Characterization of effluent fractions from ClO2 and Cl2 bleaching of unbleached and O2 bleached softwood kraft pulps

Effluents from OC(EO), OD(EO), and D(EO) laboratory bleaching of softwood kraft pulp were characterized by fractionation and analysis of the fractions. Adsorbable organic halide (AOX) and total organic carbon (TOC) were determined for the effluents and effluent fractions from each stage, with the exception of the oxygen stage. Each fraction was characterized in terms of its size, and in terms of its ratio of chlorine to carbon an environmentally significant parameter. The fractionation consisted of ether extraction followed by separation of the extract into acidic, phenolic, and neutral subfractions. Replacement of chlorine with chlorine dioxide after oxygen bleaching sharply reduced the AOX, TOC, and chlorine to carbon ratio (expressed as the number of chlorine atoms per 100 carbon atoms, Cl/C100) of the whole effluents. Larger AOX and TOC reductions were seen in the ether soluble fraction and in the phenolic subfraction of the ether soluble material. Since both Cl/C 100 and the size of the ether soluble fractions may be expected to correlate with the potential of an effluent for negative environmental effects, these observations show that replacement of chlorine by chlorine dioxide may be more beneficial than the resulting reductions in whole effluent AOX would suggest. Oxygen delignification prior to a 100% chlorine dioxide stage reduced whole effluent AOX and TOC in rough proportion to the amount of lignin removed in the oxygen stage, but did not affect the overall Cl/C100 . The reductions in AOX and TOC in the environmentally significant ether soluble neutral and phenolic fractions were slightly larger. There was also a significant reduction in Cl/C100 in these fractions. Oxygen delignification, like chlorine dioxide INTRODUCTION During the past decade, the possibility that pulp bleaching effluents may harm the environment has become a major issue with environmentalists and the general public alike. Total chlorinated organic material (measured as AOX) is currently of considerable interest. In fact, environmental pressures have led to either proposed or implemented AOX limits in several European countries and Canadian provinces (1, 2). Similar limits are likely in the U. S. AOX regulation, together with the consumer's desire to buy "environmentally friendly" products, has led pulp mills to implement AOX reduction strategies. The AOX in pulp bleaching effluents may be decreased by one or more of the following process changes: improved brownstock washing (3-5), extended kraft delignification (6-8), 02 delignification (3, 6, 9, 10), substitution of chlorine dioxide for chlorine (6, 9, 11-16), and oxidative caustic extraction (11, 17-19). Although process changes can reduce the AOX, their effect on the environment remains uncertain since AOX reduction does not necessarily imply environmental impact reduction (20, 21). Because much of the effluent AOX is believed to be innocuous and only a small fraction potentially harmful, reducing the overall AOX may or may not reduce its environmental effects. The small, potentially harmful fraction is of low molecular weight and consists of hundreds or perhaps thousands of compounds, including certain environmentally troublesome ones. Such compounds or groups of them have been isolated and identified in pulp bleaching effluents. These include chlorophenolics (12, 16, 22-28), chlorinated dioxins (12, 25, 29), chlorinated neutral compounds (28, 30, 31), chlorinated carboxylic acids (28, 32) and chloroform (23, 33). The complexity of pulp bleaching effluents complicates the task of developing a bleaching process that eliminates the potential for harmful effects on the environment. In an ideal world, a full chemical characterization would be done on effluents from a variety of process alternatives, and data on the environmenT. A. Schwantes Graduate Student Institute of Paper Science and Technology Atlanta, GA tal effects of each component would be available. It would then be a simple task to choose the alternative that results in minimum environmental effect Effluents are, however, sufficiently complex to defy full chemical characterization. For example, the toxicity of effluents can only be partially accounted for by identified components (24). A practical alternative to the impossible ideal of complete analysis, is fractionation of the effluents from candidate bleaching processes and characterization of the fractions in terms that will allow prediction of environmental effects. Such an approach was adopted in the present study. Fractionation was conducted on the basis of ether solubility, volatility and acidity, and the fractions were characterized in terms of their relative amounts and chlorine to carbon ratios. Ether solubility implies low molecular weight (34, 35) and low molecular weight material may be correlated with acute and chronic toxicity (21, 36). The ether extract contains most of the effluent's mutagenicity (30, 37, 38) and toxicity (34). For example, Dence and co-workers (34) found that 92% of the C stage toxicity and 75% of the E stage toxicity resides in the corresponding ether extracts. Furthermore, the toxicities of the extracts were concentrated in the phenolic and neutral subfractions. The amounts of these subfractions therefore assume corresponding significance. The chlorine to carbon ratio may be used as a predictor of toxicity (24, 39) and lipophilicity (40). EXPERIMENTAL APPROACH