Activated Carbon Derived from Cork Powder Waste by KOH Activation

Ind. Eng. Chem. Res. 2008, 47, 5841–5846

5841

Activated Carbon Derived from Cork Powder Waste by KOH Activation: Preparation, Characterization, and VOCs Adsorption Beatriz Cardoso, Ana S. Mestre, Ana P. Carvalho,* and Joa˜o Pires Departamento de Quı´mica e Bioquı´mica and CQB, Faculdade de Cieˆncias da UniVersidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal

The potentialities of cork powder waste as raw material for the preparation of activated carbons that can be used in the control of atmospheric pollution was studied. Three series of activated carbons were prepared by chemical activation of cork powder waste using KOH as activating agent. In each series, the ratio between the amount of the cork powder and the activating agent was changed, and the effect of the activating temperature was studied also. It was possible to obtain activated carbons with specific surface areas higher than 1300 m2/g and microporous volumes higher than 0.5 cm3/g. Regarding the surface chemistry properties, a set of carbons with pH at the point of zero charge ranging from 9.0 to 5.6, was obtained. The properties of adsorption of various noxious volatile organic compounds (VOCs) were investigated, and the sample activated with the highest amount of KOH, and at the highest activation temperature, showed promising properties in both the abatement and the selective adsorption of VOCs. Introduction Activated carbons are solids that, due to their porous structure and surface properties, are materials of fundamental importance in modern societies, so excellent reviews were written about their properties.1–4 These materials can be prepared from almost all high carbonaceous content materials, as is the case of coal and agricultural or industrial wastes. The use of wastes which are renewable and low-cost raw materials for activated carbon preparation is growing5–12 since this is an effective way to reduce the production costs. In fact, the commercially available activated carbons are relatively expensive, which, in many cases, can be a drawback to their use. The methodology used for activated carbons preparation involves physical and/or chemical activation. Physical activation is a two-step procedure: carbonization under inert atmosphere, followed by controlled gasification of the char under mildly oxidizing agents such as water or carbon dioxide. Chemical activation consists of the pyrolysis of the precursor in the presence of a chemical agent. The great advantage of this last method is that it demands less energy consumption than physical activation since there is only one calcination step, and, thus, it is made at lower temperature.1,2,4 The main disadvantage of chemical activation is that it implies a washing step to remove the reaction products that are retained in the porosity. Different compounds can be considered for producing activated carbons by chemical activation as, for instance, phosphoric acid,9,13 zinc chloride,7,11,12,14 and alkaline hydroxides.1–4,7,15 These last ones, and in particular KOH, have been reported to provide materials with high values of surface areas and pore volumes.3,4,15 The present work is part of a larger project concerning the use of the most important waste from cork industry, that is, cork powder. The annual worldwide production of cork powder is around 50 000 tons, which corresponds to, on average, 25-30% of the cork processed.16–18 Although some studies point out different potential applications of this byproduct of cork processing, cork powder is still essentially used as an energy source due to its negligible economic value and high-burning capacity.17,18 Searching for high added-value applications of * To whom correspondence should be addressed. Phone: +351 217500897. Fax: +351 217500088. E-mail: [email protected].

cork powder waste, we have assayed various types of activation methodologies to prepare activated carbons using cork powder as raw material and the potentialities of the solids obtained, for specificgas-andliquid-phaseapplications,havebeenevaluated.19–22 The present study deals with the preparation and characterization of activated carbons from cork powder waste by activation with KOH, and the evaluation of their potentialities to be used in the adsorption/abatement of noxious pollutants such as volatile organic compounds (VOCs). In fact the content of VOCs in the air has been the object of regulation as was the case of European Community Directive 2001/81 and the U.S. Clean Air Act Amendments. The use of adsorbents to control the amount of VOCs in the air can be envisaged not only in the elimination of these species by adsorption but also in their use in monitoring and analysis, acting as enrichment material for analytical purposes.23 This last application can open new niches of use for more specific adsorbents that can be produced in a relatively small scale. The materials can be prepared in smallsized activated carbon production plants that can be located near the major waste production units favoring a viable transformation of the waste into a high-added-value product. Experimental Section Preparation of Activated Carbons. Waste of the cork industry, corresponding to a very fine fraction of cork powder with particle dimensions lower than 0.297 mm, was used as the starting material. Before activation, the cork powder was treated with a 10% aqueous solution of H2SO4 (Riedel-de Hae¨n, 99%), washed with water until complete elimination of the acid (neutral solution), and dried overnight at 60 °C. This treatment was made to remove the inorganic impurities of the precursor material, allowing the reduction of the activated carbon ash content. The activation conditions with KOH (Ekza Nobel, PA) were based on the literature6,24 and on a previous optimization study from our laboratory.19 Three series of samples, labeled A-C, were prepared corresponding to, respectively, an amount of 0.25, 0.5, and 1 g of KOH/(g of pretreated cork waste). An aqueous solution of KOH (25% (w/w)) was mixed with the cork powder, transferred to a ceramic boat, and heated in a tubular oven (Carbolite, model MTF/10/25310) under a nitrogen flow

10.1021/ie800338s CCC: $40.75  2008 American Chemical Society Published on Web 07/22/2008

5842 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008

of 2 cm3/s. Approximately 0.5 g of sample was used in each calcination. The heating program was as follows: from ambient temperature to 300 °C, at a rate of 5 °C/min, being kept for 3 h at 300 °C and then heated to the final calcination temperature, at a rate of 10 °C/min. The final calcination temperatures, which were maintained during 2 h, were 500, 600, 700, and 800 °C. After cooling to room temperature (still under nitrogen flow) the samples were ground to powder in an agate mortar, to make more easy the wash with water until pH ) 7. After washing the samples were dried overnight at 100 °C. The identification of the samples, within the A, B, or C series, will be made by the final calcination temperature. For instance B700 will indicate a sample prepared with 0.5 g of KOH/(g of cork) and submitted to a final calcination temperature of 700 °C. Activated Carbons Textural and Chemical Characterization. The elemental analysis (contents in carbon, hydrogen, and nitrogen) of the carbons was made in a CHNS Analyzer (Thermofinnigan Flash, EA, 1112 series). Prior to analysis, the samples were dried overnight at 105 °C and cooled in a desiccator. Oxygen content was obtained by the difference between the total percentage (100 wt %) and the sum of percentages (wt %) of nitrogen, carbon, hydrogen, and sulfur. The pH of the point of zero charge (pHPZC) was assessed by reverse mass-titration according to the procedure detailed in a previous work.22 The pH measurements were made with a Metrohm 744 pH meter. The nitrogen (Air Liquid, 99.995%) adsorption isotherms at -196 °C were determined in a manual volumetric Pyrex-made apparatus equipped with a pressure transducer from Datametrics (model 600) for pressures until 133 kPa. This apparatus was equipped with a vacuum production system composed of rotary/ diffusion pumps and a liquid nitrogen trap, which allowed a final vacuum better than 1.33 × 10-2 Pa. Before the experiments, the samples were outgassed under vacuum at 300 °C for 2 h. From the low-temperature nitrogen adsorption data, the “apparent” specific surface area was estimated from the BET model, ABET,25,26 in the pressure range of 0.05 < p/p0 < 0.15. Microporous volumes were obtained applying the DubininRadushkevich (DR) equation,26,27 which has the form w ) w0 exp[-(A/βE0)2], where w is the amount adsorbed at a given temperature and relative pressure p/p0, w0 is the limiting adsorption, and A is the adsorption potential (A ) RT ln(p0/p)). E0 is the characteristic adsorption energy and β the affinity coefficient. The DR equation was applied in the linear form, and straight lines were obtained in all cases for relative pressures until 0.1. The mean micropore half-width (L0) was estimated from the Dubinin-Stoeckli relation:28 L0 ) (13.028 - 1.53 × 10-5E0)/E0. VOCs Adsorption. The adsorption isotherms of the VOCs studied, namely, n-hexane (Merck, 99.9%), cyclohexane (Merck, 99.9%), methyl ethyl ketone (MEK; BDH, 99.5%), and 1,1,1trichloroethane (TCA; Aldrich, 99%), were determined in a gravimetric apparatus. The microbalance used for this was from C. I. Electronics and had a precision of 10 µg. The apparatus was equipped with a vacuum system similar to that described above. Pressure readings were made with a pressure transducer from Shaevitz (range, 0-102 kPa), and the isotherms were determined at 25 ( 0.1 °C (water bath from VWR Scientific). The outgassing conditions of the samples were similar to those used in the low-temperature nitrogen adsorption. Results and Discussion Activated Carbons Characterization. The results of the elemental analysis are presented in Table 1. Sulfur is not

Table 1. Elemental Analysis and pH at the Point of Zero Charge (pHPZC) Sample A500 A600 A700 A800 B500 B600 B700 B800 C500 C600 C700 C800

C (wt %)

H (wt %)

N (wt %)

O (wt %)

pHPZC

63.4 63.5 61.5 59.5 69.9 65.8 75.5 65.1 68.6 68.1 72.5 60.5

1.8 1.7 1.6 1.2 1.8 1.5 0.8 1.3 1.8 1.3 1.0 0.8

0.8 0.8 0.7 0.3 0.5 0.5 0.3 0.3 0.6 0.5 0.5 0.3

34.0 34.0 36.2 39.0 27.8 32.2 23.4 33.3 29.0 30.1 26.0 38.4

7.5 7.4 7.6 9.0 7.6 7.1 7.9 7.7 7.3 6.0 5.6 5.9

included in the table since it was not detected in any of the samples. A high oxygen content is observed in all samples in line with what was found in another study focused in corkbased activated carbons.29 The C/O ratios vary from 1.5 to 3.2, which are quite low values and smaller than those found in the mentioned study for carbons prepared by chemical activation of cork also with KOH. The pHPZC values presented in Table 1 show that the followed methodology enables the preparation of a series of carbons with neutral surface charge varying from pH 5.6 to 9.0, that is, activated carbons with acid and basic surface properties. Although the majority of samples presented neutral surface charge in the basic region, the activated carbons prepared with the highest KOH amount and temperatures equal or higher than 600 °C (samples C600, C700, and C800) have their point of zero charge in the acid region. This is not a common feature since acidic carbons are usually obtained by oxidizing treatments

Figure 1. Adsorption isotherms of nitrogen at -196 °C for the samples from series B (closed symbols are desorption points). Table 2. Specific Surface Areas (ABET), Microporous Volumes (w0), and Characteristic Energy (E0) from the DR Equation and L0 Values from the Dubinin-Stoeckli Relationa sample A500 A600 A700 A800 B500 B600 B700 B800 C500 C600 C700 C800 a

ABET (m2/g)

w0 (cm3/g)

E0 (kJ/mol)

L0 (nm)

354 498 731 1044 427 561 1050 1190 507 723 1063 1336

0.15 0.21 0.34 0.43 0.18 0.24 0.45 0.51 0.21 0.30 0.45 0.56

17.6 22.7 20.3 22.5 18.4 25.6 26.1 19.4 24.2 25.6 23.7 19.1

0.72 0.54 0.61 0.54 0.69 0.46 0.45 0.65 0.49 0.44 0.51 0.66

All the values were obtained from the nitrogen adsorption isotherms at -196 °C.

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Figure 2. Adsorption isotherms at 25 °C in the indicated samples of (a) n-hexane and (b) cyclohexane.

after activation.1 It is also interesting to note that for the samples heated at 800 °C the higher the amount of KOH the lower the pHPZC is. This trend is less pronounced for the samples submitted to lower temperatures. The nitrogen adsorption isotherms at -196 °C on the various samples (in Figure 1 the results for series B are displayed as an example) were essentially of type I according to the IUPAC classification,25 revealing the microporous nature (pore width less than 2 nm)25 of the activated carbons prepared. This result is in line with the general observation that the activation with alkaline hydroxides, and with KOH in particular, produces samples that are essentially microporous.4 The abrupt knee of the isotherms, less marked for the samples activated at 800 °C (Figure 1), suggests a narrow distribution of the micropore sizes,30 which is also one of the advantages of using KOH as an activating agent.4 Considering either the ABET values or the microporous volumes (Table 2), for a given amount of KOH, the adsorption capacity increases with the increase in the calcination temperature. Similarly, for a given temperature, the adsorption capacities increase when the amount of KOH increases, that is, from the A to the C series. Therefore, the sample with the highest microporous volume is C800. In the case of the highest temperature values (for the various series), the microporous volumes and ABET values compare favorably with those found in samples obtained from the activation with KOH of other precursors, except in cases where the starting materials are already carbons and not carbonaceous-rich agricultural or industrial wastes.4,5,8,31,32 Taking into account the values of E0, or the related parameter L0, the trends of the results reported in Table 2 are less clear. Nonetheless, in series A and B it can be considered that the L0

Figure 3. Adsorption isotherms at 25 °C in the indicated samples of (a) methyl ethyl ketone (MEK) and (b) 1,1,1-trichloroethane (TCA). Table 3. Limiting Adsorbed Amounts Estimated from the DR Equation and Expressed in Liquid Volume (w0), for the Various VOCs w0 /(cm3/g) sample A500 A600 A700 A800 B500 B600 B700 B800 C500 C600 C700 C800

n-hexane

cyclohexane

MEK

TCA

0.14 0.14 0.35 0.15 0.18 0.22 0.40 0.45 0.21 0.22 0.29 0.58

0.03 0.10 0.35 0.20 0.06 0.08 0.35 0.31 0.09 0.15 0.39 0.42

0.19 0.21 – 0.25 0.10 0.18 – 0.34 0.10 0.30 0.27 0.52

– – – 0.02 – 0.02 – 0.25 0.04 0.11 0.39 0.58

values decrease when the temperature increases, while in the C series (where the highest amount of KOH was used) the opposite situation occurs; that is, with an increase in the temperature the mean micropore half-width also increases. In the first cases (series A and B) an initial decrease in the micropore widths by increasing the calcinations temperature was already reported in the literature.33 This fact was interpreted in terms of contraction of the carbon structure, eventually because the bonds formed at relatively low temperatures are not stable enough. The increase in the half-widths above 600 °C may be related with the fact that, according to the activation mechanism accepted in literature,4 potassium carbonate that is formed at relatively low temperatures will decomposes leading to the evolution of CO2 that will react with the structural carbon, promoting the widening of the porosity. This transformation became especially important for activation temperatures as high as 800 °C.

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Figure 4. Relation between the adsorption capacities (cm3/g) of nitrogen and VOCs for the samples of series C.

Figure 5. Separation factors for the various pairs of studied VOCs in the samples from series C activated at temperatures of 500 (×) and 800 °C (+).

Adsorption of VOCs. To evaluate the possibilities of using the activated carbons prepared in this work for the abatement of noxious volatile organic compounds, the first approach is the determination of the amount adsorbed, at a given temperature, as a function of the pressure, which is the adsorption isotherm. In this way, the adsorption isotherms at 25 °C of the various VOCs are given in Figures 2 and 3. The shape of the adsorption isotherms is of type I according to the IUPAC classification,25 as a consequence of the microporous nature of the adsorbents already discussed in the previous section. In fact, the limiting adsorbed amounts are generally attained for relative pressures lower than 0.2.

This is a first significant result since in ambient conditions VOCs are present usually at small concentration. Therefore, a good adsorbent for VOCs abatement must efficiently adsorb even at low relative pressures. The limiting adsorbed amounts, expressed in liquid volumes, by assuming the density of the adsorbates equal to the respective liquids at the same temperature, were estimated from the DR equation and are listed in Table 3. Comparing the adsorption capacities of the VOCs (Table 3) with the adsorption capacities from the low-temperature nitrogen adsorption (Table 2), the former are usually lower or, in a few cases, similar. In only one sample this is not the case (sample A700), but the difference is only 0.01 cm3/g, which can be considered experimental uncertainty. The fact that the amounts of nitrogen adsorbed are higher than those for the VOC molecules can be interpreted in terms of the inaccessibility of the VOCs to parts of the microporous structure, according to the differences in the kinetic diameters of the molecules,34 which are, for instance, 0.36, 0.43, and 0.60 nm for nitrogen, n-hexane, and cyclohexane, respectively. The trends in the limiting adsorbed amounts among the various VOC molecules are less clear since they do not follow, for example, the sequence of the molecular diameters. However, this is not an unexpected result since, for comparing the adsorbed capacities for the several VOCs, the approximation of considering the density of the adsorbed species similar to that of the respective liquids was made. This approximation can be eventually less valid in the case of the more polar molecules, such as MEK and TCA, because the enhanced polarity can promote more specific adsorbate-adsorbent interactions. Additionally, the possibility of the existence of various conformations in a given molecule can also influence its adsorption.35 The values in Table 2 can be compared with data from literature concerning commercial activated carbons such as the RB4 from Norit.35 This activated carbon has a total microporous volume, evaluated from the low-temperature nitrogen adsorption of 0.59 cm3/g, therefore slightly higher than the more favorable results in this work, and present limiting adsorption capacities of 0.42 and 0.30 cm3/g for MEK and TCA, respectively.36 The samples from series B and C, activated at the highest temperature (800 °C), present results that compare favorably with these literature data. From the results from Tables 1 and 3 there is no apparent relation between the surface chemistry properties and the maximum adsorbed amounts of the various VOCs. Considering the evolution of the adsorption capacities for the VOCs within a given series of samples, this evolution is not monotonous but a trend in the increase of the adsorption of VOCs with the increase in the activation temperature can be observed. This is illustrated in Figure 4 for the samples of series C, where the limiting adsorption values for the various VOCs are plotted against the total microporous volume of each sample obtained from the low-temperature nitrogen adsorption. In the following portion of this work, the possibilities of using the prepared activated carbons for the selective adsorption of VOCs will be ascertained. Several models, based on the DR equation, which deals with the equilibrium adsorption of binary mixtures of VOCs on activated carbons, were reviewed elsewhere.37 In the present work, and with the purpose of making a first screening of the various samples for the adsorption of selected binary mixtures of VOCs, we estimated the separation factor (S) by a model proposed by Dobruskin,38 which is based on the following DR equation:

Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 5845 0.5 -1

S ) (β2 ⁄ β1) θ

exp(b ){exp{[(ln 1 ⁄ θ)-0.5 - b ]} + 2

2

π b{1 - erf{[(ln 1 ⁄ θ) -b]} (1) In eq 1, which besides activated carbons was applied also to other microporous materials,39 θ is the degree of filling and b ) ∆βE0/2RT. The affinity coefficients (β2 and β1) of the VOC molecules were estimated, in agreement with those discussed in the literature,40 from the molar polarizations of the respective liquids, taking benzene as reference adsorbate. The values of β used were 1.142, 1.058, 0.790, and 0.999 for n-hexane, cyclohexane, MEK, and TCA, respectively. The separation factors obtained from eq 1 are given in Figure 5 for the various pairs of the VOC molecules using as an example the samples C500 and C800, that is, the samples where the highest ratio of activating agent was used at the two extremes of activation temperatures. This series was selected also because, for similar activation temperatures, in relation to series A and B, the materials from series C always presented the more favorable values of specific surface area and micropore volume, which were highest for sample C800. In all the cases in Figure 5 the separation factor is higher than one revealing that a separation of the studied molecules by selective adsorption could in fact take place. Additionally, the sample activated at 800 °C is always the most favorable when compared with the sample obtained at the lowest activation temperature. The most unfavorable situation was for the pair cyclohexane/TCA. One reason for this fact can be the relative proximity of the critical diameters of these molecules. The separations that involve MEK always present the most favorable separation factors as a most probable consequence of the highest dipole moment of this molecule (3.3 D),41 which eventually can promote a different type of adsorbent-adsorbate interactions when compared with the other studied VOC molecules. Furthermore, and in line with the latter observation, the separations that involve the MEK molecule are clearly always more favorable in the C800 sample, that is, the material with the surface richer in oxygen and in more acidic (polar) groups. For all the studied systems the separation is, in general, more favorable for lowest degrees of filling. 0.5

0.5

Conclusion A waste of the cork industry, constituted mainly by a fine fraction (less than 0.297 mm) of cork powder, was chemically activated with KOH. From the study of the ratio between the activating agent and the cork powder, as well as from the changes in the activation temperature, it was concluded that when using equal weight amounts of KOH and cork powder (series C), and for an activation temperature of 800 °C, the activated carbon obtained presents values of specific surface areas and microporous volumes that can reach 1300 m2/g and 0.56 cm3/g, respectively. From the study of the adsorption of various VOC molecules, that are representative of different types of VOCs, the adsorbed amounts, particularly in the case of sample C800, compared favorably with commercial samples. Moreover, a first evaluation of the properties of selective adsorption of this sample indicated that it has promising properties for use in the separation/reuse of noxious volatile organic compounds. Acknowledgment This work was partially funded through the pluriannual funding of CQB from FCT. A.S.M. thanks FCT for a Ph.D. grant (SFRH/BD/17942/2004).

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ReceiVed for reView February 29, 2008 ReVised manuscript receiVed May 7, 2008 Accepted May 16, 2008 IE800338S