MFO induction in fish by filtrates from chlorine dioxide bleaching of wood pulp

Spent bleaching liquors from pulp bleached with chlorine dioxide were assessed for their potency to induce hepatic mixed function oxygenase enzymes (MFO) in rainbow trout, as indicated by activity of ethoxyresorufin-o-deethyase (EROD). Filtrates were collected from two kraft mills in Central Canada to assess the potency of filtrates from hardwood and softwood bleaching. All mill-scale bleaching filtrates induced MFO activity, and filtrates from softwood pulp bleaching appeared more potent than filtrates from hardwood bleaching. Filtrates from the final bleaching stage were most potent, and filtrates from the first stage were the least potent. In laboratory bench-scale bleaching experiments, pulp from softwood and hardwood kraft mills in Eastern Canada was bleached via an industry-standard 5-stage chlorine dioxide bleaching sequence. The filtrates were collected and used in fish bioassays to assess EROD-inducing potency. Potency of bench-scale filtrates varied depending on wood furnish (i.e. softwood vs. hardwood) and the bleaching stage, with all bench-scale filtrates being much weaker EROD inducers than mill-scale filtrates. Recycled paper mill washwater is a possible source of compounds causing increased potency of the mill-scale filtrates.