Mass balance and elimination mechanism of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during the kraft pulping process.

Papermaking raw materials are usually digested by NaOH and NaS solution. The fate of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) during the kraft pulping process is still poorly known. In this study, a comprehensive investigation was conducted on the variation in PCDD/Fs in the kraft pulping section of 3 modern non-wood pulp mills adopting elemental chlorine-free (ECF) bleaching technology. Similar dioxin homologue profiles, dominated by ClDF and ClDD homologues, were observed in the raw materials, black liquors and brown pulps; and the partitioning behaviors of dioxin congeners between black liquor and brown pulp were found to be partly dependent on their octanol-water partitioning coefficients. Dioxin mass flow analysis indicated that the raw materials contributed more than 95 % to the dioxins entering the pulping section. Approximately 7 - 30 % of the input dioxins were exported by black liquor, and the brown pulp carried 44 - 51 % of the input dioxins into the subsequent bleaching section. The kraft pulping process caused a 40 - 48 % reduction in input dioxins. Alkaline hydrolysis and coupling reaction between dioxins and the aromatic fragments of lignin were proposed as two most possible mechanisms for dioxin elimination. In general, modern pulp mills have actually become industrial plants that eliminate environmental dioxins.