The proportion of high molar mass (M > 1000D*) substances (HMM) in acid and alkaline bleaching effluents from kraft pulping has been studied by means of analytical gel permeation chromatography, GPC. The amount of HMM substances in alkaline effluents was found to be 65 - 75 % and in acid stage effluents 20 % of the UV-absorbing (A280 nm) components. Lignin is the precursor of the HMM substances in the alkaline stage, and lignin and carbohydrates are the precursors of the acid stage HMM substances. For the purpose of characterization the HMM compounds were separated from low molar mass substances by preparative GPC. The IR spectra of both alkaline and acid stage substances in their neutral form had strong bands characteristic of carboxylic salts. There were no aromatic bands in the spectra of softwood HMM substances. In the spectra of hardwood substances a narrow aromatic band of low intensity was found in two alkaline stage HMM samples originating from mills using a high proportion of ClO2 to chlorine. Elemental analysis of the alkaline stage HMM substances showed that the chlorine content was as high as 9.8 % when the ClO2 charge had been small and less than 4 % with a large ClO2 charge. The low methoxyl group content and high oxygen content showed that the substances were largely oxidized. The LC50 test with water flea indicated no acute toxicity.

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