Chlorine in the Bleaching of Pulp and Paper

Abstract

The pulp and paper industry in Canada contributes significant effluent discharges to surface and marine waters. Chlorine and chlorine compounds are used in the bleaching of pulp. A large number of organochlorine chemicals of widely differing properties are, or have been, produced from this bleaching. These range from the highly hydrophobic, persistent and bioaccumulative toxic substances, such as 2,3,7,8-TCDD, to the higher molecular weight, non-hydrophobic material usually characterized by absorbable organic halogen. Chlorine dioxide and chlorine react with lignin by different chemical processes and produce different organochlorine products. Use of molecular chlorine as a bleaching agent results in the formation and release of chlorinated organic compounds, sometimes at concentrations that present an unacceptable risk to the receiving environment and the food chain. Alternatives to the use of molecular chlorine, such as chlorine dioxide, result in reductions in the quantities of organochlorines produced, and the degree of chlorine substitution in the organochlorines formed. This, in turn, leads to reduced persistence, reduced potential for bioaccumulation and food chain transfer, reduced toxicity and reductions in adverse ecological effects. However, sufficient evidence exists from responses observed at non-bleached mills to show that other (probably non-halogenated) compounds are released or formed during the production of pulp and cause responses such as induction of MFOs, changes in hormone levels, and reproductive effects.