604
Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 604-607
Lignite Sulfonation Optimized by a Modified Simplex Method Jos6 V. Ibarra" and Jesiis J. LBzaro Instituto de Carboquimica, CSIC, 50004 Zaragoza, Spain
The preparation of cation-exchange materials by H,S04 sulfonation of Utrillas Spanish lignite was studied. The influence of temperature, reaction time, and H,SO,/coal ratio (v/w) on the exchange capacity of the sulfonated lignites was investigated by a modified Simplex method. The exchange capacity of the parent lignite was increased to 3 mequiv g-'. Sulfolignite samples have high specific surface areas and suitable stabilities (low leaching and swelling) for use in water treatment procedures. The exchangeable functional groups were analyzed by discontinuous titrations and FTIR spectroscopy. Development of specific surface area was studied by the p -nitrophenol method.
Introduction Sulfonated coals were the first cation-exchange materials found to be stable a t low pH, and they have been used in the past as cation exchangers when they were substituted by synthetic resins derived from petroleum. Synthetic ion-exchange resins seem to be attractive materials for water treatment in processes such as metal removal from dilute solutions or water quality improvement (softening,deionization, desalination, etc.). However, industrial wastewater treatment with ion-exchange resins has not been widely used because of destructive effects of certain impurities, interfering ions, limited loading capacity, and, mainly, the high cost of operations (Dean et al., 1972). In the last few years attention has been paid to the development of decontamination technologies derived from naturally occurring materials that are inexpensive due to their availability. For this reason sulfonated coals again appear as alternative materials to commercial resins when the high performances of these resins are not essential and the cost is the main consideration. Numerous papers have been published in the last few years on the preparation of cation-exchangematerials from coal by sulfonation reactions, and very diverse carbonaceous materials have been used: peat (Smith et al., 1979); lignites (Seitz et al., 1960; Caio and Bigalii, 1979; Ibarra et al., 1984);bituminous coals (Cornelius and Sarkar, 1980; Shul'zhenko et al., 1981; Pandey and Chaudhuri, 1982); asphaltites and petroleum formolites (Pokonova et al., 1981; Pokonova and Persinen, 1981); and thermolized polystyrene resins (Ford et al., 1982). In contrast, sulfonated coals are being widely used a t present in East Europe in water softening for power plants and in desalination units (Smagin and Yankovskii, 1976; Kuvshinov et al., 1978; Kryzhanovskii et al., 1978; Abdullaev et al., 1978; and Krasnovarskaya et al., 1978). In a previous paper (Ibarra et al.,1984) we studied the preparation of cation-exchange materials by oleum sulfonation of Utrillas Spanish lignite. Studies were carried out by a factorial design a t two levels, and the initial cationexchange capacity of lignite (0.7 mequiv g-l) was increased to 3.7 mequiv g-l. These sulfonated lignites have also been tested in processes of metal recovery from effluents (Ibarra and Moliner, 1984) and water softening (Moliner and Ibarra, 1984a, 1985). These materials are suitable cation-exchange resins from water treatment that compare favorably with available commercial resins (Moliner and Ibarra, 1984b). Exhausted sulfolignites are easily regenerated with HC1 or NaCl dilute solutions. 0 196-432 1/ 85/ 1224-0604$0 1.50/0
Table I. Operation Levels temp, time, level "C h 0 80 1 1 120 2
ratio,
mL of H2S04/g of coal 5:1 20:l
In this paper, the sulfonation by H2S04of a Spanish lignite is studied in order to prepare cation-exchange materials from a sulfonation agent which is cheaper and more manageable than oleum. Runs were carried out according to a modified Simplex method (Deming and Morgan, 1973). This statistical method has the advantage of great simplicity, and it can lead straight to the optimum point of the process (Massart et al., 1978). Additionally, the stability (leaching and swelling) and the surface area development in the sulfonated lignites by action of the sulfuric acid are studied. Likewise, characterizations of the exchangeable functional groups and of the main chemical modifications were carried out. Experimental Section A Spanish lignite (C, 72.7; H, 5.1 wt YO,daf basis) from Utrillas basin (Teruel) that was ground and sieved to a particle size of 0.5-1.2 mm was used. Elemental analyses (C, H, N) were obtained by using a Leco Model CHN-600 elemental analyzer, and total sulfur (ST)was obtained in a Leco SC-32. Statistical Method. In this paper, a modified Simplex method was used. Three significant variables, temperature, T , reaction time, t , and H2S04/coalratio, r (v/w), were considered. The upper and lower levels for the variables (Table I) were chosen according to the published bibliography on coal sulfonation: Smith et al. (1979), Sarma and Vasudevan (1980),Cornelius and Sarkar (1980),and Ford et al. (1982). Previous tests advised limiting the reaction temperature to 155 OC in order to reduce the leaching of colored organic substances from the sulfonated lignites, SLU (LBzaro, 1984). The initial matrix of the design for the three variables was T(x,)
M=
[ 4
t(x*) 0
; 4
q
r(x3)
p
SLU-1
,";: SLU-4
where p = 0.943 and q = 0.236 for three variables. In this matrix, rows represent the coordinates of the four vertices of Simplex (SLU-1 to SLU-4) and columns are 0 1985 American Chemical Society
lnd. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985
Table 111. Elemental Analysis and Total Sulfur of the Sulfonated Lignites anal., wt %. daf
Table 11. Reaction Conditions vertex SLU-1 SLU-2 SLU-3 SLU-4 SLU-5 SLU-6 SLU-7 SLU-8 SLU-9
vertices retained
variables modification
T,OC 80
2, 2, 2, 2, 2,
3, 4 3, 4 3, 4 3, 7 3, 7
expansion expansion contraction
118 90 90 118 137 155 152 152
t,h 1.00 1.23 1.94 1.23 1.94 2.41 2.88 2.80 2.37
r, v/w 5:l 8.51 8.5:l 19:l 19:l 26:l 33:l 14.4:l 20.8:l
coded values for each variable. For the next run (SLU-5), the vertex of the worst response (SLU-1) was rejected, and the new vertex was calculated as the reflection of the rejected vertex, according to the rules of the Simplex method (Deming and Morgan, 1973; Massart et al., 1978). For run 6, rules of the modified Simplex method (Deming and Morgan, 1973; Massart et al., 1978) with expansions and contractions were used. Reaction conditions are shown in Table 11. Sulfonation Procedure. For the sulfonation of the lignite, sulfuric acid (98 wt %) was placed in an open glass reactor, and the system was heated in an oil bath with constant mechanical stirring to the required reaction temperature. Coal was added slowly (exothermicreaction), and stirring was continued for the desired time. The mixture was then poured onto ice and the sulfonated lignite was filtered, washed free of acid, and dried at 105 "C. The yield of sulfonated product was always >lo0 wt % of the parent lignite. Determination of the Total Exchange Capacity. The total exchange capacity (TEC) of the sulfolignites (carboxyl and sulfonic groups) was determined by the barium acetate method (Schafer, 1970). Measurement of Specific Surface Area. Specific surface area of the sulfolignites was measured from adsorption of p-nitrophenol (PNP) in water. Measurements were made by treating sulfolignite samples (50 mg) with 20 mL of 0.015 N PNP solution. The equilibrium concentration of PNP in solution was determined at 320 nm by (Ibarra et al., 1984) y = 0.0136
+ 8511.8~
( R = 0.999)
605
(1)
where y is the absorbance and x is the PNP concentration N). The surface area of sulfolignites was calculated from the amount of adsorbed PNP. A value of 52.5 A2 for the area occupied by a PNP molecule was used (Vasudevan et al., 1978; Giles and D'Silva, 1969). Measurement of Stability. Leaching of sulfolignites was investigated by passing buffer solutions with pH increasing from 6 to 10 and measuring the absorbance at 410 nm of the last 25 mL of each buffer effluent (Ibarra et al., 1984). Measurements of the swelling in water were made in graduated columns and were calculated as percentage change in height over the initial height of the bed after 24 h of stabilization (Smith et al., 1979; Ibarra et al., 1984). Characterization of the Exchangeable Functional Groups. To distinguish between the two functional groups that contribute to the ion-exchange capacity of the sulfolignites and to ascertain if oxidation reactions had taken place, potentiometric titrations were carried out. The titrations were made intermittently with 0.100 g of sulfolignite, 5 mL of 1N KC1 solution, different volumes (up to 25 mL) of 0.02 N NaOH solution, and enough C02-free distilled water to make a final volume of 50 mL.
sample LU SLU-1 SLU-2 SLU-4 SLU-4 SLU-5 SLU-6 SLU-7 SLU-8 SLU-9
C 72.70 64.46 63.22 64.14 63.87 62.75 62.35 62.54 63.34 62.09
H 5.09 3.08 2.28 2.89 2.90 2.23 2.06 2.07 2.05 1.96
N 0.68 0.56 0.69 0.78 0.79 0.92 0.94 0.93 0.91 0.97
0 +% org" 21.53 31.90 35.21 32.19 32.33 34.10 34.65 34.46 33.70 34.98
ash, wt %
11.64 6.45 5.23 6.24 6.30 5.43 4.91 5.10 5.20 4.93
ST, wt %, dry basis 5.33 7.26 7.52 7.33 7.12 7.11 6.97 6.34 6.23 6.40
" By difference. Table IV. Results for the Sulfolianite Samples TEC, swelling, surface area, sample mequiv g-* leaching, pHa %* m2 g-' LU 0.71 10 50 SLU-1 1.60 35 358 SLU-2 2.06 41 577 1.94 SLU-3 38 448 SLU-4 1.88 35 447 SLU-5 2.36 32 530 2.61 SLU-6 0.01 42 564 SLU-7 2.99 0.02 45 624 SLU-8 2.97 0.02 42 558 SLU-9 2.96 0.01 50 568 "Absorbance a t 410 nm of the effluent. *Increase in height.
Fourier transform infrared spectroscopy (FTIR) has been used for the characterization of the modifications introduced in the Utrillas lignite by the sulfonation reaction. Spectra were run on KBr pellets (120 mg, 1wt %) and were recorded on a Nicolet MX-10 IR spectrometer by co-adding 160 scans (interferograms) at a resolution of 1 cm-'.
Results and Discussion Table I11 shows the elemental analysis and total sulfur content of sulfonated samples. The oxygen + organic sulfur content increases and the hydrogen content decreases very markedly in sulfolignites in relation to the parent lignite. The introduction of -S03H groups in samples implies an increase in the total sulfur content (ST) of the sulfolignites (Table 111). In samples SLU-1-SLU-5 a satisfactory agreement between the increase in STvalues and the introduced sulfonic groups exists (Table V). However, the most sulfonated samples (SLU-7-SLU-9) have not the higher sulfur contents. This fact can be explained by considering that in coal there are several sulfur forms (sulfates, pyritic, and organic) that are affected in different degrees depending on the reaction conditions. Sulfates me removed from sulfonated samples as is provided by IR spectroscopy, while pyritic and organic sulfur can be partially removed in runs where oxidation reactions take place. An increase in the leaching of colored organic materials from samples as reaction conditions increase has been observed experimentally during the sulfonation. Therefore, the sulfur content of sulfolignites is not an adequate way to estimate the degree of sulfonation of these materials. Total Exchange Capacity of the Sulfolignites. Table IV shows the results obtained for the TEC and the reaction conditions for the sulfonated lignites. From run 5, rules of modified Simplex were applied with two consecutive expansions for runs 6 (y = 2) and 7 (y = 3). In
606
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985
Table V. Titration of Functional GrouDs in the Sulfolimites ~
__
inflection points, PH 7.11 8.4 5.3 8.7 5.4 8.8 5.2 8.3 5.2 8.5 5.4 8.7 5.4 8.7 5.2 8.3 5.4 85 5.3
sample LU SLU-1 SLU-2 SLU-3 SLU-4 SLU-5 SLU-6 SLU-7 SLU-8 SLU-9
-S03H 1.04 1.33 1.19 1.14 1.46 1.70 1.76 1.79 1.77
PH
9
7
5
3
1
2
6
10
V NaOH(m1)
Figure 1. pH titration curves of the sulfolignites with 0.019 N NaOH. 0 , LU; A,SLU-1; 0,SLU-5; 0, SLU-7. Inflection points are denoted by interrupted vertical lines.
run 8, it was necessary to return to the original Simplex due to the limit imposed previously on the temperature. In run 9, a contraction (P = 0.2) was carried out for the same reason. Coincidence among the values of runs 7,8, and 9 suggests that the optimum of sulfonation has been reached. It also indicates that the sulfonation process is strongly dependent on temperature. The optimum conditions for sulfonation reaction involve a compromise between the lower conditions of reaction and the greater exchange capacities. This requirement is accomplished by run 8. Exchangeable Functional Groups. The pH titration curves of the parent lignite and some sulfonated samples are shown in Figure 1, and they show the dual functionality of the exchange sites. Sulfolignites have small jumps over pH 5.3 due to the titration of sulfonic groups as well as inflections over pH 8.5 corresponding to the end point of the titration (carboxyl + sulfonic groups). The inflection points were determined according to Gran's method (Gran, 1950) by ploting AV/pH vs. the NaOH volume. Table V gives the inflection points, and titrated carboxyl and sulfonic groups of the samples. These results suggest -that oxidation reactions take place when reaction conditions are increased and that both reactions, oxidation and sulfonation, progress together. Results quoted here suggest that lignites have a limited number of sulfonation sites. An increase in the reaction conditions does not increase the content of sulfonic groups but it favors oxidation reaction. Stability. The characterization of the leaching is especially important when the sulfolignites are used in water
acidity, mequiv g-l titrated -COOH total 0.73 0.73 0.76 1.74 2.15 0.82 2.05 0.80 0.77 1.91 1.07 2.53 1.04 2.75 1.21 2.97 1.19 2.98 1.21 2.98
exchangeable 0.71 1.60 2.06 1.90 1.88 2.36 2.61 2.99 2.97 2.96
treatment because they can introduce organic materials into the effluent. The results obtained for the sulfonated samples are shown in Table IV. All samples have an adequate degree of leaching for use in ion-exchange procedures. The swelling of sulfonated samples is greater than that of the starting coal. It indicates a decrease of the macromolecular rigidity of the samples due to sulfonation reaction (Vasudevan et al., 1978). Development of Specific Surface Area in the Sulfonation Process. The sulfonation reaction leads to a large development of surface area in the samples. The values of surface area for the sulfolignites calculated by the PNP method are given in Table IV. It can be observed that the specific surface area increases when the reaction conditions are increased. The surface area values estimated for the sulfonated samples are comparable to those of commercial activated coals. So, the PNP method gave a value of 650 m2 g-' for a sample of DARKO-S 51 activated charcoal. Therefore, the sulfuric acid treatment of lignite produces a very interesting material for water treatment that combines the exchange capacity with the adsorption power. FTIR Study. The IR spectra corresponding to sulfonated lignites (Figure 2c,d) show, in relation to the parent lignite (Figure 2a), new bands at 1400,1175,and 620 cm-' assigned to stretching vibrations of -S03H group (asymmetric and symmetric) and S-0, respectively. However, the contribution of other groups such as esters, phenolics, and carboxyls to the 1400 cm-I band cannot be discarded. The spectra of sulfonated samples also show a decrease in the intensity of the bands of aliphatic C-H (2950,2920, 2850, and 1440 cm-l) as well as a progressive increase in the 1710 cm-l (C=O) and 1210 cm-' (C-0) bands. These observations tend to support the presence of oxidation reactions in the sulfonation process, commented previously, and they indicate the formation of carboxyl groups from the oxidation of methyl and methylene groups. This fact agrees with the decrease in hydrogen content of the samples determined by elemental analysis. In relation to the mineral matter in Utrillas lignite, the sulfuric acid treatment removes the gypsum (bands at 1160,1140,1115,680, and 605 cm-') from the sulfonated lignites while the halloysite (3760, 3620, 1110, 1033, 1010, 915, 750, 695, 545, and 470 cm-l) remains. In the spectra of the sulfonated lignites the bands at 1032 and 1010 cm-' (Si-0) show modifications in their relative intensities in relation to the parent lignite. The spectra corresponding to Utrillas lignite and SLU-7, both free of mineral matter (Figure 2b,e), prove the contribution of the C-0 stretching vibration to the 1032 cm-' band. The disappearance in the spectrum of SLU-7 demineralized (Figure 2e) of the bands at 860,815, and 755 cm-' (aromatic substitution) in relation to the parent lignite
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985 607
3 mequiv g-' by sulfuric acid treatment. Temperature seems to be the most influential variable on the exchange capacity. The conditions for the optimum of sulfonation are T = 150 "C, t = 3 h, and ratio 151 mL of H2S04/gof coal. The exchanger materials, so obtained, have suitable stabilities (low leaching) for their use in water treatments technologies and a high specific surface area. Acknowledgment We are grateful to the CAICYT for the support of this work, Project No 33 118.03. Registry No. H2S04,7664-93-9.
Literature Cited
&
4400
3200
xxx)
1400
800 cm-1
WAVE NUMBERS
Figure 2. FTIR spectra of t h e sulfolignites. (a) LU; (b) LU demineralized ( H C l / H F ) ; (c) SLU-l; (d) SLU-7; (e) SLU-7 demineralized ( H C l / H F ) .
(Figure 2b) agrees with the character of electrophilic aromatic substitution of the sulfonation reaction. Conclusions The results of the present work indicate that the exchange capacity of the Utrillas lignite can be increased to
Abdullaev, K. M.; Malakhov, I.A.; Potetaev, I.A.; Poletav, L. N. Teploenergetika (Moscow) 1978, 6 , 67. Caio, F. A.; Bigalli, D. Rev. Bras. Techno/. 1979, 70. 129. Cornelius, R. L.; Sarkar, S. Indian Chem. Manuf. 1980, 70, 17. Dean, J. G.; Bosqul, F. L.; Lanoutte, K. H. Environ. Sci. Techno/. 1972, 6 , 518. Deming, S. N.; Morgan, S.L. Anal. Chem. 1973, 4 5 , 278A. Ford, W. T.; Beasly, G. H.; Chong, B. P.; Neely, J. W. J. folym. Sci. Polym. Chem. Ed. 1982, 20, 1213. Giles, C. H.; D'Silva, A. P. Trans. Faraday Soc. 1989, 6 5 , 1943. Gran, G. Acta Chem. Scand. 1950, 4 , 559. Ibarra, J. V.; Mollner, R. Fuel 1984, 6 3 , 373. Ibarra, J. V.; Rebollar, M.; Gavlln, J. M. Fuel 1984, 6 3 , 1743. Krasnovarskaya, A. S.; Gryazeva, A. S.; Gofman, I.N. Energefik 1978, 7 , 32. Kryzhanovskil, B. N.; Botvlnov, V. P.; Vlasklsna. Q. V. Energefik 1978, 7, 5 . Kuvshinov, G. I.; Kirpichev, 0. I.; Apekhtina, N. M. Eiekfr. Stn. 1978, 7, 32. Uzaro, J. Tesina de Llcenciatura, Unlversldad de Zaragoza, 1984. Massart, D. L.; Dijkstra, A.; Kaufman. L. "Evaluation and Optimization of Laboratory Methods and Analytical Procedures"; Elsevier: Amsterdam, 1978. Mollner, R.; Ibarra, J. V. Ing. Ouim. 1984, 783,75. Moliner. R.; Ibarra, J. V. presented at the 3rd Mediterranean Congress on Chemlcal Englneering, Barcelona, Nov 1984b. Mollner. R.; Ibarra, J. V. Ing. Ouim. 1985, 795, 59. Pandey, M. P.; Chaudhurl, M. Wafer Res. 1982, 76, 1113. Pokonova, Yu. V.; Persinen, A. A. Z h . frlkl. Khim. (Leningrad) 1981, 5 4 , 1916. Pokonova, Yu. V.; Pol'Kin, G. E.; Roskuryakov, V. A,; Vinogradov, M. V. Z h . W k i . Khim. (Leningrad) 1981, 5 4 , 1781. Sarma, N. L. N.; Vasudevan, P. J. Indian Chem. SOC. 1980, 5 7 , 191. Schafer, H. N. S.Fue/ 1970, 49, 197. Seitz, D. S.;Minnear, F. L.; Dunbar, R. E. Ind. Eng. Chem. 1980, 52, 313. Shui'zhenko, E. A.; Lltvinenko, A. M.; Sergienko, N. I.; Man'ko, L. P.; Tyutyunlkov, Yu. 6.; Nosalevich, M. I. KO+ Khim. 1981, 70, 14. Smagln, V. N.; Yankouskil, K. A. S b . T r . - Mosk. Inzh. - Sroif Insf. lm. V . V . Kuibysheva 1970, 748, 119. Smlh, E. F.; Mark, H. E. Jr.; MacCarthy, P. I n "Recent Advances in Environmental Analysis"; Frei, R. W. Ed.; Gordon and Breach: New York, 1979; pp 241-160. Vasudevan, P.; Singh, M.; Nanda, S.; Sarma, N. L. N. J . folym. Sci. 1978, 76. 2545.
Receiued f o r reuiew D e c e m b e r 20, 1984 Accepted May 24, 1985
