Chlorinated Carboxylic Acids in Softwood Kraft Pulp Spent Bleach Liquors K. Llndstrom* and F. Osterberg
Swedish Forest Products Research Laboratory, 114 86 Stockholm, Sweden ~
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Characterization and identification of the chlorinated acidic low molecular matter of kraft pulp mill bleach plant effluents have been carried out. Characterization was performed by ultrafiltration followed by ether extraction and determination of the organically bound chlorine in the various fractions. Identification was performed by capillary gas chromatography-mass spectrometry after isolation and methylation of the acidic components. The chloroacetic acids predominate, but chlorinated acrylic, maleic, and fumaric acids are also found. A new series of compounds, chlorinated 2-oxo-3-pentenoic acids, has been identified. Totally 31 acids have been found and quantified, most of them chlorinated.
Introduction Bleaching of chemically produced pulp to get highquality paper involves the use of chlorine-containing bleaching agents. The bleaching process is generally carried out in consecutive steps in order to effectively eliminate residual lignin remaining after pulping. Alternation of bleaching (oxidation) with alkali extraction provides the most effective and economic way of eliminating the residual lignin. Chlorine (C) and chlorine dioxide (D) meet the requirements of good economy, high selectivity, and rapid reactions, as oxidizing agents in the bleaching process. Alkali extraction (E) is carried out by sodium hydroxide. A typical bleaching sequence is C-ED-E-D although a variety of other sequences are used. The CE stages constitute the prebleaching operation. About 70% of the total organic matter in the total bleaching discharge is derived from these two stages. During the oxidation process not only degradation but also chlorination may occur. The chloro organic matter formed is discharged more or less untreated and may give rise to negative environmental effects. This fact has been comprehensively reviewed in a recent publication where a list of almost 200 compounds, most of them chlorinated, is available (1). However, surprisingly few of these are acidic chlorinated compounds (carboxylic acids) although it has been reported that the major part of the low molecular organically bound chlorine (OgC1) in the ether extract of the effluents is of acidic character (2). Some of these carboxylic acids reported in the literature contain chlorine atoms (3-8),but most of the reported substances are nonchlorinated (9-14). The objective of this work is to supply further information on the chemical composition of the prebleaching effluents and thus provide a good basis for the development of modifications to the industrial process which would reduce environmental pollution. 0013-936X/86/0920-0133$01.50/0
Experimental Section Preparation of Laboratory Spent Bleach Liquors. Softwood kraft pulp (Kappa number = 33.5) was used in the preparation of laboratory spent bleach liquors of the chlorine (C) and extraction (E) stages. Distilled water was used in the preparation of the bleaching liquors. All treatments were carried out in glass vessels. The following were for the C stage: pulp consistency 3.5%; chlorine 6.7% (as active C12); time 60 min; temperature 25 OC; final pH 1.7. The following were for the E stage: pulp consistency 10%; NaOH 2.5%; time 60 min; temperature 60 "C; final pH 10.3. All reagents and materials used were of analytical grade. Separation of Extractable Low Molecular Organically Bound Chlorine (OgC1) in Fractions. One-liter portions of the spent liquors were ultrdiltrated. This was performed by using an Amicon UM-2 separation filter with a relative molecular mass (M,) cutoff at 1000. The pressure used was 400 kPa. When the retentate had decreased to about 40 mL, 100 mL of freshly distilled water was added as washing liquor, and the ultrafiltration was continued until the retentate volume again decreased to 40 mL. The washing step (100 mL) was repeated once. Half of the low molecular mass fraction (M, < 1OOO) was acidified to pH 2 with 5 mol/L sulfuric acid which was slowly added under vigorous stirring. The acidified low molecular mass fraction was extracted with 2 X 200 mL of diethyl ether. The combined organic phases were dried over anhydrous sodium sulfate and reduced to about 40 mL by evaporation in a Kuderna Danish apparatus, then transferred to a 50-mL volumetric flask, and made up to the mark with diethyl ether. Ten milliliters of this solution was removed for OgCl determination. The residue (40 mL) was extracted with 2 X 5 mL of sodium bicarbonate (0.5 mol/L) to isolate the acidic substance. The residual organic phase (-40 mL) was again extracted with 2 X 5 mL of sodium hydroxide (0.5 mol/L). This aqueous phase now contained the phenolic fraction. The remaining organic phase (neutral fraction) was concentrated in a 50-mL pear-shaped flask to 2 mL by distilling the ether through a Vigreux column. The three fractions obtained were taken for OgCl determination. OgCl Determination. The total organically bound chlorine (TOgC1) content of the spent liquors was determined as described elsewhere (15). Aliquots (2 mL) of the three different fractions obtained from the extractable low molecular fraction was applied to filter paper (Whatman's No. 3 chromatography paper, 3.5 X 8 cm) in portions with intermediate drying (16). The
0 1986 American Chemical Society
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, gltonne
OgCI, gltonne
300
200
100
n 10%
Acids
Phenolics
Neutrals
Flgure 2. Distribution of organically bound chlorine (OgCl) in different fractions of the ether extract. [Extraction of the ether is carried out first with sodium bicarbonate (acids) and then sodium hydroxide (phenols). Resldual ether contains neutrals.]
Low molecwlar Low molecular ( ~ 1 0 0 10 ether-extractable Flgure 1. Distribution of organically bound chlorine (OgCl) in different fractions of a total bleach plant effluent. pH is adjusted to 2 prior to extraction with ether. Total
chlorine contents were determined by Schoniger combustion (17). Gas Chromatographic Determination. To 1L of the spent liquor was added 1mL of a solution of bromoacetic acid (Merck-Schuchardt) in ethanol (4mg/mL) as internal standard. The solution was acidified to pH 0.8 with sulfuric acid (5 mol/L). Extraction was performed with 3 X 200 mL of diethyl ether. The combined organic phases were dried over anhydrous sodium sulfate filtered and concentrated by evaporation in a Kuderna Danish apparatus followed by concentration in a pear-shaped flask with a Vigreux column to about 1mL. One millilter of diethyl ether/methanol (5/1) was added, and the extract was methylated with diazomethane (18). The resulting extract was analyzed by gas chromatography-mass spectrometry (GC-MS). GC. A Varian 3700 gas chromatograph was used. The GC conditions were. the following: BDS glass capillary column (32 m X 0.25 mm i.d.); temperature program 3 "C/min from 50 to 220 OC; injector and detector temperature 250 OC; carrier gas helium (0.6 mL/min); split ratio 1:25. MS. A Finnigan mass spectrometer (3200 F) with an on-line computer (6000)was used for electron impact (EI) mass spectra. The E1 mass spectra were recorded every 2 s with a speed of 5 m s / m u up to 400 m u . The electron energy was 70 eV, and the electron multiplier was set at 2.5 kV. Chemical ionization (CI) mass spectra were recorded every second on a Jeol DX 300 mass spectrometer. Methane was used as reagent gas. The electron energy was 70 eV, the electron multiplier 1.6 kV, and the ion acceleration voltage 3 kV.
Results Total Bleach Plant Effluent. Figure 1illustrates the distribution of organically bound chlorine in different 134
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fractions of a total bleach plant effluent sample (sequence: (C90 + D,O)-E-D-E-D). Normally, a conventional bleaching sequence exhibits around 5 kg of TOgCl in the effluents per ton of pulp produced ( I ) , but this value can vary widely depending on local conditions. About 25% (1.2 kg) of this TOgCl appears to be low relative molecular mass material (
