Mechanistic aspects of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) formation from chlorine bleaching of non-wood pulp.

ABSTRACT

Chlorine bleaching of non-wood pulp can produce and release polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) into environment. In this study, a series of chlorination experiments were conducted to explore the source, formation pathway and formation kinetics of Cl1-8DD/Fs during the chlorine bleaching of wheat straw pulp. The 13C isotope-labeling experiment verified that PCDD/Fs could not be formed from the chlorination of phenols in pulp. Above 80% of the observed PCDFs originated from the direct chlorination of dibenzofuran (DBF) in unbleached wheat straw pulp, while approximately 60% of the observed PCDDs came from the chlorination of nonextractable precursors in the pulp. The yield of total PCDFs was 2 orders of magnitude higher than that of total PCDDs. The kinetic study indicated that DBF was more easily chlorinated than dibenzo-p-dioxin (DBD) in the pulp solution. Pseudo-first-order reaction kinetic equations fitted the formation and degradation of PCDD/Fs well. The direct chlorination of DBD/F primarily preferred a selective pattern at the 2 position, followed by the 8, 7 and 3 positions. A higher content of DBF in unbleached pulp led to a higher yield of 2,3,7,8-Cl4DF. Finally, the practical implication for dioxin formation control was discussed.