ARTICLE pubs.acs.org/IECR
Adsorption of Lignocelluloses Dissolved in Prehydrolysis Liquor of Kraft-Based Dissolving Pulp Process on Oxidized Activated Carbons Xin Liu,† Pedram Fatehi,*,†,‡ and Yonghao Ni† † ‡
Chemical Engineering Department, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 Chemical Engineering Department, Lakehead University, Thunder Bay, Ontario, Canada P7B 5E1 ABSTRACT: The prehydrolysis stage is essential in producing high-purity cellulose from the kraft-based process and is effective in removing hemicelluloses from wood chips. To convert hemicelluloses and other dissolved materials of the prehydrolysis liquor (PHL) into value-added products, they must first be recovered from the PHL. In this work, we investigated the use of activated carbon (AC) to adsorb such materials from the PHL. The adsorption of hemicelluloses on unmodified AC was significantly higher in neutral than acidic environment. To improve the adsorption performance of AC, the AC was oxidized with H2O2 or H2SO4. The H2O2- and H2SO4-modified ACs prepared under milder conditions had higher capacities than those prepared under stronger conditions for adsorbing hemicelluloses, lignin, and furfural. Additionally, the modifications increased the adsorption rate of dissolved materials on the AC, especially that of hemicelluloses. Oxidized activated carbon is a promising adsorbent for lignocellulosic materials of the PHL that can be employed in biorefinery processes conducted during the kraft-based dissolved pulp production process.
1. INTRODUCTION The forest biorefinery concept has received great attention recently. The main purpose of a forest biorefinery is to extract various biomaterials from woods and then use them for the production of fuel or biobased materials.13 One potential step toward this forest biorefinery is to apply a prehydrolysis stage for extracting hemicelluloses prior to the pulping of wood chips. The prehydrolysis stage has been reported for different pulping processes in the literature.411 In the prehydrolysis stage, some organics, such as lignin, furfural, and acetic acid, are dissolved or formed in addition to the dissolved hemicelluloses.7 However, the concentration of these organic materials in the prehydrolysis liquor (PHL) is very low (approximately 34%), which makes their separation process challenging. Use of nanofiltration for separating the hemicelluloses from the PHL was studied previously.12 Although nanofiltration is an effective method, it has a number of technical challenges, such as fouling. Solvent extraction is another method used for recovering furfural from wastewater.13 However, because of the dilute nature of the PHL, the solvent recovery process might be prohibitively expensive in some cases. In this project, we explored the potential of separating organics, including hemicelluloses, from the PHL by employing adsorption. Activated carbon has been used extensively as an efficient and versatile adsorbent in many chemical industries.14,15 It has a highly porous structure and a large specific surface area, which makes it a strong candidate for adsorbing various organic and inorganic materials in liquid or gas media.16,17 AC is used for adsorbing lignocelluloses to concentrate dissolved organics. The subsequent utilization of adsorbed organics can be directly carried out on the organics-rich AC under acidic, alkaline, or oxidative conditions, because AC is usually inert under such conditions. The AC can then be recycled and/or reused with a necessary regeneration process. To achieve the selective utilization of r 2011 American Chemical Society
lignocelluloses, these materials should first be adsorbed on the AC selectively. The adsorption capacity of activated carbon depends on its surface area, pore size, and carboxylic group content.18 Although the adsorption of furfural on AC has been reported in the literature,19,20 the AC did not have a considerable adsorption capacity for hemicelluloses.21 Therefore, if used as an absorbent for hemicelluloses, some modifications would be required to improve its adsorption performance. In one study, the oxidation of AC was carried out using nitric acid, which increased its carboxylic group on the surface of AC and improved the adsorption of Hg by 1.48 mg/g.22 In another study, the oxidation of activated carbon using H2O2 improved its adsorption capacity for metal ions dissolved in water by almost 24 times.23 It was also reported that acid modification improved the adsorption capacity of AC for Cr(VI) by approximately 2555%.24 However, the influence of the oxidation of AC on improving the adsorption performance of lignocellulosic materials dissolved in water has not yet been reported. The objectives of this work were to investigate (1) the adsorption performance of lignocellulosic materials dissolved in the prehydrolysis liquor (PHL) of a kraft pulping process on AC under various conditions and (2) the improvement in the capacity of the AC for adsorbing dissolved materials from the PHL. In the present work, industrially produced prehydrolysis liquor was received from a mill. The PHL was first acidified to reduce its lignin content. Then, the PHL was neutralized, and the adsorption of lignocellulosic materials on unmodified and modified AC samples was evaluated under various conditions. The chemical modification of AC was conducted using H2O2 or Received: May 14, 2011 Accepted: September 9, 2011 Revised: August 11, 2011 Published: September 09, 2011 11706
dx.doi.org/10.1021/ie201036q | Ind. Eng. Chem. Res. 2011, 50, 11706–11711
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Characteristics of the PHL before and after Acidification/Neutralization Pretreatment furfural
acetic acid
lignin
hemicelluloses
pH
(g/L)
(g/L)
(g/L)
(g/L)
4.5 7
2.04 2.03
8.25 8.24
26.32 11.78
25.92 23.11
Table 2. Conditions of AC Modification and Characteristics of Modified ACs sample ID
a
concentration chemical
(%)
T
time COOH S 102 a
(°C) (h) (mmol/g)
D
(m2/g)
(nm)
1
0.82
4.49
4.24
2
H2O2
20
30
4
0.85
5.42
4.04
3
H2O2
30
80
2
0.78
5.56
3.84
4
H2SO4
16
40
6
2.54
2.86
5.74
5
H2SO4
6
40
6
1.01
5.11
3.99
S = surface area.
H2SO4 under various conditions. The alterations in the physicochemical properties of the AC were systematically investigated and related to the changes in its adsorption capacity.
2. MATERIALS AND METHODS 2.1. Materials. The prehydrolysis liquor (PHL) was collected from a commercial plant that produces dissolved pulp based on the kraft process. The plant operates in eastern Canada and uses a mixture of maple (70 wt %), poplar (20 wt %), and birch (10 wt %) as raw materials. Activated carbon powder (animal bone charcoal) was obtained from Carvao Ativado do Brasil Ltda. H2O2, H2SO4, NaOH, NaHCO3, and CaO (analytical grade) were purchased from Sigma-Aldrich Co. and used as received. 2.2. Treatment of PHLs. The original pH of the PHL was 4.5. The PHL was first acidified to pH 2 using sulfuric acid to induce lignin precipitation. In a previous study, the acidification of PHL resulted in about 50% lignin removal but marginal hemicellulose removal.11 After acidification and filtration, the PHL was neutralized to pH 7 by addition of calcium oxide, leading to the formation of calcium sulfate, which was removed from the PHL sample by filtration. The results are listed in Table 1. As can be seen, after the acidification, filtration, and neutralization steps, the lignin content of the PHL decreased by 55%, and the hemicellulose content was only slightly lowered (by about 11%), whereas the furfural and acetic acid concentrations were unchanged. The acidification, neutralization, and filtration steps were performed to reduce the lignin content of the PHL so that the adsorption of lignin on the AC could be reduced in the downstream adsorption process. The reduction in lignin content of the PHL could help increase the selectivity of the process toward the adsorption of hemicelluloses on the AC. PHLs with various pH values that had been prepared in this way were kept in a refrigerator prior to further use. 2.3. Modification of AC. Modification of the AC was carried out with H2O2 or H2SO4 under various conditions, as specified in Table 2 (above). The reaction conditions used were adopted from literature results on oxidizing AC using various chemicals.18,2224 The reactions were conducted by adding 2 g of AC to 50 mL of
deionized and distilled water in Erlenmeyer flasks. After the specified chemicals had been added, the flasks were shaken at 150 rpm, but at different temperatures and for different times (Table 2). Afterward, the modified ACs were washed with 500 mL of deionized and distilled water and dried in an oven at 105 °C for 4 h. 2.4. Characterization of AC. Nitrogen adsorption/desorption isotherms were determined for the unmodified and modified ACs using a BELSORP-max instrument (BEL Inc.). About 0.1 g of the ACs was pretreated at 120 °C and 107 Torr overnight for contamination removal. Afterward, the measurement was carried out using nitrogen as the probe at 77 K overnight. The isotherm data were recorded at relative pressures (P/P0) in the range of 1070.99999. The BET (BrunauerEmmettTeller) specific surface area and average pore size of the unmodified and modified ACs were determined from the adsorption isotherms. The carboxylic group contents of unmodified and modified ACs were determined by Boehm’s method.25 This method is based on acid/base titration of carbon acidic or basic centers. In this set of experiments, 50 mL of 0.1 M solution of NaHCO3 was added to 1 g of AC in a 100 mL flask. The flask was sealed and shaken at 150 rpm for 24 h, and the mixture was then filtered. Then, 10 mL of the filtrate was titrated with HCl, and the carboxylic group content was calculated based on the HCl consumption. 2.5. Adsorption of Dissolved Lignocellulosic Materials on ACs. In one set of experiments, 20 mL samples of PHLs having two different pH values were separately added to 1 g of unmodified AC in Erlenmeyer flasks and stirred at 150 rpm for 24 h to investigate the effect of pH on the adsorption performance of hemicelluloses on the AC. In another set of experiments, 20 mL of PHL with a pH of 7 was added to 1 g of unmodified or modified ACs in a 125 mL Erlenmeyer flask and shaken at 150 rpm and room temperature for 24 h. The modified ACs with higher adsorption capacities were selected for further study. To investigate the adsorption isotherms, various amounts (from 20 to 140 g) of PHL having a pH of 7 was added to 1 g of the selected ACs and shaken at 150 rpm and room temperature for 24 h. To investigate the adsorption kinetics, 120 mL of PHL having a pH of 7 was added to 1 g of a selected AC, and shaken at 150 rpm and room temperature for various time intervals. The samples were filtered, and the filtrates were collected for analysis. 2.6. Hemicelluloses Analysis. The concentrations of hemicelluloses in the original PHL and filtrates were determined using an ion chromatography unit equipped with a CarboPac PA1 column (Dionex-300, Dionex Corporation) and a pulsed amperometric detector (PAD). The detailed procedure was published earlier.8,26 To convert oligosaccharide of PHLs to monosaccharide, additional acid hydrolysis of the sample was carried out under the conditions of 4% sulfuric acid at 121 °C in an oil bath (Neslab Instruments, Inc., Portsmouth, NH).26 The PAD settings were E1 = 0.1 V, E2 = 0.6 V, and E3 = 0.8 V. Deionized water was used as the eluant at a 1 mL/min flow rate, 0.2 N NaOH was used as the regeneration agent at a 1 mL/min flow rate, and 0.5 N NaOH as the supporting electrolyte at a 1 mL/min flow rate. The samples were filtered and diluted prior to analysis. 2.7. Lignin Analysis. The lignin contents of the original PHL and processed samples were measured based on the UV/vis spectrometric method at a wavelength of 205 nm (Tappi UM 250).26 2.8. Furfural and Acetic Acid Analyses. A Varian 300 1H NMR spectrometer was employed to determine the concentrations of 11707
dx.doi.org/10.1021/ie201036q |Ind. Eng. Chem. Res. 2011, 50, 11706–11711
Industrial & Engineering Chemistry Research
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furfural and acetic acid.2729 Calibration curves were established for both furfural and acetic acid.
3. RESULTS AND DISCUSSION 3.1. Oxidation of Activated Carbon. The AC modification conditions and characteristics of unmodified and modified ACs are listed in Table 2. As can be seen, modification of the AC with H2O2 increased the total surface area of the AC but did not change the carboxylic group content significantly. It has been proposed that the oxidation of AC under mild conditions creates small pores on the surface of the AC, thereby increasing its surface area.25 However, oxidation under strong conditions causes the small pores to collapse and be converted into larger pores. In other words, the conversion of micropores to mesopores or macropores under strong oxidation conditions leads to a decrease in the surface area of the AC.25 In a previous study, the oxidation of AC under stronger conditions of 30% H2O2 at 25 or 80 °C for 24 h was found to decrease slightly the surface area but increase the carboxylic group content of the AC.30 Included in Table 2 are also the results of modifying the AC using H2SO4 (samples 4 and 5). It is evident that H2SO4 oxidation under strong sulfuric acid conditions (sample 4) significantly increased the amount of carboxylic groups on the AC (2.54 mmol/g) and the average pore size (5.74 nm) but markedly lowered the total surface area (286 m2/g), which is due to the collapse of the small pores under the strong oxidation conditions.31,32 On the other hand, upon H2SO4 modification with 6% sulfuric acid (sample 5), the carboxyl group content was slightly increased, whereas the average pore size was reduced somewhat. The oxidation of AC with a mixture of nitric and sulfuric acids (1:1 v/v) at 22 °C for 4 h was shown to improve the oxygen content of AC by a factor of 4 while reducing the surface area by 10%.33 3.2. Effect of pH on Hemicellulose Adsorption. The adsorptions of hemicelluloses and lignin were marginal, 3 and 20 mg/g, respectively, under acidic conditions (pH 4), whereas the corresponding values were approximately 46 and 100 mg/g under neutral conditions after 1 h of treatment. The adsorptions of glucose and fructose on AC were also reported to be negligible under acidic conditions at 40 °C.21 The reason for these findings is that carboxyl groups are protonated under acidic conditions, which hinders their interactions with dissolved organics in PHL. Because the adsorption of hemicelluloses was higher under neutral conditions, the subsequent analyses were conducted at a pH of 7. The adsorption of acetic acid on the unmodified or modified ACs was marginal (
