Nature of the Compounds Produced
The chemical bleaching of pulp produces (in addition to bleached pulp) a complex mixture of degradation products of residual lignin and other components such as wood extractives. The degradation products vary greatly in properties such as molecular weight and chemical structure. Material which has a molecular weight >1000 is referred to as high molecular weight material (HMM) while that <1000 is referred to as low molecular weight material (LMM). About 50% of the organic material in the effluent from high substitution (100%) chlorine dioxide bleaching is HMM (Österberg and Lindström, 1985; Pfister and Sjöström, 1978; O'Connor et al., 1993; Dahlman et al., 1993). This compares with 70 - 80% HMM reported for effluents from elemental chlorine bleaching (Lindström and Österberg, 1984; Österberg and Lindström, 1985). The structure of the HMM is not known precisely. This may be inconsequential since its general characteristics are known. A major difference between HMM from chlorine dioxide and elemental chlorine bleaching is the chlorine content. The HMM from chlorine dioxide bleaching contains about one chlorine atom for every one-hundred carbon atoms whereas that from elemental chlorine bleaching contains seven to ten chlorines for every one-hundred carbon atoms, i.e. a factor of about five to ten-fold different ((Lindström and Österberg, 1984; Österberg and Lindström, 1985). The HMM is soluble in water by virtue of the presence of large numbers of hydrophilic functional groups, particularly carboxylic acid and alcohol groups and, it is largely non-aromatic (Lindström and Österberg, 1984; Österberg and Lindström, 1985).
Many individual chlorinated compounds are produced by bleaching with elemental chlorine dioxide and/or elemental chlorine. This mixture of organochlorine material (which is measured as Adsorbable Organic Halogen, AOX) is mostly non-hydrophobic and water soluble. Diercks and Banerjee (1993) measured the octanol-water partition coefficient (Kow) of an AOX mixture to be about 0.03, i.e. the equilibrium AOX concentration in octanol was some 30 times less than that in water. The limited solubility of AOX in octanol implies that AOX (as a sum parameter) will not partition into lipid phases in biota, i.e. it will not bioaccumulate.
Hydrophobic compounds are of most concern because of their ability to bioconcentrate in organisms and, if persistent, their movement through the food chain. Highly chlorinated compounds, such as PCDDs (polychlorinated dibenzo-p-dioxins), PCDFs (polychlorinated dibenzofurans) and polychlorinated phenols, have a greater potential for bioaccumulation and are more persistent than their lesser chlorinated counterparts because these properties are enhanced by the presence of the additional chlorine atoms.
Chlorophenols have been studied in detail, probably because they were originally identified as contributing to the acute toxicity of effluent produced with elemental chlorine bleaching. Low substituted chlorophenols are classified as "non-hydrophobic" because they have low octanol-water partition coefficients (Kows of 2, as for monochlorocatechols). Only the more highly chlorinated species are considered hydrophobic and hence bioaccumulative (log Kows ranging from 4 for trichloroguaiacols to 5 for pentachlorophenol).
Levels of chlorinated resin acids in final biotreated mill effluent from bleaching sequences employing 100% chlorine dioxide substitution have been reported to be < 0.005 to 0.2 g/tonne of pulp produced. Levels of chloroform generated in the bleach plant drop a thousand-fold from about 470 g/tonne at 10% substitution (Stinchfield and Woods, 1994) to 0.4 g/tonne at 100% substitution (Wiesemann, 1994). Chlorate ions are formed during bleaching with chlorine dioxide and can be toxic to certain aquatic plants under conditions of nitrogen limitation. Chlorate can be efficiently removed during biotreatment and levels in effluents from mills with secondary treatment are below detection thresholds (Munro et al., 1990; Pryke et al., 1993).