Degradation of Chlorinated Compounds
Transformation of chemicals is a common occurrence in the environment. Chlorinated veratroles, for example, are not products of pulp bleaching but are biotransformation products of chlorinated catechols and guaiacols (Figure 3). Chlorinated veratroles are more lipophilic and stable than the corresponding catechols and guaiacols and therefore have a greater potential for bioaccumulation (Neilson et al., 1984). As veratroles are produced from chlorinated guaiacols and catechols, reductions in levels of catechols and guaiacols released into the effluent will correspondingly reduce the levels of veratroles formed. An important benefit from a shift to 100% chlorine dioxide bleaching is that formation of highly chlorinated veratroles will be reduced or eliminated (and indeed have not been found (Pryke et al., 1994). Any veratroles that do form will contain only one or two chlorine atoms and these (and their parent phenols) are less lipophilic and more biodegradable.
Another important factor must be considered. Although most organochlorines of environmental concern have been produced by chemical synthesis or during processes such as combustion, it has been known for some decades that organochlorines are present in many organisms and play useful biochemical roles (Gribble, 1992; Engvild, 1986; Sulda and De Bernardis, 1973; Fowden, 1968). There is a large literature on the microbial degradation of organochlorines, even the relatively recalcitrant volatile organochlorines and the PCBs. There is indisputable evidence that organochlorines are produced and destroyed naturally and that, even in pristine environments, low levels of organochlorines will be present (Gribble, 1992 and Engvild, 1986) -- presumably at concentrations below the thresholds of ecotoxicological significance.