Journal of Chromatography A, 1192 (2008) 208–211
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Determination of oxalate in black liquor by headspace gas chromatography Hailong Li a,b , Xin-Sheng Chai a,b,∗ , Nikolai DeMartini b , Huaiyu Zhan a , Shiyu Fu a a b
State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China Institute of Paper Science and Technology, Georgia Institute of Technology, Atlanta, GA 30332, USA
a r t i c l e
i n f o
Article history: Received 3 December 2007 Received in revised form 19 March 2008 Accepted 20 March 2008 Available online 28 March 2008 Keywords: Headspace Gas chromatography Oxalate Manganese dioxide Carbon dioxide
a b s t r a c t This study demonstrated a headspace gas chromatographic method (HS-GC) for the determination of oxalate content in black liquor (alkaline aqueous solution of inorganic chemicals and dissolved wood species from the alkaline pulping of wood). The method described in this paper is based on the reaction between oxalic and manganese dioxide in an acidic medium, in which oxalic acid is converted to carbon dioxide that is measured with a GC using a thermal conductivity detector. The challenge in developing this method was ensuring complete conversion of oxalic acid while minimizing the contribution of side reactions between carbohydrates, lignin and manganese dioxide to the carbon dioxide measured. It was found that a complete conversion of oxalate to carbon dioxide can be achieved within 3 min at a temperature of 70 ◦ C; a MnO2 :Coxalate (concentration of H2 C2 O4 + HC2 O4 − + C2 O4 2− ) mole ratio of 60 and H2 SO4 concentration of 0.005–0.01 mol/L in the headspace vial. The method can detect concentrations as low as 0.39 g of oxalate. The standard deviation was found to be 7% while recovery experiments with black liquor showed recoveries of 93–108% which were deemed acceptable for analysis of oxalate in an industrial sample such as black liquor. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Black liquor is an aqueous solution of dissolved salts and organic compounds, produced from the chemical pulping of wood with alkaline solutions, most typically NaOH and Na2 S (kraft pulping). The dry solids (d.s.) are a mixture of lignin (∼25–30%, w/w, d.s.), aliphatic carboxylic acids (∼30%, w/w, d.s.) extractives (3–4%, w/w, d.s.), polysaccharides (2–7%, w/w) and inorganics (∼30–35%, w/w, d.s.) . The inorganics are primarily sodium, potassium, sulﬁde, hydroxide, carbonate, and sulfate from the pulping liquor and trace metals from the wood and mill chemical make-up. The oxalate concentration in black liquors from 19 North American Pulp mills was found to vary from 0.2 to 1.3% (w/w) d.s. . Krasowski and Marton  quantiﬁed the oxalic acid in bark and wood from both hardwood species and pine trees. They found the concentration of oxalic acid to be 0.1–0.3 and 0.1–0.4 kg/metric tonne in hardwood and pine trees, respectively. In the same work, the concentration in bark was found to be 9–15 and 4–10 kg/metric tonne in hardwood and pine bark, respectively. Alkali degradation of hydrocellulose will form oxalate . Oxalate is also formed during oxygen deligniﬁcation and bleaching stages due to the oxidation
∗ Corresponding author at: Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street, NW, Atlanta, GA 30332, USA. Tel.: +1 404 894 9992; fax: +1 404 894 4778. E-mail address: [email protected]
(X.-S. Chai). 0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2008.03.066
of lignin and hexenuronic acid [3,5–8]. If ﬁltrates from oxygen deligniﬁcation or bleaching are brought into the black liquor evaporators they can contribute to the oxalate in the black liquor . Sodium salts of the Na–CO3 –SO4 system and calcium carbonate are the primary sources causes of evaporator fouling [2,10], while oxalate is often a minor constituent found in scales and attributable to the entrained black liquor . However, sodium oxalate has been the primary precipitated salt in plugging reported for two mills [9,12]; the primary source of scale in a third mill  and a contributing salt in a layered scale in a high solids tank . Oxalate has also been shown to increase the rate of calcium scaling when added as either calcium oxalate or sodium oxalate . The quantiﬁcation of oxalate in black liquor is of therefore of value both industrially and academically. A redox titration method using permanganate is traditionally used for oxalate analysis. However, due to its high oxidation potential, especially in a strong acidic medium, permanganate can also react with lignin and carbohydrates typically found in black liquor. Thus, a titration method fails to quantify oxalate in mill samples. Capillary ion analysis and ion chromatographic methods have been used for the determination of many anions, including oxalate in various mill streams in the paper industry [14–17]. The ions can be separated from each other in a column and then measured by UV or electrolytic conductivity detector. The major limitation of these methods is that the repeatability in the measurement is poor: with errors of 10–30% for the inorganic anions and 15% for the organic acids (including oxalate) being observed . Kakola
H. Li et al. / J. Chromatogr. A 1192 (2008) 208–211
et al.  also reported a method for quantitative determination of the main aliphatic carboxylic acids in wood kraft black liquors by high-performance liquid chromatography with atmosphericpressure chemical ionization mass spectrometry. The headspace method presented here offers similar simplicity and rapid determination, and was chosen for its availability in our lab. Recently, we reported a novel HS-GC technique to quantify the oxalate in the oxygen deligniﬁcation liquor . It is based on a headspace GC measurement using thermal conductivity detection (TCD) of the carbon dioxide released from the reaction between oxalate and permanganate. A multiple headspace extraction technique makes it possible to conduct a kinetic measurement of carbon dioxide formation in the reaction system (headspace sample vial). Thus, a carbon dioxide formation proﬁle can be obtained, in which the amount of carbon dioxide converted from the fast reaction, i.e., between oxalate and permanganate, can be identiﬁed. The method, however, cannot be simply applied to the liquors containing a signiﬁcant amount of carbohydrates (sugars) and lignins, e.g., black liquor, because the reaction between these species and permanganate at the given conditions are also signiﬁcant. In this work, we developed an alternative HS-GC method, focusing on the derivatization reaction technique, for the determination of oxalate in black liquor. A moderate oxidation reagent, manganese dioxide, was selected and the aim was to ﬁnd a proper reaction condition that could minimize the side reaction between manganese dioxide and lignin and carbohydrate species in the black liquor.
2.3.2. HS-GC measurement A 1-mL volume of sulfuric acid solution (2 mol/L) and 0.03–0.05 g of manganese dioxide solid were added in a headspace test vial and the vial was sealed with a septum. A 500 L volume of the above ﬁltrate was injected into the sealed vial by a microsyringe. The vial was then immediately put into headspace sampler for the measurement. The manganese dioxide solid must be fully covered by the solution. Otherwise, the dust created by the headspace pressurization will accumulated during each sampling, and eventually cause a clogging problem in the headspace sampling channels. Agitation in the GC sampler was used to maximize mixing in the vial during the reaction before the CO2 concentration was measured.
3.1.1. Dosage of manganese dioxide An excess amount of manganese dioxide is necessary for complete conversion of oxalate to carbon dioxide via reaction (1). The results illustrated in Fig. 1 show that a molar ratio of 60-mole manganese dioxide to oxalate is required to achieve a complete reaction within 3 min.
2.1. Chemicals All chemicals used in the experiment were from commercial sources. The standard oxalate solution (0.200 mol/L) was prepared using sodium oxalate. A black liquor from a North American kraft pulp mill was used in the study. 2.2. Apparatus and operation All measurements were carried out using an HP-7694 Automatic Headspace Sampler and Model HP-6890 capillary gas chromatograph (Hewlett–Packard, now Agilent Technologies, Palo Alto, CA, USA) using a thermal conductivity detector. GC conditions were as follows: capillary column with an i.d. = 0.53 mm and a length of 30 m (model GS-Q, J&W Scientiﬁc, Folsom, CA, USA) at 30 ◦ C; carrier gas helium ﬂow rate of 3.1 mL/min. The temperature for TCD operation is 220 ◦ C. Headspace sampler operating conditions were as follows: oven temperature of 70 ◦ C; vial pressurized by helium and pressurization time of 0.2 min; sample-loop ﬁll time of 0.2 min; loop equilibration time of 0.05 min; loop ﬁll time of 0.2 min; the vial equilibration time was 3 min.
3. Results and discussion 3.1. Effects of reaction conditions on the conversion of oxalic acid The present method is based on measuring headspace carbon dioxide that is quantitatively converted from oxalic acid reacting with manganese dioxide , as described by H2 C2 O4 + MnO2 + 2H+ → 2CO2 + Mn2+ + 2H2 O
The reaction conditions to achieve a complete oxalate conversion are discussed below. Variables studied were: dosage of manganese dioxide, reaction temperature, reaction time, the concentration of sulfuric acid and interference other inorganic and organic species.
3.1.2. Temperature Fig. 2 shows the GC signal for CO2 after a 3 min reaction time between an acidic solution of oxalic acid and manganese dioxide at temperatures between 30 and 70 ◦ C. A high conversion (>80%) took place at a temperature of 40 ◦ C, and complete conversion was obtained at the temperature above 60 ◦ C. In this work, we chose 70 ◦ C as the reaction temperature, to ensure complete reaction within 3 min. Higher temperatures will lead to a higher water vapor pressure, which not only may deteriorate the separation perfor-
2.3. Sample preparation and measurement procedures 2.3.1. Sample preparation A 5-mL volume of black liquor (15%, w/w, d.s.) and 5 mL of hydrochloric acid solution (2 mol/L) were added into a beaker, and mixed using a magnetic stir bar. If the solid of the sample is lower, a larger sample may be necessary or if the solids are higher, a smaller sample size may be acceptable as the amount of oxalate is relative to the solids content in black liquor. The amount of hydrochloric acid used is sufﬁcient to bring the pH of the resulting solution down to below 3, to ensure complete dissolution of oxalate from the insoluble oxalate salts, remove carbonate, and precipitate lignin from the black liquor. The ﬁltrate of the resulting solution was collected for HS-GC testing.
Fig. 1. Effect of manganese dioxide dosage.
H. Li et al. / J. Chromatogr. A 1192 (2008) 208–211
Fig. 2. Effect of temperature.
mance of the GC column, but also create a corrosion problem for the sampling channel when a strong acidic medium is used . 3.1.3. Reaction time Fig. 3 shows the time effect on the extent of the reaction at 70 ◦ C and a mole ratio of manganese dioxide to oxalic acid of 60. A minimum of 3 min is required for complete conversion. In the present work, the sample vial was strongly agitated in the headspace sampler during the reaction to increase the sample contact with manganese dioxide particles. 3.1.4. Concentration of sulfuric acid According to Eq. (1), hydrogen ions are required in the reaction. A minimum of 5 mmol/L was found to provide complete reaction in 3 min at 70 ◦ C as shown in Fig. 4. There is a practical upper limit, as discussed below, to the concentration of acid as a higher concentration can lead to oxidation of organic species that the release of CO2 , thus interfering with the quantiﬁcation of oxalic acid. 3.2. Interferences 3.2.1. Inorganic species The present method is based on measuring carbon dioxide released from the reaction between oxalic acid and manganese dioxide. Carbonate in the sample can affect the oxalic acid quan-
Fig. 4. Effect of sulfuric acid concentration.
tiﬁcation because it can rapidly be converted to carbon dioxide in an acidic medium. Thus, acidiﬁcation of the sample prior to the addition of manganese dioxide is necessary to remove carbonate. The carbon dioxide in air involved in the headspace sample vial has a minor effect on oxalate determination and it can be easily corrected for using a blank sample. 3.2.2. Organic species In black liquor, there are many organic species such as dissolved alkali lignin, carbohydrates and the volatile organic compounds (mainly methanol). Table 1 shows that carbohydrates and dissolved alkali lignin have the potential effect to the present method if the concentration of acid is too high or the reaction time is too long. However, it became non-detectable (N.D., i.e., within the measurement error margin) when a short reaction time (3 min) and/or lower acidity (10 mmol/L) were selected for the reaction. 3.3. Method precision and validation Five measurements were made of a black liquor sample giving a standard deviation of less than 7%, which is smaller than that in the previous method . By adding different volumes (0–250 L) of a standard oxalic solution (0.0400 mol/L) in a set of headspace sample vials, a standard calibration curve was obtained. The equation for that curve is A = 3.46(±31.4) + 491(±5.95)C
where A and C represent the GC signal count of carbon dioxide and its absolute amount (in mol) in the headspace sample vial, respectively. Table 1 Effect of the interference species on oxalate quantiﬁcation Compounds
Methanol Xylose Glucose Alkaline lignin
Fig. 3. Effect of reaction time.
Concentration of H2 SO4 (mol/L)
Time = 3 min
Time: 8 min
40 84 100
4.00 4.00 4.00
N.D. N.D. N.D.
N.D. Detectable Detectable
2.00 0.20 0.02 0.01
Detectable Detectable Detectable N.D.
Detectable Detectable Detectable Detectable
Reaction conditions: Temp. = 70 ◦ C, MnO2 = 0.04 g; and sample size = 0.1 mL.
H. Li et al. / J. Chromatogr. A 1192 (2008) 208–211 Table 2 Method validation Sample No.
1 2 3 4 5
Oxalate content (g/L) Added
0.35 0.88 1.41 1.75 3.52
0.38 0.82 1.31 1.80 4.02
108 94 93 102 97
this acidiﬁed sample is then injected into a sealed headspace vial that contains manganese dioxide and 5 mM H2 SO4 . At 70 ◦ C, using a 60 mol ratio manganese dioxide to oxalic acid mole ratio and an acid concentration of 5 mmol, a reaction time of 3 min provided complete conversion without interference from side reactions of the organics. The standard deviation of this method was found to be 7%. Acknowledgements
200 L of sample was used in the HS-GC measurement.
The regression coefﬁcient = 0.9992 (n = 7), and the limit of quantiﬁcation in the present method is 0.39 g. To verify the present method, we prepared a set of sample solutions by accurately adding different volume (from 0 to 200 L) of a 0.200 mol/L standard oxalate solution into a 1 mL black liquor sample, in which the content of oxalate covers the range found in the black liquors in pulp mills. The original black liquor was measured as a reference and had an oxalate concentration of 0.33 g/L. Thus, the net contribution from added oxalate in these make-up samples was determined. The absolute amount of oxalate in the sample that placed in the headspace vial can be calculated by Eq. (2). The added oxalate content (Coxalate ) in these make-up black liquor samples can be obtained by Coxalate (g/L) = 88 ×
C ×R Vl
where Vl is the volume of the ﬁltrate added into the vial, and R is a sample dilution ratio. Table 2, shows a comparison between the added amounts of oxalate in the black liquors and measured by the present method. Because the recovery of the present method is in an acceptable range (93–108%) for industrial sample analysis, we conclude that the present technique is a viable method for quantifying of oxalate in mill samples. The recovery is within the 7% standard deviation except for at the lowest concentration. 4. Conclusions We have developed a HS-GC method that can be applied to the determination of oxalate content in black liquors. The lower limit for this method was found to be 0.39 g of oxalate. Pre-acidiﬁcation of the black liquor sample (∼15%, w/w, d.s.) converts oxalate to HC2 O4 − , thereby dissolving any oxalate species that may exist. Additionally, pre-acidiﬁcation allows for lignin precipitation and the formation of carbonic acid and the subsequent release of CO2 originating from the carbonate in black liquor. A 500 L volume of
The China Scholarship Committee, State Education Department, and National Basic Research Program of China (973 Program2007CB 210201) is gratefully acknowledged for helping support H.L. as a visiting researcher at Institute of Paper Science and Technology (IPST), Georgia Institute of Technology, Atlanta, USA. Additionally we would like to thank the consortium of IPST member companies for their support this work within the “Enhanced Chemical Recovery Project” at IPST. References ¨ R. Alen, ´ in: E. Sjostr ¨ om, ¨ ´ (Eds.), Analytical Methods in Wood  K. Niemela, R. Alen Chemistry, Pulping, and Papermaking, Springer, Berlin, 1999, Chapter 7.  W. Schmidt, W.J. Frederick, Chemical Recovery Conference Proceedings, TAPPI Press, Atlanta, GA, 1998, p. 367.  J.A. Krasowski, J. Marton, J. Wood Chem. Technol. 3 (1983) 445.  L. Lowendahl, G. Perterson, O. Samuelson, TAPPI J. 59 (9) (1976) 118.  A. Elsander, M. Ek, G. Gellerstedt, TAPPI J. 83 (2) (2000) 73.  J. Fiskari, J. Gullichsen, Proceedings of the 10th International Symposium on Wood and Pulping Chemistry-10th Biennial, vol. 1, ISWPC, Yokohama, June 7–10, 1999, p. 324. ¨ ˚  P. Ulmgren, R. Radestr om, Nordic Pulp Pap. Res. J. 15 (2000) 128.  N.-O. Nilvebrant, A. Reimann, Proceedings of 4th European Workshop on Lignocellulosics and Pulp, Streza, Italy, 1996, p. 485. ¨ ˚  P. Ulmgren, R. Radestr om, Nordic Pulp Pap. Res. J. 17 (2002) 275.  W.J. Frederick Jr., B. Shi, D.D. Euhus, R.W. Rousseau, TAPPI J. 3 (6) (2004) 7. ¨ Proceedings of the 10th International Symposium on Wood and  K. Niemela, Pulping Chemistry-10th Biennial, vol. 1, ISWPC, Yokohama, June 7–10, 1999, p. 418.  N.A. DeMartini, C.L. Verrill, Presented at the Pulping and Environmental Conference, Philadelphia, PA, August 28–31, 2005.  W.J. Frederick, T.M. Grace, A Study of Evaporator Scaling: Calcium Carbonate Scales, in Proj. 3234 (3), Institute of Paper Chemistry, 1977, 141 pp.  J.P. Romano, D.R. Salomom, Tappi Proceedings 1992 Pulping Conference, 1992, p. 303.  J. Krishnagopalan, M. Hill, A.L. Fricke, TAPPI J. 68 (9) (1985) 108.  D.B. Easty, M.L. Borchardt, A.A. Webb, Paperi ja Puu 67 (9) (1985) 501.  Tappi Test Method, Analysis of Pulping Liquors by Suppressed Ion Chromatography, T 699 om-87, Tappi Press, Atlanta, GA, 1987, 8 pp.  J. Kakola, R. Alen, H. Pakkanen, R. Matilainen, K. Lahti, J. Chromatogr. A 1139 (2007) 263.  X.-S. Chai, J. Samp, H.N. Song, H.X. Zhu, J. Chromatogr. A 1122 (2006) 209.  H.W. Schimpf, Essentials of Volumetric Analysis, Wiley, New York, 1917.  X.-S. Chai, J.C. Samp, J. Chromatogr. A 1157 (2007) 477.