Cation exchangers prepared from cork wastes

Bioresource Technology 44 (1993) 229-233

CATION EXCHANGERS PREPARED FROM CORK WASTES C. V a l e n z u e l a Calahorro, a A. Bernalte Garcia, a C. Preciado Barrera, b M. J. Bernalte Garcia a & M. G 6 m e z C o r z o a "Departamento de Quimica Inorgfnica bEscuela de Ingenieria Tdcnica Industrial, Universidad de Extremadura, 06071-Badajoz, Spain (Received 21 June 1992; revised version received 11 November 1992; accepted 19 November 1992)

ally alterable at relatively low temperature (Preciado, 1991 ) suggested the necessity of carrying out a preliminary pyrolysis of the raw material to ensure the elimination of those components and thus obtain a thermally stable exchanger. The total exchange capacity of the sulphonated cork charcoals obtained by the action of sulphuric acid or oleum, as well as a chemical characterisation of exchangeable functional groups, has also been studied.

Abstract

Cation exchangers have been prepared from cork charcoal obtained at temperatures of 300, 400, 500, and 600°C by sulphonating with sulphuric acid and 10 and 20% oleum at 80, 100, and 120°C. Some of the exchangers obtained had an exchange capacity around 2 meq g- 1 and very high thermal stability. Key words: Cork, sulphonation.

cationic

exchange,

charcoal,

METHODS

INTRODUCTION

As raw material, cork wastes with a particle size of 1"6-4 mm were used. Pyrolysis was performed in the experimental set-up shown in a previous paper (Valenzuela et al., 1992). A mass of about 40 g of material was heated at a rate of 5°C/rain in a nitrogen stream (500 ml/min) from room temperature to 300, 400, 500, or 600°C; the residence time at the final temperature was that necessary to complete 200 min from the beginning of the experiment. These experiments were carried out many times at each pyrolysis temperature to obtain cork charcoals sufficient for the sulphonation processes. For the sulphonation, the sulphuric acid and 10% oleum or 20% oleum (250 ml) were placed in a threenecked flask, and the system was heated in an oil bath with mechanical stirring to the reaction temperature (80, 100, or 120°C). Cork charcoal (10 g) was added slowly and stirring was continued for 90 min. The mixture was then poured onto ice, and the sulphonated cork charcoal was filtered, washed free of acid, dried at 70°C, and resieved, and the fractions > 0"2 mm in particle diameter were stored. Proximate and ultimate analyses were carried out on the cork, carbonized, and sulphonated products. For the proximate analysis, a thermogravimetric method adapted to the ASTM procedure was used (Valenzuela & Bernalte, 1985). The ultimate (elemental) analysis was carried out with a Perkin-Elmer 240C instrument.

Many different natural and synthetic products show cation-exchange properties (Helfferich, 1962). The most important of these are cation-exchange resins (Osborn, 1955; Kunin, 1972) and also coals, mineral exchangers, and synthetic inorganic exchangers. In addition, a number of other materials can be transformed into cation exchangers by chemical treatment (Osborn, 1955). The most common procedures are sulphonation and phosphorylation, which are not restricted to synthetic resins and coals. This is illustrated by different patents covering the sulphonation of olive pits (Heimann & R a t n e r , 1953) and the sulphonation of nut shells and spent ground coffee (Mohan Rao & Pillai, 1954). Other materials (Vasudevan & Sharma, 1979) that can be sulphonated or phosphorylated are tar, wood, paper, cotton, groundnut shells (Chandrasekaran & Krishnamoorthy, 1987), etc. The present work shows the results obtained by studying different sulphonation conditions of cork waste. The existence in the cork of components therm-

Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain 229

230 C. Valenzuela Calahorro, A. Bernalte Garcia, C. Preciado Barrera, M. J. Bernalte Garcia, M. G6mez Corzo T h e total exchange capacity of the sulphonated cork charcoals was determined according to Schafer's method (Schafer, 1970). Fourier-transform infra-red spectroscopy (FTIR) was used for the characterization of the main chemical modifications introduced in the cork charcoals by the sulphonation reaction. Spectra were run in KBr pellets (1 wt%) and recorded on a Perkin-Elmer Model 1720 F T I R spectrometer by co-adding 100 scans (intefferograms) at a resolution of 2 c m - 1. T h e thermal stability of sulphonated cork charcoal was studied both in a dry atmosphere and in a steamsaturated atmosphere. T h e exchange capacity was obtained with a sufficient amount of sample, put into an oven at 200°C or into an autoclave at 121°C (press u r e = 1 kg cm 2). Samples of the exchangers were taken at regular intervals from 4 to 216 h for the oven test, and from 0"5 to 16 h for the autoclave test. T h e y were later washed with deionized water before testing the exchange capacity.

TG

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Temperature(~C) Fig. 1.

Thermogram for the pyrolysis of cork.

Thermal behaviour of cork T h e T G - D T G diagram, obtained for cork in a nitrogen atmosphere, is shown in Fig. 1. It is similar to that for other lignocellulosic materials that have been previously studied in our Departament (Valenzuela et al., 1987; Figueiredo et al., 1989; Pastor, 1990). Two clearly defined effects can be observed in this thermogram: a first effect, which starts at the beginning of the thermogram and finishes at about 130°C, and a second one, which starts at 200°C and does not finish until the

R E S U L T S AND D I S C U S S I O N T h e results of proximate and elemental analyses in Tables 1 and 2, and total exchange capacity in Table 2 and Figs 4 and 5 are averages of four analyses.

Table 1. Yield and proximate and elemental analyses T

(oc)

300 400 500 600 Cork

Elemental analysis

Proximate analysis

Yield

(%)

Moisture

(%)

Volatile* Matter (%)

Fixed* Carbon (%)

Ash* (%)

c (%)*

H (%)*

N (%)*

3"5 4"6 6"6 7"7 6"3

63"5 38"1 16"3 14"4 78"9

30"6 54"4 73"5 74"0 13"0

2"4 2"9 3"6 3"9 1"8

70"4 76"2 82"2 82"5 53"9

5"9 4-3 2"2 1"5 6"5

0"8 1"6 1-6 1"7 0"7

44"4 25"0 18-1 15"3 --

T = pyrolysis temperature; undried materials.

Table 2. Proximate and elemental analyses of selected sulphonated cork charcoals (dry basis)

Sample

Char 300°C S-10-300 S-20-300 Char 400°C S-10-400 S-20-400

Proximate analysis

Elemental analysis

EC

Volatile Matter (%)

Fixed Carbon (%)

Ash (%)

C (%)

H (%)

N (%)

S (%)

meq g- l

65.8 55"7 53-5 39"9 54.8 52"8

31.7 43"3 45.6 57"0 44.1 46 "2

2.5 1.0 0-9 3.1 1.1 1.0

72.9 58"8 53"1 79.8 70.8 65-5

6.0 0.2 0"1 4.4 0"1 < 0-1

0"8 0-5 0"5 1.7 0-6 0-6

< 0.1 4.4 4.4 < 0.1 4.0 4-0

< 0.1 2-0 2"2 < 0"1 1-8 2.0

Char 300°C = Cork charcoal prepared by pyrolysis at 300°C. Char 400°C -- Cork charcoal prepared by pyrolysis at 400°C. S-10-300 = Sulphonated cork charcoal prepared from Char 300°C by sulphonation at S-20-300 = Sulphonated cork charcoal prepared from Char 300°C by sulphonation at S-10-400 = Sulphonated cork charcoal prepared from Char 4000C by sulphonation at S-20-400 = Sulphonated cork charcoal prepared from Char 400°C by sulphonation at

120°C, by using 10% oleum. 120°C, by using 20% oleum. 1200C, by using 10% oleum. 1200C, by using 20% oleum.

Cation exchangersfrom cork

231

process approaches 700°C, although, at higher temperatures than 480°C, the process develops slowly. The first effect can be assigned to removal of moisture along with different volatile light materials, such as essential oils, etc. The corresponding weight loss is 6.4 wt% of initial weight in this effect. In the second effect, three different stages can be discerned. The first one begins at 200°C and continues to about 310°C. It appears as a shoulder of the main effect and is associated with a 15.9% weight loss from the initial mass. This first stage is followed by a second one in which the pyrolysis and loss of volatile matter develop much faster and can be considered as finished at about 480°C; the corresponding weight loss at this stage is 51-8% of the initial weight. Finally, the pyrolysis develops more slowly at higher temperatures than 480°C, and a third phase, which implies a loss of initial weight of 10"2% and which proceeds to 700°C, appears in the thermogram. Fig. 2.

Pyrolysis The yields of the cork-pyrolysis process at 300, 400, 500, and 600°C are shown in Table 1, together with the proximate and elemental analyses of the cork charcoals obtained, and, for comparison, those of the initial cork. These results are averages of all the pyrolysis experiments carried out at each temperature. As was expected, the higher the pyrolysis temperature, the lower was the yield, owing to the progressive loss of volatile matter. This is shown in the proximateanalysis results, the volatile-matter content decreasing as the pyrolysis temperature increases. Conversely, an increase in the fixed-carbon content is observed. The ash content increases slightly along with the pyrolysis temperature. Although the constituent inorganic material of the ash must be the same in all the cork charcoals, as the weight of product decreases when the temperature rises the ash content will be proportionally higher. The moisture contents of the samples rise with the pyrolysis temperature; this increase may be a consequence of the cork-charcoal porosity, which would retain a higher amount of water in its structure. The carbon content rises with the temperature, and these figures are higher than those for the fixed carbon since part of the carbon content comes from volatile matter. The H content decreases as the temperature rises, which is in accordance with the lower volatile matter in the cork charcoals.

Total exchange capacity of the sulphonated cork charcoals The results for the total exchange capacity of the sulphonated cork charcoals are shown in Fig. 2. The total exchange capacity rises along with the SO 3 content of the sulphonating agent, as with the sulphonation temperature. On the contrary, the exchange capacity decreases as the pyrolysis temperature rises, as would be expected, since the more intense the pyrolysis process, the lower are the volatile-matter content of the

Exchange

capacity of the sulphonated charcoals.

cork

charcoal and the number of aromatic nuclei that might be sulphonated. The values of Fig. 2 were fitted to the following equation: EC = 2"98 + 1"21 x 10 -2 AS + 4"79 × 10 -3 T S - 5"61 × 10 -3 TC with a multiple linear correlation coefficient r = 0"967 and an estimated standard error of 0.274, and in which EC = total exchange capacity, AS = percentage SO3 in the sulphonating agent, TS = sulphonation temperature (°C), and TC = pyrolysis temperature (°C). This confirms quantitatively the qualitative suggestions obtained from Fig. 2. Allowing for the values of the independent variables TC (from 300 to 600), TS (from 80 to 120), and AS (from 0 to 20), and from coefficients of the former equation, it can be deduced that the variable of greatest influence on the exchange capacity is the cork pyrolysis temperature TC, and AS is the variable with the least influence. The exchange-capacity values are slightly lower than those obtained for sulphonated lignite coals (Ibarra, 1983) and higher than those obtained in the sulphonation of other materials prepared from other agricultural wastes, such as peanut or rice shell (Chandrasekaran & Krishnamoorthy, 1987).

Exchanger characterization After the previous results, further work was carried out on the four sulphonated cork charcoals of the highest exchange capacity, those prepared at 120°C, by using oleum of 10% and 20% and cork charcoal at 300 and 400°C as the starting material. These products were named S-AS-TC, AS and TC having the meaning previously given.

232 C. Valenzuela Calahorro, A. Bernalte Garcia, C. Preciado Barrera, M. J. Bernalte Garcia, M. G6mez Corzo The proximate and ultimate analyses for these sulphonated cork charcoals are shown in Table 2. These results are referred to dried samples, which, as a consequence of the degradation of the material and the insertion of ionic groups during the sulphonation process, retain more than 15% water. The analyses of the starting charcoals (Char 300°C and Char 400°C) are shown in Table 2 for comparison. The ash contents of the sulphonated cork charcoals are smaller than those of the starting cork charcoals, which may have been due to the extraction of inorganic compounds from the ash in the acid treatment to which the cork charcoals had been subjected. The volatilematter and fixed-carbon contents change in different ways depending on the starting charcoal. Thus, for the 300°C charcoal, as the sulphonation increases in intensity, the fixed carbon rises, whereas the volatilematter content decreases owing to the carbonization effect produced by the sulphonating agent. The reverse happens for the 400°C charcoal. This suggests that the 400°C sulphonation treatment for charcoal must lead to a superficial carbon oxidation, which produces a decrease in the fixed carbon and total carbon, whereas the 300°C charcoal sulphonation had an effect on the volatile matter (oxidizing it, since the total carbon content decreases) but provoking a different distribution of volatile matter and fixed carbon. The hydrogen and nitrogen contents decrease when the sulphonation is intensified owing to the dehydration and destruction of volatile matter (hydrogenated and nitrogenated) of the cork charcoal. Sulphur was introduced as a consequence of the sulphonation, probably as --SO3 H. However, the amount seems to depend not on the sulphonating agent used but on the starting charcoal; possibly, as has been previously pointed out, because of the greater presence in the 300°C charcoal of aromatic groups in which the sulphonation reaction is better. The exchange capacity does not depend simply on the presence of --SO3H groups, because, if we suppose all the sulphur to be in the form of such groups, the exchange capacity would have values from 1.37 to 1-25 meq g-1, lower values than those obtained experimentally and shown in Table 2. From this, we must necessarily suppose that some other functional groups with the capacity for cationic exchange exist within the sulphonated cork charcoal, which were present in the starting charcoal or created during the sulphonation reaction because of the strong oxidizing character of oleum. The FTIR spectra were used to check the modifications introduced by the sulphonation reaction. In Fig. 3, as an example, the 300°C cork-charcoal and the S-10-300 exchanger FTIR spectra are shown; these differ considerably. Increase in intensity of the bands was assigned to the vibrations v(OH), which appear centered at 3445 cm-~ for charcoal whereas for the exchangers they appear at about 3420 cm -1. The bands assigned to the vibration of --(CH2) n - groups of the charcoal, situated at 2923 and 2852 cm-1, do not appear in the sulphonated charcoal. An increase in the

intensity of the band assigned to the vibration v(C = C), in the zone of 1615 cm -t, from the charcoals, is observed in every exchanger. This may be a consequence of the dehydration reactions that occur at the same time as the sulphonation reaction. The 1730cm-l zone band, which is assigned to v ( C = O ) and indicates the presence of ketonic and carboxylic groups, appears in both charcoals and exchangers, in the former being slightly more intense. The addition of three new bands situated at 1200, 1040, and 600 cm- 1, which may be assigned to the vibrations of the sulphonic groups according to the literature (Ibarra, 1984), may be noted. These results deduced from the FTIR spectra seem to confirm what has been pointed out previously about the presence of at least two different exchanger groups in the sulphonated cork char-

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2000

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450

cm-1

65

I

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45

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2000

i000

450

cm -1

Fig. 3.

spectra of the 300°C cork charcoal and S-10-300 sample.

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Fig. 4. Evolution of exchange capacity with time for selected sulphonated samples dry-heated at 200°C.

Cation exchangers from cork

233

REFERENCES 7 g vE2

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Fig. 5. Evolution of exchange capacity with time for selected sulphonated samples treated in steam in an autoclave at 121°C. coals produced as a result of a simultaneous cork-charcoal oxidation. With regard to thermal stability, the exchangecapacity data in relation to the treatment time in a dry atmosphere in an oven at 200°C appear in Fig. 4, and for treatment in a steam-saturated atmosphere in an autoclave in Fig. 5. No influence of the treatment time upon the sulphonated cork-charcoal exchange capacity is seen, and the slight scattering of the points may be a consequence of the heterogeneity of the exchangers themselves. These results show a high thermal stability of the exchangers both in a dry atmosphere and in a steam-saturated one in which they might be hydrolyzed. This would lead us to suggest use of them at higher temperatures than the most thermally stable resins such as macroreticular polystyrene structures (Dofner, 1973). ACKNOWLEDGEMENTS We gratefully acknowledge the D G I C Y T PB 89-05 20) for their financial support.

(project

Chandrasekaran, M. B. & Krisnamoorthy, S. (1987). Studies on macroreticular cation-exchangers. J. Indian Chem. Soc., LXIV, 134. Dorfner, K. (1973). Ion Exchangers: Properties and Applications. Ann Arbor Science Publishers, MN, USA. Figueiredo, J. L., Valenzuela, C., Bernalte, A. & Encinar, J. M. (1989). Pyrolysis of olive wood. Biological Wastes, 28, 217. Heiman, H. &Ratner, R. (1953). Cation exchangers from olive pits. Bull. Res. Council Israel, 3, 96. Helfferich, F. (1962). Ion Exchange. McGraw-Hill, New York, NY, USA. Ibarra, J. V., Juan, R. & Moliner, R. (1983). Intercambiadores catirnicos a partir de lignitos. Afinidad, XL, 229. Ibarra, J. V., Rebollar, M. & Gavilan, J. M. (1984). Preparation of cation exchange materials by oleum sulphonation of an Utrillas Spanish lignite. Fuel, 63, 1743. Kunin, R. (1972). Ion Exchange Resins. R. E. Kreiger Publishing Co., Huntington, NJ, USA. Mohan Rao, G. J. & Pillai, S. C. (1954). Treatment and utilization of spent coffee grounds for preparing ion exchange material. J. Indian Inst. Sci., A36, 70. Osborn, G. H. (1955). Synthetic Ion Exchangers. Chapman & Hall, London. Pastor Villegas, J. (1990). Preparaci6n y caracterizacion de carbones activados a partir de jara. Tesis Doctoral, Facultad de Ciencias, Universidad de Extremadura, Spain. Preciado, C. (1991). Obtencirn de intercambiadores de cationes a partir de residuos de corcho. Tesis Doctoral, E.T.S. Ingenieros Industriales, U.N.E.D., Madrid, Spain. Schafer, H. N. S. (1970). Carboxyl groups and ion exchange in low-rank coals. Fuel 49, 197. Valenzuela, C. & Bernalte, A. (1985). Un m&odo termogravimrtrico rhpido para anfilisis inmediato de carbones. Boletln Geol6gico y Minero, 96, 58. Valenzuela Calahorro, C., Bernalte Garcia, A., Grmez Serrano, V. & Bernalte Garcia, M. J. (1987). Influence of particle size and pyrolysis conditions on yield, density and some textural parameters of chars prepared from holmoak wood. J. AnaL AppL Pyrolysis, 12, 61. Valenzuela Calahorro, C., Grmez Serrano, V., Hern~indez Alvaro, J. & Bernalte Garcia, A. (1992). The use of waste matter after olive grove pruning for the preparation of charcoal: The influence of the type of matter, particle size and pyrolysis. Bioresource Technol., 40, 17. Vasudevan, P. & Sharma, N. L. V. (1979). Composite cation exchangers. J. Appl. Polym. Sci., 23, 1443.