Mutagenic effects of effluents from chlorine bleaching of pulp

Effluents from the bleaching of kraft pulp were tested for mutagenicity. Samples from different mills in which softwood kraft pulp is bleached in a conventional sequence of stages were spot-tested with the Escherichia coli pol A-/pol A+ system. All samples were nontoxic and therefore no difference could be noted between the repair-proficient and the repair-deficient strain. Also no toxic mutagenic effects were seen in spot tests with Salmonella. In a quantitative test with Salmonella, using the plate incorporation assay, effluents from the chlorination (C) stage induced point mutations of the base-substitution type. No frameshift mutations were noted. In the presence of a metabolizing system the direct mutagenic effect was reduced. This reduction was essentially NADPH-independent. Tests on effluents concentrated by freeze-drying indicated, in addition to the base-substitution effect of the C-stage effluent, the presence of frameshift mutagen(s) in this effluent and a base-substitution effect of the hypochlorite stage effluent. Fractionation of the C-stage effluent showed that the mutagenic components are of low molecular weight, extractable with diethyl ether, and sensitive to alkali treatment. Most of the mutagenicity is found in the neutral ether fraction. Induction of 6-thioguanine-resistant mutants in Chinese hamster V79 cells confirmed the mutagenic properties of the C-stage effluent.