ES&T
CRITICALREVIEW Spent liquors from pulp bleaching Present knowledge of the chemical composition of these wastes is discussed, with emphasis on toxic compounds
Knut P. Kringstad Krister Lindström Swedish Forest Products Research Laboratory Box 5604, S-114 86, Stockholm, Sweden Many halogenated organic com pounds are toxic and may show con siderable resistance to biological and chemical degradation. Thus, the growing concern about the release of these materials into the environment is well understood. The industry that manufactures bleached chemical pulp consumes considerable quantities of chlorine and discharges large quantities of chlori nated organic matter into rivers, lakes, and oceans. Improved knowledge of its chemical composition and biological effects would facilitate a meaningful assessment of any risks involved in releasing such material into receiving waters. This discussion will focus on the composition and sources of pulp bleaching wastes, with special em phasis on kraft pulp bleaching and on
toxic compounds. Some alternative bleaching procedures, perhaps with less severe environmental effects, will be considered briefly. The composition of wood Wood is by far the most important raw material for the production of chemical pulp. Its main component groups are cellulose, hemicelluloses, lignin, and extractives (1). Cellulose (Figure 1) is a linear polysaccharide consisting of β-D-glucopyranose units, which are linked by (l-4)-glucosidic bonds. Wood cellu lose in its native state is composed of at least 10 000 anhydro glucose units. Cellulose molecules are bundled to gether in wood to form microfibrils. These in turn build up fibrils and, fi nally, cellulose fibers. About 40% of most wood is cellulose that has a mo lecular weight in excess of 10 000. Wood hemicelluloses are composed of different carbohydrate units. Unlike cellulose, hemicelluloses are branched to various extents; their relative mo lecular masses, with a degree of poly merization on the order of 200, are
much lower. Also, the content and types of hemicellulose in softwoods differ considerably from those in hardwoods. In the former, galactoglucomannans (15-20%), arabinoglucuronoxylan (5-10%) and arabinogalactan (2-3%) are the most com mon. In the latter, glucuronoxylan (20-30%) and glucomannan (1-5%) are the most important hemicellu loses. Lignin is essentially an aromatic polymer. It is formed in wood by an enzyme-initiated dehydrogenative polymerization of a mixture of three different 4-hydroxyarylpropenyl al cohols (Figure 2). The proportions of these alcohols vary with different wood species. Thus, softwood lignin is largely a polymerization product of coniferyl alcohol (Figure 2, II). The lower portion of Figure 2 shows a summary of prominent structures in such lignin, as suggested by Adler (2). The relative molecular mass of native lignin is considered infinite. The scheme shown in Figure 2 con tains only 16 monomeric units and does not set forth all of the known
FIGURE 1
The linear structure of cellulose
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Environ. Sci. Technol., Vol. 18, No. 8, 1984
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FIGURE 2
Lignin precursors . . .
. . . and prominent structures in softwood lignin
Source: Reference 2
structural details. It does show, however, that softwood lignin is a branched molecule in which the phenylpropane-based units are linked by different types of bonds. These include ether bonds of alkyl-aryl, alkyl-alkyl, and aryl-aryl configurations. Various types of carbon-carbon bonds are also
found. The aromatic content, expressed as monomeric phenol, is approximately 51%. Figure 2 also shows characteristic functional groups such as primary and secondary hydroxyl, etherified, free phenolic hydroxyl, and carbonyl groups of various types. In wood, lignin is probably chemi-
cally linked to hemicelluloses. In hardwoods, lignin is formed by copolymerization of coniferyl (Figure 2, II) and sinapyl (Figure 2, III) alcohols. The ratio between the two may vary from 4:1 to 1:2. Figure 3 shows the cross section of softwood fibers photographed by UV Environ. Sci. Technol., Vol. 18, No. 8, 1984
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radiation at a wavelength of 240 nm. Absorption takes place only in the lignin part of the fiber material (3). The densitometer curve taken across the two fiber walls and the middle lamella between them shows that lig nin is present in the fiber walls, as well as in the middle lamella. Lignin per forms such functions as imparting ri gidity to the fiber walls, and acting as a bonding agent between the fibers. The term "extractives" is normally used for those components of wood that can be extracted by organic sol vents such as ethanol, acetone, or dichloromethane. They include a variety of compounds that may be subdivided into aliphatic extractives, consisting nearly of fats and waxes; terpenoid compounds—present only in soft woods—to which class a number of mono-, sesqui-, and diterpenes, as well as various resin acids belong; and fi nally, phenolic extractives, which in clude hydrolyzable tannins, flavonoids, lignans, stilbenes, and tropolines (4). Examples of extractives are shown in Figure 4. The total content of extrac tives in wood varies greatly (1.5-5%), depending on the species, place of growth, and the age of the tree. Even within the same stem, the content of extractives may vary considerably. Principles of pulping Most chemical pulping is carried out either according to the kraft (sulfate) process or the sulfite process (5). The purposes of pulping are to remove lig nin in order to facilitate fiber separa tion and to improve the papermaking properties of the fibers. The kraft process (Figure 5) is presently the most important by far. It entails treating wood chips at 160180 °C with a liquor that contains so dium hydroxide and sodium sulfide, which promote cleavage of the various ether bonds in the lignin. The lignin degradation products so formed dis solve in the alkaline pulping liquor. Depending on pulping conditions, as much as 90-95% of the lignin is re moved from wood at this stage. In addition, portions of the wood polysaccharides, especially those of the hemicelluloses, are dissolved during the pulping operation. That is because cellulose and hemicelluloses are sen sitive to alkali through their aldehydic end groups, and will thus degrade by a mechanism known as the peeling re action (5). As a result, isosaccharinic acid and a number of other organic acids are formed and dissolved in the pulping liquor. The peeling reaction is ended by the so-called stopping reac tion, in which the end group rearranges 238A
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FIGURE 3
Cross section of black spruce earlywood fibers8
a
Photographed in 240-nm-wavelength light Source: Reference 3
FIGURE 4
Examples of wood extractives.
Acids Saturated Laurie acid Myristic Palmitic Stearic Arachic Behenic Lignoceric
Formula C,,H23COOH C13H27COOH C15H3,COOH C17H36COOH CjH^COOH C21H43COOH
C23H47COOH
Unsaturated Palrnitolenic Oleic Linolic Linolenic Eleostearic Pinolenic
C15H29COOH C,7H33COOH C17H31COOH 017Η29ΟΟΟΗ C17H29COOH C17H29COOH
monoterpenes. . .
α-pinene
0-piriene
A3-carene
. . . and resin acids
COOH Pimario acid Source: Reference 4
COOH Abietic acid
COOH Dehydroabietic acid
FIGURE 5
Kraft pulp mill, including a conventional bleach plant White liquor
Water
Knotter Wood
Barking drum
Chipper To recovery and tall oil production
Filter Screening
Chip pile Cooking Black liquor
Unbleached pulp -*"
Bleaching
Pulp chest
Filter
Bleached pulp
ClJ
Chlorine dioxide tower D2 stage
Alkali tower E2 stage
Chlorine dioxide tower D, stage
Alkali tower E, stage
Hypochlorite tower . H stage
Cl2 tower C-, stage
Bleachery effluent Source: Reference 5
to an alkali-stable m-saccharinic end group. Wood extractives are also dissolved or dispersed in the kraft pulping liquor. If softwood is the raw material, the extractives are to a large degree recovered as important by-products, such as sulfate turpentine and tall oil. Turpentine contains a mixture of the lower terpenes, whereas a raw tall oil consists mainly of fatty and resin acids. The content of residual extractives in unbleached kraft pulp is low. In a kraft process for bleached pulp, somewhat more than 55% of the total weight of the wood is dissolved in the pulping liquor. After separation from the pulp, the spent liquor is evaporated to a high concentration and then burned to recover energy and inorganic chemicals. By comparison, the sulfite process solubilizes lignin through sulfonation at elevated temperatures. The pulping liquor contains sulfur dioxide and an alkaline oxide such as that of sodium, magnesium, or calcium. Principles of bleaching Neither the kraft nor the sulfite process removes all lignin. About
TABLE 1
Yield loss in kg/tonne of pulp in the bleaching of various pulp types Source of yield toss
Lignin Polysaccharides Extractives Total
Krall pulp
50 19 1 70
Sulfite pulp Paper Rayon
20 22 3 45
35 12 8
55
12 70 8 90
Source: Reference 6
5-10% of the original lignin remains in the pulp, since it cannot be removed by extended pulping without seriously degrading the polysaccharide fraction. Removal of the residual lignin, which is responsible for the dark color of kraft pulps, calls for a multistage bleaching process (5). For softwood kraft pulps (Figure 5), bleaching is normally accomplished by successive treatments with chlorine ( C i ) , alkali (Ei), chlorine dioxide (Di), alkali (E2) and chlorine dioxide (D2). Often, a hypochlorite stage (H) may be inserted between the Ei and D[ stages. The largest quantity of material is
dissolved from the pulp during the C and Ei stages. The C stage consists of a treatment of a slurry of the pulp with elemental chlorine. Normally, the slurry's consistency is 3%; the charge of chlorine (depending upon the content of the residual lignin) is about 60-70 kg/tonne (metric ton) of pulp; the temperature is 15-30 °C; and the final pH is 1.5-2.0. The subsequent Ei stage is an extraction of the chlorinated pulp with alkali. Here, the consistency is usually somewhat more than 10%; the charge of alkali is about 35-40 kg/tonne of pulp; the temperature is 55-70 °C; and Environ. Sci. Technol., V o l . 18. No. 8, 1984
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FIGURE 6
Reactions between residual lignin in pulp and chlorine
Polymeric material
cally bound chlorine, the relative mo lecular mass distribution, and the sol ubility properties of the organic ma terial are important. Available figures on the content of organically bound chlorine vary to some degree (9, 12-16), but in the bleaching of a softwood kraft pulp would normally correspond to about
Estimating organic chlorine discharges World production of bleached chemi cal pulp is presently on the order of 50 million tonnes/y. The discharge of organically bound chlorine could be estimated at 250 000 tonnes/y, if one assumes that all the pulp produced is bleached by conventional means and that the total quantity of organically bound chlorine formed (including final bleaching) is 5 kg/tonne pulp. The actual discharge is probably somewhat lower, since variations in factors such as pulp types, bleaching sequences, and external treatments are not taken into account. Source: Reference 7
the final pH is about 11. The C and Ej stages constitute the prebleaching part, and the successive stages make up the final bleaching part of the bleaching process. During the conventional bleaching of a softwood kraft pulp, perhaps 70 kg of material for each tonne of pulp will dissolve from the pulp into the bleaching liquors (Table 1). About 50 kg of this material originates from the residual lignin; about 19 kg comes from the polysaccharide fraction; and approximately 1 kg is from the ex tractives portion of the pulp (6). Most of this material—75%—is dissolved during the C and E. stages. When chlorine is dissolved in water, the following equilibria form: Cl 2 + H 2 0 H+ + C 1 - + HOC1 HOC1 ^ H+ + O C 1 -
(1) (2)
At 25 °C, the equilibrium constants are K, = 3.0 X 10~ 4 and K 2 = 2.9 X ΙΟ" 8 . Chlorine may react in two different ways with organic material. In one case, it reacts as a molecular species, and in the other, it acts after decom position into a radical. The latter mechanism is believed to be important in reactions with carbohydrates. However, during pulp chlorination, chlorine reacts primarily with residual 240A
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lignin. In these reactions, it acts in its molecular form. As summarized in Figure 6, oxida tion and substitution by chlorine and the addition of hydrogen chloride are important reactions in this stage (7). These reactions lead to a substantial depolymerization of the lignin, as well as to the introduction of chlorine and various acidic groups into its structure. As a result, part of the residual lignin will dissolve in the chlorination li quor. The reactions in the alkali extrac tion stage Ei are less understood. Apart from ionizing the acidic groups formed during the C stage, which fa cilitates the solution of the chlorinated lignin, treatment with alkali will cause a substantial loss of the organically bound chlorine introduced during chlorination (8-11). A marked de crease in the content of carbonyl groups also has been observed (8). Spent liquor composition In view of this great variety of re actions, one readily perceives the ex treme complexity of the organic chemical composition of spent chlo rination and alkali extraction liquors. From an environmental standpoint, it is of interest to ascertain the nature and content of as many of the com pounds present as possible. Moreover, data regarding the content of organi
4 kg/tonne of pulp for the two types of spent liquor. Organically bound chlorine is present in a wide range of organic material found in spent liquors. This may be seen by dividing the organic material into fractions of different relative molecular mass and then de termining the organically bound chlorine in each of these fractions (12-15). Figure 7 shows results from the most recent investigations in which such determinations were made. It appears that in spent chlorination li quor, about 70% of the organically bound chlorine is present as high-rel ative-molecular-mass material (M r > 1000); whereas in alkali extraction li quor, about 95% of the organically bound chlorine belongs to this class. It should be emphasized that these values have been obtained through ultrafil tration of the spent liquor, using one special type of membrane filter (40). High-molecular-mass materials The structures of high-relativemolecular-mass materials are poorly defined at present. Determination of the elemental composition has given C9H904C1, C10H14O7Cl and C9H10O8CI (17-19) as representative formulas for high-relative-molecu lar-mass compounds in spent chlori nation liquor. For corresponding ma-
FIGURE 7
Distribution of organically bound chlorine8"
Spent chlorination liquor
Spent alkali extraction liquor
"(In percent of total organically bound chlorine) in fractions of various relative molecular mass of spent chlorination and alkali extraction liquors from the bleaching of softwood kratt pulp ^Determined by ultrafiltration Source: Reference 40
terial present in alkali extraction li quor, formulas such as C ^ H ^ C ^ O and C14H10O9CI were obtained. These results show that both types of mate rial have high chlorine-to-carbon ra tios. Studies of these materials were carried out by means of oxidative degradation involving a combination of permanganate, periodate, and hy drogen peroxide. Results suggest that the content of aromatic nuclei is sur prisingly low for both types (17, 20, 21). The same conclusions may be drawn from UV studies (12, 13), which indicate that a major part of the material consists of cross-linked, probably unsaturated aliphatic com pounds. Moreover, the oxidative deg radation experiments also show that such aromatic nuclei as are present in the material are demethylated to a considerable degree as compared with those of the original lignin, and that the nuclei carry 0-3 chloro substituents in the material from both types of liquor. High-relative-molecular-mass ma terials in spent chlorination and alkali extraction liquors are probably bio logically inactive, because they cannot penetrate cell membranes of living organisms. However, such materials are still of environmental importance, because they carry chromophoric structures that cause bleaching plant
TABLE 2
Some compounds identified in spent chlorination and alkali extraction liquors Formula
HOOC-COOH CI C=C—COOH,
Chlorination Extraction (g/tonne) (g/tonne)
Compound
Oxalic acid
130
590
Reference
15, 22
Trichloropropenoic acid
—
+ 2 3
Chloro-2-thiophenic acid
—
+ 2 3
a Ci
\3*—COOH OH
;
Jk^ CI (QJ
2,4-Dichlorophenol
0.7
2
Tetrachloroguaiacol
0.1
9
Dichlorovanillin
0.5
1.5
29, 31
C\
OH CI^AJXH, ΊΚΙ/Γ-
29,31,83
CI OH l^OCH,
Ji0)
31
CHO
CHC13
Chloroform
>=
