well-developed porosity and high adsorption capacity for both organic and inorganic substrates. Removal of Pb2 + ions from an unbuffered solution took place on specific adsorption sites, with mostly the same surface abundance, covering about one-quarter of the carbon surface. Carbon slurry pH plays no detectable role in determining the uptake efficiency for the Pb2 + ions.


apparent surface area by the method of Kaganer (m2 g−1) DR S mic surface area inside micropores 2 −1 from 2×103 V DR ) o /L (m g Vo(s) volume within supermicropo−1 a res= V DR ) o −Vo (ml g constants in the Freundlich KF and n equation Xm monolayer capacity from the Langmuir plot (mmol g−1) KL constant in Langmuir equation (l g−1) Micropores pores with dimensions up to 2.0 nm Ultramicrop- pores with dimensions up to ores (u) 0.72 nm Supermicrop- pores with dimensions between ores (s) 0.72 and 2.0 nm SMB surface area covered by methylene blue (m2 g−1) SPb2+ surface area covered by hydrated lead ions (m2 g−1) r2 correlation index from linear regression of data

5. Nomenclature References I-No. SBET Vp r¯ Sa S an V ao Vmeso V DR o Eo L

iodine number (mmol g−1) apparent surface area by BET equation (m2 g−1) total adsorption space (ml g−1) average pore radius=2Vp/SBET (nm) apparent surface area from asplots (m2 g−1) non-microporous surface area from as-plots (m2 g−1) micropore volume from as-plots (ml g−1) volume within mesopores, V0.95−V0.1 (ml g−1) micropore volume from D-R plots (ml g−1) characteristic energy of adsorption from D-R plots (kJ mol−1) average pore width from 17.25/ Eo (nm)

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