Unexpected promotion of PCDD/F formation by enzyme-aided Cl₂ bleaching in non-wood pulp and paper mill

Enzyme-aided Cl₂ bleaching is widely considered as promising replacements for conventional Cl₂ bleaching in wood pulp and paper mills. However, the effects of using enzyme-aided bleaching on the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the non-wood pulp and paper mills are unclear. A field study was performed to investigate PCDD/F formation when enzyme-aided Cl₂ bleaching was used to replace conventional Cl₂ bleaching in non-wood pulp and paper mills. Unexpectedly, the PCDD/F toxic equivalents (TEQs) in solid samples were higher when using enzyme-aided bleaching (0.49-5.4 pg TEQ/g) than that using conventional Cl₂ bleaching (0.15-2.44 pg TEQ/g). Large amounts of octachlorodibenzo-p-dioxin were formed during the enzyme-aided bleaching process. This could have been because enzyme strongly promoted the release of organic molecules bound to lignin and thus accelerated the formation of octachlorodibenzo-p-dioxin through organic molecular precursors. Although enzyme-aided Cl₂ bleaching was previously considered to be efficient for reducing PCDD/F releases and to be the best available technologies and best environmental practices for wood pulp and paper mills, the results obtained in this study suggested the necessity and urgency to evaluate the suitability of enzyme-aided Cl₂ bleaching for non-wood pulp and paper mills that intensively practiced in developing countries.