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Recovery of acetic acid from pre-hydrolysis liquor of hardwood kraft-based dissolving pulp production process by reactive extraction with triisooctylamine
Bioresource Technology 138 (2013) 253–258
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Recovery of acetic acid from pre-hydrolysis liquor of hardwood kraft-based dissolving pulp production process by reactive extraction with triisooctylamine G. Yang a,b,⇑, M. Sarwar Jahan b,c,⇑, Laboni Ahsan b, Linqiang Zheng b, Yonghao Ni b,⇑ a b c
Key Laboratory of Pulp & Paper Science and Technology (Shandong Polytechnic University), Ministry of Education, Jinan 250353, Shandong, China Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dhaka 1205, Bangladesh
h i g h l i g h t s Acetic acid was recovered from the PHL by reactive extraction with triisooctylamine. Diluent played an important role in recovering acetic acid from the PHL. Temperature and pH negatively affected the acetic acid extraction. Acetic acid concentration positively affected the acetic acid extraction.
a r t i c l e
i n f o
Article history: Received 29 December 2012 Received in revised form 22 March 2013 Accepted 24 March 2013 Available online 31 March 2013 Keywords: Pre-hydrolysis liquor Acetic acid Reactive extraction Triisooctylamine Distribution coefficient
a b s t r a c t Acetic acid was one of the main compositions of the pre-hydrolysis liquor (PHL), which was recovered by reactive extraction with triisooctylamine (TIOA) diluted with decanol. Dilution of TIOA played an important role in extracting acetic acid from the PHL. The recovery of acetic acid from the PHL by TIOA was increased from 10.34% to 66.60% with the dilution of TIOA to 20% by decanol at the HAc to TIOA molar ratio of 1, consequently, the equilibrium distribution coefficient KD increased. The effects of time, temperature and pH on the extraction process were also studied. The extraction process was very fast. The acetic acid extraction decreased from 65.13% to 57.34% with the rise of temperature to 50 °C from 20 °C. A higher pH increased the dissociation of acetic acid, as a result, decreased acetic acid extraction. The hemicelluloses in the PHL were unaffected on the extraction process of acetic acid. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Pre-hydrolysis is carried out in the kraft-based dissolving pulp process to remove hemicelluloses from the lignocellulose. The pre-hydrolysate liquor (PHL) contains both mono- and oligo-sugars, lignin, acetic acid (HAc), furfural and some degradation products (Shen et al., 2011; Liu et al., 2011; Shi et al., 2012). Effectively recovering these components will make it viable for the biorefinery concept of dissolving pulp production process. Acetic acid is generated by the cleavage of acetyl groups in wood, and it is an important industrial commodity chemical with a total production of 6.94 million tons in 2009 (http://www.marketave⇑ Corresponding authors. Address: Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick. Canada E3B 5A3. Tel.: +1 5064516860 (G. Yang). E-mail addresses: [email protected] (G. Yang), [email protected] (M.S. Jahan), [email protected] (Y. Ni). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.03.164
nue.cn/upload/ChinaMarketReports/REPORTS_1212.htm). The largest use of acetic acid is as a feedstock for the manufacture of vinyl acetate monomer (VAM), followed by terephthalic acid (TPA) which is used for the manufacture of polyethylene terephthalate (PET) bottle resins and polyester fiber (Mirasol, 2009). Presently, acetic acid is mainly produced (65%) through methanol carbonylation using non-renewable feedstock. Therefore, recovery of acetic acid from the PHL as a valuable product, certainly fits into the forest biorefinery and can open a window for producing acetic acid from a renewable resource. Many methods for separating acetic acid from different industrial waste liquor have been reported such as solvent extraction, membrane separation, adsorption (Tamada and King, 1990a,b; Hou et al., 2008). Activated carbons and polymeric resins with functional groups were also used in the recovery of acetic acid (Husson and King, 1999); however, the adsorption capacity was limited. In the conventional solvent extraction process, the equilibrium distribution coefficient, KD for acetic acid extraction is 1 or
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less, therefore, was not economically attractive (Saha et al., 2000). The distribution coefficient, KD, is defined as the ratio of organic acid in the two phases by:
KD ¼
½HAorg ½HAaq
ð1Þ
It is known that ethyl acetate has a high dissolving power for acetic acid in comparison with other solvents, and many studies were carried out on acetic acid extraction using ethyl acetate (Garner and Ellis, 1953). But these process needs lot of solvent for a diluted HAc feed, such as PHL, therefore, regeneration of solvent is energy intensive. Another drawback of ethyl acetate is its partial miscibility with water. In view of these limitations of the conventional process, it is appropriate to explore an efficient extraction process that will provide high KD value. Some literature explored the use of some organophosphorous compounds and high-molecular-weight alkyl amine, which were found to have higher efficiency than the conventional solvent extraction for the extraction of diluted carboxylic acids (Wardell and King, 1978; Mahfud et al., 2008; Vitasari et al., 2012). The amine is dissolved in a diluent that dilutes the extractant to desire composition and control the viscosity and density. It has been found that the type of diluent and the solvent to diluents ratio can affect the extraction of carboxylic acid by amine (Bizek et al., 1993). Polar diluents have been shown to be more effective diluents than non polar ones due to high partitioning coefficients (Kumar and Babu, 2009). Polar diluents such as alcohols have been shown to be most suitable for amines because they give the highest distribution coefficients resulting from formation of solvents through specific hydrogen bonding between the proton of diluent and acid amine complex. Li et al. (2003) studied on the reactive extraction of aqueous solutions of different carboxylic acids with trioctylamine in various diluents, and the degrees of extraction of the medium-strong carboxylic acids with trioctylamine were in the order of 1-octanol P chloroform > tetrachloromethane > hexane. The strong acid provides a larger loading of trioctylamine than the weak acid. Tertiary amines, such as trioctyl amine (TOA) or triisooctyl amine (TIOA), appeared to be the better choice for the extraction process. In this study, TIOA was chosen as the extractant for the recovery of acetic acid and decanol was selected for diluting the extractant. TIOA was chosen due to its lower price than the TOA. Many factors such as nature of the acid extracted, concentrations of the acid and the extractant, and the type of diluent used can influence the extraction process. The industrially produced PHL sample is a unique source for HAc and it is different from those studied on the extraction of acetic acid from waste stream/fermentation broth, in particular, the HAc concentration is only at 1%. In this study, the recovering of HAc from the PHL was investigated by following the reactive extraction process using TIOA/ decanol. The different factors such as time, temperature, pH, diluation of TIOA with decanol and acid concentration have been studied for the extraction of acetic acid from the activated carbon (AC) pretreated pre-hydrolysis liquor (PHL). The characteristics of PHL after acetic acid extraction have also been studied. 2. Experimental 2.1. Materials The industrially produced PHL samples were collected from the bottom of digester (draining the whole digester after depressurizing) at a dissolving pulp mill in Eastern Canada. The PHL was filtered with Whatman qualitative filter papers (GE Healthcare UK Limited, UK) and Nylon 66 membrane with a pore size of
0.45 lm (Supelco analytical group, USA) for removing large particles and impurities. TIOA and decanol were purchased from Sigma–Aldrich Co. (USA). CR325 W activated carbon was obtained from Carbon Resources Inc. (CA). Sulfuric acid (4 N) was purchased from Fisher Scientific Inc. (USA) and diluted to 4 wt.% prior to use. Sodium hydroxide (60 wt.%) was obtained from Ricca Chemical Company (USA) and diluted to 5 wt.% for use. 2.2. Activated carbon treatment In removing/recovering the lignin present, the PHL was treated with activated carbon (AC) at room temperature and in a sonication bath for 30 min. The weight ratio of PHL to AC was 20:1. 2.3. Sugar analysis The hydrolysis condition was 1.78% (w/w) sulfuric acid concentrations in the PHL at 130 °C for 20 min (Yang et al., 2012). Hydrolysis of PHL was carried out in a Parr reactor. Required amount of sulfuric acid was added to PHL in an ample and sealed. Sealed ample was placed inside the parr reactor contained water which was put in an oil bath (Neslab Instruments, Inc., Portsmouth, NH, USA) to convert oligosaccharide to monosaccharide. The sugar concentrations were determined by using an Ion Chromatography unit equipped with CarboPac TM PA1 column (Dionex–300, Dionex Cooperation, Canada) and a pulsed amperometric detector (PAD). The sugar content in the PHL before the acid hydrolysis represented the monomeric form while the polymeric sugars were calculated from the difference with and without the acid hydrolysis. 2.4. Acetic acid and furfural analysis A Varian 300 NMR-spectrometer was employed for determining the furfural and acetic acid concentrations as described earlier (Ni and Kang, 2007). Calibration curves were made with the standard solutions of each component to determine the unknown concentration for each of these present in the PHL. The sample was prepared with D2O to sample volume ratio of 1:4. 2.5. Lignin analysis The lignin content in the PHL was determined following Tappi Useful Method (Tappi UM250). 2.6. Reactive extraction Required amount of the organic phase (extractants) and aqueous solution (PHL samples) were charged in flasks separately. The extraction was performed by stirring with magnetic bar at 200 and 400 rpm. The extraction time and temperature were varied from 2 to 20 min and from 20 to 50 °C, respectively. The pH value of the PHL-amine mixture was adjusted by adding a diluted NaOH solution. Then the extraction mixture was centrifuged at 3000 rpm for 5 min to assist separation of organic and aqueous phases, followed by 20 min in separating funnel. The two phases were analyzed immediately after the separation. Then the concentration of HAc in the organic phase was determined by mass balance (Eq. (2)).
ð½HAorg Þeq ¼
ð½HAaq V aq Þinitial ð½HAaq V aq Þeq ðV org Þaq
ð2Þ
where Vaq: volume of aqueous phase (mL) and Vorg: volume of organic phase (mL). For the evaluating extraction efficiencies of different runs, HAc recovery (%) and distribution coefficient (KD) were determined. The
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percent weight of HAc transferred from the aqueous phase into organic phase was expressed the percentage recovery of corresponding HAc (Eq. (3)).
% Recovery ¼
ð½HAorg V org Þaq 100% ð½HAaq V aq Þinitial
ð3Þ
The equilibrium distribution coefficient (KD) was defined the ratio of the HAc weight fraction of organic phase ([HA]orgVorg) to that of aqueous phase ([HA]aqVaq), at equilibrium (Eq. (4)) (Wardell and King, 1978).
KD ¼
½HAorg V org ½HAaq V aq
! ð4Þ eq
3. Results and discussion 3.1. Characteristics of the industrial produced PHL The properties and chemical composition of the initial PHL and the activated carbon treated PHL are shown in Table 1. The hydrolysis was done with 1.78% sulfuric acid at 130 °C for 20 min (Yang et al., 2012). It can be seen that solid content in the industrial PHL was 11%. As expected xylan/xylose was the major component in the PHL, it was about 58% of the total solid content. The lignin in the PHL was 13% of the total solid content. The sum of total sugars, lignin, acetic acid and furfural was less than the solid content of the PHL (Table 1). This was due to the loss of furfural and some of the acetic acid on drying the PHL and also ash was not accounted here. The lignin dissolved in the industrial PHL can be recovered for the production of valuable chemicals or generation of energy (Shen et al., 2011). But its presence in the PHL can hinder extractants to be recycled, therefore its removal and recovery has been studied based on the adsorption concept by using activated carbon (AC), or lime, or flocculation concept (Shen et al., 2011; Liu et al., 2011a; Shi et al., 2012). As shown in Table 1, activated carbon adsorbed 89% of the lignin and 86% of the furfural from the PHL, while sugars (main xylan/xylose) and acetic acid remained almost unchanged. Acetic acid is present in the PHL, which was generated from the bound acetyl group of hemicelluloses in the wood (Li et al., 2010). After AC pretreatment, the acetic acid content in the PHL was 1.15%. The dissolving pulp mill in Eastern Canada produces 450 m3 PHL per day, which can potentially generate 5.17 T acetic acid per day. 3.2. A proposed process scheme for recovering acetic acid in PHL with amine Fig. 1 shows an extraction process scheme for recovering acetic acid from the PHL. In the proposed system TIOA is used for extractTable 1 Compositions of industrial PHL.
Rhamnose Arabinose Galactose Glucose Xylose Mannose Total sugars Solid Lignin Acetic acid Furfural
Initial PHL (g/L)
Activated carbon adsorbed PHL (g/L)
Mono-
Oligo-
Mono-
Oligo-
0.71 1.47 0.79 0.94 5.63 0.43 9.95
0.67 0 1.59 4.33 46.91 4.69 58.2
0.7 1.39 0.77 0.87 5.53 0.41 9.67
0.59 0 1.64 4.01 43.61 5.43 55.28
111.33 14.95 11.91 1.62
91.1 1.69 11.53 0.22
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ing acetic acid from the PHL and decanol is chosen as diluent. In the first sage, PHL is pretreated with activated carbon (AC) at the AC to PHL ratio 1:20. Most of the lignin and furfural present in the PHL are adsorbed on AC (Liu et al., 2011a), which can hinder the recovery of extractants (Grzenia et al., 2010). After AC pretreatment, the activated carbon can be regenerated. The thermal regeneration/reactivation (Gütsch and Sixta, 2012; Moreno-Castilla et al., 1995) might be a preferred commercializable process, in which high-heating-value lignin may be utilized as a fuel source. In a recent study, the regeneration of a lignin-enriched activated carbon was conducted via either solvent extraction or thermal regeneration, and thermal regeneration was found to be a more effective process (Gütsch and Sixta, 2012). On the other hand, the activated carbon may be alternatively treated to recover the dissolved organics, mainly lignin, for the production of platform chemicals or materials. The AC pretreated PHL contains mainly sugars and acetic acid, which are mixed with extractants (TIOA) diluted with dacanol. The formed HAc–amine complex is transferred to the organic phase. Aqueous phase contains sugars only that will be further used as feed stock for biochemicals/biomaterials/bioenergy production process. The HAc–amine complex containing organic phase is transferred to regeneration unit, where the separation of acetic acid and regeneration of extractant and diluents are done. One approach of such processes was proposed by Poole and King (1991). Acetic acid can be back-extracted by temperature swing (Baniel et al., 1981; Han et al., 2000), diluent swing, pH swing (Ma et al., 2006). 3.3. Effect of TIOA dosage and dilution The amount of TIOA and the diluents amount were the important factor in the extraction process. The results of TIOA extraction at different molar ratio and dilution are shown in Table 2. It can be seen that the acetic acid recovery was increased with increasing the molar ratio of TIOA to acetic acid, as a result, the equilibrium distribution coefficient, KD increased. The extraction efficiency of pure TIOA was very low, and the maximum acetic acid recovery was only 16% at the molar ratio of 2, where distribution coefficient was 0.19 only. The TIOA dilution with decanol increased the HAc extraction. At the molar ratio of 1, the extraction of acetic acid was increased from 10.34% to 66.60% with the TIOA dilution to 20%. At the same dilution, the HAc extraction further increased to 79.50% with the increase of molar ratio to 2. All distribution coefficient KD values at different molar ratio were above 1.9 at 20% TIOA dilution. It was obvious that TIOA dilution had a significant effect on the extraction efficiency. It was proposed that the formation of an acid–amine complex is promoted by the dipole–dipole interactions between diluent and complex; and/or by the complex-diluent hydrogen bond (Bizek et al., 1993). The results in Table 2 were consistent with those reported by others (Li et al., 1996; Mahfud et al., 2008; Vitasari et al., 2012). Hong et al. (2001) found that the extraction power of tertiary amine increased in an active diluent like octanol. Active diluent which has polar functional group can interact strongly with the acid–amine complex (Tamada and King, 1990a,b). Katikaneni and Cheryan (2002) has reported that without diluent Alamine 336 (C8–C10) had extraction efficiency of only 36%, while with octanol it was 75%. Compared to TIOA dilution, the molar ratio of TIOA to acetic acid had lower influence. At 20% TIOA dilution, the recovery of acetic acid was slightly increased at the molar ratio of 4 (data not shown). So the molar ratio of 1 was chosen for the optimal ratio to decrease the extractants cost. The extraction efficiency was decreased with the increase of TIOA dilution more than 20% as shown in Table 2. This optimum is probably related to the number of amine ion pairs formed complex with acetic acid. At low levels of
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Fig. 1. Proposed process for recovering acetic acid from industrial PHL based on the reactive extraction using TIOA/decanol.
Table 2 Effect of TIOA dilution and molar ratio on the recovery of acetic acid from the activated carbon pre-treated PHL (Extraction time, temperature and pH were 10 min, 20 °C and 4.5 respectively). TIOA dilution (%)
Molar ratio of TIOA to acetic acid 1
0 20 25 30 35
1.5
2
Recovery (%)
KD
Recovery (%)
KD
Recovery (%)
KD
10.34 66.6 65.9 65.73 62.46
0.12 1.95 1.93 1.92 1.66
13.19 76.16 72.3 70.35 68.62
0.15 3.2 2.61 2.42 2.19
15.99 79.5 77.94 76.69 72.48
0.19 3.88 3.53 3.29 2.63
diluation (20–30%), increasing amine concentration would increase the interaction between the amine and the acid; the polarity of the solvent system would remain high as the diluent would be in excess. However, the optimum composition of the solvent system depends on the diluent. For the extraction of acetic acid by Alamine-336, Yang et al. (1991) observed that the optimum concentration of dilution was 50% for 2-octanol, whereas Kertes and King (1986) obtained the optimum concentration for octanol at 35%. The optimal recovery was obtained at the TIOA dilution range of 20–30% (Table 2). This TIOA diluation might provide a lower solubility in aqueous phase, increase the amine–acid interaction and increase polarity of the solvent (Wardell and King, 1978). The above results support the conclusion that the diluent for amine had a significant effect on extraction equilibrium (Vitasari et al., 2012; Hong and Hong, 2005; Keshav et al., 2008). 3.4. Effect of time, temperature and pH Effects of time, temperature and pH in the extraction process were explored to obtain maximum extraction efficiency. The
extraction was carried out with TIOA at 30% dilution at 1 M ratio with varying time, temperature and pH and the results are shown in Table 3. It can be seen from Table 3 that the recovery of acetic acid from the AC pretreated PHL was reached to equilibrium within a short time. Within 2 min, recovery reached to 65.03, followed by only slight increase of acetic acid recovery. Similar reaction kinetics was also observed for citric acid extraction by tertiary amine (Koparan et al., 2001). Similarly, a fast acetic acid extraction was observed from the bio-oil by diluted TOA (Mahfud et al., 2008). Table 3 also shows that the temperature had a negative effect on the extraction efficiency. The acetic acid recovery was decreased from 65.73% to 57.34% with the increase of temperature from 20 to 50 °C. It may be explained by the instability of acetic acid–amine complex at high temperature (Tamada and King, 1990a,b). This can also be explained by shifting of extraction equilibrium caused by temperature (Baniel et al., 1981). Amines extract acids from the aqueous phase by forming an acid–base complex with the undissociated acid (Kertes and King, 1986). Since the concentration of undissociated acid is a function of the pH, the extraction of organic acids will greatly depend on
Table 3 Effect of time, temperature and pH on the recovery of acetic acid from the activated carbon pre-treated PHL (molar ratio of TIOA to acetic acid of 1, 30% TIOA dilution). Time (min)
Temperature (°C)
pH
Acetic acid recovery (%)
Distribution coefficient (KD)
2 4 8 12 16 20 10 10 10 10 10 10 10 10 10
20 20 20 20 20 20 20 30 40 50 20 20 20 20 20
4.5 (original) 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 5 5.5 6 7
65.03 65.34 65.5 65.99 65.64 67.9 65.13 63.36 61.78 57.34 65.13 64.51 64.17 63.01 59.92
1.86 1.89 1.9 1.94 1.91 2.16 1.87 1.72 1.62 1.34 1.87 1.82 1.79 1.7 1.5
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the pH of the aqueous phase. It also can be seen from Table 3 that increase of pH negatively affected the recovery of acetic acid compared to the initial mixture pH of 4.5. Yang et al. (1991) showed that the KD value for the extraction of acetic acid with aliphatic amine decreased with the increase of pH. Katikaneni and Cheryan (2002) studied acetic acid extraction with the Alamine-336/2ethyl-1-hexanol (1:1) from the fermentation broth solvent system and an extraction efficiency of 17% was reported, while the extraction efficiency increased to 85% with the model system. This difference can be explained by the differences in pH of the fermentation broth (pH 6.5) and the model system (pH 3.3). In summary, it can be concluded that the extraction process of acetic acid from PHL into diluted TIOA could be completed within a short time, and the increase of temperature and pH negatively affected the recovery of acetic acid from the PHL. 3.5. Effect of acetic acid concentration
Table 4 Compositions of the TIOA/decanol extracted PHL (30% TIOA dilution, 1 M ratio of TIOA to acetic acid for PHL1 and 2 M ratio of TIOA to acetic acid for PHL2). g/L
PHL1
PHL2
Solid Lignin Acetic acid Furfural
90.84 1.18 3.59 0.21
88.56 1.12 3.03 0.21
Rhamnose Arabinose Galactose Glucose Xylose Mannose Total sugars
1.22 1.3 2.3 4.86 48.31 5.86 63.85
1.31 1.46 2.47 4.77 48.4 5.97 64.38
4. Conclusions
Acetic acid concentration had a positive effect on the extraction efficiency (Wardell and King, 1978; Vitasari et al., 2012.). The AC pretreated PHL was evaporated to obtain different acetic acid concentrations of 16, 20, 24 and 28 g/L and the extraction process was carried out with 30% TIOA dilution at the TIOA/HAc molar ratio of 1 and at room temperature and the results are shown in Fig. 2. The recovery of acetic acid increased with an increase in the acetic acid concentration. The explanation is as follows: increasing the acetic acid concentration may decrease the PHL pH, which in return, increased the HAc extraction.
As the diluent for triisooctylamine (TIOA), decanol had a significant effect on extraction equilibrium. The diluted TIOA can offer a high extraction efficiency, which increased the recovery of acetic acid. The diluted TIOA gave much higher equilibrium distribution coefficient KD than the pure TIOA. The increase of temperature and pH negatively affected the HAc extraction process from the PHL. In this extraction process, sugars/oligo-sugars contents in the PHL were almost unaffected. Finally, it can be concluded that activated carbon pretreatment followed by reactive extraction for recovering HAc can purify PHL for further utilization of sugars in fermentation/biomaterial/bio-based chemicals.
3.6. Characteristics of the TIOA/decanol extracted PHL
Acknowledgements
The characteristics of the extracted PHL are crucial for the successful implementation of the biorefinery. Table 4 shows the chemical compositions of the AC pretreated PHL extracted by 30% TIOA dilution at the molar ratio of 1 and 2. As shown in Tables 1 and 4, the total solid and sugars contents of the extracted PHL remained almost constant. It can also be found that the recovery of lignin and acetic acid from the industrial PHL by activated carbon adsorption and amine extraction, respectively, not only can recover the potential energy (lignin has high heat value) and valuable products (HAc) to earn income but also be beneficial to the membrane- based concentration of sugars/oligo-sugars for the production of biomaterials/biochemicals/bioenergy.
This project was funded by an NSERC CRD grant, the National Science Foundation of China (Grant Nos. 31070525, 31270627), and the Prior Special Study of 973 Program (Grant No. 2011CB211705).
Fig. 2. Effect of the acetic acid concentration on the recovery of acetic acid (TIOA to HAc molar ratio of 1, 30% TIOA in decanol, temperature, other conditions).
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Saha, G., Chopade, S.P., Mahajani, S.M., 2000. Recovery of dilute acetic acid through esterification in a reactive distillation column. Catal. Today 16, 147–157. Shen, J., Fatehi, P., Soleimani, P., Ni, Y., 2011. Recovery of lignocellulose from prehydrolysis liquor in the lime kiln of kraft-based dissolving pulp production process by adsorption to lime mud. Bioresour. Technol. 102, 10035–10039. Shi, H., Fatehi, P., Xiao, H., Ni, Y., 2012. Optimizing the poly ethylene oxide flocculation process for isolating lignin of prehydrolysis liquor of a kraft-based dissolving pulp production process. Ind. Eng. Chem. Res. 51, 5330–5335. Tamada, J.A., King, C.J., 1990a. Extraction of carboxylic acids with amine extractants. 2. Chemical interactions and interpretation of data. Ind. Eng. Chem. Res. 29 (7), 1327–1333. Tamada, J.A., King, C.J., 1990b. Extraction of carboxylic acids with amine extractants. 3. Effect of temperature, water coextraction, and process considerations. Ind. Eng. Chem. Res. 29 (7), 1333–1338. Vitasari, C.R., Meindersma, G.W., De Haan, A.B., 2012. Glycolaldehyde co-extraction during the reactive extraction of acetic acid with tri-n-octylamine/2-ethyl-1hexanol from a wood-based pyrolysis oil-derived aqueous phase. Sep. Purif. Technol. 95, 39–43. Wardell, J.M., King, C.J., 1978. Solvent equilibria for extraction of carboxylic acid from water. J. Chem. Eng. Data 23, 144–148. Yang, G., Jahan, M.S., Liu, H., Ni, Y., 2012. Acid hydrolysis of pre-hydrolysis liquor produced from the kraft-based dissolving pulp production process. Ind. Eng. Chem. Res. 51, 13902–13907. Yang, S., White, S.A., Hsu, S., 1991. Extraction of carboxylic acids with tertiary and quaternary amines: effect of pH. Ind. Eng. Chem. Res. 30, 1335–1342.
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Recovery of acetic acid from pre-hydrolysis liquor of hardwood kraft-based dissolving pulp production process by reactive extraction with triisooctylamine G. Yang a,b,⇑, M. Sarwar Jahan b,c,⇑, Laboni Ahsan b, Linqiang Zheng b, Yonghao Ni b,⇑ a b c
Key Laboratory of Pulp & Paper Science and Technology (Shandong Polytechnic University), Ministry of Education, Jinan 250353, Shandong, China Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dhaka 1205, Bangladesh
h i g h l i g h t s Acetic acid was recovered from the PHL by reactive extraction with triisooctylamine. Diluent played an important role in recovering acetic acid from the PHL. Temperature and pH negatively affected the acetic acid extraction. Acetic acid concentration positively affected the acetic acid extraction.
a r t i c l e
i n f o
Article history: Received 29 December 2012 Received in revised form 22 March 2013 Accepted 24 March 2013 Available online 31 March 2013 Keywords: Pre-hydrolysis liquor Acetic acid Reactive extraction Triisooctylamine Distribution coefficient
a b s t r a c t Acetic acid was one of the main compositions of the pre-hydrolysis liquor (PHL), which was recovered by reactive extraction with triisooctylamine (TIOA) diluted with decanol. Dilution of TIOA played an important role in extracting acetic acid from the PHL. The recovery of acetic acid from the PHL by TIOA was increased from 10.34% to 66.60% with the dilution of TIOA to 20% by decanol at the HAc to TIOA molar ratio of 1, consequently, the equilibrium distribution coefficient KD increased. The effects of time, temperature and pH on the extraction process were also studied. The extraction process was very fast. The acetic acid extraction decreased from 65.13% to 57.34% with the rise of temperature to 50 °C from 20 °C. A higher pH increased the dissociation of acetic acid, as a result, decreased acetic acid extraction. The hemicelluloses in the PHL were unaffected on the extraction process of acetic acid. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Pre-hydrolysis is carried out in the kraft-based dissolving pulp process to remove hemicelluloses from the lignocellulose. The pre-hydrolysate liquor (PHL) contains both mono- and oligo-sugars, lignin, acetic acid (HAc), furfural and some degradation products (Shen et al., 2011; Liu et al., 2011; Shi et al., 2012). Effectively recovering these components will make it viable for the biorefinery concept of dissolving pulp production process. Acetic acid is generated by the cleavage of acetyl groups in wood, and it is an important industrial commodity chemical with a total production of 6.94 million tons in 2009 (http://www.marketave⇑ Corresponding authors. Address: Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick. Canada E3B 5A3. Tel.: +1 5064516860 (G. Yang). E-mail addresses: [email protected] (G. Yang), [email protected] (M.S. Jahan), [email protected] (Y. Ni). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.03.164
nue.cn/upload/ChinaMarketReports/REPORTS_1212.htm). The largest use of acetic acid is as a feedstock for the manufacture of vinyl acetate monomer (VAM), followed by terephthalic acid (TPA) which is used for the manufacture of polyethylene terephthalate (PET) bottle resins and polyester fiber (Mirasol, 2009). Presently, acetic acid is mainly produced (65%) through methanol carbonylation using non-renewable feedstock. Therefore, recovery of acetic acid from the PHL as a valuable product, certainly fits into the forest biorefinery and can open a window for producing acetic acid from a renewable resource. Many methods for separating acetic acid from different industrial waste liquor have been reported such as solvent extraction, membrane separation, adsorption (Tamada and King, 1990a,b; Hou et al., 2008). Activated carbons and polymeric resins with functional groups were also used in the recovery of acetic acid (Husson and King, 1999); however, the adsorption capacity was limited. In the conventional solvent extraction process, the equilibrium distribution coefficient, KD for acetic acid extraction is 1 or
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less, therefore, was not economically attractive (Saha et al., 2000). The distribution coefficient, KD, is defined as the ratio of organic acid in the two phases by:
KD ¼
½HAorg ½HAaq
ð1Þ
It is known that ethyl acetate has a high dissolving power for acetic acid in comparison with other solvents, and many studies were carried out on acetic acid extraction using ethyl acetate (Garner and Ellis, 1953). But these process needs lot of solvent for a diluted HAc feed, such as PHL, therefore, regeneration of solvent is energy intensive. Another drawback of ethyl acetate is its partial miscibility with water. In view of these limitations of the conventional process, it is appropriate to explore an efficient extraction process that will provide high KD value. Some literature explored the use of some organophosphorous compounds and high-molecular-weight alkyl amine, which were found to have higher efficiency than the conventional solvent extraction for the extraction of diluted carboxylic acids (Wardell and King, 1978; Mahfud et al., 2008; Vitasari et al., 2012). The amine is dissolved in a diluent that dilutes the extractant to desire composition and control the viscosity and density. It has been found that the type of diluent and the solvent to diluents ratio can affect the extraction of carboxylic acid by amine (Bizek et al., 1993). Polar diluents have been shown to be more effective diluents than non polar ones due to high partitioning coefficients (Kumar and Babu, 2009). Polar diluents such as alcohols have been shown to be most suitable for amines because they give the highest distribution coefficients resulting from formation of solvents through specific hydrogen bonding between the proton of diluent and acid amine complex. Li et al. (2003) studied on the reactive extraction of aqueous solutions of different carboxylic acids with trioctylamine in various diluents, and the degrees of extraction of the medium-strong carboxylic acids with trioctylamine were in the order of 1-octanol P chloroform > tetrachloromethane > hexane. The strong acid provides a larger loading of trioctylamine than the weak acid. Tertiary amines, such as trioctyl amine (TOA) or triisooctyl amine (TIOA), appeared to be the better choice for the extraction process. In this study, TIOA was chosen as the extractant for the recovery of acetic acid and decanol was selected for diluting the extractant. TIOA was chosen due to its lower price than the TOA. Many factors such as nature of the acid extracted, concentrations of the acid and the extractant, and the type of diluent used can influence the extraction process. The industrially produced PHL sample is a unique source for HAc and it is different from those studied on the extraction of acetic acid from waste stream/fermentation broth, in particular, the HAc concentration is only at 1%. In this study, the recovering of HAc from the PHL was investigated by following the reactive extraction process using TIOA/ decanol. The different factors such as time, temperature, pH, diluation of TIOA with decanol and acid concentration have been studied for the extraction of acetic acid from the activated carbon (AC) pretreated pre-hydrolysis liquor (PHL). The characteristics of PHL after acetic acid extraction have also been studied. 2. Experimental 2.1. Materials The industrially produced PHL samples were collected from the bottom of digester (draining the whole digester after depressurizing) at a dissolving pulp mill in Eastern Canada. The PHL was filtered with Whatman qualitative filter papers (GE Healthcare UK Limited, UK) and Nylon 66 membrane with a pore size of
0.45 lm (Supelco analytical group, USA) for removing large particles and impurities. TIOA and decanol were purchased from Sigma–Aldrich Co. (USA). CR325 W activated carbon was obtained from Carbon Resources Inc. (CA). Sulfuric acid (4 N) was purchased from Fisher Scientific Inc. (USA) and diluted to 4 wt.% prior to use. Sodium hydroxide (60 wt.%) was obtained from Ricca Chemical Company (USA) and diluted to 5 wt.% for use. 2.2. Activated carbon treatment In removing/recovering the lignin present, the PHL was treated with activated carbon (AC) at room temperature and in a sonication bath for 30 min. The weight ratio of PHL to AC was 20:1. 2.3. Sugar analysis The hydrolysis condition was 1.78% (w/w) sulfuric acid concentrations in the PHL at 130 °C for 20 min (Yang et al., 2012). Hydrolysis of PHL was carried out in a Parr reactor. Required amount of sulfuric acid was added to PHL in an ample and sealed. Sealed ample was placed inside the parr reactor contained water which was put in an oil bath (Neslab Instruments, Inc., Portsmouth, NH, USA) to convert oligosaccharide to monosaccharide. The sugar concentrations were determined by using an Ion Chromatography unit equipped with CarboPac TM PA1 column (Dionex–300, Dionex Cooperation, Canada) and a pulsed amperometric detector (PAD). The sugar content in the PHL before the acid hydrolysis represented the monomeric form while the polymeric sugars were calculated from the difference with and without the acid hydrolysis. 2.4. Acetic acid and furfural analysis A Varian 300 NMR-spectrometer was employed for determining the furfural and acetic acid concentrations as described earlier (Ni and Kang, 2007). Calibration curves were made with the standard solutions of each component to determine the unknown concentration for each of these present in the PHL. The sample was prepared with D2O to sample volume ratio of 1:4. 2.5. Lignin analysis The lignin content in the PHL was determined following Tappi Useful Method (Tappi UM250). 2.6. Reactive extraction Required amount of the organic phase (extractants) and aqueous solution (PHL samples) were charged in flasks separately. The extraction was performed by stirring with magnetic bar at 200 and 400 rpm. The extraction time and temperature were varied from 2 to 20 min and from 20 to 50 °C, respectively. The pH value of the PHL-amine mixture was adjusted by adding a diluted NaOH solution. Then the extraction mixture was centrifuged at 3000 rpm for 5 min to assist separation of organic and aqueous phases, followed by 20 min in separating funnel. The two phases were analyzed immediately after the separation. Then the concentration of HAc in the organic phase was determined by mass balance (Eq. (2)).
ð½HAorg Þeq ¼
ð½HAaq V aq Þinitial ð½HAaq V aq Þeq ðV org Þaq
ð2Þ
where Vaq: volume of aqueous phase (mL) and Vorg: volume of organic phase (mL). For the evaluating extraction efficiencies of different runs, HAc recovery (%) and distribution coefficient (KD) were determined. The
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percent weight of HAc transferred from the aqueous phase into organic phase was expressed the percentage recovery of corresponding HAc (Eq. (3)).
% Recovery ¼
ð½HAorg V org Þaq 100% ð½HAaq V aq Þinitial
ð3Þ
The equilibrium distribution coefficient (KD) was defined the ratio of the HAc weight fraction of organic phase ([HA]orgVorg) to that of aqueous phase ([HA]aqVaq), at equilibrium (Eq. (4)) (Wardell and King, 1978).
KD ¼
½HAorg V org ½HAaq V aq
! ð4Þ eq
3. Results and discussion 3.1. Characteristics of the industrial produced PHL The properties and chemical composition of the initial PHL and the activated carbon treated PHL are shown in Table 1. The hydrolysis was done with 1.78% sulfuric acid at 130 °C for 20 min (Yang et al., 2012). It can be seen that solid content in the industrial PHL was 11%. As expected xylan/xylose was the major component in the PHL, it was about 58% of the total solid content. The lignin in the PHL was 13% of the total solid content. The sum of total sugars, lignin, acetic acid and furfural was less than the solid content of the PHL (Table 1). This was due to the loss of furfural and some of the acetic acid on drying the PHL and also ash was not accounted here. The lignin dissolved in the industrial PHL can be recovered for the production of valuable chemicals or generation of energy (Shen et al., 2011). But its presence in the PHL can hinder extractants to be recycled, therefore its removal and recovery has been studied based on the adsorption concept by using activated carbon (AC), or lime, or flocculation concept (Shen et al., 2011; Liu et al., 2011a; Shi et al., 2012). As shown in Table 1, activated carbon adsorbed 89% of the lignin and 86% of the furfural from the PHL, while sugars (main xylan/xylose) and acetic acid remained almost unchanged. Acetic acid is present in the PHL, which was generated from the bound acetyl group of hemicelluloses in the wood (Li et al., 2010). After AC pretreatment, the acetic acid content in the PHL was 1.15%. The dissolving pulp mill in Eastern Canada produces 450 m3 PHL per day, which can potentially generate 5.17 T acetic acid per day. 3.2. A proposed process scheme for recovering acetic acid in PHL with amine Fig. 1 shows an extraction process scheme for recovering acetic acid from the PHL. In the proposed system TIOA is used for extractTable 1 Compositions of industrial PHL.
Rhamnose Arabinose Galactose Glucose Xylose Mannose Total sugars Solid Lignin Acetic acid Furfural
Initial PHL (g/L)
Activated carbon adsorbed PHL (g/L)
Mono-
Oligo-
Mono-
Oligo-
0.71 1.47 0.79 0.94 5.63 0.43 9.95
0.67 0 1.59 4.33 46.91 4.69 58.2
0.7 1.39 0.77 0.87 5.53 0.41 9.67
0.59 0 1.64 4.01 43.61 5.43 55.28
111.33 14.95 11.91 1.62
91.1 1.69 11.53 0.22
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ing acetic acid from the PHL and decanol is chosen as diluent. In the first sage, PHL is pretreated with activated carbon (AC) at the AC to PHL ratio 1:20. Most of the lignin and furfural present in the PHL are adsorbed on AC (Liu et al., 2011a), which can hinder the recovery of extractants (Grzenia et al., 2010). After AC pretreatment, the activated carbon can be regenerated. The thermal regeneration/reactivation (Gütsch and Sixta, 2012; Moreno-Castilla et al., 1995) might be a preferred commercializable process, in which high-heating-value lignin may be utilized as a fuel source. In a recent study, the regeneration of a lignin-enriched activated carbon was conducted via either solvent extraction or thermal regeneration, and thermal regeneration was found to be a more effective process (Gütsch and Sixta, 2012). On the other hand, the activated carbon may be alternatively treated to recover the dissolved organics, mainly lignin, for the production of platform chemicals or materials. The AC pretreated PHL contains mainly sugars and acetic acid, which are mixed with extractants (TIOA) diluted with dacanol. The formed HAc–amine complex is transferred to the organic phase. Aqueous phase contains sugars only that will be further used as feed stock for biochemicals/biomaterials/bioenergy production process. The HAc–amine complex containing organic phase is transferred to regeneration unit, where the separation of acetic acid and regeneration of extractant and diluents are done. One approach of such processes was proposed by Poole and King (1991). Acetic acid can be back-extracted by temperature swing (Baniel et al., 1981; Han et al., 2000), diluent swing, pH swing (Ma et al., 2006). 3.3. Effect of TIOA dosage and dilution The amount of TIOA and the diluents amount were the important factor in the extraction process. The results of TIOA extraction at different molar ratio and dilution are shown in Table 2. It can be seen that the acetic acid recovery was increased with increasing the molar ratio of TIOA to acetic acid, as a result, the equilibrium distribution coefficient, KD increased. The extraction efficiency of pure TIOA was very low, and the maximum acetic acid recovery was only 16% at the molar ratio of 2, where distribution coefficient was 0.19 only. The TIOA dilution with decanol increased the HAc extraction. At the molar ratio of 1, the extraction of acetic acid was increased from 10.34% to 66.60% with the TIOA dilution to 20%. At the same dilution, the HAc extraction further increased to 79.50% with the increase of molar ratio to 2. All distribution coefficient KD values at different molar ratio were above 1.9 at 20% TIOA dilution. It was obvious that TIOA dilution had a significant effect on the extraction efficiency. It was proposed that the formation of an acid–amine complex is promoted by the dipole–dipole interactions between diluent and complex; and/or by the complex-diluent hydrogen bond (Bizek et al., 1993). The results in Table 2 were consistent with those reported by others (Li et al., 1996; Mahfud et al., 2008; Vitasari et al., 2012). Hong et al. (2001) found that the extraction power of tertiary amine increased in an active diluent like octanol. Active diluent which has polar functional group can interact strongly with the acid–amine complex (Tamada and King, 1990a,b). Katikaneni and Cheryan (2002) has reported that without diluent Alamine 336 (C8–C10) had extraction efficiency of only 36%, while with octanol it was 75%. Compared to TIOA dilution, the molar ratio of TIOA to acetic acid had lower influence. At 20% TIOA dilution, the recovery of acetic acid was slightly increased at the molar ratio of 4 (data not shown). So the molar ratio of 1 was chosen for the optimal ratio to decrease the extractants cost. The extraction efficiency was decreased with the increase of TIOA dilution more than 20% as shown in Table 2. This optimum is probably related to the number of amine ion pairs formed complex with acetic acid. At low levels of
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Fig. 1. Proposed process for recovering acetic acid from industrial PHL based on the reactive extraction using TIOA/decanol.
Table 2 Effect of TIOA dilution and molar ratio on the recovery of acetic acid from the activated carbon pre-treated PHL (Extraction time, temperature and pH were 10 min, 20 °C and 4.5 respectively). TIOA dilution (%)
Molar ratio of TIOA to acetic acid 1
0 20 25 30 35
1.5
2
Recovery (%)
KD
Recovery (%)
KD
Recovery (%)
KD
10.34 66.6 65.9 65.73 62.46
0.12 1.95 1.93 1.92 1.66
13.19 76.16 72.3 70.35 68.62
0.15 3.2 2.61 2.42 2.19
15.99 79.5 77.94 76.69 72.48
0.19 3.88 3.53 3.29 2.63
diluation (20–30%), increasing amine concentration would increase the interaction between the amine and the acid; the polarity of the solvent system would remain high as the diluent would be in excess. However, the optimum composition of the solvent system depends on the diluent. For the extraction of acetic acid by Alamine-336, Yang et al. (1991) observed that the optimum concentration of dilution was 50% for 2-octanol, whereas Kertes and King (1986) obtained the optimum concentration for octanol at 35%. The optimal recovery was obtained at the TIOA dilution range of 20–30% (Table 2). This TIOA diluation might provide a lower solubility in aqueous phase, increase the amine–acid interaction and increase polarity of the solvent (Wardell and King, 1978). The above results support the conclusion that the diluent for amine had a significant effect on extraction equilibrium (Vitasari et al., 2012; Hong and Hong, 2005; Keshav et al., 2008). 3.4. Effect of time, temperature and pH Effects of time, temperature and pH in the extraction process were explored to obtain maximum extraction efficiency. The
extraction was carried out with TIOA at 30% dilution at 1 M ratio with varying time, temperature and pH and the results are shown in Table 3. It can be seen from Table 3 that the recovery of acetic acid from the AC pretreated PHL was reached to equilibrium within a short time. Within 2 min, recovery reached to 65.03, followed by only slight increase of acetic acid recovery. Similar reaction kinetics was also observed for citric acid extraction by tertiary amine (Koparan et al., 2001). Similarly, a fast acetic acid extraction was observed from the bio-oil by diluted TOA (Mahfud et al., 2008). Table 3 also shows that the temperature had a negative effect on the extraction efficiency. The acetic acid recovery was decreased from 65.73% to 57.34% with the increase of temperature from 20 to 50 °C. It may be explained by the instability of acetic acid–amine complex at high temperature (Tamada and King, 1990a,b). This can also be explained by shifting of extraction equilibrium caused by temperature (Baniel et al., 1981). Amines extract acids from the aqueous phase by forming an acid–base complex with the undissociated acid (Kertes and King, 1986). Since the concentration of undissociated acid is a function of the pH, the extraction of organic acids will greatly depend on
Table 3 Effect of time, temperature and pH on the recovery of acetic acid from the activated carbon pre-treated PHL (molar ratio of TIOA to acetic acid of 1, 30% TIOA dilution). Time (min)
Temperature (°C)
pH
Acetic acid recovery (%)
Distribution coefficient (KD)
2 4 8 12 16 20 10 10 10 10 10 10 10 10 10
20 20 20 20 20 20 20 30 40 50 20 20 20 20 20
4.5 (original) 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 5 5.5 6 7
65.03 65.34 65.5 65.99 65.64 67.9 65.13 63.36 61.78 57.34 65.13 64.51 64.17 63.01 59.92
1.86 1.89 1.9 1.94 1.91 2.16 1.87 1.72 1.62 1.34 1.87 1.82 1.79 1.7 1.5
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the pH of the aqueous phase. It also can be seen from Table 3 that increase of pH negatively affected the recovery of acetic acid compared to the initial mixture pH of 4.5. Yang et al. (1991) showed that the KD value for the extraction of acetic acid with aliphatic amine decreased with the increase of pH. Katikaneni and Cheryan (2002) studied acetic acid extraction with the Alamine-336/2ethyl-1-hexanol (1:1) from the fermentation broth solvent system and an extraction efficiency of 17% was reported, while the extraction efficiency increased to 85% with the model system. This difference can be explained by the differences in pH of the fermentation broth (pH 6.5) and the model system (pH 3.3). In summary, it can be concluded that the extraction process of acetic acid from PHL into diluted TIOA could be completed within a short time, and the increase of temperature and pH negatively affected the recovery of acetic acid from the PHL. 3.5. Effect of acetic acid concentration
Table 4 Compositions of the TIOA/decanol extracted PHL (30% TIOA dilution, 1 M ratio of TIOA to acetic acid for PHL1 and 2 M ratio of TIOA to acetic acid for PHL2). g/L
PHL1
PHL2
Solid Lignin Acetic acid Furfural
90.84 1.18 3.59 0.21
88.56 1.12 3.03 0.21
Rhamnose Arabinose Galactose Glucose Xylose Mannose Total sugars
1.22 1.3 2.3 4.86 48.31 5.86 63.85
1.31 1.46 2.47 4.77 48.4 5.97 64.38
4. Conclusions
Acetic acid concentration had a positive effect on the extraction efficiency (Wardell and King, 1978; Vitasari et al., 2012.). The AC pretreated PHL was evaporated to obtain different acetic acid concentrations of 16, 20, 24 and 28 g/L and the extraction process was carried out with 30% TIOA dilution at the TIOA/HAc molar ratio of 1 and at room temperature and the results are shown in Fig. 2. The recovery of acetic acid increased with an increase in the acetic acid concentration. The explanation is as follows: increasing the acetic acid concentration may decrease the PHL pH, which in return, increased the HAc extraction.
As the diluent for triisooctylamine (TIOA), decanol had a significant effect on extraction equilibrium. The diluted TIOA can offer a high extraction efficiency, which increased the recovery of acetic acid. The diluted TIOA gave much higher equilibrium distribution coefficient KD than the pure TIOA. The increase of temperature and pH negatively affected the HAc extraction process from the PHL. In this extraction process, sugars/oligo-sugars contents in the PHL were almost unaffected. Finally, it can be concluded that activated carbon pretreatment followed by reactive extraction for recovering HAc can purify PHL for further utilization of sugars in fermentation/biomaterial/bio-based chemicals.
3.6. Characteristics of the TIOA/decanol extracted PHL
Acknowledgements
The characteristics of the extracted PHL are crucial for the successful implementation of the biorefinery. Table 4 shows the chemical compositions of the AC pretreated PHL extracted by 30% TIOA dilution at the molar ratio of 1 and 2. As shown in Tables 1 and 4, the total solid and sugars contents of the extracted PHL remained almost constant. It can also be found that the recovery of lignin and acetic acid from the industrial PHL by activated carbon adsorption and amine extraction, respectively, not only can recover the potential energy (lignin has high heat value) and valuable products (HAc) to earn income but also be beneficial to the membrane- based concentration of sugars/oligo-sugars for the production of biomaterials/biochemicals/bioenergy.
This project was funded by an NSERC CRD grant, the National Science Foundation of China (Grant Nos. 31070525, 31270627), and the Prior Special Study of 973 Program (Grant No. 2011CB211705).
Fig. 2. Effect of the acetic acid concentration on the recovery of acetic acid (TIOA to HAc molar ratio of 1, 30% TIOA in decanol, temperature, other conditions).
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